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Autophagy: Cancer’s Friend or Foe?

Sujit K. Bhutia,*,1 Subhadip Mukhopadhyay,* Niharika Sinha,* Durgesh Nandini Das,* Prashanta Kumar Panda,* Samir K. Patra,* Tapas K. Maiti,† Mahitosh Mandal,‡ Paul Dent,§¶|| Xiang-Yang Wang,¶||# Swadesh K. Das,¶# Devanand Sarkar,¶||# and Paul B. Fisher¶||#,1

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Abstract

The functional relevance of autophagy in tumor formation and progression remains controversial. Autophagy can promote tumor suppression during cancer initiation and protect tumors during progression. Autophagy-associated cell death may act as a tumor suppressor, with several autophagy-related genes deleted in cancers. Loss of autophagy induces genomic instability and necrosis with inflammation in mouse tumor models. Conversely, autophagy enhances survival of tumor cells subjected to metabolic stress and may promote metastasis by enhancing tumor cell survival under environmental stress. Unraveling the complex molecular regulation and multiple diverse roles of autophagy is pivotal in guiding development of rational and novel cancer therapies.

1. INTRODUCTION

Stress stimuli, including metabolic stress, activate cellular mechanisms for adaptation that are crucial for cells to either tolerate adverse conditions or to trigger cell suicide mechanisms to eliminate damaged and potentially dangerous cells (Hanahan & Weinberg, 2011). Stress stimulates autophagy, in which double membrane vesicles form and engulf proteins, cytoplasm, protein aggregates, and organelles that are then transported to lysosomes where they are degraded, thereby providing energy (Klionsky & Emr, 2000; Mizushima, Ohsumi, & Yoshimori, 2002). Constitutive, basal autophagy also plays a significant homeostatic function, maintaining protein and organelle quality control and acting simultaneously with the ubiquitin proteasome degradation pathway to prevent the accumulation of polyubiquitinated and aggregated proteins (Klionsky & Emr, 2000). Autophagy-defective mice display signs of energy depletion and reduced amino acid concentrations in plasma and tissues and fail to survive in the neonatal starvation period, providing a clear example of autophagy-mediated maintenance of energy homeostasis (Kuma et al., 2004). Autophagy is also a pathway that is used for the elimination of pathogens (Colombo, 2007) and for the engulfment of apoptotic cells (Qu et al., 2007). Peptides generated from proteins degraded by autophagy can also be used for antigen presentation to T-cells for regulation of immunity and host defense (Crotzer & Blum, 2009; Levine, Mizushima, & Virgin, 2011). The importance of autophagy as a homeostatic and regulatory mechanism is underscored by the association of autophagy defects in the etiology of many diseases, including cancer (Levine & Kroemer, 2008).

Cancer is a multifaceted complex disease characterized by several defining properties, including avoidance of cell death (Hanahan & Weinberg, 2011). The ability of cancer cells to resist apoptotic cell death is a well-known mechanism that is the key to their survival and aggressiveness. Similarly, the phenomenon of autophagy in cancer has been studied extensively, and it is now firmly established that autophagy can provide both tumor-suppressive and tumor-promoting functions (Høyer-Hansen & Jäättelä, 2008; Maiuri et al., 2009). This review focuses on the tumor-suppressive and tumor-promoting properties of autophagy during different stages of cancer development. It provides insights into how autophagy’s tumor-suppressive properties, which are frequently observed at the initial stage of cancer development, are later transformed into tumor-promoting potential during cancer progression.

2. AUTOPHAGY AND AUTOPHAGIC DEATH

Autophagy (from the Greek word “auto,” meaning oneself and “phagy,” meaning to eat) refers to a process by which cytoplasmic constituents are delivered to the lysosome for bulk degradation (Mizushima & Klionsky, 2007; Mizushima et al., 2002). The term autophagy originated when the Nobel laureate Christian de Duve used it while attending the Ciba Foundation Symposium on Lysosomes, which took place in London on February 12–14, 1963. Autophagy is classified into three main types depending on the different pathways in which cargo is delivered to the lysosome or vacuole: chaperone-mediated autophagy, microautophagy, and macroautophagy (Yorimitsu & Klionsky, 2005). In this review, we focus on the most widely investigated autophagic process: macroautophagy (herein, referred to as autophagy) with formation of autophagosomes and autolysosomes. Autophagosomes are double-membrane cytoplasmic vesicles, which engulf various cellular constituents, including cytoplasmic organelles. Autophagosomes fuse to lysosomes to become autolysosomes, where sequestered cellular materials are digested (Mizushima et al., 2002). The molecular basis of autophagy has been extensively studied, mainly in yeasts, through investigation of autophagy-defective mutants to identify the responsible genes (designated as AuTophaGy; atg), and presently 35 atg genes have been discovered in yeast (Nakatogawa, Suzuki, Kamada, & Ohsumi, 2009). The basic mechanism of autophagy is well conserved during evolution as varied organisms, including plants, flies, yeast, and mammals, all of which contain a related group of atg genes, in spite of the fact that there are some differences between yeast and man (Klionsky, 2007).

The fundamental components of the autophagic process (Fig. 2.1) include phagophore formation, elongation and multimerization of phagosomes, cargo selection and lysosomal fusion. These components of autophagy will be discussed below.

Figure 2.1

Molecular events in the autophagy pathway. A stress response, such as nutrient withdrawal, causes cells to initiate autophagy. The stress sensor TOR kinase remains inactivated in low-nutrient condition and maintains hypophosphorylated Atg13. Atg1/Ulk1 interacts with Atg13 and Atg17 and regulates transmembrane protein Atg9 involved in lipid import from cellular organelles to act as a “phagophore” formation initiator. Next, Vps34/Beclin1 converts PI to PI3P followed by Atg5–Atg12 conjugation and interaction with Atg16L resulting in multimerization at the phagophore and formation of nascent curvature; coupled to these changes, LC3 processing helps elongation and expansion. Random or selective cargoes are targeted for degradation, followed by formation of a complete double-membrane ring called an “autophagosome.” Lysosomes dock and fuse with the autophagosome, forming an “autolysosome” where degraded cargoes generate amino and fatty acids to be transported back into the cytoplasmic pool. Autophagy acts as a primary response promoting cell viability and serving a cytoprotective role and upon stress removal the cell resumes normal function. However, extreme stress pushes the cell to cross the point of no return and commits it toward autophagic cell death (type II programmed cell death, PCD).

2.1. Phagophore formation and regulation

The initial step of phagophore membrane formation in mammals remains elusive and has not been adequately defined, whereas in the yeast system this pathway is well defined. Unlike yeast, in the mammalian system there are no reports of preautophagosomal structures (Klionsky, 2007; Yorimitsu & Klionsky, 2005). Target of rapamycin (TOR) kinase acts as a molecular sensor to various stress responses, including hypoxia, insulin signaling, and energy and nutrient depletion, playing a pivotal role in cellular growth and autophagy control (Kamada et al., 2010). Initial nutrient starvation inactivates TOR kinase, resulting in a hypophosphorylated Atg13 that shows an increased affinity for Atg1 kinase (mammalian homolog of Ulk1) and forms a complex with a scaffold-like protein Atg17 (Fig. 2.1; Mizushima, 2010). Starvation treatment enhances the crosstalk between Atg13, Atg1, and Atg17. Atg13 and Atg17 are both required for appropriate monitoring of the kinase activity of Atg1. In turn, Atg1 regulates the transmembrane protein Atg9. The kinase activity of Atg1 is dispensable; however, it controls the dynamics of Atg9 recruitment to the phagophore in an Atg17-dependent pathway (Sekito, Kawamata, Ichikawa, Suzuki, & Ohsumi, 2009; Simonsen & Tooze, 2009). Atg9 is involved in lipid import from different sources like the endoplasmic reticulum (ER), endosomes, mitochondria, golgi bodies, and nuclear envelope and also helps in the assembly of the intact phagophore membrane (Axe et al., 2008; Simonsen & Tooze, 2009; Yorimitsu & Klionsky, 2005).

2.2. Elongation and multimerization of phagophores

The phagophore is elongated when Class III PI3 kinases, for example, Vps34 (vesicular protein sorting), bind to Beclin1 (mammalian homolog of yeast Atg6) increasing its catalytic activity to produce PI3P (phosphatidyl inositol-3-phosphate) (Simonsen & Tooze, 2009). PI3P acts as an important localization signal and may facilitate fusion at the final step of autophagosome formation (Axe et al., 2008). Vps34–Beclin1 interaction is upregulated by proteins like Ambra (activating molecule in Beclin1-regulated autophagy protein 1), UVRAG (ultraviolet radiation resistance-associated gene), and Bif 1 (Bax-interacting factor 1). In contrast, this interaction is downregulated by Bcl-2, Bcl-XL, and Rubicon (RUN domain and cysteine-rich domain-containing Beclin1-interacting protein) (Funderbur, Wang, & Yue, 2010). Two ubiquitin-like conjugation systems are part of the vesicle elongation process. In one experimental system, Atg12 binds Atg7 (E1 ubiquitin-like activating enzyme) in an ATP-dependent manner. Next, Atg12 non-covalently binds Atg10 (E2-like ubiquitin carrier) linking Atg12–Atg5; Atg16 dimers conjugate with this complex via C-terminal coiled-coil domain to facilitate the creation of the expanding phagophore. The Atg5–Atg12–Atg16 complex induces curvature formation in the growing phagophore. However, this trimeric structure dissociates when the phagophore progresses into a double-membrane ring called an “autophagosome” (Geng & Klionsky, 2008). The second ubiquitin-like system, which plays a pivotal role in autophagosome formation, helps in the processing of microtubule-associated light chain 3 (LC3) (mammalian homolog of Atg8). Cysteine proteinase Atg4 (also known as autophagin) cleaves LC3 to produce LC3BI that binds E1-like Atg7 in an energy-expending pathway resulting in activation of LC3BI. Atg3, an E2-like carrier, interacts with activated LC3BI and promotes lipidation, giving rise to LC3BI–phosphatidylethanolamine (PE) conjugate or LC3BII (Kabeya et al., 2000).

The sequential order of mammalian autophagosome biogenesis begins with activation of an Ulk1/2 (UNC-51-like kinase 1/2) complex, which associates with the initiating phagophore membrane. The Vps34 complex is recruited to the phagophore and phosphorylates phosphoinositides (PIs), leading to the production of PI3P. WIPI1/WIPI2 (WD repeat protein-interacting with PIs) and DFCP1 (double FYVE domain-containing protein), two PI3P effectors, contribute to this nascent elongating membrane. The Atg12–Atg5 complex serves an E3-type enzyme function and acts as a supporting framework to which Atg8s arrive at the phagophore (Hanada et al., 2007; Tanida, Ueno, & Kominami, 2004). Atg16L joins the Atg12–Atg5 complex and helps to govern at which site of the membrane the downstream conjugation of LC3 occurs (Fujita et al., 2008). Atg8 and GATE-16 (Golgi-associated ATPase enhancer) are recruited and conjugated to PE on the phagophore membrane, which starts elongating mediated through the action of LC3–PE. GATE-16 acts downstream of LC3 in a step coupled to the disassociation of the Atg12–Atg5 with Atg16L. Subsequently, a mature autophagosome bearing a double-membrane structure is formed (Weidberg, Shvets, & Elazar, 2011).

A recent report highlights the role of Atg14L, a subunit of PI3kinase involved in localization to the ER via four N-terminal cysteines, which accumulate in omegasomes (PI3P-rich Ω-shaped structure formed at the periphery of ER) upon autophagy induction in the process of autophagosome biogenesis (Matsunaga et al., 2010). Atg14L is able to target the PI3-kinase complex to the ER, enabling PI3P generation for omegasome and autophagosome formation. Although the role of specific kinases in autophagosome is well documented and emphasized, the role of antagonistic partners in this process, that is, phosphatases, is equally important (Vergne & Deretic, 2010). The phosphatidylinositol 3-phosphate (PI3P) phosphatase Jumpy (MTMR14) associates with isolated membranes during early autophagosome biogenesis that is guided by Atg16, which helps in the subsequent development and localization of autophagic organelles (Noda, Matsunaga, Taguchi-Atarashi, & Yoshimori, 2010; Vergne et al., 2009). Jumpy coordinates the recruitment of Atg factors in an orderly manner by interacting with PI3P through WIPI-1 (Atg18) thereby affecting the distribution of Atg9 and LC3, the factors responsible for controlling growth of the autophagic membrane.

2.3. Cargo selection

In general, autophagy has been considered a random process as it appears to engulf cytoplasm indiscriminately. Electron micrographs often show autophagosomes with wide-ranging contents comprising mitochondria, ER, and golgi membranes. However, there is accumulating evidence that the growing phagophore membrane can interact selectively with protein aggregates and organelles. LC3B-II provides the role of “receptor” at the phagophore and interacts with “adaptor” molecules on the target including protein aggregates, damaged mitochondria, thereby helping in promotion of their selective uptake and degradation (Weidberg et al., 2011; Yorimitsu & Klionsky, 2005). The best-characterized molecule in this process is the multiadaptor molecule p62/SQSTM1, which binds Atg8/LC3 and promotes degradation of polyubiquitinated protein aggregates (Ichimura & Komatsu, 2010). Similarly, Atg32 has been identified in yeast as a protein that promotes selective uptake of mitochondria, a process known as mitophagy (Okamoto, Kondo-Okamoto, & Ohsumi, 2009).

2.4. Lysosomal fusion

Autophagosomes dock with lysosomes and fuse to give rise to structures known as “autolysosomes,” where the acidic lysosomal components digest all cargos. Migrating bidirectionally along microtubules, the autophagosomes have a natural propensity toward the lysosome-enriched microtubule organizing center, supervised by the function of dynein motor proteins (Kimura, Noda, & Yoshimori, 2008; Ravikumar et al., 2005; Williams et al., 2008). Small GTPases, like Rabs (Rab7), ESCRT (endosomal sorting complex required for transport), SNARE (soluble N-ethylmaleimide-sensitive factor activating protein receptor), and class C Vps proteins play key roles in the coordinated vesicular docking and fusion with target components (Atlashkin et al., 2003; Gutierrez, Munafo, Beron, & Colombo, 2004; Jager et al., 2004; Lee, Beigneux, Ahmad, Young, & Gao, 2007; Zerial & McBride, 2001). Accumulation of Rab proteins along specific intracellular niches triggers the last fusion step mediated by SNARE complexes (Martens & McMahon, 2008). It is worthwhile highlighting important proteins like ESCRT whose mutation or loss of function culminates in inhibition of autophagosome maturation. Although the role of UVRAG is most recognized as a Beclin1-interacting protein, it also has an independent function in the final maturation step by engaging the fusion machinery on autophagosomes. UVRAG is also known to engage the class C Vps proteins thereby activating Rab7, which helps to promote fusion with late endosomes and lysosomes. Another Beclin1-interacting protein Rubicon also modulates autophagosomal maturity. Rubicon remains part of a complex containing varied proteins, like UVRAG, hVps34, and hVps15, and is known to suppress autophagosomal maturation (Matsunaga et al., 2009; Zhong et al., 2009). Further research will help to clarify all of the key-interacting players involved in the crucial autophagy pathway.

Shifting the focus from the autophagosome to the lysosome, inhibiting the lysosomal H+ ATPase by chemicals like bafilomycin A1, nocodazole, or vinblastine, will prevent the fusion of autophagosomes with endosomes/lysosomes (Fass, Shvets, Degani, Hirschberg, & Elazar, 2006; Köchl, Hu, Chan, & Tooze, 2006). A recent scientific report suggests an alternative autophagic pathway in mouse cells lacking Atg5 or Atg7 when treated with stress inducers, like etoposide. The key autophagic proteins operational in this Atg5/Atg7-independent route include Ulk1 and Beclin1 and proceed in a Rab9-dependent manner (Nishida et al., 2009).

Lysosomal permeases and transporters export essential products like fatty acids and amino acids back into the cytosolic pool. This replenishing phenomenon plays a survival role for starving cells, contributing to what is called “protective autophagy” (Mizushima et al., 2002; Yorimitsu & Klionsky, 2005).

2.5. Autophagic cell death: An elusive process

The current concept of programmed cell death involves three areas including apoptosis, autophagic cell death (ACD) and necroptosis. The autophagic pathway initially functions as an adaptive response to stress; however, when the cell continues to face extreme stress over a protracted period of time it reaches a point of no return and becomes committed to undergo cell death. This type of cell death that is associated with autophagosomes and is dependent on autophagic proteins is called “Autophagic cell death.” As indicated, extended autophagy beyond the optimal survival limit culminates in ACD (Fig. 2.1). ACD has gained immense attention among scientists since the 1990s, which has progressed rapidly due to the discovery of the atg genes, establishing a caspase-independent “type II programmed cell death” (Kroemer & Levine, 2008). The Nomenclature Committee on Cell Death (NCCD, 2005) classified ACD as cell death through autophagy, referring to this process as cell death with autophagy (Kroemer et al., 2005). Later in 2008, Kroemer and Levine characterized ACD morphologically (by transmission electron microscopy) as a cell death process occurring in the absence of chromatin condensation but characterized by large-scale sequestration of cytoplasmic components into autophagosomes, imparting a characteristic vacuolated appearance to the cell.

ACD is mainly a morphological phenomenon, and there currently is no conclusive evidence that a specific mechanism of autophagic death exists (Tsujimoto & Shimizu, 2005; Yorimitsu & Klionsky, 2005). It is difficult to define the pathophysiological role of ACD; hence, the attempt to hypothesize the reason for this mode of cell death remains elusive. Pinpointing the exact function of autophagy in programmed cell death is not only challenging but also equally complicated due to the simultaneous occurrence of caspase-dependent apoptosis, often occurring in the context of autophagy. Kinetically speaking, it can be anticipated that caspase-mediated proteolytic degradation would occur faster than self-digestion by autophagy. Accordingly, the cell would be experiencing a predominant extent of apoptosis in spite of extensive autophagy (Debnath, Baehrecke, & Kroemer, 2005). However, the cell displays a highly multifaceted crosstalk between the apoptotic and ACD mechanism(s); which can promote antagonism, synergism, or mutually independent pathways of induction that are context dependent. It is difficult to conceptualize how on one hand ACD is part of the cell death mechanism, while multiple studies also emphasize a protective role of ACD since autophagic inhibition does not stop cell death but may even promote death. Whether ACD is primarily a mediator of the death mechanism, an innocent bystander, or a double-edged sword in cell survival/death processes remains to be determined. It is also possible that this duality of functions depends on the temporal induction of autophagy and the context in which this pathway is induced or suppressed. It is clear that a complete understanding of the autophagic process as well as its mediators and determinants of expression represents an evolving story that will only become clarified with additional research.

3. AUTOPHAGY IN TUMOR INITIATION AND DEVELOPMENT

Autophagy is believed to play an essential role in tumor initiation and development (Chen & Debnath, 2010; Liang & Jung, 2010). When base-line levels of autophagy fluctuation were compared, the amount of proteolysis or autophagic degradation in cancer cells was less than that of their normal counterparts (Gunn, Clark, Knowles, Hopgood, & Ballard, 1977; Kisen et al., 1993). This differential expression suggests a direct connection between tumorigenesis and decreased levels of autophagy. Intriguingly, many oncogenes and tumor suppressor genes affect autophagic pathways (Maiuri et al., 2009), and the deregulation of the autophagic process contributes to malignant transformation. For example, many tumor suppressor proteins such as p53, phosphatase and tensin homolog (PTEN), death-associated protein kinase (DAPK), tuberous sclerosis 1 (TSC1), and TSC2 that provide constitutive input signals to activate autophagy are mutated in multiple cancers.

PTEN, a dual lipid/protein phosphatase, dephosphorylates PIP3 to PIP2, preventing inhibition of autophagy by the PI3K/Akt/mTOR pathway. In human tumors, PTEN is mutated, resulting in activation of Akt that suppresses autophagy (Maehama, 2007; Yin & Shen, 2008) and also participates in increased protein translation, cell growth, and cell proliferation, which is a contributor to tumorigenesis (Hafner et al., 2007; Horn et al., 2008; LoPiccolo, Blumenthal, Bernstein, & Dennis, 2008; Vivanco & Sawyers, 2002). Additionally, p53 plays a divergent role in the regulation of autophagy. Within the nucleus, p53 can act as an autophagy-inducing transcription factor through AMPK and TSC1/TSC2 dependent activation (Tasdemir et al., 2008). In contrast, cytoplasmic p53 exerts an autophagy-inhibitory function, and its degradation is actually required for the induction of autophagy. Although the relationship between autophagy and p53 is complicated, it is clear that p53 mutation(s) cause alterations in p53-mediated autophagy that leads to cancer development. Additionally, another target of p53 that is present in nucleus, DRAM (damage-regulated autophagy modulator), has been shown to positively regulate autophagy. DRAM is essential for p53-mediated autophagy and apoptosis in response to DNA-damaging agents, and the overexpression of DRAM is sufficient to activate autophagy without affecting apoptosis (Crighton et al., 2006). The fact that DRAM is also deleted in multiple types of cancer underscores its importance and highlights the possibility that autophagy might play a fundamentally important role in cancer.

The death-associated protein kinase (DAPK), a cytoskeleton-associated calmodulin-regulated serine/threonine protein kinase, has been shown to possess multiple tumor- and metastasis-suppressor functions (Bialik & Kimchi, 2006; Eisenberg-Lerner & Kimchi, 2009). It is often decreased or lost in many human cancers, and ectopic expression is associated with p53-mediated apoptosis and decreased cell migration and invasion. Moreover, DAPK expression has been shown to suppress formation of metastatic foci in Lewis lung carcinoma in mice (Inbal, Bialik, Sabanay, Shani, & Kimchi, 2002). Recently, DAPK and DAPK-related protein kinase-1 (DRP-1) have been found to activate autophagy in MCF-7 and HeLa cells. Expression of these genes triggered membrane blebbing and ACD and conversely inhibition of DAPKs resulted in decreased autophagy (Moretti, Yang, Kim, & Lu, 2007). Metabolic stress results in a decline in ATP:ADP ratios and induction of the tumor suppressor LKB-I (STK-I), which is serine threonine kinase that is upregulated in the LKB-I/AMPK/mTOR pathway. It phosphorylates the a subunit at Thr172 (Shaw et al., 2004) and activates AMPK. AMPK either activates TSC2 and the regulatory-associated protein raptor by promoting phosphorylation of TSC2 or directly phosphorylating raptor, leading to inhibition of mTOR and autophagy induction (Corradetti, Inoki, Bardeesy, DePinho, & Guan, 2004). Moreover, the LKB-I/AMPK/mTOR axis activates p27, a cyclin-dependent kinase inhibitor, inducing cell cycle arrest for energy conservation (Liang et al., 2007).

Conversely, oncogenes including Akt, mTOR, Bcl-2, and FLICE-like inhibitory protein (FLIP) inhibit autophagic processes indicating that elevated autophagy signaling may contribute to tumor suppression (Lee et al., 2009; Morselli et al., 2009). In most cancers, the PI3K–Akt axis undergoes mutation in either upstream or downstream regulators (Shaw & Cantley, 2006). Thus, downstream activation of mTOR favors cell growth stimulation and inhibition of autophagy. Constitutive activation of Akt inhibits autophagy in vitro and in vivo in Bax and Bak double mutants. Monoallelic knockout of beclin1 or biallelic knockout of atg5 in mice promotes genomic alterations due to activation of Akt (Karantza-Wadsworth et al., 2007; Mathew, Karantza-Wadsworth, & White, 2007; Mathew, Kongara, et al., 2007). The BH3 receptor domain of Bcl-2 and the multidomain anti-apoptotic proteins bind with the amphipathic BH3 helix of Beclin1 (Oberstein, Jeffrey, & Shi, 2007), promoting its sequestration and blocking its interaction with Vps34. Overexpressed Bcl-2 along with deletion in one allele of beclin1 accelerates tumor growth in vivo (Degenhardt et al., 2006). Notably, the Bcl2 gene is overexpressed in a majority of cancers (Levine, Sinha, & Kroemer, 2008; Pattingre & Levine, 2006), and knockdown or gene silencing of Bcl-2 through antisense oligonucleotides or siRNA heteroduplexes, respectively, in MCF-7 cells results in induction of autophagy (Akar et al., 2008).

Identification of beclin1 as a haploinsufficient tumor suppressor frequently containing deletion of one allele in a large proportion of human breast, ovarian, and prostate cancers provided initial evidence for a potential direct link between autophagy and cancer. Notably, overexpression of Beclin1 in MCF-7 breast carcinoma cells induced autophagy and restricted proliferation and clonigenicity in vitro, and it also inhibited tumorigenesis in nude mice (Liang et al., 1999). The contribution of the allelic deletion of Beclin1 to carcinogenesis is supported by the observation that beclin1+/− mice have an increased incidence of lung cancer, hepatocellular carcinoma (HCC), and lymphoma (Yue, Jin, Yang, Levine, & Heintz, 2003; Qu et al., 2003). Furthermore, allelic loss of beclin1 in HCC causes accumulation of p62/SQSTM (Mathew et al., 2009). ER chaperones and damaged mitochondria result in an elevated production of ROS (reactive oxygen species). Generation of ROS promotes a cascade of events, including increased oxidative stress, DNA damage, and chromosomal instability, which ultimately lead to inhibition of the NF-kB pathway and development of HCC. Consequently, tumor-suppressive or -supportive properties of Beclin1-interacting molecules including UVRAG, Bif-1, and Rubicon have been documented (Funderbur et al., 2010). UVRAG activates Beclin1 and induces autophagosome formation. Ectopic expression of UVRAG suppresses the proliferation and tumorigenicity of HCT116 tumor cells and sensitizes these cells to undergo self-directed autophagy even without starvation treatment (Liang et al., 2006). Monoallelic loss of UVRAG is observed in various colon cancer cells and tissues. Nonsense mutations of UVRAG are observed in colon and gastric cancers resulting in inactivation of autophagy (Ionov, Nowak, Perucho, Markowitz, & Cowell, 2004; Kim et al., 2008). Moreover, Bif 1 (also known as Endophilin B1) interacts with Beclin1 through UVRAG (Takahashi et al., 2007). Downregulation of Bif 1 is frequently found in different types of cancer, and loss of Bif 1 leads to suppression of autophagy by decreasing Vps34 kinase activity, which promotes colon adenocarcinoma formation (Coppola et al., 2008). Moreover, Bif 1−/− mice are cancer prone (Takahashi et al., 2007). By contrast, Rubicon, a newly identified Beclin1-interacting protein, reduces Vps34 lipid kinase activity and downregulates autophagy (Matsunaga et al., 2009; Zhong et al., 2009) with aberrant expression in multiple types of cancer. Thus, Beclin1 acts as a nodal point and activates PI3K III, revealing its mandatory role in autophagy and tumorigenesis (Table 2.1).

Table 2.1

Autophagy genes in cancer

Autophagy process Autophagy gene Cancer Mechanism Autophagy in cancer References

Induction UKL3 Tumorigenesis Oncogene-induced cell senescence Lethal Young et al. (2009)

Nucleation Beclin1 Tumorigenesis Highly mutated in human breast, ovarian, and prostate tumors; haploinsufficient tumor suppressor Lethal Liang et al. (1999)

UVRAG Tumorigenesis Mutations detected in human colorectal, breast, and gastric tumors; haploinsufficient tumor suppressor Lethal Ionov et al. (2004) and Kim et al. (2008)

Bif-1 Tumorigenesis Bif-1−/− mice are cancer prone; decreased Lethal Coppola et al. (2008)

Ambra1 Tumorigenesis Ambra1−/− mice have severe neural tube defects Lethal Garber (2011)

Elongation Atg12–Atg5 Tumorigenesis Atg5 frameshift mutations in gastric cancers Lethal Cadwell et al. (2008)

Atg4C Tumorigenesis Atg4C−/− mice develop fibrosarcomas in response to carcinogen treatment Lethal Marino et al. (2007)

Atg7 Tumorigenesis Defects in hematopoietic stem cell functions,liver-specific Atg7−/− mice develop liver cancer Lethal Takamura et al. (2011)

Atg16 Tumorigenesis Mutations detected in Crohn’s disease Lethal Cadwell et al. (2008)

Maturation Rab7 Tumorigenesis Rab7−/− aberrant expression in human leukemia Protective Liang and Jung (2010)

Cargo selection p62 Tumorigenesis ROS accumulation through NF-kB induction leads to tumorigenesis in autophagy-deficient conditions Lethal Mathew et al. (2009)

Unknown Unknown Metastasis Resistance to TRAIL, energy metabolism, necrosis, inflammation, HMGB Lethal/protective (?) -

Unknown Unknown Tumor dormancy Resistance to TRAIL, Ras homolog member I induction Protective -

Unknown Unknown Cancer stem cells Unknown Lethal/protective (?) -

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Additional autophagic genes involved at different stages of autophagy have been identified as contributors to the oncogenic and tumor-suppressive signaling pathways of cancer (Table 2.1). For instance, Atg7 contributes to the maintenance of HSCs (hematopoietic stem cells) (Mortensen et al., 2011). Atg7-deficient LSK (Lin−Sca-1+c-Kit+) cells show defects in HSC functions with impairment of production of both lymphoid and myeloid progenitors in lethally irradiated mice. Similarly, suppression of Atg5 and Atg16L1 genes leads to tissue injury in intestinal Paneth cells culminating in Crohn’s death, a known risk factor for colorectal cancer in humans (Cadwell et al., 2008). Moreover, mice in which Atg5 and liver-specific Atg7−/− have been deleted develop liver cancer and display mitochondrial swelling, p62 accumulation, and oxidative stress and genomic damage responses in isolated hepatocytes (Takamura et al., 2011). Amino acid depletion, which hinders metabolism, occurs during the survival period of neonatal mice deficient in Atg5. Moreover, mice with Atg4C deficiency have increased tendency for carcinogen-induced tumorigenesis (Marino et al., 2007).

Cellular senescence is a state of stable dynamic cell cycle arrest that limits the proliferation of damaged cells and has been regarded as a tumor suppressor mechanism. A recent study showed that autophagy was activated during senescence, and its activation was correlated with negative feedback in the PI3K–mTOR pathway (Young et al., 2009). A subset of autophagy-related genes is upregulated during senescence. For instance, overexpression of ULK3/Atg3-induced autophagy and senescence contributed to tumor suppression. Furthermore, inhibition of autophagy delayed the senescence phenotype, including senescence-associated secreted factor.

4. AUTOPHAGY IN TUMOR PROGRESSION AND METASTASIS

Tumor metastasis is a complex, multistep process by which tumor cells from a primary site migrate to and colonize at distant organ sites (Fig. 2.2; Das et al., 2012). This process involves multiple, discrete steps including invasion of tumor cells from the primary tumor site, intravasation and survival in the blood stream, extravasation at a distant site, and finally colonization of disseminated tumor cells (DTCs) at distant sites (Bingle, Brown, & Lewis, 2002; Das et al., 2012). In primary tumors, inflammatory cells infiltrate tumor sites in response to necrosis resulting from hypoxia and metabolic stress (Degenhardt et al., 2006; Jin & White, 2007). Protective autophagy, promoted by hypoxia and metabolic stress, inhibits inflammation at primary sites that is required for initiation of metastasis. Interestingly, autophagy reduces necrosis and subsequent macrophage infiltration thereby decreasing primary tumor growth, and genetic inhibition of autophagy can cause cell death, tissue damage, and chronic inflammation (Mathew, Karantza-Wadsworth, et al., 2007; Mathew, Kongara, et al., 2007). Another important function of autophagy is clearance of cellular debris accumulated as unfolded protein, damaged organelles, and high-cargo receptor p62 in response to metabolic stress during tumor progression. When autophagy is defective, cellular toxic substances are not degraded which cause ROS production followed by DNA damage and chromosomal instability that initiate metastasis (Mathew et al., 2009). Additionally, autophagy activates a proinflammatory immune response by enhancing the release of immunostimulatory molecules including high-mobility group box protein-1 (HMGB-1) from dying tumor cells which mediate antitumor immunity (Fig. 2.3; Thorburn et al., 2008).

Figure 2.2

Model of the primary events involved in the metastatic cascade. The metastatic process is complex and involves numerous changes in cellular phenotype resulting from both genetic and epigenetic modifications of the cancer genome. The process is initiated by the spread of cancer cells from a primary tumor site to other regions in the body. Cells initiate growth as primary tumors in the epithelium and with genetic and epigenetic modifications, subsets of tumor cells develop metastatic properties allowing them to degrade the basal layer and invade the blood stream (Intravasation). A small percentage of tumor cells escape into blood vessels (Extravasation), survive in the bloodstream, adhere to new target organ sites, and ultimately form secondary tumors (metastases) in distant organ or tissue sites. A key component of both the primary and secondary expansion of the tumor and metastases is the development of a new supply of blood vessels, that is, angiogenesis. Taken from Das et al. (2012).

Figure 2.3

Role of autophagy at different stages of cancer. During the initial phase of cancer development, autophagy-related cell death has been regarded as a primary mechanism for tumor suppression. Moreover, autophagy also restricts necrosis and inflammation thus limiting invasion and dissemination of tumor cells from a primary site, resulting in restriction of metastasis at a premature step. Moreover, lethal (toxic) autophagy directly causes the release of immunomodulatory factors such as HMGB-1 from dead tumor cells, which activates immune response and restricts metastasis by inhibiting protumorigenic responses. On the other hand, altered energy metabolism and TRAIL-resistant phenomena of tumor cells are maintained through protective autophagy during tumor progression. Similarly, autophagy may promote metastasis by enhancing tumor cell fitness in response to environmental stresses, such as anoikis during metastatic progression. Protective autophagy is involved in maintaining dormant tumor cells and promoting their survival under stressful conditions. The exact role of autophagy in cancer stem cells is unclear in tumor progression. Finally, tumor cells maintain protective autophagy through activation of HIF-1 (hypoxia-inducible factors) and AMPK (5′-AMP-activated protein kinase) in apoptosis deficient and long-term metabolic stress conditions, in a full-blown cancer.

Apart from autophagy’s antimetastatic properties, it can also have an opposite effect enhancing the metastatic potential of tumor cells. It is well established that death ligand-induced apoptosis, particularly TNF-related apoptosis inducing ligand (TRAIL), plays a critical role in regulating the suppression of metastasis by T-cells and NK cells (Wang, 2008). But recently it was demonstrated that protective autophagy is upregulated in TRAIL-resistant cancer cells, which enhances viability and survival of tumor cells during metastasis (Fig. 2.3; Han et al., 2008; Herrero-Martin et al., 2009).

Another important property of metastatic cancer cells is resistance to anoikis, apoptosis associated with lack of proper extracellular matrix attachment (ECM) (Taddei, Giannoni, Fiaschi, & Chiarugi, 2012). Aberrant activation of growth factor pathways including Ras/MAPK and PI3K/Akt pathways is a common mechanism utilized by cancer cells to evade anoikis. Although autophagy-mediated cell death was initially recognized in association with anoikis, a recent study also supports the protective nature of autophagy in anoikis (Kenific, Thorburn, & Debnath, 2010). Fung, Lock, Gao, Salas, and Debnath (2008) demonstrated autophagy induced by detachment from the substratum or inhibition of the b1-integrin receptor, and inhibition of autophagy by knockdown of atg genes enhanced detachment-induced cell death. Anoikis resistance in tumor cells is protective and facilitates survival and expansion of metastatic cells. Although the detailed mechanisms of protective autophagy in anoikis resistance remain largely unknown, a recent study indicates that PERK, an ER kinase, facilitates survival of ECM-detached cells by promoting autophagy and antioxidant activity induced through ROS generation, which may provide fitness to cells without ECM contact during later stages of cancer dissemination and metastasis (Fig. 2.3; Avivar-Valderas et al., 2011).

Alterations in cellular metabolism, an important hallmark of cancer, are employed by neoplastic cells to adapt to specific growth requirements during cancer progression (Hanahan & Weinberg, 2011; Mizushima & Klionsky, 2007). Cancer cells specifically consume glucose through anaerobic glycolysis during hypoxic conditions, which is known as the “Warburg effect,” with high levels of glycolytic intermediates and lactate as reported in human colon and gastric cancers (Hirayama et al., 2009). In addition, cancer cells also display increased glutamine utilization. Autophagy induced under stressful conditions causes high degradation and recycling of proteins providing substrates for energy and carbon/nitrogen sources for biomass production required for rapidly proliferating cancer cells (Mizushima & Klionsky, 2007). Similarly, intracellular fat storage provides acetyl-CoA for mitochondria to support the TCA cycle through lipophagy to meet the elevated metabolic demands of deregulated tumor cell growth (Rabinowitz & White, 2010). Another important property of autophagy is preservation and maintenance of organelle function, especially mitochondria that are required for cell growth during tumor progression, whereas it prevents tumor growth by reducing tissue damage and necrosis during cancer initiation (Twig et al., 2008). Damaged mitochondria are the major site of ROS production in cells, and mitochondria-selective autophagy, “mitophagy,” clears depolarized mitochondria and maintains cellular homeostasis (Wu et al., 2009). Recent studies indicate that Ras-expressing cells have upregulated basal autophagy that is required to maintain the pool of functional mitochondria necessary to support growth of aggressive tumors (Guo et al., 2011).

As tumors grow with defects in apoptosis and following long-term metabolic stress conditions, they may require autophagy to survive in nutrient-limited and low-oxygen conditions, especially in the central area of the tumor, which is often poorly vascularized. Survival through autophagy is a key process enabling long-term tumor cell viability and eventual regrowth and tumor recurrence. Accordingly, induction of autophagy allows cancer cells to survive in low-nutrient and low-oxygen conditions through activation of HIF-1 (hypoxia-inducible factor) and AMPK (5′-AMP-activated protein kinase) (Eisenberg-Lerner & Kimchi, 2009; Mathew, Karantza-Wadsworth, et al., 2007; Mathew, Kongara, et al., 2007). For instance, the oncogenic gene astrocyte elevated gene-1 (Emdad et al., 2009; Kang et al., 2005; Su et al., 2002) was associated with protective autophagy through AMPK/mTOR-dependent pathway and inhibition of the protective autophagy by ATG-5 knockdown provided therapeutic benefits (Fig. 2.4; Bhutia, Dash, et al., 2010; Bhutia, Kegelman, et al., 2010). HIF-1 and AMPK are components of a concerted cellular response to maintain energy homeostasis in oxygen- and nutrient-limited tumor microenvironments.

Figure 2.4

Astrocyte elevated gene-1 (AEG-1) and protective autophagy. Model illustrating the possible molecular mechanism of AEG-1-mediated protective autophagy, which promotes escape from apoptosis and resistance to chemotherapy. Taken from Bhutia, Kegelman, et al. (2010).

5. AUTOPHAGY IN TUMOR DORMANCY

Tumor dormancy is a protracted stage in tumor progression in which tumors remain occult and asymptomatic for extended periods of time. This state can be present as one of the earliest stages in tumor development, as well as in the in micrometastasis stage, and can occur when minimal residual disease remains after surgical removal or treatment of primary tumors (Almog, 2010). Clinically, the dormant tumor cells are not easily detected representing a major problem in breast cancer, ovarian cancer, and other malignancies. Apart from analysis of autopsies of trauma victims and clinical data accumulating from patients with late recurrence or relapse, recently DTCs and circulating tumor cells (CTCs) in cancer patients provide data relative to the frequency and prevalence of tumor dormancy. Tumor dormancy can result from angiogenesis arrest, a balance between apoptosis and cell proliferation, cell cycle arrest, and immune surveillance (Pantel, Alix-Panabières, & Riethdorf, 2009). Recently, the role of autophagy in tumor dormancy has been recognized (Fig. 2.3). Tumor cells in dormant conditions are not efficiently associated with extracellular matrix and stimulate autophagy for survival and maintenance of dormancy. Impaired b1-integrin signaling, a known inducer of autophagy, has been shown to promote dormancy in MMTV-PyMT breast cancer model (White et al., 2004). In breast cancer metastases to bone, disseminated cells displayed Src-mediated TRAIL resistance and remained dormant in the bone marrow for extended periods of time (Zhang et al., 2009). As mentioned earlier, autophagy can protect cells from TRAIL-induced apoptosis. Based on this consideration, it is speculated that dormancy in the bone marrow could induce protective autophagy and support survival of dormant cells (Kenific et al., 2010). A direct link between autophagy and tumor dormancy was recently demonstrated in ovarian cancer cells. The tumor suppressor aplasia Ras homolog member I (ARHI) induced autophagic death in ovarian tumor cells in vitro. However, in xenografted tumors in mice ARHI-induced autophagic death was switched to dormant tumor cell survival in the context of the tumor microenvironment suggesting that autophagy may be a prerequisite for tumor dormancy (Lu et al., 2008). Future studies focused on defining the precise mechanism by which protective autophagy is associated with tumor dormancy is warranted and holds potential for defining new therapeutic strategies for treating cancer.

6. AUTOPHAGY IN CANCER INITIATING/STEM CELLS

The “stem cell hypothesis” embodies the concept of tumor cell heterogeneity, in which only tumor-initiating cells in the heterogeneous tumor population are capable of proliferating and differentiating into new tumor-producing cells. Stem cells form the apex of the tumor hierarchy and retain the capability to replicate and grow into a tumor in vivo. It has been argued that cancer initiating or tumor-initiating stem cells (CSCs) do not arise from normal stem cells (Clevers, 2011). They can arise from subpopulation of cancer cells, including cancer initiating stem cells, cancer progenitor cells, or differentiated cancer cells. The phenotype with respect to the self-renewal and differentiating capacity of CSCs depends upon tumor type and is quite predictable (Zhou et al., 2009). Cancer initiating/stem cells have now been identified and isolated from tumors of the hematopoietic system, skin, breast, brain, prostate, colon, head and neck, and pancreas, and reduced CSC numbers can initiate tumor growth in xenograft models (Fu et al., 2009). This sub-population within the tumor evades therapy, persists, and initiates recurrence thereby enhancing malignant spread of the disease. Several pathways over-expressed in different types of cancers including Wnt/Notch/Hedgehog have been identified and shown to be critical to the self-renewal behavior of CSCs. Moreover, CSCs show resistance to apoptosis; they have high expression of ATP-binding cassette transporters, and display enhanced DNA repair capacity making therapy extremely difficult. Apart from its functions in cancer, autophagy plays a seminal role in maintaining and modulating growth and survival of cancer initiating/stem cells.

Autophagy is downregulated in specific CSCs as compared to the remaining portion of cancer cells. For example, brain CSCs (CD133+) display decreased expression of autophagy-related proteins and are more resistant to temozolomide as compared to putative brain cancer CD133− nonstem cells (Fu et al., 2009). In contrast, a recent study showed that autophagy in CSCs provides a protective effect to current cancer therapeutics, and inhibition of protective autophagy improves therapeutic response. For example, radiation-induced autophagy in glioma CSCs and the CD133+ cells exhibited a larger degree of autophagy compared with the CD133− cells. Moreover, the CD133+ cells expressed higher levels of LC3-II, Atg5, and Atg12, and inhibition of autophagy sensitized these cells to g-radiation (Lomonaco et al., 2009). Similarly, therapy with tyrosine kinase inhibitors is associated with drug resistance in chronic myeloid leukemia (CML) CSCs, and autophagy is one of the mechanisms involved in this protective response. Suppression of autophagy using either pharmacological inhibitors or RNA interference of essential autophagy genes results in nearly complete elimination of phenotypically and functionally defined CML CSCs (Bellodi et al., 2009). However, these contradicting reports need to be reconciled by further experimentation. Understanding the role of autophagy in CSCs holds significant promise for enhancing cancer therapies (Fig. 2.3).

7. AUTOPHAGY IN CANCER THERAPY

Because cancer cells often display defective autophagic capacities, induction of ACD is viewed as a tumor suppressor mechanism. Induction of autophagic death, “type II programmed cell death,” could be a useful therapeutic approach for apoptosis-resistant cancer cells and could provide a complementary approach along with apoptosis in promoting cancer cell death. On other hand, autophagy has been shown to provide resistance to therapy-mediated tumor cell death. When tumor cells induce protective autophagy, inhibition of autophagy may provide a way of sensitizing tumor cells to therapy by activating apoptosis. ACD by anticancer drugs may occur depending on cell type and genetic background. Based on the type of treatment, different signaling pathways can be activated in the same cell and produce varied types of autophagy. Understanding whether autophagy will be “protective” or “toxic” is a key area for further development and will define whether it is appropriate to block or promote autophagy in specific cancer contexts (Chen & Karantza, 2011; Kondo, Kanzawa, Sawaya, & Kondo, 2005; White & DiPaola, 2009).

7.1. Stimulation of autophagic cell death

Therapeutic induction of ACD through overstimulation of autophagy remains an important approach for tumor cell elimination. A number of studies have reported that ACD is activated in cancer cells derived from tissues such as breast, colon, prostate, and brain, in response to various anticancer therapies. The consequence of promoting autophagy depends on multiple factors, including extent of induction, duration, and cellular context. Several chemotherapeutic drugs (alkylating agents, actinomycin D, arsenic trioxide), radiation and photodynamic therapy, hormonal therapies (tamoxifen and vitamin D analogs), cytokines (IFN-g), gene therapies (p53, mda-7/IL-24, and p27Kip1), and natural compounds (resveratrol and plant lectins) have been shown to trigger ACD in various cancer cells in vitro (Chen & Karantza, 2011). Accumulating evidence indicates that autophagic death contributes to in vivo antitumor effects. For instance, a natural BH3-mimetic, small-molecule inhibitor of Bcl2, (−)-gossypol, shows potent antitumor activity in ongoing phase II and III clinical trials for human prostate cancer. The antitumor activity by (−)-gossypol is mediated through induction of both apoptosis and autophagic death (Lian et al., 2011). ACD can occur independently or it can act synergistically or assist apoptotic cell death (Kondo et al., 2005; Maiuri et al., 2009). Interestingly, combining two therapies that trigger autophagy by targeting different pathways increased sensitivity to ACD, an alternative form of programmed cell death to promote synergistic cancer inhibitory effects (White & DiPaola, 2009). But, autophagy appears to serve as a death program primarily when the apoptotic machinery is defective, as observed in most tumors. One major drawback in using autophagy-promoting drugs may involve unwanted paradoxical effects by actually protecting tumors against cell death triggered by simultaneous anticancer therapies or by nutrient deprivation in the tumor environment (Chen & Karantza, 2011).

7.2. Inhibition of protective autophagy

Autophagy, which is decreased in cancer cells as compared to normal cells, can provide a target for enhancing cancer therapy. Although Beclin1 is a haploinsufficient tumor suppressor, deletion of the remaining Beclin1 in vitro induces growth arrest in cancer cells (Wirawan et al., 2010). Similarly, elimination of Atg5 induces growth arrest in cancer cells (Yousefi et al., 2006). An in vivo study revealed that transplantation of Bcr–Abl-expressing hematopoietic cells depleted of Atg3 to lethally irradiated mice failed to induce leukemia based on ablation of autophagy (Altman et al., 2010). This suggests that a certain level of autophagy is required for tumor growth, and autophagic inhibitors could have relevant anticancer effects even when applied alone. However, it is more likely that autophagy inhibitors will be most effective when used in combination with cytotoxic drugs that activate a protective autophagy to permit cancer cell survival upon treatment. Accordingly, it has been demonstrated that melanoma differentiation-associated gene-7/Interleukin-24 (MDA-7/IL-24), a member of IL-10 gene family, shows nearly ubiquitous antitumor properties in vitro and in vivo through induction of cancer-specific apoptosis (Dash et al., 2010; Fisher, 2005). A recent report indicates that the apoptosis potential of MDA-7/IL-24 increases by inhibiting protective autophagy with 3-methyladenosine (3-MA) in prostate cancer cells (Bhutia, Dash, et al., 2010; Bhutia, Kegelman, et al., 2010). Similarly, inhibition of autophagy by 3-MA or Atg7 knockdown induced apoptosis in colon cancer cells treated with 5-FU (Li, Hou, Faried, Tsutsumi, & Kuwano, 2010). Inhibition of protective autophagy was shown to sensitize resistant cells to TRAIL-mediated apoptosis in apoptosis-defective leukemic and colon cancer cell lines (Han et al., 2008). Additionally, protective autophagy was accompanied with Ginsenoside F2-induced apoptosis in breast cancer stem cells, and treatment with chloriquine (CQ), an autophagy inhibitor, enhanced Ginsenoside F2-mediated cell death (Mai et al., 2012).

Autophagy also plays an important role in chemoresistance of cancer to some therapeutic agents that typically induce an apoptotic response (Carew, Nawrocki, & Cleveland, 2007; Carew, Nawrocki, Kahue, et al., 2007). Although autophagy has been proposed as a “magic bullet” in fighting apoptosis-resistant cancers (Gozuacik & Kimchi, 2004), a more recent study demonstrated that rapamycin-induced autophagy could protect various tumor cells against apoptosis induced by general apoptotic stimuli (Ravikumar, Berger, Vacher, O’Kane, & Rubinsztein, 2006). Recent reports highlight that treatment of estrogen-receptor-positive breast cancer cells with the anti-estrogen tamoxifen, combined with a histone deacetylase inhibitor, maintains a subpopulation of cells with elevated autophagy that display a remarkable resistance to apoptosis. These apoptosis-resistant cells only become apoptotic after inhibition of autophagy (Thomas, Thurn, Biçaku, Marchion, & Münster, 2011).

The potential to inhibit autophagy and sensitize tumor cells to metabolic stress is another promising approach for cancer therapy. Many current cancer therapies including angiogenesis, growth factor, and receptor inhibitors when combined with autophagy inhibition produced synergistic anticancer effects (Corcelle, Puustinen, & Jäättelä, 2009; Moretti et al., 2007; White & DiPaola, 2009). In addition, autophagy is also involved in removing damaged and potentially dangerous organelles from the cell. Therefore, combining organelle-damaging drugs, such as sigma-2 receptor agonists, with an autophagy inhibitor might be an effective means of anticancer therapy (Ostenfeld et al., 2008). It is likely that ER stress inducers, including thapsigargin and tunicamycin, that trigger cell death in cancer cells will increase cell killing when autophagy is inhibited (Carew, Nawrocki, & Cleveland, 2007; Carew, Nawrocki, Kahue, et al., 2007). Protein turnover by lysosomal degradation through the autophagy pathway is functionally complementary and linked with ubiquitin proteasome protein degradation. Consequently, targeting both proteasome- and autophagy-mediated protein degradations might be an effective antitumor approach for highly metabolically active tumor cells (Ding, Ni, Gao, Hou, et al., 2007; Ding, Ni, Gao, Yoshimori, et al., 2007). The proteasome inhibitor bortezomib has the approval of the US Food and Drug Administration and has demonstrated potent efficiency in treating multiple myeloma (Roccaro et al., 2006).

The metastasis prone state of tumor cells may be particularly susceptible to autophagy inhibition as cells in isolation are expected to be more reliant on autophagy, although this possibility remains to be confirmed. In this regard, chloroquine (CQ), which inhibits lysosome acidification and thereby autophagy, in conjunction with alkylating agents, displayed remarkable efficacy in inhibiting tumor growth in mice as well as in clinical studies (Høyer-Hansen & Jäättelä, 2008; Moretti et al., 2007; White & DiPaola, 2009). Synergy between CQ and the HDAC inhibitor SAHA in killing imatinib refractory chronic myeloid leukemia cells also supports a protective role for autophagy, reinforcing the therapeutic use of autophagy inhibitors in cancer therapy (Carew, Nawrocki, & Cleveland, 2007; Carew, Nawrocki, Kahue, et al., 2007). Similarly, the synergy between CQ and the PI3K–mTOR inhibitor NVP-BEZ235 induced apoptosis in glioma xenografts (Fan et al., 2010). Likewise, CQ enhanced cyclophosphamide-induced tumor cell death in a Myc-induced murine lymphoma similar to that shown by shRNA knockdown of Atg5, and it delayed the time-to-tumor recurrence (Amaravadi et al., 2007). These studies documented that CQ, or its analog hydroxychloroquine, when used as autophagy inhibitors in combination with proapoptotic drugs, increases twofold the median survival of cancer patients (Carew, Nawrocki, & Cleveland, 2007; Carew, Nawrocki, Kahue, et al., 2007; Fimia et al., 2007; Garber, 2011; Savarino, Lucia, Giordano, & Cauda, 2006; Sotelo, Briceño, & López-González, 2006).

One underlying concern is that autophagy inhibitors approved for cancer patients might actually act as promoters of tumor development. However, the tumor-promoting effect of autophagy inhibitors, which depends on necrotic cell lysis that follows the inflammatory response, could prevent this undesirable effect upon cotreatment with immunosuppressive drugs (Chen & Karantza, 2011; Høyer-Hansen & Jäättelä, 2008).

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8. CONCLUSIONS

Although the multiple roles of autophagy in cancer require further clarification, it is obvious that autophagy is directly involved in many important physiological processes such as metabolism, response to stress, and cell death pathways in cancer cells. Both tumor suppressor genes and oncogenes are implicated in autophagy regulation, thereby linking autophagy directly to cancer development and progression. Interestingly, autophagy also limits necrosis and inflammation and may restrict the invasion and dissemination of tumor cells from a primary site, thereby inhibiting a critical and early event in metastasis (Fig. 2.3). In contrast, autophagy may paradoxically promote metastasis at later stages by protecting detached and stressed tumor cells as they travel through blood vessel and establish new colonies at distant sites (Fig. 2.3). Accordingly, it is suggested that autophagy might elicit disparate effects in tumors at different stages of progression. Therapy-stimulated accumulation of autophagosomes is by itself not sufficient to draw any conclusions whether autophagy has a lethal or protective function. Accordingly, the role of autophagy in cancer raises a number of intriguing questions. Does autophagy play any direct or indirect role in cancer development and progression? If it does, what is its exact contribution in cancer development and progression? Does autophagy regulate cancer stem cell development, and if it does is its pattern of regulation different from that in normal stem cells? Are there any genetic and cellular physiologic conditions that direct when and how autophagy facilitates cancer cells to survive or causes them to die? Can autophagy be exploited as a means of enhancing cancer therapies? Considering the potential seminal roles of autophagy in both normal and abnormal cellular physiology it is important to unravel its complex regulation. This information will be crucial if one is to exploit autophagy in the future as a potential therapeutic for advanced cancers and potentially other proliferative diseases.

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Acknowledgments

Research support was provided in part by the National Institutes of Health grants R01 CA097318 (P. B. F.), R01 CA127641 (P. B. F.), R01 CA134721 (P. B. F.), R01 CA138540 (D. S.), and P01 CA104177 (P. B. F. and P. D.), and Department of Defense (DOD) Prostate Cancer Research Program (PCRP) Synergistic Idea Development Award W81XWH-10-PCRP-SIDA (P. B. F. and X.-Y. W.), the National Foundation for Cancer Research (P. B. F.), the Samuel Waxman Foundation for Cancer Research (D. S. and P. B. F.), the James S. McDonnell Foundation (D. S.) and a Rapid Grant for Young Investigator (RGYI) award, Department of Biotechnology, Government of India (S. K. B.). D. S. is a Harrison Scholar and a Blick Scholar in the VCU MCC and the VCU SOM. P. B. F. holds the Thelma Newmeyer Corman Chair in Cancer Research in the VCU MCC.

Sumber :

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4349374/

Food wastage: Key facts and figures

FAO's new report is the first study to focus on the environmental impacts of food wastage.

The global volume of food wastage is estimated at 1.6 billion tonnes of "primary product equivalents." Total food wastage for the edible part of this amounts to 1.3 billion tonnes.

Food wastage's carbon footprint is estimated at 3.3 billion tonnes of CO2 equivalent of GHG released into the atmosphere per year.

The total volume of water used each year to produce food that is lost or wasted (250km3) is equivalent to the annual flow of Russia's Volga River, or three times the volume of Lake Geneva.

Similarly, 1.4 billion hectares of land - 28 percent of the world's agricultural area - is used annually to produce food that is lost or wasted.

Agriculture is responsible for a majority of threats to at-risk plant and animal species tracked by the International Union for Conservation of Nature.

A low percentage of all food wastage is composted: much of it ends up in landfills, and represents a large part of municipal solid waste. Methane emissions from landfills represents one of the largest sources of GHG emissions from the waste sector.

Home composting can potentially divert up to 150 kg of food waste per household per year from local collection authorities.

Developing countries suffer more food losses during agricultural production, while in middle- and high-income regions, food waste at the retail and consumer level tends to be higher.

The direct economic consequences of food wastage (excluding fish and seafood) run to the tune of $750 billion annually.

Sumber :

https://www.fao.org/news/story/en/item/196402/icode/

The Future of Sustainable Clothing – New Breakthrough in Synthetic Spider Silk Fabrication

TOPICS:EngineeringSilkSpidersSynthetic BiologyWashington University In St. Louis

MAY 3, 2023

Spider Web Darkness

Researchers have made a breakthrough in synthetic spider silk production, potentially enabling more sustainable clothing manufacturing. By using engineered mussel foot proteins to create bi-terminal Mfp fused silks (btMSilks), they achieved an eightfold increase in yield and improved strength and toughness, paving the way for an eco-friendly alternative to traditional textiles.

A team of engineers has discovered a technique for producing synthetic spider silk with high yield, while maintaining its strength and toughness, using mussel foot proteins.

The remarkable properties of spider silk have fascinated scientists for a long time, as it boasts strength surpassing steel yet remains lightweight and flexible. Fuzhong Zhang, a professor of energy, environmental, and chemical engineering at Washington University in St. Louis McKelvey School of Engineering, has made a major advancement in creating synthetic spider silk, opening doors for a new age of sustainable clothing production.

Since he engineered recombinant spider silk in 2018 using bacteria, Zhang has been focused on enhancing the production of silk threads from microbes, while preserving its desirable traits, such as increased strength and durability.

Higher yields will be critical if synthetic silk is to be used in everyday applications, particularly in the fashion industry where renewable materials are much in demand to stem the environmental impacts that come from producing an estimated 100 billion garments and 92 million tons of waste each year.

With the help of an engineered mussel foot protein, Zhang has created new spider silk fusion proteins, called bi-terminal Mfp fused silks (btMSilks). Microbial production of btMSilks have eightfold higher yields than recombinant silk proteins, and the btMSilk fibers have substantially improved strength and toughness while being lightweight. This could revolutionize clothing manufacturing by providing a more eco-friendly alternative to traditional textiles. The findings were recently published in the journal Nature Communications.

“The outstanding mechanical properties of natural spider silk come from its very large and repetitive protein sequence,” Zhang said. “However, it is extremely challenging to ask fast-growing bacteria to produce a lot of repetitive proteins.

“To solve this problem, we needed a different strategy,” he said. “We went looking for disordered proteins that can be genetically fused to silk fragments to promote molecular interaction so that strong fibers can be made without using large repetitive proteins. And we actually found them right here in work we’ve already been doing on mussel foot proteins.”

Mussels secrete these specialized proteins on their feet to stick to things. Zhang and his collaborators have engineered bacteria to produce them and engineer them as adhesives for biomedical applications. As it turns out, mussel foot proteins are also cohesive, which enables them to stick to each other well, too. By placing mussel foot protein fragments at the ends of his synthetic silk protein sequences, Zhang created a less repetitive, lightweight material that’s at least twice as strong as recombinant spider silk.

The yields on Zhang’s material increased eightfold compared with past studies, reaching 8 grams of fiber material from 1 liter of bacterial culture. This output constitutes enough fabric to test for use in real products.

“The beauty of synthetic biology is that we have lots of space to explore,” Zhang said. “We can cut and paste sequences from various natural proteins and test these designs in the lab for new properties and functions. This makes synthetic biology materials much more versatile than traditional petroleum-based materials.”

In coming work, Zhang and his team will expand the tunable properties of their synthetic silk fibers to meet the exact needs of each specialized market.

“Because our synthetic silk is made from cheap feedstock using engineered bacteria, it presents a renewable and biodegradable replacement for petroleum-derived fiber materials like nylon and polyester,” Zhang said.

Reference: “Bi-terminal fusion of intrinsically-disordered mussel foot protein fragments boosts mechanical strength for protein fibers” by Jingyao Li, Bojing Jiang, Xinyuan Chang, Han Yu, Yichao Han and Fuzhong Zhang, 14 April 2023, Nature Communications.

DOI: 10.1038/s41467-023-37563-0

Sumber :

https://scitechdaily.com/the-future-of-sustainable-clothing-new-breakthrough-in-synthetic-spider-silk-fabrication/

Scientists find link between photosynthesis and 'fifth state of matter'

MAY 3, 2023

by Louise Lerner, University of Chicago

Two pathways represented by the two sites of each chromophore. When ξ=0 , there is no cross-site interchromophore coupling, as shown. When V>0 , there is coupling, resulting in quantum interference between the sites of each chromophore. The waves demonstrate that the quantum interference can be either constructive or destructive, with constructive interference enhancing the energy-transfer efficiency. Credit: PRX Energy (2023). DOI: 10.1103/PRXEnergy.2.023002

Inside a lab, scientists marvel at a strange state that forms when they cool down atoms to nearly absolute zero. Outside their window, trees gather sunlight and turn them into new leaves. The two seem unrelated—but a new study from the University of Chicago suggests that these processes aren't so different as they might appear on the surface.

The study, published in PRX Energy on April 28, found links at the atomic level between photosynthesis and exciton condensates—a strange state of physics that allows energy to flow frictionlessly through a material. The finding is scientifically intriguing and may suggest new ways to think about designing electronics, the authors said.

"As far as we know, these areas have never been connected before, so we found this very compelling and exciting," said study co-author Prof. David Mazziotti.

Mazziotti's lab specializes in modeling the complicated interactions of atoms and molecules as they display interesting properties. There's no way to see these interactions with the naked eye, so computer modeling can give scientists a window into why the behavior happens—and can also provide a foundation for designing future technology.

In particular, Mazziotti and study co-authors Anna Schouten and LeeAnn Sager-Smith have been modeling what happens at the molecular level when photosynthesis occurs.

When a photon from the sun strikes a leaf, it sparks a change in a specially designed molecule. The energy knocks loose an electron. The electron, and the "hole" where it once was, can now travel around the leaf, carrying the energy of the sun to another area where it triggers a chemical reaction to make sugars for the plant.

Together, that traveling electron-and-hole-pair is referred to as an "exciton." When the team took a birds-eye view and modeled how multiple excitons move around, they noticed something odd. They saw patterns in the paths of the excitons that looked remarkably familiar.

In fact, it looked very much like the behavior in a material that is known as a Bose-Einstein condensate, sometimes known as "the fifth state of matter." In this material, excitons can link up into the same quantum state—kind of like a set of bells all ringing perfectly in tune. This allows energy to move around the material with zero friction. (These sorts of strange behaviors intrigue scientists because they can be the seeds for remarkable technology—for example, a similar state called superconductivity is the basis for MRI machines).

According to the models created by Schouten, Sager-Smith and Mazziotti, the excitons in a leaf can sometimes link up in ways similar to exciton condensate behavior.

This was a huge surprise. Exciton condensates have only been seen when the material is cooled down significantly below room temperature. It'd be kind of like seeing ice cubes forming in a cup of hot coffee.

"Photosynthetic light harvesting is taking place in a system that is at room temperature and what's more, its structure is disordered—very unlike the pristine crystallized materials and cold temperatures that you use to make exciton condensates," explained Schouten.

This effect isn't total—it's more akin to "islands" of condensates forming, the scientists said. "But that's still enough to enhance energy transfer in the system," said Sager-Smith. In fact, their models suggest it can as much as double the efficiency.

This opens up some new possibilities for generating synthetic materials for future technology, Mazziotti said. "A perfect ideal exciton condensate is sensitive and requires a lot of special conditions, but for realistic applications, it's exciting to see something that boosts efficiency but can happen in ambient conditions."

Mazziotti said the finding also plays into a broader approach his team has been exploring for a decade.

The interactions between atoms and molecules in processes like photosynthesis are incredibly complex—difficult even for a supercomputer to handle—so scientists have traditionally had to simplify their models in order to get a handle on them. But Mazziotti thinks some parts need to be left in: "We think local correlation of electrons are essential to capturing how nature actually works."

Sumber :

https://phys.org/news/2023-05-scientists-link-photosynthesis-state.html

New catalyst transforms carbon dioxide into sustainable byproduct

MAY 3, 2023

by Win Reynolds, Northwestern University

CO-to-acetate electrocatalyst design. Credit: Nature (2023). DOI: 10.1038/s41586-023-05918-8

The need to capture CO2 and transport it for permanent storage or conversion into valued end uses is a national priority recently identified in the Bipartisan Infrastructure Law to move toward net-zero greenhouse gas emissions by 2050.

Now, Northwestern University researchers have worked with an international team of collaborators to create acetic acid out of carbon monoxide derived from captured carbon. The innovation, which uses a novel catalyst created in the lab of professor Ted Sargent, could spur new interest in carbon capture and storage.

"Carbon capture is feasible today from a technical point of view, but not yet from an economic point of view," Sargent said. "By using electrochemistry to convert captured carbon into products with established markets, we provide new pathways to improving these economics, as well as a more sustainable source for the industrial chemicals that we still need."

The paper was published in the journal Nature.

Sargent, the paper's corresponding author, is Northwestern's Lynn Hopton Davis and Greg Davis Professor of Chemistry at the Weinberg College of Arts and Sciences and a professor of electrical and computer engineering at the McCormick School of Engineering. His team has a track record of using electrolyzers—devices in which electricity drives a desired chemical reaction forward—to convert captured carbon into key industrial chemicals, including ethylene and propanol.

Though acetic acid may be most familiar as the key component in household vinegar, recent University of Toronto Ph.D. recipient Josh Wicks, one of the paper's four co-lead authors, said this use accounts for only a small proportion of what it's used for.

"Acetic acid in vinegar needs to come from biological sources via fermentation because it's consumed by humans," Wicks said. "But about 90% of the acetic acid market is for feedstock in the manufacture of paints, coatings, adhesives and other products. Production at this scale is primarily derived from methanol, which comes from fossil fuels."

Lifecycle assessment databases showed the team that for every kilogram of acetic acid produced from methanol, the process releases 1.6 kg of CO2.

Their alternative method takes place via a two-step process: first, captured gaseous CO2 is passed through an electrolyzer, where it reacts with water and electrons to form carbon monoxide (CO). Gaseous CO is then passed through a second electrolyzer, where another catalyst transforms it into various molecules containing two or more carbon atoms.

"A major challenge that we face is selectivity," Wicks said. "Most of the catalysts used for this second step facilitate multiple simultaneous reactions, which leads to a mix of different two-carbon products that can be hard to separate and purify. What we tried to do here was set up conditions that favor one product above all others."

Vinayak Dravid, another senior author on the paper and the Abraham Harris Professor of Materials Science and Engineering, is the founding director of the Northwestern University Atomic and Nanoscale Characterization (NUANCE) Center, which allowed the team to access diverse capabilities for atomic- and electronic-scale measurements of materials.

"Modern research problems are complex and multifaceted and require diverse yet integrated capabilities to analyze materials down to the atomic scale," Dravid said. "Colleagues like Ted present us with challenging problems that stimulate our creativity to develop novel ideas and innovative characterization methods."

The team's analysis showed that using a much lower proportion of copper (approximately 1%) compared with previous catalysts would favor the production of just acetic acid. It also showed that elevating the pressure to 10 atmospheres would enable the team to achieve record-breaking efficiency.

In the paper, the team reports a faradic efficiency of 91%, meaning that 91 out of every 100 electrons pumped into the electrolyzers end up in the desired product—in this case, acetic acid.

"That's the highest faradic efficiency for any multi-carbon product at a scalable current density we've seen reported," Wicks said. "For example, catalysts targeting ethylene typically max out around 70% to 80%, so we're significantly higher than that."

The new catalyst also appears to be relatively stable: while the faradic efficiency of some catalysts tend to degrade over time, the team showed that it remained at a high level of 85% even after 820 hours of operation.

Wicks hopes that the elements that led to the team's success—including a novel target product, a slightly increased reaction pressure, and a lower proportion of copper in the catalyst—inspire other teams to think outside the box.

"Some of these approaches go against the conventional wisdom in this field, but we showed that they can work really well," he said. "At some point, we're going to have to decarbonize all the elements of chemical industry, so the more different pathways we have to useful products, whether it's ethanol, propylene or acetic acid, the better."

Sumber :

https://phys.org/news/2023-05-catalyst-carbon-dioxide-sustainable-byproduct.html

Carbon Capture Is Beginning To Take Off

By Tsvetana Paraskova - May 02, 2023, 5:00 PM CDT

Carbon capture

Carbon capture projects and carbon removal credits have received new impetus with major government support over the past year as part of the solutions to cut greenhouse gas emissions and put the world on track to reach the Paris Agreement targets. 

In the U.K., the Spring Budget in March made up to $25 billion (£20 billion) available for Carbon Capture, Utilization and Storage (CCUS), while the U.S. Inflation Reduction Act has significantly raised the incentives for carbon capture projects, including direct air capture (DAC).  

As governments move to back carbon capture projects and corporations look to reduce their carbon footprint, the market for carbon removal projects and carbon removal credits is expected to thrive in the coming years. 

The schemes face criticism from environmental advocates who say that carbon removal credits do not address the problem of emissions reduction and could lead to more greenwashing from the big polluters. 

Government Support Accelerates Carbon Capture Projects

The U.K. government pledged to provide up to £20 billion in funding for early deployment of Carbon Capture, Usage and Storage (CCUS) to help meet the government’s climate commitments. 

The government recognized the Viking CCS project as one of two leading transport and storage system contenders for the next phase of projects. This has incentivized supermajor B.P. to enter into an agreement with Harbour Energy, the biggest oil producer in the U.K. North Sea, to develop the Viking CCS project.

In the United States, the IRA increased credit values across the board, with the tax credit for carbon storage from carbon capture on industrial and power generation facilities rising from $50 to $85 per ton, and the tax incentives for storage from DAC jumping from $50 to $180 per ton. The provisions also extend the construction window by seven years to January 1, 2033. This means that projects must begin physical work by then to qualify for the credit. 

Related: WTI Crude Falls 4% As Economic Fears Trigger Selloff

The significantly higher incentives in the IRA are giving impetus to projects. 

“The CCS market has just taken off,” Nick Cooper, CEO at carbon capture and storage developer Storegga, told the Financial Times. 

“This feels a bit like the U.S. shale boom 15 years ago”.

The historic legislation “builds the foundation for a budding direct air capture industry in the U.S.,” says Aaron Benjamin, UK and Europe Lead at Direct Air Capture Coalition. 

“Above all, the IRA sends a strong signal to the rest of the world that the U.S. is backing the reality of a carbon capture and removal industry,” Benjamin added. 

New Life For Carbon Capture Projects  

The IRA and the growing commitment of companies – from banks to the fashion industry – to become carbon neutral within a decade or two are spurring construction projects in the U.S. and the U.K. 

Occidental, for example, via its subsidiary 1PointFive, held last week a groundbreaking ceremony for its first Direct Air Capture facility in the Permian basin in West Texas. The facility, STRATOS, will be the world’s largest direct air capture facility, expected to capture up to 500,000 tons of CO2 per year. It will be the first of many such plants Oxy and 1PointFive plan to build, the oil giant says. 

DAC is the most expensive application of carbon capture, the International Energy Agency (IEA) says. Capture cost estimates for DAC are estimated at between $125 and $335 per ton of CO2 for a large-scale plant built today. 

But the incentives in the IRA could bridge the gap in costs, analysts say. 

Carbon Removal Deals Abound 

Companies are signing long-term carbon credit agreements with developers of carbon capture technologies, which supports the investment case of CCS projects, according to experts.  

Just last month, major deals for carbon removal and credits were signed. 

NextGen, a joint venture of climate project developer South Pole and Mitsubishi Corporation, announced the advance purchase of 193,125 tons of carbon dioxide removals (CDRs) from carbon removal projects, including from 1PointFive’s DAC project in Texas. 

Partners Group, a global private markets firm, signed last month a 13-year agreement with Climeworks, a Swiss provider of carbon dioxide removal via direct air capture. Partners Group announced last year that it would develop a decarbonization program to achieve net-zero corporate greenhouse gas emissions by 2030.  

“While a priority of the program will be to reduce the firm’s overall emissions, removing residual emissions via capture and storage of atmospheric CO2 will also play a role in achieving the net zero goal,” Partners Group said. 

“High-quality carbon removal must be scaled to gigaton level by 2050, and multi-year agreements like this one are a crucial lever,” said Christoph Gebald, co-founder and co-CEO of Climeworks. 

“Partners Group’s commitment to high-quality carbon removals underlines the leading role of the financial services industry in this scale-up.” 

Climate groups, however, are not convinced that carbon removal deals would accelerate global emissions reduction.  

For example, the European Commission’s proposed Carbon Removal Certification Framework (CRCF) “leaves many important questions unanswered and vital issues unaddressed, and could usher in an era of greenwashed and money-wasting carbon removals,” non-profit think tank Carbon Market Watch says. 

In the EC’s draft regulation, “there is a risk for the framework to be turned into a greenwashing exercise and provide another excuse for big polluters to avoid cutting their emissions,” according to WWF.   

By Tsvetana Paraskova for Oilprice.com

Sumber :

https://oilprice.com/Energy/Energy-General/Carbon-Capture-Is-Beginning-To-Take-Off.html

Journal home page for Cell Reports Physical Science

17 May 2023, 101383

Article

Synthesis of stable single-crystalline carbon dioxide clathrate powder by pressure swing crystallization

Author links open overlay panelZhiling Xiang 1 5, Congyan Liu 1 5, Chunhui Chen 1, Xin Xiao 2, Thien S. Nguyen 3, Cafer T. Yavuz 3, Qiang Xu 2 4, Bo Liu 1 6

Summary

Reversible CO2 capture and release under ambient conditions is crucial for energy-efficient carbon capture and storage. Here, we report the pressure swing crystallization of CO2 in a single-crystalline guanidinium sulfate-based clathrate salt under practical conditions of 52 kPa and 298 K, with a high CO2 density (0.252 g cm−3) and capacity (17 wt %). The captured CO2 is released as a pure stream through moderate means of pressure or temperature stimulation, all while the desorbed Gua2SO4 is ready for another cycle. The clathrate is selective exclusively to CO2 even in the presence of common flue gas components, such as water vapor and N2, owing to the specific electrostatic interaction between the CO2 and guanidinium cations. The mechanism unraveled through single-crystal studies is distinctively different from physisorption or chemisorption, opening up a promising venue for future carbon capture and storage technologies through rapid CO2 solidification using an abundant salt.

Graphical abstract

Keywords

carbon captureadsorptionCO2 storageguanidinium sulfategas hydratessustainable chemistryflue gas treatmentdirect air capture

Introduction

Reversible carbon capture and release in an energy-efficient way is vital toward many industrial processes, particularly in efforts to address the current global warming crisis. In general, CO2 capture solutions can be classified into physisorption and chemisorption processes through temperature or pressure swing operations. But each strategy possesses its own merits and shortcomings. Physisorption of CO2 via weak interactions mainly involves porous materials with high surface areas, such as porous carbons,1,2 zeolites,3,4 metal-organic frameworks,5,6,7 covalent-organic frameworks,8 and hydrogen-bonded organic frameworks,9 giving rise to low sorption heat and easy adsorbent regeneration. Parasitic molecules like water, however, compete with CO2 and deteriorate the selectivity, capacity, and cycling performance for carbon capture and ultimately increase the energy consumption for regeneration in practical scenarios.10,11,12,13 Chemisorption of CO2, on the other hand, usually produces high heat and therefore requires intensive energy input for absorbent regeneration.14,15,16 This is because the captured CO2 is converted into carbamates and carbonates (HCO3− or CO32−) when brought into contact with aqueous amine solutions or tethered amines on porous supports. One example is guanidine, a multi-amine construct, which has been explored as a chemisorptive CO2 absorbent for direct carbon capture from air.17,18,19,20 In nature, guanidine-based CO2 capture is a chemical conversion process taking advantage of basicity of guanidine, similar to carbon capture using alkaline solutions or monoethanolamine. This chemical product required high regeneration energy, usually at temperatures higher than 120°C. An interesting example is a 3D hydrogen-bonded framework assembled from tetrahedral tetraamidinium cations and carbonates via nonelectrostatic hydrogen bond in water,21 but utility for a cyclic CO2 capture through dynamic complexation is not clear.

Gas hydrates such as CO2 hydrates are suitable for a rapid, reversible carbon capture. They are, however, often generated at low temperatures and high pressures (for example, T = 0°C and P = 1,200 kPa). In a CO2 hydrate, CO2 is enclosed in a water cage constructed through hydrogen bonds.22 In other words, CO2 forces icy water to crystallize into a hydrate framework, where it is also known to trap other guest species such as CH4, N2, or small organic molecules.23 Under increasing temperature or reduced pressure, for example bringing CO2 clathrates to ambient conditions, the water cages collapse, and trapped guest molecules escape. Laboratory syntheses of gas hydrates have been accomplished.24 The kinetics of clathrate formation, however, remain unchanged, where the conditions of low temperature and high pressure are indispensable, making CO2 hydrates impractical for applications of gas adsorption, separation, and storage. Recently, we reported a reversible structural transformation of [B(OCH3)4]3[C(NH2)3]4Cl⋅4CH3OH upon MeOH capture and release as an example of achieving dynamic behavior of gas hydrates at ambient conditions.25 Carbon capture and storage as a powder based on clathrate formation using CO2 hydrate is challenging and remains elusive, to the best of our knowledge.

Herein, we report a stable CO2 clathrate powder formation under ambient conditions through co-crystallization of CO2 with guanidinium sulfate (Gua2SO4, where Gua is guanidinium) from an aqueous Gua2SO4 solution (Figure 1). As revealed by our single-crystal studies, crystallization of CO2@Gua2SO4 is triggered by dominant electrostatic interactions between CO2 and guanidinium ions, which are encased among the strong hydrogen bond interactions between guanidinium cations and sulfate. CO2@Gua2SO4 readily releases CO2 through structure collapsing in ambient conditions, and the resultant Gua2SO4 is ready for another cycle without requiring further regeneration (Figure 1). The volume and weight densities of CO2 in CO2@Gua2SO4 are determined to be 0.252 g cm−3 and 17 wt %, respectively, revealing its tremendous potential for carbon capture and storage in practical conditions.

Figure 1. Schematic demonstration of reversible clathrate formations with pressure swing for CO2 capture and release process

(A) CO2 hydrate where CO2 molecules are trapped in water clusters at high pressures and low temperatures.

(B) A clathrate from guanidinium sulfate and CO2, CO2@Gua2SO4, where CO2 can be captured at pressures as low as 32 kPa and temperatures at flue gas conditions (35°C and below).

Results

Synthesis and crystal structure of CO2@Gua2SO4

Upon charging an aqueous Gua2SO4 solution with CO2, we observe spontaneous formation of a single-crystalline CO2@Gua2SO4 (see experimental procedures for details; Figure S1–S7, 2A, and 2B). The crystal structure of CO2@Gua2SO4 is determined by single-crystal X-ray diffraction experiments at 100 K (Figure S2; Table S1). In CO2@Gua2SO4, each guanidinium ion adopts two sets of hydrogen bonds (H-bonds) connecting three SO42− ions (Figure 2C), while each sulfate ion connects six guanidinium ions via multiple H-bonds (Figure 2F). The extended H-bond system (Figure S3; Table S2) in 3D results in a H-bonded framework crystallized at a tetragonal space group in which four CO2 molecules are accommodated in each unit cell (Figures 2E, 2F, and S4). As shown in Figure S5, an irregular cage comprised of five sulfate and eight guanidinium ions can be identified, in which one CO2 molecule is accommodated, and the cages are stacked by plane sharing. In contrast, free Gua2SO4 crystallizes with a cubic space group in a dense-stacking mode, in which the H-bond connection modes between Gua+ and SO42− ions are notably different (Figure S7).

Figure 2. Formation and single-crystal structure of the clathrate, CO2@Gua2SO4

(A) Photos of CO2@Gua2SO4 precipitation under CO2 atmosphere and magnetic stirring. Test tube diameter is 2.8 cm.

(B) Optical image of CO2@Gua2SO4 crystal. Scale bar is 500 μm.

(C) Hydrogen-bond interactions of guanidinium cations with three SO42− ions.

(D) Hydrogen-bond interactions of SO42− with six guanidinium ions.

(E and F) 3D hydrogen-bonded framework of CO2@Gua2SO4 omitting CO2 (E) and (F) with captured CO2. Dash lines represent hydrogen bonds. Hydrogen-bond interaction is deduced from single-crystal X-ray diffraction measurements. Hydrogens are omitted for clarity. Color code: gray, carbon; blue, nitrogen; red, oxygen; yellow, sulfur.

Surprisingly enough, in CO2@Gua2SO4, a H-bond interaction between CO2 and Gua2SO4 is not favored because the distances and angles are out of range for a typical H-bond interaction (Figure S8), and this creates a tremendous opportunity for a reversible CO2 capture. In a molecule of CO2, C and O atoms bear partially positive and negative charges, respectively. The C atom exists as a carbocation and N atoms bear partial negative charge in a guanidinium cation. The distance among these oppositely charged atoms ranges from 3.4 to 3.9 Å (Figure 3A) so that the electrostatic interaction is more pronounced. One CO2 molecule interacts with three guanidinium ions with a “triple-team structure,” ensuring an effective coulombic force. The distance between positive C in CO2 and negative O in SO42− is determined to be 5.026 Å (Figure 3B), indicating a much weaker electrostatic interaction. We ascribe these multiple electrostatic interactions as the main driving forces for CO2-induced crystallization of CO2@Gua2SO4 from aqueous solution under CO2 atmosphere. The solid-state magic-angle spin 13C nuclear magnetic resonance (NMR) spectrum of CO2@Gua2SO4 displays a sharp chemical shift at 124.9 ppm (Figure 3C), equal to the chemical shift of CO2 in a physisorption state.26,27 This is consistent with the result from crystal structure analysis as mentioned above. The other peak at 159.2 ppm is assigned to the carbocation in the guanidinium. A similar peak with stretching vibration of gas phase CO2 appears in the infrared (IR) spectrum of CO2@Gua2SO4 at 2,335 cm−1, which also suggests the weak interaction of CO2 with guanidinium and SO42− ions (Figure 3D). It is worth noting that when reacting with hydroxyl and amine functional groups, CO2 is often converted into carbonate ester or carbamate.14,15,16,28 Owing to the positive charge, guanidinium cations with three amine groups interact with CO2 via an electrostatic interaction instead of a chemical reaction. This moderate but abundant interaction is strong enough to pull guanidiniums and SO42− together for CO2 clathrate formation and crystallization in aqueous solution. In synthetic terms, the CO2 inclusion mechanism unraveled by single-crystal structure analysis manifests that amino groups could interact with CO2 via electrostatic interactions instead of a chemical reaction, owing to the delocalization of lone-pair electrons from amino functionalities to carbocations of guanidinium ions. The results point to a substantial parameter space to tune the interactions between CO2 and sorbents for optimized carbon capture in terms of energetics and economics.

Figure 3. Interaction of CO2 with guanidinium and sulfate ions

(A) Electrostatic interactions between CO2 and guanidinium cations.

(B) Distance between CO2 and sulfate ions in CO2@Gua2SO4 based on single-crystal X-ray diffraction data.

(C) Solid-state magic-angle spin 13C NMR spectrum of CO2@Gua2SO4 and Gua2SO4.

(D) ATR-IR spectra of CO2@Gua2SO4 and Gua2SO4.

In order to screen counter anions on guanidinium, we have tested a series of guanidinium salts for CO2 clathrate potential, including nitrate, chloride, and phosphate (see details in experimental procedures). None, however, resulted in any precipitate under the same conditions using guanidinium cations. Chloride ion is not favorable for H-bond formation,29 while nitrate ion and guanidinium generate a plane structure, as both ions are planar,30 and therefore not beneficial for CO2 clathrate formation. The complicated hydrolysis process of phosphate in aqueous solution may account for the failure of CO2 co-precipitation.31 It is safe to conclude that the complex H-bond interactions between guanidinium and sulfate are crucial for crystal formation and CO2 capture.

Optimization of CO2 clathrate formation

In an attempt to explore the boundary conditions of CO2@Gua2SO4 formation, we varied pH, Gua2SO4 concentration, CO2 pressure, and temperature. Since basic aqueous solutions directly react with CO2, we tuned the pH of Gua2SO4 solutions from 1 to 7 using a H2SO4 solution. We found that pH value has little influence over CO2@Gua2SO4 formation (Figure 4A). Considering the possible influence of anion size on boundary conditions, HCl and HNO3 were also used to adjust the pH. The results showed that the sizes of the anions exert negligible effects on the formation conditions of CO2@Gua2SO4 (Figure S9). Increasing CO2 pressure led to a higher production rate of CO2@Gua2SO4 (Figure S10). The saturated Gua2SO4 concentration is determined to be 72 and 75.7 wt % in water at 273 and 298 K, respectively. In order to eliminate the influence of the Gua2SO4 concentration variation during the continuous precipitation of CO2@Gua2SO4, we used a saturated solution in equilibrium with excess Gua2SO4 powder to investigate the temperature-pressure relationship under phase equilibrium conditions. At equilibrium, the CO2 pressure-temperature relationship fitted well to the Clapeyron-Clausius equation (Figures 4B, S11, and S12), revealing the boundary conditions of CO2@Gua2SO4 formation and indicating a wide range of conditions for capturing CO2 using aqueous Gua2SO4. CO2@Gua2SO4 started to crystallize at a CO2 pressure of 32 and 52 kPa at 273 and 298 K, respectively (Figure 4B), whereas conventional CO2 hydrates are formed under 1,500 kPa at 275 K, about 50 times higher than that of CO2@Gua2SO4 formation.32 The sorption enthalpy change for CO2@Gua2SO4 is calculated to be 15.37 kJ/mol (Figure S12). This value is considerably lower than the enthalpy changes of most CO2 absorbents as summarized in Figure S1319,33,34,35,36,37 and falls into the physisorptive domain (<40 kJ/mol), suggesting minute heat requirements during CO2 capture and release and corresponding to the ease of clathrate formation and collapse. In contrast, the enthalpy change for conventional CO2 hydrate decomposition is 57.1 kJ/mol, owing to the necessity of breaking ample H-bonds among H2O molecules,32 about 3.7 times higher than that of CO2@Gua2SO4.

Figure 4. CO2 sorption behavior in a Gua2SO4 aqueous solution

(A) pH effect on CO2@Gua2SO4 formation.

(B) Pressure-temperature correlation of CO2-Gua2SO4 aqueous system under equilibrium. CO2 hydrate data is plotted as a reference.29

(C) CO2 sorption profile in aqueous Gua2SO4 with or without stirring. CO2 pressure changes of the control experiments, (i) CO2 sorption in pure water with stirring, (ii) N2 sorption in pure water, and (iii) aqueous Gua2SO4 with or without stirring, are given as a reference.

(D) Cyclic performance of CO2 sorption in aqueous Gua2SO4. The margin of error points at a ±3.5% variation.

(E) The pressure changes vs. time plot of CO2 adsorption and desorption in water and aqueous Gua2SO4 with a temperature swing between 273 (adsorption) and 303 K (desorption). Dashed lines are for pure water, and solid lines represent aqueous Gua2SO4.

(F) PXRD data for structural evolution from CO2@Gua2SO4 to Gua2SO4 in air. PXRD patterns of CO2@Gua2SO4 simulated from crystallographic data and as-prepared Gua2SO4 are given for reference (peaks marked as blue and orange belong to CO2@Gua2SO4 and Gua2SO4, respectively).

The CO2 in CO2@Gua2SO4 solidifies at near-ambient conditions with four CO2 molecules accommodated in each unit cell with a cell volume of 1,159 Å3, corresponding to a CO2 volume density of 0.252 g cm−3. This is equal to about 140 m3 CO2 in one cubic meter of CO2@Gua2SO4. Gravimetrically, the CO2 loading in CO2@Gua2SO4 is calculated to be 17 wt %. As shown in Figure S14, 2.3 g CO2 was stored in a 20 mL glass bottle, which corresponds to 1.2 L CO2 gas at standard conditions at room temperature. In contrast, to store the same amount of CO2 gas in a 20 mL bottle, the pressure will have to reach 6,000 kPa at 0°C. Hence, CO2@Gua2SO4 is an ideal candidate for CO2 storage and transport. Other reported CO2 clathrates, in general, are formed under low-temperature and/or high-pressure conditions, as summarized in Table S3. The inability to operate in ambient conditions renders these clathrates not feasible for flue gas treatment, as large deviation of temperature and/or pressure from ambient conditions means substantial energy input for a practical carbon capture operation. Gua2SO4 is perfectly optimized for such a job, as it stores 140 m3 CO2 in 1 m3 clathrate with a gravimetric capacity of 17% (uptake at 52 kPa and room temperature [RT] and release at ambient conditions), positioning it even better than most common porous materials as summarized in Table S4. And the structure does this in a simple pressure swing adsorption (PSA) cycle, eliminating energy-intensive processes and showing quantitative capture of CO2 in the presence of N2 and H2O (as discussed in the next section). It is important to note that only liquid amine solutions (industrial standard) could do such a performance with the kinetics that a process needs.

Kinetics of CO2 clathrate formation

In an isochoric experiment, after removing air by CO2 flushing, the stainless-steel autoclave containing Gua2SO4 solution (1.5 g/g, 60 wt %) was charged with CO2 to 1,000 kPa at 273 K with constant stirring (Figure S15). The pressure drop was monitored against time, and we found that the pressure decreases in an approximatively linear fashion until 60 min and gets stable at about 100 min (Figure 4C). Replacing Gua2SO4 solution with pure water leads to a much smaller pressure drop. The latter is about one-fifth of the aqueous Gua2SO4 solution under the same conditions and results from CO2 dissolution in water and the temperature drop from RT to 273 K. N2, however, does not dissolve well and shows similar behavior in both pure water and Gua2SO4 solutions, proving that N2 cannot form any clathrates under the same conditions. Therefore, we concluded that the structure alternate between Gua2SO4 and CO2@Gua2SO4 exclusively for CO2, even in the presence of N2 and water. The final conversion yield of CO2@Gua2SO4 is around 85 wt % of Gua2SO4 according to the weight increase of the solution.

The kinetics of CO2@Gua2SO4 formation closely follows the Gua2SO4 concentration, CO2 pressure, temperature, and stirring. In principle, the formation of CO2@Gua2SO4 through the steps of gas dissolution, diffusion, nucleation, and growth is quite similar to a conventional CO2 hydrate formation process.38 In a static experiment without stirring, the CO2 pressure drop is very low and similar to the case of gaseous CO2 dissolution in water under the same period (Figure 4C). A crystalline layer was observed over the solution, preventing further gas diffusion. We recorded the crystallization process by sustaining a CO2 pressure at 300 kPa while constantly stirring (Video S1). The solution is initially clear, with CO2 dissolving and diffusing. We note that the nucleation first occurs near the vortex of stirring, which correlates to high CO2 concentrations in the vicinity. Once the nucleation is initiated, the solution becomes turbid very quickly, owing to the rapid crystal growth. Therefore, it is safe to conclude that CO2 dissolution and diffusion are the rate-determining steps for CO2@Gua2SO4 formation.

https://www.sciencedirect.com/science/article/pii/S2666386423001510

Video S1. CO2@Gua2SO4 formation under 0.3 MPa (3 bar) CO2 pressure. The movie is set to play at an increased speed of 20 times

Even though physisorptive CO2 capture processes are well known to be durable with long cycle life, we repeated the sorption process for ten cycles, and the data exhibit excellent reproducibility, as Gua2SO4 is stable and nonvolatile and does not degrade or become lost during the continuous operations (Figures 4D, S16, and S17A). In a temperature swing experiment, we warmed up the autoclave to 303 K after adsorption reached equilibrium at 273 K and observed the pressure increasing (Figure 4E). This is clearly associated with CO2@Gua2SO4 collapse and CO2 release. The final pressure is slightly higher than initial value owing to the higher final temperature of the autoclave. The results reveal a reversible formation and decomposition of a salt-CO2 clathrate accompanied by CO2 capture and release near RT.

Cyclic CO2 capture

The precipitates of CO2@Gua2SO4 can be separated via centrifugation or a simple filtration. In open air, dry CO2@Gua2SO4 decomposes with CO2 release, as verified by powder X-ray diffraction (PXRD), but the decomposition in solid state is sluggish (Figure 4F). The PXRD of the fresh CO2@Gua2SO4 sample matched well with the pattern simulated from single-crystal (SC) XRD data, with some deviation at 15.0° owing to its deposition method at sample preparation and the difference of measurement temperature (Figure S6). Note that the main peaks at 14.6° and 15.3° of CO2@Gua2SO4 gradually fade, while new peaks at low angles of 11.2°, 12.3°, and 15.0° belonging to the Gua2SO4 phase appear and grow with prolonged exposure time in air. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses of the CO2@Gua2SO4 sample in air are shown in Figure S17B. The weight loss of ca. 13.7% before 86.5°C is originated from the removal of CO2, which is slightly less than 17% (theoretical capacity of CO2 in CO2@Gua2SO4). It can be attributed to CO2@Gua2SO4 slightly decomposing with CO2 release during sample preparation and measurements. Upon adding water to a CO2@Gua2SO4 powder, we observe rapid CO2 release with vigorous bubbling, and the solids disappear because Gua2SO4 salt, the only decomposition product, is highly soluble (Video S2). Judging from the shrinking solids, we project the CO2 release proceeds from the outside in. The formed Gua2SO4 on the surface of CO2@Gua2SO4 could block further CO2 release and slow down the decomposition rate, but as Gua2SO4 is dissolved out with water, the CO2 release remains unaltered. The decarbonized Gua2SO4 solid or solution is ready for reuse without requiring any further regeneration step. The released CO2 purity from CO2@Gua2SO4 is 100% except when regeneration was done by water and therefore saturated with water vapor and can be used directly in the CO2 market including in refrigerant, beverage, pharmaceutical, and food storage sectors.

https://www.sciencedirect.com/science/article/pii/S2666386423001510

Video S2. Accelerated CO2@Gua2SO4 collapse and CO2 release with water addition.

CO2 removal from flue gas

Since CO2 capture is conducted in an aqueous solution and moisture exerts no influence over CO2 capture, we tested CO2 removal performance of a Gua2SO4-saturated solution with a dry simulated flue gas of different molar ratios of N2 and CO2. The CO2 percentage in a flue gas is typically in the range of 5%–15% depending on the industrial processes and sources.39 With limited CO2 partial pressure, we pressurized flue gas in order to reach a high CO2 presence. A schematic illustration of setup is shown in Figure 5A. Figure 5B and Table S5 summarize CO2 capture performances at varying total flue gas pressures. Obviously, the higher pressure leads to higher CO2 removal. In the case of a CO2 percentage at 15%, total flue gas needs to be pressurized to 5,000–6,000 kPa to meet DOE targets in terms of a 90% CO2 capture. When the CO2 percentage is increased to 30% (i.e., from the cement industry), the flue gas pressure can be as low as 2,000–3,000 kPa for a 90% CO2 removal. We also note that the CO2 content in natural gas streams can reach over 40% in some cases.40 The high CO2 content will lower the total pressure of mixtures and therefore reduce the CO2 removal cost based on the physisorptive clathrate mechanism of the CO2@Gua2SO4.

Figure 5. CO2 removal performance of Gua2SO4-saturated solution with a dry simulated flue gas

(A) Schematic illustration of the setup.

(B) CO2 removal percentages against total flue gas pressure with CO2 mole percentages at 15 and 30 mol %.

Discussion

In this work, we reported the first ambient example of mimicking CO2 hydrate structure with CO2 as guest molecules. A simple Gua2SO4 co-crystallizes with CO2 into a stable clathrate (CO2@Gua2SO4). The clathrate releases CO2 on demand, and both adsorption and desorption can occur at mild conditions. The H-bonded framework assembled from guanidinium and sulfate mimics the water cage in a CO2 hydrate, effectively trapping four CO2 molecules per unit cell of the clathrate. Electrostatic interactions between guanidinium ion and CO2 are revealed through a single crystal (SC) study instead of the van der Waals interactions commonly observed for CO2 and water cages of CO2 hydrates.

The in situ process of co-crystallization introduced here is fundamentally different from CO2 adsorption in a preformed H-bonded organic framework and is quite unlike CO2 physisorption in other porous materials. A CO2 clathrate imitating the CO2 hydrate, therefore, exhibits unique advantages toward carbon capture. First, Gua2SO4 exclusively captures CO2 without water or moisture interference, swiftly overcoming the fatal weakness of any physisorption processes. Second, CO2 release through structure collapsing could be triggered at ambient conditions, requiring little energy input for absorbent regeneration while defeating the parasitic energy dilemma of chemisorption. Third, Gua2SO4 is stable and noncorrosive, a highly desirable feature when compared with ethanol amine, ammonia, and other basic solutions that are commonly used in carbon capture. Fourth, a stable CO2@Gua2SO4 in powder form is also beneficial for storage and transportation of CO2, benefiting from its remarkably high volume per weight capacity. In addition, by the lessons learned in this study, we can tune dynamic H-bonded frameworks with enriched structural variation, enabling us further to regulate and control the properties for further improving CO2 capture in terms of stability, recyclability, sorption capacity, and selectivity, while lowering regeneration energy penalty and cost.

Experimental procedures

Resource availability

Lead contact

Further information and request for the resources are available from the lead contact, Bo Liu (liuchem@ustc.edu.cn).

Materials availability

No unique materials were generated by this study.

Materials

All chemicals and reagents were purchased from commercial suppliers and used without further purification. Deionized water was used as solvent. Ethanol and sulfuric acid were purchased from Sinopharm Chemical Reagent. Guanidinium carbonate (Gua2CO3) was purchased from Shanghai Adamas Reagent. Nitrogen gas (99.999%), carbon dioxide (99.9%), and carbon dioxide-nitrogen mixture gas (15 mol % CO2+85 mol % N2 and 30 mol % CO2+70 mol % N2) were purchased from Nanjing Special Gas Plant.

Characterization

PXRD measurements were conducted on a Rigaku MiniFlex 600 diffractometer using Cu Kα radiation (λ = 1.5418 Å). The FTIR spectrum was measured by a Nicolet iS5 spectrophotometer with an attenuated total reflectance (ATR) module (Thermo Fisher Scientific, Waltham, MA, USA). Solid-state NMR spectra were recorded by a Bruker AVANCE NEO 600WB spectrometer with a magic-angle spin (MAS) rate of 15 kHz. The 13C resonance frequency was 150.93 MHz at the experiment. 13C single-pulse spectrum was recorded with 16 s pulse delay. TGAs were performed from 25°C to 800°C at a heating rate of 10°C/min in air on a TGA Q500 integration thermal analyzer (For CO2@Gua2SO4, the heating rate from 20°C to 300°C was 5°C/min and from 300°C to 700°C, 10°C/min). Gas chromatography (GC) was performed on a Shimadzu GC 2014 gas chromatograph fitted with a Porapak Q column and TCD detector. Ar gas (99.999%) was used as the carrier gas.

Synthesis of Gua2SO4

Gua2SO4 was synthesized simply by neutralizing guanidinium carbonate with H2SO4. Gua2CO3 (180 g, 1 mol) and deionized water (250 mL) were first mixed in a 1,000 mL beaker. Concentrated H2SO4 (98%, 54 mL, 1 mol) was then added dropwise into the beaker at a rate of one drop per second. The final pH was adjusted to 7 using only H2SO4. Absolute ethanol was added as nonsolvent under constant stirring to precipitate the product. The colorless powder was collected by vacuum filtration, followed by washing with absolute ethanol and drying in an oven at 100°C before use. Yield was calculated to be 99% based on guanidinium. The production of Gua2SO4 was verified by common characterization tools, particularly PXRD (Figure S1).

Synthesis of CO2@Gua2SO4 SC

A CO2@Gua2SO4 SC was grown by treating an aqueous Gua2SO4 solution with CO2 at low temperature. Gua2SO4 (3 g) was dissolved in water (5 mL). After complete dissolution, the solution was added in a glass vial, which was placed in an autoclave. The autoclave was rinsed using CO2 to completely replace the air, and the final CO2 pressure in the autoclave was fixed at 700 kPa. The autoclave was kept at 2°C by a cooling jacket. Colorless plate-like SCs at centimeter sizes were obtained after 12 h.

SC XRD measurements

Since the crystals of CO2@Gua2SO4 readily decompose at atmospheric conditions, a fresh crystal from the reactor was placed on top of a glass fiber under a stream of liquid nitrogen and rapidly mounted onto a sample goniometer for centering. SC XRD data were collected by a Rigaku Oxford Diffraction Super-Nova diffractometer using Mo-Kα radiation (λ = 0.71073 Å) at 100 K. The data collection and processing were carried out with CrysAlisPro software. The crystal structure was solved by direct method and refined by full-matrix least squares based on F2 using an SHELXTL 14XL program package. Hydrogen atoms were fixed geometrically at their positions and allowed to ride on parent atoms. Crystallographic and structure refinements data for CO2@Gua2SO4 are summarized in Table S1.

PXRD of CO2@Gua2SO4

RT PXRD spectra of CO2@Gua2SO4 slightly deviated from the simulated one especially at (004) plane (Figures S5 and S6) because the measurements were conducted at different temperatures. At 100 K, the guest CO2 molecules stopped moving and therefore were forced to align with guanidinium to maximize the H-bonding interactions. At RT, however, the CO2 molecules were in free motion, leading to an expected disorder. Detailed analysis from the crystal structure indicates that CO2 molecules are predominantly located in the (004) plane. Therefore, the peak position of the (004) plane in PXRD at RT shifts to a higher 2θ when CO2 is released (Figure S5).

The effect of counter ions on formation of CO2 clathrate

A series of guanidinium salts were selected to be tested, including guanidine hydrochloride, guanidine nitrate, and guanidine dihydrogen phosphate. Saturated solutions of guanidinium salts were added into glass vials, which were separately placed in an autoclave. The air in the autoclave was exchanged with CO2 at least three times before the CO2 pressure inside was fixed at 1,000 kPa (10 bar). The autoclave was kept at 0°C, controlled by a cooling jacket. After 24 h with continuous stirring, glass vials were checked for any precipitation.

The pH influence on CO2 adsorption

The pH value of Gua2SO4 aqueous solution is determined to be 7. The pH values of a series of Gua2SO4 solutions (73.7 wt %) were set at 1, 2, 3, 4, 5, and 6 using H2SO4, HCl, and HNO3 solutions. Solutions were then charged into separate glass vials and weighed. The vials were put in an autoclave that was already connected to a CO2 cylinder. The air in the autoclave was exchanged with CO2 at least three times before the CO2 pressure inside was fixed at 1,000 kPa (10 bar). The temperature of the autoclave was kept at 0°C using a cooling jacket. After 12 h of constant stirring, the pressure was recorded, and the vial containing Gua2SO4 aqueous solution was weighed again to calculate the adsorbed amount of CO2. Note that the dissolved CO2 was ignored in calculations, as the CO2 dissolved in solution is negligible (1.44 mg/g at 25°C) compared with that of the amount adsorbed by Gua2SO4.

The effect of pressure on the CO2 uptake of Gua2SO4

A Gua2SO4 aqueous solution (60 wt %, 1.5 g/g) was charged into a glass vial and weighed. The vial was put into autoclave that was connected to a CO2 cylinder. The air in the autoclave was exchanged with CO2 at least three times before the CO2 pressure inside was set to the desired values. The temperature of the autoclave was kept at 0°C using a cooling jacket. After 12 h of continuous stirring, the pressure was recorded, and the vial containing Gua2SO4 aqueous solution was weighed again to calculate the conversion rate. Note that the dissolved CO2 was ignored in the conversion calculations, as the CO2 dissolved in solution is negligible compared with that of the amount adsorbed by Gua2SO4.

CO2 equilibrium pressures of CO2-Gua2SO4 aqueous solutions at various temperatures

A saturated Gua2SO4 solution at 40°C (10 mL) and 2 g Gua2SO4 powder was mixed in a 100 mL autoclave under constant stirring. The temperature of the autoclave was kept at 0°C using a cooling jacket. The autoclave was evacuated under vacuum and filled with CO2. The flushing was repeated three times to completely remove air from the autoclave. Then, the autoclave was charged with CO2 to 100 kPa (1 bar). The pressure drop was monitored until readings were steady. The final pressure value was taken as the equilibrium pressure at the set temperature (0°C, 5°C, 10°C, 15°C, 20°C, 25°C, and 30°C). The water vapor pressure of a saturated Gua2SO4 solution was also measured under the same conditions. CO2 equilibrium pressure was obtained by subtracting the water vapor pressure at the same temperature from the recorded total pressure and plotted against temperature (Figure S11).

Cycling sorption experiments

2.5 g Gua2SO4 was dissolved in a glass vial with 2 mL water. The vial was put in a 100 mL autoclave, and the temperature was kept at 0°C using a jacket-cooling system. The air in the autoclave was exchanged with CO2 at least three times before the CO2 pressure inside was set to 1,000 kPa (10 bar). The pressure was monitored using a digital pressure meter (5 s per point) with constant stirring. After the pressure stabilized, the chamber was vented, and white slurry was observed in the vial. Ultrasound was then applied to decompose the product and release CO2, with a rush of bubbles indicating the decomposition process. The procedure was repeated to test the cyclic performance of CO2 adsorption for at least ten runs. After the final cycle, absolute ethanol was added to the solution, and a precipitate was obtained, which was subsequently dried at 100°C. The PXRD of the precipitate is consistent with that of the pristine Gua2SO4 (Figure S14). The conversion rate was determined gravimetrically by weighing the Gua2SO4 aqueous solution before and after CO2 adsorptions. The conversion rate can also be calculated according to the CO2 pressure drop during the sorption process. Control experiments were conducted following the same procedure but using pure water instead of Gua2SO4 aqueous solution, and N2 gas instead of CO2, and with or without stirring.

Isochoric adsorption-desorption experiments

2.5 g Gua2SO4 was dissolved in a glass vial with 2 mL water. The vial was put in a 100 mL autoclave, and the temperature was kept at 0°C using a jacket-cooling system. The air in the autoclave was exchanged with CO2 at least three times before the CO2 pressure inside was set to 1,000 kPa (10 bar). The pressure was monitored using a digital pressure meter (5 s per point) with constant stirring. After 3 h, the pressure became stable. Then, the temperature of the autoclave was increased to 30°C using a water bath, and the pressure inside the autoclave was recorded.

CO2 removal from flue gas

Owing to the relatively high viscousity of concentrated Gua2SO4 solutions, an autoclave equipped with mechanical stirring was used in this experiment to speed up CO2 dissolution and diffusion. Gua2SO4 (27 g) and water (7 mL) were charged into the autoclave. The air in the autoclave was first exchanged with flue gas three times before setting the gas pressure inside to a specific value. The temperature of the autoclave was kept at 0°C using a cooling jacket. CO2 content after the sorption for 24 h was analyzed by a GC. The removal percentage of flue gas under different pressure is shown in Table S5.

Acknowledgments

We acknowledge support from the Chinese Academy of Sciences, the National Key Research and Development Program of China (2021YFA1500402), the National Natural Science Foundation of China (NSFC; 21571167, 51502282, and 22075266), and the Fundamental Research Funds for the Central Universities (WK2060190053 and WK2060190100).

Author contributions

B.L. conceived of the idea, and B.L., Q.X., and C.T.Y. supervised the project together. Z.X. designed and carried out the experiments. C.L., X.X., and T.S.N. contributed to data analysis and manuscript preparation. C.C. collected SC XRD data and solved the structure. B.L., Q.X., and C.T.Y. wrote the manuscript together. All authors discussed the results and assisted with manuscript preparation.

Declaration of interests

C.T.Y. is an advisory board member at Cell Reports Physical Science. USTC filed one Chinese patent (application #202210271224.0) from the data reported in this study.

Sumber :

https://www.sciencedirect.com/science/article/pii/S2666386423001510