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Apoptosis vs Autophagy EVERYTHING YOU NEED TO KNOW CELLULAR BIOLOGY MCATAutophagy and cell death -
This review summarizes the evidence linking autophagy to cell survival and cell death, the complex interplay between autophagy and apoptosis pathways, and the role of autophagy-dependent survival and death pathways in clinical diseases. Cell biologists have long recognized the possibility that eukaryotic cells may undergo nonapoptotic forms of programmed cell death.
Autophagy, a lysosomal pathway involving the bulk degradation of cytoplasmic contents, has been identified as a prime suspect in such death, and recent studies have implicated the autophagy pathway as a cause of nonapoptotic cellular demise.
However, most evidence linking autophagy to cell death is circumstantial. Now, with new tools to assess causality, it is an opportune time to revisit the case of autophagy in cell death.
During autophagy, an isolation membrane forms, presumably arising from a vesicular compartment known as the preautophagosomal structure, invaginates, and sequesters cytoplasmic constituents including mitochondria, endoplasmic reticulum, and ribosomes Figure 1. The edges of the membrane fuse to form a double or multimembranous structure, known as the autophagosome or autophagic vacuole.
The outer membrane of the autophagosome fuses with the lysosome in mammalian cells or vacuole in yeast and plants to deliver the inner membranous vesicle to the lumen of the degradative compartment. Degradation of the sequestered material generates nucleotides, amino acids, and free fatty acids that are recycled for macromolecular synthesis and ATP generation.
The autophagy pathway and its role in cellular adaptation to nutrient deprivation. Starvation or growth factor deprivation results in a decrease in intracellular nutrients and activation of nutrient-sensing signaling pathways reviewed in ref.
Autophagy involves the sequestration of cytoplasmic material by an isolation membrane derived from the preautophagosomal structure to form a double-membrane vacuole, the autophagosome. The autophagosome undergoes fusion with a late endosome or lysosome, to form an autolysosome, in which the sequestered material is degraded.
Degradation of membrane lipids and proteins by the autolysosome generates free fatty acids and amino acids that can be reused by the cell to maintain mitochondrial ATP energy production and protein synthesis and thereby promote cell survival.
Disruption of this pathway by autophagy gene inactivation prevents cell survival in diverse organisms Table 2. The same molecular machinery and overlapping dynamic membrane rearrangement events that occur during starvation may also be used in other settings to degrade unwanted cytoplasmic contents, including damaged mitochondria, protein aggregates, and intracellular pathogens.
See text for discussion. TCA cycle, tricarboxylic acid cycle. Autophagy occurs at low basal levels in all cells to perform homeostatic functions e. Nutritional status, hormonal factors, and other cues like temperature, oxygen concentrations, and cell density are important in the control of autophagy.
Two evolutionarily conserved nutrient sensors play roles in autophagy regulation: a the target of rapamycin TOR kinase is the major inhibitory signal that shuts off autophagy during nutrient abundance reviewed in ref.
Downstream of TOR kinase, there are approximately 17 gene products essential for autophagy and related pathways in yeast, referred to as the ATG genes 5 , and most yeast ATG genes have orthologs in higher eukaryotes reviewed in ref.
The ATG genes encode proteins needed for the induction of autophagy, and the generation, maturation, and recycling of autophagosomes. The molecular mechanisms of autophagy. The autophagy Atg proteins can be divided into 4 functional groups, including A a protein kinase autophagy regulatory complex that responds to upstream signals, including nutrient limitation; B a lipid kinase signaling complex that mediates vesicle nucleation; C ubiquitin-like protein conjugation pathways that are required for vesicle expansion and completion; and D a retrieval pathway required for the disassembly of Atg protein complexes from matured autophagosomes.
Shown are the yeast Atg proteins that participate in each functional group. Yeast Atg proteins with known orthologs in higher eukaryotes are underlined. PI, phosphatidylinositol; PI3-P, phosphatidylinositol 3-phosphate; PE, phosphatidylethanolamine. In classical apoptosis, or type I programmed cell death, there is early collapse of cytoskeletal elements but preservation of organelles until late in the process.
In contrast, in autophagic, or type II, programmed cell death, there is early degradation of organelles but preservation of cytoskeletal elements until late stages. Whereas apoptotic cell death is caspase-dependent and characterized by internucleosomal DNA cleavage, caspase activation and DNA fragmentation occur very late if at all in autophagic cell death Figure 3.
In contrast with necrosis, both apoptotic and autophagic cell death are characterized by the lack of a tissue inflammatory response. Ultrastructural examples of apoptotic and autophagic cell death.
Electron micrographs of a FasL-treated Jurkat cell undergoing cell death with apoptotic features A and of a tamoxifen-treated MCF7 human breast carcinoma cell undergoing cell death with autophagic features B. In A , note chromatin condensation cell in center and cytoplasmic vacuolization cell in upper right.
In B , note absence of chromatin condensation and presence of numerous autophagosomes. Images in A and B reproduced with permission from Nature Cell Biology 98 and Landes Bioscience 90 , respectively. Large numbers of autophagic vacuoles have been observed in dying cells of animals of diverse taxa reviewed in refs.
The consensus view has been that autophagic cell death occurs primarily when the developmental program e. Recently, studies have also described autophagic cell death in diseased mammalian tissues and in tumor cell lines treated with chemotherapeutic agents Table 1. In many of these cases, morphologic features of autophagic and apoptotic cell death or of autophagic and necrotic cell death are observed in the same cell.
What is the evidence that autophagy is a death execution mechanism in autophagic cell death? If cell death is truly due to autophagy, then pharmacologic or genetic inhibition of autophagy should prevent the death.
Yet, for most of the developmental, disease-associated, and toxic stimulus-induced deaths that are presumed to be autophagic Table 1 , the evidence for its role is only correlative. Moreover, in certain cases of autophagic cell death, the available evidence calls into question a causative role of autophagy.
For example, in Drosophila , autophagic cell death but not autophagy observed during salivary gland regression is prevented by mutations in the ecdysone-regulated transcription factors BR-C and E74A In the slime mold Dictyostelium , a null mutation in the autophagy gene atg1 blocks vacuolization but not cell death in an in vitro model of autophagic cell death Thus, in these model systems, autophagy per se is neither sufficient nor required for autophagic cell death.
Furthermore, the caspase inhibitor p35 blocks metamorphic cell death in Drosophila without complete inhibition of autophagy, suggesting that it is caspase-mediated apoptosis, rather than autophagy, that plays a key role in this death process There is, however, some evidence in certain in vitro settings that pharmacologic or genetic inhibition of autophagy can prevent cell death.
The pharmacologic inhibitor of autophagy 3-methyladenine 3—MA , a nucleotide derivative that blocks class III PI3K activity 12 — 14 , delays or partially inhibits death in starved hepatocytes from carcinogen-treated rats 15 , in anti-estrogen—treated human mammary carcinoma cells 16 , in chloroquine-treated cortical neurons 17 , in nerve growth factor—deprived sympathetic neurons 18 , in serum- and potassium-deprived cerebellar granule cells 19 , in serum-deprived PC12 cells 20 , and in TNF-treated human T lymphoblastic leukemia cells However, in several of these studies, autophagy occurred in cells thought to die by apoptosis, and it was presumed that autophagy triggered apoptosis, rather than playing a direct role in the death process.
Moreover, 3-MA can inhibit kinases other than class III PI3K 18 , some of which may independently affect death signaling, as well as inhibit the permeability transition in mitochondria Thus, it is not possible to directly implicate autophagy in death execution from these 3-MA inhibitor studies.
Two recent studies provide the first genetic evidence that the autophagy pathway is capable of killing cells Table 2.
RNA interference RNAi directed against 2 autophagy genes, atg7 and beclin 1 , blocked cell death in mouse L cells treated with the caspase inhibitor zVAD Notably, in both of these studies, atg gene RNAi blocked the death of cells whose apoptotic pathway had been crippled.
Although these findings exclude the possibility that autophagy is triggering death through apoptosis induction, they raise the question of whether autophagy is a death mechanism in cells whose apoptotic machinery is intact.
Interestingly, in etoposide-treated wild-type MEFs which die by apoptosis , only minimal autophagic activity and no inhibition of death by 3-MA is seen, indicating that autophagy is not involved in the death process unless apoptosis is blocked These data are consistent with the theory previously proposed by Lockshin and Zakeri that cells preferentially die by apoptosis but will die by any alternative available route, including autophagy, if exposed to harsh enough stimuli 9.
A related possibility is that apoptotic death is faster than autophagic death and, therefore, autophagy is only witnessed playing a role in cell death in apoptotic-deficient cells.
To prove that autophagy is an important cell death pathway in normal cells, it will be necessary to demonstrate cell death resistance phenotypes in apoptotic-competent cells lacking autophagy genes.
A contradictory, but equally plausible, explanation for the presence of autophagy in dying cells is that activation of autophagy is a cellular survival strategy. This concept was first proposed in 26 and considered radical 8 but now is supported by studies demonstrating increased death in cells or organisms lacking gene products essential for autophagy Table 2.
The pro-survival function of autophagy is an evolutionarily ancient process, conserved from yeast to mammals, and best characterized in nutrient deficiency. During nutrient deficiency, degradation of membrane lipids and proteins by the autolysosome generates free fatty acids and amino acids that can be reused to fuel mitochondrial ATP energy production and maintain protein synthesis Figure 1.
Presumably, this recycling function of autophagy is linked mechanistically to its ability to sustain life during starvation. Unicellular organisms with null mutations in autophagy genes are viable in normal growth conditions; however, unlike their wild-type counterparts, they die rapidly during starvation Table 2 27 — In plants, deletion of autophagy genes e.
Mice lacking Atg5, an acceptor molecule for the ubiquitin-like molecule Atg12, die during the neonatal period, when the placental blood supply is interrupted and they undergo a form of starvation This is particularly likely in tissues such as the heart and diaphragm that have sudden increases in energy needs and exhibit increased autophagy immediately following birth.
Similarly, the inability of Caenorhabditis elegans with RNAi-silenced autophagy genes e. Autophagy genes may also be critical for maintaining cellular bioenergetics and survival when cells are unable to take up external nutrients i.
In the absence of growth factors such as IL-3, there is decreased surface expression of nutrient transporters, decreased nutrient uptake, and an intracellular deficiency of nutrients As in nutrient starvation in yeast, autophagy is a self-limited survival strategy during growth factor deprivation.
However, at any point before death, the addition of growth factor reverses the catabolic process and maintains cell viability. These observations are consistent with the concept that autophagy is a self-limited survival strategy, rather than a primary or irreversible death execution program.
It will also be interesting to examine whether autophagy genes play a similar cytoprotective role during withdrawal of hormonal support or growth factors besides IL Perhaps, rather than contributing to death execution, autophagy delays initiation of the apoptotic death pathway in cells deprived of trophic support.
Although studies have not been performed in apoptosis-competent cells deprived of trophic factors, autophagy genes prevent the onset of apoptosis during nutrient deprivation. RNAi against beclin 1 , atg5 , atg10 , and atg12 enhances starvation-induced, but not staurosporine-induced, apoptotic cell death Thus, the mechanism by which autophagy genes promote survival during nutrient deprivation may involve suppression of the canonical apoptotic death pathway.
The mechanisms by which autophagy promotes cell survival are not restricted to its role in maintaining cellular energy homeostasis during starvation. Autophagy is also involved in removing damaged mitochondria and other organelles, in degrading intracellular pathogens, and in degrading protein aggregates too large to be removed by the ubiquitin-proteasomal system.
These functions of autophagy could promote cellular survival during aging, infectious diseases, and neurodegenerative processes. In addition to a cell-autonomous role for autophagy in promoting survival, autophagy may regulate programmed cell death during physiologic processes in vivo.
For example, during the plant innate immune response, silencing of autophagy genes beclin 1 , vps34 , atg3 , and atg7 does not alter the death of infected cells or pathogen spread but results in uncontrolled spread of programmed cell death beyond sites of pathogen infection This suggests that autophagy limits cell death to the site of infection, allowing plant innate immunity to contain pathogen spread without death of innocent bystander cells.
It is not known whether autophagy alters the production of death-promoting signals, prevents the movement of death-promoting signals into uninfected tissues, or protects uninfected tissues against death induced by these signals, or whether a similar function of autophagy plays a role in the spatial restriction of development and stress-induced programmed cell death in other eukaryotic organisms.
A third explanation for high levels of autophagy in dying cells is that it is a clean-up or self-clearance mechanism in cells committed to die by apoptosis or necrosis. This theory might explain why only selected populations of dying apoptotic cells have morphologic features of autophagy.
The dogma is that most apoptotic cells are engulfed by phagocytes, with the lysosomes of the phagocyte responsible for the final degradation of dead cell bodies. However, in some forms of developmental programmed cell death e. This need might contribute to the overlap between signaling pathways that activate apoptosis and autophagy.
It has been shown that the proapoptotic signaling molecule TNF-related apoptosis-inducing ligand TRAIL regulates autophagy in an in vitro model of mammary gland formation Here, TRAIL-dependent induction of autophagy occurs in parallel with apoptosis.
Suppression of either apoptosis alone or TRAIL signaling does not prevent lumen formation, but simultaneous inhibition of apoptosis and TRAIL signaling prevents cell clearance.
These findings suggest that both apoptosis and autophagy may be involved in cavitation during mammary gland morphogenesis. However, to confirm a role for autophagy, it will be important to observe whether luminal filling occurs if autophagy genes are inactivated in cells with intact TRAIL signaling.
It is not clear whether autophagy is required for luminal cell death or for removal of cells committed to death by an apoptotic pathway. Several proapoptotic signals induce autophagy — e.
Conversely, antiapoptotic signaling pathways suppress autophagy — e. Coordinated regulation of apoptosis and autophagy is also reflected in the results of genome-wide analyses of transcriptional changes during developmental programmed cell death of the Drosophila salivary gland 45 , The mitochondrion may integrate cell death signals and autophagy activation.
Mitochondria generate apoptotic signals but are removed when damaged by autophagy; therefore, mitochondria represent a nexus at which autophagy and apoptosis pathways may interact. One example is the yeast gene, UTH1 , that encodes an outer mitochondrial membrane protein involved in mitochondrial biogenesis and stress responses.
Uth1 mutants are defective in degrading mitochondria during autophagy 47 and survive and proliferate when expressing the mammalian proapoptotic cell death gene bax or when treated with the autophagy inducer rapamycin These findings led Camougrand and colleagues to suggest that Uth1p mediates mitochondrial autophagy and autophagic death.
However, it is not yet clear whether rapamycin induces cell death versus cell cycle arrest in wild-type yeast, and whether the phenotype of rapamycin-treated uth1 mutant yeasts is due to direct effects of UTH1 and the autophagy pathway in death regulation.
In mammalian cells, Bcl-2 family members in the outer mitochondrial membrane modulate autophagy. Bcl-2 downregulation increases autophagy in a caspase-independent manner in human leukemic cells 49 , and Bcl-2 overexpression inhibits both autophagy and caspase-independent death in growth factor—deprived neural progenitor cells and in serum- and potassium-deprived cultured cerebellar granule cells 19 , Examples of possible targets include the bulk engulfment of cytoplasmic contents including cytosol and organelles or forms of selective autophagy that promote the clearance of mitochondria e.
mitophagy or ubiquitinated proteins e. Autophagic flux is dependent on lysosomal function, and it is likely that the lysosome will contribute to autophagy-dependent cell death. The degradation of larval midgut cells that are deficient for Mad and tkv has revealed unique features of autophagy-dependent cell death.
The Mad and tkv mutant cells are rapidly degraded with markers of autophagy and lysosome activity suggesting that the demise of the cell is due to the bulk degradation of cellular components, presumably by the lysosomal enzymes [ 73 ].
Another recent study suggests that the lysosome may contribute to cell death due to autophagy [ 74 ]. However, it remains to be determined if the degradation of specific organelles initiates autophagy-dependent cell death or if the bulk degradation of cytosolic components by autophagy is the primary cause of cell death.
The contribution of the lysosome to autophagy-dependent cell death also warrants further investigation. Another mechanism that has been proposed is that autophagy may selectively degrade specific survival factors to induce cell death.
For example, the Drosophila inhibitor of apoptosis protein dBruce localises to autophagosomes and is necessary for caspase-dependent cell death of the ovary [ 75 ]. The developmental cell death in the oocyte is triggered by the autophagy-mediated degradation of the anti-apoptotic dBruce that enables the subsequent activation of caspase-dependent apoptosis.
The autophagy machinery required to mediate cell death may differ to that promoting cell survival. There may be differences in the rate of autophagic flux, the length of time the pathway is active and whether the engulfed contents are being recycled or degraded.
All these factors may contribute to distinct requirements for autophagy machinery components. The genetic dissection of the requirement of the Atg genes to autophagy-dependent Drosophila midgut degradation revealed that only a subset of the individual components are required Table 1.
In the Drosophila midgut, RNAi-mediated screening of the Atg machinery revealed that a subset of the multi-subunit complexes that are required for starvation-induced autophagy in the fat body are required for autophagy-dependent cell death in the midgut [ 67 ].
The components of the initiation complex Atg1, Atg13, Atg17 and Atg and PtdIns 3 P binding proteins Atg9, Atg2 and Atg18 , as well as specific components of class III PI3K complex Vps15 and Vps34 , and Atg8a are required for autophagy-dependent midgut degradation [ 67 ].
However, components of the two conjugation systems, including Atg7 the E1 enzyme and Atg3 the E2 enzyme , are not required for this process although these genes are essential for starvation-induced autophagy in the fat body [ 66 , 67 ].
Autophagy-dependent cell death in the Drosophila larval midgut appears to be regulated differently from autophagy in cell survival context.
These findings highlight the cell type- and context-specific differences in the utilisation of autophagic machinery. This may contribute to altered rates of autophagy flux or to maintain the activity of the pathway for an extended time. There is increasing evidence that a number, if not all, of Atg proteins have functions in addition to their role in autophagy [ 38 ].
Individual autophagy pathway components also play roles in cell death that are independent of their role in autophagy [ 77 , 78 ]. Following genotoxic stress, the increase in ATG5 expression in the nucleus promotes mitotic catastrophe [ 79 ].
The conjugation of Atg12 to Atg3 during intrinsic mitochondrial-mediated apoptosis does not occur during starvation-induced autophagy [ 80 ]. Further characterisation of autophagy-dependent or -independent functions of a particular Atg protein will aid in distinguishing the contributions to various biological processes including cell death.
The ubiquitin system has multiple roles in the regulation of autophagy [ 81 , 82 ]. Many components of the autophagy machinery are modified by ubiquitin to regulate autophagic flux, from initiation to termination. Ubiquitination is also important for targeting of cargoes in several forms of selective autophagy [ 83 ].
Cargo receptors bind to specific ubiquitinated substrates to direct the selective incorporation of the cargo into autophagosomes. In Drosophila , ubiquitination is important for tissue-specific autophagy-dependent cell death. Ubiquitination is essential for midgut autophagy and cell size reduction that accompanies midgut degradation [ 66 ].
As described above, the components of the conjugation machinery, Atg7 and Atg3, are not required for autophagy-dependent midgut degradation. To identify the putative E1-activating enzymes that function in place of Atg7 , an RNAi-mediated screen in Drosophila larval midgut identified Uba1 as a specific regulator of autophagy and cell size reduction during midgut cell death [ 66 ].
The function of Uba1 is downstream of Atg1 but does not replace Atg7. This raises the possibility that additional components of the ubiquitin system e. E2s, E3s may be necessary for autophagy-dependent cell death of the midgut.
The different forms of autophagy appear to require distinct regulatory components. Given the highly context-specific requirement for autophagy-dependent cell death, there may be specific signals to initiate the cell death function of autophagy.
As mentioned above, the steroid hormone ecdysone plays an important role in triggering the degradation of the obsolete larval midgut and salivary gland in Drosophila [ 62 , 64 , 69 ]. Increased levels of ecdysone bind to the heterodimeric nuclear receptor complex, ecdysone receptor EcR and ultraspiracle USP in a spatio-temporal manner to regulate the expression of genes involved in proliferation, differentiation and PCD [ 84 , 85 , 86 , 87 ].
Following puparium formation, the rise in ecdysone levels triggers expression of genes regulating both apoptosis and autophagy resulting in salivary gland cell death [ 68 , 69 , 88 ]. Coincident with the increase in ecdysone during midgut degradation, many Atg genes are transcriptionally upregulated immediately prior to larval midgut degradation in an EcR-dependent manner [ 73 ].
The knockdown of EcR in the midgut blocks the increase in Atg genes preventing autophagy induction and delaying midgut degradation [ 73 ]. Despite the upregulation of Atg gene expression in response to ecdysone, the direct transcriptional regulators responsible still remain to be identified.
The role of ecdysone in regulating the expression of autophagy genes appears to be conserved in other insects. In Lepidoptera silk worm , the induction of autophagy and apoptosis gene expression during metamorphosis occurs in response to ecdysone 20E [ 93 ].
In other Diptera sand flies , the upregulation of Atg gene expression coincides with increased ecdysone during midgut removal; however, it is yet to be established if they are regulated by ecdysone [ 94 ].
There may be additional roles for ecdysone in the regulation of autophagy pathway activity as in H. armigera conjugation of ATG12—ATG5 occurs in response to ecdysone in a concentration- and time-dependent manner [ 57 ]. Induction of autophagy under conditions of nutrient limitation and growth factor withdrawal generally promotes cell survival.
A central regulator coordinating autophagy to nutrients and growth factors is the mechanistic target of rapamycin mTOR kinase [ 95 ]. The function of mTOR is dependent on two distinct complexes distinguished by unique components with TOR complex 1 TORC1 central in regulating autophagy containing Raptor regulatory associated protein of TOR , and TOR complex 2 TORC2 containing Rictor rapamycin insensitive companion of TOR [ 96 ].
In growth-limiting conditions such as starvation, TORC1 is no longer active enabling Atg1 activation and the initiation of autophagy [ 98 ].
During Drosophila larval development, mTOR is required for cellular growth in the salivary glands and midgut. Similar to conditions of nutrient limitation where TORC1 inactivation results in the induction of autophagy, inactivation of TORC1 is required for autophagy-dependent midgut cell death [ 67 ].
The knockdown of Tor and raptor but not TORC2 component rictor induced premature autophagy and midgut degradation [ 67 ]. Thus, TORC1 inactivation is required for the induction of autophagy in both cell survival and cell death.
This suggests that while the molecular machinery mediating autophagy-dependent cell death is distinct to that required for cell survival, the initiation of autophagy may occur similarly. It also appears that these same pathways can also regulate autophagy in the context of cell death.
In Drosophila , the maintenance of growth signalling by class I PI3K pathway blocks autophagy induction preventing the degradation of larval salivary gland and midgut tissue during metamorphosis [ 70 , 99 ].
The downregulation of PI3K activity by the expression of PTEN, TSC1 and TSC2 was sufficient to induce premature induction of autophagy and midgut degradation [ 99 ]. Thus, the developmental cell death of the fat body, salivary glands and midgut all require autophagy and downregulation of PI3K signalling [ 70 , 99 , ].
This suggests that despite the opposing roles of autophagy in cell survival and cell death, the induction of autophagy occurs in response to similar signals. The upstream signals that regulate the induction of autophagy including growth arrest and inactivation of TORC1 are similar; however, the molecular machinery mediating cell death may be distinct.
As described above, oncogenic Ras has a complex relationship between autophagy, cell growth and cell death. Depending on the context, activated Ras can induce autophagy promoting cell growth and survival or, under different conditions, it can result in cell death due to increased autophagy [ 47 , 49 ].
In mammals, caspase-independent H-Ras mediated cell death requires the BH3-only protein Noxa and Beclin-1, with knockdown of Noxa or Beclin-1 reducing autophagy and promoting survival. In the presence of Noxa, Beclin-1 dissociates from anti-apoptotic protein Mcl-1, promoting autophagy induction.
This indicates that in response to oncogenic Ras, Noxa may participate in the activation of the autophagy machinery to promote cell death by a unique caspase-independent mechanism [ 47 ]. However, the expression of Ras in the Drosophila larval salivary gland and midgut tissue maintains growth signalling and blocks autophagy induction preventing tissue degradation [ 70 , 99 ].
In the absence of Dpp signalling premature autophagy induction and rapid cell death occurred in midgut cells, whereas maintaining Dpp signalling inhibited autophagy induction [ 73 ].
While Dpp plays an important role in the timing of autophagy-dependent midgut degradation, other key morphogens Hh and Wg are not required for autophagy-dependent cell death [ ].
As sustained Dpp signalling in the midgut prevents the expression of ecdysone responsive genes and impairs ecdysone production, Dpp may act in inter-organ communication to coordinate ecdysone biosynthesis and timing of autophagy-dependent midgut degradation [ 73 ].
In mammals, members of the TGF-β superfamily induce apoptosis and autophagy in vitro in some primary and transformed cell lines [ , , ].
However, the physiological role of TGF-β signalling in autophagy-dependent cell death in mammals remains to be established. The discovery of additional genes that function during autophagy in specific contexts suggests that the canonical autophagy machinery may not be the same in all cells or tissues.
The requirements for the degradation of the larval salivary glands compared to the larval midgut are distinct, despite apparent similarity in the requirement of autophagy for the degradation of these tissues. For instance, the Hippo pathway is required for the correct removal of salivary glands but not for midgut cell death [ 99 , ].
A number of genes have a context-specific role in the regulation of autophagy during salivary gland degradation [ , , , , ].
The engulfment receptor Draper Drpr, MEGF10 in mammals is required for autophagy upstream of Atg1 for PCD of the salivary glands [ ].
Draper is present on the surface of salivary gland cells and acts in a cell autonomous manner to promote degradation and clearance of the tissue [ ]. In addition, the non-cell autonomous function of Macroglobulin complement-related Mcr is required for the regulation of autophagy that is dependent on Drpr in neighbouring cells [ ].
In salivary glands, miR and calcium signalling regulate autophagy and cell death [ ]. The miR targets inositol 1,4,5-triphosphate kinase 2 ip3k2 , regulates inositol 1,4,5-trisphosphate IP3 signalling and release of calcium from the ER [ ]. This subsequently affects the activity of the calcium-binding messenger protein Calmodulin.
The Ras-like protein A Ral , a Ras superfamily small GTPases, and the exocyst components have a context-specific role in autophagy during salivary gland PCD [ ].
In mammals, Ral regulates stress-induced autophagy by interacting with the components of the exocyst [ ]. However, Ral is dispensable for starvation-induced autophagy in the Drosophila fat body [ ]. A role for dynein light chain 1 ddlc1 in protein clearance by autophagy is required for salivary gland degradation [ ].
The proton-coupled pyruvate transporter, Hermes, acts as a negative regulator of mTOR signalling required for autophagy during salivary gland cell death [ ]. The roles of these genes in salivary gland PCD may reflect the requirement of the cell death pathways in this tissue and highlights the importance of investigating regulatory mechanisms in a context-specific manner.
Tissue-specific regulators of autophagy for cell death during midgut degradation, and not during salivary gland degradation or starvation-induced autophagy, have also been identified. As described above, the E1, Uba1, was found to be necessary for midgut cell death, but not for salivary gland cell death [ 66 ].
This raised the possibility that additional components of the ubiquitin-conjugation system such as E2s and E3s may regulate autophagy-dependent cell death of the midgut. The ubiquitin-binding UBA domain containing protein, Vps13D is a specific regulator of autophagy-dependent cell death in the midgut and does not affect starvation or stress-induced autophagy [ 72 ].
Knockdown of Vps13D or deletion of the UBA domain prevents the clearance of mitochondria during midgut degradation, a phenotype similar to blocking autophagy, with enlarged mitochondria [ 66 , 72 ].
The function of VPS13D in the regulation of mitochondrial morphology appears to be conserved as VPS13D mutant human cells also have more large and rounded mitochondria. However, it remains to be established if Vps13D also functions in autophagy-dependent cell death in the larval midgut and if such a function is conserved in cell death in mammals.
The receptor tyrosine phosphatase 52F PTP52F is required for midgut degradation. The knockdown of PTP52F delayed midgut degradation that could be rescued by the expression of the ATPase VCP valosin-containing protein orthologue Ter94 [ ]. VCP participates in many cellular processes including cell cycle regulation and ubiquitin-proteasome system [ ], as well as in several steps of the autophagy pathway, including autophagosome maturation, autophagosome—lysosome fusion, and is required for the clearance of damaged lysosomes by autophagy [ ].
This suggests that Ter94 may have other roles or may play a more general role in autophagy. Whether PTP52F function is limited to autophagy-dependent cell death or if it also plays a role in the regulation of starvation-induced autophagy for cell survival remains to be determined.
To identify novel regulators in mammalian cells, a screen using resveratrol-induced autophagy-dependent cell death of lung carcinoma identified GBA1, a glucocerebrosidase enzyme that converts glucosylceramide to glucose and ceramide in the lysosome [ 52 ].
The knockdown of GBA1 blocked resveratrol-induced autophagy-dependent cell death and reduced ceramide production. The function of GBA1 in autophagy-dependent cell death is evolutionarily conserved in Drosophila with Gba1a knockdown delaying midgut cell death [ 52 , 53 ].
As described above, the function of autophagy in promoting cell death is context dependent. The signals and molecular machinery that distinguish autophagy-dependent cell death to that of autophagy required for cell survival is only beginning to be elucidated. There is evidence that the autophagy machinery required during cell death is distinct to the machinery required during cell survival, but it remains to be established how conserved this is and what additional components function in cell death and if they are cell specific.
The induction of autophagy occurs in response to multiple signals and several of the key regulators are similar in both surviving and dying cells. The molecular mechanisms that lead to the demise of the cell during autophagy-dependent cell death are poorly understood.
There are a number of specific forms of autophagy that are defined based on the cargo selected for degradation, and whether autophagy-dependent cell death involves the degradation of selective cargo or bulk degradation of cytoplasmic components remains to be established.
Given the context-specific role and distinct molecular machinery mediating autophagy-dependent cell death, it will be important to investigate the regulatory mechanisms in the physiological settings.
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Present address: Department of Cel Oncology and Ceath Medicine, University of Texas MD Anderson Cancer Center, Houston, TX Autophagy and cell death, Deaty. Shani Bialik veath, Autophagy and cell death K. DasariAutiphagy Autophagy and cell death Autophagy-dependent eeath death — where, how and Dispelling popular nutrition myths a cell eats itself to death. J Cell Sci 15 September ; 18 : jcs However, this phenomenon later came into question, because the presence of autophagosomes in dying cells does not necessarily signify that autophagy is the cause of demise, but rather may reflect the efforts of the cell to prevent it. Resolution of this issue comes from a more careful definition of autophagy-dependent cell death ADCD as a regulated cell death that is shown experimentally to require different components of the autophagy machinery without involvement of alternative cell death pathways. Cytokines and Growth Ajtophagy. Autophagy and cell death Autlphagy fundamental cellular processes that play critical roles in Deah physiology and Gut health and gut-brain axis of multicellular organisms. Autophagy is Autophagy and cell death conserved housekeeping process utilized by cells to maintain homeostasis by conveying Autohagy components Autophagy and cell death lysosomal compartments for degradation and recycling. Cell death can either be regulated, proceeding through precise signaling pathways or accidental resulting from unexpected cellular injury. While apoptosis is the earliest discovered regulated cell death pathway, several non-apoptotic regulated cell death pathways including pyroptosis, ferroptosis and necroptosis have gained prominence more recently. Learn more about autophagy, apoptotic cell death, and non-apoptotic cell death pathways and discover how Proteintech products can accelerate your autophagy and cell death research. Contribution of Necroptosis to Myofiber Death in Idiopathic Inflammatory Myopathies.
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