Autophagy is required for mitochondrial function,lipid metabolism,growth, and fate of KRASG12D-driven lung tumors

Evidence suggests that the role of autophagy in tumorigenesis is context dependent. Using genetically engineered mouse models (GEMMs) for human non-small-cell lung cancer (NSCLC), we found that deletion of the essential autophagy gene, Atg7, in KRAS^G12D-driven NSCLC inhibits tumor growth and converts adenomas and adenocarcinomas to benign oncocytomas characterized by the accumulation of respiration-defective mitochondria. Atg7 is required to preserve mitochondrial fatty acid oxidation (FAO) to maintain lipid homeostasis upon additional loss of Trp53 in NSCLC. Furthermore, cell lines derived from autophagy-deficient tumors depend on glutamine to survive starvation. This suggests that autophagy is essential for the metabolism, growth, and fate of NSCLC.     

EGFRvIII expression triggers a metabolic dependency and therapeutic vulnerability sensitive to autophagy inhibition

Expression of EGFRvIII is frequently observed in glioblastoma and is associated with increased cellular proliferation, enhanced tolerance to metabolic stresses, accelerated tumor growth, therapy resistance and poor prognosis. We observed that expression of EGFRvIII elevates the activation of macroautophagy/autophagy during starvation and hypoxia and explored the underlying mechanism and consequence. Autophagy was inhibited (genetically or pharmacologically) and its consequence for tolerance to metabolic stress and its therapeutic potential in (EGFRvIII⁺) glioblastoma was assessed in cellular systems, (patient derived) tumor xenopgrafts and glioblastoma patients. Autophagy inhibition abrogated the enhanced proliferation and survival advantage of EGFRvIII⁺ cells during stress conditions, decreased tumor hypoxia and delayed tumor growth in EGFRvIII⁺ tumors. These effects can be attributed to the supporting role of autophagy in meeting the high metabolic demand of EGFRvIII⁺ cells. As hypoxic tumor cells greatly contribute to therapy resistance, autophagy inhibition revokes the radioresistant phenotype of EGFRvIII⁺ tumors in (patient derived) xenograft tumors. In line with these findings, retrospective analysis of glioblastoma patients indicated that chloroquine treatment improves survival of all glioblastoma patients, but patients with EGFRvIII⁺ glioblastoma benefited most. Our findings disclose the unique autophagy dependency of EGFRvIII⁺ glioblastoma as a therapeutic opportunity. Chloroquine treatment may therefore be considered as an additional treatment strategy for glioblastoma patients and can reverse the worse prognosis of patients with EGFRvIII⁺ glioblastoma.     

Autophagy inhibits oxidative stress and tumor suppressors to exert its dual effect on hepatocarcinogenesis

The role of autophagy in carcinogenesis is controversial and apparently complex. By using mice with hepatocyte-specific knockout of Atg5, a gene essential for autophagy, we longitudinally studied the role of autophagy in hepatocarcinogenesis. We found that impairing autophagy in hepatocytes would induce oxidative stress and DNA damage, followed by the initiation of hepatocarcinogenesis, which could be suppressed by the antioxidant N-acetylcysteine. Interestingly, these mice developed only benign tumors with no hepatocellular carcinoma (HCC), even after the treatment with diethylnitrosamine, which induced HCC in wild-type mice. The inability of mice to develop HCC when autophagy was impaired was associated with the induction of multiple tumor suppressors including p53. Further analysis indicated that the induction of p53 was associated with the DNA-damage response. Tumorigenesis studies using an established liver tumor cell line confirmed a positive role of autophagy in tumorigenesis and a negative role of p53 in this process when autophagy was impaired. Our studies thus demonstrate that autophagy is required to maintain healthy mitochondria and to reduce oxidative stress and DNA damage to prevent the initiation of hepatocarcinogenesis. However, once hepatocarcinogenesis has been initiated, its presence is also required to suppress the expression of tumor suppressors to promote the development of HCC.Autophagy (i.e., macroautophagy) is important for cells to remove protein aggregates and damaged organelles. Its dysfunction can cause a variety of diseases including cancers.^{1, 2} However, its role in carcinogenesis is apparently complex, as it has been shown in different reports to positively or negatively regulate carcinogenesis.^{3, 4} Autophagy apparently can function as a tumor suppressor, as the gene encoding Beclin-1, a component of the phosphatidylinositol-3-kinase class III (PI3KC3) complex that is essential for the initiation of autophagy, is often monoallelically deleted or mutated in breast, ovarian and prostate cancers.⁵ Frameshift mutations in Atg2B, Atg5, Atg9B and Atg12 autophagy genes are also often found in gastric and colorectal cancers with microsatellite instability.⁶ The tumor suppressor role of autophagy is further supported by the studies using mouse models. It has been shown that the monoallelic deletion of the Beclin-1 gene in mice induced tumor lesions in various tissues,⁷ Atg4C-knockout (KO) mice had increased susceptibility to carcinogens for the development of fibrosarcomas⁸ and the systemic mosaic KO of Atg5 and the liver-specific KO of Atg7 in mice led to the development of benign liver adenomas.^{9, 10}Autophagy has also been shown to promote tumor growth. It has been shown that autophagy can enhance the survival of tumor cells in the hypoxic regions of solid tumors.¹¹ It has also been shown that in cells expressing oncogenic Ras, autophagy is required to promote tumorigenesis by maintaining oxidative metabolism or facilitating glycolysis.^{12, 13} Moreover, it has also been demonstrated that the suppression of autophagy by the expression of FIP200, a component of the ULK1-Atg13-FIP200-Atg101 complex that is essential for the induction of autophagy, could suppress mammary tumorigenesis induced by the polyomavirus middle T antigen in mice.¹⁴ These observations indicated a protumorigenic role of autophagy.In this report, we used mice with liver-specific KO of Atg5 (L-Atg5-KO) to study the role of autophagy in carcinogenesis. We found that abolishing the expression of Atg5 impaired autophagy in the liver and led to oxidative DNA damage and the development of benign hepatic tumors with no visible carcinoma. This inability to develop hepatocellular carcinoma (HCC) was correlated with the induction of tumor suppressors, which negatively regulate the progression of tumorigenesis when autophagy was impaired.     

Autophagy promotes BrafV600E-driven lung tumorigenesis by preserving mitochondrial metabolism

The role of autophagy in cancer is complex and context-dependent. Here we describe work with genetically engineered mouse models of non-small cell lung cancer (NSCLC) in which the tumor-suppressive and tumor-promoting function of autophagy can be visualized in the same system. We discovered that early tumorigenesis in Braf^V600E-driven lung cancer is accelerated by autophagy ablation due to unmitigated oxidative stress, as observed with loss of Nfe2l2/Nrf2-mediated antioxidant defense. However, this growth advantage is eventually overshadowed by progressive mitochondrial dysfunction and metabolic insufficiency, and is associated with increased survival of mice bearing autophagy-deficient tumors. Atg7 deficiency alters progression of Braf^V600E-driven tumors from adenomas (Braf^V600E; atg7^?/?) and adenocarcinomas (trp53^?/?; Braf^V600E; atg7^?/?) to benign oncocytomas that accumulated morphologically and functionally defective mitochondria, suggesting that defects in mitochondrial metabolism may compromise continued tumor growth. Analysis of tumor-derived cell lines (TDCLs) revealed that Atg7-deficient cells are significantly more sensitive to starvation than Atg7–wild-type counterparts, and are impaired in their ability to respire, phenotypes that are rescued by the addition of exogenous glutamine. Taken together, these data suggest that Braf^V600E-driven tumors become addicted to autophagy as a means to preserve mitochondrial function and glutamine metabolism, and that inhibiting autophagy may be a powerful strategy for Braf^V600E-driven malignancies.     

Atg1 allows second-signaled autophagic cell death in Dictyostelium

We investigated the role of Atg1 in autophagic cell death (ACD) in a Dictyostelium monolayer model. The model is especially propitious, not only because of genetic tractability and absence of apoptosis machinery, but also because induction of ACD requires two successive exogenous signals, first the combination of starvation and cAMP, second the differentiation factor DIF-1. This enables one to analyze separately first-signal-induced autophagy and subsequent second-signal-induced ACD. We used mutants of atg1, a gene that plays an essential role in the initiation of autophagy. Upon starvation/cAMP, in contrast to parental cells, atg1 mutant cells showed irreversible lesions, clearly establishing a protective role for Atg1. Upon subsequent exposure to DIF-1 or to more ACD-specific second signals, starved parental cells progressed to ACD, but starved atg1 mutant cells did not, showing that Atg1 was required for ACD. Thus, in the same cells Atg1 was required in two apparently opposite ways, upon first-signaling for cell survival and upon second-signaling for ACD. Our findings strongly suggest that Atg1, thus presumably autophagy, protects the cells from starvation-induced cell death, allowing subsequent induction of ACD by the second signal. ACD is therefore not only “with” autophagy (since

it showed signs of autophagy throughout), but is also “allowed by” autophagy. This does not exclude a role for autophagy also after second signaling. These results may account for discrepancies reported in the literature, encourage searches for second signals in different developmental models of ACD, and incite caution in autophagy-related therapeutic attempts.     

Defective autophagy in vascular smooth muscle cells enhances cell death and atherosclerosis

Macroautophagy/autophagy is considered as an evolutionarily conserved cellular catabolic process. In this study, we aimed to elucidate the role of autophagy in vascular smooth muscle cells (SMCs) on atherosclerosis. SMCs cultured from mice with SMC-specific deletion of the essential autophagy gene atg7 (Atg7cKO) showed reduced serum-induced cell growth, increased cell death, and decreased cell proliferation rate. Furthermore, 7-ketocholestrerol enhanced apoptosis and the expression of CCL2 (chemokine [C-C motif] ligand 2) with the activation of TRP53, the mouse ortholog of human and rat TP53, in SMCs from Atg7cKO mice. In addition, Atg7cKO mice crossed with Apoe (apolipoprotein E)-deficient mice (apoeKO; Atg7cKO:apoeKO) showed reduced medial cellularity and increased TUNEL-positive cells in the descending aorta at 10 weeks of age. Intriguingly, Atg7cKO: apoeKO mice fed a Western diet containing 1.25% cholesterol for 14 weeks showed a reduced survival rate. Autopsy of the mice demonstrated the presence of aortic rupture. Analysis of the descending aorta in Atg7cKO:apoeKO mice showed increased plaque area, increased TUNEL-positive area, decreased SMC-positive area, accumulation of macrophages in the media, and adventitia and perivascular tissue, increased CCL2 expression in SMCs in the vascular wall, medial disruption, and aneurysm formation. In conclusion, our data suggest that defective autophagy in SMCs enhances atherosclerotic changes with outward arterial remodeling.     

An unexpected role for autophagy in degranulation of mast cells

Mast cells play a crucial role in allergic inflammatory reactions through releasing cytosolic granules upon antigen stimulation. However, the mechanisms underlying maturation and release of secretory granules are not fully understood. We found that autophagy is constitutively induced in mast cells under full nutrition conditions, and type II LC3 (LC3-II), a marker for autophagosomes, localizes on secretory granules. While deletion of Atg7 does not impair the development of bone marrow-derived mast cells (BMMCs), Atg7-deficient BMMCs show severe impairment of degranulation, but not cytokine production, upon antigen stimulation. Moreover we found that LC3-II, but not LC3-I, colocalizes with CD63, a marker for secretory lysosomes and is released extracellularly along with degranulation in wild-type BMMCs, but not Atg7-deficient BMMCs. Finally, passive cutaneous anaphylaxis reactions are almost completely abolished in mast celldeficient mice reconstituted with Atg7-deficient BMMCs. Collectively, these results suggest that autophagy is not essential for the development, but plays a crucial role in degranulation, of mast cells.     

Dual role of Atg1 in regulation of autophagy-specific PAS assembly in Saccharomyces cerevisiae

Most autophagy-related (Atg) proteins are assembled at the phagophore assembly site or pre-autophagosomal structure (PAS), which is a potential site for vesicle formation during vegetative or starvation conditions. To understand the initial step of vesicle formation, it is important to know how Atg proteins are recruited to the PAS. Atg11 facilitates PAS assembly for the cytoplasm to vacuole targeting (Cvt) pathway in vegetative conditions. To examine autophagy-specific PAS formation, an ATG11 deletion mutant was used to eliminate the PAS formation that occurs in vegetative conditions. We found that Atg1, Atg13 and Atg17 play a similar role for PAS formation under autophagy-inducing conditions as seen for Atg11 during vegetative growth. In particular, Atg1 is proposed to have dual roles for autophagy-specific PAS recruitment. Atg1 plays a structural role for efficient recruitment of Atg proteins to the PAS, which is mediated by interaction with Atg13 and Atg17. In contrast, Atg1 kinase activity is needed for dissociation of Atg proteins from the PAS during autophagy inducing conditions, a function which is also critical for autophagy activity.

Addendum to: Cheong H, Nair U, Geng J Klionsky DK. The Atg1 kinase complex Is involved in the regulation of protein recruitment to initiate sequestering vesicle formation for nonspecific autophagy in Saccharomyces cerevisiae. Mol Biol Cell 2008; 19:668-81.     

Autophagy plays a critical role in kidney tubule maintenance,aging and ischemia-reperfusion injury

Autophagy is responsible for the degradation of protein aggregates and damaged organelles. Several studies have reported increased autophagic activity in tubular cells after kidney injury. Here, we examine the role of tubular cell autophagy in vivo under both physiological conditions and stress using two different tubular-specific Atg5-knockout mouse models. While Atg5 deletion in distal tubule cells does not cause a significant alteration in kidney function, deleting Atg5 in both distal and proximal tubule cells results in impaired kidney function. Already under physiological conditions, Atg5-null tubule cells display a significant accumulation of p62 and oxidative stress markers. Strikingly, tubular cell Atg5-deficiency dramatically sensitizes the kidneys to ischemic injury, resulting in impaired kidney function, accumulation of damaged mitochondria as well as increased tubular cell apoptosis and proliferation, highlighting the critical role that autophagy plays in maintaining tubular cell integrity during stress conditions.     

Autophagy,proteases and the sense of balance

The knowledge of the molecular mechanisms underlying autophagy has considerably improved after the isolation and characterization of autophagy-defective mutants in the yeast Saccharomyces cerevisiae. Two ubiquitin-like conjugation systems are required for yeast autophagy. One of them requires the participation of Atg8 synthesized as a precursor protein, which is cleaved after a Gly residue by a cysteine proteinase called Atg4. The new Gly-terminal residue from Atg8 is activated by Atg7 (an E1-like enzyme) then transferred to Atg3 (an E2-like enzyme) and finally conjugated with membrane-bound phosphatidylethanolamine (PE) through an amide bond. The complex Atg8–PE is also deconjugated by the protease Atg4, facilitating the release of Atg8 from membranes. This modification system, which is essential for the membrane rearrangement dynamics that accompany the initiation and execution of autophagy, is conserved in higher eukaryotes including mammals. We have previously identified and cloned the four human orthologues of the yeast proteinase Atg4, whereas parallel studies have revealed that there are at least six orthologues of yeast Atg8 in mammals (LC3A, LC3B, LC3C, GABARAP, ATG8L/GABARAPL1 and GATE-16/GABARAPL2). Thus, in mammals, the Atg4-Atg8 proteolytic system is composed of four proteinases (autophagins) that may target at least six distinct substrates, contrasting with the simplified yeast system in which one single protease cleaves a sole substrate. Currently, it is unclear why mammals have developed this array of closely related enzymes, as other essential autophagy genes such as Atg3, Atg5 or Atg7 are represented in mammalian cells by a single orthologue. It has been suggested that the multiplication of Atg4 orthologues may reflect a regulatory heterogeneity of functionally redundant proteins or, alternatively, derive from the acquisition of new functions that are not related to autophagy. Our first approach to elucidate this question was based on the generation of autophagin-3/Atg4C-deficient mice, which however presented a minor phenotype. With the generation of autophagin-1/Atg4B-deficient mice, recently reported, we have progressed in our attempt to identify the in vivo physiological and pathological roles of autophagins.     

Autophagic reliance promotes metabolic reprogramming in oncogenic KRAS-driven tumorigenesis

Defects in basal autophagy limit the nutrient supply from recycling of intracellular constituents. Despite our understanding of the prosurvival role of macroautophagy/autophagy, how nutrient deprivation, caused by compromised autophagy, affects oncogenic KRAS-driven tumor progression is poorly understood. Here, we demonstrate that conditional impairment of the autophagy gene Atg5 (atg5-KO) extends the survival of KRAS^G12V-driven tumor-bearing mice by 38%. atg5-KO tumors spread more slowly during late tumorigenesis, despite a faster onset. atg5-KO tumor cells displayed reduced mitochondrial function and increased mitochondrial fragmentation. Metabolite profiles indicated a deficiency in the nonessential amino acid asparagine despite a compensatory overexpression of ASNS (asparagine synthetase), key enzyme for de novo asparagine synthesis. Inhibition of either autophagy or ASNS reduced KRAS^G12V-driven tumor cell proliferation, migration, and invasion, which was rescued by asparagine supplementation or knockdown of MFF (mitochondrial fission factor). Finally, these observations were reflected in human cancer-derived data, linking ASNS overexpression with poor clinical outcome in multiple cancers. Together, our data document a widespread yet specific asparagine homeostasis control by autophagy and ASNS, highlighting the previously unrecognized role of autophagy in suppressing the metabolic barriers of low asparagine and excessive mitochondrial fragmentation to permit malignant KRAS-driven tumor progression.     

Autophagy deficiency in myeloid cells increases susceptibility to obesity-induced diabetes and experimental colitis

Autophagy, which is critical for the proper turnover of organelles such as endoplasmic reticulum and mitochondria, affects diverse aspects of metabolism, and its dysregulation has been incriminated in various metabolic disorders. However, the role of autophagy of myeloid cells in adipose tissue inflammation and type 2 diabetes has not been addressed. We produced mice with myeloid cell-specific deletion of Atg7 (autophagy-related 7), an essential autophagy gene (Atg7 conditional knockout [cKO] mice). While Atg7 cKO mice were metabolically indistinguishable from control mice, they developed diabetes when bred to ob/w mice (Atg7 cKO-ob/ob mice), accompanied by increases in the crown-like structure, inflammatory cytokine expression and inflammasome activation in adipose tissue. Mφs (macrophages) from Atg7 cKO mice showed significantly higher interleukin 1 β release and inflammasome activation in response to a palmitic acid plus lipopolysaccharide combination. Moreover, a decrease in the NAD⁺:NADH ratio and increase in intracellular ROS content after treatment with palmitic acid in combination with lipopolysaccharide were more pronounced in Mφs from Atg7 cKO mice, suggesting that mitochondrial dysfunction in autophagy-deficient Mφs leads to an increase in lipid-induced inflammasome and metabolic deterioration in Atg7 cKO-ob/ob mice. Atg7 cKO mice were more susceptible to experimental colitis, accompanied by increased colonic cytokine expression, T helper 1 skewing and systemic bacterial invasion. These results suggest that autophagy of Mφs is important for the control of inflammasome activation in response to metabolic or extrinsic stress, and autophagy deficiency in Mφs may contribute to the progression of metabolic syndrome associated with lipid injury and colitis.     

Autophagy is dispensable for Kmt2a/Mll-Mllt3/Af9 AML maintenance and anti-leukemic effect of chloroquine

Recently, macroautophagy/autophagy has emerged as a promising target in various types of solid tumor treatment. However, the impact of autophagy on acute myeloid leukemia (AML) maintenance and the validity of autophagy as a viable target in AML therapy remain unclear. Here we show that Kmt2a/Mll-Mllt3/Af9 AML (MA9-AML) cells have high autophagy flux compared with normal bone marrow cells, but autophagy-specific targeting, either through Rb1cc1-disruption to abolish autophagy initiation, or via Atg5-disruption to prevent phagophore (the autophagosome precursor) membrane elongation, does not affect the growth or survival of MA9-AML cells, either in vitro or in vivo. Mechanistically, neither Atg5 nor Rb1cc1 disruption impairs endolysosome formation or survival signaling pathways. The autophagy inhibitor chloroquine shows autophagy-independent anti-leukemic effects in vitro but has no efficacy in vivo likely due to limited achievable drug efficacy in blood. Further, vesicular exocytosis appears to mediate chloroquine resistance in AML cells, and exocytotic inhibition significantly enhances the anti-leukemic effect of chloroquine. Thus, chloroquine can induce leukemia cell death in vitro in an autophagy-independent manner but with inadequate efficacy in vivo, and vesicular exocytosis is a possible mechanism of chloroquine resistance in MA9-AML. This study also reveals that autophagy-specific targeting is unlikely to benefit MA9-AML therapy.     

Autophagy in non-small cell lung carcinogenesis

In a mouse model of non-small cell lung carcinogenesis, we recently found that the inactivation of the essential autophagy gene Atg5 causes an acceleration of the early phases of oncogenesis. Thus, hyperplastic lesions and adenomas are more frequent at early stages after adenoviral delivery of Cre recombinase via inhalation, when Cre—in addition to activating the KRas^G12D oncogene—inactivates both alleles of the Atg5 gene. The accelerated oncogenesis of autophagy-deficient tumors developing in KRas;Atg5^fl/fl mice (as compared with autophagy-competent KRas;Atg5^fl/+ control tumors) correlates with an increased infiltration by FOXP3⁺ regulatory T cells (Tregs). Depletion of such Tregs by means of specific monoclonal antibodies inhibits the accelerated oncogenesis of autophagy-deficient tumors down to the level observed in autophagy-competent controls. Subsequent analyses revealed that the combination of KRas activation and Atg5 inactivation favors the expression of ENTPD1/CD39, an ecto-ATPase that initiates the conversion of extracellular ATP, which is immunostimulatory, into adenosine, which is immunosuppressive. Pharmacological inhibition of ENTPD1 or blockade of adenosinergic receptors reduces the infiltration of KRas;Atg5^fl/fl tumors by Tregs and reverses accelerated oncogenesis. Altogether these data favor a model according to which autophagy deficiency favors oncogenesis via changes in the tumor microenvironment that ultimately entail the Treg-mediated inhibition of anticancer immunosurveillance.     

Nuclear factor-κB signaling pathway is involved in phospholipase Cε-regulated proliferation in human renal cell carcinoma cells

Glioblastoma is the most aggressive cerebral gliomas. Despite advances in therapies, the prognosis is still very poor. Therefore, novel therapeutic strategies are required. As a proteasome inhibitor, bortezomib has shown its efficacy as an active antitumor agent against a variety of tumors. However, inhibition of proteasome activity leads to cell death and also induces cell autophagy, and due to the dual roles of autophagy in the survival and death of tumor cells, the effect of inhibition of autophagy on glioblastoma cells remains to be explored. We therefore assessed whether bortezomib is capable of inducing autophagy, and investigated the antitumor effect of bortezomib combined with autophagy inhibitors on human glioblastoma U251 and U87 cells. Cell viability was measured by MTT assay. The expressions of autophagy and apoptosis-related proteins were determined by Western blot analysis. U251 and U87 cells proliferation was inhibited in a dose-dependent manner. Both apoptosis and autophagy induced by bortezomib were observed in human glioblastoma U87 and U251 cells. However, when U251 and U87 cells were co-treated with bortezomib and autophagy inhibitors 3-MA or Atg7 siRNA, the autophagy inhibitors blocked the autophagy in the cells and resulted in a further inhibition of cell proliferation and a further increase in cell apoptosis as compared with that treated with bortezomib alone. These findings indicated that combination of bortezomib and autophagy inhibitors may shed new light on glioblastoma treatment.     

Autophagy proteins are not universally required for phagosome maturation

Phagocytosis plays a central role in immunity and tissue homeostasis. After internalization of cargo into single-membrane phagosomes, these compartments undergo a maturation sequences that terminates in lysosome fusion and cargo degradation. Components of the autophagy pathway have recently been linked to phagosome maturation in a process called LC3-associated phagocytosis (LAP). In this process, autophagy machinery is thought to conjugate LC3 directly onto the phagosomal membrane to promote lysosome fusion. However, a recent study has suggested that ATG proteins may in fact impair phagosome maturation to promote antigen presentation. Here, we examined the impact of ATG proteins on phagosome maturation in murine cells using FCGR2A/FcγR-dependent phagocytosis as a model. We show that phagosome maturation is not affected in Atg5-deficient mouse embryonic fibroblasts, or in Atg5- or Atg7-deficient bone marrow-derived macrophages using standard assays of phagosome maturation. We propose that ATG proteins may be required for phagosome maturation under some conditions, but are not universally required for this process.     

Drosophila Atg7: Required for stress resistance,longevity and neuronal homeostasis,but not for metamorphosis

Autophagy, the lysosomal degradation and recycling of self material, has been implicated in a number of developmental and pathological conditions including aging, cancer, neurodegeneration, and insect metamorphosis. Surprisingly, Atg7 mutant flies are able to complete metamorphosis with only a slight delay, despite strongly reduced autophagy levels. Similarly, developmental elimination of the larval midgut proceeds with normal morphology, suggesting that animals can compensate for reduced autophagy during development. Atg7 mutant adults are hypersensitive to starvation and oxidative stress, live shorter, and accumulate ubiquitin-positive aggregates in the brain that lead to a progressive decline of neuronal function and cell death. These results suggest that in Drosophila, normal levels of autophagy may play a more important role in the homeostasis of certain terminally differentiated cells and stress survival than during development.

Addendum to: Juhász G, Érdi B, Sass M, Neufeld TP. Atg7-dependent autophagy promotes neuronal health, stress tolerance, and longevity but is dispensable for metamorphosis in Drosophila. Genes Dev 2007; 21:3061-6.     

Autophagy gene ATG7 regulates ultraviolet radiation-induced inflammation and skin tumorigenesis

Macroautophagy (hereafter autophagy) is a cellular “self-eating” process that is implicated in many human cancers, where it can act to either promote or suppress tumorigenesis. However, the role of autophagy in regulation of inflammation during tumorigenesis remains unclear. Here we show that autophagy is induced in the epidermis by ultraviolet (UV) irradiation and autophagy gene Atg7 promoted UV-induced inflammation and skin tumorigenesis. Atg7 regulated UV-induced cytokine expression and secretion, and promoted Ptgs2/Cox-2 expression through both a CREB1/CREB-dependent cell autonomous mechanism and an IL1B/IL1β-dependent non-cell autonomous mechanism. Adding PGE₂ increased UV-induced skin inflammation and tumorigenesis, reversing the epidermal phenotype in mice with Atg7 deletion in keratinocytes. Similar to ATG7 knockdown in human keratinocytes, ATG5 knockdown inhibited UVB-induced expression of PTGS2 and cytokines. Furthermore, ATG7 loss increased the activation of the AMPK pathway and the phosphorylation of CRTC1, and led to endoplasmic reticulum (ER) accumulation and reduction of ER stress. Inducing ER stress and inhibiting calcium influx into the ER by thapsigargin reverses the inflammation and tumorigenesis phenotype in mice with epidermal Atg7 deletion. Taken together, these findings demonstrate that deleting autophagy gene Atg7 leads to a suppression of carcinogen-induced protumorigenic inflammatory microenvironment and tumorigenesis of the epithelium.