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.     

The yin and yang of autophagy in acute kidney injury

Antagonizing the strongly activated pathway of autophagy in renal ischemic injury has been associated with poor outcome. In our recent study we used mice with a selective deletion of Atg5 in the S3 proximal tubule segment, which is most susceptible to ischemic damage. In line with the notion that autophagy is a prosurvival mechanism our studies revealed an early accelerated cell death of heavily damaged tubular cells in the S3 segment of these mice. Interestingly, this expedited loss of cells was associated with better long-term outcome as reflected by less inflammation, improved tubular repair, and function and reduced accumulation of senescent cells. While these data confirm the role of tubular autophagy as a prosurvival mechanism in ischemic kidney injury, they also show that autophagy may enable severely damaged cells to persist and exert deleterious effects. Such ambivalent effects might be of relevance if modulating autophagy is considered as a therapeutic option.     

The impact of autophagy on the development of senescence in primary tubular epithelial cells

Autophagy and senescence are 2 distinct pathways that are importantly involved in acute kidney injury and renal repair. Recent data indicate that the 2 processes might be interrelated. To investigate the potential link between autophagy and senescence in the kidney we isolated primary tubular epithelial cells (PTEC) from wild-type mice and monitored the occurrence of cellular senescence during autophagy activation and inhibition. We found that the process of cell isolation and transfer into culture was associated with a strong basal autophagic activation in PTEC. Specific inhibition of autophagy by silencing autophagy-related 5 (Atg5) counteracted the occurrence of senescence hallmarks under baseline conditions. Reduced senescent features were also observed in Atg5 silenced PTEC after γ-irradiation and during H-Ras induced oncogenic senescence, but the response was less uniform in these stress models. Senescence inhibition was paralleled by better preservation of a mature epithelial phenotype in PTEC. Interestingly, treatment with rapamycin, which acts as an activator of autophagy, also counteracted the occurrence of senescence features in PTEC. While we interpret the anti-senescent effect of rapamycin as an autophagy-independent effect of mTOR-inhibition, the more specific approach of Atg5 silencing indicates that overactivated autophagy can have pro-senescent effects in PTEC. These results highlight the complex interaction between cell culture dependent stress mechanisms, autophagy and senescence.     

Role of mitofusin 2 in the renal stress response

The role of mitofusin 2 (MFN2), a key regulator of mitochondrial morphology and function in the renal stress response is unknown. To assess its role, the MFN2 floxed gene was conditionally deleted in the kidney of mice (MFN2 cKO) by Pax2 promoter driven Cre expression (Pax2Cre). MFN2 cKO caused severe mitochondrial fragmentation in renal epithelial cells that are critical for normal kidney tubular function. However, despite a small (20%) decrease in nephron number, newborn cKO pups had organ or tubular function that did not differ from littermate Cre-negative pups. MFN2 deficiency in proximal tubule epithelial cells in primary culture induced mitochondrial fragmentation but did not significantly alter ATP turnover, maximal mitochondrial oxidative reserve capacity, or the low level of oxygen consumption during cyanide exposure. MFN2 deficiency also did not increase apoptosis of tubule epithelial cells under non-stress conditions. In contrast, metabolic stress caused by ATP depletion exacerbated mitochondrial outer membrane injury and increased apoptosis by 80% in MFN2 deficient vs. control cells. Despite similar stress-induced Bax 6A7 epitope exposure in MFN2 deficient and control cells, MFN2 deficiency significantly increased mitochondrial Bax accumulation and was associated with greater release of both apoptosis inducing factor and cytochrome c. In conclusion, MFN2 deficiency in the kidney causes mitochondrial fragmentation but does not affect kidney or tubular function during development or under non-stress conditions. However, MFN2 deficiency exacerbates renal epithelial cell injury by promoting Bax-mediated mitochondrial outer membrane injury and apoptosis.     

Mitochondrial accumulation under oxidative stress is due to defects in autophagy

Mitochondrial dynamics maintains normal mitochondrial function by degrading damaged mitochondria and generating newborn mitochondria. The accumulation of damaged mitochondria influences the intracellular environment by promoting mitochondrial dysfunction, and thus initiating a vicious cycle. Oxidative stress induces mitochondrial malfunction, which is involved in many cardiovascular diseases. However, the mechanism of mitochondrial accumulation in cardiac myoblasts remains unclear. We observed mitochondrial dysfunction and an increase in mitochondrial mass under the oxidative conditions produced by tert‐butyl hydroperoxide (tBHP) in cardiac myoblast H9c2 cells. However, in contrast to the increase in mitochondrial mass, mitochondrial DNA (mtDNA) decreased, suggesting that enhanced mitochondrial biogenesis may be not the primary cause of the mitochondrial accumulation. Therefore, we investigated changes in a number of proteins involved in autophagy. Beclin1, Atg12–Atg5 conjugate, Atg7 contents decreased but LC3‐II accumulated in tBHP‐treated H9c2 cells. Moreover, the capacity for acid hydrolysis decreased in H9c2 cells. We also demonstrated a decrease in DJ‐1 protein under the oxidative conditions that deregulate mitochondrial dynamics. These results reveal that autophagy became defective under oxidative stress. We therefore suggest that defects in autophagy mediate mitochondrial accumulation under these conditions. J. Cell. Biochem. 114: 212–219, 2012. © 2012 Wiley Periodicals, Inc.     

Atg5-mediated autophagy deficiency in proximal tubules promotes cell cycle G2/M arrest and renal fibrosis

Macroautophagy/autophagy protects against cellular stress. Renal sublethal injury-triggered tubular epithelial cell cycle arrest at G₂/M is associated with interstitial fibrosis. However, the role of autophagy in renal fibrosis is elusive. Here, we hypothesized that autophagy activity in tubular epithelial cells is pivotal for inhibition of cell cycle G₂/M arrest and subsequent fibrogenic response. In both renal epithelial cells stimulated by angiotensin II (AGT II) and the murine kidney after unilateral ureteral obstruction (UUO), we observed that occurrence of autophagy preceded increased production of COL1 (collagen, type I). Pharmacological enhancement of autophagy by rapamycin suppressed COL1 accumulation and renal fibrosis. In contrast, genetic ablation of autophagy by proximal tubular epithelial cell-specific deletion of Atg5, with reduction of the LC3-II protein level and degradation of SQSTM1/p62, showed marked cell cycle arrest at the G₂/M phase, robust COL1 deposition, and severe interstitial fibrosis in a UUO model, as compared with wild-type mice. In vitro, AGT II exposure triggered autophagy preferentially in the G₁/S phase, and increased COL1 expression in the G₂/M phase in renal epithelial cells. Stimulation of Atg5-deficient primary proximal tubular cells with AGT II also resulted in elevated G₂/M arrest and COL1 production. Pharmacological or genetic inhibition of autophagy increased AGT II-mediated G₂/M arrest. Enhanced expression of ATG5, but not the autophagy-deficient ATG5 mutant K130R, rescued the G₂/M arrest, suggesting the regulation of cell cycle progression by ATG5 is autophagy dependent. In conclusion, Atg5-mediated autophagy in proximal epithelial cells is a critical host-defense mechanism that prevents renal fibrosis by blocking G₂/M arrest.     

Loss of autophagy diminishes pancreatic beta cell mass and function with resultant hyperglycemia

Autophagy is a cellular degradation-recycling system for aggregated proteins and damaged organelles. Although dysregulated autophagy is implicated in various diseases including neurodegeneration, its role in pancreatic beta cells and glucose homeostasis has not been described. We produced mice with beta cell-specific deletion of Atg7 (autophagy-related 7). Atg7 mutant mice showed impaired glucose tolerance and decreased serum insulin level. beta cell mass and pancreatic insulin content were reduced because of increased apoptosis and decreased proliferation of beta cells. Physiological studies showed reduced basal and glucose-stimulated insulin secretion and impaired glucose-induced cytosolic Ca2+ transients in autophagy-deficient beta cells. Morphologic analysis revealed accumulation of ubiquitinated protein aggregates colocalized with p62, which was accompanied by mitochondrial swelling, endoplasmic reticulum distension, and vacuolar changes in beta cells. These results suggest that autophagy is necessary to maintain structure, mass and function of pancreatic beta cells, and its impairment causes insulin deficiency and hyperglycemia because of abnormal turnover and function of cellular organelles.     

The role of autophagy in cardiomyocytes in the basal state and in response to hemodynamic stress

Autophagy, an evolutionarily conserved process for the bulk degradation of cytoplasmic components, serves as a cell survival mechanism in starving cells. Although altered autophagy has been observed in various heart diseases, including cardiac hypertrophy and heart failure, it remains unclear whether autophagy plays a beneficial or detrimental role in the heart. Here, we report that the cardiac-specific loss of autophagy causes cardiomyopathy in mice. In adult mice, temporally controlled cardiac-specific deficiency of Atg5 (autophagy-related 5), a protein required for autophagy, led to cardiac hypertrophy, left ventricular dilatation and contractile dysfunction, accompanied by increased levels of ubiquitination. Furthermore, Atg5-deficient hearts showed disorganized sarcomere structure and mitochondrial misalignment and aggregation. On the other hand, cardiac-specific deficiency of Atg5 early in cardiogenesis showed no such cardiac phenotypes under baseline conditions, but developed cardiac dysfunction and left ventricular dilatation one week after treatment with pressure overload. These results indicate that constitutive autophagy in the heart under baseline conditions is a homeostatic mechanism for maintaining cardiomyocyte size and global cardiac structure and function, and that upregulation of autophagy in failing hearts is an adaptive response for protecting cells from hemodynamic stress.     

Endoplasmic Reticulum Stress-Induced Autophagy Provides Cytoprotection from Chemical Hypoxia and Oxidant Injury and Ameliorates Renal Ischemia-Reperfusion Injury

We examined whether endoplasmic reticulum (ER) stress-induced autophagy provides cytoprotection from renal tubular epithelial cell injury due to oxidants and chemical hypoxia in vitro, as well as from ischemia-reperfusion (IR) injury in vivo. We demonstrate that the ER stress inducer tunicamycin triggers an unfolded protein response, upregulates ER chaperone Grp78, and activates the autophagy pathway in renal tubular epithelial cells in culture. Inhibition of ER stress-induced autophagy accelerated caspase–3 activation and cell death suggesting a pro-survival role of ER stress-induced autophagy. Compared to wild-type cells, autophagy-deficient MEFs subjected to ER stress had enhanced caspase–3 activation and cell death, a finding that further supports the cytoprotective role of ER stress-induced autophagy. Induction of autophagy by ER stress markedly afforded cytoprotection from oxidants H₂O₂ and tert-Butyl hydroperoxide and from chemical hypoxia induced by antimycin A. In contrast, inhibition of ER stress-induced autophagy or autophagy-deficient cells markedly enhanced cell death in response to oxidant injury and chemical hypoxia. In mouse kidney, similarly to renal epithelial cells in culture, tunicamycin triggered ER stress, markedly upregulated Grp78, and activated autophagy without impairing the autophagic flux. In addition, ER stress-induced autophagy markedly ameliorated renal IR injury as evident from significant improvement in renal function and histology. Inhibition of autophagy by chloroquine markedly increased renal IR injury. These studies highlight beneficial impact of ER stress-induced autophagy in renal ischemia-reperfusion injury both in vitro and in vivo.     

Persistent activation of autophagy in kidney tubular cells promotes renal interstitial fibrosis during unilateral ureteral obstruction

Renal fibrosis is the final, common pathway of end-stage renal disease. Whether and how autophagy contributes to renal fibrosis remains unclear. Here we first detected persistent autophagy in kidney proximal tubules in the renal fibrosis model of unilateral ureteral obstruction (UUO) in mice. UUO-associated fibrosis was suppressed by pharmacological inhibitors of autophagy and also by kidney proximal tubule-specific knockout of autophagy-related 7 (PT-Atg7 KO). Consistently, proliferation and activation of fibroblasts, as indicated by the expression of ACTA2/α-smooth muscle actin and VIM (vimentin), was inhibited in PT-Atg7 KO mice, so was the accumulation of extracellular matrix components including FN1 (fibronectin 1) and collagen fibrils. Tubular atrophy, apoptosis, nephron loss, and interstitial macrophage infiltration were all inhibited in these mice. Moreover, these mice showed a specific suppression of the expression of a profibrotic factor FGF2 (fibroblast growth factor 2). In vitro, TGFB1 (transforming growth factor β 1) induced autophagy, apoptosis, and FN1 accumulation in primary proximal tubular cells. Inhibition of autophagy suppressed FN1 accumulation and apoptosis, while enhancement of autophagy increased TGFB1-induced-cell death. These results suggest that persistent activation of autophagy in kidney proximal tubules promotes renal interstitial fibrosis during UUO. The profibrotic function of autophagy is related to the regulation on tubular cell death, interstitial inflammation, and the production of profibrotic factors.     

Autophagy delays apoptosis in renal tubular epithelial cells in cisplatin cytotoxicity

One of the major side effects of cisplatin chemotherapy is toxic acute kidney injury due to preferential accumulation of cisplatin in renal proximal tubule epithelial cells and the subsequent injury to these cells. Apoptosis is known as a major mechanism of cisplatin-induced cell death in renal tubular cells. We have also recently demonstrated that autophagy induction is an immediate response of renal tubular epithelial cell exposure to cisplatin. Inhibition of cisplatin-induced autophagy blocks the formation of autophagosomes and enhances cisplatin-induced caspase-3, -6, and -7 activation, nuclear fragmentation and apoptosis. The switch from autophagy to apoptosis by autophagic inhibitors suggests that autophagy induction was responsible for a pre-apoptotic lag phase observed on exposure of renal tubular cells to cisplatin. Our studies provide evidence that autophagy induction in response to cisplatin mounts an adaptive response that suppresses and delays apoptosis. The beneficial effect of autophagy has a potential clinical significance in minimizing or preventing cisplatin nephrotoxicity.     

The role of autophagy in doxorubicin-induced cardiotoxicity

Doxorubicin (Dox) is an effective chemotherapeutic agent, however, its use is limited by cardiotoxicity. The mechanisms causing cardiotoxicity have not been clearly elucidated, but known to involve, at least in part, oxidative stress, mitochondrial dysfunction and apoptosis. More recently, it has been suggested that dysregulation of autophagy may also play an important role in Dox-induced cardiotoxicity. Autophagy has dual functions. Under physiological conditions, autophagy is essential for optimal cellular function and survival by ridding the cell of damaged or unwanted proteins and organelles. Under pathological conditions, autophagy may be stimulated in order to protect the cell from stress stimuli or, alternatively, to contribute to cell death. Thus, appropriate regulation of autophagy can be a matter of life or death. The role of autophagy in Dox-induced cardiotoxicity has recently been explored, however, conflicting reports on the effects of Dox on autophagy and its role in cardiotoxicity exist. Most, but not all, of the studies conclude that Dox upregulates cardiac autophagy and contributes to the pathogenesis of Dox-induced toxicity. Dox may induce autophagy by suppressing the expression of GATA4 and/or S6K1, which may directly or indirectly regulate expression of essential autophagy genes such as Atg12, Atg5, Beclin1 and Bcl-2. Interestingly, the Dox-induced autophagic response may be species specific as Dox treatment has been shown to stimulate autophagy in rat models, but suppress autophagy in mouse models. Additional studies will elucidate this possibility.     

Inducible disruption of autophagy in the lung causes airway hyper-responsiveness

Autophagy is a highly conserved process primarily known for its role in cellular adaptation to nutritional stress. This bulk protein degradation pathway relocates nutrients during starvation. Recent studies, however, have revealed essential roles of autophagy in various organs under normal conditions. Especially, autophagy is now recognized as the pathway responsible for the elimination of damaged proteins resulting from environmental stress. Lungs are constantly exposed to high oxygen tension and environmental chemicals. To investigate the importance of autophagy in lung physiology, we used an inducible system to ablate Atg7 expression, which is a protein essential for autophagy, in the respiratory epithelial cells of adult mice. We found that Atg7 deficiency caused swelling of bronchiolar epithelial cells and accumulation of p62, which links substrate proteins to the autophagy machinery. Bronchiolar epithelial cells, isolated by micro-dissection of lung tissues, had elevated expression of cytoprotective genes that are typically activated by Nrf2. Interestingly, Atg7-deficient lungs displayed hyper-responsiveness to cholinergic stimuli without apparent inflammatory signs. Swollen bronchiolar epithelial cells may have lead to mechanical airway constriction and lowered the threshold for the increase of airway resistance. This study demonstrates the critical role of autophagy in the lungs for the maintenance of pulmonary homeostasis.     

Autophagy delays apoptosis in renal tubular epithelial cells in cisplatin cytotoxicity

One of the major side effects of cisplatin chemotherapy is toxic acute kidney injury due to preferential accumulation of cisplatin in renal proximal tubule epithelial cells and the subsequent injury to these cells. Apoptosis is known as a major mechanism of cisplatin-induced cell death in renal tubular cells. We have also recently demonstrated that autophagy induction is an immediate response of renal tubular epithelial cell exposure to cisplatin. Inhibition of cisplatin-induced autophagy blocks the formation of autophagosomes and enhances cisplatin-induced caspase-3, -6, and -7 activation, nuclear fragmentation, and apoptosis. The switch from autophagy to apoptosis by autophagic inhibitors suggests that autophagy induction was responsible for a pre-apoptotic lag phase observed on exposure of renal tubular cells to cisplatin. Our studies provide evidence that autophagy induction in response to cisplatin mounts an adaptive response that suppresses and delays apoptosis. The beneficial effect of autophagy has a potential clinical significance in minimizing or preventing cisplatin nephrotoxicity.

Addedum to: Yang C, Kaushal V, Shah SV, Kaushal GP. Autophagy and apoptosis are associated in cisplatin injury to renal tubular epithelial cell injury. Am J Physiol Renal Physiol 2008; 294:F777-87.     

Reciprocal regulation between ER stress and autophagy in renal tubular fibrosis and apoptosis

Both endoplasmic reticulum (ER) stress and autophagy have been implicated in chronic kidney injury and renal fibrosis. However, the relationship and regulatory mechanisms between ER stress and autophagy under this condition remain largely unknown. In this study, we first established a mouse model of ER stress-induced chronic kidney injury by 2 weekly injections of a low dose of tunicamycin (TM), a classical ER stress inducer. This model showed the induction of ER stress, autophagy, fibrosis and apoptosis in kidney tissues. In vitro, TM also induced ER stress, autophagy, fibrosis and apoptosis in HK-2 human kidney proximal tubular cells and BUMPT-306 mouse kidney proximal tubular cells. In these cells, autophagy inhibitor suppressed TM-induced fibrotic changes and apoptosis, suggesting an involvement of autophagy in ER stress-associated chronic kidney injury. PERK inhibitor ameliorated autophagy, fibrotic protein expression and apoptosis in TM-treated cells, indicating a role of the PERK/eIF2α pathway in autophagy activation during ER stress. Similar results were shown in TGF-β1-treated HK-2 cells. Interestingly, in both TM- or TGF-β1-treated kidney proximal tubular cells, inhibition of autophagy exaggerated ER stress, suggesting that autophagy induced by ER stress provides a negative feedback mechanism to reduce the stress. Together, these results unveil a reciprocal regulation between ER stress and autophagy in chronic kidney injury and fibrosis.Subject terms: Acute kidney injury, Chronic kidney disease     

Autophagy in FOXD1 stroma-derived cells regulates renal fibrosis through TGF-β and NLRP3 inflammasome pathway

Renal fibrosis is the final common pathway of various renal injuries and it leads to chronic kidney disease. Recent studies reported that FOXD1-lineage pericyte plays a critical role in tubulointerstitial fibrosis (TIF). However the regulatory mechanisms remain unclear. Autophagy is a cellular process of degradation of damaged cytoplasmic components that regulates cell death and proliferation. To investigate the role of autophagy in FOXD1-lineage pericytes on renal TIF, we generated the FOXD1-lineage stromal cell-specific Atg7 deletion (Atg7^△FOXD1) mice. FOXD1-lineage stromal cell-specific Atg7 deletion enhanced renal TIF through Smad-dependent transforming growth factor (TGF)-β signaling after unilateral ureteral obstruction (UUO). FOXD1-lineage stromal cell-specific Atg7 deletion increased the accumulation of interstitial myofibroblasts and enhanced the differentiation of pericytes into myofibroblasts after UUO. Peritubular capillary rarefaction was accelerated in Atg7^△FOXD1 mice after UUO. Atg7^△FOXD1 mice increased the accumulation of SQSTM1/p62-positive aggregates in the obstructed kidney and resulted in increased expression of NLRP3 inflammasome, interleukin (IL) 1-β and caspase-1 signaling pathway, which enhanced apoptosis of interstitial cells after UUO. In summary, our data showed that autophagy in FOXD1-lineage stromal cells plays a protective role in renal TIF through regulating the Smad4 dependent TGF-β an NLRP3 inflammasome signaling pathway.     

Maintaining T lymphocyte homeostasis: another duty of autophagy

First identified as a pathway for nutrient recovery during periods of starvation, the role of autophagy has expanded to the clearance of "toxic" intracellular material including ubiquitin-positive protein aggregates, damaged organelles as well as microbial pathogens in various cell types. We have examined the role of autophagy in the development and function of the adaptive immune system. Genes encoding autophagy machinery are expressed in T lymphocytes, and autophagy occurs in primary CD4+ and CD8+ T cells. By generating fetal liver chimeric mice, we found that thymocyte development is largely normal but the mature T cell compartment is severely reduced in the absence of the essential autophagy gene Atg5. Consistent with a critical role for autophagy in promoting T cell survival, Atg5-/- CD8+ T cells display high levels of apoptosis. Surprisingly, Atg5-deficient T cells were also unable to efficiently proliferate after T-cell receptor (TCR) stimulation. These findings suggest that autophagy regulates T lymphocyte homeostasis by promoting both survival and proliferation. In addition, T cells offer a new, physiologically relevant system to study the regulation and function of autophagy pathways in vivo.     

Autophagy protects auditory hair cells against neomycin-induced damage

Aminoglycosides are toxic to sensory hair cells (HCs). Macroautophagy/autophagy is an essential and highly conserved self-digestion pathway that plays important roles in the maintenance of cellular function and viability under stress. However, the role of autophagy in aminoglycoside-induced HC injury is unknown. Here, we first found that autophagy activity was significantly increased, including enhanced autophagosome-lysosome fusion, in both cochlear HCs and HEI-OC-1 cells after neomycin or gentamicin injury, suggesting that autophagy might be correlated with aminoglycoside-induced cell death. We then used rapamycin, an autophagy activator, to increase the autophagy activity and found that the ROS levels, apoptosis, and cell death were significantly decreased after neomycin or gentamicin injury. In contrast, treatment with the autophagy inhibitor 3-methyladenine (3-MA) or knockdown of autophagy-related (ATG) proteins resulted in reduced autophagy activity and significantly increased ROS levels, apoptosis, and cell death after neomycin or gentamicin injury. Finally, after neomycin injury, the antioxidant N-acetylcysteine could successfully prevent the increased apoptosis and HC loss induced by 3-MA treatment or ATG knockdown, suggesting that autophagy protects against neomycin-induced HC damage by inhibiting oxidative stress. We also found that the dysfunctional mitochondria were not eliminated by selective autophagy (mitophagy) in HEI-OC-1 cells after neomycin treatment, suggesting that autophagy might not directly target the damaged mitochondria for degradation. This study demonstrates that moderate ROS levels can promote autophagy to recycle damaged cellular constituents and maintain cellular homeostasis, while the induction of autophagy can inhibit apoptosis and protect the HCs by suppressing ROS accumulation after aminoglycoside injury.     

Essential roles of Atg5 and FADD in autophagic cell death: dissection of autophagic cell death into vacuole formation and cell death

Autophagic cell death is characterized by the accumulation of vacuoles in physiological and pathological conditions. However, its molecular event is unknown. Here, we show that Atg5, which is known to function in autophagy, contributes to autophagic cell death by interacting with Fas-associated protein with death domain (FADD). Down-regulation of Atg5 expression in HeLa cells suppresses cell death and vacuole formation induced by IFN-gamma. Inversely, ectopic expression of Atg5 using adenoviral delivery induces autophagic cell death. Deletion mapping analysis indicates that procell death activity resides in the middle and C-terminal region of Atg5. Cells harboring the accumulated vacuoles triggered by IFN-gamma or Atg5 expression become dead, and vacuole formation precedes cell death. 3-Methyladenine or expression of Atg5(K130R) mutant blocks both cell death and vacuole formation triggered by IFN-gamma, whereas benzyloxycarbonyl-VAD-fluoromethyl ketone (Z-VAD-fmk) inhibits only cell death but not vacuole formation. Atg5 interacts with FADD via death domain in vitro and in vivo, and the Atg5-mediated cell death, but not vacuole formation, is blocked in FADD-deficient cells. These results suggest that Atg5 plays a crucial role in IFN-gamma-induced autophagic cell death by interacting with FADD.     

Essential role for the ATG4B protease and autophagy in bleomycin-induced pulmonary fibrosis

Autophagy is a critical cellular homeostatic process that controls the turnover of damaged organelles and proteins. Impaired autophagic activity is involved in a number of diseases, including idiopathic pulmonary fibrosis suggesting that altered autophagy may contribute to fibrogenesis. However, the specific role of autophagy in lung fibrosis is still undefined. In this study, we show for the first time, how autophagy disruption contributes to bleomycin-induced lung fibrosis in vivo using an Atg4b-deficient mouse as a model. Atg4b-deficient mice displayed a significantly higher inflammatory response at 7 d after bleomycin treatment associated with increased neutrophilic infiltration and significant alterations in proinflammatory cytokines. Likewise, we found that Atg4b disruption resulted in augmented apoptosis affecting predominantly alveolar and bronchiolar epithelial cells. At 28 d post-bleomycin instillation Atg4b-deficient mice exhibited more extensive and severe fibrosis with increased collagen accumulation and deregulated extracellular matrix-related gene expression. Together, our findings indicate that the ATG4B protease and autophagy play a crucial role protecting epithelial cells against bleomycin-induced stress and apoptosis, and in the regulation of the inflammatory and fibrotic responses.