Targeted deletion of autophagy-related 5 (atg5) impairs adipogenesis in a cellular model and in mice

Androgens promote prostate cancer cell growth through induction of autophagy

Androgens regulate both the physiological development of the prostate and the pathology of prostatic diseases. However, the mechanisms by which androgens exert their regulatory activities on these processes are poorly understood. In this study, we have determined that androgens regulate overall cell metabolism and cell growth, in part, by increasing autophagy in prostate cancer cells. Importantly, inhibition of autophagy using either pharmacological or molecular inhibitors significantly abrogated androgen-induced prostate cancer cell growth. Mechanistically, androgen-mediated autophagy appears to promote cell growth by augmenting intracellular lipid accumulation, an effect previously demonstrated to be necessary for prostate cancer cell growth. Further, autophagy and subsequent cell growth is potentiated, in part, by androgen-mediated increases in reactive oxygen species. These findings demonstrate a role for increased fat metabolism and autophagy in prostatic neoplasias and highlight the potential of targeting underexplored metabolic pathways for the development of novel therapeutics.

GRP78 plays an essential role in adipogenesis and postnatal growth in mice

To investigate the role of GRP78 in adipogenesis and metabolic homeostasis, we knocked down GRP78 in mouse embryonic fibroblasts and 3T3-L1 preadipocytes induced to undergo differentiation into adipocytes. We also created an adipose Grp78-knockout mouse utilizing the aP2 (fatty acid binding protein 4) promoter-driven Cre-recombinase. Adipogenesis was monitored by molecular markers and histology. Tissues were analyzed by micro-CT and electron microscopy. Glucose homeostasis and cytokine analysis were performed. Our results indicate that GRP78 is essential for adipocyte differentiation in vitro. aP2-cre-mediated GRP78 deletion leads to lipoatrophy with ∼90% reduction in gonadal and subcutaneous white adipose tissue and brown adipose tissue, severe growth retardation, and bone defects. Despite severe abnormality in adipose mass and function, adipose Grp78-knockout mice showed normal plasma triglyceride levels, and plasma glucose and insulin levels were reduced by 40-60% compared to wild-type mice, suggesting enhanced insulin sensitivity. The endoplasmic reticulum is grossly expanded in the residual mutant white adipose tissue. Thus, these studies establish that GRP78 is required for adipocyte differentiation, glucose homeostasis, and balanced secretion of adipokines. Unexpectedly, the phenotypes and metabolic parameters of the mutant mice, which showed early postnatal mortality, are uniquely distinct from previously characterized lipodystrophic mouse models.

Thymidine analogues suppress autophagy and adipogenesis in cultured adipocytes

Lipoatrophy in HIV patients can result from prolonged exposure to thymidine analogues. Mitochondrial toxicity leading to dysregulated adipogenesis and increased cell death has been proposed as a leading factor in the etiology of peripheral fat loss. We hypothesized that thymidine analogues interfere with autophagy, a lysosomal degradation pathway, which is important for mitochondrial quality control, cellular survival, and adipogenesis. We assessed the effects of zidovudine (AZT), stavudine (d4T), and lamivudine (3TC) on autophagy in eukaryotic cells and adipocytes (3T3-F442A) by fluorescence microscopy and flow cytometry. The effects were compared to interventions with established genetic and pharmacological inhibitors of autophagy and correlated to assessments of cell viability, proliferation, and differentiation. AZT and d4T, but not 3TC, inhibited both constitutive and induced autophagic activity in adipocytes. This inhibition was associated with accumulation of dysfunctional mitochondria, increased reactive oxygen species (ROS) production, increased apoptosis, decreased proliferation, and impaired adipogenic conversion. Autophagy inhibition was dose and time dependent and detectable at therapeutic drug concentrations. Similar phenotypic changes were obtained when genetic or pharmacological inhibition of autophagy was employed. Our data suggest that thymidine analogues disturb adipocyte function through inhibition of autophagy. This novel mechanism potentially contributes to peripheral fat loss in HIV-infected patients.

Autophagy inhibition due to thymidine analogues as novel mechanism leading to hepatocyte dysfunction and lipid accumulation

OBJECTIVES

Prolonged nucleoside reverse transcriptase inhibitors (NRTI) exposure can lead to microvesicular steatosis. We hypothesized that thymidine analogues might interfere with autophagy in hepatocytes, a lysosomal degradation pathway implicated in cell survival and regulation of hepatocyte lipid metabolism.

DESIGN

Using HepG2 and HUH7 cell lines and primary human hepatocytes, we performed a comprehensive analysis of NRTI-mediated effects on autophagy.

METHODS

The impact of zidovudine (ZDV), stavudine (d4T) and lamivudine (3TC) on constitutive and induced autophagy was analyzed by fluorescent and electron microscopy, western blotting and flow cytometry. Effects on hepatocyte autophagy were correlated to cellular viability, mitochondrial dysfunction and intracellular lipid accumulation.

RESULTS

ZDV and d4T, but not 3TC, significantly inhibited both constitutive as well as stimulated autophagic activity in a dose-dependent and time-dependent manner. Inhibition of autophagy at therapeutic drug concentrations led to accumulation of dysfunctional mitochondria, increased ROS production, increased apoptosis, decreased proliferation and increased intracellular lipid accumulation. These NRTI effects could be readily resembled by pharmacological and genetic inhibition of hepatocyte autophagy.

CONCLUSION

Our data suggest that thymidine analogues inhibit autophagy in hepatocytes, which in turn leads to increased ROS production, lipid accumulation and hepatic dysfunction. This novel mechanism could contribute to nonalcoholic fatty liver disease in HIV-infected patients.

Regulation of adipocyte autophagy--the potential anti-obesity mechanism of high density lipoprotein and ApolipoproteinA-I

Obesity is reaching epidemic worldwide and is risk factor for cardiovascular disease and type 2 diabetes. Although plasma high density lipoprotein (HDL) and apolipoprotein A-I (apoA-I) are inversely correlated to obesity, whether HDLs have anti-obesity effect remains unclear until a recent study reporting the direct anti-obesity effect of apoA-I and its mimetic peptide. However, the mechanism is not fully understood. Increasing adipose energy expenditure through attainment of brown adipocyte phenotype in white adipose tissue is considered a potential strategy to combat obesity. Specific inhibition of autophagy in adipose tissue is associated with reduced adiposity which is attributed to the attainment of brown adipocyte phenotype in white adipose tissue and the increased energy expenditure. HDL and apoA-I could activate PI3K-Akt-mTORC1 signaling which negatively regulates autophagy. The links between HDL/apoA-I and autophagy brings a new understanding on the anti-obesity effect of HDL and apoA-I.

Autophagosomes accumulate in differentiated and hypertrophic adipocytes in a p53-independent manner

Autophagy is induced by several kinds of stress, including oxidative, genotoxic, endoplasmic reticulum and nutrient stresses. The tumor suppressor p53, which is a stress sensor, plays a critical role in the regulation of autophagy. Although p53 is required for starvation (nutrient deficient stress)-induced autophagy, it is still not clear whether p53 is also required for the autophagy observed in differentiated and hypertrophic adipocytes, which accumulate excessive amounts of nutrients in the form of triglycerides. In this study, we demonstrated that starvation induces autophagy in p53-proficient adipocytes, but not in p53-deficient adipocytes as previously reported. On the other hand, autophagy was equally observed in both p53-deficient and -proficient differentiated and hypertrophic adipocytes. Similar results were obtained by in vivo analysis using white adipose tissue of high-fat diet-induced obese mice. Moreover, unexpectedly, the autophagy observed in the differentiated and hypertrophic adipocytes involved increased accumulation of autophagosomes and decreased autophagic flux. Thus, we concluded that in differentiated and hypertrophic adipocytes autophagosomes accumulate in a p53-independent manner, and this accumulation is caused by reduced autophagic flux.

Unmasking the janus faces of autophagy in obesity-associated insulin resistance and cardiac dysfunction

Autophagy is an intracellular, lysosomal-dependent process involved in the turnover of long-lived proteins, damaged organelles and other subcellular structures. The autophagic process is known to play an essential role in the maintenance of cellular homeostasis. Results from recent studies also indicate an important role for the autophagic process in the pathogenesis of human diseases, including cancer, cardiovascular diseases, obesity, diabetes mellitus and ageing. Because of the pivotal role of autophagy in the regulation of adipogenesis, obesity and insulin sensitization, research efforts have focused on elucidating the role of autophagy in metabolic syndrome. Mammalian target of rapamycin (mTOR) is a key regulator of cell growth and is characterized by a complex signalling mechanism that affects protein synthesis and autophagy. Results from experimental and clinical studies reveal some interesting, but conflicting, findings regarding the mTOR signalling pathway and autophagy in adipocytes in metabolic syndrome. Although the pivotal role of autophagy in obesity and other metabolic diseases has been established, its involvement in obesity-induced cardiac dysfunction remains unknown, as do the upstream signalling regulators of autophagy. The present minireview discusses the molecular mechanisms of autophagy in the regulation of cardiac function in overweight and obesity. Further studies using appropriate models are needed to better unravel the complex intracellular mechanisms involved in the regulation of autophagy in obesity-induced cardiac dysfunction.

Autophagy in adipose tissue biology

Obesity, which predisposes individuals to type II diabetes and cardiovascular diseases, results from accumulation of white adipose tissue (WAT). WAT comprises mainly white adipocytes that have a unique cellular structure in which almost the entire intracellular space is occupied by one single lipid droplet. The cytoplasm envelopes this lipid droplet and occupies negligible space. Differentiation of WAT, or adipogenesis, requires dramatic cytoplasmic reorganization, including a dynamic change in mitochondrial mass. Autophagy is a major cytoplasmic degradation pathway and a primary pathway for mitochondrial degradation. Recent studies indicate that autophagy is implicated in adipogenesis. In this review, we summarize our current knowledge on autophagy in adipose tissue biology, with the emphasis on its role in mitochondrial degradation. Adipose tissue is a central component for whole-body energy homeostasis regulation. Advancement in this research area may provide novel venues for the intervention of obesity and obesity related diseases.

Autophagy during vertebrate development

Autophagy is an evolutionarily conserved catabolic process by which cells degrade their own components through the lysosomal machinery. In physiological conditions, the mechanism is tightly regulated and contributes to maintain a balance between synthesis and degradation in cells undergoing intense metabolic activities. Autophagy is associated with major tissue remodeling processes occurring through the embryonic, fetal and early postnatal periods of vertebrates. Here we survey current information implicating autophagy in cellular death, proliferation or differentiation in developing vertebrates. In developing systems, activation of the autophagic machinery could promote different outcomes depending on the cellular context. Autophagy is thus an extraordinary tool for the developing organs and tissues.

p62/SQSTM1/A170: physiology and pathology

p62/SQSTM1/A170 (hereafter referred to as p62) is a stress-inducible intracellular protein known to regulate various signal transduction pathways involved in cell survival and cell death. Comprehensive analysis of LC3 (an autophagosome localizing protein)-binding proteins resulted in the recognition of autophagy and p62. While autophagy modulates the level of p62 protein, p62 can suppress autophagy via activation of mTORC1. Moreover, growing lines of evidence point to the important role of p62 in directing ubiquitinated cargos toward autophagy as well as compaction of those cargos. Furthermore, this protein functions as a signaling hub for various signal transduction pathways, such as NF-κB signaling, apoptosis, and Nrf2 activation, whose dysregulation is associated with Paget disease of bone and tumorigenesis. In this review, we discuss the pathophysiological significance of p62 and its role in autophagy.

Impaired osteoblastogenesis in a murine model of dominant osteogenesis imperfecta: a new target for osteogenesis imperfecta pharmacological therapy

The molecular basis underlying the clinical phenotype in bone diseases is customarily associated with abnormal extracellular matrix structure and/or properties. More recently, cellular malfunction has been identified as a concomitant causative factor and increased attention has focused on stem cells differentiation. Classic osteogenesis imperfecta (OI) is a prototype for heritable bone dysplasias: it has dominant genetic transmission and is caused by mutations in the genes coding for collagen I, the most abundant protein in bone. Using the Brtl mouse, a well-characterized knockin model for moderately severe dominant OI, we demonstrated an impairment in the differentiation of bone marrow progenitor cells toward osteoblasts. In mutant mesenchymal stem cells (MSCs), the expression of early (Runx2 and Sp7) and late (Col1a1 and Ibsp) osteoblastic markers was significantly reduced with respect to wild type (WT). Conversely, mutant MSCs generated more colony-forming unit-adipocytes compared to WT, with more adipocytes per colony, and increased number and size of triglyceride drops per cell. Autophagy upregulation was also demonstrated in mutant adult MSCs differentiating toward osteogenic lineage as consequence of endoplasmic reticulum stress due to mutant collagen retention. Treatment of the Brtl mice with the proteasome inhibitor Bortezomib ameliorated both osteoblast differentiation in vitro and bone properties in vivo as demonstrated by colony-forming unit-osteoblasts assay and peripheral quantitative computed tomography analysis on long bones, respectively. This is the first report of impaired MSC differentiation to osteoblasts in OI, and it identifies a new potential target for the pharmacological treatment of the disorder.

Macroautophagy and cell responses related to mitochondrial dysfunction, lipid metabolism and unconventional secretion of proteins

Macroautophagy has important physiological roles and its cytoprotective or detrimental function is compromised in various diseases such as many cancers and metabolic diseases. However, the importance of autophagy for cell responses has also been demonstrated in many other physiological and pathological situations. In this review, we discuss some of the recently discovered mechanisms involved in specific and unspecific autophagy related to mitochondrial dysfunction and organelle degradation, lipid metabolism and lipophagy as well as recent findings and evidence that link autophagy to unconventional protein secretion.

Regulation of lipid stores and metabolism by lipophagy

Intracellular lipids are stored in lipid droplets (LDs) and metabolized by cytoplasmic neutral hydrolases to supply lipids for cell use. Recently, an alternative pathway of lipid metabolism through the lysosomal degradative pathway of autophagy has been described and termed lipophagy. In this form of lipid metabolism, LD triglycerides (TGs) and cholesterol are taken up by autophagosomes and delivered to lysosomes for degradation by acidic hydrolases. Free fatty acids generated by lipophagy from the breakdown of TGs fuel cellular rates of mitochondrial β-oxidation. Lipophagy therefore functions to regulate intracellular lipid stores, cellular levels of free lipids such as fatty acids and energy homeostasis. The amount of lipid metabolized by lipophagy varies in response to the extracellular supply of nutrients. The ability of the cell to alter the amount of lipid targeted for autophagic degradation depending on nutritional status demonstrates that this process is selective. Intracellular lipids themselves regulate levels of autophagy by unclear mechanisms. Impaired lipophagy can lead to excessive tissue lipid accumulation such as hepatic steatosis, alter hypothalamic neuropeptide release to affect body mass, block cellular transdifferentiation and sensitize cells to death stimuli. Future studies will likely identify additional mechanisms by which lipophagy regulates cellular physiology, making this pathway a potential therapeutic target in a variety of diseases.

Inflammation, endoplasmic reticulum stress, autophagy, and the monocyte chemoattractant protein-1/CCR2 pathway

Numerous inflammatory cytokines have been implicated in the pathogenesis of cardiovascular diseases. Monocyte chemoattractant protein (MCP)-1/CCL2 is expressed by mainly inflammatory cells and stromal cells such as endothelial cells, and its expression is upregulated after proinflammatory stimuli and tissue injury. MCP-1 can function as a traditional chemotactic cytokine and also regulates gene transcription. The recently discovered novel zinc-finger protein, called MCPIP (MCP-1-induced protein), initiates a series of signaling events that causes oxidative and endoplasmic reticulum (ER) stress, leading to autophagy that can result in cell death or differentiation, depending on the cellular context. After a brief review of the basic processes involved in inflammation, ER stress, and autophagy, the recently elucidated role of MCP-1 and MCPIP in inflammatory diseases is reviewed. MCPIP was found to be able to control inflammatory response by inhibition of nuclear factor-κB activation through its deubiquitinase activity or by degradation of mRNA encoding a set of inflammatory cytokines through its RNase activity. The potential inclusion of such a novel deubiquitinase in the emerging anti-inflammatory strategies for the treatment of inflammation-related diseases such as cardiovascular diseases and type 2 diabetes is briefly discussed.

The physiological role of mitophagy: new insights into phosphorylation events

Mitochondria play an essential role in oxidative phosphorylation, fatty acid oxidation, and the regulation of apoptosis. However, this organelle also produces reactive oxygen species (ROS) that continually inflict oxidative damage on mitochondrial DNA, proteins, and lipids, which causes further production of ROS. To oppose this oxidative stress, mitochondria possess quality control systems that include antioxidant enzymes and the repair or degradation of damaged mitochondrial DNA and proteins. If the oxidative stress exceeds the capacity of the mitochondrial quality control system, it seems that autophagy degrades the damaged mitochondria to maintain cellular homeostasis. Indeed, recent evidence from yeast to mammals indicates that the autophagy-dependent degradation of mitochondria (mitophagy) contributes to eliminate dysfunctional, aged, or excess mitochondria. In this paper, we describe the molecular processes and regulatory mechanisms of mitophagy in yeast and mammalian cells.

FAT SIGNALS--lipases and lipolysis in lipid metabolism and signaling

Lipolysis is defined as the catabolism of triacylglycerols stored in cellular lipid droplets. Recent discoveries of essential lipolytic enzymes and characterization of numerous regulatory proteins and mechanisms have fundamentally changed our perception of lipolysis and its impact on cellular metabolism. New findings that lipolytic products and intermediates participate in cellular signaling processes and that "lipolytic signaling" is particularly important in many nonadipose tissues unveil a previously underappreciated aspect of lipolysis, which may be relevant for human disease.

A mammalian autophagosome maturation mechanism mediated by TECPR1 and the Atg12-Atg5 conjugate

Autophagy is a major catabolic pathway in eukaryotes associated with a broad spectrum of human diseases. In autophagy, autophagosomes carrying cellular cargoes fuse with lysosomes for degradation. However, the molecular mechanism underlying autophagosome maturation is largely unknown. Here we report that TECPR1 binds to the Atg12-Atg5 conjugate and phosphatidylinositol 3-phosphate (PtdIns[3]P) to promote autophagosome-lysosome fusion. TECPR1 and Atg16 form mutually exclusive complexes with the Atg12-Atg5 conjugate, and TECPR1 binds PtdIns(3)P upon association with the Atg12-Atg5 conjugate. Strikingly, TECPR1 localizes to and recruits Atg5 to autolysosome membrane. Consequently, elimination of TECPR1 leads to accumulation of autophagosomes and blocks autophagic degradation of LC3-II and p62. Finally, autophagosome maturation marked by GFP-mRFP-LC3 is defective in TECPR1-deficient cells. Thus, we propose that the concerted interactions among TECPR1, Atg12-Atg5, and PtdIns(3)P provide the fusion specificity between autophagosomes and lysosomes and that the assembly of this complex initiates the autophagosome maturation process.

Rab32 is important for autophagy and lipid storage in Drosophila

Lipids are essential components of all organisms. Within cells, lipids are mainly stored in a specific type of organelle, called the lipid droplet. The molecular mechanisms governing the dynamics of lipid droplets have been little explored. The protein composition of lipid droplets has been analyzed in numerous proteomic studies, and a large number of lipid droplet-associated proteins have been identified, including Rab small GTPases. Rab proteins are known to participate in many intracellular membranous events; however, their exact role in lipid droplets is largely unexplored. Here we systematically investigate the roles of Drosophila Rab family proteins in lipid storage in the larval adipose tissue, fat body. Rab32 and several other Rabs were found to affect the size of lipid droplets as well as lipid levels. Further studies showed that Rab32 and Rab32 GEF/Claret may be involved in autophagy, consequently affecting lipid storage. Loss-of-function mutants of several components in the autophagy pathway result in similar effects on lipid storage. These results highlight the potential functions of Rabs in regulating lipid metabolism.

ER Stress and Lipid Metabolism in Adipocytes

The role of endoplasmic reticulum (ER) stress is a rapidly emerging field of interest in the pathogenesis of metabolic diseases. Recent studies have shown that chronic activation of ER stress is closely linked to dysregulation of lipid metabolism in several metabolically important cells including hepatocytes, macrophages, β-cells, and adipocytes. Adipocytes are one of the major cell types involved in the pathogenesis of the metabolic syndrome. Recent advances in dissecting the cellular and molecular mechanisms involved in the regulation of adipogenesis and lipid metabolism indicate that activation of ER stress plays a central role in regulating adipocyte function. In this paper, we discuss the current understanding of the potential role of ER stress in lipid metabolism in adipocytes. In addition, we touch upon the interaction of ER stress and autophagy as well as inflammation. Inhibition of ER stress has the potential of decreasing the pathology in adipose tissue that is seen with energy overbalance.

Atg5 and Ambra1 differentially modulate neurogenesis in neural stem cells

Neuroepithelial cells undergoing differentiation efficiently remodel their cytoskeleton and shape in an energy-consuming process. The capacity of autophagy to recycle cellular components and provide energy could fulfill these requirements, thus supporting differentiation. However, little is known regarding the role of basal autophagy in neural differentiation. Here we report an increase in the expression of the autophagy genes Atg7, Becn1, Ambra1 and LC3 in vivo in the mouse embryonic olfactory bulb (OB) during the initial period of neuronal differentiation at E15.5, along with a parallel increase in neuronal markers. In addition, we observed an increase in LC3 lipidation and autophagic flux during neuronal differentiation in cultured OB-derived stem/progenitor cells. Pharmacological inhibition of autophagy with 3-MA or wortmannin markedly decreased neurogenesis. These observations were supported by similar findings in two autophagy-deficient genetic models. In Ambra1 loss-of-function homozygous mice (gt/gt) the expression of several neural markers was decreased in the OB at E13.5 in vivo. In vitro, Ambra1 haploinsufficient cells developed as small neurospheres with an impaired capacity for neuronal generation. The addition of methylpyruvate during stem/progenitor cell differentiation in culture largely reversed the inhibition of neurogenesis induced by either 3-MA or Ambra1 haploinsufficiency, suggesting that neural stem/progenitor cells activate autophagy to fulfill their high energy demands. Further supporting the role of autophagy for neuronal differentiation Atg5-null OB cells differentiating in culture displayed decreased TuJ1 levels and lower number of cells with neurites. These results reveal new roles for autophagy-related molecules Atg5 and Ambra1 during early neuronal differentiation of stem/progenitor cells.

Autophagy: for better or for worse

Autophagy is a lysosomal degradation pathway that degrades damaged or superfluous cell components into basic biomolecules, which are then recycled back into the cytosol. In this respect, autophagy drives a flow of biomolecules in a continuous degradation-regeneration cycle. Autophagy is generally considered a pro-survival mechanism protecting cells under stress or poor nutrient conditions. Current research clearly shows that autophagy fulfills numerous functions in vital biological processes. It is implicated in development, differentiation, innate and adaptive immunity, ageing and cell death. In addition, accumulating evidence demonstrates interesting links between autophagy and several human diseases and tumor development. Therefore, autophagy seems to be an important player in the life and death of cells and organisms. Despite the mounting knowledge about autophagy, the mechanisms through which the autophagic machinery regulates these diverse processes are not entirely understood. In this review, we give a comprehensive overview of the autophagic signaling pathway, its role in general cellular processes and its connection to cell death. In addition, we present a brief overview of the possible contribution of defective autophagic signaling to disease.

Osteoclast precursor differentiation by MCPIP via oxidative stress, endoplasmic reticulum stress, and autophagy

Osteoclasts (OCs) are responsible for bone resorption in inflammatory joint diseases. Monocyte chemotactic protein-1 (MCP-1) has been shown to induce differentiation of monocytes to OC precursors, but nothing is known about the underlying mechanisms. Here, we elucidate how MCPIP, induced by MCP-1, mediates this differentiation. Knockdown of MCPIP abolished MCP-1-mediated expression of OC markers, tartrate-resistant acid phosphatase, and serine protease cathepsin K. Expression of MCPIP induced p47(PHOX) and its membrane translocation, reactive oxygen species formation, and induction of endoplasmic reticulum (ER) stress chaperones, up-regulation of autophagy marker, Beclin-1, and lipidation of LC3, and induction of OC markers. Inhibition of oxidative stress attenuated ER stress and autophagy, and suppressed expression of OC markers. Inhibition of ER stress by a specific inhibitor or by knockdown of IRE1 blocked autophagy and induction of OC markers. ER stress inducers, tunicamycin and thapsigargin, induced expression of OC markers. Autophagy inhibition by 3'-methyladenine, LY294002, wortmannin or by knockdown of Beclin-1 or Atg 7 inhibited MCPIP-induced expression of OC markers. These results strongly suggest that MCP-1-induced differentiation of OC precursor cells is mediated via MCPIP-induced oxidative stress that causes ER stress leading to autophagy, revealing a novel mechanistic insight into the role of MCP-1 in OCs differentiation.

ATRA-induced upregulation of Beclin 1 prolongs the life span of differentiated acute promyelocytic leukemia cells

Acute promyelocytic leukemia (APL) results from a blockade of granulocyte differentiation at the promyelocytic stage. All-trans retinoic acid (ATRA) induces clinical remission in APL patients by enhancing the rapid differentiation of APL cells and the clearance of PML-RARα, APL's hallmark oncoprotein. In the present study, we demonstrated that both autophagy and Beclin 1, an autophagic protein, are upregulated during the course of ATRA-induced neutrophil/granulocyte differentiation of an APL-derived cell line named NB4 cells. This induction of autophagy is associated with downregulation of Bcl-2 and inhibition of mTOR activity. Small interfering RNA-mediated knockdown of BECN1 expression enhances apoptosis triggered by ATRA in NB4 cells but does not affect the differentiation process. These results provide evidence that the upregulation of Beclin 1 by ATRA constitutes an anti-apoptotic signal for maintaining the viability of mature APL cells, but has no crucial effect on the granulocytic differentiation. This finding may help to elucidate the mechanisms involved in ATRA resistance of APL patients, and in the ATRA syndrome caused by an accumulation of mature APL cells.

Autophagy promotes survival of retinal ganglion cells after optic nerve axotomy in mice

Autophagy is an essential recycling pathway implicated in neurodegeneration either as a pro-survival or a pro-death mechanism. Its role after axonal injury is still uncertain. Axotomy of the optic nerve is a classical model of neurodegeneration. It induces retinal ganglion cell death, a process also occurring in glaucoma and other optic neuropathies. We analyzed autophagy induction and cell survival following optic nerve transection (ONT) in mice. Our results demonstrate activation of autophagy shortly after axotomy with autophagosome formation, upregulation of the autophagy regulator Atg5 and apoptotic death of 50% of the retinal ganglion cells (RGCs) after 5 days. Genetic downregulation of autophagy using knockout mice for Atg4B (another regulator of autophagy) or with specific deletion of Atg5 in retinal ganglion cells, using the Atg5(flox/flox) mice reduces cell survival after ONT, whereas pharmacological induction of autophagy in vivo increases the number of surviving cells. In conclusion, our data support that autophagy has a cytoprotective role in RGCs after traumatic injury and may provide a new therapeutic strategy to ameliorate retinal diseases.

Autophagy regulates cholesterol efflux from macrophage foam cells via lysosomal acid lipase

The lipid droplet (LD) is the major site of cholesterol storage in macrophage foam cells and is a potential therapeutic target for the treatment of atherosclerosis. Cholesterol, stored as cholesteryl esters (CEs), is liberated from this organelle and delivered to cholesterol acceptors. The current paradigm attributes all cytoplasmic CE hydrolysis to the action of neutral CE hydrolases. Here, we demonstrate an important role for lysosomes in LD CE hydrolysis in cholesterol-loaded macrophages, in addition to that mediated by neutral hydrolases. Furthermore, we demonstrate that LDs are delivered to lysosomes via autophagy, where lysosomal acid lipase (LAL) acts to hydrolyze LD CE to generate free cholesterol mainly for ABCA1-dependent efflux; this process is specifically induced upon macrophage cholesterol loading. We conclude that, in macrophage foam cells, lysosomal hydrolysis contributes to the mobilization of LD-associated cholesterol for reverse cholesterol transport.

Functions of autophagy in hepatic and pancreatic physiology and disease

Autophagy is a lysosomal pathway that degrades and recycles intracellular organelles and proteins to maintain energy homeostasis during times of nutrient deprivation and to remove damaged cell components. Recent studies have identified new functions for autophagy under basal and stressed conditions. In the liver and pancreas, autophagy performs the standard functions of degrading mitochondria and aggregated proteins and regulating cell death. In addition, autophagy functions in these organs to regulate lipid accumulation in hepatic steatosis, trypsinogen activation in pancreatitis, and hepatitis virus replication. This review discusses the effects of autophagy on hepatic and pancreatic physiology and the contribution of this degradative process to diseases of these organs. The discovery of novel functions for this lysosomal pathway has increased our understanding of the pathophysiology of diseases in the liver and pancreas and suggested new possibilities for their treatment.

Regulation of lipid droplets by autophagy

Autophagy is a lysosomal pathway by which intracellular organelles and proteins are degraded to supply the cell with energy and to maintain cellular homeostasis. Recently, lipid droplets (LDs) have been identified as a substrate for macroautophagy. In addition to the classic pathway of lipid metabolism by cytosolic lipases, LDs are sequestered in autophagosomes that fuse with lysosomes for the breakdown of LD components by lysosomal enzymes. The ability of autophagy to respond to changes in nutrient supply allows the cell to alter LD metabolism to meet the cell's energy demands. Pathophysiological changes in autophagic function can alter cellular lipid metabolism and promote disease states. Autophagy therefore represents a new cellular target for abnormalities in lipid metabolism and accumulation.

Autophagic degradation of mitochondria in white adipose tissue differentiation

Recent work has revealed that autophagy plays a significant role in the process of white adipocyte differentiation. In both in vitro and in vivo model systems, autophagy inactivation by targeted deletion of essential autophagy genes results in alterations in white adipocyte structure. In both models, postdifferentiation cells exhibit atypical morphology, with many small lipid droplets and large numbers of mitochondria, rather than the single large lipid droplet and relatively few mitochondria observed in normal white adipocytes. The role of autophagy as the primary means of the degradation of mitochondria has long been studied, and it is likely that a deficiency in the degradation of mitochondria contributes to the unusual phenotypes observed in mice with autophagy-deficient adipose tissue, including reduced adiposity, resistance to diet-induced obesity, and increased insulin sensitivity. What is not yet known is whether the process of mitochondria-specific autophagy, often referred to as "mitophagy," is specifically induced during adipogenesis or if a general increase in the nonspecific autophagic degradation of mitochondria plays a role in normal adipose differentiation. Despite remaining questions, these findings not only establish the critical role of autophagy in white adipose tissue development, but also suggest that the manipulation of autophagy in adipose tissue may provide novel therapeutic opportunities for metabolic diseases.

Autophagy in human keratinocytes: an early step of the differentiation?

Studies have established that autophagy constitutes an efficient process to recycle cellular components and certain proteins. The phenomenon was demonstrated primarily in response to nutrient starvation, and there are increasing evidences that it is implied in differentiation. Keratinocyte differentiation was going along an activation of lysosomal enzymes and organelle clearance, and terminal steps are sometimes described as a specialized form of cell death leading to corneocytes. We examined whether initiation of the process in human keratinocyte HaCaT involves autophagy. The KSFM™ culture medium was substituted by M199, which contains a low glucose concentration but a high calcium level (known to induce differentiation). Metabolic stress reduced enhanced cell number in G(1) phase, without apoptotic features (ΔΨmt and membrane integrity are unchanged). Morphological changes were associated with a lower integrin ß1 expression and modifications of protein levels involved in keratinocyte differentiation (involucrin, keratin K10 and ΔNp63α). Whereas autophagic signalling was supported by SIRT1 and pAMPK (T172) increase according to time kinetic, which led to the disappearance of mTOR phosphorylated on S2448 residue. The significant Bcl-X(L) level reduction with stress promoted autophagy, by the release of Beclin-1, whereas ATG5-ATG12 and LC3-II that are involved in autophagosome formation were enhanced significantly. Then, the level of lysosomal protein cathepsin B rose to execute autophagy. Kinetic studies established that autophagy would constitute an early signalling process required for keratinocyte commitment in differentiation pathway.

The complex interplay between autophagy, apoptosis, and necrotic signals promotes T-cell homeostasis

Intense research efforts over the last two decades have focused on establishing the significance of apoptotic signaling in adaptive immunity. Without doubt, caspase-dependent apoptosis plays vital roles in many immune processes, including lymphocyte development, positive and negative selection, homeostasis, and self-tolerance. Cell biologists have developed new insights into cell death, establishing that other modes of cell death exist, such as programmed necrosis and type II/autophagic cell death. Additionally, immunologists have identified a number of immunological processes that are highly dependent upon cellular autophagy, including antigen presentation, lymphocyte development and function, pathogen recognition and destruction, and inflammatory regulation. In this review, we provide detailed mechanistic descriptions of cellular autophagy and programmed necrosis induced in response to death receptor ligation, including methods to identify them, and compare and contrast these processes with apoptosis. The crosstalk between these three processes is emphasized as newly formulated evidence suggests that this interplay is vital for efficient T-cell clonal expansion. This new evidence indicates that in addition to apoptosis, autophagy and programmed necrosis play significant roles in the termination of T-cell-dependent immune responses.

Autophagy in mammalian development and differentiation

It has been known for many decades that autophagy, a conserved lysosomal degradation pathway, is highly active during differentiation and development. However, until the discovery of the autophagy-related (ATG) genes in the 1990s, the functional significance of this activity was unknown. Initially, genetic knockout studies of ATG genes in lower eukaryotes revealed an essential role for the autophagy pathway in differentiation and development. In recent years, the analyses of systemic and tissue-specific knockout models of ATG genes in mice has led to an explosion of knowledge about the functions of autophagy in mammalian development and differentiation. Here we review the main advances in our understanding of these functions.

Autophagy in health and disease. 2. Regulation of lipid metabolism and storage by autophagy: pathophysiological implications

Autophagy is a lysosomal degradative pathway critical for the removal and breakdown of cellular components such as organelles and proteins. Despite striking similarities in the regulation and function of autophagy and lipid metabolism, the two processes have only recently been shown to be interrelated. This review details new findings of critical functions for autophagy in lipid metabolism and storage. Studies in hepatocytes and liver have demonstrated that macroautophagy mediates the breakdown of lipids stored in lipid droplets and that an inhibition of autophagy leads to the development of a fatty liver. In contrast, in adipocytes the loss of macroautophagy decreases the amount of lipid stored in adipose tissue through effects on white and brown adipocyte differentiation. Other investigations have indicated that the relationship between autophagy and lipids is bidirectional, with changes in cellular lipid content altering autophagic function. These newly described links between autophagy and lipid metabolism and storage have provided new insights into the mechanisms of both processes. The findings also suggest possible new therapeutic approaches to the problems of lipid overaccumulation and impaired autophagy that occur with aging and the metabolic syndrome.

Adipose-specific deletion of autophagy-related gene 7 (atg7) in mice reveals a role in adipogenesis

White adipocytes have a unique structure in which nearly the entire cell volume is occupied by one large lipid droplet. However, the molecular and cellular processes involved in the cytoplasmic remodeling necessary to create this structure are poorly defined. Autophagy is a membrane trafficking process leading to lysosomal degradation. Here, we investigated the effect of the deletion of an essential autophagy gene, autophagy-related gene 7 (atg7), on adipogenesis. A mouse model with a targeted deletion of atg7 in adipose tissue was generated. The mutant mice were slim and contained only 20% of the mass of white adipose tissue (WAT) found in wild-type mice. Interestingly, approximately 50% of the mutant white adipocytes were multilocular. The mutant white adipocytes were smaller with a larger volume of cytosol and contained more mitochondria. These cells exhibited altered fatty acid metabolism with increased rates of beta-oxidation and reduced rates of hormone-induced lipolysis. Consistently, the mutant mice had lower fed plasma concentrations of fatty acids and the levels decreased at faster rates upon insulin stimuli. These mutant mice exhibited increased insulin sensitivity. The mutant mice also exhibited markedly decreased plasma concentrations of leptin but not adiponectin, lower plasma concentrations of triglyceride and cholesterol, and they had higher levels of basal physical activity. Strikingly, these mutant mice were resistant to high-fat-diet-induced obesity. Taken together, our results indicate that atg7, and by inference autophagy, plays an important role in normal adipogenesis and that inhibition of autophagy by disrupting the atg7 gene has a unique anti-obesity and insulin sensitization effect.