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

Weight Loss Upregulates the Small GTPase DIRAS3 in Human White Adipose Progenitor Cells, Which Negatively Regulates Adipogenesis and Activates Autophagy via Akt-mTOR Inhibition

Long-term weight-loss (WL) interventions reduce insulin serum levels, protect from obesity, and postpone age-associated diseases. The impact of long-term WL on adipose-derived stromal/progenitor cells (ASCs) is unknown. We identified DIRAS3 and IGF-1 as long-term WL target genes up-regulated in ASCs in subcutaneous white adipose tissue of formerly obese donors (WLDs). We show that DIRAS3 negatively regulates Akt, mTOR and ERK1/2 signaling in ASCs undergoing adipogenesis and acts as a negative regulator of this pathway and an activator of autophagy. Studying the IGF-1–DIRAS3 interaction in ASCs of WLDs, we demonstrate that IGF-1, although strongly up-regulated in these cells, hardly activates Akt, while ERK1/2 and S6K1 phosphorylation is activated by IGF-1. Overexpression of DIRAS3 in WLD ASCs completely inhibits Akt phosphorylation also in the presence of IGF-1. Phosphorylation of ERK1/2 and S6K1 is lesser reduced under these conditions. In conclusion, our key findings are that DIRAS3 down-regulates Akt–mTOR signaling in ASCs of WLDs. Moreover, DIRAS3 inhibits adipogenesis and activates autophagy in these cells.

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Long-term weight loss (WL) induces DIRAS3 and IGF-1 in ASCs of sWAT in formerly obese humans.

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DIRAS3 selectively down-regulates IGF-1R-Akt–mTOR signaling in ASCs and channels the IGF-1 stimulus to the ERK1/2 branch.

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DIRAS3 inhibits adipogenesis and activates autophagy in ASCs.

Long-term weight loss (WL) interventions reduce insulin serum levels, protect from obesity and postpone age-associated diseases. The impact of WL on adipose-derived stromal/progenitor cells (ASCs), stem cell-like cells in human subcutaneous white adipose tissue (sWAT), is not understood. We found that WL induced GTP-binding RAS-like 3 (DIRAS3) and insulin-like growth factor 1 (IGF-1), regulators of the IGF-1–mTOR signal transduction pathway, in ASCs in sWAT of formerly obese humans. We demonstrate that DIRAS3 selectively down-regulates IGF-1R–Akt–mTOR signaling in ASCs upon WL even in the presence of high IGF-1 level and that DIRAS3 inhibits adipogenesis and activates autophagy in these cells.

Hedgehog associated to microparticles inhibits adipocyte differentiation via a non-canonical pathway

Hedgehog (Hh) is a critical regulator of adipogenesis. Extracellular vesicles are natural Hh carriers, as illustrated by activated/apoptotic lymphocytes specifically shedding microparticles (MP) bearing the morphogen (MP(Hh+)). We show that MP(Hh+) inhibit adipocyte differentiation and orientate mesenchymal stem cells towards a pro-osteogenic program. Despite a Smoothened (Smo)-dependency, MP(Hh+) anti-adipogenic effects do not activate a canonical Hh signalling pathway in contrast to those elicited either by the Smo agonist SAG or recombinant Sonic Hedgehog. The Smo agonist GSA-10 recapitulates many of the hallmarks of MP(Hh+) anti-adipogenic effects. The adipogenesis blockade induced by MP(Hh+) and GSA-10 was abolished by the Smo antagonist LDE225. We further elucidate a Smo/Lkb1/Ampk axis as the non-canonical Hh pathway used by MP(Hh+) and GSA-10 to inhibit adipocyte differentiation. Our results highlight for the first time the ability of Hh-enriched MP to signal via a non-canonical pathway opening new perspectives to modulate fat development.

Autophagy: a new target for nonalcoholic fatty liver disease therapy

Nonalcoholic fatty liver disease (NAFLD) has gained importance in recent decades due to drastic changes in diet, especially in Western countries. NAFLD occurs as a spectrum from simple hepatic steatosis, steatohepatitis to cirrhosis, and even hepatocellular carcinoma. Although the molecular mechanisms underlying the development of NAFLD have been intensively investigated, many issues remain to be resolved. Autophagy is a cell survival mechanism for disposing of excess or defective organelles, and has become a hot spot for research. Recent studies have revealed that autophagy is linked to the development of NAFLD and regulation of autophagy has therapeutic potential. Autophagy reduces intracellular lipid droplets by enclosing them and fusing with lysosomes for degradation. Furthermore, autophagy is involved in attenuating inflammation and liver injury. However, autophagy is regarded as a double-edged sword, as it may also affect adipogenesis and adipocyte differentiation. Moreover, it is unclear as to whether autophagy protects the body from injury or causes diseases and even death, and the association between autophagy and NAFLD remains controversial. This review is intended to discuss, comment, and outline the progress made in this field and establish the possible molecular mechanism involved.

The roles of autophagy in vascular smooth muscle cells

Autophagy, which is an evolutionarily conserved mechanism and links to several cellular pathways, impacts vascular smooth muscle cells (VSMCs) survival and function. Activation of autophagy by intercellular and/or extracellular stimuli has protective effects on VSMCs against cell death, while on the contrary, overloading autophagy has been recognized as a deleterious process by excessive self-digestion. Alterations in autophagy has been documented in VSMC in response to various stimuli, resulting in modulation of VSMC functions, including proliferation, migration, matrix secretion, contraction/relaxation, and differentiation. Each of these changes in VSMC functions plays a critical role in the development of vascular diseases. Importantly, emerging evidence demonstrates that autophagy deficiency in VSMCs would contribute to atherosclerosis and restenosis, shedding novel light on therapeutic target of the vascular disorders. Herein, this review summarizes the recent progress associated with the roles of autophagy in VSMC and offers the perspectives to several challenges and future directions for further studies.

Autophagy, a new target for non-alcoholic fatty liver disease therapy

The importance of non-alcoholic fatty liver disease (NAFLD) has increased among human liver diseases with the change of diet structure in recent decades worldwide. NAFLD occurs as a spectrum from simple hepatic steatosis, steatohepatitis to cirrhosis, and even hepatocellular carcinoma. Although the molecular mechanisms of NAFLD have been intensively investigated, effective therapeutic methods are still lacking. Autophagy is a cell survival mechanism for self-digesting excess or defective organelles, which are used for materials and energy recycling. Recent studies have revealed that autophagy is closely linked to NAFLD. Autophagy reduces intracellular lipid droplets by enclosing them and fusing with lysosomes for degradation. Furthermore, autophagy is involved in attenuating inflammation and fibrosis, even hepatoma. However, autophagy is regarded as a double-edged sword, and the relationship between autophagy and NAFLD is controversial.

The Role of Organelle Stresses in Diabetes Mellitus and Obesity: Implication for Treatment

The type 2 diabetes pandemic in recent decades is a huge global health threat. This pandemic is primarily attributed to the surplus of nutrients and the increased prevalence of obesity worldwide. In contrast, calorie restriction and weight reduction can drastically prevent type 2 diabetes, indicating a central role of nutrient excess in the development of diabetes. Recently, the molecular links between excessive nutrients, organelle stress, and development of metabolic disease have been extensively studied. Specifically, excessive nutrients trigger endoplasmic reticulum stress and increase the production of mitochondrial reactive oxygen species, leading to activation of stress signaling pathway, inflammatory response, lipogenesis, and pancreatic beta-cell death. Autophagy is required for clearance of hepatic lipid clearance, alleviation of pancreatic beta-cell stress, and white adipocyte differentiation. ROS scavengers, chemical chaperones, and autophagy activators have demonstrated promising effects for the treatment of insulin resistance and diabetes in preclinical models. Further results from clinical trials are eagerly awaited.

Cellular mechanisms of MR regulation of adipose tissue physiology and pathophysiology

In addition to the well-documented expression and activity of the mineralocorticoid receptor (MR) in the kidney, in the last decade research on MR has also revealed its important role in regulating functions of extrarenal tissues, including adipose tissue, where MR is involved in adipocyte fundamental processes such as differentiation, autophagy and adipokine secretion. MR expression is increased in adipose tissue of murine models of obesity and in obese human subjects, suggesting that over-activation of the mineralocorticoid signaling leads to dysfunctional adipocyte and associated metabolic disorders. Notably, pharmacological blockade of MR prevents metabolic dysfunctions observed in obese mice and suggests a potential therapeutic use of MR antagonists in the treatment of obesity and metabolic syndrome. However, the molecular pathways affected by MR blockade have been poorly investigated. This review summarizes the functions of MR in the adipocyte, discusses potential signaling pathways mediating MR action, and describes post-translational modifications regulating its activity.

DAPK2 Downregulation Associates With Attenuated Adipocyte Autophagic Clearance in Human Obesity

Adipose tissue dysfunction in obesity has been linked to low-grade inflammation causing insulin resistance. Transcriptomic studies have identified death-associated protein kinase 2 (DAPK2) among the most strongly downregulated adipose tissue genes in human obesity, but the role of this kinase is unknown. We show that mature adipocytes rather than the stromal vascular cells in adipose tissue mainly expressed DAPK2 and that DAPK2 mRNA in obese patients gradually recovered after bariatric surgery-induced weight loss. DAPK2 mRNA is also downregulated in high-fat diet-induced obese mice. Adenoviral-mediated DAPK2 overexpression in 3T3-L1 adipocytes did not affect lipid droplet size or cell viability but did increase autophagic clearance in nutrient-rich conditions, dependent on protein kinase activity. Conversely, DAPK2 inhibition in human preadipocytes by small interfering RNA decreased LC3-II accumulation rates with lysosome inhibitors. This led us to assess autophagic clearance in adipocytes freshly isolated from subcutaneous adipose tissue of obese patients. Severe reduction in autophagic flux was observed in obese adipocytes compared with control adipocytes, inversely correlated to fat cell lipids. After bariatric surgery, adipocyte autophagic clearance partially recovered proportional to the extent of fat cell size reduction. This study links adipocyte expression of an autophagy-regulating kinase, lysosome-mediated clearance and fat cell lipid accumulation; it demonstrates obesity-related attenuated autophagy in adipocytes, and identifies DAPK2 dependence in this regulation.

The role of C/EBP-α expression in human liver and liver fibrosis and its relationship with autophagy

AIM

To investigate the expression of CCAAT enhancer binding protein-α (C/EBP-α) in normal human liver and liver fibrosis and its probable association with autophagy.

METHODS

Double label immunohistochemistry was used to detect the location of C/EBP-α in hepatocytes and hepatic stellate cells (HSCs). The expression of C/EBP-α, Atg5, and Atg6 was also evaluated by immunohistochemistry in paraffin sections of human liver. HSC-T6 cells were treated with rapamycin and 3-methyladenine (3MA) to induce or inhibit autophagy, and the expression of C/EBP-α protein was detected by Western blotting.

RESULTS

Double label immunohistochemistry showed that C/EBP-α was predominantly located in hepatocytes and that its expression was significantly decreased in fibrosis compared with normal liver. Atg5 expression was increased in fibrosis but was located primarily in liver septa and peri-vascular areas, which was consistent with the distribution of HSCs. In contrast, Atg6 was not expressed in normal or fibrotic liver. Treatment of HSC-T6 cells in culture with rapamycin or 3MA decreased or increased C/EBP-α expression, respectively, as shown by Western blotting.

CONCLUSION

C/EBP-α was primarily expressed in hepatocytes in normal liver, but its expression decreased significantly in liver fibrosis. Autophagy might play a role in liver fibrosis through its association with C/EBP-α, but this hypothesis warrants further investigation.

Autophagy at the crossroads of catabolism and anabolism

Autophagy is a conserved catabolic process that degrades cytoplasmic constituents and organelles in the lysosome. Starvation-induced protein degradation is a salient feature of autophagy but recent progress has illuminated how autophagy, during both starvation and nutrient-replete conditions, can mobilize diverse cellular energy and nutrient stores such as lipids, carbohydrates and iron. Processes such as lipophagy, glycophagy and ferritinophagy enable cells to salvage key metabolites to sustain and facilitate core anabolic functions. Here, we discuss the established and emerging roles of autophagy in fuelling biosynthetic capacity and in promoting metabolic and nutrient homeostasis.

Autophagy governs erythroid differentiation both in vitro and in vivo

OBJECTIVES

Although the importance of autophagy in determination of cell fate has been much explored in recent years, its definite role in regulating erythroid differentiation remains unclear.

METHODS

In this study, human erythroleukemic cell line K562 was employed as a cell model for studying erythroid differentiation in vitro. Starvation and rapamycin were used to induce autophagy, whereas Baf-A1 and CRISPR/Cas9/Atg7 were used to inhibit late and early phase of autophagy, respectively. The mice model of autophagy activation and autophagy deletion were established, and red blood cell counts and flow cytometry were used to analyze erythroid differentiation in vivo.

RESULTS

The results showed that the transcriptional levels of α-globin and γ-globin, and the ratio of hemoglobin-positive cells all significantly increased in K562 cells with starvation and rapamycin treatment, which were blocked by autophagy inhibitor, Baf-A1 or Atg7 gene knockout. In the autophagy-enhanced mouse model, the number of mature erythrocytes significantly increased, while in Atg7-deleted mouse model, erythroid differentiation was severely blocked.

CONCLUSION

It is concluded that autophagy is indispensible in the development, maturation, and homeostasis of erythroid cells both in vitro and in vivo. Our findings support the potential strategy for erythroleukemia treatment and production of erythroblasts in vitro by modulating autophagy.

ProteINSIDE to Easily Investigate Proteomics Data from Ruminants: Application to Mine Proteome of Adipose and Muscle Tissues in Bovine Foetuses

Genomics experiments are widely acknowledged to produce a huge amount of data to be analysed. The challenge is to extract meaningful biological context for proteins or genes which is currently difficult because of the lack of an integrative workflow that hinders the efficiency and the robustness of data mining performed by biologists working on ruminants. Thus, we designed ProteINSIDE, a free web service (www.proteinside.org) that (I) provides an overview of the biological information stored in public databases or provided by annotations according to the Gene Ontology, (II) predicts proteins that are secreted to search for proteins that mediate signalisation between cells or tissues, and (III) analyses protein-protein interactions to identify proteins contributing to a process or to visualize functional pathways. Using lists of proteins or genes as a unique input, ProteINSIDE is an original all-in-one tool that merges data from these searches to present a fast overview and integrative analysis of genomic and proteomic data from Bovine, Ovine, Caprine, Human, Rat, and Murine species. ProteINSIDE was bench tested with 1000 proteins identifiers from each species by comparison with DAVID, BioMyn, AgBase, PrediSi, and Phobius. Compared to DAVID or BioMyn, identifications and annotations provided by ProteINSIDE were similar from monogastric proteins but more numerous and relevant for ruminants proteins. ProteINSIDE, thanks to SignalP, listed less proteins potentially secreted with a signal peptide than PrediSi and Phobius, in agreement with the low false positive rate of SignalP. In addition ProteINSIDE is the only resource that predicts proteins secreted by cellular processes that do not involve a signal peptide. Lastly, we reported the usefulness of ProteINSIDE to bring new biological hypotheses of research from proteomics data: the biological meaning of the uptake of adiponectin by the foetal muscle and a role for autophagy during ontogenesis of adipose and muscle tissues.

Autophagy is involved in the initiation and progression of Graves' orbitopathy

BACKGROUND

Differentiation of orbital fibroblasts into mature adipocytes and subsequent accumulation of adipose tissue has been shown in the progression of Graves' orbitopathy (GO). Autophagy is involved in adipogenesis, but little is known about the role of autophagy in the initiation and progression of GO. The aim of this study is to investigate the role of autophagy in the pathogenesis of GO.

METHODS

Orbital adipose/connective tissue explants from patients with GO and from normal subjects, as well as isolated orbital fibroblasts, were analyzed. Adipogenesis was induced using differentiating medium with or without hydrogen peroxide, and autophagy was manipulated using bafilomycin A1 and Atg5-targeted short hairpin RNA (shRNA). Autophagosomes were identified by electron microscopy. Expression of autophagy-related genes and adipogenesis-related transcription factors were analyzed by real time reverse transcription-polymerase chain reaction and/or Western blot analysis. Lipid droplet accumulation was examined by Oil Red O staining.

RESULTS

Autophagic vacuoles were more abundant in GO cells than in non-GO cells (p<0.05). Expression of autophagy-related genes was significantly higher in GO tissues and cells than in their non-GO counterparts, respectively. Interleukin-1β increased LC3-II, p62, and Atg7 protein in GO cells. Autophagosome accumulation was shown at day 4 of adipogenesis and decreased by day 10, along with lipid droplet formation. Expression of LC3 and p62 proteins increased within 48 hours of differentiation and diminished gradually from day 4 to 10. Bafilomycin A1 treatment and Atg5 knockdown by shRNA inhibited lipid droplet accumulation and suppressed expression of adipogenic markers.

CONCLUSIONS

Autophagy was increased in GO tissue and cells compared to non-GO tissue and cells, suggesting that autophagy plays a role in GO pathogenesis. Autophagy manipulation may be a therapeutic target for GO.

Emerging roles of autophagy in metabolism and metabolic disorders

The global prevalence of metabolic disorders is an immediate threat to human health. Genetic features, environmental aspects and lifestyle changes are the major risk factors determining metabolic dysfunction in the body. Autophagy is a housekeeping stress-induced lysosomal degradation pathway, which recycles macromolecules and metabolites for new protein synthesis and energy production and regulates cellular homeostasis by clearance of damaged protein or organelles. Recently, a dramatically increasing number of literatures has shown that defects of the autophagic machinery is associated with dysfunction of multiple metabolic tissues including pancreatic β cells, liver, adipose tissue and muscle, and is implicated in metabolic disorders such as obesity and insulin resistance. Here in this review, we summarize the representative works on these topics and discuss the versatile roles of autophagy in the regulation of cellular metabolism and its possible implication in metabolic diseases.

Glycolipid Metabolism Disorder in the Liver of Obese Mice Is Improved by TUDCA via the Restoration of Defective Hepatic Autophagy

Objective. Tauroursodeoxycholic acid (TUDCA) has been considered an important regulator of energy metabolism in obesity. However, the mechanism underlying how TUDCA is involved in insulin resistance is not fully understood. We tested the effects of TUDCA on autophagic dysfunction in obese mice. Material and Methods. 500 mg/kg of TUDCA was injected into obese mice, and metabolic parameters, autophagy markers, and insulin signaling molecular were assessed by Western blotting and real-time PCR. Results. The TUDCA injections in the obese mice resulted in a reduced body weight gain, lower blood glucose, and improved insulin sensitivity compared with obese mice that were injected with vehicle. Meanwhile, TUDCA treatment not only reversed autophagic dysfunction and endoplasmic reticulum stress, but also improved the impaired insulin signaling in the liver of obese mice. Additionally, the same results obtained with TUDCA were evident in obese mice treated with the adenoviral Atg7. Conclusions. We found that TUDCA reversed abnormal autophagy, reduced ER stress, and restored insulin sensitivity in the liver of obese mice and that glycolipid metabolism disorder was also improved via the restoration of defective hepatic autophagy.

Role of the ubiquitin-proteasome system and autophagy in regulation of insulin sensitivity in serum-starved 3T3-L1 adipocytes

The ubiquitin-proteasome system (UPS) and autophagy are two conserved intracellular proteolytic pathways, responsible for degradation of most cellular proteins in living cells. Currently, both the UPS and autophagy have been suggested to be associated with pathogenesis of insulin resistance and diabetes. However, underlying mechanism remains largely unknown. The purpose of the present study is to investigate the impact of the UPS and autophagy on insulin sensitivity in serum-starved 3T3-L1 adipocytes. Our results show that serum depletion resulted in activation of the UPS and autophagy, accompanied with increased insulin sensitivity. Inhibition of the UPS with bortezomib (BZM), a highly selective, reversible 26S proteasome inhibitor induced compensatory activation of autophagy but did not affect significantly insulin action. Genetic and pharmacological inhibition of autophagy dramatically mitigated serum starvation-elevated insulin sensitivity. In addition, autophagy inhibition compromised UPS function and led to endoplasmic reticulum (ER) stress and unfolded protein response (UPR). Inability of the UPS by BMZ exacerbated autophagy inhibition-induced ER stress and UPR. These results suggest that protein quality control maintained by the UPS and autophagy is required for preserving insulin sensitivity. Importantly, adaptive activation of autophagy plays a critical role in serum starvation-induced insulin sensitization in 3T3-L1 adipocytes.

Autophagy and cardiometabolic risk factors

Autophagy is an essential cellular pathway by which protein aggregates, long-lived proteins, or defective organelles are sequestered in double membrane vesicles and then degraded upon fusion of those vesicles with lysosomes. Although autophagy plays a critical role in maintaining intracellular homeostasis and keeping the cell in a healthy state, this key pathway can become dysregulated in various cardiometabolic disorders, such as; obesity, dyslipidemia, inflammation, and insulin resistance. In these conditions, autophagy may actually worsen the pathological state instead of protecting the cell or organism. In this review, we discuss how dysregulated autophagy may be linked to increases in cardiovascular risk factors, and how manipulation of the autophagic machinery might reduce those risks.

Transcriptional regulation of adipocyte differentiation: a central role for CCAAT/enhancer-binding protein (C/EBP) β

A detailed understanding of the processes controlling adipogenesis is instrumental in the fight against the obesity epidemic. Adipogenesis is controlled by a transcriptional cascade composed of a large number of transcriptional factors, among which CCAAT/enhancer-binding protein (C/EBP) β plays an essential role. During 3T3-L1 adipocyte differentiation, C/EBPβ is induced early to transactivate the expression of C/EBPα and peroxisome proliferator-activated receptor γ (PPARγ), two master transcription factors for terminal adipocyte differentiation. Studies in recent years have revealed many new target genes of C/EBPβ, implicating its participation in many other processes during adipogenesis, such as mitotic clonal expansion, epigenetic regulation, unfolded protein response, and autophagy. Moreover, the function of C/EBPβ is highly regulated by post-translational modifications, which are crucial for the proper activation of the adipogenic program. Advances toward elucidation of the function and roles of the post-translational modification of C/EBPβ during adipogenesis will greatly improve our understanding of the molecular mechanisms governing adipogenesis.

The generation of neutrophils in the bone marrow is controlled by autophagy

Autophagy has been demonstrated to have an essential function in several cellular hematopoietic differentiation processes, for example, the differentiation of reticulocytes. To investigate the role of autophagy in neutrophil granulopoiesis, we studied neutrophils lacking autophagy-related (Atg) 5, a gene encoding a protein essential for autophagosome formation. Using Cre-recombinase mediated gene deletion, Atg5-deficient neutrophils showed no evidence of abnormalities in morphology, granule protein content, apoptosis regulation, migration, or effector functions. In such mice, however, we observed an increased proliferation rate in the neutrophil precursor cells of the bone marrow as well as an accelerated process of neutrophil differentiation, resulting in an accumulation of mature neutrophils in the bone marrow, blood, spleen, and lymph nodes. To directly study the role of autophagy in neutrophils, we employed an in vitro model of differentiating neutrophils that allowed modulating the levels of ATG5 expression, or, alternatively, intervening pharmacologically with autophagy-regulating drugs. We could show that autophagic activity correlated inversely with the rate of neutrophil differentiation. Moreover, pharmacological inhibition of p38 MAPK or mTORC1 induced autophagy in neutrophilic precursor cells and blocked their differentiation, suggesting that autophagy is negatively controlled by the p38 MAPK-mTORC1 signaling pathway. On the other hand, we obtained no evidence for an involvement of the PI3K-AKT or ERK1/2 signaling pathways in the regulation of neutrophil differentiation. Taken together, these findings show that, in contrast to erythropoiesis, autophagy is not essential for neutrophil granulopoiesis, having instead a negative impact on the generation of neutrophils. Thus, autophagy and differentiation exhibit a reciprocal regulation by the p38-mTORC1 axis.

Circulating PGRN is significantly associated with systemic insulin sensitivity and autophagic activity in metabolic syndrome

Progranulin (PGRN) is a secreted protein that has recently emerged as an important regulatory adipokine of glucose metabolism and insulin sensitivity. We report here that serum PGRN concentrations were significantly higher in patients with metabolic syndrome (MS) than in subjects without MS and correlated positively with body mass index, waist circumference, fasting insulin, fasting plasma glucose, glycated hemoglobin A1c, triglyceride, and homeostasis model assessment of insulin resistance, and were inversely related to high-density lipoprotein cholesterol and homeostasis model assessment of β cell function. Subgroup analysis in 32 subjects showed that elevated expression levels of PGRN were positively correlated with increased autophagy markers LC3 and Atg7 proteins in omental adipose tissue of subjects with MS. Consistent with these findings, the enhanced PGRN levels were also observed in multiple insulin-resistant cellular models, whereas PGRN-deficient adipocytes were more susceptible to insulin action and refractory to tunicamycin-induced autophagic disorders. PGRN remarkably attenuated insulin sensitivity, increased autophagic activity, and triggered endoplasmic reticulum (ER) stress in cultured human adipocytes, whereas these effects were nullified by reduction of ER stress with phenylbutyric acid chemical chaperone treatment. In addition, PGRN-induced ER stress and impaired insulin sensitivity were improved in TNFR1(-/-) cells, indicating a causative role of TNF receptor in the action of PGRN. Collectively, our findings suggest that circulating PGRN is significantly associated with systemic insulin sensitivity and autophagic activity in adipose tissue and support the notion that PGRN functions as a potential link between chronic inflammation and insulin resistance.

Mitochondrial function/dysfunction in white adipose tissue

The role of mitochondria in white adipocytes has long been neglected due in part to their lower abundance in these cells. However, accumulating evidence suggests that mitochondria are vital for maintaining metabolic homeostasis in white adipocytes because of their involvement in adipogenesis, fatty acid synthesis and esterification, branched-chain amino acid catabolism and lipolysis. It is therefore not surprising that white adipose tissue function can be perturbed by altering mitochondrial components or oxidative capacity. Moreover, studies in humans and animals with significantly altered fat mass, such as in obesity or lipoatrophy, indicate that impaired mitochondrial function in adipocytes may be linked directly to the development of metabolic diseases such as diabetes and insulin resistance. However, recent studies that specifically targeted mitochondrial function in adipocytes indicated dissociation between impaired mitochondrial oxidative capacity and systemic insulin sensitivity.

Autophagy in adipose tissue and the beta cell: implications for obesity and diabetes

Autophagy is a lysosomal degradation pathway recycling intracellular long-lived proteins and damaged organelles, thereby maintaining cellular homeostasis. In addition to inflammatory processes, autophagy has been implicated in the regulation of adipose tissue and beta cell functions. In obesity and type 2 diabetes autophagic activity is modulated in a tissue-dependent manner. In this review we discuss the regulation of autophagy in adipose tissue and beta cells, exemplifying tissue-specific dysregulation of autophagy and its implications for the pathophysiology of obesity and type 2 diabetes. We will highlight common themes and outstanding gaps in our understanding, which need to be addressed before autophagy could be envisioned as a therapeutic target for the treatment of obesity and diabetes.

Autophagy inhibition in early but not in later stages prevents 3T3-L1 differentiation: Effect on mitochondrial remodeling

Autophagy is essential for successful white adipocyte differentiation but the data regarding the timing and relevance of autophagy action during different phases of adipogenesis are limited. We subjected 3T3-L1 preadipocytes to a standard differentiation protocol and inhibited the autophagy within time-limited periods (days 0-2; 2-4; 4-6; 6-8) by asparagine or 3-methyladenine. In the normal course of events, both autophagy flux and the mRNA expression of autophagy related genes (Atg5, Atg12, Atg16, beclin 1) is most intensive at the beginning of differentiation (days 0-4) and then declines. The initiation of differentiation is associated with a 50% reduction of the mitochondrial copy number on day 2 followed by rapid mitochondrial biogenesis. Preadipocytes and differentiated adipocytes differ in the mRNA expression of genes involved in electron transport (Nufsd1, Sdhb, Uqcrc1); ATP synthesis (ATP5b); fatty acid metabolism (CPT1b, Acadl); mitochondrial transporters (Hspa9, Slc25A1) and the TCA cycle (Pcx, Mdh2) as well as citrate synthase activity. Autophagy inhibition during the first two days of differentiation blocked both phenotype changes (lipid accumulation) and the gene expression pattern, while having no or only a marginal effect over any other time period. Similarly, autophagy inhibition between days 0-2 inhibited mitotic clonal expansion as well as mitochondrial network remodeling. In conclusion, we found that autophagy is essential and most active during an initial stage of adipocyte differentiation but it is dispensable during its later stages. We propose that the degradation of preadipocyte cytoplasmic structures, predominantly mitochondria, is an important function of autophagy during this phase and its absence prevents remodeling of the mitochondrial gene expression pattern and mitochondrial network organization.

Spermidine feeding decreases age-related locomotor activity loss and induces changes in lipid composition

Spermidine is a natural polyamine involved in many important cellular functions, whose supplementation in food or water increases life span and stress resistance in several model organisms. In this work, we expand spermidine's range of age-related beneficial effects by demonstrating that it is also able to improve locomotor performance in aged flies. Spermidine's mechanism of action on aging has been primarily related to general protein hypoacetylation that subsequently induces autophagy. Here, we suggest that the molecular targets of spermidine also include lipid metabolism: Spermidine-fed flies contain more triglycerides and show altered fatty acid and phospholipid profiles. We further determine that most of these metabolic changes are regulated through autophagy. Collectively, our data suggests an additional and novel lipid-mediated mechanism of action for spermidine-induced autophagy.

Ethanol Extract of Alismatis rhizome Inhibits Adipocyte Differentiation of OP9 Cells

The rhizome of Alisma orientale (Alismatis rhizome) has been used in Asia for promoting diuresis to eliminate dampness from the lower-jiao and to expel heat. In this study, an ethanol extract of the rhizome of Alisma orientale (AOE) was prepared and its effects on adipocyte differentiation of OP9 cells were investigated. Treatment with AOE in a differentiation medium for 5 days resulted in dose-dependent inhibition of lipid droplet formation in OP9 cells. Furthermore, AOE significantly inhibited adipocyte differentiation by downregulating the expression of the master transcription factor of adipogenesis, peroxisome proliferation-activity receptor γ (PPARγ), and related genes, including CCAAT/enhancer binding protein β (C/EBPβ), fatty acid-binding protein (aP2), and fatty acid synthase (FAS). AOE exerted its inhibitory effects primarily during the early adipogenesis stage (days 1-2), at which time it also exerted dose-dependent inhibition of the expression of C/EBPβ, a protein related to the inhibition of mitotic clonal expansion. Additionally, AOE decreased the expression of autophagy-related proteins, including beclin 1, and the autophagy-related genes, (Atg) 7 and Atg12. Our results indicate that AOE's inhibitory effects on adipocyte differentiation of OP9 cells are mediated by reduced C/EBPβ expression, causing inhibition of mitotic clonal expansion and autophagy.

iASPP is a novel autophagy inhibitor in keratinocytes

The protein iASPP (encoded by PPP1R13L) is an evolutionarily conserved p53 inhibitor, the expression of which is often upregulated in human cancers. We have recently shown that iASPP is a crucial regulator of epidermal homeostasis. Here, we report that iASPP also acts as autophagy inhibitor in keratinocytes. Our data show that depletion of iASPP protects keratinocytes from apoptosis by modulating the expression of Noxa (also known as PMAIP1). In our model, iASPP expression can affect the fission-fusion cycle, mass and shape of mitochondria. iASPP-silenced keratinocytes display disorganization of cytosolic compartments and increased metabolic stress caused by deregulation of mTORC1 signaling. Moreover, increased levels of lipidated LC3 protein confirmed the activation of autophagy in iASPP-depleted cells. We have identified a novel mechanism modulating autophagy in keratinocytes that relies upon iASPP expression specifically reducing the interaction of Atg5-Atg12 with Atg16L1, an interaction that is essential for autophagosome formation or maturation. Using organotypic culture, we further explored the link between autophagy and differentiation, and we showed that impairing autophagy affects epidermal terminal differentiation. Our data provide an alternative mechanism to explain how epithelial integrity is maintained against environmental stressors and might also improve the understanding of the etiology of skin diseases that are characterized by defects in differentiation and DNA damage responses.

Inhibition of autophagy enhances the effects of the AKT inhibitor MK-2206 when combined with paclitaxel and carboplatin in BRAF wild-type melanoma

This study investigates the mechanism of action behind the long-term responses (12-16 months) of two BRAF WT melanoma patients to the AKT inhibitor MK-2206 in combination with paclitaxel and carboplatin. Although single agent MK-2206 inhibited phospho-AKT signaling, it did not impact in vitro melanoma growth or survival. The combination of MK-2206 with paclitaxel and carboplatin was cytotoxic in long-term colony formation and 3D spheroid assays, and induced autophagy. Autophagy was initially protective with autophagy inhibitors and deletion of ATG5 found to enhance cytotoxicity. Although prolonged autophagy induction (>6 days) led to caspase-dependent apoptosis, drug resistant clones still emerged. Autophagy inhibition enhanced the cell death response through reactive oxygen species and could be reversed by anti-oxidants. We demonstrate for the first time that AKT inhibition in combination with chemotherapy may have clinical activity in BRAF WT melanoma and show that an autophagy inhibitor may prevent resistance to these drugs.

Autophagy impairment aggravates the inhibitory effects of high glucose on osteoblast viability and function

Autophagy is a highly regulated homoeostatic process involved in the lysosomal degradation of damaged cell organelles and proteins. This process is considered an important pro-survival mechanism under diverse stress conditions. A diabetic milieu is known to hamper osteoblast viability and function. In the present study, we explored the putative protective role of autophagy in osteoblastic cells exposed to an HG (high glucose) medium. HG was found to increase protein oxidation and triggered autophagy by a mechanism dependent on reactive oxygen species overproduction in osteoblastic MC3T3-E1 cells. MC3T3-E1 cell survival was impaired by HG and worsened by chemical or genetic inhibition of autophagy. These findings were mimicked by H2O2-induced oxidative stress in these cells. Autophagy impairment led to both defective mitochondrial morphology and decreased bioenergetic machinery and inhibited further osteoblast differentiation in MC3T3-E1 cells upon exposure to HG. These novel findings indicate that autophagy is an essential mechanism to maintain osteoblast viability and function in an HG environment.

Mechanism and Physiological Significance of Growth Factor-Related Autophagy

Growth factors, typically defined as natural substances capable of stimulating cell growth and differentiation, are vital regulators for the survival of metazoan cells. In this review, we will focus on growth factor signaling pathways that are closely related to autophagy induction and discuss the critical roles of this fascinating cellular process in intracellular energy homeostasis, cell fate determination, and pathophysiological regulation.

Suppression of autophagy by FIP200 deletion leads to osteopenia in mice through the inhibition of osteoblast terminal differentiation

Autophagy is a conserved lysosomal degradation process that has important roles in both normal human physiology and disease. However, the function of autophagy in bone homeostasis is not well understood. Here, we report that autophagy is activated during osteoblast differentiation. Ablation of focal adhesion kinase family interacting protein of 200 kD (FIP200), an essential component of mammalian autophagy, led to multiple autophagic defects in osteoblasts including aberrantly increased p62 expression, deficient LC3-II conversion, defective autophagy flux, absence of GFP-LC3 puncta in FIP200-null osteoblasts expressing transgenic GFP-LC3, and absence of autophagosome-like structures by electron microscope examination. Osteoblast-specific deletion of FIP200 led to osteopenia in mice. Histomorphometric analysis revealed that the osteopenia was the result of cell-autonomous effects of FIP200 deletion on osteoblasts. FIP200 deletion led to defective osteoblast terminal differentiation in both primary bone marrow and calvarial osteoblasts in vitro. Interestingly, both proliferation and differentiation were not adversely affected by FIP200 deletion in early cultures. However, FIP200 deletion led to defective osteoblast nodule formation after initial proliferation and differentiation. Furthermore, treatment with autophagy inhibitors recapitulated the effects of FIP200 deletion on osteoblast differentiation. Taken together, these data identify FIP200 as an important regulator of bone development and reveal a novel role of autophagy in osteoblast function through its positive role in supporting osteoblast nodule formation and differentiation.

Distinct functions of Ulk1 and Ulk2 in the regulation of lipid metabolism in adipocytes

ULK1 (unc-51 like kinase 1) is a serine/threonine protein kinase that plays a key role in regulating the induction of autophagy. Recent studies using autophagy-defective mouse models, such as atg5- or atg7-deficient mice, revealed an important function of autophagy in adipocyte differentiation. Suppression of adipogenesis in autophagy-defective conditions has made it difficult to study the roles of autophagy in metabolism of differentiated adipocytes. In this study, we established autophagy defective-differentiated 3T3-L1 adipocytes, and investigated the roles of Ulk1 and its close homolog Ulk2 in lipid and glucose metabolism using the established adipocytes. Through knockdown approaches, we determined that Ulk1 and Ulk2 are important for basal and MTORC1 inhibition-induced autophagy, basal lipolysis, and mitochondrial respiration. However, unlike other autophagy genes (Atg5, Atg13, Rb1cc1/Fip200, and Becn1) Ulk1 was dispensable for adipogenesis without affecting the expression of CCAAT/enhancer binding protein ? (CEBPA) and peroxisome proliferation-activated receptor gamma (PPARG). Ulk1 knockdown reduced fatty acid oxidation and enhanced fatty acid uptake, the metabolic changes that could contribute to adipogenesis, whereas Ulk2 knockdown had opposing effects. We also found that the expression levels of insulin receptor (INSR), insulin receptor substrate 1 (IRS1), and glucose transporter 4 (SLC2A4/GLUT4) were increased in Ulk1-silenced adipocytes, which was accompanied by upregulation of insulin-stimulated glucose uptake. These results suggest that ULK1, albeit its important autophagic role, regulates lipid metabolism and glucose uptake in adipocytes distinctly from other autophagy proteins.

Regulation and function of mitophagy in development and cancer

Beyond its role in recycling intracellular components nonselectively to sustain survival in response to metabolic stresses, autophagy can also selectively degrade specific cargoes such as damaged or dysfunctional organelles to maintain cellular homeostasis. Mitochondria, known as the power plant of cells, are the critical and dynamic organelles playing a fundamental role in cellular metabolism. Mitophagy, the selective autophagic elimination of mitochondria, has been identified both in yeast and in mammalian cells. Moreover, defects in mitophagy may contribute to a variety of human disorders such as neurodegeneration and myopathies. However, the role of mitophagy in development and cancer remains largely unclear. In this review, we summarize our current knowledge of the regulation and function of mitophagy in development and cancer.

Autophagy is involved in adipogenic differentiation by repressesing proteasome-dependent PPARγ2 degradation

Animal studies have shown that autophagy is essential in the process of obesity. Here, we performed daily injection of the autophagy inhibitor chloroquine (CQ) in mice and found that systemic administration of CQ blocks high-fat diet-induced obesity. To investigate the potential underlying molecular mechanism, we employed genetic and pharmacological interventions in cultured preadipocytes to investigate the role of autophagy in the control of the expression of the adipogenic regulator peroxisome proliferatior-activated receptor-γ (PPARγ). We show that adipogenic differentiation of 3T3-L1 preadipocytes is associated with activation of autophagy and increased PPARγ2 protein level. Treatment with CQ, shRNA-mediated knockdown, or genetic engineering-induced deletion of autophagy-related gene 5 (Atg5) promoted proteasome-dependent PPARγ2 degradation and attenuated adipogenic differentiation. Therefore, activated autophagy increases PPARγ2 stability and promotes adipogenic differentiation, and inhibition of autophagy may prevent high-fat diet-induced obesity and the consequential type 2 diabetes.

Lysosome/lipid droplet interplay in metabolic diseases

Lysosomes and lipid droplets are generally considered as intracellular compartments with divergent roles in cell metabolism, lipid droplets serving as lipid reservoirs in anabolic pathways, whereas lysosomes are specialized in the catabolism of intracellular components. During the last few years, new insights in the biology of lysosomes has challenged this view by providing evidence for the importance of lysosome recycling as a sparing mechanism to maintain cellular fitness. On the other hand the understanding of lipid droplets has evolved from an inert intracellular deposit toward the status of an intracellular organelle with dynamic roles in cellular homeostasis beyond storage. These unrelated aspects have also recently converged in the finding of unexpected lipid droplet/lysosome communication through autophagy, and the discovery of lysosome-mediated lipid droplet degradation called lipopagy. Furthermore, adipocytes which are professional cells for lipid droplet formation were also shown to be active in peptide antigen presentation a pathway requiring lysosomal activity. The potential importance of lipid droplet/lysosome interplay is discussed in the context of metabolic diseases and the setting of chronic inflammation.

Transactivation of Atg4b by C/EBPβ promotes autophagy to facilitate adipogenesis

Autophagy is a highly conserved self-digestion pathway involved in various physiological and pathophysiological processes. Recent studies have implicated a pivotal role of autophagy in adipocyte differentiation, but the molecular mechanism for its role and how it is regulated during this process are not clear. Here, we show that CCAAT /enhancer-binding protein β (C/EBPβ), an important adipogenic factor, is required for the activation of autophagy during 3T3-L1 adipocyte differentiation. An autophagy-related gene, Atg4b, is identified as a de novo target gene of C/EBPβ and is shown to play an important role in 3T3-L1 adipocyte differentiation. Furthermore, autophagy is required for the degradation of Klf2 and Klf3, two negative regulators of adipocyte differentiation, which is mediated by the adaptor protein p62/SQSTM1. Importantly, the regulation of autophagy by C/EBPβ and the role of autophagy in Klf2/3 degradation and in adipogenesis are further confirmed in mouse models. Our data describe a novel function of C/EBPβ in regulating autophagy and reveal the mechanism of autophagy during adipocyte differentiation. These new insights into the molecular mechanism of adipose tissue development provide a functional pathway with therapeutic potential against obesity and its related metabolic disorders.

Reciprocal effects of rab7 deletion in activated and neglected T cells

Mouse models lacking proteins essential for autophagosome formation have demonstrated that autophagy plays a critical role in T cell development and activation. To better understand the function of autophagy in quiescent and activated lymphocytes, we have generated a mouse deficient in rab7 selectively in T cells and compared the effects of blocking autophagy at an early (atg5(-/-)) or late (rab7(-/-)) stage on T cell biology. rab7(-/-) murine embryonic fibroblasts (MEFs) and T cells generated from these mice exhibit a profound block in autophagosome degradation and are as sensitive as atg5(-/-) cells to extracellular nutrient limitation. Rab7(flox/flox)CD4-Cre(+) mice lacking the RAB7 protein in both CD4 and CD8 T cells had reduced numbers of peripheral T cells, but this defect was not as severe as in Atg5(flox/flox)CD4-Cre(+) mice despite efficient rab7 deletion and inhibition of autophagic flux. This difference may stem from the reduced ROS generation and enhanced survival of rab7(-/-) T cells compared with wild-type and atg5(-/-) T cells in the absence of cytokine stimulation. rab7(-/-) and atg5(-/-) T cells exhibited similar defects in proliferation both following antibody-mediated T cell receptor (TCR) cross-linking and using a more physiologic activation protocol, allogeneic stimulation. Interestingly, autophagy was not required to provide building blocks for the upregulation of nutrient transporter proteins immediately following activation. Together, these studies suggest that autophagosome degradation is required for the survival of activated T cells, but that loss of rab7 is better tolerated in naïve T cells than the loss of atg5.

The response to DNA damage during differentiation: pathways and consequences

Damage to genomic DNA triggers a prompt set of signaling events known as the DNA damage response (DDR) which coordinates DNA repair, cell cycle arrest and ultimately cell death or senescence. Although activation of adequate DNA damage signaling and repair systems depends on the type of lesion and the cell-cycle phase in which it occurs, emerging evidence indicates that DNA repair and DDR function differently in different cellular contexts. Depending on the time maintenance and function of a specific cell type the risk of accumulating DNA damage may vary. For instance, damage to stem cells if not repaired can lead to mutation amplification or propagation through the processes of self-renewal and differentiation, respectively, whereas damage to post-mitotic cells can affect mostly tissue homeostasis. Stem cells are therefore expected to address DNA damage differently from their somatic counterparts. In this review the information available on the common and distinct mechanisms of control of genome integrity utilized by different cell types along the self-renewal/differentiation program will be reviewed, with special emphasis on their roles in the prevention of aging and disease.

Autophagy in stem cells

Autophagy is a highly conserved cellular process by which cytoplasmic components are sequestered in autophagosomes and delivered to lysosomes for degradation. As a major intracellular degradation and recycling pathway, autophagy is crucial for maintaining cellular homeostasis as well as remodeling during normal development, and dysfunctions in autophagy have been associated with a variety of pathologies including cancer, inflammatory bowel disease and neurodegenerative disease. Stem cells are unique in their ability to self-renew and differentiate into various cells in the body, which are important in development, tissue renewal and a range of disease processes. Therefore, it is predicted that autophagy would be crucial for the quality control mechanisms and maintenance of cellular homeostasis in various stem cells given their relatively long life in the organisms. In contrast to the extensive body of knowledge available for somatic cells, the role of autophagy in the maintenance and function of stem cells is only beginning to be revealed as a result of recent studies. Here we provide a comprehensive review of the current understanding of the mechanisms and regulation of autophagy in embryonic stem cells, several tissue stem cells (particularly hematopoietic stem cells), as well as a number of cancer stem cells. We discuss how recent studies of different knockout mice models have defined the roles of various autophagy genes and related pathways in the regulation of the maintenance, expansion and differentiation of various stem cells. We also highlight the many unanswered questions that will help to drive further research at the intersection of autophagy and stem cell biology in the near future.

Non-autophagic roles of autophagy-related proteins

Autophagy and autophagy-related processes are fundamentally important in human health and disease. These processes are viewed primarily as cellular degradative pathways that recycle macromolecules and dysfunctional or redundant organelles into amino acids, sugars and lipids, especially during starvation. However, the ubiquitin-like autophagy proteins and other components of the autophagic machinery additionally participate in cellular reprogramming. We highlight these non-autophagic roles of autophagy proteins with the aim of drawing attention to this growing, but unexplored, research topic. We focus on the non-autophagic functions of autophagy proteins in cell survival and apoptosis, modulation of cellular traffic, protein secretion, cell signalling, transcription, translation and membrane reorganization.

The role of autophagy in Drosophila metamorphosis

Macroautophagy (autophagy) is a conserved catabolic process that targets cytoplasmic components to lysosomes for degradation. Autophagy is required for cellular homeostasis and cell survival in response to starvation and stress, and paradoxically, it also plays a role in programmed cell death during development. The mechanisms that regulate the relationship between autophagy, cell survival, and cell death are poorly understood. Here we review research in Drosophila that has provided insights into the regulation of autophagy by steroid hormones and nutrient restriction and discuss how autophagy influences cell growth, nutrient utilization, cell survival, and cell death.

Autophagy: Emerging roles in lipid homeostasis and metabolic control

Current evidence implicates autophagy in the regulation of lipid stores within the two main organs involved in maintaining lipid homeostasis, the liver and adipose tissue. Critical to this role in hepatocytes is the breakdown of cytoplasmic lipid droplets, a process referred to as lipophagy. Conversely, autophagy is required for adipocyte differentiation and the concurrent accumulation of lipid droplets. Autophagy also affects lipid metabolism through contributions to lipoprotein assembly. A number of reports have now implicated autophagy in the degradation of apolipoprotein B, the main structural protein of very-low-density-lipoprotein. Aberrant autophagy may also be involved in conditions of deregulated lipid homeostasis in metabolic disorders such as the metabolic syndrome. First, insulin signalling and autophagy activity appear to diverge in a mechanism of reciprocal regulation, suggesting a role for autophagy in insulin resistance. Secondly, upregulation of autophagy may lead to conversion of white adipose tissue into brown adipose tissue, thus regulating energy expenditure and obesity. Thirdly, upregulation of autophagy in hepatocytes could increase breakdown of lipid stores controlling triglyceride homeostasis and fatty liver. Taken together, autophagy appears to play a very complex role in lipid homeostasis, affecting lipid stores differently depending on the tissue, as well as contributing to pathways of lipoprotein metabolism.