Autophagy in adipose tissue

Crosstalk between autophagy and insulin resistance: evidence from different tissues

Insulin is a critical hormone that promotes energy storage in various tissues, as well as anabolic functions. Insulin resistance significantly reduces these responses, resulting in pathological conditions, such as obesity and type 2 diabetes mellitus (T2DM). The management of insulin resistance requires better knowledge of its pathophysiological mechanisms to prevent secondary complications, such as cardiovascular diseases (CVDs). Recent evidence regarding the etiological mechanisms behind insulin resistance emphasizes the role of energy imbalance and neurohormonal dysregulation, both of which are closely regulated by autophagy. Autophagy is a conserved process that maintains homeostasis in cells. Accordingly, autophagy abnormalities have been linked to a variety of metabolic disorders, including insulin resistance, T2DM, obesity, and CVDs. Thus, there may be a link between autophagy and insulin resistance. Therefore, the interaction between autophagy and insulin function will be examined in this review, particularly in insulin-responsive tissues, such as adipose tissue, liver, and skeletal muscle.

High levels of intracellular endotrophin in adipocytes mediate COPII vesicle supplies to autophagosome to impair autophagic flux and contribute to systemic insulin resistance in obesity

BACKGROUND AND AIMS

Extracellular matrix (ECM) homeostasis plays a crucial role in metabolic plasticity and endocrine function of adipose tissue. High levels of intracellular endotrophin, a cleavage peptide of type VI collagen alpha 3 chain (Col6a3), have been frequently observed in adipocyte in obesity and diabetes. However, how endotrophin intracellularly traffics and influences metabolic homeostasis in adipocyte remains unknown. Therefore, we aimed to investigate the trafficking of endotrophin and its metabolic effects in adipocytes depending on lean or obese condition.

METHODS

We used doxycycline-inducible adipocyte-specific endotrophin overexpressed mice for a gain-of-function study and CRISPR-Cas9 system-based Col6a3-deficient mice for a loss-of-function study. Various molecular and biochemical techniques were employed to examine the effects of endotrophin on metabolic parameters.

RESULTS

In adipocytes during obesity, the majority of endosomal endotrophin escapes lysosomal degradation and is released into the cytosol to mediate direct interactions between SEC13, a major component of coat protein complex II (COPII) vesicles, and autophagy-related 7 (ATG7), leading to the increased formation of autophagosomes. Autophagosome accumulation disrupts the balance of autophagic flux, resulting in adipocyte death, inflammation, and insulin resistance. These adverse metabolic effects were ameliorated by either suppressing ATG7 with siRNA ex vivo or neutralizing endotrophin with monoclonal antibodies in vivo.

CONCLUSIONS

High levels of intracellular endotrophin-mediated autophagic flux impairment in adipocyte contribute to metabolic dysfunction such as apoptosis, inflammation, and insulin resistance in obesity.

Association of cell death mechanisms and fibrosis in visceral white adipose tissue with pathological alterations in the liver of morbidly obese patients with NAFLD

The role of visceral white adipose tissue (vWAT) in the progression of non-alcoholic liver disease (NAFLD) with its sub entities non-alcoholic fatty liver and steatohepatitis (NAFL; NASH) is underinvestigated. We thus explored mechanisms of fibrosis and regulated cell death in vWAT and liver tissue. In NAFLD, women displayed significantly more fibrosis in vWAT than men, and collagen 1α mRNA expression was significantly upregulated. The degrees of fibrosis in vWAT and liver tissue correlated significantly. The size of vWAT-resident adipocytes in NAFLD correlated negatively with the local degree of fibrosis. The extent of apoptosis, as measured by circulating M30, positively correlated with the degree of fibrosis in vWAT; necrosis-associated HMGB1 mRNA expression was significantly downregulated in vWAT and liver tissue; (iii) necroptosis-related RIPK-3 mRNA expression was significantly upregulated in vWAT; and autophagy-related LC3 mRNA expression was significantly downregulated in vWAT, while upregulated in the liver. Thus, the different cell death mechanisms in the vWAT in NAFLD are regulated independently while not ruling out their interaction. Fibrosis in vWAT may be associated with reduced adipocyte size and thus partially protective against NAFLD progression.

Abbreviations: ATG5: autophagy related 5; BAS: bariatric surgery; BMI: body mass index; ELISA: enzyme-linked immunosorbent assay; EtOH: ethanol; FFAs: free fatty acids; HCC: hepatocellular carcinoma; HMGB1: high-mobility group box 1 protein; IHC: immunohistochemistry; IL: interleukin; LC3: microtubule-associated proteins 1A/1B light chain 3B; M30: neoepitope K18Asp396-NE displayed on the caspase-cleaved keratin 18 fragment; M65: epitope present on both caspase-cleaved and intact keratin 18; NAFL: non-alcoholic fatty liver; NAFLD: non-alcoholic fatty liver disease; NAS: NAFLD activity score; NASH: non-alcoholic steatohepatitis; NLRP3: nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain containing 3; qRT-PCR: quantitative real-time polymerase-chain reaction; r: Pearson’s correlation coefficient (r); r_s: Spearman’s rank correlation coefficient; RIPK3: receptor-interacting serine/threonine-protein kinase 3; T2DM: type 2 diabetes mellitus (T2DM); TUNEL: terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling; vWAT: visceral WAT; WAT: white adipose tissue

Metformin and Vitamin D Modulate Inflammation and Autophagy during Adipose-Derived Stem Cell Differentiation

Adipose-derived stem cells (ADSCs) came out from the regenerative medicine landscape for their ability to differentiate into several phenotypes, contributing to tissue regeneration both in vitro and in vivo. Dysregulation in stem cell recruitment and differentiation during adipogenesis is linked to a chronic low-grade inflammation and macrophage infiltration inside the adipose tissue, insulin resistance, cardiovascular disease and obesity. In the present paper we aimed to evaluate the role of metformin and vitamin D, alone or in combination, in modulating inflammation and autophagy in ADSCs during adipogenic commitment. ADSCs were cultured for 21 days in the presence of a specific adipogenic differentiation medium, together with metformin, or vitamin D, or both. We then analyzed the expression of FoxO1 and Heat Shock Proteins (HSP) and the secretion of proinflammatory cytokines IL-6 and TNF-α by ELISA. Autophagy was also assessed by specific Western blot analysis of ATG12, LC3B I, and LC3B II expression. Our results showed the ability of the conditioned media to modulate adipogenic differentiation, finely tuning the inflammatory response and autophagy. We observed a modulation in HSP mRNA levels, and a significant downregulation in cytokine secretion. Taken together, our findings suggest the possible application of these molecules in clinical practice to counteract uncontrolled lipogenesis and prevent obesity and obesity-related metabolic disorders.

The autophagy inducer spermidine protects against metabolic dysfunction during overnutrition

Autophagy, a process catabolizing intracellular components to maintain energy homeostasis, impacts aging and metabolism. Spermidine, a natural polyamine and autophagy activator, extends lifespan across a variety of species, including mice. In addition to protecting cardiac and liver tissue, spermidine also affects adipose tissue through unexplored mechanisms. Here, we examined spermidine in the links between autophagy and systemic metabolism. Consistently, daily injection of spermidine delivered even at late life is sufficient to cause a trend in lifespan extension in wild type mice. We further found that spermidine has minimal metabolic effects in young and old mice under normal nutrition. However, spermidine counteracts HFD (high-fat diet)-induced obesity by increasing lipolysis in visceral fat. Mechanistically, spermidine increases the hepatokine FGF21 expression in liver without reducing food intake. Spermidine also modulates FGF21 in adipose tissues, elevating FGF21 expression in subcutaneous fat, but reducing it in visceral fat. Despite this, FGF21 is not required for spermidine action, since Fgf21  -/- mice were still protected from HFD. Furthermore, the enhanced lipolysis by spermidine was also independent of autophagy in adipose tissue, given that adipose-specific autophagy deficient (Beclin-1  flox/+  Fabp4-cre) mice remained spermidine-responsive under HFD. Our results suggest that the metabolic effects of spermidine occurs through systemic changes in metabolism, involving multiple mechanistic pathways.

Lipolytic and Lipophagic Effects of Pinellia ternata Pharmacopuncture on Localized Adiposity

Localized adiposity is not only a common aesthetic issue but also a health risk factor. Pharmacopuncture can be a therapeutic option for the imbalance of regional fat distribution. The tuber of Pinellia ternata has been prescribed as antitussive and expectorant as a traditional Korean medicine. This study investigated the effects of pharmacopuncture with P. ternata water extract (PT) on localized adiposity. Male C57BL/6J mice were fed on a high-fat diet (HFD) for 6 weeks. 100 μL of 10 mg/mL of PT was injected into the left-side inguinal fat pad, while saline was injected into the right-side inguinal fat pad as self-control. Treatments were performed 3 times per week for 4 weeks. The inguinal fat weight was analyzed by dual-energy X-ray absorptiometry. PT pharmacopuncture significantly decreased the weight of the inguinal fat pad. The adipocyte size was reduced with increases of lipolytic enzymes and lipophagy-related factors by PT pharmacopuncture. There was marked inhibition of lipid accumulation content in 3T3-L1 adipocytes by PT treatment. The expressions of adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL), autophagy-related gene (ATG) 5, ATG7, and LC3 were markedly increased by PT treatments in vivo and in vitro. This study suggests that pharmacopuncture of Pinellia ternata has ameliorative effects on adiposity by lipid catabolic effects via activating both lipolysis and lipophagy in a localized region.

Leptin stimulates autophagy/lysosome-related degradation of long-lived proteins in adipocytes

Obesity, a condition most commonly associated with hyper-leptinemia, is also characterized by increased expression of autophagy genes and likely autophagic activity in human adipose tissue (AT). Indeed, circulating leptin levels were previously shown to positively associate with the expression levels of autophagy genes such as Autophagy related gene-5 (ATG5). Here we hypothesized that leptin acts in an autocrine-paracrine manner to increase autophagy in two major AT cell populations, adipocytes and macrophages. We followed the dynamics of autophagosomes following acute leptin administration with or without a leptin receptor antagonist (SMLA) using high-throughput live-cell imaging in murine epididymal adipocyte and macrophage (RAW264.7) cell-lines. In macrophages leptin exerted only a mild effect on autophagy dynamics, tending to attenuate autophagosomes growth rate. In contrast, leptin-treated adipocytes exhibited a moderate, ~20% increase in the rate of autophagosome growth, an effect that was blocked by SMLA. This finding corresponded to mild increases in mRNA and protein expression of key autophagy genes. Interestingly, a long-lived proteins degradation assay uncovered a robust, >2-fold leptin-mediated stimulation of the autophagy/lysosome-related (bafilomycin-inhibited) activity, which was entirely blocked by SMLA. Collectively, leptin regulates autophagy in a cell-type specific manner. In adipocytes, autophagosome dynamics is moderately enhanced, but even more pronounced stimulation is seen in autophagy-related long-lived protein degradation. These findings suggest a causal link between obesity-associated hyperleptinemia and elevated adipocyte and AT autophagy-related processes.

Atg7 Knockdown Reduces Chemerin Secretion in Murine Adipocytes

CONTEXT

In individuals with obesity, adipocyte endocrine function is affected by altered autophagy. Genetic variants in autophagy-related gene 7 (ATG7) correlated with serum chemerin (RARRES2) concentrations.

OBJECTIVES

To investigate a functional interplay between chemerin and ATG7, how it may relate to autophagy-mediated adipocyte dysfunction in obesity, and the relevance of genetic ATG7 variants in chemerin physiology.

DESIGN

Adipose ATG7 mRNA expression and adiposity measures were available in two human study cohorts. The effect of a high-calorie diet on adipose Rarres2 and Atg7 expression was investigated in mice. In 3T3-L1 adipocytes, the effect of Atg7 knockdown on chemerin expression and secretion was studied. The influence of single nucleotide polymorphisms on ATG7 transcription and chemerin physiology was investigated using a luciferase assay.

SETTING

Mouse model, clinical trials, in vitro studies.

PARTICIPANTS

Native American (n = 83) and white (n = 100) cohorts.

MAIN OUTCOME MEASURE

Adipocyte chemerin expression and secretion.

RESULTS

In mice fed a high-calorie diet, adipose Atg7 mRNA expression did not parallel an increase in Rarres2 mRNA expression. ATG7 mRNA expression in human subcutaneous adipose tissue correlated with body mass index, fat mass (r > 0.27; P < 0.01), and adipocyte cell size (r > 0.24; P < 0.02). Atg7 knockdown in 3T3-L1 adipocytes decreased chemerin secretion by 22% (P < 0.04). Rs2606729 in ATG7 was predicted to alter ATG7 transcription and induced higher luciferase activity in vitro (P < 0.0001).

CONCLUSIONS

Human adipose ATG7 mRNA expression relates to measures of adiposity. Atg7 knockdown reduces chemerin secretion from adipocytes in vitro, supportive of a functional interplay between ATG7 and chemerin in autophagy-mediated adipocyte dysfunction.

Autophagy in Metabolic Age-Related Human Diseases

Autophagy is a highly conserved homeostatic cellular mechanism that mediates the degradation of damaged organelles, protein aggregates, and invading pathogens through a lysosome-dependent pathway. Over the last few years, specific functions of autophagy have been discovered in many tissues and organs; however, abnormal upregulation or downregulation of autophagy has been depicted as an attribute of a variety of pathologic conditions. In this review, we will describe the current knowledge on the role of autophagy, from its regulation to its physiological influence, in metabolic age-related disorders. Finally, we propose to discuss the therapeutic potential of pharmacological and nutritional modulators of autophagy to treat metabolic diseases.

Autophagy mediates calcium-sensing receptor-induced TNFα production in human preadipocytes

Obesity is a major current public health problem worldwide due to the severe co-morbid conditions that this disease entails. The development of obesity-related cardiometabolic disorders is in direct association with adipose tissue inflammation that leads to its functional impairment. Activation of the Calcium-Sensing Receptor (CaSR) in adipose tissue contributes to inflammation and adipose dysfunction. Autophagy, a process of cell component degradation, is closely related to inflammation in many diseases, however, whether autophagy is associated with CaSR-induced inflammation remains unknown. Using LS14 and SW872 preadipose cell lines as well as primary human preadipocytes, we show that CaSR activation with the allosteric activator cinacalcet induces autophagosome formation. Cinacalcet-induced LC3II content elevation was precluded by knockdown of the CaSR and enhanced by CaSR overexpression, indicating a specific effect. Autophagy inhibition using 3-methyladenine prevented CaSR-induced TNFα production, indicating that autophagy contributes to CaSR-induced inflammation in human preadipocytes. Our results suggest that modulation of CaSR-induced autophagy is an attractive target in obese inflamed adipose tissue, to prevent the development of diseases triggered by adipose dysfunction. We describe a novel mechanism and possible new target to modulate and prevent adipose inflammation and hence the resulting disease-generating adipose tissue dysfunction.

Up-regulated autophagy: as a protective factor in adipose tissue of WOKW rats with metabolic syndrome

BACKGROUND

Wistar Ottawa Karlsburg W (RT1u) rats (WOKW) are a model of the metabolic syndrome (MetS). Adipose tissue (AT) and peripheral nerves of WOKW rats exhibit up-regulated autophagy and inflammation corresponding with decreased apoptosis rate. The aim of this study was to characterize AT in WOKW rats in relation to autophagic activity.

METHODS

mRNA and protein expression of adiponectin, pro-inflammatory and pro-apoptotic markers including MCP1, TNFα, cleaved caspase-3 and RNF157, a new candidate gene regulated through autophagy, were analyzed in adipocytes isolated from visceral and subcutaneous AT of 5-month old WOKW rats with MetS and LEW.1W controls in response to pharmacological inhibition of autophagy. Immunohistochemistry was performed to detect adiponectin and RNF157 protein in cultured adipocytes.

RESULTS

Inhibition of autophagy by LY294002 was associated with a fourfold up-regulation of adiponectin expression and a decrease of RNF157 protein and pro-inflammatory markers-MCP-1 and TNFα predominantly in visceral adipocytes of obese WOKW rats compared to LEW.1W rats. Moreover, inhibition of autophagic activity correlates with an activation of cleaved caspase-3 apoptotic signaling pathway.

CONCLUSIONS

Up-regulated autophagy in obese WOKW rats contributes to the regulation of visceral AT function and involves an altered balance between pro-inflammatory and protective adipokine expression. Our data suggest that activation of AT autophagy protects against adipocyte apoptosis at least under conditions of obesity related MetS in WOKW rats.

Effects of Weight Loss on Glutathione Peroxidase 3 Serum Concentrations and Adipose Tissue Expression in Human Obesity

BACKGROUND/AIMS

Altered expression and circulating levels of glutathione peroxidase 3 (GPX3) have been observed in obesity and type 2 diabetes (T2D) across species. Here, we investigate whether GPX3 serum concentrations and adipose tissue (AT) GPX3 mRNA expression are related to obesity and weight loss.

METHODS

GPX3 serum concentration was measured in 630 individuals, including a subgroup (n = 293) for which omental and subcutaneous (SC) GPX3 mRNA expression has been analyzed. GPX3 analyses include three interventions: 6 months after bariatric surgery (n = 80) or combined exercise/hypocaloric diet (n = 20) or two-step bariatric surgery (n = 24) studies.

RESULTS

Bariatric surgery-induced weight loss (-25.8 ± 8.4%), but not a moderate weight reduction of -8.8 ± 6.5% was associated with significantly reduced GPX3 serum concentrations. GPX3 mRNA is significantly higher expressed in AT from individuals with normal glucose metabolism compared to T2D patients. SC AT GPX3 expression is significantly higher in lean compared to obese as well as in insulin-sensitive compared insulin-resistant individuals with obesity. Weight loss after bariatric surgery causes a significant increase in SC AT GPX3 expression. AT GPX3 expression significantly correlates with age, BMI, fat distribution, insulin sensitivity (only SC AT), but not with circulating GPX3.

CONCLUSION

Our data support the notion that SC AT GPX3 expression is associated with obesity, fat distribution and related to whole body insulin resistance.

Raspberry Ketone Reduced Lipid Accumulation in 3T3-L1 Cells and Ovariectomy-Induced Obesity in Wistar Rats by Regulating Autophagy Mechanisms

This study aimed to determine the antiobesity effects of raspberry ketone (RK), one of the major aromatic compounds contained in raspberry, and its underlying mechanisms. During adipogenesis of 3T3-L1 cells, RK (300 μM) significantly reduced lipid accumulation and downregulated the expression of CCAAT/enhancer-binding protein α (C/EBPα), peroxisome proliferation-activated receptor γ (PPARγ), fatty acid-binding protein 4 (FABP4), and fatty acid synthase (FAS). RK also reduced the expression of light chain 3B (LC3B), autophagy-related protein 12 (Atg12), sirtuin 1 (SIRT1), and phosphorylated-tuberous sclerosis complex 2 (TSC2), whereas it increased the level of p62 and phosphorylated-mammalian target of rapamycin (mTOR). Daily administration of RK decreased the body weight (ovariectomy [Ovx] + RK, 352.6 ± 5 vs Ovx, 386 ± 5.8 g; P < 0.05), fat mass (Ovx + RK, 3.2 ± 0.05 vs Ovx, 5.0 ± 0.4 g; P < 0.05), and fat cell size (Ovx + RK, 6.4 ± 0.6 vs Ovx, 11.1 ± 0.7 × 10³ μm²; P < 0.05) in Ovx-induced obesity in rats. The expression of PPARγ, C/EBPα, FAS, and FABP4 was significantly reduced in the Ovx + RK group compared with that in the Ovx group. Similar patterns were observed in autophagy-related proteins and endoplasmic reticulum stress proteins. These results suggest that RK inhibited lipid accumulation by regulating autophagy in 3T3-L1 cells and Ovx-induced obese rats.

Maresin 1 inhibits TNF-alpha-induced lipolysis and autophagy in 3T3-L1 adipocytes

Obesity is associated with high levels of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), which promotes inflammation in adipose tissue. The omega-3 PUFAs, and their derived lipid mediators, such as Maresin 1 (MaR1) have anti-inflammatory effects on adipose tissue. This study aimed to analyze if MaR1 may counteract alterations induced by TNF-α on lipolysis and autophagy in mature 3T3-L1 adipocytes. Our data revealed that MaR1 (1-100 nM) inhibited the TNF-α-induced glycerol release after 48 hr, which may be related to MaR1 ability of preventing the decrease in lipid droplet-coating protein perilipin and G0/G1 Switch 2 protein expression. MaR1 also reversed the decrease in total hormone sensitive lipase (total HSL), and the ratio of phosphoHSL at Ser-565/total HSL, while preventing the increased ratio of phosphoHSL at Ser-660/total HSL and phosphorylation of extracellular signal-regulated kinase 1/2 induced by TNF-α. Moreover, MaR1 counteracted the cytokine-induced decrease of p62 protein, a key autophagy indicator, and also prevented the induction of LC3II/LC3I, an important autophagosome formation marker. Current data suggest that MaR1 may ameliorate TNF-α-induced alterations on lipolysis and autophagy in adipocytes. This may also contribute to the beneficial actions of MaR1 on adipose tissue and insulin sensitivity in obesity.

Molecular mechanisms underlying metabolic syndrome: the expanding role of the adipocyte

The metabolic syndrome (MetS) is defined as a cluster of 3 or more metabolic and cardiovascular risk factors and represents a serious problem for public health. Altered function of adipose tissue has a significant impact on whole-body metabolism and represents a key driver for the development of these metabolic derangements, collectively referred as to MetS. In particular, increased visceral and ectopic fat deposition play a major role in the development of insulin resistance and MetS. A large body of evidence demonstrates that aging and MetS share several metabolic alterations. Of importance, molecular pathways that regulate lifespan affect key processes of adipose tissue physiology, and transgenic mouse models with adipose-specific alterations in these pathways show derangements of adipose tissue and other metabolic features of MetS, which highlights a causal link between dysfunctional adipose tissue and deleterious effects on whole-body homeostasis. This review analyzes adipose tissue-specific dysfunctions, including metabolic alterations that are related to aging, that have a significant impact on the development of MetS.-Armani, A., Berry, A., Cirulli, F., Caprio, M. Molecular mechanisms underlying metabolic syndrome: the expanding role of the adipocyte.

Transcriptional Dysregulation of Adipose Tissue Autophagy in Obesity

There is growing interest in understanding how dysregulated autophagy may contribute to pathogenesis of disease. Most frequently, disease states are associated with diminished autophagy, mostly attributed to genetic variation in autophagy genes and/or to dysfunctional posttranscriptional mechanisms. In human adipose tissue (AT), in obesity, expression of autophagy genes is upregulated and autophagy is likely activated, associating with adipose dysfunction. This review explores the emerging role of transcriptional mechanisms regulating AT autophagy in obesity.

Thermogenic activation represses autophagy in brown adipose tissue

BACKGROUND

Brown adipose tissue (BAT) thermogenesis is an adaptive process, essential for energy expenditure and involved in the control of obesity. Obesity is associated with abnormally increased autophagy in white adipose tissue. Autophagy has been proposed as relevant for brown-vs-white adipocyte differentiation; however, its role in the response of BAT to thermogenic activation is unknown.

METHODS

The effects of thermogenic activation on autophagy in BAT were analyzed in vivo by exposing mice to 24 h cold condition. The effects of norepinephrine (NE), cAMP and modulators of lysosomal activity were determined in differentiated brown adipocytes in the primary culture. Transcript expression was quantified by real-time PCR, and specific proteins were determined by immunoblot. Transmission electron microscopy, as well as confocal microscopy analysis after incubation with specific antibodies or reagents coupled to fluorescent emission, were performed in BAT and cultured brown adipocytes, respectively.

RESULTS

Autophagy is repressed in association with cold-induced thermogenic activation of BAT in mice. This effect was mimicked by NE action in brown adipocytes, acting mainly through a cAMP-dependent protein kinase A pathway. Inhibition of autophagy in brown adipocytes leads to an increase in UCP1 protein and uncoupled respiration, suggesting a repressing role for autophagy in relation to the activity of BAT thermogenic machinery. Under basal conditions, brown adipocytes show signs of active lipophagy, which is suppressed by a cAMP-mediated thermogenic stimulus.

CONCLUSIONS

Our results show a noradrenergic-mediated inverse relationship between autophagy and thermogenic activity in BAT and point toward autophagy repression as a component of brown adipocyte adaptive mechanisms to activate thermogenesis.

Elevated autophagy gene expression in adipose tissue of obese humans: A potential non-cell-cycle-dependent function of E2F1

Autophagy genes' expression is upregulated in visceral fat in human obesity, associating with obesity-related cardio-metabolic risk. E2F1 (E2F transcription factor 1) was shown in cancer cells to transcriptionally regulate autophagy. We hypothesize that E2F1 regulates adipocyte autophagy in obesity, associating with endocrine/metabolic dysfunction, thereby, representing non-cell-cycle function of this transcription factor. E2F1 protein (N=69) and mRNA (N=437) were elevated in visceral fat of obese humans, correlating with increased expression of ATG5 (autophagy-related 5), MAP1LC3B/LC3B (microtubule-associated protein 1 light chain 3 β), but not with proliferation/cell-cycle markers. Elevated E2F1 mainly characterized the adipocyte fraction, whereas MKI67 (marker of proliferation Ki-67) was elevated in the stromal-vascular fraction of adipose tissue. In human visceral fat explants, chromatin-immunoprecipitation revealed body mass index (BMI)-correlated increase in E2F1 binding to the promoter of MAP1LC3B, but not to the classical cell cycle E2F1 target, CCND1 (cyclin D1). Clinically, omental fat E2F1 expression correlated with insulin resistance, circulating free-fatty-acids (FFA), and with decreased circulating ADIPOQ/adiponectin, associations attenuated by adjustment for autophagy genes. Overexpression of E2F1 in HEK293 cells enhanced promoter activity of several autophagy genes and autophagic flux, and sensitized to further activation of autophagy by TNF. Conversely, mouse embryonic fibroblast (MEF)-derived adipocytes from e2f1 knockout mice (e2f1^−/−) exhibited lower autophagy gene expression and flux, were more insulin sensitive, and secreted more ADIPOQ. Furthermore, e2f1^−/− MEF-derived adipocytes, and autophagy-deficient (by Atg7 siRNA) adipocytes were resistant to cytokines-induced decrease in ADIPOQ secretion. Jointly, upregulated E2F1 sensitizes adipose tissue autophagy to inflammatory stimuli, linking visceral obesity to adipose and systemic metabolic-endocrine dysfunction.

Targeting fatty acid metabolism to improve glucose metabolism

Disturbances in fatty acid metabolism in adipose tissue, liver, skeletal muscle, gut and pancreas play an important role in the development of insulin resistance, impaired glucose metabolism and type 2 diabetes mellitus. Alterations in diet composition may contribute to prevent and/or reverse these disturbances through modulation of fatty acid metabolism. Besides an increased fat mass, adipose tissue dysfunction, characterized by an altered capacity to store lipids and an altered secretion of adipokines, may result in lipid overflow, systemic inflammation and excessive lipid accumulation in non-adipose tissues like liver, skeletal muscle and the pancreas. These impairments together promote the development of impaired glucose metabolism, insulin resistance and type 2 diabetes mellitus. Furthermore, intrinsic functional impairments in either of these organs may contribute to lipotoxicity and insulin resistance. The present review provides an overview of fatty acid metabolism-related pathways in adipose tissue, liver, skeletal muscle, pancreas and gut, which can be targeted by diet or food components, thereby improving glucose metabolism.

Adipocyte dysfunction, inflammation and metabolic syndrome

Obesity is frequently associated with chronic inflammation, metabolic and vascular alterations which predispose to the development of the Metabolic Syndrome (MetS). However, the individual obesity-related risk for the MetS is not determined by increased fat mass alone. Heterogeneity of body composition, fat distribution and adipose tissue (AT) function may underly the variable risk to develop metabolic and cardiovascular diseases associated with increased body fat mass. Importantly, an inability to increase AT mass by adipocyte hyperplasia may lead to adipocyte hypertrophy and could induce dysfunction of adipose tissue characterized by decreased insulin sensitivity, hypoxia, increased parameters of intracellular stress, increased autophagy and apoptosis and tissue inflammation. As a result, adipocytes and other AT cells release signals (e.g. adipokines, cells, metabolites) resulting in a proinflammatory, diabetogenic and atherogenic serum profile. These adverse signals may contribute to further AT inflammation and secondary organ damage in target tissues such as liver, brain, endothelium, vasculature, endocrine organs and skeletal muscle. Recently, a specific adipocyte volume threshold has been shown to predict the risk for obesity-associated type 2 diabetes. Most likely, impaired adipocyte function is caused by genetic, behavioural and environmental factors which are not entirely understood. Elucidating the mechanisms of adipocyte dysfunction may lead to the identification of novel treatment targets for obesity and the MetS.

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.

Role of autophagy in the pathophysiology of nonalcoholic fatty liver disease: A controversial issue

Autophagy is a mechanism involved in cellular homeostasis under basal and stressed conditions delivering cytoplasmic content to the lysosomes for degradation to macronutrients. The potential role of autophagy in disease is increasingly recognised and investigated in the last decade. Nowadays it is commonly accepted that autophagy plays a role in the hepatic lipid metabolism. Hence, dysfunction of autophagy may be an underlying cause of non-alcoholic fatty liver disease. However, controversy of the exact role of autophagy in the lipid metabolism exists: some publications report a lipolytic function of autophagy, whereas others claim a lipogenic function. This review aims to give an update of the present knowledge on autophagy in the hepatic lipid metabolism, hepatic insulin resistance, steatohepatitis and hepatic fibrogenesis.

Adipose tissue dysfunction contributes to obesity related metabolic diseases

Obesity significantly increases the risk of developing type 2 diabetes, hypertension, coronary heart disease, stroke, fatty liver disease, dementia, obstructive sleep apnea and several types of cancer. Adipocyte and adipose tissue dysfunction represent primary defects in obesity and may link obesity to metabolic and cardiovascular diseases. Adipose tissue (AT) dysfunction manifests by a proinflammatory adipokine secretion pattern that mediate auto/paracrine and endocrine communication and by inflammatory cell infiltration, particularly in intra-abdominal fat. Impaired AT function is caused by the interaction of genetic, behavioral and environmental factors which lead to adipocyte hypertrophy, ectopic fat accumulation, hypoxia, AT stresses, impaired AT mitochondrial function and inflammatory processes within adipose tissue. Recently, increased autophagy has been linked to obesity and AT dysfunction and may represent a mechanism to compensate for AT stresses. A better understanding of mechanisms causing or maintaining AT dysfunction may provide new therapeutic strategies in the treatment of obesity-induced metabolic diseases.

The polygenetically inherited metabolic syndrome of male WOKW rats is associated with enhanced autophagy in adipose tissue

BACKGROUND

Recent studies revealed that autophagy is up-regulated in obese individuals, as evidenced by increased expression of autophagy related genes. As argued elsewhere, it is possible that initially insulin resistance functions as an adaptive mechanism to increase autophagy in order to protect cells against death. We have shown that Wistar Ottawa Karlsburg W (RT1(u)) rats (WOKW) develop a metabolic syndrome with insulin resistance in adipose tissue, closely resembling the human disease. Therefore, the aim of this study was to characterize the autophagy phenotype in WOKW rats to clarify the interrelation between insulin resistance and autophagy in adipose tissue.

METHODS

Subcutaneous and epidydimal adipose tissue samples of 5-months-old WOKW and healthy LEW.1 W male rats were investigated and protein levels (Western blot and immunhistochemistry) of key autophagy genes, including Atg5, Atg7, LC3-II/LC3-I and apoptosis marker cleaved caspase-3 were analyzed.

RESULTS

WOKW rats displayed a significant increase of autophagy related proteins (Atg5, Atg7) in adipose tissue compared with LEW.1 W. This increase was predominantly found in epididymal adipose tissue. Furthermore, the LC3-II/LC3-I ratio as a marker of autophagosomes was significantly up-regulated in subcutaneous adipose tissue of WOKW rats. Cleaved caspase-3 was just slightly detectable in visceral adipose tissue and not detected in subcutaneous fat.

CONCLUSION

Insulin resistance in adipose tissue of obese WOKW rats is associated with up-regulation of differing autophagy markers in visceral and subcutaneous fat depots. This fact not only qualifies the WOKW rat for further detailed analysis of genetic determinants of metabolic syndrome but also highlights its suitability for autophagy research.