Isoproterenol, TNFalpha, and insulin downregulate adipose triglyceride lipase in 3T3-L1 adipocytes

Eye on the horizon: The metabolic landscape of the RPE in aging and disease

To meet the prodigious bioenergetic demands of the photoreceptors, glucose and other nutrients must traverse the retinal pigment epithelium (RPE), a polarised monolayer of cells that lie at the interface between the outer retina and the choroid, the principal vascular layer of the eye. Recent investigations have revealed a metabolic ecosystem in the outer retina where the photoreceptors and RPE engage in a complex exchange of sugars, amino acids, and other metabolites. Perturbation of this delicate metabolic balance has been identified in the aging retina, as well as in age-related macular degeneration (AMD), the leading cause of blindness in the Western world. Also common in the aging and diseased retina are elevated levels of cytokines, oxidative stress, advanced glycation end-products, increased growth factor signalling, and biomechanical stress - all of which have been associated with metabolic dysregulation in non-retinal cell types and tissues. Herein, we outline the role of these factors in retinal homeostasis, aging, and disease. We discuss their effects on glucose, mitochondrial, lipid, and amino acid metabolism in tissues and cell types outside the retina, highlighting the signalling pathways through which they induce these changes. Lastly, we discuss promising avenues for future research investigating the roles of these pathological conditions on retinal metabolism, potentially offering novel therapeutic approaches to combat age-related retinal disease.

The Role of Adipocytes Recruited as Part of Tumor Microenvironment in Promoting Colorectal Cancer Metastases

Adipose tissue dysfunction, which is associated with an increased risk of colorectal cancer (CRC), is a significant factor in the pathophysiology of obesity. Obesity-related inflammation and extracellular matrix (ECM) remodeling promote colorectal cancer metastasis (CRCM) by shaping the tumor microenvironment (TME). When CRC occurs, the metabolic symbiosis of tumor cells recruits adjacent adipocytes into the TME to supply energy. Meanwhile, abundant immune cells, from adipose tissue and blood, are recruited into the TME, which is stimulated by pro-inflammatory factors and triggers a chronic local pro-inflammatory TME. Dysregulated ECM proteins and cell surface adhesion molecules enhance ECM remodeling and further increase contractibility between tumor and stromal cells, which promotes epithelial-mesenchymal transition (EMT). EMT increases tumor migration and invasion into surrounding tissues or vessels and accelerates CRCM. Colorectal symbiotic microbiota also plays an important role in the promotion of CRCM. In this review, we provide adipose tissue and its contributions to CRC, with a special emphasis on the role of adipocytes, macrophages, neutrophils, T cells, ECM, and symbiotic gut microbiota in the progression of CRC and their contributions to the CRC microenvironment. We highlight the interactions between adipocytes and tumor cells, and potential therapeutic approaches to target these interactions.

The critical role of BTRC in hepatic steatosis as an ATGL E3 ligase

Non-alcoholic fatty liver disease (NAFLD), characterized by hepatic steatosis, is one of the commonest causes of liver dysfunction. Adipose triglyceride lipase (ATGL) is closely related to lipid turnover and hepatic steatosis as the speed-limited triacylglycerol lipase in liver lipolysis. However, the expression and regulation of ATGL in NAFLD remain unclear. Herein, our results showed that ATGL protein levels were decreased in the liver tissues of high-fat diet (HFD)-fed mice, naturally obese mice, and cholangioma/hepatic carcinoma patients with hepatic steatosis, as well as in the oleic acid-induced hepatic steatosis cell model, while ATGL mRNA levels were not changed. ATGL protein was mainly degraded through the proteasome pathway in hepatocytes. Beta-transducin repeat containing (BTRC) was upregulated and negatively correlated with the decreased ATGL level in these hepatic steatosis models. Consequently, BTRC was identified as the E3 ligase for ATGL through predominant ubiquitination at the lysine 135 residue. Moreover, adenovirus-mediated knockdown of BTRC ameliorated steatosis in HFD-fed mouse livers and oleic acid-treated liver cells via upregulating the ATGL level. Taken together, BTRC plays a crucial role in hepatic steatosis as a new ATGL E3 ligase and may serve as a potential therapeutic target for treating NAFLD.

IL-27 increases energy storage in white adipocytes by enhancing glucose uptake and fatty acid esterification

The cytokine interleukin (IL)-27 has been reported to induce thermogenesis in white adipocytes. However, it remains unknown whether IL-27-mediated adipocyte energy dissipation is paralleled by an elevated energy supply from lipids and/or carbohydrates. We hypothesized that IL-27 increases lipolysis and glucose uptake in white adipocytes, thereby providing substrates for thermogenesis. Unexpectedly, we found that treatment of 3T3-L1 adipocytes with IL-27 reduced intra- and extracellular free fatty acid (FFA) concentrations and that phosphorylation of hormone-sensitive lipase (HSL) was not affected by IL-27. These results were confirmed in subcutaneous white adipocytes. Further, application of IL-27 to 3T3-L1 adipocytes increased intracellular triglyceride (TG) content but not mitochondrial ATP production nor expression of enzymes involved in beta-oxidation indicating that elevated esterification rather than oxidation causes FFA disappearance. In addition, IL-27 significantly increased GLUT1 protein levels, basal glucose uptake as well as glycolytic ATP production, suggesting that increased glycolytic flux due to IL-27 provides the glycerol backbone for TG synthesis. In conclusion, our findings suggest IL-27 increases glucose uptake and TG deposition in white adipocytes.

Transcriptional regulation of adipogenin expression in liver steatosis by hepatic peroxisome proliferator-activated receptor gamma

The nuclear receptors peroxisome proliferator-activated receptor gamma (PPARγ) and adipogenin (ADIG) play vital roles in lipid metabolism. However, the interaction between PPARγ and ADIG during liver steatosis remains unclear. In this study, we aimed to investigate the role of PPARγ in the transcriptional regulation of hepatic ADIG expression. Adig was found to be highly expressed in various fatty liver mouse models. Although hepatic Adig was expressed at high levels in the fatty liver of type 2 diabetic ob/ob mice and was upregulated by PPARγ agonist treatment, it was expressed at significantly low levels in liver-specific Pparg-knockout mice. Moreover, hepatic Adig expression was observed in other mouse models of liver steatosis, such as the leptin receptor mutant db/db and alcohol-fed mice. Adig was also highly expressed in the white and brown adipose tissues, skeletal muscles, and heart of ob/ob mice. Reporter and electromobility shift assays showed that PPARγ positively regulates Adig transcriptional activity by directly binding to a functional PPARγ-responsive element in the promoter region. Our results indicate that Adig is a novel target gene of hepatic PPARγ in liver steatosis.

TNFα Effects on Adipocytes Are Influenced by the Presence of Lysine Methyltransferases, G9a (EHMT2) and GLP (EHMT1)

Impaired adipocyte function contributes to systemic metabolic dysregulation, and altered fat mass or function increases the risk of Type 2 diabetes. EHMTs 1 and 2 (euchromatic histone lysine methyltransferases 1 and 2), also known as the G9a-like protein (GLP) and G9a, respectively, catalyze the mono- and di-methylation of histone 3 lysine 9 (H3K9) and also methylate nonhistone substrates; in addition, they can act as transcriptional coactivators independent of their methyltransferase activity. These enzymes are known to contribute to adipocyte development and function, and in vivo data indicate a role for G9a and GLP in metabolic disease states; however, the mechanisms involved in the cell-autonomous functions of G9a and GLP in adipocytes are largely unknown. Tumor necrosis factor alpha (TNFα) is a proinflammatory cytokine typically induced in adipose tissue in conditions of insulin resistance and Type 2 diabetes. Using an siRNA approach, we have determined that the loss of G9a and GLP enhances TNFα-induced lipolysis and inflammatory gene expression in adipocytes. Furthermore, we show that G9a and GLP are present in a protein complex with nuclear factor kappa B (NF-κB) in TNFα-treated adipocytes. These novel observations provide mechanistic insights into the association between adipocyte G9a and GLP expression and systemic metabolic health.

Associations between the levels of circulating inflammatory adipokines and the risk of type 2 diabetes in Chinese male individuals: A case-control study

BACKGROUND

Whether the levels of circulating inflammatory adipokines affect the progression of type 2 diabetes (T2D) remains unclear. This study aimed to assess the association between circulating inflammatory adipokine levels and risk of T2D.

METHODS

This case-control study involved 130 individuals consisting of 66 healthy controls (Control group) and 64 patients with T2D (T2D group) in Lishui Municipal Central Hospital from January 2017 to June 2017. Multivariate logistic regression analysis was applied to assess the associations between circulating inflammatory adipokine levels and the risk of T2D.

RESULTS

There were significant differences in the levels of adiponectin (p = 0.013) and visfatin (p < 0.001) between the T2D and Control groups. In contrast, no significant differences in leptin (p = 0.113), TNF-α (p = 0.632), and IL-6 (p = 0.156) levels were found between the groups. Multivariate logistic regression indicated that elevated visfatin level was associated with an increased risk of T2D (OR: 3.543; 95% CI: 1.771-7.088; p < 0.001), while adiponectin (OR: 1.946; 95% CI: 0.925-4.094; p = 0.079), leptin (OR: 3.723; 95% CI: 0.788-17.583; p = 0.097), TNF-α (OR: 1.081; 95% CI: 0.911-1.281; p = 0.373), and IL-6 (OR: 0.878; 95% CI: 0.657-1.173; p = 0.379) were not associated with the risk of T2D.

CONCLUSIONS

This study found elevated visfatin levels are associated with an increased risk of T2D, while adiponectin, leptin, TNF-α, and IL-6 are not. These findings should be further verified by a large-scale prospective study.

α-Linolenic acid rich-chia seed modulates visceral adipose tissue collagen deposition, lipolytic enzymes expression, insulin signaling and GLUT-4 levels in a diet-induced adiposity rodent model

Given obesity and its associated metabolic disorders have reached epidemic proportions, the study of therapeutic strategies targeting white adipose tissue (WAT) are of main research interest. We previously shown that α-linolenic acid-rich chia seed was able to ameliorate a wide range of metabolic disorders including body fat accretion in sucrose-rich diet (SRD)-fed rats, an experimental model of visceral adiposity and insulin resistance. However, the mechanisms involved are not fully clarified. The aim of this study was to evaluate the effect of chia seed administration upon WAT remodeling and key enzymes that controls lipolysis, insulin signaling (tAKT, pAKT), and GLUT-4 levels in different visceral fat pad depots (epididymal -eWAT- and retroperitoneal -rWAT- adipose tissues) of SRD-fed rats. Results showed that chia seed reduces adipocytes hypertrophy, the increased lipid content and collagen deposition in both WAT. These changes were accompanied by a significant reduction of HSL and ATGL protein levels in eWAT and HSL protein levels in rWAT. Moreover, chia seed restored the altered expression pattern of the pAKT observed in SRD-fed rats, and modulated GLUT-4 levels. Chia seed could be a dietary intervention of great relevance with potential beneficial effects in the management of body fat excess and WAT function.

Enterocyte-specific ATGL overexpression affects intestinal and systemic cholesterol homeostasis

Enterocytes of the small intestine (SI) play an important role in maintaining systemic lipid levels by regulating dietary lipid absorption and postprandial lipoprotein secretion. An excessive amount of dietary-derived triglycerides (TGs) taken up by the apical side of enterocytes or basolaterally internalized lipoprotein remnants can be transiently stored in cytosolic lipid droplets (cLDs). As mice lacking adipose TG lipase (ATGL) in the SI display massive accumulation of cLDs but also delayed cholesterol absorption, we hypothesized that SI-specific overexpression of ATGL (Atgl iTg) might have beneficial effects on lipid homeostasis in the gut and possibly throughout the body. Here, we demonstrate that Atgl iTg mice had only modestly increased enzymatic activity despite drastically elevated Atgl mRNA levels (up to 120-fold) on chow diet, and was highly induced upon high-fat/high-cholesterol diet (HF/HCD) feeding. Atgl iTg mice showed markedly reduced intestinal TG concentrations after acute and chronic lipid challenge without affecting chylomicron TG secretion. Circulating plasma cholesterol levels were significantly lower in Atgl iTg mice under different feeding conditions, contrasting the accelerated uptake of dietary cholesterol into the circulation after HF/HCD feeding. In the fasted state, gene expression analysis revealed modulation of PPARα and liver X receptor (LXR) target genes by an increased fatty acid release, whereas the decreased plasma cholesterol concentrations in refed mice were more likely due to changes in HDL synthesis and secretion. We conclude that ATGL, in addition to its role in TG catabolism, plays a critical role in whole-body cholesterol homeostasis by modulating PPARα and LXR signaling in intestinal enterocytes.

Adipose Triglyceride Lipase in Hepatic Physiology and Pathophysiology

The liver is extremely active in oxidizing triglycerides (TG) for energy production. An imbalance between TG synthesis and hydrolysis leads to metabolic disorders in the liver, including excessive lipid accumulation, oxidative stress, and ultimately liver damage. Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme that catalyzes the first step of TG breakdown to glycerol and fatty acids. Although its role in controlling lipid homeostasis has been relatively well-studied in the adipose tissue, heart, and skeletal muscle, it remains largely unknown how and to what extent ATGL is regulated in the liver, responds to stimuli and regulators, and mediates disease progression. Therefore, in this review, we describe the current understanding of the structure–function relationship of ATGL, the molecular mechanisms of ATGL regulation at translational and post-translational levels, and—most importantly—its role in lipid and glucose homeostasis in health and disease with a focus on the liver. Advances in understanding the molecular mechanisms underlying hepatic lipid accumulation are crucial to the development of targeted therapies for treating hepatic metabolic disorders.

Modulation of endothelium function by fatty acids

The endothelium acts as the barrier that prevents circulating lipids such as lipoproteins and fatty acids into the arterial wall; it also regulates normal functioning in the circulatory system by balancing vasodilation and vasoconstriction, modulating the several responses and signals. Plasma lipids can interact with endothelium via different mechanisms and produce different phenotypes. Increased plasma-free fatty acids (FFAs) levels are associated with the pathogenesis of atherosclerosis and cardiovascular diseases (CVD). Because of the multi-dimensional roles of plasma FFAs in mediating endothelial dysfunction, increased FFA level is now considered an essential link in the onset of endothelial dysfunction in CVD. FFA-mediated endothelial dysfunction involves several mechanisms, including dysregulated production of nitric oxide and cytokines, metaflammation, oxidative stress, inflammation, activation of the renin-angiotensin system, and apoptosis. Therefore, modulation of FFA-mediated pathways involved in endothelial dysfunction may prevent the complications associated with CVD risk. This review presents details as to how endothelium is affected by FFAs involving several metabolic pathways.

The Role of Lipid Sensing Nuclear Receptors (PPARs and LXR) and Metabolic Lipases in Obesity, Diabetes and NAFLD

Obesity and type 2 diabetes mellitus (T2DM) are metabolic disorders characterized by metabolic inflexibility with multiple pathological organ manifestations, including non-alcoholic fatty liver disease (NAFLD). Nuclear receptors are ligand-dependent transcription factors with a multifaceted role in controlling many metabolic activities, such as regulation of genes involved in lipid and glucose metabolism and modulation of inflammatory genes. The activity of nuclear receptors is key in maintaining metabolic flexibility. Their activity depends on the availability of endogenous ligands, like fatty acids or oxysterols, and their derivatives produced by the catabolic action of metabolic lipases, most of which are under the control of nuclear receptors. For example, adipose triglyceride lipase (ATGL) is activated by peroxisome proliferator-activated receptor γ (PPARγ) and conversely releases fatty acids as ligands for PPARα, therefore, demonstrating the interdependency of nuclear receptors and lipases. The diverse biological functions and importance of nuclear receptors in metabolic syndrome and NAFLD has led to substantial effort to target them therapeutically. This review summarizes recent findings on the roles of lipases and selected nuclear receptors, PPARs, and liver X receptor (LXR) in obesity, diabetes, and NAFLD.

The NLRP3 inflammasome regulates adipose tissue metabolism

Adipose tissue regulates metabolic homeostasis by participating in endocrine and immune responses in addition to storing and releasing lipids from adipocytes. Obesity skews adipose tissue adipokine responses and degrades the coordination of adipocyte lipogenesis and lipolysis. These defects in adipose tissue metabolism can promote ectopic lipid deposition and inflammation in insulin-sensitive tissues such as skeletal muscle and liver. Sustained caloric excess can expand white adipose tissue to a point of maladaptation exacerbating both local and systemic inflammation. Multiple sources, instigators and propagators of adipose tissue inflammation occur during obesity. Cross-talk between professional immune cells (i.e. macrophages) and metabolic cells (i.e. adipocytes) promote adipose tissue inflammation during metabolic stress (i.e. metaflammation). Metabolic stress and endogenous danger signals can engage pathogen recognition receptors (PRRs) of the innate immune system thereby activating pro-inflammatory and stress pathways in adipose tissue. The Nod-like receptor protein 3 (NLRP3) inflammasome can act as a metabolic danger sensor to a wide range of pathogen- and damage-associated molecular patterns (PAMPs and DAMPs). Activation of the NLRP3 inflammasome facilitates caspase-1 dependent production of the pro-inflammatory cytokines IL-1β and IL-18. Activation of the NLRP3 inflammasome can promote inflammation and pyroptotic cell death, but caspase-1 is also involved in adipogenesis. This review discusses the role of the NLRP3 inflammasome in adipose tissue immunometabolism responses relevant to metabolic disease. Understanding the potential sources of NLRP3 activation and consequences of NLRP3 effectors may reveal therapeutic opportunities to break or fine-tune the connection between metabolism and inflammation in adipose tissue during obesity.

5α,6α-Epoxyphytosterols and 5α,6α-Epoxycholesterol Increase Nitrosative Stress and Inflammatory Cytokine Production in Rats on Low-Cholesterol Diet

Objective

Oxidized cholesterol derivatives are compounds with proven atherogenic and mutagenic effects. However, little is known about the effect of oxidized plant sterol derivatives (oxyphytosterols), whose structure is similar to the one of oxycholesterols. Our previous studies indicate that they have a similar profile of action, e.g., both exacerbate disorder of lipid metabolism and oxidative stress in experimental animals. The aim of the present study was to assess the effect of epoxycholesterol and epoxyphytosterols (mainly sitosterol) on the severity of nitrosative stress and the concentration of selected proinflammatory cytokines in blood and liver tissue of rats on a low-cholesterol diet. Material and Methods. Forty-five male Wistar rats were fed with feed containing 5α,6α-epoxyphytosterols (ES group, n: 15), 5α,6α-epoxycholesterol (ECh group, n: 15), and oxysterol-free feed (C group, n: 15) for 90 days (daily dose of oxysterols: 10 mg/kg). At the end of the experiment, nitrotyrosine, TNF-α, IL-1β, IL-6, and lipid metabolism parameters were determined in blood serum. Furthermore, nitrotyrosine, TNF-α, cholesterol, and triglyceride content were determined in liver homogenates.

Results

Serum nitrotyrosine, IL-1β, and TNF-α concentrations as well as TNF-α content in the liver were significantly higher in both groups exposed to oxysterols (ECh and ES groups) as compared to the C group. The serum IL-6 level and nitrotyrosine content in the liver were significantly higher in the ECh group, as compared to the C and ES groups. There was evidence to support the dyslipidemic effect of studied compounds.

Conclusions

The results indicate that oxidized plant sterols have a similar toxicity profile to that of oxycholesterols, including nitrosative stress induction, proinflammatory effect, and impaired lipid metabolism.

Hepatic lipid droplet breakdown through lipolysis during hibernation in Chinese Soft-Shelled Turtle (Pelodiscus sinensis)

Hibernation is an adaptive survival strategy in response to cold and foodless winter. To determine the underlying mechanisms of seasonal adaptions, transcriptome sequencing studies have been conducted in bears, ground squirrels and bats. Despite advances in identifying differentially expressed genes involved in metabolism, the precise mechanisms of these physiological adaptions remain unclear. In the present study, we examined liver of Chinese Soft-Shelled Turtle (Pelodiscus sinensis) and found that the contents of lipid droplet (LD) and triglyceride (TG) were significantly decreased during hibernation. Increases in mRNA expression levels of lipolysis-related genes and decreased levels of lipogenesis-related genes during hibernation indicated that LD hydrolysis was stimulated during hibernation. To continuously release fatty acids (FAs) from LD, adipose triglyceride lipase (ATGL) was recruited and accumulated on the surface of LDs via activation of Cyclic Adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling. Meanwhile, increased phosphorylation of the LD-associated protein, perilipin-5, activated the enzyme activity of ATGL via interaction between comparative gene identification-58 (CGI-58) and ATGL. Taken together, our results indicated that ATGL accumulation on the LD surface and its induced enzyme activity during hibernation promoted LD breakdown in the liver of Chinese Soft-Shelled Turtle (Pelodiscus sinensis), thereby enhancing mitochondrial β-oxidation to maintain energy hemostasis.

The role of pro-inflammatory cytokines in lipid metabolism of metabolic diseases

Adipose tissue has been considered as a crucial source of certain pro-inflammatory cytokines; conversely, these pro-inflammatory cytokines are involved in regulating the proliferation and apoptosis of adipocytes, promoting lipolysis, inhibiting lipid synthesis and decreasing blood lipids, etc. In recent decades, extensive studies have indicated that pro-inflammatory cytokines play important roles in the development of lipid metabolism of metabolic diseases, including obesity, atherosclerosis, steatohepatitis and hyperlipoproteinemia. However, the involved pro-inflammatory cytokines types and the underlying mechanisms remain largely unknown. The "re-discovery" of cancer as a metabolic disorder largely occurred in the last five years. Although pro-inflammatory cytokines have been intensively investigated in cancer research, there are very few studies about the roles of pro-inflammatory cytokines in the lipid metabolism of cancer. In the current review, we provide an overview of the progress that has been made in the roles of different pro-inflammatory cytokines in lipid metabolism of metabolic diseases including cancer.

Induction of fat apoptosis by a non-thermal device: Mechanism of action of non-invasive high-intensity electromagnetic technology in a porcine model

Objectives

While controlled thermal changes in subcutaneous tissue have been used to trigger apoptosis of fat cells and have been proven clinically efficacious, another mechanism of electromagnetic stress suggests that fat apoptosis could be achieved by a non‐thermal manner as well. This animal model study investigates the use of a non‐invasive high‐intensity magnetic field device to induce apoptosis in fat cells.

Methods

Yorkshire pigs (N = 2) received one treatment (30 minutes) in the abdominal area using a High‐Intensity Focused Electromagnetic (HIFEM) device. Punch biopsy samples of fat tissue and blood samples were collected at the baseline, 1 and 8 hours after the treatment. Biopsy samples were sectioned and evaluated for the levels of an apoptotic index (AI) by the TUNEL method. Statistical significance was examined using the rANOVA and Tukey's test (α 5%). Biopsy samples were also assessed for molecular biomarkers. Blood samples were evaluated to determine changes related to fat and muscle metabolism. Free fatty acids (FFA), triacylglycerol (TG), glycerol and glucose (Glu) were used as the main biomarkers of fat metabolism. Creatinine, creatinine kinase (CK), lactate dehydrogenase (LDH) and interleukin 6 (IL6) served as the main biomarkers to evaluate muscle metabolism.

Results

In treated pigs, a statistically significant increase in the apoptotic index (AI) (P = 1.17E‐4) was observed. A significant difference was found between AI at baseline (AI = 18.75%) and 8‐hours post‐treatment (AI = 35.95%). Serum levels of fat and muscle metabolism indicated trends (FFA −0.32 mmol · l⁻¹, −28.1%; TG −0.24 mmol · l⁻¹, −51.8%; Glycerol −5.68 mg · l⁻¹, −54.8%; CK +67.58 μkat · l⁻¹, +227.8%; LDH +4.9 μkat · l⁻¹,+35.4%) suggesting that both adipose and muscle tissue were affected by HIFEM treatment. No adverse events were noted to skin and surrounding tissue.

Conclusions

Application of a high‐intensity electromagnetic field in a porcine model results in adipocyte apoptosis. The analysis of serum levels suggests that HIFEM treatment influences fat and muscle metabolism. Lasers Surg. Med. 51:47–53, 2019. © 2018 The Authors. Lasers in Surgery and Medicine Published by Wiley Periodicals, Inc.

Gold nanoparticles as cell regulators: beneficial effects of gold nanoparticles on the metabolic profile of mice with pre-existing obesity

Background

We have previously shown that intraperitoneal injection of gold nanoparticles (AuNPs, 20–30 nm) into mice, decreases high-fat diet (HFD) induced weight gain and glucose intolerance, via suppression of inflammatory responses in both fat and liver tissues. This study investigates whether AuNPs provide similar benefit to mice with pre-existing obesity. Male C57BL/6 mice were fed a HFD for 15 weeks. AuNPs (OB-EAu 0.0785 μg/g/day, OB-LAu 0.785 μg/g/day, OB-HAu7.85 μg/g/day, ip) were administered to subgroups of HFD-fed mice over the last 5 weeks. Control group was fed standard chow and administered vehicle injection.

Results

Only the OB-LAu group demonstrated significant weight loss (12%), while all AuNP treated groups showed improved glycaemic control and reduced blood lipid levels. In the fat tissue, mRNA expression of pro-inflammatory markers were unchanged following AuNP treatment, while glucose and lipid metabolic markers were improved in OB-LAu and OB-HAu mice. In the liver, AuNP treatment downregulated inflammatory markers and improved lipid metabolic markers, with marked effects in OB-EAu and OB-LAu groups.

Conclusions

AuNP treatment can improve glucose and fat metabolism in mice with long-term obesity, however weight loss was only observed in a single specific dose regime. AuNP therapy is a promising new technology for managing metabolic disorders in the obese.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0414-6) contains supplementary material, which is available to authorized users.

An ethanolic extract of Artemisia scoparia inhibits lipolysis in vivo and has antilipolytic effects on murine adipocytes in vitro

An ethanolic extract of Artemisia scoparia (SCO) has metabolically favorable effects on adipocyte development and function in vitro and in vivo. In diet-induced obese mice, SCO supplementation significantly reduced fasting glucose and insulin levels. Given the importance of adipocyte lipolysis in metabolic health, we hypothesized that SCO modulates lipolysis in vitro and in vivo. Free fatty acids and glycerol were measured in the sera of mice fed a high-fat diet with or without SCO supplementation. In cultured 3T3-L1 adipocytes, the effects of SCO on lipolysis were assessed by measuring glycerol and free fatty acid release. Microarray analysis, qPCR, and immunoblotting were used to assess gene expression and protein abundance. We found that SCO supplementation of a high-fat diet in mice substantially reduces circulating glycerol and free fatty acid levels, and we observed a cell-autonomous effect of SCO to significantly attenuate tumor necrosis factor-α (TNFα)-induced lipolysis in cultured adipocytes. Although several prolipolytic and antilipolytic genes were identified by microarray analysis of subcutaneous and visceral adipose tissue from SCO-fed mice, regulation of these genes did not consistently correlate with SCO's ability to reduce lipolytic metabolites in sera or cell culture media. However, in the presence of TNFα in cultured adipocytes, SCO induced antilipolytic changes in phosphorylation of hormone-sensitive lipase and perilipin. Together, these data suggest that the antilipolytic effects of SCO on adipose tissue play a role in the ability of this botanical extract to improve whole body metabolic parameters and support its use as a dietary supplement to promote metabolic resiliency.

Of mice and men: The physiological role of adipose triglyceride lipase (ATGL)

Adipose triglyceride lipase (ATGL) has been discovered 14 years ago and revised our view on intracellular triglyceride (TG) mobilization – a process termed lipolysis. ATGL initiates the hydrolysis of TGs to release fatty acids (FAs) that are crucial energy substrates, precursors for the synthesis of membrane lipids, and ligands of nuclear receptors. Thus, ATGL is a key enzyme in whole-body energy homeostasis. In this review, we give an update on how ATGL is regulated on the transcriptional and post-transcriptional level and how this affects the enzymes' activity in the context of neutral lipid catabolism. In depth, we highlight and discuss the numerous physiological functions of ATGL in lipid and energy metabolism. Over more than a decade, different genetic mouse models lacking or overexpressing ATGL in a cell- or tissue-specific manner have been generated and characterized. Moreover, pharmacological studies became available due to the development of a specific murine ATGL inhibitor (Atglistatin®). The identification of patients with mutations in the human gene encoding ATGL and their disease spectrum has underpinned the importance of ATGL in humans. Together, mouse models and human data have advanced our understanding of the physiological role of ATGL in lipid and energy metabolism in adipose and non-adipose tissues, and of the pathophysiological consequences of ATGL dysfunction in mice and men.

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Summary of mouse models with genetic or pharmacological manipulation of ATGL.

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Summary of patients with mutations in the human gene encoding ATGL.

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In depth discussion of the role of ATGL in numerous physiological processes in mice and men.

Insulin Regulates Lipolysis and Fat Mass by Upregulating Growth/Differentiation Factor 3 in Adipose Tissue Macrophages

Previous genetic studies in mice have shown that functional loss of activin receptor-like kinase 7 (ALK7), a type I transforming growth factor-β receptor, increases lipolysis to resist fat accumulation in adipocytes. Although growth/differentiation factor 3 (GDF3) has been suggested to function as a ligand of ALK7 under nutrient-excess conditions, it is unknown how GDF3 production is regulated. Here, we show that a physiologically low level of insulin converts CD11c^- adipose tissue macrophages (ATMs) into GDF3-producing CD11c⁺ macrophages ex vivo and directs ALK7-dependent accumulation of fat in vivo. Depletion of ATMs by clodronate upregulates adipose lipases and reduces fat mass in ALK7-intact obese mice, but not in their ALK7-deficient counterparts. Furthermore, depletion of ATMs or transplantation of GDF3-deficient bone marrow negates the in vivo effects of insulin on both lipolysis and fat accumulation in ALK7-intact mice. The GDF3-ALK7 axis between ATMs and adipocytes represents a previously unrecognized mechanism by which insulin regulates both fat metabolism and mass.

A Powerful Test for SNP Effects on Multivariate Binary Outcomes using Kernel Machine Regression

Evaluating multiple binary outcomes is common in genetic studies of complex diseases. These outcomes are often correlated because they are collected from the same individual and they may share common marker effects. In this paper, we propose a procedure to test for effect of a SNP-set on multiple, possibly correlated, binary responses. We develop a score-based test using a nonparametric modeling framework that jointly models the global effect of the marker set. We account for the nonlinear effects and potentially complicated interaction between markers using reproducing kernels. Our testing procedure only requires estimation under the null hypothesis and we use multivariate generalized estimating equations (GEEs) to estimate the model components to account for the correlation among the outcomes. We evaluate finite sample performance of our test via simulation study and demonstrated our methods using the CATIE antibody study data and the CoLaus Study data.

Coordinated transcriptional control of adipocyte triglyceride lipase (Atgl) by transcription factors Sp1 and peroxisome proliferator-activated receptor γ (PPARγ) during adipocyte differentiation

The breakdown of stored fat deposits into its components is a highly regulated process that maintains plasma levels of free fatty acids to supply energy to cells. Insulin-mediated transcription of Atgl, the enzyme that mediates the rate-limiting step in lipolysis, is a key point of this regulation. Under conditions such as obesity or insulin resistance, Atgl transcription is often misregulated, which can contribute to overall disease progression. The mechanisms by which Atgl is induced during adipogenesis are not fully understood. We utilized computational approaches to identify putative transcriptional regulatory elements in Atgl and then tested the effect of these elements and the transcription factors that bind to them in cultured preadipocytes and mature adipocytes. Here we report that Atgl is down-regulated by the basal transcription factor Sp1 in preadipocytes and that the magnitude of down-regulation depends on interactions between Sp1 and peroxisome proliferator-activated receptor γ (PPARγ). In mature adipocytes, when PPARγ is abundant, PPARγ abrogated transcriptional repression by Sp1 at the Atgl promoter and up-regulated Atgl mRNA expression. Targeting the PPARγ-Sp1 interaction could be a potential therapeutic strategy to restore insulin sensitivity by modulating Atgl levels in adipocytes.

Tumor Necrosis Factor-α Promotes Phosphoinositide 3-Kinase Enhancer A and AMP-Activated Protein Kinase Interaction to Suppress Lipid Oxidation in Skeletal Muscle

Tumor necrosis factor-α (TNF-α) is an inflammatory cytokine that plays a central role in obesity-induced insulin resistance. It also controls cellular lipid metabolism, but the underlining mechanism is poorly understood. We report in this study that phosphoinositide 3-kinase enhancer A (PIKE-A) is a novel effector of TNF-α to facilitate its metabolic modulation in the skeletal muscle. Depletion of PIKE-A in C2C12 myotubes diminished the inhibitory activities of TNF-α on mitochondrial respiration and lipid oxidation, whereas PIKE-A overexpression exacerbated these cellular responses. We also found that TNF-α promoted the interaction between PIKE-A and AMP-activated protein kinase (AMPK) to suppress its kinase activity in vitro and in vivo. As a result, animals with PIKE ablation in the skeletal muscle per se display an upregulation of AMPK phosphorylation and a higher preference to use lipid as the energy production substrate under high-fat diet feeding, which mitigates the development of diet-induced hyperlipidemia, ectopic lipid accumulation, and muscle insulin resistance. Hence, our data reveal PIKE-A as a new signaling factor that is important for TNF-α-initiated metabolic changes in skeletal muscle.

Almond Skin Polyphenol Extract Inhibits Inflammation and Promotes Lipolysis in Differentiated 3T3-L1 Adipocytes

Studies have shown that polyphenols reduce the risk of inflammation-related diseases and upregulates energy expenditure in adipose tissue. Here, we investigated the mechanism of the anti-inflammatory and antiobesity effects of almond skin polyphenol extract (ASP) in differentiated 3T3-L1 adipocytes. The antioxidant effects of ASP were measured based on DPPH radical scavenging activity, Trolox equivalent antioxidant capacity, and total phenolic content. Differentiated 3T3-L1 cells were treated with ASP. Subsequently, lipolysis proteins and transcription factors of adipogenesis were measured. The proinflammatory mediators monocyte chemotactic protein-1 (MCP-1) and chemokine ligand 5 (CCL-5) were determined by enzyme-linked immunosorbent assay. We found that ASP significantly promoted phosphorylation of AMP-activated protein kinase (AMPK), increased activity of adipose triglyceride lipase and hormone-sensitive lipase, and inhibited adipogenesis-related transcription factors. In addition, ASP inhibited the tumor necrosis factor-α (TNF-α)-induced cell inflammatory response via downregulation of MCP-1 and CCL-5 secretion. This study suggests that ASP regulates lipolysis through activation of AMPK, reduced adipogenesis, and suppresses proinflammatory cytokines in adipocytes.

Critical role of miR-125b in lipogenesis by targeting stearoyl-CoA desaturase-1 (SCD-1)

Alteration of gene expression tightly regulates lipogenesis. Stearoyl-CoA desaturase-1 (SCD-1), a key enzyme in lipogenesis, catalyzes the conversion of SFA to MUFA, and inhibition of its activity impairs lipid synthesis. As posttranscriptional regulators, microRNAs are involved in many pathways of lipid metabolism; however, their effect on SCD-1 has not been reported. In this study, miR-125b was identified as a potential regulator of SCD-1 using bioinformatics analysis. Here, we validated SCD-1 as the target of miR-125b using a dual luciferase assay. During adipogenesis, a synthetic mimic or inhibitor was used to overexpress or reduce the expression of miR-125b in porcine adipocytes. Overexpression of miR-125b reduced the accumulation of lipid droplets and triglycerides concentration and repressed SCD-1 protein expression and MUFA composition. The inhibitor had the reverse effect. Small interfering RNA against tested in adipocytes further proved the direct correlation between miR-125b and SCD-1. Moreover, in vivo experiments in mice showed that injection of miR-125b expression vector decreased the hepatic triglycerides concentration relative to saline. This study indicated that miR-125b regulates lipogenesis by targeting SCD-1; therefore, miR-125b might be applied in therapy of lipid metabolism disorders.

Protein Kinase A Subunit Balance Regulates Lipid Metabolism in Caenorhabditis elegans and Mammalian Adipocytes

Protein kinase A (PKA) is a cyclic AMP (cAMP)-dependent protein kinase composed of catalytic and regulatory subunits and involved in various physiological phenomena, including lipid metabolism. Here we demonstrated that the stoichiometric balance between catalytic and regulatory subunits is crucial for maintaining basal PKA activity and lipid homeostasis. To uncover the potential roles of each PKA subunit, Caenorhabditis elegans was used to investigate the effects of PKA subunit deficiency. In worms, suppression of PKA via RNAi resulted in severe phenotypes, including shortened life span, decreased egg laying, reduced locomotion, and altered lipid distribution. Similarly, in mammalian adipocytes, suppression of PKA regulatory subunits RIα and RIIβ via siRNAs potently stimulated PKA activity, leading to potentiated lipolysis without increasing cAMP levels. Nevertheless, insulin exerted anti-lipolytic effects and restored lipid droplet integrity by antagonizing PKA action. Together, these data implicate the importance of subunit stoichiometry as another regulatory mechanism of PKA activity and lipid metabolism.

Contribution of lipase deficiency to mitochondrial dysfunction and insulin resistance in hMADS adipocytes

BACKGROUND/OBJECTIVES

Adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) are key enzymes involved in intracellular lipid catabolism. We have previously shown decreased expression and activity of these lipases in adipose tissue of obese insulin resistant individuals. Here we hypothesized that lipase deficiency might impact on insulin sensitivity and metabolic homeostasis in adipocytes not just by enhancing lipid accumulation, but also by altering lipid and carbohydrate catabolism in a peroxisome proliferator-activated nuclear receptor (PPAR)-dependent manner.

METHODS

To address our hypothesis, we performed a series of in vitro experiments in a human white adipocyte model, the human multipotent adipose-derived stem (hMADS) cells, using genetic (siRNA) and pharmacological knockdown of ATGL and/or HSL.

RESULTS

We show that ATGL and HSL knockdown in hMADS adipocytes disrupted mitochondrial respiration, which was accompanied by a decreased oxidative phosphorylation (OxPhos) protein content. This lead to a reduced exogenous and endogenous palmitate oxidation following ATGL knockdown, but not in HSL deficient adipocytes. ATGL deficiency was followed by excessive triacylglycerol accumulation, and HSL deficiency further increased diacylglycerol accumulation. Both single and double lipase knockdown reduced insulin-stimulated glucose uptake, which was attributable to impaired insulin signaling. These effects were accompanied by impaired activation of the nuclear receptor PPARα, and restored on PPARα agonist treatment.

CONCLUSIONS

The present study indicates that lipase deficiency in human white adipocytes contributes to mitochondrial dysfunction and insulin resistance, in a PPARα-dependent manner. Therefore, modulation of adipose tissue lipases may provide a promising strategy to reverse insulin resistance in obese and type 2 diabetic patients.

Isoproterenol Increases Uncoupling, Glycolysis, and Markers of Beiging in Mature 3T3-L1 Adipocytes

Beta-adrenergic activation stimulates uncoupling protein 1 (UCP1), enhancing metabolic rate. In vitro, most work has studied brown adipocytes, however, few have investigated more established adipocyte lines such as the murine 3T3-L1 line. To assess the effect of beta-adrenergic activation, mature 3T3-L1s were treated for 6 or 48 hours with or without isoproterenol (10 and 100 μM) following standard differentiation supplemented with thyroid hormone (T3; 1 nM). The highest dose of isoproterenol increased lipid content following 48 hours of treatment. This concentration enhanced UCP1 mRNA and protein expression. The increase in UCP1 following 48 hours of isoproterenol increased oxygen consumption rate. Further, coupling efficiency of the electron transport chain was disturbed and an enhancement of glycolytic rate was measured alongside this, indicating an attempt to meet the energy demands of the cell. Lastly, markers of beige adipocytes (protein content of CD137 and gene transcript of CITED1) were also found to be upregulated at 48 hours of isoproterenol treatment. This data indicates that mature 3T3-L1 adipocytes are responsive to isoproterenol and induce UCP1 expression and activity. Further, this finding provides a model for further pharmaceutical and nutraceutical investigation of UCP1 in 3T3-L1s.

Hepatic and visceral adipose tissue 11βHSD1 expressions are markers of body weight loss after bariatric surgery

OBJECTIVE

Cortisolemia and 11βHSD1 in liver and adipose tissue are altered in obesity. However, their participation in the development of obesity remains unclear. This study analyzed these parameters in the transition from morbid to type 1 obesity after bariatric surgery.

METHODS

A group of 34 patients with morbid obesity and 22 nonobese subjects were recruited. Initial hypothalamus-pituitary-adrenal (HPA) basal activity and 11βHSD1 mRNA expression in liver, subcutaneous (SAT), and visceral adipose tissue (VAT) were evaluated. A year after bariatric surgery (weight loss of 48 kg), these parameters were reappraised in plasma, SAT, and liver.

RESULTS

Body weight loss was accompanied by a downshift in basal HPA activity and 11βHSD1 expression in SAT. In patients with morbid obesity, 11βHSD1 expression correlated positively with BMI in VAT and negatively in liver at 6 and 12 months after surgery. In SAT, a correlation was observed with body weight only when patients showed type 1 obesity. Insulin, glucose, and HOMA correlated positively with all the HPA indicators and 11βHSD1 expression in SAT.

CONCLUSIONS

Body weight loss after bariatric surgery is accompanied by a downshift in basal HPA activity. Hepatic and VAT 11βHSD1 expressions in morbid obesity are predictors of body weight loss.

DAG tales: the multiple faces of diacylglycerol--stereochemistry, metabolism, and signaling

The neutral lipids diacylglycerols (DAGs) are involved in a plethora of metabolic pathways. They function as components of cellular membranes, as building blocks for glycero(phospho)lipids, and as lipid second messengers. Considering their central role in multiple metabolic processes and signaling pathways, cellular DAG levels require a tight regulation to ensure a constant and controlled availability. Interestingly, DAG species are versatile in their chemical structure. Besides the different fatty acid species esterified to the glycerol backbone, DAGs can occur in three different stereo/regioisoforms, each with unique biological properties. Recent scientific advances have revealed that DAG metabolizing enzymes generate and distinguish different DAG isoforms, and that only one DAG isoform holds signaling properties. Herein, we review the current knowledge of DAG stereochemistry and their impact on cellular metabolism and signaling. Further, we describe intracellular DAG turnover and its stereochemistry in a 3-pool model to illustrate the spatial and stereochemical separation and hereby the diversity of cellular DAG metabolism.

Progesterone-induced down-regulation of hormone sensitive lipase (Lipe) and up-regulation of G0/G1 switch 2 (G0s2) genes expression in inguinal adipose tissue of female rats is reflected by diminished rate of lipolysis

Decreased lipolytic activity in adipose tissue may be one of the reasons behind excess accumulation of body fat during pregnancy. The aim of this study was to analyze the effect of progesterone on the expression of: (a) Lipe (encoding hormone-sensitive lipase, HSL), (b) Pnpla2 (encoding adipose triglyceride lipase, ATGL), (c) abhydrolase domain containing 5 (Abhd5), and (d) G0/G1 switch 2 (G0s2) genes in white adipose tissue (WAT), as potential targets for progesterone action during the course of pregnancy. Administration of progesterone to female rats, which was reflected by approximately 2.5-fold increase in circulating progesterone concentration, is associated with a decrease in Lipe gene expression in the inguinal WAT. The expression of Pnpla2 gene in all main fat depots of females and males remained unchanged after progesterone administration. Administration of progesterone resulted in an increase in the expression of Abhd5 gene (whose product increases ATGL activity) and G0s2 gene (whose product decreases ATGL activity) in the inguinal WAT of female rats. Mifepristone, a selective antagonist of progesterone receptor, abolished the effect of progesterone on Lipe, Abhd5 and G0s2 genes expression in the inguinal WAT. The decrease in Lipe and the increase in Abhd5 and G0s2 genes expression was associated with lower rate of stimulated lipolysis. Administration of progesterone exerted no effect on Lipe, Abhd5 and G0s2 genes expression and stimulated lipolysis in the retroperitoneal WAT of females, as well as in the inguinal, epididymal and retroperitoneal WAT of males. In conclusion, our findings suggest that progesterone decreases the rate of lipolysis in the inguinal WAT of female rats, inhibiting the activity of both ATGL (by stimulating synthesis of G0S2 - specific inhibitor of the enzyme) and HSL (due to inhibition of Lipe gene expression).

Cardiotrophin-1 stimulates lipolysis through the regulation of main adipose tissue lipases

Cardiotrophin-1 (CT-1) is a cytokine with antiobesity properties and with a role in lipid metabolism regulation and adipose tissue function. The aim of this study was to analyze the molecular mechanisms involved in the lipolytic actions of CT-1 in adipocytes. Recombinant CT-1 (rCT-1) effects on the main proteins and signaling pathways involved in the regulation of lipolysis were evaluated in 3T3-L1 adipocytes and in mice. rCT-1 treatment stimulated basal glycerol release in a concentration- and time-dependent manner in 3T3-L1 adipocytes. rCT-1 (20 ng/ml for 24 h) raised cAMP levels, and in parallel increased protein kinase (PK)A-mediated phosphorylation of perilipin and hormone sensitive lipase (HSL) at Ser660. siRNA knock-down of HSL or PKA, as well as pretreatment with the PKA inhibitor H89, blunted the CT-1-induced lipolysis, suggesting that the lipolytic action of CT-1 in adipocytes is mainly mediated by activation of HSL through the PKA pathway. In ob/ob mice, acute rCT-1 treatment also promoted PKA-mediated phosphorylation of perilipin and HSL at Ser660 and Ser563, and increased adipose triglyceride lipase (desnutrin) content in adipose tissue. These results showed that the ability of CT-1 to regulate the activity of the main lipases underlies the lipolytic action of this cytokine in vitro and in vivo, and could contribute to CT-1 antiobesity effects.

Measurement of lipolysis

Lipolysis is defined as the hydrolytic cleavage of ester bonds in triglycerides (TGs), resulting in the generation of fatty acids (FAs) and glycerol. The two major TG pools in the body of vertebrates comprise intracellular TGs and plasma/nutritional TGs. Accordingly, this leads to the discrimination between intracellular and intravascular/gastrointestinal lipolysis, respectively. This chapter focuses exclusively on intracellular lipolysis, referred to as lipolysis herein. The lipolytic cleavage of TGs occurs in essentially all cells and tissues of the body. In all of them, the resulting FAs are utilized endogenously for energy production or biosynthetic pathways with one exception, white adipose tissue (WAT). WAT releases FAs and glycerol to supply nonadipose tissues at times of nutrient deprivation. The fundamental role of lipolysis in lipid and energy homeostasis requires the accurate measurement of lipase activities and lipolytic rates. The recent discovery of new enzymes and regulators that mediate the hydrolysis of TG has made these measurements more complex. Here, we describe detailed methodology for how to measure lipolysis and specific enzymes' activities in cells, organs, and their respective extracts.

Roles of chronic low-grade inflammation in the development of ectopic fat deposition

Pattern of fat distribution is a major determinant for metabolic homeostasis. As a depot of energy, the storage of triglycerides in adipose tissue contributes to the normal fat distribution. Decreased capacity of fat storage in adipose tissue may result in ectopic fat deposition in nonadipose tissues such as liver, pancreas, and kidney. As a critical biomarker of metabolic complications, chronic low-grade inflammation may have the ability to affect the process of lipid accumulation and further lead to the disorder of fat distribution. In this review, we have collected the evidence linking inflammation with ectopic fat deposition to get a better understanding of the underlying mechanism, which may provide us with novel therapeutic strategies for metabolic disorders.

TNF-α reduces g0s2 expression and stimulates lipolysis through PPAR-γ inhibition in 3T3-L1 adipocytes

Tumor necrosis factor-α (TNF-α) is a multifunctional cytokine that acts as a mediator of obesity-linked insulin resistance (IR). It is commonly accepted that macrophage-derived TNF-α acts in a paracrine manner on adjacent adipocytes, induces lipolysis, which contributes to obesity-linked hyperglycemia. Several studies suggested that G0/G1 switch gene 2 (g0s2) was up-regulated during adipogenesis, and its protein could be degraded in response to TNF-α stimulation. The aim of the present work was to investigate the transcriptional regulation of g0s2 by TNF-α stimulation. In this study, 3T3-L1 pre-adipocytes were differentiated, and treated with TNF-α for 24h. The effects of TNF-α on lipolysis and lipase expression were then examined. Our results revealed that TNF-α exerted dose- and time-dependent lipolytic effects, which could be partially reversed by overexpression of g0s2 and peroxisome proliferator-activated receptor-γ (ppar-γ). In addition, TNF-α treatment significantly reduced the expression of adiponectin, ppar-γ, hormone-sensitive Lipase (hsl), adipose triglyceride lipase (atgl) as well as ATGL co-factors. Interestingly, TNF-α significantly decreased adiponectin and PPAR-γ protein levels, while treatment with the proteasomal inhibitor MG-132 maintained PPAR-γ levels. Degradation of PPAR-γ almost completely abolished the binding of PPAR-γ to the g0s2 promoter in adipocytes treated with TNF-α. We propose that proteasomal degradation of PPAR-γ and the reduction of g0s2 content are permissive for prolonged TNF-α induced lipolysis.

Regulation of adipocyte lipolysis

In adipocytes the hydrolysis of TAG to produce fatty acids and glycerol under fasting conditions or times of elevated energy demands is tightly regulated by neuroendocrine signals, resulting in the activation of lipolytic enzymes. Among the classic regulators of lipolysis, adrenergic stimulation and the insulin-mediated control of lipid mobilisation are the best known. Initially, hormone-sensitive lipase (HSL) was thought to be the rate-limiting enzyme of the first lipolytic step, while we now know that adipocyte TAG lipase is the key enzyme for lipolysis initiation. Pivotal, previously unsuspected components have also been identified at the protective interface of the lipid droplet surface and in the signalling pathways that control lipolysis. Perilipin, comparative gene identification-58 (CGI-58) and other proteins of the lipid droplet surface are currently known to be key regulators of the lipolytic machinery, protecting or exposing the TAG core of the droplet to lipases. The neuroendocrine control of lipolysis is prototypically exerted by catecholaminergic stimulation and insulin-induced suppression, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and perilipin. Interestingly, in recent decades adipose tissue has been shown to secrete a large number of adipokines, which exert direct effects on lipolysis, while adipocytes reportedly express a wide range of receptors for signals involved in lipid mobilisation. Recently recognised mediators of lipolysis include some adipokines, structural membrane proteins, atrial natriuretic peptides, AMP-activated protein kinase and mitogen-activated protein kinase. Lipolysis needs to be reanalysed from the broader perspective of its specific physiological or pathological context since basal or stimulated lipolytic rates occur under diverse conditions and by different mechanisms.

MiRNA-181a regulates adipogenesis by targeting tumor necrosis factor-α (TNF-α) in the porcine model

Adipogenesis is tightly regulated by altering gene expression, and TNF-α is a multifunctional cytokine that plays an important role in regulating lipogenesis. MicroRNAs are strong post-transcriptional regulators of cell differentiation. In our previous work, we found high expression of miR-181a in a fat-rich pig breed. Using bioinformatic analysis, miR-181a was identified as a potential regulator of TNF-α. Here, we validated TNF-α as the target of miR-181a by a dual luciferase assay. In response to adipogenesis, a mimic or inhibitor was used to overexpress or reduce miR-181a expression in porcine pre-adipocytes, which were then induced into mature adipocytes. Overexpression of miR-181a accelerated accumulation of lipid droplets, increased the amount of triglycerides, and repressed TNF-α protein expression, while the inhibitor had the opposite effect. At the same time, TNF-alpha rescued the increased lipogenesis by miR181a mimics. Additionally, miR-181a suppression decreased the expression of fatty synthesis associated genes PDE3B (phosphodiesterase 3B), LPL (lipoprotein lipase), PPARγ (proliferator-activated receptor-γ), GLUT1(glucose transporter), GLUT4, adiponectin and FASN (fatty acid synthase), as well as key lipolytic genes HSL (hormone-sensitive lipase) and ATGL (adipose triglyceride lipase) as revealed by quantitative real-time PCR. Our study provides the first evidence of the role of miR-181a in adipocyte differentiation by regulation of TNF-α, which may became a new therapeutic target for anti-obesity drugs.

IL-6 and epinephrine have divergent fiber type effects on intramuscular lipolysis

IL-6 is an exercise-regulated myokine that has been suggested to increase lipolysis in fast-twitch skeletal muscle. However, it is not known if a similar effect is present in slow-twitch muscle. Furthermore, epinephrine increases IL-6 secretion from skeletal muscle, suggesting that IL-6 could play a role in mediating the lipolytic effects of catecholamines. The purpose of this study was to determine whether IL-6 stimulates skeletal muscle lipolysis in a fiber type dependent manner and is required for epinephrine-stimulated lipolysis in murine skeletal muscle. Soleus and extensor digitorum longus (EDL) muscles from male C57BL/6J wild-type and IL-6(-/-) mice were incubated with 1 μM (183 ng/ml) epinephrine or 75 ng/ml recombinant IL-6 (rIL-6) for 60 min. IL-6 treatment increased 5'-AMP-activated protein kinase and signal transducer and activator of transcription 3 phosphorylation and glycerol release in isolated EDL but not soleus muscles from C57BL/6J mice. Conversely, epinephrine increased glycerol release in soleus but not EDL muscles from C57BL/6J mice. Basal lipolysis was elevated in soleus muscle from IL-6(-/-) mice, and this was associated with increases in adipose triglyceride lipase (ATGL) and its coactivator comparative gene identification-58 (CGI-58). The increase in ATGL content does not appear to be due to a loss of IL-6's direct effects, because ex vivo treatment with IL-6 failed to alter the expression of ATGL mRNA in soleus muscle. In summary, IL-6 stimulates lipolysis in glycolytic but not oxidative muscle, whereas the opposite fiber type effect is seen with epinephrine. The absence of IL-6 indirectly upregulates lipolysis, and this is associated with increases in ATGL and its coactivator CGI-58.

Insulin inhibits lipolysis in adipocytes via the evolutionarily conserved mTORC1-Egr1-ATGL-mediated pathway

One of the basic functions of insulin in the body is to inhibit lipolysis in adipocytes. Recently, we have found that insulin inhibits lipolysis and promotes triglyceride storage by decreasing transcription of adipose triglyceride lipase via the mTORC1-mediated pathway (P. Chakrabarti et al., Diabetes 59:775-781, 2010), although the mechanism of this effect remained unknown. Here, we used a genetic screen in Saccharomyces cerevisiae in order to identify a transcription factor that mediates the effect of Tor1 on the expression of the ATGL ortholog in yeast. This factor, Msn4p, has homologues in mammalian cells that form a family of early growth response transcription factors. One member of the family, Egr1, is induced by insulin and nutrients and directly inhibits activity of the ATGL promoter in vitro and expression of ATGL in cultured adipocytes. Feeding animals a high-fat diet increases the activity of mTORC1 and the expression of Egr1 while decreasing ATGL levels in epididymal fat. We suggest that the evolutionarily conserved mTORC1-Egr1-ATGL regulatory pathway represents an important component of the antilipolytic effect of insulin in the mammalian organism.

Adipose triglyceride lipase (ATGL) clone, expression pattern, and regulation by different lipid sources and lipid levels in large yellow croaker (Pseudosciaena crocea R.)

Adipose triglyceride lipase (ATGL) was recently identified as a triglyceride (TG)-specific lipase. In this study, we first obtained from large yellow croaker fish a 1,820-bp (GenBank ID: HQ916211) ATGL cDNA fragment with a 141-bp 5'UTR, a 1,485-bp open reading frame, and a 194-bp 3'UTR. The predicted fish ATGL had 494 amino acids (GenBank ID: ADY89608) and a calculated molecular weight of 55.1 kDa. ATGL was highly expressed in liver and, to a lesser degree, in heart, muscle, and abdominal fat. ATGL gene expression was high at 4.5 g and then decreased at 157.9 g and increased again at 474.2 g. The effects of lipid levels and lipid sources on ATGL expression in vivo were also investigated. A high-lipid diet decreased ATGL expression in fish significantly (P < 0.01). Fish in soybean oil group exhibited significantly lower ATGL expression than fish in the fish oil and beef tallow groups (P < 0.01). These data enhance our understanding of ATGL in fish lipid metabolism.

The role of triglyceride lipases in cancer associated cachexia

Cancer associated cachexia (CAC) is a complex multiorgan syndrome frequently associated with various forms of cancer. Affected patients suffer from a dramatic loss of skeletal muscle and adipose tissue. Most cases are accompanied by anorexia, and nutritional supplements are not sufficient to stop or reverse its course. CAC impairs many forms of therapeutic interventions and accounts for 15-20% of all deaths of cancer patients. Recently, several studies have recognized the importance of lipid metabolism and triglyceride hydrolysis as a major metabolic pathway involved in the initiation and/or progression of CAC. In this review, we explore the contributions of the triglyceride lipases to CAC and discuss various factors modulating lipase activity.

Sirtuin 1 affects the transcriptional expression of adipose triglyceride lipase in porcine adipocytes

To investigate whether sirtuin 1 (Sirt1) could affect the transcriptional expression of the adipose triglyceride lipase (ATGL) gene, we treated porcine adipocytes with the general Sirt1 activator resveratrol (RES) with the Sirt1 inhibitor nicotinamide (NAM) or a knockdown of Sirt1 by Sirt1-specific small interfering RNA (siRNA). The RES (50 μM) activated Sirt1 gene expression and increased ATGL gene expression and glycerol release (P < 0.01). The Sirt1 inhibitor NAM or knockdown with Sirt1 siRNA further proved that the ATGL mRNA abundances were decreased (P < 0.05) after inhibition with Sirt1 in adipocytes. Furthermore, we found the opposite Sirt1 regulation pattern for PPARγ to that of ATGL in adipocytes. In summary, Sirt1 regulates the transcriptional expression of ATGL in adipocytes, and PPARγ appears to have an important role in this process. These results add to our understanding of the role of Sirt1 in adipose mobilization.

The effect of insulin, TNFα and DHA on the proliferation, differentiation and lipolysis of preadipocytes isolated from large yellow croaker (Pseudosciaena Crocea R.)

Fish final product can be affected by excessive lipid accumulation. Therefore, it is important to develop strategies to control obesity in cultivated fish to strengthen the sustainability of the aquaculture industry. As in mammals, the development of adiposity in fish depends on hormonal, cytokine and dietary factors. In this study, we investigated the proliferation and differentiation of preadipocytes isolated from the large yellow croaker and examined the effects of critical factors such as insulin, TNFα and DHA on the proliferation, differentiation and lipolysis of adipocytes. Preadipocytes were isolated by collagenase digestion, after which their proliferation was evaluated. The differentiation process was optimized by assaying glycerol-3-phosphate dehydrogenase (GPDH) activity. Oil red O staining and electron microscopy were performed to visualize the accumulated triacylglycerol. Gene transcript levels were measured using SYBR green quantitative real-time PCR. Insulin promoted preadipocytes proliferation, stimulated cell differentiation and decreased lipolysis of mature adipocytes. TNFα and DHA inhibited cell proliferation and differentiation. While TNFα stimulated mature adipocyte lipolysis, DHA showed no lipolytic effect on adipocytes. The expressions of adipose triglyceride lipase (ATGL), fatty acid synthase (FAS), lipoprotein lipase (LPL) and peroxisome proliferator-activated receptor α, γ (PPARα, PPARγ) were quantified during preadipocytes differentiation and adipocytes lipolysis to partly explain the regulation mechanisms. In summary, the results of this study indicated that although preadipocytes proliferation and the differentiation process in large yellow croaker are similar to these processes in mammals, the effects of critical factors such as insulin, TNFα and DHA on fish adipocytes development are not exactly the same. Our findings fill in the gaps in the basic data regarding the effects of critical factors on adiposity development in fish and will facilitate the further study of molecular mechanism by which these factors act in fish and the application of this knowledge to eventually control obesity in cultured species.