The adipose tissue triglyceride lipase ATGL/PNPLA2 is downregulated by insulin and TNF-alpha in 3T3-L1 adipocytes and is a target for transactivation by PPARgamma

Novel targets of β-TrCP cooperatively accelerate carbohydrate and fatty acid consumption

Cellular energy is primarily produced from glucose and fat through glycolysis and fatty acid oxidation (FAO) followed by the tricarboxylic acid cycle in mitochondria; energy homeostasis is carefully maintained via numerous feedback pathways. In this report, we uncovered a new master regulator of carbohydrate and lipid metabolism. When ubiquitin E3 ligase β-TrCP2 was inducibly knocked out in β-TrCP1 knockout adult mice, the resulting double knockout mice (DKO) lost fat mass rapidly. Biochemical analyses of the tissues and cells from β-TrCP2 KO and DKO mice revealed that glycolysis, FAO, and lipolysis were dramatically upregulated. The absence of β-TrCP2 increased the protein stability of metabolic rate-limiting enzymes including 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFKFB3), adipose triglyceride lipase (ATGL), carnitine palmitoyltransferase 1A (CPT1A), and carnitine/acylcarnitine translocase (CACT). Our data suggest that β-TrCP is a potential regulator for total energy homeostasis by simultaneously controlling glucose and fatty acid metabolism and that targeting β-TrCP could be an effective strategy to treat obesity and other metabolic disorders.

The association between plasma angiopoietin-like protein 4, glucose and lipid metabolism during pregnancy, placental function, and risk of delivering large-for-gestational-age neonates

BACKGROUND

Large-for-gestational-age (LGA) neonates have increased risk of adverse pregnancy outcomes and adult metabolic diseases. We aimed to investigate the relationship between plasma angiopoietin-like protein 4 (ANGPTL4), a protein involved in lipid and glucose metabolism during pregnancy, placental function, growth factors, and the risk of LGA.

METHODS

We conducted a prospective cohort study and recruited women with singleton pregnancies at the National Taiwan University Hospital between 2013 and 2018. First trimester maternal plasma ANGPTL4 concentrations were measured.

RESULTS

Among 353 pregnant women recruited, the LGA group had higher first trimester plasma ANGPTL4 concentrations than the appropriate-for-gestational-age group. Plasma ANGPTL4 was associated with hemoglobin A1c, post-load plasma glucose, plasma triglyceride, plasma free fatty acid concentrations, plasma growth hormone variant (GH-V), and birth weight, but was not associated with cord blood growth factors. After adjusting for age, body mass index, hemoglobin A1c, and plasma triglyceride concentrations, plasma ANGPTL4 concentrations were significantly associated with LGA risk, and its predictive performance, as measured by the area under the receiver operating characteristic curve, outperformed traditional risk factors for LGA.

CONCLUSIONS

Plasma ANGPTL4 is associated with glucose and lipid metabolism during pregnancy, plasma GH-V, and birth weight, and is an early biomarker for predicting the risk of LGA.

Comprehensive genetic study of the insulin resistance marker TG:HDL-C in the UK Biobank

Insulin resistance (IR) is a well-established risk factor for metabolic disease. The ratio of triglycerides to high-density lipoprotein cholesterol (TG:HDL-C) is a surrogate marker of IR. We conducted a genome-wide association study of the TG:HDL-C ratio in 402,398 Europeans within the UK Biobank. We identified 369 independent SNPs, of which 114 had a false discovery rate-adjusted P value < 0.05 in other genome-wide studies of IR making them high-confidence IR-associated loci. Seventy-two of these 114 loci have not been previously associated with IR. These 114 loci cluster into five groups upon phenome-wide analysis and are enriched for candidate genes important in insulin signaling, adipocyte physiology and protein metabolism. We created a polygenic-risk score from the high-confidence IR-associated loci using 51,550 European individuals in the Michigan Genomics Initiative. We identified associations with diabetes, hyperglyceridemia, hypertension, nonalcoholic fatty liver disease and ischemic heart disease. Collectively, this study provides insight into the genes, pathways, tissues and subtypes critical in IR.

Peroxisome proliferator-activated receptors: A key link between lipid metabolism and cancer progression

Lipids represent the essential components of membranes, serve as fuels for high-energy processes, and play crucial roles in signaling and cellular function. One of the key hallmarks of cancer is the reprogramming of metabolic pathways, especially abnormal lipid metabolism. Alterations in lipid uptake, lipid desaturation, de novo lipogenesis, lipid droplets, and fatty acid oxidation in cancer cells all contribute to cell survival in a changing microenvironment by regulating feedforward oncogenic signals, key oncogenic functions, oxidative and other stresses, immune responses, or intercellular communication. Peroxisome proliferator-activated receptors (PPARs) are transcription factors activated by fatty acids and act as core lipid sensors involved in the regulation of lipid homeostasis and cell fate. In addition to regulating whole-body energy homeostasis in physiological states, PPARs play a key role in lipid metabolism in cancer, which is receiving increasing research attention, especially the fundamental molecular mechanisms and cancer therapies targeting PPARs. In this review, we discuss how cancer cells alter metabolic patterns and regulate lipid metabolism to promote their own survival and progression through PPARs. Finally, we discuss potential therapeutic strategies for targeting PPARs in cancer based on recent studies from the last five years.

Modulation of angiogenic switch in reprogramming browning and lipid metabolism in white adipocytes

Thermogenic activation via trans-and de novo browning of white adipocytes is a promising strategy to accelerate lipid metabolism for regulating obesity-related disorders. In this study, we investigated the intricate interplay between angiogenic regulation and browning in white adipocytes using the bioactive compound, resveratrol (Rsv). Rsv has previously been documented for its regulatory influence on the trans and de novo browning of white adipocytes. Our findings revealed that concurrent activation of angiogenesis is prerequisite for inducing browning within the microenvironment of white adipocytes when exposed to browning activators. Additionally, we observed a significant browning effect on white adipocytes when the local adipose tissue environment was prompted to undergo angiogenesis, notably facilitated by a proangiogenic molecule known as Vascular endothelial growth factor (VEGF). Intriguingly, this effect was reversed when angiogenesis was inhibited by treatment with the antiangiogenic agent thalidomide. Furthermore, the study revealed the role of VEGF in paracrine activation of white adipocytes resulting in the induction of browning in both 3T3-L1 cell lines and primary mouse white adipocytes. The cross-talk between angiogenesis and browning was found to be initiated via the transcriptional activation of Estrogen receptor α (ERα) triggering the VEGF/VEGFR2 signaling pathway leading to browning and a reconfiguration of lipid metabolism within adipocytes. In conclusion, this study sheds light on the intricate cross-talk between angiogenesis and browning of white adipocytes. Notably, the findings underscore the reciprocal relationship between these processes, wherein inhibition of one process exerts discernible effects on the other.

CTCF modulates adipocyte lipolysis via directly regulating the expression of Beclin 1 with the cooperation of PPARγ

Dysregulated lipolysis is a risk factor contributing to metabolic diseases and autophagy is known to be important in lipolysis. CTCF is involved in diverse cellular processes including adipogenesis, yet its role in lipolysis or autophagy remains unknown. We identified lipolytic genes were downregulated in CTCF knockdown adipocytes based on the RNA-seq data. Further validation showed that CTCF knockdown restrained adipocyte lipolysis while overexpression of CTCF had opposite effects. Similarly, overexpression and knockdown studies demonstrated that CTCF was a positive regulator of autophagy. Treatment with autophagy inducer relieved the suppression of lipolysis caused by CTCF knockdown, while autophagy inhibitor treatment alleviated lipolysis stimulated by CTCF overexpression, indicating that CTCF regulates adipocyte lipolysis through autophagy. Mechanistically, CTCF interacted with PPARγ to coordinately enhanced lipolytic capacity. Data of chip-seq, chip-qPCR and further experiments confirmed that CTCF and PPARγ separately stimulated transactivation of autophagy regulatory protein Beclin 1, while co-expression of the two displayed synergistic effects to regulate autophagy flux. Expectedly, overexpression of Beclin 1 abolished the blockage of lipolysis and autophagy caused by CTCF knockdown. Collectively, CTCF cooperates with PPARγ to regulate autophagy via directly modulating BECLIN 1 transcription, thereby leading to increased adipocyte lipolysis.

Unbiased gene expression analysis of the delayed fracture healing observed in Zucker diabetic fatty rats

Aims

Impaired fracture repair in patients with type 2 diabetes mellitus (T2DM) is not fully understood. In this study, we aimed to characterize the local changes in gene expression (GE) associated with diabetic fracture. We used an unbiased approach to compare GE in the fracture callus of Zucker diabetic fatty (ZDF) rats relative to wild-type (WT) littermates at three weeks following femoral osteotomy.

Methods

Zucker rats, WT and homozygous for leptin receptor mutation (ZDF), were fed a moderately high-fat diet to induce T2DM only in the ZDF animals. At ten weeks of age, open femoral fractures were simulated using a unilateral osteotomy stabilized with an external fixator. At three weeks post-surgery, the fractured femur from each animal was retrieved for analysis. Callus formation and the extent of healing were assessed by radiograph and histology. Bone tissue was processed for total RNA extraction and messenger RNA (mRNA) sequencing (mRNA-Seq).

Results

Radiographs and histology demonstrated impaired fracture healing in ZDF rats with incomplete bony bridge formation and an influx of intramedullary inflammatory tissue. In comparison, near-complete bridging between cortices was observed in Sham WT animals. Of 13,160 genes, mRNA-Seq analysis identified 13 that were differentially expressed in ZDF rat callus, using a false discovery rate (FDR) threshold of 10%. Seven genes were upregulated with high confidence (FDR = 0.05) in ZDF fracture callus, most with known roles in inflammation.

Conclusion

These findings suggest that elevated or prolonged inflammation contributes to delayed fracture healing in T2DM. The identified genes may be used as biomarkers to monitor and treat delayed fracture healing in diabetic patients.

Adipose METTL14-Elicited N6 -Methyladenosine Promotes Obesity, Insulin Resistance, and NAFLD Through Suppressing β Adrenergic Signaling and Lipolysis

White adipose tissue (WAT) lipolysis releases free fatty acids as a key energy substance to support metabolism in fasting, cold exposure, and exercise. Atgl, in concert with Cgi-58, catalyzes the first lipolytic reaction. The sympathetic nervous system (SNS) stimulates lipolysis via neurotransmitter norepinephrine that activates adipocyte β adrenergic receptors (Adrb1-3). In obesity, adipose Adrb signaling and lipolysis are impaired, contributing to pathogenic WAT expansion; however, the underling mechanism remains poorly understood. Recent studies highlight importance of N6 -methyladenosine (m6A)-based RNA modification in health and disease. METTL14 heterodimerizes with METTL3 to form an RNA methyltransferase complex that installs m6A in transcripts. Here, this work shows that adipose Mettl3 and Mettl14 are influenced by fasting, refeeding, and insulin, and are upregulated in high fat diet (HFD) induced obesity. Adipose Adrb2, Adrb3, Atgl, and Cgi-58 transcript m6A contents are elevated in obesity. Mettl14 ablation decreases these transcripts' m6A contents and increases their translations and protein levels in adipocytes, thereby increasing Adrb signaling and lipolysis. Mice with adipocyte-specific deletion of Mettl14 are resistant to HFD-induced obesity, insulin resistance, glucose intolerance, and nonalcoholic fatty liver disease (NAFLD). These results unravel a METTL14/m6A/translation pathway governing Adrb signaling and lipolysis. METTL14/m6A-based epitranscriptomic reprogramming impairs adipose Adrb signaling and lipolysis, promoting obesity, NAFLD, and metabolic disease.

Alcohol, White Adipose Tissue, and Brown Adipose Tissue: Mechanistic Links to Lipogenesis and Lipolysis

According to data from the World Health Organization, there were about 3 million deaths caused by alcohol consumption worldwide in 2016, of which about 50% were related to liver disease. Alcohol consumption interfering with the normal function of adipocytes has an important impact on the pathogenesis of alcoholic liver disease. There has been increasing recognition of the crucial role of adipose tissue in regulating systemic metabolism, far beyond that of an inert energy storage organ in recent years. The endocrine function of adipose tissue is widely recognized, and the significance of the proteins it produces and releases is still being investigated. Alcohol consumption may affect white adipose tissue (WAT) and brown adipose tissue (BAT), which interact with surrounding tissues such as the liver and intestines. This review briefly introduces the basic concept and classification of adipose tissue and summarizes the mechanism of alcohol affecting lipolysis and lipogenesis in WAT and BAT. The adipose tissue-liver axis is crucial in maintaining lipid homeostasis within the body. Therefore, this review also demonstrates the effects of alcohol consumption on the adipose tissue-liver axis to explore the role of alcohol consumption in the crosstalk between adipose tissue and the liver.

Fatty acid desaturation by stearoyl-CoA desaturase-1 controls regulatory T cell differentiation and autoimmunity

The imbalance between pathogenic and protective T cell subsets is a cardinal feature of autoimmune disorders such as multiple sclerosis (MS). Emerging evidence indicates that endogenous and dietary-induced changes in fatty acid metabolism have a major impact on both T cell fate and autoimmunity. To date, however, the molecular mechanisms that underlie the impact of fatty acid metabolism on T cell physiology and autoimmunity remain poorly understood. Here, we report that stearoyl-CoA desaturase-1 (SCD1), an enzyme essential for the desaturation of fatty acids and highly regulated by dietary factors, acts as an endogenous brake on regulatory T-cell (Treg) differentiation and augments autoimmunity in an animal model of MS in a T cell-dependent manner. Guided by RNA sequencing and lipidomics analysis, we found that the absence of Scd1 in T cells promotes the hydrolysis of triglycerides and phosphatidylcholine through adipose triglyceride lipase (ATGL). ATGL-dependent release of docosahexaenoic acid enhanced Treg differentiation by activating the nuclear receptor peroxisome proliferator-activated receptor gamma. Our findings identify fatty acid desaturation by SCD1 as an essential determinant of Treg differentiation and autoimmunity, with potentially broad implications for the development of novel therapeutic strategies and dietary interventions for autoimmune disorders such as MS.

Receptors that bind to PEDF and their therapeutic roles in retinal diseases

Retinal neovascular, neurodegenerative, and inflammatory diseases represented by diabetic retinopathy are the main types of blinding eye disorders that continually cause the increased burden worldwide. Pigment epithelium-derived factor (PEDF) is an endogenous factor with multiple effects including neurotrophic activity, anti-angiogenesis, anti-tumorigenesis, and anti-inflammatory activity. PEDF activity depends on the interaction with the proteins on the cell surface. At present, seven independent receptors, including adipose triglyceride lipase, laminin receptor, lipoprotein receptor-related protein, plexin domain-containing 1, plexin domain-containing 2, F1-ATP synthase, and vascular endothelial growth factor receptor 2, have been demonstrated and confirmed to be high affinity receptors for PEDF. Understanding the interactions between PEDF and PEDF receptors, their roles in normal cellular metabolism and the response the initiate in disease will be accommodating for elucidating the ways in which inflammation, angiogenesis, and neurodegeneration exacerbate disease pathology. In this review, we firstly introduce PEDF receptors comprehensively, focusing particularly on their expression pattern, ligands, related diseases, and signal transduction pathways, respectively. We also discuss the interactive ways of PEDF and receptors to expand the prospective understanding of PEDF receptors in the diagnosis and treatment of retinal diseases.

Cdo1 promotes PPARγ-mediated adipose tissue lipolysis in male mice

Cysteine dioxygenase 1 (Cdo1) is a key enzyme in taurine synthesis. Here we show that Cdo1 promotes lipolysis in adipose tissue. Adipose-specific knockout of Cdo1 in mice impairs energy expenditure, cold tolerance and lipolysis, exacerbates diet-induced obesity (DIO) and decreases adipose expression of the key lipolytic genes encoding ATGL and HSL, with little effect on adipose taurine levels. White-adipose-specific overexpression of ATGL and HSL blunts the role of adipose Cdo1 deficiency in promoting DIO. Mechanistically, Cdo1 interacts with PPARγ and facilitates the recruitment of Med24, the core subunit of mediator complex, to ATGL and HSL gene promoters, thereby transactivating their expression. Further, mice with transgenic overexpression of Cdo1 show better cold tolerance, ameliorated DIO and higher lipolysis capacity. Thus, we uncover an unexpected and important role of Cdo1 in regulating adipose lipolysis.

Egr1 plays a major role in the transcriptional response of white adipocytes to insulin and environmental cues

It is believed that insulin regulates metabolic functions of white adipose tissue primarily at the post-translational level via the PI3K-Akt-mediated pathway. Still, changes in transcription also play an important role in the response of white adipocytes to insulin and environmental signals. One transcription factor that is dramatically and rapidly induced in adipocytes by insulin and nutrients is called Early Growth Response 1, or Egr1. Among other functions, it directly binds to promoters of leptin and ATGL stimulating the former and inhibiting the latter. Furthermore, expression of Egr1 in adipocytes demonstrates cell autonomous circadian pattern suggesting that Egr1 not only mediates the effect of insulin and nutrients on lipolysis and leptin production but also, coordinates insulin action with endogenous circadian rhythms of adipose tissue.

The Role of Gut Microbiota Modulation Strategies in Obesity: The Applications and Mechanisms

Nowadays, obesity is a leading public health problem worldwide. The growing prevalence of obesity significantly accounts for other cardio-metabolic diseases, including hypertension and diabetes. Several studies have shown that obesity is strongly associated with genetic, environmental, lifestyle, and dietary factors, especially the disordered profiles of gut microbiota (GM). The present review concluded mechanistic studies and potential correspondent treatments for obesity. Specifically, the anti-obesity effects of food-derived compounds manipulating GM were highlighted. The potential limitations of bioactive compounds on absorption in the intestinal tract were also discussed. Thus, the future direction of fecal microbiota transplantation (FMT) as an approach to support modulating host GM (considered to be a potential therapeutic target for obesity) was discussed. This review shed light on the role of GM modulation strategies for the prevention/treatment of obesity.

White to brown adipocyte transition mediated by Apigenin via VEGF-PRDM16 signaling

The dysregulated energy metabolism in white adipose tissues results in derangement of biological signaling resulting in obesity. Lack of vascularization in these white adipose tissues is one of the major reasons for dysregulated energy metabolism. Not much work has been done in this direction to understand the role of angiogenesis in white adipose tissue metabolism. In the present study, we evaluated the effect of angiogenic modulator in the metabolism of white adipocyte (WAC). Bioactive Apigenin was selected and its angiogenic ability was studied. Apigenin was shown to be highly proangiogenic hence the effect of Apigenin on de novo and trans-differentiation of WAT was studied. Apigenin showed enhanced de novo differentiation and trans-differentiation of mouse WAC into brown-like phenotype. To understand the effect of Apigenin on adipose tissue vasculature, coculture studies were conducted. Cross talk between endothelial cell and adipocytes were observed in coculture studies. Gene expression studies of cocultured cells revealed that browning of WAC occurred by triggering the expression of Vascular endothelial growth factor A. The study provides a new insight for inducing metabolic shift in WACs by modulation of angiogenesis in WAC microenvironment by the upregulation of PRDM16 cascade to trigger browning for the treatment of obesity.

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.

Lipolysis: cellular mechanisms for lipid mobilization from fat stores

The perception that intracellular lipolysis is a straightforward process that releases fatty acids from fat stores in adipose tissue to generate energy has experienced major revisions over the last two decades. The discovery of new lipolytic enzymes and coregulators, the demonstration that lipophagy and lysosomal lipolysis contribute to the degradation of cellular lipid stores and the characterization of numerous factors and signalling pathways that regulate lipid hydrolysis on transcriptional and post-transcriptional levels have revolutionized our understanding of lipolysis. In this review, we focus on the mechanisms that facilitate intracellular fatty-acid mobilization, drawing on canonical and noncanonical enzymatic pathways. We summarize how intracellular lipolysis affects lipid-mediated signalling, metabolic regulation and energy homeostasis in multiple organs. Finally, we examine how these processes affect pathogenesis and how lipolysis may be targeted to potentially prevent or treat various diseases.

Coffee Bioactive N-Methylpyridinium Attenuates Tumor Necrosis Factor (TNF)-α-Mediated Insulin Resistance and Inflammation in Human Adipocytes

Although coffee consumption has been historically associated with negative health outcomes, recent evidence suggests a lower risk of metabolic syndrome, obesity and diabetes among regular coffee drinkers. Among the plethora of minor organic compounds assessed as potential mediators of coffee health benefits, trigonelline and its pyrolysis product N-methylpyridinium (NMP) were preliminary shown to promote glucose uptake and exert anti-adipogenic properties. Against this background, we aimed at characterizing the effects of trigonelline and NMP in inflamed and dysfunctional human adipocytes. Human Simpson-Golabi-Behmel syndrome (SGBS) adipocytes were treated with NMP or, for comparison, trigonelline, for 5 h before stimulation with tumor necrosis factor (TNF)-α. NMP at concentrations as low as 1 µmol/L reduced the stimulated expression of several pro-inflammatory mediators, including C-C Motif chemokine ligand (CCL)-2, C-X-C Motif chemokine ligand (CXCL)-10, and intercellular adhesion Molecule (ICAM)-1, but left the induction of prostaglandin G/H synthase (PTGS)2, interleukin (IL)-1β, and colony stimulating factor (CSF)1 unaffected. Furthermore, NMP restored the downregulated expression of adiponectin (ADIPOQ). These effects were functionally associated with downregulation of the adhesion of monocytes to inflamed adipocytes. Under the same conditions, NMP also reversed the TNF-α-mediated suppression of insulin-stimulated Ser473 Akt phosphorylation and attenuated the induction of TNF-α-stimulated lipolysis restoring cell fat content. In an attempt to preliminarily explore the underlying mechanisms of its action, we show that NMP restores the expression of the master regulator of adipocyte differentiation peroxisome proliferator-activated receptor (PPAR)γ and downregulates activation of the pro-inflammatory mitogen-activated protein jun N-terminal kinase (JNK). In conclusion, NMP reduces adipose dysfunction in pro-inflammatory activated adipocytes. These data suggest that bioactive NMP in coffee may improve the inflammatory and dysmetabolic milieu associated with obesity.

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.

NOTUM promotes thermogenic capacity and protects against diet-induced obesity in male mice

We recently showed that NOTUM, a liver-secreted Wnt inhibitor, can acutely promote browning of white adipose. We now report studies of chronic overexpression of NOTUM in liver indicating that it protects against diet-induced obesity and improves glucose homeostasis in mice. Adeno-associated virus (AAV) vectors were used to overexpress GFP or mouse Notum in the livers of male C57BL/6J mice and the mice were fed an obesifying diet. After 14 weeks of high fat, high sucrose diet feeding, the AAV-Notum mice exhibited decreased obesity and improved glucose tolerance compared to the AAV-GFP mice. Gene expression and immunoblotting analysis of the inguinal fat and brown fat revealed increased expression of beige/brown adipocyte markers in the AAV-Notum group, suggesting enhanced thermogenic capacity by NOTUM. A β3 adrenergic receptor agonist-stimulated lipolysis test suggested increased lipolysis capacity by NOTUM. The levels of collagen and C–C motif chemokine ligand 2 (CCL2) in the epididymal white adipose tissue of the AAV-Notum mice were significantly reduced, suggesting decreased fibrosis and inflammation, respectively. RNA sequencing analysis of inguinal white adipose of 4-week chow diet-fed mice revealed a highly significant enrichment of extracellular matrix (ECM) functional cluster among the down-regulated genes in the AAV-Notum group, suggesting a potential mechanism contributing to improved glucose homeostasis. Our in vitro studies demonstrated that recombinant human NOTUM protein blocked the inhibitory effects of WNT3A on brown adipocyte differentiation. Furthermore, NOTUM attenuated WNT3A’s effects on upregulation of TGF-β signaling and its downstream targets. Overall, our data suggest that NOTUM modulates adipose tissue function by promoting thermogenic capacity and inhibiting fibrosis through inhibition of Wnt signaling.

Physiological and pathophysiological concentrations of fatty acids induce lipid droplet accumulation and impair functional performance of tissue engineered skeletal muscle

Fatty acids (FA) exert physiological and pathophysiological effects leading to changes in skeletal muscle metabolism and function, however, in vitro models to investigate these changes are limited. These experiments sought to establish the effects of physiological and pathophysiological concentrations of exogenous FA upon the function of tissue engineered skeletal muscle (TESkM). Cultured initially for 14 days, C2C12 TESkM was exposed to FA-free bovine serum albumin alone or conjugated to a FA mixture (oleic, palmitic, linoleic, and α-linoleic acids [OPLA] [ratio 45:30:24:1%]) at different concentrations (200 or 800 µM) for an additional 4 days. Subsequently, TESkM morphology, functional capacity, gene expression and insulin signaling were analyzed. There was a dose response increase in the number and size of lipid droplets within the TESkM (p < .05). Exposure to exogenous FA increased the messenger RNA expression of genes involved in lipid storage (perilipin 2 [p < .05]) and metabolism (pyruvate dehydrogenase lipoamide kinase isozyme 4 [p < .01]) in a dose dependent manner. TESkM force production was reduced (tetanic and single twitch) (p < .05) and increases in transcription of type I slow twitch fiber isoform, myosin heavy chain 7, were observed when cultured with 200 µM OPLA compared to control (p < .01). Four days of OPLA exposure results in lipid accumulation in TESkM which in turn results in changes in muscle function and metabolism; thus, providing insight ito the functional and mechanistic changes of TESkM in response to exogenous FA.

Influenza virus infection affects insulin signaling, fatty acid-metabolizing enzyme expressions, and the tricarboxylic acid cycle in mice

Although the severity of influenza virus infections has been associated with host energy metabolism, the related mechanisms have not yet been clarified. Here we examined the effects of influenza virus infection on host energy metabolism in mice. After infecting mice with intranasal applications of 500 plaque-forming units of A/Puerto Rico/8/34 (H1N1; PR8) virus, the serum levels of most intermediates in the tricarboxylic acid (TCA) cycle and related metabolic pathways were significantly reduced. These data suggest that substrate supply to the TCA cycle is reduced under these conditions, rather than specific metabolic reactions being inhibited. Then, we focused on glucose and fatty acid metabolism that supply substrates to the TCA cycle. Akt phosphorylation following insulin injections was attenuated in the livers of PR8 virus-infected mice. Furthermore, glucose tolerance tests revealed that the PR8 virus-infected mice showed higher blood glucose levels than the vehicle-inoculated control mice. These results suggest that influenza virus infection impairs insulin signaling, which regulates glucose uptake. However, increases in the hepatic expressions of fatty acid-metabolizing enzymes suggest that fatty acids accumulate in liver cells of infected mice. Collectively, our data indicate that influenza virus infection dysregulates host energy metabolism. This line of investigation provides novel insights into the pathogenesis of influenza.

Thermogenic crosstalk occurs between adipocytes from different species

Visceral obesity increases risks for all-cause mortality worldwide. A small population of thermogenic adipocytes expressing uncoupling protein-1 (Ucp1) regulates energy dissipation in white adipose tissue (WAT) depots. Thermogenic adipocytes subsets decrease obesity in mice, but their efficacy has not been tested in obese large animals. Here we enclosed murine subcutaneous adipocytes with and without engineered thermogenic response in biocompatible microcapsules and implanted them into the left and right side of the visceral falciform depot in six obese dogs. After 28 days of treatment, dogs have markedly reduced waist circumference, body weight, and fat mass. Ucp1 expression in canine WAT was increased at sites implanted with thermogenic vs. wild type murine adipocytes. This site-specific thermogenic remodeling of canine tissue by thermogenic murine adipocytes suggests evolutionary conserved paracrine regulation of energy dissipation across species. These findings have translational potential aimed to reduce deleterious WAT depots in humans and pets.

C1q/TNF-related protein 2 (CTRP2) deletion promotes adipose tissue lipolysis and hepatic triglyceride secretion

The highly conserved C1q/TNF-related protein (CTRP) family of secreted hormones has emerged as important regulators of insulin action and of sugar and fat metabolisms. Among these, the specific biological function of CTRP2 remains elusive. Here, we show that the expression of human CTRP2 is positively correlated with body mass index (BMI) and is up-regulated in obesity. We used a knockout (KO) mouse model to determine CTRP2 function and found that Ctrp2-KO mice have significantly elevated metabolic rates and energy expenditure leading to lower body weights and lower adiposity. CTRP2 deficiency up-regulated the expression of lipolytic enzymes and protein kinase A signaling, resulting in enhanced adipose tissue lipolysis. In cultured adipocytes, CTRP2 treatment suppressed triglyceride (TG) hydrolysis, and its deficiency enhanced agonist-induced lipolysis in vivo CTRP2-deficient mice also had altered hepatic and plasma lipid profiles. Liver size and hepatic TG content were significantly reduced, but plasma TG was elevated in KO mice. Both plasma and hepatic cholesterol levels, however, were reduced in KO mice. Loss of CTRP2 also enhanced hepatic TG secretion and contributed to impaired plasma lipid clearance following an oral lipid gavage. Liver metabolomic analysis revealed significant changes in diacylglycerols and phospholipids, suggesting that increased membrane remodeling may underlie the altered hepatic TG secretion we observed. Our results provide the first in vivo evidence that CTRP2 regulates lipid metabolism in adipose tissue and liver.

PNPLA2 influences secretion of triglyceride-rich lipoproteins by human hepatoma cells

Patatin-like phospholipase domain-containing proteins (PNPLAs) are involved in triglyceride hydrolysis and lipid-droplet homeostasis in mice, but the physiological significance of the PNPLAs for triglyceride metabolism in human hepatocytes is unclear. Here, we investigate the roles of PNPLA2, PNPLA3, and PNPLA4 in triglyceride metabolism of human Huh7 and HepG2 hepatoma cells using gene-specific inhibition methods. siRNA inhibition of PNPLA3 or PNPLA4 is not associated with changes in triglyceride hydrolysis, secretion of triglyceride-rich lipoproteins (TRLs), or triglyceride accumulation. However, PNPLA2 siRNA inhibition, both in the absence and presence of oleate-containing medium, or treatment with the PNPLA2 inhibitor Atglistatin reduced intracellular triglyceride hydrolysis and decreased TRL secretion. In contrast, PNPLA2 inhibition showed no effects on lipid-droplet homeostasis, which is the primary physiological function of PNPLA2 in nonhepatic tissues. Moreover, confocal microscopy analysis found no clear evidence for the localization of PNPLA2 around lipid droplets. However, significant colocalization of PNPLA2 with the endoplasmic reticulum marker protein disulfide-isomerase was found in HepG2 and Huh7 cells with Rcoloc values of 0.61 ± 0.06 and 0.81 ± 0.05, respectively. In conclusion, PNPLA2 influences TRL secretion, but is not involved in lipid-droplet homeostasis in human hepatoma cells, a physiological role that is quite distinct from the metabolic function of PNPLA2 in nonhepatic tissues.

Arctigenin improves lipid metabolism by regulating AMP-activated protein kinase and downstream signaling pathways

Although it has been reported that arctigenin (ARG) can reduce the body weight and inhibit adipogenic differentiation by activating AMP-activated protein kinase (AMPK), the exact signals responsible for the ARG-mediated antiobesity mechanism through AMPK are not well understood. In this study, we investigated the potential improvement of AGR on lipid metabolism using a high-fat diet (HFD)-induced hyperlipidemia rats and 3T3-L1 mature adipocytes. The levels of AMPK and its downstream factors were examined by Western blot analysis and real-time fluorescent quantitative polymerase chain reaction. We observed that ARG lowered the HFD-induced body weight and the levels of serum lipid. Moreover, ARG clearly alleviated fat deposition in the liver and reduced epididymal fat accumulation. ARG also suppressed lipogenesis and lipolysis but promoted fatty acid β-oxidation in adipocytes. Most importantly, ARG increased the phosphorylation of AMPK and acetyl-CoA carboxylase (ACC) and upregulated the messenger RNA levels of downstream genes related to fatty acid β-oxidation, such as carnitine palmitoyltransferase 1 and acyl-CoA oxidase 1 but downregulated the expression of peroxisome proliferator-activated receptor γ (PPARγ), sterol regulatory element-binding transcription factor 1 (SREBP1c) and their targets, including lipogenesis-related genes such as CCAAT/enhancer-binding protein α, lipoprotein lipase, adipocyte protein 2, and fatty acid synthase (FAS), as well as lipolysis-related genes such as adipose triglyceride lipase and hormone-sensitive lipase. The activity of FAS was also decreased by ARG. We conclude that AMPK activation is important for the pharmacological effects of ARG. ARG may improve lipid metabolism by regulating the AMPK-ACC and AMPK-PPARγ/SREBP1c signaling pathways.

Sphk2-/- mice are protected from obesity and insulin resistance

Sphingosine kinases phosphorylate sphingosine to sphingosine 1‑phosphate (S1P), which functions as a signaling molecule. We have previously shown that sphingosine kinase 2 (Sphk2) is important for insulin secretion. To obtain a better understanding of the role of Sphk2 in glucose and lipid metabolism, we have characterized 20- and 52-week old Sphk2^-/- mice using glucose and insulin tolerance tests and by analyzing metabolic gene expression in adipose tissue. A detailed metabolic characterization of these mice revealed that aging Sphk2^-/- mice are protected from metabolic decline and obesity compared to WT mice. Specifically, we found that 52-week old male Sphk2^-/- mice had decreased weight and fat mass, and increased glucose tolerance and insulin sensitivity compared to control mice. Indirect calorimetry studies demonstrated an increased energy expenditure and food intake in 52-week old male Sphk2^-/- versus control mice. Furthermore, expression of adiponectin gene in adipose tissue was increased and the plasma levels of adiponectin elevated in aged Sphk2^-/- mice compared to WT. Analysis of lipid metabolic gene expression in adipose tissue showed increased expression of the Atgl gene, which was associated with increased Atgl protein levels. Atgl encodes for the adipocyte triglyceride lipase, which catalyzes the rate-limiting step of lipolysis. In summary, these data suggest that mice lacking the Sphk2 gene are protected from obesity and insulin resistance during aging. The beneficial metabolic effects observed in aged Sphk2^-/- mice may be in part due to enhanced lipolysis by Atgl and increased levels of adiponectin, which has lipid- and glucose-lowering effects.

Osteocalcin triggers Fas/FasL-mediated necroptosis in adipocytes via activation of p300

The uncarboxylated form of osteocalcin (GluOC) regulates glucose and lipid metabolism in mice. We previously showed that low-dose (≤10 ng/ml) GluOC induces the expression of adiponectin and peroxisome proliferator-activated receptor γ (PPARγ) via a cAMP-PKA-ERK-CREB signaling pathway in 3T3-L1 adipocytes. We also noticed that high-dose (≥20 ng/ml) GluOC inhibits the expression of adiponectin and PPARγ in these cells. We have here explored the mechanism underlying these effects of high-dose GluOC. High-dose GluOC triggered morphological changes in 3T3-L1 adipocytes suggestive of the induction of cell death. It activated the putative GluOC receptor GPRC6A and thereby induced the production of cAMP and activation of protein kinase A (PKA), similar to signaling by low-dose GluOC with the exception that the catalytic subunit of PKA also entered the nucleus. Cytosolic PKA induced phosphorylation of cAMP response element-binding protein (CREB) at serine-133 via extracellular signal-regulated kinase (ERK). Nuclear PKA appeared to mediate the inhibitory phosphorylation of salt-inducible kinase 2 (SIK2) at serine-358 and thereby to alleviate the inhibitory phosphorylation of the CREB co-activator p300 at serine-89. The activation of CREB and p300 resulted in increased expression of the transcription factor FoxO1 and consequent upregulation of Fas ligand (FasL) at the plasma membrane. The interaction of FasL with Fas on neighboring adipocytes triggered the phosphorylation at threonine-357/serine-358 and homotrimerization of mixed-lineage kinase domain-like protein (MLKL), a key regulator of necroptosis, as well as Ca²⁺ influx via transient receptor potential melastatin 7 (TRPM7), the generation of reactive oxygen species and lipid peroxides, and dephosphorylation of dynamin-related protein 1 (DRP1) at serine-637, resulting in mitochondrial fragmentation. Together, our results indicate that high-dose GluOC triggers necroptosis through upregulation of FasL at the plasma membrane in a manner dependent of activation of CREB-p300, followed by the activation of Fas signaling in neighboring adipocytes.

Decreased PEDF Expression Promotes Adipogenic Differentiation through the Up-Regulation of CD36

Adipogenesis is a tightly regulated cellular process that involves the action of multiple signaling pathways. Characterization of regulators that are associated with adipose development is crucial to understanding the mechanisms underlying obesity and other metabolic disorders. Pigment epithelium-derived factor (PEDF) is a secreted glycoprotein that was first described as a neurotrophic factor. The role of PEDF in lipid metabolism was established when adipose triglyceride lipase (ATGL), a major triglyceride hydrolase, was characterized as its binding partner. In this study, we investigated the downstream effects of PEDF on adipogenic differentiation using rat adipose-derived stem cells (AdSCs) and the mouse pre-adipocyte cell line 3T3-L1. Knocking down PEDF in differentiating cells resulted in elevated levels of ATGL and CD36, as well as other adipogenic markers, with a concomitant increase in adipocyte number. CD36, a scavenger receptor for a variety of ligands, regulated proliferation and lipogenic gene expression during adipogenesis. The CD36 increase due to PEDF down-regulation might be a result of elevated PPARγ. We further demonstrated that PEDF expression was regulated by dexamethasone, a synthetic glucocorticoid that is widely used for adipogenesis at the transcriptional level. Taken together, our findings highlight that PEDF negatively regulates adipogenesis through the regulation of various signaling intermediates, and it may play a crucial role in lipid metabolic disorders.

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.

•

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

•

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

•

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

Identification of Phf16 and Pnpla3 as new adipogenic factors regulated by phytochemicals

Adipocyte differentiation is controlled by multiple signaling pathways. To identify new adipogenic factors, C3H10T1/2 adipocytes were treated with previously known antiadipogenic phytochemicals (resveratrol, butein, sulfuretin, and fisetin) for 24 hours. Commonly regulated genes were then identified by transcriptional profiling analysis. Three genes (chemokine (C-X-C motif) ligand 1 [ Cxcl1], heme oxygenase 1 [ Hmox1], and PHD (plant homeo domain) finger protein 16 [ Phf16]) were upregulated while two genes (G0/G1 switch gene 2 [ G0s2] and patatin-like phospholipase domain containing 3 [ Pnpla3]) were downregulated by these four antiadipogenic compounds. Tissue expression profiles showed that the G0s2 and Pnpla3 expressions were highly specific to adipose depots while the other three induced genes were ubiquitously expressed with significantly higher expression in adipose tissues. While Cxcl1 expression was decreased, expressions of the other four genes were significantly increased during adipogenic differentiation of C3H10T1/2 cells. Small interfering RNA-mediated knockdown including Phf16 and Pnpla3 indicated that these genes might play regulatory roles in lipid accumulation and adipocyte differentiation. Specifically, the silencing of two newly identified adipogenic genes, Phf16 or Pnpla3, suppressed lipid accumulation and expression of adipocyte markers in both 3T3-L1 and C3H10T1/2 cells. Taken together, these data showed previously uncovered roles of Phf16 and Pnpla3 in adipogenesis, highlighting the potential of using phytochemicals for further investigation of adipocyte biology.

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.

Dietary energy level affects adipose depot mass but does not impair in vitro subcutaneous adipose tissue response to short-term insulin and tumor necrosis factor-α challenge in nonlactating, nonpregnant Holstein cows

We assessed effects of overfeeding energy to nonlactating and nonpregnant Holstein cows during a length of time similar to a typical dry period on body lipid storage and the abundance of genes related to insulin signaling, inflammation, and ubiquitination in subcutaneous adipose tissue (SAT) in vitro challenged with insulin and recombinant bovine tumor necrosis factor-α. Fourteen cows were randomly assigned to either a high-energy (OVE; net energy for lactation = 1.60 Mcal/kg of dry matter; n = 7) or control (CON; net energy for lactation = 1.30 Mcal/kg of dry matter; n = 7) diet for 6 wk. Immediately after slaughter, liver, kidneys, and mammary gland were separated and weighed. The adipose tissue mass in the omental, mesenteric, and perirenal depots was dissected and weighed. Subcutaneous adipose tissue was collected from the tail-head region and was used as follows: control, bovine insulin (INS) at 1 µmol/L, tumor necrosis factor-α at 5 ng/mL (TNF), and their combination. Despite a lack of difference in final body condition score, OVE cows had greater energy intake and were heavier than CON cows. Furthermore, overfeeding led to greater mass of mesenteric and perirenal adipose, liver, and mammary gland. Overall, SAT incubated with INS had an upregulation of insulin receptor (INSR), interleukin-10 (IL10), small ubiquitin-like modifier 3 (SUMO3), and ubiquitin conjugating enzyme E2I (UBC9), whereas TNF upregulated peroxisome proliferator-activated receptor gamma (PPARG), diacylglycerol O-acyltransferase 2 (DGAT2), interleukin-6 (IL6), nuclear factor kappa B subunit 1 (NFKB1), small ubiquitin-like modifier 2 (SUMO2), and UBC9. Regardless of in vitro treatment, feeding OVE upregulated PPARG, fatty acid synthase (FASN), and insulin induced gene 1 (INSIG1). Abundance of PPARG was greater in SAT of OVE cows cultured individually with INS and TNF. The interaction between diet and in vitro treatment revealed that sterol regulatory element binding transcription factor 1 (SREBF1) had greater abundance in SAT from the CON group in response to culture with INS, whereas SAT from OVE cows had greater SREBF1 abundance in response to culture with TNF. The mRNA abundance of IL6 and NFKB1 was greater in response to TNF treatment and overall in CON cows. Furthermore, SAT from these cows had greater IL10 abundance when cultured with INS and TNF. Overall, data highlighted that overfeeding energy increases adipose tissue mass in part by stimulating transcription of key genes associated with insulin signaling, adipogenesis, and lipogenesis. Because SAT thickness or mass was not measured, the lack of effect of overfeeding on body condition score limits its use to predict overall body lipid storage. An overt inflammatory response in SAT after a 6-wk period of over-consumption of energy could not be discerned.

Effects of fetal calf serum on cGMP pathway and oocyte lipid metabolism in vitro

Intracellular levels of cyclic nucleotides, such as cGMP, are involved in the regulation of adipocyte lipolysis. Cumulus-oocyte complexes (COCs) express enzymes that both synthesise (guanylate cyclase) and degrade (phosphodiesterase (PDE) 5A) cGMP. Because serum interferes with lipid metabolism, its effects on the cGMP pathway and lipid content in bovine COCs were examined. COCs were matured in medium containing fetal calf serum (FCS; 2% or 10%) or 0.4% bovine serum albumin (BSA; control). At both 2% and 10%, FCS decreased cGMP levels in COCs compared with BSA (0.64 and 1.04 vs 9.46 fmol per COC respectively; P<0.05) and decreased transcript levels of guanylate cyclase 1, soluble, beta 3 (GUCY1B3), whereas PDE5A levels were increased. FCS also affected the expression of genes related to lipolysis, increasing relative expression of perilipin 2 (PLIN2) and carnitine palmitoyltransferase 1B (CPT1B) in cumulus cells. Effects of FCS and cGMP on the lipid content of oocytes and embryos were evaluated by Nile red staining. COCs were matured with 10% FCS, FCS+10^-5 M sildenafil (SDF), a PDE5 inhibitor, or 0.4% BSA. The lipid content was increased in oocytes matured in FCS compared with BSA (fluorescence intensity 20.1 vs 17.61 respectively; P<0.05), whereas the lipid content in oocytes matured in FCS+SDF (fluorescence intensity 16.33) was similar to that in the BSA-treated group (P>0.05). In addition, lipid content was higher in embryos from oocytes matured with FCS than BSA (fluorescence intensity 31.12 vs 22.31 respectively; P<0.05), but was increased even further in the FCS+SDF-treated group (fluorescence intensity 40.35; P<0.05), possibly due to a compensatory mechanism during embryo culture without SDF for the reduction in lipid content during IVM. The present study provides, for the first time, evidence that the cGMP pathway may be involved in lipid metabolism in bovine COCs and that this pathway is affected by FCS.

PPARγ activation mitigates glucocorticoid receptor-induced excessive lipolysis in adipocytes via homeostatic crosstalk

Proper balance between lipolysis and lipogenesis in adipocytes determines the release of free fatty acids (FFA) and glycerol, which is crucial for whole body lipid homeostasis. Although, dysregulation of lipid homeostasis contributes to various metabolic complications such as insulin resistance, the regulatory mechanism remains elusive. This study clarified the individual and combined roles for glucocorticoid receptor (GCR) and peroxisome proliferator-activated receptor (PPAR)γ pathways in lipid metabolism of adipocytes. In mature 3T3-L1 adipocytes, GCR activation using dexamethasone upregulated adipose triglyceride lipase (ATGL) and downregulated phosphoenolpyruvate carboxykinase (PEPCK), resulting in enhanced glycerol release into the medium. In contrast, PPARγ ligand pioglitazone modestly upregulated ATGL and hormone sensitive lipase (HSL), but markedly enhanced PEPCK and glycerol kinase (GK), thereby suppressed glycerol release. Dexamethasone showed permissive like effect on PPARγ target genes including perilipin A and aP2, therefore co-administration of dexamethasone and pioglitazone demonstrated synergistic upregulation of these enzymes excepting PEPCK, of which downregulation by dexamethasone was abolished by pioglitazone to the level above control. Thus, the excessive glycerol release was prevented as the net outcome of the co-administration. Consistently, the bodipy stain demonstrated that dexamethasone reduced the amount of cytosolic lipid, which was preserved in co-treated adipocytes. Moreover, silencing of PPARγ suppressed the synergistic effects of co-treatment on the lipolytic and lipogenic genes, and therefore the GCR pathway indeed involves PPARγ. In conclusion, crosstalk between GCR and PPARγ is largely synergistic but counter-regulatory in lipogenic genes, of which enhancement prevents excessive glycerol and possibly FFA release by glucocorticoids into the circulation.

Mutation yellow in agouti loci prevents age-related increase of skeletal muscle genes regulating free fatty acids oxidation

The lethal yellow mutation in agouti loci (Ay mutation) reduces the activity of melanocortin (MC) receptors and causes hyperphagia, obesity and type two diabetes mellitus in aging mice (Ay mice). It is unknown if changes in distinct elements of the metabolic system such as white adipose tissue (WAT) and brown adipose tissue (BAT), and skeletal muscle will manifest before the development of obesity. The aim of this work was to measure the relative gene expression of key proteins that regulate carbohydrate-lipid metabolism in WAT, BAT and skeletal muscle in Ay mice before the development of obesity. C57Bl/6J mice bearing a dominant autosomal mutation Ay (Ay /a mice) and mice of the standard genotype (a/a mice, control) have been studied in three age groups: 10, 15 and 30 weeks. The relative mRNA level of genes was measured by real-time PCR in skeletal muscles (uncoupling protein 3 (Ucp3) and carnitine palmitoyl transferase 1b (Cpt1b) (free fatty acids oxidation), solute carrier family 2 (facilitated glucose transporter), member 4 (Slc2a4) (glucose uptake)), in WAT lipoprotein lipase (Lpl) (triglyceride deposition), hormone-sensitive lipase (Lipe) (lipid mobilization), and Slc2a4 (glucose uptake)), and in BAT: uncoupling protein 1 (Ucp1) (energy expenditure). The expression of Cpt1b was reduced in young Ay mice (10 weeks), there was no transient peak of transcription of Cpt1b, Ucp3 in skeletal muscle tissue and Lipe, Slc2a4 in WAT in early adult Ay mice (15 weeks), which was noted in а/а mice. Reduction of the transcriptional activity of the studied genes in skeletal muscle and white adipose tissue can initiate the development of melanocortin obesity in Ay mice.

Adipose Tissue Macrophages in Rheumatoid Arthritis: Prevalence, Disease-Related Indicators, and Associations With Cardiometabolic Risk Factors

OBJECTIVE

Adipose tissue macrophages (ATMs) are a potent source of inflammatory cytokines, with profound effects on adipose tissue function, yet their potential role in rheumatoid arthritis (RA) pathobiology is largely unstudied.

METHODS

Periumbilical subcutaneous adipose tissue was obtained from 36 RA patients and 22 non-RA controls frequency matched on demographics and body mass index. Samples were stained for the macrophage marker CD68, and the average proportions of ATMs, crown-like structures (periadipocyte aggregates of 3 or more ATMs), and fibrosis were compared between groups.

RESULTS

The adjusted proportion of ATMs among all nucleated cells was 76% higher in RA than in non-RA samples (37.7 versus 21.3%, respectively; P < 0.001), and the adjusted average number of crown-like structures was more than 1.5-fold higher in the RA group than in controls (0.58 versus 0.23 crown-like structure/high-power field, respectively; P = 0.001). ATMs were significantly more abundant in early RA and in those with anti-cyclic citrullinated peptide seropositivity. Users of methotrexate, leflunomide, and tumor necrosis factor inhibitors had a significantly lower proportion of ATMs compared with nonusers. Crown-like structures were significantly higher in patients with rheumatoid factor seropositivity and in those with C-reactive protein levels ≥10 mg/liter, and significantly lower among those treated with statins. Linear ATMs were significantly associated with whole-body insulin resistance, but not with serum lipids.

CONCLUSIONS

ATMs and crown-like structures were more abundant in RA patients and were associated with systemic inflammation, autoimmunity, and whole-body insulin resistance, suggesting possible contributions to the RA disease process. Lower levels of ATMs and crown-like structures associated with specific RA treatments suggest that adipose tissue inflammation may be ameliorated by immunomodulation.

Role of Peroxisome Proliferator-Activated Receptor (PPARγ) in Metabolic Disorders in SGA with Catch-Up Growth

OBJECTIVE

Abnormal fat metabolism is a major disorder in adults who were small for gestational age (SGA). Peroxisome prolferator-activated receptor (PPARγ) participates in adipocyte differentiation and the regulation of lipid metabolism. This study explored the role of PPARγ in the regulation of fat catch-up growth (CUG) and the lipid metabolism of SGA individuals.

METHODS

The CUG-SGA rats were treated with pioglitazone. The weight of the visceral adipose tissue, serum lipid levels, and PPARγ expression in the visceral adipose tissue were detected at 4, 8, and 12 weeks of age.

RESULTS

The PPARγ expression in the visceral adipose tissue in the CUG-SGA group was lower than that in the appropriate for gestational age (AGA) group at 4, 8, and 12 weeks (P < 0.05). The serum triglycerides in the CUG-SGA group were elevated compared with that in the AGA group at 4 and 12 weeks (P = 0.005; P = 0.037); however, they were significantly decreased after 8 weeks of pioglitazone intervention (P = 0.001).

CONCLUSIONS

PPARγ expression in the visceral adipose tissue was lower in SGA rats and may be related to the regulation of adipocyte differentiation. The early increased PPARγ expression by pioglitazone might reduce serum triglycerides and decrease the CUG of the visceral adipose tissue in SGA.

Comparative Assessment of Serum Adipokines Zinc-α2-glycoprotein and Adipose Triglyceride Lipase, and Cardiovascular Risk Factors Between Normal Weight and Obese Patients with Hemodialysis

BACKGROUND

Little is known about the potential relationship of obesity, adipose tissue and novel adipokines with cardiometabolic risk factors in end-stage renal disease. Zinc-α2-glycoprotein (ZAG) and adipose triglyceride lipase (ATGL) are novel adipokines with proposed desirable effects on inflammation, and lipid and glucose metabolism. The aim of this study was to investigate serum concentrations of ZAG and ATGL, and the relationship of these adipokines with cardiovascular risk factors in normal weight (NW) and obese (OB) patients undergoing hemodialysis.

METHODS

Patients with regular hemodialysis including 44 normal weight (18.5<BMI<25 kg/m²) and 44 obese (BMI≥30 kg/m²) were enrolled. Serum lipid profile, high-sensitivity C-reactive protein (hsCRP) and nitric oxide metabolites along with ZAG and ATGL concentrations were assessed.

RESULTS

ZAG concentrations were significantly lower in OB compared to NW group (100 ± 34 vs. 106 ± 31 ng/ml; p = 0.007). No significant difference was observed in ATGL between the two groups. A significant inverse correlation between ZAG and HDL (r = ‒0.236, p = 0.048) and a marginal inverse correlation between ATGL and HDL (r = ‒0.211, p = 0.078) were observed in all patients. ZAG had positive correlations with triglyceride/HDL (r = 0.279, p = 0.019), cholesterol/HDL (r = 0.319, p = 0.007), and LDL/HDL (r = 0.26, p = 0.029) ratios. Among cardiovascular risk factors, only LDL/HDL ratio and hsCRP were significantly higher in OB patients (p = 0.009 and p = 0.038, respectively).

CONCLUSIONS

Serum concentrations of ZAG, but not ATGL, were significantly lower in the OB group. It appears that obesity overrides the role of hemodialysis in determining ZAG concentration. In contrast, uremic condition might overshadow the role of obesity in determining levels of traditional cardiovascular risk factors.

Anti-obesity effect of a traditional Chinese dietary habit-blending lard with vegetable oil while cooking

Obesity, which is associated with dietary habits, has become a global social problem and causes many metabolic diseases. In China, both percentages of adult obesity and overweight are far lower compared to western countries. It was designed to increase the two levels of daily intake in human, namely 3.8% and 6.5%, which are recommendatory intake (25 g/d) and Chinese citizens’ practical intake (41.4 g/d), respectively. The mice were respectively fed with feeds added with soybean oil, lard or the oil blended by both for 12 weeks. In the mice fed with diet containing 3.8% of the three oils or 6.5% blended oil, their body weight, body fat rate, cross-sectional area of adipocytes, adipogenesis and lipogenesis in adipose were decreased, whereas hydrolysis of triglyserides in adipose was increased. This study demonstrated that the oil mixture containing lard and soybean oil had a remarkable anti-obesity effect. It suggests that the traditional Chinese dietary habits using oils blended with lard and soybean oil, might be one of the factors of lower percentages of overweight and obesity in China, and that the increasing of dietary oil intake and the changing of its component resulted in the increasing of obesity rate in China over the past decades.

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.

The relationship of serum adipokines with malnutrition inflammation score in haemodialysis

BACKGROUND

Protein-energy wasting is a prevalent disorder in haemodialysis. Zinc-α2-glycoprotein (ZAG) and adipose triglyceride lipase (ATGL) are novel adipokines with recognized lipolytic effects and proposed role in metabolic homoeostasis. This study was conducted to investigate the association of ZAG and ATGL concentrations with malnutrition-inflammation score (MIS) and metabolic profile of patients with haemodialysis.

MATERIALS AND METHODS

Eighty-eight patients under regular haemodialysis were divided based on MIS to normal to mild wasting (NMW; n = 35) or moderate wasting (MW; n = 53) group. Anthropometric measurements along with fasting serum concentrations of ZAG, ATGL, free fatty acids (FFAs), albumin, transferrin, total iron-binding capacity (TIBC), hs-CRP, lipid profile and glucose metabolism were assessed.

RESULTS

Adipose triglyceride lipase concentration was significantly higher in MW than NMW group (10·89 ± 5·7 vs. 8·02 ± 3·37 mIU/mL; P = 0·008). The ZAG and FFAs were not significantly different between two groups. ATGL was directly correlated with FFAs in all of the patients (r = 0·284, P = 0·007) and MW (r = 0·32, P = 0·021), and marginally in NMW (r = 0·31, P = 0·057) groups. ATGL and odds of having mild or moderate wasting were significantly correlated (OR = 1·21, P = 0·033). A positive association was observed between ATGL with TG (r = 0·31, P = 0·049) and also with transferrin and TIBC (r = 0·44, P = 0·001) only in MW group. An inverse relationship was observed between ATGL and HDL in all of the participants (r=-0·222, P = 0·04). No significant correlation was observed between ZAG and other parameters.

CONCLUSIONS

The serum concentrations of ATGL, but not ZAG, were significantly higher in MW compared to NMW group. Each unit increase in ATGL concentrations was correlated with 21% increase in the odds of wasting severity. ATGL might play a role in wasting pathogenesis and metabolic profile in haemodialysis.

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.

Mild mitochondrial uncoupling induces HSL/ATGL-independent lipolysis relying on a form of autophagy in 3T3-L1 adipocytes

Obesity is characterized by an excessive triacylglycerol accumulation in white adipocytes. Various mechanisms allowing the tight regulation of triacylglycerol storage and mobilization by lipid droplet-associated proteins as well as lipolytic enzymes have been identified. Increasing energy expenditure by inducing a mild uncoupling of mitochondria in adipocytes might represent a putative interesting anti-obesity strategy as it reduces the adipose tissue triacylglycerol content (limiting alterations caused by cell hypertrophy) by stimulating lipolysis through yet unknown mechanisms, limiting the adverse effects of adipocyte hypertrophy. Herein, the molecular mechanisms involved in lipolysis induced by a mild uncoupling of mitochondria in white 3T3-L1 adipocytes were characterized. Mitochondrial uncoupling-induced lipolysis was found to be independent from canonical pathways that involve lipolytic enzymes such as HSL and ATGL. Finally, enhanced lipolysis in response to mitochondrial uncoupling relies on a form of autophagy as lipid droplets are captured by endolysosomal vesicles. This new mechanism of triacylglycerol breakdown in adipocytes exposed to mild uncoupling provides new insights on the biology of adipocytes dealing with mitochondria forced to dissipate energy.

Insulin, not glutamine dipeptide, reduces lipases expression and prevents fat wasting and weight loss in Walker 256 tumor-bearing rats

Cachexia is the main cause of mortality in advanced cancer patients. We investigated the effects of insulin (INS) and glutamine dipeptide (GDP), isolated or associated, on cachexia and metabolic changes induced by Walker 256 tumor in rats. INS (NPH, 40 UI/kg, sc) or GDP (1.5g/kg, oral gavage) was once-daily administered during 11 days after tumor cell inoculation. GDP, INS or INS+GDP treatments did not influence the tumor growth. However, INS and INS+GDP prevented retroperitoneal fat wasting and body weight loss of tumor-bearing rats. In consistency, INS and INS+GDP prevented the increased expression of triacylglycerol lipase (ATGL) and hormone sensitive lipase (HSL), without changing the expression of tumor necrosis factor α (TNF-α) and interleukin-6 (IL-6) in the retroperitoneal adipose tissue of tumor-bearing rats. INS and INS+GDP also prevented anorexia and hyperlactatemia of tumor-bearing rats. However, INS and INS+GDP accentuated the loss of muscle mass (gastrocnemius, soleus and long digital extensor) without affecting the myostatin expression in the gastrocnemius muscle and blood corticosterone. GDP treatment did not promote beneficial effects. It can be concluded that treatment with INS (INS or INS+GDP), not with GDP, prevented fat wasting and weight loss in tumor-bearing rats without reducing tumor growth. These effects might be attributed to the reduction of lipases expression (ATGL and LHS) and increased food intake. The results show the physiological function of INS in the suppression of lipolysis induced by cachexia mediators in tumor-bearing rats.

Adipocyte STAT5 deficiency promotes adiposity and impairs lipid mobilisation in mice

AIMS/HYPOTHESIS

Dysfunction of lipid metabolism in white adipose tissue can substantially interfere with health and quality of life, for example in obesity and associated metabolic diseases. Therefore, it is important to characterise pathways that regulate lipid handling in adipocytes and determine how they affect metabolic homeostasis. Components of the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway are involved in adipocyte physiology and pathophysiology. However, the exact physiological importance of the STAT family member STAT5 in white adipose tissue is yet to be determined. Here, we aimed to delineate adipocyte STAT5 functions in the context of lipid metabolism in white adipose tissue.

METHODS

We generated an adipocyte specific knockout of Stat5 in mice using the Adipoq-Cre recombinase transgene followed by in vivo and in vitro biochemical and molecular studies.

RESULTS

Adipocyte-specific deletion of Stat5 resulted in increased adiposity, while insulin resistance and gluconeogenic capacity was decreased, indicating that glucose metabolism can be improved by interfering with adipose STAT5 function. Basal lipolysis and fasting-induced lipid mobilisation were diminished upon STAT5 deficiency, which coincided with reduced levels of the rate-limiting lipase of triacylglycerol hydrolysis, adipose triglyceride lipase (ATGL, encoded by Pnpla2) and its coactivator comparative gene identification 58 (CGI-58). In a mechanistic analysis, we identified a functional STAT5 response element within the Pnpla2 promoter, indicating that Pnpla2 is transcriptionally regulated by STAT5.

CONCLUSIONS/INTERPRETATION

Our findings reveal an essential role for STAT5 in maintaining lipid homeostasis in white adipose tissue and provide a rationale for future studies into the potential of STAT5 manipulation to improve outcomes in metabolic diseases.

Adipose Snail1 Regulates Lipolysis and Lipid Partitioning by Suppressing Adipose Triacylglycerol Lipase Expression

Lipolysis provides metabolic fuel; however, aberrant adipose lipolysis results in ectopic lipid accumulation and lipotoxicity. While adipose triacylglycerol lipase (ATGL) catalyzes the first step of lipolysis, its regulation is not fully understood. Here, we demonstrate that adipocyte Snail1 suppresses both ATGL expression and lipolysis. Adipose Snail1 levels are higher in fed mice than in fasted mice and higher in obese mice as opposed to lean mice. Insulin increases Snail1 levels in both murine and human adipocytes, wherein Snail1 binds to the ATGL promoter to repress its expression. Importantly, adipocyte-specific deletion of Snail1 increases adipose ATGL expression and lipolysis, resulting in decreased fat mass and increased liver fat content in mice fed either a normal chow diet or a high-fat diet. Thus, we have identified a Snail1-ATGL axis that regulates adipose lipolysis and fatty acid release, thereby governing lipid partitioning between adipose and non-adipose tissues.

LncRNA SRA promotes hepatic steatosis through repressing the expression of adipose triglyceride lipase (ATGL)

Nonalcoholic fatty liver disease (NAFLD), the most common form of chronic liver disease, manifests as an over-accumulation of hepatic fat. We have recently shown that mice with genetic knockout of a long non-coding RNA (lncRNA) steroid receptor RNA activator (SRA) (SRAKO) are resistant to high fat diet-induced obesity with a phenotype that includes improved glucose tolerance and attenuated hepatic steatosis. The underlying mechanism was investigated in the present study. We found that hepatic levels of SRA and adipose triglyceride lipase (ATGL), a major hepatic triacylglycerol (TAG) hydrolase, were inversely regulated by fasting in mice, and the expression of liver ATGL was induced by SRAKO under normal and high fat diet (HFD) feeding. Loss of SRA in primary hepatocytes or a hepatocyte cell line upregulates, but forced expression of SRA inhibits ATGL expression and free fatty acids (FFA) β-oxidation. SRA inhibits ATGL promoter activity, primarily by inhibiting the otherwise-inductive effects of the transcription factor, forkhead box protein O1 (FoxO1). Our data reveal a novel function of SRA in promoting hepatic steatosis through repression of ATGL expression.