Adipose tissue sensitization to insulin induced by troglitazone and MEDICA 16 in obese Zucker rats in vivo

Metabolic reprogramming by Acly inhibition using SB-204990 alters glucoregulation and modulates molecular mechanisms associated with aging

ATP-citrate lyase is a central integrator of cellular metabolism in the interface of protein, carbohydrate, and lipid metabolism. The physiological consequences as well as the molecular mechanisms orchestrating the response to long-term pharmacologically induced Acly inhibition are unknown. We report here that the Acly inhibitor SB-204990 improves metabolic health and physical strength in wild-type mice when fed with a high-fat diet, while in mice fed with healthy diet results in metabolic imbalance and moderated insulin resistance. By applying a multiomic approach using untargeted metabolomics, transcriptomics, and proteomics, we determined that, in vivo, SB-204990 plays a role in the regulation of molecular mechanisms associated with aging, such as energy metabolism, mitochondrial function, mTOR signaling, and folate cycle, while global alterations on histone acetylation are absent. Our findings indicate a mechanism for regulating molecular pathways of aging that prevents the development of metabolic abnormalities associated with unhealthy dieting. This strategy might be explored for devising therapeutic approaches to prevent metabolic diseases.

Regulation of Insulin Receptor Pathway and Glucose Metabolism by CD36 Signaling

During reduced energy intake, skeletal muscle maintains homeostasis by rapidly suppressing insulin-stimulated glucose utilization. Loss of this adaptation is observed with deficiency of the fatty acid transporter CD36. A similar loss is also characteristic of the insulin-resistant state where CD36 is dysfunctional. To elucidate what links CD36 to muscle glucose utilization, we examined whether CD36 signaling might influence insulin action. First, we show that CD36 deletion specific to skeletal muscle reduces expression of insulin signaling and glucose metabolism genes. It decreases muscle ceramides but impairs glucose disposal during a meal. Second, depletion of CD36 suppresses insulin signaling in primary-derived human myotubes, and the mechanism is shown to involve functional CD36 interaction with the insulin receptor (IR). CD36 promotes tyrosine phosphorylation of IR by the Fyn kinase and enhances IR recruitment of P85 and downstream signaling. Third, pretreatment for 15 min with saturated fatty acids suppresses CD36-Fyn enhancement of IR phosphorylation, whereas unsaturated fatty acids are neutral or stimulatory. These findings define mechanisms important for muscle glucose metabolism and optimal insulin responsiveness. Potential human relevance is suggested by genome-wide analysis and RNA sequencing data that associate genetically determined low muscle CD36 expression to incidence of type 2 diabetes.

Depot-Specific Changes in Fat Metabolism with Aging in a Type 2 Diabetic Animal Model

Visceral fat accretion is a hallmark of aging and is associated with aging-induced metabolic dysfunction. PPARγ agonist was reported to improve insulin sensitivity by redistributing fat from visceral fat to subcutaneous fat. The purpose of this study was to investigate the underlying mechanisms by which aging affects adipose tissue remodeling in a type 2 diabetic animal model and through which PPARγ activation modulates aging-related fat tissue distribution. At the ages of 21, 31 and 43 weeks, OLETF rats as an animal model of type 2 diabetes were evaluated for aging-related effects on adipose tissue metabolism in subcutaneous and visceral fat depots. During aging, the ratio of visceral fat weight to subcutaneous fat weight (V/S ratio) increased. Aging significantly increased the mRNA expression of genes involved in lipogenesis such as lipoprotein lipase, fatty acid binding protein aP2, lipin 1, and diacylglycerol acyltransferase 1, which were more prominent in visceral fat than subcutaneous fat. The mRNA expression of adipose triglyceride lipase, which is involved in basal lipolysis and fatty acid recycling, was also increased, more in visceral fat compared to subcutaneous fat during aging. The mRNA levels of the genes associated with lipid oxidation were increased, whereas the mRNA levels of genes associated with energy expenditure showed no significant change during aging. PPARγ agonist treatment in OLETF rats resulted in fat redistribution with a decreasing V/S ratio and improved glucose intolerance. The genes involved in lipogenesis decreased in visceral fat of the PPARγ agonist-treated rats. During aging, fat distribution was changed by stimulating lipid uptake and esterification in visceral fat rather than subcutaneous fat, and by altering the lipid oxidation.

Targeting nuclear receptors with marine natural products

Nuclear receptors (NRs) are important pharmaceutical targets because they are key regulators of many metabolic and inflammatory diseases, including diabetes, dyslipidemia, cirrhosis, and fibrosis. As ligands play a pivotal role in modulating nuclear receptor activity, the discovery of novel ligands for nuclear receptors represents an interesting and promising therapeutic approach. The search for novel NR agonists and antagonists with enhanced selectivities prompted the exploration of the extraordinary chemical diversity associated with natural products. Recent studies involving nuclear receptors have disclosed a number of natural products as nuclear receptor ligands, serving to re-emphasize the translational possibilities of natural products in drug discovery. In this review, the natural ligands of nuclear receptors will be described with an emphasis on their mechanisms of action and their therapeutic potentials, as well as on strategies to determine potential marine natural products as nuclear receptor modulators.

Activin receptor-like kinase 7 suppresses lipolysis to accumulate fat in obesity through downregulation of peroxisome proliferator-activated receptor γ and C/EBPα

We previously identified a quantitative trait locus for adiposity, non-insulin-dependent diabetes 5 (Nidd5), on mouse chromosome 2. In the current study, we identified the actual genetic alteration at Nidd5 as a nonsense mutation of the Acvr1c gene encoding activin receptor-like kinase 7 (ALK7), one of the type I transforming growth factor-β receptors, which results in a COOH-terminal deletion of the kinase domain. We further showed that the ALK7 dysfunction causes increased lipolysis in adipocytes and leads to decreased fat accumulation. Conversely, ALK7 activation inhibits lipolysis by suppressing the expression of adipose lipases. ALK7 and activated Smads repress those lipases by downregulating peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT/enhancer binding protein (C/EBP) α. Although PPARγ and C/EBPα act as adipogenic transcription factors during adipocyte differentiation, they are lipolytic in sum in differentiated adipocytes and are downregulated by ALK7 in obesity to accumulate fat. Under the obese state, ALK7 deficiency improves glucose tolerance and insulin sensitivity by preferentially increasing fat combustion in mice. These findings have uncovered a net lipolytic function of PPARγ and C/EBPα in differentiated adipocytes and point to the ALK7-signaling pathway that is activated in obesity as a potential target of medical intervention.

Long-term pioglitazone treatment enhances lipolysis in rat adipose tissue

OBJECTIVES

The insulin-sensitizing effects of thiazolidinediones are believed to depend at least in part on reductions in circulating levels of nonesterified fatty acids (NEFA). The mechanisms that mediate the reductions in NEFA are not fully understood and could involve reductions in adipose tissue lipolysis, increases in glyceroneogenesis and NEFA reesterification in triglycerides in adipose tissue and increases in NEFA metabolism by oxidative tissues.

METHODS

In a congenic strain of spontaneously hypertensive rats that fed a high-sucrose diet to promote features of the metabolic syndrome, we studied the effects of chronic pioglitazone treatment over 4 months on adipose tissue lipolysis and NEFA metabolism.

RESULTS

We observed significant increases in basal and adrenaline-stimulated NEFA and glycerol release, and near-total suppression of NEFA reesterification in epididymal adipose tissue isolated from rats chronically treated with pioglitazone. However, pioglitazone-treated rats also exhibited significant increases in mitochondrial DNA levels in adipose tissue (3.2-fold increase, P=0.001) and potentially greater sensitivity to the antilipolytic effects of insulin than untreated controls. In addition, chronic pioglitazone treatment was associated with increased palmitate oxidation in soleus muscle, reduced fasting levels of serum NEFA and triglycerides, as well as reduced serum levels of insulin and increased serum levels of adiponectin.

CONCLUSIONS

Despite suppressing NEFA reesterification and increasing basal and adrenaline-stimulated lipolysis, chronic pioglitazone treatment may decrease circulating NEFA levels in part by increasing adipose tissue sensitivity to the antilipolytic effects of insulin and by enhancing NEFA oxidation in skeletal muscle.

PPARgamma regulates adipose triglyceride lipase in adipocytes in vitro and in vivo

Peroxisome proliferator-activated receptor-gamma (PPARgamma) regulates adipocyte genes involved in adipogenesis and lipid metabolism and is the molecular target for thiazolidinedione (TZD) antidiabetic agents. Adipose triglyceride lipase (ATGL) is a recently described triglyceride-specific lipase that is induced during adipogenesis and remains highly expressed in mature adipocytes. This study evaluates the ability of PPARgamma to directly regulate ATGL expression in adipocytes in vitro and in vivo. In fully differentiated 3T3-L1 adipocytes, ATGL mRNA and protein are increased by TZD and non-TZD PPARgamma agonists in a dose- and time-dependent manner. Rosiglitazone-mediated induction of ATGL mRNA is rapid and is not inhibited by the protein synthesis inhibitor cycloheximide, indicating that intervening protein synthesis is not required for this effect. Rosiglitazone-mediated induction of ATGL mRNA and protein is inhibited by the PPARgamma-specific antagonist GW-9662 and is also significantly reduced following siRNA-mediated knockdown of PPARgamma, supporting the direct transcriptional regulation of ATGL by PPARgamma. In vivo, ATGL mRNA and protein are increased by rosiglitazone treatment in white and brown adipose tissue of mice with and without obesity due to high-fat diet or leptin deficiency. Thus, PPARgamma positively regulates ATGL mRNA and protein expression in mature adipocytes in vitro and in adipose tissue in vivo, suggesting a role for ATGL in mediating PPARgamma's effects on lipid metabolism.

Mechanisms of the depot specificity of peroxisome proliferator-activated receptor gamma action on adipose tissue metabolism

In this study, we aimed to establish the mechanisms whereby peroxisome proliferator-activated receptor gamma (PPARgamma) agonism brings about redistribution of fat toward subcutaneous depots and away from visceral fat. In rats treated with the full PPARgamma agonist COOH (30 mg x kg(-1) x day(-1)) for 3 weeks, subcutaneous fat mass was doubled and that of visceral fat was reduced by 30% relative to untreated rats. Uptake of triglyceride-derived nonesterified fatty acids was greatly increased in subcutaneous fat (14-fold) and less so in visceral fat (4-fold), with a concomitant increase, restricted to subcutaneous fat only, in mRNA levels of the uptake-, retention-, and esterification-promoting enzymes lipoprotein lipase, aP2, and diacylglycerol acyltransferase 1. Basal lipolysis and fatty acid recycling were stimulated by COOH in both subcutaneous fat and visceral fat, with no frank quantitative depot specificity. The agonist increased mRNA levels of enzymes of fatty acid oxidation and thermogenesis much more strongly in visceral fat than in subcutaneous fat, concomitantly with a stronger elevation in O2 consumption in the former than in the latter. Mitochondrial biogenesis was stimulated equally in both depots. These findings demonstrate that PPARgamma agonism redistributes fat by stimulating the lipid uptake and esterification potential in subcutaneous fat, which more than compensates for increased O2 consumption; conversely, lipid uptake is minimally altered and energy expenditure is greatly increased in visceral fat, with consequent reduction in fat accumulation.

PPARgamma agonism increases rat adipose tissue lipolysis, expression of glyceride lipases, and the response of lipolysis to hormonal control

AIMS/HYPOTHESIS

The aim of this study was to investigate the effect and mechanisms of action of in vivo peroxisome proliferator-activated receptor gamma (PPARgamma) activation on white adipose tissue (WAT) lipolysis and NEFA metabolism.

MATERIALS AND METHODS

Study rats were treated for 7 days with 15 mg/kg of rosiglitazone per day; control rats were not treated. After a 6-h fast, lipolysis and levels of mRNA for lipases were assessed in explants from various adipose depots.

RESULTS

Rosiglitazone markedly increased basal and noradrenaline (norepinephrine)-stimulated glycerol and NEFA release from WAT explants, and amplified their inhibition by insulin. Primary adipocytes isolated from PPARgamma agonist-treated rats were also more responsive to noradrenaline stimulation expressed per cell, ruling out a contribution of an altered number of mature adipocytes in explants. Rosiglitazone concomitantly increased levels of mRNA transcripts for adipose triglyceride lipase (ATGL) and monoglyceride lipase (MGL) in subcutaneous and visceral WAT, and mRNA for hormone-sensitive lipase (HSL) in subcutaneous WAT. Lipase expression increased within 12 h of in vitro exposure of naïve explants to rosiglitazone, suggesting direct transcriptional activation. In parallel, chronic in vivo treatment with rosiglitazone lowered plasma NEFAs and in WAT its expected stimulatory action on glycerol and NEFA recycling, and on the expression of genes involved in NEFA uptake and retention by WAT, such processes counteracting net NEFA export.

CONCLUSIONS/INTERPRETATION

These findings demonstrate that, in the face of its plasma NEFA-lowering action, PPARgamma agonism stimulates WAT lipolysis, an effect that is compensated by lipid-retaining pathways. The results further suggest that PPARgamma agonism stimulates lipolysis by increasing the lipolytic potential, including the expression levels of the genes encoding adipose triglyceride lipase and monoglyceride lipase.

Transcriptional suppression of human microsomal triglyceride transfer protein by hypolipidemic insulin sensitizers

Microsomal triglyceride transfer protein (MTP) catalyzes the assembly and secretion of liver triglyceride-rich lipoproteins. The human MTP (hMTP) promoter activity is reported here to be suppressed by HNF-4alpha ligand antagonists (e.g., Medica analogs) or by PPARgamma ligand agonists (e.g., thiazolidinediones), thus accounting for their hypolipidemic activity in humans. Suppression of liver hMTP by Medica analogs or by thiazolidinediones was mediated by the TAAA sequence that serves as non-canonical TATA box of the hMTP core promoter. MTP suppression was evident in the specific context of the wild type hMTP core promoter, but not in the context of the mutated rodent-conforming hMTP core promoter governed by a canonical TATA box conjoined with its proximal (-50/-38)DR-1 element. hMTP suppression by Medica analogs or thiazolidinediones mediated by hMTP TAAA was independent of HNF-4alpha or PPARgamma. hMTP suppression by Medica analogs, but not by thiazolidinediones, was further complemented by inhibition of HNF-4alpha transcriptional activity transduced by the distal (-83/-70)DR-1 element of hMTP promoter. hMTP promoter activity was unaffected by PPARalpha activation. Furthermore, in contrast to hMTP, the promoter activity of the rodent-conforming hMTP was robustly activated by Wy-14,643-activated PPARalpha or by thiazolidinedione-activated PPARgamma. Transcriptional activation by PPARalpha or PPARgamma of the rodent-conforming, but not the wild type hMTP gene promoter, resulted from the species-specific context of the respective proximal DR-1 elements. Hence, suppression of hMTP transcription by hypolipidemic insulin sensitizers requires the specific context of hMTP core promoter. In light of the species-specific context of MTP core promoters, the rodent MTP promoter may not substitute for the human promoter when searching for hypolipidemic MTP suppressors.

Role of PP2C in cardiac lipid accumulation in obese rodents and its prevention by troglitazone

In obese rodents, excess myocardial lipid accumulation (lipotoxicity) of myocardium may cause cardiomyopathy that in the obese Zucker diabetic fatty (ZDF) fa/fa rat can be prevented by treatment with troglitazone (TGZ). To determine the underlying mechanisms, we measured total 5'-AMP-activated kinase (AMPK) protein and its activated, phosphorylated form, P-AMPK. P-AMPK was significantly reduced in both ZDF fa/fa rat and ob/ob mouse hearts compared with lean, wild-type controls. TGZ treatment of obese ZDF rats, which lowered cardiac lipid content, increased P-AMPK. Expression of protein phosphatase 2C (PP2C), which inactivates AMPK activity by dephosphorylation, was increased in untreated ZDF fa/fa rat hearts, but fell with TGZ treatment, suggesting that PP2C can influence AMPK activity. In cultured myocardiocytes, fatty acids reduced P-AMPK, suggesting a feed-forward effect of lipid overload. Our findings highlight a role of PP2C and AMPK in the derangements of cardiac lipid metabolism in obesity and provide new insights as to the mechanisms of the liporegulatory disorder leading to lipotoxic cardiomyopathy.

Transcriptional regulation of human microsomal triglyceride transfer protein by hepatocyte nuclear factor-4alpha

Microsomal triglyceride transfer protein (MTP) catalyzes the assembly of triglyceride (TG)-rich apolipoprotein B-containing liver (e.g., VLDL) and intestinal (e.g., chylomicron) lipoproteins. The human MTP gene promoter is reported here to associate in vivo with endogenous hepatocyte nuclear factor-4alpha (HNF-4alpha) and to be transactivated or transsuppressed by overexpressed or by dominant negative HNF-4alpha, respectively. Human MTP (hMTP) transactivation by HNF-4alpha is accounted for by the concerted activity of distal (-83/-70) and proximal (-50/-38) direct repeat 1 elements of the hMTP promoter that bind HNF-4alpha. Transactivation by HNF-4alpha is specifically antagonized by chicken ovalbumin upstream promoter. Transcriptional activation of hMTP by HNF-4alpha is mediated by HNF-4alpha domains engaged in ligand binding and ligand-driven transactivation and is further complemented by HNF-4alpha/HNF-1alpha synergism that involves the HNF-4alpha activation function 1 (AF-1) domain. hMTP transactivation by HNF-4alpha is specifically inhibited by beta,beta-tetramethyl-hexadecanedioic acid acting as an HNF-4alpha antagonist ligand. hMTP transactivation by HNF-4alpha may account for the activation or inhibition of MTP expression and the production of TG-rich lipoproteins by agonist (e.g., saturated fatty acids) or antagonist [e.g., (n-3) PUFA, hypolipidemic fibrates, or Methyl-substituted dicarboxylic acid (Medica) compounds] HNF-4alpha ligands.