过氧化物酶体增殖物激活受体β亚型激动剂GW501516对胰岛素抵抗模型小鼠的胰岛素增敏作用(英文)

目的在长期(4个月)高脂高糖饮食诱导的胰岛素抵抗小鼠模型上,评价过氧化物酶体增殖物激活受体β(PPARβ)亚型激动剂GW501516对胰岛素抵抗的改善作用,并对可能的相关机制进行探讨。方法C57BL/6J小鼠采用高脂高糖饮食(35%脂肪,30%麦芽糖)诱导4个月,待产生明显的糖脂代谢紊乱。实验分为正常对照、饮食导致的肥胖(DIO)模型与DIO模型+GW501516(10mg·kg-1·d-1)给药组。隔天监测体重与进食量情况,以葡萄糖氧化酶法检测血糖,并进行口服葡萄糖耐量试验和血脂(甘油三酯、总胆固醇和高密度脂蛋白)含量的检测。以组织学方法检测肝脏异位脂积聚及病理变化情况。为确证其相关作用机制,采用RT-PCR方法检测骨骼肌内PPARβ下游糖脂代谢靶基因的表达。结果GW501516有效改善模型小鼠的胰岛素抵抗,显著降低口服糖耐量曲线下面积〔DIO模型组,(32.4±4.6)mmol·h·L-1, DIO +GW501516组,(23.4±2.5)mmol·h·L-1,n=7 ~8, P<0.05〕,降低空腹血糖,增加血清高密度脂蛋白含量,减轻模型小鼠的肝脂肪变性。此外,RT-PCR结果表明,骨骼肌卡尼汀(肉碱)软脂酰转移酶1b,解偶联蛋白(UCP)2,UCP3明显上调,同时葡萄糖转运蛋白也明显上调。结论GW501516显著改善模型小鼠的胰岛素抵抗,恢复其空腹血糖值,降低肝脏内异位脂积聚,其治疗作用机制可能与①促进骨骼肌内脂肪酸氧化和能量的解偶联,②促进骨骼肌内的糖摄取有关,提示PPARβ可能是胰岛素抵抗及代谢综合征的有效治疗靶标。     

Peroxisome proliferator-activated receptor delta (PPARdelta), a novel target site for drug discovery in metabolic syndrome.

The development of new treatments for metabolic syndrome is urgent project for decreasing the prevalence of coronary heart disease and diabetes mellitus in the advanced countries. Peroxisome proliferator-activated receptor (PPAR)alpha and gamma agonists have shed light on the treatment of hypertriglyceridemia and type 2 diabetes mellitus, respectively. Among PPARs, analysis of the PPARdelta functions is lagging behind because specific PPARdelta agonists have not been developed. The appearance of new PPARdelta agonists is brightening the prospects for elucidating the physiological role of PPARdelta. PPARdelta is a new target for the treatment of metabolic syndrome. In particular, the fact that fatty acid oxidation and energy dissipation in skeletal muscle and adipose tissue by PPARdelta agonists lead to improved lipid profile, reduced adiposity and insulin sensitivity is a breakthrough. It seems that treatment of PPARdelta agonists operate similarly to the caloric restriction and prolonged exercise. We suggest that the physiological role of PPARdelta may be an indicator for switching from glucose metabolism to fatty acid metabolism. To receive new benefits of PPARdelta agonists against metabolic syndrome by increasing fatty acid consumption in skeletal muscle and adipose tissue, we need to unveil more details on the functions of PPARdelta itself and its agonists in the future.     

Cardiovascular disease and PPARdelta: targeting the risk factors

Metabolism, in part, is regulated by the peroxisome proliferator-activated receptors (PPARs). The PPARs act as nutritional lipid sensors and three mammalian PPAR subtypes designated PPARalpha (NR1C1), PPARgamma (NR1C3) and PPARdelta (NR1C2) have been identified. This subgroup of nuclear hormone receptors binds DNA and controls gene expression at the nexus of pathways that regulate lipid and glucose homeostasis, energy storage and expenditure in an organ-specific manner. Recent evidence has demonstrated activation of PPARdelta in the major mass peripheral tissue (ie, adipose and skeletal muscle). It enhances glucose tolerance, insulin-stimulated glucose disposal, lipid catabolism, energy expenditure, cholesterol efflux and oxygen consumption. These effects positively influence the blood-lipid profile. Furthermore, PPARdelta activation produces a predominant type I/slow twitch/oxidative muscle fiber phenotype that leads to increased endurance, insulin sensitivity and resistance to obesity. PPARdelta has rapidly emerged as a potential target in the battle against dyslipidemia, insulin insensitivity, type II diabetes and obesity, with therapeutic efficacy in the treatment of cardiovascular disease risk factors. GW-501516 is currently undergoing phase II safety and efficacy trials in human volunteers for the treatment of dyslipidemia. The outcome of these clinical trials are eagerly awaited against a background of conflicting reports about cancer risks in genetically predisposed animal models. This review focuses on the potential pharmacological utility of selective PPARdelta agonists in the context of risk factors associated with metabolic and cardiovascular disease.     

Effects of bezafibrate, PPAR pan-agonist, and GW501516, PPARdelta agonist, on development of steatohepatitis in mice fed a methionine- and choline-deficient diet

We evaluated the effects of bezafibrate, a peroxisome proliferator-activated receptor (PPAR) pan-agonist, and GW501516, a PPARdelta agonist, on mice fed a methionine- and choline-deficient (MCD) diet, a model of non-alcholic steatohepatitis (NASH), to investigate (a) the efficacy of bezafibrate against non-alcholic steatohepatitis and (b) the relation between non-alcholic steatohepatitis and the functional role of PPARdelta. Bezafibrate (50 or 100 mg/kg/day) and GW501516 (10 mg/kg/day) were administered by gavage once a day for 5 weeks. Hepatic lipid contents, plasma triglyceride, high density lipoprotein (HDL)-cholesterol and alanine aminotransferase (ALT) concentrations were evaluated, as were histopathological changes in the liver and hepatic mRNA expression levels. Bezafibrate and GW501516 inhibited the MCD-diet-induced elevations of hepatic triglyceride and thiobarbituric acid-reactants contents and the histopathological increases in fatty droplets within hepatocytes, liver inflammation and number of activated hepatic stellate cells. In this model, bezafibrate and GW501516 increased the levels of hepatic mRNAs associated with fatty acid beta-oxidation [acyl-CoA oxidase (ACO), carnitine palmitoyltransferase-1 (CPT-1), liver-fatty acid binding protein (L-FABP) and peroxisomal ketothiolase], and reduced the levels of those associated with inflammatory cytokines or chemokine [transforming growth factor (TGF)-beta1, interleukin (IL)-6, IL-1beta, monocyte chemoattractant protein (MCP)-1, tumor necrosis factor (TNF) alpha and nuclear factor (NF)-kappaB1]. In addition, bezafibrate characteristically reduced the elevation in the level of plasma ALT, but enhanced that in plasma adiponectin and increased the mRNA expression levels of its receptors (adiponectin receptors 1 and 2). These results suggest that (a) bezafibrate (especially) and GW501516 might improve hepatic steatosis via an improvement in fatty acid beta-oxidation and a direct prevention of inflammation, (b) treatment with a PPARdelta agonist might improve non-alcholic steatohepatitis, (c) bezafibrate may improve non-alcholic steatohepatitis via activation not only of PPARalpha but also of PPARdelta, because bezafibrate is a PPAR pan-agonist.     

PPARδ agonists have opposing effects on insulin resistance in high fat-fed rats and mice due to different metabolic responses in muscle

BACKGROUND AND PURPOSE

The peroxisome proliferator-activated receptor (PPAR)δ has been considered a therapeutic target for diabetes and obesity through enhancement of fatty acid oxidation. The present study aimed to characterize the effects of PPARδ agonists during insulin resistance of the whole body, muscle and liver.

EXPERIMENTAL APPROACH

Wistar rats and C57BL/J6 mice were fed a high fat diet (HF) and then treated with PPARδ agonists NNC61-5920 and GW501516. The effects on insulin resistance were evaluated by hyperinsulinaemic clamp or glucose tolerance tests combined with glucose tracers.

KEY RESULTS

In HF rats, 3 weeks of treatment with NNC61-5920 reduced the glucose infusion rate (by 14%, P < 0.05) and glucose disposal into muscle (by 20–30%, P < 0.01) during hyperinsulinaemic clamp. Despite increased mRNA expression of carnitine palmitoyltransferase-1, pyruvate dehydrogenase kinase 4 and uncoupling protein 3 in muscle, plasma and muscle triglyceride levels were raised (P < 0.01). Similar metabolic effects were observed after extended treatment with NNC61-5920 and GW501516 to 6 weeks. However, HF mice treated with NNC61-5920 improved their plasma lipid profile, glucose tolerance and insulin action in muscle. In both HF rats and mice, NNC61-5920 treatment attenuated hepatic insulin resistance and decreased expression of stearoyl-CoA desaturase 1, fatty acid translocase protein CD36 and lipoprotein lipase in liver.

CONCLUSIONS AND IMPLICATIONS

PPARδ agonists exacerbated insulin resistance in HF rats in contrast to their beneficial effects on metabolic syndrome in HF mice. These opposing metabolic consequences result from their different effects on lipid metabolism and insulin sensitivity in skeletal muscle of these two species.     

Atorvastatin improves insulin sensitivity in mice with obesity induced by monosodium glutamate

Aim:

To examine the mechanisms underlying the effects of atorvastatin on glucose and lipid metabolism.

Methods:

Mice with insulin resistance and obesity induced by monosodium glutamate (MSG) were used. Atorvastatin (80 mg·kg⁻¹·d⁻¹) or vehicle control treatment was given orally once a day for 30 days. Plasma levels of total cholesterol, triglycerides, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and free fatty acids were monitored. Serum insulin and glucose concentrations were used to calculate the insulin resistance index and insulin sensitivity index using a homeostasis model. Body length, waistline circumference, intraperitoneal adipose tissue mass, and total body mass were measured. Semi-quantitative RT-PCR and Western analysis were used to determine the expression of inflammatory factors and proteins involved in inflammation signaling pathways.

Results:

Atorvastatin improved insulin sensitivity, ameliorated glucose tolerance, and decreased plasma levels of total cholesterol, triglycerides, LDL-C, HDL-C and free fatty acids. Semi-quantitative RT-PCR and Western analysis revealed increased expression of interleukin 6 (IL-6) and tumor necrosis factor α (TNF-α) in serum and adipose tissue in MSG obese mice. Atorvastatin treatment decreased expression of IL-6, TNF-α, nuclear factor κB (NF-κB) and I-kappa-B (IκB) kinase-β, but increased the expression of IκB, in adipose tissue.

Conclusion:

Atorvastatin is a potential candidate for the prevention and therapy of diseases associated with insulin resistance such as type 2 diabetes mellitus and cardiovascular disease. One possible mechanism underlying the effects of atorvastatin on glucose and lipid metabolism may be to ameliorate a state of chronic inflammation.     

非酒精性脂肪肝与代谢综合征的关系

目的探讨非酒精性脂肪肝(NAFL)脂代谢紊乱、胰岛素抵抗的情况及其与代谢综合征的关系.方法对284例NAFL患者测体重指数、血压、空腹血糖、胰岛素、肝功、血脂和血尿酸.以96例非脂肪肝的体检者为对照组.两组间年龄和性别构成差异无显著性.结果NAFL组体重指数、血压、空腹血糖、甘油三醋、总胆固醇、谷丙转氨酶和血尿酸水平显著高于对照组(P＜0.05);两组间总胆固醇和总胆红素差异无显著性(P＞0.05).NAFL组空腹血胰岛素、胰岛素抵抗指数(HOMA-IR)水平明显升高(P＜0.01).NAFL组合并肥胖、高血压、高甘油三酯血症和(或)低高密度脂蛋白胆固醇血症、高血糖和代谢综合征显著高于对照组(70.77%vs 20.83%、54.93% vs 29.17%、62.32%vs 21.88%、48.59% vs 15.63%、39.08% vs 8.33%).两组中119例有代谢综合征,其中有NAFL占93.28%,肥胖占88.23%,高血压占87.40%,血脂紊乱占92.44%,高血糖占65.55%.结论NAFL患者存在明显的代谢综合征各组分集聚的特征,代谢综合征患病率明显升高.因此NAFL是代谢综合征的一个危险因素.     

Effects of CB1 receptor blockade on monosodium glutamate induced hypometabolic and hypothalamic obesity in rats

Effects of cannabinoid receptor 1 (CB1R) blockade were observed by comparing 9-day and 6-week SR141716 treatments in monosodium glutamate (MSG)-induced hypometabolic and hypothalamic obesity (HO) in rats for the first time and molecular mechanisms were investigated. Compared with normal rats, the MSG rats display typical symptoms of the metabolic syndrome, i.e., excessive abdominal obesity, hypertriglyceridemia, hyperinsulinemia, insulin resistance, and hepatic steatosis, but with lower food intake. Although both the 9-day and 6-week treatments with the specific CB1R antagonist SR141716 effectively lowered body weight, intraperitoneal adipose tissue mass, serum triglyceride (TG), and insulin level, the effect of chronic treatment is more impressive. Moreover, serum cholesterol, free fatty acids (FFA), fasted and postprandial blood glucose, and insulin insensitivity were more effectively improved by 6-week exposure to SR141716, whereas hypophagia was only effective within the initial 2 weeks. In addition, hepatic steatosis as well as hepatic and adipocyte morphology was improved. Western blot analysis revealed that the markedly increased CB1R expression and decreased insulin receptor (INR) expression in liver and adipose tissues were effectively corrected by SR141716. Consistent with this, deregulated gene expression of lipogenesis and lipolysis as well as glucose metabolic key enzymes were also restored by SR141716. In conclusion, based on present data we found that: (1) alteration of the hypothalamus in MSG rats leads to a lower expression of INR in crucially insulin-targeted tissues and hyperinsulinemia that was reversed by SR141716, (2) the abnormally increased expression of CB1R in liver and adipose tissues plays a vital role in the pathophysiological process of MSG rats, and (3) chronic CB1R blockade leads to a sustained improvement of the metabolic dysfunctions of MSG rats.     

Effect of GCP-02, a PPARalpha/gamma dual activator, on glucose and lipid metabolism in insulin-resistant mice

This paper reports on the effect of GCP-02, a dual activator of the peroxisome proliferator-activated receptors alpha/gamma (PPARalpha/gamma), on glucose and lipid metabolism in insulin-resistant obese mice induced by monosodium glutamate. The mice were divided into four groups on the basis of treatment: control group, rosiglitazone (positive control) (7 micromol/kg), and low- and high-dosage GCP-02 (7 micromol/kg and 3.5 micromol/kg, respectively). Drugs were given orally once a day for 19 days, and mice underwent testing for insulin tolerance, oral glucose tolerance and gluconeogenesis, and plasma cholesterol, triglyceride and free fatty acid levels. Mice were sacrificed, and body length and weight were measured; intraperitoneal adipose, heart and liver weighed; and plasma alanine aminotransferase (ALT) level and aspartate aminotransferase (AST) activity measured. Liver, soleus muscle and myocardium were assayed for glycogen, triglyceride and free fatty acid content and myocardia tested for superoxide dismutase (SOD) activity and malonaldehyde content. RT-PCR revealed expression of insulin receptor substrate 1 and 2 (IRS1, IRS2) and related genes in liver. GCP-02 had a more powerful effect than rosiglitazone on improving insulin sensitivity, ameliorating glucose tolerance, suppressing L-alanine-induced gluconeogenesis, and decreasing plasma levels of cholesterol, triglyceride and free fatty acid. It reduced body weight in control mice, significantly lowered hepatic content of glycogen, triglyceride and free fatty acid and myocardial content of triglyceride, and increased myocardial SOD activity. IRS2 mRNA was down-regulated in control mice but up-regulated by GCP-02. Thus, GCP-02 is a potential candidate for the prevention and therapy of diseases associated with insulin resistance such as type 2 diabetes mellitus and cardiovascular disease.     

Insulin resistance in obesity as the underlying cause for the metabolic syndrome

The metabolic syndrome affects more than a third of the US population, predisposing to the development of type 2 diabetes and cardiovascular disease. The 2009 consensus statement from the International Diabetes Federation, American Heart Association, World Heart Federation, International Atherosclerosis Society, International Association for the Study of Obesity, and the National Heart, Lung, and Blood Institute defines the metabolic syndrome as 3 of the following elements: abdominal obesity, elevated blood pressure, elevated triglycerides, low high-density lipoprotein cholesterol, and hyperglycemia. Many factors contribute to this syndrome, including decreased physical activity, genetic predisposition, chronic inflammation, free fatty acids, and mitochondrial dysfunction. Insulin resistance appears to be the common link between these elements, obesity and the metabolic syndrome. In normal circumstances, insulin stimulates glucose uptake into skeletal muscle, inhibits hepatic gluconeogenesis, and decreases adipose-tissue lipolysis and hepatic production of very-low-density lipoproteins. Insulin signaling in the brain decreases appetite and prevents glucose production by the liver through neuronal signals from the hypothalamus. Insulin resistance, in contrast, leads to the release of free fatty acids from adipose tissue, increased hepatic production of very-low-density lipoproteins and decreased high-density lipoproteins. Increased production of free fatty acids, inflammatory cytokines, and adipokines and mitochondrial dysfunction contribute to impaired insulin signaling, decreased skeletal muscle glucose uptake, increased hepatic gluconeogenesis, and β cell dysfunction, leading to hyperglycemia. In addition, insulin resistance leads to the development of hypertension by impairing vasodilation induced by nitric oxide. In this review, we discuss normal insulin signaling and the mechanisms by which insulin resistance contributes to the development of the metabolic syndrome.     

PPARγ activation redirects macrophage cholesterol from fecal excretion to adipose tissue uptake in mice via SR-BI

PPARγ agonists, used in the treatment of Type 2 diabetes, can raise HDL-cholesterol, therefore could potentially stimulate macrophage-to-feces reverse cholesterol transport (RCT). We aimed to test whether PPARγ activation promotes macrophage RCT in vivo. Macrophage RCT was assessed in mice using cholesterol loaded/³H-cholesterol labeled macrophages. PPARγ agonist GW7845 (20 mg/kg/day) did not change ³H-tracer plasma appearance, but surprisingly decreased fecal ³H-free sterol excretion by 43% (P < 0.01) over 48 h. Total free cholesterol efflux from macrophages to serum (collected from control and GW7845 groups) was not different, although ABCA1-mediated efflux was significantly higher with GW7845. To determine the effect of PPARγ activation on HDL cholesterol uptake by different tissues, the metabolic fate of HDL labeled with ³H-cholesteryl ether (CE) was also measured. We observed two-fold increase in HDL derived ³H-CE uptake by adipose tissue (P < 0.005) with concomitant 22% decrease in HDL derived ³H-CE uptake by the liver (P < 0.05) in GW7845 treated wild type mice. This was associated with a significant increase in SR-BI protein expression in adipose tissue, but not liver. The same experiment in SR-BI knockout mice, showed no difference in HDL derived ³H-CE uptake by adipose tissue or liver. In conclusion, PPARγ activation decreases the fecal excretion of macrophage derived cholesterol in mice. This is not due to inhibition of cholesterol efflux from macrophages, but rather involves redirection of effluxed cholesterol from liver towards adipose tissue uptake via SR-BI. This represents a novel mechanism for regulation of RCT and may extend the therapeutic implications of these ligands.     

PPARδ激动剂GW501516调节小鼠骨骼肌线粒体生物合成和纤维类型转换的作用

目的观察和比较PPARδ激动剂GW501516补充和(或)耐力训练对骨骼肌内源性PPARδ及PGC-1α表达、骨骼肌线粒体生物合成和骨骼肌肌纤维类型的影响。方法♂C57BL/6小鼠随机分为5组:正常对照组(C)、耐力训练组(T)、低剂量GW501516组(LG,3mg·kg-1·d-1,po)、低剂量激动剂干预+训练组(TG)、高剂量GW501516组(HG,10mg·kg-1·d-1,po)。15周实验期结束后,取腓肠肌以Western blot法测定骨骼肌组织PPARδ、PGC-1α、COXⅣ和MHCⅠ、MHCⅡ以及MHCⅡa蛋白表达;骨骼肌ATP酶染色区分纤维类型以及透射电镜观察。结果①与C、T组比较,电镜下可见激动剂干预各组(LG、TG)线粒体数目明显增多,排列整齐;②T、LG和TG组腓肠肌PPARδ、PGC-1α和COXⅣ蛋白表达量均较C组提高,且激动剂和训练在3种指标变化中均有交互作用;③小鼠腓肠肌ATP酶染色发现,与C组相比,T组的Ⅰ型肌纤维比例无变化;LG、TG组的Ⅰ型肌纤维增加,尤其以TG组明显;④T、LG、TG组的腓肠肌MHCⅡa蛋白表达量较C组提高,LG、TG组腓肠肌MHCⅡ蛋白表达量较C组降低的同时表现出MHCI蛋白的明显增加。激动剂和耐力训练在小鼠腓肠肌MHCs蛋白含量变化中有交互作用。结论①耐力训练与补充GW501516均可诱导骨骼肌线粒体生物合成;②GW501516补充可促进骨骼肌纤维类型由Ⅱ型向Ⅰ型的转变,单纯运动训练未发生肌纤维类型的转变;③GW501516补充与训练结合,在上调骨骼肌线粒体生物合成及肌纤维类型转变的过程中有叠加作用。     

Effects of the K+ channel opener KRN4884 on the cardiovascular metabolic syndrome model in rats

We examined the effects of the potassium channel opener KRN4884 (5-amino-N-[2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine ) on cardiovascular metabolic syndrome (i.e., syndrome X), in rats. High-fructose diet rats developed hypertension, hypertriglyceridemia, increased total cholesterol/HDL (high-density lipoprotein)-cholesterol ratio, and hyperinsulinemia, KRN4884 (0.3-3.0 mg/kg, twice a day for 14 days, p.o.) alleviated the risk factors in fructose-fed rats. Furthermore, fructose-fed rats exhibited impairment of glucose tolerance and excess insulin secretion when loaded with glucose orally. Treatment with KRN4884 (1.0 mg/kg, twice a day for 14 days, p.o.) improved the glucose intolerance and inhibited hypersecretion of insulin in the glucose-loaded, fructose-fed rats. In contrast, KRN4884 (0.3-1.0 mg/kg, twice a day for 10 days, p.o.) did not affect serum triglyceride, cholesterol, glucose, or insulin concentrations in normal rats. LPL (lipoprotein lipase) activities in skeletal muscle and adipose tissue, and HTGL (hepatic triglyceride lipase) activity in liver were measured after administration of KRN4884 or vehicle twice a day for 14 days in fructose-fed rats. KRN4884 caused a significant increase in LPL activity in muscle and tended to increase LPL activity in adipose tissue in fructose-fed rats. HTGL was decreased in fructose-fed rats as compared with normal controls and was unaffected by KRN4884. These findings suggested that KRN4884 enhances insulin sensitivity and LPL activity, which are related to glucose and lipid metabolism and may be useful for the treatment of syndrome X.     

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands inhibit growth of UACC903 and MCF7 human cancer cell lines

The development of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) ligands for the treatment of diseases including metabolic syndrome, diabetes and obesity has been hampered due to contradictory findings on their potential safety. For example, while some reports show that ligand activation of PPARbeta/delta promotes the induction of terminal differentiation and inhibition of cell growth, other reports suggest that PPARbeta/delta ligands potentiate tumorigenesis by increasing cell proliferation. Some of the contradictory findings could be due in part to differences in the ligand examined, the presence or absence of serum in cell cultures, differences in cell lines or differences in the method used to quantify cell growth. For these reasons, this study examined the effect of ligand activation of PPARbeta/delta on cell growth of two human cancer cell lines, MCF7 (breast cancer) and UACC903 (melanoma) in the presence or absence of serum using two highly specific PPARbeta/delta ligands, GW0742 or GW501516. Culturing cells in the presence of either GW0742 or GW501516 caused upregulation of the known PPARbeta/delta target gene angiopoietin-like protein 4 (ANGPTL4). Inhibition of cell growth was observed in both cell lines cultured in the presence of either GW0742 or GW501516, and the presence or absence of serum had little influence on this inhibition. Results from the present studies demonstrate that ligand activation of PPARbeta/delta inhibits the growth of both MCF7 and UACC903 cell lines and provide further evidence that PPARbeta/delta ligands are not mitogenic in human cancer cell lines.     

The mechanisms by which PPARgamma and adiponectin regulate glucose and lipid metabolism

Obesity, a state of increased adipose tissue mass, is a major cause for type 2 diabetes, hyperlipidemia, and hypertension, resulting in clustering of risk factors for atherosclerosis. Heterozygous PPARgamma knockout mice and KKA(y) mice administered with a PPARgamma antagonist were protected from high-fat diet-induced adipocyte hypertrophy and insulin resistance. Moderate reduction of PPARgamma activity prevented adipocyte hypertrophy, thereby diminution of TNFalpha, resistin, and FFA and upregulation of adiponectin and leptin. These alterations led to reduction of tissue TG content in muscle/liver, thereby ameliorating insulin resistance. Insulin resistance in the lipoatrophic mice and KKA(y) mice were ameliorated by replenishment of adiponectin. Moreover, adiponectin transgenic mice ameliorated insulin resistance and diabetes, but not the obesity of ob/ob mice. Furthermore, targeted disruption of the adiponectin gene caused moderate insulin resistance and glucose intolerance. In muscle, adiponectin activated AMP kinase and PPARgamma pathways, thereby increasing beta-oxidation of lipids, leading to decreased TG content, which ameliorated muscle insulin resistance. In the liver, adiponectin also activated AMPK, thereby downregulating PEPCK and G6Pase, leading to decreased glucose output from the liver. In conclusion, PPARgamma plays a central role in the regulation of adipocyte hypertrophy and insulin sensitivity. The upregulation of the adiponectin pathway by PPARgamma may play a role in the increased insulin sensitivity of heterozygous PPARgamma knockout mice, and activation of adiponectin pathway may provide novel therapeutic strategies for obesity-linked disorders such as type 2 diabetes and metabolic syndrome.     

AMP activated protein kinase: a next generation target for total metabolic control

Metabolic syndrome is characterized by a cluster of metabolic disorders, such as reduced glucose tolerance, hyperinsulinemia, hypertension, visceral obesity and lipid disorders. The benefit of exercise in maintaining total metabolic control is well known and recent research indicates that AMP-activated protein kinase (AMPK) may play an important role in exercise-related effects. AMPK is considered as a master switch in regulating glucose and lipid metabolism. AMPK is an enzyme that works as a fuel gauge, being activated in conditions of high phosphate depletion. In the liver, activation of AMPK results in decreased production of plasma glucose, cholesterol, triglyceride and enhanced fatty acid oxidation. AMPK is also robustly activated by skeletal muscle contraction and myocardial ischemia, and is involved in the stimulation of glucose transport and fatty acid oxidation by these stimuli. In adipose tissue, activated AMPK inhibits deposition of fat, but enhances breakdown and burning of stored fat, resulting in reduction of body weight. The two leading diabetic drugs, namely metformin and rosiglitazone, and adipokines, such as adiponectin and leptin, show their metabolic effects partially through AMPK. These data suggest that AMPK may be a key player in the development of new treatments for obesity, Type 2 diabetes and the metabolic syndrome. In this review, the author provide insight into the role of AMPK as a probable target for treatment of metabolic syndrome.     

Insulin therapy stimulates lipid synthesis and improves endocrine functions of adipocytes in dietary obese C57BL/6 mice

Aim:

To evaluate whether insulin intervention could affect the metabolic and endocrine functions of adipose tissue.

Methods:

C57BL/6 mice were fed on a high-fat-diet for 12−16 weeks to induce insulin resistance. Insulin intervention was administered in the high-fat-diet mice for 4 weeks at 12 weeks (early insulin treatment) or 16 weeks (late insulin treatment). Intraperitoneal glucose tolerance tests were performed before and after insulin treatment. Expression levels of factors involved in the triglyceride synthesis and endocrine functions of adipose tissue including phosphoenolpyruvate carboxykinase (PEPCK-C), fatty acid synthase (FAS), aquaporin 7 (AQP7), adiponectin, visfatin, and interleukin-6 (IL-6) were determined by Western blot.

Results:

In the obese mice, glucose tolerance was impaired; triglyceride content was increased in the liver tissue; protein expression of FAS and adiponectin was decreased; expression of visfatin was increased in adipose tissue. After 4-week insulin treatment, glucose tolerance was improved; triglyceride content was decreased in the liver and skeletal muscle; expression of PEPCK-C, FAS, and adiponectin was increased in the adipose tissue; IL-6 and AQP7 expression was reduced in the fat. Early insulin treatment had better effect in increasing the expression of FAS and PEPCK-C and decreasing the expression of IL-6.

Conclusion:

These results indicate that insulin can target adipocytes for improvement of insulin sensitivity through stimulating triglyceride synthesis and partly improving endocrine functions.