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.     

Dual roles of adiponectin/Acrp30 in vivo as an anti-diabetic and anti-atherogenic adipokine

Genome-wide scanning is a powerful tool to identify susceptible chromosome loci, however, individual chromosomal regions still have many candidate genes. Although cDNA microarray analysis provides valuable information for identifying genes involved in pathogenesis, expression levels of many genes are changed. A novel approach for identification of therapeutic targets is the combination of genome-wide scanning and the use of DNA chips, as shown in Fig. (1). Using DNA chips, we screened for secreted molecules, the expressions of which were changed in adipose tissues from mice rendered insulin resistance. Decreased expression of one of these molecules, adiponectin/Acrp30, correlates strongly with insulin resistance. Interestingly, recent genome-wide scans have mapped a susceptibility locus for type 2 diabetes and metabolic syndrome to chromosome 3q27, where adiponectin gene is located. Decreasing serum adiponectin levels are associated with increased risk for type 2 diabetes. Interestingly, adiponectin was decreased in insulin resistant rodent models both of obesity and lipoatrophy, and replenishment of adiponectin ameliorated their insulin resistance. Moreover, adiponectin transgenic mice ameliorated insulin resistance and diabetes Adiponectin knockout mice showed insulin resistance and glucose intolerance. In muscle and liver, adiponectin activated AMP kinase and PPARalpha pathways thereby increasing beta-oxidation of lipids, leading to decreased TG content, which ameliorated insulin resistance under a high-fat diet. Despite similar plasma glucose and lipid levels on an apoE deficient background, adiponectin transgenic apoE deficient mice showed amelioration of atherosclerosis, which was associated with decreased expressions of class A scavenger receptor and tumor necrosis factor alpha. Finally, cDNA encoding adiponectin receptors (AdipoR1 and R2) have been identified by expression cloning, which facilitates the understanding of molecular mechanisms of adiponectin actions and obesity-linked diseases such as diabetes and atherosclerosis and the designing of novel antidiabetic and anti-atherogenic drugs with AdipoR1 and R2 as molecular targets.     

Effect of DHEA on endocrine functions of adipose tissue, the involvement of PPAR gamma

Dehydroepiandrosterone (DHEA), an adrenal steroid, is known to decrease body fat. Thus, it may also alter the endocrine functions of adipose tissue. The aim of this study was to determine if administration of DHEA might influence adiponectin gene expression and secretion from adipose tissue. We demonstrate here the inducing effect of exogenously administered DHEA on adiponectin gene expression in epididymal WAT and adiponectin levels in serum of rats fed a DHEA-containing diet (0.6%, w/w) for 2 weeks, accompanied by a reduction in epididymal adipose tissue mass. A corresponding increase in peroxisome proliferator-activated receptor gamma (PPAR(gamma)) mRNA expression suggests that PPAR(gamma) may be involved in the up-regulation of adiponectin gene expression after DHEA treatment. The presented observations indicate that the positive effects of DHEA, which seems to play a protective role against insulin resistance and atherosclerosis, may be in fact indirect and due to up-regulation of adiponectin gene expression and stimulation of adiponectin secretion from adipose tissue.     

Effects of PPAR gamma agonists on cardiovascular function in obese, non-diabetic patients

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors, and when activated by their ligands, they induce perixosome proliferation. Three receptors have been identified: PPAR gamma, PPAR delta, and PPAR alpha, all with different tissue expression. PPAR gamma is predominantly expressed in adipose tissue and regulates the formation of fat cells and their function. The effect of PPAR gamma activation is to enhance the action of insulin in insulin-sensitive tissue by increasing peripheral glucose disposal and decreasing hepatic glucose production. The thiazolidinediones (TZDs) are a class of medications used for treatment and possibly the prevention of type 2 diabetes, which are potent agonists for the PPAR gamma receptor. Because the thiazolidinediones target insulin resistance, these agents may improve many of the risk factors associated with obesity and insulin resistance including dyslipidemia, hypertension, impaired fibrinolysis, and atherosclerosis. The impact of the thiazolidinediones on cardiovascular mortality is currently unclear but it appears that the thiazolidinediones exert numerous non-glycemic effects that may improve cardiovascular outcomes. Several non-TZD PPAR gamma agonists and combined PPAR gamma/alpha effect on cardiovascular disease are also being evaluated. These drugs have anti-inflammatory and vascular properties and are currently the subject of numerous studies targeting the primary and secondary prevention of macrovascular disease in patients with diabetes and insulin resistance and might be developed as anti-atherogenic agents on the basis of their actions.     

PPAR- γ agonist in treatment of diabetes: cardiovascular safety considerations

The peroxisome proliferator-activated receptors (PPARs) are nuclear fatty acid receptors, which contain a type II zinc finger DNA binding motif and a hydrophobic ligand binding pocket. These receptors are thought to play an important role in metabolic diseases such as obesity, insulin resistance, and coronary artery disease. Three subtypes of PPAR receptors have been described: PPARα, PPARδ/β, and PPARγ. PPARα is found in the liver, muscle, kidney, and heart. In the liver, its role is to up-regulate genes involved in fatty acid uptake, binding, β-oxidation and electron transport, and oxidative phosphorylation in subcutaneous fat but not in skeletal muscle. PPARδ/β is expressed in many tissues but markedly in brain, adipose tissue, and skin. PPARγ has high expression in fat, low expression in the liver, and very low expression in the muscle. The thiazolidinediones (TZD) are synthetic ligands of PPARγ. By activating a number of genes in tissues, PPARγ increases glucose and lipid uptake, increases glucose oxidation, decreases free fatty acid concentration, and decreases insulin resistance. Although, there is a rationale for the use of TZDs in patients with type 2 diabetes mellitus, clinical studies have produced conflicting data. While currently used TZDs are clearly associated with heart failure (HF) worsening; with regards to cardiovascular outcomes, pioglitazone seems to be related to a trend toward reduction in cardiovascular morbidity and mortality, whereas rosiglitazone may actually increase risk of cardiovascular events. We review the existing literature on TZDs and discuss role and cardiovascular safety of these agents for the contemporary treatment of diabetes. Other side effects of these agents i.e. increase in osteoporosis and possible risk of bladder cancer is also discussed.     

2型糖尿病合并高血压患者血清脂联素水平的变化及其意义

目的探讨2型糖尿病合并高血压患者血清脂联素水平的变化及其意义。方法随机选取2型糖尿病患者120例,肥胖者68例与非肥胖者52例,2型糖尿病患者合并高血压120例,肥胖者62例与非肥胖者58例,正常对照组50名。分别检测以上三组患者的空腹脂联素、瘦素、肿瘤坏死因子(TNF)-α、胰岛素、空腹和馒头餐后2h血糖、糖化血红蛋白(HbA1c)、体重指数(BMI)、血压、总胆固醇、甘油三酯(TG),计算胰岛素敏感指数(ISI),比较脂联素水平变化,并与其他因素进行相关分析及逐步回归分析。结果2型糖尿病合并高血压组和单纯2型糖尿病组血清脂联素水平低于正常对照组(P<0.01),2型糖尿病合并高血压组血清脂联素水平低于单纯2型糖尿病组(P<0.01)。多元线性相关分析显示,2型糖尿病合并高血压组中,脂联素与BMI、收缩压、空腹血糖、胰岛素、甘油三酯、瘦素、TNF-α之间存在显著负相关,与ISI呈独立正相关,进入回归方程的因素为脂联素、瘦素、TNF-α、BMI、TG、收缩压;2型糖尿病非高血压组中,脂联素与空腹血糖、胰岛素、甘油三酯、瘦素、TNF-α之间呈显著负相关,与ISI呈独立正相关,进入回归方程的因素为脂联素、瘦素、TNF-α、TG。结论在2型糖尿病患者中,血清脂联素水平明显降低,合并高血压者血清脂联素水平更低,脂联素水平与高血压之间存在一定的相关性。     

西格列汀对肥胖大鼠代谢指标及脂肪因子chemerin脂联素水平的影响

目的 研究西格列汀对肥胖大鼠代谢指标及脂肪因子chemerin、脂联素(ADPN)水平的影响及其可能机制,并探讨chemerin及ADPN与肥胖的关系。方法 将30只大鼠随机分为正常对照组(NC组)、肥胖对照组(HF组)和肥胖西格列汀干预组(SP组),ELISA法分别测定干预前后各组大鼠生化指标,Western blot 法检测大鼠脂肪、肝脏、肌肉及肾脏组织chemerin及ADPN蛋白表达水平。结果 与NC组相比,HF组大鼠血清chemerin、体质量(BW),空腹血糖(FBG)、胰岛素(FINS)、总胆固醇(TG)、甘油三酯(TC)、低密度脂蛋白胆固醇(LDL-C),胰岛素抵抗指数(HOMO-IR)增高,血清ADPN及高密度脂蛋白胆固醇(HDL-C)降低(P<0.05),HF组大鼠脂肪、肝脏、肌肉及肾脏组织的chemerin蛋白表达量较NC组对应组织的表达量增加(P<0.05),脂肪、肝脏及肌肉组织的ADPN表达量减少(P<0.05)。与HF组相比,SP组大鼠FBG、FINS、TC、TG、LDL-C、HOMO-IR、血清chemerin水平降低, HDL-C及血清ADPN水平升高(P<0.05),脂肪、肝脏、肾脏及肌肉组织的chemerin蛋白表达量减少,脂肪、肌肉组织的ADPN表达增加(P<0.05)。干预前chemerin、ADPN与BW、FBG、TG、HDL-C、LDL-C、FINS、HOMO-IR有相关性,干预后chemerin、ADPN与BW、LDL-C、FINS、HOMO-IR有相关性,ADPN与HDL-C呈正相关。结论 chemerin及ADPN参与糖脂代谢过程,西格列汀可改善血清学各代谢指标,并通过影响chemerin及ADPN来改善肥胖大鼠胰岛素敏感性。     

Challenge of creating single-agents for the treatment of type 1 and 2 diabetes by targeting retinoid X receptor

It might be seen as reckless to challenge to create single-agents for the treatment of both type 1 diabetes caused by the destruction of the Langerhans β cells in pancreas by excessive autoimmunity, and type 2 diabetes caused by the obesity. However, we hypothesized that retinoid X receptor (RXR) agonists, which are researched for the treatment of type 2 diabetes, will also be useful like metformin, which shows insulin-sparing effect in type 1 diabetes. This is because PPARγ/RXR is known to be a target of thiazolidinediones (TZDs), which are used for the treatment of insulin resistance, LXR/RXR is reported to be involved in glucose/lipid metabolism, and these heterodimers can be activated by RXR agonists alone (permissive mechanism). However, repeated administration of RXR agonists can elevate blood triglyceride and induce hypothyroidism. In this study, we performed systematic conversion of the alkoxy side chain of 5a (6-[ethyl-(3-isopropoxy-4-isopropylphenyl)amino] nicotinic acid: NEt-3IP) and evaluated the RXR-, PPAR/RXR- and LXR/RXR-agonist activities of the products. The cyclopropylmethoxy analog (5c) showed similar RXR- and LXR/RXR-agonistic activities to the benzyloxy analog (5i) and n-propoxy analog (5k), but exhibited more potent PPAR/RXR-agonistic activity than 5i or 5k. Differential modulation of RXR heterodimer-activating ability by conversion of the alkoxy group located in the lipophilic domain of the RXR-agonist common structure is expected to be a useful approach in the design of new RXR agonists for the treatment of hyperlipidemia.     

Diacerein versus adipoRon as adiponectin modulators in experimentally-induced end-stage type 2 diabetes mellitus in rats

The objective of the present study is to evaluate and compare the possible anti-diabetic effects of adipoRon and diacerein in type 2 diabetes mellitus (T2DM) rats. T2DM is marked by impaired oxidative, inflammatory and metabolic signaling. Indeed, T2DM progression is associated with elevated HbA1C%, low adiponectin and insulin concentration. Moreover, in this study epididymal adipose tissue and soleus muscle MDA contents significantly escalated, while serum TAC and epididymal adipose Nrf2 significantly declined. Nevertheless, serum TNF-α, epididymal NLRP3, NF-κB, PPARγ and CD68 expression rose significantly with a parallel significant reduction in serum IL-10 and soleus muscle expression of IRS1. Both adipoRon and diacerein significantly improved adiponectin and insulin secretion with augmentation of anti-oxidant defenses and diminution of oxidative burden, with obvious anti-inflammatory consequences (p < 0.05). Thus, adipoRon and diacerein positively modulated adiponectin expression with down-regulation of NF-κB/NLRP3/PPARγ expression with subsequent improvement in glycemic control, inflammatory and oxidative signaling.     

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.     

Monosodium glutamate neonatal intoxication associated with obesity in adult stage is characterized by chronic inflammation and increased mRNA expression of peroxisome proliferator-activated receptors in mice

The monosodium glutamate (MSG) neonatal administration in mice provides a model of obesity with impaired glucose tolerance (IGT) and insulin resistance. However, the inflammatory profile of cytokines produced from fat tissue and its relationship to the metabolic dysfunction induced by MSG have not yet been revealed. The aim of this study was to establish the inflammatory profile attributed to MSG by measuring the expression of adipokines in visceral fat and serum of 19-week-old mice as well as the peroxisome proliferator-activated receptors alpha and gamma (PPARα and γ). Some metabolic and biochemical parameters were also quantified. The MSG increased mRNA expression of interleukin-6 (IL-6), tumour necrosis factor-alpha (TNFα), resistin and leptin, but adiponectin did not exhibit any changes. In addition, impaired glucose tolerance, increased levels of insulin, resistin and leptin were observed in serum. Both PPARα and PPARγ were activated in MSG-induced obese mice, which might explain its inflammatory profile. However, liver transaminases were severely depressed, indicating that MSG may also induce liver injury, contributing to inflammation. The MSG neonatal neuro-intoxication in mice may thus provide a model of obesity and inflammation characterized by the dual activation of PPARα and PPARγ, which might offer new insights into the mechanism of inflammatory diabetes in obesity leading to steatohepatitis, as well as a suitable model to study the role of new therapeutic agents to prevent or reduce insulin resistance, the inflammatory state and liver steatosis.     

Insulin-sensitizing effects of a novel alpha-methyl- alpha -phenoxylpropionate derivative in vitro

AIM: To examine the insulin sensitizing effects of a novel alpha-methyl-alpha- phenoxylpropionate derivative YY20 in insulin-sensitive cell lines. METHODS: The peroxisome proliferator-activated receptor gamma (PPAR gamma) agonist bioactivities of YY20 were detected by a preadipocyte differentiation assay. RT-PCR and Western blotting analysis were used to detect the expression of the target gene or protein. The effects of YY20 on insulin-mediated glucose consumption were determined in the HepG2 human hepatocellular carcinoma line. RESULTS: YY20 could enhance the differentiation of preadipocytes to adipocytes and upregulate the gene expression of PPAR gamma 2, as well as the protein expression of insulin receptor substrate- 1 (IRS-1), glucose transporter-4 (GLUT4), and adiponectin (ACRP30). The effects on GLUT4 and ACRP30 could be reversed by the PPAR gamma inhibitor SR-202. Furthermore, YY20 efficiently reduced glucose consumptions in HepG2 cells after 24 h culture, and the effects were related to insulin and YY20 concentrations. CONCLUSION: YY20, a potential insulin-sensitizing agent like rosiglitazone, could enhance glucose consumption in HepG2 cells in a concentration- and insulindependent manner. It may improve the insulin resistance associated with type 2 diabetes.     

2型糖尿病患者中脂联素和腹腔脂肪型丝氨酸蛋白酶抑制剂vaspin的水平及与胰岛素抵抗的关系

目的 观察2型糖尿病患者中脂联素、vaspin与胰岛素抵抗的关系.方法 通过260例初诊2型糖尿病患者(126例男性、134例女性)和260名健康成年人研究脂联素、vaspin和肥胖、胰岛素抵抗的关系.非参数Spearman相关和多元回归分析观察脂联素、vaspin和胰岛素抵抗、肥胖、代谢变量的关系.结果 2型糖尿病人...     

Dual acting and pan-PPAR activators as potential anti-diabetic therapies

The thiazolidinedione PPAR-γ activator drugs rosiglitazone and pioglitazone suppress insulin resistance in type 2 diabetic patients. They lock lipids into adipose tissue triglyceride stores, thereby preventing lipid metabolites from causing insulin resistance in liver and skeletal muscle and β-cell failure. They also reduce the secretion of inflammatory cytokines such as TNFα and increase the plasma level of adiponectin, which increases insulin sensitivity in liver and skeletal muscle. However, they have only a modest effect on dyslipidaemia, and they increase fat mass and plasma volume. Fibrate PPAR-α activator drugs decrease plasma triglycerides and increase HDL-cholesterol levels. PPAR-δ activators increase the capacity for fat oxidation in skeletal muscle.Clinical experience with bezafibrate, which activates PPAR-δ and -α, and studies on the PPAR-α/δ activator tetradecylthioacetic acid, the PPAR-δ activator GW501516, and combinations of the PPAR-α activator fenofibrate with rosiglitazone or pioglitazone have encouraged attempts to develop single molecules that activate two or all three PPARs. Most effort has focussed on dual PPAR-α/γ activators. These reduce both hyperglycaemia and dyslipidaemia, but their development has been terminated by issues such as increased weight gain, oedema, plasma creatinine and myocardial infarction or stroke. In addition, the FDA has stated that many PPAR ligands submitted to it have caused increased numbers of tumours in carcinogenicity studies.Rather than aiming for full potent agonists, it may be best to identify subtype-selective partial agonists or compounds that selectively activate PPAR signalling pathways and use these in combination. Nutrients or modified lipids that are low-affinity agonists may also have potential.     

脂肪因子表达对胰岛素抵抗及代谢综合征的影响

脂肪组织分泌的多种脂肪细胞因子,如肿瘤坏死因子(TNF)-α、白介素(IL)-6、瘦素、脂联素等均可介导或参与慢性炎症反应,导致胰岛素抵抗,进而发展为代谢综合征。在肥胖、炎症-胰岛素抵抗-代谢综合征这一病理生理过程中,脂肪细胞的内分泌调节功能障碍扮演了重要角色。本文综述目前已知的部分脂肪因子与胰岛素抵抗的关系,旨在为肥胖症、糖尿病及代谢综合征发病机制的研究和治疗策略提供线索。     

Involvement of adipokines in rimonabant‐mediated insulin sensitivity in ob/ob mice

Objectives It has been recently reported that blockade of type 1 cannabinoid (CB1) receptors by specific antagonists or genetic manipulation alleviates dyslipidaemia, hyperglycaemia and insulin resistance in animal models of obesity and type 2 diabetes. However, the precise role of adipokines in the insulin‐sensitising effects of the CB1 antagonist rimonabant is not clear. Methods ob/ob mice were treated with different doses of rimonabant and then subjected to an oral glucose tolerance test. The expression of different adipokines in white adipose tissue was analysed by quantitative real‐time PCR. Key findings Rimonabant (30 mg/kg) significantly inhibited body weight and fat pad weight gain (P < 0.05) and improved glucose tolerance. Gene expression analysis indicated that tumour necrosis factor‐α, visfatin and retinol binding protein‐4 were downregulated in the adipose tissue of ob/ob mice treated with rimonabant compared with controls, whereas adiponectin was significantly upregulated. Conclusions Rimonabant‐mediated alteration of adipokines in white adipose tissues may play a role in improving insulin sensitivity in obese animals.     

Targeting PPARβ/δ for the treatment of type 2 diabetes mellitus

INTRODUCTION: The nuclear receptors Peroxisome Proliferator-Activated Receptors (PPAR)α and PPARγ are therapeutic targets for hypertriglyceridemia and insulin resistance, respectively. Evidence is now emerging that the PPARβ/δ isotype is a potential pharmacological target for the treatment of insulin resistance and type 2 diabetes mellitus. AREAS COVERED: In this review, the capacity of PPARβ/δ to prevent the development of insulin resistance and type 2 diabetes mellitus is discussed. Special emphasis is placed on preclinical studies and the molecular mechanisms responsible for its actions in the main cell types involved in these pathologies: adipocytes, β-cells, skeletal muscle cells and hepatocytes. EXPERT OPINION: While several concerns remain for the development of PPARβ/δ agonists, these drugs have demonstrated their efficacy in the treatment of insulin resistance and type 2 diabetes mellitus in preclinical studies, as well as in a few short clinical studies in humans. Although this data is promising, additional studies must be performed to confirm the efficacy and safety of these drugs in the treatment of type 2 diabetes mellitus.     

选择性PPARγ调节剂治疗糖尿病的研究新进展

过氧化物酶体增殖物激活受体γ是一类主要调控糖脂代谢与脂肪细胞分化的核受体,与肥胖、胰岛素抵抗和糖尿病的发生发展密切相关,其激动剂作为胰岛素增敏剂治疗2型糖尿病也被临床广泛应用。近年来,随着对PPARγ信号通路和胰岛素增敏剂的深入研究,使我们对选择性PPARγ调节剂类胰岛素增敏剂的认识不断完善和更新。该文主要阐述选择性PPARγ调节剂类胰岛素增敏剂的作用机制和药物研究的最新进展。