Molecular mechanisms involved in hepatic steatosis and insulin resistance

Increased hepatic lipid content is associated with hepatic as well as whole‐body insulin resistance and is typical for individuals with type 2 diabetes mellitus. However, whether insulin resistance causes hepatic steatosis or whether hepatic steatosis per se reduces insulin sensitivity remains unclear. Multiple metabolic pathways lead to the development of hepatic steatosis, including enhanced free fatty acid release from adipose tissues (lipolysis), increased de novo fatty acid synthesis (lipogenesis), decreased mitochondrial β‐oxidation and decreased very low‐density lipoprotein secretion. Although the molecular mechanisms leading to the development of hepatic steatosis in the pathogenesis of type 2 diabetes mellitus are complex, several recent animal models have shown that modulating important enzymes involved in hepatic fatty acid and glycerolipid synthesis might be a key for treating hepatic insulin resistance. We highlight recent advances in the understanding of the molecular mechanisms leading to the development of hepatic steatosis and insulin resistance. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2011.00111.x, 2011)     

Molecular mechanisms of insulin resistance in chronic hepatitis C

It is now widely recognized that chronic hepatitis C （CHC） is associated with insulin resistance （IR） and type 2 diabetes, so can be considered a metabolic disease. IR is most strongly associated with hepatitis C virus （HCV） genotype 1, in contrast to hepatic steatosis, which is associated with genotype 3 infection. Apart from the well-described complications of diabetes, IR in CHC predicts faster progression to fibrosis and cirrhosis that may culminate in liver failure and hepatocellular carcinoma. More recently, it has been recognized that IR in CHC predicts a poor response to antiviral therapy. The molecular mechanisms for the association between IR and HC＇V infection are not well defined. This review will elaborate on the clinical associations between CHC and IR and summarize current knowledge regarding the molecular mechanisms that potentially mediate HCV-associated IR.     

Molecular and signaling mechanisms of atherosclerosis in insulin resistance.

Although the prevalence of cardiovascular complications is increased in insulin-resistant individuals, the underlying causes of this link have been elusive. Recent work suggests that several intracellular signal transduction pathways are inappropriately activated by hyperinsulinemia, hyperglycemia, increased free fatty acids, dyslipidemia, various inflammatory cytokines and adipokines--factors that are increased in insulin resistance. Once activated, substantial cross talk occurs between these pathways, especially a self-reinforcing cascade of vascular inflammation and cell dysfunction, greatly increasing the risk and severity of atherosclerosis in the insulin-resistant individual. We review several key cell-signalling pathways, describe how they are activated in they insulin-resistant state and the damage they induce, and discusses possible therapeutic approaches to limit vascular damage.     

Molecular mechanisms of skeletal muscle insulin resistance in type 2 diabetes

Type 2 (non-insulin-dependent) diabetes mellitus afflicts millions of people worldwide and is one of the main causes of morbidity and mortality. Current therapeutic agents to treat Type 2 diabetes are insufficient and thus, newer approaches are desperately needed. Type 2 diabetes is manifested by progressive metabolic impairments in tissues such as skeletal muscle, adipose tissue and liver, such that these tissues become less responsive to insulin. Skeletal muscle is quantitatively the most important tissue involved in maintaining glucose homeostasis under insulin-stimulated conditions, and is a major site of insulin resistance in Type 2 diabetic patients. At the cellular level, glucose transport into skeletal muscle is the rate-limiting step for whole body glucose uptake and a primary site of insulin resistance in Type 2 diabetes. Thus, skeletal muscle is a key insulin target tissue that harbours intrinsic defects that impinges upon whole body glucose homeostasis. Here, we review the current knowledge of signalling events that regulate glucose transport in human skeletal muscle.     

Neuroendocrine mechanisms in insulin resistance

Dysregulated hormonal, metabolic and neural signalling within and between organs can contribute to development of metabolic diseases including type 2 diabetes. Insulin-antagonistic effects of hormones, cytokines and excess metabolic substrates such as glucose and fatty acids may be exerted via common mechanisms involving for example reactive oxygen species (ROS) accumulation and associated inflammatory responses. Visceral adiposity is a central component of the metabolic syndrome and it is also strongly associated with insulin resistance. Both visceral obesity and insulin resistance are important risk factors for the development of type 2 diabetes. In the development of insulin resistance, it is likely that intra-abdominal adipose tissue plays a critical role in a complex endocrine and neural network involving several tissues. This review paper focuses on neuroendocrine 'stress' factors that target insulin-responsive tissues, in particular adipose tissue. We propose that there are common pathways by which dysregulation in different endocrine systems may contribute to the development of type 2 diabetes.     

Molecular mechanisms of lipid-induced insulin resistance in muscle, liver and vasculature

Increased body fat content correlates with insulin resistance and is a key feature of type 2 diabetes. Excessive intake of fat results in deposition of lipids not only in fat tissue but also in skeletal muscle and liver. Subsequently, both plasma and intracellular concentrations of free fatty acids and their metabolites rise and activate signal transduction pathways, which will induce inflammation and impair insulin signalling. Furthermore, elevated circulating lipids impair endothelial function and fibrinolysis, which contributes to the development of vascular disease. Thus, therapeutic strategies aiming at reduction of (intracellular) lipid availability in skeletal muscle and liver and pharmacological modulation of the signalling pathways activated by increased lipid stores represent promising targets for future treatment of insulin resistance and prevention of its complications. This review focuses on the effects of increased lipid availability on the regulation of glucose metabolism in skeletal muscle and liver as well as on vascular function.     

Molecular mechanisms of insulin resistance and the role of the adipocyte

Insulin resistance is a common feature of obesity and predisposes the affected individuals to a variety of diseases, including hypertension, dyslipidemias, cardiovascular problems and type 2 diabetes mellitus. However, the molecular mechanisms underlying abnormal insulin action and these other pathological states are not well understood. We have been focusing on cytokines, particularly TNFalpha and fatty acid binding proteins, as potential sites to study the molecular basis of these disorders. The role of TNFalpha in insulin resistance and other pathologies associated with obesity, have been examined in several experimental systems including obese mice with homozygous null mutations at the TNFalpha or TNF receptor loci. Analysis of these animals demonstrated that the genetic absence of TNF signaling in obesity: (i) significantly improves insulin receptor signaling capacity and consequently insulin sensitivity; (ii) prevents brown adipose tissue atrophy and beta3-adrenoreceptor deficiency and improves thermo-adaptive responses, (iii) decreases the elevated PAI-1 and TGFbeta production; and (iv) lowers hyperlipidemia and hyperleptinemia. Hence, abnormal TNFalpha action in adipocytes disturbs many aspects of metabolic homeostasis in obesity.     

胰岛素抵抗的分子遗传学病因

目的探讨中国人群胰岛素受体底物１（ＩＲＳ１）基因、β３肾上腺素受体（β３ＡＲ）基因突变与胰岛素抵抗的关系。方法对２８１例病人通过糖耐量试验分为糖耐量正常组、糖耐量低减组及糖尿病组，分别对之进行ＩＲＳ１基因、β３ＡＲ基因多态性分析。结果糖尿病者和糖耐量正常者相比，ＩＲＳ１基因、β３ＡＲ基因的基因型频率及等位基因频率存在明显差异；多元回归分析发现：胰岛素水平和ＩＲＳ１基因、β３ＡＲ基因多态性显著相关。非条件多因素Ｌｏｇｉｓｔｉｃ回归模型发现：糖尿病和β３ＡＲ基因多态性有明显关系。结论β３ＡＲ基因突变可能是与胰岛素抵抗有关疾病的共有危险因素。     

Molecular mechanisms of corticosteroid resistance

Most patients with asthma are successfully treated with conventional therapy. Nevertheless, there is a small proportion of asthmatic patients, including present cigarette smokers and former cigarette smokers, who fail to respond well to therapy with high-dose glucocorticoids (GCs) or with supplementary therapy. In addition, high doses of steroids have a minimal effect on the inexorable decline in lung function in COPD patients and only a small effect on reducing exacerbations. GC insensitivity, therefore, presents a profound management problem in these patients. GCs act by binding to a cytosolic GC receptor (GR), which is subsequently activated and is able to translocate to the nucleus. Once in the nucleus, the GR either binds to DNA and switches on the expression of antiinflammatory genes or acts indirectly to repress the activity of a number of distinct signaling pathways such as nuclear factor (NF)-kappaB and activator protein (AP)-1. This latter step requires the recruitment of corepressor molecules. Importantly, this latter interaction is mutually repressive in that high levels of NF-kappaB and AP-1 attenuate GR function. A failure to respond may therefore result from reduced GC binding to GR, reduced GR expression, enhanced activation of inflammatory pathways, or lack of corepressor activity. These events can be modulated by oxidative stress, T-helper type 2 cytokines, or high levels of inflammatory mediators, all of which may lead to a reduced clinical outcome. Understanding the molecular mechanisms of GR action, and inaction, may lead to the development of new antiinflammatory drugs or may reverse the relative steroid insensitivity that is characteristic of patients with these diseases.     

Antibody-mediated insulin resistance treated by cessation of insulin administration

A 45-year-old Japanese man was referred to our hospital because of hyperglycemia despite the administration of as much as 120 U/day of human insulin. He had no history of injecting animal insulin. Free insulin was below 5 microU/ml, but a high titer of total insulin (about 3,000 microU/ml) was observed, suggesting the presence of antibodies against human insulin. Scatchard analysis showed an increased insulin binding capacity in the plasma characterized by a higher affinity for insulin. He was successfully treated by cessation of insulin administration. A Scatchard analysis series showed that a reduction in the insulin binding capacity of antibodies paralleled the improvement in glycemic control.     

糖皮质激素诱导胰岛素抵抗的分子机制

糖皮质激素是体内重要的胰岛素拮抗激素,可诱导胰岛素抵抗。糖皮质激素可以干扰骨骼肌细胞的葡萄糖摄取和利用,抑制脂肪组织中葡萄糖转运蛋白（GLUT）-4的胞内分布及糖转运活性从而抑制糖摄取。并可通过调节脂肪细胞因子如脂联素、抵抗素、瘦素、内脏脂肪素的分泌,影响胰岛素的敏感性。此外,糖皮质激素还可抑制胰岛β细胞功能,使胰岛素分泌减少,并触发胰岛细胞凋亡。     

结核杆菌异烟肼耐药的分子机制

在抗结核治疗策略中,异烟肼是最重要的一线药物.在过去50年的抗结核治疗中,异烟肼发挥了非常重要的作用,而耐异烟肼结核菌的出现使这种效果减弱.本文从异烟肼的抗结核机制人手,对异烟肼耐药的分子机制作一综述.     

Postprandial dyslipidemia in insulin resistance: mechanisms and role of intestinal insulin sensitivity

Insulin resistance is strongly associated with metabolic dyslipidemia, which is largely a postprandial phenomenon. Though previously regarded as a consequence of delayed triglyceride-rich lipoprotein clearance, emerging evidence present intestinal overproduction of apoB-48-containing lipoproteins as a major contributor to postprandial dyslipidemia. The majority of mechanistic information is however derived from animal models, namely the fructose-fed Syrian Golden hamster, and extension to human studies to date has been limited. Work in our laboratory has established that aberrant insulin signalling exists in the enterocyte, and that inflammation appears to induce intestinal insulin resistance. The intestine is a major site of lipid synthesis in the body, and upregulated intestinal de novo lipogenesis and cholesterogenesis have been noted in insulin resistant and diabetic states. There is also enhanced dietary lipid absorption attributable to changes in ABCG5/8, NPC1L1, CD36/FAT, and FATP4. Proteins that are involved in chylomicron assembly and secretion, including MTP, MGAT, DGAT, apoAI-V, and Sar1 GTPase, show evidence of increased expression and activity levels. Increased circulating free fatty acids, typically observed in insulin resistant states, may serve to deliver lipid substrates to the intestine for enhanced chylomicron assembly and secretion. To compound the dysregulation of intestinal lipid metabolism, there are changes in the secretion of gut-derived peptides, which include GLP-1, GLP-2, and GIP. Thus, accumulating evidence presents intestinal lipoprotein secretion as a highly regulated process that is sensitive to perturbations in whole body energy homeostasis, and is severely perturbed in insulin resistant states.     

Islet adaptation to insulin resistance: mechanisms and implications for intervention

Insulin sensitivity and insulin secretion are reciprocally related such that insulin resistance is adapted by increased insulin secretion to maintain normal glucose and lipid homeostasis. The relation between insulin sensitivity and secretion is curvilinear and mathematically best described as a hyperbolic relation. Several potential mediators have been suggested to be signals for the beta cells to respond to insulin resistance such as glucose, free fatty acids, autonomic nerves, fat-derived hormones and the gut hormone glucagon-like peptide-1 (GLP-1). Failure of these signals or of the pancreatic beta cells to adequately adapt insulin secretion in relation to insulin sensitivity results in inappropriate insulin levels, impaired glucose intolerance (IGT) and type 2 diabetes. Therefore, treatment of IGT and type 2 diabetes should aim at restoring the normal relation between insulin sensitivity and secretion. Such treatment includes stimulation of insulin secretion (sulphonylureas, repaglinide and nateglinide) and insulin sensitivity (metformin and thiazolidinediones), as well as treatment aimed at supporting the signals mediating the islet adaptation (cholinergic agonists and GLP-1). Both, for correct understanding of diabetes pathophysiology and for development of novel treatment modalities, therefore, the non-linear inverse relation between insulin sensitivity and secretion needs to be acknowledged.