Abstract-Endothelial dysfunction contributes to cardiovascular diseases, including hypertension, atherosclerosis, and coronary artery disease, which are also characterized by insulin resistance. Insulin resistance is a hallmark of metabolic disorders, including type 2 diabetes mellitus and obesity, which are also characterized by endothelial dysfunction. Metabolic actions of insulin to promote glucose disposal are augmented by vascular actions of insulin in endothelium to stimulate production of the vasodilator nitric oxide (NO). Indeed, NO-dependent increases in blood flow to skeletal muscle account for 25% to 40% of the increase in glucose uptake in response to insulin stimulation. Phosphatidylinositol 3-kinase-dependent insulin-signaling pathways in endothelium related to production of NO share striking similarities with metabolic pathways in skeletal muscle that promote glucose uptake. Other distinct nonmetabolic branches of insulin-signaling pathways regulate secretion of the vasoconstrictor endothelin-1 in endothelium. Metabolic insulin resistance is characterized by pathway-specific impairment in phosphatidylinositol 3-kinase-dependent signaling, which in endothelium may cause imbalance between production of NO and secretion of endothelin-1, leading to decreased blood flow, which worsens insulin resistance. Therapeutic interventions in animal models and human studies have demonstrated that improving endothelial function ameliorates insulin resistance, whereas improving insulin sensitivity ameliorates endothelial dysfunction. Taken together, cellular, physiological, clinical, and epidemiological studies strongly support a reciprocal relationship between endothelial dysfunction and insulin resistance that helps to link cardiovascular and metabolic diseases. In the present review, we discuss pathophysiological mechanisms, including inflammatory processes, that couple endothelial dysfunction with insulin resistance and emphasize important therapeutic implications. Key Words: diabetes mellitus Ⅲ endothelium Ⅲ hypertension Ⅲ insulin I nsulin resistance is typically defined as decreased sensitivity and/or responsiveness to metabolic actions of insulin that promote glucose disposal. This important feature of diabetes, obesity, glucose intolerance, and dyslipidemia is also a prominent component of cardiovascular disorders, including hypertension, coronary artery disease, and atherosclerosis, which are characterized by endothelial dysfunction. 1 Conversely, endothelial dysfunction is present in diabetes, obesity, and dyslipidemias. 2 Moreover, it is firmly established that these metabolic disorders are major risk factors for cardiovascular diseases. 3 In addition to its essential metabolic actions, insulin has important vascular actions that involve stimulation of the production of nitric oxide (NO) from endothelium, leading to vasodilation, increased blood flow, and augmentation of glucose disposal in skeletal muscle. 4 Elucidation of insulin-signaling pathways regulating endothelial production of NO reveals strik...

Insulin resistance plays an important role in the pathophysiology of diabetes and is associated with obesity and other cardiovascular risk factors. The "gold standard" glucose clamp and minimal model analysis are two established methods for determining insulin sensitivity in vivo, but neither is easily implemented in large studies. Thus, it is of interest to develop a simple, accurate method for assessing insulin sensitivity that is useful for clinical investigations. We performed both hyperinsulinemic isoglycemic glucose clamp and insulin-modified frequently sampled iv glucose tolerance tests on 28 nonobese, 13 obese, and 15 type 2 diabetic subjects. We obtained correlations between indexes of insulin sensitivity from glucose clamp studies (SI(Clamp)) and minimal model analysis (SI(MM)) that were comparable to previous reports (r = 0.57). We performed a sensitivity analysis on our data and discovered that physiological steady state values [i.e. fasting insulin (I(0)) and glucose (G(0))] contain critical information about insulin sensitivity. We defined a quantitative insulin sensitivity check index (QUICKI = 1/[log(I(0)) + log(G(0))]) that has substantially better correlation with SI(Clamp) (r = 0.78) than the correlation we observed between SI(MM) and SI(Clamp). Moreover, we observed a comparable overall correlation between QUICKI and SI(Clamp) in a totally independent group of 21 obese and 14 nonobese subjects from another institution. We conclude that QUICKI is an index of insulin sensitivity obtained from a fasting blood sample that may be useful for clinical research.

tance contributes to the pathophysiology of diabetes and is a hallmark of obesity, metabolic syndrome, and many cardiovascular diseases. Therefore, quantifying insulin sensitivity/resistance in humans and animal models is of great importance for epidemiological studies, clinical and basic science investigations, and eventual use in clinical practice. Direct and indirect methods of varying complexity are currently employed for these purposes. Some methods rely on steady-state analysis of glucose and insulin, whereas others rely on dynamic testing. Each of these methods has distinct advantages and limitations. Thus, optimal choice and employment of a specific method depends on the nature of the studies being performed. Established direct methods for measuring insulin sensitivity in vivo are relatively complex. The hyperinsulinemic euglycemic glucose clamp and the insulin suppression test directly assess insulin-mediated glucose utilization under steady-state conditions that are both labor and time intensive. A slightly less complex indirect method relies on minimal model analysis of a frequently sampled intravenous glucose tolerance test. Finally, simple surrogate indexes for insulin sensitivity/ resistance are available (e.g., QUICKI, HOMA, 1/insulin, Matusda index) that are derived from blood insulin and glucose concentrations under fasting conditions (steady state) or after an oral glucose load (dynamic). In particular, the quantitative insulin sensitivity check index (QUICKI) has been validated extensively against the reference standard glucose clamp method. QUICKI is a simple, robust, accurate, reproducible method that appropriately predicts changes in insulin sensitivity after therapeutic interventions as well as the onset of diabetes. In this Frontiers article, we highlight merits, limitations, and appropriate use of current in vivo measures of insulin sensitivity/resistance. glucose clamp; quantitative insulin sensitivity check index; minimal model; homeostatis model assessment INSULIN IS AN ESSENTIAL PEPTIDE HORMONE whose metabolic actions maintain whole body glucose homeostasis and promote efficient glucose utilization (3). Insulin stimulates increased glucose disposal in skeletal muscle and adipose tissue, whereas it inhibits gluconeogenesis in liver to help regulate glucose homeostasis. In addition to these classical insulin target tissues, there are many other important physiological targets of insulin, including the brain, pancreatic ␤-cells, heart, and vascular endothelium, that help to coordinate and couple metabolic and cardiovascular homeostasis under healthy conditions (3,54,72,79). Insulin has concentration-dependent saturable actions to increase whole body glucose disposal. The maximal effect of insulin defines "insulin responsiveness," whereas the insulin concentration required for a half-maximal response defines "insulin sensitivity" (Fig. 1A).Insulin resistance is typically defined as decreased sensitivity or responsiveness to metabolic actions of insulin, such as insulin-mediated glucose d...

Insulin has important vascular actions to stimulate production of nitric oxide from endothelium. This leads to capillary recruitment, vasodilation, increased blood flow, and subsequent augmentation of glucose disposal in classical insulin target tissues (e.g., skeletal muscle). Phosphatidylinositol 3-kinase-dependent insulin-signaling pathways regulating endothelial production of nitric oxide share striking parallels with metabolic insulin-signaling pathways. Distinct MAPK-dependent insulin-signaling pathways (largely unrelated to metabolic actions of insulin) regulate secretion of the vasoconstrictor endothelin-1 from endothelium. These and other cardiovascular actions of insulin contribute to coupling metabolic and hemodynamic homeostasis under healthy conditions. Cardiovascular diseases are the leading cause of morbidity and mortality in insulin-resistant individuals. Insulin resistance is typically defined as decreased sensitivity and/or responsiveness to metabolic actions of insulin. This cardinal feature of diabetes, obesity, and dyslipidemia is also a prominent component of hypertension, coronary heart disease, and atherosclerosis that are all characterized by endothelial dysfunction. Conversely, endothelial dysfunction is often present in metabolic diseases. Insulin resistance is characterized by pathway-specific impairment in phosphatidylinositol 3-kinase-dependent signaling that in vascular endothelium contributes to a reciprocal relationship between insulin resistance and endothelial dysfunction. The clinical relevance of this coupling is highlighted by the findings that specific therapeutic interventions targeting insulin resistance often also ameliorate endothelial dysfunction (and vice versa). In this review, we discuss molecular mechanisms underlying cardiovascular actions of insulin, the reciprocal relationships between insulin resistance and endothelial dysfunction, and implications for developing beneficial therapeutic strategies that simultaneously target metabolic and cardiovascular diseases.

Adiponectin is secreted by adipose cells and mimics many metabolic actions of insulin. However, mechanisms by which adiponectin acts are poorly understood. The vascular action of insulin to stimulate endothelial production of nitric oxide (NO), leading to vasodilation and increased blood flow is an important component of insulin-stimulated whole body glucose utilization. Therefore, we hypothesized that adiponectin may also stimulate

Receptor kinase activity is necessary to mediate production of NO through the insulin receptor. Both PI3K and Akt contribute importantly to this process, whereas the contribution of Ras is small.

Hypertension is associated with insulin-resistant states such as diabetes and obesity. Nitric oxide (NO) contributes to regulation of blood pressure. To gain insight into potential mechanisms linking hypertension with insulin resistance we directly measured and characterized NO production from human umbilical vein endothelial cells (HUVEC) in response to insulin using an amperometric NO-selective electrode. Insulin stimulation of HUVEC resulted in rapid, dose-dependent production of NO with a maximal response of ‫ف‬ 100 nM NO (200,000 cells in 2 ml media; ED 50 ‫ف‬ 500 nM insulin). Although HUVEC have many more IGF-1 receptors than insulin receptors ( ‫ف‬ 400,000, and ‫ف‬ 40,000 per cell respectively), a maximally stimulating dose of IGF-1 generated a smaller response than insulin (40 nM NO; ED 50 ‫ف‬ 100 nM IGF-1). Stimulation of HUVEC with PDGF did not result in measurable NO production. The effects of insulin and IGF-1 were completely blocked by inhibitors of either tyrosine kinase (genestein) or nitric oxide synthase (L-NAME). Wortmannin (an inhibitor of phosphatidylinositol 3-kinase [PI 3-kinase]) inhibited insulin-stimulated production of NO by ‫ف‬ 50%. Since PI 3-kinase activity is required for insulin-stimulated glucose transport, our data suggest that NO is a novel effector of insulin signaling pathways that are also involved with glucose metabolism. ( J. Clin. Invest. 1996. 98:894-898.)

Insulin resistance contributes importantly to the pathophysiology of type 2 diabetes mellitus. One mechanism mediating insulin resistance may involve the phosphorylation of serine residues in insulin receptor substrate-1 (IRS-1), leading to impairment in the ability of IRS-1 to activate downstream phosphatidylinositol 3-kinase-dependent pathways. Insulin-resistant states and serine phosphorylation of IRS-1 are associated with the activation of the inhibitor B kinase (IKK) complex. However, the precise molecular mechanisms by which IKK may contribute to the development of insulin resistance are not well understood. In this study, using phosphospecific antibodies against rat IRS-1 phosphorylated at Ser 307 (equivalent to Ser 312 in human IRS-1), we observed serine phosphorylation of IRS-1 in response to TNF-␣ or calyculin A treatment that paralleled surrogate markers for IKK activation. The phosphorylation of human IRS-1 at Ser 312 in response to tumor necrosis factor-␣ was significantly reduced in cells pretreated with the IKK inhibitor 15 deoxy-prostaglandin J 2 as well as in cells derived from IKK knock-out mice. We observed interactions between endogenous IRS-1 and IKK in intact cells using a co-immunoprecipitation approach. Moreover, this interaction between IRS-1 and IKK in the basal state was reduced upon IKK activation and increased serine phosphorylation of IRS-1. Data from in vitro kinase assays using recombinant IRS-1 as a substrate were consistent with the ability of IRS-1 to function as a direct substrate for IKK with multiple serine phosphorylation sites in addition to Ser 312 . Taken together, our data suggest that IRS-1 is a novel direct substrate for IKK and that phosphorylation of IRS-1 at Ser 312 (and other sites) by IKK may contribute to the insulin resistance mediated by activation of inflammatory pathways.Many factors implicated in the development of insulin resistance such as TNF-␣ 1 (1, 2), free fatty acids (3, 4), and serine phosphatase inhibitors (5, 6) are able to activate the inhibitor B kinase (IKK) complex and its downstream effector, NFB (7-10). Interestingly, insulin-sensitizing drugs such as thiazolidinediones inhibit NFB activity (11). Adiponectin, a cytokine secreted by adipose cells whose plasma levels are negatively correlated with insulin resistance (12), inhibits IKK activity in cells (13). Moreover, diet-induced insulin resistance is ameliorated in IKK2-deficient mice (14). Because IKK and NFB are major components of the intracellular inflammatory pathway, a cross-talk between metabolic and inflammatory signaling pathways may play an important role in the development of insulin resistance and the pathophysiology of major public health problems such as diabetes and obesity. However, molecular mechanisms by which IKK may specifically interact with metabolic insulin signaling pathways are not well understood. IKK is a serine kinase that controls the activation of NFB, a ubiquitous transcription factor closely associated with inflammation (15-18). In addition to inflammation, NF...