Characterization of selective resistance to insulin signaling in the vasculature of obese Zucker (fa/fa) rats

Vascular neointimal formation and signaling pathway activation in response to stent injury in insulin-resistant and diabetic animals

Diabetes and insulin resistance are associated with increased disease risk and poor outcomes from cardiovascular interventions. Even drug-eluting stents exhibit reduced efficacy in patients with diabetes. We now report the first study of vascular response to stent injury in insulin-resistant and diabetic animal models. Endovascular stents were expanded in the aortae of obese insulin-resistant and type 2 diabetic Zucker rats, in streptozotocin-induced type 1 diabetic Sprague-Dawley rats, and in matched controls. Insulin-resistant rats developed thicker neointima (0.46+/-0.08 versus 0.37+/-0.06 mm2, P=0.05), with decreased lumen area (2.95+/-0.26 versus 3.29+/-0.15 mm2, P=0.03) 14 days after stenting compared with controls, but without increased vascular inflammation (ED1+ tissue macrophages). Insulin-resistant and diabetic rat vessels did exhibit markedly altered signaling pathway activation 1 and 2 weeks after stenting, with up to a 98% increase in p-ERK (anti-phospho ERK) and a 54% reduction in p-Akt (anti-phospho Akt) stained cells. Western blotting confirmed a profound effect of insulin resistance and diabetes on Akt and ERK signaling in stented segments. p-ERK/p-Akt ratio in stented segments uniquely correlated with neointimal response (R2=0.888, P=0.04) in insulin-resistant and type 1 and 2 diabetic rats, but not in lean controls. Transfemoral aortic stenting in rats provides insight into vascular responses in insulin resistance and diabetes. Shifts in ERK and Akt signaling related to insulin resistance may reflect altered tissue repair in diabetes accompanied by a shift in metabolic:proliferative balance. These findings may help explain the increased vascular morbidity in diabetes and suggest specific therapies for patients with insulin resistance and diabetes.

Insulin inhibits glucagon-induced glycogenolysis normally in perivenous hepatocytes of Wistar fatty rats

Wistar fatty (WF) rats are obese, hyperinsulinemic and hyperglycemic, and thus a model of type 2 diabetes mellitus. Since we have found that insulin specifically inhibits glucagon-induced glycogenolysis in perivenous hepatocytes (PVH) from normal rats, we examined the inhibitory effect of insulin on glucagon-induced glycogenolysis in PVH of hyperinsulinemic WF rats. Basal glucose release was 64.0+/-4.1 nmol/mgprotein/30 min from PVH of lean littermates (WL rats) and 137.0+/-19.3 nmol/mgprotein/30 min from that of WF rats (p<0.01). These were proportional to the glycogen content in PVH of WL and WF rats (56.7+/-7.2 and 131.0+/-20.3 microg/mgprotein, p<0.01), and increased to 109.0+/-8.8 and 225.8+/-17.9nmol/mgprotein/30min, respectively, with 0.1 nmol/l glucagon. When 10 nmol/l insulin was coincubated, 0.1 nmol/l glucagon-induced increase in glucose release decreased to 93.3+/-10.9 nmol/mgprotein/30 min in PVH of WL rats (p<0.01) and to 181+/-20.7 nmol/mgprotein/30 min in PVH of WF rats (p<0.01). Thus, insulin antagonized glucagon-induced glycogenolysis in PVH similarly between WL and WF rats, to 56.7+/-13.3% and to 46.1+/-7.5%, respectively. Thus, the antagonizing effect of insulin on glucagon-induced increase in glycogenolysis was preserved in PVH of hyperinsulinemic and hyperglycemic WF rats.

Gene expression profiling of the left ventricles in a rat model of intrinsic aerobic running capacity

Our previous work found DA rats superior for intrinsic aerobic running capacity (ARC) and several cardiac function indexes compared with Copenhagen (COP) rats, and identified ARC quantitative trait loci (QTLs) on rat chromosomes 16 (RNO16) and 3 (RNO3). The purpose of this study was to use these inbred rat strains as a genetic substrate for differential cardiac gene expression to identify candidate genes for the observed ARC QTLs. RNA expression was examined globally in left ventricles of 15-wk-old DA, F1(COP x DA), and COP rats using microarrays to identify candidate genes for ARC QTLs. We identified 199 differentially expressed probe sets and determined their chromosomal locations. Six differentially expressed genes and expressed sequence tags (ESTs) mapped near ARC QTL regions, including PDZ and LIM domain 3 (Pdlim3). Differential expression of these genes/ESTs was confirmed by quantitative RT-PCR. The Ingenuity Pathways program identified 13 biological networks containing 50 (of the 199) differentially expressed probe sets and 85 additional genes. Four of these eighty-five genes mapped near ARC QTL-containing regions, including insulin receptor substrate 2 (Irs2) and acyl-CoA synthetase long-chain family member 1 (Acsl1). Most (148/199) differentially expressed probe sets showed left ventricular expression patterns consistent with the alleles exerting additive effects, i.e., F1(COP x DA) rat RNA expression was intermediate between DA and COP rats. This study identified several potential ARC QTL candidate genes and molecular networks, one of them related to energy expenditure involving Pik3r1 mRNA expression that may, in part, explain the observed strain differences in ARC and cardiac performance.

Redox modulation of insulin signaling and endothelial function

Reactive oxygen and nitrogen species (ROS and RNS) recently emerged as critical signaling molecules in cardiovascular research. Several studies over the past decade have shown that physiological effects of vasoactive factors are mediated by these reactive species and, conversely, that altered redox mechanisms are implicated in the occurrence of metabolic and cardiovascular diseases. Oxidant stress occurs when ROS and/or RNS production exceeds the cell natural antioxidant systems, and pathological events ensue. Cardiovascular risk factors are associated with an imbalance of the redox equilibrium toward oxidative stress, leading to endothelial activation and proinflammatory processes implicated in atherogenesis and metabolic disorders. Recent studies indicate that insulin and insulin-sensitizing drugs activate antiinflammatory pathways that may limit oxidant stress in insulin target tissues. The main goal of this brief review is to discuss recent progress in the field of cellular redox signaling as it pertains to insulin modulation of vascular endothelial function in cardiovascular diseases.

Putative roles of kinin receptors in the therapeutic effects of angiotensin 1-converting enzyme inhibitors in diabetes mellitus

The role of endogenous kinins and their receptors in diabetes mellitus is being confirmed with the recent developments of molecular and genetic animal models. Compelling evidence suggests that the kinin B(2) receptor is organ-protective and partakes to the therapeutic effects of angiotensin 1-converting enzyme inhibitors (ACEI) and angiotensin AT(1) receptor antagonists. Benefits derive primarily from vasodilatory, antihypertensive, antiproliferative, antihypertrophic, antifibrotic, antithrombotic and antioxidant properties of kinin B(2) receptor activation. Mechanisms include the formation of nitric oxide and prostacyclin and the inhibition of NAD(P)H oxidase activity involving classical and novel signalling pathways. Kinin B(2) receptor also ameliorates insulin resistance by increasing glucose uptake and supply, and by inducing glucose transporter-4 translocation either directly or through phosphorylation of insulin receptor. The kinin B(1) receptor, which is induced by the cytokine network, growth factors and hyperglycaemia, mediates hyperalgesia, vascular hyperpermeability and leukocytes infiltration in diabetic animals. However, emerging data highlight reno- and cardio-protective effects mediated by kinin B(1) receptor under chronic ACEI therapy in diabetes mellitus. Thus, the Janus-faced of kinin receptors needs to be taken into account in future drug development. For instance, locally acting kinin B(1)/B(2) receptor agonists if used in a safe therapeutic window may represent a more rationale strategy in the prevention and management of diabetic complications. Because kinin B(2) receptor antagonists may further increase insulin resistance, the persisting dogma that restricts the development of kinin receptor analogues to antagonists (that is still relevant to abrogate pain and inflammation) needs to be revisited.

Diabetes Induces Endothelial Dysfunction but Does Not Increase Neointimal Formation in High-Fat Diet Fed C57BL/6J Mice

Studies of diabetic vascular disease have traditionally used murine models of type 1 diabetes and genetic models of type 2 diabetes. Because the majority of patients with type 2 diabetes have diet induced obesity, we sought to study the effect of diabetes on arterial disease in a mouse model of diet induced obesity/diabetes. C57Bl/6 mice fed a high-fat diet for 9 weeks developed type 2 diabetes characterized by elevated body weight, hyperglycemia, and hyperinsulinemia. Arteries from diabetic mice exhibited a marked decrease in endothelium-dependent vasodilation, a modest decrease in endothelium independent vasodilation, and an increase in sensitivity to adrenergic vasoconstricting agents. Insulin stimulated protein kinase B (akt) and endothelial nitric oxide synthase (eNOS) phosphorylation were preserved in arteries from diabetic mice; however, eNOS protein dimers were markedly diminished. Arterial nitrotyrosine staining indicated that increased levels of peroxynitrite contributed to eNOS dimer disruption in the diabetic mice. The abnormal vasomotion was not an acute response to the high-fat diet, as short term high-fat diet feeding had no effect on endothelium dependent dilation. A trend toward smaller neointimal lesions was noted in high-fat diet fed mice after femoral artery wire denudation injury. In summary, disrupted eNOS dimer formation rather than impaired insulin mediated eNOS phosphorylation contributed to the endothelial dysfunction in diet induced obese/diabetic mice. The lack of an increase in neointimal formation indicates that additional diabetes associated parameters (such as hyperlipidemia and atherosclerotic vascular disease) may need to be present to increase neointimal formation in this model.

Insulin Signaling in Arteries Prevents Smooth Muscle Apoptosis

OBJECTIVE

Insulin is an antiapoptotic factor of cultured vascular cells, but it is not clear whether it also exerts antiapoptotic effects on vascular cells in vivo. We studied insulin receptor signaling in the arteries of normal and diabetic rats to establish whether insulin exhibits antiapoptotic activity toward vascular smooth muscle cells in vivo as well as in vitro.

METHODS AND RESULTS

Western blot analysis and real-time polymerase chain reaction revealed alpha- and beta-subunits of the insulin receptor in association with insulin receptor substrate-1 and phosphatidylinositol 3-kinase in the media of the aorta and carotid artery. The insulin receptor signaling pathway was partially activated under physiological conditions, further activated by intravenous insulin injection, and was attenuated in streptozotocin-induced diabetic rats. Lipopolysaccharide injection induced more apoptosis of vascular smooth muscle cells in diabetic rats than in control rats, whereas insulin prevented apoptosis in the aortic wall. An in vitro study suggested that the antiapoptotic effect of insulin was mediated by phosphatidylinositol 3-kinase.

CONCLUSIONS

Insulin is an antiapoptotic factor of vascular smooth muscle cells in vitro and in vivo. Decreased insulin activity on the artery may increase smooth muscle cell death and cause unstable plaque formation associated with diabetes.

Coordinated regulation of insulin signaling by the protein tyrosine phosphatases PTP1B and TCPTP

The protein tyrosine phosphatase PTP1B is a negative regulator of insulin signaling and a therapeutic target for type 2 diabetes. Our previous studies have shown that the closely related tyrosine phosphatase TCPTP might also contribute to the regulation of insulin receptor (IR) signaling in vivo (S. Galic, M. Klingler-Hoffmann, M. T. Fodero-Tavoletti, M. A. Puryer, T. C. Meng, N. K. Tonks, and T. Tiganis, Mol. Cell. Biol. 23:2096-2108, 2003). Here we show that PTP1B and TCPTP function in a coordinated and temporally distinct manner to achieve an overall regulation of IR phosphorylation and signaling. Whereas insulin-induced phosphatidylinositol 3-kinase/Akt signaling was prolonged in both TCPTP-/- and PTP1B-/- immortalized mouse embryo fibroblasts (MEFs), mitogen-activated protein kinase ERK1/2 signaling was elevated only in PTP1B-null MEFs. By using phosphorylation-specific antibodies, we demonstrate that both IR beta-subunit Y1162/Y1163 and Y972 phosphorylation are elevated in PTP1B-/- MEFs, whereas Y972 phosphorylation was elevated and Y1162/Y1163 phosphorylation was sustained in TCPTP-/- MEFs, indicating that PTP1B and TCPTP differentially contribute to the regulation of IR phosphorylation and signaling. Consistent with this, suppression of TCPTP protein levels by RNA interference in PTP1B-/- MEFs resulted in no change in ERK1/2 signaling but caused prolonged Akt activation and Y1162/Y1163 phosphorylation. These results demonstrate that PTP1B and TCPTP are not redundant in insulin signaling and that they act to control both common as well as distinct insulin signaling pathways in the same cell.

Insulin resistance in spontaneously hypertensive rats is associated with endothelial dysfunction characterized by imbalance between NO and ET-1 production

Insulin stimulates production of NO in vascular endothelium via activation of phosphatidylinositol (PI) 3-kinase, Akt, and endothelial NO synthase. We hypothesized that insulin resistance may cause imbalance between endothelial vasodilators and vasoconstrictors (e.g., NO and ET-1), leading to hypertension. Twelve-week-old male spontaneously hypertensive rats (SHR) were hypertensive and insulin resistant compared with control Wistar-Kyoto (WKY) rats (systolic blood pressure 202 +/- 11 vs. 132 +/- 10 mmHg; fasting plasma insulin 5 +/- 1 vs. 0.9 +/- 0.1 ng/ml; P < 0.001). In WKY rats, insulin stimulated dose-dependent relaxation of mesenteric arteries precontracted with norepinephrine (NE) ex vivo. This depended on intact endothelium and was blocked by genistein, wortmannin, or N(omega)-nitro-l-arginine methyl ester (inhibitors of tyrosine kinase, PI3-kinase, and NO synthases, respectively). Vasodilation in response to insulin (but not ACh) was impaired by 20% in SHR (vs. WKY, P < 0.005). Preincubation of arteries with insulin significantly reduced the contractile effect of NE by 20% in WKY but not SHR rats. In SHR, the effect of insulin to reduce NE-mediated vasoconstriction became evident when insulin pretreatment was accompanied by ET-1 receptor blockade (BQ-123, BQ-788). Similar results were observed during treatment with the MEK inhibitor PD-98059. In addition, insulin-stimulated secretion of ET-1 from primary endothelial cells was significantly reduced by pretreatment of cells with PD-98059 (but not wortmannin). We conclude that insulin resistance in SHR is accompanied by endothelial dysfunction in mesenteric vessels with impaired PI3-kinase-dependent NO production and enhanced MAPK-dependent ET-1 secretion. These results may reflect pathophysiology in other vascular beds that directly contribute to elevated peripheral vascular resistance and hypertension.

Insulin-stimulated endothelial nitric oxide release is calcium independent and mediated via protein kinase B

Insulin exerts a vasodilator effect by stimulating endothelial nitric oxide (NO) production. Studies in cultured cells suggest that insulin might activate endothelial nitric oxide synthase (eNOS) by an atypical, calcium-independent mechanism. This study investigates the mechanism of insulin-stimulated endothelial NO production in intact aortic wall. Real time fluorescence imaging with 4,5-diaminofluorescin diacetate (DAF-2 DA) or 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM DA) and FURA 2-AM was used to simultaneously visualise NO and intracellular calcium concentrations at multiple locations in the endothelium and vascular smooth muscle of isolated rat and mouse aorta after exposure to insulin. Inhibitors of intracellular insulin signalling were used to determine the pathway for insulin-stimulated NO production. Unlike acetylcholine, which stimulated endothelial NO production with a typical increase in free intracellular calcium, insulin (10(-8) to 10(-6)M) stimulated endothelial NO production without elevating intracellular calcium levels. Insulin-stimulated NO production was concentration dependent and detected within 30s of application. Peak increases in NO occurred between 60 and 120 s and declined slowly thereafter. Separate measurements of NO production by fluorescence of 2,3-diaminonaphthalene (DAN) noted that selective inhibitors of phosphatidylinositol 3-kinase (PI3K) and protein kinase B (PKB) inhibited insulin-stimulated NO production, whereas these inhibitors alone did not alter NO production or acetylcholine-stimulated NO production. Insulin-stimulated NO production by endothelium is an acute calcium-independent effect mediated via the PI3K-PKB signalling pathway.

Reduced cardiac expression of plasminogen activator inhibitor 1 and transforming growth factor beta1 in obese Zucker rats by perindopril

OBJECTIVE

To determine whether angiotensin converting enzyme inhibition by perindopril can reduce cardiac transforming growth factor beta1 (TGFbeta1) and plasminogen activator inhibitor 1 (PAI-1) and therefore control collagen accumulation in an animal model with the metabolic syndrome such as the obese Zucker rat (OZR).

ANIMALS

Male OZR (group 1, n = 10); OZR treated with perindopril (group 2, n = 10); and lean Zucker rats (group 3, n = 10).

METHODS

During six months, group 2 received 3 mg/kg/day of perindopril orally and group 1 and group 3 were given a vehicle. Hearts were processed for pathology studies including immunohistochemical analysis with antibodies to PAI-1, TGFbeta1, collagen type I, and collagen type III.

RESULTS

Group 2 had lower blood pressure (126.7 (2) v 148.6 (2.7) mm Hg, p < 0.01) than untreated OZR and had decreased cardiac PAI-1 (3.6 (0.4) v 13.5 (1.7)% of positive area/field, p < 0.01), TGFbeta1 in myocytes (0.13 (0.1) v 9.14 (4.7)%/area, p < 0.01) and in interstitium (19.8 (6.8) v 178.9 (27.4) positive cells/area, p < 0.01), collagen I (3 (0.8) v 13.3 (1)%/area, p < 0.01), collagen III (5 (0.6) v 9.5 (0.9)%/area, p < 0.01), and collagen I to collagen III ratio (0.59 (0.13) v 1.40 (0.15) p < 0.01) compared with untreated OZR.

CONCLUSION

These results suggest that perindopril reduces cardiac PAI-1 and TGFbeta1 and ameliorates cardiac fibrosis in a rat model with multiple cardiovascular risk factors.

Proatherosclerotic mechanisms involving protein kinase C in diabetes and insulin resistance

In diabetes and insulin resistance, activation of protein kinase C (PKC) in vascular cells may be a key link between elevated plasma and tissue concentrations of glucose and nonesterified fatty acids and abnormal vascular cell signaling. Initial studies of PKC activation in diabetes focused on microvascular complications, but increasing evidence supports that PKC plays a role in several mechanisms promoting atherosclerosis. This review explains how PKC is thought to be activated in diabetes and insulin resistance through de novo synthesis of diacylglycerol. Furthermore, the review summarizes studies that implicate PKC in promoting proatherogenic mechanisms or inhibiting antiatherogenic mechanisms, including studies of endothelial dysfunction; gene induction and activation of vascular NAD(P)H oxidase; endothelial nitric oxide synthase expression and function; endothelin-1 expression; growth, migration, and apoptosis of vascular smooth muscle cells; induction of adhesion molecules; and oxidized low-density lipoprotein uptake by monocyte-derived macrophages.

S-nitrosylation-dependent inactivation of Akt/protein kinase B in insulin resistance

Inducible nitric-oxide synthase (iNOS) has been implicated in many human diseases including insulin resistance. However, how iNOS causes or exacerbates insulin resistance remains largely unknown. Protein S-nitrosylation is now recognized as a prototype of a redox-dependent, cGMP-independent signaling component that mediates a variety of actions of nitric oxide (NO). Here we describe the mechanism of inactivation of Akt/protein kinase B (PKB) in NO donor-treated cells and diabetic (db/db) mice. NO donors induced S-nitrosylation and inactivation of Akt/PKB in vitro and in intact cells. The inhibitory effects of NO donor were independent of phosphatidylinositol 3-kinase and cGMP. In contrast, the concomitant presence of oxidative stress accelerated S-nitrosylation and inactivation of Akt/PKB. In vitro denitrosylation with reducing agent reactivated recombinant and cellular Akt/PKB from NO donor-treated cells. Mutated Akt1/PKBalpha (C224S), in which cysteine 224 was substituted by serine, was resistant to NO donor-induced S-nitrosylation and inactivation, indicating that cysteine 224 is a major S-nitrosylation acceptor site. In addition, S-nitrosylation of Akt/PKB was increased in skeletal muscle of diabetic (db/db) mice compared with wild-type mice. These data suggest that S-nitrosylation-mediated inactivation may contribute to the pathogenesis of iNOS- and/or oxidative stress-involved insulin resistance.

The PI3-K/Akt pathway: roles related to alterations in vasomotor responses in diabetic models

Macro- and microvascular disease states currently represent the principal causes of morbidity and mortality in patients with type I or type II diabetes mellitus. Abnormal vasomotor responses and impaired endothelium-dependent vasodilation have been demonstrated in various beds in different animal models of diabetes and in humans with type I or type II diabetes. Several mechanisms leading to endothelial dysfunction have been reported, including changes in substrate avail ability, impaired release of NO, and increased destruction of NO. The principal mediators of diabetes-associated endothelial dysfunction are (a) increases in oxidized low density lipoprotein, endothelin-1, angiotensin II, oxidative stress, and (b) decreases in the actions of insulin or growth factors in endothelial cells. An accumulating body of evidence indicates that abnormal regulation of the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway may be one of several factors contributing to vascular dysfunction in diabetes. The PI3-K pathway, which activates serine/threonine protein kinase Akt, enhances NO synthase phosphorylation and NO production. Several studies suggest that in diabetes the relative ineffectiveness of insulin and the hyperglycemia act together to reduce activity in the insulin-receptor substrates (IRS)/PI3-K/Akt pathway, resulting in impairments of both IRS/PI3-K/Akt-mediated endothelial function and NO production. This article summarizes the PI3-K/Akt pathway-mediated contraction and relaxation responses induced by various agents in the blood vessels of diabetic animals.

How does blood glucose control with insulin save lives in intensive care?

Patients requiring prolonged intensive care are at high risk for multiple organ failure and death. Insulin resistance and hyperglycemia accompany critical illness, and the severity of this "diabetes of stress" reflects the risk of death. Recently it was shown that preventing hyperglycemia with insulin substantially improves outcome of critical illness. This article examines some potential mechanisms underlying prevention of glucose toxicity as well as the effects of insulin independent of glucose control. Unraveling the molecular mechanisms will provide new insights into the pathogenesis of multiple organ failure and open avenues for novel therapeutic strategies.

Impairment of PI3-K/Akt Pathway Underlies Attenuated Endothelial Function in Aorta of Type 2 Diabetic Mouse Model

The phosphatidylinositol 3-kinase (PI3-K) pathway, which activates serine/threonine protein kinase Akt, enhances endothelial nitric oxide synthase (eNOS) phosphorylation and nitric oxide (NO) production. We investigated the involvement of the PI3-K/Akt pathway in the relaxation responses to acetylcholine (ACh) and clonidine in a new type 2 diabetic model (streptozotocin plus nicotinamide-induced diabetic mice). Plasma glucose and insulin levels were significantly elevated in our model, and intravenous glucose tolerance tests revealed clear abnormalities in glucose tolerance and insulin responsiveness. Although in our model the ACh-induced relaxation and NO x − (NO 2 − +NO 3 − )/cGMP production were unchanged, the clonidine-induced and insulin-induced relaxations and NO x − /cGMP production were all greatly attenuated. In control mice, the clonidine-induced and insulin-induced relaxations were each abolished by LY294002 and by Wortmannin (inhibitors of PI3-K), and also by Akt-inhibitor treatment. The ACh-induced relaxation was unaffected by such treatments in either group of mice. The expression level of total Akt protein was significantly decreased in the diabetic mice aorta, but those for the p85 and p110γ subunits of PI3-K were not. The clonidine-induced Ser-473 phosphorylation of Akt through PI3-K was significantly decreased in our model; however, that induced by ACh was not. These results suggest that relaxation responses and NO production mediated via the PI3-K/Akt pathway are decreased in this type 2 diabetic model. This may be a major cause of endothelial dysfunction (and the resulting hypertension) in type 2 diabetes.

The role of insulin and the adipocytokines in regulation of vascular endothelial function

Vascular integrity in the healthy endothelium is maintained through the release of a variety of paracrine factors such as NO (nitric oxide). Endothelial dysfunction, characterized by reduced NO bioavailability, is associated with obesity, insulin resistance and Type II diabetes. Insulin has been demonstrated to have direct effects on the endothelium to increase NO bioavailability. Therefore altered insulin signalling in the endothelium represents a candidate mechanism underlying the association between insulin resistance and endothelial dysfunction. In recent years, it has become apparent that insulin sensitivity is regulated by the adipocytokines, a group of bioactive proteins secreted by adipose tissue. Secretion of adipocytokines is altered in obese individuals and there is increasing evidence that the adipocytokines have direct effects on the vascular endothelium. A number of current antidiabetic strategies have been demonstrated to have beneficial effects on endothelial function and to alter adipocytokine concentrations in addition to their effects on glucose homoeostasis. In this review we will explore the notion that the association between insulin resistance and endothelial dysfunction is accounted for by adipocytokine action on the endothelium. In addition, we examine the effects of weight loss, exercise and antidiabetic drugs on adipocytokine availability and endothelial function.

Vasoconstrictor effects of insulin in skeletal muscle arterioles are mediated by ERK1/2 activation in endothelium

Insulin exerts both NO-dependent vasodilator and endothelin-dependent vasoconstrictor effects on skeletal muscle arterioles. The intracellular enzymes 1-phosphatidylinositol 3-kinase (PI3-kinase) and Akt have been shown to mediate the vasodilator effects of insulin, but the signaling molecules involved in the vasoconstrictor effects of insulin in these arterioles are unknown. Our objective was to identify intracellular mediators of acute vasoconstrictor effects of insulin on skeletal muscle arterioles. Rat cremaster first-order arterioles ( n = 40) were isolated, and vasoreactivity to insulin was studied using a pressure myograph. Insulin induced dose-dependent vasoconstriction of skeletal muscle arterioles (up to −22 ± 3% of basal diameter; P < 0.05) during PI3-kinase inhibition with wortmannin (50 nmol/l). Insulin-induced vasoconstriction was abolished by inhibition of extracellular signal-regulated kinase 1/2 (ERK1/2) with PD-98059 (40 μmol/l). In addition, inhibition of ERK1/2 without PI3-kinase inhibition uncovered insulin-mediated vasodilatation in skeletal muscle arterioles (up to 37 ± 10% of baseline diameter; P < 0.05). Effects of insulin on ERK1/2 activation in arterioles were then investigated by Western blot analysis. Insulin induced a transient 2.4-fold increase in ERK1/2 phosphorylation (maximal at ∼15 min) in skeletal muscle arterioles ( P < 0.05). Removal of the arteriolar endothelium abolished insulin-induced vasoconstriction, which suggests that activation of ERK1/2 in endothelial cells is involved in acute insulin-mediated vasoconstriction. To investigate this, acute effects of insulin on ERK1/2 phosphorylation were studied in human microvascular endothelial cells. In support of the findings in skeletal muscle arterioles, insulin induced a 1.9-fold increase in ERK1/2 phosphorylation (maximal at ∼15 min) in microvascular endothelial cells ( P < 0.05). We conclude that acute vasoconstrictor effects of insulin in skeletal muscle arterioles are mediated by activation of ERK1/2 in endothelium. This ERK1/2-mediated vasoconstrictor effect antagonizes insulin-induced, PI3-kinase-dependent vasodilatation in skeletal muscle arterioles. These findings provide a novel mechanism by which insulin may determine blood flow and glucose disposal in skeletal muscle.

Molecular mechanisms of insulin resistance that impact cardiovascular biology

Insulin resistance is concomitant with type 2 diabetes, obesity, hypertension, and other features of the metabolic syndrome. Because insulin resistance is associated with cardiovascular disease, both scientists and physicians have taken great interest in this disorder. Insulin resistance is associated with compensatory hyperinsulinemia, but individual contributions of either of these two conditions remain incompletely understood and a subject of intense investigation. One possibility is that in an attempt to overcome the inhibition within the metabolic insulin-signaling pathway, hyperinsulinemia may continue to stimulate the mitogenic insulin-signaling pathway, thus exerting its detrimental influence. Here we discuss some of the effects of insulin resistance and mechanisms of potentially detrimental influence of hyperinsulinemia in the presence of metabolic insulin resistance.

Free fatty acid levels modulate microvascular function: relevance for obesity-associated insulin resistance, hypertension, and microangiopathy

To test the hypothesis that free fatty acids (FFAs) modulate microvascular function and that this contributes to obesity-associated insulin resistance, hypertension, and microangiopathy, we examined the effects of both FFA elevation in lean women and FFA lowering in obese women on skin microvascular function. A total of 16 lean and 12 obese women underwent, respectively, Intralipid plus heparin (or saline) infusion and overnight acipimox (or placebo) treatment. We measured capillary recruitment with capillaroscopy and endothelium-(in)dependent vasodilation by iontophoresis of acetylcholine and sodium nitroprusside before and during hyperinsulinemia (40 mU . m(-2) . min(-1)). FFA elevation impaired capillary recruitment and acetylcholine-mediated vasodilation before (44.6 +/- 16.8 vs. 56.9 +/- 18.9%, P < 0.05; and 338 +/- 131 vs. 557 +/- 162%, P < 0.01, respectively) and during (54.0 +/- 21.3 vs. 72.4 +/- 25.4%, P < 0.01; and 264 +/- 186 vs. 685 +/- 199%, P < 0.01, respectively) hyperinsulinemia. FFA lowering improved capillary recruitment before (50.9 +/- 14.6 vs. 37.4 +/- 9.3%, P < 0.01) and during (66.8 +/- 20.6 vs. 54.8 +/- 15.4%, P < 0.05) hyperinsulinemia. Changes in FFA levels were inversely associated with changes in capillary recruitment and insulin sensitivity in lean (r = -0.46, P = 0.08; and r = -0.56, P = 0.03) and in obese (r = -0.70, P = 0.02; and r = -0.62, P = 0.04) women. Regression analyses showed that changes in capillary recruitment statistically explained approximately 29% of the association between changes in FFA levels and insulin sensitivity. In conclusion, FFA levels modulate microvascular function and may contribute to obesity-associated insulin resistance, hypertension, and microangiopathy.

How does blood glucose control with insulin save lives in intensive care?

Patients requiring prolonged intensive care are at high risk for multiple organ failure and death. Insulin resistance and hyperglycemia accompany critical illness, and the severity of this "diabetes of stress" reflects the risk of death. Recently it was shown that preventing hyperglycemia with insulin substantially improves outcome of critical illness. This article examines some potential mechanisms underlying prevention of glucose toxicity as well as the effects of insulin independent of glucose control. Unraveling the molecular mechanisms will provide new insights into the pathogenesis of multiple organ failure and open avenues for novel therapeutic strategies.

Insulin activates vascular endothelial growth factor in vascular smooth muscle cells: influence of nitric oxide and of insulin resistance

BACKGROUND

We aimed to evaluate whether insulin influences vascular endothelial growth factor (VEGF) synthesis and secretion in cultured vascular smooth muscle cells (VSMCs) via nitric oxide (NO) and whether these putative effects are lost in insulin-resistant states.

MATERIALS AND METHODS

In VSMC derived from human arterioles and from aortas of insulin-sensitive Zucker fa/+rats and insulin-resistant Zucker fa/fa rats incubated with different concentrations of human regular insulin with or without inhibitors of phosphatidylinositol 4,5-bisphosphate 3-kinase (PI3-K), mitogen-activated protein kinase (MAPK), nitric oxide synthase (NOS) and guanosine 3',5'cyclic monophosphate(cGMP)-dependent protein kinase (PKG), we measured protein expression (Western blot) and secretion (ELISA) of VEGF.

RESULTS

We found that in VSMCs from humans and from insulin-sensitive Zucker fa/+rats, insulin increases VEGF protein expression and secretion, with mechanisms blunted by wortmannin and LY294002 (PI3-K inhibitors), PD98059 (MAPK inhibitor), L-NMMA (NOS inhibitor) and Rp-8pCT-cGMPs (PKG inhibitor). Also the NO donor sodium nitroprusside (SNP) and the cGMP analogue 8-Bromo-cGMP increase VEGF protein expression and secretion, with mechanisms inhibited by wortmannin and PD98059. The insulin effects on VEGF are impaired in VSMCs from Zucker fa/fa rats, which also present a reduced insulin ability to increase NO.

CONCLUSIONS

In VSMCs from humans and insulin-sensitive Zucker fa/+rats: (i) insulin increases VEGF protein expression and secretion via both PI3-K and MAPK; (ii) the insulin effects on VEGF are mediated by nitric oxide. The insulin action on both nitric oxide and VEGF is impaired in VSMCs from Zucker fa/fa rats, an animal model of metabolic and vascular insulin-resistance.

Increased insulin receptor substrate 1 serine phosphorylation and stress-activated protein kinase/c-Jun N-terminal kinase activation associated with vascular insulin resistance in spontaneously hypertensive rats

Insulin resistance is associated with cardiovascular disease. Impaired insulin receptor substrate (IRS)-mediated signal transduction is a major contributor to insulin resistance. Recently, IRS-1 phosphorylation at serine 307 by stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) has been highlighted as a molecular event that causes insulin resistance. We investigated IRS-1-mediated insulin signaling, IRS-1 phosphorylation at serine 307, and SAPK/JNK activation status in the aorta of spontaneously hypertensive rats (SHR) by immunoprecipitation and immunoblotting. Insulin-stimulated tyrosine phosphorylation of insulin receptor and IRS-1 in SHR was decreased to 55% (P<0.01) and 40% (P<0.01) of the levels in Wistar-Kyoto rats (WKY), respectively. Insulin-stimulated IRS-1-associated phosphatidylinositol 3-kinase activation in SHR was reduced to 28% of the level in WKY (P<0.0001). Immunoblot analysis revealed that phosphorylated IRS-1 at serine 307 in SHR was increased to 261% (P<0.001) of the level in WKY. Phosphorylated (activated) SAPK/JNK in SHR was increased to 223% of the level in WKY (P<0.01). Serine-phosphorylated IRS-1 that was immunoprecipitated from the aorta of SHR was capable of inhibiting in vitro tyrosine phosphorylation by recombinant insulin receptor compared with WKY-derived IRS-1. These findings demonstrate that insulin resistance in the aorta of SHR was associated with elevated IRS-1 phosphorylation at serine 307 and increased SAPK/JNK activation. The present study suggests that increased SAPK/JNK activation may play an important role in the pathogenesis of vascular insulin resistance via inhibitory serine phosphorylation of IRS-1.

Endothelial dysfunction, inflammation, and insulin resistance: a focus on subjects at risk for type 2 diabetes

Subjects with obesity, family history of type 2 diabetes, polycystic ovary syndrome, previous gestational diabetes, dyslipidemia, hypertension, impaired glucose tolerance or impaired fasting glucose, and those with metabolic syndrome are at risk for the development of type 2 diabetes. Some of them are also at risk for cardiovascular disease. Some underlying abnormalities such as insulin resistance, endothelial dysfunction, and low-grade chronic inflammation are frequently present and closely associated in all these groups. The flow of substrates, hormones, and cytokines from visceral fat to skeletal muscle and to the endothelial cells, along with some genetic abnormalities that lead to impaired insulin action in the peripheral tissues and to impaired insulin-stimulated nitric oxide production in endothelial cells, may play a role in establishing these shared metabolic and vascular derangements. Weight loss, thiazolidinediones, and metformin improve vascular function in subjects at risk for type 2 diabetes and may prove to reduce cardiovascular events in these individuals.

Insulin Regulation of Sterol Regulatory Element-binding Protein-1 Expression in L-6 Muscle Cells and 3T3 L1 Adipocytes

Sterol regulatory element-binding proteins (SREBPs) are transcription factors that regulate enzymes required for cholesterol and fatty acid synthesis. Expression of SREBP-1 is enhanced by insulin; however, the actual insulin-signaling cascades employed are yet unclear. We determined the roles of the phosphatidylinositol (PI) 3-kinase and mitogen-activated protein (MAP) kinase-dependent pathways in the effect of mediating insulin on SREBP-1 in L-6 skeletal muscle cells and 3T3 L1 adipocytes, using wortmannin or LY294002 to inhibit the PI 3-kinase pathway, and PD98059 to inhibit the MAP kinase-dependent pathway. In myocytes, insulin increased SREBP-1 protein in a dose-dependent manner. 1 and 10 nm insulin significantly increased expression of total cellular SREBP-1 protein at 24 and 48 h, nuclear SREBP-1 protein at 24 h, and SREBP-1a mRNA at 24 h. Although wortmannin and LY294002 had no effect on this aspect of insulin action, PD98059 completely blocked each of these responses. Transfection of a dominant negative mutant of Ras similarly blocked the insulin effect on SREBP-1. In contrast, in adipocytes, the insulin effect on SREBP-1 was mediated via the PI 3-kinase and not the MAP kinase pathway. In conclusion, although insulin increases skeletal muscle SREBP-1 expression in a dose-dependent fashion via the MAP kinase-dependent signaling pathway, insulin action on adipocyte SREBP-1 is mediated via the PI 3-kinase signaling pathway. In the state of insulin resistance, characterized by selective inhibition of the PI 3-kinase pathway, the usual stimulation of lipogenesis by insulin in adipocytes may be inhibited, whereas intramyocellular lipogenesis via the MAP kinase pathway of insulin may continue unabated.

Pathogenesis of type 2 diabetes mellitus

This article provides an overview of the pathogenesis of type 2 diabetes mellitus. Discussion begins by describing normal glucose homeostasis and ingestion of a typical meal and then discusses glucose homeostasis in diabetes. Topics covered include insulin secretion in type 2 diabetes mellitus and insulin resistance, the site of insulin resistance, the interaction between insulin sensitivity and secretion, the role of adipocytes in the pathogenesis of type 2 diabetes, cellular mechanisms of insulin resistance including glucose transport and phosphorylation, glycogen and synthesis,glucose and oxidation, glycolysis, and insulin signaling.

Stimulatory effect of leptin on nitric oxide production is impaired in dietary-induced obesity

OBJECTIVE

We investigated the effect of leptin on nitric oxide production in lean and rats made obese by a high-calorie diet.

RESEARCH METHODS AND PROCEDURES

The animals were placed in metabolic cages, and urine was collected in 2-hour periods after leptin (1 mg/kg intraperintoneally) or vehicle administration. Blood was obtained 0.5, 1, 2, 4, or 6 hours after injection.

RESULTS

Leptin had no effect on systolic blood pressure in either lean or obese animals. Plasma concentration of NO metabolites (nitrites + nitrates, NOx) increased in lean rats by 31.5%, 58.0%, and 27.9% at 1, 2, and 4 hours after leptin injection, respectively. In the obese group, plasma NOx increased only at 2 hours (+36.5%). Leptin increased urinary NOx excretion by 31.8% in the first 2-hour period after injection in lean but not in obese rats. In lean animals, leptin elevated plasma cyclic 3',5'-guanosine monophosphate (cGMP) at 1, 2, and 4 hours by 35.3%, 96.3%, and 57.3%, respectively. In the obese group, plasma cGMP was higher only at 2 and 4 hours (+44.6% and +32.1%, respectively). Urinary excretion of cGMP increased in lean animals by 67.1% in the first period and by 50.4% in the second period. In the obese group, leptin induced a 53.9% increase in urinary cGMP excretion only in the first 2-hour period.

DISCUSSION

The stimulatory effect of leptin on NO production is impaired in dietary-induced obesity; however, leptin does not increase blood pressure in obese animals, suggesting that other NO-independent depressor mechanisms are stimulated.

Angiotensin II impairs the insulin signaling pathway promoting production of nitric oxide by inducing phosphorylation of insulin receptor substrate-1 on Ser312 and Ser616 in human umbilical vein endothelial cells

It has been suggested that serine (Ser) phosphorylation of insulin receptor substrate-1 (IRS-1) decreases the ability of IRS-1 to be phosphorylated on tyrosine, thereby attenuating insulin signaling. There is evidence that angiotensin II (AII) may impair insulin signaling to the IRS-1/phosphatydilinositol 3-kinase (PI 3-kinase) pathway by enhancing Ser phosphorylation. Insulin stimulates NO production by a pathway involving IRS-1/PI3-kinase/Akt/endothelial NO synthase (eNOS). We addressed the question of whether AII affects insulin signaling involved in NO production in human umbilical vein endothelial cells and tested the hypothesis that the inhibitory effect of AII on insulin signaling was caused by increased site-specific Ser phosphorylation in IRS-1. Exposure of human umbilical vein endothelial cells to AII resulted in inhibition of insulin-stimulated production of NO. This event was associated with impaired IRS-1 phosphorylation at Tyr612 and Tyr632, two sites essential for engaging the p85 subunit of PI3-kinase, resulting in defective activation of PI 3-kinase, Akt, and eNOS. This inhibitory effect of AII was reversed by the type 1 receptor antagonist losartan. AII increased c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) 1/2 activity, which was associated with a concomitant increase in IRS-1 phosphorylation at Ser312 and Ser616, respectively. Inhibition of JNK and ERK1/2 activity reversed the negative effects of AII on insulin-stimulated NO production. Our data suggest that AII, acting via the type 1 receptor, increases IRS-1 phosphorylation at Ser312 and Ser616 via JNK and ERK1/2, respectively, thus impairing the vasodilator effects of insulin mediated by the IRS-1/PI 3-kinase/Akt/eNOS pathway.

Endothelial dysfunction in obesity and insulin resistance: a road to diabetes and heart disease

Obesity, insulin resistance, and endothelial dysfunction closely coexist throughout the natural history of type 2 diabetes. They all can be identified not only in people with type 2 diabetes, but also in various groups at risk for the disease, such as individuals with impaired glucose tolerance, family history of type 2 diabetes, hypertension, dyslipidemia, prior gestational diabetes, or polycystic ovary syndrome. Whereas their evident association cannot fully establish a cause-effect relationship, fascinating mechanisms that bring them closer together than ever before are rapidly emerging. Central or abdominal obesity leads to insulin resistance and endothelial dysfunction through fat-derived metabolic products, hormones, and cytokines. Insulin resistance leads to endothelial dysfunction through the frequent association with traditional cardiovascular risk factors and through some more direct novel mechanisms. Some specific and shared insulin signaling abnormalities in muscle, fat, and endothelial cells, as well as some new genetic and nontraditional factors, may contribute to this interesting association. Some recent clinical studies demonstrate that nonpharmacological and pharmacological strategies targeting obesity and/or insulin resistance ameliorate endothelial function and low-grade inflammation. All these findings have added a new dimension to the association of obesity, insulin resistance, and endothelial dysfunction that may become a key target in the prevention of type 2 diabetes and cardiovascular disease.

Malfunction of Vascular Control in Lifestyle-Related Diseases: Mechanisms Underlying Endothelial Dysfunction in the Insulin-Resistant State

It is tempting to speculate that increased vasoconstriction and loss of endothelium-dependent vasodilation might be etiological factors of elevated blood pressure in the insulin-resistant state. Vascular contraction induced by angiotensin II and the expression of NAD(P)H oxidase were increased in the aorta of insulin-resistant mice. In addition, both angiotensin II type 1 receptor expression and superoxide anion production were up-regulated in these mice. Another mechanism for imparing endothelial function is the uncoupling of endothelial nitric oxide synthase (eNOS). It has become clear from studies on the aorta of insulin-resistant rat that insulin resistance may be a pathogenic factor for endothelial dysfunction through impaired eNOS activity and increased oxidative breakdown of NO (nitric oxide) due to an enhanced formation of superoxide anion (NO/superoxide anion imbalance), which are caused by relative deficiency of tetrahydrobiopterin, a cofactor of NOS, in vascular endothelial cells. Supplementation of tetrahydrobiopterin restored endothelial function and relieved oxidative tissue damage through activation of eNOS in those rats. These results indicate that generation of superoxide anion from NAD(P)H oxidases and an uncoupled eNOS may be pathogenic factors for impaired endothelial function and hypertension in the insulin-resistant state.

Insulin and insulin resistance: impact on blood pressure and cardiovascular disease

Cardiovascular disease is a major cause of mortality in individuals with diabetes. Many factors, including hypertension, contribute to the high prevalence of CVD in this population. Hypertension occurs approximately twice as frequently in patients with diabetes compared with patients without diabetes. Conversely, recent data suggest that hypertensive persons are more likely to develop diabetes than normotensive persons. In addition, up to 75% of CVD in patients with diabetes may be attributed to hypertension, leading to recommendations for more aggressive blood pressure control (ie, < 130/85 mm Hg) in persons with coexistent diabetes and hypertension. Increasing obesity further contributes to both diabetes and hypertension and significantly increases CVD morbidity and mortality. Other important risk factors for CVD in these patients include atherosclerosis, dyslipidemia, microalbuminuria, endothelial dysfunction, platelet hyperaggregability, coagulation abnormalities, and diabetic cardiomyopathy. The current knowledge regarding these risk factors has been reviewed, placing special emphasis on the metabolic syndrome, hypertension, microalbuminuria, and the role of obesity in these disorders. Although not discussed in detail, it is acknowledged that both hygienic measures (weight loss and aerobic exercise) and treatment strategies that include aspirin, statins, INS sensitizers, and antihypertensive agents that reduce renin-angiotensin-aldosterone system activity have been shown to reduce inflammation, coagulation abnormalities, endothelial function, proteinuria, and in some cases reduce CVD and renal disease progression. Additional therapeutic agents are currently being developed specifically to improve INS sensitivity and other CVD risk factors that are components of the cardiometabolic syndrome.

Thiazolidinedione regulation of smooth muscle cell proliferation

Vascular smooth muscle cells (VSMCs) in the media of adult arteries are normally quiescent, proliferate at low frequency, and are arrested in the G(0)/G(1) phase of the cell cycle. Proliferation of VSMCs occurs in response to arterial injury and plays a crucial role in the atherosclerotic process and in the pathogenesis of restenosis. Patients with type 2 diabetes mellitus are at increased risk for postangioplasty restenosis, which results from excessive intimal hyperplasia. Insulin sensitizers of the thiazolidinedione (TZD) class inhibit growth of VSMCs by attenuating the activity of important cell-cycle regulators. The TZDs inhibit progression from G(1) to S phase in the cell cycle by blocking growth factor-induced phosphorylation of retinoblastoma tumor suppressor protein (Rb). In animal models of restenosis, TZDs inhibit intimal hyperplasia after mechanical injury in both insulin-sensitive and insulin-resistant vessels. Preliminary clinical studies using troglitazone demonstrate less intimal hyperplasia with this TZD after implantation of coronary stents in individuals with type 2 diabetes. Further large trials are needed to confirm that treatment with a TZD can protect against postangioplasty restenosis.

Enhanced insulin signaling via Shc in human breast cancer

Insulin is a mild mitogen and has been shown to potentiate mitogenic influence of other growth factors. Because hyperinsulinemia and/or overexpression of insulin receptors have been linked to development, progression, and outcome of breast cancer, we attempted to evaluate the mechanism of these associations. We have compared the expression of insulin receptors and the magnitude of insulin signaling in breast tumors and adjacent normal mammary tissue samples obtained from 20 patients. We observed that insulin binding more than doubled in the tumors as compared with the normal tissue (P <.01 by paired t test). Insulin signaling to Shc, judged by the magnitude of its phosphorylation, was also significantly enhanced in the tumors. In contrast, the phosphorylation of the insulin-receptor substrate-1 (IRS-1), Akt, and mitogen-activated protein (MAP) kinase were identical in the tumorous and normal mammary tissues. Finally, tumors displayed significantly increased amounts of farnesylated p21 Ras and geranylgeranylated Rho-A (P <.01), consistent with Shc-dependent activation of farnesyl (FTase) and geranylgeranyl transferases (GGTase) in the tumor tissue. We conclude that the mechanism of the mitogenic influence of insulin in breast cancer may include increased expression of insulin receptors, preferential hyperphosphorylation of Shc, and increased amounts of prenylated p21 Ras and Rho-A in tumor tissue as compared with adjacent normal mammary tissue.

Glucocorticoids and insulin sensitivity: dissociation of insulin's metabolic and vascular actions

Insulin sensitivity in tissues such as a skeletal muscle and fat is closely correlated with insulin action in the vasculature, but the mechanism underlying this is unclear. We investigated the effect of dexamethasone on insulin-stimulated glucose disposal and vasodilation in healthy males to test the hypothesis that a reduction in glucose disposal would be accompanied by a reduction in insulin action in the vasculature. We performed a double-blind, placebo-controlled, cross-over trial comparing insulin sensitivity (measured by the euglycemic hyperinsulinemic clamp) and vascular insulin action (measured by small vessel wire myography) in young healthy males allocated to placebo or 1 mg dexamethasone twice daily for 6 d, each in random order. Six days of dexamethasone therapy was associated with a 30% (95% confidence interval, 19.1-40.0%) fall in insulin sensitivity. Despite this, there was no difference in insulin-mediated vasodilation between phases. Dexamethasone had no effect on circulating markers of endothelial function, such as D-dimer, von Willebrand factor, and tissue plasminogen activator. By short-term exposure to high dose dexamethasone we were able to differentially affect the metabolic and vascular actions of insulin. This implies that, using this model, there is physiological uncoupling of the effects of insulin in different tissues.

Selective impairment of insulin signalling in the hypothalamus of obese Zucker rats

AIM/HYPOTHESIS

By acting in the brain, insulin suppresses food intake. However, little is known with regard to insulin signalling in the hypothalamus in insulin-resistant states.

METHODS

Western blotting, immunohistochemistry and polymerase chain reaction assays were combined to compare in vivo hypothalamic insulin signalling through the PI3-kinase and MAP kinase pathways between lean and obese Zucker rats.

RESULTS

Intracerebroventricular insulin infusion reduced food intake in lean rats to a greater extent than that observed in obese rats, and pre-treatment with PI3-kinase inhibitors prevented insulin-induced anorexia. The relative abundance of IRS-2 was considerably higher than that of IRS-1 in hypothalamus of both lean and obese rats. Insulin-stimulated phosphorylation of IR, IRS-1/2, the associations of PI 3-kinase to IRS-1/2 and phosphorylation of Akt in hypothalamus were decreased in obese rats compared to lean rats. These effects seem to be mediated by increased phosphoserine content of IR, IRS-1/2 and decreased protein levels of IRS-1/2 in obese rats. In contrast, insulin stimulated the phosphorylation of MAP kinase equally in lean and obese rats.

CONCLUSION/INTERPRETATION

This study provides direct measurements of insulin signalling in hypothalamus, and documents selective resistance to insulin signalling in hypothalamus of Zucker rats. These findings provide support for the hypothesis that insulin could have anti-obesity actions mediated by the PI3-kinase pathway, and that impaired insulin signalling in hypothalamus could play a role in the development of obesity in this animal model of insulin-resistance.

Microvascular complications of impaired glucose tolerance

Impaired glucose tolerance (IGT) serves as a marker for the state of insulin resistance and predicts both large- and small-vessel vascular complications, independent of a patient's progression to diabetes. Patients with IGT are at significantly increased risk for death and morbidity due to myocardial infarction, stroke, and large-vessel occlusive disease. IGT is more predictive of cardiovascular morbidity than impaired fasting glucose, probably because it is a better surrogate for the state of insulin resistance. IGT is also independently associated with traditional microvascular complications of diabetes, including retinopathy, renal disease, and polyneuropathy, which are the topics of this review. Inhibition of nitric oxide-mediated vasodilation, endothelial injury due to increased release of free fatty acids and adipocytokines from adipocytes, and direct metabolic injury of endothelial and end-organ cells contribute to vascular complications. Early detection of IGT allows intensive diet and exercise modification, which has proven significantly more effective than drug therapy in normalizing postprandial glucose and inhibiting progression to diabetes. To what degree intervention will limit recognized complications is not known.

Tumor Necrosis Factor-α Inhibits Insulin’s Stimulating Effect on Glucose Uptake and Endothelium-Dependent Vasodilation in Humans

Background— Inflammatory mechanisms could be involved in the pathogenesis of both insulin resistance and atherosclerosis. Therefore, we aimed at examining whether the proinflammatory cytokine tumor necrosis factor (TNF)-α inhibits insulin-stimulated glucose uptake and insulin-stimulated endothelial function in humans.

Methods and Results— Healthy, lean male volunteers were studied. On each study day, 3 acetylcholine (ACh) or sodium nitroprusside (SNP) dose-response studies were performed by infusion into the brachial artery. Before and during the last 2 dose-response studies, insulin and/or TNF-α were coinfused. During infusion of insulin alone for 20 minutes, forearm glucose uptake increased by 220±44%. This increase was completely inhibited during coinfusion of TNF-α (started 10 min before insulin) with a more pronounced inhibition of glucose extraction than of blood flow. Furthermore, TNF-α inhibited the ACh forearm blood flow response ( P <0.001), and this inhibition was larger during insulin infusion ( P =0.01) but not further increased by N G -monomethyl- l -arginine acetate ( P =0.2). Insulin potentiated the SNP response less than the ACh response and the effect of TNF-α was smaller ( P <0.001); TNF-α had no effect on the SNP response without insulin infusion. Thus, TNF-α inhibition of the combined response to insulin and ACh was likely mediated through inhibition of NO production.

Conclusion— These results support the concept that TNF-α could play a role in the development of insulin resistance in humans, both in muscle and in vascular tissue.

Fructose-fed rats are protected against ischemia/reperfusion injury

This study examines the relationship between insulin resistance (IR) induced by fructose feeding (FF) and susceptibility to myocardial ischemia/reperfusion injury (MI/R). Six-week-old male Sprague-Dawley rats were randomized into control (CON; n = 59) or FF (n = 58) groups. After 4 weeks, rats were further randomized into one of the following groups: placebo, ischemic preconditioning (IPC), 5-hydroxydecanoic acid (5-HD) (10 mg/kg), or 5-HD + IPC. Moreover, to determine the role of fructose, a second model of IR (Zucker obese) and rats fed fructose diet for 3 days (FF-3) were also subjected to MI/R. In all experiments, rats were subjected to 30 min of myocardial ischemia and 4 h of reperfusion. In rats randomized to placebo, infarct size was significantly reduced by FF (24 +/- 5%) compared with CON (54 +/- 1%, p < 0.05). Pretreatment with 5-HD did not alter the infarct size in CON (45 +/- 5%) but inhibited the protection afforded by FF (53 +/- 7%). IPC reduced the infarct size to an equivalent level in both groups, whereas 5-HD administration prior to IPC blunted the IPC effect. In Zucker obese rats, infarct size was significantly larger (57 +/- 4%) compared with lean controls (37 +/- 4%, p < 0.05). In FF-3 rats, infarct size was also decreased (20 +/- 2%, p < 0.01) compared with CON. This study suggests that fructose feeding affords protection against MI/R that is related to or mimics preconditioning. This protection is not consistent with other models of IR and is likely related to the fructose diet itself.

Expression profiling identifies genes that continue to respond to insulin in adipocytes made insulin-resistant by treatment with tumor necrosis factor-alpha

We have employed microarray technology using RNA from normal 3T3-L1 adipocytes and from 3T3-L1 adipocytes made insulin-resistant by treatment with tumor necrosis factor-alpha to identify a new class of insulin-responsive genes. These genes continued to respond normally to insulin even though the adipocytes themselves were metabolically insulin-resistant, i.e. they displayed a significantly decreased rate of insulin-stimulated glucose uptake. Approximately 12,000 genes/expressed sequence tags (ESTs) were screened. Of these, 40 genes/ESTs were identified that became insulin-resistant as expected (e.g. Socs-3, junB, and matrix metalloproteinase-11). However, 61 genes/ESTs continued to respond normally to insulin. Although some of these genes were previously shown to be regulated by insulin (e.g. Glut-1 and beta3-adrenergic receptor), other novel insulin-sensitive genes were also identified (e.g. Egr-1, epiregulin, Fra-1, and ABCA1). Real-time reverse transcription-PCR analysis confirmed the expression patterns of several of the differentially expressed genes. One gene that remained insulin-sensitive in the insulin-resistant adipocytes is the transcription factor Egr-1. Using an antisense strategy, we show that tissue factor and macrophage colony-stimulating factor, two cardiovascular risk factors, are downstream EGR-1 target genes in the adipocyte. Taken together, these data support the hypothesis that some signaling pathways remain insulin-sensitive in metabolically insulin-resistant adipocytes. These pathways may promote abnormal gene expression in hyperinsulinemic states like obesity and type II diabetes and thus may contribute to pathologies associated with these conditions.

Insulin affects vascular smooth muscle cell phenotype and migration via distinct signaling pathways

Insulin maintains vascular smooth muscle cell (VSMC) quiescence yet can also promote VSMC migration. The mechanisms by which insulin exerts these contrasting effects were examined using alpha-smooth muscle actin (alpha-SMA) as a marker of VSMC phenotype because alpha-SMA is highly expressed in quiescent but not migratory VSMC. Insulin alone maintained VSMC quiescence and modestly stimulated VSMC migration. Wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, decreased insulin-stimulated expression of alpha-SMA mRNA by 26% and protein by 48% but had no effect on VSMC migration. PD98059, a mitogen-activated protein kinase (MAPK) kinase inhibitor, decreased insulin-induced VSMC migration by 52% but did not affect alpha-SMA levels. Platelet-derived growth factor (PDGF) promoted dedifferentiation of VSMC, and insulin counteracted this effect. Furthermore, insulin increased alpha-SMA mRNA and protein levels to 111 and 118%, respectively, after PDGF-induced dedifferentiation, an effect inhibited by wortmannin. In conclusion, insulin's ability to maintain VSMC quiescence and reverse the dedifferentiating influence of PDGF is mediated via the PI3K pathway, whereas insulin promotes VSMC migration via the MAPK pathway. Thus, with impaired PI 3-kinase signaling and intact MAPK signaling, as seen in insulin resistance, insulin may lose its ability to maintain VSMC quiescence and instead promote VSMC migration.

A major health hazard: the metabolic syndrome

The clustering of several metabolic and cardiovascular disease risk factors has been termed the metabolic syndrome. The metabolic syndrome seems to result from a collision between susceptible "thrifty genes" and a society characterized by an increased prevalence of obesity and a sedentary lifestyle. The typical patient is characterized by abdominal obesity, a varying degree of glucose intolerance, dyslipidemia and often hypertension. The components of the metabolic syndrome are associated with insulin resistance, disturbances of coagulation and fibrinolysis, endothelial dysfunction and elevated markers of sub-clinical inflammation. The current review focuses mainly on the new definitions of the syndrome, the results of recent epidemiological studies and the consequences of the metabolic syndrome as an important risk factor for cardiovascular disease, premature death and diabetes. The metabolic syndrome constitutes a major challenge for public health professionals in the field of preventive medicine since more than 40 million U.S. adults seem to be affected by the syndrome. Lifestyle changes could have a profound influence on the syndrome and its development.

Insulin resistance and endothelial dysfunction

Insulin has multiple metabolic actions, including effects on blood vessels. Insulin normally increases blood flow by a mechanism which involves generation of nitric oxide (NO) via the arginine-NO pathway. Although insulin itself is a weak and physiologically unimportant vasodilatator, it appears to markedly potentiate endothelium-dependent vasodilatation. Therefore, anything that impairs insulin action in endothelial cells can be expected to be associated with endothelial dysfunction, i.e. loss of NO bioactivity in the vessel wall. Consistent with the idea that insulin resistance and endothelial dysfunction frequently coexist, all insulin-resistant conditions examined to date have been associated with endothelial dysfunction. However, the latter can also be caused by factors other than insulin resistance-such as a high concentration of low-density lipoprotein (LDL) cholesterol. Therapies which reverse insulin resistance-such as exercise, insulin and inhibitors of the renin-angiotensin-aldosterone (RAA) axis-also reverse endothelial dysfunction, which may thus be an inherent feature of insulin resistance.

Characterization of multiple signaling pathways of insulin in the regulation of vascular endothelial growth factor expression in vascular cells and angiogenesis

The effects of insulin on vascular endothelial growth factor (VEGF) expression in cultured vascular cells and in angiogenesis were characterized. Insulin increased VEGF mRNA levels in mouse aortic smooth muscle cells from 10(-9) to 10(-7) m with an initial peak of 3.7-fold increases at 1 h and a second peak of 2.8-fold after 12 h. The first peak of VEGF expression was inhibited by LY294002, an inhibitor of phosphatidylinositol (PI) 3-kinase, and by the overexpression of dominant negative forms of p85 subunit of PI 3-kinase or Akt. Inhibitors of MEK kinase, PD98059, or overexpression of dominant negative forms of Ras was ineffective. In contrast, the chronic effect of insulin on VEGF expression was partially inhibited by both LY294002 or PD98059 as well as by the overexpression of dominant negatives of PI 3-kinase or Ras. The importance of PI 3-kinase-Akt pathway on VEGF expression was confirmed in mouse aortic smooth muscle cells isolated from insulin receptor substrate -1 knockout (IRS-1-/-) mice that showed parallel reductions of 46-49% in insulin-stimulated VEGF expression and PI 3-kinase-Akt activation. Insulin-induced activation of PI 3-kinase-Akt on hypoxia-induced VEGF expression and neovascularization was reduced by 40% in the retina of neonatal hypoxia model using IRS-1-/- mice. Thus, unlike other cells, insulin can regulate VEGF expression by both IRS-1/PI 3-kinase-Akt cascade and Ras-MAPK pathways in aortic smooth muscle cells. The in vivo results provide direct evidence that insulin can modulate hypoxia-induced angiogenesis via reduction in VEGF expression in vivo.

Role of endothelial dysfunction in insulin resistance

The endothelium regulates vascular tone through the release of vasodilating and vasoconstricting substances. The most important of these vasodilating substances is nitric oxide (NO), which is also vascular protective and inhibits inflammation, oxidation, vascular smooth muscle cell proliferation, and migration. Damage to the endothelium causes endothelial dysfunction with impaired release of NO and loss of its antiatherogenic protection. Traditional risk factors for coronary artery disease, including diabetes, hypercholesterolemia, hypertension, and low levels of high-density lipoprotein cholesterol, are associated with endothelial dysfunction and thus promote the atherogenic process. More recently, insulin resistance in the absence of overt diabetes or the metabolic syndrome has been associated with endothelial dysfunction. This association provides evidence that the atherosclerotic process may actually begin earlier in the spectrum of insulin resistance, ultimately resulting in a progression of the metabolic syndrome to prediabetes and then to type 2 diabetes. Aggressive treatment of dyslipidemia and hypertension, even before the onset of type 2 diabetes, would appear prudent in decreasing the progression of the atherosclerotic process. The thiazolidinediones are peroxisome proliferator-activated receptor-gamma agonists that improve glucose and lipid metabolism. These agents have recently been shown to improve endothelial function in the early stages of insulin resistance. Results from ongoing trials with thiazolidinediones will reveal whether they will also reduce cardiovascular end points.

Tetrahydrobiopterin restores endothelial dysfunction induced by an oral glucose challenge in healthy subjects

An oral glucose challenge causes transient impairment of endothelial function, probably because of increased oxidative stress. During oxidative stress, endothelial nitric oxide (NO) synthase (eNOS) becomes uncoupled because of decreased bioavailability of tetrahydrobiopterin (BH4), an essential cofactor of eNOS. Therefore, we examined whether an acute supplement of BH4 could restore endothelial dysfunction induced by an oral glucose challenge. Healthy subjects were examined in 53 experiments. Forearm blood flow was measured by venous occlusion plethysmography. Dose-response studies were obtained during intra-arterial infusion of serotonin to elicit endothelium-dependent, NO-specific vasodilation and during sodium nitroprusside (SNP) infusion to elicit endothelium-independent vasodilation. Subjects were examined before (fasting) and 1 and 2 h after an oral glucose challenge (75 g) with serotonin (n = 10) and SNP (n = 8). On different days (6R)-5,6,7,8-tetrahydro-l-biopterin dihydrochloride (6R-BH4; n = 10), the active cofactor of eNOS or its stereoisomer (6S)-5,6,7,8-tetrahydro-l-biopterin sulfate (6S-BH4; n = 10), which is inactive as a cofactor, was added 10 min (500 microg/min) before and during the 1-h postchallenge serotonin dose-response study. In vitro studies showed that 6R-BH4 and 6S-BH4 were equipotent antioxidants. Serotonin response was reduced by 24 +/- 7% (at the highest dose) at 1 h postchallenge compared with fasting (P = 0.001) and was restored 2 h postchallenge. The reduction was reversed by the administration of 6R-BH4 but not by 6S-BH4. SNP responses were slightly increased 1 and 2 h postchallenge (increased by 15 +/- 13% at third dose 2 h postchallenge, P = 0.0001). An oral glucose challenge causes transient, NO-specific, endothelial dysfunction, which may be reversed by BH4. Transient postprandial endothelial dysfunction may be partly explained by reduced bioavailability of BH4 and NO.

Molecular and physiologic actions of insulin related to production of nitric oxide in vascular endothelium

Insulin has important vascular actions that regulate blood flow, in addition to its classical actions to coordinate glucose homeostasis. Insulin-stimulated production of nitric oxide in vascular endothelium results in capillary recruitment and vasodilation that diverts and increases blood flow to skeletal muscle and consequently increases glucose disposal. Thus, vascular actions of insulin may be essential for coupling hemodynamic and metabolic homeostasis. A complete biochemical signaling pathway linking the insulin receptor to activation of endothelial nitric oxide synthase in vascular endothelium has recently been elucidated. Moreover, the time course and dose response for capillary recruitment in response to physiologic concentrations of insulin parallels that of insulin-mediated glucose uptake in vivo. Taken together, these observations suggest a molecular mechanism that may help to explain how insulin resistance contributes to cardiovascular components of the metabolic syndrome and vascular complications of diabetes.

Lack of insulin receptor substrate-2 causes progressive neointima formation in response to vessel injury

BACKGROUND

Insulin resistance is associated with atherosclerosis, but its mechanism is unknown. It has been reported that insulin receptor substrate (IRS)-1 deficient (IRS-1-/-) mice showed insulin resistance without type 2 diabetes, whereas the IRS-2 deficient (IRS-2-/-) mice showed insulin resistance with type 2 diabetes.

METHODS AND RESULTS

We investigated neointima formation in the IRS-1-/- and IRS-2-/- mice at 8 and 20 weeks. The IRS-2-/- mice showed much greater neointima formation than the IRS-1-/- and wild-type mice at 8 weeks. At 20 weeks, the IRS-2-/- mice had greater neointima formation than the IRS-1-/- mice, which showed more enhanced neointima formation than the wild-type mice. The IRS-1-/- and IRS-2-/- mice had dyslipidemia, hypertension, and insulin resistance. The IRS-2-/- mice had more metabolic abnormalities than the IRS-1-/- mice at 8 and 20 weeks. IRS-2 expression was detected, but IRS-1 expression was not detected in the vessels.

CONCLUSIONS

The neointima formation in the IRS-1-/- and IRS-2-/- mice appears to be related to abnormalities induced by the altered metabolic milieu in insulin-resistant states. Moreover, because neointima formation was much greater in the IRS-2-/- mice than in the IRS-1-/- mice at 8 and 20 weeks, it is suggested that a lack of IRS-2 renders the vasculature more susceptible to injury in the abnormal metabolic milieu, and IRS-2 may have a protective effect on neointima formation. We conclude that IRS-2 is protective and retards the development of neointima formation in insulin-resistant states.