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Biomedical subjects

Michael J Quon

Publications and source records attributed to Michael J Quon.

At least 37 records · Page 2Linked to original sources
PubMed2005

Beneficial vascular and metabolic effects of peroxisome proliferator-activated receptor-alpha activators.

Fibric acid is a synthetic ligand of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-alpha that is highly expressed in skeletal muscle and heart, where it promotes beta-oxidation of fatty acids to mediate hypolipidemic actions. PPAR-alpha regulates expression of key proteins involved in atherogenesis, vascular inflammation, plaque instability, and thrombosis. Thus, PPAR-alpha may exert direct antiatherogenic actions in the vascular wall. Endothelial dysfunction associated with the metabolic syndrome and other insulin-resistant states is characterized by impaired insulin-stimulated nitric oxide production from the endothelium and decreased blood flow to skeletal muscle. Thus, improvement in insulin sensitivity leads to improved endothelial function. This may be an additional mechanism whereby fibrates decrease the incidence of coronary heart disease. Adiponectin is a protein secreted specifically by adipose cells that may couple regulation of insulin sensitivity with energy metabolism and serve to link obesity with insulin resistance. In this review, we discuss the mechanisms underlying the vascular and metabolic effects of fibrates that may act synergistically to prevent or regress atherosclerosis and coronary heart disease.

Atherosclerosis↗
PubMed2005

High-dose oral vitamin C partially replenishes vitamin C levels in patients with Type 2 diabetes and low vitamin C levels but does not improve endothelial dysfunction or insulin resistance.

Endothelial dysfunction is a hallmark of Type 2 diabetes related to hyperglycemia and oxidative stress. Nitric oxide-dependent vasodilator actions of insulin may augment glucose disposal. Thus endothelial dysfunction may worsen insulin resistance. Intra-arterial administration of vitamin C improves endothelial dysfunction in diabetes. In the present study, we investigated effects of high-dose oral vitamin C to alter endothelial dysfunction and insulin resistance in Type 2 diabetes. Plasma vitamin C levels in 109 diabetic subjects were lower than healthy (36 +/- 2 microM) levels. Thirty-two diabetic subjects with low plasma vitamin C (<40 microM) were subsequently enrolled in a randomized, double-blind, placebo-controlled study of vitamin C (800 mg/day for 4 wk). Insulin sensitivity (determined by glucose clamp) and forearm blood flow in response to ACh, sodium nitroprusside (SNP), or insulin (determined by plethysmography) were assessed before and after 4 wk of treatment. In the placebo group (n = 17 subjects), plasma vitamin C (22 +/- 3 microM), fasting glucose (159 +/- 12 mg/dl), insulin (19 +/- 7 microU/ml), and SI(Clamp) [2.06 +/- 0.29 x 10(-4) dl x kg(-1) x min(-1)/(microU/ml)] did not change significantly after placebo treatment. In the vitamin C group (n = 15 subjects), basal plasma vitamin C (23 +/- 2 microM) increased to 48 +/- 6 microM (P < 0.01) after treatment, but this was significantly less than that expected for healthy subjects (>80 microM). No significant changes in fasting glucose (156 +/- 11 mg/dl), insulin (14 +/- 2 microU/ml), SI(Clamp) [2.71 +/- 0.46 x 10(-4) dl x kg(-1) x min(-1)/(microU/ml)], or forearm blood flow in response to ACh, SNP, or insulin were observed after vitamin C treatment. We conclude that high-dose oral vitamin C therapy, resulting in incomplete replenishment of vitamin C levels, is ineffective at improving endothelial dysfunction and insulin resistance in Type 2 diabetes.

Administration, Oral↗
PubMed2005

Additive beneficial effects of fenofibrate combined with atorvastatin in the treatment of combined hyperlipidemia.

OBJECTIVES: We compared vascular and metabolic responses (and adverse responses) to statin and fibrate therapies alone or in combination in patients with combined hyperlipidemia. BACKGROUND: The mechanisms of action for statins and fibrates are distinct. METHODS: Fifty-six patients were given atorvastatin 10 mg and placebo, atorvastatin 10 mg and fenofibrate 200 mg, or fenofibrate 200 mg and placebo daily during each two-month treatment period of a randomized, double-blind, placebo-controlled crossover trial with two washout periods of two months' each. RESULTS: Lipoproteins were changed to a greater extent with combined therapy when compared with atorvastatin or fenofibrate alone. Flow-mediated dilator response to hyperemia and plasma high-sensitivity C-reactive protein and fibrinogen levels were changed to a greater extent with combined therapy when compared with atorvastatin or fenofibrate alone (p < 0.001, p = 0.182, and p = 0.015 by analysis of variance [ANOVA], respectively). The effects of combined therapy or fenofibrate alone on plasma adiponectin levels and insulin sensitivity (determined by the Quantitative Insulin-Sensitivity Check Index [QUICKI]) were significantly greater than those of atorvastatin alone (p = 0.022 for adiponectin and p = 0.049 for QUICKI by ANOVA). No patients were withdrawn from the study as the result of serious adverse effects. CONCLUSIONS: Combination therapy is safe and has beneficial additive effects on endothelial function in patients with combined hyperlipidemia.

Arteriosclerosis↗
PubMed2005

Vascular and metabolic effects of combined therapy with ramipril and simvastatin in patients with type 2 diabetes.

Mechanisms underlying biological effects of statin and angiotensin-converting enzyme inhibitor therapies differ. Therefore, we compared vascular and metabolic responses to these therapies either alone or in combination in patients with type 2 diabetes. This was a randomized, double-blind, placebo-controlled crossover trial with 3 treatment arms (each 2 months) and 2 washout periods (each 2 months). Fifty patients with type 2 diabetes were given simvastatin 20 mg and placebo, simvastatin 20 mg and ramipril 10 mg, or ramipril 10 mg and placebo daily during each 2-month treatment period. Ramipril alone or combined therapy significantly reduced blood pressure when compared with simvastatin alone. When compared with ramipril alone, simvastatin alone or combined therapy significantly improved the lipoprotein profile. All 3 treatment arms significantly improved flow-mediated dilator response to hyperemia and reduced plasma levels of malondialdehyde relative to baseline measurements. However, these parameters were changed to a greater extent with combined therapy when compared with simvastatin or ramipril alone (P<0.001 by ANOVA). When compared with simvastatin or ramipril alone, combined therapy significantly reduced high-sensitivity C-reactive protein levels (P=0.004 by ANOVA). Interestingly, combined therapy or ramipril alone significantly increased plasma adiponectin levels and insulin sensitivity relative to baseline measurements. These changes were significantly greater than in the group treated with simvastatin alone (P<0.015 by ANOVA). Ramipril combined with simvastatin had beneficial vascular and metabolic effects when compared with monotherapy in patients with type 2 diabetes.

Angiotensin-Converting Enzyme Inhibitors↗
PubMed2005

Phosphorylation of Ser24 in the pleckstrin homology domain of insulin receptor substrate-1 by Mouse Pelle-like kinase/interleukin-1 receptor-associated kinase: cross-talk between inflammatory signaling and insulin signaling that may contribute to insulin resistance.

Inflammation contributes to insulin resistance in diabetes and obesity. Mouse Pelle-like kinase (mPLK, homolog of human IL-1 receptor-associated kinase (IRAK)) participates in inflammatory signaling. We evaluated IRS-1 as a novel substrate for mPLK that may contribute to linking inflammation with insulin resistance. Wild-type mPLK, but not a kinase-inactive mutant (mPLK-KD), directly phosphorylated full-length IRS-1 in vitro. This in vitro phosphorylation was increased when mPLK was immunoprecipitated from tumor necrosis factor (TNF)-alpha-treated cells. In NIH-3T3(IR) cells, wild-type mPLK (but not mPLK-KD) co-immunoprecipitated with IRS-1. This association was increased by treatment of cells with TNF-alpha. Using mass spectrometry, we identified Ser(24) in the pleckstrin homology (PH) domain of IRS-1 as a specific phosphorylation site for mPLK. IRS-1 mutants S24D or S24E (mimicking phosphorylation at Ser(24)) had impaired ability to associate with insulin receptors resulting in diminished tyrosine phosphorylation of IRS-1 and impaired ability of IRS-1 to bind and activate PI-3 kinase in response to insulin. IRS-1-S24D also had an impaired ability to mediate insulin-stimulated translocation of GLUT4 in rat adipose cells. Importantly, endogenous mPLK/IRAK was activated in response to TNF-alpha or interleukin 1 treatment of primary adipose cells. In addition, using a phospho-specific antibody against IRS-1 phosphorylated at Ser(24), we found that interleukin-1 or TNF-alpha treatment of Fao cells stimulated increased phosphorylation of endogenous IRS-1 at Ser(24). We conclude that IRS-1 is a novel physiological substrate for mPLK. TNF-alpha-regulated phosphorylation at Ser(24) in the pleckstrin homology domain of IRS-1 by mPLK/IRAK represents an additional mechanism for cross-talk between inflammatory signaling and insulin signaling that may contribute to metabolic insulin resistance.

Adipose Tissue↗
PubMed2005

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.

Animals↗
PubMed2005

Assessing the predictive accuracy of QUICKI as a surrogate index for insulin sensitivity using a calibration model.

The quantitative insulin-sensitivity check index (QUICKI) has an excellent linear correlation with the glucose clamp index of insulin sensitivity (SI(Clamp)) that is better than that of many other surrogate indexes. However, correlation between a surrogate and reference standard may improve as variability between subjects in a cohort increases (i.e., with an increased range of values). Correlation may be excellent even when prediction of reference values by the surrogate is poor. Thus, it is important to evaluate the ability of QUICKI to accurately predict insulin sensitivity as determined by the reference glucose clamp method. In the present study, we used a calibration model to compare the ability of QUICKI and other simple surrogates to predict SI(Clamp). Predictive accuracy was evaluated by both root mean squared error of prediction as well as a more robust leave-one-out cross-validation-type root mean squared error of prediction (CVPE). Based on data from 116 glucose clamps obtained from nonobese, obese, type 2 diabetic, and hypertensive subjects, we found that QUICKI and log (homeostasis model assessment [HOMA]) were both excellent at predicting SI(Clamp) (CVPE = 1.45 and 1.51, respectively) and significantly better than HOMA, 1/HOMA, and fasting insulin (CVPE = 3.17, P < 0.001; 1.67, P < 0.02; and 2.85, P < 0.001, respectively). QUICKI and log(HOMA) also had the narrowest distribution of residuals (measured SI(Clamp) - predicted SI(Clamp)). In a subset of subjects (n = 78) who also underwent a frequently sampled intravenous glucose tolerance test with minimal model analysis, QUICKI was significantly better than the minimal model index of insulin sensitivity (SI(MM)) at predicting SI(Clamp) (CVPE = 1.54 vs. 1.98, P = 0.001). We conclude that QUICKI and log(HOMA) are among the most accurate surrogate indexes for determining insulin sensitivity in humans.

Adult↗
PubMed2005

Essential role for membrane lipid rafts in interleukin-1beta-induced nitric oxide release from insulin-secreting cells: potential regulation by caveolin-1+.

We recently reported that the activation of H-Ras represents one of the signaling steps underlying the interleukin-1beta (IL-1beta)-mediated metabolic dysfunction of the islet beta-cell. In the present study, we examined potential contributory roles of membrane-associated, cholesterol-enriched lipid rafts/caveolae and their constituent proteins (e.g., caveolin-1 [Cav-1]) as potential sites for IL-1beta-induced nitric oxide (NO) release in the isolated beta-cell. Disruption of lipid rafts (e.g., with cyclodextrin) markedly reduced IL-1beta-induced gene expression of inducible NO synthase (iNOS) and NO release from beta-cells. Immunologic and confocal microscopic evidence also suggested a transient but significant stimulation of tyrosine phosphorylation of Cav-1 in beta-cells briefly (for 15 min) exposed to IL-1beta that was markedly attenuated by three structurally distinct inhibitors of protein tyrosine phosphorylation. Overexpression of an inactive mutant of Cav-1 lacking the tyrosine phosphorylation site (Y14F) or an siRNA-mediated Cav-1 knock down also resulted in marked attenuation of IL-1beta-induced iNOS gene expression and NO release from these cells, thus further implicating Cav-1 in this signaling cascade. IL-1beta treatment also increased (within 20 min) the translocation of H-Ras into lipid rafts. Here we provide the first evidence to suggest that tyrosine phosphorylation of Cav-1 and subsequent interaction among members of the Ras signaling pathway within the membrane lipid microdomains represent early signaling mechanisms of IL-1beta in beta-cells.

Animals↗
PubMed2005

Beneficial effects of fenofibrate to improve endothelial dysfunction and raise adiponectin levels in patients with primary hypertriglyceridemia.

OBJECTIVE: Improvement in endothelial function is predicted to improve insulin sensitivity, and this may be one mechanism by which fenofibrate decreases the incidence of coronary heart disease. We hypothesize fenofibrate improves endothelial function by enhancing insulin sensitivity. RESEARCH DESIGN AND METHODS: We administered placebo or fenofibrate 200 mg daily for 8 weeks to 46 patients with primary hypertriglyceridemia (24 had metabolic syndrome). This study was randomized, double blind, placebo controlled, and crossover in design. RESULTS: Compared with placebo, fenofibrate decreased total cholesterol, non-HDL cholesterol, apolipoprotein B, and triglycerides and increased HDL cholesterol and apolipoprotein A-I (all P < 0.001) while tending to decrease LDL cholesterol (P = 0.069). Fenofibrate significantly improved percent flow-mediated dilator response to hyperemia by 48 +/- 5% (P < 0.001) and lowered plasma levels of high-sensitivity C-reactive protein (hsCRP) relative to baseline measurements from 0.80 to 0.70 mg/l (P = 0.001) and fibrinogen levels by 16 +/- 3% (P < 0.001). Compared with placebo, fenofibrate therapy significantly increased plasma levels of adiponectin by 14 +/- 5% (P = 0.008) and increased insulin sensitivity (assessed by quantitative insulin sensitivity check index [QUICKI]) by 6 +/- 2% (P = 0.048). There were significant correlations between percent changes in adiponectin levels and percent changes in flow-mediated dilation (r = 0.401, P = 0.006), hsCRP (r = -0.443, P = 0.002), or QUICKI (r = 0.292, P = 0.049). Multivariate regression analysis showed that only changes in adiponectin levels persisted as an independent predictor of changes in flow-mediated dilation (r = 0.504, P = 0.013). Overall, we observed similar results in 24 patients with metabolic syndrome. CONCLUSIONS: Fenofibrate therapy significantly improved percent flow-mediated dilator response to hyperemia, reduced inflammation marker levels, increased adiponectin levels, and improved insulin sensitivity in hypertriglyceridemic or metabolic syndrome patients.

Adiponectin↗
PubMed2004

Additive beneficial effects of losartan combined with simvastatin in the treatment of hypercholesterolemic, hypertensive patients.

BACKGROUND: Biological mechanisms underlying statin and angiotensin II type 1 receptor blocker therapies differ. Therefore, we compared vascular and metabolic responses to these therapies either alone or in combination in hypercholesterolemic, hypertensive patients. METHODS AND RESULTS: This was a randomized, double-blind, placebo-controlled crossover trial with 3 treatment arms (each 2 months) and 2 washout periods (each 2 months). Forty-seven hypertensive, hypercholesterolemic patients were given simvastatin 20 mg and placebo, simvastatin 20 mg and losartan 100 mg, or losartan 100 mg and placebo daily during each 2-month treatment period. Losartan alone or combined therapy significantly reduced blood pressure compared with simvastatin alone. Compared with losartan alone, simvastatin alone or combined therapy significantly changed lipoproteins. All 3 treatment arms significantly improved flow-mediated dilator response to hyperemia and decreased plasma malondialdehyde and monocyte chemoattractant protein-1 levels relative to baseline measurements. However, these parameters were changed to a greater extent with combined therapy compared with simvastatin or losartan alone (both P<0.001 and P=0.030 for monocyte chemoattractant protein-1 by ANOVA). Combined therapy or losartan alone significantly increased plasma adiponectin levels and insulin sensitivity (determined by QUICKI) relative to baseline measurements. These changes were significantly greater than in the group treated with simvastatin alone (P<0.001 for adiponectin, P=0.029 for QUICKI by ANOVA). CONCLUSIONS: Simvastatin combined with losartan improves endothelial function and reduces inflammatory markers to a greater extent than monotherapy with either drug in hypercholesterolemic, hypertensive patients.

Adiponectin↗
PubMed2004

The luteinizing hormone-releasing hormone inhibits the anti-apoptotic activity of insulin-like growth factor-1 in pituitary alphaT3 cells by protein kinase Calpha-mediated negative regulation of Akt.

The luteinizing hormone-releasing hormone (LHRH) receptor is a G protein-coupled receptor involved in the synthesis and release of pituitary gonadotropins and in the proliferation and apoptosis of pituitary cells. Insulin-like growth factor-1 receptor (IGF-1R) is a tyrosine kinase receptor that has a mitogenic effect on pituitary cells. In this study, we used the alphaT3 gonadotrope cell line as a model to characterize the IGF-1R signaling pathways and to investigate whether this receptor interacts with the LHRH cascade. We found that IGF-1 activated the IGF-1R, insulin receptor substrate (IRS)-1, phosphatidylinositol 3-kinase, and Akt in a time-dependent manner in alphaT3 cells. The MAPK (ERK1/2, p38, and JNK) pathways were only weakly activated by IGF-1. In contrast, LHRH strongly stimulated the MAPK pathways but had no effect on Akt activation. Cotreatment with IGF-1 and LHRH had various effects on these signaling pathways. 1) It strongly increased IGF-1-induced tyrosine phosphorylation of IRS-1 and IRS-1-associated phosphatidylinositol 3-kinase through activation of the epidermal growth factor receptor. 2) It had an additive effect on ERK1/2 activation without modifying the phosphorylation of p38 and JNK1/2. 3) It strongly reduced IGF-1 activation of Akt. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays and cell cycle analysis revealed that, in addition to having an additive effect on ERK1/2 activation, cotreatment with IGF-1 and LHRH also had an additive effect on cell proliferation. The LHRH-induced inhibition of Akt stimulated by IGF-1 was completely blocked by Safingol, a protein kinase C (PKC) alpha-specific inhibitor, and by a dominant negative form of PKCalpha. Finally, we showed that the inhibitory effect of LHRH on IGF-1-induced PKCalpha-mediated Akt activation was associated with a marked reduction in Bad phosphorylation and a substantial decrease in the ability of IGF-1 to rescue alphaT3 cells from apoptosis induced by serum starvation. Our results demonstrate for the first time that several interactions take place between IGF-1 and LHRH receptors in gonadotrope cells.

Animals↗
PubMed2004

Inhibition of insulin sensitivity by free fatty acids requires activation of multiple serine kinases in 3T3-L1 adipocytes.

Insulin receptor substrate (IRS) has been suggested as a molecular target of free fatty acids (FFAs) for insulin resistance. However, the signaling pathways by which FFAs lead to the inhibition of IRS function remain to be established. In this study, we explored the FFA-signaling pathway that contributes to serine phosphorylation and degradation of IRS-1 in adipocytes and in dietary obese mice. Linoleic acid, an FFA used in this study, resulted in a reduction in insulin-induced glucose uptake in 3T3-L1 adipocytes. This mimics insulin resistance induced by high-fat diet in C57BL/6J mice. The reduction in glucose uptake is associated with a decrease in IRS-1, but not IRS-2 or GLUT4 protein abundance. Decrease in IRS-1 protein was proceeded by IRS-1 (serine 307) phosphorylation that was catalyzed by serine kinases inhibitor kappaB kinase (IKK) and c-JUN NH2-terminal kinase (JNK). IKK and JNK were activated by linoleic acid and inhibition of the two kinases led to prevention of IRS-1 reduction. We demonstrate that protein kinase C (PKC) theta is expressed in adipocytes. In 3T3-L1 adipocytes and fat tissue, PKCtheta was activated by fatty acids as indicated by its phosphorylation status, and by its protein level, respectively. Activation of PKCtheta contributes to IKK and JNK activation as inhibition of PKCtheta by calphostin C blocked activation of the latter kinases. Inhibition of either PKCtheta or IKK plus JNK by chemical inhibitors resulted in protection of IRS-1 function and insulin sensitivity in 3T3-L1 adipocytes. These data suggest that: 1) activation of PKCtheta contributes to IKK and JNK activation by FFAs; 2) IKK and JNK mediate PKCtheta signals for IRS-1 serine phosphorylation and degradation; and 3) this molecular mechanism may be responsible for insulin resistance associated with hyperlipidemia.

Adipocytes↗
PubMed2004

Insulin and the insulin receptor in experimental models of learning and memory.

Insulin is best known for its action on peripheral insulin target tissues such as the adipocyte, muscle and liver to regulate glucose homeostasis. In the central nervous system (CNS), insulin and the insulin receptor are found in specific brain regions where they show evidence of participation in a variety of region-specific functions through mechanisms that are different from its direct glucose regulation in the periphery. While the insulin/insulin receptor associated with the hypothalamus plays important roles in regulation of the body energy homeostasis, the hippocampus- and cerebral cortex-distributed insulin/insulin receptor has been shown to be involved in brain cognitive functions. Emerging evidence has suggested that insulin signaling plays a role in synaptic plasticity by modulating activities of excitatory and inhibitory receptors such as glutamate and GABA receptors, and by triggering signal transduction cascades leading to alteration of gene expression that is required for long-term memory consolidation. Furthermore, deterioration of insulin receptor signaling appears to be associated with aging-related brain degeneration such as the Alzheimer's dementia and cognitive impairment in aged subjects suffering type 2 diabetes mellitus.

Animals↗
PubMed2004

Impaired insulin secretion in the Turner metabolic syndrome.

An increased prevalence of impaired glucose homeostasis (IGH) and diabetes mellitus is reported in monosomy X, or Turner syndrome (TS). To determine whether IGH is an intrinsic feature of this syndrome, independent of obesity or hypogonadism, we compared results of a standard oral glucose challenge in age- and body mass index-matched women with TS and with karyotypically normal premature ovarian failure (POF). Fasting glucose levels were normal in both groups, but glucose values after oral glucose challenge were higher in TS [2-h glucose, 135 +/- 36 mg/dl (7.5 +/- 2.0 mmol/liter) in TS and 97 +/- 18 mg/dl (5.4 +/- 1.0 mmol/liter) in POF; P < 0.0001]. Glucose-stimulated insulin secretion was lower in TS; e.g. the initial insulin response (DeltaI/DeltaG(30)) was decreased by 60% compared with POF (P < 0.0001). We also compared responses to a standard iv glucose tolerance test in women with TS and in age- and body mass index-matched normal women and found that the insulin area under the curve was 50% lower in women with TS (P = 0.003). Insulin sensitivity measured by the quantitative insulin sensitivity check index was higher in women with TS compared with both control groups. Thus, IGH is not secondary to obesity or hypogonadism in TS, but it is a distinct entity characterized by decreased insulin secretion, suggesting that haploinsufficiency for X-chromosome gene(s) impairs beta-cell function and predisposes to diabetes mellitus in TS.

Adult↗
PubMed2003

Negative regulation of insulin-stimulated mitogen-activated protein kinase signaling by Grb10.

Grb10 is a Pleckstrin homology and Src homology 2 (SH2) domain-containing protein that binds to the tyrosine-phosphorylated insulin receptor in response to insulin stimulation. Loss of Grb10 function in mice results in fetal and placental overgrowth; however, the molecular mechanism remains unknown. In the present study, we show that overexpression of Grb10 in Chinese hamster ovary cells expressing the insulin receptor or in 3T3-L1 adipocytes reduced insulin-stimulated phosphorylation of MAPK. Overexpression of Grb10 in rat primary adipocytes also inhibited insulin-stimulated phosphorylation of the MAPK downstream substrate Elk1. To determine the mechanism by which Grb10 inhibited insulin-stimulated MAPK signaling, we examined whether Grb10 affects the phosphorylation of MAPK upstream signaling components. We found that overexpression of Grb10 inhibited the insulin-stimulated phosphorylation of Shc, a positive regulator of the MAPK signaling pathway. The inhibitory effect was diminished when the SH2 domain of Grb10 was deleted. The negative role of Grb10 in insulin signaling was established by suppression of endogenous Grb10 by RNA interference in HeLa cells overexpressing the insulin receptor, which enhanced insulin-stimulated phosphorylation of MAPK, Shc, and Akt. Taken together, our findings suggest that Grb10 functions as a negative regulator in the insulin-stimulated MAPK signaling pathway. In addition, the inhibitory effect of Grb10 on the MAPK pathway is most likely due to a direct block of insulin-stimulated Shc tyrosine phosphorylation.

Adaptor Proteins, Signal Transducing↗