[Show abstract][Hide abstract] ABSTRACT: Green tea is rich in polyphenol flavonoids including catechins. Epigallocatechin 3-gallate (EGCG) is the most abundant and potent green tea catechin. EGCG has been extensively studied for its beneficial health effects as a nutriceutical agent. Based upon its chemical structure, EGCG is often classified as an antioxidant. However, treatment of cells with EGCG results in production of hydrogen peroxide and hydroxyl radicals in the presence of Fe (III). Thus, EGCG functions as a pro-oxidant in some cellular contexts. Recent investigations have revealed many other direct actions of EGCG that are independent from anti-oxidative mechanisms. In this review, we discuss these novel molecular mechanisms of action for EGCG. In particular, EGCG directly interacts with proteins and phospholipids in the plasma membrane and regulates signal transduction pathways, transcription factors, DNA methylation, mitochondrial function, and autophagy to exert many of its beneficial biological actions.
[Show abstract][Hide abstract] ABSTRACT: Insulin resistance, a hallmark of metabolic disorders, is a risk factor for diabetes and cardiovascular disease. Impairment of insulin responsiveness in vascular endothelium contributes to insulin resistance. The reciprocal relationship between insulin resistance and endothelial dysfunction augments the pathophysiology of metabolism and cardiovascular functions. The most abundant green tea polyphenol, epigallocatechin-3-gallate (EGCG), has been shown to have vasodilator action in vessels by activation of endothelial nitric oxide synthase (eNOS). However, it is not known whether EGCG has a beneficial effect in high fat diet (HFD)-induced endothelial dysfunction. Male C57BL/6J mice were fed either a normal chow diet (NCD) or HFD with or without EGCG supplement (50 mg/kg/day) for 10 weeks. Mice fed a HFD with EGCG supplement gained less body weight and showed improved insulin sensitivity. In vehicle treated HFD mice, endothelial function was impaired in response to insulin but not to acetylcholine, while EGCG treated HFD group showed improved insulin-stimulated vasodilation. Interestingly, EGCG intake reduced macrophage infiltration into aortic tissues in HFD mice. Pre-treatment with EGCG restored the insulin-stimulated phosphorylation of eNOS, insulin receptor substrate-1 (IRS-1) and protein kinase B (Akt), which was inhibited by palmitate (200 μM, 5 hr) in primary bovine aortic endothelial cells. From these results, we conclude that supplementation of EGCG improves glucose tolerance, insulin sensitivity and endothelial function. The results suggest that EGCG may have beneficial health effects in glucose metabolism and endothelial function through modulating HFD-induced inflammatory response.
[Show abstract][Hide abstract] ABSTRACT: Epigallocatechin gallate (EGCG) is a major polyphenol in green tea which has beneficial effects in prevention of cardiovascular disease. Autophagy is a cellular process that protects cells from stressful conditions. To determine whether beneficial effect of EGCG is mediated by a mechanism involving autophagy, the role of EGCG-stimulated autophagy in the context of ectopic lipid accumulation were investigated. Treatment with EGCG increased formation of LC3-II and autophagosomes in primary bovine aortic endothelial cells (BAEC). Activation of CaMKKβ was required for EGGC-induced LC3-II formation, as evidenced by the fact that EGCG-induced LC3-II formation was significantly impaired by knock-down of CaMKKβ. This effect is most likely due to cytosolic Ca(++) load. To determine whether EGCG affects palmitate-induced lipid accumulation, the effects of EGCG on autophagic flux and co-localization of lipid droplets and autophagolysosomes were examined. EGCG normalized the palimitate-induced impairment of autophagic flux. Accumulation of lipid droplets by palmitate was markedly reduced by EGCG. Blocking autophagosomal degradation opposed the effect of EGCG in ectopic lipid accumulation, suggesting the action of EGCG is through autophagosomal degradation. The mechanism for this could be due to the increased co-localization of lipid droplets and autophagolysosome. Co-localization of lipid droplets with LC3 and lysosome was dramatically increased when the cells were treated with EGCG and palmitate compared to the cells treated with palmitate alone. Collectively, the findings suggest that EGCG regulates ectopic lipid accumulation through a facilitated autophagic flux, and further implicate that EGCG may be a potential therapeutic reagent to prevent cardiovascular complications.
[Show abstract][Hide abstract] ABSTRACT: Obesity is characterized by a chronic pro-inflammatory state that leads to endothelial dysfunction. Saturated fatty acids (SFAs) stimulate toll-like receptors that promote metabolic insulin resistance. However, it is not known whether TLR2 mediates impairment of vascular actions of insulin in response to high fat diet (HFD) to cause endothelial dysfunction. SiRNA knock-down of TLR2 in primary endothelial cells opposed palmitate-stimulated expression of pro-inflammatory cytokines and splicing of X box protein (XBP)-1. Inhibition of unfolding protein response (UPR) reduced SFA-stimulated expression of TNF- . Thus, SFA stimulates UPR and pro-inflammatory response through activation of TLR2 in endothelial cells. Knock-down of TLR2 also opposed impairment of insulin-stimulated phosphorylation of eNOS and subsequent production of NO. Importantly, insulin-stimulated vasorelaxation of mesenteric arteries from TLR2 knock-out mice was preserved even on HFD (by contrast with results from arteries examined in wild type mice on HFD). We conclude that TLR2 in vascular endothelium mediates HFD-stimulated pro-inflammatory responses and UPR that accompany impairment of vasodilator actions of insulin leading to endothelial dysfunction. These results are relevant to understanding the pathophysiology of cardiovascular complications of diabetes and obesity.
[Show abstract][Hide abstract] ABSTRACT: Vascular endothelial dysfunction is determined by both genetic and environmental factors that cause decreased bioavailability of the vasodilator nitric oxide. This is a hallmark of atherosclerosis, hypertension, and coronary heart disease, which are major complications of metabolic disorders, including diabetes and obesity. Several therapeutic interventions, including changes in lifestyle as well as pharmacologic treatments, are useful for improving endothelial dysfunction in the face of lipotoxicity. This review discusses the current understanding of molecular and physiologic mechanisms underlying lipotoxicity-mediated endothelial dysfunction as well as relevant therapeutic approaches to ameliorate dyslipidemia and consequent endothelial dysfunction that have the potential to improve cardiovascular and metabolic outcomes.
[Show abstract][Hide abstract] ABSTRACT: Adiponectin is an adipocyte-secreted hormone that exists as trimers, hexamers, and larger species collectively referred to as higher molecular weight (HMW) adiponectin. Whether hexamers or HMW adiponectin serve as precursors for trimers outside of circulation is currently unknown. Here, we demonstrate that adiponectin trimers can be generated from larger oligomers secreted from primary rat adipose cells or differentiated 3T3-L1 adipocytes. Purified hexameric, but not HMW, adiponectin converted to trimers in conditioned media separated from 3T3-L1 adipocytes or, more efficiently, when enclosed in dialysis membrane in presence of adipocytes. Several lines of evidence indicate the conversion is mediated by an extracellular redox system. First, N-terminal epitope-tagged hexamers converted to trimers without proteolytic removal of the tag. Second, appearance of trimers was associated with conversion of disulfide-bonded dimers to monomers. Third, thiol-reactive agents inhibited conversion to trimers. Consistent with a redox-based mechanism, purified hexamers reductively converted to trimers in defined glutathione redox buffer with reduction potential typically found in the extracellular environment while the HMW adiponectin remained stable. In addition, conversion of hexamers to trimers was enhanced by NADPH, but not by NADP+. Collectively, these data strongly suggest the presence of an extracellular redox system capable of converting adiponectin oligomers.
[Show abstract][Hide abstract] ABSTRACT: Elevated tissue levels of angiotensin II (ANG II) are associated with impairment of insulin actions in metabolic and cardiovascular tissues. ANG II-stimulated activation of mammalian target of rapamycin (mTOR)/p70 S6 kinase (p70S6K) in cardiovascular tissues is implicated in cardiac hypertrophy and vascular remodeling. However, the role of ANG II-stimulated mTOR/p70S6K in vascular endothelium is poorly understood. In the present study, we observed that ANG II stimulated p70S6K in bovine aortic endothelial cells. ANG II increased phosphorylation of insulin receptor substrate-1 (IRS-1) at Ser(636/639) and inhibited the insulin-stimulated phosphorylation of endothelial nitric oxide synthase (eNOS). An inhibitor of mTOR, rapamycin, attenuated the ANG II-stimulated phosphorylation of p70S6K and phosphorylation of IRS-1 (Ser(636/639)) and blocked the ability of ANG II to impair insulin-stimulated phosphorylation of eNOS, nitric oxide production, and mesenteric-arteriole vasodilation. Moreover, point mutations of IRS-1 at Ser(636/639) to Ala prevented the ANG II-mediated inhibition of insulin signaling. From these results, we conclude that activation of mTOR/p70S6K by ANG II in vascular endothelium may contribute to impairment of insulin-stimulated vasodilation through phosphorylation of IRS-1 at Ser(636/639). This ANG II-mediated impairment of vascular actions of insulin may help explain the role of ANG II as a link between insulin resistance and hypertension.
[Show abstract][Hide abstract] ABSTRACT: Hesperidin, a citrus flavonoid, and its metabolite hesperetin may have vascular actions relevant to their health benefits. Molecular and physiological mechanisms of hesperetin actions are unknown.
We tested whether hesperetin stimulates production of nitric oxide (NO) from vascular endothelium and evaluated endothelial function in subjects with metabolic syndrome on oral hesperidin therapy. DESIGN, SETTING, AND INTERVENTIONS: Cellular mechanisms of action of hesperetin were evaluated in bovine aortic endothelial cells (BAEC) in primary culture. A randomized, placebo-controlled, double-blind, crossover trial examined whether oral hesperidin administration (500 mg once daily for 3 wk) improves endothelial function in individuals with metabolic syndrome (n = 24).
We measured the difference in brachial artery flow-mediated dilation between placebo and hesperidin treatment periods.
Treatment of BAEC with hesperetin acutely stimulated phosphorylation of Src, Akt, AMP kinase, and endothelial NO synthase to produce NO; this required generation of H(2)O(2). Increased adhesion of monocytes to BAEC and expression of vascular cell adhesion molecule-1 in response to TNF-α treatment was reduced by pretreatment with hesperetin. In the clinical study, when compared with placebo, hesperidin treatment increased flow-mediated dilation (10.26 ± 1.19 vs. 7.78 ± 0.76%; P = 0.02) and reduced concentrations of circulating inflammatory biomarkers (high-sensitivity C-reactive protein, serum amyloid A protein, soluble E-selectin).
Novel mechanisms for hesperetin action in endothelial cells inform effects of oral hesperidin treatment to improve endothelial dysfunction and reduce circulating markers of inflammation in our exploratory clinical trial. Hesperetin has vasculoprotective actions that may explain beneficial cardiovascular effects of citrus consumption.
The Journal of Clinical Endocrinology and Metabolism 02/2011; 96(5):E782-92. DOI:10.1210/jc.2010-2879 · 6.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Insulin resistance is associated with obesity and may be accompanied by left ventricular diastolic dysfunction and myocardial remodeling. Decreased insulin metabolic signaling and increased oxidative stress may promote these maladaptive changes. In this context, the β-blocker nebivolol has been reported to improve insulin sensitivity, increase endothelial NO synthase activity, and reduce NADPH oxidase-induced superoxide generation. We hypothesized that nebivolol would attenuate diastolic dysfunction and myocardial remodeling by blunting myocardial oxidant stress and promoting insulin metabolic signaling in a rodent model of obesity, insulin resistance, and hypertension. Six-week-old male Zucker obese and age-matched Zucker lean rats were treated with nebivolol (10 mg • kg • day) for 21 days, and myocardial function was assessed by cine MRI. Compared with untreated Zucker lean rats, untreated Zucker obese rats exhibited prolonged diastolic relaxation time (27.7±2.5 versus 40.9±2.0 ms; P<0.05) and reduced initial diastolic filling rate (6.2±0.5 versus 2.8±0.6 μL/ms; P<0.05) in conjunction with increased homeostatic model assessment of insulin resistance (7±2 versus 95±21; P<0.05), interstitial and pericapillary fibrosis, abnormal cardiomyocyte histoarchitecture, 3-nitrotyrosine, and NADPH oxidase-dependent superoxide. Nebivolol improved diastolic relaxation (32.8±0.7 ms; P<0.05 versus untreated Zucker obese), reduced fibrosis, and remodeling in Zucker obese rats, in concert with reductions in nitrotyrosine, NADPH oxidase-dependent superoxide, and improvements in the insulin metabolic signaling, endothelial NO synthase activation, and weight gain (381±7 versus 338±14 g; P<0.05). Results support the hypothesis that nebivolol reduces myocardial structural maladaptive changes and improves diastolic relaxation in concert with improvements in insulin sensitivity and endothelial NO synthase activation, concomitantly with reductions in oxidative stress.
[Show abstract][Hide abstract] ABSTRACT: Epigallocatechin gallate (EGCG), a green tea polyphenol, promotes vasodilation by phosphatidylinositol 3-kinase-dependent activation of Akt and endothelial nitric oxide synthase to stimulate production of nitric oxide. Reduction in endothelin-1 (ET-1) synthesis may also increase bioavailability of nitric oxide. We hypothesized that the phosphatidylinositol 3-kinase-dependent transcription factor FOXO1 may mediate effects of EGCG to regulate expression of ET-1 in endothelial cells. EGCG treatment (10 microm, 8 h) of human aortic endothelial cells reduced expression of ET-1 mRNA, protein, and ET-1 secretion. We identified a putative FOXO binding domain in the human ET-1 promoter 51 bp upstream from the transcription start site. Trans-activation of a human ET-1 (hET-1) promoter luciferase reporter was enhanced by coexpression of a constitutively nuclear FOXO1 mutant, whereas expression of a mutant FOXO1 with disrupted DNA binding domain did not trans-activate the hET-1 promoter. Disrupting the hET-1 putative FOXO binding domain by site-directed mutagenesis ablated promoter activity in response to overexpression of wild-type FOXO1. EGCG stimulated time-dependent phosphorylation of Akt (S(473)), FOXO1 (at Akt phosphorylation site T(24)), and AMP-activated protein kinase alpha (AMPK alpha) (T(172)). EGCG-induced nuclear exclusion of FOXO1, FOXO1 binding to the hET-1 promoter, and reduction of ET-1 expression was partially inhibited by the AMPK inhibitor Compound C. Basal ET-1 protein expression was enhanced by short interfering RNA knock-down of Akt and reduced by short interfering RNA knock-down of FOXO1 or adenovirus-mediated expression of dominant-negative Foxo1. We conclude that EGCG decreases ET-1 expression and secretion from endothelial cells, in part, via Akt- and AMPK-stimulated FOXO1 regulation of the ET-1 promoter. These findings may be relevant to beneficial cardiovascular actions of green tea.
[Show abstract][Hide abstract] ABSTRACT: Protein kinase C-zeta, a downstream effector of phosphatidylinositol 3-kinase (PI3K), phosphorylates insulin receptor substrate (IRS)-1 on serine residues impairing activation of PI3K in response to insulin. Because IRS-1 is upstream from PI3K, this represents a negative feedback mechanism that may contribute to signal specificity in insulin action. To determine whether similar feedback pathways exist for other IRS isoforms, we evaluated IRS-2, -3, and -4 as substrates for PKC-zeta. In an in vitro kinase assay, purified recombinant PKC-zeta phosphorylated IRS-1, -3 and -4 but not IRS-2. Similar results were obtained with an immune-complex kinase assay demonstrating that wild-type, but not kinase-deficient mutant PKC-zeta, phosphorylated IRS-1, -3, and -4 but not IRS-2. We evaluated functional consequences of serine phosphorylation of IRS isoforms by PKC-zeta in NIH-3T3(IR) cells cotransfected with epitope-tagged IRS proteins and either PKC-zeta or empty vector control. Insulin-stimulated IRS tyrosine phosphorylation was impaired by overepxression of PKC-zeta for IRS-1, -3, and -4 but not IRS-2. Significant insulin-stimulated increases in PI3K activity was coimmunoprecipitated with all IRS isoforms. In cells overexpressing PKC-zeta there was marked inhibition of insulin-stimulated PI3K activity associated with IRS-1, -3 and -4 but not IRS-2. That is, PI3K activity associated with IRS-2 in response to insulin was similar in control cells and cells overexpressing PKC-zeta. We conclude that IRS-3 and -4 are novel substrates for PKC-zeta that may participate in a negative feedback pathway for insulin signaling similar to IRS-1. The inability of PKC-zeta to phosphorylate IRS-2 may help determine specific functional roles for IRS-2.
[Show abstract][Hide abstract] ABSTRACT: Insulin resistance is characteristic of obesity, type 2 diabetes, and components of the cardiometabolic syndrome, including hypertension and dyslipidemia, that collectively contribute to a substantial risk for cardiovascular disease. Metabolic actions of insulin in classic insulin target tissues (eg, skeletal muscle, fat, and liver), as well as actions in nonclassic targets (eg, cardiovascular tissue), help to explain why insulin resistance and metabolic dysregulation are central in the pathogenesis of the cardiometabolic syndrome and cardiovascular disease. Glucose and lipid metabolism are largely dependent on mitochondria to generate energy in cells. Thereby, when nutrient oxidation is inefficient, the ratio of ATP production/oxygen consumption is low, leading to an increased production of superoxide anions. Reactive oxygen species formation may have maladaptive consequences that increase the rate of mutagenesis and stimulate proinflammatory processes. In addition to reactive oxygen species formation, genetic factors, aging, and reduced mitochondrial biogenesis all contribute to mitochondrial dysfunction. These factors also contribute to insulin resistance in classic and nonclassic insulin target tissues. Insulin resistance emanating from mitochondrial dysfunction may contribute to metabolic and cardiovascular abnormalities and subsequent increases in cardiovascular disease. Furthermore, interventions that improve mitochondrial function also improve insulin resistance. Collectively, these observations suggest that mitochondrial dysfunction may be a central cause of insulin resistance and associated complications. In this review, we discuss mechanisms of mitochondrial dysfunction related to the pathophysiology of insulin resistance in classic insulin-responsive tissue, as well as cardiovascular tissue.
Circulation Research 03/2008; 102(4):401-14. DOI:10.1161/CIRCRESAHA.107.165472 · 11.09 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ramipril and candesartan have distinct mechanisms of action to improve endothelial function. Therefore, we hypothesized that combination therapy has additive beneficial effects to simultaneously improve endothelial dysfunction and adipocytokine profiles in patients with hypertension.
Thirty-four patients were given ramipril 10 mg and placebo, ramipril 10 mg and candesartan 16 mg, or candesartan 16 mg and placebo daily in a randomized, double-blind, placebo-controlled cross-over trial with three treatment arms and two washout periods (each 2 months). Ramipril, candesartan, or combination therapy reduced blood pressure, improved flow-mediated dilation, and increased plasma adiponectin levels when compared with baseline values. However, combination therapy improved these outcome measures to a greater extent than either ramipril or candesartan alone (P < 0.001 and P = 0.016 for systolic and diastolic blood pressure, P < 0.001 and P = 0.048 for flow-mediated dilation and adiponectin levels by ANOVA). In addition, combination therapy reduced plasma leptin levels to a greater extent than either ramipril or candesartan alone (P = 0.042 by ANOVA). There were correlations between percent changes in adiponectin levels and percent changes in insulin sensitivity (determined by QUICKI) (r = 0.319, P = 0.066) following ramipril therapy, percent changes in QUICKI (r = 0.374, P = 0.029) following combination therapy, and percent changes in QUICKI (r = 0.607, P < 0.001) following candesartan therapy.
Ramipril in combination with candesartan improves blood pressure, endothelial function, and adipocytokine profiles to a greater extent than monotherapy with either drug in hypertensive patients.
European Heart Journal 07/2007; 28(12):1440-7. DOI:10.1093/eurheartj/ehm101 · 14.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Epigallocatechin gallate (EGCG), a bioactive polyphenol in green tea, may augment metabolic and vascular actions of insulin. Therefore, we investigated effects of EGCG treatment to simultaneously improve cardiovascular and metabolic function in spontaneously hypertensive rats (SHR; model of metabolic syndrome with hypertension, insulin resistance, and overweight). In acute studies, EGCG (1-100 microM) elicited dose-dependent vasodilation in mesenteric vascular beds (MVB) isolated from SHR ex vivo that was inhibitable by N(omega)-nitro-L-arginine methyl ester (L-NAME; nitric oxide synthase antagonist) or wortmannin [phosphatidylinositol (PI) 3-kinase inhibitor]. In chronic studies, 9-wk-old SHR were treated by gavage for 3 wk with EGCG (200 mg.kg(-1).day(-1)), enalapril (30 mg.kg(-1).day(-1)), or vehicle. A separate group of SHR receiving L-NAME (80 mg/l in drinking water) was treated for 3 wk with either EGCG or vehicle. Vasodilator actions of insulin were significantly improved in MVB from EGCG- or enalapril-treated SHR (when compared with vehicle-treated SHR). Both EGCG and enalapril therapy significantly lowered systolic blood pressure (SBP) in SHR. EGCG therapy of SHR significantly reduced infarct size and improved cardiac function in Langendorff-perfused hearts exposed to ischemia-reperfusion (I/R) injury. In SHR given L-NAME, beneficial effects of EGCG on SBP and I/R were not observed. Both enalapril and EGCG treatment of SHR improved insulin sensitivity and raised plasma adiponectin levels. We conclude that acute actions of EGCG to stimulate production of nitric oxide from endothelium using PI 3-kinase-dependent pathways may explain, in part, beneficial effects of EGCG therapy to simultaneously improve metabolic and cardiovascular pathophysiology in SHR. These findings may be relevant to understanding potential benefits of green tea consumption in patients with the metabolic syndrome.
[Show abstract][Hide abstract] ABSTRACT: Green tea consumption is associated with reduced cardiovascular mortality in some epidemiological studies. Epigallocatechin gallate (EGCG), a bioactive polyphenol in green tea, mimics metabolic actions of insulin to inhibit gluconeogenesis in hepatocytes. Because signaling pathways regulating metabolic and vasodilator actions of insulin are shared in common, we hypothesized that EGCG may also have vasodilator actions to stimulate production of nitric oxide (NO) from endothelial cells. Acute intra-arterial administration of EGCG to mesenteric vascular beds isolated ex vivo from WKY rats caused dose-dependent vasorelaxation. This was inhibitable by L-NAME (NO synthase inhibitor), wortmannin (phosphatidylinositol 3-kinase inhibitor), or PP2 (Src family kinase inhibitor). Treatment of bovine aortic endothelial cells (BAEC) with EGCG (50 microm) acutely stimulated production of NO (assessed with NO-specific fluorescent dye DAF-2) that was inhibitable by l-NAME, wortmannin, or PP2. Stimulation of BAEC with EGCG also resulted in dose- and time-dependent phosphorylation of eNOS that was inhibitable by wortmannin or PP2 (but not by MEK inhibitor PD98059). Specific knockdown of Fyn (but not Src) with small interfering RNA inhibited both EGCG-stimulated phosphorylation of Akt and eNOS as well as production of NO in BAEC. Treatment of BAEC with EGCG generated intracellular H(2)O(2) (assessed with H(2)O(2)-specific fluorescent dye CM-H(2)DCF-DA), whereas treatment with N-acetylcysteine inhibited EGCG-stimulated phosphorylation of Fyn, Akt, and eNOS. We conclude that EGCG has endothelial-dependent vasodilator actions mediated by intracellular signaling pathways requiring reactive oxygen species and Fyn that lead to activation of phosphatidylinositol 3-kinase, Akt, and eNOS. This mechanism may explain, in part, beneficial vascular and metabolic health effects of green tea consumption.
[Show abstract][Hide abstract] ABSTRACT: Ghrelin is an orexigenic peptide hormone secreted by the stomach. In patients with metabolic syndrome and low ghrelin levels, intra-arterial ghrelin administration acutely improves their endothelial dysfunction. Therefore, we hypothesized that ghrelin activates endothelial nitric oxide synthase (eNOS) in vascular endothelium, resulting in increased production of nitric oxide (NO) using signaling pathways shared in common with the insulin receptor. Similar to insulin, ghrelin acutely stimulated increased production of NO in bovine aortic endothelial cells (BAEC) in primary culture (assessed using NO-specific fluorescent dye 4,5-diaminofluorescein) in a time- and dose-dependent manner. Production of NO in response to ghrelin (100 nM, 10 min) in human aortic endothelial cells was blocked by pretreatment of cells with NG-nitro-L-arginine methyl ester (nitric oxide synthase inhibitor), wortmannin [phosphatidylinositol (PI) 3-kinase inhibitor], or (D-Lys3)-GHRP-6 (selective antagonist of ghrelin receptor GHSR-1a), as well as by knockdown of GHSR-1a using small-interfering (si) RNA (but not by mitogen/extracellular signal-regulated kinase inhibitor PD-98059). Moreover, ghrelin stimulated increased phosphorylation of Akt (Ser473) and eNOS (Akt phosphorylation site Ser1179) that was inhibitable by knockdown of GHSR-1a using siRNA or by pretreatment of cells with wortmannin but not with PD-98059. Ghrelin also stimulated phosphorylation of mitogen-activated protein (MAP) kinase in BAEC. However, unlike insulin, ghrelin did not stimulate MAP kinase-dependent secretion of the vasoconstrictor endothelin-1 from BAEC. We conclude that ghrelin has novel vascular actions to acutely stimulate production of NO in endothelium using a signaling pathway that involves GHSR-1a, PI 3-kinase, Akt, and eNOS. Our findings may be relevant to developing novel therapeutic strategies to treat diabetes and related diseases characterized by reciprocal relationships between endothelial dysfunction and insulin resistance.
[Show abstract][Hide abstract] ABSTRACT: 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. Obesity-related disorders including the metabolic syndrome, diabetes, atherosclerosis, hypertension, and coronary artery disease are associated with decreased plasma levels of adiponectin, insulin resistance, and endothelial dysfunction. Adiponectin has insulin-sensitizing effects as well as antiatherogenic properties. Lifestyle modifications and some drug therapies to treat atherosclerosis, hypertension, and coronary heart disease have important effects to simultaneously increase adiponectin levels, decrease insulin resistance, and improve endothelial dysfunction. In this review, we discuss insights into the relationships between adiponectin levels, insulin resistance, and endothelial dysfunction that are derived from various therapeutic interventions. The effects of lifestyle modifications and cardiovascular drugs on adiponectin levels and insulin resistance suggest plausible mechanisms that may be important for treating atherosclerosis and coronary heart disease.
Journal of the American College of Cardiology 03/2007; 49(5):531-8. DOI:10.1016/j.jacc.2006.08.061 · 15.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The IRS-1 PH and PTB domains are essential for insulin-stimulated IRS-1 Tyr phosphorylation and insulin signaling, while Ser/Thr phosphorylation of IRS-1 disrupts these signaling events. To investigate consensus PKC phosphorylation sites in the PH-PTB domains of human IRS-1, we changed Ser24, Ser58, and Thr191 to Ala (3A) or Glu (3E), to block or mimic phosphorylation, respectively. The 3A mutant abrogated the inhibitory effect of PKCdelta on insulin-stimulated IRS-1 Tyr phosphorylation, while reductions in insulin-stimulated IRS-1 Tyr phosphorylation, cellular proliferation, and Akt activation were observed with the 3E mutant. When single Glu mutants were tested, the Ser24 to Glu mutant had the greatest inhibitory effect on insulin-stimulated IRS-1 Tyr phosphorylation. PKCdelta-mediated IRS-1 Ser24 phosphorylation was confirmed in cells with PKCdelta catalytic domain mutants and by an RNAi method. Mechanistic studies revealed that IRS-1 with Ala and Glu point mutations at Ser24 impaired phosphatidylinositol-4,5-bisphosphate binding. In summary, our data are consistent with the hypothesis that Ser24 is a negative regulatory phosphorylation site in IRS-1.
Biochemical and Biophysical Research Communications 11/2006; 349(3):976-86. DOI:10.1016/j.bbrc.2006.08.158 · 2.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dehydroepiandrosterone (DHEA) is an adrenal steroid and nutritional supplement that may improve insulin sensitivity. Although steroid hormones classically act by regulating transcription, they may also signal through cell surface receptors to mediate nongenomic actions. Because DHEA may augment insulin sensitivity, we hypothesized that DHEA mimics vascular actions of insulin to acutely activate signaling pathways in endothelium-mediating production of nitric oxide (NO) and endothelin 1 (ET-1). Treatment of bovine aortic endothelial cells with either insulin or DHEA (100 nm, 5 min) stimulated significant increases in NO production (assessed with NO-selective fluorescent dye diaminofluorescein 2). These responses were abolished by pretreatment of cells with L-NAME (nitro-L-arginine methyl ester; NO synthase inhibitor) or wortmannin [phosphatidylinositol (PI) 3-kinase inhibitor]. Under similar conditions, insulin- or DHEA-stimulated phosphorylation of Akt (Ser473) and endothelial nitric oxide synthase (Ser1179) was inhibited by pretreatment of cells with wortmannin (but not MAPK kinase inhibitor PD98059). Acute DHEA treatment also caused phosphorylation of MAPK (Thr202/Tyr204) that was inhibitable by PD98059 (but not wortmannin). DHEA treatment of bovine aortic endothelial cells (100 nM, 5 min) stimulated a 2-fold increase in ET-1 secretion that was abolished by pretreatment of cells with PD98059 (but not wortmannin). We conclude that DHEA has acute, nongenomic actions in endothelium to stimulate production of the vasodilator NO via PI 3-kinase-dependent pathways and secretion of the vasoconstrictor ET-1 via MAPK-dependent pathways. Altering the balance between PI 3-kinase- and MAPK-dependent signaling in vascular endothelium may determine whether DHEA has beneficial or harmful effects relevant to the pathophysiology of diabetes.