Xiuyun Hou

AMPK Phosphorylates and Inhibits SREBP Activity to Attenuate Hepatic Steatosis and Atherosclerosis in Diet-Induced Insulin-Resistant Mice

AMPK has emerged as a critical mechanism for salutary effects of polyphenols on lipid metabolic disorders in type 1 and type 2 diabetes. Here we demonstrate that AMPK interacts with and directly phosphorylates sterol regulatory element binding proteins (SREBP-1c and -2). Ser372 phosphorylation of SREBP-1c by AMPK is necessary for inhibition of proteolytic processing and transcriptional activity of SREBP-1c in response to polyphenols and metformin. AMPK stimulates Ser372 phosphorylation, suppresses SREBP-1c cleavage and nuclear translocation, and represses SREBP-1c target gene expression in hepatocytes exposed to high glucose, leading to reduced lipogenesis and lipid accumulation. Hepatic activation of AMPK by the synthetic polyphenol S17834 protects against hepatic steatosis, hyperlipidemia, and accelerated atherosclerosis in diet-induced insulin-resistant LDL receptor-deficient mice in part through phosphorylation of SREBP-1c Ser372 and suppression of SREBP-1c- and -2-dependent lipogenesis. AMPK-dependent phosphorylation of SREBP may offer therapeutic strategies to combat insulin resistance, dyslipidemia, and atherosclerosis.

SIRT1 Regulates Hepatocyte Lipid Metabolism through Activating AMP-activated Protein Kinase

Resveratrol may protect against metabolic disease through activating SIRT1 deacetylase. Because we have recently defined AMPK activation as a key mechanism for the beneficial effects of polyphenols on hepatic lipid accumulation, hyperlipidemia, and atherosclerosis in type 1 diabetic mice, we hypothesize that polyphenol-activated SIRT1 acts upstream of AMPK signaling and hepatocellular lipid metabolism. Here we show that polyphenols, including resveratrol and the synthetic polyphenol S17834, increase SIRT1 deacetylase activity, LKB1 phosphorylation at Ser428, and AMPK activity. Polyphenols substantially prevent the impairment in phosphorylation of AMPK and its downstream target, ACC (acetyl-CoA carboxylase), elevation in expression of FAS (fatty acid synthase), and lipid accumulation in human HepG2 hepatocytes exposed to high glucose. These effects of polyphenols are largely abolished by pharmacological and genetic inhibition of SIRT1, suggesting that the stimulation of AMPK and lipid-lowering effect of polyphenols depend on SIRT1 activity. Furthermore, adenoviral overexpression of SIRT1 stimulates the basal AMPK signaling in HepG2 cells and in the mouse liver. AMPK activation by SIRT1 also protects against FAS induction and lipid accumulation caused by high glucose. Moreover, LKB1, but not CaMKKβ, is required for activation of AMPK by polyphenols and SIRT1. These findings suggest that SIRT1 functions as a novel upstream regulator for LKB1/AMPK signaling and plays an essential role in the regulation of hepatocyte lipid metabolism. Targeting SIRT1/LKB1/AMPK signaling by polyphenols may have potential therapeutic implications for dyslipidemia and accelerated atherosclerosis in diabetes and age-related diseases.

Polyphenols Stimulate AMP-Activated Protein Kinase, Lower Lipids, and Inhibit Accelerated Atherosclerosis in Diabetic LDL Receptor–Deficient Mice

Because polyphenols may have beneficial effects on dyslipidemia, which accelerates atherosclerosis in diabetes, we examined the effect of polyphenols on hepatocellular AMP-activated protein kinase (AMPK) activity and lipid levels, as well as hyperlipidemia and atherogenesis in type 1 diabetic LDL receptor–deficient mice (DMLDLR−/−). In HepG2 hepatocytes, polyphenols, including resveratrol (a major polyphenol in red wine), apigenin, and S17834 (a synthetic polyphenol), increased phosphorylation of AMPK and its downstream target, acetyl-CoA carboxylase (ACC), and they increased activity of AMPK with 200 times the potency of metformin. The polyphenols also prevented the lipid accumulation that occurred in HepG2 cells exposed to high glucose, and their ability to do so was mimicked and abrogated, respectively, by overexpression of constitutively active and dominant-negative AMPK mutants. Furthermore, treatment of DMLDLR−/− mice with S17834 prevented the decrease in AMPK and ACC phosphorylation and the lipid accumulation in the liver, and it also inhibited hyperlipidemia and the acceleration of aortic lesion development. These studies 1) reveal that inactivation of hepatic AMPK is a key event in the pathogenesis of hyperlipidemia in diabetes, 2) point to a novel mechanism of action of polyphenols to lower lipids by activating AMPK, and 3) emphasize a new therapeutic avenue to benefit hyperlipidemia and atherosclerosis specifically in diabetes via activating AMPK.

Hepatic overexpression of SIRT1 in mice attenuates endoplasmic reticulum stress and insulin resistance in the liver

Endoplasmic reticulum (ER) stress has been implicated in the pathophysiology of human type 2 diabetes (T2DM). Although SIRT1 has a therapeutic effect on metabolic deterioration in T2DM, the precise mechanisms by which SIRT1 improves insulin resistance remain unclear. Here, we demonstrate that adenovirus‐mediated overexpression of SIRT1 in the liver of diet‐induced insulin‐resistant low‐density lipoprotein receptor‐deficient mice and of genetically obese ob/ob mice attenuates hepatic steatosis and ameliorates systemic insulin resistance. These beneficial effects were associated with decreased mammalian target of rapamycin complex 1 (mTORC1) activity, inhibited the unfolded protein response (UPR), and enhanced insulin receptor signaling in the liver, leading to decreased hepatic gluconeogenesis and improved glucose tolerance. The tunicamycin‐in‐duced splicing of X‐box binding protein‐1 and expression of GRP78 and CHOP were reduced by resveratrol in cultured cells in a SIRT1‐dependent manner. Conversely, SIRT1‐deficient mouse embryonic fibroblasts challenged with tunicamycin exhibited markedly increased mTORC1 activity and impaired ER homeostasi and insulin signaling. These effects were abolished by mTORC1 inhibition by rapamycin in human HepG2 cells. These studies indicate that SIRT1 serves as a negative regulator of UPR signaling in T2DM and that SIRT1 attenuates hepatic steatosis, ameliorates insulin resistance, and restores glucose homeostasis, largely through the inhibition of mTORC1 and ER stress.—Li, Y., Xu, S., Giles, A., Nakamura, K., Lee, J. W., Hou, X., Donmez, G., Li, J., Luo, Z., Walsh, K., Guarente, L., Zang, M. Hepatic overexpression of SIRT1 in mice attenuates endoplasmic reticulum stress and insulin resistance in the liver. FASEB J. 25, 1664–1679 (2011). www.fasebj.org

Modulation by Peroxynitrite of Akt- and AMP-activated Kinase-dependent Ser1179 Phosphorylation of Endothelial Nitric Oxide Synthase

Peroxynitrite (ONOO−), a nitric oxide-derived oxidant, uncouples endothelial nitric oxide synthase (eNOS) and increases enzymatic production of superoxide anions (O 2 ⨪ ) (Zou, M. H., Shi, C., and Cohen, R. A. (2002) J. Clin. Invest. 109, 817–826). Here we studied how ONOO−influences eNOS activity. In cultured bovine aortic endothelial cells (BAEC), ONOO− increased basal and agonist-stimulated Ser1179 phosphorylation of eNOS, whereas it decreased nitric oxide production and bioactivity. However, ONOO−strongly inhibited the phosphorylation and activity of Akt, which is known to phosphorylate eNOS-Ser1179. Moreover, expression of an Akt dominant-negative mutant did not prevent ONOO−-enhanced eNOS-Ser1179 phosphorylation. In contrast to Akt, ONOO− significantly activated 5′-AMP-activated kinase (AMPK), as evidenced by its increased Thr172 phosphorylation as well as increased Ser92 phosphorylation of acetyl-coenzyme A carboxylase, a downstream target of AMPK. Associated with the increased release of O 2 ⨪ , ONOO− significantly increased the co-immunoprecipitation of eNOS with AMPK. Further, overexpression of the AMPK-constitutive active adenovirus significantly enhanced ONOO− up-regulated eNOS-Ser(P)1179. In contrast, overexpression of a dominant-negative AMPK mutant attenuated the ONOO−-enhanced eNOS-Ser1179phosphorylation as well as O 2 ⨪ release. We conclude that ONOO− inhibits Akt and increases AMPK-dependent Ser1179 phosphorylation of eNOS resulting in enhanced O 2 ⨪ release.

The Polyphenols Resveratrol and S17834 Prevent the Structural and Functional Sequelae of Diet-Induced Metabolic Heart Disease in Mice

Background— Diet-induced obesity is associated with metabolic heart disease characterized by left ventricular hypertrophy and diastolic dysfunction. Polyphenols such as resveratrol and the synthetic flavonoid derivative S17834 exert beneficial systemic and cardiovascular effects in a variety of settings including diabetes mellitus and chronic hemodynamic overload. Methods and Results— We characterized the structural and functional features of a mouse model of diet-induced metabolic syndrome and used the model to test the hypothesis that the polyphenols prevent myocardial hypertrophy and diastolic dysfunction. Male C57BL/6J mice were fed a normal diet or a diet high in fat and sugar (HFHS) with or without concomitant treatment with S17834 or resveratrol for up to 8 months. HFHS diet–fed mice developed progressive left ventricular hypertrophy and diastolic dysfunction with preservation of systolic function in association with myocyte hypertrophy and interstitial fibrosis. In HFHS diet–fed mice, there was increased myocardial oxidative stress with evidence of oxidant-mediated protein modification via tyrosine nitration and 4-OH-2-nonenol adduction. HFHS diet–fed mice also exhibited increases in plasma fasting glucose, insulin, and homeostasis model assessment of insulin resistance indicative of insulin resistance. Treatment with S17834 or resveratrol prevented left ventricular hypertrophy and diastolic dysfunction. For S17834, these beneficial effects were associated with decreases in oxidant-mediated protein modifications and hyperinsulinemia and increased plasma adiponectin. Conclusions— Resveratrol and S17834 administered concurrently with a HFHS diet prevent the development of left ventricular hypertrophy, interstitial fibrosis, and diastolic dysfunction. Multiple mechanisms may contribute to the beneficial effects of the polyphenols, including a reduction in myocardial oxidative stress and related protein modifications, amelioration of insulin resistance, and increased plasma adiponectin. The polyphenols resveratrol and S17834 may be of value in the prevention of diet-induced metabolic heart disease.

Upregulation of Nox4 by TGFβ1 Oxidizes SERCA and Inhibits NO in Arterial Smooth Muscle of the Prediabetic Zucker Rat

Rationale: Vascular smooth muscle cell (SMC) migration is an important pathological process in several vascular occlusive diseases, including atherosclerosis and restenosis, both of which are accelerated by diabetes mellitus. Objective: To determine the mechanisms of abnormal vascular SMC migration in type 2 diabetes, the obese Zucker rat (ZO), a model of obesity and insulin resistance, was studied. Methods and Results: In culture, ZO aortic SMCs showed a significant increase in Nox4 mRNA and protein levels compared with the control lean Zucker rat (ZL). The sarco-/endoplasmic reticulum Ca2+ ATPase (SERCA) nitrotyrosine-294,295 and cysteine-674 (C674)-SO3H were increased in ZO SMCs, indicating oxidant stress. Unlike ZL SMC, nitric oxide (NO) failed to inhibit serum-induced SMC migration in ZO. Transfection of Nox4 small interference RNA or overexpression of SERCA2b wild type, but not C674S mutant SERCA, restored the response to NO. Knockdown of Nox4 also decreased SERCA oxidation in ZO SMCs. In addition, transforming growth factor-&bgr;1 via Smad2 was necessary and sufficient to upregulate Nox4, oxidize SERCA, and block the antimigratory action of NO in ZO SMCs. Corresponding to the results in cultured SMCs, immunohistochemistry confirmed that Nox4 and SERCA C674-SO3H were significantly increased in ZO aorta. After common carotid artery injury, knockdown of Nox4 by adenoviral Nox4 short hairpin RNA decreased Nox4 and SERCA C674-SO3H staining and significantly decreased injury-induced neointima. Conclusion: These studies indicate that the upregulation of Nox4 by transforming growth factor-&bgr;1 in ZO SMCs is responsible for the impaired response to NO by a mechanism involving the oxidation of SERCA C674. Knockdown of Nox4 inhibits oxidation of SERCA, as well as neointima formation, after ZO common carotid artery injury.

Redox Regulation of Sirtuin-1 by S-Glutathiolation

Sirtuin-1 (SIRT1) is an NAD(+)-dependent protein deacetylase that is sensitive to oxidative signals. Our purpose was to determine whether SIRT1 activity is sensitive to the low molecular weight nitrosothiol, S-nitrosoglutathione (GSNO), which can transduce oxidative signals into physiological responses. SIRT1 formed mixed disulfides with GSNO-Sepharose, and mass spectrometry identified several cysteines that are modified by GSNO, including Cys-67 which was S-glutathiolated. GSNO had no effect on basal SIRT1 deacetylase activity, but inhibited stimulation of activity by resveratrol (RSV) with an IC(50) of 69 microM. These observations indicate that S-glutathiolation of SIRT1 by low concentrations of reactive glutathione can modulate its enzymatic activity.

S-Glutathiolation of p21ras by Peroxynitrite Mediates Endothelial Insulin Resistance Caused by Oxidized Low-Density Lipoprotein

Objectives—To understand the mechanism by which oxidants are linked to insulin resistance, bovine aortic endothelial cells were exposed to oxidized low-density lipoproteins (oxLDL) or peroxynitrite. Methods and Results—OxLDL transiently increased phosphorylation of Erk and Akt within 5 minutes, but 60 minutes later, resulted in decreased insulin-induced Akt phosphorylation. OxLDL promoted a 2- to 5-fold increase in oxidant generation as measured by dihydrorhodamine or dihydroethidium oxidation that was ascribed to peroxynitrite. Exogenous peroxynitrite (25 to 100 &mgr;mol/L) or oxidized glutathione mimicked the effects of oxLDL. OxLDL increased the S-glutathiolation of p21ras, and adenoviral transfection with either a mutant p21ras (C118S) lacking the predominant site of S-glutathiolation or a dominant-negative mutant restored insulin-induced Akt phosphorylation. The requirement for oxidant-mediated S-glutathiolation and activation of p21ras in mediating insulin resistance was further implicated by showing that insulin signaling was restored by Mek inhibitors or by overexpression of glutaredoxin-1. Furthermore, oxLDL increased Erk-dependent phosphorylation of insulin receptor substrate-1 serine-616 that was prevented by inhibiting oxidant generation, Erk activation, or by the p21ras C118S mutant. Conclusions—This study provides direct evidence for a novel molecular mechanism by which oxidants can induce insulin resistance via S-glutathiolation of p21ras and Erk-dependent inhibition of insulin signaling.

A Redox-resistant Sirtuin-1 Mutant Protects against Hepatic Metabolic and Oxidant Stress

Background: Sirtuin-1 improves metabolic disease, but oxidants may inhibit it. Results: Metabolic stress increased glutathione adducts, inactivated endogenous Sirtuin-1, and promoted apoptosis. A novel Sirtuin-1 oxidation-insensitive mutant or glutaredoxin-1 prevented metabolic dysregulation and apoptosis. Conclusion: A novel Sirtuin-1 mutant circumvents oxidation and more effectively inhibits metabolic dysregulation and apoptosis. Significance: Oxidative inactivation of Sirtuin-1 contributes to metabolic disease. Sirtuin-1 (SirT1), a member of the NAD+-dependent class III histone deacetylase family, is inactivated in vitro by oxidation of critical cysteine thiols. In a model of metabolic syndrome, SirT1 activation attenuated apoptosis of hepatocytes and improved liver function including lipid metabolism. We show in SirT1-overexpressing HepG2 cells that oxidants (nitrosocysteine and hydrogen peroxide) or metabolic stress (high palmitate and high glucose) inactivated SirT1 by reversible oxidative post-translational modifications (OPTMs) on three cysteines. Mutating these oxidation-sensitive cysteines to serine preserved SirT1 activity and abolished reversible OPTMs. Overexpressed mutant SirT1 maintained deacetylase activity and attenuated proapoptotic signaling, whereas overexpressed wild type SirT1 was less protective in metabolically or oxidant-stressed cells. To prove that OPTMs of SirT1 are glutathione (GSH) adducts, glutaredoxin-1 was overexpressed to remove this modification. Glutaredoxin-1 overexpression maintained endogenous SirT1 activity and prevented proapoptotic signaling in metabolically stressed HepG2 cells. The in vivo significance of oxidative inactivation of SirT1 was investigated in livers of high fat diet-fed C57/B6J mice. SirT1 deacetylase activity was decreased in the absence of changes in SirT1 expression and associated with a marked increase in OPTMs. These results indicate that glutathione adducts on specific SirT1 thiols may be responsible for dysfunctional SirT1 associated with liver disease in metabolic syndrome.