Elsevier

Metabolism

Volume 55, Issue 7, July 2006, Pages 928-934
Metabolism

Oxidative stress and dysregulation of NAD(P)H oxidase and antioxidant enzymes in diet-induced metabolic syndrome

https://doi.org/10.1016/j.metabol.2006.02.022Get rights and content

Abstract

Previously, we have demonstrated that chronic consumption of a high-fat, high-refined sugar (HFS) diet results in metabolic syndrome which is marked by obesity, insulin resistance, hyperlipidemia, and hypertension in Fischer rats. Metabolic syndrome in this model is associated with oxidative stress, avid nitric oxide (NO) inactivation by reactive oxygen species (ROS), diminished NO bioavailability, and dysregulation of NO synthase isotypes. Although occurrence of oxidative stress and its impact on NO metabolism are well established, the molecular source(s) of ROS in this model is unknown. In an attempt to explore this issue, we measured protein expressions of the key ROS-producing enzyme, NAD(P)H oxidase, and the main antioxidant enzymes, superoxide dismutase (CuZn SOD and Mn SOD), catalase, glutathione peroxidase (GPX), and heme oxygenase-2 (HO-2), in the kidney and aorta of Fischer rats fed an HFS or low-fat, complex-carbohydrate diet for 7 months. In addition, plasma lipid peroxidation product (malondialdehyde) as well as endothelium-dependent and -independent vasorelaxation (aorta rings) was determined. The results showed a significant upregulation of gp91phox subunit of NAD(P)H oxidase and downregulations of SOD isoforms, GPX, and HO-2 in the kidney and aorta of the HFS-fed animals. This was associated with increased plasma malondialdehyde concentration and impaired vasodilatory response to acetylcholine, but not the NO donor, Na nitroprusside. The latter findings confirm the presence of oxidative stress and endothelial dysfunction in the HFS-fed rats. Oxidative stress and endothelial dysfunction in the diet-induced metabolic syndrome are accompanied by upregulation of NAD(P)H oxidase, pointing to increased ROS production capacity, and downregulation of SOD isoforms, GPX, and HO-2, the key enzymes in the antioxidant defense system.

Introduction

Metabolic syndrome, otherwise known as syndrome X or insulin resistance syndrome, is defined by the presence of insulin resistance, hyperinsulinemia, and some combination of obesity, dyslipidemia, inflammation, endothelial dysfunction, and hypertension [1], [2], [3], [4], [5], [6], [7], [8], [9]. The incidence of metabolic syndrome has reached epidemic proportion worldwide due primarily to prevailing sedentary lifestyle and unhealthy dietary habits. Consequently, metabolic syndrome has emerged as a major cause of diabetes, cardiovascular, and kidney diseases in the industrialized societies [10], [11].

In a series of earlier studies, we demonstrated that chronic consumption of a high-fat, high-refined sugar (HFS) diet induces obesity, insulin resistance, dyslipidemia, and hypertension in genetically normal rats [12], [13]. Hypertension in this model is associated with oxidative stress [14], avid nitric oxide (NO) inactivation, depressed NO bioavailability [15], and downregulations of NO synthase (NOS) isoforms, AKT (eNOS activator), and soluble guanylate cyclase (downstream target of NO) [14]. It is of note that diminished NO production and NOS expression found in rats with diet-induced metabolic syndrome are also seen in the obese Zucker rats with hereditary metabolic syndrome [16]. The reduced NO production capacity in the obese Zucker rats is associated with inflammation and activation of inflammatory pathways [17].

Oxidative stress plays a major role in the pathogenesis of endothelial dysfunction, hypertension, inflammation, and atherosclerotic cardiovascular disease. Although the presence of oxidative stress and its impact on NO metabolism in rats with diet-induced metabolic syndrome are well established [14], [15], the molecular sources of the reactive oxygen species (ROS) in this model are uncertain. Oxidative stress can result from either excess ROS production and/or deficient antioxidant capacity. The present study was designed to test the hypothesis that oxidative stress in rats with diet-induced metabolic syndrome may be due to upregulation of NAD(P)H oxidase (a major source of ROS in the kidney and cardiovascular tissues) and downregulation of the main antioxidant enzymes, superoxide dismutases, glutathione peroxidase (GPX), catalase (CAT), and heme oxygenase.

Section snippets

Animals and diets

All protocols were approved and conducted in accordance with the University of California, Los Angeles, Animal Research Committee. Two-month-old female Fischer 344 rats were obtained from Harlan Sprague Dawley (San Diego, Calif). We have used this rat model in previous studies, as the female Fischer rat normally shows little weight gain after the maturation phase [12], [18]. The animals (6 per group) were housed in a light-controlled (12-hour light cycle starting at 7 am) and

Blood pressure and MDA

Tail arterial blood pressure at the conclusion of the 7-month study period was slightly higher in the HFS group (131 ± 6 mm Hg) than in the LFCC group (120 ± 2 mm Hg, P > .05). Likewise, plasma MDA level in the HFS group (1.91 ± 0.08 μmol/L) was significantly greater than in the LFCC group (1.37 ± 0.06 μmol/L, P < .05) denoting the presence of oxidative stress in the HFS rats.

NAD(P)H oxidase

Data are illustrated in Fig. 1. The gp91phox protein abundance in the thoracic aorta was significantly greater in the

Discussion

Several studies have shown that diets high in fat and refined sugars (sucrose or fructose) cause endothelial dysfunction and ultimately hypertension [15], [22], [23], [24], [25]. Endothelial dysfunction and hypertension in these and other models are associated with and, at least in part, due to oxidative stress [26], [27]. Oxidative stress in the kidney and vascular tissues can promote endothelial dysfunction by several mechanisms. For instance, oxidative stress limits the bioavailability of NO

Acknowledgment

Christian Roberts was supported by a National Research Scholarship Award postdoctoral fellowship (NIH F32 HL68406-01) during this project.

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