The effect of vitamin E, probucol, and lovastatin on oxidative status and aortic fatty lesions in hyperlipidemic-diabetic hamsters

Abstract

Diabetes mellitus is associated with an increased risk of premature atherosclerosis, which may be due in part to an increased rate of low density lipoprotein (LDL) oxidation. Previous studies have shown that vitamin E, probucol, and lovastatin can reduce the oxidative susceptibility of LDL in normoglycemic animal models; however, few studies have investigated this in conjunction with aortic fatty streak lesion formation in diabetic hyperlipidemic models. Forty-eight Syrian hamsters were made diabetic by intraperitoneal injection of low dose streptozotocin. Diabetic animals (12 animals/groups) received a high saturated fat and cholesterol diet for 12.5 weeks. At 2.5 week of dietary treatments, the diet was supplemented with either: (1) 500 IU/day vitamin E (D+E); (2) 1% probucol w/w of the diet (D+P); (3) 25 mg/kg lovastatin (D+L); or (4) diabetic control (D). An age-matched group of hamsters (n=6) receiving the same diet but not made diabetic (ND) was used as control. At the end of the study, aortic arch foam cell-rich fatty streak lesion, plasma glucose, total cholesterol (TC), high density lipoprotein cholesterol (HDL-C), non-HDL-C, triglycerides (TG), phospholipids, α-tocopherol, plasma lipid peroxide and the susceptibility of LDL to copper-catalyzed oxidation were determined. Diabetes increased plasma glucose, and when combined with an atherogenic diet resulted in a further increase of plasma lipids. Vitamin E, probucol, and lovastatin significantly reduced plasma TG in the diabetic hamsters fed the atherogenic diet. Vitamin E treatment increased TC, probucol reduced HDL-C without affecting TC; whereas lovastatin reduced TC and selectively decreased non-HDL-C, and significantly reduced fatty streak lesion formation in the aortic arch. While vitamin E and probucol were effective in reducing several indices of oxidative stress including plasma lipid peroxides, cholesterol oxidation products and in vitro LDL oxidation, they had no effect on fatty streak lesion formation. Our results indicate that the LDL in diabetic animals is more susceptible to oxidation than in non-diabetic hamsters and that not only vitamin E and probucol but also lovastatin provide antioxidant protection. It appears that in this combined model of diabetes and hypercholesterolemia, progression of fatty streak lesion formation is mainly associated with changes in TC and non-HDL-C as affected by lovastatin, and is less dependent on the extent of LDL oxidation, changes in plasma TG level and oxidative stress status.

Introduction

Patients with diabetes mellitus have a high incidence of coronary heart disease (CHD) [1], and over 50% of their mortality rate may be attributed to CHD. In light of this, the National Cholesterol Education Program Expert Panel suggests that the goal of lowering low density lipoprotein (LDL) cholesterol (i.e. <100 mg/dl or 2.6 mmol/l) in diabetic patients should be the same as subjects with CHD [2]. Moreover, the diabetic state is characterized by an increased rate of lipoprotein oxidation, which may play an important role in the development of atherosclerosis [3]. Diabetes can affect lipoprotein metabolism in multiple ways. The diabetic’s lipoprotein profile is characterized by changes in lipoprotein concentrations and composition [4]. Diabetic patients show increases in triglyceride and decreases in high-density lipoprotein-xholesterol (HDL-C) levels [5]. Macrophages readily take up the oxidized LDL by the scavenger receptors, the expression of which is not regulated by intracellular levels of cholesterol leading to foam cell formation within the fatty streaks [6]. Oxidized LDL has numerous effects on arterial wall cells, which could increase the propensity of the artery for atherosclerosis. This includes cytotoxicity stimulation of adhesion molecule expression, monocyte chemotaxis, inhibition of nitric oxide production and disrupted cholesterol homeostasis [7], [8], [9].

There is increasing evidence that the complications of diabetes are associated with oxidative stress induced by generation of free radicals [10], [11]. Glycation and oxidation processes are closely linked and occur simultaneously during diabetes, and high levels of oxidized LDL are expected to be present under these conditions. Glycated LDL also resembles minimally modified LDL and is prone to oxidation. A number of reports from different laboratories described shortened oxidative lag-phase during copper-catalyzed oxidation of glycated LDL and LDL from diabetic patients. Increased TBARS levels were found in LDL and erythrocyte membranes of NIDDM and IDDM diabetic patients after exposure to phenylhydrazine [12]. TBARS levels were also higher in vitro glycated LDL after incubation with copper ions [13], [14], and xanthine/-xanthine oxidase [14]. Vitamin E supplementation of glycated LDL during in vitro oxidation decreases its susceptibility to oxidation [15]. Oxidative reactions also can contribute to enhanced glycation. However, a number of studies failed to show an effect of vitamin E on LDL glycation [3], [15], [16]. Further, the initially increased levels of lipoperoxidation products in the LDL molecule also may contribute to long-term glycosylation and oxidation. Plasma lipid peroxides are elevated in streptozotocin (STZ)-induced diabetic rats [17]. Some of the clinical manifestations of diabetes in experimental animals have been shown to be diminished by antioxidant and/or hypolipidemic drug treatments [18].

In recent years, the golden Syrian hamster has been used as a valuable model for studying lipoprotein cholesterol metabolism because of its similarities to humans with regards to: (1) cholesterol and bile acid metabolism [19]; (2) non-high density lipoprotein cholesterol (non-HDL-C) and triglyceride response to an atherogenic diet [20]; (3) development of atherosclerotic lesions similar to those found in early stages of human disease [21]; and 4) exclusive hepatic production of apolipoprotein (apo) B-100 [22]. In addition, the hamster is a useful model for studying lipoprotein metabolism in diabetes [23], and oxidative markers associated with atherosclerosis in this model have not been well investigated. Since accelerated atherosclerosis is one of the main complications of diabetes, in the current study, we used an STZ-induced diabetes model combined with the hyperlipidemic Syrian golden hamster model induced by dietary cholesterol. We investigated the efficacy of the antioxidant vitamin E and the hypolipidemic drugs probucol and lovastatin on plasma lipids and lipoproteins profile, plasma lipid peroxides, aortic arch foam cell-rich lesion formation, and in vitro LDL oxidation.