Salutary effects of attenuation of angiotensin II on coronary perivascular fibrosis associated with insulin resistance and obesity

Abstract

Obesity and insulin resistance confer increased risk for accelerated coronary disease and cardiomyopathic phenomena. We have previously shown that inhibition of angiotensin-converting enzyme (ACE) prevents coronary perimicrovascular fibrosis in genetically obese mice that develop insulin resistance. This study was performed to elucidate mechanism(s) implicated and to determine the effects of attenuation of angiotensin II (Ang) II. Genetically obese ob/ob mice were given ACE inhibitor (temocapril) or Ang II type 1 (AT₁) receptor blocker (olmesartan) from 10 to 20 weeks. Cardiac expressions of plasminogen activator inhibitor (PAI)-1, the major physiologic inhibitor of fibrinolysis, and transforming growth factor (TGF)-β₁, a prototypic profibrotic molecule, were determined and extent of perivascular coronary fibrosis was measured. Twenty-week-old obese mice exhibited increased plasma levels of PAI-1 and TGF-β₁ compared with the values in lean counterpart. Perivascular coronary fibrosis in arterioles and small arteries was evident in obese mice that also showed increased left ventricular collagen as measured by hydroxyproline assay. Immunohistochemistry confirmed the deposition of perivascular type 1 collagen. Markedly increased PAI-1 and TGF-β were seen immunohistochemically in coronary vascular wall and confirmed by western blotting. When obese mice were treated with temocapril or olmesartan from 10 to 20 weeks, both were equally effective and prevented increases in perivascular fibrosis, plasma PAI-1 and TGF-β₁, left ventricular collagen and mural immunoreactivity for PAI-1, TGF-β and collagen type 1. The c-Jun NH₂-terminal kinase (JNK) activity was elevated in the left ventricle of obese mice (western) and blocked by temocapril and olmesartan. Ang II-mediated upregulation of PAI-1 and TGF-β₁ with collagen deposition may explain the mechanism of perivascular fibrosis in obese mice. ACE inhibition and blockade of AT₁ receptor may prevent coronary perivascular fibrosis and collagen deposition even before development of overt diabetes. JNK activation may be a mediator of obesity-related cardiac dysfunction and a potential therapeutic target.

Introduction

Obesity is frequently associated with hyperglycemia, hyperinsulinemea, insulin resistance and non-insulin-dependent diabetes mellitus (NIDDM) [1] and confers increased risk for accelerated macrovascular and microvascular coronary disease and cardiomyopathic phenomena [2]. The molecular changes in obesity that promote these conditions are unclear. Perivascular arteriolar fibrosis is often present in patients with myocardial ischemia despite the absence of marked stenosis [3]. We have previously reported that coronary perimicrovascular fibrosis is augmented by increased cardiac expression of plasminogen activator inhibitor (PAI)-1, the major physiologic inhibitor of fibrinolysis, in genetically obese mice (ob/ob) with insulin resistance, hyperglycemia, hyperinsulinemea and NIDDM [4].

The renin–angiotensin system (RAS) is a major determinant for cardiac remodeling. RAS is important in regulating circulating and vascular PAI-1 expression, thereby affecting fibrinolytic balance. Angiotensin II (Ang II) stimulates fibrous tissue formation by promoting transforming growth factor (TGF)-β₁ synthesis via Ang II type 1 (AT₁) receptor [5]. Early induction of TGF-β₁ by chronic inhibition of nitric oxide (NO) synthesis contributes to cardiac fibrosis in rat [6], suggesting that Ang II may play major roles in coronary perivascular fibrosis observed in genetically obese mice. The present study was designed to gain insight into the molecular mechanisms responsible for coronary microvascular changes in ob/ob mice. Angiotensin-converting enzyme (ACE) inhibitor (ACEI) and AT₁ receptor blocker (ARB) were found to be potent therapeutic tools for prevention of cardiovasculopathy associated with obesity and insulin resistance.