Effect of chronic AT(1) receptor blockade on cardiac Smad overexpression in hereditary cardiomyopathic hamsters

Abstract

Objective: As the pharmacological suppression of angiotensin has been associated with cardioprotective effects in cardiomyopathy, our primary aim was to determine whether the expression of Smad protein components of the cardiac TGF-beta signaling cascade is modulated by chronic AT(1) receptor blockade. Furthermore, we examined the relationship between cardiac Smad protein expression and altered collagen turnover in the cardiomyopathic heart.

Methods: Male UM-X7. 1 cardiomyopathic (CMP) Syrian hamsters at early (65 days) and late (200 days) stages of cardiomyopathy were subjected to 4 week losartan (15 mg/kg/day) treatment. Expression of left ventricular (LV) receptor-activated (Smad 2) and common-mediator (Smad 4) Smads from control (F1-beta strain) hamsters, non-treated cardiomyopathic (CMP), and losartan-treated CMP animals was assessed. Collagen turnover, including fibrillar collagen synthesis/accretion and cardiac MMP activity was assessed.

Results: Elevated mRNA abundance of fibrillar collagens and ANF were present in cardiomyopathic hearts and these trends were normalized in the early stage losartan-treated group. 4-Hydroxyproline and zymographic assays confirmed fibrosis and elevated MMP-1 and -2 activities in CMP hearts. Losartan treatment was associated with a modest reduction of cardiac 4-hydroxyproline concentration, and a significant reduction of both MMP-1 and MMP-2 activities. While TGF-beta(1) mRNAs were elevated in both CMP groups vs. controls, total TGF-beta protein content was not different in CMP vs. controls. In LV preparations containing nuclear extract, elevated Smad 2 and Smad 4 protein expression was noted in cardiomyopathic hearts vs. controls. Losartan treatment of late-stage CMP hamsters was associated with a significant reduction in Smad 2 and a modest reduction of Smad 4 protein expression vs. untreated CMP samples.

Conclusions: Altered cardiac Smad expression, present in both early and late stage cardiomyopathy, is positively correlated with the occurrence of cardiac fibrosis and elevated collagen turnover in failing CMP hearts. Four week AT(1) blockade is associated with normalized expression of cardiac Smad 2 proteins, and these changes occur in parallel with some aspects of collagen turnover in failing cardiomyopathic hearts.