The Role of Angiotensin II in Post-Translational Regulation of Fibrillar Collagens in Fibrosed and Failing Rat Heart

Abstract

An explanation of the molecular mechanisms that trigger the development of pathological cardiac fibrosis, myocyte hypertrophy, and heart failure associated with common ailments such as chronic postinfarction has been sought at the benchtop and clinic for the past forty years and is a current topic of intensive investigative activity. Among the information garnered from these studies is that (1) growth factors including angiotensin (Ang) II are involved in modulation of gene products specifically expressed by cardiac fibroblasts in vitro and (2) their enhanced presence has been associated with myocardial stress, inappropriate cardiac growth, and fibrosis in vivo. Although increased deposition of collagen proteins has been described after myocardial infarction (MI), little is known of (1) time-dependent transcriptional alteration of specific cardiac collagen subtypes or (2) the degradative mechanisms for cardiac collagens in right and left ventricular myocardium remote to large left ventricular infarction. We have investigated collagen mRNA abundance and the deposition of specific collagen subtypes in noninfarcted left and right rat heart muscle at different times after MI. We have also assessed the activity of different myocardial matrix metalloproteinases (MMPs) using zymography to gain some information about degradative pathways for collagen. Furthermore, we assessed passive compliance properties of the right ventricle in experimental hearts. Finally, the efficacy of an angiotensin-converting enzyme (ACE) inhibitor (ramipril) and that of an Ang II receptor type I antagonist (losartan) was compared in experimental animals. We observed that the mRNA abundance of types I and III collagen were increased 3 days after myocardial infarction in both viable left and uninfarcted right ventricular tissues and were maintained at relatively high levels throughout the duration of our studies. Stiffness of the right ventricular myocardium was significantly increased in the 56 days experimental group when compared with that of control values; this was positively correlated with increased immunoreactive collagens in surviving right (and left) cardiac tissue of 14, 28, and 56 day experimental groups. The elevation of fibrillar collagen mRNA abundance in noninfarcted muscle from ventricular chambers was not normalized by treatments with either ramipril or losartan. MMP activity was increased in viable left ventricle at 14, 28, and 56 days and at 14 days in the right ventricle in experimental animals when compared with controls. Finally, we observed that AT1 receptor blockade in infarcted hearts was associated with normalization of elevated cardiac 4-prolyl hydroxylase protein expression compared with nontreated post-MI rats. Thus, increased expression of collagen types I and III occurs in noninfarcted heart after MI, followed thereafter by deposition of collagen proteins. Increased MMP activity is present in chronic experimental hearts; MMP activation may be important in remodeling of the noninfarcted cardiac stroma. Because losartan treatment was not associated with any normalization of elevated collagen mRNA abundance, a clear causal relationship between the upregulation of fibrillar collagen mRNA abundance and the AT1 receptor was not apparent. On the other hand, the reduction of cardiac fibrosis mediated by ACE inhibition and losartan treatment may reside at the posttranslational level in cardiac collagen metabolism.