Stretch-induced paracrine hypertrophic stimuli increase TGF-β1 expression in cardiomyocytes

Cardiac hypertrophy refers to the abnormal growth of cardiomyocytes, and is often caused by valvular heart disease and hypertension. It involves the activation of growth, including increased protein synthesis and changes in gene expression. Transforming growth factor-beta(1) (TGF-beta(1)) may play a central role in protecting the heart during the hypertrophic response by helping to restore normal functions of the affected myocardium. We tested the hypothesis that cardiomyocytes respond to stretch-induced paracrine hypertrophic stimuli with increased expression of TGF-beta(1). To that purpose, we investigated whether angiotensin II (AII), endothelin-1 (ET-1) and TGF-beta, secreted by stretched cardiac and vascular cells, are involved in the paracrine mechanisms of stretch-induced changes of TGF-beta1 mRNA expression in stationary (i.e. non-stretched) cardiomyocytes. Our results indicated that TGF-beta(1) mRNA expression in stationary cardiomyocytes was increased by AII release from cardiomyocytes that had been stretched for 30-60 min. Furthermore, it is likely that ET-beta(1) and TGF-beta were released by stretched cardiac fibroblasts and endothelial cells to induce TGF-beta(1) mRNA expression in stationary cardiomyocytes. Stretched vascular smooth muscle cells did not influence TGF-beta(1) mRNA expression in stationary cardiomyocytes. These results indicate that AII, ET-1 and TGF-beta, released by cardiac cell types, act as paracrine mediators of TGF-beta(1) mRNA expression in cardiomyocytes. Therefore, we conclude that in stretched myocardium the cardiomyocytes, cardiac fibroblasts and endothelial cells take part in intercellular interactions contributing to cardiomyocyte hypertrophy.