Regulation of Proliferative Response of Cardiac Fibroblasts by Transforming Growth Factor-β1

Cardiac fibroblasts constitute greater than 90% of the non-myocyte cells in the heart. Previously, it was established that cardiac fibroblasts are predisposed to transformation into a phenotype with muscle-specific features and that transforming growth factor-β1(TGF-β1) is a specific inducer of this event. In this study the hypothesis that TGF-β1-induced phenotypic modulation of cardiac fibroblasts is associated with their altered proliferative capacity is tested. Therefore the effects of TGF-β1on DNA synthesis in cardiac fibroblasts under normal conditions of cell culture and in response to a potent mitogen, basic fibroblasts growth factor (bFGF) were determined. The results showed that TGF-β1at 15 ng/ml (a concentration that induces fibroblast “transformation”) had a regulatory effect on proliferative capacity of cardiac fibroblasts which varied as the function of cell density in culture. In subconfluent and confluent cultures, pre-treatment of cardiac fibroblasts with TGF-β1for 24 h resulted in a dramatic shift in the bFGF-induced stimulation of DNA synthesis. TGF-β1-induced inhibition of DNA synthesis in cardiac fibroblasts coincided with their phenotypic modulation as evidenced by the expression of sarcomeric actin mRNA and morphological changes. Cross-linking studies with [125I]-labeled TGF-β1showed the presence of conventional types I, II and III TGF-β1receptor complexes on cardiac fibroblasts and their binding to TGF-β1under the experimental conditions. In summary, these data indicate that the proliferative capacity of cardiac fibroblasts is controlled by TGF-β1. They further suggest that the TGF-β1-induced phenotypic modulation of cardiac fibroblasts may be extended to include their altered proliferative capacity.