Autocrine Regulation of TGF β Expression in Adult Cardiomyocytes

G. Taimor, K.-D. Schlüter, K. Frischkopf, M. Flesch, S. Rosenkranz and H. M. Piper. Autocrine Regulation of TGF β Expression in Adult Cardiomyocytes. Journal of Molecular and Cellular Cardiology (1999)31 , 2127–2136. As shown before, TGF β acts in an autocrine manner on the induction of hypertrophic responsiveness to β -adrenoceptor stimulation in cultured ventricular cardiomyocytes of adult rat. We now investigated how TGF β expression and activation is regulated in these cultures and how β -adrenoceptor stimulation influences TGF β -mRNA expression. It was found that freshly isolated cardiomyocytes secrete latent TGF β in the culture medium. Supplementation of the cultures with 20% FCS resulted in activation of the secreted TGFβ to 4.1±0.2 ng/ml active TGF β after 6 days. Presence of the protease inhibitor aprotinin (50μ g/ml) reduced TGF β activity by 44±5% (n=5, P<0.05). In cultures supplemented with 5% FCS, TGF β was not activated. Active TGF β downregulated its mRNA-expression: after 6 days TGF β₁-mRNA was reduced to 55.1±11.0%, TGFβ₂ -mRNA to 30.1±16.5%, and TGF β₃-mRNA to 0.3±0.4% in 20% FCS-cultures as compared to their expression in freshly isolated cells (n=4, P<0.05). TGFβ -mRNA expression did not change in cultures without active TGF β. Isoprenaline (1 μ m) increased TGF β₁-mRNA only in cultures which had been pre-exposed to active TGF β. This effect was also seen when hearts from normal mice were compared with hearts from transgenic mice overexpressing TGFβ₁ : only in hearts from transgenic animals perfusion with isoprenaline increased TGFβ₁ -mRNA. In conclusion, isolated cardiomyocytes release latent TGF β, which is activated by external proteases. Active TGF β downregulates its own mRNA expression. Preexposure to TGF β is necessary for a β -adrenoceptor-mediated increase in TGF β₁-mRNA in cardiomyocytes.