Autocrine regulation of TGF beta expression in adult cardiomyocytes

As shown before, TGF beta acts in an autocrine manner on the induction of h ypertrophic responsiveness to beta-adrenoceptor stimulation in cultured ven tricular cardiomyocytes of adult rat. We now investigated how TGF beta expr ession and activation is regulated in these cultures and how beta-adrenocep tor stimulation influences TGF beta-mRNA expression. It was found that freshly isolated cardiomyocytes secrete latent TGF beta i n the culture medium. Supplementation of the cultures with 20% FCS resulted in activation of the secreted TGF beta to 4.1 +/- 0.2 ng/ml active TGF bet a after 6 days. Presence of the protease inhibitor aprotinin (50 mu g/ml) r educed TGF beta activity by 44 +/- 5% (n = 5, P<0.05). In cultures suppleme nted with 5% FCS. TGF beta was not activated. Active TGF beta downregulated its mRNA-expression: after 5 days TGF beta(1)-mRNA was reduced to 55.1+/-1 1.0% TGF beta(2)-mRNA to 30.1+/-16.5%, and TGF beta(3)-mRNA to 0.3 +/- 0.4% in 20% FCS-cultures as compared to their expression in freshly isolated ce lls (n = 4, p<0.05). TGF beta-mRNA expression did not change in cultures wi thout active TGF beta. Isoprenaline (1 mu M) increased TGF beta(1)-mRNA onl y in cultures which had been pre-exposed to active TGF beta. This effect wa s also seen when hearts from normal mice were compared with hearts from tra nsgenic mice overexpressing TGF beta(1): only in hearts from transgenic ani mals perfusion with isoprenaline increased TGF beta(1)-mRNA. In conclusion, isolated cardiomyocytes release latent TGF beta. which is ac tivated by external proteases, Active TGF beta downregulates its own mRNA e xpression, Preexposure to TGF beta is necessary for a beta-adrenoceptor-med iated increase in TGF beta(1)-mRNA in cardiomyocytes. (C) 1999 Academic Pre ss.