TRANSFORMING GROWTH-FACTOR-BETA-1 MODULATES ADENYLYL-CYCLASE SIGNALING ELEMENTS AND EPIDERMAL GROWTH-FACTOR SIGNALING IN CARDIOMYOCYTES

Studies presented in this report were designed to investigate the effe cts of transforming growth factor-beta 1 (TGF-beta 1) on epidermal gro wth factor (EGF)- mediated stimulation of cAMP accumulation in cardiac myocytes and elucidate the mechanism(s) involved in this modulation. TGF-beta 1 (20 pM) treatment of cardiac myocytes, in a time-dependent manner, decreased the ability of EGF (100 nM) to increase cAMP accumul ation. Significant attenuation of EGF-elicited cAMP accumulation was o bserved 2 h after exposure to TGF-beta 1 and 18 h after addition of TG F-beta 1, the ability of EGF to increase cAMP accumulation was complet ely obliterated. TGF-beta 1 neither decreased immunoprecipitable EGF r eceptors in membranes from cardiomyocytes nor altered the specific bin ding of [I-125] EGF to cardiomyocyte membranes. However, TGF-beta 1 de creased the ability of EGF to phosphorylate membrane proteins on tyros ine residues. TGF-beta 1 treatment of cardiomyocytes also decreased th e ability of forskolin to augment cAMP accumulation in intact cells an d stimulate adenylyl cyclase activity. Similarly, in membranes of TGF- beta 1-treated cells, neither isoproterenol nor EGF stimulated adenyly l cyclase activity. Interestingly, as assessed by the ability of A1F(4 )(-) to stimulate adenylyl cyclase, TGF-beta 1 did not alter the coupl ing between G(s) and catalytic subunits. Likewise, TGF-beta 1 did not alter the functional activity of the inhibitory regulatory element of the system, G(i). Western analysis of cellular proteins revealed that TGF-beta 1 did not alter the amounts of G(s alpha) G(i alpha 2) and G( i alpha 3). We conclude that TGF-beta 1 attenuates EGF-elicited cAMP a ccumulation in cardiomyocytes, in part, by decreasing the EGF receptor kinase function and that TGF-beta 1-mediated alterations in the activ ity of adenylyl cyclase catalytic subunit also contribute toward the r egulation of adenylyl cyclase by various agonists. (C) 1995 Wiley-Liss , Inc.