Transforming growth factor‐β1 modulates adenylyl cyclase signaling elements and epidermal growth factor signaling in cardiomyocytes

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Abstract
Studies presented in this report were designed to investigate the effects of transforming growth factor‐b̃1 (TGF‐b̃1) on epidermal growth factor (EGF)‐ mediated stimulation of cAMP accumulation in cardiac myocytes and elucidate the mechanism(s) involved in this modulation. TGF‐b̃1 (20 pM) treatment of cardiac myocytes, in a time‐dependent manner, decreased the ability of EGF (100 nM) to increase cAMP accumulation. Significant attenuation of EGF‐elicited cAMP accumulation was observed 2 h after exposure to TGF‐b̃1 and 18 h after addition of TGF‐b̃1, the ability of EGF to increase cAMP accumulation was completely obliterated. TGF‐β1 neither decreased immunoprecipitable EGF receptors in membranes from cardiomyocytes nor altered the specific binding of [125I] EGF to cardiomyocyte membranes. However, TGF‐b̃1 decreased the ability of EGF to phosphorylate membrane proteins on tyrosine residues. TGF‐b̃1 treatment of cardiomyocytes also decreased the ability of forskolin to augment cAMP accumulation in intact cells and stimulate adenylyl cyclase activity. Similarly, in membranes of TGF‐b̃1‐treated cells, neither isoproterenol nor EGF stimulated adenylyl cyclase activity. Interestingly, as assessed by the ability of A1F4− to stimulate adenylyl cyclase, TGF‐b̃1 did not alter the coupling between Gs and catalytic subunits. Likewise, TGF‐b̃1 did not alter the functional activity of the inhibitory regulatory element of the system, Gi. Western analysis of cellular proteins revealed that TGF‐b̃1 did not alter the amounts of Gsα, Giα2, and Giα3. We conclude that TGF‐b̃1 attenuates EGF‐elicited cAMP accumulation in cardiomyocytes, in part, by decreasing the EGF receptor kinase function and that TGF‐b̃1‐mediated alterations in the activity of adenylyl cyclase catalytic subunit also contribute toward the regulation of adenylyl cyclase by various agonists.