Adrenoceptor agonists play an important role in cardiac hypertrophy. In cardiomyocytes, activation of alpha- and beta-adrenoceptors induces a variety of hypertrophic responses via activation of protein kinase C (PKC) and protein kinase A (PKA), respectively. Although PKC evokes activation of the Raf-1 kinase (Raf-1)/mitogen-activated protein (MAP) kinase cascade, PKA has been shown to inhibit the activation of Raf-1 and MAP kinases induced by growth factors in various cell types. The present study was performed to elucidate the role of PKA and PKC in cardiomyocyte hypertrophy. PKA activators such as forskolin (FSK), isobutylmethylxanthine, dibutyryl cAMP and isoproterenol, significantly activated Raf-1 and MAP kinases with a peak at 2 and 8 min, respectively, followed by an increase in protein synthesis in cardiac myocytes. Similar responses were observed when cardiomyocytes were stimulated with PKC activators such as 12-O-tetradecanoylphorbol-13-acetate (TPA), angiotensin II, phenylephrine and mechanical stretch. After depleting extracellular Ca2+ with EGTA, FSK did not activate MAP kinases, while down-regulation of PKC by long exposure with TPA did not influence FSK-induced MAP kinase activation. Furthermore, FSK and TPA synergistically activated Raf-1. Similar synergistic activation of MAP kinases was observed when other PKC activators were added to cardiac myocytes with FSK at the same time. In conclusion, unlike other cell types, PKA activates Raf-1 and MAP kinases followed by an increase in protein synthesis in cardiac myocytes.
Copyright 1997 Academic Press Limited.