PROTEIN-KINASE-A AND PROTEIN-KINASE-C SYNERGISTICALLY ACTIVATE THE RAF-1 KINASE MITOGEN-ACTIVATED PROTEIN-KINASE CASCADE IN NEONATAL RAT CARDIOMYOCYTES

Adrenoceptor agonists play an important role in cardiac hypertrophy. I n 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 a ctivation 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 p resent study was performed to elucidate the role of PKA and PKC in car diomyocyte hypertrophy, PKA activators such as forskolin (FSK), isobut ylmethylxanthine, dibutyryl cAMP and isoproterenol, significantly acti vated Raf-1 and MAP kinases with a peak at 2 and 8 min, respectively, followed by an increase in protein synthesis in cardiac myocytes. Simi lar responses were observed when cardiomyocytes were stimulated with P KC activators such as 12-0-tetra-decanoylphorbol-13-acetate (TPA), ang iotensin II, phenylephrine and mechanical stretch, After depleting ext racellular Ca2+ with EGTA, FSK did not activate MAP kinases, while dow n-regulation of PKC by long exposure with TPA did not influence FSK-in duced MAP kinase activation, Furthermore, FSK and TPA synergistically activated Raf-1. Similar synergistic activation of MAP kinases was obs erved when other PKC activators were added to cardiac myocytes with FS K at the same time. In conclusion, unlike other cell types, PKA activa tes Raf-1 and MAP kinases followed by an increase in protein synthesis in cardiac myocytes. (C) 1997 Academic Press Limited.