ENDOTHELIN-1 AND FIBROBLAST GROWTH-FACTORS STIMULATE THE MITOGEN-ACTIVATED PROTEIN-KINASE SIGNALING CASCADE IN CARDIAC MYOCYTES - THE POTENTIAL ROLE OF THE CASCADE IN THE INTEGRATION OF 2 SIGNALING PATHWAYS LEADING TO MYOCYTE HYPERTROPHY

Maximally effective concentrations of endothelin-1 (ET-1), acidic FGF (aFGF), or 12-O-tetradecanoylphorbol-13-acetate (TPA) activated mitoge n-activated protein kinase (MAPK) by 3-4-fold in crude extracts of myo cytes cultured from neonatal rat heart ventricles. Maximal activation was achieved after 5 min. Thereafter, MAPK activity stimulated by ET-1 or aFGF declined to control values within 1-2 h, whereas activation b y TPA was more sustained. Two peaks of MAPK activity (a 42- and a 44-k Da MAPK) were resolved in cells exposed to ET-1 or aFGF by fast protei n liquid chromatography on a Mono Q column. One major and one minor pe ak of MAPK kinase (MAPKK) was stimulated by ET-1 or aFGF. Cardiac myoc ytes expressed protein kinase C (PKC)-alpha, -delta, -epsilon, and -ze ta as shown by immunoblotting. Exposure to 1 muM TPA for 24 h down-reg ulated PKC-alpha, -delta, and -epsilon, but not PKC-zeta. This maneuve r wholly abolished the activation of MAPK on re-exposure to TPA but di d not affect the response to aFGF. The effect of ET-1 was partially do wn-regulated. ET-1 stimulated phospho[H-3]inositide hydrolysis 18-fold , whereas aFGF stimulated by only 30%. Agonists which initially utiliz e dissimilar signaling pathways may therefore converge at the level of MAPKK/MAPK and this may be relevant to the hypertrophic response of t he heart.