DIFFERENTIAL REGULATION OF PARALLEL MITOGEN-ACTIVATED PROTEIN-KINASESIN CARDIAC MYOCYTES REVEALED BY PHOSPHATASE INHIBITION

Previous studies have suggested that the contribution of inducible pho sphatases to ERK MAPK deactivation is both cell-type- and agonist-spec ific, The aim of this study was to define the role of inducible phosph atases in ERK MAPK regulation in cardiac myocytes. We examined the kin etics of activation/deactivation of ERK MAPKs following the exposure o f cardiac myocytes to endothelin-1 or phorbol ester. Deactivation was prevented by inhibition of protein synthesis indicating a contribution of inducible phosphatases. In contrast, okadaic acid failed to prolon g ERK MAPK activation, but activated three myelin basic protein kinase s (MBPKs, 55, 62, and 87 kDa) and two c-Jun kinases (46 and 55 kDa). A lthough the identity of the MBPKs is unknown, the c-Jun kinases corres ponded to JNK MAPKs. Simultaneous exposure of cardiac myocytes to okad aic acid and osmotic shock potentiated JNK MAPK activation. Thus, indu cible phosphatases regulate ERK MAPK deactivation, whereas okadaic aci d-sensitive phosphatases regulate JNK MAPKs and three novel MBPKs. (C) 1998 Academic Press.