Platelet-derived growth factor-induced disruption of gap junctional communication and phosphorylation of connexin43 involves protein kinase C and mitogen-activated protein kinase

Previously we showed a rapid and transient inhibition of gap junctional communication (GIC) by platelet-derived growth Factor (PDGF) in T51B rat liver epithelial cells expressing wild-type platelet-derived growth factor beta receptors (PDCFr beta). This action of PDGF correlated with the hyperphosphorylation of the gap junction protein connexin43 (Cx43) and required PDGFr beta tyrosine kinase activity, suggesting the participation of protein kinases and phosphatases many of which are activated by PDGF treatment. In the present study, two such kinases, namely protein kinase C (PKC) and mitogen-activated protein kinase (MAPK), are investigated for their possible involvement in PDCF-induced closure of junctional channels and Cx43-phosphorylation. Down-regulation of PKC-isoforms by 12-O-tetradecanoylphorbol-13-acetate or pretreatment with the PKC inhibitor calphostin C, completely blocked PDGF action on GJC and Cx43. Activation of MAPK correlated with PDGF-induced Down phosphorylation, and prevention of MAPK activation by PD98059 eliminated the PDGF effects. Interestingly, elimination of CIC recovery by cycloheximide was associated with a sustained activated-MAPK level. Based on these results we postulate that the activation of PKC and MAPK are required in PDCF-mediated Down phosphorylation and functional closure. (C) 1998 Wiley-Liss, Inc.