Mitogen-activated protein kinase and phosphorylation of connexin43 are notsufficient for the disruption of gap junctional communication by platelet-derived growth factor and tetradecanoylphorbol acetate

Disruption of gap junctional communication (GIC) by various compounds, incl uding growth factors and tumor promoters, is believed to be modulated by th e phosphorylation of a gap junctional protein, connexin43 (Cx43). We have p reviously demonstrated a platelet-derived growth factor (PDGF)-induced bloc kade of CIC and phosphorylation of Cx43 in T51B rat liver epithelial cells expressing wild-type PDGF receptor beta (PDGFr beta). Both of these actions of PDGF required participation of protein kinase C (PKC) and mitogen-activ ated protein kinase (MAPK). Similar requirements of MAPK were suggested in the modulation of GIC by other agents, including epidermal growth factor (E GF) and lysophosphatidic acid (LPA). Since many of these agents activate ad ditional protein kinases, our present study examined whether activation of MAPK was sufficient for Cx43 phosphorylation and GJC blockade. By utilizing a variety of MAPK activators, we now show that activation of MAPK is not a lways associated with either Cx43 phosphorylation or disruption of GIG, whi ch suggests a requirement for additional factors. Furthermore, pretreatment with hydrogen peroxide (H2O2), a potent MAPK activator but inefficient GJC /Cx43 modulator, abrogated PDCF- or TPA-induced disruption of GJC. While a 5 min H2O2 pretreatment abolished both PDGF- and TPA-induced Cx43 phosphory lation and GJC blockade, a simultaneous H2O2 treatment interfered only with GJC closure but not with the phosphorylation of Cx43 induced by PDGF and T PA. This finding indicates that, in addition to the Cx43 phosphorylation st ep, inhibition of GJC requires interaction with other components. H2O2-medi ated abrogation of PDGF/TPA signaling can be neutralized by the antioxidant N-acetylcysteine (NAC) or by the tyrosine kinase inhibitor genistein. Take n together, our results suggest that disruption of GJC is not solely mediat ed by either activated MAPK or Cx43 phosphorylation but requires the partic ipation of additional kinases and regulatory components. This complex mode of regulation is perhaps essential for the proposed functional role of GJC. J. Cell. Physiol. 179:87-96, 1999. (C) 1999 Wiley-Liss, Inc.