Role of PKC and MAP kinase in EGF- and TPA-induced connexin43 phosphorylation and inhibition of gap junction intercellular communication in rat liver epithelial cells

Gap junction intercellular communication (GJIC) is involved in the regulation of many cellular processes. The gap junction channels are made up of connexins and the flow of polar low molecular weight molecules through these channels is inhibited by several groups of substances, such as tumour promoters and growth factors. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), chlordane and the growth factor epidermal growth factor (EGF) are potent inhibitors of GJIC in several cell types, including the rat liver epithelial cell line IAR6.1. The induced inhibition of communication by TPA and EGF in IAR6.1 cells is associated with hyperphosphorylation of connexin43, the connexin responsible for GJIC. Two enzyme inhibitors, PD98059, a specific inhibitor of MEK kinase, and GF109203X, a selective inhibitor of protein kinase C (PKC), were used to study the signalling pathways involved in the effect of EGF and TPA on GJIC, with the following conclusions. The inhibition of cell communication in IAR6.1 cells by EGF is likely to be mediated by direct phosphorylation of connexin43 by MAP kinase. TPA blocks GJIC mainly by the direct action of PKC, but also partly through cross-talk with the MAP kinase pathway. Connexin43 hyperphosphorylation induced by TPA is, as for EGF, mediated through MAP kinase, while PKC seems to block GJIC either through other substrates or induces a type of connemn43 phosphorylation that causes no significant electrophoresis mobility shift.