INHIBITION OF GAP JUNCTIONAL INTERCELLULAR COMMUNICATION BY TUMOR PROMOTERS IN CONNEXIN43 AND CONNEXIN32-EXPRESSING LIVER-CELLS - CELL-SPECIFICITY AND ROLE OF PROTEIN-KINASE-C

In this study, we investigated whether the tumor promoters, 12-O-tetra decanoylphorbol-13-acetate (TPA), phenobarbital (PB), and 1,1-bis(p-ch lorophenyl)-2,2,2-trichloroethane (DDT), inhibited gap junctional inte rcellular communication (GJIC) in a cell-specific or connexin-specific manner and whether protein kinase C was involved, To do this, we used highly communicating WB-F344 rat liver epithelial cells, which expres s connexin43 as their predominant gap junction protein, WB-aB1 cells, which are a GJIC-incompetent mutant line of WB-F344 cells and that exp ress connexin43, WE-a/32-10 cells, which are a highly communicating de rivative of WB-aB1 cells generated by stable transduction with a conne xin32 retroviral expression vector, and primary cultured rat hepatocyt es, which express conexin32 predominantly, Treatment of WB-F344 and WB -a/32-10 cells, but not hepatocytes, with TPA inhibited GJIC (assayed by Lucifer Yellow dye microinjection), This inhibition involved protei n kinase C because (i) inhibition was prevented by co-treatment of the cells with a specific protein kinase C inhibitor, bis-indolylmaleimid e, and (ii) treatment with TPA for 24 h had no effect on dyecoupling i n agreement with the downregulation of protein kinase C, TPA also caus ed the internalization of Cx43-containing gap junctions and the format ion of a hyperphosphorylated form of Cx43, Cx43-P3, in WB-F344 cells o nly, but TPA had no effect on Cx32-containing gap junctions or protein mobility, In contrast, PB inhibited GJIC only in hepatocytes and DDT inhibited GJIC in all three types of cells; bis-indolylmaleimide did n ot block the effects of either agent, These results indicate that the inhibitory actions of TPA and PB on GJIC are cell-specific rather than ,connexin-specific and that TPA inhibits connexin43 and connexin32-med iated GJIC through a protein kinase C-dependent mechanism.