INHIBITION OF GAP JUNCTIONAL INTERCELLULAR COMMUNICATION IN SYRIAN-HAMSTER EMBRYO CELLS BY TPA, RETINOIC ACID AND DDT

12-O-Tetradecanoylphorbol-13-acetate (TPA), trans-retinoic acid (RA) a nd DDT inhibit gap junctional intercellular communication in Syrian ha mster embryo (SHE) cells. The inhibition is rapid and takes place with in minutes. Northern blot analysis shows that SHE cells express connex in 43 and that exposure to these compounds for up to 20 h has no effec t on connexin 43 mRNA level. Immune cytochemistry shows that the conne xon structures in SHE cells are scattered over the cell, and not confi ned to the cell-cell boundaries as is the case in the rat liver epithe lial cell line IAR20. RA and TPA induce the disappearance of the conne xon structures in parallel to the induced inhibition of communication in SHE cells. The disappearance of the connexon spots takes place with no apparent effect on the cellular content of connexin protein measur ed by immunoblotting, and is probably caused by disaggregation of the connexon structures rather than disappearance or degradation of the co nnexin protein. DDT shows little or no apparent effect on connexin imm unostaining in SHE cells, indicating a different mechanism of action. In the IAR20 cells, exposure to TPA and RA also results in loss of imm unostainable connexon structures while exposure to DDT results in relo calization of the connexons away from the cell-cell borders. Immunoblo tting of connexin 43 in SHE cells results in three major bands with ap parent mel. wts of 40-50 kDa where the two higher mel. wt bands repres ent phosphorylated connexin 43 protein. Exposure of the cells to the c ommunication inhibiting compounds results in reduction or loss of the highest mel. wt phosphorylated band, indicating a relation between a s pecific connexin phosphorylation and aggregation of connexin 43 protei n to functional communicating gap junctions. The results suggest the p resence of various post-transcriptional control mechanisms in the regu lation of connexin function which are vulnerable to exogenous stimuli.