E. Rivedal et al., INHIBITION OF GAP JUNCTIONAL INTERCELLULAR COMMUNICATION IN SYRIAN-HAMSTER EMBRYO CELLS BY TPA, RETINOIC ACID AND DDT, Carcinogenesis, 15(4), 1994, pp. 689-694
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.