Two inhibitors of gap junctional intercellular communication, TPA and endosulfan: different effects on phosphorylation of connexin 43 in the rat liver epithelial cell line, IAR 20.

The skin tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and the chlorinated insecticide, endosulfan, are two potent inhibitors of gap junctional intercellular communication. In the present study the effects of TPA and endosulfan on cell communication have been investigated in IAR 20 rat liver epithelial cells, as well as the effects of these compounds on connexin 43 (cx43), the main gap junction protein in this cell line. The results clearly demonstrate that at non-toxic doses both compounds inhibited the cell communication by at least 90% within 5 min. The communication was partially restored after 4 h of TPA exposure and almost fully restored by 24 h, whereas in endosulfan-exposed cells the communication was completely down-regulated for the whole exposure-period of 24 h. Immunoblots of IAR 20 cell extracts indicated that TPA initially caused an increased phosphorylation of cx43. A normal phosphorylation pattern was observed after 4 h when the cell communication was restored. Immunoblot analysis after endosulfan-exposure showed a slightly increased phosphorylation of cx43 after 10 min treatment, gradually followed by dephosphorylation during the rest of the 24 h treatment period. Immunostaining of IAR 20 cells showed that both compounds caused a rapid disappearance of cx43 from the cell membrane. After 4 h of exposure immunofluorescent cx43-plaques started to reappear in the cell membrane, although less pronounced in endosulfan-treated cells. However, after 24 h of endosulfan-exposure a high number of cx43-spots was demonstrated. These results indicate that different mechanisms are responsible for the inhibition of gap junctional intercellular communication induced by TPA and by endosulfan, at least during the later part of the 24 h exposure-period. TPA causes a marked hyperphosphorylation of cx43, whereas endosulfan increases phosphorylation initially only slightly but longer exposure-periods lead to hypophosphorylation. Thus, phosphorylation as well as dephosphorylation seem to be important factors involved in the regulation of the function of cx43 in this cell line.