Effect of tumor promoting stimuli on gap junction permeability and connexin 43 expression in ARL 18 rat liver cell line

The ARL18 rat liver cell line has previously been used for screening tumor promoters in the metabolic cooperation assay (Williams 1980; Williams et al. 1981; Telang et al. 1982). These cells display high levels of gap junctional communication, as assessed functionally and immunologically. Intracellularly injected Lucifer Yellow diffused extensively and there was rapid fluorescent recovery after photobleaching. Moreover, expression of connexin 43 (Cx43) was high as evaluated by immunocyto-chemisty of cell monolayers and Western blot analysis of total cell homogenates. Western blot analysis revealed multiple forms of Cx43, which presumably correspond to known dephosphorylated and phosphorylated states of this protein. Gap junction permeability and Cx43 expression in ARL 18 cells were studied after exposure to the tumor promoters 12-0-tetradecanoyl-phorbol-13-acetate (TPA), and 1,1,1-trichloro-2,2-bis(p-chlorphenyl)-ethane (DDT), and after wounding the cell monolayer. TPA and DDT strongly inhibited gap junction permeability; whereas monolayer wounding did not affect the degree of fluorescent recovery after injury, either in the cells on the edge of the wound or in distal regions. No changes in the cellular distribution of Cx43 were observed after any of these treatments, although Western blots revealed a decrease in total Cx43 after 24-h exposure to DDT (10 μg/ml) and a slight increase after TPA treatment (30 min, 0.1 μg/ml). Relative abundance of different phosphorylated Cx43 forms was increased after 1 h exposure to DDT (10 μg) and 30 min exposure to TPA (0.1 μg/ml). Thus, various changes in Cx43 are correlated with gap junction closure induced by tumor promoters including changes in the total amount of Cx43 and increase in the proportion of Cx43 that is in phosphorylated forms. The possible role that connexin phosphorylation might play in gap junction regulation is discussed.