The mechanism by which the liver tumor promoter 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) inhibits gap junctional intercellular communication (GJIC) in WB-F344 rat liver epithelial cells could involve gap junction loss and/or decreased gap junction channel permeability. We examined these two possibilities in the present study. Immunohistochemical studies using antibodies specific to connexin43, the major gap junction protein expressed by these cells, revealed that gap junction number and size were reduced during exposure to DDT. The reductions in gap junctions (33-91%) correlated with dose-dependent (1-10 microM) and time-dependent (0.5-4 h) decreases in cell-to-cell fluorescent dye-coupling (64-85%), as well as cellular levels of phosphorylated connexin43. These effects were reversible following removal of the tumor promoter from the culture medium, although cycloheximide reduced the level of gap junction reformation. The losses in gap junctions were not due to decreased connexin43 gene expression since steady-state levels of connexin43 mRNA were not similarly affected by DDT. Fenarimol (10 microM), a structural analog of DDT, did not inhibit GJIC and had no effect on gap junction structure or connexin43 expression. These data suggest that the inhibition of GJIC by DDT resulted from the removal of gap junctions from the plasma membrane and their degradation rather than simply a decrease in their permeability.

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The role of altered ras oncoprotein (Ras) farnesylation and membrane association in the growth inhibitory effects of several monoterpenes (limonene, perillic acid, perillyl alcohol, menthol, pinene and cineole) was investigated in rat liver epithelial cells. All of the above compounds except cineole inhibited the growth of viral Ha-ras-transformed rat liver epithelial cells (WB-ras cells) at concentrations of 0.25-2.5 mM. These cells, however, were not necessarily more sensitive to these compounds compared to non-transformed and viral raf-transformed rat liver epithelial cells. Growth inhibition by limonene, perillic acid and pinene was only partially restored (20-50%) by supplementing the culture medium with 2 mM mevalonic acid. Western blot analyses of cytosolic and membranous fractions of WB-ras cells treated with monoterpenes indicated no change in Ras distribution. In contrast, lovastatin, a potent inhibitor of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase and Ras farnesylation, specifically reduced WB-ras cell growth and increased cytosolic levels of Ras. Thus, monoterpene-induced growth inhibition of rat liver epithelial cells was dissimilar to lovastatin and did not appear to involve altered Ras plasma membrane association.

Many studies have correlated reductions in gap junctional intercellular communication (GJIC) with altered cellular growth, tumor promotion, and neoplastic transformation. To test directly whether reduced GJIC affects cellular growth, GJIC was inhibited in murine BALB/c 3T3 fibroblasts by treatment with a phosphorothioate-modified antisense oligonucleotide targeted against the connexin43 translation start codon, and in vitro cell growth was monitored. The cells were incubated with the oligonucleotide (0.1-0.5 microM) in liposomes in serumless culture medium for 16 h; washed and refed with serum-containing medium; and analyzed for dye-coupling, connexin43 protein and mRNA levels, and cell growth over the next 5 d. The antisense oligonucleotide inhibited dye-coupling and reduced connexin43 protein levels in a concentration-dependent manner but had no effect on connexin43 mRNA levels. Cell growth rate was not affected, but saturation density was increased approximately threefold by the oligonucleotide. These data support a role for GJIC in the establishment of contact inhibition of in vitro cell growth.

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