Reversible Alteration in the Expression of the Gap Junctional Protein Connexin 32 During Tumor Promotion in Rat Liver and Its Role during Cell Proliferation

Although numerous biochemical markers can identify putative preneoplastic altered hepatic foci (AHF) in rat liver, no consistent pattern of expression during hepatocarcinogenesis has emerged. Using quantitative stereologic analyses we demonstrated that decreased expression of the major hepatocyte gap junction protein, connexin 32 (Cx32), in rat AHF is a consistent observation in several protocols of multistage hepatocarcinogenesis. This change was observed after initiation by either ethylnitrosourea (ENU) or diethylnitrosamine (DEN), followed by promotion with phenobarbital (PB), dioxin, chlorendic acid, C.I. Solvent Yellow, or tamoxifen. AHF generated by Wy-14,643, ciprofibrate, and a choline/methionine-deficient dietary regimen also showed decreased Cx32 expression. The decrease of Cx32 in AHF was rapidly reversible after withdrawal of PB, and this change preceded a reduction in placental isozyme of glutathione-S-transferase (GST) expression in the same AHF. Within 20 days of withdrawal, fewer than 4% of GST-positive AHF were Cx32 deficient, while the volume of total AHF decreased 30%. Chronic PB treatment also resulted in a reversible decrease in Cx32 specifically in mid- and centro-lobular hepatocytes. Continuous thymidine labeling demonstrated that Cx32 could be uncoupled from the cell cycle, suggesting that some liver promoters may act directly to alter the expression of Cx32. These observations suggest that a decrease in Cx32 content was a relatively common epigenetic change in AHF induced during hepatocarcinogenesis by a number of initiating and promoting agents but that this change was not sufficient for carcinogenesis. This change, however, may be necessary for the mechanism(s) of tumor promotion, since Cx32-positive AHF did not proliferate as readily as Cx32-deficient AHF.