Gap-junctional intercellular communication (GJIC) in normal rat liver cells involves at least three different connexins (Cxs)--Cx32, Cx26, Cx43--depending on the cell type, position in the lobule, or both. Whereas rat hepatocyte primary cultures expressed Cx32 and Cx26 as observed in vivo, cell lines derived from normal rat liver (WB-F344, Clone 9, RLEC, and BRL) expressed Cx43 and to a lesser extent Cx26. Hepatoma cells propagated in vitro were either deficient in GJIC and Cx expression (7777, 8994, H4IIE-C3) or communicated via gap junctions composed of Cx43 protein (N1S1-67, 9618A). Analysis of neoplasms that resulted from injection of hepatoma cells into rat femoral muscle showed differences in Cx expression when compared with cells grown in vitro. Whereas hepatoma cells 7777 and H4IIE-C3 failed to express Cx mRNAs in culture, these cells transplanted in vivo expressed levels of Cx32 mRNA comparable to those in normal liver. However, detectable Cx32 immunostaining was observed in less than 5% of the neoplastic cells in vivo. These results indicate that Cx32 protein was posttranscriptionally downregulated in 7777 and H4IIE-C3 tumor cells. Unexpectedly, 9618A cells expressed Cx43 mRNA and protein in cell culture but expressed Cx32 mRNA in vivo. In contrast, N1S1 transplants continued to express Cx43 mRNA and protein in vivo. Unlike the punctate Cx43 staining observed in suspension cultures of N1S1 cells, diffuse intracellular Cx43 staining was observed in N1S1-derived neoplasms in vivo, although the electrophoretic pattern of Cx43 isolated from N1S1 tumors grown in vivo (43 kDa) was different from that observed in suspension cell cultures (43 and 45 kDa). Thus, the findings reported here demonstrate that Cx expression in hepatoma cells depends on the environment, whether in vivo or in vitro, in which the cells are propagated.