Infection of rat liver epithelial cells with v‐Ha‐ras: Correlation between oncogene expression, gap junctional communication, and tumorigenicity

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Abstract
The role of the v‐Ha‐ras oncogene in tumorigenesis in an in vitro/in vivo model system was studied by investigating the expression of the Ha‐ras gene, gap junctional intercellular communication, and tumorigenicity as endpoints. Infection of a Fischer 344 rat liver epithelial cell line (WB 344) with a retrovirus containing the v‐Ha‐ras oncogene resulted in altered cell morphology and decreased contact sensitivity. Gap junctional intercellular communication in v‐Ha‐ras infected WB cells (WBHa‐ras), assessed by fluorescence redistribution after photobleaching (FRAP), microinjection/dye transfer, and scrape‐loading/dye transfer techniques, was markedly decreased compared with the level in control WB cells. Injection of 107 WBHa‐ras cells into the portal vein of male F344 rats caused multiple focal hepatic lesions within 1 and 2 wk, merging to large invading tumors after 3 and 4 wk. Examination of the methylation pattern of the Ha‐ras gene in WBHa‐ras and control WB cells showed that the infected Ha‐ras gene was relatively hypomethylated in comparison to the normal cellular Ha‐ras gene, indicating a greater potential for expression. There was an increased level of Ha‐ras mRNA in hepatomas as compared with both adjacent nontumor liver tissue and liver tissue obtained from normal animals. Three cell lines derived from three different primary hepatic tumors induced by an injection of WBHa‐ras cells in a F344 rat displayed similar growth characteristics, levels of gap junctional communication, and methylation patterns as the original WBHa‐ras cells. The results of these studies have established a strong positive correlation between expression of the Ha‐ras oncogene, reduced gap junctional intercellular communication, decreased contact sensitivity, and tumorigenicity of the v‐Ha‐ras‐infected rat liver epithelial cells.