Homologous and heterologous gap-junctional intercellular communication in v-raf-, v-myc-, and v-raf/v-myc-transduced rat liver epithelial cell lines

Abstract

We examined gap-junctional intercellular communication (GJIC) in a series of normal and v-raf-, v-myc-, and v-raf/v-myc-transduced rat liver epithelial (RLE) cell lines using the scrape loading-dye transfer and fluorescence-recovery-after-photobleaching (FRAP) assays. Whereas the normal RLE cell line, the control helper virus-transduced cell line, and the v-myc-transduced cell line all showed excellent GJIC, the v-raf-transduced cell lines displayed decreasing levels of GJIC associated with their increasing tumorigenicity. The v-raf/v-myc-transformed cell lines showed the lowest levels of GJIC and were also the most tumorigenic. Heterologous GJIC of these oncogene-transduced cell lines was also compared with that in the normal RLE cells. A modified FRAP assay, using fluorescent-microbead labelling to identify the oncogene-transduced cell from surrounding normal cells, was used to quantify the heterologous GJIC. The v-raf/v-myc-transformed RLE cells had no heterologous communication with the normal RLE cells, whereas v-raf- and v-myc-transduced cell lines maintained heterologous GJIC. Northern analysis showed that connexin 43 was the only gap-junction protein message expressed in these cell lines; connexin 32 and connexin 26 were not expressed. The levels of connexin 43 mRNA expression were relatively unchanged in all cell lines, suggesting that the reduction in GJIC was primarily at the posttranslational level. These findings suggest that reduction of homologous GJIC in v-raf- and v-raf/v-myc-transformed RLE cells is linked to their tumorigenic potential. Furthermore, the loss of heterologous GJIC, which we observed only in the v-raf/v-myc-transformed cells, might release such cells from the growth-regulating effects of surrounding normal cells, possibly contributing to their enhanced tumorigenic potential.