Overexpression of c-fos induces expression of the retinoblastoma tumor suppressor gene Rb in transfected cells.

Hypophosphorylated Rb, the product of the tumor suppressor gene associated with hereditary retinoblastoma, is thought to act as a suppressor of cell growth and proliferation during G1 phase. We investigated whether Rb expression was dependent on the expression level of the immediate early cell growth gene, c-fos, which is transiently expressed as cells re-enter G1 phase from quiescence. To explore the functional relationship between c-fos and Rb, a eukaryotic expression plasmid was constructed containing the c-fos gene under control of the SV40 promoter complex. This plasmid was co-transfected with plasmids encoding pRSV cat and G418 resistance, into HeLa S3 cells, and clonal populations of transfected (RSfos) cells selected. High levels of c-fos expression in transfected cells were confirmed by both western and northern blot. Rb protein content per cell was determined by flow cytometry using an Rb-specific primary antibody and a FITC-conjugated secondary antibody. Higher expression of Rb (2-6 fold/cell) in approximately 20% of RSfos transfected cells was observed in comparison to parental HeLa cells. Rb content per cell increased approximately 2-fold during the cell cycle in both parental HeLa cells and RSfos cell clones which overexpressed Rb. Rb accumulation occurred in a manner consistent with normal mass accumulation during the cell cycle. Overexpression of Rb in RSfos cells was also confirmed by western blot analysis. Because one possible function of RB may be to act as a brake on cell growth, it is possible that overexpression of RB acts as an inhibitory counter activity to overexpression of the growth promoting activity of c-fos. This possible balancing activity of Rb was further suggested when Rb protein expression levels were measured in different clonal lines of RSfos transfected cells overexpressing increasing levels of c-fos. Overexpression of Rb was proportional to the level of overexpression of c-fos in each clonal cell line. Such evidence suggests that Rb was proportional to the level of overexpression of c-fos in each clonal cell line. Such evidence suggests that Rb expression may be regulated in a manner which balances the transcription stimulatory effects of c-fos overexpression and its effects on the transcription of other genes during the cell cycle.