Failure of senescent cells to phosphorylate the RB protein.

The product of the RB susceptibility gene has been shown to be differentially phosphorylated during the cell cycle, suggesting a role in the regulation of cell cycle progression. We examined the expression and phosphorylation status of the RB protein in senescent Syrian hamster embryo cells. Both phosphorylated and unphosphorylated forms of the RB protein were observed in cells at early passages; however, only unphosphorylated RB protein was found in senescent cells. When nonsenescent cells at low population doubling levels were made quiescent by reducing the serum concentration of the media, the RB protein in these cells was mostly in the unphosphorylated form. When stimulated with serum, phosphorylation of the RB protein occurred between 10-20 h after stimulation, which corresponded with the induction of DNA synthesis. Senescent cells, in contrast, did not show any phosphorylation of the RB protein in response to serum. In addition, cell lines that had escaped cellular senescence at various stages of neoplastic progression were examined; all 25 cell lines examined expressed RB protein, which was phosphorylated normally. These results suggest that the RB protein plays a role in cellular senescence with phosphorylation status determining this role. Factors controlling this phosphorylation are potential key factors in controlling cellular life span in culture.