The retinoblastoma tumor suppressor inhibits cellular proliferation through two distinct mechanisms: inhibition of cell cycle progression and induction of cell death

Studies aimed at examining the precise function(s) of the retinoblastoma tu mor suppressor protein, RE, have been hindered by the rapid phosphorylation and inactivation of ectopically expressed RE which occurs in the majority of cell types, Therefore, ectopically expressed RE is a poor inhibitor of c ellular proliferation, We have designed constitutively active RE proteins, PSM-RB, that cannot be inactivated by phosphorylation, Using these proteins , we show that unlike wild-type RE, PSM-RB proteins inhibit cell cycle prog ression in a broad range of tumor cell types. Furthermore, unlike p16(ink4a ), PSM-RB is also a potent inhibitor of cell cycle progression in RB-defici ent tumor cells. Surprisingly, we identified a tumor cell line that is resi stant to the cell cycle inhibitory effects of PSM-RB. This finding challeng es the hypothesis that RE must be inactivated in all cells for cell cycle p rogression to occur. Further characterization of this 'resistant' tumor lin e revealed that proliferation of these cells is still inhibited by PSM-RB. We show that this is due to PSM-RB-induced cell death. As such, these studi es are the first to show that RE inhibits cellular proliferation through at least two distinct mechanisms - inhibition of cell cycle progression and i nduction of cell death.