Induction of DNA synthesis and apoptosis by regulated inactivation of a temperature‐sensitive retinoblastoma protein

The retinoblastoma protein, pRb, controls entry into the S phase of the cell cycle and acts as a tumor suppressor in many tissues. Re‐introduction of pRb into tumor cells lacking this protein results in growth arrest, due in part to transcriptional repression of genes required for S phase. Several studies suggest that pRb may also be involved in terminal cell cycle exit as a result of the instigation of senescence or differentiation programs. To understand better these multiple growth‐inhibitory properties of pRb, a temperature‐sensitive mutant of pRb has been produced. This tspRb induces G 1 arrest and morphological changes efficiently at the permissive temperature of 32.5°C, but is weakly functional at 37°C. Consistent with this, tspRb is compromised in nuclear association and E2F regulation at the non‐permissive temperature, but regains these properties at 32.5°C. Serial activation and inactivation of tspRb in SAOS‐2 cells does not allow proliferation, but rather leads to apoptotic cell death. Transient activation of pRb may kill tumor cells by establishing a conflict between persistent proliferation‐inhibitory signals and renewed deregulation of pRb targets such as E2F, and may thus be a more potent means of eliminating these cells than through simple re‐introduction of the tumor suppressor gene.