INHIBITION OF APOPTOSIS BY THE RETINOBLASTOMA GENE-PRODUCT

Tissue homeostasis and the prevention of neoplasia require regulatory co-ordination between cellular proliferation and apoptosis. Several ce llular proteins, including c-myc and E2F, as well as viral proteins su ch as E1A, have dual functions as positive regulators of apoptosis and proliferation. The product of the retinoblastoma tumor suppressor gen e, pRb, binds these proteins and is known to function in growth suppre ssion. To examine whether pRb may function as a negative regulator of both proliferation and apoptosis, we analyzed apoptosis induced in tra nsfected derivatives of the human osteosarcoma cell line SAOS-2. Ioniz ing radiation induced apoptosis in a time- and dose-dependent manner i n SAOS-2 cells, which lack pRb expression. In both a transient and sta ble transfection assay, SAOS-2 derivatives expressing wild-type (wt) p Rb exhibited increased viability and decreased apoptosis following tre atment at a variety of radiation doses. Expression in SAOS-2 of a muta nt pRb that fails to complex with several known binding partners of pR b, including E1A and E2F, did not protect SAOS-2 cells from apoptosis. Radiation exposure induced a G(2) arrest in SAOS-2 and in derivatives expressing pRb. Inhibition of DNA synthesis and cell cycle progressio n by aphidicolin treatment failed to protect SAOS-2 cells or pRb-expre ssing isolates from undergoing apoptosis. Our data document a novel fu nction for pRb in suppressing apoptosis and suggest that several prote ins shown to induce apoptosis, including E1A, E2F and c-myc, may do so by interfering with the protective function of pRb.