DIFFERENTIATION-INDUCED BY RB EXPRESSION AND APOPTOSIS INDUCED BY P53EXPRESSION IN AN OSTEOSARCOMA CELL-LINE

Multiple genetic alterations, including concurrent inactivation of RB and p53, occur frequently in several human cancers. To investigate the biological significance of RB and p53 gene inactivations, a wild-type RB or p53 cDNA expression vector regulated by tetracycline was introd uced by stable transfection into an osteosarcoma cell line Saos-2, in which both the RB and p53 genes were inactivated. Induction of introdu ced RB expression resulted in suppression of cell growth, increased pe rcentage of cells at the G(0)/G(1) phase, and enlargement of the cells . Furthermore, activity of alkaline phosphatase was increased and expr ession of fibronectin was decreased, suggesting the induction of cell differentiation by RB expression. Induction of p53 expression also res ulted in significant suppression of cell growth with slight accumulati on of cells at the G(0)/G(1) and G(2)/M phases. The cells mere detache d from culture dishes and the dead cell fraction increased. Furthermor e, condensation of chromatin and DIVA fragmentation were observed, sug gesting the induction of apoptosis by p53. These results suggest that RB and p53 play different roles in carcinogenesis of osteoblast; RB in activation releases cells from G(0)/G(1) arrest and suppresses cell di fferentiation while p53 inactivation assists the cells to proliferate by repressing both apoptosis and cell cycle arrest at G(0)/G(1) and G( 2)/M.