MDM2 TRANSFORMATION IN THE ABSENCE OF P53 AND ABROGATION OF THE P107 G1 CELL-CYCLE ARREST

The p53 tumour-suppressor guards the genome in response to genotoxic s tress by transcriptional regulation of genes involved in cell-cycle co ntrol, DNA replication, repair and apoptosis such as p21, GADD45, bar and mdm2 (Cox and Lane, 1995), Mdm2 is classically considered to be an inhibitor of p53, that forms an auto-regulatory loop (Momand et al., 1992; Oliner et al., 1993; Wu el al., 1993; Chen et al., 1994; Chen an d Levine, 1995). It immortalises cells containing wild type p53 and tr ansforms them together with Ras (Finlay, 1993). We show that, in the a bsence of p53, mdm2 confers a growth advantage to cells (ie ''transfor ms'' them) and can overcome a G1 cell-cycle arrest induced by p107, a member of the pRb tumour-suppressor family (Adams and Kaelin, 1995), T he minimum ''transforming'' and p107 inhibiting region of Mdm2 corresp onds to its p53 binding domain, p53 inhibits transformation by Mdm2, a pparently without requiring transcription, p53 can be considered to be a suppressor of Mdm2, a positive effector of the cell cycle, Mdm2 ove r-expression in tumours is reminiscent of p53 mutations with gain of f unction, in that Mdm2 both transforms cells and inhibits p53 activity.