INDEPENDENT PATHWAYS OF P53 INDUCTION BY CISPLATIN AND X-RAYS IN A CISPLATIN-RESISTANT OVARIAN TUMOR-CELL LINE

The p53 tumor suppressor gene is critical in regulating cell prolifera tion following DNA damage, and disruption of p53 protein function by m utation has been implicated as a factor responsible for resistance of tumor cells to chemotherapeutic agents, Our studies were initiated by asking whether the translational product of the p53 gene is associated with cisplatin resistance in the 2780CP human ovarian tumor model, We have demonstrated by single-strand conformation polymorphism analysis and sequencing that p53 in parental cisplatin-sensitive A2780 cells w as wild type, In 2780CP cells, however, a mutation was found in exon 5 at codon 172 (Val to Phe), Interestingly, exposure to X-rays resulted in p53 induction in both A2780 and 2780CP tumor models, The p53 incre ases by the ionizing radiation were accompanied by concomitant increas es in levels of the p53-regulated p21(Waf1/Cip1) protein and led to ar rest of cells in G(1) phase of the cell cycle, A yeast functional assa y confirmed that p53 in A2780 was wild type, but, more importantly, it provided evidence that the p53 mutation in 2780CP cells was temperatu re sensitive and heterozygous, These experiments demonstrate that sens itive and resistant cells have normal p53 functions, despite the prese nce of p53 mutation in the 2780CP model, In parallel investigations us ing the Western technique, exposure of A2780 cells to clinically relev ant concentrations of cisplatin (1-20 mu M) resulted in time-and dose- dependent increases in p53, together with coordinate increases in p21( Waf1/Cip1). In contrast, cisplatin did not induce these proteins in 27 80CP cells to any significant degree. The results indicate that a defe ct exists in the signal transduction pathway for p53 induction followi ng cisplatin-induced DNA damage in 2780CP cells, and this may represen t a significant mechanism of cisplatin resistance. Furthermore, induct ion of p53 in 2780CP cells by X-rays, but not cisplatin, strongly sugg ests that independent pathways are involved in p53 regulation for the two DNA-damaging agents.