AN ABNORMALITY IN THE P53 PATHWAY FOLLOWING GAMMA-IRRADIATION IN MANYWILD-TYPE P53 HUMAN-MELANOMA LINES

DNA-damaging agents such as ionizing radiation (IR) activate the tumor suppressor p53, and, in turn, p53 transactivates a number of downstre am effector genes such as GADD45, CIP1/WAF1, and MDM2. The induction o f these downstream genes following IR appears to be strictly dependent upon the presence of wild-type functional p53 known to evoke G(1) arr est. In this study, we characterized 56 cell lines from 9 different tu mor types viith predetermined p53 genotype by measuring the induction of GADD45, CIP1/WAF1, and MDM2 relative mRNA levels after IR. A higher fraction of melanoma lines had wild-type (wt) p53 (5/8, or 63%) compa red to the nonmelanoma lines (11/48, or 23%). Most wt p53 (nonmelanoma ) cell lines (11/12, or 92%) showed clear induction of both GADD45 and CIP1/WAF1. On the other hand, many wt p53 melanoma lines (4/5, or 80% ) showed normal induction of CIP1/WAF1, but little or no induction of GADD45. Despite this defect in GADD45 induction, we found that all wt p53 melanoma lines exhibited strong G(1) arrest and increased levels o f p53 protein after IR. The results demonstrated that radiation-induce d G(1) arrest could occur by the p53-CIP1/WAF1 pathway without appreci able induction of GADD45 in melanoma lines. Time course experiments de monstrated prolonged induced expression of CIP1/WAF1 mRNA transcripts in melanoma lines in which GADD45 induction was lacking, suggesting so me sort of compensatory mechanism involving CIP1/WAF1, in cell lines w ith defective GADD45 induction. We could reproduce this compensatory e ffect in RKO colon carcinoma cells in which GADD45 expression was bloc ked by constitutive antisense vectors. These findings reveal that defe ctive induction of GADD45 following IR is common in human melanoma cel l lines.