ABERRANT P21(WAF1)-DEPENDENT GROWTH ARREST AS THE POSSIBLE MECHANISM OF ABNORMAL RESISTANCE TO ULTRAVIOLET-LIGHT CYTOTOXICITY IN LI-FRAUMENI SYNDROME FIBROBLAST STRAINS HETEROZYGOUS FOR TP53 MUTATIONS

The purpose of this study is to better understand the roles of the p53 tumor suppressor protein and the product of the p53-regulated gene p2 1(WAF1) in the response of diploid human dermal fibroblast cultures to 254 nm ultraviolet (UV) light. We report that Li - Fraumeni syndrome (LFS) fibroblast strains heterozygous for TP53 mutation at either codo n 245 or 234 exhibit markedly reduced or no expression of p21(WAF1) fo llowing UV irradiation, respectively. These strains also exhibit defec tive nucleotide excision repair and pronounced inhibition of RNA synth esis following UV exposure, both of which are molecular hallmarks of c ells derived from patients with the UV-sensitive syndrome xeroderma pi gmentosum, In sharp contrast to xeroderma pigmentosum cells, however, the repair-deficient LFS cells show abnormal resistance, rather than h ypersensitivity, to the killing effect of UV light. We further demonst rate that exposure of normal human fibroblasts to biologically relevan t fluences (less than or equal to 15 J/m(2)) of UV does not induce apo ptotic cell death, indicating that UV resistant phenotype displayed by LFS strains is not associated with deregulated apoptosis, In normal f ibroblasts, such treatment results in a moderate (similar to threefold ) up-regulation of p53 protein, induction of the p21(WAF1) gene, and a senescence-like growth arrest. On the other hand, exposure to greater than or equal to,20 J/m(2) UV results in a striking up-regulation of p53, inhibition of p21(WAF1) expression, and activation of an apoptoti c pathway. We conclude that: (i) p21(WAF1)-mediated senescence is the principal mode of cell death induced by less than or equal to 15 J/m(2 ) UV light in normal human fibroblasts; (ii) there is a threshold effe ct for p53-dependent apoptosis and that, in normal human cells, this t hreshold level is induced upon exposure to similar to 20 J/m(2) UV; (i ii) the p53 signaling pathway is malfunctional in the TP53 heterozygou s LFS strains examined; and (iv) the enhanced resistance to UV-induced cell killing displayed by these LFS strains is a consequence of dimin ished growth arrest, which is presumably mediated by p21(WAF1) nd not abnormalities in an apoptotic pathway.