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

Abstract

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 p21^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 codon 245 or 234 exhibit markedly reduced or no expression of p21^WAF1 following UV irradiation, respectively. These strains also exhibit defective nucleotide excision repair and pronounced inhibition of RNA synthesis following UV exposure, both of which are molecular hallmarks of cells derived from patients with the UV-sensitive syndrome xeroderma pigmentosum. In sharp contrast to xeroderma pigmentosum cells, however, the repair-deficient LFS cells show abnormal resistance, rather than hypersensitivity, to the killing effect of UV light. We further demonstrate that exposure of normal human fibroblasts to biologically relevant fluences (⩽15 J/m²) of UV does not induce apoptotic cell death, indicating that UV resistant phenotype displayed by LFS strains is not associated with deregulated apoptosis. In normal fibroblasts, such treatment results in a moderate (∼threefold) up-regulation of p53 protein, induction of the p21^WAF1 gene, and a senescence-like growth arrest. On the other hand, exposure to ⩾20 J/m² UV results in a striking up-regulation of p53, inhibition of p21^WAF1 expression, and activation of an apoptotic pathway. We conclude that: (i) p21^WAF1-mediated senescence is the principal mode of cell death induced by ⩽15 J/m² UV light in normal human fibroblasts; (ii) there is a threshold effect for p53-dependent apoptosis and that, in normal human cells, this threshold level is induced upon expsoure to ∼20 J/m² UV; (iii) the p53 signaling pathway is malfunctional in the TP53 heterozygous LFS strains examined; and (iv) the enhanced resistance to UV-induced cell killing displayed by these LFS strains is a consequence of diminished growth arrest, which is presumably mediated by p21^WAF1 and not abnormalities in an apoptotic pathway.