Inverse correlation between p53 protein levels and DNA repair efficiency in human fibroblast strains treated with 4-nitroquinoline 1-oxide: evidence that lesions other than DNA strand breaks trigger the p53 response

Ionizing radiation-induced stabilization and the resultant transient accumu lation of the p53 tumor suppressor protein is impaired in cells from ataxia telangiectasia (AT) patients, indicating a key role for ATM, the gene muta ted in AT, upstream in the radiation-responsive p53 signaling pathway, Acti vation of this pathway is generally assumed to be triggered by DNA strand b reaks produced directly following genotoxic stress or indirectly during exc ision repair of DNA lesions. The aim of this study was to identify the trig gering signal for induction of p53 in diploid human dermal fibroblasts trea ted with 4-nitroquinoline 1-oxide (4NQO), a model environmental carcinogen that produces both DNA strand breaks (like ionizing radiation) and alkali-s table bulky DNA lesions (like UV light), 4NQO treatment of fibroblasts cult ured from normal and AT donors and those from patients with the UV-hypersen sitivity disorder xeroderma pigmentosum (XP, complementation groups A, E an d G) resulted in up-regulation of p53 protein. In normal fibroblasts, there was no temporal relationship between the incidence of DNA strand breaks an d levels of p53 protein; >90% of strand breaks and alkali-labile sites were repaired over 2 h following treatment with 1 mu M 4NQO, whereas similar to 3 h of post-treatment incubation was required to demonstrate a significant rise in p53 protein. In contrast, exposure of normal fibroblasts to gamma- rays resulted in a rapid up-regulation of p53 and the level peaked at 2 h p ost-irradiation. XP cells with a severe deficiency in the nucleotide excisi on repair pathway showed abnormally high levels of p53 protein in response to 4NQO treatment, indicating that lesions other than incision-associated D NA strand breaks trigger p53 up-regulation. We observed a consistent, inver se correlation between the ability of the various fibroblast cultures to in duce p53 following 4NQO treatment and their DNA repair efficiencies. Treatm ent with 0.12 mu M 4NQO, for example, caused a >2-fold up-regulation of p53 in excision repair-deficient (AT, xPA and XPG) strains without eliciting a ny effect on p53 levels in repair-proficient (normal and XPE) strains. We c onclude that up-regulation of p53 by 4NQO is mediated solely by an ATM-inde pendent mechanism and that the p53 response is primarily triggered by persi stent alkali-stable 4NQO-DNA adducts.