Proficient Global Nucleotide Excision Repair in Human Keratinocytes but not in Fibroblasts Deficient in p53

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Abstract
The p53 tumor suppressor protein is important for many cellular responses to DNA damage in mammalian cells, but its role in regulating DNA repair in human keratinocytes is undefined. We compared the nucleotide excision repair (NER) response of human fibroblasts and keratinocytes deficient in p53. Fibroblasts expressing human papillomavirus 16 E6 oncoprotein had impaired repair of UV radiation–induced cyclobutane pyrimidine dimers in association with reduced levels of p53 and XPC, which is involved in DNA damage recognition. In contrast, keratinocytes expressing E6 alone or concurrently with the E7 oncoprotein, while possessing reduced levels of p53 but normal levels of XPC, continued to repair pyrimidine dimers as efficiently as control cells with normal p53 levels. Despite preservation of DNA repair, E6 and E6/E7 keratinocytes were hypersensitive to UV radiation. E6 fibroblasts exhibited markedly reduced basal and induced levels of mRNA encoding DDB2, another protein implicated in early events in global NER. In contrast, E6 or E6/E7 keratinocytes possessed basal DDB2 mRNA levels that were not significantly altered relative to control cells, although little induction occurred following UV radiation. Intact global NER was also confirmed in SCC25 cells possessing inactivating mutations in p53 as well as in cells treated with pifithrin-α, a chemical inhibitor of p53 that decreased sensitivity of cells to UV radiation. Collectively, these results indicate that human keratinocytes, unlike fibroblasts, do not require p53 to maintain basal global NER activity, but p53 may still be important in mediating inducible responses following DNA damage.

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