Abstract
Psoralen plus UVA light (PUVA) is commonly used to treat psoriasis, a common skin disorder associated with rapid proliferation of cells. PUVA exerts its antiproliferative activity through formation of DNA monoadducts and interstrand cross-links (ICLs). However, this treatment may lead to skin malignancies as a direct result of inducing carcinogenic DNA damage. Inactivation of the p53 tumor suppressor gene is an important event in the development of skin cancer. p53 is rapidly phosphorylated and stabilized in response to DNA damage, and the induction of apoptosis by p53 is an important mechanism by which p53 exerts its tumor-suppressive activity. To better understand the mechanism by which PUVA treatment induces p53, we exposed human skin fibroblasts with PUVA under conditions that differentially produce monoadducts and ICLs and found that psoralen-induced ICLs induced phosphorylation of the Ser-15 site of p53 and apoptosis much more effectively than psoralen-induced monoadducts. The induction of p53 phosphorylation by psoralen ICLs did not require factors believed to be involved in the repair of psoralen ICLs [xeroderma pigmentosum (XP)-A, XP-C, XP-F, Cockayne's syndrome-B, Fanconi anemia] but did require the ataxia-telangiectasia and Rad3-related but not the ataxia-telangiectasia mutated kinase. Psoralen-induced ICLs blocked transcription and replication more efficiently than monoadducts, and induction of p53 and apoptosis correlated with doses causing interference with transcription rather than DNA replication. Our finding that cells underwent apoptosis preferentially during S-phase suggests that the combined blockade of transcription and DNA replication by psoralen ICLs during S-phase elicits a strong apoptotic response.
Footnotes
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This work was supported by the National Institutes of Health National Cancer Institute [Grant P30-CA46592]; the National Institutes of Health National Institute of General Medical Sciences [Grant GM007767] [Program in Cellular and Molecular Biology, the University of Michigan Comprehensive Cancer Center Core] and by the Department of Radiation Oncology, University of Michigan.
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ABBREVIATIONS: HMT, 4′-hydroxymethyl-4,5′,8-trimethylpsoralen; PUVA, psoralen + ultraviolet A; ICL, DNA interstrand cross-link; ATM, ataxia-telangiectasia mutated; ATR, ataxia-telangiectasia and Rad3-related; XP, xeroderma pigmentosum; CS, Cockayne's syndrome; FA, Fanconi anemia; NER, nucleotide excision repair; TCR, transcription-coupled repair; RPA, replication protein A; ERK, extracellular signal-regulated kinase; PBS, phosphate-buffered saline; PI3, phosphatidylinositol-3; PI, propidium iodide; BrdU, bromodeoxyuridine; TCA, trichloroacetic acid; DNA-PK, DNA-dependent protein kinase.
- Received September 1, 2008.
- Accepted December 8, 2008.
- The American Society for Pharmacology and Experimental Therapeutics
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