Activation of the p53 DNA damage response pathway after inhibition of DNA methyltransferase by 5-aza-2 '-deoxycytidine

Transcriptional silencing of tumor suppressor genes by DNA methylation occu rs in cancer cell lines and in human tumors. This has led to the pursuit of DNA methyltransferase inhibition as a drug target. 5-Aza-2'-deoxycytidine [5-aza-CdR (decitabine)], a potent inhibitor of DNA methyltransferase, is a drug currently in clinical trials for the treatment of solid tumors and le ukemia. The efficacy of 5-aza-CdR may be related to the induction of methyl ation-silenced tumor suppressor genes, genomic hypomethylation, and/or enzy me-DNA adduct formation. Here, we test the hypothesis that 5-aza-CdR treatm ent is perceived as DNA damage, as assessed by the activation of the tumor suppressor p53. We show that 1) colon tumor cell lines expressing wild-type p53 are more sensitive to 5-aza-CdR mediated growth arrest and cytotoxicit y; 2) the response to 5-aza-CdR treatment includes the induction and activa tion of wild-type but not mutant p53 protein; and 3) the induction of the d ownstream p53 target gene p21 is partially p53-dependent. The induction of p53 protein after 5-aza-CdR treatment did not correlate with an increase in p53 transcripts, indicating that hypomethylation at the p53 promoter does not account for the p53 response. It is relevant that 5-aza-CdR has shown t he greatest promise in clinical trials for the treatment of chronic myeloge nous leukemia, a malignancy in which functional p53 is often retained. Our data raise the hypothesis that p53 activation may contribute to the clinica l efficacy and/or toxicity of 5-aza-CdR.