DNMT1 as a Molecular Target in a Multimodality-Resistant Phenotype in Tumor Cells

We have previously shown that hydrogen peroxide–resistant permanent (OC-14) cells are resistant to the cytotoxicity of several exogenous oxidative and anticancer agents including H2O2, etoposide, and cisplatin; and we refer to this process as an oxidative multimodality-resistant phenotype (MMRP). Furthermore, OC-14 cells contain increased activator protein 1 activity, and inhibition of activator protein 1 reversed the MMRP. In this study, we show that permanent Rat-1 cell lines genetically altered to overexpress c- Fos also displayed a similar MMRP to H2O2, etoposide, and cisplatin as OC-14 cells. Gene expression analysis of the OC-14 cells and c- Fos –overexpressing cells showed increased DNMT1 expression. Where OC-14 and c- Fos –overexpressing cells were exposed to 5-aza-2′-deoxycytidine, which inhibits DNMT activity, a significant but incomplete reversal of the MMRP was observed. Thus, it seems logical to suggest that DNMT1 might be at least one target in the MMRP. Rat-1 cells genetically altered to overexpress DNMT1 were also shown to be resistant to the cytotoxicity of H2O2, etoposide, and cisplatin. Finally, somatic HCT116 knockout cells that do not express either DNMT1 (DNMT1−/−) or DNMT3B (DNMT3B−/−) were shown to be more sensitive to the cytotoxicity of H2O2, etoposide, and cisplatin compared with control HCT116 cells. This work is the first example of a role for the epigenome in tumor cell resistance to the cytotoxicity of exogenous oxidative (H2O2) or systemic (etoposide and cisplatin) agents and highlights a potential role for DNMT1 as a potential molecular target in cancer therapy. (Mol Cancer Res 2008;6(2):243–9)