Epigenetic alterations such as histone acetylation and DNA methylation play an important role in the regulation of gene expression for cell cycles and apoptosis that may affect the chemosensitivity of cancers. Previously, we have reported that the combination of suberoylanilide hydroxamic acid (SAHA), a newly developed histone deacetylase inhibitor, with cisplatin (CDDP) possessed synergistic cytotoxicity against human oral squamous cell carcinoma (OSCC) cell line HSC-3. In this study, we used a novel DNA methyltransferase inhibitor, zebularine (Zeb), to investigate the epigenetic influence on the sensitivity of carcinoma cell lines to 5-fluorouracil (5-FU) or CDDP by evaluating apoptotic inducibility. Treatment with CDDP or 5-FU either alone or in combination with Zeb or SAHA continued for 48 or 72 h. In HSC-3 cells, Zeb had chemosensitive efficacy with CDDP, but not 5-FU, whereas SAHA showed efficacy with both CDDP and 5-FU. We showed that Zeb has strong anti-proliferative activity against HSC-3 cells, shown by decreased cellular growth and G2/M cell cycle phase accumulation. Furthermore, DNA methylation could be a regulatory mechanism for dihydropyrimidine dehydrogenase (DPD), known to be a principal factor in 5-FU resistance. CDHP (5-chloro-2,4-dihydroxypyridine), an inhibitor of DPD, had an enhancing effect on the apoptotic ability of 5-FU alone or 5-FU/Zeb combination. In conclusion, the present study suggests that low-dose (IC20) Zeb may sensitize cancer cells to CDDP, which may be an important characteristic for solid cancer treatment, and that DPD and other agents activated by Zeb in cancer cells could be an inhibitory factor in the response to apoptosis induced by 5-FU.

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