Effect of combined therapy with low-dose 5-aza-2′-deoxycytidine and irinotecan on colon cancer cell line HCT-15

Background: Aberrant promoter hypermethylation is an epigenetic change that silences the expression of crucial genes, resulting in inactivation of the apoptotic pathway in various cancers. This hypermethylation can be restored by the demethylating agent 5-aza-2 '-deoxycytidine (DAC). DAC might increase the tumor sensitivity to chemotherapy through demethylation and restoration of gene expression. We investigated the effect of combined therapy with DAC and irinotecan (CPT-11) on the human colon cancer cell line HCT-15. Methods: Human colon cancer cell line HCT-15 was treated with DAC and/or CPT-11 both in vitro and in vivo. The changes in mRNA expression of several apoptosis-related genes were investigated by reverse transcriptase-polymerase chain reaction (PCR). Promoter methylation was detected by methylation-specific PCR and combined bisulfite restriction analysis. Suppression of tumor growth was observed during the treatment with DAC and/or CPT-11 and apoptosis in the tumors was investigated by TUNEL (terminal deoxynucleotidyl transferase dUTP nick-end labeling) assay. Results: Promoter methylation of p14(ARF), p16(INK4a), BNIP3, and XAF1 was confirmed, and DAC restored mRNA expression of these genes. Demethylation and restoration of gene expression was observed with low-dose DAC, and demethylation status was sustained for several weeks. Combined therapy with DAC and CPT-11 produced marked suppression in tumor growth compared with DAC or CPT-11 alone, both in vitro and in vivo. Conclusions: Pretreatment with low-dose DAC may have the potential to be used as a "biosensitizer" of DNA-damaging agents such as CPT-11 when the apoptotic pathway is inactivated as a result of aberrant promoter methylation in the cancer.