Depletion of O⁶-methylguanine-DNA methyltransferase by O ⁶-benzylguanine enhances 5-FU cytotoxicity in colon and oral cancer cell lines

O⁶-methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme whose expression is controlled by its promoter methylation. A cell that expresses a low amount of MGMT is known to be more sensitive to the antiproliferative effects of alkylating agents. We have previously shown that the colorectal cancer patients treated with 5-fluorouracil (5-FU) as adjuvant chemotherapy had a better prognosis when the tumor revealed hypermethylation in its MGMT promoter. Therefore, we sought to investigate the relationship between the expression levels of MGMT and the antitumor effect of 5-FU in vitro by using two colon adenocarcinoma and four oral cancer cell lines with a variety of MGMT expression. We also investigated the effects of MGMT depletion by O ⁶-benzylguanine (O⁶-BG), a potent inhibitor of MGMT. The 5-FU treatment uniformly depleted protein and mRNA expression of MGMT in all cell lines examined. Cell lines expressing low levels of MGMT were sensitive to 5-FU. On the other hand, cells expressing high levels of MGMT were less sensitive to 5-FU. The 5-FU treatment exhibited a better antiproliferative effect on the cells expressing high levels of MGMT by the pretreatment of O ⁶-BG. Depletion of MGMT by O⁶-BG enhanced the antitumor effect of 5-FU. Assessment of the levels of MGMT expression in cancer cells and the control of its expression could contribute to the effective chemotherapy by 5-FU especially in patients who previously were considered as low-responsive individuals whose tumors have high levels of MGMT.