O6-methylguanine-DNA methyltransferase hypermethylation modulated by 17β-estradiol in lung cancer cells

Background: Our recent report indicated that MGMT hypermethylation is more common in squamous cell carcinomas (SCC) in males, and smokers than in adenocarcinomas (ADC) in females, and nonsmokers. More interestingly, MGMT hypermethylation in SCC and ADC was pronouncedly influenced by gender factor, not by smoking status. We questioned whether 17β-estradiol could modulate the machinery of promoter methylation to cause the gender difference of MGMT hypermethylation in lung cancer. Materials and Methods: Two MGMT hypermethylated Ch27 and H1355 lung cancer cell lines were treated with or without 17β-estradiol and the status of hypermethylation was examined by methylated specific methylation (MSP) as compared with both cells treated with demethylating agents, 5-AZA-dC (AZA) or TSA. Results: Our data showed that 17β-estradiol, similar to AZA, diminished the MGMT hypermethylation and restored MGMT mRNA expression, which was not observed in the case of TSA. Western blotting showed that 17β-estradiol markedly reduced DNMT1 expression in Ch27 and H1355 cells, but slightly reduced HDAC1 expression. Consequently, acetylated H3 and H4 histone levels were slightly increased by 17β-estradiol in both cell types. In addition, ChIP analysis revealed that 17β- estradiol simultaneously diminished the binding activity of both proteins on the MGMT promoter of both cell lines. Conclusion: 17β-Estradiol decreased DNMT1 and HDAC1 protein expressions and their binding activity on MGMT promoter, and this may partially contribute to the gender difference of MGMT hypermethylation in lung cancer. The silencing of MGMT is most likely attributed to hypermethylation of the MGMT promoter in several types of human carcinomas including lung cancer (1, 2). An MGMT transgenic mice model has demonstrated that K-ras mutation in lung tumors is significantly reduced in MGMT transgenic mice compared to that of non-transgenic mice, suggesting that MGMT inactivation by promoter methylation plays an important role in lung carcinogenesis (3). Our recent reports indicated that MGMT hypermethylation in lung cancer is more common in males, patients with squamous cell carcinomas (SCC), and smokers than in females, patients with adenocarcinomas (ADC), and nonsmokers. After stratification by gender, smoking status and tumor type, male nonsmokers in both tumor types had more prevalent MGMT hypermethylation than did female nonsmokers (53% vs. 31% for ADC, p=0.043; 65% vs. 29% for SCC, p=0.045), but the difference in MGMT hypermethylation was not observed between male smokers and nonsmokers in either tumor types (40% vs. 53% for ADC, p=0.326; 65% vs. 65% for SCC, p=0.990; (4)). More interestingly, MGMT hypermethylation may be associated with an increased occurrence of p53 mutation including G:C to A:T transition and other types of p53 mutations in lung cancer in males, but not in females. This result was consistent with our p53 mutation database showing that lung cancer in nonsmoking males had higher p53 mutation frequency than in corresponding female cases. Our previous report indicated that a lower prevalence of estrogen receptor (ER) hypermethylation in lung cancer in females compared with that of males, might be due to increased acetylation of histone 3 (H3) and histone 4 (H4) of