NNK-induced DNA methyltransferase 1 in lung tumorigenesis in A/J mice and inhibitory effects of (-)-epigallocatechin-3-gallate.

DNA methyltransferase 1 (DNMT1), a key enzyme mediating DNA methylation, is known to be elevated in various cancers, including the mouse lung tumors induced by the tobacco-specific carcinogen 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). However, it is not known whether DNMT1 expression is induced right after NNK treatment and how DNMT1 expression varies throughout lung tumorigenesis. In the present study, we found that administration of NNK to A/J mice caused elevation of DNMT1 in bronchial epithelial cells at Days 1, 3, and 14 after NNK treatment. DNMT1 elevation at Day 1 was accompanied by an increase in phospho-histone H2AX (γ-H2AX) and phospho-AKT (p-AKT). At Weeks 5 to 20, NNK-induced DNMT1 in lung tissues was in lower levels than the early stages, but was highly elevated in lung tumors at Week 20. In addition, the early induction of p-AKT and γ-H2AX as well as cleaved caspase-3 in NNK-treated lung tissues was not detected at Weeks 5 to 20 but was elevated in lung tumors. In concordance with DNMT1 elevation, promoter hypermethylation of tumor suppressor genes Cdh13, Prdm2, and Runx3 was observed in lung tissues at Day 3 and in lung tumors. Treatment by EGCG attenuated DNMT1, p-AKT, and γ-H2AX inductions at Days 1 and 3 and inhibited lung tumorigenesis.