DNA methyltransferase 1 (DNMT1) catalyzes DNA methylation and is overexpressed in many human diseases, including cancer. The tobacco-specific carcinogen NNK also induces DNA methylation. However, the role of DNMT1-mediated methylation in tobacco carcinogenesis remains unclear. Here we used human and mouse lung cancer samples and cell lines to determine a mechanism whereby NNK induced DNMT1 expression and activity. We determined that in a human lung cell line, glycogen synthase kinase 3β (GSK3β) phosphorylated DNMT1 to recruit β-transducin repeat–containing protein (βTrCP), resulting in DNMT1 degradation, and that NNK activated AKT, inhibiting GSK3β function and thereby attenuating DNMT1 degradation. NNK also induced βTrCP translocation to the cytoplasm via the heterogeneous nuclear ribonucleoprotein U (hnRNP-U) shuttling protein, resulting in DNMT1 nuclear accumulation and hypermethylation of the promoters of tumor suppressor genes. Fluorescence immunohistochemistry (IHC) of lung adenomas from NNK-treated mice and tumors from lung cancer patients that were smokers were characterized by disruption of the DNMT1/βTrCP interaction and DNMT1 nuclear accumulation. Importantly, DNMT1 overexpression in lung cancer patients who smoked continuously correlated with poor prognosis. We believe that the NNK-induced DNMT1 accumulation and subsequent hypermethylation of the promoter of tumor suppressor genes may lead to tumorigenesis and poor prognosis and provide an important link between tobacco smoking and lung cancer. Furthermore, this mechanism may also be involved in other smoking-related human diseases.
Ruo-Kai Lin, Yi-Shuan Hsieh, Pinpin Lin, Han-Shui Hsu, Chih-Yi Chen, Yen-An Tang, Chung-Fan Lee, Yi-Ching Wang
NNK enhances DNMT1 protein stability through the AKT signaling pathway, which is associated with the ubiquitin-proteosome system.