Epigenetic disorders give rise to several human diseases, including various cancers, neuron disorders, psychosis, and cardiovascular diseases, many of which are mediated by altered expression and activity of DNA methyltransferase 1 (DNMT1; refs. 1–8). DNMT1 is involved in DNA methylation (4). Cancer cells undergo changes in 5′-methylcytosine distribution, including global DNA hypomethylation and region-specific hypermethylation of promoter CpG islands associated with tumor suppressor genes (TSGs; ref. 9). Aberrant promoter hypermethylation of CpG islands associated with TSGs can lead to transcriptional silencing and result in tumorigenesis (4). DNMT1 is reported to be especially overexpressed in lung and liver cancer patients that are smokers (10, 11).
The key ingredient of tobacco smoke carcinogen, nitrosamine 4-(methylnitro-samino)-1-(3-pyridyl)-1-butanone (also known as nicotine-derived nitrosamine ketone; NNK) systemically induces tumors of the lung in rats, mice, and hamsters and also plays a major role in lung carcinogenesis (12, 13). As previously reported in mouse and rat studies, NNK exposure not only leads to gene mutation, but also induces hypermethylation of multiple TSG promoters in liver or lung tumors, such as cyclin-dependent kinase inhibitor 2A (p16, inhibits CDK4) (p16INK4A), death-associated protein kinase 1 (Dapk1), retinoic acid receptor β (Rarb), and runt-related transcription factor 3 (Runx) (14–17). Clinical studies indicated that smoking is associated with promoter hypermethylation at more than 20 TSGs in lung tumors (18, 19). However, the mechanism underlying the promoter hypermethylation of TSGs relevant to tobacco carcinogenesis has not been elucidated.
NNK has been shown to induce alterations in many signaling pathways, such as the EGFR, AKT, MAPK, ERK1/2, and NFκB pathways (13, 20–22). Therefore, we proposed that NKK induces overexpression of DNMTs through some signaling pathways, thereby leading to hypermethylation at multiple TSG promoters. Lung cancer is one of the most common cancers worldwide and is the leading cause of cancer mortality in industrialized countries (23). About 85%–90% of lung cancer cases are caused by cigarette smoking (12). Using lung cancer as a model, we performed cell, animal, and clinical studies to analyze the molecular mechanisms of DNMT1 overexpression in relation to NNK.