NNK induces epigenetic alteration during cancer formation and inhibits tumor suppressor genes

NCKUFeb 9 2010

In the history of humans fighting against cancer, the relationship between tobaccos and cancer has been an important subject for the academic and clinical research. By understanding the mechanism of why cancer is caused by tobaccos, scientists around the world all anticipate to solve the mystery of cancer and find the way of prevention and cure.

With the financial support from the National Research Program for Genomic Medicine (NRPGM) and the National Science Council, under the leadership of Prof. Yi-Ching Wang, Department of Pharmacology, NCKU, and with the joint teamwork of Dr. Pinpin Lin, Division of Environmental Health and Occupational Medicine, National Health Research Institute, Dr. Han-Shui Hsu, Division of Thoracic Surgery, Veterans General Hospital, and Dr. Chih-Yi Chen, Cancer Center, China Medical University Hospital, the paper launched a new approach to the prevention and cure of the human tumor disease and it was made the cover story of the February issue of the world’s leading biomedical journal, The Journal of Clinical Investigation. The research results showed that nicotine-derived nitrosamine ketone (NNK), known as a key ingredient in the tobacco carcinogen, would not only cause DNA and genetic alteration; it would also induce epigenetic alteration in the process of cancer formation, inhibiting the tumor suppressor genes. The US Science News thinks very highly of this particular research, as they have already interviewed Prof. Yi-Ching Wang and written a commentary on it. It is evident that Prof. Wang’s paper has considerable significance and contribution in the field of international medical research.

Through the discovery of the team led by Prof. Yi-Ching Wang, the study proved that the DNA methyltranferase 1 (DNMT1) of the promoter would cause a deficiency in the tumor suppressor genes of the lung cancer patients, and it also verified the correlation between accumulation of DNMT1 and patients who smoked continuously. Yet the detailed mechanism for this phenomenon is still uncertain. Based on the model of cells, clinical and animal treatment, this particular study discussed the mechanism which caused the phenomenon of the overexpression of DNMT1, leading to the inhibition of tumor suppressor genes. The results of the cell treatment showed that NNK, the ingredient in tobacco carcinogen, would weaken degradation capacity of β-transducin repeat-containing protein (βTrCP) through AKT pathway, enabling the increase in the stability of DNMT1 protein. Moreover, NNK would activate heterogeneous nuclear ribonuclear protein U (hnRNP-U) to allow the translocation of the βTrCP proteins from the nucleus to the cytoplasm, inducing the accumulation of the DNMT1 proteins. The NNK-induced DNMT proteins would bind with the tumor suppressor genes, inducing the overexpression of DNA methyltranferase 1. In the animal model experiments, after the lung tissues of mice were resected and stained with H&E for microscopic evaluation, it was discovered that the βTrCP proteins mainly located in cytoplasm and the DNMT1 nuclear were misplaced. In the case of clinical research, the research team used immunohistochemistry (IHC) assay to detect the expression levels of DNMT1, βTrCP and hnRNP-U proteins of the lung tissues in 124 cancer patients. In comparison to the expression level of DNMT1 protein in patients who smoked continuously, the expression level in patients who were not smoking but had smoked and later quitted was apparently lower. The survival rate of patients who smoked continuously and had had surgery was lower as well. For the first time this molecular mechanism study confirmed that βTrCP degradation pathway would indeed affect the expression level of DNMT1 protein, and NNK would trigger the translocation of the βTrCP and DNMT1 protein, further enabling the excessive activation of the DNMT1 protein in the nucleus.

Furthermore, Prof. Yi-Ching Wang pointed out lung cancer was ranked the first among national cancer deaths. Recently, there have been approximately 7,500 people who died from lung cancer annually; however, the molecular carcinogenic mechanism is yet to be clarified. In Taiwan, whether men or women, lung cancer is ranked the top of the cancer mortality, and the reason for lung cancer is associated with the long-term exposure to carcinogenic substances in the environment. Nicotine-derived nitrosamine ketone (NNK) was originally considered only to lead to genetic mutation, but the research study further revealed how NNK would induce epigenetic alteration in the process of cancer formation, inhibiting the tumor suppressor genes and leading to lung cancer. In addition to acquiring the authentication in cell biology fundamental research and clinical lung cancer samples, the study provided strong scientific evidence for non-smoking or quit-smoking propaganda and proposed an important goal for lung cancer treatment, which is the development of DNMT methyltranferase enzyme inhibitor. In the future, the research team will commit to the treatment of lung cancer patients and the invention of new drugs based on this founding.