Inhibition of Nucleotide Excision Repair by Cigarette Smoke

The link between cigarette smoke and an increased incidence of lung cancer is welt-established. However, despite a sizeable volume of research, the mechanism(s) by which cigarette smoke exposure causes cancer are still not well-understood. Many components of smoke can introduce DNA damage. The persistence of such DNA damage can result in mutations in key oncogenes and tumor suppressor genes leading to cancer development. Cellular repair processes that remove various DNA lesions are crucial for reducing mutations. Nucleotide excision repair (NER) removes a wide spectrum of structurally diverse DNA lesions, including many adducts produced by agents in tobacco smoke and also the photoproducts generated by UV irradiation. In addition to directly damaging DNA, exposure to tobacco smoke can cause other alterations in cells. We have recently demonstrated that exposure of human lung fibroblasts to cigarette smoke condensate (CSC) substantially inhibits removal of UV-induced DNA lesions. Based on these results, we hypothesize that one or more components of tobacco smoke inhibits the NER pathway, thereby decreasing the ability of cells to remove DNA damage induced by smoking itself and potentiating its carcinogenic effects. This inhibition of NER in lung tissues exposed to cigarette smoke would exacerbate the effects of the DNA damage introduced by smoking. Specifically, lesions normally recognized and removed by NER would persist longer, resulting in elevated mutation frequency and increased lung cancer incidence. In addition, we have also demonstrated that treatment of lung cells with sodium arsenite dramatically reduces NER efficiency. Arsenic compounds are present at significant concentrations in cigarette smoke and environmental exposure to arsenic has been linked to the development of lung cancer. Exposure to arsenic and other metals through smoking in combination with other environmental sources may cause an even more pronounced inhibition of NER, leading to increased development of lung cancer. Conceivably, these relationships between smoking, arsenic and other metals play an unforeseen role in the extremely high incidence of lung cancer in areas of Kentucky and elsewhere. The experiments presented in this proposal investigate the inhibition of NER by CSC and metals present in smoke. This project is a collaborative effort at the University of Kentucky between three investigators with collective experience in tobacco smoke toxicology and DNA damage and repair. Our general goals are to: 1) characterize the inhibitory effect of CSC on NER in cells likely to be involved in the etiology of different types of lung cancer and determine if CSC inhibits the removal of types of DNA damage introduced by agents present in cigarette smoke and 2) identify agent(s) in CSC that participate in this inhibition of NER, with particular emphasis on the role of arsenic and other metals. Hence, our proposed studies could 1) spur clinical/translational studies that investigate how exposure to agents in the environment impact cigarette smoke-induced lung cancer and 2) lead to the development of intervention strategies and/or therapeutic agents that would reduce lung cancer.