K-ras mutations in lung tumors from NNK-treated mice with lipopolysaccharide-elicited lung inflammation.

Background: Chronic lung inflammation has been associated with an increased risk of lung cancer. However, it is unclear whether such an event affects the incidence of mutations in the K-ras oncogene frequently found in lung tumors and suggested to be involved in lung tumorigenesis. This study investigated potential impacts of inflammation on the incidence of lung tumors and K-ras mutations using a mouse model. Materials and Methods: FVB/N mice were treated with lipopolysaccharide (LPS) for 16 weeks with or without co- treatment with 4-(methyl-nitrosoamino)-1-(3-pyridyl)-1-butanone (NNK) during the first 4 weeks. Results: There was a significant increase in lung inflammatory responses in mice treated with LPS and with LPS+NNK, compared with mice treated with NNK or with vehicle. The average number of lung tumors per mouse was 3.87 (between 1 and 6) and 0.73 (between 0 and 3) in mice treated with LPS+NNK and NNK alone, respectively (p<0.0001). No lung tumors were observed in mice treated with LPS or vehicle. A higher proportion of lung tumors from mice treated with LPS+NNK had K-ras mutations, compared with the mice treated with NNK alone (81.03% versus 45.45%, p<0.05). Conclusion: LPS-elicited chronic lung inflammation significantly increases the risk of NNK-mediated lung tumorigenesis in FVB/N mice through K-ras gene activation by point mutations. Lung cancer, the leading cause of cancer death in the United States and worldwide (1, 2), is a complex disease that develops through a series of sequential genetic and epigenetic alterations in oncogenes and tumor suppressor genes, causing subtle alterations in growth control (3-5). Epidemiologic data strongly associate exposure to exogenous factors, chiefly tobacco smoking, with increased risk of lung cancer as some of the molecular changes in lung tumors are similar to those caused by genotoxic carcinogens found in tobacco smoke (6, 7). Aside from direct carcinogens, inhaled smoke contains other constituents that can stimulate chronic lung inflammatory responses (8-10) and these events have been associated with an increased lung cancer risk. For instance, lung diseases, including chronic obstructive pulmonary disease, have been shown to be an independent predictor of lung cancer risk (11-15). Other studies have attempted to gain insight into the mechanisms of inflammation-mediated increased risk of lung cancer. It has been shown that the transcription factor, nuclear factor kappa beta (NK-κB), and other genes play an important role in the promotion, growth, and metastasis of lung tumors (16-24). Our previous studies demonstrated that mutations in the K-ras oncogene occurred in a high proportion of lung tumors from mice treated with 4-(methylnitrosamino)-1-(3-pyridyl)- 1-butanone (NNK), a potent lung carcinogen found in tobacco smoke, and these mutations have been suggested as early events in lung tumorigenesis (25). It is currently unclear whether chronic lung inflammation may affect the incidence of lung tumors and/or the frequency of K-ras mutations in these tumors in a mouse model. In this study, we used a mouse model for treatment with the known lung carcinogen, NNK, in the presence and absence of pulmonary inflammation elicited by exposure to lipopolysaccaharide (LPS), a major pro-inflammatory glycolipid component of the gram-negative bacterial cell wall (8-10). We compared results on inflammatory responses, lung tumor incidence, and types and frequencies of mutations of the K-ras gene between mice treated with NNK alone and those treated with LPS and NNK combined.