Lung cancer is the most common human neoplasm in the U.S. and, increasingly, in much of the world. While smoking is known to be the major etiological factor, the causative cellular and molecular mechanisms are complex and not well understood. Currently, we are focusing on three aspects of causation and behavior of adenocarcinoma, the most common form of lung cancer: the role of the K-ras gene; involvements of reactive oxygen species and the molecules that regulate these; and contributions of a signaling pathway triggered by the ErbB3 receptor. The oncogene K-ras is often mutated in adenocarcinoma of the lung (as well as other common carcinomas), but the wild-type form is tumor suppressive. Important questions, then, are: why is mutant K-ras actively oncogenic? How is wild-type K-ras tumor suppressive? Answers to these questions could aid in prevention of up to 50 percent of human lung adenocarcinomas, and an even higher percentage of cancers of the colon and pancreas. We have found that transfection of mutant K-ras in lung epithelial cells causes increases in reactive oxygen species and DNA damage. High levels of ROS are also associated with DNA damage and malignant phenotype in human lung adenocarcinoma cell lines. Also there is a positive correlation between levels of mutant K-ras protein and cellular peroxides, which are in turn negatively correlated with four isoforms of peroxiredoxins. These results support use of antioxidants for prevention/intervention of lung cancer. With regard to mechanisms of regulation of wild-type K-ras in nontransformed lung epithelial cells, conditioned medium from growth-arrested cells activates K-ras and reduces cell growth rate, and specific inhibitor studies implicate the epidermal growth factor receptor. Characterization of this role of the EGFR is in progress. Preliminary results indicate that alternate pathways from EGFR via the adaptor Grb2 may lead to growth arrest (via K-ras) or mitosis (via PI3kinase). Understanding of control of a toggle-switch role for the EGFR is critical. In another aspect of this project, the majority of human and mouse lung adenocarcinoma cell lines, but not nontransformed cells, express the ErbB3 receptor, which signals through phosphatidylinositol 3-kinase, Akt, GSK3beta, and cyclin D1 to stimulate the cell cycle and also cell invasiveness and migration. These behaviors can be blocked with siRNA to ErbB3 or the several Akt isoforms. Thus, siRNA treatment may be an approach to therapy. siRNA to ErbB3 or Akt markedly suppressed the growth of humnan lung adenocarcinoma xenografts, promising novel therapeutic potential. This result has been confirmed in a two additional more extensive experiments, and confirmatory biochemistry and molecular biology is in progress.
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