The objective of our laboratory is to understand the molecular determinants of cellular survival that allow tumor cells to escape programmed cell death (apoptosis) when they are exposed to chemotherapy or irradiation. Identifying specific molecules that promote survival will provide new, attractive targets for the development of compounds that abrogate survival signals and enhance therapeutic effectiveness. Cellular survival is determined by factors both within the cell and outside the cell, including the contribution of extracellular influences such as soluble growth factors and extracellular matrix molecules. Both growth factors and extracellular matrix molecules stimulate survival through activation of enzymatic pathways within the cell that involve proteins that either add phosphate to downstream substrates (kinase) or remove phosphate (phosphatases). The best described survival pathways depend on activity from kinases such as P13K, Akt, PKC, and MAPK, that become activated when they themselves are phosphorylated. Activation can occur after binding of extracellular growth factors to their cognate receptors, or in the case of some tumor cells, activation is independent of extracellular growth factors and is constitutive. Recently, we have described a role for three signaling pathways that contribute to the survival and therapeutic resistance of lung cancer cells: the PI3K/Akt pathway, the MEK/ERK pathway, and pathways involving isoforms of PKC. Because lung cancer cells appear to be most dependent upon the Akt pathway, we have focused in the last year on this pathway. We have shown that the PI3K/Akt pathway is activated in cultures of human bronchial and small airway epithelial cells by two components of tobacco smoke- nicotine and a tobacco-specific carcinogen. This activation is rapid and was observed at concentrations achievable in smokers. Importantly, Akt activation by these tobacco components led to increased survival of these normal cells. These results suggested that Akt activation might have a permissive role for lung tumorigenesis. Moreover, they suggested that Akt activation might be an early event after exposure to tobacco. We are currently addressing the role of Akt in the formation and therapeutic resistance of lung cancer.
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