Nicotine inactivation of the proapoptotic function of Bax through phosphorylation

Nicotine-induced cell survival is associated with chemoresistance of human lung cancer cells, but our understanding of the intracellular mechanism(s) is fragmentary. Bax is a major proapoptotic member of the Bcl2 family and a molecule required for apoptotic cell death. Growth factor (i.e. granulocyte-macrophage colony-stimulating factor)-induced phosphorylation of Bax has been reported to negatively regulate its proapoptotic function. Because Bax is ubiquitously expressed in both small cell lung cancer and non-small cell lung cancer cells, nicotine may mimic growth factor( s) to regulate the activity of Bax. We found that nicotine potently induces Bax phosphorylation at Ser-184, which results in abrogation of the proapoptotic activity of Bax and increased cell survival. AKT, a known physiological Bax kinase, is activated by nicotine, co-localizes with Bax in the cytoplasm, and can directly phosphorylate Bax in vitro. Treatment of cells with the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 or specific depletion of AKT expression by RNA interference can block both nicotine-induced Bax phosphorylation and cell survival. Importantly, nicotine-induced Bax phosphorylation potently blocks stress-induced translocation of Bax from cytosol to mitochondria, impairs Bax insertion into mitochondrial membranes, and reduces the half-life of Bax protein (i.e. from 9-12 h to < 6 h). Because knockdown of Bax expression by gene silencing results in prolonged cell survival following treatment with cisplatin in the absence or presence of nicotine, Bax may be an essential component in the nicotine survival signaling pathway. Thus, nicotine-induced survival and chemoresistance of human lung cancer cells may occur in a novel mechanism involving activation of PI3K/AKT that directly phosphorylates and inactivates the proapoptotic function of Bax.