Interaction of hypoxia and nicotine acetylcholine receptor signaling network reveals a novel mechanism for lung adenocarcinoma progression in never-smokers

High incident of lung cancer among never smokers and their disease pathogenesis is an unexplained phenomenon. We have analyzed 1727 lung cancer patient data to understand the impact of smoking on overall survival of lung cancer patients and have observed a difference of only 47 days between smokers and never smokers in adenocarcinoma patients suggesting that the disease is equally fatal in never-smokers irrespective of gender. In this study, we have investigated the possible collaboration between the nAChR and hypoxia signaling pathway to elucidate a mechanism of disease progression in never-smokers. We report a previously unidentified increase in both acetylcholine and nAChR-7 levels in non small cell lung cancer cells in hypoxia. Similar increase in ubiquitously expressed nAChR-7 transcripts was also observed in other cancer lines. A direct binding of HIF-1 with the hypoxia response element (HRE) present at -48 position preceding the transcriptional start site in nAChR-7 promoter region was established. Significantly, the increased acetylcholine levels in hypoxia drove a feedback loop via modulation of PI3K/AKT pathway to stabilize HIF-1 in hypoxia. Further, Bungarotoxin, an antagonist of nAChR-7 significantly reversed hypoxia mediated metastasis and induction of HIF-1 in these cells. Our study gives a plausible explanation for the equally worse prognosis of lung adenocarcinoma in never-smokers wherein the nAChR signaling is enhanced in hypoxia by acetylcholine, in the absence of nicotine.