When “No-Smoking” is not enough: Hypoxia and nicotine acetylcholine receptor signaling may drive lung adenocarcinoma progression in never-smokers

Abstract

The question of how lung cancer progresses in never-smokers remains largely unanswered. In our analysis of data from 1727 lung cancer patients, we observed a difference of only 47 days in the overall survival between lung adenocarcinoma patients who were smokers vis-a-vis never-smokers - the disease has a poor prognosis irrespective of the smoking status, or gender. We have investigated the possible collaboration between the nAChR and hypoxia signaling pathway to explicate a mechanism of disease progression in never-smokers using patient-derived tumor cells. We found a previously unidentified increase in both acetylcholine and nAChR-α7 levels in non-small cell lung cancer cells in hypoxia. A similar increase in ubiquitously expressed nAChR-α7 transcripts was also observed in other cancer lines and primary tumor tissues. A direct binding of HIF-1α with the hypoxia-response element (HRE) present at −48 position preceding the transcriptional start site in nAChR-α7 promoter region was established. Crucially, the increased acetylcholine levels in hypoxia drove a feedback loop via modulation of PI3K/AKT pathway to stabilize HIF-1α in hypoxia. Further, hypoxia-mediated metastasis and induction of HIF-1α in these cells was significantly reversed by bungarotoxin, an antagonist of nAChR-α7. The nAChR-AKT-HIF network needs to be further investigated to conclusively prove its mechanism and to explore its therapeutic potential. 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.