Simultaneous exposure to nicotine and hyperoxia causes tumors in hamsters

BACKGROUND : We have shown that the nicotine-derived nitrosamine 4-(methyl-nitrosamino)-3-(pyridyl)-1-butanone (NNK) causes a high incidence of neuroendocrine lung tumors in male Syrian golden hamsters when administered to animals maintained in an atmosphere of 60% hyperoxia. In vitro studies with fetal hamster pulmonary neuroendocrine cells (PNE cells) and human neuroendocrine lung cancer cell lines revealed that nicotine and NNK are both potent mitogens for normal and neoplastic PNE cells when the cells were maintained in an atmosphere of high CO 2 . These effects were completely inhibited by antagonists of nicotinic acetylcholine receptors (nAChR). NNK displaced 3 H-(-)L-nicotine from the nAchR in radioreceptor assays with cell membrane fractions from hamster lungs enriched in PNE cells. We therefore hypothesized that NNK acts as an agonist of the nAchR in PNE cells and that stimulation of this receptor in an environment of impaired pulmonary oxygenation is an important molecular event leading to the development of lung tumors with a neuroendocrine phenotype. EXPERIMENTAL DESIGN : To test this hypothesis, we exposed male Syrian golden hamsters maintained in 60% hyperoxia to s.c. injections of nicotine for the duration of their life. To allow for a survival time long enough to assess a potential carcinogenic effect of this treatment, animals demonstrating symptoms of respiratory distress were returned to ambient air for 24 hours throughout the experiment. RESULTS : A low but significant number of the animals exposed to hyperoxia and nicotine developed tumors of the nasal cavity, lungs, and adrenal glands. All of the tumor-bearing animals had survived 40 weeks or longer. The lung tumors demonstrated focal areas of positive immunoreactivity to neuron-specific enolase (NSE) and 5-hydroxy-tryptamine (5-HT, serotonin), both of which are markers of neuroendocrine differentiation. Hamsters maintained in ambient air and receiving identical injections with nicotine as well as animals maintained in hyperoxia and injected with saline did not develop tumors in any organs. All hamsters exposed to hyperoxia and surviving more than 12 weeks had thickened alveolar walls and emphysema. CONCLUSIONS : Our data support the hypothesis that chronic stimulation of the nAChR in an environment of impaired pulmonary oxygenation contributes to the carcinogenic burden associated with exposure to cigarette smoke and provides selective growth advantage for lung tumors with a neuroendocrine phenotype.