Interaction of tobacco-specific toxicants with the neuronal alpha(7) nicotinic acetylcholine receptor and its associated mitogenic signal transduction pathway: potential role in lung carcinogenesis and pediatric lung disorders.

Pulmonary neuroendocrine cells function as hypoxia-sensitive chemoreceptors, and they release peptides and biogenic amines that are important mediators of pulmonary neonatal adaptation. Some of these products additionally act as autocrine growth factors. Increased numbers of pulmonary neuroendocrine cells have been observed in several smoking-associated pediatric lung disorders such as bronchopulmonary dysplasia, cystic fibrosis, sudden infant death syndrome, and asthma. Disturbed pulmonary neuroendocrine function has been implicated in the etiology of this disease complex. One of the most common smoking-associated lung cancer types, small cell lung carcinoma, expresses phenotypic and functional features of pulmonary neuroendocrine cells. We, as well as others, have shown that the release of the autocrine growth factors 5-hydroxytryptamine (5-HT, serotonin) and mammalian bombesin/gastrin releasing peptide (MB/GRP) by cell lines derived from human small cell lung carcinoma or fetal hamster pulmonary neuroendocrine cells are regulated by a neuronal nicotinic acetylcholine receptor comprised of alpha(7) subunits. In radio-receptor assays, nicotine and the nicotine-derived carcinogenic nitrosamines NNNN. Binding of nicotine or NNK to the alpha(7) receptor resulted in calcium influx and overexpression and activation of the serine-threonine protein kinase Raf-1. In turn, this event lead to overexpression and activation of the mitogen activated (MAP) kinases extracellular signal regulated kinase 1 (ERK1) and extracellular signal regulated kinase 2 (ERK2) and stimulation of DNA synthesis accompanied by an increase in cell numbers in fetal pulmonary neuroendocrine cells and small cell carcinoma cells. Exposure of fetal pulmonary neuroendocrine cells for 6 days to NNK caused a prominant up-regulation of Raf-1. Our findings suggest that chronic exposure to nicotine and NNK in pregnant women who smoke may up-regulate the alpha(7) nicotinic receptor as well as components of its associated mitogenic signal transduction pathway, thus increasing the susceptibilities of the infants for the development of pediatric lung disorders. Similarly, up-regulation of one or several components of this nicotinic receptor pathway in smokers may be an important factor for the development of small cell lung carcinoma.