The nicotine-derived tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) induces lung cancer in all animal species tested and is thought to contribute significantly to the high lung cancer burden associated with smoking. NNK has recently been identified as a high affinity ligand for neuronal nicotinic acetylcholine receptors comprised of alpha7 subunits (alpha7 nAChR), and expressed in human small cell lung carcinoma (SCLC). As agonist-binding to this receptor in mammalian cells often results in membrane depolarization and activation of voltage-operated Ca2+-channels (VOCCs), we hypothesized that NNK may exert similar effects in SCLC. Using flow cytometry to monitor the influx of Ca2+, reverse transcription polymerase chain reaction (RT-PCR) to determine the expression of VOCC-specific messenger RNA, as well as analysis of DNA synthesis or determination of cell number, our data demonstrate that binding of NNK to the alpha7 nAChR in SCLC cells caused influx of Ca2+ via VOCCs of the L-, N-, and P-type. In turn, this led to a significant increase in DNA synthesis and cell number which was inhibited by a site-selective antagonist for the alpha7 nAChR and by Ca2+-channel blockers of the L-, N-, or P-types of VOCCs. Our findings suggest that the chronic activation of VOCC-mediated Ca2+ influx by NNK in smokers is an important event that may affect numerous Ca2+-dependent signal transduction pathways, thus contributing significantly to the development of SCLC.

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