Receptor-mediated tobacco toxicity:: acceleration of sequential expression of α5 and α7 nicotinic receptor subunits in oral keratinocytes exposed to cigarette smoke

Tobacco products and nicotine alter the cell cycle and lead to squamatization of oral keratinocytes (KCs) and squamous cell carcinoma. Activation of nicotinic acetylcholine receptors (nAChRs) elicits Ca(2+) influx that varies in magnitude between different nAChR subtypes. Normal differentiation of KCs is associated with sequential expression of the nAChR subtypes with increasing Ca(2+) permeability, such as alpha 5-containing alpha 3 nAChR and alpha 7 nAChR. Exposure to environmental tobacco smoke (ETS) or an equivalent concentration of nicotine accelerated by severalfold the alpha 5 and alpha 7 expression in KCs, which could be abolished by mecamylamine and et-bungarotoxin with different efficacies, suggesting the following sequence of autoregulation of the expression of nAChR subtypes: alpha 3(beta 2/beta 4) > alpha 3(beta 2/beta 4)alpha 5 > alpha 7 > alpha 7. This conjecture was corroborated by results of quantitative assays of subunit mRNA and protein levels, using nAChR-specific pharmacologic antagonists and small interfering RNAs. The genomic effects of ETS and nicotine involved the transcription factor GATA-2 that showed a multifold increase in quantity and activity in exposed KCs. Using protein kinase inhibitors and dominant negative and constitutively active constructs, we characterized the principal signaling cascades mediating a switch in the nAChR subtype. Cumulative results indicated that the alpha 3(beta 2/beta 4) to alpha 3(beta 2/beta 4)alpha 5 nAChR transition predominantly involved protein kinase C, alpha 3(beta 2/beta 4)alpha 5 to alpha 7 nAChR transition-Ca(2+)/calmodulin-dependent protein kinase II and p38 MAPK, and alpha 7 self-up-regulation-the p38 MAPK/Akt pathway, and JAK-2. These results provide a mechanistic insight into the genomic effects of ETS and nicotine on KCs and characterize signaling pathways mediating autoregulation of stepwise overexpression of nAChR subtypes with increasing Ca(2+) permeability in exposed cells. These observations have salient clinical implications, because a switch in the nAChR subunit composition can bring about a corresponding switch in receptor function, leading to profound pathobiologic effects observed in KCs exposed to tobacco products.