Nicotine-induced upregulation of human neuronal nicotinic α7-receptors is potentiated by modulation of cAMP and PKC in SH-EP1-hα7 cells

Chronic nicotine exposure induces upregulation of nicotinic receptors, but the mechanisms underlying this phenomenon are not well understood. The aim of this study was to examine the role of different second messenger systems in the nicotine-induced upregulation of α7-nicotinic receptors in SH-EP1-hα7 human epithelial cells. We show here that chronic exposure to nicotine results in accumulation of cAMP. Furthermore, an enhanced cAMP signalling potentiates nicotine-induced upregulation of α7-nicotinic receptors measured by [3H]methyllycaconitine ([3H]MLA) binding suggesting that cAMP is involved in the α7-nicotinic receptor upregulation. Down-regulation of protein kinase C (PKC) with a phorbol ester abolishes the nicotine-induced upregulation of α7-nicotinic receptors. Furthermore, overexpression of PKCα in SH-EP1-hα7 cells results in potentiation of nicotine-evoked upregulation indicating that PKC has a role in regulation of α7-nicotinic receptor number. The Ca2+-calmodulin kinase II (CaMKII) and extracellular signal regulated kinase 1/2 (ERK1/2) appear not to participate in α7-nicotinic receptor upregulation since the specific inhibitors of these kinases did not have an effect on the nicotine-induced upregulation. Taken together this study provides evidence that nicotine induces accumulation of cAMP and that the upregulation mechanisms of α7-nicotinic receptors are potentiated both by cAMP and PKC. As nicotine-evoked upregulation of heteromeric nicotinic receptors in SH-SY5Y cells was unaffected by the treatment with drugs affecting cAMP signalling or PKC activity, our results suggest that the upregulation mechanisms of homomeric α7-nicotinic receptors and heteromeric nicotinic receptors differ from each other.