Studies on signal transduction mechanisms from nicotinic acetylcholine receptors to nucleus

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Considering the crosstalk between NGF/cAMP signaling and nicotinic acetylcholine receptor (nAChR)-mediating signaling pathways, we first investigated regulation of nAChR expression by NGF and cAMP in three sublines of PC12 cells, PC12h cells, PC12 cells expressing dominant negative Ras and the parential PC12 cells (PC12-wild). cAMP analogue down-regulated level of nAChR α3 subunit mRNA (α3mRNA) in PC12h and PC12-wild cells through protein kinase A.NGF up-regulated the α3mRNA level in PC12h cells and down-regulated the α3mRNA in PC12-wild cells, which is mediated by Ras-MAP kinase cascade. Membrane depolarization with high K^+ had no effect on the α3mRNA level in PC12h cells. Based on these results, we proposed that at least two unknown downstream factors of MAP kinase (ERK) and CREB regulate α3mRNAs in PC12 cells.Next we investigated mechnisms of nicotine-induced MAP kinase (ERK) and CREB phosphorylation in PC12h cells. Nicotine-induced ERK phosphorylation was transient and its level was lower than that of ERK phosphorylation induced by high K^+ depolarization and NGF.α7 nAChR subunit-selective antagonists had no effect on nicotine-induced ERK phosphorylation. L-type voltage-sensitive calcium channel antagonists inhibited nicotine-induced ERK phosphorylation. These results suggest that no α7-containing nAChRs was involved in nicotine-induced ERK phosphorylation. Inhibition of nicotine-induced ERK phosphorylation by a calmodulin antagonist, CaM kinase inhibitor, MAP kinase kinase inhibitor and expression of dominant inhibitory Ras shows that CaM kinase and Ras-MAP kinase cascade are involved in nicotine-induced ERK phosphorylation. Furthermore, it has been suggested that most of nicotine-induced CREB phosphorylation is mediated by nicotine-induced ERK phosphorylation in PC12h cells.