α7 Nicotinic Acetylcholine Receptor Regulates the Function and Viability of L Cells

Enteroendocrine L cells secrete the incretin hormone glucagon-like peptide-1 (GLP-1), and they also express the alpha 7 nicotinic acetylcholine receptor (alpha 7nAChR), which may regulate GLP-1 secretion. Here, GTS-21, a selective alpha 7nAChR agonist, was used to examine the effect of alpha 7nAChR activation in L-cell lines, mouse intestinal primary cell cultures, and C57BL/6 mice. GTS-21 stimulated GLP-1 secretion in vitro, and this effect was attenuated by an alpha 7nAChR antagonist or by alpha 7nAChR-specific small interfering RNA. Under in vitro cell culture conditions of glucotoxicity, GTS-21 restored GLP-1 secretion and improved L-cell viability while also acting in vivo to raise levels of circulating GLP-1 in mice. To assess potential signaling mechanisms underlying these actions of GTS-21, we first monitored Ca2+, cAMP, and phosphatidylinositol 3-kinase (PI3K) activity. As expected for a GLP-1 secretagogue promoting Ca2+ influx through alpha 7nAChR cation channels, [Ca2+](i) increased in response to GTS-21, but [cAMP](i) was unchanged. Surprisingly, pharmacological inhibition of growth factor signaling pathways revealed that GTS-21 also acts on the PI3K-protein kinase B-mammalian target of rapamycin pathway to promote L-cell viability. Moreover, the Ca2+ chelator BAPTA-AM counteracted GTS-21. stimulated PI3K activity, thereby indicating unexpected crosstalk of L-cell Ca2+ and growth factor signaling pathways. Collectively, these data demonstrate that alpha 7nAChR activation enhances GLP-1 secretion by increasing levels of cytosolic Ca2+ while also revealing Ca2+ and PI3K-dependent processes of alpha 7nAChR activation that promote L-cell survival.