A7 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 a7 nicotinic acetylcholine receptor (a7nAChR), which may regulate GLP-1 secretion. Here, GTS-21, a selective a7nAChR agonist, was used to examine the effect of a7nAChR 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 a7nAChR antagonist or by a7nAChR-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 Ca ²⁺ , cAMP, and phosphatidylinositol 3-kinase (PI3K) activity. As expected for a GLP-1 secretagogue promoting Ca ²⁺ influx through a7nAChR cation channels, [Ca ²⁺ ] _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 Ca ²⁺ chelator BAPTA-AM counteracted GTS-21‒stimulated PI3K activity, thereby indicating unexpected crosstalk of L-cell Ca ²⁺ and growth factor signaling pathways. Collectively, these data demonstrate that a7nAChR activation enhances GLP-1 secretion by increasing levels of cytosolic Ca ²⁺ while also revealing Ca ²⁺ - and PI3K-dependent processes of a7nAChR activation that promote L-cell survival.