Insulin receptor signaling for the proliferation of pancreatic β-cells: involvement of Ca2+ second messengers, IP3, NAADP and cADPR.

Insulin has an autocrine/paracrine role through insulin receptors in pancreatic β-cells. Herein, we show the insulin receptor signaling pathway underlying CD38/ADPR-cyclase activation for NAADP/cADPR formation to induce Ca2+ rise, ultimately resulting in β-cell proliferation. Binding of insulin on insulin receptors leads to the activation of IRS/Akt/PI3K/PLC. Activation of PLC generates IP3 and DAG; the former induces Ca (2+) release, resulting in activation of CD38/ADPR-cyclase for cADPR production via cGMP-dependent mechanism and the latter activates PKC, resulting in activation of ADPR-cyclase for NAADP synthesis. The NAADP-induced Ca (2+) signal is required for IP3-induced Ca (2+) release from the ER. CD38 plays an important role in insulin receptor signaling in β-cells by reflecting a declined sustained Ca (2+) signal, cADPR levels, and β-cell proliferation in response to insulin in CD38 (-/-) islets. However, evidence indicates that a hitherto-unidentified ADPR cyclase in addition to CD38 participates in insulin-induced signaling through cADPR and NAADP synthesis. In conclusion, insulin receptor signaling in β-cells employs three Ca (2+) signaling messengers, IP3, NAADP, and cADPR through a complex but concerted action of signaling molecules for Ca2+ signaling, which is involved in the proliferation of the islets.