Mechanisms of mitogenic and anti-apoptotic signaling by glucose-dependent insulinotropic polypeptide in β(INS-1)-cells

Abstract

Glucose-dependent insulinotropic polypeptide (GIP) acts as a glucose-dependent growth factor for β-cells. Here we show that GIP and glucose also act synergistically as anti-apoptotic factors for β-cells, using the well-differentiated β-cell line, INS-1. Mitogenic and anti-apoptotic signaling of GIP were dependent upon pleiotropic activation of protein kinase A (PKA)/cAMP regulatory element binder (CREB), mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3-kinase)/PKB signaling modules. The signaling modules activated by GIP were dependent on glucose metabolism and calcium influx and were tightly linked by multiple activating and inhibiting cross-talk. These interactions included: (i) a central role of tyrosine phosphorylation for stimulation of PKA/CREB, MAPK and P13-kinase/PKB, (ii) inhibition of PKA/CREB by the MAPK pathway at the level of MAPK kinase-1 or downstream, (iii) activation of MAPK signaling by PI3-kinase and PKA at the level of extracellular-signal regulated kinase 1/2 or upstream, and (iv) activation of PKB by MAPK and PKA signaling at the level of PKB or upstream. Furthermore, we demonstrated inhibition of CREB signaling by Ca2+/ calmodulin kinase I/IV. These results indicated that GIP acts as a mitogenic and anti-apoptotic factor for β-cells by pleiotropic activation of tightly linked signaling pathways in β-cells.