Regulation of Insulin Secretion by Phosphatidylinositol-4,5-Bisphosphate

The role of PIP2 in pancreatic beta cell function was examined here using the beta cell line MIN6B1. Blocking PIP2 with PH-PLC-GFP or PIP5KI gamma RNAi did not impact on glucose-stimulated secretion although susceptibility to apoptosis was increased. Over-expression of PIP5KI gamma improved cell survival and inhibited secretion with accumulation of endocytic vacuoles containing F-actin, PIP2, transferrin receptor, caveolin 1, Arf6 and the insulin granule membrane protein phogrin but not insulin. Expression of constitutively active Arf6 Q67L also resulted in vacuole formation and inhibition of secretion, which was reversed by PH-PLC-GFP co-expression. PIP2 co-localized with gelsolin and F-actin, and gelsolin co-expression partially reversed the secretory defect of PIP5KI gamma-over-expressing cells. RhoA/ROCK inhibition increased actin depolymerization and secretion, which was prevented by over-expressing PIP5KI gamma, while blocking PIP2 reduced constitutively active RhoA V14-induced F-actin polymerization. In conclusion, although PIP2 plays a pro-survival role in MIN6B1 cells, excessive PIP2 production because of PIP5KI gamma over- expression inhibits secretion because of both a defective Arf6/PIP5KI gamma-dependent endocytic recycling of secretory membrane and secretory membrane components such as phogrin and the RhoA/ROCK/PIP5KI gamma-dependent perturbation of F-actin cytoskeleton remodelling.