Essential role of phosphoinositide 3-kinase in leptin-induced K_ATP channel activation in the rat CRI-G1 insulinoma cell line

The mechanism by which leptin increases ATP-sensitive K⁺ (K_ATP) channel activity was investigated using the insulin-secreting cell line, CRI-G1. Wortmannin and LY 294002, inhibitors of phosphoinositide 3-kinase (PI3-kinase), prevented activation of K_ATP channels by leptin. The inositol phospholipids phosphatidylinositol bisphosphate and phosphatidylinositol trisphosphate (PtdIns(3,4,5)P₃) mimicked the effect of leptin by increasing K_ATP channel activity in whole-cell and inside-out current recordings. LY 294002 prevented phosphatidylinositol bisphosphate, but not PtdIns(3,4,5)P₃, from increasing K_ATP channel activity, consistent with the latter lipid acting as a membrane-associated messenger linking leptin receptor activation and K_ATP channels. Signaling cascades, activated downstream from PI 3-kinase, utilizing PtdIns(3,4,5)P₃ as a second messenger and commonly associated with insulin and cytokine action (MAPK, p70 ribosomal protein-S6 kinase, stress-activated protein kinase 2, p38 MAPK, and protein kinase B), do not appear to be involved in leptin-mediated activation of K_ATP channels in this cell line. Although PtdIns(3,4,5)P₃ appears a plausible and attractive candidate for the messenger that couples K_ATP channels to leptin receptor activation, direct measurement of PtdIns(3,4,5)P₃ demonstrated that insulin, but not leptin, increased global cellular levels of PtdIns(3,4,5)P₃. Possible mechanisms to explain the involvement of PI 3-kinases in K_ATP channel regulation are discussed.