Class II Phosphoinositide 3-Kinase Regulates Exocytosis of Insulin Granules in Pancreatic β Cells

Abstract

Phosphoinositide 3-kinases (PI3Ks) are critical regulators of pancreatic β cell mass and survival, whereas their involvement in insulin secretion is more controversial. Furthermore, of the different PI3Ks, the class II isoforms were detected in β cells, although their role is still not well understood. Here we show that down-regulation of the class II PI3K isoform PI3K-C2α specifically impairs insulin granule exocytosis in rat insulinoma cells without affecting insulin content, the number of insulin granules at the plasma membrane, or the expression levels of key proteins involved in insulin secretion. Proteolysis of synaptosomal-associated protein of 25 kDa, a process involved in insulin granule exocytosis, is impaired in cells lacking PI3K-C2α. Finally, our data suggest that the mRNA for PI3K-C2α may be down-regulated in islets of Langerhans from type 2 diabetic compared with non-diabetic individuals. Our results reveal a critical role for PI3K-C2α in β cells and suggest that down-regulation of PI3K-C2α may be a feature of type 2 diabetes.

Phosphoinositides: Tiny Lipids With Giant Impact on Cell Regulation

“…PI3KC2␣ was found to regulate exocytosis of insulin in pancreatic beta cells (370), but was also activated by insulin via the small GTP binding protein TC10 (970) and played a role in insulin-induced Glut4 translocation (370). A role of PI3K-C2␣ was also suggested in neurosecretion (1034), and in this context, it is notable that TC10 was recently found to regulate axon cone specification by inducing translocation of the exo70 exocyst complex to membranes (391).…”

“…PI3KC2␥ was claimed to play a role in regenerating liver (1171), but there is little knowledge concerning the involvement of this enzyme in any biological process. As pointed out above, it is increasingly evident that class II PI 3-kinases make PtdIns3P as their signaling molecule (370,970,1700). If the enzyme indeed makes primarily PtdIns3P, it is likely that it does so in very specific molecular contexts that differ from those regulated by the class III Vps34 enzymes.…”

“…However, in this case, PI3K, specifically the PI3K␦ (30) and PI3K␥ isoforms (157,847), act as important amplifiers to control PLC␥ activation, and hence, Ca 2ϩ signaling and PtdIns(3,4,5)P 3 may not be directly involved in the control of the exocytic machinery (30, 157,847). Recent studies implicated PI3K-C2␣ in the control of exocytosis of insulin-containing vesicles in pancreatic ␤-cells (370,879) and neurosecretory cells (1034,1700). It should be remembered that PI 3-kinase inhibitors can also indirectly affect secretion by altering the various Ca 2ϩ influx pathways (see above) or actin dynamics as shown in pancreatic ␤-cells (282, 1222).…”

“…It is also important to note that PX domains also bind proteins (1540,1643). Recent studies also suggest that class II PI3Ks produce some of the PtdIns3P in higher eukaryotes (370,970,1700), and the molecular cloning of the type II PI4Ks (108, 1058) revealing their presence in endosomes (83, 1060) also suggested the involvement of these latter enzymes in endocytic trafficking in mammalian cells (83, 1060). PtdIns3P controls several aspects of the biology of endosomes.…”

Protein Kinases and Pancreatic Islet Function

“…Leibiger and colleagues showed that PI3K-C2 is able to enhance glucose-stimulated insulin release in a feed-forward (126) mechanism, in response to stimuli from insulin itself, via the activation of PKB by PI3P. Subsequently, it was shown that PI3K-C2 may have a role in the late stages of insulin granule release through its action on the synaptosomal-associated protein of 25 kDA (SNAP25), which is independent of nutrient control (130). Insulin granules dock at the plasma mebrane via the interaction of SNAP25 within a protein complex (161)(162)(163), with granule fusion and insulin release occuring post proteolysis of SNAP25 (164).…”