Isolated pancreatic islets express an ATP-stimulated,Ca2+- independent(ASCI)phospholipase A2(PLA2) enzyme, and we postulate that ASCI-PLA2 participates in early events in glucose-induced insulin secretion by facilitating Ca2+ entry into beta-cells via generation of nonesterified arachidonic acid. Inhibition of ASCI-PLA2 with a haloenol lactone suicide substrate suppresses the glucose-induced rise in beta- cell cytosolic Ca2+, and the effect is reversed by exogenous arachidonate. We postulate that ASCI-PLA2 participates in capacitative Ca2+ entry, a process described both in beta-cells and in cells of the vascular wall and which is involved in regulation of insulin secretion and in governing vascular myocyte proliferation. ASCI-PLA2 is expressed in vascular myocytes and endothelial cells, and we postulate that activity is unregulated by hyperglycemia and participates in regulation of proliferation.
Aim 1 is therefore to characterize the role of ASCI- PLA2 in capacitative Ca2+ entry into beta-cells and the regulation of cytosolic Ca2+ and insulin secretion, and Aim 2 is to characterize the role of ASCI-PLA2 in capacitative Ca2+ entry into vascular myocytes and endothelial cells and the regulation of proliferation. Islets also express Ca2+-dependent 14kDa secretory PLA2(sPLA2)enzymes that are secreted from beta-cells upon stimulation with insulin secretagogues. Precedents with other cells cause us to postulate that sPLA2 participates in the final common pathway of exocytosis by modifying plasma membranes so as to facilitate fusion with secretory granules. Some effects of sPLA2 are mediated by 180 kDa membrane receptors, which are expressed by islets and by vascular myocytes and which influence myocyte proliferation.
Aim 3 is therefore to characterize the role of sPLA2 in distal events in exocytosis in beta-cells and in vascular myocyte proliferation. We have found that fusion of islet secretory granules and plasma membranes is catalyzed by an isoform of glyceraldehyde-3-phosphate dehydrogenase(GAPDH), which specifically recognizes membranes rich in plasmenylethanolamine species known to be expressed in islet plasma membranes and secretory granules. A critical signal in glucose-induced insulin secretion is known to arise at the level of GAPDH, and we postulate that this signal involves activation of the fusion-catalyzing GAPDH isoform.
Aim 4 is to characterize the role of GAPDH isoforms in catalyzing fusion of beta-cell plasma membranes and secretory granules.
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