Type IB secretory phospholipase A₂ is contained in insulin secretory granules of pancreatic islet β-cells and is co-secreted with insulin from glucose-stimulated islets

Stimulation of pancreatic islets with D-glucose induces insulin secretion from secretory granules contained within the islet β-cells. Accumulating evidence suggests that secretory phospholipases A (sPLA₂) may play a role in the distal events of secretory processes in many different cell types. Since intact pancreatic islets have been reported to contain sPLA₂, it was of interest to determine the cellular and subcellular localization of the sPLA₂ enzymes in pancreatic islets. Our findings indicate that rat pancreatic islets express mRNA for both types IB and IIA sPLA₂ enzymes and mRNA for an sPLA₂ membrane receptor. Immunoblotting analyses with antibodies directed against type IB sPLA₂ or against type IIA sPLA₂ indicate that the type IB isoform is much more abundant than the type IIA isoform in islets. Studies with purified populations of islet β-cells prepared from dispersed islet cells by fluorescence-activated cell sorting indicate that both sPLA₂ activity and type IB sPLA₂ immunoreactive protein are substantially more abundant in β-cells than in non-β-cells. Subcellular fractionation studies indicate that sPLA₂ activity and type IB sPLA₂ immunoreactive protein are contained in insulin secretory granules. Stimulation of intact islets with insulin secretagogues results in the co-secretion of insulin and of sPLA₂ activity and type IB sPLA₂ immunoreactive protein into the incubation medium. These findings raise the possibility that type IB sPLA₂ participates in the secretory process of pancreatic islet β-cells.