Cloning and expression of a group IV cytosolic Ca2+-dependent phospholipase A2 from rat pancreatic islets. Comparison of the expressed activity with that of an islet group VI cytosolic Ca2+-independent phospholipase A2

Abstract

Stimulation of pancreatic islets with glucose induces phospholipid hydrolysis and accumulation of nonesterified arachidonic acid, which may play signaling or effector roles in insulin secretion. Of enzymes that catalyze phospholipid hydrolysis, islet β-cells express low molecular weight secretory phospholipases A₂ (PLA₂) and a Group VI, Ca²⁺-independent PLA₂ (iPLA₂). Previous studies indicate that islets also express a protein recognized by antibodies against a Group IV, cytosolic, Ca²⁺-dependent PLA₂ (cPLA₂). To further examine the possible expression of cPLA₂ by islets, we screened a rat islet cDNA library with a probe that recognizes cPLA₂ sequence, and isolated a full-length cPLA₂ cDNA. The rat islet cPLA₂-deduced amino acid sequence is 96% identical to those of human and mouse cPLA₂. Transfection of COS-7 cells with cPLA₂ cDNA in an expression vector induced expression of Ca²⁺-dependent PLA₂ activity and of a protein recognized by anti-cPLA₂ antibody. Comparison of recombinant islet cPLA₂ and iPLA₂ activities expressed in transfected COS-7 cells indicated that iPLA₂ but not cPLA₂ is stimulated by ATP. Both activities are similarly sensitive to inhibition by arachidonyltrifluoromethyl ketone, but iPLA₂ is more effectively inhibited by a haloenol lactone suicide substrate than cPLA₂. RT-PCR experiments with RNA from purified islet β-cells and from an α-cell-enriched population prepared by fluorescence-activated cell-sorting indicated that cPLA₂ mRNA is more abundant in the β-cell population. Immunoblotting analyses indicate that islets express cPLA₂-immunoreactive protein, and that interleukin-1 does not affect its expression. The cPLA₂ is thus one of at least three classes of PLA₂ enzymes with distinct properties expressed in β-cells.