The Role of Cytosolic Phospholipase A₂ in Insulin Secretion

Abstract

Cytosolic phospholipase A₂ (cPLA₂) comprises a widely expressed family of enzymes, some members of which have the properties required of signal transduction elements in electrically excitable cells. Thus, α- and β-isoforms of cPLA₂ are activated by the increases in intracellular Ca²⁺ concentration ([Ca²⁺]_i) achieved in depolarized cells. Activation is associated with a redistribution of the enzyme within the cell; activation of cPLA₂ generates arachidonic acid (AA), a biologically active unsaturated fatty acid that can be further metabolized to generate a plethora of biologically active molecules. Studies using relatively nonselective pharmacological inhibitors have implicated cPLA₂ in insulin secretory responses to stimuli that elevate β-cell [Ca²⁺]_i; therefore, we have investigated the role of cPLA₂ in β-cell function by generating β-cell lines that under- or overexpress the α-isoform of cPLA₂. The functional phenotype of the modified cells was assessed by observation of cellular ultrastructure, by measuring insulin gene expression and insulin protein content, and by measuring the effects of insulin secretagogues on cPLA₂ distribution, on changes in [Ca²⁺]_i, and on the rate and pattern of insulin secretion. Our results suggest that cPLA₂ is not required for the initiation of insulin secretion from β-cells, but that it plays an important role in the maintenance of β-cell insulin stores. Our data also demonstrate that excessive production of, or exposure to, AA is deleterious to normal β-cell secretory function through metabolic dysfunction.