Mechanisms that account for the selective release of arachidonic acid from intact cells by secretory phospholipase A 2

Abstract

The current study examined mechanisms that account for the selective release of arachidonic acid (AA) from cells by secretory phospholipase A 2 (sPLA 2). Initial studies demonstrated that low concentrations of group I and group III PLA 2 isotypes and an sPLA 2-enriched extract from bone marrow-derived mast cells (BMMC) selectively released AA from mast cells. Much higher concentrations of group II PLA 2 were required to release comparable quantities of AA. Group I PLA 2 also selectively released AA from another mast cell line (CFTL-15) and a monocytic cell line (THP-1). In contrast, high concentrations of group I PLA 2 were required to release fatty acids from a promyelocytic cell line (HL-60) and this release was not selective for AA. Binding studies revealed that cell types (BMMC, CFTL-15 and THP-1) which selectively released AA also had the capacity to specifically bind group I PLA 2. However, group II PLA 2, which did not selectively release AA from cells, also did not specifically bind to these same cell types. Additional studies revealed that sPLA 2 binding to the mast cell receptor was attenuated after stimulation with antigen or ionophore A23187. Reverse transcriptase–polymerase chain reaction analyses indicated the presence of mRNA for the sPLA 2 receptor in BMMC, CFTL-15 and THP-1 and the absence of this mRNA in HL-60. Final studies demonstrated that p-aminophenyl-α- d-mannopyranoside BSA, a known ligand of the sPLA 2 receptor, also selectively released AA from mast cells but not from HL-60 cells. These experiments indicated that receptor occupancy alone (without PLA 2 activity) is sufficient to induce the release of AA from mast cells. Together, these data reveal that specific isotypes of sPLA 2 have the capacity to selectively release AA from certain cells by their capacity to bind to sPLA 2 receptors on the cell surface.