Mechanisms that account for the selective release of arachidonic acid from intact cells by secretory phospholipase A2

Abstract

The current study examined mechanisms that account for the selective release of arachidonic acid (AA) from cells by secretory phospholipase A₂ (sPLA₂). Initial studies demonstrated that low concentrations of group I and group III PLA₂ isotypes and an sPLA₂-enriched extract from bone marrow-derived mast cells (BMMC) selectively released AA from mast cells. Much higher concentrations of group II PLA₂ were required to release comparable quantities of AA. Group I PLA₂ 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 PLA₂ 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 PLA₂. However, group II PLA₂, which did not selectively release AA from cells, also did not specifically bind to these same cell types. Additional studies revealed that sPLA₂ 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 sPLA₂ 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 sPLA₂ receptor, also selectively released AA from mast cells but not from HL-60 cells. These experiments indicated that receptor occupancy alone (without PLA₂ activity) is sufficient to induce the release of AA from mast cells. Together, these data reveal that specific isotypes of sPLA₂ have the capacity to selectively release AA from certain cells by their capacity to bind to sPLA₂ receptors on the cell surface.