An arachidonoyl (polyenoic)-specific phospholipase A2 activity regulates the synthesis of platelet-activating factor in granulocytic HL-60 cells.

Human promyelocytic leukemia cells (HL-60) were used as a cell model to determine how arachidonic acid stimulates the synthesis of platelet-activating factor (PAF) synthesized via the remodeling pathway. In these studies HL-60 cells were cultured over 30 passages in fatty acid-free medium to deplete them of arachidonic acid. Even though the phospholipid classes from these cells contained no arachidonate, they could still be differentiated into granulocytes by dimethyl sulfoxide (1.25%). When the differentiated HL-60 cells, depleted of arachidonic acid, were stimulated with calcium ionophore A23187 in the presence of Ca2+ and [3H]acetate, only minimal amounts of [3H]PAF were produced. In contrast, if the differentiated HL-60 cells were supplemented with 10 microM arachidonic acid for 24 h and then stimulated with the ionophore, there was a large amount of [3H]PAF formed. The increase in PAF synthesis depended on the length of time the cells were supplemented with arachidonic acid; only a small increase in PAF synthesis occurred during the early hours of supplementation whereas stimulation of PAF synthesis was maximal (3-5-fold) after a 24-h period of the 20:4 supplementation. Other polyenoic fatty acid supplements (20:5, 22:4, and 22:6 for 24 h) also stimulated PAF production in the ionophore-treated HL-60 cells depleted of 20:4, but the amount of PAF was significantly less than found for the supplements of 20:4 under identical experimental conditions. Also noteworthy is that undifferentiated cells supplemented with 20:4 or their unsupplemented controls could not be stimulated by the calcium ionophore to produce PAF. Addition of indomethacin (cyclooxygenase inhibitor), A63162 (5'-lipoxygenase inhibitor), or eicosatetraynoic acid (cyclooxygenase/lipoxygenase inhibitor) to the incubations caused little change in the production of [3H]PAF in the differentiated cells supplemented with 20:4 for 24 h. On the other hand, the addition of mepacrine, bromophenacyl bromide, or U26384 (phospholipase A2 inhibitors) resulted in very large decreases (80-90% lower than controls) in the amount of [3H]PAF produced under the same conditions. Analysis of the molecular species of [3H]alkylacyl-GroPCho (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine, the precursor of PAF in the remodeling pathway) in 20:4-supplemented cells prelabeled with [3H]alkyl-lyso-GroPCho revealed that only the alkylarachidonoyl-GroPCho species were preferentially decreased after stimulation with the A23187 ionophore. These results demonstrate that arachidonate must be at the sn-2 position of alkylacyl-GroPCho in order for it to serve as a precursor of PAF.(ABSTRACT TRUNCATED AT 400 WORDS)