Regulation of arachidonic acid, eicosanoid, and phospholipase A₂ levels in murine mast cells by recombinant stem cell factor

The current study evaluates the capacity of recombinant rat stem cell factor (rrSCF) to regulate enzymes that control AA release and eicosanoid generation in mouse bone marrow-derived mast cells (BMMCs). Initial studies indicated that rrSCF provided for 24 h inhibited the release of AA into supernatant fluids of antigen- and ionophore A23187-stimulated BMMCs. Agonist-induced increases in cellular levels of AA were also inhibited, albeit to a lesser degree by rrSCF. To determine the inhibitory mechanism, several steps (e.g., mobilization of cytosolic calcium, release of BMMC granules, and regulation of phospholipase A₂ [PLA₂] activity) that could influence AA release were measured in rrSCF-treated cells. rrSCF did not alter the capacity of BMMCs to mobilize cytosolic calcium or release histamine in response to antigen or ionophore. BMMCs released large amounts of PLA₂ with characteristics of the group II family in response to antigen and ionophore A23187. rrSCF treatment of BMMCs reduced the secretion of this PLA₂ activity by BMMCs. Partial purification of acid-extractable PLA₂ from rrSCF-treated and untreated BMMCs suggested that rrSCF decreased the quantity of acid-stable PLA₂ within the cell. In contrast to group II PLA₂, the quantity of cPLA₂ (as determined by Western blot analysis) increased in response to rrSCF. To assess the ramifications of rrSCF-induced reductions in AA and group II PLA₂, eicosanoid formation was measured in antigen- and ionophore-stimulated BMMCs. rrSCF-inhibited (100 ng/ml, 24 h) prostaglandin D₂ (PGD₂), thromboxane B₂, and leukotriene B₄ by 48.4 ± 7.7%, 61.1 ± 10.0% and 38.1 ± 3.6%, respectively, in antigen-stimulated cells. Similar patterns of inhibition were observed in ionophore-stimulated BMMCs. The addition of a group I PLA₂ or exogenous AA to BMMCs reversed the inhibition of eicosanoid generation induced by rrSCF. Together, these data indicate that rrSCF differentially regulates group II and cytosolic PLA₂ activities in BMMCs. The resultant reductions in eicosanoid generation suggest that group II PLA₂ provides a portion of AA that is used for eicosanoid biosynthesis by BMMCs.