COX-1 driven biosynthesis of PGD2 during activation of human mast cells prevents formation of other prostanoids

Background: The major mast cell prostanoid PGD2 is an interesting target for therapeutic intervention because of its influence on bronchoconstriction, vasodilation and eosinophilic recruitment. We investigated the biochemical impact of inhibition of PGD2 biosynthesis in four human mast cell models. Methods: Mast cells (LAD2, cord blood derived mast cells (CBMC), peripheral blood mast cells (PBMC) and human lung mast cells (HLMC)) were activated by anti-IgE or ionophore A23187. Expression of COX-1, COX-2 and hematopoietic prostaglandin D synthase (hPGDS) was examined by western blot. The formation of different prostanoids was measured by UPLC-MS/MS, with release of PGD2 also measured by enzyme immunoassay. Pathways of biosynthesis were determined by pharmacological interventions and addition of the prostanoid precursor PGH2. Results: The biosynthesis of PGD2 and other prostanoids was exclusively catalyzed by COX-1. During inhibition of hPGDS, cell activation was associated with an increase in PGE2 and the TXA2 metabolite TXB2. Combined inhibition of hPGDS and thromboxane synthase led to a greater PGE2 increase. Furthermore, addition of PGH2 while inhibiting hPGDS also increased levels of TXB2 and PGE2. TXB2 was produced only in the presence of cells, whereas PGE2 was formed readily by non-enzymatic degradation. Conclusion: Mast cell production of prostanoids is COX-1 dependent. Downstream inhibition of the major prostanoid producing enzyme hPGDS will lead to shunting of PGH2 to form other prostanoids. Physiologically, the high hPGDS activity prevents the formation of other prostanoids in mast cells.