Inhibition by adenosine 3′:5′-monophosphate of eicosanoid and platelet-activating factor biosynthesis in the mouse PT-18 mast cell

A mouse spleen-derived mast cell line (PT-18) was employed to examine the mechanisms of adenosine 3′:5′-monophosphate (cAMP)-mediated inhibition of antigen-induced lipid mediator biosynthesis. Specifically, we tested the hypothesis that increasing cAMP in mast cells inhibits lipid mediator biosynthesis by a mechanism independent of effects on histamine release (degranulation) or changes in cytosolic calcium concentration. Forskolin inhibited antigen-induced prostaglandin D₂ (PGD₂), leukotriene C₄ (LTC₄), and leukotriene B₄ (LTB₄) production by 30-50%. In contrast, forskolin had no inhibitory effect on antigen-induced increases in cytosolic calcium concentration, as monitored by the calcium indicator fura-2, or histamine release from the cells. The combination of the phosphodiesterase inhibitor isobutylmethylxanthine with forskolin inhibited the antigen-induced production of PGD₂ and LTC₄ by 90-100% and histamine release by about 60%. These responses were accompanied by a virtual abolition of the antigen-induced increase in cytosolic calcium. To test further the hypothesis that increasing c AMP can lead to inhibition of lipid mediator biosynthesis in the absence of effects on cytosolic calcium, we employed the calcium ionophores A23187 and ionomycin. Forskolin alone or in combination with isobutylmethylxanthine had no effect on ionophore-induced increases in cytosolic calcium but effectively inhibited leukotriene biosynthesis. In addition, increasing cyclic AMP led to an inhibition of ionophoreinduced production of platelet-activating factor and liberation of arachidonic acid. These data suggest that a relatively modest increase in cAMP-dependent protein kinase activity in mast cells leads to inhibition of the lipase-catalyzed cleavage of arachidonic acid from membrane phospholipids in the absence of measurable effects on either histamine release or changes in cytosolic calcium concentration. This effect results in a selective inhibition of the biosynthesis of lipid mediators including LTC₄, LTB₄, PGD₂, and platelet-activating factor.