Abstract

Boswellic acids inhibit the transformation of arachidonic acid to leukotrienes via 5-lipoxygenase but can also enhance the liberation of arachidonic acid in human leukocytes and platelets. Using human platelets, we explored the molecular mechanisms underlying the boswellic acid-induced release of arachidonic acid and the subsequent metabolism by platelet-type 12-li-poxygenase (p12-LO). Both β-boswellic acid and 3-O-acetyl-11-keto-boswellic acid (AKBA) markedly enhanced the release of arachidonic acid via cytosolic phospholipase A₂ (cPLA₂), whereas for generation of 12-hydro(pero)xyeicosatetraenoic acid [12-H(P)ETE], AKBA was less potent than β-boswellic acid and was without effect at higher concentrations (≥30 μM). In contrast to thrombin, β-boswellic acid-induced release of ara-chidonic acid and formation of 12-H(P)ETE was more rapid and occurred in the absence of Ca²⁺. The Ca²⁺-independent release of arachidonic acid and 12-H(P)ETE production elicited by β-boswellic acid was not affected by pharmacological inhibitors of signaling molecules relevant for agonist-induced arachidonic acid liberation and metabolism. It is noteworthy that in cell-free assays, β-boswellic acid increased p12-LO catalysis approximately 2-fold in the absence but not in the presence of Ca²⁺, whereas AKBA inhibited p12-LO activity. No direct modulatory effects of boswellic acids on cPLA₂ activity in cell-free assays were evident. Therefore, immobilized KBA (linked to Sepharose beads) selectively precipitated p12-LO from platelet lysates but failed to bind cPLA₂. Taken together, we show that boswellic acids induce the release of arachidonic acid and the synthesis of 12-H(P)ETE in human platelets by unique Ca²⁺-independent routes, and we identified p12-LO as a selective molecular target of boswellic acids.