Received for publication March 22, 2006.
Revised June 9, 2006.
Accepted for publication June 20, 2006.

Boswellic acids stimulate arachidonic acid release and 12-lipoxygenase activity in human platelets independent of Ca²⁺ and differentially interact with platelet-type 12-lipoxygenase

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. Utilizing human platelets, we explored the molecular mechanisms underlying the boswellic acid-induced release of arachidonic acid and the subsequent metabolism by platelet-type 12-lipoxygenase (p12-LO). Both, -boswellic acid as well as 3-O-acetyl-11-keto-boswellic acid (AKBA) markedly enhanced the release of arachidonic acid via cytosolic phospholipase (PL)A₂, whereas for generation of 12-hydro(pero)xyeicosatetraenoic acid (12-H(P)ETE), AKBA was less potent than -boswellic acid and was without effect at higher concentrations ( 30 µM). In contrast to thrombin, -boswellic acid-induced release of arachidonic acid and formation of 12-H(P)ETE was more rapid and occurred also in the absence of Ca²⁺. The Ca²⁺-independent release of arachidonic acid and 12-H(P)ETE production elicited by -boswellic acid was not affected by pharmacological inhibitors of signaling molecules relevant for agonist-induced arachidonic acid liberation and metabolism. Notably, in cell-free assays, -boswellic acid increased p12-LO catalysis about two-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. Accordingly, 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.

Key words: Prostanoid, Phospholipase A2's, Calcium (G Protein Coupled Signals), MAP Kinase, HETEs, Leukotrienes, Lipoxygenases, Platelets