Stimulation of Leukotriene Synthesis in Intact Polymorphonuclear Cells by the 5-Lipoxygenase Inhibitor 3-oxo-Tirucallic Acid

Abstract

Commercially available extracts from Boswellia serrata resin used as anti-inflammatory drugs or phytonutrients show paradoxical concentration-dependent potentiating and inhibitory actions on 5-lipoxygenase (5-LO) product synthesis in stimulated PMNs. In our attempt to characterize the stimulating constituents, we identified the tetracyclic triterpene 3-oxo-tirucallic acid (3-oxo-TA), which, in the range from 2.5 to 15 μM, enhanced 5-LO product formation in ionophore-challenged polymorphonuclear cells (PMNs) (e.g., from 1981 ± 177 to 3042 ± 208 pmol at 10 μM 3-oxo-TA), and initiated Ca²⁺ mobilization, MEK-1/2 phosphorylation, 5-LO translocation, and 5-LO product formation in resting cells (534 ± 394 pmol/5 × 10⁶ PMNs). In cell-free 5-LO assays, 3-oxo-TA acted only inhibitory (IC₅₀value of about 3 μM), demonstrating the pivotal role of intact cell structure for its activating property. In 3-oxo-TA-challenged PMNs, the mitogen-activated protein kinase kinase (MEK)-1/2 inhibitor PD098059 abolished 5-LO product formation, along with inhibition of MEK-1/2 phosphorylation and 5-LO translocation. The 3-acetoxy derivative of 3-oxo-TA acted like 3-oxo-TA in intact PMNs, whereas 3-hydroxy-TA barely stimulated MEK phosphorylation in resting cells and showed only inhibition on ionophore-induced 5-LO product synthesis. Steroid-type tetracycles neither induced 5-LO activation nor had enhancing or inhibitory effects. In summary, defined natural tetracyclic triterpenes, which act as inhibitors of the 5-LO in the cell-free assay, initiate 5-LO activation by a MEK-inhibitor sensitive mechanism and potentiate stimulated product synthesis in intact cells. Because TAs contribute significantly to the overall biological effects ofB. serrata resin extracts, special precaution for standardization is recommended when using B. serratapreparations as drugs or dietary supplements.