Inhibition of 5-Lipoxygenase-Derived Leukotrienes and Hemiketals as a Novel Anti-Inflammatory Mechanism of Urolithins

Abstract

Scope: Urolithins (Uro), gut microbial metabolites derived from ellagic acid (EA), reach significant concentrations in the human colon. Uro-A exerts anti-inflammatory activity in animal models of inflammatory bowel diseases (IBDs). It is hypothesized that Uro can modulate the biosynthesis of leukocyte-derived inflammatory eicosanoids from the 5-lipoxygenase (5-LOX), cyclooxygenase-2 (COX-2), and 5-LOX/COX-2 pathways, relevant in the onset and progression of IBDs, including 5-hydroxyeicosatetraenoic acids (5-HETEs), leukotriene-B₄ (LTB₄ ), prostaglandin E₂ (PGE₂ ), and hemiketals (HKE₂ and HKD₂ ).

Methods and results: Leukocytes, obtained from six healthy donors, are stimulated with lipopolysaccharide and calcium ionophore A23187. Uro, at concentrations found in the human colon (1-15 µm), decrease eicosanoid biosynthesis and COX-2 levels in the activated leukocytes. In contrast, EA and conjugated Uro (glucuronides and sulfates) are inactive. Uro-A and isourolithin-A reduce the formation of the 5-LOX/COX-2 products HKE₂ and HKD₂ through the COX-2 pathway (down-regulation of COX-2 and PGE2), whereas Uro-C reduces 5-HETE and LTB₄ via inhibition of 5-LOX.

Conclusions: The results show that physiologically relevant colonic Uro target eicosanoid biosynthetic pathways. The effect on HKs and LTB₄ formation is unprecedented and expands the knowledge on anti-inflammatory mechanisms of Uro against IBDs.

Keywords: 5-lipoxygenase; cyclooxygenase-2; eicosanoid; inflammatory bowel disease; urolithins.