Selective Inhibition of COX-2 by a Standardized CO₂ Extract of Humulus lupulus in vitro and its Activity in a Mouse Model of Zymosan-Induced Arthritis

Abstract

A standardized CO₂ extract from Humulus lupulus L. (hop extract) was investigated for its selective COX-1/2 inhibitory properties. An in vitro model of inflammation using lipopolysaccharide (LPS)-stimulated human peripheral blood mononuclear cells (PBMC) was used as a model to investigate the effect of hop extract on PGE₂ production. COX-1/2 selective inhibition by the hop extract was investigated in a COX-1 whole blood assay (WBA) and a COX-2 WBA. To evaluate the in vivo activity of hop extract, it was administered orally to C57BL/6 mice in which inflammation of the right joint was induced by injecting zymosan intra-articularly. Ex vivo PGE₂ production of LPS-stimulated blood cells was determined. Also, the effect of hop extract on healthy and arthritic cartilage was investigated as well as effects on inflammatory joint swelling. Hop extract inhibited PGE₂ production by LPS-stimulated PBMC without compromising the metabolic activity of these cells. Furthermore, hop extract showed a decline in PGE₂ production in the COX-2 whole blood assay (WBA) with an IC₅₀ of 20.4 μg/mL, while in the COX-1 WBA no inhibition of PGE₂ production was observed. This indicates a COX-2 selective inhibition. The COX-1 inhibitor SC-560 inhibited PGE₂ production in the COX-1 WBA but not in the COX-2 WBA. At 2 μM, celecoxib inhibited PGE₂ production in the COX-2 WBA by 92 % and in the COX-1 WBA by 50 %. When hop extract was administered orally to C57BL/6 mice in which joint inflammation was induced with zymosan, PGE₂ production in ex vivo LPS-stimulated whole blood was significantly decreased by 24 %, suggesting that hop extract becomes bioavailable. Furthermore, oral administration of hop extract showed no negative or positive effects on healthy cartilage proteoglycan synthesis, or on zymosan-induced arthritic cartilage proteoglycan synthesis. However, no effect of oral administration of 1.25 mg hop extract daily was observed on joint swelling. In conclusion, this standardized CO₂ extract of Humulus lupulus could be a useful agent for intervention strategies targeting inflammatory disorders and/or inflammatory pain.

Humulus lupulus - hops - PGE₂ - cyclooxygenase - COX-1 - COX-2 - whole blood assay

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