Effects of naturally occurring prenylated flavonoids on enzymes metabolizing arachidonic acid: Cyclooxygenases and lipoxygenases1
Introduction
Flavonoids, naturally occurring plant products, when consumed medicinally have mild if any side-effects and are, therefore, referred to as “tender drugs.” They demonstrate various pharmacological activities in vitro and in vivo including anticancer, anti-inflammatory, and antiallergic activities. Among these actions, the anti-inflammatory activity of flavonoids may be mediated by the inhibition of the AA-metabolizing enzymes, COX/LOX, as well as by their antioxidative properties.
Various flavonoids modulate the activities of these eicosanoid-generating enzymes. It was demonstrated previously that several flavonoids including luteolin, galangin and epicatechin at relatively high concentrations inhibit COX and LOX [1]. Some chalcones having a 3,4-dihydroxyl substitution inhibit COX and 12-LOX from mouse epidermis at concentrations of 0.1 to 10 μM [2]. And several flavones including chrysin, 3-hydroxyflavone and galangin have been reported to be COX inhibitors as determined by measuring TXB2 formation from A-23187-stimulated leukocytes [3]. Recently, amentoflavone (biflavone) was found to be a selective inhibitor of COX-1 from guinea pig epidermis, comparable to indomethacin [4]. Many flavonoids have been found to be LOX inhibitors. For instance, flavonols such as kaempferol, fisetin, morin, and quercetin are 5-LOX inhibitors [5]. It has also been shown that in terms of their ic50 values most flavonol derivatives were stronger inhibitors of 12-LOX than of COX-1 as measured in sheep platelet homogenates [6]. All of these previous investigations suggest that flavones are relatively selective inhibitors of COX-1, while flavonols are stronger inhibitors of LOX. There have been only a few reports studying the effects of flavonoids on COX-2, an inducible isoform of COX. It was demonstrated recently that flavan-3-ols such as (+)-catechin, 4′-O-methyl-ent-gallocatechin and mearnsitrin weakly inhibit COX-2 at pharmacologically unobtainable concentrations (>100 μM); no selectivity over COX-1 was observed [7], [8].
Prenylated flavonoids have limited distribution in the plant kingdom, i.e. the Moraceae family. Most of these plants have been used as anti-inflammatory agents in Chinese medicine, and their major constituents are various types of prenylated flavonoids [9]. These molecules are chemical entities having an isoprenyl (3,3-dimethylallyl), a geranyl (E-3,7-dimethyl-2,6-octadienyl), a 1,1-dimethylallyl, and/or a lavandulyl (5-methyl-2-isoprophenyl-hex-4-enyl) moiety as part of their flavonoid backbone structure. Hence, they usually are more hydrophobic than the conventional flavonoids, suggesting easy penetration through the cell membrane and skin barrier when used topically [10]. Thus, they may have advantages for topical use as anti-inflammatory agents if the prenylated ones have inhibitory activities on AA-metabolizing enzymes. It was reported previously that several prenylated flavonoids such as morusin, kuwanon C, kuwanon G, kuwanon H and sanggenon C inhibit COX-1 at a relatively high concentration [11], [12]. Sanggenon D was found to inhibit both COX-1 and 12-LOX. It was also demonstrated that morusin and artonin E are potent inhibitors of purified 5-, 12-, and 15-LOXs at micromolar concentrations, having the greatest potency on 5-LOX [13]. However, these previous studies did not extensively investigate the various chemically different groups of prenylated flavonoids, i.e. sophoraflavanones, kenusanones, and echinoisoflavanones. Moreover, there has been no report describing the effects of prenylated flavonoids on COX-2 activity. Therefore, in the present investigation, the effects of 19 chemically different groups of prenylated flavonoids on COX-1, COX-2, 5-LOX, and 12-LOX activities were studied and compared with the conventional flavonoids.
Section snippets
Chemicals
[1-14C]AA (54.6 mCi/mmol) was obtained from NEN. NS-398 (N-[2-cyclohexyloxy-4-nitrophenyl]methane sulfonamide) was obtained from Biomol. MTT, LPS (Escherichia coli 0127:B8), aspirin, indomethacin, and NDGA were purchased from the Sigma Chemical Co. DMEM and other cell culture reagents including FBS were products of Gibco BRL. TLC plates (20 × 20 cm) were products of the Merck Co. PGE2, PGF2α, TXB2, and 5-, 12-, and 15-HETE were the products of Cayman Chem. Apigenin and quercetin were purchased
Results
For testing the effects of prenylated flavonoids on COX-1 and on 5- and 12-LOX, the homogenates of bovine platelets and PMNs were used as the enzyme sources. For the COX-2 source, the cell homogenate of aspirin-pretreated LPS-induced RAW 264.7 macrophages was used. Under each assay condition, indomethacin (a COX-1/COX-2 inhibitor) potently inhibited the formation of TXB2 and PGE2, while NDGA (an LOX inhibitor) inhibited the formation of 12-HETE and 5-HETE, its effect being more potent on 5-HETE
Discussion
Many researchers have studied the effects of flavonoids on COXs/LOXs in the hope of finding an active ingredient in plant-derived drugs and finding lead compounds for pharmacological use. In this study, 19 prenylated flavonoid derivatives were isolated from several plant extracts and evaluated for their inhibitory activities on AA-metabolizing enzymes, including COX-2.
The prenylated flavonoids tested showed varying degrees of inhibition on COXs and LOXs depending upon their chemical structures.
Acknowledgements
This study was financially supported by Grant 98-0403-11-01-3 from the interdisciplinary research program of the KOSEF, which is gratefully acknowledged.
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2021, European Journal of PharmacologyCitation Excerpt :Leguminosae) have been traditionally used in East Asian countries (Kuroyanagi et al., 1999) as an anti-inflammatory herbal medicine for the treatment of pyogenic infection, sores, acne, enteritis, chronic bronchitis, chronic hepatitis, atopic dermatitis, and other inflammation-related diseases (He et al., 2015). Among the reported components from Sophora flavescens Ait., prenylated flavonoids are a group of unique flavonoids which have isoprenyl, geranyl, 1,1-dimethylallyl, and/or lavandulyl moiety linked to their backbone structures (Chi et al., 2001). This kind of flavonoid distributes mostly in Leguminosae or Moraceae plants (Li et al., 2020).
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Abbreviations: AA, arachidonic acid; COX, cyclooxygenase; LOX, lipoxygenase; PG, prostaglandin; TX, thromboxane; HETE, hydroxyeicosatetraenoic acid; NDGA, nordihydroguaiaretic acid; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; LPS, lipopolysaccharide; DMEM, Dulbecco’s modified Eagle’s medium; FBS, fetal bovine serum; and PMN, polymorphonuclear leukocyte.