J Pharmacol Sci 96, 000 – 000 (2004) Journal of Pharmacological Sciences ©2004 The Japanese Pharmacological Society Critical Review Anti-inflammatory Plant Flavonoids and Cellular Action Mechanisms Hyun Pyo Kim1,*, Kun Ho Son2, Hyeun Wook Chang3, and Sam Sik Kang4 1College of Pharmacy, Kangwon National University, Chunchon 200-701, Korea 2Department of Food and Nutrition, Andong National University, Andong 760-749, Korea 3Collge of Pharmacy, Yeungnam University, Gyongsan 712-749, Korea 4Natural Products Research Institute, Seoul National University, Seoul 110-460, Korea Received September 6, 2004 Abstract. Plant flavonoids show anti-inflammatory activity in vitro and in vivo. Although not fully understood, several action mechanisms are proposed to explain in vivo anti-inflammatory action. One of the important mechanisms is an inhibition of eicosanoid generating enzymes including phospholipase A2, cyclooxygenases, and lipoxygenases, thereby reducing the concen- trations of prostanoids and leukotrienes. Recent studies have also shown that certain flavonoids, especially flavone derivatives, express their anti-inflammatory activity at least in part by modu- lation of proinflammatory gene expression such as cyclooxygenase-2, inducible nitric oxide synthase, and several pivotal cytokines. Due to these unique action mechanisms and significant in vivo activity, flavonoids are considered to be reasonable candidates for new anti-inflammatory drugs. To clearly establish the therapeutic value in inflammatory disorders, in vivo anti-inflam- matory activity, and action mechanism of varieties of flavonoids need to be further elucidated. This review summarizes the effect of flavonoids on eicosanoid and nitric oxide generating enzymes and the effect on expression of proinflammatory genes. In vivo anti-inflammatory activity is also discussed. As natural modulators of proinflammatory gene expression, certain flavonoids have a potential for new anti-inflammatory agents. Keywords: flavonoid, inflammation, gene expression, phospholipase, cyclooxygenase Inflammation and flavonoids research candidates is plant constituents used in Chinese medicine. Inflammation is clinically defined as a pathophysio- Among many different groups of natural products, logical process characterized by redness, edema, fever, flavonoids, are a group of chemical entities of benzo-- pain, and loss of function. Although the currently used pyrone derivatives widely distributed in the Plant steroidal anti-inflammatory drugs (SAID) and non- Kingdom. They are mainly classified as chalcones, steroidal anti-inflammatory drugs (NSAID) treat acute flavan-3-ols, flavanones, flavones and flavonols, iso- inflammatory disorders, these conventional drugs have flavones, and biflavonoids (Fig. 1). They have relatively not been successful to cure chronic inflammatory dis- simple chemical structures, but more than 4,000 deriva- orders such as rheumatoid arthritis (RA) and atopic tives have been reported from nature, indicating their dermatitis (AD). Since the critical etiology and exacer- chemical diversities. bating mechanisms are not completely understood, it is Flavonoids, also known as nature’s tender drugs, difficult to develop a magic bullet for chronic inflam- possess various biological/pharmacological activities matory disorders. Therefore, there is a need for new and including anticancer, antimicrobial, antiviral, anti- safe anti-inflammatory agents and one of the ongoing inflammatory, immunomodulatory, and antithrombotic activities (1). Of these biological activities, the anti- inflammatory capacity of flavonoids has long been *Corresponding author. FAX: +82-33-255-9271 utilized in Chinese medicine and the cosmetic industry E-mail: [email protected] as a form of crude plant extracts. Many investigations Invited article have proven that varieties of flavonoid molecules 1 2 HP Kim et al Fig. 1. The representative flavonoids in nature. Anti-inflammatory Flavonoids 3 possess anti-inflammatory activity on various animal Cellular action mechanisms models of inflammation. Especially, some flavonoids were found to inhibit chronic inflammation of several The effect on PLA2 experimental animal models. Thus, it may be valuable The inhibitory activity of several flavonoid deriva- to continuously evaluate the anti-inflammatory activity tives against AA metabolizing enzymes was initially of flavonoids, not only for establishing anti-inflam- reported in 1980 (3). Thereafter, numerous investigators matory mechanisms, but also for developing a new class have studied the inhibitory effect of flavonoids on these of anti-inflammatory agents. enzymes. AA (a precursor of eicosanoids) is released There have been several proposed cellular action mostly from membrane lipids in cells. The enzyme mechanisms explaining in vivo anti-inflammatory acti- responsible for this release is PLA2, although some vity of flavonoids. They possess antioxidative and portion is attributed to the combined action of phospho- radical scavenging activities. They could regulate lipase C and diacylglycerol lipase. Up to date, many cellular activities of the inflammation-related cells: isoforms of PLA2 have been discovered (4). They are mast cells, macrophages, lymphocytes, and neutrophils. mainly classified into three large categories, secretory For instance, some flavonoids inhibit histamine release PLA2 (sPLA2), cytosolic PLA2 (cPLA2), and calcium- from mast cells and others inhibit T-cell proliferation. independent PLA2 (iPLA2). These PLA2s are distributed These properties of flavonoids have been recently in wide varieties of tissues and cells. In some conditions, summarized (2). In addition, certain flavonoids modu- they are coupled to COXs depending on the cells and late the enzyme activities of arachidonic acid (AA) agonists used (4). For instance, group IIA sPLA2 was metabolizing enzymes such as phospholipase A2 (PLA2), found in arthritic synovial fluid, and group IV cPLA2 are cyclooxygenase (COX), and lipoxygenase (LOX) and coupled to COXs and 5-LOX to produce eicosanoids. the nitric oxide (NO) producing enzyme, nitric oxide On the other hand, group VI iPLA2 is thought to serve a synthase (NOS). An inhibition of these enzymes by housekeeping role in phospholipid remodeling. There- flavonoids reduces the production of AA, prostaglandins fore, a modulation of sPLA2 and/or cPLA2 activity is (PG), leukotrienes (LT), and NO, crucial mediators of important to control the inflammatory process. inflammation. Thus, the inhibition of these enzymes The first flavonoid inhibitor of PLA2 found was exerted by flavonoids is definitely one of the important quercetin, which inhibited PLA2 from human neutro- cellular mechanisms of anti-inflammation. Furthermore, phils (5). Quercetn was repeatedly found to inhibit in recent years, many lines of evidence support the PLA2 from several sources. It inhibited PLA2 from idea that certain flavonoids are the modulators of gene rabbit peritoneal neutrophils with an IC50 of 57 – 100 expression, especially the modulators of proinflam- M (6). It was also demonstrated that quercetin selec- matory gene expression, thus leading to the attenuation tively inhibited group II sPLA2 from Vipera russelli of the inflammatory response. At present, it is not known with less inhibition of PLA2 from porcine pancreas, to what extent these proinflammatory gene expressions PLA2-IB (7). While flavanones including flavanone, contribute to the inflammatory response. However, it is hesperetin, and naringenin showed less inhibition, evident that flavonoids show anti-inflammatory activity, flavonols such as kaempferol, quercetin, and myricetin at least in part, by the suppression of these proinflam- were found to considerably inhibit snake venom PLA2, matory gene expressions. indicating an importance of the C-ring-2,3-double bond In the present review, we have summarized the (8). The IC50 values of these flavonols were 75 – findings of anti-inflammatory flavonoid research. 115 M, not easily obtainable concentrations in the Especially, this review is focused on two most important body even by pharmacological treatment. topics: the effect on AA metabolizing enzymes and NOS On the other hand, several polyhydroxylated flavo- and the effect on expression of pivotal proinflammatory noids were found to strongly inhibit group II human enzymes/cytokines. In vivo anti-inflammatory activity recombinant PLA2 with less inhibition against Naja naja of flavonoids is also discussed, but the anti-inflam- PLA2, PLA2-IIB (9). The IC50 values of quercetagetin, matory properties of tannins, anthocyanins, and sily- kaempferol-3-galactoside, and scutellarein (Fig. 2) are marin are not discussed because the chemistry and 10 – 30 M. Along with these flavonoids, the most biological activity of tannins and anthocyanins are quite potent flavonoid inhibitors of PLA2-IIA so far being different from the conventional flavonoids, and sily- found are biflavonoids. Several biflavonoids such as marin is not a true flavonoid, but a flavonolignan. ochnaflavone, amentoflavone, ginkgetin, and iso- ginkgetin were for the first time revealed to inhibit sPLA2-IIA from rat platelets at micromolar concentra- tions with some selectivity over PLA2-IB (10). The 4 HP Kim et al Fig. 2. Some flavonoids acting on eicosanoid and NO generating enzymes. IC50 values were within 10 M. Ochnaflavone inhibited (unpublished results). sPLA2-IIA noncompetitively. The observation that All these findings have shown that certain biflavo- another biflavonoid, morelloflavone, possessed