Radical-scavenging and Anti-inflammatory Activity of Quercetin and Related Compounds and Their Combinations Against RAW264.7 Cells Stimulated with Porphyromonas gingivalis

Background/Aim: The flavonoid quercetin exerts significant anti-inflammatory activity against chronic infections, including periodontal disease. However, it is unclear whether combination of quercetin with other flavonoids enhances antioxidant and anti-inflammatory activity. To clarify the molecular mechanism responsible for the anti-inflammatory activity of quercetin, we investigated the antioxidant, cytotoxicity and anti-inflammatory activity of quercetin and its related compounds, catechin and epicatechin, and their combinations. Materials and Methods: Radical-scavenging activities were determined by the 1,1diphenyl-2-picrylhydrazyl (DPPH) assay, and cytotoxicity against RAW264.7 cells was determined using a cell counting kit (CCK-8). The inhibitory effects of these compounds on the mRNA expression of cyclooxygenase-2 (Cox2), tumor necrosis factor-alpha (Tnfα) and nitric oxide synthase 2 (Nos2), in RAW264.7 cells stimulated with Porphyromonas gingivalis (Pg) fimbriae, was also determined using real-time polymerase chain reaction analysis. The phenolic O-H bond dissociation enthalpy (BDE) and quantum chemical parameters were calculated on the basis of density function theory (DFT) BLYP/6-31G*. Results: The DPPH radical-scavenging activity (EC50) of quercetin, catechin and epicatechin was 5.5, 7.7 and 6.2 μM, respectively, whereas the cytotoxicity (LC50) was 4.45, 4.80 and 4.95 mM, respectively. Quercetin had slightly higher cytotoxicity and anti-DPPH activity than catechin and epicatechin. The BDE for the three flavonoids at the 4'-OH in the B ring, which is the initial active site, was about 75 kcal/mol. Furthermore, various combinations of quercetin with catechin or epicatechin exerted an antagonistic effect on anti-DPPH activity. Gene expression of Cox2, Tnfα and Nos2 stimulated by exposure to Pg-fimbriae was markedly suppressed by quercetin, but was not modulated by its combination with epicatechin. The 50% inhibitory concentration of quercetin for Cox2 expression was approximately 10 μM, while that of catechin and epicatechin was approximately 500 μM. Values of the quantum chemical parameters softness (σ) and electronegativity (χ) were highest for quercetin among the three flavonoids tested. Conclusion: The potent anti-inflammatory activity of quercetin appears to be attributable to its high σ and χ values. Quercetin may be applicable as a preventive agent against inflammatory periodontal disease as a manifestation of systemic disease. Conditions such as diabetes mellitus (1), cardiovascular disease (2) and preterm low birth weight (3) are associated with an increased risk of chronic infections such as periodontitis. Porphyromonas gingivalis (Pg) is a key bacterium strongly associated with early-onset, progressive and refractory periodontal disease and subsequent alveolar bone loss. Pg-fimbriae, in addition to lipopolysaccharide (LPS), are also involved in the pathogenesis of periodontal disease through production of various pro-inflammatory cytokines, which promote the differentiation of osteoclasts (4-6). Pg-fimbriae are peritrichous filamentous appendages, 701 Correspondence to: Dr. Yukio Murakami, Division of Oral Diagnosis and General Dentistry, Department of Diagnostic and Therapeutic Sciences, Meikai University School of Dentistry, 1-1 Keyakidai, Sakado City, Saitama 350-0283, Japan. Tel: +81 492855511, Fax: +81 492876657, e-mail: [email protected]