Polymethoxyflavones and Bone Metabolism

Abstract

Phytochemicals, such as flavonoids, are bioactive compounds produced by plants, including citrus fruits, that exhibit antioxidant effects on mammalian cells and tissues. Polymethoxyflavones (PMFs) are a family of flavonoids found in the pulp and peel of citrus fruits, and have been reported to have potent antioxidant activity implicated in the prevention of human diseases. Several studies have shown that PMFs have a protective effect on bone resorption in mouse models of diseases, including osteoporosis, rheumatoid arthritis, and periodontal disease. PMFs significantly suppressed the differentiation of osteoclasts (bone resorptive cells) through indirect and direct mechanisms. The indirect effect of PMFs is the suppression of inflammatory mediator production, such as prostaglandin E2 (PGE2), and the reduction of osteoclastic inducers, such as the receptor activator of NF-κB ligand (RANKL), in osteoblasts (bone-forming cells). The direct effect of PMF suppresses osteoclast differentiation and function by inhibiting the NF-κB signaling pathway. In silico molecular docking studies indicated that PMFs target the ATP-binding pocket of IKKβ and inhibit the NF-κB signaling pathway. These findings suggest that PMFs protect against bone destruction by interfering with the NF-κB pathway in osteoblasts and osteoclasts. In this review, we summarize the latest findings regarding the effects of PMFs on various bone resorption-related diseases in mouse models.

Keywords: bone metabolism; bone resorption; osteoblast; osteoclast; polymethoxyflavone; prostaglandin E2.

Figure 1

Chemical structures of PMFs. The flavone backbone (A). Several PMFs were identified, as follows: (B) Tangeretin. (C) Sinensetin. (D) Nobiletin. (E) Heptamethoxyflavone. (F) Sudachitin. (G) Syringetin. (H) 4′-Demethyltangeretin. (I) 4′-Demethylnobiletin.

Figure 2

Bone resorptive molecules stimulate PGE₂-mediated bone resorption. Several molecules (e.g., LPS, IL-1, and TNFα) activate the NF-κB pathway in osteoblasts. NF-κB transcriptionally upregulates PGE synthases, including COX-2 and mPGES-1, leading to the production of PGE₂. Subsequently, the autocrine/paracrine effect of PGE₂ via EP4 enhances the expression of RANKL, leading to osteoclast differentiation.

Figure 3

Effects of 4′-demethylnobiletin and 4′-demethyltangeretin on osteoclast differentiation. Images show TRAP-positive osteoclasts. The compounds 4′-DN and 4′-DT exhibited potent inhibitory effects on IL-1-induced osteoclast differentiation relative to nobiletin and tangeretin, respectively. These data were obtained from a previous study: Hirata M, Tominari T, and Inada M, et al. Effects of 4′-Demethylnobiletin and 4′-Demethyltangeretin on Osteoclast Differentiation In Vitro and in a Mouse Model of Estrogen-Deficient Bone Resorption [10].

Figure 4

Docking simulation between IKKβ protein and 4′-DN or 4′-DT. In silico molecular docking test was performed using AutoDock Vina. Three-dimensional docking models of IKKβ protein and 4′-DN (upper) or 4′-DT (lower). These data were obtained from a previous report: Hirata M, Tominari T, and Inada M, et al. Effects of 4′-Demethylnobiletin and 4′-Demethyltangeretin on Osteoclast Differentiation In Vitro and a Mouse Model of Estrogen-Deficient Bone Resorption [10].

Figure 5

Schematic model for the inhibition of osteoclast differentiation by PMFs. Hydrophobic PMFs can cross the cell membrane and bind to IKKβ protein, blocking the NF-κB pathway. Thus, PMFs attenuated NF-κB-driven expression of inflammatory cytokines and PGE synthases, thereby decreasing the expression of RANKL.