Propolis Blocks Osteoclastogenesis Downstream of the Cyclooxygenase

Objective: Previous studies have indicated that propolis exerts biological effects by inhibiting expression and/or activity of cyclooxygenases. This study examines the effects of an ethanol extract of Brazilian propolis on osteoclast formation and activity, and whether the inhibition of cyclooxygenase (COX) activity decreases osteoclast maturation. Methods: Mouse marrow was stimulated to generate osteoclasts by treatment with calcitriol. An EP4 agonist was used to bypass the cyclooxygenase pathway. Osteoclasts were identified by staining for tartrate-resistant acid phosphatase (TRAP) activity. Osteoclast activity was monitored by formation of actin rings and by the formation of pits on bone. Results: Consistent with previous reports we found that osteoclastogenesis stimulated by calcitriol in mouse marrow was sharply reduced in the presence of COX inhibitors and that simultaneous treatment with prostaglandin or an EP4 agonist rescued osteoclast formation in the presence of the COX inhibitor. Propolis reduced calcitriol-stimulated osteoclastogenesis (P<0.05). Treatment of cultures with prostaglandin or EP4 agonist failed to rescue osteoclast formation in the presence of propolis. Treatment of pure osteoclast precursors with propolis reduced the growth rate of the cells and reduced giant cell formation induced by recombinant RANKL (Anova, p<0.05). Treatment of resorbing osteoclasts on bone slices for 6H with propolis sharply reduced the number of cells exhibiting actin rings, a marker for osteclastic bone resorption. Pit formation was inhibited by propolis when mature mouse marrow osteoclast cultures on bone slices were treated with propolis during 3 Days of culture. Conclusions: These data indicate that propolis contains elements that block osteoclastogenesis and osteoclast activity downstream of COX activity. Propolis affects the cytoskeletal organization of mature resorbing osteoclasts. Future studies will seek the specific molecule(s) from propolis that are responsible for the observed effects on osteoclast function