Introduction: Bone morphogenetic protein (BMP)-2 is a potent stimulator of new bone formation with a potential role in dental regenerative therapies and improving dental implants. There are studies showing that effects ofBMP-2 on bone formation may be mediated in part by prostaglandins (PGs) [1] or enhanced by exogenous addition of PGs (Sasaoka, et al, BBRC 2004; Inoue et al, Bone 1996). Since prostaglandin E2 (PGE2) is a potent stimulator of osteoclast (OC) formation and since BMP-2 at high doses can induce cyclooxygenase-2 (COX-2) expression and PGE2 production, we examined the effects ofBMP-2 alone and in combination with PGE2 on osteoclast formation in vitro. Materials and Methods: Bone marrow (BM) was flushed from C57B1/6 mouse tibia, plated in 48, 12 and 6 well plates at 1X106 cells/cm2, and cultured for 6-10 d. Treatment groups (n=3 wells) were control (vehicle), PGE2 (1 gM), BMP-2 (100 ng/ml), and PGE2 + BMP-2. Osteoclasts, defined as multinucleated cells (MNCs) (>3 nuclei/cell) that stained positive for tartrate resistant acid phosphase (TRAP), were counted, mRNA levels for receptor activator ofNFb ligand (RANKL), which is necessary for osteoclast differentiation, and for osteoprotegrin (OPG), the decoy receptor for RANKL that inhibits osteoclast formation, were measured by quantitative (q) PCR. Results: In 12-well plates, no OCs formed in control cultures and only a few in BMP-2 treated groups (range 0-2). PGE2 and PGE2+BMP-2 both caused similar significant increases (p<0.05) in OC counts on days 7 (24.3+7.4 and 14.7+/- 11.7) and 8 (42.0+5.3 and 48.7+4.5), respectively. On day 9, however, the combination ofPGE2+BMP-2 (76.3+20.5) showed a significant synergistic increase in number of OC cells compared to PGE2 (21.3+/-3.3) alone. There was a significant increase in RANKL mRNA on day 7 (2-fold) and a significant decrease in OPG mRNA on days 7 (50%), 8 (75%) and 9 (90%) in the PGE2+BMP-2 group compared to the PGE2 treatment group. The ratio of RANKL to OPG mRNA was significantly higher by 4-times and 43-times on days 7 and 8, respectively, for the combination group. Similar results were observed in 6-well and 48-well BM cell cultures. Conclusion: We conclude that BMP-2 alone did not increase OC, but did increase OC formation in the presence of exogenous PGE2. The effect of the BMP-2+PGE2 combination correlated with an increase in RANKL and a decrease in OPG expression. It is possible that the increased activation of resorption in vivo by PGE2 will permit BMP-2 to have a greater anabolic effect by increasing the number of remodeling sites at which BMP-2 can act.
