Prostaglandin E-2 directly inhibits bone-resorbing activity of isolated mature osteoclasts mainly through the EP4 receptor

Prostaglandins (PGs) are well known to be important local factors in regula ting bone formation and resorption. PGE(2) is a potent stimulator of bone r esorption because of enhancing osteoclast formation by its indirect action through stromal cells. However, the direct action of PGE(2) on functionally mature osteoclasts is still controversial. In this study using highly puri fied rabbit mature osteoclasts, we examined the direct effect of PGE(2) on osteoclastic bone-resorbing activity and its mechanism. PGE(2) inhibited re sorption pit formation on a dentine slice by the purified osteoclasts in a dose- and time-dependent manner. The inhibitory effect appeared as early as 4 hours after the PGE(2) addition. Forskolin and 12-0-tetradecanoyl phorbo l-13-acetate (TPA), respective activators of adenylate cyclase and protein kinase C, also decreased the osteoclastic bone-resorbing activity. PGE(2) i ncreased the content of intracellular cAMP in a dose range effective for th e inhibition of bone resorption, whereas the prostanoid did not alter the i ntracellular level of inositol triphosphate. The inhibition of osteoclastic bone resorption by PGE(2) was amplified and diminished by a cAMP phosphodi esterase inhibitor (isobutyl methylxanthine) and a protein kinase A inhibit or (Rp-cAMP), respectively. Of four different subtypes of PGE(2) receptors (EPs), EP4 mRNA was predominantly expressed in isolated osteoclasts, wherea s the other types of EP mRNA were detected in only small amounts. These res ults suggest that the PGE(2) inhibitory effect was mediated by an adenylate cyclase system coupled with EP4, This possible association of PGE(2) with EP4 in mature osteoclasts was supported by the finding that a specific agon ist of EP4 (AE-604) inhibited the bone-resorbing activity and elevated the intracellular cAMP content. However, butaprost, a selective EP2 agonist, al so mimicked the PGE(2) effects on isolated osteoclasts although EP2 mRNA ex pression was minimal. In conclusion, PGE(2) directly inhibits bone-resorbin g activity of functionally mature osteoclasts by activation of the adenylat e cyclase system, perhaps mainly through EP4.