NEW EXPERIMENTAL SYSTEM : CO-EXISTANCE OF BONE FORMATION AND RESORPTION IN VITRO

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Administration of prostaglandin E_2 (PGE_2) has anabolic effects in bone, we examined the effects of PGE_2 on bone-like mineralized tissue formation in rat bone marrow stromal cell culture. Addition of PGE_2 at early stage of culture increased the amount of mineralized tissue whereas addition of PGE_2 at later stage, just before mineralization, inhibited mineralized tissue formation, indicating that effects of PGE_2 is depending on the stage of osteoblastic differentiation. Then, we tried to find the signal transduction pathway of the anabolic effect of PGE_2 at the early stage of this culture system, and our experiment revealed the involvement of cAMP-dependent signal transduction in this phenomenon. On the other hand, PGE_2 increases intracellular Ca, and stimulates or inhibits cAMP production through PGE receptor subtypes ; EP_1, EP_2 and EP_3, respectively. These receptor subtypes have been recently cloned. We investigated the expression of these receptor subtypes in bone tissue. RT-PCR revealed that EP_1, EP_2 and EP_3 were expressed in rat calvaria and that osteoblastic cells (MC3T3-E1) expressed EP_1 and EP_2. In situ hybridization analysis using cryosection of neonatal calvariae revealed that EP_2 was expressed by osteoblasts and cells not in contact with bone, probably including preosteoblasts. EP_2 expression was observed at an early stage in calvarial development, and also in rat bone marrow cell culture. It has been established that PGE_2 response accompanying cAMP production is one of the characteristics of osteoblasts. Our results indicate that this phenotype appears at an early stage of osteoblastic differentiation and bone development.