Prostaglandins E (especially PGE 2 ) stimulate bone formation and increase bone mass in several species including man. The mechanism for this effect, the target cells, and the receptors involved are not known. Specific cell-surface receptors for PGE 2 (EP 1 - 4 ) have been cloned and characterized. EP 4 was reported to be the major receptor in embryonic and neonatal bone tissue in mice, especially in preosteoblasts; however, no data are available regarding its expression in adult bone. This study examines the expression of EP 4 in bone tissue of young adult rats, in which PGE 2 is markedly anabolic, and in various osteoblastic cell lines. Using northern blot analysis, we found that osteoblastic cell lines RCT-1, RCT-3, TRAB-11, and RP-1, primary osteoblastic cells harvested from fetal rat calvaria, as well as tibiae and calvariae of 5-week-old rats express 3.8 kb EP 4 messenger RNA (mRNA). Treatment of periosteal cells (RP-1) in vitro with 10 - 6 mol/L PGE 2 increased the levels of both EP 4 mRNA and EP 4 protein, peaking at 1-2 h. Similarly, systemic administration of an anabolic dose of PGE 2 (3-6 mg/kg) to young adult rats upregulated the expression of EP 4 in the tibia and calvaria, also peaking at 1-2 h. Using in situ hybridization, we found increased expression of EP 4 in bone marrow cells of the tibial metaphysis in response to systemic PGE 2 treatment. The preosteoblastic nature of these EP 4 -expressing cells was suggested by the fact that dexamethasone-treated bone marrow stromal cells in culture express EP 4 mRNA, which is upregulated by PGE 2 . Northern blot analysis failed to detect both basal and PGE 2 -induced EP 2 mRNA in the bone samples or cell lines tested. Taken together, these data implicate EP 4 as the major cyclic AMP-related PGE 2 receptor subtype expressed in bone tissue and osteoblastic cells and indicate that this receptor is upregulated by its ligand, PGE 2 .