Expression of the prostaglandin E2 (PGE2) receptor subtype EP4 and its regulation by PGE2 in osteoblastic cell lines and adult rat bone tissue

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) (EP1-4) have been cloned and characterized. EP4 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 EP4 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 EP4 messenger RNA (mRNA), Treatment of periosteal cells (RP-1) in vitro with 10(-6) mol/L PGE(2) increased the levels of both EP4 mRNA and EP4 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 EP4 in the tibia and calvaria, also peaking at 1-2 h, Using in situ hybridization, we found increased expression of EP4 in bone marrow cells of the tibial metaphysis in response to systemic PGE(2) treatment. The preosteoblastic nature of these EP4-expressing cells was suggested by the fact that dexamethasone-treated bone marrow stromal cells in culture express EP4 mRNA, which is upregulated by PGE(2). Northern blot analysis failed to detect both basal and PGE(2)-induccd EP2 mRNA in the bone samples or cell lines tested. Taken together, these data implicate EP4 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). (Bone 28: 275-281; 2001) (C) 2001 by Elsevier Science Inc, All rights reserved.