EP2 and EP4 receptors differentially mediate MAPK pathways underlying anabolic actions of prostaglandin E2 on bone formation in rat calvaria cell cultures

Abstract

Of the four prostaglandin (PG) E receptor subtypes (EP1-EP4), EP2 and EP4 have been proposed to mediate the anabolic action of PGE(2) on bone formation but comparative evaluation studies of EPs on bone formation do not necessarily share a common mechanism, implying that their additional features including downstream MAPK pathways may be beneficial to resolve this issue. We systematically assessed the roles of EPs in the rat calvaria (RC) cell culture model by using four selective EP agonists (EPAs). Consistent with relative expression levels of the respective receptors, multiple phenotypic traits of bone formation in vitro, including proliferation of nodule-associated cells, osteoblast marker expression and mineralized nodule formation were upregulated not only by PGE(2) but equally by EP2A and EP4A, but not by EP1A and EP3A. EP2A and EP4A were effective when cells were treated chronically or pulse-treated during nascent nodule formation. EP2A and EP4A equally stimulated the endogenous PGE(2) production, while EP2A caused a greater increase in cAMP production and c-Fos gene expression compared to EP4A. EP2A and EP4A activated predominantly p38 MAPK and ERK respectively, while c-Jun N-terminal kinase (JNK) was equally activated by both agonists. SB203580 (p38 MAPK inhibitor) blocked the PGE(2) effect on mineralized nodule formation, while U0126 (ERK inhibitor) and dicumarol (JNK inhibitor) were less effective. PGE(2)-dependent phosphorylation of the MAPKs was affected not only by protein kinase (PK)A and PKC inhibitors but also by adenylate cyclase and PKC activators. Co-treatment of RC cells with EP2A or EP4A and bone morphogenetic protein (BMP)2, whose effects on bone nodule formation is known to be, in part, mediated through the PKA and p38 MAPK pathways, resulted in an additive effect on mineralized nodule formation. Further, PGE(2), EP2A and EP4A did not increase BMP2/4 mRNA levels in RC cells, and EP2-induced phosphorylation of p38 MAPK was not eliminated by Noggin. These results suggest that, in the RC cell model, the anabolic actions of PGE(2) on mineralized nodule formation are mediated at least in part by activation of the EP2 and EP4 receptor subtype-specific MAPK pathways, independently of BMP signaling, in cells associated with nascent bone nodules.

Serotonergic 5-HT2B Receptor Controls Tissue-nonspecific Alkaline Phosphatase Activity in Osteoblasts via Eicosanoids and Phosphatidylinositol-specific Phospholipase C

“…Eicosanoids are known to be local regulators of bone metabolism (37). Some pathways sustaining the anabolic action of prostaglandin EP2 and EP4 receptors have been recently uncovered (38). Our unpublished results indicate that EP4 receptors are induced along the osteogenic differentiation of C1 cells.…”

<b>Effect of Prostaglandin E₂on Human Dental Follicle Cells during Osteogenic </b><b>Differentiation </b>

“…In contrast, findings from in vitro studies utilizing hMSCs derived from bone marrow and adipose tissue suggest that PGE 2 may also have a negative influence on osteogenesis (10,19). Our findings showed that continuous treatment with showing that bone formation is enhanced by a selective EP agonists (24)and also that PGE 2 -mediated bone formation is abolished only in EP2 or EP4 knockout mice (25,26). It also suggested that PGE 2 mediated its effects through EP2 and EP4 in inhibiting matrix mineralization of hMSCs derived from bone marrow (10).…”

A Cyclooxygenase-2/Prostaglandin E2 Pathway Augments Activation-Induced Cytosine Deaminase Expression within Replicating Human B Cells

“…Two intracellular enzymes known to be downstream of PGE 2 →EP2 signaling, PKA (82) and p38 MAPK (83)(84)(85)(86), have a prior demonstrated role in promoting AID mRNA expression. Thus, PKA and downstream CREB were reported to promote IL-4-driven, STAT6-dependent AID mRNA in mouse B cells (87); in addition, p38 MAPK was found necessary for BAFF synergy with IL-4 in promoting AID mRNA synthesis in human B cells (88).…”