title: Prostaglandin E2 acts via bone marrow macrophages to block PTH-stimulated osteoblast differentiation in vitro.
abstract: Intermittent PTH is the major anabolic therapy for osteoporosis while continuous PTH causes bone loss. PTH acts on the osteoblast (OB) lineage to regulate bone resorption and formation. PTH also induces cyclooxygenase-2 (COX-2), producing prostaglandin E2 (PGE(2)) that can act on both OBs and osteoclasts (OCs). Because intermittent PTH is more anabolic in Cox-2 knockout (KO) than wild type (WT) mice, we hypothesized COX-2 might contribute to the effects of continuous PTH by suppressing PTH-stimulated differentiation of mesenchymal stem cells into OBs. We compared effects of continuous PTH on bone marrow stromal cells (BMSCs) and primary OBs (POBs) from Cox-2 KO mice, mice with deletion of PGE(2) receptors (Ptger(4) and Ptger(2) KO mice), and WT controls. PTH increased OB differentiation in BMSCs only in the absence of COX-2 expression or activity. In the absence of COX-2, PTH stimulated differentiation if added during the first week of culture. In Cox-2 KO BMSCs, PTH-stimulated differentiation was prevented by adding PGE(2) to cultures. Co-culture of POBs with M-CSF-expanded bone marrow macrophages (BMMs) showed that the inhibition of PTH-stimulated OB differentiation required not only COX-2 or PGE(2) but also BMMs. Sufficient PGE(2) to mediate the inhibitory effect was made by either WT POBs or WT BMMs. The inhibitory effect mediated by COX-2/PGE(2) was transferred by conditioned media from RANKL-treated BMMs and could be blocked by osteoprotegerin, which interferes with RANKL binding to its receptor on OC lineage cells. Deletion of Ptger(4), but not Ptger(2), in BMMs prevented the inhibition of PTH-stimulated OB differentiation. As expected, PGE(2) also stimulated OB differentiation, but when given in combination with PTH, the stimulatory effects of both were abrogated. These data suggest that PGE(2), acting via EP4R on BMMs committed to the OC lineage, stimulated secretion of a factor or factors that acted to suppress PTH-stimulated OB differentiation. This suppression of OB differentiation could contribute to the bone loss seen with continuous PTH in vivo.
authors: New England Musculoskeletal Institute, University of Connecticut Health Center, Farmington, CT 06030, USA.
authors: Shilpa
authors: S
authors: Choudhary
authors: Katherine
authors: K
authors: Blackwell
authors: Olga
authors: O
authors: Voznesensky
authors: Abhijit
authors: A
authors: Deb Roy
authors: Carol
authors: C
authors: Pilbeam
mesh_terms: D050156:Adipogenesis / Q000187:drug effects / Q000235:genetics; D000818:Animals; D001854:Bone Marrow Cells / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D002417:Cattle; D002454:Cell Differentiation / Q000187:drug effects* / Q000235:genetics; D002478:Cells, Cultured; D017077:Culture Media, Conditioned / Q000494:pharmacology; D051546:Cyclooxygenase 2 / Q000378:metabolism; D015232:Dinoprostone / Q000494:pharmacology*; D005786:Gene Expression Regulation / Q000187:drug effects; D006413:Hematopoietic System / Q000166:cytology; D008264:Macrophages / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D010006:Osteoblasts / Q000166:cytology* / Q000187:drug effects / Q000201:enzymology; D015675:Osteocalcin / Q000235:genetics / Q000378:metabolism; D010281:Parathyroid Hormone / Q000494:pharmacology*; D053245:RANK Ligand / Q000494:pharmacology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D058307:Receptors, Prostaglandin E, EP2 Subtype / Q000172:deficiency / Q000378:metabolism; D058309:Receptors, Prostaglandin E, EP4 Subtype / Q000172:deficiency / Q000378:metabolism; D017154:Stromal Cells / Q000166:cytology / Q000187:drug effects / Q000201:enzymology
chemical_list: D017077:Culture Media, Conditioned; D010281:Parathyroid Hormone; C545645:Ptger2 protein, mouse; C545651:Ptger4 protein, mouse; D053245:RANK Ligand; D012333:RNA, Messenger; D058307:Receptors, Prostaglandin E, EP2 Subtype; D058309:Receptors, Prostaglandin E, EP4 Subtype; D015675:Osteocalcin; C077623:Ptgs2 protein, mouse; D051546:Cyclooxygenase 2; D015232:Dinoprostone
keywords: Bone marrow stromal cells; Cyclooxygenase-2; EP2 receptor; EP4 receptor; Osteoclasts; Osteoprotegerin
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