Effects of selective prostaglandins E2 receptor agonists on cultured calvarial murine osteoblastic cells

G. AR, Al Kawas S, Samsudin AR, A. C. SA. Impact of Bone Morphogenetic Protein 7 and Prostaglandin receptors on osteoblast healing and organization of collagen

PURPOSE

This study seeks to investigate the impact of co-administering either a Prostaglandin EP2 receptor agonist or an EP1 receptor antagonist alone with a low dose BMP7 on in vitro healing process, collagen content and maturation of human osteoblasts.

METHODOLOGY

Human osteoblast cells were used in this study. These cells were cultured and subjected to different concentrations of Prostaglandin EP2 receptor agonist, EP1 receptor antagonist, BMP7, Control (Ct) (Vehicle alone), and various combinations treatments. Cell viability at 24, 48 and 72 hours (h) was evaluated using the XTT assay. A wound healing assay was conducted to observe the migration ability of human osteoblast cells. Additionally, Sirius red staining and Fourier-Transform Infrared Spectroscopy Imaging (FT-IR) was employed to analyze various parameters, including total protein concentration, collagen production, mature collagen concentration, and mineral content.

RESULTS

The combination of low dose BMP7 and Prostaglandin EP2 receptor agonist resulted to the lowest cell viability when compared to both the Ct and individual treatments. In contrast, the Prostaglandin EP1 receptor antagonist alone showed the highest cellular viability at 72 h. In the wound healing assay, the combined treatment of low dose BMP7 with the Prostaglandin EP2 receptor agonist and EP1 receptor antagonist showed a decrease in human osteoblast healing after 24 h. Analysis of FT-IR data indicated a reduction in total protein content, collagen maturity, collagen concentration and mineral content in combination treatment compared to the single or Ct treatments.

CONCLUSION

The combination of a Prostaglandin EP2 receptor agonist or an EP1 receptor antagonist when combined with low dose BMP7 significantly hinders both human osteoblast healing and collagen maturity/concentration in comparison to low dose BMP7 treatment alone.

Increased Osteoblast GαS Promotes Ossification by Suppressing Cartilage and Enhancing Callus Mineralization During Fracture Repair in Mice

GαS, the stimulatory G protein α-subunit that raises intracellular cAMP levels by activating adenylyl cyclase, plays a vital role in bone development, maintenance, and remodeling. Previously, using transgenic mice overexpressing GαS in osteoblasts (GS-Tg), we demonstrated the influence of osteoblast GαS level on osteogenesis, bone turnover, and skeletal responses to hyperparathyroidism. To further investigate whether alterations in GαS levels affect endochondral bone repair, a postnatal bone regenerative process that recapitulates embryonic bone development, we performed stabilized tibial osteotomy in male GS-Tg mice at 8 weeks of age and examined the progression of fracture healing by micro-CT, histomorphometry, and gene expression analysis over a 4-week period. Bone fractures from GS-Tg mice exhibited diminished cartilage formation at the time of peak soft callus formation at 1 week post-fracture followed by significantly enhanced callus mineralization and new bone formation at 2 weeks post-fracture. The opposing effects on chondrogenesis and osteogenesis were validated by downregulation of chondrogenic markers and upregulation of osteogenic markers. Histomorphometric analysis at times of increased bone formation (2 and 3 weeks post-fracture) revealed excess fibroblast-like cells on newly formed woven bone surfaces and elevated osteocyte density in GS-Tg fractures. Coincident with enhanced callus mineralization and bone formation, GS-Tg mice showed elevated active β-catenin and Wntless proteins in osteoblasts at 2 weeks post-fracture, further substantiated by increased mRNA encoding various canonical Wnts and Wnt target genes, suggesting elevated osteoblastic Wnt secretion and Wnt/β-catenin signaling. The GS-Tg bony callus at 4 weeks post-fracture exhibited greater mineral density and decreased polar moment of inertia, resulting in improved material stiffness. These findings highlight that elevated GαS levels increase Wnt signaling, conferring an increased osteogenic differentiation potential at the expense of chondrogenic differentiation, resulting in improved mechanical integrity. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Prostate cancer expressing membrane-bound TGF-α induces bone formation mediated by the autocrine effect of prostaglandin E2 in osteoblasts

Prostate cancer highly metastasizes to bone, and such cancer is associated with severe bone resorption and bone formation at the site of metastasis. Prostaglandin E₂ (PGE₂) promotes bone resorption in inflammatory diseases; however, the roles in prostate cancer-induced bone formation are still unclear. In the present study, we investigated the effects of membrane-bound TGF-α on prostate cancer-induced bone formation through autocrine PGE₂ signaling in osteoblasts. In the prostate cancer explant experiment into tibiae, injected prostate cancer cells induced bone formation with the increased expression of osteogenic genes, such as Runx2 and Wnt5a, and prostaglandin synthase Ptgs2. In osteoblasts, PGE₂ increased the number of calcified bone nodules with enhanced expression of Runx2 and Wnt5a. We also screened the factors involved in cancer progression, and 11 EGF family members were found to be expressed in the human prostate cancer cell line PC3. PC3 highly expressed amphiregulin, HB-EGF, and especially TGF-α. Treatment with recombinant TGF-α increased the Ptgs2 expression and PGE₂ production in osteoblasts, which promoted the formation of calcified bone nodules, suggesting that the interaction between PC3 and osteoblasts promoted PGE₂ production. In co-culture of osteoblasts and fixed PC3 cells, the phosphorylation of EGFR and ERK and subsequent Ptgs2 expression and PGE₂ production were increased, an effect that was attenuated by treatment with inhibitors of EGFR and ERK. These results indicate that membrane-bound TGF-α enhances ERK signaling while also inducing PGE₂-mediated bone formation in osteoblasts, thus suggesting that prostate cancer regulates both PGE₂-mediated bone resorption and bone formation at the site of bone metastasis of prostate cancer.

Ca2+-independent phospholipase A2β-derived PGE2 contributes to osteogenesis

Bone modeling can be modulated by lipid signals such as arachidonic acid (AA) and its cyclooxygenase 2 (COX2) metabolite, prostaglandin E₂ (PGE₂), which are recognized mediators of optimal bone formation. Hydrolysis of AA from membrane glycerophospholipids is catalyzed by phospholipases A₂ (PLA₂s). We reported that mice deficient in the Ca²⁺- independent PLA₂beta (iPLA₂β), encoded by Pla2g6, exhibit a low bone phenotype, but the cause for this remains to be identified. Here, we examined the mechanistic and molecular roles of iPLA₂β in bone formation using bone marrow stromal cells and calvarial osteoblasts from WT and iPLA₂β-deficient mice, and the MC3T3-E1 osteoblast precursor cell line. Our data reveal that transcription of osteogenic factors (Bmp2, Alpl, and Runx2) and osteogenesis are decreased with iPLA₂β-deficiency. These outcomes are corroborated and recapitulated in WT cells treated with a selective inhibitor of iPLA₂ β (10 μM S-BEL), and rescued in iPLA₂β-deficient cells by additions of 10 μM PGE₂. Further, under osteogenic conditions we find that PGE₂ production is through iPLA₂β activity and that this leads to induction of Runx2 and iPLA₂β transcription. These findings reveal a strong link between osteogenesis and iPLA₂β-derived lipids and raise the intriguing possibility that iPLA₂β-derived PGE₂ participates in osteogenesis and in the regulation of Runx2 and also iPLA₂β.

Complexity of enthesitis and new bone formation in ankylosing spondylitis: current understanding of the immunopathology and therapeutic approaches

Despite increasing availability of treatments for spondyloarthritis (SpA) including tumour necrosis factor (TNF) and interleukin-17 (IL-17) inhibitors, there is no established treatment that abates new bone formation (NBF) in ankylosing spondylitis (AS), a subset of SpA. Recent research on TNF has revealed the increased level of transmembrane TNF in the joint tissue of SpA patients compared to that of rheumatoid arthritis patients, which appears to facilitate TNF-driven osteo-proliferative changes in AS. In addition, there is considerable interest in the central role of IL-23/IL-17 axis in type 3 immunity and the therapeutic potential of blocking this axis to ameliorate enthesitis and NBF in AS. AS immunopathology involves a variety of immune cells, including both innate and adoptive immune cells, to orchestrate the immune response driving type 3 immunity. In response to external stimuli of inflammatory cytokines, local osteo-chondral progenitor cells activate intra-cellular anabolic molecules and signals involving hedgehog, bone morphogenetic proteins, receptor activator of nuclear factor kappa-B ligand, and Wnt pathways to promote NBF in AS. Here, we provide an overview of the current immunopathology and future directions for the treatment of enthesitis and NBF associated with AS.

Juvenile Spondyloarthritis: What More Do We Know About HLA-B27, Enthesitis, and New Bone Formation?

Juvenile spondyloarthritis (JSpA) refers to a diverse spectrum of immune-mediated inflammatory arthritides whose onset occurs in late childhood and adolescence. Like its adult counterpart, JSpA is typified by a strong association with human leukocyte antigen-B27 (HLA-B27) and potential axial involvement, while lacking rheumatoid factor (RF) and distinguishing autoantibodies. A characteristic manifestation of JSpA is enthesitis (inflammation of insertion sites of tendons, ligaments, joint capsules or fascia to bone), which is commonly accompanied by bone resorption and new bone formation at affected sites. In this Review, advances in the role of HLA-B27, enthesitis and its associated osteoproliferation in JSpA pathophysiology and treatment options will be discussed. A deeper appreciation of how these elements contribute to the JSpA disease mechanism will better inform diagnosis, prognosis and therapy, which in turn translates to an improved quality of life for patients.

International Union of Basic and Clinical Pharmacology. CIX. Differences and Similarities between Human and Rodent Prostaglandin E2 Receptors (EP1-4) and Prostacyclin Receptor (IP): Specific Roles in Pathophysiologic Conditions

Prostaglandins are derived from arachidonic acid metabolism through cyclooxygenase activities. Among prostaglandins (PGs), prostacyclin (PGI2) and PGE2 are strongly involved in the regulation of homeostasis and main physiologic functions. In addition, the synthesis of these two prostaglandins is significantly increased during inflammation. PGI2 and PGE2 exert their biologic actions by binding to their respective receptors, namely prostacyclin receptor (IP) and prostaglandin E2 receptor (EP) 1-4, which belong to the family of G-protein-coupled receptors. IP and EP1-4 receptors are widely distributed in the body and thus play various physiologic and pathophysiologic roles. In this review, we discuss the recent advances in studies using pharmacological approaches, genetically modified animals, and genome-wide association studies regarding the roles of IP and EP1-4 receptors in the immune, cardiovascular, nervous, gastrointestinal, respiratory, genitourinary, and musculoskeletal systems. In particular, we highlight similarities and differences between human and rodents in terms of the specific roles of IP and EP1-4 receptors and their downstream signaling pathways, functions, and activities for each biologic system. We also highlight the potential novel therapeutic benefit of targeting IP and EP1-4 receptors in several diseases based on the scientific advances, animal models, and human studies. SIGNIFICANCE STATEMENT: In this review, we present an update of the pathophysiologic role of the prostacyclin receptor, prostaglandin E2 receptor (EP) 1, EP2, EP3, and EP4 receptors when activated by the two main prostaglandins, namely prostacyclin and prostaglandin E2, produced during inflammatory conditions in human and rodents. In addition, this comparison of the published results in each tissue and/or pathology should facilitate the choice of the most appropriate model for the future studies.

Prostaglandins and Bone

Prostaglandins (PGs) are highly bioactive fatty acids. PGs, especially prostaglandin E₂ (PGE₂), are abundantly produced by cells of both the bone-forming (osteoblast) lineage and the bone-resorbing (osteoclast) lineage. The inducible cyclooxygenase, COX-2, is largely responsible for most PGE₂ production in bone, and once released, PGE₂ is rapidly degraded in vivo. COX-2 is induced by multiple agonists - hormones, growth factors, and proinflammatory factors - and the resulting PGE₂ may mediate, amplify, or, as we have recently shown for parathyroid hormone (PTH), inhibit responses to these agonists. In vitro, PGE₂ can directly stimulate osteoblast differentiation and, indirectly via stimulation of RANKL in osteoblastic cells, stimulate the differentiation of osteoclasts. The net balance of these two effects of PGE₂ in vivo on bone formation and bone resorption has been hard to predict and, as expected for such a widespread local factor, hard to study. Some of the complexity of PGE₂ actions on bone can be explained by the fact that there are four receptors for PGE₂ (EP1-4). Some of the major actions of PGE₂ in vitro occur via EP2 and EP4, both of which can stimulate cAMP signaling, but there are other distinct signaling pathways, important in other tissues, which have not yet been fully elucidated in bone cells. Giving PGE₂ or agonists of EP2 and EP4 to accelerate bone repair has been examined with positive results. Further studies to clarify the pathways of PGE₂ action in bone may allow us to identify new and more effective ways to deliver the therapeutic benefits of PGE₂ in skeletal disorders.

Prostaglandin receptor EP4 expression by Th17 cells is associated with high disease activity in ankylosing spondylitis

BACKGROUND

Th17 cells are involved in the pathogenesis of ankylosing spondylitis (AS). However, the mechanism underlying enhanced Th17 cell accumulation in AS remains unknown. The prostaglandin E₂ receptor EP2/EP4 signaling pathway plays a critical role in the development of autoimmune Th17 cells. Interestingly, recent genome-wide association studies (GWAS) have identified five risk alleles for AS in PTGER4, the gene encoding for EP4. The aim of this study was to reveal a possible link between EP4 and disease activity in patients with AS.

METHODS

Th17 cells from patients with AS were analyzed for the transcriptional expression of prostaglandin receptor genes by quantitative RT-PCR. Th17 cells from patients with rheumatoid arthritis (RA) and from healthy individuals served as controls. EP4 receptor expression in Th17 cells was assessed ex vivo by flow cytometry and by western blot. Functional analysis using EP4-specific agonists was performed to reveal how EP4 regulates Th17 cells.

RESULTS

EP4 is significantly overexpressed in Th17 cells from patients with AS compared to Th17 cells from healthy individuals or patients with RA or psoriatic arthritis (PsA). EP4 upregulation is unique to Th17 cells and is not found in other CD4⁺ T cell subsets. Specific activation of EP4 drives Th17 cell development and promotes EP4 expression in a positive feedback loop in AS but not in RA or PsA. Mechanistically, EP4 acts via upregulation of the interleukin-23 receptor (IL-23R), by suppressing the RORγt inhibitor FoxO1 and by enhancing STAT3 phosphorylation. Increased EP4 expression levels in Th17 cells from AS patients correlate with high disease activity as defined by a Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) score ≥ 4 (r = 0.7591, p = 0.0016).

CONCLUSIONS

EP4 is a potential marker of disease activity in patients with AS. Aberrant EP4 expression might contribute to pathogenic Th17 cell accumulation and represent a new target for the treatment of AS.

Expression of Prostaglandin E2 Receptors in Acquired Middle Ear Cholesteatoma

Objectives

To investigate the expression of prostaglandin E₂ receptor subtypes, E-prostanoid (EP) 1–4 receptors, in acquired cholesteatoma and its possible role in the pathologic process of this disorder.

Methods

Specimens of human acquired cholesteatoma were obtained from 29 patients and 19 skin biopsies of normal external auditory canal were as controls. The mRNA and protein expression of EP receptors was assessed by quantitative real-time polymerase chain reaction, immunohistochemistry and Western blot.

Results

In acquired cholesteatoma, EP1–EP4 receptors were mainly expressed on squamous epithelium and subepithelial infiltrated inflammatory cells. In external auditory canal skin, EP1–EP4 receptors were mainly expressed on squamous epithelium and glandular epithelium. The expression of EP4 receptor on mRNA and protein levels were significant lower in acquired cholesteatoma compared with controls. EP1–EP3 receptors had no significant difference between the experimental and control group.

Conclusion

Low expression of EP4 may play a crucial role in the pathologic process of inflammation reaction and bone destruction in acquired cholesteatoma, but not EP1, EP2, or EP3 receptors.

Prostaglandin E2 inhibits matrix mineralization by human bone marrow stromal cell-derived osteoblasts via Epac-dependent cAMP signaling

The osteoinductive properties of prostaglandin E₂ (PGE₂) and its signaling pathways have led to suggestions that it may serve as a potential therapeutic strategy for bone loss. However, the prominence of PGE₂ as an inducer of bone formation is attributed primarily to findings from studies using rodent models. In the current study, we investigated the effects of PGE₂ on human bone marrow stromal cell (hBMSC) lineage commitment and determined its mode of action. We demonstrated that PGE₂ treatment of hBMSCs significantly altered the expression profile of several genes associated with osteoblast differentiation (RUNX2 and ALP) and maturation (BGLAP and MGP). This was attributed to the activation of specific PGE₂ receptors, and was associated with increases in cAMP production and sustained AKT phosphorylation. Pharmacological inhibition of exchange protein directly activated by cAMP (Epac), but not protein kinase A (PKA), recovered the mineralization functions of hBMSC-derived osteoblasts treated with PGE₂ and restored AKT phosphorylation, along with the expression levels of RUNX2, ALP, BGLAP and MGP. Our findings therefore provide insights into how PGE₂ influences hBMSC-mediated matrix mineralization, and should be taken into account when evaluating the role of PGE₂ in human bone metabolism.

Celecoxib treatment of fibrous dysplasia (FD) in a human FD cell line and FD-like lesions in mice with protein kinase A (PKA) defects

Osteochondromyxomas (OMX) in the context of Carney complex (CNC) and fibrous dysplasia (FD)-like lesions (FDLL) in mice, as well as isolated myxomas in humans may be caused by inactivation of PRKAR1A, the gene coding for the type 1a regulatory subunit (R1α) of cAMP-dependent protein kinase (PKA). OMXs and FDLL in mice lacking Prkar1a grow from abnormal proliferation of adult bone stromal cells (aBSCs). Prkar1a and Prkaca (coding for Cα) haploinsufficiency leads to COX2 activation and prostaglandin E2 (PGE2) production that, in turn, activates proliferation of aBSCs. Celecoxib is a cyclooxygenase-2 (COX2) inhibitor. We hypothesized that COX-2 inhibition may have an effect in FD and FDLL. In vitro treatment of a human cell line prepared from a FD patient with Celecoxib resulted in decreased PGE2 and cell proliferation. Treatment of mice haploinsufficient for R1α and Cα with 1500 mg/kg Celecoxib led to decreased PGE2 and proliferation and increased apoptosis, with a corresponding gene expression profile, resulting in dramatic reduction of tumor growth. Furthermore, the treatment improved the organization of cortical bone that was adjacent to the tumor. We conclude that, in vitro and in vivo, Celecoxib had an inhibitory effect on FD cell proliferation and in mouse FDLL structure, respectively. We speculate that COX-2 inhibitors offer an attractive alternative to current treatments for benign tumors such as OMX and FD that, apart from tumor suppression, may mechanically stabilize affected bones.

Compensatory cellular reactions to nonsteroidal anti-inflammatory drugs on osteogenic differentiation in canine bone marrow-derived mesenchymal stem cells

The suppressive effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on the bone healing process have remained controversial, since no clinical data have clearly shown the relationship between NSAIDs and bone healing. The aim of this study was to assess the compensatory response of canine bone marrow-derived mesenchymal stem cells (BMSCs) to several classes of NSAIDs, including carprofen, meloxicam, indomethacin and robenacoxib, on osteogenic differentiation. Each of the NSAIDs (10 µM) was administered during 20 days of the osteogenic process with human recombinant IL-1β (1 ng/ml) as an inflammatory stimulator. Gene expression of osteoblast differentiation markers (alkaline phosphatase and osteocalcin), receptors of PGE₂ (EP2 and EP4) and enzymes for prostaglandin (PG) E2 synthesis (COX-1, COX-2, cPGES and mPGES-1) was measured by using quantitative reverse transcription-polymerase chain reaction. Protein production levels of alkaline phosphatase, osteocalcin and PGE₂ were quantified using an alkaline phosphatase activity assay, osteocalcin immunoassay and PGE₂ immunoassay, respectively. Histologic analysis was performed using alkaline phosphatase staining, von Kossa staining and alizarin red staining. Alkaline phosphatase and calcium deposition were suppressed by all NSAIDs. However, osteocalcin production showed no significant suppression by NSAIDs. Gene expression levels of PGE₂-related receptors and enzymes were upregulated during continuous treatment with NSAIDs, while certain channels for PGE₂ synthesis were utilized differently depending on the kind of NSAIDs. These data suggest that canine BMSCs have a compensatory mechanism to restore PGE₂ synthesis, which would be an intrinsic regulator to maintain differentiation of osteoblasts under NSAID treatment.

Selective and non-selective cyclooxygenase inhibitors delay stress fracture healing in the rat ulna

Anti-inflammatory drugs are widely used to manage pain associated with stress fractures (SFxs), but little is known about their effects on healing of those injuries. We hypothesized that selective and non-selective anti-inflammatory treatments would retard the healing of SFx in the rat ulna. SFxs were created by cyclic loading of the ulna in Wistar rats. Ulnae were harvested 2, 4 or 6 weeks following loading. Rats were treated with non-selective NSAID, ibuprofen (30 mg/kg/day); selective COX-2 inhibition, [5,5-dimethyl-3-3 (3 fluorophenyl)-4-(4 methylsulfonal) phenyl-2 (5H)-furanone] (DFU) (2.0 mg/kg/day); or the novel c5a anatagonist PMX53 (10 mg/kg/day, 4 and 6 weeks only); with appropriate vehicle as control. Quantitative histomorphometric measurements of SFx healing were undertaken. Treatment with the selective COX-2 inhibitor, DFU, reduced the area of resorption along the fracture line at 2 weeks, without affecting bone formation at later stages. Treatment with the non-selective, NSAID, ibuprofen decreased both bone resorption and bone formation so that there was significantly reduced length and area of remodeling and lamellar bone formation within the remodeling unit at 6 weeks after fracture. The C5a receptor antagonist PMX53 had no effect on SFx healing at 4 or 6 weeks after loading, suggesting that PMX53 would not delay SFx healing. Both selective COX-2 inhibitors and non-selective NSAIDs have the potential to compromise SFx healing, and should be used with caution when SFx is diagnosed or suspected.

Prostaglandins in bone: bad cop, good cop?

Prostaglandins (PGs) are multifunctional regulators of bone metabolism that stimulate both bone resorption and formation. PGs have been implicated in bone resorption associated with inflammation and metastatic bone disease, and also in bone formation associated with fracture healing and heterotopic ossification. Recent studies have identified roles for inducible cyclooxygenase (COX)-2 and PGE(2) receptors in these processes. Although the effects of PGs have been most often associated with cAMP production and protein kinase A activation, PGs can engage an extensive G-protein signaling network. Further analysis of COX-2 and PG receptors and their downstream G-protein signaling in bone could provide important clues to the regulation of skeletal cell growth in both health and disease.

Phosphodiesterase 3 and 4 negatively regulate receptor activator of nuclear factor-kappaB ligand-mediated osteoclast formation by prostaglandin E2

Prostaglandin E2 (PGE2) stimulates osteoclast formation by increasing receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) mRNA expression via cAMP-protein kinase A (PKA) pathways in osteoblasts. Since phosphodiesterases (PDEs) balance the concentration of intracellular cAMP stimulated by PGE2, we investigated the role of PDEs in PGE2-mediated osteoclast formation using various cAMP-specific PDEs inhibitors. In the presence of PGE(2), PDE3 and 4 inhibitors were shown to dose-dependently increase the osteoclast formation in cocultures of mouse bone marrow cells and calvarial osteoblasts. In agreement with this finding, they stimulated PGE2-induced cAMP production followed by increased RANKL mRNA expression in osteoblasts, suggesting that PDE3 and 4 negatively regulate PGE2-mediated RANKL expression in osteoblasts. RT-PCR analysis revealed that PDE3A, 3B, 4A, 4B and 4D are expressed in osteoblasts. The PDE8 inhibitor did not increase osteoclast formation, although it stimulated PGE2-induced RANKL mRNA expression in osteoblasts. The four subtypes of PGE receptors are designated EP1, EP2, EP3, and EP4. PDE3 and 4 inhibitors were found to increase EP1/3, EP4 and/or EP2 agonist-stimulated RANKL expression, indicating that PDE3 and PDE4 negatively regulate PGE2-induced RANKL mRNA expression through four EPs. Taken together, these data suggest that PDE3 and PDE4 could have important pharmacological and clinical implications in bone-related diseases.

Effects of global or targeted deletion of the EP4 receptor on the response of osteoblasts to prostaglandin in vitro and on bone histomorphometry in aged mice

Because global deletion of the prostaglandin EP4 receptor results in neonatal lethality, we generated a mouse with targeted EP4 receptor deletion using Cre-LoxP methodology and a 2.3 kb collagen I a1 promoter driving Cre recombinase that is selective for osteoblastic cells. We compared wild type (WT), global heterozygote (G-HET), targeted heterozygote (T-HET) and knockout (KO) mice. KO mice had one targeted and one global deletion of the EP4 receptor. All mice were in a mixed background of C57BL/6 and CD-1. Although there were one third fewer G-HET or KO mice at weaning compared to WT and T-HET mice, G-HET and KO mice appeared healthy. In cultures of calvarial osteoblasts, prostaglandin E(2) (PGE(2)) increased alkaline phosphatase (ALP) activity in cells from WT mice, and this effect was significantly decreased in cells from either G-HET or T-HET mice and further decreased in cells from KO mice. A selective agonist for EP4 receptor increased ALP activity and osteocalcin mRNA levels in cells from WT but not KO mice. A selective COX-2 inhibitor, NS-398, decreased osteoblast differentiation in WT but not KO cells. At 15 to 18 months of age there were no differences in serum creatinine, calcium, PTH, body weight or bone mineral density among the different genotypes. Static and dynamic histomorphometry showed no consistent changes in bone volume or bone formation. We conclude that expression of the EP4 receptor in osteoblasts is critical for anabolic responses to PGE(2) in cell culture but may not be essential for maintenance of bone remodeling in vivo.

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

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.

Effect of deletion of the prostaglandin EP2 receptor on the anabolic response to prostaglandin E2 and a selective EP2 receptor agonist

Studies using prostaglandin E receptor (EP) agonists indicate that prostaglandin (PG) E(2) can have anabolic effects through both EP4 and EP2 receptors. We previously found that the anabolic response to a selective EP4 receptor agonist (EP4A, Ono Pharmaceutical) was substantially greater than to a selective EP2 receptor agonist (EP2A) in cultured murine calvarial osteoblastic cells. To further define the role of the EP2 receptor in PG-mediated effects on bone cells, we examined the effects of EP2A and PGE(2) on both calvarial primary osteoblasts (POB) and marrow stromal cells (MSC) cultured from mice with deletion of one (Het) or both (KO) alleles of the EP2 receptor compared to their wild-type (WT) littermates. Deletion of EP2 receptor was confirmed by quantitative real-time PCR, Western blot and immunohistochemistry. The 1 month-old mice used to provide cells in these studies did not show any significant differences in their femurs by static histomorphometry. EP2A was found to enhance osteoblastic differentiation as measured by alkaline phosphatase mRNA expression and activity as well as osteocalcin mRNA expression and mineralization in the WT cell cultures from both marrow and calvariae. The effects were somewhat diminished in cultures from Het mice and abrogated in cultures from KO mice. PGE(2) effects were greater than those of EP2A, particularly in POB cultures and were only moderately diminished in Het and KO cell cultures. We conclude that activation of the EP2 receptor is able to enhance differentiation of osteoblasts, that EP2A is a true selective agonist for this receptor and that PGE(2) has an additional anabolic effect likely mediated by the EP4 receptor.