An EP2 receptor-selective prostaglandin E2 agonist induces bone healing

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.

Critical issues in translational and clinical research for the study of new technologies to enhance bone repair

Osteoporosis increases fracture risk, especially in metaphyseal bone. Fractures seriously impair function and quality of life and incur large direct and indirect costs. Although the prevention of fractures is certainly the option, a fast and uneventful healing process is optimal when fractures do occur. Many new therapeutic strategies have been developed to accelerate fracture-healing or to diminish the complication rate during the course of fracture-healing. However, widely accepted guidelines are needed to demonstrate the positive or negative interactions of bioactive substances, drugs, and other agents that are being used to promote fracture-healing. For each study design, the primary study goal should be indicated. Outcome variables should include both objective and subjective parameters. The guidelines should be harmonized between European and American regulatory authorities to ensure comparability of results of studies and to foster global harmonization of regulatory requirements.

Effects of gestational age on prostaglandin EP receptor expression and functional involvement during in vitro contraction of the guinea pig uterus

Prostaglandin E(2) (PGE(2)) exerts diverse biological effects through four G-protein-coupled cell surface receptor subtypes, EP1-4. This study's objective was to characterize EP1-4 receptor mRNA expression within pregnant guinea pig myometrium during early implantation stage (gestation day [GD] 6) and late stage gestation (GD 50) and evaluate in vitro contractile activity of receptor subtype selective agonists. Using RT-PCR, qualitative gene expression patterns of EP2, EP3, and EP4 mRNA were detected in the myometrium and remained unchanged between the gestational ages. EP1 mRNA remained undetected in pregnant tissue. In vitro contractile activity was evaluated in GD 6 and GD 50 myometrium using vehicle and EP agonists PGE(2), 17-phenyl trinor PGE(2), sulprostone, misoprostol, and CP-533,536. All spasmogens in pregnant myometrium were EP1/EP3 selective agonists, though likely acting via EP3 receptors in this test model. CP-533,536--a highly selective EP2 receptor agonist--and the vehicle failed to induce myometrial contraction at both gestational ages.

Effects of a monoclonal antibody raised against nerve growth factor on skeletal pain and bone healing after fracture of the C57BL/6J mouse femur

A closed femur fracture pain model was developed in the C57BL/6J mouse. One day after fracture, a monoclonal antibody raised against nerve growth factor (anti-NGF) was delivered intraperitoneally and resulted in a reduction in fracture pain-related behaviors of approximately 50%. Anti-NGF therapy did not interfere with bone healing as assessed by mechanical testing and histomorphometric analysis.

INTRODUCTION

Current therapies to treat skeletal fracture pain are limited. This is because of the side effect profile of available analgesics and the scarcity of animal models that can be used to understand the mechanisms that drive this pain. Whereas previous studies have shown that mineralized bone, marrow, and periosteum are innervated by sensory and sympathetic fibers, it is not understood how skeletal pain is generated and maintained even in common conditions such as osteoarthritis, low back pain, or fracture.

MATERIALS AND METHODS

In this study, we characterized the pain-related behaviors after a closed femur fracture in the C57BL/6J mouse. Additionally, we assessed the effect of a monoclonal antibody that binds to and sequesters nerve growth factor (anti-NGF) on pain-related behaviors and bone healing (mechanical properties and histomorphometric analysis) after fracture.

RESULTS

Administration of anti-NGF therapy (10 mg/kg, days 1, 6, and 11 after fracture) resulted in a reduction of fracture pain-related behaviors of approximately 50%. Attenuation of fracture pain was evident as early as 24 h after the initial dosing and remained efficacious throughout the course of fracture pain. Anti-NGF therapy did not modify biomechanical properties of the femur or histomorphometric indices of bone healing.

CONCLUSIONS

These findings suggest that therapies that target NGF or its cognate receptor(s) may be effective in attenuating nonmalignant fracture pain without interfering with bone healing.

Non-steroidal anti-inflammatory drugs, cyclooxygenase-2 and the bone healing process

Traditional non-steroidal anti-inflammatory drugs (NSAID) and selective cyclooxygenase-2 (COX-2) inhibitors are widely used in the treatment of pain, including bone fracture pain and orthopaedic post-operative pain. The gastrointestinal and cardiovascular adverse effects of NSAIDs are acknowledged, but their effects on bone are less widely known. Prostaglandins play an important role in the regulation of osteoblast and osteoclast functions, and inhibition of prostaglandin production retards bone formation. Therefore, NSAIDs could be expected to have significant consequences in divergent clinical situations where bone formation or remodelling is a contributing factor. The present survey reviews current experimental and clinical evidence related to two of those conditions (i.e. on ectopic bone formation and on bone fracture healing). NSAIDs are used clinically to prevent ectopic bone formation (also known as heterotopic ossification) (e.g. after total hip arthroplasty or trauma). The efficacy of NSAIDs in the avoidance of heterotopic ossification has been documented in controlled clinical trials, but the inherent risks (e.g. on healing processes and on loosening of prostheses) need further studies. At the same time, NSAIDs are widely used in the treatment of fracture pain, and their inhibitory effects on the ongoing bone healing process have raised concerns. Results of fracture healing studies in animals treated with NSAIDs or in mice lacking COX-2 gene show that inhibition or deficiency of COX-2 impairs the bone healing process. The limited clinical data also support the assumption that inhibition of COX-2 by non-selective or COX-2-selective NSAIDs delays fracture healing. However, the clinical significance of the effect in various patient groups needs to be carefully assessed and further investigations are needed to characterize the patients at the highest risk for NSAID-induced delayed fracture healing and its complications. In the meantime, use of NSAIDs in fracture patients should be cautious, keeping in mind the benefits of pain relief and inhibition of ectopic bone formation on one hand, and the risks of non-union and retarded union on the other hand.

Misoprostol enhances early fracture healing: a preliminary biochemical study on rats

The purpose of this study was to demonstrate if misoprostol, a methyl derivative of prostaglandin E1, enhanced fracture healing in 54 male adult Sprague-Dawley rats. The base level of serum alkaline phosphatase (ALP) in 6 randomly selected rats was measured. Rats were then randomly separated into 3 groups and their tibias fractured. First and second groups received misoprostol for 4 weeks, 100 and 300 microg/kg/day respectively via oral route. The third group had no misoprostol. p<0.05 was considered significant. Elevation of ALP level in the 2nd week was significant in group 1, it was not in group 2 or 3; in the 4th week it was significant in all groups. In conclusion dosage dependent osteoinductive effect of misoprostol was shown in the early bone healing period. Biochemical findings in the latter period did not show any inhibitory effect of misoprostol on bone healing. Further studies, probably biomechanical, may be required for the final verdict.

Enhancing effects of a prostaglandin EP4 receptor agonist on recombinant human bone morphogenetic protein-2 mediated spine fusion in a rabbit model

STUDY DESIGN

An experimental animal study aimed at achieving posterolateral intertransverse process fusion with rhBMP-2 in combination with the local delivery of an EP4 receptor agonist.

OBJECTIVE

To determine whether an EP4 receptor agonist (EP4A) can reduce the amount of BMP required to achieve posterolateral spinal fusion in rabbits.

SUMMARY OF BACKGROUND DATA

In the clinic, BMP retaining implants are used for spinal fusion and the treatment of pseudarthrosis after long bone fracture. However, the requirement of high doses of BMP-2 for bone formation in humans makes the implants expensive and limits their use in the clinic. Previous studies in our laboratory using a new delivery system involving a synthetic polymer/beta-TCP powder composite had shown it was possible to reduce the total BMP-2 amount to 30 microg per fusion in a rabbit model. To further reduce the dose of BMP required for a successful fusion, we explored the use of a chemical compound to enhance the bone-inducing action of BMP-2.

METHODS

In order to prepare 1 implant to bridge the unilateral L5 and L6 transverse processes, 300 mg of polymer gel (PLA-DX-PEG block copolymer), 300 mg of beta-TCP powder, rhBMP-2 (7.5, 3.75, or 0 microg), with or without EP4A (45 microg) were mixed and manually shaped to resemble a rod. Through a posterolateral approach, 2 implants were placed on both sides (1 per side) by surgery in order to bridge the transverse processes of adult New Zealand white rabbits (n = 48). The lumbar vertebrae were recovered 6 weeks after surgery. The posterolateral fusion was examined by manual palpation, radiography, biomechanical testing, and histology.

RESULTS

All of 8 rabbits that received 7.5 microg of BMP-2 and EP4A consistently showed fusion by significant amount of new bone formation. However, solid fusion was seen in only 3 of 8 rabbits that received 7.5 microg of BMP-2 without the EP4 receptor agonist.

CONCLUSION

Local administration of an EP4 receptor agonist enhanced the bone-inducing activity of BMP-2 in a rabbit posterolateral lumbar spinal fusion model and as a result, the dose of BMP-2 required for this outcome was reduced by 50% compared with our previous report. The coadministration of this compound via a local delivery system may help to reduce the costs of spine fusion with use of BMP-2 in the clinic.

Strontium ranelate promotes osteoblastic differentiation and mineralization of murine bone marrow stromal cells: involvement of prostaglandins

Strontium ranelate is a new anti-osteoporosis treatment. This study showed that strontium ranelate stimulated PGE(2) production and osteoblastic differentiation in murine marrow stromal cells, which was markedly reduced by inhibition of COX-2 activity or disruption of COX-2 gene expression. Hence, some anabolic effects of strontium ranelate may be mediated by the induction of COX-2 and PGE(2) production.

INTRODUCTION

Strontium ranelate is an orally active drug that reduces vertebral and hip fracture risk by increasing bone formation and reducing bone resorption. Strontium ranelate effects on bone formation are the result of increased osteoblastic differentiation and activity, but the mechanisms governing these effects are unknown. Based on previous work, we hypothesized that strontium ranelate increases cyclooxygenase (COX)-2 expression and that, consequently, the prostaglandin E(2) (PGE(2)) produced could mediate some effects of strontium ranelate on osteoblasts.

MATERIALS AND METHODS

Marrow stromal cells (MSCs) from COX-2 wildtype (WT) and knockout (KO) mice were cultured with and without low-dose dexamethasone. Osteoblastic differentiation was characterized by alkaline phosphatase (ALP) activity, real-time PCR for ALP and osteocalcin (OCN) mRNA expression, and alizarin red staining for mineralization. Medium PGE(2) was measured by radioimmunoassay or enzyme immunoassay.

RESULTS AND CONCLUSIONS

In MSCs from COX-2 WT mice, strontium ranelate significantly increased ALP activity, ALP and OCN mRNA expression, and mineralization after 14 or 21 days of culture. A short treatment at the beginning of the culture (0-7 days) with strontium ranelate was as effective as continuous treatment. Strontium ranelate (1 and 3 mM Sr(+2)) dose-dependently increased PGE(2) production, with maximum PGE(2) production occurring during the first week of culture. NS-398, a selective COX-2 inhibitor, blocked the strontium ranelate stimulation of PGE(2) production and significantly inhibited the strontium ranelate stimulation of ALP activity. In MSCs from COX-2 KO mice, the strontium ranelate stimulation of ALP and OCN mRNA expression and mineralization were markedly reduced compared with COX-2 WT cultures. Similar effects of strontium ranelate on osteoblastic markers and on PGE(2) production were seen when MSCs were cultured with or without low-dose dexamethasone (10 nM). We conclude that PGE(2) produced by the strontium ranelate induction of COX-2 expression plays a role in strontium ranelate-induced osteoblastic differentiation in MSCs in vitro.

Prostaglandin E(2) receptors in bone formation

Prostaglandins, PGE(2) in particular, have diverse actions on various organs, including inflammation, bone healing, bone formation, embryo implantation, induction of labour and vasodilatation, among others. However, systemic side effects have limited their clinical utility. The pharmacological activities of PGE(2) are mediated through four G protein-coupled receptor subtypes, EP1-EP4. Recent studies have shown that EP2 and EP4 receptors play important roles in regulating bone formation and resorption. EP2 and EP4 receptor-selective agonists have been shown to stimulate local or systemic bone formation, augment bone mass and accelerate the healing of fractures or bone defects in animal models upon local or systemic administration, thus, potentially offering new therapeutic options for enhancing bone formation and bone repair in humans. This review will focus on the studies related to bone formation and bone healing in the EP receptor knockout (KO) mice and the EP2 or EP4 receptor-selective agonist treated animal models.

Dose and time-dependent effects of cyclooxygenase-2 inhibition on fracture-healing

BACKGROUND

Fracture-healing is impaired in mice lacking a functional cyclooxygenase-2 (COX-2) gene or in rats continuously treated with COX-2 inhibitors. These observations indicate that COX-2 is a critical regulator of fracture repair. Nonsteroidal anti-inflammatory drugs are commonly used to treat pain associated with musculoskeletal trauma and disease. Nonsteroidal anti-inflammatory drugs inhibit COX-2 function and in so doing can impair fracture-healing. The goal of the present study was to determine how variations in nonsteroidal anti-inflammatory drug therapy ultimately affect fracture-healing.

METHODS

Closed femoral fractures were made in female Sprague-Dawley rats. The rats were treated with different doses of celecoxib (a COX-2-selective nonsteroidal anti-inflammatory drug) or were treated for different periods before or after fracture with celecoxib. Eight weeks after the fracture, healing was assessed with radiography and destructive torsional mechanical testing. The effect of celecoxib treatment on fracture callus prostaglandin E2 and F(2alpha) levels was determined as a measure of cyclooxygenase activity.

RESULTS

Celecoxib doses as small as 2 mg/kg/day reduced fracture callus mechanical properties and caused a significant increase in the proportion of nonunions. Similarly, treatment with celecoxib at a dose of 4 mg/kg/day for just five days reduced fracture callus mechanical properties and significantly increased the proportion of nonunions. Conversely, celecoxib therapy prior to fracture or initiated fourteen days after fracture did not significantly increase the proportion of nonunions. Celecoxib treatment at a dose of 4 mg/kg/day reduced fracture callus prostaglandin E2 and F(2alpha) levels by >60%.

CONCLUSIONS

COX-2-selective nonsteroidal anti-inflammatory drug therapy during the early stages of fracture repair significantly reduced fracture callus mechanical properties at later stages of healing and increased the proportion of nonunions in this animal model.

New technologies for the enhancement of skeletal repair

Although fracture healing is a well-optimized biological process that leads to healing, approximately 10-20% of fractures result in impaired or delayed healing and these fractures may benefit from the use of biotechnologies to enhance skeletal repair. Peptide signaling molecules such as the bone morphogenetic proteins have been shown to stimulate the healing of fresh fractures, nonunions, and spinal fusions and side effects from their use appear to be minimal. Other growth factors currently being studied for local application include growth and differentiation factor-5 (GDF-5), vascular endothelial growth factor (VEGF), transforming growth factor beta (TGFbeta), and platelet-derived growth factor (PDGF). Molecules such as prostaglandin E receptor agonists and the thrombin-related peptide, TP508, have shown promise in animal models of fracture repair. Gene therapy using various growth factors or combinations of factors might also aid in fracture repair, particularly as new methods for delivery that do not require viral vectors are developed. Systemic therapy with agents such as parathyroid hormone (PTH), growth hormone (GH), and the HMG-CoA reductase inhibitors are also under investigation. As these and other technologies are shown to be safe and effective, their use will become a part of the standard of care in managing skeletal injuries.

Anabolic agents and the bone morphogenetic protein pathway

A major unmet need in the medical field today is the availability of suitable treatments for the ever-increasing incidence of osteoporosis and the treatment of bone deficit conditions. Although therapies exist which prevent bone loss, the options are extremely limited for patients once a substantial loss of skeletal bone mass has occurred. Patients who have reduced bone mass are predisposed to fractures and further morbidity. The FDA recently approved PTH (1-34) (Teriparatide) for the treatment of postmenopausal osteoporosis after both preclinical animal and clinical human studies indicated it induces bone formation. This is the only approved bone anabolic agent available but unfortunately it has limited use, it is relatively expensive and difficult to administer. Consequently, the discovery of low cost orally available bone anabolic agents is critical for the future treatment of bone loss conditions. The intricate process of bone formation is co-ordinated by the action of many different bone growth factors, some stored in bone matrix and others released into the bone microenvironment from surrounding cells. Although all these factors play important roles, the bone morphogenetic proteins (BMPs) clearly play a central role in both bone cartilage formation and repair. Recent research into the regulation of the BMP pathway has led to the discovery of a number of small molecular weight compounds as candidate bone anabolic agents. These agents may usher in a new wave of more innovative and versatile treatments for osteoporosis as well as orthopedic and dental indications.

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

We compared the direct effects of selective EP4 and EP2 receptor agonists (EP4A and EP2A) with prostaglandin E(2) (PGE(2)) on the differentiation of cultured murine calvarial osteoblastic cells. EP4A increased alkaline phosphatase activity and osteocalcin mRNA levels in these cultures similar to PGE(2). This effect was seen with both "direct plating" immediately after isolating the cells, or "indirect plating" in which the cells were grown to confluence and replated. EP2A had a smaller effect, significant only in "indirect plating" experiments. All three agents decreased the DNA and protein content in indirect plating experiments, but not in direct plating experiments. We conclude that the anabolic effect of PGE(2) in calvarial osteoblastic cell cultures is largely mediated by activation of the EP4 receptor, while activation of the EP2 receptor is less effective.

Fracture healing in the elderly patient

Clinical experience gives rise to the impression that there are differences in fracture healing in different age groups. It is evident that fractures heal more efficiently in children than in adults. However, minimal objective knowledge exists to evaluate this assumption. Temporal, spatial, and cellular quantitative and qualitative interrelationships, as well as signaling molecules and extracellular matrix have not been comprehensively and adequately elucidated for fracture healing in the geriatric skeleton. The biological basis of fracture healing will provide a context for revealing the pathophysiology of delayed or even impaired bone regeneration in the elderly. We will summarize experimental studies on age-related changes at the cellular and molecular level that will add to the pathophysiological understanding of the compromised bone regeneration capacity believed to exist in the elderly patient. We will suggest why this understanding would be useful for therapeutics focused on bone regeneration, in particular fracture healing at an advanced age.

Drug delivery strategies for cathepsin inhibitors in joint diseases

Cathepsins play important roles in the development of joint and bone diseases such as osteoporosis, rheumatoid arthritis (RA) and osteoarthritis (OA). Cathepsin inhibitors are presently in development and clinical testing for use as novel disease-modifying drugs for the improved treatment of osteoporosis. They may also be applicable for the treatment of joint diseases. However, some barriers still hamper their clinical applications in these indications. Based on pathophysiological features of RA and OA, the authors discuss six potential drug delivery strategies for the effective delivery of cathepsin inhibitors or other antiarthritic drugs to the arthritic joint tissue. Successful application of these strategies may significantly contribute to a more effective and safe treatment of RA and OA.

Role of EP2 and EP4 receptor-selective agonists of prostaglandin E2 in acute and chronic kidney failure

We tested the efficacy of three selective agonists of prostaglandin E(2) (PGE(2)) receptor, EP2 (CP-536,745-01), EP2/4 (CP-043,305-02), and EP4 (CP-044,519-02), in two models of acute and chronic kidney failure. In the nephrotoxic mercury chloride (HgCl(2)) rat model of acute kidney failure systemically administered EP4 agonist reduced the serum creatinine values and increased the survival rate. Although the EP2 or the EP2/4 agonist did not change the serum creatinine values, the EP2 receptor agonist increased the survival rate. Histological evaluation of kidneys from EP4-treated rats indicated less proximal tubular necrosis and less apoptotic cells. In a rat model of chronic renal failure, the three receptor agonists decreased the serum creatinine and increased the glomerular filtration rate at 9 weeks following therapy. Kidneys treated with the EP4 agonist had less glomerular sclerosis, better preservation of proximal and distal tubules and blood vessels, increased convoluted epithelium proliferation and less apoptotic cells. Nephrectomy had no influence on the expression of the EP4 receptor, whereas EP2 receptor expression was reduced by 50% and then corrected following treatment with EP2 and EP2/4 receptor agonists. These findings suggest that PGE(2) has an important role in acute kidney failure via the EP4 receptor, whereas in chronic kidney failure both EP2 and EP4 receptors are equally important in preserving the progression of chronic kidney failure. Thus, agonism of EP2 and EP4 receptors may provide a basis for treating acute and chronic kidney failure.

Bone strength: current concepts

Bones serve several mechanical functions, including acoustic amplification in the middle ear, shielding vital organs from trauma, and serving as levers for muscles to contract against. Bone is a multiphase material made up of a tough collagenous matrix intermingled with rigid mineral crystals. The mineral gives bone its stiffness. Without sufficient mineralization, bones will plastically deform under load. Collagen provides toughness to bone making it less brittle so that it better resists fracture. Bone adapts to mechanical stresses largely by changing its size and shape, which are major determinants of its resistance to fracture. Tissue is added in regions of high mechanical stress providing an efficient means for improving bone strength. Experiments have shown that small additions of bone mineral density (BMD) (5-8%) caused by mechanical loading can improve bone strength by over 60% and extend bone fatigue life by 100-fold. Consequently, it is clear that bone tissue possesses a mechanosensing apparatus that directs osteogenesis to where it is most needed for improving bone strength. The biological processes involved in bone mechanotransduction are poorly understood and further investigation of the molecular mechanisms involved might uncover drug targets for osteoporosis. Several pathways are emerging from current research, including membrane ion channels, ATP signaling, second messengers, such as prostaglandins and nitric oxide, insulin-like growth factors, and Wnt signaling.

rhBMP-2 delivered in a calcium phosphate cement accelerates bridging of critical-sized defects in rabbit radii

BACKGROUND

Treatment of segmental bone loss remains a challenge in skeletal repairs. This study was performed to evaluate the efficacy of the use of recombinant bone morphogenetic protein-2 (rhBMP-2) delivered in an injectable calcium phosphate cement (alpha bone substitute material [alpha-BSM]) to bridge critical-sized defects in the rabbit radius.

METHODS

Unilateral 20-mm mid-diaphyseal defects were created in the radii of thirty-six skeletally mature New Zealand White rabbits. The defects in twelve rabbits each were filled with 0.166 mg/mL rhBMP-2/alpha-BSM cement, 0.033 mg/mL rhBMP-2/alpha-BSM cement, or buffer/alpha-BSM cement. Six rabbits from each group were killed at four weeks, and six were killed at eight weeks. Serial radiographs were made to monitor defect-bridging and residual alpha-BSM carrier. A semiquantitative histological scoring system was used to evaluate defect-bridging. Histomorphometry was used to quantify residual alpha-BSM; trabecular bone area; trabecular bone volume fraction; and cortical length, width, and area.

RESULTS

At four weeks, there had been more rapid resorption of alpha-BSM and filling of the defects with trabecular bone in the group treated with 0.166 mg/mL rhBMP-2/alpha-BSM than in the other two groups. Histomorphometry confirmed an increased trabecular area and volume fraction in this group compared with the other two groups. In both rhBMP-2/alpha-BSM-treated groups, the majority of the trabecular bone was formed by a direct process adjacent to the resorbing alpha-BSM. At eight weeks, complete cortical bridging and regeneration of the marrow space were present in all of the defects treated with 0.166 mg/mL rhBMP-2/alpha-BSM. That group also had reduced residual alpha-BSM and trabecular area and volume, compared with the other two groups, at eight weeks as a result of a rapid remodeling process.

CONCLUSIONS

Treatment of a critical-sized defect in a rabbit radius with 0.166 mg/mL rhBMP-2/alpha-BSM injectable cement can result in bridging with cortical bone and a regenerated bone-marrow space by eight weeks. Site-specific remodeling appears to be responsible for corticalization and marrow regeneration.

CLINICAL RELEVANCE

RhBMP-2 delivered in a calcium phosphate cement may be useful to achieve bridging of critical-sized defects in patients. Its injectable properties may allow minimally invasive use. Delayed percutaneous administration would also be possible when augmentation is desired following an initial surgical procedure or when soft-tissue injuries preclude adequate initial treatment.

CGRP inhibits osteoprotegerin production in human osteoblast-like cells via cAMP/PKA-dependent pathway

The osteoprotegerin (OPG)/receptor activator of nuclear factor-kappaB ligand (RANKL)/receptor activator of nuclear factor-kappaB (RANK) system was evaluated as a potential target of CGRP anabolic activity on bone. Primary cultures of human osteoblast-like cells (hOB) express calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1, and, because CGRP stimulates cAMP (one of the modulators of OPG production in osteoblasts), it was investigated whether it affects OPG secretion and expression in hOB. CGRP treatment of hOB (10(-11) M-10(-7) M) dose-dependently inhibited OPG secretion with an EC(50) of 1.08 x 10(-10) M, and also decreased its expression. This action was blocked by the antagonist CGRP(8-37). Forskolin, a stimulator of cAMP production, and dibutyryl cAMP also reduced the production of OPG. CGRP (10(-8) M) enhanced protein kinase A (PKA) activity in hOB, and hOB exposure to the PKA inhibitor, H89 (2 x 10(-6) M), abolished the inhibitory effect of CGRP on OPG secretion. Conditioned media from CGRP-treated hOB increased the number of multinucleated tartrate-resistant acid phosphatase-positive cells and the secretion of cathepsin K in human peripheral blood mononuclear cells compared with the conditioned media of untreated hOB. These results show that the cAMP/PKA pathway is involved in the CGRP inhibition of OPG mRNA and protein secretion in hOB and that this effect favors osteoclastogenesis. CGRP could thus modulate the balance between osteoblast and osteoclast activity, participating in the fine tuning of all of the bone remodeling phases necessary for the subsequent anabolic effect.

COX-2 has a Critical Role During Incorporation of Structural Bone Allografts

Nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit cyclooxygenase (COX) activity, reduced pain and are commonly used in patients with skeletal injury. In this article we will also present data to show that selective COX-2 inhibitor delays allograft healing and incorporation. In contrast, local delivery of prostaglandin E2 (PGE2) enhanced bone formation at cortical bone graft junction. A 4-mm mid-diaphyseal segmental femoral defect was created and then repaired by frozen bone allograft of the same size. A 22-gauge metal pin was placed in the intramedullary cavity to stabilize the bone graft. Healing was evaluated weekly by X ray and by a semiquantitative histomorphometric analysis at 5 weeks postsurgery. Celecoxib (25 mg/kg/day) and Ketorolac (4 mg/kg/day) were administered daily for 2 weeks or 5 weeks. PGE2 was infused locally at a dose of 800 nmol/kg per day via osmotic minipump for 4 weeks. Inhibition of cyclooxygenase by daily administration of the Celecoxib or Ketorolac for 5 weeks reduced new bone ingrowth by about 60% (P < 0.05). The percentage of bony bridging in both drug-treated groups was significantly decreased at 5 weeks. Temporal administration of Celecoxib for 2 weeks also significantly reduced bone formation by 45% and withdrawal of the Celecoxib only led to slight recovery of bone formation at the graft side. In contrast to the inhibitory effects of NSAIDS, PGE2 infusion at the cortical bone junction increased bone formation by about twofold. These results demonstrated that COX-2 is essential for bone allograft incorporation. Furthermore, our data support the notion that COX-2-dependent PGE2 produced at the early stage of bone healing is prerequisite for efficient skeletal repair.

A nonprostanoid EP4 receptor selective prostaglandin E2 agonist restores bone mass and strength in aged, ovariectomized rats

CP432 is a newly discovered, nonprostanoid EP4 receptor selective prostaglandin E2 agonist. CP432 stimulates trabecular and cortical bone formation and restores bone mass and bone strength in aged ovariectomized rats with established osteopenia.

INTRODUCTION

The purpose of this study was to determine whether a newly discovered, nonprostanoid EP4 receptor selective prostaglandin E2 (PGE2) agonist, CP432, could produce bone anabolic effects in aged, ovariectomized (OVX) rats with established osteopenia.

MATERIALS AND METHODS

CP432 at 0.3, 1, or 3 mg/kg/day was given for 6 weeks by subcutaneous injection to 12-month-old rats that had been OVX for 8.5 months. The effects on bone mass, bone formation, bone resorption, and bone strength were determined.

RESULTS

Total femoral BMD increased significantly in OVX rats treated with CP432 at all doses. CP432 completely restored trabecular bone volume of the third lumbar vertebral body accompanied with a dose-dependent decrease in osteoclast number and osteoclast surface and a dose-dependent increase in mineralizing surface, mineral apposition rate, and bone formation rate-tissue reference in OVX rats. CP432 at 1 and 3 mg/kg/day significantly increased total tissue area, cortical bone area, and periosteal and endocortical bone formation in the tibial shafts compared with both sham and OVX controls. CP432 at all doses significantly and dose-dependently increased ultimate strength in the fifth lumber vertebral body compared with both sham and OVX controls. At 1 and 3 mg/kg/day, CP432 significantly increased maximal load in a three-point bending test of femoral shaft compared with both sham and OVX controls.

CONCLUSIONS

CP432 completely restored trabecular and cortical bone mass and strength in established osteopenic, aged OVX rats by stimulating bone formation and inhibiting bone resorption on trabecular and cortical surfaces.

Early changes in prostaglandins precede bone formation in a rabbit model of heterotopic ossification

We have tested the hypothesis that the formation of heterotopic ossification (HO) in a rabbit model is correlated with a local increase in specific prostaglandins that may modulate mechanisms of ossification. Rabbits were sacrificed at 1 to 21 days following the daily forcible flexion of immobilized knees. The extraction and analysis of prostaglandins (PG) E2, F2alpha, D2, 6-keto-F1alpha, and thromboxane B2 in vastus intermedius muscles of manipulated legs revealed increases compared to control hindlimbs for all five prostaglandins, albeit of differing magnitude. The earliest increase was observed for PGF2alpha after 24 h (to 2.6-fold of control) with peak levels observed at day ten (185-fold of control). PGE2 was increased above control from 2 to 21 days following manipulation, with a peak level of 33-fold of control after 10 days. In a separate arm of the study, the role of PGE2 was investigated through the use of pharmacological antagonist of the PGE2 receptors and one of its second messengers, cAMP. Rabbits were preadministered the PGE2/PGD receptor antagonist AH 6809 or the cAMP antagonist Rp-cAMP prior to undergoing the regimen of limb immobilization and passive exercise. Both AH 6809 and Rp-cAMP were found to prevent the later development of radiographically documented heterotopic ossification in 15 out of 16 animals, thus identifying prostaglandins as being required for the development of ectopic bone. In this latter group, all but one pharmacologically treated animal showed an absence of HO at 3, 4, 5, or 6 weeks. These findings suggest an obligate cascade of prostaglandins for HO that offers the potential for novel prophylactic therapies, including those that target receptors for specific prostaglandins.

Use of bone morphogenetic proteins in traumatology

An estimated 5-10% of all fractures show impaired healing, leading to delayed union, or non-union. Chemical, or physical methods to accelerate bone healing are of great interest to the orthopaedic and trauma community. Research over the last 20 years has established that successful fracture healing is steered by specific growth factors. Of these, the bone morphogenetic proteins (BMPs) are probably the most important. The signalling pathway of these proteins is tightly regulated, overseeing a finely orchestrated cascade of events that occur after a fracture. The promising results of BMPs in preclinical studies have recently cleared the way for their use in specific fractures, or non-unions in clinical practice. The purpose of this work is to give a brief overview of BMPs and to review the clinical data currently available on the use of BMPs in fracture healing.

Effects of essential fatty acid deficiency on periodontal tissue adaptation to spontaneous tooth migration

Essential fatty acids (EFAs) play a significant role in bone metabolism. Herein we studied the adaptation of alveolar bone to physiologic tooth drift in young rats deprived of essential fatty acids from birth. Reductions in femur size and trabecular bone volume reflected body growth impairment. Along the alveolar wall, osteoclastic resorption and bone formation were depressed, disrupting the adaptive deformation of the tooth socket to ongoing migration. As a result, the periodontal ligament narrowed considerably, and further adaptation was achieved through root resorption. Essential fatty acid deficiency (EFAD), did not affect precursor recruitment or differentiation in the periodontal ligament (PDL), but caused redistribution of nonspecific-esterase (NSE)-positive osteoclast precursors and tartrate-resistant acid phosphatase (TRAP)-positive pre-osteoclasts between the bone compartment (which was depleted) and the root compartment (which was enriched). EFAD had also a marked effect on the PDL vasculature; the number of vessels was reduced, whereas their size was markedly increased. As a whole, our results show that EFAD disturbs alveolar bone adaptation to drift, but that a reaction (detrimental to root integrity) prevents root collision with the bone surface, thereby preserving the PDL as a source of precursor cells for bone and cementum homeostasis. Moreover, our results confirm that although alveolar bone resorption is arachidonic acid-dependent, the factors activating root resorption are different.

METABOLISM, EXCRETION, AND PHARMACOKINETICS OF (3-{[4-TERT-BUTYL-BENZYL)-(PYRIDINE-3-SULFONYL)-AMINO]-METHYL}-PHENOXY)-ACETIC ACID, AN EP2 RECEPTOR-SELECTIVE PROSTAGLANDIN E2 AGONIST, IN MALE AND FEMALE SPRAGUE-DAWLEY RATS

Metabolism, excretion, and pharmacokinetics of a highly selective EP2 agonist, CP-533,536 (3-{[4-tert-butyl-benzyl)-(pyridine-3-sulfonyl)-amino]-methyl}-phenoxy)-acetic acid), were investigated in male and female Sprague-Dawley rats following an intravenous administration of a single 15 mg/kg dose of [(14)C]CP-533,536. At 144 h after the dose, the cumulative excretion of radioactivity averaged 98.2 +/- 3.44% and 97.0 +/- 4.82% in male and female rats, respectively. The radioactivity was predominantly excreted in feces, reaching 87% of the dose. Mean exposure [area under the concentration-time curve (AUC(0-infinity))] for both CP-533,536 and total radioactivity was higher in female rats than in male rats, whereas the plasma clearance of CP-533,536 and metabolites was lower in female rats compared to male rats. CP-533,536 was extensively metabolized in both male and female rats. The major oxidative pathway was due to the oxidation of the tert-butyl side chain to form the omega-hydroxy metabolite M4 (males, 19.7%; females, 6.5%). M4 was further oxidized to form the omega-carboxy metabolite M3 (males, 32.8%; females 1.66%) or conjugated via sulfation to form metabolite M6 (males 12.7%; females 36.2%). Other metabolites were due to N-oxidation of the pyridine ring (M5) and aromatic hydroxylation (M12), and conjugation with glucuronic acid. The secondary metabolites were due to N-dealkylation of the methyl-phenoxyacetic acid moiety and phase II conjugation. CP-536,536 accounted for about 63 and 72% of the AUC of the total radioactivity for male and female rats, respectively. Gender-related differences in the metabolism and pharmacokinetics were observed. omega-Carboxy metabolite M3 was the major metabolite in male rats, whereas M3-sulfate was identified as the major metabolite in female rats.

Prostaglandin E2 receptors EP2 and EP4 are down-regulated during differentiation of mouse osteoclasts from their precursors

Prostaglandin E2 (PGE2) has been proposed to be a potent stimulator of bone resorption. However, PGE2 itself has been shown to directly inhibit bone-resorbing activity of osteoclasts. We examined the role of PGE2 in the function of mouse osteoclasts formed in vitro. Bone marrow macrophage osteoclast precursors expressed PGE2 receptors EP1, EP2, EP3beta, and EP4, and the expression of EP2 and EP4 was down-regulated during osteoclastic differentiation induced by receptor activator of NF-kappaB ligand and macrophage colony-stimulating factor. In contrast, functional EP1 was continuously expressed in mature osteoclasts. PGE2 as well as calcitonin caused intracellular Ca2+ influx in osteoclasts. However, PGE2 and 17-phenyltrinol-PGE2 (an EP1 agonist) failed to inhibit actin-ring formation and pit formation by osteoclasts cultured on dentine slices. When EP4 was expressed in osteoclasts using an adenovirus carrying EP4 cDNA, both actin-ring and pit-forming activities of osteoclasts were inhibited in an infectious unit-dependent manner. Treatment of EP4-expressing osteoclasts with PGE2 further inhibited their actin-ring and pit-forming activities. Such inhibitory effects of EP4-mediated signals on osteoclast function are similar to those that are calcitonin receptor-mediated. Thus, osteoclast precursors down-regulate their own EP2 and EP4 levels during their differentiation into osteoclasts to escape inhibitory effects of PGE2 on bone resorption.

Salicylic acid-derived poly(anhydride-esters) inhibit bone resorption and formationin vivo

The objective of this study was to investigate the effects of a novel polymer that biodegrades into salicylic acid (SA) on the healing of critical sized long bone defects. Microspheres of the homopolymer, or a copolymer containing 50% less of the SA, were packed into 5-mm mid-diaphyseal defects in rat femurs. Control animals received collagen sponge implants. After 4 and 8 weeks of implantation, bone healing was evaluated using microradiography and quantitative histomorphometry. Four weeks postsurgery, significantly less new bone was formed in both of the polymer groups (p<0.038). Reduced bone loss was also noted with the polymers at this time, although it was not statistically significant. However, at 8 weeks postsurgery, a statistically significant reduction in bone loss was observed in both of the polymer groups compared with controls (p<0.0072). Both polymers seemed to elicit identical tissue responses because there were no differences detected between the homopolymer and copolymer materials at either time point. These results indicate that locally released SA can significantly reduce both bone loss and bone formation in this animal model.

PGE2 signal through EP2 promotes the growth of articular chondrocytes

EP2 was identified as the major PGE2 receptor expressed in articular cartilage. An EP2 agonist increased intracellular cAMP in articular chondrocytes, stimulating DNA synthesis in both monolayer and 3D cultures. Hence, the EP2 agonist may be a potent therapeutic agent for degenerative cartilage diseases.

INTRODUCTION

Prostaglandin E2 (PGE2) exhibits pleiotropic effects in various types of tissue through four types of receptors, EP1-4. We examined the expression of EPs and effects of agonists for each EP on articular chondrocytes.

MATERIALS AND METHODS

The expression of each EP in articular chondrocytes was examined by immunohistochemistry and RT-PCR. A chondrocyte cell line, MMA2, was established from articular cartilage of p53(-/-) mice and used to analyze the effects of agonists for each EP. A search for molecules downstream of the PGE2 signal through the EP2 agonist was made by cDNA microarray analysis. The growth-promoting effect of the EP2 agonist on chondrocytes surrounded by cartilage matrix was examined in an organ culture of rat femora.

RESULTS AND CONCLUSION

EP2 was identified as the major EP expressed in articular cartilage. Treatment of MMA2 cells with specific agonists for each EP showed that only the EP2 agonist significantly increased intracellular cAMP levels in a dose-dependent manner. Gene expression profiling of MMA2 revealed a set of genes upregulated by the EP2 agonist, including several growth-promoting and apoptosis-protecting genes such as the cyclin D1, fibronectin, integrin alpha5, AP2alpha, and 14-3-3gamma genes. The upregulation of these genes by the EP2 agonist was confirmed in human articular chondrocytes by quantitative mRNA analysis. On treatment with the EP2 agonist, human articular chondrocytes showed an increase in the incorporation of 5-bromo-2-deoxyuracil (BrdU), and the organ culture of rat femora showed an increase of proliferating cell nuclear antigen (PCNA) staining in articular chondrocytes surrounded by cartilage matrix, suggesting growth-promoting effects of the PGE2 signal through EP2 in articular cartilage. These results suggested that the PGE2 signal through EP2 enhances the growth of articular chondrocytes, and the EP2 agonist is a candidate for a new therapeutic compound for the treatment of degenerative cartilage diseases.

Prostaglandin E2 receptor subtype EP-2 is not involved in the induction of non-pregnant guinea pig uterine contractions associated with terminal pregnancy

Prostaglandin E2 (PGE2) exerts its biological effects through 4 different receptor subtypes, EP-1, EP-2, EP-3, and EP-4. Recently we have demonstrated the importance of the prostaglandin E2 receptor subtype EP-2 in the healing of bone defects and fractures. This discovery led to the identification of CP-533,536, an EP-2 selective agonist, a promising therapeutic alternative for the enhancement of bone healing and the treatment of fractures (J Bone Miner Res 18 (2003) 2033). PGE2 has a myriad of effects throughout the body including the induction of uterine contractions, which results in termination of pregnancies. Our objective in this study was to determine the role of the EP-2 receptor and specifically that of CP-533,536, an EP-2 specific agonist, to induce uterine contractions and terminate pregnancy in guinea pigs, an animal model of human pregnancy. Preliminary experiments confirmed earlier reports that the guinea pig uterus was more sensitive than that of the rat. The guinea pig uterus contains the four PGE2 receptor subtypes, and ex vivo treatment of the uterus with PGE2 as expected causes profound uterine contractions. However, using receptor selective prostaglandin agonists including CP-533,536 we showed that the EP-1 and 3 receptors not the EP-2 receptor is responsible for the induction of uterine contractions of PGE2. Further, CP-533,536 did not antagonize the ability of PGE2 to induce uterine contractions in this model.

Recombinant human bone morphogenetic protein-2 delivered in an injectable calcium phosphate paste accelerates osteotomy-site healing in a nonhuman primate model

BACKGROUND

In recent clinical trials demonstrating the efficacy of recombinant human bone morphogenetic protein-2 (rhBMP-2) for the acceleration of bone-healing, investigators used carriers requiring open surgery for administration. In this study, we used a nonhuman primate fibular osteotomy model to evaluate injectable rhBMP-2/carrier formulations that can be administered in closed fractures.

METHODS

The fibular osteotomy model was first characterized by evaluating surgically harvested fibular segments containing untreated osteotomy sites (controls) from seventy adult male Cynomolgus monkeys at eight weeks (twenty-four monkeys), ten weeks (thirty-four), twelve weeks (six), and fourteen weeks (six). Fibular segments, from twenty-four animals, in which an osteotomy had not been performed served as normal controls (intact). The contralateral limb of twenty-four of the animals was then used to evaluate the effect of rhBMP-2 administered, three hours after the osteotomy, in eight carrier formulations (buffer, calcium phosphate paste, and hyaluronan gel, hyaluronan paste, and gelatin foam formulated with and without tricalcium phosphate granules). Each carrier was used in three monkeys. At ten weeks, the fibulae with the treated osteotomy sites were harvested and were compared with the contralateral, untreated osteotomized fibulae (paired control). The most promising carrier, calcium phosphate paste (alpha bone substitute material, or alpha-BSM), was then evaluated in eleven additional animals. The outcomes included the findings on radiographs made weekly until the time of fibular harvest, the callus area, the biomechanical properties, and the histologic findings.

RESULTS

Radiographic and histologic studies confirmed complete bridging of the control osteotomy sites in most animals by fourteen weeks. The mean torsional stiffness and maximum torque of the control osteotomy sites were 42.7% and 53.7%, 55.2% and 60.4%, 66.7% and 66.4% of the mean torsional stiffness and maximum torque of the intact fibulae at eight, ten, and twelve weeks, respectively, but they were not substantially different from the mean torsional stiffness and maximum torque of the intact fibulae at fourteen weeks (82.3% and 79.8%). In the carrier screening study, outcome measures of healing were more consistently enhanced in the rhBMP-2/alpha-BSM-treated osteotomy sites. In the confirmatory study, the mean callus area, torsional stiffness, and maximum torque were 86%, 72%, and 68% greater in the rhBMP-2/alpha-BSM-treated osteotomy sites than in the paired-control osteotomy sites at ten weeks (p < 0.001). The torsional stiffness and maximum torque in the rhBMP-2/alpha-BSM-treated osteotomy sites were equal to those in the intact fibulae, whereas those parameters in the paired-control osteotomy sites were only 55% and 58%, respectively, of the torsional stiffness and maximum torque of the intact fibulae. Histologic analysis confirmed complete osseous bridging of the rhBMP-2/alpha-BSM-treated osteotomy sites but incomplete bridging of the paired-control osteotomy sites at ten weeks.

CONCLUSIONS

A single percutaneous injection of rhBMP-2/alpha-BSM accelerates the healing of fibular osteotomy sites in nonhuman primates by approximately 40% compared with the healing of untreated osteotomy sites.

A prostanoid receptor EP4 agonist enhances ectopic bone formation induced by recombinant human bone morphogenetic protein-2

The anabolic effects of prostaglandin E(2) on bone are effected through the activation of EP4, a G protein-coupled receptor. In the present study, we examined the effects of a prostanoid receptor-selective agonist (ONO-4819) in an experimental system of ectopic bone formation using recombinant human bone morphogenetic protein-2 (rhBMP-2). Collagen pellets containing rhBMP-2 were implanted onto the back muscles of mice and then treated with ONO-4819 administered every 8 h by subcutaneous injection. The ossicles elicited ectopically by rhBMP-2 in mice treated with 30 microg/kg ONO-4819 were significantly larger in size and had a higher bone mineral density and bone mineral content when compared to the controls. We also noted that the anabolic effect of ONO-4819 was seen only in the early phase of the rhBMP-2-induced bone-forming process. These experimental results indicate that the EP4 receptor agonist enhances the rhBMP-2-induced bone formation through a selective effect on early stage mesenchymal cells, which in turn may result in increased responsiveness of the host animals to rhBMP-2.

Upregulation of orphan nuclear receptor Nur77 following PGF(2alpha), Bimatoprost, and Butaprost treatments. Essential role of a protein kinase C pathway involved in EP(2) receptor activated Nur77 gene transcription

1. Using gene chip technology, we first identified that PGF(2alpha) (FP agonist) and Butaprost (EP(2) agonist) induced about a five-fold upregulation of Nur77 mRNA expression in hFP-HEK 293/EBNA and hEP(2)-HEK293/EBNA cells. Northern Blot analysis revealed that PGF(2alpha)- and Butaprost-induced upregulation of Nur77 expression are dose- and time-dependent. 2. Both PGF(2alpha) and Butaprost upregulated Nur77 gene expression through the protein kinase C (PKC) pathway. These data are the first showing a link between EP(2) receptor stimulation and protein kinase C activation. Calcineurin was found to be involved downstream of the PKC pathway in PGF(2alpha)-induced Nur77 expression, but not in Butaprost-induced Nur77 expression. 3. We also used Nur77 as a marker gene to compare the effects of PGF(2alpha), Butaprost, and Bimatoprost (a prostamide) on Nur77 expression in human primary trabecular meshwork and ciliary smooth muscle (SM) cells, which are target cells for antiglaucoma drugs. The results showed that PGF(2alpha) and Butaprost, but not Bimatoprost, induced upregulation of Nur77 expression in human TM cells. PGF(2alpha), but not Bimatoprost, dramatically induced upregulation of Nur77 mRNA expression in human ciliary SM cells, whereas Butaprost slightly upregulated Nur77 mRNA expression in SM cells. 4. Nur77 promoter deletion analysis indicated that PGF(2alpha), but not Bimatoprost, activated Nur77 promoter-luciferase reporter in hFP-HEK 293/EBNA cells. Butaprost was less efficacious in inducing Nur77 promoter-luciferase reporter activity in hEP(2)-HEK293/EBNA cells relative to PGF(2alpha) in the comparable assay. The data for Nur77 promoter functional analysis were matched to the Northern blot analysis. 5. It appears that PGF(2alpha) and Butaprost activate Nur77 transcription mechanisms through the activation of FP and EP(2) receptor-coupled signaling pathways, whereas Bimatoprost stimulates neither FP nor EP(2) receptors.

Prostaglandin E2 receptor (EP4) selective agonist (ONO-4819.CD) accelerates bone repair of femoral cortex after drill-hole injury associated with local upregulation of bone turnover in mature rats

Prostaglandin E(2) (PGE(2)) is essential for fracture healing. Systemic administration of EP4 ligands such as PGE(2) and other synthetic EP4 agonists appears to transduce anabolic signals by binding to receptor EP4. Therefore, the present study was designed to test whether administration of EP4 agonist accelerates the healing of drill-hole injury in the femoral diaphysis. After surgery, a total of 128 Wistar rats, at the age of 12 weeks, were assigned to basal control (n = 8), and three groups with respective doses of 0 (vehicle control), 10 (low-dose), and 30 (high-dose) microg/kg body weight of the agent were subcutaneously injected twice a day. Femoral specimens were obtained at 0, 5, 7, 14, 21, and 28 days. In EP4 agonist-treated groups, the total bone volume of the regenerating bone in the defect did not significantly differ, but the regenerated cortical bone volume measured by histomorphometry and cortical bone mineral content (Ct. BMC) by pQCT dose-dependently increased at 14 and 21 days compared to the control. In the high-dose group, the value of osteoclast surface significantly increased compared with that in the control at 14 days. Expression levels of osteocalcin and TRAP mRNAs in the injured tissue increased at 14 days. Expression levels of EP4, BMP-2, and RANKL mRNAs increased at 7 days in the high-dose group. The bone mineral values of the lumbar bone at 28 days, measured by DXA, did not differ in the three groups. These data indicated that systemic administration of EP4 agonist ONO-4819.CD accelerated cortical bone healing after drill-hole injury by upregulating the local turnover of the regenerating bone.

EP2 and EP4 prostanoid receptor signaling

The EP(2) and EP(4) prostanoid receptors are two of the four subtypes of receptors for prostaglandin E(2) (PGE(2)). They are in the family of G-protein coupled receptors and both receptors were initially characterized as coupling to Gs and increasing intracellular cAMP formation. Recently, however, we have shown that both receptors can stimulate T-cell factor (Tcf) mediated transcriptional activity. The EP(2) receptor does this primarily through cAMP-dependent protein kinase (PKA), whereas the EP(4) utilizes phosphatidylinositol 3-kinase (PI3K) as well as PKA. In addition, we have shown that the EP(4) receptor, but not the EP(2), can activate the extracellular signal-regulated kinases (ERKs) 1 and 2 by way of PI3K leading to the induction of early growth response factor-1 (EGR-1), a transcription factor traditionally associated with wound healing. This induction of EGR-1 expression has significant implications concerning the potential role of PGE(2) in cancer and inflammatory disorders.

Angiogenesis and bone repair

The intimate connection, both physical and biochemical, between blood vessels and bone cells has long been recognized. Genetic, biochemical, and pharmacological studies have identified and characterized factors involved in the conversation between endothelial cells (EC) and osteoblasts (OB) during both bone formation and repair. The long-awaited FDA approval of two growth factors, BMP-2 and OP-1, with angiogenic and osteogenic activity confirms the importance of these two processes in human skeletal healing. In this review, the role of osteogenic factors in the adaptive response and interactive function of OB and EC during the multi-step process of bone repair will be discussed.