Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

The potential benefits and inherent risks of vibration as a non-drug therapy for the prevention and treatment of osteoporosis

The delivery of mechanical signals to the skeleton using vibration is being considered as a non-drug treatment of osteoporosis. Delivered over a range of magnitudes and frequencies, vibration has been shown to be both anabolic and anti-catabolic to the musculoskeletal tissues, yet caution must be emphasized as these mechanical signals, particularly chronic exposure to higher intensities, is a known pathogen to many physiological systems. In contrast, accumulating preclinical and clinical evidence indicates that low intensity vibration (LIV) improves bone quality through regulating the activity of cells responsible for bone remodeling, as well as biasing the differentiation fate of their mesenchymal and hematopoietic stem cell progenitors. In vitro studies provide insights into the biologic mechanisms of LIV, and indicate that cells respond to these low magnitude signals through a distinct mechanism driven not by matrix strain but acceleration. These cell, animal, and human studies may represent the foundation of a safe, non-drug means to protect and improve the musculoskeletal system of the elderly, injured, and infirmed.

Human immune cells' behavior and survival under bioenergetically restricted conditions in an in vitro fracture hematoma model

The initial inflammatory phase of bone fracture healing represents a critical step for the outcome of the healing process. However, both the mechanisms initiating this inflammatory phase and the function of immune cells present at the fracture site are poorly understood. In order to study the early events within a fracture hematoma, we established an in vitro fracture hematoma model: we cultured hematomas forming during an osteotomy (artificial bone fracture) of the femur during total hip arthroplasty (THA) in vitro under bioenergetically controlled conditions. This model allowed us to monitor immune cell populations, cell survival and cytokine expression during the early phase following a fracture. Moreover, this model enabled us to change the bioenergetical conditions in order to mimic the in vivo situation, which is assumed to be characterized by hypoxia and restricted amounts of nutrients. Using this model, we found that immune cells adapt to hypoxia via the expression of angiogenic factors, chemoattractants and pro-inflammatory molecules. In addition, combined restriction of oxygen and nutrient supply enhanced the selective survival of lymphocytes in comparison with that of myeloid derived cells (i.e., neutrophils). Of note, non-restricted bioenergetical conditions did not show any similar effects regarding cytokine expression and/or different survival rates of immune cell subsets. In conclusion, we found that the bioenergetical conditions are among the crucial factors inducing the initial inflammatory phase of fracture healing and are thus a critical step for influencing survival and function of immune cells in the early fracture hematoma.

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.

High throughput transcriptome profiling of lithium stimulated human mesenchymal stem cells reveals priming towards osteoblastic lineage

Human mesenchymal stem cells (hMSCs) present in the bone marrow are the precursors of osteoblasts, chondrocytes and adipocytes, and hold tremendous potential for osteoregenerative therapy. However, achieving directed differentiation into osteoblasts has been a major concern. The use of lithium for enhancing osteogenic differentiation has been documented in animal models but its effect in humans is not clear. We, therefore, performed high throughput transcriptome analysis of lithium-treated hMSCs to identify altered gene expression and its relevance to osteogenic differentiation. Our results show suppression of proliferation and enhancement of alkaline phosphatase (ALP) activity upon lithium treatment of hMSCs under non-osteogenic conditions. Microarray profiling of lithium-stimulated hMSC revealed decreased expression of adipogenic genes (CEBPA, CMKLR1, HSD11B1) and genes involved in lipid biosynthesis. Interestingly, osteoclastogenic factors and immune responsive genes (IL7, IL8, CXCL1, CXCL12, CCL20) were also downregulated. Negative transcriptional regulators of the osteogenic program (TWIST1 and PBX1) were suppressed while genes involved in mineralization like CLEC3B and ATF4 were induced. Gene ontology analysis revealed enrichment of upregulated genes related to mesenchymal cell differentiation and signal transduction. Lithium priming led to enhanced collagen 1 synthesis and osteogenic induction of lithium pretreated MSCs resulted in enhanced expression of Runx2, ALP and bone sialoprotein. However, siRNA-mediated knockdown of RRAD, CLEC3B and ATF4 attenuated lithium-induced osteogenic priming, identifying a role for RRAD, a member of small GTP binding protein family, in osteoblast differentiation. In conclusion, our data highlight the transcriptome reprogramming potential of lithium resulting in higher propensity of lithium "primed" MSCs for osteoblastic differentiation.

Clinically applicable antianginal agents suppress osteoblastic transformation of myogenic cells and heterotopic ossifications in mice

Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant disorder characterized by progressive heterotopic ossification. FOP is caused by a gain-of-function mutation in ACVR1 encoding the bone morphogenetic protein type II receptor, ACVR1/ALK2. The mutant receptor causes upregulation of a transcriptional factor, Id1. No therapy is available to prevent the progressive heterotopic ossification in FOP. In an effort to search for clinically applicable drugs for FOP, we screened 1,040 FDA-approved drugs for suppression of the Id1 promoter activated by the mutant ACVR1/ALK2 in C2C12 cells. We found that that two antianginal agents, fendiline hydrochloride and perhexiline maleate, suppressed the Id1 promoter in a dose-dependent manner. The drugs also suppressed the expression of native Id1 mRNA and alkaline phosphatase in a dose-dependent manner. Perhexiline but not fendiline downregulated phosphorylation of Smad 1/5/8 driven by bone morphogenetic protein (BMP)-2. We implanted crude BMPs in muscles of ddY mice and fed them fendiline or perhexiline for 30 days. Mice taking perhexiline showed a 38.0 % reduction in the volume of heterotopic ossification compared to controls, whereas mice taking fendiline showed a slight reduction of heterotopic ossification. Fendiline, perhexiline, and their possible derivatives are potentially applicable to clinical practice to prevent devastating heterotopic ossification in FOP.

The role of oxygen during fracture healing

Oxygen affects the activity of multiple skeletogenic cells and is involved in many processes that are important for fracture healing. However, the role of oxygen in fracture healing has not been fully studied. Here we systematically examine the effects of oxygen tension on fracture healing and test the ability of hyperoxia to rescue healing defects in a mouse model of ischemic fracture healing. Mice with tibia fracture were housed in custom-built gas chambers and groups breathed a constant atmosphere of 13% oxygen (hypoxia), 21% oxygen (normoxia), or 50% oxygen (hyperoxia). The influx of inflammatory cells to the fracture site, stem cell differentiation, tissue vascularization, and fracture healing were analyzed. In addition, the efficacy of hyperoxia (50% oxygen) as a treatment regimen for fracture nonunion was tested. Hypoxic animals had decreased tissue vascularity, decreased bone formation, and delayed callus remodeling. Hyperoxia increased tissue vascularization, altered fracture healing in un-complicated fractures, and improved bone repair in ischemia-induced delayed fracture union. However, neither hypoxia nor hyperoxia significantly altered chondrogenesis or osteogenesis during early stages of fracture healing, and infiltration of macrophages and neutrophils was not affected by environmental oxygen after bone injury. In conclusion, our results indicate that environmental oxygen levels affect tissue vascularization and fracture healing, and that providing oxygen when fractures are accompanied by ischemia may be beneficial.

Effects of a selective cyclooxygenase-2 inhibitor (celecoxib) on fracture healing in rats

BACKGROUND

Several studies suggested that celecoxib interferes with bone healing while others contradict these findings. This study was conducted to investigate the effects of celecoxib on bone healing in rats femur mold with a dose based on body surface area conversion.

MATERIALS AND METHODS

72 adult female Sprague Dawley rats were randomly divided into three groups after the internal fixation operation of nondisplaced transverse mid diaphyseal fractures of the right femurs. Each group was treated with 1% methylcellulose, celecoxib (21 mg/kg/d) for 1 week, or celecoxib (21 mg/kg/d) for 4 weeks after surgeries respectively. Bone healing scores and callus formation were evaluated by radiographs at 3, 4, 6 weeks after surgeries. Half of these rats were sacrificed for histological analysis at 4 weeks after surgery. The remaining fractured femurs were evaluated by biomechanical tests at 6 weeks after surgery.

RESULTS

The mean radiographic scores for fracture healing of both short and long term groups were lower than that of the control group and the differences among the three groups were statistically significant (P < 0.05) at 3, 4, 6 weeks after surgery. The mean bone trabecula density of both groups was smaller than that of the control group and the differences were also statistically significant (P < 0.05) at 4 week. The maximum load, total energy and stiffness in both the short term and long term groups were significantly decreased compared with those in the control group (P < 0.05) at 6 week.

CONCLUSION

Both short term and long term sustained use of celecoxib in rat models has significantly inhibitory effects on rat fracture healing.

MMP9 regulates the cellular response to inflammation after skeletal injury

Like other tissue injuries, bone fracture triggers an inflammatory response, which plays an important role in skeletal repair. Inflammation is believed to have both positive and negative effects on bone repair, but the underlying cellular mechanisms are not well understood. To assess the role of inflammation on skeletal cell differentiation, we used mouse models of fracture repair that stimulate either intramembranous or endochondral ossification. In the first model, fractures are rigidly stabilized leading to direct bone formation, while in the second model, fracture instability causes cartilage and bone formation. We compared the inflammatory response in these two mechanical environments and found changes in the expression patterns of inflammatory genes and in the recruitment of inflammatory cells and osteoclasts. These results suggested that the inflammatory response could influence skeletal cell differentiation after fracture. We then exploited matrix metalloproteinase 9 (MMP9) that is expressed in inflammatory cells and osteoclasts, and which we previously showed is a potential regulator of cell fate decisions during fracture repair. Mmp9(-/-) mice heal stabilized fractures via endochondral ossification, while wild type mice heal via intramembranous ossification. In parallel, we observed increases in macrophages and T cells in the callus of Mmp9(-/-) compared to wild type mice. To assess the link between the profile of inflammatory cells and skeletal cell fate functionally, we transplanted Mmp9(-/-) mice with wild type bone marrow, to reconstitute a wild type hematopoietic lineage in interaction with the Mmp9(-/-) stroma and periosteum. Following transplantation, Mmp9(-/-) mice healed stabilized fractures via intramembranous ossification and exhibited a normal profile of inflammatory cells. Moreover, Mmp9(-/-) periosteal grafts healed via intramembranous ossification in wild type hosts, but healed via endochondral ossification in Mmp9(-/-) hosts. We observed that macrophages accumulated at the periosteal surface in Mmp9(-/-) mice, suggesting that cell differentiation in the periosteum is influenced by factors such as BMP2 that are produced locally by inflammatory cells. Taken together, these results show that MMP9 mediates indirect effects on skeletal cell differentiation by regulating the inflammatory response and the distribution of inflammatory cells, leading to the local regulation of periosteal cell differentiation.

Current insights on the regenerative potential of the periosteum: molecular, cellular, and endogenous engineering approaches

While century old clinical reports document the periosteum's remarkable regenerative capacity, only in the past decade have scientists undertaken mechanistic investigations of its regenerative potential. At a Workshop at the 2012 Annual Meeting of Orthopaedic Research Society, we reviewed the molecular, cellular, and tissue scale approaches to elucidate the mechanisms underlying the periosteum's regenerative potential as well as translational therapies engineering solutions inspired by its remarkable regenerative capacity. The entire population of osteoblasts within periosteum, and at endosteal and trabecular bone surfaces within the bone marrow, derives from the embryonic perichondrium. Periosteal cells contribute more to cartilage and bone formation within the callus during fracture healing than do cells of the bone marrow or endosteum, which do not migrate out of the marrow compartment. Furthermore, a current healing paradigm regards the activation, expansion, and differentiation of periosteal stem/progenitor cells as an essential step in building a template for subsequent neovascularization, bone formation, and remodeling. The periosteum comprises a complex, composite structure, providing a niche for pluripotent cells and a repository for molecular factors that modulate cell behavior. The periosteum's advanced, "smart" material properties change depending on the mechanical, chemical, and biological state of the tissue. Understanding periosteum development, progenitor cell-driven initiation of periosteum's endogenous tissue building capacity, and the complex structure-function relationships of periosteum as an advanced material are important for harnessing and engineering ersatz materials to mimic the periosteum's remarkable regenerative capacity.

Could heterotopic ossification be prevented by varying dietary n-3/n-6 polyunsaturated fatty acid ratio: a novel perspective to its treatment?

Heterotopic ossification (HO) is a common complication following with musculoskeletal trauma and surgical procedures. It usually decreases joint mobility and eventually causes loss of joint function. Despite nonsteroidal anti-inflammatory drugs (NSAIDs), the inhibitor of cyclooxygenase(COX), have been proven to prevent HO effectively via prostaglandin E2 synthesis regulation and modulation of tissue responsiveness to pro-inflammatory signaling, HO prevention is still a matter of debate for clinicians to avoid the side effect of NSAIDs. Interestingly, it is suggested that PGE2 production and pro-inflammatory microenvironment in body could be modified by varying the ratio of the precursor fatty acids in the diet. On account of the effect of dietary (n-6)/(n-3) PUFAs ratio on both COX metabolism and pro-inflammatory cytokines mediated biological responsiveness, we hypothesized lowering dietary (n-6)/(n-3) PUFAs ratio may not only directly reduce the substrate of COX-2 and COX-2 activity, but also partially ameliorate tissue inflammatory responsiveness to cytokines correlated with HO development,exerting an inhibitory effect on PGE2 synthesis to prevent HO formation. The negative role of lowering dietary (n-6)/(n-3) PUFAs ratio on angiogenesis, cytokines-induced apoptosis, inflammatory responsiveness and osteogenesis could also contribute to its action on HO development. If our hypothesis is proved to be corrected, it could be an innovative method to treat HO.

PGE2 and BMP-2 in bone and cartilage metabolism: 2 intertwining pathways

Osteoarthritis and lesions to cartilage tissue are diseases that frequently result in impaired joint function and patient disability. The treatment of osteoarthritis, along with local bone defects and systemic skeletal diseases, remains a significant clinical challenge for orthopaedic surgeons. Several bone morphogenetic proteins (BMPs) are known to have osteoinductive effects, whereof BMP-2 and BMP-7 are already approved for clinical applications. There is growing evidence that the metabolism of bone as well as the cartilage damage associated with the above disease processes are strongly inter-related with the interactions of the inflammation-related pathways (in particular prostaglandin E2 (PGE2)) and osteogenesis (in particular bone morphogenetic protein-2 (BMP-2)). There is strong evidence that the pathways of prostaglandins and bone morphogenetic proteins are intertwined, and they have recently come into focus in several experimental and clinical studies. This paper focuses on PGE2 and BMP-2 intertwining pathways in bone and cartilage metabolism, and summarizes the recent experimental and clinical data.

Periosteal BMP2 activity drives bone graft healing

Bone graft incorporation depends on the orchestrated activation of numerous growth factors and cytokines in both the host and the graft. Prominent in this signaling cascade is BMP2. Although BMP2 is dispensable for bone formation, it is required for the initiation of bone repair; thus understanding the cellular mechanisms underlying bone regeneration driven by BMP2 is essential for improving bone graft therapies. In the present study, we assessed the role of Bmp2 in bone graft incorporation using mice in which Bmp2 has been removed from the limb prior to skeletal formation (Bmp2(cKO)). When autograft transplantations were performed in Bmp2cKO mice, callus formation and bone healing were absent. Transplantation of either a vital wild type (WT) bone graft into a Bmp2(cKO) host or a vital Bmp2(cKO) graft into a WT host also resulted in the inhibition of bone graft incorporation. Histological analyses of these transplants show that in the absence of BMP2, periosteal progenitors remain quiescent and healing is not initiated. When we analyzed the expression of Sox9, a marker of chondrogenesis, on the graft surface, we found it significantly reduced when BMP2 was absent in either the graft itself or the host, suggesting that local BMP2 levels drive periosteal cell condensation and subsequent callus cell differentiation. The lack of integrated healing in the absence of BMP2 was not due to the inability of periosteal cells to respond to BMP2. Healing was achieved when grafts were pre-soaked in rhBMP2 protein, indicating that periosteal progenitors remain responsive in the absence of BMP2. In contrast to the requirement for BMP2 in periosteal progenitor activation in vital bone grafts, we found that bone matrix-derived BMP2 does not significantly enhance bone graft incorporation. Taken together, our data show that BMP2 signaling is not essential for the maintenance of periosteal progenitors, but is required for the activation of these progenitors and their subsequent differentiation along the osteo-chondrogenic pathway. These results indicate that BMP2 will be among the signaling molecules whose presence will determine success or failure of new bone graft strategies.

Therapeutic controversies in spondyloarthritis: nonsteroidal anti-inflammatory drugs

Nonsteroidal anti-inflammatory drugs (NSAIDs) are considered a first-line therapy in patients with axial spondyloarthritis (axSpA), including ankylosing spondylitis. NSAIDs reduce pain and stiffness effectively in most patients, are able to reduce systemic and local inflammation, and can inhibit progression of structural damage in the spine. However, effective control of symptoms and retardation of radiographic progression often require continuous and long-term treatment, which raises safety concerns. This article discusses controversies related to the current role of NSAIDs in axSpA treatment, risks and benefits of this treatment, and current trends for individualized treatment.

Perioperative glucocorticosteroid treatment delays early healing of a mandible wound by inhibiting osteogenic differentiation

AIM

The purpose of this study is to investigate the effects of dexamethasone on repair of a critical size defect of the mandible in male Sprague-Dawley rats.

MATERIALS AND METHODS

Fifty rats were divided into 2 groups: saline control and dexamethasone-treated groups. A 1 mm × 3 mm full-thickness bone defect was created at the inferior border of the mandible. Saline or dexamethasone was administered once a day for 5 days after postoperative palinesthesia. On days 1, 3, 6, 10 and 17, after cessation of drug administration, 5 samples from each group were analysed. The bone defect healing process was examined and analysed by stereology, radiology, histology and histochemical staining for total collagen, tartrate-resistant acid phosphatase staining for osteoclasts and immunohistochemical staining for the COX-2, RUNX2 and osteocalcin antigens.

RESULTS

The dexamethasone-treated rats exhibited significantly lower radiopacity properties compared to the control rats. Histological staining revealed that the osteogenic differentiation and maturation of a callus in the defect region was significantly delayed from day 1 to day 10 in the dexamethasone group after cessation of drug administration compared to the control group. Consistent with the histological data, the level of total collagen protein was significantly lower in the dexamethasone group than in the control group. However, there was no significant difference between the 2 groups at day 17. Immunohistochemical analysis of COX-2, RUNX2 and osteocalcin expression showed that, at day 1, COX-2 and RUNX2 expression in the dexamethasone group was significantly lower than in the control group. There was no significant difference in osteocalcin expression between the two groups at each time point. There was no significant difference in the number of osteoclasts between the two groups.

CONCLUSION

In a model of bone healing of a mandible defect, dexamethasone-treated rats exhibited impaired osteogenic differentiation and maturation due to the inhibition of COX-2, osteogenic gene, RUNX2 and collagen protein expression, which resulted in delayed bone repair. Although perioperative short-term therapy did not exhibit long-term effects on wound healing of the maxillofacial bone, the application of glucocorticoids should be cautiously considered in the clinic.

Anti-inflammatory treatment increases angiogenesis during early fracture healing

OBJECTIVES

Both inflammation and angiogenesis are crucial for normal fracture healing. The goal of this work was to determine how anti-inflammatory treatment affects angiogenesis during early stages of fracture repair.

METHODS

Tibia fractures were created in adult mice and animals were treated with indomethacin (2 mg/kg/day), a non-steroidal anti-inflammatory drug, or PBS once a day beginning from 1 day before fracture and continuing to 6 days after fracture. Animals were killed at 7, 14, and 28 days after injury for histomorphometric analysis of fracture healing. A second group of animals were killed at 3 and 7 days after injury to measure tissue levels of VEGF and interleukin-1 beta (IL-1β). A third group of animals were killed at 3 and 7 days after injury for stereology analysis of macrophage and neutrophil infiltration and tissue vascularization.

RESULTS

Indomethacin significantly decreased bone and cartilage formation at 7 days after fracture compared to controls. Indomethacin decreased the tissue levels of IL-1β at 3 days after fracture but did not affect the recruitment of macrophages or neutrophils to injured limbs. Indomethacin-treated fractures had similar length density and surface density of vasculature as the controls at 3 days after injury. At 7 days after fracture, vasculature in indomethacin-treated fractures exhibited higher length density and surface density than that in controls. By 28 days after injury, indomethacin-treated fractures still exhibited defects in fracture repair.

CONCLUSIONS

Anti-inflammatory treatments using indomethacin impair bone and cartilage formation and increase tissue vascularization in the callus during early fracture healing.

Fracture healing in protease-activated receptor-2 deficient mice

Protease-activated receptor-2 (PAR-2) provides an important link between extracellular proteases and the cellular initiation of inflammatory responses. The effect of PAR-2 on fracture healing is unknown. This study investigates the in vivo effect of PAR-2 deletion on fracture healing by assessing differences between wild-type (PAR-2(+/+)) and knock-out (PAR-2(-/-)) mice. Unilateral mid-shaft femur fractures were created in 34 PAR-2(+/+) and 28 PAR-2(-/-) mice after intramedullary fixation. Histologic assessments were made at 1, 2, and 4 weeks post-fracture (wpf), and radiographic (plain radiographs, micro-computed tomography (µCT)) and biomechanical (torsion testing) assessments were made at 7 and 10 wpf. Both the fractured and un-fractured contralateral femur specimens were evaluated. Polar moment of inertia (pMOI), tissue mineral density (TMD), bone volume fraction (BV/TV) were determined from µCT images, and callus diameter was determined from plain radiographs. Statistically significant differences in callus morphology as assessed by µCT were found between PAR-2(-/-) and PAR-2(+/+) mice at both 7 and 10 wpf. However, no significant histologic, plain radiographic, or biomechanical differences were found between the genotypes. The loss of PAR-2 was found to alter callus morphology as assessed by µCT but was not found to otherwise effect fracture healing in young mice.

Reduced osteoblast activity in the mice lacking TR4 nuclear receptor leads to osteoporosis

BACKGROUND

Early studies suggested that TR4 nuclear receptor might play important roles in the skeletal development, yet its detailed mechanism remains unclear.

METHODS

We generated TR4 knockout mice and compared skeletal development with their wild type littermates. Primary bone marrow cells were cultured and we assayed bone differentiation by alkaline phosphatase and alizarin red staining. Primary calvaria were cultured and osteoblastic marker genes were detected by quantitative PCR. Luciferase reporter assays, chromatin immunoprecipitation (ChIP) assays, and electrophoretic mobility shift assays (EMSA) were performed to demonstrate TR4 can directly regulate bone differentiation marker osteocalcin.

RESULTS

We first found mice lacking TR4 might develop osteoporosis. We then found that osteoblast progenitor cells isolated from bone marrow of TR4 knockout mice displayed reduced osteoblast differentiation capacity and calcification. Osteoblast primary cultures from TR4 knockout mice calvaria also showed higher proliferation rates indicating lower osteoblast differentiation ability in mice after loss of TR4. Mechanism dissection found the expression of osteoblast markers genes, such as ALP, type I collagen alpha 1, osteocalcin, PTH, and PTHR was dramatically reduced in osteoblasts from TR4 knockout mice as compared to those from TR4 wild type mice. In vitro cell line studies with luciferase reporter assay, ChIP assay, and EMSA further demonstrated TR4 could bind directly to the promoter region of osteocalcin gene and induce its gene expression at the transcriptional level in a dose dependent manner.

CONCLUSIONS

Together, these results demonstrate TR4 may function as a novel transcriptional factor to play pathophysiological roles in maintaining normal osteoblast activity during the bone development and remodeling, and disruption of TR4 function may result in multiple skeletal abnormalities.

The effect of nonsteroidal anti-inflammatory drug administration on acute phase fracture-healing: a review

BACKGROUND

The analgesic efficacy of nonsteroidal anti-inflammatory drugs (NSAIDs) is well established, and these agents often form an integral part of posttraumatic pain management. However, potentially deleterious effects of resulting prostaglandin suppression on fracture-healing have been suggested.

METHODS

A systematic literature review involving searches of electronic databases and online sources was performed to identify articles exploring the influence of NSAIDs on fracture-healing.

RESULTS

A structured search approach identified 316 papers as potentially relevant to the topic, and these were manually reviewed. The majority described small-scale studies that were retrospective or observational in nature, with limited control of potentially confounding variables, or presented little key information that was not also present in other studies.

CONCLUSIONS

Although increasing evidence from animal studies suggests that cyclooxygenase-2 (COX-2) inhibition suppresses early fracture-healing, in vivo studies involving human subjects have not provided convincing evidence to substantiate this concern. We found no robust evidence to attest to a significant and appreciable patient detriment resulting from the short-term use of NSAIDs following a fracture. The balance of evidence in the available literature appears to suggest that a short-duration NSAID regimen is a safe and effective supplement to other modes of post-fracture pain control, without a significantly increased risk of sequelae related to disrupted healing.

Activation of NF-κB/p65 facilitates early chondrogenic differentiation during endochondral ossification

BACKGROUND

NF-κB/p65 has been reported to be involved in regulation of chondrogenic differentiation. However, its function in relation to key chondrogenic factor Sox9 and onset of chondrogenesis during endochondral ossification is poorly understood. We hypothesized that the early onset of chondrogenic differentiation is initiated by transient NF-κB/p65 signaling.

METHODOLOGY/PRINCIPAL FINDINGS

The role of NF-κB/p65 in early chondrogenesis was investigated in different in vitro, ex vivo and in vivo endochondral models: ATDC5 cells, hBMSCs, chicken periosteal explants and growth plates of 6 weeks old mice. NF-κB/p65 activation was manipulated using pharmacological inhibitors, RNAi and activating agents. Gene expression and protein expression analysis, and (immuno)histochemical stainings were employed to determine the role of NF-κB/p65 in the chondrogenic phase of endochondral development. Our data show that chondrogenic differentiation is facilitated by early transient activation of NF-κB/p65. NF-κB/p65-mediated signaling determines early expression of Sox9 and facilitates the subsequent chondrogenic differentiation programming by signaling through key chondrogenic pathways.

CONCLUSIONS/SIGNIFICANCE

The presented data demonstrate that NF-κB/p65 signaling, as well as its intensity and timing, represents one of the transcriptional regulatory mechanisms of the chondrogenic developmental program of chondroprogenitor cells during endochondral ossification. Importantly, these results provide novel possibilities to improve the success of cartilage and bone regenerative techniques.

The EP4-ERK-dependent pathway stimulates osteo-adipogenic progenitor proliferation resulting in increased adipogenesis in fetal rat calvaria cell cultures

We previously reported that fetal rat calvaria (RC) cells are osteo-adipogenic bipotential and that PGE(2) receptors EP2 and EP4 are involved in bone nodule formation via both common and distinct MAPK pathways in RC cell cultures. Because PGE(2) participates in multiple biological processes including adipogenesis, it is of interest to determine the additional role(s) of PGE(2) in RC cells. PGE(2) increased the number of adipocyte colonies when RC cells were treated during proliferation but not other development stages. Of four EP agonists tested, the EP4 agonist ONO-AE1-437 (EP4A) was the most effective in promoting adipogenesis. Concomitantly, EP4A increased the number of cells with BrdU labeling and gene expression of CCAAT/enhancer binding protein (C/EBP)δ and c-fos but not peroxisome proliferator-activated receptor γ2 and C/EBPα. Amongst MAPK inhibitors, U0126, an inhibitor of MEK1/2, abrogated the EP4A-dependent effects. Our results suggest that the PGE(2)-EP4-ERK pathway increases the number of osteo-adipogenic bipotential progenitor cells, with a resultant increase in adipogenesis in RC cell cultures.

Response of a preosteoblastic cell line to cyclic tensile stress conditioning and growth factors for bone tissue engineering

Bone regeneration can be accelerated by utilizing mechanical stress and growth factors (GFs). However, a limited understanding exists regarding the response of preosteoblasts to tensile stress alone or with GFs. We measured cell proliferation and expression of heat-shock proteins (HSPs) and other bone-related proteins by preosteoblasts following cyclic tensile stress (1%-10% magnitude) alone or in combination with bone morphogenetic protein-2 (BMP-2) and transforming growth factor-β1 (TGF-β1). Tensile stress (3%) with GFs induced greater gene upregulation of osteoprotegerin (3.3 relative fold induction [RFI] compared to sham-treated samples), prostaglandin E synthase 2 (2.1 RFI), and vascular endothelial growth factor (VEGF) (11.5 RFI), compared with samples treated with stimuli alone or sham-treated samples. The most significant increases in messenger RNA expression occurred with GF addition to either static-cultured or tensile-loaded (1% elongation) cells for the following genes: HSP47 (RFI=2.53), cyclooxygenase-2 (RFI=72.52), bone sialoprotein (RFI=11.56), and TGF-β1 (RFI=8.05). Following 5% strain with GFs, VEGF secretion increased 64% (days 3-6) compared with GF alone and cell proliferation increased 23% compared with the sham-treated group. GF addition increased osteocalcin secretion but decreased matrix metalloproteinase-9 significantly (days 3-6). Tensile stress and GFs in combination may enhance bone regeneration by initiating angiogenic and anti-osteoclastic effects and promote cell growth.

Effects of long-term administration of carprofen on healing of a tibial osteotomy in dogs

OBJECTIVE

To evaluate effects of long-term administration of carprofen on healing of a tibial osteotomy in dogs.

ANIMALS

12 healthy female Beagles.

PROCEDURES

A mid-diaphyseal transverse osteotomy (stabilized with an intramedullary pin) of the right tibia was performed in each dog. The carprofen group (n = 6 dogs) received carprofen (2.2 mg/kg, PO, q 12 h) for 120 days; the control group (6) received no treatment. Bone healing and change in callus area were assessed radiographically over time. Dogs were euthanized 120 days after surgery, and tibiae were evaluated biomechanically and histologically.

RESULTS

The osteotomy line was not evident in the control group on radiographs obtained 120 days after surgery. In contrast, the osteotomy line was still evident in the carprofen group. Callus area was significantly less in the carprofen group, compared with the area in the control group, at 20, 30, and 60 days after surgery. At 120 days after surgery, stiffness, elastic modulus, and flexural rigidity in the carprofen group were significantly lower than corresponding values in the control group. Furthermore, histologic evaluation revealed that the cartilage area within the callus in the carprofen group was significantly greater than that in the control group.

CONCLUSIONS AND CLINICAL RELEVANCE

Long-term administration of carprofen appeared to inhibit bone healing in dogs that underwent tibial osteotomy. We recommend caution for carprofen administration when treating fractures that have delays in healing associated with a reduction in osteogenesis as well as fractures associated with diseases that predispose animals to delays of osseous repair.

Effects of interleukin-6 ablation on fracture healing in mice

This study examined the impact of an interleukin-6 (IL-6) knockout on fracture healing in terms of histological and biomechanical responses. Following IACUC approval, tibial fractures were produced in 4- to 6-week-old IL-6 knockouts (n = 35) and wild-type mice (n = 36) and harvested along with contralateral limbs at 2 and 6 weeks postsurgery. Histology quantified stage of healing, lymphocyte infiltration, TRAP+ cells, and osteocalcin deposition. Bend testing established maximum load and stiffness. Based on normality assessments, Mann-Whitney U or independent t-tests were used for data analysis using a p-value threshold of 0.05. Stage of healing, lymphocyte infiltration, and osteocalcin deposition were similar for all time points (p ≥ 0.243). TRAP+ cell counts were reduced approximately 10-fold in the knockout at 2 weeks (p = 0.015) but were similar at 6 weeks (p = 0.689). Force-to-failure in knockouts was approximately 40% that of wild-type mice at 2 weeks (p = 0.040) but similar at 6 weeks (p = 0.735). Knockout bone was about 25% less stiff at 2 weeks but approximately 60% stiffer at 6 weeks (p ≥ 0.110). The absence of IL-6 during early fracture healing significantly reduced osteoclastogenesis and impaired callus strength. By 6 weeks, most histological and biomechanical parameters were similar to fractures in wild-type bone.

Molecular mechanisms involved in the mitogenic effect of lactoferrin in osteoblasts

Lactoferrin, an iron-binding glycoprotein present in milk and other exocrine secretions in mammals, is anabolic to bone at physiological concentrations. Lactoferrin stimulates the proliferation, differentiation and survival of osteoblasts, as well as potently inhibiting osteoclastogenesis in bone marrow cultures. In the current study we further investigated the mechanism of action of lactoferrin in osteoblasts. We used low-density arrays to measure the level of expression of 45 genes in MC3T3-E1 osteoblast-like cells treated with lactoferrin, and identified transient, dose-dependent increases in the transcription levels of interleukin-6, of the pro-inflammatory factor prostaglandin-endoperoxide synthase 2 (Ptgs2), and of the transcription factor nuclear factor of activated T cells (Nfatc1). We demonstrated similar changes in primary osteoblast cultures from human and rat. Levels of prostaglandin E2 were increased in conditioned media collected from osteoblasts treated with lactoferrin, indicating that the activity of the enzyme cyclooxygenase 2 (COX2), which is encoded by Ptgs2, was also up-regulated. Using a luciferase reporter construct we showed that lactoferrin induced transcription from the NFAT consensus sequence. We found that inhibiting either COX2 or NFATc1 activity blocked the mitogenic effect of lactoferrin in osteoblasts and that inhibition of NFATc1 activity partially blocked the transcriptional activation of Ptgs2. Our study has provided the first evidence that COX2 and NFATc1 activities are increased by lactoferrin, and demonstrated a role for each of these molecules as mediators of the mitogenic effects of lactoferrin in osteoblasts.

Gsα enhances commitment of mesenchymal progenitors to the osteoblast lineage but restrains osteoblast differentiation in mice

The heterotrimeric G protein subunit Gsα stimulates cAMP-dependent signaling downstream of G protein-coupled receptors. In this study, we set out to determine the role of Gsα signaling in cells of the early osteoblast lineage in vivo by conditionally deleting Gsα from osterix-expressing cells. This led to severe osteoporosis with fractures at birth, a phenotype that was found to be the consequence of impaired bone formation rather than increased resorption. Osteoblast number was markedly decreased and osteogenic differentiation was accelerated, resulting in the formation of woven bone. Rapid differentiation of mature osteoblasts into matrix-embedded osteocytes likely contributed to depletion of the osteoblast pool. In addition, the number of committed osteoblast progenitors was diminished in both bone marrow stromal cells (BMSCs) and calvarial cells of mutant mice. In the absence of Gsα, expression of sclerostin and dickkopf1 (Dkk1), inhibitors of canonical Wnt signaling, was markedly increased; this was accompanied by reduced Wnt signaling in the osteoblast lineage. In summary, we have shown that Gsα regulates bone formation by at least two distinct mechanisms: facilitating the commitment of mesenchymal progenitors to the osteoblast lineage in association with enhanced Wnt signaling; and restraining the differentiation of committed osteoblasts to enable production of bone of optimal mass, quality, and strength.

A role for the primary cilium in paracrine signaling between mechanically stimulated osteocytes and mesenchymal stem cells

Bone turnover is a mechanically regulated process, coordinated in part by the network of mechanosensitive osteocytes residing within the tissue. The recruitment and bone forming activity of the mesenchymal derived osteoblast is determined by numerous factors including mechanical loading. It is therefore somewhat surprising that although mechanically regulated signaling between the coordinating osteocytes and mesenchymal stem cells (MSCs) should exist, to date it has not been directly demonstrated. In this study, conditioned media from mechanically stimulated osteocytes (MLO-Y4 cell line) was collected and added to MSCs (C3H10T1/2 cell line). The addition of mechanically stimulated osteocyte conditioned media resulted in a significant upregulation of the osteogenic genes OPN and COX-2 in MSCs compared to statically cultured conditioned media, demonstrating a novel paracrine signaling mechanism between the two cell types. The same mechanically conditioned media did not alter gene expression in osteoblasts (MC3T3 cell line), and mechanically stimulated osteoblast conditioned media did not alter gene expression in MSCs demonstrating that this signaling is unique to osteocytes and MSCs. Finally, the upregulation in osteogenic genes in MSCs was not observed if primary cilia formation was inhibited prior to mechanical stimulation of the osteocyte. In summary, the results of this study indicate that soluble factors secreted by osteocytes in response to mechanical stimulation can enhance osteogenic gene expression in MSCs demonstrating a novel, unique signaling mechanism and introduces a role for the primary cilium in flow mediated paracrine signaling in bone thereby highlighting the cilium as a potential target for therapeutics aimed at enhancing bone formation.

NSAIDS inhibit in vitro MSC chondrogenesis but not osteogenesis: implications for mechanism of bone formation inhibition in man

The non-steroidal anti-inflammatory drugs (NSAIDs) are widely used for analgesia but may inhibit bone formation. We investigated whether the reported NSAID effect on bone is related to inhibition of bone marrow mesenchymal stem cell (MSC) proliferation and osteogenic and chondrogenic differentiation and evaluated both cyclooxygenase (COX)-1 and COX-2 specific drugs. The effects of seven COX-1 and COX-2 inhibitors on MSC proliferation and osteogenic and chondrogenic differentiation were tested using Vybrant, sodium 3'-[1-(phenylaminocarbonyl)- 3,4-tetrazolium]-bis (4-methoxy-6-nitro) benzene sulfonic acid hydrate (XTT), functional and quantitative assays of MSC differentiation. The MSC expression of COX-1 and COX-2 and prostaglandin E2 (PGE-2) levels were evaluated serially during lineage differentiation by quantitative PCR and ELISA. None of the NSAIDs at broad range of concentration (range 10(-3) to 100 μg/ml) significantly affected MSC proliferation. Surprisingly, MSC osteogenic differentiation inhibition was not evident. However, NSAIDs affected chondrogenic potential with a reduction in sulphated glycosaminoglycans (sGAG) content by 45% and 55% with diclofenac and ketorolac, respectively (P < 0.05 compared to controls). Parecoxib and meloxicam, more COX-2 specific reagents inhibited sGAG to a lesser degree, 22% and 27% respectively (P < 0.05 compared to controls). Cartilage pellet immunohistochemistry confirmed the above results. Pellet chondrogenesis was associated with increased COX-1 expression levels but not COX-2, and COX-1 specific drugs suppressed MSC PGE-2 more than COX-2 specific inhibitors. These findings suggest that NSAIDs may inhibit bone formation via blockage of MSC chondrogenic differentiation which is an important intermediate phase in normal endochondral bone formation.

Angiogenesis in bone regeneration

Angiogenesis is a key component of bone repair. New blood vessels bring oxygen and nutrients to the highly metabolically active regenerating callus and serve as a route for inflammatory cells and cartilage and bone precursor cells to reach the injury site. Angiogenesis is regulated by a variety of growth factors, notably vascular endothelial growth factor (VEGF), which are produced by inflammatory cells and stromal cells to induce blood vessel in-growth. A variety of studies with transgenic and gene-targeted mice have demonstrated the importance of angiogenesis in fracture healing, and have provided insights into regulatory processes governing fracture angiogenesis. Indeed, in animal models enhancing angiogenesis promotes bone regeneration, suggesting that modifying fracture vascularization could be a viable therapeutic approach for accelerated/improved bone regeneration clinically.

Allergy and the bone: unexpected relationships

OBJECTIVE

To examine the relationships between allergy and bone metabolism as represented by the effects of antihistamines and leukotriene modifiers on bone resorption and formation.

DATA SOURCES

The major databases MEDLINE and Scopus were searched using the keywords bone metabolism, bone resorption, bone formation, antihistamines, h1 antagonist, leukotriene antagonist, and leukotriene modifier.

STUDY SELECTION

The studies were independently evaluated by all the coauthors, who judged their pertinence to this review.

RESULTS

Two of the most intriguing aspects in the field are the possible effects of leukotriene modifiers on fracture repair and the hypothesized role of antihistamines in contrasting osteoporosis. Another rapidly expanding field of research is that related to the immune-modulating effects of vitamin D because serum levels of vitamin D have been shown to correlate with pulmonary function, asthma onset, and the development of allergic diseases.

CONCLUSION

Although unexpected, data are now available suggesting a strict connection among allergy, its treatments, and bone metabolism.

The early fracture hematoma and its potential role in fracture healing

Research regarding the potency and potential of the fracture hematoma has begun to receive increasing attention. However, currently there is a paucity of relevant literature on the capability and composition of the fracture hematoma. This review briefly summarizes the regenerative fracture healing process and the close interplay between the skeletal and immune systems. The role of immune cells in wound healing is also discussed to clarify their involvement in immunological processes during regeneration. We attempt to describe the current state of knowledge regarding the fracture hematoma as the initial stage of the regenerative process of fracture healing. The review discusses how a better understanding of immune reactions in the hematoma may have implications for bone tissue engineering strategies. We conclude the review by emphasizing how additional investigations of the initial phase of healing will allow us to better differentiate between deleterious and beneficial aspects of inflammation, thereby facilitating improved fracture treatment strategies.

High-dose ketorolac affects adult spinal fusion: a meta-analysis of the effect of perioperative nonsteroidal anti-inflammatory drugs on spinal fusion

STUDY DESIGN

Meta-analysis of literature.

OBJECTIVE

To evaluate the effect of perioperative nonsteroidal anti-inflammatory drugs (NSAIDs) on the success rate of adult spinal fusion.

SUMMARY OF BACKGROUND DATA

NSAIDs are commonly used to treat postsurgical orthopedic pain. Studies on animal models have shown a significant inhibiting effect of NSAIDs on osteogenesis process, on which spinal fusion also depends. Recently, great interest has been shown in the effect of NSAIDs on the success rate of adult spinal fusion. Clinical trials have tested the effect of perioperative NSAIDs in spinal fusion procedures. A cumulative result of these studies would give more credit to the final conclusions.

METHODS

A systematic search of electronic databases and references from eligible articles was conducted. Comparative studies reporting on the results of primary spinal fusion including treatment group of NSAIDs perioperatively were regarded eligible. A pooled estimate of effect size was produced using both random and fixed effect model.

RESULTS

Five retrospective comparative studies (n = 1403 participants) were included in the present study. The mean age of these patients was more than 40 years and none of them had NSAIDs for longer than 14 days following spinal fusion surgery. High-dose ketorolac showed a statistically significant adverse effect on spinal fusion (P = 0.001, RR = 2.87, 95% CI = 1.53 = - 5.38) with no statistical heterogeneity (I = 3%, P = 0.38), whereas normal-dose NSAIDs (ketorolac, diclofenac sodium, celecoxib, or rofecoxib) did not appear to produce inferior results than the no-NSAIDs group (P = 0.30, RR = 1.39, 95% CI = 0.74 - 2.61) with no statistical heterogeneity (I² = 0%, P = 0.50).

CONCLUSION

Although randomized controlled trials would be optimal for meta-analyses, the data of this review revealed that short-time (<14 days) exposure to normal-dose NSAIDs (ketorolac, diclofenac sodium, celecoxib, or rofecoxib) were safe after spinal fusion, whereas short-time (<14 days) exposure to high-dose ketorolac increased the risk of nonunion, which meant that the effect of perioperative NSAIDs on spinal fusion might be dose-dependent. Further studies would be needed to find out whether long-time exposure to normal-dose NSAIDs could also increase the risk of nonunion and which type of NSAIDs would like to have a worse effect on spinal fusion.

EP1(-/-) mice have enhanced osteoblast differentiation and accelerated fracture repair

As a downstream product of cyclooxygenase 2 (COX-2), prostaglandin E(2) (PGE(2)) plays a crucial role in the regulation of bone formation. It has four different receptor subtypes (EP1 through EP4), each of which exerts different effects in bone. EP2 and EP4 induce bone formation through the protein kinase A (PKA) pathway, whereas EP3 inhibits bone formation in vitro. However, the effect of EP1 receptor signaling during bone formation remains unclear. Closed, stabilized femoral fractures were created in mice with EP1 receptor loss of function at 10 weeks of age. Healing was evaluated by radiographic imaging, histology, gene expression studies, micro-computed tomographic (µCT), and biomechanical measures. EP1(-/-) mouse fractures have increased formation of cartilage, increased fracture callus, and more rapid completion of endochondral ossification. The fractures heal faster and with earlier fracture callus mineralization with an altered expression of genes involved in bone repair and remodeling. Fractures in EP1(-/-) mice also had an earlier appearance of tartrate-resistant acid phosphatase (TRAcP)-positive osteoclasts, accelerated bone remodeling, and an earlier return to normal bone morphometry. EP1(-/-) mesenchymal progenitor cells isolated from bone marrow have higher osteoblast differentiation capacity and accelerated bone nodule formation and mineralization in vitro. Loss of the EP1 receptor did not affect EP2 or EP4 signaling, suggesting that EP1 and its downstream signaling targets directly regulate fracture healing. We show that unlike the PGE(2) receptors EP2 and EP4, the EP1 receptor is a negative regulator that acts at multiple stages of the fracture healing process. Inhibition of EP1 signaling is a potential means to enhance fracture healing.

Noncanonical BMP signaling regulates cyclooxygenase-2 transcription

Activation of p38 MAPK has been shown to be relevant for a number of bone morphogenetic protein (BMP) physiological effects. We report here the involvement of noncanonical phosphorylated mothers against decapentaplegic (Smad) signaling in the transcriptional induction of Cox2 (Ptgs2) by BMP-2 in mesenchymal cells and organotypic calvarial cultures. We demonstrate that different regulatory elements are required for regulation of Cox2 expression by BMP-2: Runt-related transcription factor-2 and cAMP response element sites are essential, whereas a GC-rich Smad binding element is important for full responsiveness. Efficient transcriptional activation requires cooperation between transcription factors because mutation of any element results in a strong decrease of BMP-2 responsiveness. BMP-2 activation of p38 leads to increased recruitment of activating transcription factor-2, Runx2, Smad, and coactivators such as p300 at the responsive sites in the Cox2 proximal promoter. We demonstrate, by either pharmacological or genetic analysis, that maximal BMP-2 effects on Cox2 and JunB expression require the function of p38 and its downstream effector mitogen/stress-activated kinase 1. Altogether our results strongly suggest that cooperative effects between canonical and noncanonical BMP signaling allow the fine-tuning of BMP transcriptional responses on specific target genes.

N-formyl-methionyl-leucyl-phenylalanine (fMLP) promotes osteoblast differentiation via the N-formyl peptide receptor 1-mediated signaling pathway in human mesenchymal stem cells from bone marrow

Binding of N-formyl-methionyl-leucyl-phenylalanine (fMLP) to its specific cell surface receptor, N-formyl peptide receptor (FPR), triggers different cascades of biochemical events, eventually leading to cellular activation. However, the physiological role of fMLP and FPR during differentiation of mesenchymal stem cells is unknown. In this study, we attempted to determine whether fMLP regulates differentiation of mesenchymal stem cells derived from bone marrow. Analysis by quantitative-PCR and flow cytometry showed significantly increased expression of FPR1, but not FPR2 and FPR3, during osteoblastic differentiation. fMLP, a specific ligand of FPR1, promotes osteoblastic commitment and suppresses adipogenic commitment under differentiation conditions. Remarkably, fMLP-stimulated osteogenesis is associated with increased expression of osteogenic markers and mineralization, which were blocked by cyclosporine H, a selective FPR1 antagonist. In addition, fMLP inhibited expression of peroxisome proliferator-activated receptor-γ1, a major regulator of adipocytic differentiation. fMLP-stimulated osteogenic differentiation was mediated via FPR1-phospholipase C/phospholipase D-Ca(2+)-calmodulin-dependent kinase II-ERK-CREB signaling pathways. Finally, fMLP promoted bone formation in zebrafish and rabbits, suggesting its physiological relevance in vivo. Collectively, our findings provide novel insight into the functional role of fMLP in bone biology, with important implications for its potential use as a therapeutic agent for treatment of bone-related disorders.

Expression of endogenous BMP-2 in periosteal progenitor cells is essential for bone healing

Bone morphogenic protein 2 (BMP-2) plays a key role in skeletal development, repair and regeneration. To gain a better understanding of the role of BMP-2 in periosteum-mediated bone repair, we deleted BMP-2 postnatally at the initiation stage of healing utilizing a Tamoxifen-inducible CreER mouse model. To mark the mutant cells, we further generated a BMP-2(f/f); CreER; RosaR mouse model that enabled the activation of a LacZ reporter gene upon treatment of Tamoxifen. We demonstrated that deletion of BMP-2 at the onset of healing abolished periosteum-mediated bone/cartilage callus formation. In a chimeric periosteal callus with cells derived from both wild type and the mutant, over 90% of the mutant mesenchymal progenitors remained undifferentiated. Within differentiated bone and cartilage tissues, only a few cells could be identified as mutants. Using a bone graft transplantation approach, we further showed that transplantation of a mutant bone graft into a wild type host failed to rescue the deficient differentiation of the mutant cells at day 10 post-grafting. These data strongly suggest that the endogenous expression of BMP-2 plays a critical role in osteogenic and chondrogenic differentiation of periosteal progenitors during repair. To determine whether BMP-2 deficient cells remained responsive to exogenous BMP-2, we isolated periosteal mesenchymal progenitors from BMP-2 deficient bone autografts. The isolated cells demonstrated a 90% reduction of endogenous BMP-2 expression, accompanied by significant decrease in cellular proliferation and a near blockade of osteogenic differentiation. The addition of exogenous BMP-2 partially rescued impaired proliferation and further enhanced osteogenic differentiation in a dose dependent manner. Taken together, our data show that the initiation of the cortical bone repair in vivo is controlled by endogenous BMP-2. Future studies are necessary to determine the mechanisms by which the BMP-2 pathway is activated in periosteal progenitor cells at the onset of cortical bone repair.

Indomethacin promotes adipogenesis of mesenchymal stem cells through a cyclooxygenase independent mechanism

Regulation of mesenchymal stem cell (MSC) lineage selection is important for the generation of bone mass. Inhibition of cyclooxygenase-2 (COX2) may increase adipogenesis at the cost of decreasing osteoprogenitor output. Here we investigated the role of COX2 and its products during MSC differentiation. Indomethacin stimulated adipogenesis (increased aP2, adiponectin and lipid droplets) of CH310T1/2 stem cells as well as marrow-derived MSCs to a degree similar to the PPARγ2 ligand, rosiglitazone. Unlike rosiglitazone, indomethacin significantly upregulated PPARγ2 expression. Indomethacin and the COX2 specific inhibitor celecoxib suppressed PGE2 production, but celecoxib did not induce adipogenesis. As well, addition of PGE2 failed to reverse indomethacin induced adipogenesis, indicating that indomethacin's effects were prostaglandin independent. In MSCs over-expressing PPARγ2 and RXRα, indomethacin did not increase PPAR-induced transcription, while rosiglitazone and 15d-PGJ2 did (1.7- and 1.3-fold, respectively, P < 0.001). We considered whether indomethacin might directly affect C/EBPβ proximally to PPARγ2 induction. Indomethacin significantly increased C/EBPβ expression and protein within 24 h of addition. These results indicate that indomethacin promotes adipogenesis by increasing C/EBPβ and PPARγ2 expression in a prostaglandin-independent fashion. This effect of indomethacin is pertinent to potential deleterious effects of this commonly used anti-inflammatory drug on bone remodeling and tissue healing.

A role for nuclear factor interleukin-3 (NFIL3), a critical transcriptional repressor, in down-regulation of periovulatory gene expression

The LH surge triggers dramatic transcriptional changes in genes associated with ovulation and luteinization. The present study investigated the spatiotemporal expression of nuclear factor IL-3 (NFIL3), a transcriptional regulator of the basic leucine zipper transcription factor superfamily, and its potential role in the ovary during the periovulatory period. Immature female rats were injected with pregnant mare's serum gonadotropin, treated with human chorionic gonadotropin (hCG), and ovaries or granulosa cells were collected at various times after hCG. Nfil3 mRNA was highly induced both in intact ovaries and granulosa cells after hCG treatment. In situ hybridization demonstrated that Nfil3 mRNA was highly induced in theca-interstitial cells at 4-8 h after hCG, localized to granulosa cells at 12 h, and decreased at 24 h. Overexpression of NFIL3 in granulosa cells inhibited the induction of prostaglandin-endoperoxide synthase 2 (Ptgs2), progesterone receptor (Pgr), epiregulin (Ereg), and amphiregulin (Areg) and down-regulated levels of prostaglandin E2. The inhibitory effect on Ptgs2 induction was reversed by NFIL3 small interfering RNA treatment. In theca-interstitial cells the expression of hydroxyprostaglandin dehydrogenase 15-(nicotinamide adenine dinucleotide) (Hpgd) was also inhibited by NFIL3 overexpression. Data from luciferase assays demonstrated that NFIL3 overexpression decreased the induction of the Ptgs2 and Areg promoter activity. EMSA and chromatin immunoprecipitation analyses indicated that NFIL3 binds to the promoter region containing the DNA-binding sites of cAMP response element binding protein and CCAAT enhancer binding protein-β. In summary, hCG induction of NFIL3 expression may modulate the process of ovulation and theca-interstitial and granulosa cell differentiation by regulating expression of PTGS2, PGR, AREG, EREG, and HPGD, potentially through interactions with cAMP response element binding protein and CCAAT enhancer binding protein-β on their target gene promoters.

The effect of Cox-2 specific inhibition on direct fracture healing in the rabbit tibia

OBJECTIVE

The purpose of this study was to evaluate the effect of Cox-2 administration on direct (primary) fracture healing.

METHODS

A transverse tibial osteotomy was created in adult male rabbits and rigidly fixed in compression using a 2.7-mm dynamic compression plate. Animals were randomized to receive either rofecoxib (12.5 mg orally per day) or placebo. Animals were killed at 4 weeks and fracture healing assessed by mechanical testing.

RESULTS

There were no significant differences between the control and Cox-2 treated animals in terms of mechanical strength at 4 weeks. There was a high complication rate of peri-implant fractures during the daily medication administration.

CONCLUSION

The immediate administration of a Cox-2 specific inhibitor did not impair primary (direct) bone healing at the dose administered in this rabbit tibial osteotomy model.

Accelerated fracture healing in mice lacking the 5-lipoxygenase gene

BACKGROUND AND PURPOSE

Cyclooxygenase-2 (COX-2) promotes inflammation by synthesizing pro-inflammatory prostaglandins from arachidonic acid. Inflammation is an early response to bone fracture, and ablation of COX-2 activity impairs fracture healing. Arachidonic acid is also converted into leukotrienes by 5-lipoxygenase (5-LO). We hypothesized that 5-LO is a negative regulator of fracture healing and that in the absence of COX-2, excess leukotrienes synthesized by 5-LO will impair fracture healing.

METHODS

Fracture healing was assessed in mice with a targeted 5-LO mutation (5-LO(KO) mice) and control mice by radiographic and histological observations, and measured by histomorphometry and torsional mechanical testing. To assess effects on arachidonic acid metabolism, prostaglandin E2, F2α, and leukotriene B4 levels were measured in the fracture calluses of control, 5-LO(KO) COX-1(KO), and COX-2(KO) mice by enzyme linked immunoassays.

RESULTS

Femur fractures in 5-LO(KO) mice rapidly developed a cartilaginous callus that was replaced with bone to heal fractures faster than in control mice. Femurs from 5-LO(KO) mice had substantially better mechanical properties after 1 month of healing than did control mice. Callus leukotriene levels were 4-fold higher in mice homozygous for a targeted mutation in the COX-2 gene (COX-2(KO)), which indicated that arachidonic acid was shunted into the 5-LO pathway in the absence of COX-2.

INTERPRETATION

These experiments show that 5-LO negatively regulates fracture healing and that shunting of arachidonic acid into the 5-LO pathway may account, at least in part, for the impaired fracture healing response observed in COX-2(KO) mice.

Mechanical activation of β-catenin regulates phenotype in adult murine marrow-derived mesenchymal stem cells

Regulation of skeletal remodeling appears to influence the differentiation of multipotent mesenchymal stem cells (MSC) resident in the bone marrow. As murine marrow cultures are contaminated with hematopoietic cells, they are problematic for studying direct effects of mechanical input. Here we use a modified technique to isolate marrow-derived MSC (mdMSC) from adult mice, yielding a population able to differentiate into adipogenic and osteogenic phenotypes that is devoid of hematopoietic cells. In pure mdMSC populations, a daily strain regimen inhibited adipogenic differentiation, suppressing expression of PPARγ and adiponectin. Strain increased β-catenin and inhibition of adipogenesis required this effect. Under osteogenic conditions, strain activated β-catenin signaling and increased expression of WISP1 and COX2. mdMSC were also generated from mice lacking caveolin-1, a protein known to sequester β-catenin: caveolin-1((-/-)) mdMSC exhibited retarded differentiation along both adipogenic and osteogenic lineages but retained mechanical responses that involved β-catenin activation. Interestingly, caveolin-1((-/-)) mdMSC failed to express bone sialoprotein and did not form mineralized nodules. In summary, mdMSC from adult mice respond to both soluble factors and mechanical input, with mechanical activation of β-catenin influencing phenotype. As such, these cells offer a useful model for studies of direct mechanical regulation of MSC differentiation and function.