Association of microsomal prostaglandin E synthase 1 deficiency with impaired fracture healing, but not with bone loss or osteoarthritis, in mouse models of skeletal disorders

In Vitro Bioassay for Damage-Associated Molecular Patterns Arising from Injured Oral Cells

Gingival fibroblasts are a significant source of paracrine signals required to maintain periodontal homeostasis and to mediate pathological events linked to periodontitis and oral squamous cell carcinomas. Among the potential paracrine signals are stanniocalcin-1 (STC1), involved in oxidative stress and cellular survival; amphiregulin (AREG), a growth factor that mediates the cross-talk between immune cells and epithelial cells; chromosome 11 open reading frame 96 (C11orf96) with an unclear biologic function; and the inflammation-associated prostaglandin E synthase (PTGES). Gingival fibroblasts increasingly express these genes in response to bone allografts containing remnants of injured cells. Thus, the gene expression might be caused by the local release of damage-associated molecular patterns arising from injured cells. The aim of this study is consequently to use the established gene panel as a bioassay to measure the damage-associated activity of oral cell lysates. To this aim, we have exposed gingival fibroblasts to lysates prepared from the squamous carcinoma cell lines TR146 and HSC2, oral epithelial cells, and gingival fibroblasts. We report here that all lysates significantly increased the transcription of the entire gene panel, supported for STC1 at the protein level. Blocking TGF-β receptor 1 kinase with SB431542 only partially reduced the forced expression of STC1, AREG, and C11orf96. SB431542 even increased the PTGES expression. Together, these findings suggest that the damage signals originating from oral cells can change the paracrine activity of gingival fibroblasts. Moreover, the expression panel of genes can serve as a bioassay for testing the biocompatibility of materials for oral application.

A review on mPGES-1 inhibitors: From preclinical studies to clinical applications

Prostaglandin E₂ (PGE₂) is a lipid mediator of inflammation and cancer progression. It is mainly formed via metabolism of arachidonic acid by cyclooxygenases (COX) and the terminal enzyme microsomal prostaglandin E synthase-1 (mPGES-1). Widely used non-steroidal anti-inflammatory drugs (NSAIDs) inhibit COX activity, resulting in decreased PGE₂ production and symptomatic relief. However, NSAIDs block the production of many other lipid mediators that have important physiological and resolving actions, and these drugs cause gastrointestinal bleeding and/or increase the risk for severe cardiovascular events. Selective inhibition of downstream mPGES-1 for reduction in only PGE₂ biosynthesis is suggested as a safer therapeutic strategy. This review covers the recent advances in characterization of new mPGES-1 inhibitors in preclinical models and their future clinical applications.

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

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

G-CSF-induced sympathetic tone provokes fever and primes antimobilizing functions of neutrophils via PGE2

Granulocyte colony-stimulating factor (G-CSF) is widely used for peripheral blood stem/progenitor mobilization. G-CSF causes low-grade fever that is ameliorated by nonsteroidal anti-inflammatory drugs (NSAIDs), suggesting the activation of arachidonic acid (AA) cascade. How G-CSF regulated this reaction was assessed. G-CSF treatment in mice resulted in fever, which was canceled in prostaglandin E synthase (mPGES-1)-deficient mice. Mobilization efficiency was twice as high in chimeric mice lacking mPGES-1, specifically in hematopoietic cells, suggesting that prostaglandin E₂ (PGE₂) from hematopoietic cells modulated the bone marrow (BM) microenvironment. Neutrophils from steady-state BM constitutively expressed mPGES-1 and significantly enhanced PGE₂ production in vitro by β-adrenergic stimulation, but not by G-CSF, which was inhibited by an NSAID. Although neutrophils expressed all β-adrenergic receptors, only β3-agonist induced this phenomenon. Liquid chromatography-tandem mass spectrometry traced β-agonist-induced PGE₂ synthesis from exogenous deuterium-labeled AA. Spontaneous PGE₂ production was highly efficient in Gr-1^high neutrophils among BM cells from G-CSF-treated mice. In addition to these in vitro data, the in vivo depletion of Gr-1^high neutrophils disrupted G-CSF-induced fever. Furthermore, sympathetic denervation eliminated both neutrophil priming for PGE₂ production and fever during G-CSF treatment. Thus, sympathetic tone-primed BM neutrophils were identified as one of the major PGE₂ producers. PGE₂ upregulated osteopontin, specifically in preosteoblasts, to retain progenitors in the BM via EP4 receptor. Thus, the sympathetic nervous system regulated neutrophils as an indispensable PGE₂ source to modulate BM microenvironment and body temperature. This study provided a novel mechanistic insight into the communication of the nervous system, BM niche components, and hematopoietic cells.

Endoplasmic reticulum stress-induced apoptosis contributes to articular cartilage degeneration via C/EBP homologous protein

OBJECTIVE

When endoplasmic reticulum (ER) stress, i.e., the excessive accumulation of unfolded proteins in ER, endangers homeostasis, apoptosis is induced by C/EBP homologous protein (Chop). In osteoarthritis (OA) cartilage, Chop expression and apoptosis increase as degeneration progresses. We investigated the role of Chop in murine chondrocyte apoptosis and in the progression of cartilage degeneration.

METHOD

We induced experimental OA in Chop-knockout (Chop(-/-)) mice by medial collateral ligament transection and meniscectomy and compared cartilage degeneration, apoptosis, and ER stress in Chop(-/-)- and wild-type (Chop(+/+)) mice. In our in vitro experiments we treated murine Chop(-/-) chondrocytes with the ER stress inducer tunicamycin (TM) and evaluated apoptosis, ER stress, and chondrocyte function.

RESULTS

In vivo, the degree of ER stress was similar in Chop(-/-)- and Chop(+/+) mice. However, in Chop(-/-) mice apoptosis and cartilage degeneration were lower by 26.4% and 42.4% at 4 weeks, by 26.8% and 44.9% at 8 weeks, and by 26.9% and 32.3% at 12 weeks after surgery than Chop(+/+) mice, respectively. In vitro, the degree of ER stress induction by TM was similar in Chop(-/-)- and Chop(+/+) chondrocytes. On the other hand, apoptosis was 55.3% lower and the suppression of collagen type II and aggrecan mRNA was 21.0% and 23.3% less, and the increase of matrix metalloproteinase-13 mRNA was 20.0% less in Chop(-/-)- than Chop(+/+) chondrocytes.

CONCLUSION

Our results indicate that Chop plays a direct role in chondrocyte apoptosis and that Chop-mediated apoptosis contributes to the progression of cartilage degeneration in mice.

Fracture healing and lipid mediators

Lipid mediators regulate bone regeneration during fracture healing. Prostaglandins and leukotrienes are well-known lipid mediators that regulate inflammation and are synthesized from the Ω-6 fatty acid, arachidonic acid. Cyclooxygenase (COX-1 or COX-2) and 5-lipoxygenase (5-LO) catalyze the initial enzymatic steps in the synthesis of prostaglandins and leukotrienes, respectively. Inhibition or genetic ablation of COX-2 activity impairs fracture healing in animal models. Genetic ablation of COX-1 does not affect the fracture callus strength in mice, suggesting that COX-2 activity is primarily responsible for regulating fracture healing. Inhibition of cyclooxygenase activity with nonsteroidal anti-inflammatory drugs (NSAIDs) is performed clinically to reduce heterotopic ossification, although clinical evidence that NSAID treatment impairs fracture healing remains controversial. In contrast, inhibition or genetic ablation of 5-LO activity accelerates fracture healing in animal models. Even though prostaglandins and leukotrienes regulate inflammation, loss of COX-2 or 5-LO activity appears to primarily affect chondrogenesis during fracture healing. Prostaglandin or prostaglandin analog treatment, prostaglandin-specific synthase inhibition and prostaglandin or leukotriene receptor antagonism also affect callus chondrogenesis. Unlike the Ω-6-derived lipid mediators, lipid mediators derived from Ω-3 fatty acids, such as resolvin E1 (RvE1), have anti-inflammatory activity. In vivo, RvE1 can inhibit osteoclastogenesis and limit bone resorption. Although Ω-6 and Ω-3 lipid mediators have clear-cut effects on inflammation, the role of these lipid mediators in bone regeneration is more complex, with apparent effects on callus chondrogenesis and bone remodeling.

Mapping pathogenesis of arthritis through small animal models

Animal models have been used for a number of decades to study arthritis and have contributed greatly to unravelling mechanisms of pathogenesis and validating new targets for treatment. All animal models have sets of limitations and over the years there has been natural refinement of existing models as well as creation of new ones. The success of genetic modification in mice has fuelled an exponential increase in the use of murine models for arthritis research and has significantly increased our understanding of disease processes. This review focuses on those rodent models of RA and OA that have current utility and are widely used by the research community. We highlight the subtle but important differences in existing models by positioning them on a pathogenesis map whereby model selection is determined by the specific aspect of disease to be studied. We discuss the evolving challenges in in vivo arthritis studies and our perceived gaps for future new model development. The document includes technical and cost implications of performing the described models, and the ethical considerations of such approaches.

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.

Harnessing and modulating inflammation in strategies for bone regeneration

Inflammation is an immediate response that plays a critical role in healing after fracture or injury to bone. However, in certain clinical contexts, such as in inflammatory diseases or in response to the implantation of a biomedical device, the inflammatory response may become chronic and result in destructive catabolic effects on the bone tissue. Since our previous review 3 years ago, which identified inflammatory signals critical for bone regeneration and described the inhibitory effects of anti-inflammatory agents on bone healing, a multitude of studies have been published exploring various aspects of this emerging field. In this review, we distinguish between regenerative and damaging inflammatory processes in bone, update our discussion of the effects of anti-inflammatory agents on bone healing, summarize recent in vitro and in vivo studies demonstrating how inflammation can be modulated to stimulate bone regeneration, and identify key future directions in the field.

Long bone fracture repair in mice harboring GFP reporters for cells within the osteoblastic lineage

GFP reporter mice previously developed to assess levels of osteoblast differentiation were employed in a tibial long bone fracture model using a histological method that preserves fluorescent signals in non-decalcified sections of bone. Two reporters, based on Col1A1 (Col3.6GFPcyan) and osteocalcin (OcGFPtpz) promoter fragments, were bred into the same mice to reflect an early and late stage of osteoblast differentiation. Three observations were apparent from this examination. First, the osteoprogenitor cells that arise from the flanking periosteum proliferate and progress to fill the fracture zone. These cells differentiate to osteoblasts, chondrocytes, to from the outer cortical shell. Second, the hypertrophic chondrocytes are dispersed and the cartilage matrix mineralized by the advancing Col3.6+ osteoblasts. The endochondral matrix is removed by the following osteoclasts. Third, a new cortical shell develops over the cartilage core and undergoes a remodeling process of bone formation on the inner surface and resorption on the outer surface. The original fractured cortex undergoes resorption as the outer cortical shell remodels inward to become the new diaphyseal bone. The fluorescent microscopy and GFP reporter mice used in this study provide a powerful tool for appreciating the molecular and cellular processes that control these fundamental steps in fracture repair, and may provide a basis for understanding fracture nonunion.

Antiphospholipid syndrome treatment beyond anticoagulation: are we there yet?

Persistently positive antiphospholipid antibodies in association with thromboses and/or pregnancy morbidity is the hallmark of the antiphospholipid syndrome. The management of antiphospholipid antibody-positive patients has been focused on utilizing anti-thrombotic medications such as heparin or warfarin. Given that our understanding of the molecular mechanisms of antiphospholipid antibody-mediated thrombosis has been growing, it is highly likely that the current ‘anti-thrombotic’ approach to these patients will be replaced by an ‘immunomodulatory’ approach in the near future. This review article will address the experimental and/or clinical evidence behind some of these potential ‘immunomodulatory’ approaches (tissue factor inhibition, P38 mitogen-activated protein kinase inhibition, nuclear factor-κB inhibition, platelet glycoprotein receptor inhibition, hydroxychloroquine, statins, inhibition of β2GPI and/or anti-β2GPI binding to target cells, complement inhibition, and B cell inhibition) in antiphospholipid syndrome.

Prostaglandin E(2) synthase inhibition as a therapeutic target

BACKGROUND

Most NSAIDs function by inhibiting biosynthesis of PGE(2) by inhibition of COX-1 and/or COX-2. Since COX-1 has a protective function in the gastro-intestinal tract (GIT), non-selective inhibition of both cycloxy genases leads to moderate to severe gastro-intestinal intolerance. Attempts to identify selective inhibitors of COX-2, led to the identification of celecoxib and rofecoxib. However, long-term use of these drugs has serious adverse effects of sudden myocardial infarction and thrombosis. Drug-mediated imbalance in the levels of prostaglandin I(2) (PGI(2)) and thromboxane A(2) (TXA(2)) with a bias towards TXA(2) may be the primary reason for these events. This resulted in the drugs being withdrawn from the market, leaving a need for an effective and safe anti-inflammatory drug.

METHODS

Recently, the focus of research has shifted to enzymes downstream of COX in the prosta glandin biosynthetic pathway such as prostaglandin E(2) synthases. Microsomal prostaglandin E(2) synthase-1 (mPGES-1) specifically isomerizes PGH(2) to PGE(2), under inflammatory conditions. In this review, we examine the biology of mPGES-1 and its role in disease. Progress in designing molecules that can selectively inhibit mPGES-1 is reviewed.

CONCLUSION

mPGES-1 has the potential to be a target for anti-inflammatory therapy, devoid of adverse GIT and cardiac effects and warrants further investigation.

PGE2 signal via EP2 receptors evoked by a selective agonist enhances regeneration of injured articular cartilage

OBJECTIVE

The effect of the prostaglandin E2 (PGE2) signal through prostaglandin E receptor 2 (EP2) receptors on the repair of injured articular cartilage was investigated using a selective agonist for EP2.

METHODS

Chondral and osteochondral defects were prepared on the rabbit femoral concave in both knee joints, and gelatin containing polylactic-co-glycolic acid microspheres conjugated with or without the EP2 agonist was placed nearby. Animals were sacrificed at 4 or 12 weeks post-operation, and regenerated cartilage tissues and subchondral structure remodeling were evaluated by histological scoring. The quality of regenerated tissues was also evaluated by the immunohistochemical staining of EP2, type II collagen, and proliferating cell nuclear antigen (PCNA). As an evaluation of side effects, the inflammatory reaction of the synovial membrane was analyzed based on histology and the mRNA expression of matrix metalloproteinase3 (MMP3), tissue inhibitor of metalloproteinase 3 (TIMP3), and interleukin-1 beta (IL-1 beta). Also, the activity of MMP3 and the amount of tumor necrosis factor-alpha (TNF-alpha) and C-reactive protein in joint fluid were measured.

RESULTS

In both models, the EP2 agonist enhanced the regeneration of the type II collagen-positive tissues containing EP2- and PCNA-positive chondrocytes, and the histological scale of regenerated tissue and subchondral bone was better than that of on the control side, particularly at 12 weeks post-operation. No inflammatory reaction in the synovial membrane was observed, and no induction of pro-inflammatory cytokines was found in joint fluid.

CONCLUSION

Selective stimulation of the PGE2 signal through EP2 receptors by a specific agonist promoted regeneration of cartilage tissues with a physiological osteochondral boundary, suggesting the potential usefulness of this small molecule for the treatment of injured articular cartilages.

Heme oxygenase-1 induction modulates microsomal prostaglandin E synthase-1 expression and prostaglandin E(2) production in osteoarthritic chondrocytes

Pro-inflammatory cytokines such as interleukin-1beta (IL-1beta) may participate in the pathogenesis of cartilage damage in osteoarthritis (OA) through the production of catabolic enzymes and inflammatory mediators. Induction of heme oxygenase-1 (HO-1) has previously been shown to exert anti-inflammatory effects in different cell types. We have investigated whether HO-1 induction may modify chondrocyte viability and the production of relevant mediators such as oxidative stress and prostaglandin E(2) (PGE(2)) elicited by IL-1beta in OA chondrocytes. Chondrocytes were isolated from OA cartilage and used in primary culture. Cells were stimulated with IL-1beta in the absence or presence of the HO-1 inducer cobalt protoporphyrin IX (CoPP). Gene expression was assessed by quantitative real-time PCR, protein levels by ELISA and Western blot, apoptosis by laser scanning cytometry using annexin V-FITC and TUNEL assays, and oxidative stress by LSC with dihydrorhodamine 123. HO-1 induction by CoPP enhanced chondrocyte viability and aggrecan content while inhibiting apoptosis and oxidative stress generation. PGE(2) is produced in OA chondrocytes stimulated by IL-1beta by the coordinated induction of cyclooxygenase-2 and microsomal PGE synthase 1 (mPGES-1). The production of PGE(2) was decreased by HO-1 induction as a result of diminished mPGES-1 protein and mRNA expression. Transfection with HO-1 small interfering RNA counteracted CoPP effects. In addition, the activation of nuclear factor-kappaB and early growth response-1 was significantly reduced by CoPP providing a basis for its anti-inflammatory effects. These results confirm the protective role of HO-1 induction in OA chondrocytes and suggest the potential interest of this strategy in degenerative joint diseases.