15d-PGJ(2) is acting as a 'dual agent' on the regulation of COX-2 expression in human osteoarthritic chondrocytes

Krüppel-like factor 15 deficiency exacerbates osteoarthritis through reduced expression of peroxisome proliferator-activated receptor gamma signaling in mice

OBJECTIVE

Krüppel-like zinc finger transcription factors (KLFs) play diverse roles in mammalian cell differentiation and development. In this study, we investigated the function of KLF15 in the progression of osteoarthritis (OA).

METHODS

0Destabilization of the medial meniscus (DMM) surgery was performed in 10-week-old male wild-type control (WT) mice and cartilage-specific KLF15 knockout (KO) mice. Histological analysis, immunohistochemistry, and terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick-end labeling staining were performed. Morphological changes were measured using microcomputed tomography. Six mice from each group were analyzed (total number of mice analyzed: 60). In vitro, immunofluorescence, quantitative reverse transcription-polymerase chain reaction, and western blot analyses were performed.

RESULTS

KLF15 KO DMM mice exhibited significant cartilage degradation compared to WT mice. According to the Osteoarthritis Research Society International cartilage OA-histopathology scoring system, the mean sum score in KLF15 KO mice was significantly higher than that in WT mice at 8 weeks after surgery. Immunohistochemistry results revealed KLF15 KO mice exhibited reduced peroxisome proliferator-activated receptor gamma (PPARγ) expression, increased pIKKα/β, a disintegrin-like and metalloproteinase with thrombospondin motifs (ADAMTS) 5, and Matrix metalloproteinases (MMP13) expression, and reduced Forkhead box O (FOXO1) and Light chain 3B (LC3B) expression. Inhibition of PPARγ phosphorylation accelerated the effects of interleukin (IL) 1β-treatment in both KLF15 KO and WT chondrocytes, and activation of PPARγ expression canceled the IL1β-induced catabolic effects.

CONCLUSION

Our results indicated that the OA phenotype of KLF15 KO DMM mice was influenced by reduced PPARγ expression, including enhanced pIKKα/β, ADAMTS5, and MMP13 expression, reduced autophagy, and increased apoptosis. KLF15 regulation may constitute a possible therapeutic strategy for the treating OA.

Otto Aufranc Award: Identification of Key Molecular Players in the Progression of Hip Osteoarthritis Through Transcriptomes and Epigenetics

BACKGROUND

This study aimed: (1) to compare the transcriptome profile of articular cartilage in cam-FAI (early stage) to advanced OA secondary to cam-FAI (late stage) and (2) to investigate epigenetic changes through the expression of DNA methylation enzymes DNMT3B, DNMT1, and DNMT3A and peroxisome proliferator-activated receptor gamma (PPARγ) in human cartilage samples during the progression of hip OA.

METHODS

Full-thickness cartilage samples were collected from the anterolateral head-neck junction (impingement zone) of 22 patients (9 early-FAI and 13 late-FAI). RNA sequencing and in vitro cartilage cultures with histological analysis and immunohistochemistry staining for PPARγ and DNMT3B were performed. Target gene validation was confirmed with RT-PCR.

RESULTS

Fifty genes and 42 pathways were identified differentially between early and late-FAI (fold change <-1.5 or >1.5, P < .01). PPARγ and DNMT3B were gradually suppressed with disease progression. Contrarily, disease progression induced expression of DNMT1/3A.

CONCLUSION

By comparing comprehensive gene expression in early and late stage hip degeneration at the whole-genome level, distinct transcriptome profiles for early and late stage disease were identified along with key molecular contributors to the progression of hip OA. Preservation of endogenous PPARγ may have therapeutic potential to delay or prevent hip OA.

Harnessing Inflammation Resolution in Arthritis: Current Understanding of Specialized Pro-resolving Lipid Mediators' Contribution to Arthritis Physiopathology and Future Perspectives

The concept behind the resolution of inflammation has changed in the past decades from a passive to an active process, which reflects in novel avenues to understand and control inflammation-driven diseases. The time-dependent and active process of resolution phase is orchestrated by the endogenous biosynthesis of specialized pro-resolving lipid mediators (SPMs). Inflammation and its resolution are two forces in rheumatic diseases that affect millions of people worldwide with pain as the most common experienced symptom. The pathophysiological role of SPMs in arthritis has been demonstrated in pre-clinical and clinical studies (no clinical trials yet), which highlight their active orchestration of disease control. The endogenous roles of SPMs also give rise to the opportunity of envisaging these molecules as novel candidates to improve the life quality of rhematic diseases patients. Herein, we discuss the current understanding of SPMs endogenous roles in arthritis as pro-resolutive, protective, and immunoresolvent lipids.

On the Mechanisms of Action of the Low Molecular Weight Fraction of Commercial Human Serum Albumin in Osteoarthritis

The low molecular weight fraction of commercial human serum albumin (LMWF5A) has been shown to successfully relieve pain and inflammation in severe osteoarthritis of the knee (OAK). LMWF5A contains at least three active components that could account for these antiinflammatory and analgesic effects. We summarize in vitro experiments in bone marrow-derived mesenchymal stem cells, monocytic cell lines, chondrocytes, peripheral blood mononuclear cells, fibroblast-like synoviocytes, and endothelial cells on the biochemistry of anti-inflammatory changes induced by LMWF5A. We then look at four of the major pathways that cut across cell-type considerations to examine which biochemical reactions are affected by mTOR, COX-2, CD36, and AhR pathways. All three components show anti-inflammatory activities in at least some of the cell types. The in vitro experiments show that the effects of LMWF5A in chondrocytes and bone marrow- derived stem cells in particular, coupled with recent data from previous clinical trials of single and multiple injections of LMWF5A into OAK patients demonstrated improvements in pain, function, and Patient Global Assessment (PGA), as well as high responder rates that could be attributed to the multiple mechanism of action (MOA) pathways are summarized here. In vitro and in vivo data are highly suggestive of LMWF5A being a disease-modifying drug for OAK.

The function of the oxylipin 12-oxophytodienoic acid in cell signaling, stress acclimation, and development

Forty years ago, 12-oxophytodienoic acid (12-OPDA) was reported as a prostaglandin (PG)-like metabolite of linolenic acid found in extracts of flaxseed. Since then, numerous studies have determined the role of 12-OPDA in regulating plant immunity, seed dormancy, and germination. This review summarizes our current knowledge of the regulation of 12-OPDA synthesis in the chloroplast and 12-OPDA-dependent signaling in gene expression and targeting protein functions. We describe the properties of OPDA that are linked to the activities of PGs, which are derived from arachidonic acid and act as tissue hormones in animals, including humans. The similarity of OPDA with bioactive PGs is particularly evident for the most-studied cyclopentenone, PG 15-dPGJ2. In addition to chemical approaches towards 12-OPDA synthesis, bio-organic synthesis strategies for 12-OPDA and analogous substances have recently been established. The resulting availability of OPDA will aid the identification of additional effector proteins, help in elucidating the mechanisms of OPDA sensing and transmission, and will foster the analysis of the physiological responses to OPDA in plants. There is a need to determine the compartmentation and transport of 12-OPDA and its conjugates, over long distances as well as short. It will be important to further study OPDA in animal and human cells, for example with respect to beneficial anti-inflammatory and anti-cancer activities.

Metabolic stress-induced joint inflammation and osteoarthritis

Osteoarthritis (OA) is a heterogeneous disorder with several risk factors. Among them, obesity has a major impact on both loading and non-loading joints. Mechanical overload and activity of systemic inflammatory mediators derived from adipose tissue (adipokines, free fatty acids (FFA), reactive oxygen species (ROS)) provide clues to the increased incidence and prevalence of OA in obesity. Recently, research found greater OA prevalence and incidence in obese patients with cardiometabolic disturbances than "healthy" obese patients, which led to the description of a new OA phenotype - metabolic syndrome (MetS)-associated OA. Indeed, individual metabolic factors (diabetes, dyslipidemia, and hypertension) may increase the risk of obesity-induced OA. This review discusses hypotheses based on pathways specific to a metabolic factor in MetS-associated OA, such as the role of advanced glycation end products (AGEs) and glucose toxicity. A better understanding of these phenotypes based on risk factors will be critical for designing trials of this specific subset of OA.

Inflammatory Pathways in Knee Osteoarthritis: Potential Targets for Treatment

Osteoarthritis (OA) of the knee is a wide-spread, debilitating disease that is prominent in Western countries. It is associated with old age, obesity, and mechanical stress on the knee joint. By examining the recent literature on the effect of the anti-inflammatory prostaglandins 15d-PGJ2 and Δ12-PGJ2, we propose that new therapeutic agents for this disease could facilitate the transition from the COX-2-dependent pro-inflammatory synthesis of the prostaglandin PGE2 (catalyzed by mPGES-1), to the equally COX-2-dependent synthesis of the aforementioned anti-inflammatory prostaglandins. This transition could be instrumental in halting the breakdown of cartilage via matrix metalloproteinases (MMPs) and aggrecanases, as well as promoting the matrix regeneration and synthesis of cartilage by chondrocytes. Another desirable property of new OA therapeutics could involve the recruitment of mesenchymal stem cells to the damaged cartilage and bone, possibly resulting in the generation of chondrocytes, synoviocytes, and, in the case of bone, osteoblasts. Moreover, we propose that research promoting this transition from pro-inflammatory to anti-inflammatory prostaglandins could aid in the identification of new OA therapeutics.

Advanced glycation end products induce peroxisome proliferator-activated receptor γ down-regulation-related inflammatory signals in human chondrocytes via Toll-like receptor-4 and receptor for advanced glycation end products

Accumulation of advanced glycation end products (AGEs) in joints is important in the development of cartilage destruction and damage in age-related osteoarthritis (OA). The aim of this study was to investigate the roles of peroxisome proliferator-activated receptor γ (PPARγ), toll-like receptor 4 (TLR4), and receptor for AGEs (RAGE) in AGEs-induced inflammatory signalings in human OA chondrocytes. Human articular chondrocytes were isolated and cultured. The productions of metalloproteinase-13 and interleukin-6 were quantified using the specific ELISA kits. The expressions of related signaling proteins were determined by Western blotting. Our results showed that AGEs enhanced the productions of interleukin-6 and metalloproteinase-13 and the expressions of cyclooxygenase-2 and high-mobility group protein B1 and resulted in the reduction of collagen II expression in human OA chondrocytes. AGEs could also activate nuclear factor (NF)-κB activation. Stimulation of human OA chondrocytes with AGEs significantly induced the up-regulation of TLR4 and RAGE expressions and the down-regulation of PPARγ expression in a time- and concentration-dependent manner. Neutralizing antibodies of TLR4 and RAGE effectively reversed the AGEs-induced inflammatory signalings and PPARγ down-regulation. PPARγ agonist pioglitazone could also reverse the AGEs-increased inflammatory signalings. Specific inhibitors for p38 mitogen-activated protein kinases, c-Jun N-terminal kinase and NF-κB suppressed AGEs-induced PPARγ down-regulation and reduction of collagen II expression. Taken together, these findings suggest that AGEs induce PPARγ down-regulation-mediated inflammatory signalings and reduction of collagen II expression in human OA chondrocytes via TLR4 and RAGE, which may play a crucial role in the development of osteoarthritis pathogenesis induced by AGEs accumulation.

Expression of peroxisome proliferator-activated receptors α, β, γ, and H- and L-prostaglandin D synthase during osteoarthritis in the spontaneous hartley guinea pig and experimental dog models

OBJECTIVE

To investigate the expression of peroxisome proliferator-activated receptors (PPAR) α, β, and γ, and hematopoietic and lipocalin-type prostaglandin D synthase (H- and L-PGDS) over the course of osteoarthritis (OA) in the spontaneous Hartley guinea pig and the anterior cruciate ligament transection dog models.

METHODS

Guinea pigs were sacrificed at 2 (control group), 4, 8, and 12 months of age (n = 5 per group). Non-operated (control) and operated dogs were sacrificed at 4, 8, and 12 weeks postsurgery. Cartilage was evaluated histologically using the Osteoarthritis Research Society International (OARSI) guidelines. The expression of PPAR-α, β, γ, and H- and L-PGDS was evaluated by real-time PCR and immunohistochemistry. The nonparametric Spearman test was used for correlation analysis.

RESULTS

PPAR-α, β, and γ were detected in medial tibial plateau from control animals in both the spontaneous and surgical models. Levels of PPAR-α and β did not change over the course of OA, whereas PPAR-γ levels decreased during progression of disease. We also observed that the expression of H-PGDS remained unchanged, whereas L-PGDS increased over the course of OA. PPAR-γ levels correlated negatively, whereas L-PGDS levels correlated positively, with the histological score of OA.

CONCLUSION

The level of PPAR-γ decreased, whereas level of L-PGDS increased during the progression of OA. These data suggest that reduced expression of PPAR-γ may contribute to the pathogenesis of OA, whereas enhanced expression of L-PGDS may be part of a reparative process.

Nuclear receptors in bone physiology and diseases

During the last decade, our view on the skeleton as a mere solid physical support structure has been transformed, as bone emerged as a dynamic, constantly remodeling tissue with systemic regulatory functions including those of an endocrine organ. Reflecting this remarkable functional complexity, distinct classes of humoral and intracellular regulatory factors have been shown to control vital processes in the bone. Among these regulators, nuclear receptors (NRs) play fundamental roles in bone development, growth, and maintenance. NRs are DNA-binding transcription factors that act as intracellular transducers of the respective ligand signaling pathways through modulation of expression of specific sets of cognate target genes. Aberrant NR signaling caused by receptor or ligand deficiency may profoundly affect bone health and compromise skeletal functions. Ligand dependency of NR action underlies a major strategy of therapeutic intervention to correct aberrant NR signaling, and significant efforts have been made to design novel synthetic NR ligands with enhanced beneficial properties and reduced potential negative side effects. As an example, estrogen deficiency causes bone loss and leads to development of osteoporosis, the most prevalent skeletal disorder in postmenopausal women. Since administration of natural estrogens for the treatment of osteoporosis often associates with undesirable side effects, several synthetic estrogen receptor ligands have been developed with higher therapeutic efficacy and specificity. This review presents current progress in our understanding of the roles of various nuclear receptor-mediated signaling pathways in bone physiology and disease, and in development of advanced NR ligands for treatment of common skeletal disorders.

The role of peroxisome proliferator-activated receptors in colorectal cancer

Colorectal cancer is one of the most common cancers in the world. Dietary fat intake is a major risk factor for colorectal cancer. Some nuclear hormone receptors play an important role in regulating nutrient metabolism and energy homeostasis. Among these receptors, special attention has been focused on the role of peroxisome proliferator-activated receptors (PPARs) in colorectal cancer, because PPARs are involved in regulation of lipid and carbohydrate metabolism. PPARs are ligand-activated intracellular transcription factors. The PPAR subfamily consists of three subtypes encoded by distinct genes named PPARα, PPARβ/δ, and PPARγ. PPARγ is the most extensively studied subtype of PPARs. Even though many investigators have studied the expression and clinical implications of PPARs in colorectal cancer, there are still many controversies about the role of PPARs in colorectal cancer. In this paper, the recent progresses in understanding the role of PPARs in colorectal cancer are summarized.

Effects of diacerein at the molecular level in the osteoarthritis disease process

In osteoarthritis (OA), the alterations in joint tissues are numerous and involve morphological, biochemical and metabolic changes and an upregulation of the inflammatory pathways. The focus of this article is a brief narrative review of the effects of diacerein, an antirheumatic drug from the anthraquinone chemical class, and its active metabolite, rhein, on the factors that participate in the complex interaction between OA tissues and cells leading to the progression of joint structural changes.

Microsomal Prostaglandin E2 Synthase-1

The prostanoids and leukotrienes (LTs) formed from arachidonic acid (AA) via the cyclooxygenase (COX)-1/2 and 5-lipoxygenase (5-LO) pathway, respectively, mediate inflammatory responses, chronic tissue remodelling, cancer, asthma and autoimmune disorders, but also possess homeostatic functions in the gastrointestinal tract, uterus, brain, kidney, vasculature and host defence. Based on the manifold functions of these eicosanoids, the clinical use of non-steroidal anti-inflammatory drugs (NSAIDs), a class of drugs that block formation of all prostanoids, is hampered by severe side-effects including gastrointestinal injury, renal irritations and cardiovascular risks. Therefore, anti-inflammatory agents interfering with eicosanoid biosynthesis require a well-balanced pharmacological profile to minimize these on-target side-effects. Current anti-inflammatory research aims at identifying compounds that can suppress the massive formation of pro-inflammatory prostaglandin (PG)E2 without affecting homeostatic PGE2 and PGI2 synthesis. The inducible microsomal prostaglandin E2 synthase-1 (mPGES-1) is one promising target enzyme. We will give an overview about the structure, regulation and function of mPGES-1 and then present novel inhibitors of mPGES-1 that may possess a promising pharmacological profile.

Egr-1 contributes to IL-1-mediated down-regulation of peroxisome proliferator-activated receptor γ expression in human osteoarthritic chondrocytes

INTRODUCTION

Peroxisome proliferator-activated receptor (PPAR)γ has been shown to exhibit anti-inflammatory and anti-catabolic properties and to be protective in animal models of osteoarthritis (OA). We have previously shown that interleukin-1β (IL-1) down-regulates PPARγ expression in human OA chondrocytes. However, the mechanisms underlying this effect have not been well characterized. The PPARγ promoter harbors an overlapping Egr-1/specificity protein 1 (Sp1) binding site. In this study, our objective was to define the roles of Egr-1 and Sp1 in IL-1-mediated down-regulation of PPARγ expression.

METHODS

Chondrocytes were stimulated with IL-1 and the expression levels of Egr-1 and Sp1 mRNAs and proteins were evaluated using real-time reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting, respectively. The role of de novo protein synthesis was evaluated using the protein synthesis inhibitor cycloheximide (CHX). The recruitment of Sp1 and Egr-1 to the PPARγ promoter was evaluated using chromatin immunoprecipitation (ChIP) assays. The PPARγ promoter activity was analyzed in transient transfection experiments. The roles of Egr-1 and Sp1 were further evaluated using small interfering RNA (siRNA) approaches. The level of Egr-1 in cartilage was determined using immunohistochemistry.

RESULTS

Down-regulation of PPARγ expression by IL-1 requires de novo protein synthesis and was concomitant with the induction of the transcription factor Egr-1. Treatment with IL-1 induced Egr-1 recruitment and reduced Sp1 occupancy at the PPARγ promoter. Overexpression of Egr-1 potentiated, whereas overexpression of Sp1 alleviated, the suppressive effect of IL-1 on the PPARγ promoter, suggesting that Egr-1 may mediate the suppressive effect of IL-1. Consistently, Egr-1 silencing prevented IL-1-mediated down-regulation of PPARγ expression. We also showed that the level of Egr-1 expression was elevated in OA cartilage compared to normal cartilage.

CONCLUSIONS

Our results indicate that induction and recruitment of Egr-1 contributed to the suppressive effect of IL-1 on PPARγ expression. They also suggest that modulation of Egr-1 levels in the joint may have therapeutic potential in OA.

Graphene for controlled and accelerated osteogenic differentiation of human mesenchymal stem cells

Current tissue engineering approaches combine different scaffold materials with living cells to provide biological substitutes that can repair and eventually improve tissue functions. Both natural and synthetic materials have been fabricated for transplantation of stem cells and their specific differentiation into muscles, bones, and cartilages. One of the key objectives for bone regeneration therapy to be successful is to direct stem cells' proliferation and to accelerate their differentiation in a controlled manner through the use of growth factors and osteogenic inducers. Here we show that graphene provides a promising biocompatible scaffold that does not hamper the proliferation of human mesenchymal stem cells (hMSCs) and accelerates their specific differentiation into bone cells. The differentiation rate is comparable to the one achieved with common growth factors, demonstrating graphene's potential for stem cell research.

Anti-inflammatory activity of monogalactosyldiacylglycerol in human articular cartilage in vitro: activation of an anti-inflammatory cyclooxygenase-2 (COX-2) pathway

Introduction

The mono- and digalactosyldiacylglycerol (MGDG and DGDG) galactolipids have been purified from the thermophilic blue-green alga Phormidium sp. ETS-05 that colonizes the therapeutic thermal mud of Abano Terme and Montegrotto Terme, Italy. Both compounds present a marked composition in polyunsaturated fatty acids, mainly omega-3. The therapeutic thermal mud is applied mainly to osteoarthritic cartilage patients. In the present study the effect of MGDG treatment on proteins and factors expressed by human articular cartilage cells in culture and on pathways activated in inflammatory conditions was studied.

Methods

Primary cultures of human articular chondrocytes were used at cell passage number 1 (P1). Cells were treated in serum-free medium with inflammatory cytokines in the presence and in the absence of MGDG. Western blot was performed on collected medium and on cell layers. At least three different experiments were performed on primary cultures. The quantitation of the MGDG effect was performed by densitometric scanning of Western blots. p38 Mitogen Activated Protein Kinase (p38) activation, Nuclear Factor-kappaB (NF-kB) activation and Prostaglandin E₂ (PGE₂) quantitation were performed by commercially available assays. Results are given as the mean values ± SD. All statistical analyses were performed using GraphPad software. The two-tailed Student's t -test was performed.

Results

We report that MGDG: 1) represses the expression of interleukin-6 (IL-6) and interleukin-8 (IL-8) induced by interleukin-1alpha (IL-1α) or IL-1α + tumor necrosis factor α (TNFα) interfering with the p38 and NF-kB pathways; 2) is not toxic for the cells and does not affect the cell phenotype; 3) strongly enhances COX-2 expression induced by IL-1α or IL-1α + TNFα; 4) represses mPGES expression induced by IL-1α and the synthesis of PGE₂ and induces the synthesis of 15-deoxy-Δ 12,14-prostaglandin J₂ (15ΔPGJ₂). In addition, the COX-2 product 15ΔPGJ₂ added to the cells: 1) strongly represses IL-6 and IL-8 induced by IL-1α; 2) represses mPGES expression induced by IL-1α and the synthesis of PGE₂.

Conclusions

All together these data suggest that MGDG has an anti-inflammatory activity in human articular cartilage and possibly activates an anti-inflammatory loop triggered by COX-2 via 15ΔPGJ₂ production, indicating a possible role of COX-2 in resolution of inflammation. The purified compound is a novel anti-inflammatory agent potentially active for human articular cartilage pathologies related to inflammation.

Multiple phospholipase A2 enzymes participate in the inflammatory process in osteoarthritic cartilage

OBJECTIVE

The aim of this study was to determine the involvement of pro-inflammatory phospholipase A2 (PLA2) enzymes in human chondrocytes from patients with osteoarthritis (OA).

METHODS

PLA2 involvement in OA chondrocytes was analysed by (a) arachidonic acid (AA) and oleic acid release, (b) PLA2 mRNA analysis, and (c) prostaglandin E2 (PGE2) production in cultured OA chondrocytes in response to various cytokines and platelet activating factor (PAF).

RESULTS

Pro-inflammatory cytokines and PAF stimulation led to increased AA release, interleukin (IL)-1β and tumour necrosis factor (TNF) being the strongest inducers. The pattern of oleic acid release was similar to but less prominent than AA release, suggesting that predominantly arachidonyl selective enzymes were activated. IL-1β, TNF, IL-6, and IL-8 upregulated secretory group IIA, IID, and V phospholipase A2 (sPLA2-IIA, -IID, -V) and cytosolic group IVA phospholipase A2 (cPLA2-IVA) expression, where induction of chondrocyte sPLA2-IID is a novel finding. Furthermore, IL-1β, TNF, and IL-6 also induced COX2 expression. PAF induced expression of group IIA, IID and IVA PLA2, and COX2. In line with its anti-inflammatory properties, IL-4 was unable to induce either AA release or expression of PLA2s or COX2. IL-1β and TNF strongly increased PGE2 production, with IL-1β as the most prominent inducer.

CONCLUSION

Multiple PLA2 isoforms are expressed and influenced by pro-inflammatory stimuli in OA chondrocytes. Hence, several PLA2 enzymes may contribute to chondrocyte function by their upregulation and activation, and increased AA release and PGE2 production may therefore be important effectors in OA pathophysiology. PLA2 enzymes and cPLA2-IVA in particular are thus possible therapeutic targets in OA.

Peroxisome proliferator-activated receptor gamma in osteoarthritis

Osteoarthritis (OA) is among the most prevalent chronic human health disorders and the most common form of arthritis. It is a leading cause of disability in developed countries. This disease is characterized by cartilage deterioration, synovitis, and remodeling of the subchondral bone. There is not yet a satisfactory treatment to stop or arrest this disease process. Although several candidates for therapeutic approaches have been put forward, recent studies suggest that activation of the transcription factor peroxisome proliferator-activated receptor gamma (PPARγ) is an interesting target for this disease. PPARγ is a ligand-activated transcription factor and member of the nuclear receptor superfamily. Agonists of PPARγ inhibit inflammation and reduce synthesis of cartilage degradation products both in vitro and in vivo, and reduce the development/progression of cartilage lesions in OA animal models. This review will highlight the recent experimental studies on the presence of PPARγ in articular tissues and its effect on inflammatory and catabolic responses in chondrocytes and synovial fibroblasts, as well as the protective effects of PPARγ ligands in arthritis experimental models. Finally, the role of PPARγ polymorphism in the pathogenesis of OA and related musculoskeletal diseases will also be discussed.

[Pathophysiological relevance of peroxisome proliferators activated receptors (PPAR) to joint diseases - the pro and con of agonists]

Peroxisome proliferators activated receptors (PPAR) are ligand-inducible nuclear transacting factors comprising three subtypes, PPARalpha, PPARbeta/delta and PPARgamma, which play a key role in lipids and glucose homeostasis. All PPAR subtypes have been identified in joint or inflammatory cells and their activation resulted in a transcriptional repression of pro-inflammatory cytokines (IL-1, TNFalpha), early inflammatory genes (NOS(2), COX-2, mPGES-1) or matrix metalloproteases (MMP-1, MMP-13), at least for the gamma subtype. PPAR full agonists were also shown to stimulate IL-1 receptor antagonist (IL-1Ra) production by cytokine-stimulated articular cells in a subtype-dependent manner. These anti-inflammatory and anti-catabolic properties were confirmed in animal models of joint diseases where PPAR agonists reduced synovial inflammation while preventing cartilage destruction or inflammatory bone loss, although many effects required much higher doses than needed to restore insulin sensitivity or to lower circulating lipid levels. However, these promising effects of PPAR full agonists were hampered by their ability to reduce the growth factor-dependent synthesis of extracellular matrix components or to induce chondrocyte apoptosis, by the possible contribution of immunosuppressive properties to their anti-arthritic effects, by the increased adipocyte differentiation secondary to prolonged stimulation of PPARgamma, and by a variable contribution of PPAR subtypes depending on the system. Clinical data are scarce in rheumatoid arthritis (RA) patients whereas thousands of patients worldwilde, treated with PPAR agonists for type 2 diabetes or dyslipidemia, are paradoxically prone to suffer from osteoarthritis (OA). Whereas high dosage of full agonists may expose RA patients to cardiovascular adverse effects, the proof of concept that PPAR agonists have therapeutical relevance to OA may benefit from an epidemiological follow-up of joint lesions in diabetic or hyperlipidemic patients treated for long periods of time with glitazones or fibrates. Additionally, cellular and animal studies are required to assess whether partial agonists of PPAR (SPPARMs) may preserve therapeutical properties with potentially less safety concern.

Ajulemic acid, a synthetic cannabinoid acid, induces an antiinflammatory profile of eicosanoids in human synovial cells

AIMS

To better understand mechanisms whereby Ajulemic acid (AjA), a synthetic antiinflammatory cannabinoid, promotes resolution of acute and chronic inflammation in animal models, we investigated its influence on cyclooxygenase 2 (COX2) expression and eicosanoid production in human fibroblast-like synovial cells (FLS).

MAIN METHODS

FLS isolated from tissue obtained at joint replacement surgery or cultured from synovial fluid were treated for 60 min with AjA (10-30 microM), then stimulated with tumor necrosis factor alpha (TNFalpha). COX2 mRNA was measured by hybridization/colorimetric assay of whole cell lysates collected 4 h after stimulation. To determine effects on arachidonic acid release, FLS were incubated with (14)C-arachidonic acid for 20 h then treated with AjA (8-32 microM). Arachidonic acid release was measured by scintillation counting. Prostaglandins (PG) were measured by enzyme linked immunosorbent assay (ELISA) in cell supernatants collected 4 and 24 h after stimulation.

KEY FINDINGS

AjA increased the steady state levels of COX2 mRNA in and arachidonic acid release from FLS. Treatment of FLS with AjA increased 15-deoxy-delta(12,14)-PGJ(2) (15d-PGJ(2)) production in a concentration dependent manner, but did not affect PGE(2) production significantly.

SIGNIFICANCE

The capacity of AjA to increase selectively and markedly 15d-PGJ(2), an eicosanoid which facilitates resolution of inflammation, suggests that AjA may have value as a therapeutic agent for the treatment of rheumatoid arthritis (RA) and other diseases characterized by acute and chronic inflammation.

The role of 15-deoxy-delta(12,14)-prostaglandin J(2), an endogenous ligand of peroxisome proliferator-activated receptor gamma, in tumor angiogenesis

Peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear hormone receptor, is a ligand-activated transcription factor involved in adipogenesis, glucose homeostasis and lipid metabolism. 15-Deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)), an endogenous ligand of PPARgamma, has multifaceted cellular functions. Angiogenesis plays an important role in the pathophysiology of ischemic and neoplastic disorders, especially cancer. 15d-PGJ(2) is involved in regulation of angiogenic mediators including vascular endothelial growth factor and hence participates in the blood vessel formation by means of angiogenesis. However, depending on the experimental conditions, this cyclopentenone prostaglandin can exert opposite effects on angiogenesis. 15d-PGJ(2) inhibits angiogenesis via suppression of pro-inflammatory enzymes and cytokines, while it also stimulates angiogenesis via induction of heme oxygenase-1, endothelial nitric-oxide synthase, and hypoxia inducible factor-1alpha. The aim of this review is to highlight such dual effects of 15d-PGJ(2) on angiogenesis and underlying molecular mechanisms.

Peroxisome proliferator-activated receptor gamma1 expression is diminished in human osteoarthritic cartilage and is downregulated by interleukin-1beta in articular chondrocytes

Peroxisome proliferator-activated receptor γ (PPARγ) is a nuclear receptor involved in the regulation of many cellular processes. We and others have previously shown that PPARγ activators display anti-inflammatory and chondroprotective properties in vitro and improve the clinical course and histopathological features in an experimental animal model of osteoarthritis (OA). However, the expression and regulation of PPARγ expression in cartilage are poorly defined. This study was undertaken to investigate the quantitative expression and distribution of PPARγ in normal and OA cartilage and to evaluate the effect of IL-1β, a prominent cytokine in OA, on PPARγ expression in cultured chondrocytes. Immunohistochemical analysis revealed that the levels of PPARγ protein expression were significantly lower in OA cartilage than in normal cartilage. Using real-time RT-PCR, we demonstrated that PPARγ1 mRNA levels were about 10-fold higher than PPARγ2 mRNA levels, and that only PPARγ1 was differentially expressed: its levels in OA cartilage was 2.4-fold lower than in normal cartilage (p < 0.001). IL-1 treatment of OA chondrocytes downregulated PPARγ1 expression in a dose- and time-dependent manner. This effect probably occurred at the transcriptional level, because IL-1 decreases both PPARγ1 mRNA expression and PPARγ1 promoter activity. TNF-α, IL-17, and prostaglandin E₂ (PGE₂), which are involved in the pathogenesis of OA, also downregulated PPARγ1 expression. Specific inhibitors of the mitogen-activated protein kinases (MAPKs) p38 (SB203580) and c-Jun N-terminal kinase (SP600125), but not of extracellular signal-regulated kinase (PD98059), prevented IL-1-induced downregulation of PPARγ1 expression. Similarly, inhibitors of NF-κB signaling (pyrrolidine dithiocarbamate, MG-132, and SN-50) abolished the suppressive effect of IL-1. Thus, our study demonstrated that PPARγ1 is downregulated in OA cartilage. The pro-inflammatory cytokine IL-1 may be responsible for this downregulation via a mechanism involving activation of the MAPKs (p38 and JNK) and NF-κB signaling pathways. The IL-1-induced downregulation of PPARγ expression might be a new and additional important process by which IL-1 promotes articular inflammation and cartilage degradation.

15-Deoxy-Δ12,14-prostaglandin J2 as a potential endogenous regulator of redox-sensitive transcription factors

15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) has been known to display multifaceted cellular functions, including anti-inflammatory and cytoprotective effects. However, depending on the concentrations and intracellular microenvironment, this cyclopentenone prostaglandin can exert opposite effects. Because of the alpha,beta-unsaturated carbonyl moiety present in its cyclopentenone ring structure, 15d-PGJ(2) can act as a Michael reaction acceptor and readily interacts with critical cellular nucleophiles, such as cysteine thiol groups in proteins. Many of the biological effects induced by 15d-PGJ(2) involve redox-transcription factors as the potential targets. Thus, 15d-PGJ(2) can modulate the transcriptional activities of nuclear factor-kappaB (NF-kappaB), activator protein-1 (AP-1), nuclear factor-erythroid 2p45 (NF-E2)-related factors (Nrf2), hypoxia inducible factor (HIF), etc. 15d-PGJ(2) is also well known as an endogenous ligand of peroxisome proliferator-activated receptor gamma (PPARgamma). However, the regulation of the aforementioned redox-sensitive transcription factors by 15d-PGJ(2) is not necessarily mediated via PPARgamma activation, but rather involves covalent modification or oxidation of their critical cysteine residues acting as a redox-sensor. This commentary describes the biological and physiological functions of 15d-PGJ(2) and underlying biochemical and molecular mechanisms with emphasis on the modulation of redox-sensitive transcription factors and their regulators.

Evaluation of coexpression of microsomal prostaglandin E synthase-1 and cyclooxygenase-2 in interleukin-1- stimulated equine articular chondrocytes

OBJECTIVE

To characterize expression of cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase-1 (mPGES-1) and regulation of prostaglandin E2 (PGE2) production by equine articular chondrocytes.

SAMPLE POPULATION

Articular cartilage from the metacarpophalangeal joints of 7 adult horses.

PROCEDURE

Equine chondrocyte monolayer cultures were stimulated with different concentrations (2.5, 5, 10, and 20 ng/mL) of recombinant human interleukin-1beta (rhIL-1beta) for 24 hours and then with rhIL-1beta (5 ng/mL) for 3, 6, 9, 12, and 24 hours. Concentration of PGE2 in the media was measured via radioimmunoassay. Total RNA was extracted from harvested chondrocytes, and regulation of COX-2 and mPGES-1 mRNA was studied via reverse transcriptase-polymerase chain reaction assay and Southern blot analysis with equine-specific probes. Western blot analyses were performed on cellular extracts to characterize expression of COX-2 and mPGES-1 protein.

RESULTS

Stimulation with 5, 10, and 20 ng of rhIL-1beta/mL caused a significant increase in PGE2 concentrations in the culture media, and incubation of cells with rhIL-1beta (5 ng/mL) for 6 to 24 hours increased PGE2 production significantly. The increase in prostaglandin production was associated with an induction of COX-2 and mPGES-1 transcripts. There also was an rhIL-1beta-dependent induction in COX-2 and mPGES-1 protein expression.

CONCLUSIONS AND CLINICAL RELEVANCE

Collectively, results indicated that the rhIL-1beta-dependent increase in PGE2 production in equine chondrocytes in monolayer culture was associated with coordinated upregulation of COX-2 and mPGES-1 expression. The pathophysiologic consequences of upregulated COX-2 and mPGES-1 expression and of PGE2 synthesis in rhIL-1beta-stimulated equine chondrocytes remain to be elucidated.

Contrasting effects of peroxisome-proliferator-activated receptor (PPAR)gamma agonists on membrane-associated prostaglandin E2 synthase-1 in IL-1beta-stimulated rat chondrocytes: evidence for PPARgamma-independent inhibition by 15-deoxy-Delta12,14prostaglandin J2

Microsomal prostaglandin E synthase (mPGES)-1 is a newly identified inducible enzyme of the arachidonic acid cascade with a key function in prostaglandin (PG)E2 synthesis. We investigated the kinetics of inducible cyclo-oxygenase (COX)-2 and mPGES-1 expression with respect to the production of 6-keto-PGF1alpha and PGE2 in rat chondrocytes stimulated with 10 ng/ml IL-1beta, and compared their modulation by peroxisome-proliferator-activated receptor (PPAR)gamma agonists. Real-time PCR analysis showed that IL-1beta induced COX-2 expression maximally (37-fold) at 12 hours and mPGES-1 expression maximally (68-fold) at 24 hours. Levels of 6-keto-PGF1alpha and PGE2 peaked 24 hours after stimulation with IL-1beta; the induction of PGE2 was greater (11-fold versus 70-fold, respectively). The cyclopentenone 15-deoxy-Delta12,14prostaglandin J2 (15d-PGJ2) decreased prostaglandin synthesis in a dose-dependent manner (0.1 to 10 microM), with more potency on PGE2 level than on 6-keto-PGF1alpha level (-90% versus -66% at 10 microM). A high dose of 15d-PGJ2 partly decreased COX-2 expression but decreased mPGES-1 expression almost completely at both the mRNA and protein levels. Rosiglitazone was poorly effective on these parameters even at 10 microM. Inhibitory effects of 10 microM 15d-PGJ2 were neither reduced by PPARgamma blockade with GW-9662 nor enhanced by PPARgamma overexpression, supporting a PPARgamma-independent mechanism. EMSA and TransAM analyses demonstrated that mutated IkappaBalpha almost completely suppressed the stimulating effect of IL-1beta on mPGES-1 expression and PGE2 production, whereas 15d-PGJ2 inhibited NF-kappaB transactivation. These data demonstrate the following in IL-1-stimulated rat chondrocytes: first, mPGES-1 is rate limiting for PGE2 synthesis; second, activation of the prostaglandin cascade requires NF-kappaB activation; third, 15d-PGJ2 strongly inhibits the synthesis of prostaglandins, in contrast with rosiglitazone; fourth, inhibition by 15d-PGJ2 occurs independently of PPARgamma through inhibition of the NF-kappaB pathway; fifth, mPGES-1 is the main target of 15d-PGJ2.

The anti-inflammatory prostaglandin 15d-PGJ2 decreases oxidative/nitrosative mediators in brain after acute stress in rats

RATIONALE

Immobilisation stress is followed by accumulation of oxidative/nitrosative mediators in brain after the release of tumour necrosis factor-alpha (TNFalpha) and other cytokines, nuclear factor kappa B (NFkappaB) activation, nitric oxide synthase-2 (NOS-2) and cyclooxygenase-2 (COX-2) expression in the brain.

OBJECTIVES

This study was conducted to assess if some of the anti-inflammatory products of COX can modify the accumulation of oxidative/nitrosative species seen in brain after stress and to study the mechanisms by which this effect is achieved.

METHODS

Young-adult male Wistar rats were subjected to a single session of immobilisation during 6 h.

RESULTS

In stressed animals, brain levels of the anti-inflammatory 15d-PGJ2 increases concomitantly with COX-2 expression. Inhibition of COX-2 with NS-398 prevents stress-induced 15d-PGJ2 increase. Injection of supraphysiological doses of 15d-PGJ2 (80-120 microg/kg) decreases stress-induced increase in NOS-2 activity as well as the stress-induced increase in NO metabolites. On the other hand, 15d-PGJ2 decreases stress-induced malondialdehyde (an indicator of lipid peroxidation) accumulation in cortex and prevents oxidation of the main anti-oxidant glutathione. The mechanisms involved in the anti-oxidative properties of 15d-PGJ2 in stress involve NFkappaB blockade (by preventing stress-induced IkappaBalpha decrease) as well as inhibition of TNFalpha release in stressed animals. At the doses tested, 15d-PGJ2 decreases COX-2 expression and PGE2 release during stress, suggesting an alternative mechanism for this endogenous compound.

CONCLUSIONS

These findings demonstrate a role for this anti-inflammatory pathway in the brain response to stress and open the possibility for preventing accumulation of oxidative/nitrosative species and subsequent brain damage.

Osteoarthritis - an untreatable disease?

Osteoarthritis is a painful and disabling disease that affects millions of patients. Its aetiology is largely unknown, but is most likely multi-factorial. Osteoarthritis poses a dilemma: it often begins attacking different joint tissues long before middle age, but cannot be diagnosed until it becomes symptomatic decades later, at which point structural alterations are already quite advanced. In this review, osteoarthritis is considered as a disease of the whole joint that may result from multiple pathophysiological mechanisms, one of which is the dysregulation of lipid homeostasis. No proven disease-modifying therapy exists for osteoarthritis and current treatment options for chronic osteoarthritic pain are insufficient, but new pharmacotherapeutic options are emerging.

15d-PGJ2: the anti-inflammatory prostaglandin?

15-Deoxy-Delta-12,14-prostaglandin J2 (15d-PGJ2) is the most recently discovered prostaglandin. This cyclopentanone, the dehydration end product of PGD2, differs from other prostaglandins in several respects. There is no specific prostaglandin synthase (PGS) leading to 15d-PGJ2 production and no specific 15d-PGJ2 receptor has been identified to date. Instead, 15d-PGJ2 has been shown to act via PGD2 receptors (DP1 and DP2) and through interaction with intracellular targets. In particular, 15d-PGJ2 is recognized as the endogenous ligand for the intranuclear receptor PPARgamma. This property is responsible for many of the 15d-PGJ2 anti-inflammatory functions. In this review, we summarize the current understanding of 15d-PGJ2 synthesis, biology and main effects both in molecular physiology and pathological states.

Inhibition of interleukin-1?-induced cyclooxygenase 2 expression in human synovial fibroblasts by 15-deoxy-?12,14-prostaglandin J2 through a histone deacetylase-independent mechanism

OBJECTIVE

The cyclooxygenase (COX) metabolite, 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)), has been reported to inhibit the expression of a number of genes involved in the pathogenesis of arthritis. However, its effects on COX-2 remain controversial. We undertook this study to investigate the effects of 15d-PGJ(2) on interleukin-1beta (IL-1beta)-induced COX-2 expression in human synovial fibroblasts (HSFs).

METHODS

HSFs were cultured with IL-1beta in the absence or presence of 15d-PGJ(2), and the levels of COX-2 protein and messenger RNA (mRNA) expression were evaluated using Western blotting and real-time reverse transcriptase-polymerase chain reaction, respectively. COX-2 promoter activity was analyzed in transient transfection experiments. Chromatin immunoprecipitation assays were performed to evaluate the level of histone acetylation and the recruitment of histone deacetylases (HDACs) 1, 2, and 3 and histone acetylase (HAT) p300 to the COX-2 promoter.

RESULTS

IL-1beta-induced COX-2 protein and mRNA expression, as well as COX-2 promoter activation, were inhibited by 15d-PGJ(2). Troglitazone, a selective peroxisome proliferator-activated receptor gamma (PPARgamma) ligand, enhanced COX-2 expression, while GW9662, a specific PPARgamma antagonist, relieved the suppressive effect of 15d-PGJ(2). IL-1beta-induced histone H3 acetylation was selectively blocked by 15d-PGJ(2). The reduction of histone H3 acetylation did not correlate with the recruitment of HDACs to the COX-2 promoter. Also, treatment with the specific HDAC inhibitor, trichostatin A, did not relieve the suppressive effect of 15d-PGJ(2), indicating that HDACs are not involved in the inhibitory effect of 15d-PGJ(2). Furthermore, 15d-PGJ(2) blocked IL-1beta-induced recruitment of p300 to the COX-2 promoter, which may be the mechanism for decreased histone H3 acetylation and COX-2 expression. In accordance with this, overexpression of p300, but not of a mutant p300 lacking HAT activity, relieved the inhibitory effect of 15d-PGJ(2) on COX-2 promoter activation.

CONCLUSION

These data suggest that 15d-PGJ(2) can inhibit IL-1beta-induced COX-2 expression by an HDAC-independent mechanism, probably by interfering with HAT p300.

The regulation of chondrocyte function by proinflammatory mediators: prostaglandins and nitric oxide

Within the mature articular cartilage matrix, which has no blood or nerve supply, chondrocytes show little metabolic activity with low turnover of matrix components. Under conditions of stress because of biomechanical factors, however, chondrocytes are capable of producing mediators that are associated with inflammation, including cytokines such as interleukin-1 and tumor necrosis factor-alpha, which in turn stimulate the production of prostaglandins and nitric oxide. Chondrocytes also express receptors for these mediators, which accumulate at high local concentrations and can act in an autocrine-paracrine fashion to feedback-regulate chondrocyte responses. Prostaglandin E2 can exert catabolic or anabolic effects depending on the microenvironment. Nitric oxide can promote cellular injury and increase chondrocyte susceptibility to cytokine-induced apoptosis. Because cross-talk between these mediators produces complex modulation of catabolic and anabolic pathways, further studies in vitro and in vivo are required to elucidate their precise roles in osteoarthritis.

Signaling transduction: target in osteoarthritis

PURPOSE OF REVIEW

The pathophysiology of osteoarthritis is the result of an imbalance between anabolic and catabolic pathways. This imbalance is the result of the activation of joint cells by inflammatory mediators, matrix components, and mechanical stress. All these mediators act through specific receptors that transmit the signals to the nucleus to activate the transcription of matrix metalloproteinases and inflammatory genes. Targeting these signaling pathways in osteoarthritis is considered a novel approach to modulate this imbalance.

RECENT FINDINGS

Although many signaling pathways are necessary for physiologic cell life, it is now well established that a few are more specifically induced in an inflammatory environment. In osteoarthritis, the nuclear factor-kappaB and mitogen-activated protein kinase pathways have been shown to play a predominant role in the expression of metalloproteinases and inflammatory genes and proteins. Also involved in the activation of osteoarthritic cells are other molecules interacting with one or several signaling pathways, such as nitric oxide, peroxisome proliferator-activated receptor-gamma ligands, or C/EBP transcriptional factors. Based on this knowledge, specific inhibitors for some of these signaling pathways have been designed and include p38 mitogen-activated protein kinase or nuclear factor-kappaB inhibitors. Experimental studies evaluating cartilage degradation in arthritis models are promising, although fewer have been done specifically in osteoarthritis models.

SUMMARY

Targeting signaling pathways in osteoarthritis did not seem feasible a few years ago because of the complexity of the multiple intracellular pathways, mainly physiologic, defined by a high degree of redundancy and cross-talk. However, important advances in the knowledge of chondrocyte and synoviocyte signaling in osteoarthritis have been achieved in recent years and suggest that inhibitors of specific signaling pathways could shortly provide effective treatments for this disease.

A potential role of 15-deoxy-delta(12,14)-prostaglandin J2 for induction of human articular chondrocyte apoptosis in arthritis

The cyclopentenone prostaglandin (PG) J2 is formed within the cyclopentenone ring of the endogenous prostaglandin PG D2 by a nonenzymatic reaction. The PG J family is involved in mediating various biological effects including the regulation of cell cycle progression and inflammatory responses. Here we demonstrate the potential role of 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PG J2) in human articular chondrocyte apoptosis. 15d-PG J2 was released by human articular chondrocytes and found in joint synovial fluids taken from osteoarthritis or rheumatoid arthritis patients. Proinflammatory cytokines such as interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) up-regulated chondrocyte release of 15d-PG J2. PG D2 synthase mRNA expression was up-regulated by IL-1beta, TNF-alpha, or nitric oxide. 15d-PG J2 induced apoptosis of chondrocytes from osteoarthritis or rheumatoid arthritis patients as well as control nonarthritic subjects in a time- and dose-dependent manner and in a peroxisome proliferator-activated receptor gamma-dependent manner. Peroxisome proliferator-activated receptor gamma expression was up-regulated by IL-1beta and TNF-alpha. Inhibition of NF-kappaB, and the activation of p38 MAPK were also found to be involved in 15d-PG J2-induced chondrocyte apoptosis. Such signal pathways led to the activation of the downstream pro-apoptotic molecule p53 and caspase cascades. Together, these results suggest that 15d-PGJ2 may play an important role in the pathogenesis of arthritic joint destruction via a regulation of chondrocyte apoptosis.

Cyclooxygenase-2 and prostaglandins in articular tissues

OBJECTIVES

To provide an overview on: 1) the expression of cyclooxygenase (COX)-2 in articular tissues; 2) the role of prostaglandin E2 (PGE2) in these tissue functions; and 3) clinical trials with COX-2-selective nonsteroidal anti-inflammatory drugs (NSAIDs) (coxibs).

METHODS

MEDLINE search was performed using the key words "cyclooxygenase," "prostaglandin," "osteoarthritis" (OA), and "rheumatoid arthritis" (RA). Selected publications related to clinical trials with coxibs also are included.

RESULTS

COX-2 is upregulated in inflamed joint tissues and is responsible for elevated PGE2 production. The overexpression of COX-2 is likely induced by proinflammatory mediators such as interleukin-1beta (IL-1beta) and tumor necrosis factor (TNF) alpha. However, the exact molecular mechanisms through which the expression of COX-2 is regulated remain to be elucidated. Several studies suggest that PGE2 is involved in inflammation, apoptosis, angiogenesis, and possibly structural changes that characterize arthritic diseases. NSAIDs are prescribed for the treatment of OA and RA and provide effective relief from symptoms; however, serious gastrointestinal complications occur with their use. The clinical efficacy of NSAIDs is primarily related to the inhibition of COX-2, whereas much of the toxicity is related to COX-1 inhibition. Selective COX-2 inhibitors (coxibs) that spare COX-1 at therapeutic doses are more effective than placebo and as effective as other NSAIDs for relief of symptoms of OA and RA, and have significantly improved gastrointestinal safety and tolerability. However, some studies showed that COX-2-selective inhibitors still have classic NSAID complications.

CONCLUSIONS

Overexpression of COX-2 protein in articular tissues is a characteristic feature of arthritic diseases. However, the molecular mechanisms involved in the regulation of COX-2 expression and activity are still unclear. Elucidating the mechanisms of COX-2 expression and PGE2 production and action will help identify novel and more selective potential drug targets in the treatment of arthritic diseases.

Role of eicosanoids in structural degradation in osteoarthritis

PURPOSE OF REVIEW

Osteoarthritis is characterized mainly by degenerative changes in joint cartilage, ultimately resulting in loss of cartilage, and alterations in the subchondral bone. Osteoarthritis osteoblasts show a number of metabolic alterations that may interfere with normal cell metabolism and signaling, possibly leading to altered extracellular matrix composition. This review examines the role of eicosanoids in this structural degradation.

RECENT FINDINGS

Prostaglandins exert diverse modulatory roles in osteoarthritis, with prostaglandin E2 known to play an important role in inflammation. Prostaglandins and leukotriene B4 have been shown to regulate proinflammatory cytokine and interstitial collagenase synthesis in human osteoarthritis synovial membrane explants. Human osteoarthritis osteoblasts produce variable levels of prostaglandin E2 and leukotriene B4 compared with normal osteoblasts. Prostaglandin E2 levels can distinguish two types of patients with osteoarthritis: osteoblasts from one group produce low levels of prostaglandin E2 and interleukin-6, and the other shows an increase in production. In contrast, osteoarthritis osteoblasts that produce high levels of prostaglandin E2 produce low levels of leukotriene B4 and vice versa. This observation could be explained by the selective metabolism of arachidonic acid via the 5-lipoxygenase or cyclooxygenase pathways in osteoarthritis osteoblasts.

SUMMARY

Prostaglandins play a significant role not only in joint physiology, but also in the pathogenesis of joint disorders. In addition, it has been identified that osteoarthritis subchondral osteoblasts can synthesize leukotriene B4, indicating a role of leukotrienes in bone remodeling associated with osteoarthritis. A therapeutic intervention that blocks lipoxygenase/cyclooxygenase pathways, thereby inhibiting production of prostaglandins and leukotrienes, may therefore be very attractive for the treatment of osteoarthritis patients.