Induction of cyclooxygenase-2 by mechanical stress through a nitric oxide-regulated pathway

Mechanotransduction pathways in articular chondrocytes and the emerging role of estrogen receptor-α

In the synovial joint, mechanical force creates an important signal that influences chondrocyte behavior. The conversion of mechanical signals into biochemical cues relies on different elements in mechanotransduction pathways and culminates in changes in chondrocyte phenotype and extracellular matrix composition/structure. Recently, several mechanosensors, the first responders to mechanical force, have been discovered. However, we still have limited knowledge about the downstream molecules that enact alterations in the gene expression profile during mechanotransduction signaling. Recently, estrogen receptor α (ERα) has been shown to modulate the chondrocyte response to mechanical loading through a ligand-independent mechanism, in line with previous research showing that ERα exerts important mechanotransduction effects on other cell types, such as osteoblasts. In consideration of these recent discoveries, the goal of this review is to position ERα into the mechanotransduction pathways known to date. Specifically, we first summarize our most recent understanding of the mechanotransduction pathways in chondrocytes on the basis of three categories of actors, namely mechanosensors, mechanotransducers, and mechanoimpactors. Then, the specific roles played by ERα in mediating the chondrocyte response to mechanical loading are discussed, and the potential interactions of ERα with other molecules in mechanotransduction pathways are explored. Finally, we propose several future research directions that may advance our understanding of the roles played by ERα in mediating biomechanical cues under physiological and pathological conditions.

Spectral characterization of cell surface motion for mechanistic investigations of cellular mechanobiology

Understanding the specific mechanisms responsible for anabolic and catabolic responses to static or dynamic force are largely poorly understood. Because of this, most research groups studying mechanotransduction due to dynamic forces employ an empirical approach in deciding what frequencies to apply during experiments. While this has been shown to elucidate valuable information regarding how cells respond under controlled provocation, it is often difficult or impossible to determine a true optimal frequency for force application, as many intracellular complexes are involved in receiving, propagating, and responding to a given stimulus. Here we present a novel adaptation of an analytical technique from the fields of civil and mechanical engineering that may open the door to direct measurement of mechanobiological cellular frequencies which could be used to target specific cell signaling pathways leveraging synergy between outside-in and inside-out mechanotransduction approaches. This information could be useful in identifying how specific proteins are involved in the homeostatic balance, or disruption thereof, of cells and tissue, furthering the understanding of the pathogenesis and progression of many diseases across a wide variety of cell types, which may one day lead to the development of novel mechanobiological therapies for clinical use.

A Progress Report and Roadmap for Microphysiological Systems and Organ-On-A-Chip Technologies to Be More Predictive Models in Human (Knee) Osteoarthritis

Osteoarthritis (OA), a chronic debilitating joint disease affecting hundreds of million people globally, is associated with significant pain and socioeconomic costs. Current treatment modalities are palliative and unable to stop the progressive degeneration of articular cartilage in OA. Scientific attention has shifted from the historical view of OA as a wear-and-tear cartilage disorder to its recognition as a whole-joint disease, highlighting the contribution of other knee joint tissues in OA pathogenesis. Despite much progress in the field of microfluidic systems/organs-on-a-chip in other research fields, current in vitro models in use do not yet accurately reflect the complexity of the OA pathophenotype. In this review, we provide: 1) a detailed overview of the most significant recent developments in the field of microsystems approaches for OA modeling, and 2) an OA-pathophysiology-based bioengineering roadmap for the requirements of the next generation of more predictive and authentic microscale systems fit for the purpose of not only disease modeling but also of drug screening to potentially allow OA animal model reduction and replacement in the near future.

NO and hepatocellular cancer

NO has broad and sometimes dichotomous roles in cancer. The effects of NO in tumours depend on the type and localization of NOS isoforms, concentration and duration of NO exposure, and cellular sensitivity to NO. Hepatocellular carcinoma (HCC) is a common and lethal disease for which no effective therapy other than surgical resection exists. Over two decades of research has yielded evidence that NO generated by the inducible NOS (iNOS or NOS2) contributes to HCC progression in at least a subset of patients with HCC. The co-expression of iNOS with COX-2 may portend a particularly aggressive cancer phenotype in HCC and at the same time reveal an opportunity for pharmacological intervention. In this review, we focus on what is known about the influence of NO in HCC neoplastic transformation, proliferation and apoptosis, angiogenesis, invasion, and metastasis, cancer stem cells, and the host immune response against the tumour. We discuss the implications of recent findings for targeting the NO pathways in HCC.

Suspension state promotes metastasis of breast cancer cells by up-regulating cyclooxygenase-2

Hematogenous metastasis requires tumor cells to detach from primary tumor into blood/lymphatic circulation and extravasate. Tumor cells in the blood circulation system, named circulating tumor cells (CTCs), are in a suspension state, with unique cytoskeletal structure and molecular phenotype different from primary tumor cells. The aim of this study is to assess the impact of suspension state on the metastatic potential of breast cancer cells (BCCs) and study its underlying mechanism.

Methods: BCCs were cultured on low-adhesion plates to mimic the suspension state. Conventional adherent culture BCCs were used as the control. This study examined the metastatic potential of adherent and suspension BCCs in vitro and in vivo. RNA sequencing analysis, siRNA, and inhibitors were used to determine the underlying molecular mechanism.

Results: The suspension state significantly increased the metastatic potential of BCCs, but slightly suppressed their tumor growth. RNA sequencing analysis revealed that the suspension state resulted in an acquisition of unique molecular signature enriched in pro-metastatic and tumor-suppressive genes. Specifically, prostaglandin-endoperoxide synthase 2 (PTGS2), which encodes protein cyclooxygenase-2 (COX-2), was identified as a highly up-regulated gene in suspension state compared with adherent cultured BCCs. Inhibition of the catalytic activity of COX-2 by celecoxib markedly suppressed suspension-increased migration and invasion of BCCs. In addition, knock-down of COX-2 by siRNA reduced the experimental lung metastasis formation of suspension cultured BCCs, which was associated with a remarkable decrease in retention and survival of BCCs in lungs of mice in the early stage of metastasis. Activation of Ca²⁺/calcineurin (CaN)/nuclear factor of activated T cells (NFAT) pathway and disruption of cytoskeleton contributed to the COX-2 up-expression by suspension state.

Conclusions: Our results demonstrate that suspension state plays an important role in the metastatic potential of CTCs, and suggest a potential application of COX-2 inhibitor for anti-metastasis.

Biomechanics, obesity, and osteoarthritis. The role of adipokines: When the levee breaks

Osteoarthritis is a high-incidence painful and debilitating disease characterized by progressive degeneration of articular joints, which indicates a breakdown in joint homeostasis favoring catabolic processes. Biomechanical loading, associated with inflammatory and metabolic imbalances of joint, strongly contributes to the initiation and progression of the disease. Obesity is a primary risk factor for disease onset, and mechanical factors increased the risk for disease progression. Moreover, inflammatory mediators, in particular, adipose tissue-derived cytokines (better known as adipokines) play a critical role linking obesity and osteoarthritis. The present article summarizes the knowledge about the role of adipokines in cartilage and bone function, highlighting their contribution to the imbalance of joint homeostasis and, consequently, pathogenesis of osteoarthritis. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:594-604, 2018.

Matrix metalloproteinase activity and prostaglandin E2 are elevated in the synovial fluid of meniscus tear patients

PURPOSE

Meniscus tears are a common knee injury and are associated with the development of post-traumatic osteoarthritis (OA). The purpose of this study is to evaluate potential OA mediators in the synovial fluid and serum of meniscus tear subjects compared to those in the synovial fluid of radiographic non-OA control knees.

MATERIALS AND METHODS

Sixteen subjects with an isolated unilateral meniscus injury and six subjects who served as reference controls (knee Kellgren-Lawrence grade 0-1) were recruited. Twenty-one biomarkers were measured in serum from meniscus tear subjects and in synovial fluid from both groups. Meniscus tear subjects were further stratified by tear type to assess differences in biomarker levels.

RESULTS

Synovial fluid total matrix metalloproteinase (MMP) activity and prostaglandin E2 (PGE2) were increased 25-fold and 290-fold, respectively, in meniscus tear subjects as compared to reference controls (p < 0.05). Synovial fluid MMP activity and PGE2 concentrations were positively correlated in meniscus tear subjects (R = 0.83, p < 0.0001). In meniscus tear subjects, synovial fluid levels of MMP activity, MMP-2, MMP-3, sGAG, COMP, IL-6, and PGE2 were higher than serum levels (p < 0.05). Subjects with complex meniscus tears had higher synovial fluid MMP-10 (p < 0.05) and reduced serum TNFα and IL-8 (p < 0.05) compared to other tear types.

CONCLUSIONS

Given the degradative and pro-inflammatory roles of MMP activity and PGE2, these molecules may alter the biochemical environment of the joint. Our findings suggest that modulation of PGE2 signaling, MMP activity, or both following a meniscus injury may be targets to promote meniscus repair and prevent OA development.

Gentiopicroside prevents interleukin-1 beta induced inflammation response in rat articular chondrocyte

ETHNOPHARMACOLOGICAL RELEVANCE

In traditional Chinese medicine, Gentiana macrophylla Pall have been prescribed for the treatment of pain and inflammatory conditions. In addition, it is a common Tibetan medicinal herb used for the treatment of tonsillitis, urticaria, and rheumatoid arthritis (RA), while the flowers of G. macrophylla Pall have been traditionally treated as an anti-inflammatory agent to clear heat in Mongolian medicine. The secoiridoid glycosides and their derivatives are the primary active components of G. macrophylla and have been demonstrated to be effective as anti-inflammatory agents.

MATERIALS AND METHODS

Solvent extraction and D101 macroporous resin columns were employed to concentratethe gentiopicroside. Gentiopicroside cytotoxicity was assessed by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay; the toxicity of gentiopicroside in chondrocytes was reconfirmed using Hoechst staining. Western blotting, reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry were utilized to explore the protective effects and mechanisms of gentiopicroside prevents interleukin-1 beta induced inflammation response in rat articular chondrocyte.

RESULTS

The MTT assay demonstrated that 50, 500, and 1,500 μg/mL of gentiopicroside exhibited no significant toxicity to chondrocytes (P>0.05) after 24h. Using immunohistochemistry, ELISA, RT-PCR, Western blot method to explore the protective effect and mechanism of gentiopicroside on chondrocytes induced by IL-1β. The results showed some pathways of IL-1β signal transduction were inhibited by gentiopicroside in rat chondrocytes: p38, ERK and JNK. Meanwhile, gentiopicroside showed inhibition in the IL-1β-induced release of MMPs while increasing Collagen type II expression.

CONCLUSIONS

The current study demonstrated that gentiopicroside exhibited a potent protective effect on IL-1β induced inflammation response in rat articular chondrocyte. Thus, gentiopicroside could be a potential therapeutic strategy for treatment of OA.

The mechanobiology of articular cartilage: bearing the burden of osteoarthritis

Articular cartilage injuries and degenerative joint diseases are responsible for progressive pain and disability in millions of people worldwide, yet there is currently no treatment available to restore full joint functionality. As the tissue functions under mechanical load, an understanding of the physiologic or pathologic effects of biomechanical factors on cartilage physiology is of particular interest. Here, we highlight studies that have measured cartilage deformation at scales ranging from the macroscale to the microscale, as well as the responses of the resident cartilage cells, chondrocytes, to mechanical loading using in vitro and in vivo approaches. From these studies, it is clear that there exists a complex interplay among mechanical, inflammatory, and biochemical factors that can either support or inhibit cartilage matrix homeostasis under normal or pathologic conditions. Understanding these interactions is an important step toward developing tissue engineering approaches and therapeutic interventions for cartilage pathologies, such as osteoarthritis.

In vivo cartilage strain increases following medial meniscal tear and correlates with synovial fluid matrix metalloproteinase activity

Meniscal tears are common injuries, and while partial meniscectomy is a frequent treatment option, general meniscus loss is a risk factor for the development of osteoarthritis. The goal of this study was to measure the in vivo tibiofemoral cartilage contact patterns in patients with meniscus tears in relation to biomarkers of cartilage catabolism in the synovial fluid of these joints. A combination of magnetic resonance imaging and biplanar fluoroscopy was used to determine the in vivo motion and cartilage contact mechanics of the knee. Subjects with isolated medial meniscus tears were analyzed while performing a quasi-static lunge, and the contralateral uninjured knee was used as a control. Synovial fluid was collected from the injured knee and matrix metalloproteinase (MMP) activity, sulfated glycosaminoglycan, cartilage oligomeric matrix protein, prostaglandin E2, and the collagen type II cleavage biomarker C2C were measured. Contact strain in the medial compartment increased significantly in the injured knees compared to contralateral control knees. In the lateral compartment, the contact strain in the injured knee was significantly increased only at the maximum flexion angle (105°). The average cartilage strain at maximum flexion positively correlated with total MMP activity in the synovial fluid. These findings show that meniscal injury leads to loss of normal joint function and increased strain of the articular cartilage, which correlated to elevated total MMP activity in the synovial fluid. The increased strain and total MMP activity may reflect, or potentially contribute to, the early development of osteoarthritis that is observed following meniscal injury.

Use of cartilage derived from murine induced pluripotent stem cells for osteoarthritis drug screening

OBJECTIVE

The discovery of novel disease-modifying drugs for osteoarthritis (OA) is limited by the lack of adequate genetically defined cartilage tissues for application in high-throughput screening systems. We addressed this need by synthesizing cartilage from induced pluripotent stem cells (iPSCs) to establish and validate an in vitro model of OA.

METHODS

Native or iPSC-derived mouse cartilage samples were treated with the cytokine interleukin-1α (IL-1α) for 3 days to model the inflammatory environment of OA. The biochemical content, mechanical properties, and gene expression of the resulting tissues were assayed. In addition, the inflammatory and catabolic environment of the media was assessed. To establish high-throughput capability, we used a 96-well plate format and conducted a screen of previously identified candidate OA drugs. Glycosaminoglycan (GAG) release into the medium was used as the primary output for screening.

RESULTS

Treatment of iPSC-derived or native cartilage with IL-1α induced characteristic features of OA in a rapid and dose-dependent manner. In addition to the loss of GAGs and tissue mechanical properties, IL-1α treatment induced the expression of matrix metalloproteinases and increased the production of the inflammatory mediators nitric oxide and prostaglandin E2 . In the high-throughput screen validation, all candidate OA therapeutic agents provided some benefit, but only the NF-κB inhibitor SC514 effectively reduced cartilage loss in response to IL-1α.

CONCLUSION

This work demonstrates the utility of iPSCs for studying cartilage pathology and provides a platform for identifying novel, patient-specific therapeutic agents that prevent cartilage degradation and modify the course of OA development.

Obesity and osteoarthritis, more than just wear and tear: pivotal roles for inflamed adipose tissue and dyslipidaemia in obesity-induced osteoarthritis

OA is a degenerative joint disease characterized by articular cartilage degradation, osteophyte formation, synovitis, and subchondral bone sclerosis. One of OAs main risk factors is obesity. To date, it is not fully understood how obesity results in OA. Historically, this link was ascribed to excessive joint loading as a result of increased body weight. However, the association between obesity and OA in non-weight-bearing joints suggests a more complex aetiology for obesity-induced OA. In the present review, the link between obesity and OA is discussed. First, the historical view of altered joint loading leading to wear and tear of the joint is addressed. Subsequently, the effects of a disturbed lipid metabolism, low-grade inflammation, and adipokines on joint tissues are discussed and linked to OA. Taken together, inflamed adipose tissue and dyslipidaemia play pivotal roles in obesity-induced OA. It becomes increasingly clear that the link between obesity and OA transcends excessive loading.

Progression of Gene Expression Changes following a Mechanical Injury to Articular Cartilage as a Model of Early Stage Osteoarthritis

An impact injury model of early stage osteoarthritis (OA) progression was developed using a mechanical insult to an articular cartilage surface to evaluate differential gene expression changes over time and treatment. Porcine patellae with intact cartilage surfaces were randomized to one of three treatments: nonimpacted control, axial impaction (2000 N), or a shear impaction (500 N axial, with tangential displacement to induce shear forces). After impact, the patellae were returned to culture for 0, 3, 7, or 14 days. At the appropriate time point, RNA was extracted from full-thickness cartilage slices at the impact site. Quantitative real-time PCR was used to evaluate differential gene expression for 18 OA related genes from four categories: cartilage matrix, degradative enzymes and inhibitors, inflammatory response and signaling, and cell apoptosis. The shear impacted specimens were compared to the axial impacted specimens and showed that shear specimens more highly expressed type I collagen (Col1a1) at the early time points. In addition, there was generally elevated expression of degradative enzymes, inflammatory response genes, and apoptosis markers at the early time points. These changes suggest that the more physiologically relevant shear loading may initially be more damaging to the cartilage and induces more repair efforts after loading.

Celecoxib exerts protective effects on extracellular matrix metabolism of mandibular condylar chondrocytes under excessive mechanical stress

OBJECTIVE

Excessive mechanical stress is considered a major cause of temporomandibular joint osteoarthritis (TMJ-OA). High magnitude cyclic tensile strain (CTS) up-regulates pro-inflammatory cytokines and matrix metalloproteinases (MMPs) in chondrocytes, while selective cyclooxygenase (COX)-2 inhibition has been shown to be beneficial to cytokine-induced cartilage damage. However, the effect of selective COX-2 inhibitors on mechanically stimulated chondrocytes remains unclear. This study evaluated the effect of celecoxib, a selective COX-2 inhibitor, on extracellular matrix (ECM) metabolism of mandibular condylar chondrocytes under CTS.

METHODS

Porcine mandibular chondrocytes were subjected to CTS of 0.5 Hz, 10% elongation with celecoxib for 24 h. The gene expressions of COX-2, MMPs, aggrecanase (ADAMTS), type II collagen and aggrecan were examined by real-time PCR. Also, prostaglandin E2 (PGE2) concentrations were determined using enzyme immunoassay kit. The levels of MMP and transcription factor NF-κB were measured by western blot while MMP activity was determined by casein zymography.

RESULTS

The presence of celecoxib normalized the release of PGE2 and diminished the CTS-induced COX-2, MMP-1, MMP-3, MMP-9 and ADAMTS-5 gene expressions while recovered the downregulated type II collagen and aggrecan gene expressions. Concurrently, celecoxib showed inhibition of NF-κB and suppression of MMP production and activity.

CONCLUSIONS

Celecoxib exerts protective effects on mandibular condylar chondrocytes under CTS stimulation by diminishing degradation and restoring synthesis of ECM.

Tissue-engineered cartilage with inducible and tunable immunomodulatory properties

The pathogenesis of osteoarthritis is mediated in part by inflammatory cytokines including interleukin-1 (IL-1), which promote degradation of articular cartilage and prevent human mesenchymal stem cell (MSC) chondrogenesis. In this study, we combined gene therapy and functional tissue engineering to develop engineered cartilage with immunomodulatory properties that allow chondrogenesis in the presence of pathologic levels of IL-1 by inducing overexpression of IL-1 receptor antagonist (IL-1Ra) in MSCs via scaffold-mediated lentiviral gene delivery. A doxycycline-inducible vector was used to transduce MSCs in monolayer or within 3D woven PCL scaffolds to enable tunable IL-1Ra production. In the presence of IL-1, IL-1Ra-expressing engineered cartilage produced cartilage-specific extracellular matrix, while resisting IL-1-induced upregulation of matrix metalloproteinases and maintaining mechanical properties similar to native articular cartilage. The ability of functional engineered cartilage to deliver tunable anti-inflammatory cytokines to the joint may enhance the long-term success of therapies for cartilage injuries or osteoarthritis.

Effects of cartilage impact with and without fracture on chondrocyte viability and the release of inflammatory markers

Post-traumatic arthritis (PTA) frequently develops after intra-articular fracture of weight bearing joints. Loss of cartilage viability and post-injury inflammation have both been implicated as possible contributing factors to PTA progression. To further investigate chondrocyte response to impact and fracture, we developed a blunt impact model applying 70%, 80%, or 90% surface-to-surface compressive strain with or without induction of an articular fracture in a cartilage explant model. Following mechanical loading, chondrocyte viability, and apoptosis were assessed. Culture media were evaluated for the release of double-stranded DNA (dsDNA) and immunostimulatory activity via nuclear factor kappa B (NF-κB) activity in Toll-like receptor (TLR) -expressing Ramos-Blue reporter cells. High compressive strains, with or without articular fracture, resulted in significantly reduced chondrocyte viability. Blunt impact at 70% strain induced a loss in viability over time through a combination of apoptosis and necrosis, whereas blunt impact above 80% strain caused predominantly necrosis. In the fracture model, a high level of primarily necrotic chondrocyte death occurred along the fracture edges. At sites away from the fracture, viability was not significantly different than controls. Interestingly, both dsDNA release and NF-κB activity in Ramos-Blue cells increased with blunt impact, but was only significantly increased in the media from fractured cores. This study indicates that the mechanism of trauma determines the type of chondrocyte death and the potential for post-injury inflammation.

Synovial fluid concentrations and relative potency of interleukin-1 alpha and beta in cartilage and meniscus degradation

Cartilage degeneration with osteoarthritis (OA) is believed to involve the activities of interleukin-1 (IL-1), which exists as alpha and beta isoforms. The goal of this study was to measure the concentrations of both isoforms of IL-1 in the synovial fluid of normal and spontaneously osteoarthritic porcine knees, and to test the hypothesis that physiologic concentrations of IL-1α and IL-1β exhibit different potencies in activating calcium signaling, the production of matrix metalloproteinases and nitric oxide, and the loss of proteoglycans and tissue mechanical properties in cartilage and meniscus. Median concentrations of IL-1α were 0.043 ng/ml with mild OA and 0.288 ng/ml with moderate OA, whereas IL-1β concentrations were 0.109 ng/ml with mild OA and 0.122 ng/ml with moderate OA. Both isoforms induced calcium signaling in chondrocytes and meniscal cells at all concentrations. Overall, cartilage and meniscus catabolism was significantly more sensitive to IL-1α than IL-1β at concentrations of 1 ng/ml or less, while few differences were observed between the two forms at 10 ng/ml. These data provide a range of physiologic IL-1 concentrations that can serve as a framework for the comparison of various in vitro studies, as well as providing further insight for the development of anti-cytokine therapies for OA.

Biomechanical factors in osteoarthritis

Biomechanical factors play an important role in the health of diarthrodial joints. Altered joint loading - associated to obesity, malalignment, trauma or joint instability - is a critical risk factor for joint degeneration, whereas exercise and weight loss have generally been shown to promote beneficial effects for osteoarthritic joints. The mechanisms by which mechanical stress alters the physiology or pathophysiology of articular cartilage or other joint tissues likely involve complex interactions with genetic and molecular influences, particularly local or systemic inflammation secondary to injury or obesity. Chondrocytes perceive physical signals from their environment using a variety of mechanisms, including ion channels, integrin-mediated connections to the extracellular matrix that involve membrane, cytoskeletal and intracellular deformation. An improved understanding of the biophysical and molecular pathways involved in chondrocyte mechanotransduction can provide insight into the development of novel therapeutic approaches for osteoarthritis.

The inhibition by interleukin 1 of MSC chondrogenesis and the development of biomechanical properties in biomimetic 3D woven PCL scaffolds

Tissue-engineered constructs designed to treat large cartilage defects or osteoarthritic lesions may be exposed to significant mechanical loading as well as an inflammatory environment upon implantation in an injured or diseased joint. We hypothesized that a three-dimensionally (3D) woven poly(ε-caprolactone) (PCL) scaffold seeded with bone marrow-derived mesenchymal stem cells (MSCs) would provide biomimetic mechanical properties in early stages of in vitro culture as the MSCs assembled a functional, cartilaginous extracellular matrix (ECM). We also hypothesized that these properties would be maintained even in the presence of the pro-inflammatory cytokine interleukin-1 (IL-1), which is found at high levels in injured or diseased joints. MSC-seeded 3D woven scaffolds cultured in chondrogenic conditions synthesized a functional ECM rich in collagen and proteoglycan content, reaching an aggregate modulus of ~0.75 MPa within 14 days of culture. However, the presence of pathophysiologically relevant levels of IL-1 limited matrix accumulation and inhibited any increase in mechanical properties over baseline values. On the other hand, the mechanical properties of constructs cultured in chondrogenic conditions for 4 weeks prior to IL-1 exposure were protected from deleterious effects of the cytokine. These findings demonstrate that IL-1 significantly inhibits the chondrogenic development and maturation of MSC-seeded constructs; however, the overall mechanical functionality of the engineered tissue can be preserved through the use of a 3D woven scaffold designed to recreate the mechanical properties of native articular cartilage.

Mechanical stress-induced interleukin-1beta expression through adenosine triphosphate/P2X7 receptor activation in human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Mechanical stress is an important factor in maintaining homeostasis of the periodontium. Interleukin-1beta (IL-1β) and adenosine triphosphate (ATP) are considered potent inflammatory mediators. In macrophages, ATP-activated P2X7 receptor is involved in IL-1β processing and release. Our previous works demonstrated mechanical stress-induced expression of osteopontin and RANKL through the ATP/P2Y1 receptor in human periodontal ligament (HPDL) cells. This study was designed to examine the effect of mechanical stress on IL-1β expression in HPDL cells, as well as the mechanism and involvement of ATP and the P2 purinergic receptor.

MATERIAL AND METHODS

Cultured HPDL cells were treated with continuous compressive loading. IL-1β expression was analyzed at both mRNA and protein levels, using RT-PCR and ELISA, respectively. Cell viability was examined using the MTT assay. ATP was also used to stimulate HPDL cells. Inhibitors, antagonists and the small interfering RNA (siRNA) technique were used to investigate the role of ATP and the specific P2 subtypes responsible for IL-1β induction along with the intracellular mechanism.

RESULTS

Mechanical stress could up-regulate IL-1β expression through the release of ATP in HPDL cells. ATP alone was also capable of increasing IL-1β expression. The induction of IL-1β was markedly inhibited by inhibitors and by siRNA targeting the P2X7 receptor. ATP-stimulated IL-1β expression was also diminished by intracellular calcium inhibitors.

CONCLUSION

Our work clearly indicates the capability of HPDL cells to respond directly to mechanical stimulation. The results signified the important roles of ATP/P2 purinergic receptors, as well as intracellular calcium signaling, in mechanical stress-induced inflammation via up-regulation of the proinflammatory cytokine, IL-1β, in HPDL cells.

Induction of the chemokine IL-8/Kc by the articular cartilage: possible influence on osteoarthritis

PURPOSE

IL-8 and its murine equivalent keratinocyte chemoattractant (Kc), chemokines produced by chondrocytes, contribute to the pathophysiology of osteoarthritis. However, the mechanisms leading to their production are poorly known. We aimed to investigate whether mechanical (compression), inflammatory (IL-1β) and metabolic (visfatin) stresses may induce the release of Kc when applied on murine cartilage.

METHODS

Mouse cartilage explants were subjected to intermittent compression for 4, 6 and 24h. Primary cultures of immature murine articular chondrocytes were obtained by enzymatic digestion of articular cartilage from 6-days-old newborns mice. The effect of compression, IL-1β (10, 50, 100pg/mL) and of visfatin (5μg/mL) on the release of Kc was assessed by ELISA. IL-8 levels in conditioned media from human OA joint tissues (cartilage or synovium) were also assessed.

RESULTS

In comparison with non-compressed explants, loading increased Kc release of 3.2-, 1.9- and 2.0-fold at 4, 6 and 24h respectively (P<0.004, n=9). IL-1β triggered an increase of Kc release by primary cultured chondrocytes of 4.1-, 15.5- and 35.2-fold at 10, 50 and 100pg/mL of IL-1β respectively (P<0.05, n=4). Likewise, visfatin (5μg/mL) induced an increase in Kc release of 56.5±25.2 fold (P=0.002, n=6). IL-8 was released in conditioned media by synovium as well as by cartilage.

CONCLUSION

We show for the first time that IL-8/Kc is highly responsive to mechanical, inflammatory and metabolic stresses, strengthening the hypothesis that IL-8/Kc could be added to the cytokines which may have a deleterious impact in osteoarthritis.

The effects of adipokines on cartilage and meniscus catabolism

Obesity is one of the primary risk factors for osteoarthritis. Increased adiposity is associated not only with alterations in joint loading, but also with increased systemic and joint concentrations of adipose tissue-derived cytokines, or "adipokines", that promote a state of chronic, low-grade inflammation that may act in concert with other cytokines in the joint to increase joint degeneration. However, the direct effect of adipokines, such as leptin, visfatin, and interleukin-6 (IL-6), on joint tissues, such as articular cartilage and meniscus, are not fully understood. In this study, we examined the hypothesis that these adipokines act synergistically with interleukin-1 (IL-1) to increase catabolism and the production of proinflammatory mediators in cartilage and meniscus. Explants of porcine cartilage and meniscus were treated with physiologically relevant concentrations of leptin, IL-6, or visfatin, alone or in combination with IL-1. Visfatin and IL-1 promoted the catabolic degradation of both cartilage and meniscus, as evidenced by increased metalloproteinase activity, nitric oxide production, and proteoglycan release. However, leptin or IL-6 at physiologic concentrations had no effect on the breakdown of these tissues. These findings suggest that the effects of obesity-induced osteoarthritis may not be through a direct effect of leptin or IL-6 on cartilaginous tissues, but support a potential role for increased visfatin levels in this regard. These data provide an important first step in understanding the role of adipokines in regulating cartilage and meniscus metabolism; however, these adipokines may have different effects in the context of the whole joint and must be evaluated further.

Advanced glycation end products induce the expression of interleukin-6 and interleukin-8 by receptor for advanced glycation end product-mediated activation of mitogen-activated protein kinases and nuclear factor-κB in human osteoarthritis chondrocytes

OBJECTIVE

To investigate whether advanced glycation end products (AGEs) induce the expression of IL-6 and IL-8 through the receptor for AGEs (RAGE)-activated pathways in human OA chondrocytes.

METHODS

OA chondrocytes were stimulated with AGE-modified BSA (AGE-BSA). Gene expression of IL-6 and IL-8 was quantified by TaqMan assays and the production was determined using ELISAs. Immunoblotting was used to analyse the activation of mitogen-activated protein kinases (MAPKs) and the degradation of IκBα. Activation of NF-κB was determined using an ELISA. Pharmacological studies to elucidate the involved pathways were executed using transfection with small interfering RNAs (siRNAs), inhibitors of MAPKs and NF-κB.

RESULTS

AGE-BSA induced the expression of IL-6 and IL-8 in OA chondrocytes, which was inhibited by pre-treatment with soluble RAGE (sRAGE) or RAGE knockdown by siRNAs. Treatment with SB202190 (p38-MAPK inhibitor) or PD98059 (ERK inhibitor) inhibited AGE-BSA-induced IL-6 and IL-8 expression. However, SP600125 (JNK inhibitor) had no effect on AGE-BSA-induced IL-6 expression but inhibited the expression of IL-8. Treatment with NF-κB inhibitors suppressed AGE-BSA-induced IL-6 and IL-8 expression.

CONCLUSIONS

This is the first study to demonstrate that AGEs induce the expression of IL-6 and IL-8 in OA chondrocytes. A novel finding of our studies is that in OA chondrocytes, AGE-BSA-induced expression of IL-6, but not of IL-8, was independent of the JNK pathway. Activation of NF-κB was an absolute requirement for both IL-6 and IL-8 expression. These results demonstrate that AGE-BSA-induced expression of IL-6 and IL-8 via RAGE is mediated through different MAPK signalling pathways in OA and possibly in other degenerative diseases.

2-Deoxy-D-glucose regulates dedifferentiation through beta-catenin pathway in rabbit articular chondrocytes

2-deoxy-D-glucose (2DG) is known as a synthetic inhibitor of glucose. 2DG regulates various cellular responses including proliferation, apoptosis and differentiation by regulation of glucose metabolism in cancer cells. However, the effects of 2DG in normal cells, including chondrocytes, are not clear yet. We examined the effects of 2DG on dedifferentiation with a focus on the beta-catenin pathway in rabbit articular chondrocytes. The rabbit articular chondrocytes were treated with 5 mM 2DG for the indicated time periods or with various concentrations of 2DG for 24 h, and the expression of type II collagen, c-jun and beta-catenin was determined by Western blot, RT-PCR, immunofluorescence staining and immunohistochemical staining and reduction of sulfated proteoglycan synthesis detected by Alcain blue staining. Luciferase assay using a TCF (T cell factor)/LEF (lymphoid enhancer factor) reporter construct was used to demonstrate the transcriptional activity of beta-catenin. We found that 2DG treatment caused a decrease of type II collagen expression. 2DG induced dedifferentiation was dependent on activation of beta-catenin, as the 2DG stimulated accumulation of beta-catenin, which is characterized by translocation of beta-catenin into the nucleus determined by immunofluorescence staining and luciferase assay. Inhibition of beta-catenin degradation by inhibition of glycogen synthase kinase 3-beta with lithium chloride (LiCl) or inhibition of proteasome with z-Leu-Leu-Leu-CHO (MG132) accelerated the decrease of type II collagen expression in the chondrocytes. 2DG regulated the post-translational level of beta-catenin whereas the transcriptional level of beta-catenin was not altered. These results collectively showed that 2DG regulates dedifferentiation via beta-catenin pathway in rabbit articular chondrocytes.

Diet-induced obesity differentially regulates behavioral, biomechanical, and molecular risk factors for osteoarthritis in mice

Introduction

Obesity is a major risk factor for the development of osteoarthritis in both weight-bearing and nonweight-bearing joints. The mechanisms by which obesity influences the structural or symptomatic features of osteoarthritis are not well understood, but may include systemic inflammation associated with increased adiposity. In this study, we examined biomechanical, neurobehavioral, inflammatory, and osteoarthritic changes in C57BL/6J mice fed a high-fat diet.

Methods

Female C57BL/6J mice were fed either a 10% kcal fat or a 45% kcal fat diet from 9 to 54 weeks of age. Longitudinal changes in musculoskeletal function and inflammation were compared with endpoint neurobehavioral and osteoarthritic disease states. Bivariate and multivariate analyses were conducted to determine independent associations with diet, percentage body fat, and knee osteoarthritis severity. We also examined healthy porcine cartilage explants treated with physiologic doses of leptin, alone or in combination with IL-1α and palmitic and oleic fatty acids, to determine the effects of leptin on cartilage extracellular matrix homeostasis.

Results

High susceptibility to dietary obesity was associated with increased osteoarthritic changes in the knee and impaired musculoskeletal force generation and motor function compared with controls. A high-fat diet also induced symptomatic characteristics of osteoarthritis, including hyperalgesia and anxiety-like behaviors. Controlling for the effects of diet and percentage body fat with a multivariate model revealed a significant association between knee osteoarthritis severity and serum levels of leptin, adiponectin, and IL-1α. Physiologic doses of leptin, in the presence or absence of IL-1α and fatty acids, did not substantially alter extracellular matrix homeostasis in healthy cartilage explants.

Conclusions

These results indicate that diet-induced obesity increases the risk of symptomatic features of osteoarthritis through changes in musculoskeletal function and pain-related behaviors. Furthermore, the independent association of systemic adipokine levels with knee osteoarthritis severity supports a role for adipose-associated inflammation in the molecular pathogenesis of obesity-induced osteoarthritis. Physiologic levels of leptin do not alter extracellular matrix homeostasis in healthy cartilage, suggesting that leptin may be a secondary mediator of osteoarthritis pathogenesis.

Convergence of nitric oxide and lipid signaling: anti-inflammatory nitro-fatty acids

The signaling mediators nitric oxide ( NO) and oxidized lipids, once viewed to transduce metabolic and inflammatory information via discrete and independent pathways, are now appreciated as interdependent regulators of immune response and metabolic homeostasis. The interactions between these two classes of mediators result in reciprocal control of mediator synthesis that is strongly influenced by the local chemical environment. The relationship between the two pathways extends beyond coregulation of NO and eicosanoid formation to converge via the nitration of unsaturated fatty acids to yield nitro derivatives (NO(2)-FA). These pluripotent signaling molecules are generated in vivo as an adaptive response to oxidative inflammatory conditions and manifest predominantly anti-inflammatory signaling reactions. These actions of NO(2)-FA are diverse, with these species serving as a potential chemical reserve of NO, reacting with cellular nucleophiles to posttranslationally modify protein structure, function, and localization. In this regard these species act as potent endogenous ligands for peroxisome proliferator-activated receptor gamma. Functional consequences of these signaling mechanisms have been shown in multiple model systems, including the inhibition of platelet and neutrophil functions, induction of heme oxygenase-1, inhibition of LPS-induced cytokine release in monocytes, increased insulin sensitivity and glucose uptake in adipocytes, and relaxation of preconstricted rat aortic segments. These observations have propelled further in vitro and in vivo studies of mechanisms of NO(2)-FA signaling and metabolism, highlighting the therapeutic potential of this class of molecules as anti-inflammatory drug candidates.

Static and dynamic compressive strains influence nitric oxide production and chondrocyte bioactivity when encapsulated in PEG hydrogels of different crosslinking densities

OBJECTIVE

Mechanical loading is an important regulator of chondrocytes; however, many of the mechanisms involved in chondrocyte mechanotransduction still remain unclear. Here, poly(ethylene glycol) (PEG) hydrogels are proposed as a model system to elucidate chondrocyte response due to cell deformation, which is controlled by gel crosslinking (rho(x)).

METHODS

Bovine articular chondrocytes (50 x 10(6)cells/mL) were encapsulated in gels with three rho(x)s and subjected to static (15% strain) or dynamic (0.3 Hz or 1 Hz, 15% amplitude strain) loading for 48 h. Cell deformation was examined by confocal microscopy. Cell response was assessed by total nitric oxide (NO) production, proteoglycan (PG) synthesis ((35)SO(4)(2-)-incorporation) and cell proliferation (CP) ([(3)H]-thymidine incorporation). Oxygen consumption was assessed using an oxygen biosensor.

RESULTS

An increase in rho(x) led to lower water contents, higher compressive moduli, and higher cell deformations. Chondrocyte response was dependent on both loading regime and rho(x). For example, under a static strain, NO was not affected, while CP and PG synthesis were inhibited in low rho(x) and stimulated in high rho(x). Dynamic loading resulted in either no effect or an inhibitory effect on NO, CP, and PG synthesis. Overall, our results showed correlations between NO and CP and/or PG synthesis under static and dynamic (0.3 Hz) loading. This finding was attributed to the hypoxic environment that resulted from the high cell-seeding density.

CONCLUSION

This study demonstrates gel rho(x) and loading condition influence NO, CP, and PG synthesis. Under a hypoxic environment and certain loading conditions, NO appears to have a positive effect on chondrocyte bioactivity.

Involvement of the p38 MAPK-NF-kappaB signal transduction pathway and COX-2 in the pathobiology of meniscus degeneration in humans

Meniscal tears are attributed to either trauma or degeneration processes. Clinical data suggest that meniscal degeneration (MD) is associated with knee osteoarthritis; however, the molecular events underpinning the pathogenesis of MD in humans remain elusive. Here we immunohistochemically examined the expression of p38 MAPK, its phosphorylated/activated form (p-p38), its target NF-kappaB (p50-p65 dimer), and COX-2 in ruptured menisci and investigated their involvement in MD development. Our findings demonstrate increased expression of the p38-NF-kappaB axis elements and COX-2 in disintegrated fibrocartilage, suggesting a role of these molecules in the pathobiochemistry of MD and consequential rupture.

Dynamic compression counteracts IL-1beta induced inducible nitric oxide synthase and cyclo-oxygenase-2 expression in chondrocyte/agarose constructs

Background

Nitric oxide and prostaglandin E₂ (PGE₂play pivotal roles in both the pathogenesis of osteoarthritis and catabolic processes in articular cartilage. These mediators are influenced by both IL-1β and mechanical loading, and involve alterations in the inducible nitric oxide synthase (iNOS) and cyclo-oxygenase (COX)-2 enzymes. To identify the specific interactions that are activated by both types of stimuli, we examined the effects of dynamic compression on levels of expression of iNOS and COX-2 and involvement of the p38 mitogen-activated protein kinase (MAPK) pathway.

Methods

Chondrocyte/agarose constructs were cultured under free-swelling conditions with or without IL-1β and/or SB203580 (inhibitor of p38 MAPK) for up to 48 hours. Using a fully characterized bioreactor system, constructs were subjected to dynamic compression for 6, 12 and 48 hours under similar treatments. The activation or inhibition of p38 MAPK by IL-1β and/or SB203580 was analyzed by western blotting. iNOS, COX-2, aggrecan and collagen type II signals were assessed utilizing real-time quantitative PCR coupled with molecular beacons. Release of nitrite and PGE₂ was quantified using biochemical assays. Two-way analysis of variance and the post hoc Bonferroni-corrected t-test were used to examine data.

Results

IL-1β activated the phosphorylation of p38 MAPK and this effect was abolished by SB203580. IL-1β induced a transient increase in iNOS expression and stimulated the production of nitrite release. Stimulation by either dynamic compression or SB203580 in isolation reduced the IL-1β induced iNOS expression and nitrite production. However, co-stimulation with both dynamic compression and SB203580 inhibited the expression levels of iNOS and production of nitrite induced by the cytokine. IL-1β induced a transient increase in COX-2 expression and stimulated the cumulative production of PGE₂ release. These effects were inhibited by dynamic compression or SB203580. Co-stimulation with both dynamic compression and SB203580 restored cytokine-induced inhibition of aggrecan expression. This is in contrast to collagen type II, in which we observed no response with the cytokine and/or SB203580.

Conclusion

These data suggest that dynamic compression directly influences the expression levels of iNOS and COX-2. These molecules are current targets for pharmacological intervention, raising the possibility for integrated pharmacological and biophysical therapies for the treatment of cartilage joint disorders.

Cyclooxygenase inhibition lowers prostaglandin E2 release from articular cartilage and reduces apoptosis but not proteoglycan degradation following an impact load in vitro

This study investigated the release of prostaglandin E₂ (PGE₂) from cartilage following an impact load in vitro and the possible chondroprotective effect of cyclooxygenase-2 (COX-2) inhibition using non-steroidal anti-inflammatory drugs (NSAIDs).

Explants of human articular cartilage were subjected to a single impact load in a drop tower, and then cultured for 6 days in the presence of either a selective COX-2 inhibitor (celecoxib; 0.01, 0.1, 1.0 and 10 μM) or a non-selective COX inhibitor (indomethacin; 0.1 and 10 μM). The concentrations of PGE₂ and glycosaminoglycans (GAGs), a measure of cartilage breakdown, were measured in the explant culture medium at 3 and 6 days post-impact. Apoptotic cell death was measured in frozen explant sections by the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) method.

PGE₂ levels were increased by more than 20-fold in the medium of explants at both 3 (p = 0.012) and 6 days (p = 0.004) following impact, compared with unloaded controls. In the presence of celecoxib and indomethacin, the PGE₂ levels were reduced in a dose-related manner. These inhibitors, however, had no effect in reducing the impact-induced release of GAGs from the cartilage matrix. Addition of celecoxib and indomethacin significantly reduced the number of trauma-induced apoptotic chondrocytes in cartilage explant sections.

In this study, a marked increase in PGE₂ was measured in the medium following an impact load on articular cartilage, which was abolished by the selective COX-2 inhibitor, celecoxib, and non-selective indomethacin. These inhibitors reduced chondrocyte apoptosis but no change was observed in the release of GAGs from the explants, suggesting that the COX/PGE₂ pathway is not directly responsible for cartilage breakdown following traumatic injury. Our in vitro study demonstrates that it is unlikely that COX-2 inhibition alone would slow down or prevent the development of secondary osteoarthritis.

Mechanobiology of adult and stem cells

Mechanical forces, including gravity, tension, compression, hydrostatic pressure, and fluid shear stress, play a vital role in human physiology and pathology. They particularly influence extracellular matrix (ECM) gene expression, ECM protein synthesis, and production of inflammatory mediators of many load-sensitive adult cells such as fibroblasts, chondrocytes, smooth muscle cells, and endothelial cells. Furthermore, the mechanical forces generated by cells themselves, known as cell traction forces (CTFs), also influence many biological processes such as wound healing, angiogenesis, and metastasis. Thus, the quantitative characterization of CTFs by qualities such as magnitude and distribution is useful for understanding physiological and pathological events at the tissue and organ levels. Recently, the effects of mechanical loads on embryonic and adult stem cells in terms of self-renewal, differentiation, and matrix protein expression have been investigated. While it seems certain that mechanical loads applied to stem cells regulate their self-renewal and induce controlled cell lineage differentiation, the detailed molecular signaling mechanisms responsible for these mechano-effects remain to be elucidated. Challenges in the fields of both adult- and stem-cell mechanobiology include devising novel experimental and theoretical methodologies to examine mechano-responses more closely to various forms of mechanical forces and mechanotransduction mechanisms of these cells in a more physiologically accurate setting. Such novel methodologies will lead to better understanding of various pathological diseases, their management, and translational applications in the ever expanding field of tissue engineering.

Transforming growth factor alpha suppression of articular chondrocyte phenotype and Sox9 expression in a rat model of osteoarthritis

OBJECTIVE

To define the roles of transforming growth factor alpha (TGFalpha) in cartilage degradation.

METHODS

Primary rat articular chondrocytes and articular osteochondral explants were cultured with TGFalpha to assess the effects of TGFalpha on chondrocyte physiology and phenotype.

RESULTS

TGFalpha altered chondrocyte morphology through reorganization of the actin cytoskeleton and formation of stress fibers. Expression of anabolic genes, including aggrecan, type II collagen, and cartilage link protein, was reduced in response to TGFalpha. Proliferation of chondrocytes and formation of articular chondrocyte clusters was stimulated by TGFalpha. Expression of matrix metalloproteinase 13 and cathepsin C was increased by TGFalpha. We demonstrated the down-regulation of Sox9 messenger RNA and protein levels by TGFalpha. This was associated with reduced levels of phosphorylated and total SOX9 in cartilage explants upon TGFalpha treatment. In contrast, another growth factor identified in our microarrays, Kitl, had no effects on the chondrocyte parameters tested. To examine correlations between the increased levels of TGFalpha in experimental knee osteoarthritis (OA) with the levels of TGFalpha in humans with knee OA, a microarray analysis of mRNA from 13 normal and 12 late-stage OA cartilage samples was performed. Seven OA samples showed TGFA mRNA levels similar to those in the normal controls, but expression was markedly increased in the other 5 OA samples. These data confirm that TGFA transcript levels are increased in a subset of patients with OA.

CONCLUSION

This study adds TGFalpha to the list of dysregulated cytokines present in degrading cartilage in OA. Since TGFalpha inhibits articular chondrocyte anabolic capacity, increases catabolic factors, and contributes to the development of chondrocyte clusters, TGFalpha may be a potential target for therapeutic strategies in the treatment of OA.

Purification of the receptor for the N-acetyl-d-glucosamine specific adhesin of Mannheimia haemolytica from bovine neutrophils

The GlcNAc-specific adhesin from Mannheimia haemolytica (MhA) has been shown to participate in pathogenicity of mannheimiosis due to its capacity to adhere to tracheal epithelial cells and activate the oxidative burst of bovine neutrophils. In this work, we purified the MhA receptor from bovine neutrophils (MhAr) by affinity chromatography on MhA-Sepharose. The MhAr, which corresponded to approximately 2% of the protein from cell lysate, is a glycoprotein mainly composed of Glu, Ala, Ser, Gly, and Asp, without cysteine. The glycan portion, which corresponds to 20% by weight, is composed of GalNAc, GlcNAc, Man, Gal, and NeuAc. The receptor is a 165-kDa glycoprotein, as determined by molecular sieve chromatography under native conditions; SDS-PAGE analysis shows a heterodimer of 83 and 80 kDa subunits. This work suggests that the GlcNAc-containing receptor plays a relevant role by activating bovine neutrophils through non-opsonic mechanisms.

Regional differences in prostaglandin E2 and nitric oxide production in the knee meniscus in response to dynamic compression

Injury or loss of the knee meniscus is associated with altered joint stresses that lead to progressive joint degeneration. The goal of this study was to determine if dynamic mechanical compression influences the production of inflammatory mediators by meniscal cells. Dynamic compression increased prostaglandin E2 (PGE(2)) and nitric oxide (NO) production over a range of stress magnitudes (0.0125-0.5 MPa) in a manner that depended on stress magnitude and zone of tissue origin. Inner zone explants showed greater increases in PGE(2) and NO production as compared to outer zone explants. Meniscal tissue expressed NOS2 and NOS3 protein, but not NOS1. Mechanically induced NO production was blocked by NOS inhibitors, and the non-selective NOS inhibitor L-NMMA augmented PGE(2) production in the outer zone only. These findings suggest that the meniscus may serve as an intra-articular source of pro-inflammatory mediators, and that alterations in the magnitude or distribution of joint loading could significantly influence the production of these mediators in vivo.

Nitric oxide synthase and cyclooxygenase interactions in cartilage and meniscus: relationships to joint physiology, arthritis, and tissue repair

Rheumatoid arthritis and osteoarthritis are painful and debilitating diseases with complex pathophysiology. There is growing evidence that pro-inflammatory cytokines (e.g., interleukin-1 and tumor necrosis factor alpha) and mediators (e.g., prostaglandins, leukotrienes, and nitric oxide) play critical roles in the development and perpetuation of tissue inflammation and damage in joint tissues such as articular cartilage and meniscus. While earlier studies have generally focused on cells of the synovium (especially macrophages), there is increasing evidence that chondrocytes and meniscal cells actively contribute to inflammatory processes. In particular, it is now apparent that mechanical forces engendered by joint loading are transduced to biological signals at the cellular level and that these signals modulate gene expression and biochemical processes. Here we give an overview of the interplay of cytokines and mechanical stress in the production of cyclooxygenases and prostaglandins; lipoxygenases and leukotrienes; and nitric oxide synthases and nitric oxide in arthritis, with particular focus on the interactions of these pathways in articular cartilage and meniscus.

Prostaglandin E2 synthesis in cartilage explants under compression: mPGES-1 is a mechanosensitive gene

Knee osteoarthritis (OA) results, at least in part, from overloading and inflammation leading to cartilage degradation. Prostaglandin E2 (PGE₂) is one of the main catabolic factors involved in OA. Its synthesis is the result of cyclooxygenase (COX) and prostaglandin E synthase (PGES) activities whereas NAD+-dependent 15 hydroxy prostaglandin dehydrogenase (15-PGDH) is the key enzyme implicated in the catabolism of PGE₂. For both COX and PGES, three isoforms have been described: in cartilage, COX-1 and cytosolic PGES are constitutively expressed whereas COX-2 and microsomal PGES type 1 (mPGES-1) are inducible in an inflammatory context. COX-3 (a variant of COX-1) and mPGES-2 have been recently cloned but little is known about their expression and regulation in cartilage, as is also the case for 15-PGDH. We investigated the regulation of the genes encoding COX and PGES isoforms during mechanical stress applied to cartilage explants. Mouse cartilage explants were subjected to compression (0.5 Hz, 1 MPa) for 2 to 24 hours. After determination of the amount of PGE₂ released in the media (enzyme immunoassay), mRNA and proteins were extracted directly from the cartilage explants and analyzed by real-time RT-PCR and western blotting respectively. Mechanical compression of cartilage explants significantly increased PGE₂ production in a time-dependent manner. This was not due to the synthesis of IL-1, since pretreatment with interleukin 1 receptor antagonist (IL1-Ra) did not alter the PGE₂ synthesis. Interestingly, COX-2 and mPGES-1 mRNA expression significantly increased after 2 hours, in parallel with protein expression, whereas COX-3 and mPGES-2 mRNA expression was not modified. Moreover, we observed a delayed overexpression of 15-PGDH just before the decline of PGE₂ synthesis after 18 hours, suggesting that PGE₂ synthesis could be altered by the induction of 15-PGDH expression. We conclude that, along with COX-2, dynamic compression induces mPGES-1 mRNA and protein expression in cartilage explants. Thus, the mechanosensitive mPGES-1 enzyme represents a potential therapeutic target in osteoarthritis.

The development of posttraumatic arthritis after articular fracture

Posttraumatic arthritis (PTA) is one of the most frequent causes of disability after trauma involving weight-bearing joints and is estimated to be responsible for approximately 10% of the 21 million Americans who have osteoarthritis. Despite a number of similarities in the pathology and end-stage disease of PTA with primary osteoarthritis, the mechanisms involved in the onset and progression of joint degeneration after articular fracture are poorly understood. The largest area of study regarding articular fractures and the development of arthritic changes has focused on the role of adequate surgical reduction of the articular surfaces. However, it is now apparent that a number of complex and interacting biomechanical, biochemical, and, possibly, genetic factors contribute to the development of osteoarthritic changes in the joint after joint trauma, ranging from the cell and molecular level to the joint and systemic level. In this paper, we discuss the potential roles of the initial impact and fracture as well as the subsequent alterations in joint loading, biomechanical and metabolic properties of the cartilage, local and systemic inflammatory cytokines, and viability of chondrocytes in the progression of PTA. An improved understanding of the mechanisms involved in the development of PTA will hopefully lead to the improvement of surgical and nonsurgical therapies for this disease.

Interaction between inhibitors of inducible nitric oxide synthase and cyclooxygenase in adjuvant-induced arthritis in female albino rats: an isobolographic study

We studied the interaction of S-methylisothiourea (a selective inducible nitric oxide synthase inhibitor) with rofecoxib (a selective cyclooxygenase-2 inhibitor) and mefenamic acid (a non-selective cyclooxygenase inhibitor) in adjuvant-induced arthritis in female albino Wistar rats, applying the isobolographic analysis. Each drug was effective in reducing the progressive increase in paw volume less than 50% except rofecoxib, when used alone. Log dose-response curve was obtained for each drug along with the corresponding ED(25). Following isobolographic analysis, combination of S-methylisothiourea with rofecoxib and mefenamic acid revealed supra-additive or synergistic interaction. Experimental ED(25) of the combinations was significantly lower than the theoretical ED(25) of the corresponding drug combination which substantiated the synergistic type of interaction between inducible nitric oxide synthase and cyclooxygenase in adjuvant-induced arthritis in female albino rats. Results suggest that NO regulates the cyclooxygenase enzyme activity as the activity of cyclooxygenase enzymes in the LPS-stimulated leukocyte lysates was significantly low or hardly detectable in the presence of varying concentrations of S-methylisothiourea. Simultaneous inhibition of inducible nitric oxide synthase and cyclooxygenase appears to offer an alternative approach for ameliorating the progression of arthritis.

Biomechanical signals exert sustained attenuation of proinflammatory gene induction in articular chondrocytes

OBJECTIVES

Physical therapies are commonly used for limiting joint inflammation. To gain insight into their mechanisms of actions for optimal usage, we examined persistence of mechanical signals generated by cyclic tensile strain (CTS) in chondrocytes, in vitro. We hypothesized that mechanical signals induce anti-inflammatory and anabolic responses that are sustained over extended periods.

METHODS

Articular chondrocytes obtained from rats were subjected to CTS for various time intervals followed by a period of rest, in the presence of interleukin-1beta (IL-1beta). The induction for cyclooxygenase (COX-2), inducible nitric oxide synthase (iNOS), matrix metalloproteinase (MMP)-9, MMP-13 and aggrecan was analyzed by real-time polymerase chain reaction (PCR), Western blot analysis and immunofluorescence.

RESULTS

Exposure of chondrocytes to constant CTS (3% CTS at 0.25 Hz) for 4-24 h blocked more than 90% (P<0.05) of the IL-1beta-induced transcriptional activation of proinflammatory genes, like iNOS, COX-2, MMP-9 and MMP-13, and abrogated inhibition of aggrecan synthesis. CTS exposure for 4, 8, 12, 16, or 20 h followed by a rest for 20, 16, 12, 8 or 4h, respectively, revealed that 8h of CTS optimally blocked (P<0.05) IL-1beta-induced proinflammatory gene induction for ensuing 16 h. However, CTS for 8h was not sufficient to inhibit iNOS expression for ensuing 28 or 40 h.

CONCLUSIONS

Data suggest that constant application of CTS blocks IL-1beta-induced proinflammatory genes at transcriptional level. The signals generated by CTS are sustained after its removal, and their persistence depends upon the length of CTS exposure. Furthermore, the sustained effects of mechanical signals are also reflected in their ability to induce aggrecan synthesis. These findings, once extrapolated to human chondrocytes, may provide insight in obtaining optimal sustained effects of physical therapies in the management of arthritic joints.

The N-acetyl-D-glucosamine specific adhesin from Mannheimia haemolytica activates bovine neutrophils oxidative burst

In this work we identified specific bovine leukocytes that were bound by the Mannheimia haemolytica adhesin molecule (MhA) and the biological effect on the leukocytes. Histochemical staining and flow cytometry showed that MhA bind neutrophils (90%) and monocytes (5%). MhA induced an oxidative response in purified neutrophils; this effect was 1.5-fold higher than the effect observed with control cells activated with Zymosan. Cellular binding by MhA was inhibited with GlcNAc and its oligomers, as well as by glycoproteins containing tri- and tetra-antennary N-glycosydically linked glycans. MhA-induced oxidative burst was significantly inhibited by GlcNAc, iodoacetamide, superoxide dismutase, and piroxicam (p<0.05). Our findings suggest that among bovine leukocytes, neutrophils are the main target for MhA, inducing production of oxidative radicals by non-opsonic mechanism that seem to play an important role in tissue damage during mannheimiosis.

Celecoxib inhibits nitric oxide production in chondrocytes of ligament-damaged osteoarthritic rat joints

The purpose of this study was to demonstrate the direct effects of celecoxib, one of the selective cyclo-oxygenase (COX)-2 inhibitors, on nitric oxide (NO) and prostaglandinE2 (PGE2) synthesis in cultured osteoarthritic chondrocyte comparing with those of indomethacin. Articular chondrocytes were isolated from rat osteoarthritic knee joint with damaged anterior cruciate ligament and also from the sham knee joint. Chondrocytes were preincubated with or without IL-1 alpha, and were exposed to celecoxib, indomethacin (non-selective COX inhibitor), or nothing. The amounts of NO and PGE2 in culture supernatants of chondrocytes were measured by EIA or the Griess reaction. In a series of experiments preincubated with or without IL-1 alpha and exposed to nothing, PGE2 and NO levels were significantly higher in osteoarthritic chondrocytes than in sham chondrocytes. Celecoxib and indomethacin inhibited the increase of PGE2 in osteoarthritic chondrocytes. Celecoxib inhibited and indomethacin did not inhibit the increase of NO levels in osteoarthritic chondrocytes.

Overexpression of nitric oxide synthases in tendon overuse

Tendon disorders with a chronic nature, including the rotator cuff, are extremely common, and represent a major clinical problem. Mechanical overload has been proposed as an important etiologic factor in tendinopathies. Nitric oxide (NO), a free radical produced by nitric oxide synthases (NOSs), is a potent regulator and stimulator of biological processes including tendon degeneration and healing. It is also involved in response to mechanical stimuli in different tissues. In an animal model of acutely injured tendon healing temporal and differential expression of NOS isoforms has been demonstrated, suggesting that different patterns of NOSs expression may have different biological functions. Therefore, we hypothesized that tendon overuse may result in a differential upregulation of NOSs, particularly iNOS. An animal model of supraspinatus tendon overuse was utilized, which consisted of treadmill running. A group of animals of the same strain and age subjected to normal cage activity were used as controls. Following a 4-week exercise protocol supraspinatus tendons were harvested, RNA was extracted, and subjected to competitive reverse transcription and polymerase chain reaction (RT-PCR) to determine the expression levels of inducible-, endothelial-, and neuronal-NOS isoforms (i-, e-, and nNOS). The mRNA expression of all three NOS isoforms increased in the supraspinatus tendons as a result of overuse exercise. iNOS and eNOS mRNA expression increased fourfold (p < 0.01), and there was an increase, but statistically not significant, in nNOS mRNA expression in the overused tendons when compared with the controls. This study is the first to show that NOS isoforms are upregulated in rotator cuff tendon as a result of chronic overuse, and suggests the involvement of nitric oxide in the response of tendon tissue to increased mechanical stress.

The influence of mechanical compression on the induction of osteoarthritis-related biomarkers in articular cartilage explants

OBJECTIVE

Macromolecules of the articular cartilage extracellular matrix released into synovial fluid, blood, or urine can serve as potentially useful biomarkers of the severity of osteoarthritis (OA). Biomechanical factors play an important role in OA pathogenesis, yet their influence on biomarker production is not well understood. The goal of this study was to examine the hypothesis that dynamic mechanical stress influences the release of these biomarkers from articular cartilage.

METHODS

Explants of porcine cartilage were subjected to dynamic compression at 0.5 Hz for 24h at stresses ranging from 0.006 to 0.1 MPa. The concentrations of cartilage oligomeric matrix protein (COMP), keratan sulfate (KS measured as the 5 D 4 epitope), total sulfated glycosaminoglycan (S-GAG), and the KS (keratanase-digestible) and chondroitin sulfate (CS) (chondroitinase-digestible) fractions of S-GAG were measured. Radiolabel incorporation was used to determine the rates of proteoglycan and protein synthesis.

RESULTS

The magnitudes of mechanical stress applied in this study induced nominal tissue strains of 4-23%, consistent with a range of physiological to hyperphysiologic strains measured in situ. COMP release increased in proportion to the magnitude of dynamic mechanical stress, while KS, CS and total S-GAG release increased in a bimodal pattern with increasing stress. Protein and proteoglycan synthesis were significantly decreased at the highest level of stress.

CONCLUSION

Mechanical stress differentially regulates the turnover of distinct pools of cartilage macromolecules. These findings indicate that mechanical factors, independent of exogenous cytokines or other stimulatory factors, can influence the production and release of OA-related biomarkers from articular cartilage.

The influence of oxygen tension on the induction of nitric oxide and prostaglandin E2 by mechanical stress in articular cartilage

OBJECTIVES

Articular cartilage is an avascular tissue that exists at low oxygen tension. Oxygen tension can influence the production of the pro-inflammatory mediators nitric oxide (NO) and prostaglandin E2 (PGE(2)) in cartilage, which are increased in osteoarthritis (OA). The synthesis of these molecules can be stimulated by mechanical stress, which is an important risk factor for OA. The objective of this study was to determine the influence of oxygen tension on the induction of NO and PGE(2) production in articular cartilage in response to mechanical stress.

DESIGN

Intermittent mechanical compression (0.05MPa, 0.5Hz for 24h) was applied to full thickness skeletally mature porcine articular cartilage explants at either 20%, 5%, or 1% O(2). NO, PGE(2) and peroxynitrite formation were measured, and the effect of the selective nitric oxide synthase 2 inhibitor 1400W was tested.

RESULTS

Incubating articular cartilage at 5% O(2) significantly increased (P<0.001) baseline NO production, as compared with 1% or 20% O(2). Peroxynitrite formation was lower at reduced oxygen tension. Mechanical compression significantly increased (P<0.001) NO production at 20% O(2) but not at 5% or 1% O(2), and significantly increased (P<0.001) PGE(2) production at 20% O(2) (50 fold) and 5% O(2) (4 fold) but not at 1% O(2). 1400W blocked mechanically induced NO production and further increased PGE(2) production at 5% O(2) (P<0.05).

CONCLUSIONS

Oxygen tension influences the endogenous production of NO and PGE(2) in cartilage and can have a significant effect on the induction of these inflammatory mediators in response to mechanical compression.

Eicosanoids, osteoarthritis, and crystal deposition diseases

PURPOSE OF REVIEW

Eicosanoids are produced by chondrocytes, synoviocytes, and subchondral osteoblasts within the osteoarthritic joint and are involved in normal joint physiology as well as in the pathogenesis of joint disorders such as osteoarthritis. Calcium-containing crystals are found in most osteoarthritic joints and have been implicated in osteoarthritis. Recent advances in the understanding of the potential role of eicosanoids in the pathogenesis of osteoarthritis and in potential therapeutic targeting of eicosanoid pathways are reviewed.

RECENT FINDINGS

The ability of interleukin-1beta to upregulate microsomal prostaglandin E2 synthase-1 in synovial fibroblasts and chondrocytes of patients with osteoarthritis has been demonstrated. A potential role for prostaglandin E2 in downregulating interleukin-1beta-induced inflammatory responses has also been described. Basic calcium phosphate crystals can upregulate cyclooxygenase-1 and cocylooxygenase-2 expression, both of which contributed to the observed increase in prostaglandin E2 production in human fibroblasts. Novel potential mechanisms of inhibition of eicosanoid synthesis are also discussed. Last, further evidence of amelioration of osteoarthritis in animal models by the dual 5-lipoxygenase/cyclooxygenase inhibitor licofelone has been reported.

SUMMARY

The inhibition of prostaglandin synthesis has long been a ornerstone of the pharmacologic treatment of osteoarthritis. Nevertheless, prostaglandins may have potentially beneficial as well as deleterious effects in osteoarthritis. In addition, other eicosanoids such as leukotrienes have also been implicated in the pathogenesis of osteoarthritis. Therefore, more selective inhibition of prostaglandin pathways and/or inhibition of leukotriene activity may prove to be effective therapeutic strategies in osteoarthritis.

Effects of proinflammatory cytokines on canine articular chondrocytes in a three-dimensional culture

OBJECTIVE

To determine the effects of interleukin (IL)-1 and tumor necrosis factor (TNF)-alpha on canine chondrocytes cultured in an agarose-based 3-dimensional (3-D) system.

SAMPLE POPULATION

Humeral head articular cartilage chondrocytes obtained from 6 adult dogs.

PROCEDURE

Chondrocytes were cultured in a 3-D system for < or = 12 days in serum-free medium with IL 1alpha, IL-1beta, or TNF-alpha at concentrations of 20, 50, or 100 ng/mL. After 1, 3, 6, and 12 days, glycosaminoglycan (GAG) concentrations in 3-D constructs; nitric oxide and prostaglandin E2 (PGE2) concentrations in media samples; and relative expressions of selected genes, including metalloproteinase (MMP)-13 and tissue inhibitor of metalloproteinase (TIMP)-1 and TIMP-2, were evaluated. Control specimens were comprised of chondrocytes cultured without proinflammatory cytokines.

RESULTS

In control 3-D constructs, GAG content was significantly higher than for all other constructs. Compared with control values, relative expressions of MMP-13, TIMP-1, and TIMP-2 genes in the IL-1beta (50 ng/mL) group were significantly higher at day 1; at all evaluations, media concentrations of nitric oxide were significantly higher in all TNF-alpha-treated cultures; and concentrations of PGE2 in media samples were significantly higher in the IL-1beta (50 ng/mL) and IL-1beta (100 ng/mL) groups at days 1 and 3, in the IL-1beta (100 ng/mL) group at day 6, and in all TNF-alpha groups at days 1, 3, and 6.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that TNF-alpha more readily induces production of nitric oxide and PGE2 by canine chondrocytes, compared with IL-1beta. In vitro, IL-1alpha appeared to have a minimal effect on canine chondrocytes.