HIF-2α as a possible therapeutic target of osteoarthritis

Angiogenesis in heterotopic ossification: From mechanisms to clinical significance

Heterotopic ossification (HO) refers to the formation of pathologic bone in nonskeletal tissues (including muscles, tendons or other soft tissues). HO typically occurs after a severe injury and can occur in any part of the body. HO lesions are highly vascularized. Angiogenesis, which is the formation of new blood vessels, plays an important role in the pathophysiology of HO. Surgical resection is considered an effective treatment for HO. However, it is difficult to completely remove new vessels, which can lead to the recurrence of HO and is often accompanied by significant problems such as intraoperative hemorrhage, demonstrating the important role of angiogenesis in HO. Here, we broadly summarize the current understanding of how angiogenesis contributes to HO; in particular, we focus on new insights into the cellular and signaling mechanisms underlying HO angiogenesis. We also review the development and current challenges associated with antiangiogenic therapy for HO.

Emerging technology has a brilliant future: the CRISPR-Cas system for senescence, inflammation, and cartilage repair in osteoarthritis

Osteoarthritis (OA), known as one of the most common types of aseptic inflammation of the musculoskeletal system, is characterized by chronic pain and whole-joint lesions. With cellular and molecular changes including senescence, inflammatory alterations, and subsequent cartilage defects, OA eventually leads to a series of adverse outcomes such as pain and disability. CRISPR-Cas-related technology has been proposed and explored as a gene therapy, offering potential gene-editing tools that are in the spotlight. Considering the genetic and multigene regulatory mechanisms of OA, we systematically review current studies on CRISPR-Cas technology for improving OA in terms of senescence, inflammation, and cartilage damage and summarize various strategies for delivering CRISPR products, hoping to provide a new perspective for the treatment of OA by taking advantage of CRISPR technology.

Increased HIF-2α activity in the nucleus pulposus causes intervertebral disc degeneration in the aging mouse spine

Hypoxia-inducible factors (HIFs) are essential to the homeostasis of hypoxic tissues. Although HIF-2α, is expressed in nucleus pulposus (NP) cells, consequences of elevated HIF-2 activity on disc health remains unknown. We expressed HIF-2α with proline to alanine substitutions (P405A; P531A) in the Oxygen-dependent degradation domain (HIF-2αdPA) in the NP tissue using an inducible, nucleus pulposus-specific K19CreERT allele to study HIF-2α function in the adult intervertebral disc. Expression of HIF-2α in NP impacted disc morphology, as evident from small but significantly higher scores of degeneration in NP of 24-month-old K19CreERT; HIF-2αdPA (K19-dPA) mice. Noteworthy, comparisons of grades within each genotype between 14 months and 24 months indicated that HIF-2α overexpression contributed to more pronounced changes than aging alone. The annulus fibrosus (AF) compartment in the 14-month-old K19-dPA mice exhibited lower collagen turnover and Fourier transform-infrared (FTIR) spectroscopic imaging analyses showed changes in the biochemical composition of the 14- and 24-month-old K19-dPA mice. Moreover, there were changes in aggrecan, chondroitin sulfate, and COMP abundance without alterations in NP phenotypic marker CA3, suggesting the overexpression of HIF-2α had some impact on matrix composition but not the cell phenotype. Mechanistically, the global transcriptomic analysis showed enrichment of differentially expressed genes in themes closely related to NP cell function such as cilia, SLIT/ROBO pathway, and HIF/Hypoxia signaling at both 14- and 24-month. Together, these findings underscore the role of HIF-2α in the pathogenesis of disc degeneration in the aged spine.

Poly-3-hydroxybutyrate-co-3-hydroxyvalerate(PHBV)-Polyethylene glycol 20k(PEG20k) as a promising delivery system for PT2399 in the treatment of disc degeneration

Disc degeneration often leads to a highly prevalent symptom known as low back pain. Healthy nucleus pulposus tissue exhibited a hypoxic environment devoid of blood vessels, while degenerated nucleus pulposus experienced hypoxic deterioration and the formation of new blood vessels. In this study, the expression of important genes like HIF-2α was found to vary between normal and degenerated nucleus pulposus cells when compared to the hypoxic surroundings. The aim of this study was to examine how HIF-2α is controlled in nucleus pulposus cells under hypoxic conditions and its role in angiogenic mechanisms. To assess the impact of gradual inhibition of HIF-2α on disc degeneration, we utilized PHBV-based synthetic materials loaded with inhibitors of HIF-2α. Specifically, we employed LPS and PT2399 loaded PHBV-PEG20k (PP20) to intervene with human nucleus pulposus cells. Additionally, we treated APD rat models with PT2399 loaded PP20 to evaluate its effects. The expression levels of target markers in nucleus pulposus cells were detected using PCR, WB, and immunofluorescence. Additionally, the effect of drugs on disc degeneration was identified through HE staining. The findings indicated that HIF-2α, CAIX, PPP1R15A, VEGFA, and EGLN3 could potentially serve as new indicators of disc degeneration. Additionally, HIF-2α might contribute to the progression of disc degeneration through involvement in angiogenesis and the regulation of hypoxia. Furthermore, the utilization of PT2399 loaded PHBV-PEG20k (PP20) could potentially offer a fresh alternative for treating disc degeneration.

In vitro and in vivo investigations on arsenic-induced cartilage degeneration in osteoarthritis

Heavy metals found in the environment, including arsenic (As) pose significant risks to human health and present a risk factor for osteoarthritis (OA). This study researched the impact of As on cartilage degeneration by focusing on the role of As in causing OA in mice. We employed chemical inhibition and inductively coupled plasma mass spectrometry analyses to identify the effect of As on chondrocytes as well as studying its accumulation in organs after oral administration in mice. Additionally, the study examined the effect of intra-articular As treatment on the levels of crucial catabolic factors, namely Hif-2α (Epas1) and Zip8 (Slc39a8), during OA progression. Mice that were administered As orally in conjunction with surgically induced joint instability, had heightened cartilage destruction compared to wild-type mice. Quantitative analysis revealed a significant increase in Hif-2α and Zip8 mRNA expression (p = 0.0352,0.0004 respectively) and protein expression (p = 0.0101,0.008 respectively) post oral administration. Our findings illustrated the role of As in influencing crucial cellular functions that are triggered by reactive oxygen species. These events consequently activate the Akt/Hif-2α/NF-κB pathways, leading to disruptions in articular cartilage homeostasis. This study provides a comprehensive understanding of the impact of As on the development of osteoarthritis.

Blood-Induced Arthropathy: A Major Disabling Complication of Haemophilia

Haemophilic arthropathy (HA) is one of the most serious complications of haemophilia. It starts with joint bleeding, leading to synovitis which, in turn, can cause damage to the cartilage and subchondral bone, eventually inducing degenerative joint disease. Despite significant improvements in haemophilia treatment over the past two decades and recent guidelines from ISTH and WFH recommending FVIII trough levels of at least 3 IU/dL during prophylaxis, patients with haemophilia still develop joint disease. The pathophysiology of HA is complex, involving both inflammatory and degenerative components. Early diagnosis is key for proper management. Imaging can detect joint subclinical changes and influence prophylaxis. Magnetic resonance imagining (MRI) and ultrasound are the most frequently used methods in comprehensive haemophilia care centres. Biomarkers of joint health have been proposed to determine osteochondral joint deterioration, but none of these biomarkers has been validated or used in clinical practice. Early prophylaxis is key in all severe haemophilia patients to prevent arthropathy. Treatment is essentially based on prophylaxis intensification and chronic joint pain management. However, there remain significant gaps in the knowledge of the mechanisms responsible for HA and prognosis-influencing factors. Better understanding in this area could produce more effective interventions likely to ultimately prevent or attenuate the development of HA.

Increased HIF-2α Activity in the Nucleus Pulposus Causes Intervertebral Disc Degeneration in the Aging Mouse Spine

Hypoxia-inducible factors (HIFs) are essential to the homeostasis of hypoxic tissues. Although HIF-2α, is expressed in nucleus pulposus (NP) cells, consequences of elevated HIF-2 activity on disc health remains unknown. We expressed HIF-2α with proline to alanine substitutions (P405A;P531A) in the Oxygen-dependent degradation domain (HIF-2αdPA) in the NP tissue using an inducible, nucleus pulposus-specific K19 CreERT allele to study HIF-2α function in the adult intervertebral disc. Expression of HIF-2α in NP impacted disc morphology, as evident from small but significantly higher scores of degeneration in NP of 24-month-old K19 CreERT ; HIF-2α dPA (K19-dPA) mice. Noteworthy, comparisons of grades within each genotype between 14 months and 24 months indicated that HIF-2α overexpression contributed to more pronounced changes than aging alone. The annulus fibrosus (AF) compartment in the 14-month-old K19-dPA mice exhibited lower collagen turnover and Fourier transform-infrared (FTIR) spectroscopic imaging analyses showed changes in the biochemical composition of the 14-and 24-month-old K19-dPA mice. Moreover, there were changes in aggrecan, chondroitin sulfate, and COMP abundance without alterations in NP phenotypic marker CA3, suggesting the overexpression of HIF-2α had some impact on matrix composition but not the cell phenotype. Mechanistically, the global transcriptomic analysis showed enrichment of differentially expressed genes in themes closely related to NP cell function such as cilia, SLIT/ROBO pathway, and HIF/Hypoxia signaling at both 14- and 24-months. Together, these findings underscore the role of HIF-2α in the pathogenesis of disc degeneration in the aged spine.

Posteromedial capsular anatomy of the tibia for consideration of the medial meniscal support structure using a multidimensional analysis

Medial meniscal extrusion (MME) is a structural abnormality that leads to early knee osteoarthritis; however, its formation remains debated. For anatomical consideration of the mechanism underlying MME formation, we examined the capsular attachment on the posteromedial tibia and its layered association with the semimembranosus. Fourteen knees of eight body donors were analyzed in this study; six knees were grouped for macroscopic analysis, whereas four knees each were grouped for histological and phosphotungstic acid-enhanced micro-computed tomography analyses. The capsular attachment varied in width according to location and was not distant from the articular cartilage and posterior root. A portion of the posteromedial joint capsule formed the semimembranosus tendinous sheath. The dense fibrous membrane superficial to the semimembranosus, which was continuous from its tendinous sheath, existed as one of the layers of the joint capsule. The aforementioned findings were confirmed in all specimens. Based on the capsular attachment and its layered association with the semimembranosus, the conventional posteromedial knee ligaments may be only a part of the joint capsule divided into two layers by the semimembranosus. If the coordinated action of the joint capsule and semimembranosus partially contributes to the medial meniscus stability, such a structural problem may affect MME formation.

Network-based modelling of mechano-inflammatory chondrocyte regulation in early osteoarthritis

Osteoarthritis (OA) is a debilitating joint disease characterized by articular cartilage degradation, inflammation and pain. An extensive range of in vivo and in vitro studies evidences that mechanical loads induce changes in chondrocyte gene expression, through a process known as mechanotransduction. It involves cascades of complex molecular interactions that convert physical signals into cellular response(s) that favor either chondroprotection or cartilage destruction. Systematic representations of those interactions can positively inform early strategies for OA management, and dynamic modelling allows semi-quantitative representations of the steady states of complex biological system according to imposed initial conditions. Yet, mechanotransduction is rarely integrated. Hence, a novel mechano-sensitive network-based model is proposed, in the form of a continuous dynamical system: an interactome of a set of 118 nodes, i.e., mechano-sensitive cellular receptors, second messengers, transcription factors and proteins, related among each other through a specific topology of 358 directed edges is developed. Results show that under physio-osmotic initial conditions, an anabolic state is reached, whereas initial perturbations caused by pro-inflammatory and injurious mechanical loads leads to a catabolic profile of node expression. More specifically, healthy chondrocyte markers (Sox9 and CITED2) are fully expressed under physio-osmotic conditions, and reduced under inflammation, or injurious loadings. In contrast, NF-κB and Runx₂, characteristic of an osteoarthritic chondrocyte, become activated under inflammation or excessive loading regimes. A literature-based evaluation shows that the model can replicate 94% of the experiments tested. Sensitivity analysis based on a factorial design of a treatment shows that inflammation has the strongest influence on chondrocyte metabolism, along with a significant deleterious effect of static compressive loads. At the same time, anti-inflammatory therapies appear as the most promising ones, though the restoration of structural protein production seems to remain a major challenge even in beneficial mechanical environments. The newly developed mechano-sensitive network model for chondrocyte activity reveals a unique potential to reflect load-induced chondroprotection or articular cartilage degradation in different mechano-chemical-environments.

Mechanism of HIFs in osteoarthritis

Osteoarthritis (OA) is a common disabling disease which has a high incidence rate in the elderly. Studies have found that many factors are involved in the pathogenesis of OA. Hypoxia-inducible factors (HIFs) are core regulators that induce hypoxia genes, repair the cellular oxygen environment, and play an important role in the treatment of OA. For example, HIF-1α can maintain the stability of the articular cartilage matrix, HIF-2α is able to cause chondrocyte apoptosis and intensify in-flammatory response, and HIF-3α may be the target gene of HIF-1α and HIF-2α, thereby playing a negative regulatory role. This review examines the mechanism of HIFs in cartilage extracellular matrix degradation, apoptosis, inflammatory reaction, autophagy and then further expounds on the roles of HIFs in OA, consequently providing theoretical support for the pathogenesis of OA and a new target for OA treatment.

Conditional Deletion of HIF-2α in Mouse Nucleus Pulposus Reduces Fibrosis and Provides Mild and Transient Protection From Age-Dependent Structural Changes in Intervertebral Disc

Hypoxia-inducible factors (HIFs) are critical to the development and homeostasis of hypoxic tissues. Although HIF-2α, one of the main HIF-α isoforms, is expressed in nucleus pulposus (NP) cells, its functions remain unknown. We deleted HIF-2α in the NP tissue using a notochord-specific FoxA2Cre allele to study HIF-2α function in the adult intervertebral disc. Unlike observations in HIF-1αcKO mice, fate mapping studies using Rosa26-mTmG reporter showed that HIF-2α loss in NP did not negatively impact cell survival or affect compartment development. Rather, loss of HIF-2α resulted in slightly better attributes of NP morphology in 14-month-old HIF-2αcKO mice as evident from lower scores of degeneration. These 14-month-old HIF-2αcKO mice also exhibited significant reduction in NP tissue fibrosis and lower collagen turnover in the annulus fibrosis (AF) compartment. Imaging-Fourier transform-infrared (FTIR) analyses showed decreased collagen and protein content in the NP and maintained chondroitin sulfate levels in 14-month-old HIF-2αcKO . Mechanistically, global transcriptomic analysis showed enrichment of differentially expressed genes with Gene Ontology (GO) terms related to metabolic processes and cell development, molecular functions concerned with histone and protein binding, and associated pathways, including oxidative stress. Noteworthy, these morphological differences were not apparent in 24-month-old HIF-2αcKO , indicating that aging is the dominant factor in governing disc health. Together these data suggest that loss of HIF-2α in the NP compartment is not detrimental to the intervertebral disc development but rather mitigates NP tissue fibrosis and offers mild but transient protection from age-dependent early degenerative changes. © 2022 American Society for Bone and Mineral Research (ASBMR).

Molecular Mechanism of Ferroptosis in Orthopedic Diseases

Ferroptosis is a new iron-dependent programmed cell death process that is directly mediated by the accumulation of lipid peroxides and reactive oxygen species. Numerous studies have shown that ferroptosis is important in regulating the occurrence and development of bone-related diseases, but the underlying mechanisms are not completely clear. Herein, we review the progress of the mechanism of ferroptosis in bone marrow injury, osteoporosis, osteoarthritis, and osteosarcoma and attempt to deeply understand the regulatory targets of ferroptosis, which will open up a new way for the prevention and treatment of orthopedic diseases.

Hypoxia-Inducible Factors Signaling in Osteogenesis and Skeletal Repair

Sufficient oxygen is required to maintain normal cellular and physiological function, such as a creature’s development, breeding, and homeostasis. Lately, some researchers have reported that both pathological hypoxia and environmental hypoxia might affect bone health. Adaptation to hypoxia is a pivotal cellular event in normal cell development and differentiation and in pathological settings such as ischemia. As central mediators of homeostasis, hypoxia-inducible transcription factors (HIFs) can allow cells to survive in a low-oxygen environment and are essential for the regulation of osteogenesis and skeletal repair. From this perspective, we summarized the role of HIF-1 and HIF-2 in signaling pathways implicated in bone development and skeletal repair and outlined the molecular mechanism of regulation of downstream growth factors and protein molecules such as VEGF, EPO, and so on. All of these present an opportunity for developing therapies for bone regeneration.

HIF-1α in Osteoarthritis: From Pathogenesis to Therapeutic Implications

Osteoarthritis is a common age-related joint degenerative disease. Pain, swelling, brief morning stiffness, and functional limitations are its main characteristics. There are still no well-established strategies to cure osteoarthritis. Therefore, better clarification of mechanisms associated with the onset and progression of osteoarthritis is critical to provide a theoretical basis for the establishment of novel preventive and therapeutic strategies. Chondrocytes exist in a hypoxic environment, and HIF-1α plays a vital role in regulating hypoxic response. HIF-1α responds to cellular oxygenation decreases in tissue regulating survival and growth arrest of chondrocytes. The activation of HIF-1α could regulate autophagy and apoptosis of chondrocytes, decrease inflammatory cytokine synthesis, and regulate the chondrocyte extracellular matrix environment. Moreover, it could maintain the chondrogenic phenotype that regulates glycolysis and the mitochondrial function of osteoarthritis, resulting in a denser collagen matrix that delays cartilage degradation. Thus, HIF-1α is likely to be a crucial therapeutic target for osteoarthritis via regulating chondrocyte inflammation and metabolism. In this review, we summarize the mechanism of hypoxia in the pathogenic mechanisms of osteoarthritis, and focus on a series of therapeutic treatments targeting HIF-1α for osteoarthritis. Further clarification of the regulatory mechanisms of HIF-1α in osteoarthritis may provide more useful clues to developing novel osteoarthritis treatment strategies.

Integration and Visualization of Regulatory Elements and Variations of the EPAS1 Gene in Human

Endothelial PAS domain-containing protein 1 (EPAS1), also HIF2α, is an alpha subunit of hypoxia-inducible transcription factor (HIF), which mediates cellular and systemic response to hypoxia. EPAS1 has an important role in the transcription of many hypoxia-responsive genes, however, it has been less researched than HIF1α. The aim of this study was to integrate an increasing number of data on EPAS1 into a map of diverse OMICs elements. Publications, databases, and bioinformatics tools were examined, including Ensembl, MethPrimer, STRING, miRTarBase, COSMIC, and LOVD. The EPAS1 expression, stability, and activity are tightly regulated on several OMICs levels to maintain complex oxygen homeostasis. In the integrative EPAS1 map we included: 31 promoter-binding proteins, 13 interacting miRNAs and one lncRNA, and 16 post-translational modifications regulating EPAS1 protein abundance. EPAS1 has been associated with various cancer types and other diseases. The development of neuroendocrine tumors and erythrocytosis was shown to be associated with 11 somatic and 20 germline variants. The integrative map also includes 12 EPAS1 target genes and 27 interacting proteins. The study introduced the first integrative map of diverse genomics, transcriptomics, proteomics, regulomics, and interactomics data associated with EPAS1, to enable a better understanding of EPAS1 activity and regulation and support future research.

D-mannose alleviates osteoarthritis progression by inhibiting chondrocyte ferroptosis in a HIF-2α-dependent manner

OBJECTIVES

Chondrocyte ferroptosis contributes to osteoarthritis (OA) progression, and D-mannose shows therapeutic value in many inflammatory conditions. Here, we investigated whether D-mannose interferes in chondrocyte ferroptotic cell death during osteoarthritic cartilage degeneration.

MATERIALS AND METHODS

In vivo anterior cruciate ligament transection (ACLT)-induced OA mouse model and an in vitro study of chondrocytes in an OA microenvironment induced by interleukin-1β (IL-1β) exposure were employed. Combined with Epas1 gene gain- and loss-of-function, histology, immunofluorescence, quantitative RT-PCR, Western blot, cell viability and flow cytometry experiments were performed to evaluate the chondroprotective effects of D-mannose in OA progression and the role of hypoxia-inducible factor 2 alpha (HIF-2 α) in D-mannose-induced ferroptosis resistance of chondrocytes.

RESULTS

D-mannose exerted a chondroprotective effect by attenuating the sensitivity of chondrocytes to ferroptosis and alleviated OA progression. HIF-2α was identified as a central mediator in D-mannose-induced ferroptosis resistance of chondrocytes. Furthermore, overexpression of HIF-2α in chondrocytes by Ad-Epas1 intra-articular injection abolished the chondroprotective effect of D-mannose during OA progression and eliminated the role of D-mannose as a ferroptosis suppressor.

CONCLUSIONS

D-mannose alleviates osteoarthritis progression by suppressing HIF-2α-mediated chondrocyte sensitivity to ferroptosis, indicating D-mannose to be a potential therapeutic strategy for ferroptosis-related diseases.

Molecular Insights into the Oxygen-Sensing Pathway and Erythropoietin Expression Regulation in Erythropoiesis

Erythropoiesis is regulated by several factors, including the oxygen-sensing pathway as the main regulator of erythropoietin (EPO) synthesis in the kidney. The release of EPO from the kidney and its binding to the EPO receptor (EPOR) on erythrocyte progenitor cells in the bone marrow results in increased erythropoiesis. Any imbalance in these homeostatic mechanisms can lead to dysregulated erythropoiesis and hematological disorders. For example, mutations in genes encoding key players of oxygen-sensing pathway and regulation of EPO production (HIF-EPO pathway), namely VHL, EGLN, EPAS1 and EPO, are well known causative factors that contribute to the development of erythrocytosis. We aimed to investigate additional molecular mechanisms involved in the HIF-EPO pathway that correlate with erythropoiesis. To this end, we conducted an extensive literature search and used several in silico tools. We identified genes encoding transcription factors and proteins that control transcriptional activation or repression; genes encoding kinases, deacetylases, methyltransferases, conjugating enzymes, protein ligases, and proteases involved in post-translational modifications; and genes encoding nuclear transport receptors that regulate nuclear transport. All these genes may modulate the stability or activity of HIF2α and its partners in the HIF-EPO pathway, thus affecting EPO synthesis. The theoretical information we provide in this work can be a valuable tool for a better understanding of one of the most important regulatory pathways in the process of erythropoiesis. This knowledge is necessary to discover the causative factors that may contribute to the development of hematological diseases and improve current diagnostic and treatment solutions in this regard.

Non-surgical osteoarthritis therapy, intra-articular drug delivery towards clinical applications

Osteoarthritis (OA）is a common orthopaedic disease in middle-aged and aged people. To date, no disease-modifying drug is available to prevent the progression of OA. Surgical treatment of OA has complications such as pain and high costs with increased risk of post-operative infections. An intra-articular drug delivery is a conservative treatment method to apply therapeutic composites directly into the OA joint cavity. This method has an advantage to improve the bioavailability of therapeutics and hence is a widely preferred choice to test novel disease-modifying drug targets for OA. Herein, we summarised and discussed the current status of intra-articular therapy for OA treatment as well as outlined drug delivery of small molecular, protein and gene delivery for OA therapy. Currently, new targeted nano-based drug delivery systems, including nanoparticles, exosomes and hydrogel formulations under investigation for OA treatment via intra-articular injection are also addressed. The emerging trend demonstrates that intra-articular drug delivery has vast prospects for the clinical selective treatment of OA. The rational application of intra-articular injection of drugs and biological agents will be of great significance for alleviating the patients with OA, improving their quality of life, delaying surgery, and reducing the disease burden of OA.

Accelerating functional gene discovery in osteoarthritis

Osteoarthritis causes debilitating pain and disability, resulting in a considerable socioeconomic burden, yet no drugs are available that prevent disease onset or progression. Here, we develop, validate and use rapid-throughput imaging techniques to identify abnormal joint phenotypes in randomly selected mutant mice generated by the International Knockout Mouse Consortium. We identify 14 genes with functional involvement in osteoarthritis pathogenesis, including the homeobox gene Pitx1, and functionally characterize 6 candidate human osteoarthritis genes in mouse models. We demonstrate sensitivity of the methods by identifying age-related degenerative joint damage in wild-type mice. Finally, we phenotype previously generated mutant mice with an osteoarthritis-associated polymorphism in the Dio2 gene by CRISPR/Cas9 genome editing and demonstrate a protective role in disease onset with public health implications. We hope this expanding resource of mutant mice will accelerate functional gene discovery in osteoarthritis and offer drug discovery opportunities for this common, incapacitating chronic disease.

Chemically modified curcumin (CMC2.24) alleviates osteoarthritis progression by restoring cartilage homeostasis and inhibiting chondrocyte apoptosis via the NF-κB/HIF-2α axis

The disorders of cartilage homeostasis and chondrocyte apoptosis are major events in the pathogenesis of osteoarthritis (OA). Herein, we aim to assess the chondroprotective effect and underlying mechanisms of a novel chemically modified curcumin, CMC2.24, in modulating extracellular matrix (ECM) homeostasis and inhibiting chondrocyte apoptosis. Rats underwent the anterior cruciate ligament transection, and medial menisci resection was treated by intra-articular injection with CMC2.24. In an in vitro study, rat chondrocytes were pretreated with CMC2.24 before stimulation with sodium nitroprusside (SNP). Results from in vivo studies demonstrated that the intra-articular administration of CMC2.24 ameliorated osteoarthritic cartilage destruction by promoting collagen 2a1 production and inhibited cartilage degradation and apoptosis by suppressing hypoxia-inducible factor-2a (Hif-2α), matrix metalloproteinase-3, runt-related transcription factor 2, cleaved caspase-3, and vascular endothelial growth factor and the phosphorylation of IκBα and NF-κB p65. The in vitro results revealed that CMC2.24 exhibited a strong inhibitory effect on SNP-induced chondrocyte catabolism and apoptosis. The SNP-enhanced expression of Hif-2α, a catabolic and apoptotic factor, decreased in a dose-dependent manner after CMC2.24 treatment. CMC2.24 pretreatment effectively inhibited SNP-induced IκBα and NF-κB p65 phosphorylation in rat chondrocytes, whereas pretreatment with the NF-κB antagonist BMS-345541 significantly enhanced the effects of CMC2.24. Overall, these results demonstrated that CMC2.24 attenuates OA progression by modulating ECM homeostasis and chondrocyte apoptosis by suppressing the NF-κB/Hif-2α axis, thus providing a new perspective for therapeutic strategies in OA. KEY MESSAGES: • Intra-articular injection of CMC2.24 ameliorated osteoarthritic cartilage destruction. • CMC2.24 promoted cell viability and decreased SNP-induced apoptotic gene expression. • SNP-induced activation of Hif-2α is inhibited by CMC2.24. • CMC2.24 inhibits NF-κB/Hif-2α axis activation to modulate ECM homeostasis and inhibit chondrocyte apoptosis.

Icariin increases chondrocyte vitality by promoting hypoxia-inducible factor-1α expression and anaerobic glycolysis

BACKGROUND

Articular cartilage is a unique avascular tissue in which chondrocytes are embedded in extracellular matrix (ECM). The decreased ECM resulting from the loss of articular chondrocyte viability leads to degenerative diseases such as osteoarthritis (OA). This study aims to investigate the effect of icariin (ICA) on ECM synthesis and chondrocyte viability.

METHODS

Micromass culture, alcian blue, and Safran O (SO)/fast green staining were used to investigate chondrocyte viability and ECM synthesis in chondrocytes treated with ICA. The expression of hypoxia-inducible factor-1α (HIF-1α), SOX9, and anaerobic glycolysis enzymes were detected by western blot and reverse transcription-quantitative polymerase chain reaction.

RESULTS

ICA, an active flavonoid component of Herba epimedii, was demonstrated to increase chondrocyte viability and ECM synthesis. HIF-1α is a key mediator of chondrocyte response to fluctuations in oxygen availability during cartilage development or damage, and its expression was unregulated by ICA treatment. Meanwhile, ICA treatment increased SOX9 expression, which is a key regulator of ECM synthesis. Furthermore, ICA treatment increased the expression of glucose transporter 1 (GLUT1), glucose-6-phosphate dehydrogenase (G6PD), phosphoglycerate kinase 1 (PGK1), and pyruvate dehydrogenase kinase 1 (PDK1), which contribute to glucose transfer and anaerobic glycolysis.

CONCLUSIONS

The present study revealed that ICA treatment facilitates chondrocyte vitality by promoting HIF-1α expression and anaerobic glycolysis. Therefore, ICA could be a novel clinical treatment for OA.

Oral administration of N-acetyl cysteine prevents osteoarthritis development and progression in a rat model

The number of osteoarthritis patients is increasing with the rise in the number of elderly people in developed countries. Osteoarthritis, which causes joint pain and deformity leading to loss of activities of daily living, is often treated surgically. Here we show that mechanical stress promotes accumulation of reactive oxygen species (ROS) in chondrocytes in vivo, resulting in chondrocyte apoptosis and leading to osteoarthritis development in a rat model. We demonstrate that mechanical stress induces ROS accumulation and inflammatory cytokine expression in cultured chondrocytes in vitro and that both are inhibited by treatment with the anti-oxidant N-acetyl cysteine (NAC). In vivo, osteoarthritis development in a rat osteoarthritis model was also significantly inhibited by oral administration of NAC. MMP13 expression and down-regulation of type II collagen in chondrocytes, both of which indicate osteoarthritis, as well as chondrocyte apoptosis in osteoarthritis rats were inhibited by NAC. Interestingly, osteoarthritis development in sham-operated control sides, likely due to disruption of normal weight-bearing activity on the control side, was also significantly inhibited by NAC. We conclude that osteoarthritis development in rats is significantly antagonized by oral NAC administration. Currently, no oral medication is available to prevent osteoarthritis development. Our work suggests that NAC may represent such a reagent and serve as osteoarthritis treatment.

Osthole ameliorates cartilage degradation by downregulation of NF-κB and HIF-2α pathways in an osteoarthritis murine model

Osteoarthritis (OA) is a common and disabling joint disease mainly characterized by cartilage degradation, with the knees most commonly affected. No effective treatment for the cartilage degradation of OA exists. Preliminary studies have revealed the protective and osteogenic effects of osthole, a natural coumarin first isolated from Cnidium monnieri (Fructus Cnidii); however, no evidence of osthole in an OA-related model has been published to date. This study further explored the effects of osthole in a monoiodoacetate (MIA)-induced OA-related animal model and focused on the molecular mechanism(s) behind the anti-inflammatory and cartilage protective effects of osthole. This study revealed that the cartilage protective effect of osthole in a MIA-induced osteoarthritis (OA) murine model can be explained by downregulation of COX-2 and RUNX2 by inhibition of NF-κB and HIF-2α up-regulated by OA induction, resulting in downregulation of MMP-13, Syndecan IV and ADAMTS-5. In addition, osthole might have anti-inflammatory and analgesic effects due to COX-2 inhibition. Osthole can be considered as a potential component of the treatment of OA, for it possesses a cartilage protective effect, as well as anti-inflammation, analgesic, and movement improving effects. Further preclinical and human clinical studies are needed to examine the efficacy and safety profile of long-term therapy.

Cartilage and Bone Destruction in Arthritis: Pathogenesis and Treatment Strategy: A Literature Review

Arthritis is inflammation of the joints accompanied by osteochondral destruction. It can take many forms, including osteoarthritis, rheumatoid arthritis, and psoriatic arthritis. These diseases share one commonality—osteochondral destruction based on inflammation. The background includes a close interaction between osseous tissues and immune cells through various inflammatory cytokines. However, the tissues and cytokines that play major roles are different in each disease, and as a result, the mechanism of osteochondral destruction also differs. In recent years, there have been many findings regarding not only extracellular signaling pathways but also intracellular signaling pathways. In particular, we anticipate that the intracellular signals of osteoclasts, which play a central role in bone destruction, will become novel therapeutic targets. In this review, we have summarized the pathology of arthritis and the latest findings on the mechanism of osteochondral destruction, as well as present and future therapeutic strategies for these targets.

Up-regulated HIF-2α contributes to the Osteoarthritis development through mediating the primary cilia loss

BACKGROUNDS

Up-regulated HIF-2α (hypoxia induced factor 2) had been demonstrated to contribute to Osteoarthritis (OA) development via inducing the expression of matrix-degrading enzymes. However, the HIF-2α also could promote primary cilia loss through HIF-2α/AURKA (Aurora kinase A)/NEDD9 pathway. And the primary cilia dysfunction is another characteristic of the OA. Thus, we investigated here whether the HIF-2α also contributes the OA development through mediating the primary cilia loss.

METHODS

The primary chondrocytes were isolated from the experimental OA mice induced by destabilization of the medial meniscus (DMM). Chondrocytes were cultured under normoxia (21% O₂) or hypoxia (2% O₂) conditions. The HIF-1α and HIF-2α expressions were assessed by western blot. The cilia formation was counted by immuno-staining the acetylated tubulin. The contribution of HIF-1α or HIF-2α to the primary cilia loss was assessed by knocking-down the HIF-1α or HIF-2α individually. The HIF-2α/AURKA/NEDD9 pathway was validated through over-expressing or knocking-down specific components of the pathway and then counting the primary cilia number. Finally, the pathway was further confirmed in the OA mice.

RESULTS

Hypoxia could induce the expression of both HIF-1α and HIF-2α, and also reduce the number of primary cilia on the chondrocytes isolated from the experimental OA mice. Knocking-down or over-expressing HIF-1α or HIF-2α individually showed that the HIF-2α could induce the primary cilia reduction rather than the HIF-1α. Manipulating the HIF-2α expression could positively affect the AURKA and NEDD9 expression. Manipulating the AURKA and NEDD9 expressions could reverse the function of HIF-2α on primary cilia. In the mice, knocking-down both AURKA and NEDD9 could alleviate the OA development significantly.

CONCLUSION

Up-regulated HIF-2α contributes to the Osteoarthritis development through mediating the primary cilia loss, which might be developed as therapeutic targets for OA treatment.

MicroRNA in osteoarthritis: physiopathology, diagnosis and therapeutic challenge

BACKGROUND

Osteoarthritis (OA) is the most orthopedic condition. The pattern of gene expression and the transcription factors that exert control of chondrogenesis have been extensively studied.

SOURCES OF DATA

A systematic search (up to July 2018) of articles assessing the role of microRNA (miRNA) in physiopathology, diagnosis and therapy of OA was performed, with the purpose of giving a critical perspective of the possibilities for diagnostic and therapeutic use of miRNA in the management of OA.

AREAS OF AGREEMENT

miRNAs are small noncoding RNAs that can regulate gene expression in human cells. miRNAs can be expressed in a different fashion in osteoarthritic compared to nonosteoarthritic cartilage.

AREAS OF CONTROVERSY

The mechanisms that produce alteration of gene expression in OA are still not completely understood. miRNAs may be involved in the diagnosis of OA as well as in its treatment.

GROWING POINTS

There are complex interactions between miRNAs and their multiple target genes. These interactions may be important in gene regulation and the control of homeostatic pathways in OA.

AREAS TIMELY FOR DEVELOPING RESEARCH

miRNA could be useful for diagnostic or management purposes, but the issue of delivery of miRNA targeting agents needs to be overcome before miRNA can be applied in clinical practice.

The Importance of Physioxia in Mesenchymal Stem Cell Chondrogenesis and the Mechanisms Controlling Its Response

Articular cartilage covers the surface of synovial joints and enables joint movement. However, it is susceptible to progressive degeneration with age that can be accelerated by either previous joint injury or meniscectomy. This degenerative disease is known as osteoarthritis (OA) and it greatly affects the adult population. Cell-based tissue engineering provides a possible solution for treating OA at its earliest stages, particularly focal cartilage lesions. A candidate cell type for treating these focal defects are Mesenchymal Stem Cells (MSCs). However, present methods for differentiating these cells towards the chondrogenic lineage lead to hypertrophic chondrocytes and bone formation in vivo. Environmental stimuli that can stabilise the articular chondrocyte phenotype without compromising tissue formation have been extensively investigated. One factor that has generated intensive investigation in MSC chondrogenesis is low oxygen tension or physioxia (2⁻5% oxygen). In vivo articular cartilage resides at oxygen tensions between 1⁻4%, and in vitro results suggest that these conditions are beneficial for MSC expansion and chondrogenesis, particularly in suppressing the cartilage hypertrophy. This review will summarise the current literature regarding the effects of physioxia on MSC chondrogenesis with an emphasis on the pathways that control tissue formation and cartilage hypertrophy.

Anti-inflammatory role of TPCA-1 encapsulated nanosomes in porcine chondrocytes against TNF-α stimulation

In this study, we evaluated the hypothesis that immunonanosomes carrying the drug [5-(p-Fluorophenyl)-2-ureido]thiophene-3-carboxamide (TPCA-1) will help in reducing nuclear factor-kappaB (NF-κB)-associated inflammation in porcine chondrocytes against tumor necrosis factor-alpha (TNF-α)-induced stress. The nanosomes were tagged with monoclonal anti-type II collagen (MabCII) antibody to specifically target the exposed type II collagen in cartilage matrix. TPCA-1 at a concentration of 10 µM significantly reduced expression of the matrix-degrading enzyme, Matrix metalloproteinase-13 (MMP-13) and blocked the p65 nuclear translocation. In comparison to the TPCA-1 solution alone, the TPCA-1 nanosomes were found to be more effective in reducing the cellular toxicity, oxidative stress and inflammation in chondrocytes treated with TNF-α. In addition, TPCA-1 nanosomes were more effective in reducing the gene expression of hypoxia-inducible factor-2alpha (HIF-2α) that in turn is associated with the regulation of MMP-13 gene. TPCA-1 nanosomes significantly reduced expression of both these genes. The data also showed that TPCA-1 did not attenuate the down-regulated gene expression levels of anabolic genes aggrecan (ACAN) and collagen type II alpha (COL2A1). In conclusion, this study showed that TPCA-1 nanosomes carrying a dose of 10 µM TPCA-1 can effectively increase the survival of cultured porcine chondrocytes against TNF-α-induced stress. The findings of this study could be used to develop nanosome-based drug delivery systems (DDSs) for animal model of OA. Moreover, the approach presented here can be further utilized in other studies for targeted delivery of the drug of interest at a cellular level.

Novel role of CCN3 that maintains the differentiated phenotype of articular cartilage

Knowledge of the microenvironment of articular cartilage in health and disease is the key to accomplishing fundamental disease-modifying treatments for osteoarthritis. The proteins comprising the CCN Family are matricellular proteins with a remarkable relevance within the context of cartilage metabolism. CCN2 displays a great capability for regenerating articular cartilage, and CCN3 has been shown to activate the expression of genes related to articular chondrocytes and to repress genes related to endochondral ossification in epiphyseal chondrocytes. Moreover, mice lacking CCN3 protein have been shown to display ostearthritic changes in their knee articular cartilage. In this study, we employed a monoiodoacetic acid (MIA)-induced osteoarthritic model to investigate whether osteoarthritic changes in the cartilage are reciprocally accompanied by CCN3 down-regulation and an inducible overexpression system to evaluate the effects of CCN3 on articular chondrocytes in vitro. Finally, we also investigated the effects of exogenous CCN3 in vivo during the early stages of MIA-induced osteoarthritis. We discovered that CCN3 is expressed by articular chondrocytes in normal rat knees, whereas it is rapidly down-regulated in osteoarthritic knees. In vitro, we also discovered that CCN3 increases the proteoglycan accumulation, the gene expression of type II collagen, tenascin-C and lubricin, as well as the protein production of tenascin-C and lubricin in articular chondrocytes. In vivo, it was discovered that exogenous CCN3 increased tidemark integrity and produced an increased production of lubricin protein. The potential utility of CCN3 as a future therapeutic agent and possible strategies to improve its therapeutic functions are also discussed.

Pentosan polysulfate inhibits IL-1β-induced iNOS, c-Jun and HIF-1α upregulation in canine articular chondrocytes

Osteoarthritic (OA) chondrocytes are shown to express inducible nitric oxide synthase (iNOS) which produces high concentrations of nitric oxide (NO), particularly when stimulated with proinflammatory cytokines. NO is involved in OA cartilage degradation. On the other hand, c-Jun N-terminal Kinase (JNK) pathway mediates the activation and transcription of c-Jun, which is required for interleukin-1 (IL-1)-induction of matrix metalloproteinases-13 (MMP-13) in OA pathogenesis. Therefore, the selective inhibition of iNOS and c-Jun is a promising target for treatment and prevention of OA. The purpose of the study was to investigate the inhibitory effects of pentosan polysulfate (PPS) on IL-1β-induced iNOS, c-Jun and HIF-α isoforms upregulation in canine articular chondrocytes (CACs). Primary (P0) chondrocytes were isolated and cultured from femoral head cartilages of three (3) dogs. First passage (P1) chondrocytes were preincubated with 0, 1, 5, 15 and 40 μg/mL of PPS for 4 hr before treatment with 10 ng/mL rhIL-1β for a further 8 hr. In addition, we evaluated the effects of single and multiple cytokine with or without LPS on iNOS protein induction. PPS significantly inhibited (P < 0.05) IL-1β-induced iNOS, c-Jun and HIF-1α mRNA upregulation in a dose-dependent pattern. iNOS mRNA was significantly inhibited at 15 and 40 μg/mL whereas c-Jun and HIF-1α were significantly downregulated at 5, 15 and 40 μg/mL of PPS compared to chondrocytes treated with only rhIL-1β. Intriguingly, CACs were recalcitrant to single IL-1β, TNF-α or LPS-induction of iNOS protein including to a combination of IL-1β+TNF-α, IL-1β+LPS except to TNF-α+LPS and IL-1β+TNF-α+LPS suggestive of a protective mechanism from iNOS detrimental effects on perpetuating OA. IL-1β+TNF-α+LPS-induced iNOS protein expression was significantly abrogated by PPS. We demonstrate for the first time that PPS is a novel inhibitor of IL-1β-induced iNOS, c-Jun, and HIF-1α mRNA upregulation and iNOS protein induction which may be beneficial for prevention and treatment OA.

Depletion of Gangliosides Enhances Articular Cartilage Repair in Mice

Elucidation of the healing mechanisms in damaged tissues is a critical step for establishing breakthroughs in tissue engineering. Articular cartilage is clinically one of the most successful tissues to be repaired with regenerative medicine because of its homogeneous extracellular matrix and few cell types. However, we only poorly understand cartilage repair mechanisms, and hence, regenerated cartilage remains inferior to the native tissues. Here, we show that glycosylation is an important process for hypertrophic differentiation during articular cartilage repair. GM3, which is a precursor molecule for most gangliosides, was transiently expressed in surrounding damaged tissue, and depletion of GM3 synthase enhanced cartilage repair. Gangliosides also regulated chondrocyte hypertrophy via the Indian hedgehog pathway. These results identify a novel mechanism of cartilage healing through chondrocyte hypertrophy that is regulated by glycosylation. Manipulation of gangliosides and their synthases may have beneficial effects on articular cartilage repair.

Expression of hypoxia-inducible factor-1α in synovial fluid and articular cartilage is associated with disease severity in knee osteoarthritis

The aim of the present study was to examine hypoxia-inducible factor 1α (HIF-1α) levels in the synovial fluid and articular cartilage of patients with primary knee osteoarthritis (OA) and to investigate their association with the severity of disease. A total of 36 patients with knee OA and ten healthy controls were enrolled. Anteroposterior knee radiographs and/or Mankin scores were assessed to determine the disease severity of the affected knee. Radiographic grading of OA in the knee was performed according to Kellgren-Lawrence criteria. HIF-1α levels in synovial fluid were measured using enzyme-linked immunosorbent assay, whereas HIF-1α levels in articular cartilage were assessed with immunohistochemical methods. Compared with healthy controls, OA patients exhibited an increased HIF-1α concentration in synovial fluid (218.17±25.12 vs. 156.66±7.74 pg/ml; P<0.001) and articular cartilage (P<0.05). Furthermore, synovial fluid HIF-1α levels demonstrated a positive correlation with articular cartilage HIF-1α levels (Pearson's P=0.815; P<0.001). Subsequent analysis showed that synovial fluid HIF-1α levels were significantly correlated with the severity of disease (Spearman's ρ=0.933; P<0.001). Furthermore, articular cartilage levels of HIF-1α also correlated with disease severity (Spearman's ρ=-0.967; P<0.001). The findings of the present study suggested that HIF-1α in synovial fluid and articular cartilage is associated with progressive joint damage and is likely to be a useful biomarker for determining disease severity and progression in knee OA.

Discoidin Domain Receptor 2 as a Potential Therapeutic Target for Development of Disease-Modifying Osteoarthritis Drugs

Osteoarthritis (OA) is the most common form of arthritis disorders, but the identification of therapeutic targets to effectively prevent OA has been increasingly difficult. The goal of this investigation is to provide experimental evidence that discoidin domain receptor 2 (DDR2) may be an ideal target for the development of disease-modifying OA drugs. Ddr2 was conditionally deleted from articular cartilage of adult mouse knee joints. Aggrecan-CreERT2;floxed Ddr2 mice, which were generated by crossing Aggrecan-CreERT2 mice with floxed Ddr2 mice, then received tamoxifen injections at the age of 8 weeks. The mice were then subjected to destabilization of the medial meniscus (DMM) surgery. At 8 and 16 weeks after DMM, mice were euthanized for the collection of knee joints. In a separate experiment, Aggrecan-CreERT2;floxed Ddr2 mice were subjected to DMM at the age of 10 weeks. The mice then received tamoxifen injections at 8 weeks after DMM. The mice were euthanized for the collection of knee joints at 16 weeks after DMM. The progressive process of articular cartilage degeneration was significantly delayed in the knee joints of Ddr2-deficient mice in comparison to their control littermates. Articular cartilage damage in the knee joints of the mice was associated with increased expression profiles of both Ddr2 and matrix metalloproteinase 13. These findings suggest that DDR2 may be an ideal target for the development of disease-modifying OA drugs.

Recent advances in the understanding of molecular mechanisms of cartilage degeneration, synovitis and subchondral bone changes in osteoarthritis

Osteoarthritis (OA), the most common form of degenerative joint disease, is linked to high morbidity. It is predicted to be the single greatest cause of disability in the general population by 2030. The development of disease-modifying therapy for OA currently face great obstacle mainly because the onset and development of the disease involve complex molecular mechanisms. In this review, we will comprehensively summarize biological and pathological mechanisms of three key aspects: degeneration of articular cartilage, synovial immunopathogenesis, and changes in subchondral bone. For each tissue, we will focus on the molecular receptors, cytokines, peptidases, related cell, and signal pathways. Agents that specifically block mechanisms involved in synovial inflammation, degeneration of articular cartilage, and subchondral bone remodeling can potentially be exploited to produce targeted therapy for OA. Such new comprehensive agents will benefit affected patients and bring exciting new hope for the treatment of OA.

The Relationship between HIF-2α and VEGF with Radiographic Severity in the Primary Osteoarthritic Knee

PURPOSE

The aim of this study was to determine the relationship of hypoxia-inducible factor-2 (HIF-2α) and vascular endothelial growth factor (VEGF) with radiographic severity in primary osteoarthritis (OA) of the knee. Expression of these two factors in cartilage samples from OA knee joints was examined at mRNA and protein levels.

MATERIALS AND METHODS

Knee joints were examined using plain radiographs, and OA severity was assessed using the Kellgren and Lawrence (KL) grading system. Specimens were collected from 29 patients (31 knees) who underwent total knee replacement because of severe medial OA of the knee (KL grades 3 and 4), 16 patients who underwent knee arthroscopy (KL grade 2), and 5 patients with traumatic knees (KL grade 0). HIF-2α and VEGF expression was quantified by real-time polymerase chain reaction and western blotting.

RESULTS

Cartilage degeneration correlated with the radiographic severity grade. OA severity, determined using the Mankin scale, correlated positively with the KL grade (r=0.8790, p<0.01), and HIF-2α and VEGF levels with the radiographic severity of knee OA (r=0.7001, p<0.05; r=0.6647, p<0.05).

CONCLUSION

In OA cartilage, HIF-2α and VEGF mRNA and protein levels were significantly and positively correlated. The expression of both factors correlated positively with the KL grade. HIF-2α and VEGF, therefore, may serve as biochemical markers as well as potential therapeutic targets in knee OA.

Dickkopf-3 is upregulated in osteoarthritis and has a chondroprotective role

OBJECTIVE

Dickkopf-3 (Dkk3) is a non-canonical member of the Dkk family of Wnt antagonists and its upregulation has been reported in microarray analysis of cartilage from mouse models of osteoarthritis (OA). In this study we assessed Dkk3 expression in human OA cartilage to ascertain its potential role in chondrocyte signaling and cartilage maintenance.

METHODS

Dkk3 expression was analysed in human adult OA cartilage and synovial tissues and during chondrogenesis of ATDC5 and human mesenchymal stem cells. The role of Dkk3 in cartilage maintenance was analysed by incubation of bovine and human cartilage explants with interleukin-1β (IL1β) and oncostatin-M (OSM). Dkk3 gene expression was measured in cartilage following murine hip avulsion. Whether Dkk3 influenced Wnt, TGFβ and activin cell signaling was assessed in primary human chondrocytes and SW1353 chondrosarcoma cells using qRT-PCR and luminescence assays.

RESULTS

Increased gene and protein levels of Dkk3 were detected in human OA cartilage, synovial tissue and synovial fluid. DKK3 gene expression was decreased during chondrogenesis of both ATDC5 cells and humans MSCs. Dkk3 inhibited IL1β and OSM-mediated proteoglycan loss from human and bovine cartilage explants and collagen loss from bovine cartilage explants. Cartilage DKK3 expression was decreased following hip avulsion injury. TGFβ signaling was enhanced by Dkk3 whilst Wnt3a and activin signaling were inhibited.

CONCLUSIONS

We provide evidence that Dkk3 is upregulated in OA and may have a protective effect on cartilage integrity by preventing proteoglycan loss and helping to restore OA-relevant signaling pathway activity. Targeting Dkk3 may be a novel approach in the treatment of OA.

Interaction of HIF1α and β-catenin inhibits matrix metalloproteinase 13 expression and prevents cartilage damage in mice

Low oxygen tension (hypoxia) regulates chondrocyte differentiation and metabolism. Hypoxia-inducible factor 1α (HIF1α) is a crucial hypoxic factor for chondrocyte growth and survival during development. The major metalloproteinase matrix metalloproteinase 13 (MMP13) is also associated with chondrocyte hypertrophy in adult articular cartilage, the lack of which protects from cartilage degradation and osteoarthritis (OA) in mice. MMP13 is up-regulated by the Wnt/β-catenin signaling, a pathway involved in chondrocyte catabolism and OA. We studied the role of HIF1α in regulating Wnt signaling in cartilage and OA. We used mice with conditional knockout of Hif1α (∆Hif1α(chon)) with joint instability. Specific loss of HIF1α exacerbated MMP13 expression and cartilage destruction. Analysis of Wnt signaling in hypoxic chondrocytes showed that HIF1α lowered transcription factor 4 (TCF4)-β-catenin transcriptional activity and inhibited MMP13 expression. Indeed, HIF1α interacting with β-catenin displaced TCF4 from MMP13 regulatory sequences. Finally, ΔHif1α(chon) mice with OA that were injected intraarticularly with PKF118-310, an inhibitor of TCF4-β-catenin interaction, showed less cartilage degradation and reduced MMP13 expression in cartilage. Therefore, HIF1α-β-catenin interaction is a negative regulator of Wnt signaling and MMP13 transcription, thus reducing catabolism in OA. Our study contributes to the understanding of the role of HIF1α in OA and highlights the HIF1α-β-catenin interaction, thus providing new insights into the impact of hypoxia in articular cartilage.

Intra-articular resveratrol injection prevents osteoarthritis progression in a mouse model by activating SIRT1 and thereby silencing HIF-2α

We investigated the feasibility of the intra-articular injection of resveratrol for preventing the progression of existing cartilage degeneration in a mouse model of osteoarthritis (OA). The effects of resveratrol on the expression of silent information regulator 2 type 1 (SIRT1), hypoxia-inducible factor-2α (HIF-2α) and catabolic factors in OA cartilage was explored. OA was induced in the mouse knee via destabilization of the medial meniscus (DMM). Resveratrol was injected weekly into the operated knee beginning 4 weeks after surgery. The OA phenotype was evaluated via histological and immunohistochemical analyses at 8 weeks after DMM. Western blot analysis was performed to identify whether resveratrol modulated the interleukin (IL)-1β-induced expression of HIF-2α in human chondrocytes. Histologically, resveratrol treatment preserved the structural homeostasis of the articular cartilage and the subchondral bone. Following resveratrol injection, the expression of collagen type II was retained, but the expression of inducible nitric oxide synthase and matrix metalloproteinase-13 was reduced in OA cartilage. Moreover, the administration of resveratrol significantly induced the activation of SIRT1 and the inhibition of HIF-2α expression in mouse OA cartilage and in IL-1β-treated human chondrocytes. These findings indicate that the intra-articular injection of resveratrol significantly prevents the destruction of OA cartilage by activating SIRT1 and thereby suppressing the expression of HIF-2α and catabolic factors.

Chondrocyte hypertrophy in skeletal development, growth, and disease

Most of our bones form through the process of endochondral ossification, which is tightly regulated by the activity of the cartilage growth plate. Chondrocyte maturation through the various stages of growth plate physiology ultimately results in hypertrophy. Chondrocyte hypertrophy is an essential contributor to longitudinal bone growth, but recent data suggest that these cells also play fundamental roles in signaling to other skeletal cells, thus coordinating endochondral ossification. On the other hand, ectopic hypertrophy of articular chondrocytes has been implicated in the pathogenesis of osteoarthritis. Thus, a better understanding of the processes that control chondrocyte hypertrophy in the growth plate as well as in articular cartilage is required for improved management of both skeletal growth disorders and osteoarthritis. This review summarizes recent findings on the regulation of hypertrophic chondrocyte differentiation, the cellular mechanisms involved in hypertrophy, and the role of chondrocyte hypertrophy in skeletal physiology and pathophysiology.

Molecular actions of ovarian cancer G protein-coupled receptor 1 caused by extracellular acidification in bone

Extracellular acidification occurs under physiologic and pathologic conditions, such as exercise, ischemia, and inflammation. It has been shown that acidosis has various adverse effects on bone. In recent years there has been increasing evidence which indicates that ovarian cancer G protein-coupled receptor 1 (OGR1) is a pH-sensing receptor and mediates a variety of extracellular acidification-induced actions on bone cells and other cell types. Recent studies have shown that OGR1 is involved in the regulation of osteoclast differentiation, survival, and function, as well as osteoblast differentiation and bone formation. Moreover, OGR1 also regulates acid-induced apoptosis of endplate chondrocytes in intervertebral discs. These observations demonstrate the importance of OGR1 in skeletal development and metabolism. Here, we provide an overview of OGR1 regulation ofosteoclasts, osteoblasts, and chondrocytes, and the molecular actions of OGR1 induced by extracellular acidification in the maintenance of bone health.

HAF drives the switch of HIF-1α to HIF-2α by activating the NF-κB pathway, leading to malignant behavior of T24 bladder cancer cells

Hypoxia is a characteristic feature of solid tumors, leading to malignant behavior. During this process, HIF family members (HIFs) and the NF-κB pathway are activated. In addition, the hypoxia-associated factor (HAF) is reported to participate in the regulation of HIFs. However, the precise relationship among HIFs, HAF and the NF-κB pathway in bladder cancer (BC) remains unknown. In the current investigation, T24 BC cells were exposed to hypoxia, or by plasmid transfection to overexpress HAF or RelA (P65) to demonstrate their roles. The results indicate that hypoxia leads to the elevation of HAF plus activation of the NF-κB pathway, accompanied by the switch of HIF-1α to HIF-2α, resulting in the enhanced ability of malignancy in T24 cells. In order to further demonstrate the significance of this switch, HIF-1α and HIF-2α were co-transfected into T24 cells with HIF-β, respectively. The following results indicate that the T24^hif-2α/β cells show enhanced ability of malignancy, accompanied by the maintenance of stem-cell markers, but the T24^hif-1α/β cells show higher expression of metabolism-related genes. Boyden assays and wound-healing assays indicate the enhanced ability of malignancy for T24^hif-2α/β. Thus, we conclude that on the hypoxic microenvironment, the switching of HIF-1α to HIF-2α, which is driven by HAF through activating the NF-κB pathway, contributes to the malignancy of T24 cells, accompanied by the maintenance of stem-cell markers. This provides us an avenue for understanding the progression of bladder cancer.

An association study between hypoxia inducible factor-1alpha (HIF-1α) polymorphisms and osteonecrosis

Bone hypoxia resulting from impaired blood flow is the final pathway for the development of osteonecrosis (ON). The aim of this study was to evaluate if HIF-1α, the major transcription factor triggered by hypoxia, is genetically implicated in susceptibility to ON. For this we analyzed frequencies of three known HIF-1α polymorphisms: one in exon 2 (C111A) and two in exon 12 (C1772T and G1790A) and their association with ON in a Greek population. Genotype analysis was performed using PCR-RFLP and rare alleles were further confirmed with sequencing. We found that genotype and allele frequency of C1772T and G1790A SNP of HIF-1α (SNPs found in our cohort) were not significantly different in ON patients compared to control patients. Furthermore these SNPs could not be associated with the different subgroups of ON. At the protein level we observed that the corresponding mutations (P582S and A588T, respectively) are not significant for protein function since the activity, expression and localization of the mutant proteins is practically indistinguishable from wt in HEK293 and Saos-2 cells. These results suggest that these missense mutations in the HIF-1α gene are not important for the risk of developing ON.

iTRAQ-based proteomics reveals novel biomarkers of osteoarthritis

Objective:

We performed comprehensive proteomic analyses of articular cartilage by using the isobaric tags for relative and absolute quantitation (iTRAQ) method, and searched for candidate biomarkers for osteoarthritis (OA).

Methods:

Articular cartilage was collected from patients with OA or femoral neck fracture for the control group. Molecular variations were detected by the iTRAQ method, and quantitative analyses were performed by western blot.

Results:

Using the iTRAQ method, we identified 76 proteins with different expression levels in OA patients and the control group. Among these proteins, we selected LECT2 (leukocyte cell-derived chemotaxin-2), BAALC (brain and acute leukemia, cytoplasmic), and PRDX6 (peroxiredoxin-6), which had not been reported as biomarkers for OA.

Conclusions:

Use of these proteins in combination with conventional OA biomarkers may better reflect the grade and prognosis of OA.

Regulated transcription of human matrix metalloproteinase 13 (MMP13) and interleukin-1β (IL1B) genes in chondrocytes depends on methylation of specific proximal promoter CpG sites

The role of DNA methylation in the regulation of catabolic genes such as MMP13 and IL1B, which have sparse CpG islands, is poorly understood in the context of musculoskeletal diseases. We report that demethylation of specific CpG sites at -110 bp and -299 bp of the proximal MMP13 and IL1B promoters, respectively, detected by in situ methylation analysis of chondrocytes obtained directly from human cartilage, strongly correlated with higher levels of gene expression. The methylation status of these sites had a significant impact on promoter activities in chondrocytes, as revealed in transfection experiments with site-directed CpG mutants in a CpG-free luciferase reporter. Methylation of the -110 and -299 CpG sites, which reside within a hypoxia-inducible factor (HIF) consensus motif in the respective MMP13 and IL1B promoters, produced the most marked suppression of their transcriptional activities. Methylation of the -110 bp CpG site in the MMP13 promoter inhibited its HIF-2α-driven transactivation and decreased HIF-2α binding to the MMP13 proximal promoter in chromatin immunoprecipitation assays. In contrast to HIF-2α, MMP13 transcriptional regulation by other positive (RUNX2, AP-1, ELF3) and negative (Sp1, GATA1, and USF1) factors was not affected by methylation status. However, unlike the MMP13 promoter, IL1B was not susceptible to HIF-2α transactivation, indicating that the -299 CpG site in the IL1B promoter must interact with other transcription factors to modulate IL1B transcriptional activity. Taken together, our data reveal that the methylation of different CpG sites in the proximal promoters of the human MMP13 and IL1B genes modulates their transcription by distinct mechanisms.

Macro view of microRNA function in osteoarthritis

Osteoarthritis (OA), the most common musculoskeletal disorder, is complex, multifaceted, and characterized by degradation of articular cartilage and alterations in other joint tissues. Although some pathogenic pathways have been characterized, current knowledge is incomplete and effective approaches to the prevention or treatment of OA are lacking. Understanding novel molecular mechanisms that are involved in the maintenance and destruction of articular cartilage, including extracellular regulators and intracellular signalling mechanisms in joint cells that control cartilage homeostasis, has the potential to identify new therapeutic targets in OA. MicroRNAs control tissue development and homeostasis by fine-tuning gene expression, with expression patterns specific to tissues and developmental stages, and are increasingly implicated in the pathogenesis of complex diseases such as cancer and cardiovascular disorders. The emergent roles of microRNAs in cartilage homeostasis and OA pathogenesis are summarized in this Review, alongside potential clinical applications.

Osteoarthritis year 2011 in review: biology

This review is focused on advances in understanding the biology of joint homeostasis and osteoarthritis (OA) pathogenesis mechanisms that have led to proof of concept studies on new therapeutic approaches. The three selected topics include angiogenesis in joint tissues, biomechanics and joint lubrication and mitochondrial dysfunction. This new information represents progress in the integration of mechanisms that control multiple aspects of OA pathophysiology.

Osteoarthritis of the knee and hip. Part I: aetiology and pathogenesis as a basis for pharmacotherapy

OBJECTIVES

Osteoarthritis (OA) of the knee and hip is among the most frequent and debilitating arthritic conditions. Aside from surgical intervention in severe cases, conventional treatment involves relieving painful symptoms with non-steroidal anti-inflammatory drugs (NSAIDs), narcotic and non-narcotic (weak) analgesics and physical therapy. To obtain insight into the extent of pathological changes in hip and knee OA we reviewed current literature on the pathogenesis of this state as a basis for current pharmacotherapy options.

KEY FINDINGS

Key features of the pathological joint changes in OA include: cartilage destruction by pro-inflammatory cytokines, matrix metalloproteinases and prostaglandins, which promote a catabolic environment; subchondral bone remodelling and resorption; hypertrophic differentiation of chondrocytes; neovascularisation of synovial tissue; and focal calcification of joint cartilage. Despite the central involvement of hyaline cartilage in OA pathogenesis, the source of pain likely stems from the richly innervated synovium, subchondral bone and periosteum components of the joint. Tissue damage during joint degeneration generates nociceptive stimuli. The presence of inflammatory mediators, including bradykinin, prostaglandins and leukotrienes, lowers the threshold of the Aδ and C pain fibres, resulting in a heightened response to painful stimuli.

SUMMARY

It is our opinion that it is important to base and centre the management of OA patients on the severity of patient-important outcomes, rather than purely an assessment of damage to the joint. The joint damage, as interpreted from radiographs, is not necessarily representative of the symptoms experienced. The management of OA primarily comprises pharmacological therapy, surgical interventions and various non-pharmacological interventions.