Human adult chondrocytes express hepatocyte growth factor (HGF) isoforms but not HgF: potential implication of osteoblasts on the presence of HGF in cartilage

Effects of genus Epimedium in the treatment of osteoarthritis and relevant signaling pathways

Osteoarthritis (OA) is a common chronic degenerative joint disease in clinical practice with a high prevalence, especially in the elderly. Traditional Chinese Medicine (TCM) believes that OA belongs to the category of "Bi syndrome" and the "bone Bi syndrome". The etiology and pathogenesis lie in the deficiency of the liver and kidney, the deficiency of Qi and blood, and external exposure to wind, cold, and dampness. Epimedium is a yang-reinforcing herb in TCM, which can tonify the liver and kidney, strengthen muscles and bones, dispel wind, cold and dampness, and can treat both the symptoms and the root cause of "bone Bi syndrome". In addition, Epimedium contains a large number of ingredients. Through modern science and technology, more than 270 compounds have been found in Epimedium, among which flavonoids are the main active ingredients. Therefore, our study will review the effects and mechanisms of genus Epimedium in treating OA from two aspects: (1) Introduction of Epimedium and its main active ingredients; (2) Effects of Epimedium and its active ingredients in treating OA and relevant signaling pathways, in order to provide more ideas for OA treatment.

Feasibility of ultrashort echo time (UTE) T2* cartilage mapping in the hip: a pilot study

BACKGROUND

Ultrashort echo time (UTE) T2* is sensitive to molecular changes within the deep calcified layer of cartilage. Feasibility of its use in the hip needs to be established to determine suitability for clinical use.

PURPOSE

To establish feasibility of UTE T2* cartilage mapping in the hip and determine if differences in regional values exist.

MATERIAL AND METHODS

MRI scans with UTE T2* cartilage maps were prospectively acquired on eight hips. Hip cartilage was segmented into whole and deep layers in anterosuperior, superior, and posterosuperior regions. Quantitative UTE T2* maps were analyzed (independent one-way ANOVA) and reliability was calculated (ICC).

RESULTS

UTE T2* mean values (anterosuperior, superior, posterosuperior): full femoral layer (19.55, 18.43, 16.84 ms) (P=0.004), full acetabular layer (19.37, 17.50, 16.73 ms) (P=0.013), deep femoral layer (18.68, 17.90, 15.74 ms) (P=0.010), and deep acetabular layer (17.81, 16.18, 15.31 ms) (P=0.007). Values were higher in anterosuperior compared to posterosuperior regions (mean difference; 95% confidence interval [CI]): full femur layer (2.71 ms; 95% CI 0.91-4.51: P=0.003), deep femur layer (2.94 ms; 95% CI 0.69-5.19; P=0.009), full acetabular layer (2.63 ms 95% CI 0.55-4.72; P=0.012), and deep acetabular layer (2.50 ms; 95% CI 0.69-4.30; P=0.006). Intra-reader (ICC 0.89-0.99) and inter-reader reliability (ICC 0.63-0.96) were good to excellent for the majority of cartilage layers.

CONCLUSION

UTE T2* cartilage mapping was feasible in the hip with mean values in the range of 16.84-19.55 ms in the femur and 16.73-19.37 ms in the acetabulum. Significantly higher values were present in the anterosuperior region compared to the posterosuperior region.

The Potential Role of Hepatocyte Growth Factor in Degenerative Disorders of the Synovial Joint and Spine

This paper aims to provide a comprehensive review of the changing role of hepatocyte growth factor (HGF) signaling in the healthy and diseased synovial joint and spine. HGF is a multifunctional growth factor that, like its specific receptor c-Met, is widely expressed in several bone and joint tissues. HGF has profound effects on cell survival and proliferation, matrix metabolism, inflammatory response, and neurotrophic action. HGF plays an important role in normal bone and cartilage turnover. Changes in HGF/c-Met have also been linked to pathophysiological changes in degenerative joint diseases, such as osteoarthritis (OA) and intervertebral disc degeneration (IDD). A therapeutic role of HGF has been proposed in the regeneration of osteoarticular tissues. HGF also influences bone remodeling and peripheral nerve activity. Studies aimed at elucidating the changing role of HGF/c-Met signaling in OA and IDD at different pathophysiological stages, and their specific molecular mechanisms are needed. Such studies will contribute to safe and effective HGF/c-Met signaling-based treatments for OA and IDD.

Rapidly Destructive Coxarthrosis as a Potential Side Effect of Crizotinib in a Patient with ROS1-Positive Lung Adenocarcinoma

A 75-year-old woman was diagnosed with c-ros oncogene 1 (ROS1)-positive lung adenocarcinoma. She was treated with crizotinib 750 mg/day for 4.5 years, with partial tumor response. However, the patient subsequently presented with right hip pain and difficulty in walking. She underwent magnetic resonance imaging (MRI), which detected T2 prolongation in the right femoral bone head, synovial fluid retention, and bone joint fissure narrowing. The patient was diagnosed with rapidly destructive coxarthrosis (RDC) and received a total hip arthroplasty. This represents a rare case of RDC as a potential side effect of crizotinib in a patient with ROS1-positive lung adenocarcinoma. MRI should therefore be recommended in patients receiving crizotinib who experience continuing severe hip pain and difficulty in walking. Further investigations are warranted to elucidate the pathogenesis of RDC associated with crizotinib treatment.

Hoffa's Fat Pad Abnormality in the Development of Knee Osteoarthritis

Over the past two decades, many hypotheses have been put forward to explain the cause of knee osteoarthritis. Scientific reports bring up the role of adipose tissue in the activation of the inflammatory mechanisms, which is a characteristic feature of osteoarthritis natural history. Adipose tissue produces and releases cytokines, interleukins, and growth factors by means of paracrine, endocrine, and autocrine mechanisms. Hoffa's fat pad (infrapatellar adipose tissue) plays a viable role in the initiation and progression of osteoarthritis due to its role in the activation and release of pro-inflammatory mediators. The degenerative joint disease is considered an inflammatory process. Therefore, in this article we overview the importance of Hoffa's fat pad in the development and progression of osteoarthritis.

Overexpression of TIMP-3 in Chondrocytes Produces Transient Reduction in Growth Plate Length but Permanently Reduces Adult Bone Quality and Quantity

Bone development and length relies on the growth plate formation, which is dependent on degradative enzymes such as MMPs. Indeed, deletion of specific members of this enzyme family in mice results in important joint and bone abnormalities, suggesting a role in skeletal development. As such, the control of MMP activity is vital in the complex process of bone formation and growth. We generated a transgenic mouse line to overexpress TIMP3 in mouse chondrocytes using the Col2a1-chondrocyte promoter. This overexpression in cartilage resulted in a transient shortening of growth plate in homozygote mice but bone length was restored at eight weeks of age. However, tibial bone structure and mechanical properties remained compromised. Despite no transgene expression in adult osteoblasts from transgenic mice in vitro, their differentiation capacity was decreased. Neonates, however, did show transgene expression in a subset of bone cells. Our data demonstrate for the first time that transgene function persists in the chondro-osseous lineage continuum and exert influence upon bone quantity and quality.

Basic science of osteoarthritis

Osteoarthritis (OA) is a prevalent, disabling disorder of the joints that affects a large population worldwide and for which there is no definitive cure. This review provides critical insights into the basic knowledge on OA that may lead to innovative end efficient new therapeutic regimens. While degradation of the articular cartilage is the hallmark of OA, with altered interactions between chondrocytes and compounds of the extracellular matrix, the subchondral bone has been also described as a key component of the disease, involving specific pathomechanisms controlling its initiation and progression. The identification of such events (and thus of possible targets for therapy) has been made possible by the availability of a number of animal models that aim at reproducing the human pathology, in particular large models of high tibial osteotomy (HTO). From a therapeutic point of view, mesenchymal stem cells (MSCs) represent a promising option for the treatment of OA and may be used concomitantly with functional substitutes integrating scaffolds and drugs/growth factors in tissue engineering setups. Altogether, these advances in the fundamental and experimental knowledge on OA may allow for the generation of improved, adapted therapeutic regimens to treat human OA.

Introduction to Cartilage

Cartilage is a supporting connective tissue that, together with the bone, forms the framework supporting the body as a whole. There are many distinct types of cartilage, which exhibit numerous similarities as well as differences. Among them, articular cartilage is the best known and the most studied type. Articular cartilage is the thin layer of connective tissue that covers the articulating ends of bones in synovial (diarthrodial) joints. It provides a smooth surface for joint movement and acts as a load-bearing medium that protects the bone and distributes stress. The intense interest in articular cartilage is motivated by the critical role its degradation plays in arthritis and related joint diseases, which are the number one cause of disability in humans. This chapter discusses the physical, chemical and cellular properties of cartilage that give the tissue its extraordinary load-bearing characteristics.

Characterization of osteoarthritic human knees indicates potential sex differences

BACKGROUND

The prevalence of osteoarthritis is higher in women than in men in every age group, and overall prevalence increases with advancing age. Sex-specific differences in the properties of osteoarthritic joint tissues may permit the development of sex-specific therapies. Sex hormones regulate cartilage and bone development and homeostasis in a sex-dependent manner. Recent in vitro studies show that the vitamin D3 metabolite 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] also has sex-specific effects on musculoskeletal cells, suggesting that vitamin D3 metabolites may play a role in osteoarthritis-related sex-specific differences. The purpose of this study was to determine if sex-specific differences exist in synovial fluid and knee tissues isolated from male and female patients with severe knee osteoarthritis. We determined the presence of vitamin D3 metabolites, inflammatory cytokines, growth factors, and matrix metalloproteinases (MMPs) in synovial fluid and assessed responses of articular chondrocytes and subchondral osteoblasts to 17β-estradiol, dihydrotestosterone, and 1α,25(OH)2D3.

METHODS

Samples from knee joints of 10 Caucasian male and 10 Caucasian female patients with advanced osteoarthritis aged 65 to 75 years were obtained from total knee arthroplasty. Vitamin D metabolites, cytokines, MMPs, and growth factors in the synovial fluid were measured. Primary cultures of chondrocytes were isolated from fibrillated articular cartilage adjacent to osteoarthritis lesions and minimally affected cartilage distal to the lesion. Osteoblasts were isolated from the subchondral bone. Expression of receptors for 17β-estradiol and 1α,25(OH)2D3 was assessed by real-time PCR. Chondrocytes and osteoblasts were treated with 10(-8) M 17β-estradiol, dihydrotestosterone, or 1α,25(OH)2D3 and effects on gene expression and protein synthesis determined.

RESULTS

Histology of the articular cartilage confirmed advanced osteoarthritis. Sex differences were found in synovial fluid levels of vitamin D metabolites, cytokines, and metalloproteinases as well as in the cellular expression of receptors for 17β-estradiol and 1α,25(OH)2D3. Male cells were more responsive to 1α,25(OH)2D3 and dihydrotestosterone, whereas 17β-estradiol-affected female cells.

CONCLUSIONS

These results demonstrate that there are underlying sex differences in knee tissues affected by osteoarthritis. Our findings do not address osteoarthritis etiology but have implications for different prevention methods and treatments for men and women. Further research is needed to better understand these sex-based differences.

A new strategy to tackle severe knee osteoarthritis: Combination of intra-articular and intraosseous injections of Platelet Rich Plasma

INTRODUCTION

Knee osteoarthritis (KOA) is a mechanically induced, cytokine and enzyme-mediated disorder involving all the joint tissue of the knee. Rebuilding a physiological-homeostatic network at the tissue level following knee organ failure, such as in severe KOA, is a daunting task with therapeutic targets encompassing the articular cartilage, synovium and subchondral bone. Intraarticular infiltration of plasma rich in growth factors (PRP) has emerged as a promising symptomatic approach, although it is insufficient to reach the subchondral bone.

AREAS COVERED

This review addresses current molecular and cellular data in joint homeostasis and osteoarthritis pathophysiology. In particular, it focuses on changes that subchondral bone undergoes in knee osteoarthritis and evaluates recent observations on the crosstalk among articular cartilage, subchondral bone and synovial membrane. In addition, we review some mechanistic aspects that have been proposed and provide the rationale for using PRP intraosseously in KOA.

EXPERT OPINION

The knee joint is a paradigm of autonomy and connectedness of its anatomical structures and tissues from which it is made. We propose an innovative approach to the treatment of severe knee osteoarthritis consisting of a combination of intraarticular and intraosseous infiltrations of PRP, which might offer a new therapeutic tool in KOA therapy.

Carnosol Inhibits Pro-Inflammatory and Catabolic Mediators of Cartilage Breakdown in Human Osteoarthritic Chondrocytes and Mediates Cross-Talk between Subchondral Bone Osteoblasts and Chondrocytes

Aim

The aim of this work was to evaluate the effects of carnosol, a rosemary polyphenol, on pro-inflammatory and catabolic mediators of cartilage breakdown in chondrocytes and via bone-cartilage crosstalk.

Materials and Methods

Osteoarthritic (OA) human chondrocytes were cultured in alginate beads for 4 days in presence or absence of carnosol (6 nM to 9 μM). The production of aggrecan, matrix metalloproteinase (MMP)-3, tissue inhibitor of metalloproteinase (TIMP)-1, interleukin (IL)-6 and nitric oxide (NO) and the expression of type II collagen and ADAMTS-4 and -5 were analyzed. Human osteoblasts from sclerotic (SC) or non-sclerotic (NSC) subchondral bone were cultured for 3 days in presence or absence of carnosol before co-culture with chondrocytes. Chondrocyte gene expression was analyzed after 4 days of co-culture.

Results

In chondrocytes, type II collagen expression was significantly enhanced in the presence of 3 μM carnosol (p = 0.008). MMP-3, IL-6, NO production and ADAMTS-4 expression were down-regulated in a concentration-dependent manner by carnosol (p<0.01). TIMP-1 production was slightly increased at 3 μM (p = 0.02) and ADAMTS-5 expression was decreased from 0.2 to 9 μM carnosol (p<0.05). IL-6 and PGE₂ production was reduced in the presence of carnosol in both SC and NSC osteoblasts while alkaline phosphatase activity was not changed. In co-culture experiments preincubation of NSC and SC osteoblasts wih carnosol resulted in similar effects to incubation with anti-IL-6 antibody, namely a significant increase in aggrecan and decrease in MMP-3, ADAMTS-4 and -5 gene expression by chondrocytes.

Conclusions

Carnosol showed potent inhibition of pro-inflammatory and catabolic mediators of cartilage breakdown in chondrocytes. Inhibition of matrix degradation and enhancement of formation was observed in chondrocytes cocultured with subchondral osteoblasts preincubated with carnosol indicating a cross-talk between these two cellular compartments, potentially mediated via inhibition of IL-6 in osteoblasts as similar results were obtained with anti-IL-6 antibody.

The pro-inflammatory cytokine 14-3-3ε is a ligand of CD13 in cartilage

Osteoarthritis is a whole-joint disease characterized by the progressive destruction of articular cartilage involving abnormal communication between subchondral bone and cartilage. Our team previously identified 14-3-3ε protein as a subchondral bone soluble mediator altering cartilage homeostasis. The aim of this study was to investigate the involvement of CD13 (also known as aminopeptidase N, APN) in the chondrocyte response to 14-3-3ε. After identifying CD13 in chondrocytes, we knocked down CD13 with small interfering RNA (siRNA) and blocking antibodies in articular chondrocytes. 14-3-3ε-induced MMP-3 and MMP-13 was significantly reduced with CD13 knockdown, which suggests that it has a crucial role in 14-3-3ε signal transduction. Aminopeptidase N activity was identified in chondrocytes, but the activity was unchanged after stimulation with 14-3-3ε. Direct interaction between CD13 and 14-3-3ε was then demonstrated by surface plasmon resonance. Using labeled 14-3-3ε, we also found that 14-3-3ε binds to the surface of chondrocytes in a manner that is dependent on CD13. Taken together, these results suggest that 14-3-3ε might directly bind to CD13, which transmits its signal in chondrocytes to induce a catabolic phenotype similar to that observed in osteoarthritis. The 14-3-3ε-CD13 interaction could be a new therapeutic target in osteoarthritis.

Elevated hepatocyte growth factor levels in osteoarthritis osteoblasts contribute to their altered response to bone morphogenetic protein-2 and reduced mineralization capacity

PURPOSE

Clinical and in vitro studies suggest that subchondral bone sclerosis due to abnormal osteoblasts is involved in the progression of osteoarthritis (OA). Human osteoblasts isolated from sclerotic subchondral OA bone tissue show an altered phenotype, a decreased canonical Wnt/ß-catenin pathway, and a reduced mineralization in vitro as well as in vivo. These alterations were linked with an abnormal response to BMP-2. OA osteoblasts release factors such as the hepatocyte growth factor (HGF) that contribute to cartilage loss whereas chondrocytes do not express HGF. HGF can stimulate BMP-2 expression in human osteoblasts, however, the role of HGF and its effect in OA osteoblasts remains unknown. Here we investigated whether elevated endogenous HGF levels in OA osteoblasts are responsible for their altered response to BMP-2.

METHODS

We prepared primary human subchondral osteoblasts using the sclerotic medial portion of the tibial plateaus of OA patients undergoing total knee arthroplasty, or from tibial plateaus of normal individuals obtained at autopsy. The expression of HGF was evaluated by qRT-PCR and the protein production by western blot analysis. HGF expression was reduced with siRNA technique whereas its activity was inhibited using the selective inhibitor PHA665752. Alkaline phosphatase activity (ALPase) and osteocalcin release were measured by substrate hydrolysis and EIA respectively. Canonical Wnt/β-catenin signaling (cWnt) was evaluated both by target gene expression using the TOPflash TCF/lef luciferase reporter assay and western blot analysis of β-catenin levels in response to Wnt3a stimulation. Mineralization in response to BMP-2 was evaluated by alizarin red staining.

RESULTS

The expression of HGF was increased in OA osteoblasts compared to normal osteoblasts and was maintained during their in vitro differentiation. OA osteoblasts released more HGF than normal osteoblasts as assessed by western blot analysis. HGF stimulated the expression of TGF-β1. BMP-2 dose-dependently (1 to 100 ng/ml) stimulated both ALPase and osteocalcin in normal osteoblasts whereas, it inhibited them in OA osteoblasts. HGF-siRNA treatments reversed this response in OA osteoblasts and restored the BMP-2 response. cWnt is reduced in OA osteoblasts compared to normal, and HGF-siRNA treatments increased cWnt in OA osteoblasts almost to normal. Smad1/5/8 phosphorylation in response to BMP-2, which is reduced in OA osteoblasts, was corrected when these cells were treated with PHA665752. The BMP-2-dependent mineralization of OA osteoblasts, which is also reduced compared to normal, was only partially restored by PHA665752 treatment whereas 28 days treatment with HGF reduced the mineralization of normal osteoblasts.

CONCLUSION

OA osteoblasts expressed more HGF than normal osteoblasts. Increased endogenous HGF production in OA osteoblasts stimulated the expression of TGF-β1 and reduced their response to BMP-2. Inhibiting HGF expression or HGF signaling restored the response to BMP-2 and Smad1/5/8 signaling. In addition, decreased HGF signaling partly corrects the abnormal mineralization of OA osteoblasts while increased HGF prevents the normal mineralization of normal osteoblasts. In summary, we hypothesize that sustained elevated HGF levels in OA osteoblasts drive their abnormal phenotype and is implicated in OA pathophysiology.

Hepatocyte Growth Factor Isoforms in Tissue Repair, Cancer, and Fibrotic Remodeling

Hepatocyte growth factor (HGF), also known as scatter factor (SF), is a pleotropic factor required for normal organ development during embryogenesis. In the adult, basal expression of HGF maintains tissue homeostasis and is up-regulated in response to tissue injury. HGF expression is necessary for the proliferation, migration, and survival of epithelial and endothelial cells involved in tissue repair in a variety of organs, including heart, lung, kidney, liver, brain, and skin. The administration of full length HGF, either as a protein or using exogenous expression methodologies, increases tissue repair in animal models of tissue injury and increases angiogenesis. Full length HGF is comprised of an N-terminal hairpin turn, four kringle domains, and a serine protease-like domain. Several naturally occurring alternatively spliced isoforms of HGF were also identified. The NK1 variant contains the N-terminal hairpin and the first kringle domain, and the NK2 variant extends through the second kringle domain. These alternatively spliced forms of HGF activate the same receptor, MET, but they differ from the full length protein in their cellular activities and their biological functions. Here, we review the species-specific expression of the HGF isoforms, their regulation, the signal transduction pathways they activate, and their biological activities.

Hypoxia and vitamin D differently contribute to leptin and dickkopf-related protein 2 production in human osteoarthritic subchondral bone osteoblasts

Introduction

Bone remodelling and increased subchondral densification are important in osteoarthritis (OA). Modifications of bone vascularization parameters, which lead to ischemic episodes associated with hypoxic conditions, have been suspected in OA. Among several factors potentially involved, leptin and dickkopf-related protein 2 (DKK2) are good candidates because they are upregulated in OA osteoblasts (Obs). Therefore, in the present study, we investigated the hypothesis that hypoxia may drive the expression of leptin and DKK2 in OA Obs.

Methods

Obs from the sclerotic portion of OA tibial plateaus were cultured under either 20% or 2% oxygen tension in the presence or not of 50 nM 1,25-dihydroxyvitamin D₃ (VitD₃). The expression of leptin, osteocalcin, DKK2, hypoxia-inducible factor 1α (Hif-1α) and Hif-2α was measured by real-time polymerase chain reaction and leptin production was measured by enzyme-linked immunosorbent assay (ELISA). The expression of Hif-1α, Hif-2α, leptin and DKK2 was reduced using silencing RNAs (siRNAs). The signalling pathway of hypoxia-induced leptin was investigated by Western blot analysis and with mitogen-activated protein kinase (MAPK) inhibitors.

Results

The expression of leptin and DKK2 in Obs was stimulated 7-fold and 1.8-fold, respectively (P <0.05) under hypoxia. Interestingly, whereas VitD₃ stimulated leptin and DKK2 expression 2- and 4.2-fold, respectively, under normoxia, it stimulated their expression by 28- and 6.2-fold, respectively, under hypoxia (P <0.05). The hypoxia-induced leptin production was confirmed by ELISA, particularly in the presence of VitD₃ (P <0.02). Compared to Obs incubated in the presence of scramble siRNAs, siHif-2α inhibited VitD₃-stimulated leptin mRNA and protein levels by 70% (P =0.004) and 60% (P <0.02), respectively, whereas it failed to significantly alter the expression of DKK2. siHif-1α has no effect on these genes. Immunoblot analysis showed that VitD₃ greatly stabilized Hif-2α under hypoxic conditions. The increase in leptin expression under hypoxia was also regulated, by p38 MAPK (P <0.03) and phosphoinositide 3-kinase (P <0.05). We found that the expression of leptin and DKK2 were not related to each other under hypoxia.

Conclusions

Hypoxic conditions via Hif-2 regulation trigger Obs to produce leptin, particularly under VitD₃ stimulation, whereas DKK2 is regulated mainly by VitD₃ rather than hypoxia.

Bone-cartilage interface crosstalk in osteoarthritis: potential pathways and future therapeutic strategies

Currently, osteoarthritis (OA) is considered a disease of the entire joint, which is not simply a process of wear and tear but rather abnormal remodelling and joint failure of an organ. The bone-cartilage interface is therefore a functioning synergistic unit, with a close physical association between subchondral bone and cartilage suggesting the existence of biochemical and molecular crosstalk across the OA interface. The crosstalk at the bone-cartilage interface may be elevated in OA in vivo and in vitro. Increased vascularisation and formation of microcracks associated with abnormal bone remodelling in joints during OA facilitate molecular transport from cartilage to bone and vice versa. Recent reports suggest that several critical signalling pathways and biological factors are key regulators and activate cellular and molecular processes in crosstalk among joint compartments. Therapeutic interventions including angiogenesis inhibitors, agonists/antagonists of molecules and drugs targeting bone remodelling are potential candidates for this interaction. This review summarised the premise for the presence of crosstalk in bone-cartilage interface as well as the current knowledge of the major signalling pathways and molecular interactions that regulate OA progression. A better understanding of crosstalk in bone-cartilage interface may lead to development of more effective strategies for treating OA patients.

Three-dimensional osteogenic and chondrogenic systems to model osteochondral physiology and degenerative joint diseases

Tissue engineered constructs have the potential to function as in vitro pre-clinical models of normal tissue function and disease pathogenesis for drug screening and toxicity assessment. Effective high throughput assays demand minimal systems with clearly defined performance parameters. These systems must accurately model the structure and function of the human organs and their physiological response to different stimuli. Musculoskeletal tissues present unique challenges in this respect, as they are load-bearing, matrix-rich tissues whose functionality is intimately connected to the extracellular matrix and its organization. Of particular clinical importance is the osteochondral junction, the target tissue affected in degenerative joint diseases, such as osteoarthritis (OA), which consists of hyaline articular cartilage in close interaction with subchondral bone. In this review, we present an overview of currently available in vitro three-dimensional systems for bone and cartilage tissue engineering that mimic native physiology, and the utility and limitations of these systems. Specifically, we address the need to combine bone, cartilage and other tissues to form an interactive microphysiological system (MPS) to fully capture the biological complexity and mechanical functions of the osteochondral junction of the articular joint. The potential applications of three-dimensional MPSs for musculoskeletal biology and medicine are highlighted.

Homeostatic mechanisms in articular cartilage and role of inflammation in osteoarthritis

Osteoarthritis (OA) is a whole joint disease, in which thinning and disappearance of cartilage is a critical determinant in OA progression. The rupture of cartilage homeostasis whatever its cause (aging, genetic predisposition, trauma or metabolic disorder) induces profound phenotypic modifications of chondrocytes, which then promote the synthesis of a subset of factors that induce cartilage damage and target other joint tissues. Interestingly, among these factors are numerous components of the inflammatory pathways. Chondrocytes produce cytokines, chemokines, alarmins, prostanoids, and adipokines and express numerous cell surface receptors for cytokines and chemokines, as well as Toll-like receptors. These receptors activate intracellular signaling pathways involved in inflammatory and stress responses of chondrocytes in OA joints. This review focuses on mechanisms responsible for the maintenance of cartilage homeostasis and highlights the role of inflammatory processes in OA progression.

Interplay between cartilage and subchondral bone contributing to pathogenesis of osteoarthritis

Osteoarthritis (OA) is a common debilitating joint disorder, affecting large sections of the population with significant disability and impaired quality of life. During OA, functional units of joints comprising cartilage and subchondral bone undergo uncontrolled catabolic and anabolic remodeling processes to adapt to local biochemical and biological signals. Changes in cartilage and subchondral bone are not merely secondary manifestations of OA but are active components of the disease, contributing to its severity. Increased vascularization and formation of microcracks in joints during OA have suggested the facilitation of molecules from cartilage to bone and vice versa. Observations from recent studies support the view that both cartilage and subchondral bone can communicate with each other through regulation of signaling pathways for joint homeostasis under pathological conditions. In this review we have tried to summarize the current knowledge on the major signaling pathways that could control the cartilage-bone biochemical unit in joints and participate in intercellular communication between cartilage and subchondral bone during the process of OA. An understanding of molecular communication that regulates the functional behavior of chondrocytes and osteoblasts in both physiological and pathological conditions may lead to development of more effective strategies for treating OA patients.

Adipose mesenchymal stem cells protect chondrocytes from degeneration associated with osteoarthritis

Our work aimed at evaluating the role of adipose stem cells (ASC) on chondrocytes from osteoarthritic (OA) patients and identifying the mediators involved. We used primary chondrocytes, ASCs from different sources and bone marrow mesenchymal stromal cells (MSC) from OA donors. ASCs or MSCs were co-cultured with chondrocytes in a minimal medium and using cell culture inserts. Under these conditions, ASCs did not affect the proliferation of chondrocytes but significantly decreased camptothecin-induced apoptosis. Both MSCs and ASCs from different sources allowed chondrocytes in the cocultures maintaining a stable expression of markers specific for a mature phenotype, while expression of hypertrophic and fibrotic markers was decreased. A number of factors known to regulate the chondrocyte phenotype (IL-1β, IL-1RA, TNF-α) and matrix remodeling (TIMP-1 and -2, MMP-1 and -9, TSP-1) were not affected. However, a significant decrease of TGF-β1 secretion by chondrocytes and induction of HGF secretion by ASCs was observed. Addition of a neutralizing anti-HGF antibody reversed the anti-fibrotic effect of ASCs whereas hypertrophic markers were not modulated. In summary, ASCs are an interesting source of stem cells for efficiently reducing hypertrophy and dedifferentiation of chondrocytes, at least partly via the secretion of HGF. This supports the interest of using these cells in therapies for osteo-articular diseases.

Why subchondral bone in osteoarthritis? The importance of the cartilage bone interface in osteoarthritis

Osteoarthritis is a whole joint disease characterised by the disappearance of the cartilage associated with subchondral bone sclerosis, formation of osteophytes and a mild inflammation of the synovial membrane. Although all these events have been independently studied, functional interactions between these different joint tissues should exist, especially between subchondral bone and cartilage. Moreover, recent studies show that cartilage and subchondral bone act as a single functional unit. This review highlights this novel concept.

Role of bone architecture and anatomy in osteoarthritis

When considering the pathogenesis of osteoarthritis (OA), it is important to review the contribution of bone in addition to the contribution of cartilage and synovium. Although bone clearly plays a role in determining the distribution of biomechanical forces across joints, which in turn plays a role in the initiation of OA, it has also more recently been appreciated that bone may contribute in a biological sense to the pathogenesis of OA. Far from being a static structure, bone is a dynamic tissue undergoing constant remodeling, and it is clear from a number of radiographic and biochemical studies that bone and cartilage degradation occurs hand in hand. Whether the initial instigating event in OA occurs in cartilage or bone is not known, but it is clear that bony changes occur very early in the pathogenesis of OA and often predate radiographic appearance of the disease. This review focuses on the structural variants of both hip and knee that have been associated with OA and the ultrastructural bone changes in these sites occurring in early OA pathogenesis. This article is part of a Special Issue entitled "Osteoarthritis".

Osteochondral alterations in osteoarthritis

Osteoarthritis (OA) is a major cause of pain and disability in the aging population, but its pathogenesis remains incompletely understood. Alterations beneath the articular cartilage at the osteochondral junction are attracting interest as possible mediators of pain and structural progression in OA. Osteochondral changes occur early during the development of OA and may aggravate pathology elsewhere in the joint. Loss of osteochondral integrity removes the barrier between intra-articular and subchondral compartments, exposing subchondral bone and its nerves to abnormal chemical and biomechanical influence. Osteochondral plasticity results in a merging of tissue compartments across the junction. Loss of the clearly differentiated demarcation between bone and articular cartilage is associated with invasion of articular cartilage by blood vessels and sensory nerves, and advancing endochondral ossification. Increased subchondral bone turnover is intimately associated with these alterations at the osteochondral junction. Cells signal across the osteochondral junction, and this cross-talk may be both a consequence of, and contribute to these pathological changes. Bone turnover, angiogenesis and nerve growth are also features of other diseases such as osteoporosis and cancers, for which therapeutic interventions are already advanced in their development. Here we review pathological changes at the osteochondral junction and explore their potential therapeutic implications for OA. This article is part of a Special Issue entitled "Osteoarthritis".

Influence of hepatocyte growth factor on autologous osteochondral transplants in an animal model

PURPOSE

Several studies have investigated the influence of different growth factors on hyaline cartilage regeneration. In a rabbit model, hepatocyte growth factor (HGF) was proven to increase the amount of hyaline-like chondrocytes in a mixed fibro-cartilaginous regenerate of small defects. The aim of the current study was to evaluate whether intra-articular administration of HGF influences the ingrowth of osteochondral grafts in a sheep model.

TYPE OF STUDY

Animal experiment.

METHODS

Both knee joints of eight sheep were opened surgically and osteochondral grafts were harvested and simultaneously transplanted to the opposite condyle of the same joint. The sheep were divided into two groups of four sheep, resulting in 16 grafts per group. In one group, HGF was administered by bilateral intra-articular injections given three times a week for 4 weeks. The control group received isotonic sodium chloride injections. The animals were killed after 3 months.

RESULTS

Histological evaluation showed a complete ingrowth of the osseous part of the osteochondral grafts. A healing or ingrowth at the level of the cartilage could not be observed. Histological evaluation of the transplanted grafts according to the modified Mankin score revealed less degeneration in the cartilage of the HGF group, as compared to the control group. In the HGF group, less cloning of chondrocytes and less irregularities of the articular surface were observed. Importantly, no deleterious effects, such as osteophyte formation, cartilage thickening or synovial proliferation, were found.

CONCLUSION

HGF positively influenced the cellularity of the transplanted osteochondral graft, but could not diminish the fissures in the marginal zone of the grafts.

CLINICAL RELEVANCE

Marginal zone fissures and degeneration in the absence of HGF may undermine long-term results of autologous osteochondral grafts.

Estrogen receptor-related receptor α regulation by interleukin-1β in prostaglandin E(2)- and cAMP-dependent pathways in osteoarthritic chondrocytes

OBJECTIVE

We reported previously that the orphan nuclear receptor, estrogen receptor-related receptor α (ERRα), is expressed in articular chondrocytes and is dysregulated in a mouse model of inflammatory arthritis. The aim of this study, therefore, was to determine whether ERRα is also dysregulated in patients with osteoarthritis (OA).

METHODS

ERRα messenger RNA (mRNA) and protein were quantified in normal and OA cartilage samples and in OA chondrocytes in vitro, with and without short-term treatment with a variety of OA-associated factors and signaling pathway agonists and inhibitors.

RESULTS

ERRα expression was lower in OA than in normal articular cartilage. Interleukin-1β (IL-1β) markedly up-regulated ERRα expression in OA chondrocytes in vitro, and agonist or inhibitor treatment indicated that the up-regulation was dependent on cyclooxygenase 2 (COX-2; NS398), prostaglandin E(2), cAMP (8-bromo-cAMP), and protein kinase A (PKA; KT5720). Treatment with the ERRα inverse agonist XCT790 decreased the expression of SOX9 and the up-regulation of ERRα by IL-1β, suggesting autoregulation of ERRα in the IL-1β pathway. Matrix metalloproteinase 13 (MMP-13) expression was also decreased by treatment with XCT790 plus IL-1β versus IL-1β alone, and the down-regulation of MMP-13 mRNA and protein observed with XCT790 alone suggests that the up-regulation of MMP-13 by IL-1β is ERRα-dependent.

CONCLUSION

We report the first evidence that ERRα expression is regulated by IL-1β in COX-2-, cAMP-, and PKA-dependent pathways in OA chondrocytes. We confirmed that SOX9 is an ERRα target gene in human, as in rodent, chondrocytes and identified MMP-13 as a potential new target gene, which suggests that ERRα may both respond to the healing signal and contribute to extracellular degradation in OA cartilage.

Osteochondral plate angiogenesis: a new treatment target in osteoarthritis

Healthy adult joint cartilage contains neither blood vessels nor nerves. Osteoarthritic cartilage, in contrast, may be invaded by blood vessels from the subchondral bone. The mechanisms underlying cartilage angiogenesis in osteoarthritis are unclear but may involve hypertrophic chondrocyte differentiation. Active research is under way to identify the factors involved in cartilage angiogenesis. Here, we discuss the pathophysiological mechanisms of osteoarthritic cartilage angiogenesis based on evidence from a systematic literature review of articles retrieved via PubMed and ISI Web of Knowledge. Our conclusions suggest new research perspectives and treatment options.

The infrapatellar fat pad should be considered as an active osteoarthritic joint tissue: a narrative review

INTRODUCTION

Osteoarthritis (OA) of the knee joint is caused by genetic and hormonal factors and by inflammation, in combination with biomechanical alterations. It is characterized by loss of articular cartilage, synovial inflammation and subchondral bone sclerosis. Considerable evidence indicates that the menisci, ligaments, periarticular muscles and the joint capsule are also involved in the OA process. This paper will outline the theoretical framework for investigating the infrapatellar fat pad (IPFP) as an additional joint tissue involved in the development and progression of knee-OA.

METHODS

A literature search was performed in Pubmed from 1948 until October 2009 with keywords InFrapatellar fat pad, Hoffa fat pad, intraarticular adipose tissue, knee, cartilage, bone, cytokine, adipokine, inflammation, growth factor, arthritis, and OA.

RESULTS

The IPFP is situated intracapsularly and extrasynovially in the knee joint. Besides adipocytes, the IPFP from patients with knee-OA contains macrophages, lymphocytes and granulocytes, which are able to contribute to the disease process of knee-OA. Furthermore, the IPFP contains nociceptive nerve fibers that could in part be responsible for anterior pain in knee-OA. These nerve fibers secrete substance P, which is able to induce inflammatory responses and cause vasodilation, which may lead to IPFP edema and extravasation of the immune cells. The IPFP secretes cytokines, interleukins, growth factors and adipokines that influence cartilage by upregulating the production of matrix metalloproteinases (MMPs), stimulating the expression of pro-inflammatory cytokines and inhibiting the production of cartilage matrix proteins. They may also stimulate the production of pro-inflammatory mediators, growth factors and MMPs in synovium.

CONCLUSION

These data are consistent with the hypothesis that the IPFP is an osteoarthritic joint tissue capable of modulating inflammatory and destructive responses in knee-OA.

Targeting subchondral bone for treating osteoarthritis: what is the evidence?

Over the past few decades, significant progress has been made with respect to new concepts about the pathogenesis of osteoarthritis (OA). This article summarises some of the knowledge we have today on the involvement of the subchondral bone in OA. It provides substantial evidence that changes in the metabolism of the subchondral bone are an integral part of the OA disease process and that these alterations are not merely secondary manifestations, but are part of a more active component of the disease. Thus, a strong rationale exists for therapeutic approaches that target subchondral bone resorption and/or formation, and data evaluating the drugs targeting bone remodelling raise the hope that new treatment options for OA may become available.

Local leptin production in osteoarthritis subchondral osteoblasts may be responsible for their abnormal phenotypic expression

INTRODUCTION

Leptin is a peptide hormone with a role in bone metabolism and rheumatic diseases. The subchondral bone tissue plays a prominent role in the pathophysiology of osteoarthritis (OA), related to abnormal osteoblast (Ob) differentiation. Although leptin promotes the differentiation of Ob under normal conditions, a role for leptin in OA Ob has not been demonstrated. Here we determined if endogenous leptin produced by OA Ob could be responsible for the expression of the abnormal phenotypic biomarkers observed in OA Ob.

METHODS

We prepared primary normal and OA Ob from subchondral bone of tibial plateaus removed for knee surgery of OA patients or at autopsy. We determined the production of leptin and of the long, biologically active, leptin receptors (OB-Rb) using reverse transcriptase-polymerase chain reaction, ELISA and Western blot analysis. We determined the effect of leptin on cell proliferation by BrdU incorporation and 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays, and we determined by Western blot analysis phospho 42/44 MAPK (p42/44 Erk1/2) and phospho p38 levels. We then determined the effect of the addition of exogenous leptin, leptin receptor antagonists, inhibitors of leptin signaling or siRNA techniques on the phenotypic features of OA Ob. Phenotypic features of Ob were determined by measuring alkaline phosphatase activity (ALP), osteocalcin release (OC), collagen type 1 production (CICP) and of Transforming Growth Factor-beta1 (TGF-beta1).

RESULTS

Leptin expression was increased approximately five-fold and protein levels approximately two-fold in OA Ob compared to normal. Leptin stimulated its own expression and the expression of OB-Rb in OA Ob. Leptin dose-dependently stimulated cell proliferation of OA Ob and also increased phosphorylated p42/44 Erk1/2 and p38 levels. Inactivating antibodies against leptin reduced ALP, OC, CICP and TGF-beta1 levels in OA Ob. Tyrphostin (AG490) and piceatannol (Pce), inhibitors of leptin signaling, reproduced this effect. Inhibition of endogenous leptin levels using siRNA for leptin or inhibiting leptin signaling using siRNA for OB-Rb expression both reduced ALP and OC about 60%. Exogenous leptin addition stimulated ALP, yet this failed to further increase OC or CICP.

CONCLUSIONS

These results suggest that abnormal production of leptin by OA Ob could be responsible, in part, for the elevated levels of ALP, OC, collagen type 1 and TGF-beta1 observed in these cells compared to normal. Leptin also stimulated cell proliferation, and Erk 1/2 and p38 signaling. Taken together, these data suggest leptin could contribute to abnormal osteoblast function in OA.

Transcriptional activation of human MMP-13 gene expression by c-Maf in osteoarthritic chondrocyte

Matrix metalloproteinase (MMP)-13 has pivotal roles in the pathogenesis of Osteoarthritis (OA) and it is necessary to understand the regulatory mechanisms of MMP-13 expression. MMP-13 gene expression is regulated primarily at the transcriptional level. In this study, we investigated the role of c-maf in regulating MMP-13 transcription. Using transient transfection system with an c-maf construct, and MMP-13 promoter-luciferase constructs with specific mutations in transcription factor binding sites, we found that c-maf can significantly enhance MMP-13 promoter activity via the AP-1 site, By gene suppression with RNAi technology, we could show that c-maf downregulation leads to a reduced expression of MMP13. Chromatin immunoprecipitation assays reveal that c-maf binds to the MMP-13 gene promoter to a region of the MMP-13 promoter containing the AP-1 site. Taken together, these studies demonstrate a new level of transcriptional regulation of MMP-13 expression by the c-maf.

Cartilage in normal and osteoarthritis conditions

The preservation of articular cartilage depends on keeping the cartilage architecture intact. Cartilage strength and function depend on both the properties of the tissue and on their structural parameters. The main structural macromolecules are collagen and proteoglycans (aggrecan). During life, cartilage matrix turnover is mediated by a multitude of complex autocrine and paracrine anabolic and catabolic factors. These act on the chondrocytes and can lead to repair, remodeling or catabolic processes like those that occur in osteoarthritis. Osteoarthritis is characterized by degradation and loss of articular cartilage, subchondral bone remodeling, and, at the clinical stage of the disease, inflammation of the synovial membrane. The alterations in osteoarthritic cartilage are numerous and involve morphologic and metabolic changes in chondrocytes, as well as biochemical and structural alterations in the extracellular matrix macromolecules.

The shunt from the cyclooxygenase to lipoxygenase pathway in human osteoarthritic subchondral osteoblasts is linked with a variable expression of the 5-lipoxygenase-activating protein

Osteoarthritis (OA) is characterized by articular cartilage degradation and hypertrophic bone changes with osteophyte formation and abnormal bone remodeling. Two groups of OA patients were identified via the production of variable and opposite levels of prostaglandin E₂ (PGE₂) or leukotriene B₄ (LTB₄) by subchondral osteoblasts, PGE₂ levels discriminating between low and high subgroups. We studied whether the expression of 5-lipoxygenase (5-LO) or 5-LO-activating protein (FLAP) is responsible for the shunt from prostaglandins to leukotrienes. FLAP mRNA levels varied in low and high OA groups compared with normal, whereas mRNA levels of 5-LO were similar in all osteoblasts. Selective inhibition of cyclooxygenase-2 (COX-2) with NS-398-stimulated FLAP expression in the high OA osteoblasts subgroup, whereas it was without effect in the low OA osteoblasts subgroup. The addition of PGE₂ to the low OA osteoblasts subgroup decreased FLAP expression but failed to affect it in the high OA osteoblasts subgroup. LTB₄ levels in OA osteoblasts were stimulated about twofold by 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) plus transforming growth factor-β (TGF-β), a situation corresponding to their effect on FLAP mRNA levels. Treatments with 1,25(OH)₂D₃ and TGF-β also modulated PGE₂ production. TGF-β stimulated PGE₂ production in both OA osteoblast groups, whereas 1,25(OH)₂D₃ alone had a limited effect but decreased the effect of TGF-β in the low OA osteoblasts subgroup. This modulation of PGE₂ production was mirrored by the synthesis of COX-2. IL-18 levels were only slightly increased in a subgroup of OA osteoblasts compared with normal; however, no relationship was observed overall between IL-18 and PGE₂ levels in normal and OA osteoblasts. These results suggest that the shunt from the production of PGE₂ to LTB₄ is through regulation of the expression of FLAP, not 5-LO, in OA osteoblasts. The expression of FLAP in OA osteoblasts is also modulated differently by 1,25(OH)₂D₃ and TGF-β depending on their endogenous low and high PGE₂ levels.

Abnormal insulin-like growth factor 1 signaling in human osteoarthritic subchondral bone osteoblasts

Insulin-like growth factor (IGF)-1 is a key factor in bone homeostasis and could be involved in bone tissue sclerosis as observed in osteoarthritis (OA). Here, we compare the key signaling pathways triggered in response to IGF-1 stimulation between normal and OA osteoblasts (Obs). Primary Obs were prepared from the subchondral bone of tibial plateaus of OA patients undergoing knee replacement or from normal individuals at autopsy. Phenotypic characterization of Obs was evaluated with alkaline phosphatase and osteocalcin release. The effect of IGF-1 on cell proliferation, alkaline phosphatase and collagen synthesis was evaluated in the presence or not of 50 ng/ml IGF-1, whereas signaling was studied with proteins separated by SDS-PAGE before western blot analysis. We also used immunoprecipitation followed by western blot analysis to detect interactions between key IGF-1 signaling elements. IGF-1 receptor (IGF-1R), Shc, Grb2, insulin receptor substrate (IRS)-1, and p42/44 mitogen-activated protein kinase (MAPK) levels were similar in normal and OA Obs in the presence or absence of IGF-1. After IGF-1 stimulation, the phosphorylation of IGF-1R in normal and OA Obs was similar; however, the phosphorylation of IRS-1 was reduced in OA Ob. In addition, the PI3K pathway was activated similarly in normal and OA Obs while that for p42/44 MAPK was higher in OA Obs compared to normal. p42/44 MAPK can be triggered via an IRS-1/Syp or Grb2/Shc interaction. Interestingly, Syp was poorly phosphorylated under basal conditions in normal Obs and was rapidly phosphorylated upon IGF-1 stimulation, yet Syp showed a poor interaction with IRS-1. In contrast, Syp was highly phosphorylated in OA Obs and its interaction with IRS-1 was very strong initially, yet rapidly dropped with IGF-1 treatments. The interaction of Grb2 with IRS-1 progressively increased in response to IGF-1 in OA Obs whereas this was absent in normal Ob. IGF-1 stimulation altered alkaline phosphatase in Ob, an effect reduced in the presence of PD98059, an inhibitor of p42/44 MAPK signaling, whereas neither IGF-1 nor PD98059 had any significant effect on collagen synthesis. In contrast, cell proliferation was higher in OA Obs compared to normal under basal conditions, and IGF-1 stimulated more cell proliferation in OA Obs than in normal Ob, an effect totally dependent on p42/44 MAPK activiy. The altered response of OA Obs to IGF-1 may be due to abnormal IGF-1 signaling in these cells. This is mostly linked with abnormal IRS-1/Syp and IRS-1/Grb2 interaction in these cells.

Chondroitin and glucosamine sulfate in combination decrease the pro-resorptive properties of human osteoarthritis subchondral bone osteoblasts: a basic science study

Early in the pathological process of osteoarthritis (OA), subchondral bone remodelling, which is related to altered osteoblast metabolism, takes place. In the present study, we explored in human OA subchondral bone whether chondroitin sulfate (CS), glucosamine sulfate (GS), or both together affect the major bone biomarkers, osteoprotegerin (OPG), receptor activator of nuclear factor-kappa B ligand (RANKL), and the pro-resorptive activity of OA osteoblasts. The effect of CS (200 mug/mL), GS (50 and 200 mug/mL), or both together on human OA subchondral bone osteoblasts, in the presence or absence of 1,25(OH)2D3 (vitamin D3) (50 nM), was determined on the bone biomarkers alkaline phosphatase and osteocalcin, on the expression (mRNA) and production (enzyme-linked immunosorbent assay) of bone remodelling factors OPG and RANKL, and on the pro-resorptive activity of these cells. For the latter experiments, human OA osteoblasts were incubated with differentiated peripheral blood mononuclear cells on a sub-micron synthetic calcium phosphate thin film. Data showed that CS and GS affected neither basal nor vitamin D3-induced alkaline phosphatase or osteocalcin release. Interestingly, OPG expression and production under basal conditions or vitamin D3 treatment were upregulated by CS and by both CS and GS incubated together. Under basal conditions, RANKL expression was significantly reduced by CS and by both drugs incubated together. Under vitamin D3, these drugs also showed a decrease in RANKL level, which, however, did not reach statistical significance. Importantly, under basal conditions, CS and both compounds combined significantly upregulated the expression ratio of OPG/RANKL. Vitamin D3 decreased this ratio, and GS further decreased it. Both drugs reduced the resorption activity, and statistical significance was reached for GS and when CS and GS were incubated together. Our data indicate that CS and GS do not overly affect cell integrity or bone biomarkers. Yet CS and both compounds together increase the expression ratio of OPG/RANKL, suggesting a positive effect on OA subchondral bone structural changes. This was confirmed by the decreased resorptive activity for the combination of CS and GS. These data are of major significance and may help to explain how these two drugs exert a positive effect on OA pathophysiology.

Update on the biology of the chondrocyte and new approaches to treating cartilage diseases

Osteoarthritis (OA) is a joint disease that involves degeneration of articular cartilage, limited intraarticular inflammation manifested by synovitis and changes in the subchondral bone. The aetiology of OA is largely unknown, but since it may involve multiple factors, including mechanical, biochemical and genetic factors, it has been difficult to identify unique targets for therapy. Chondrocytes, which are the unique cellular component of adult articular cartilage, are capable of responding to structural changes in the surrounding cartilage matrix. Since the initial stages of OA involve increased cell proliferation and synthesis of matrix proteins, proteinases and cytokines in the cartilage, laboratory investigations have focused on the chondrocyte as a target for therapeutic intervention. The capacity of the adult articular chondrocyte to regenerate the normal cartilage matrix architecture is limited, however, and the damage becomes irreversible unless the destructive process is interrupted. Current pharmacological interventions that address chronic pain are insufficient and no proven disease-modifying therapy is available. Identification of methods for early diagnosis is of key importance, since therapeutic interventions aimed at blocking or reversing structural damage will be more effective when there is the possibility of preserving normal homeostasis. At later stages, cartilage tissue engineering with or without gene therapy with anabolic factors will also require therapy to inhibit inflammation and block damage to newly repaired cartilage. This review will focus on experimental approaches currently under study that may lead to elucidation of effective strategies for therapy in OA, with emphasis on mediators that affect the function of chondrocytes and interactions with surrounding tissues.

Identification in human osteoarthritic chondrocytes of proteins binding to the novel regulatory site AGRE in the human matrix metalloprotease 13 proximal promoter

OBJECTIVE

Matrix metalloprotease 13 (MMP-13) plays a major role in osteoarthritic (OA) processes. We previously identified the AG-rich element (AGRE) regulatory site (GAAAAGAAAAAG) in the proximal promoter of this gene. Electrophoretic mobility shift assays (EMSAs) done with nuclear extracts from OA chondrocytes showed the presence of 2 AGRE protein-binding complexes, the formation of which depended on the pathophysiologic state (high or low) of the cells; the low OA (L-OA) chondrocytes have low MMP-13 basal levels and high interleukin-1beta (IL-1beta) inducibility, and the high OA (H-OA) chondrocytes have high MMP-13 basal levels and low IL-1beta inducibility. In this study, we sought to determine the importance of individual AGRE bases in promoter activity and to identify AGRE binding proteins from L-OA and H-OA chondrocyte complexes.

METHODS

Promoter activity was determined following transient transfection into human OA chondrocytes. AGRE binding proteins were identified by mass spectroscopy.

RESULTS

Individual mutations of the AGRE site differentially modulated promoter activity, indicating that the intact AGRE site is required for optimal MMP-13 expression. Damage-specific DNA binding protein 1 (DDB-1) was identified in the L-OA chondrocyte-binding complex. EMSA experiments performed with the mutation of the left AGRE site (GTGCTGAAAAAG) and nuclear extracts of L-OA chondrocytes reproduced the pattern seen in the H-OA chondrocytes. Mass spectroscopy identified p130cas as one of the proteins in this complex. Supershift experiments showed the presence of p130cas and nuclear matrix transcription factor 4 (NMP-4) in the wild-type AGRE/H-OA chondrocyte complex.

CONCLUSION

These data suggest that the binding of p130(cas) and NMP-4 to the AGRE site regulates MMP-13 expression and may trigger the change in human chondrocytes from the L-OA state to the H-OA state.

Involvement of hepatocyte growth factor in the development of bone metastasis of a mouse mammary cancer cell line, BALB/c-MC

Some cancers frequently affect the skeleton, and the bone microenvironment supports growth of certain cancer cells. After tumors metastasize to bone, they stimulate osteoclastogenesis and expand in the bone tissue. Hepatocyte growth factor (HGF), which was originally identified as a potent mitogen for hepatocytes, promotes tumor growth, invasion and metastasis. HGF is mainly produced by cells of mesenchymal origin, and osteoblasts/osteocytes and bone marrow stromal cells originate from mesenchymal cells. However, it is not clear what effect HGF has on tumor progression in bone metastasis. In the present study, we investigated the roles of HGF in bone metastasis using the mouse mammary cancer cell line BALB/c-MC. Cancer cells injected into hearts of mice metastasized to bone in their hind limbs. HGF immunoreactivity was detected in the stroma surrounding the tumor nests, and blood vessels expressing CD31 (a marker of endothelial cells) were observed in the HGF-positive area. To identify the cells producing HGF, we measured concentration of HGF in culture media. HGF concentration was elevated in osteoblast cultures (3.13+/-0.25 ng/ml), whereas HGF was undetectable (<0.4 ng/ml) in BALB/c-MC and bone marrow cell cultures. HGF concentration in osteoblast cultures increased 2.5-fold in response to 10(-6) M PGE(2). Addition of HGF to BALB/c-MC cultures caused doubling of the cell number. Moreover, Western blot analysis revealed expression of c-Met/HGF receptor by BALB/c-MC. In the Matrigel invasion chamber assay, addition of HGF to the bottom well increased the rate at which BALB/c-MC invaded the bottom well through the membrane. Furthermore, when osteoblasts were cultured in the bottom well, the number of BALB/c-MC cells that invaded the bottom well through the membrane increased 3.7-fold, compared to assays without osteoblasts. Addition of NK4, an inhibitor of HGF, completely abolished the enhancement of the invasive potential of the BALB/c-MC cells in the presence of osteoblasts. These findings suggest that HGF produced by osteoblasts induces migration of cancer cells from sinusoidal capillaries to bone marrow space and stimulates growth of cancer cells in the bone microenvironment. Thus, osteoblasts appear to promote bone metastasis of some cancers via HGF-c-Met signaling.

Modulation of insulin-like growth factor 1 levels in human osteoarthritic subchondral bone osteoblasts

Human osteoarthritis (OA) is characterized by cartilage loss, bone sclerosis, osteophyte formation and inflammation of the synovial membrane. We previously reported that OA osteoblasts (Ob) show abnormal phenotypic characteristics possibly responsible for bone sclerosis and that two subgroups of OA patients can be identified by low or high endogenous production of prostaglandin E2 (PGE2) by OA Ob. Here, we determined that the elevated PGE2 levels in the high OA subgroup were linked with enhanced cyclooxygenase-2 (COX-2) protein levels compared to normal and low OA Ob. A linear relationship was observed between endogenous PGE2 levels and insulin-like growth factor 1 (IGF-1) levels in OA Ob. As parathyroid hormone (PTH) and PGE2 are known stimulators of IGF-1 production in Ob, we next evaluated their effect in OA Ob. Both subgroups increased their IGF-1 production similarly in response to PGE2, while the high OA subgroup showed a blunted response to PTH compared to the low OA group. Conversely, only the high OA group showed a significant inhibition of IGF-1 production when PGE2 synthesis was reduced with Naproxen, a non-steroidal antiinflammatory drug (NSAID) that inhibits cyclooxygenases (COX). The PGE2-dependent stimulation of IGF-1 synthesis was due in part to the cAMP/protein kinase A pathway since both the direct inhibition of this pathway with H-89 and the inhibition of EP2 or EP4 receptors, linked to cAMP production, reduced IGF-1 synthesis. The production of the most abundant IGF-1 binding proteins (IGFBPs) in bone tissue, IGFBP-3, -4, and -5, was lower in OA compared to normal Ob independently of the OA group. Under basal condition, OA Ob expressed similar IGF-1 mRNA to normal Ob; however, PGE2 stimulated IGF-1 mRNA expression more in OA than normal Ob. These data suggest that increased IGF-1 levels correlate with elevated endogenous PGE2 levels in OA Ob and that higher IGF-1 levels in OA Ob could be important for bone sclerosis in OA.

Osteoblasts from the sclerotic subchondral bone downregulate aggrecan but upregulate metalloproteinases expression by chondrocytes. This effect is mimicked by interleukin-6, -1beta and oncostatin M pre-treated non-sclerotic osteoblasts

OBJECTIVE

To determine the effects of osteoarthritic (OA) subchondral osteoblasts on the metabolism of human OA chondrocytes in alginate beads.

METHODS

Human chondrocytes were isolated from OA cartilage and cultured in alginate beads for 4 days in the absence or in the presence of osteoblasts isolated from non-sclerotic (N) or sclerotic (SC) zones of human OA subchondral bone in monolayer (co-culture system). Before co-culture, osteoblasts were incubated for 72 h with or without 1.7ng/ml interleukin (IL)-1beta, 100 ng/ml IL-6 with its soluble receptor (50 ng/ml) or 10 ng/ml oncostatin M (OSM). Aggrecan (AGG) and matrix metalloproteases (MMP)-3 and -13 mRNA levels in chondrocytes were quantified by real-time polymerase chain reaction. AGG production was assayed by a specific enzyme amplified sensitivity immunoassay.

RESULTS

SC, but not N, osteoblasts induced a significant inhibition of AGG production and AGG gene expression by human OA chondrocytes in alginate beads, and significantly increased MMP-3 and MMP-13 gene expression by chondrocytes. When they were pre-incubated with IL-1beta, IL-6 or OSM, N osteoblasts inhibited AGG synthesis and increased MMP-3 and -13 gene expression by chondrocytes in alginate beads in a same order of magnitude as SC osteoblasts.

CONCLUSIONS

These results demonstrate that SC OA subchondral osteoblasts could contribute to cartilage degradation by stimulating chondrocytes to produce more MMP and also by inhibiting AGG synthesis.

Subchondral bone osteoblasts induce phenotypic changes in human osteoarthritic chondrocytes

OBJECTIVE

To determine the influence of osteoarthritic (OA) phenotype of subchondral osteoblasts on the phenotype of human chondrocytes.

METHODS

Human chondrocytes were isolated from OA cartilage and cultured in alginate beads for 4 or 10 days in the absence or in the presence of osteoblasts in monolayer. The osteoblasts were either isolated from non-sclerotic (N) or sclerotic (SC) zones of human subchondral bone. Before co-culture, osteoblasts were incubated for 72 h with or without 1.7 ng/ml interleukin (IL)-1beta, 100 ng/ml IL-6 with its soluble receptor (50 ng/ml) or 10 ng/ml oncostatin M. SOX9, type I, II and X collagen (COL1, COL2, COL10), osteoblasts-stimulating factor (OSF)-1, bone alkaline phosphatase (ALP), parathyroid hormone related peptide (PTHrP) and its receptor (PTH-R) messenger RNA (mRNA) levels in chondrocytes were quantified by real-time polymerase chain reaction.

RESULTS

In comparison with chondrocytes cultured alone in alginate beads, chondrocytes after 4 days in co-culture with N or SC osteoblasts expressed significantly less SOX9 and COL2 mRNA. The decrease of SOX9 and COL2 gene expression was significantly more pronounced in the presence of SC than in the presence of N osteoblasts (P<0.001). OSF-1 mRNA level in chondrocyte was increased by both N and SC osteoblasts, but to a larger extent by SC osteoblasts (P<0.001). PTHrP expression in chondrocytes was 21-fold increased by N osteoblasts but four-fold inhibited by SC osteoblasts. PTHrP secretion was also increased by N but reduced by SC osteoblasts. SC, but not N osteoblasts, induced a significant decrease of PTH-R gene expression in chondrocyte. In our experimental conditions, chondrocytes did not express COL1, COL10 or ALP, even after 10 days of co-culture with osteoblasts.

CONCLUSIONS

In co-culture, SC subchondral osteoblasts decrease SOX9, COL2, PTHrP and PTH-R gene expression by chondrocytes but increase that of OSF-1. These findings suggest that SC osteoblasts could initiate chondrocyte phenotype shift towards hypertrophic differentiation and subsequently further matrix mineralization.

Hepatocyte growth factor/scatter factor is not a potent regulator of anabolic and catabolic gene expression in adult human articular chondrocytes

Objective. Hepatocyte growth factor (HGF) has been reported to be present in articular cartilage and to be a potentially important inducing factor of anabolic and catabolic activity in chondrocytes. The aim of this study was to determine the expression levels of full length-functional-hgf and its receptor c-met in normal and osteoarthritic cartilage and the effect of HGF on anabolic and catabolic gene expression in adult human articular chondrocytes. Methods. Isolated adult human articular chondrocytes were stimulated for 48h with HGF (1, 10, and 100ng/ml). Synthesis of proteoglycans was determined by [(35)S]sulfate incorporation. mRNA levels for anabolic and catabolic genes as well as c-met and (functional) hgf were quantified using real-time PCR. Additionally, in situ mRNA expression levels of hgf and c-met were quantitatively measured from RNA directly isolated from normal and osteoarthritic adult human articular cartilage. Results. Proteoglycan synthesis in adult human articular chondrocytes was not stimulated by HGF nor was a selection of catabolic genes (collagenases and aggrecanases). Normal adult articular chondrocytes expressed only very low levels of hgf mRNA. Slightly higher levels of hgf were detected in chondrocytes isolated from osteoarthritic cartilage. Significant c-met expression was detected in both sample types. Conclusion. Despite the expression of its receptor c-met and its presence in articular cartilage, HGF does not appear to be a potent player in cartilage matrix turnover, at least not in terms of anabolic and catabolic gene expression in normal adult articular cartilage.