The infrapatellar fat pad induces inflammatory and degradative effects in articular cells but not through leptin or adiponectin.

OBJECTIVES: Based on a novel approach suggesting a role of adipose tissue in osteoarthritis (OA), we aimed to determine whether the infrapatellar fat pad (IFP) may affect joint cell functions through adipokines. METHODS: The conditioned media of IFP and subcutaneous adipose tissue from OA patients were used to determine the production of leptin and adiponectin, and to stimulate chondrocytes and fibroblast-like synoviocytes (FLS). Blocking experiments were carried out to evaluate the contribution of adipokines to IFP effects. The gene expression of inflammatory and degradative proteins, growth factors and components of the extracellular matrix, and the production of inflammatory mediators and metalloproteases were determined to evaluate cell response to fat-conditioned media. RESULTS: IFP releases elevated amounts of leptin and adiponectin independently of the body mass index and the gender. The conditioned media from IFP strongly induce the expression of inflammatory genes in both articular cells and the expression of degradative genes in chondrocytes, but remain ineffective in regulating the expression of aggrecan, type 2 collagen or growth factors. Blocking leptin or adiponectin does not change the cell response to IFP. A great variability between patients is found when compared the inflammatory activity of paired samples of IFP and subcutaneous adipose tissue. CONCLUSIONS: IFP may trigger both cartilage destruction and inflammation of the synovium, but not through leptin or adiponectin. The data suggest also that IFP may have specific inflammatory phenotypic features independent from the general phenotype found in obesity.

Synovial fluid levels of adipokines in osteoarthritis: Association with local factors of inflammation and cartilage maintenance.

The role of body weight in the pathogenesis of osteoarthritis (OA) - previously considered the sole factor in the association between obesity and OA - is being re-evaluated as the contribution of adiposity to the cartilage degenerative process becomes clearer. The current study has been undertaken to better understand the role of adipose-derived proteins, namely adipokines, in OA. For this purpose, we investigated in patients with OA the relationships between the joint levels of leptin, adiponectin and resistin and those of factors involved in inflammation and cartilage maintenance. The sandwich enzyme-linked immunosorbent assays were used to determine in the synovial fluid (SF) from 35 OA patients, the concentrations of adipokines, interleukin-6 (IL-6) and transforming growth factor-ß (TGF-ß). The soluble form of leptin receptor (sOb-R) was also examined to evaluate the biological active free form of leptin. Correlation analysis indicate that IL-6 levels are positively related to the levels of resistin and adiponectin. Surprisingly, the free form of leptin, but not the total leptin, is negatively associated with IL-6. Beside, adiponectin is the single adipokine that is correlated with TGF-ß. Interestingly, a sexual dimorphism is observed in the study as correlations between adipokines and IL-6 or TGF-ß are found only with female OA patients. Taken together, these findings suggest that only adiponectin may contribute to the metabolic changes associated with OA. The three adipokines may also be involved in inflammation, but with opposite effects. Both resistin and adiponectin may exhibit pro-inflammatory activity while the free form of leptin may down-regulate the inflammation.

Association between the chondrocyte phenotype and the expression of adipokines and their receptors: evidence for a role of leptin but not adiponectin in the expression of cartilage-specific markers.

Although extensive evidence support the key role of adipokines in cartilage homeostasis, contradictory data have been found for their expression and their effects in chondrocytes. This study was then undertaken to determine whether a phenotypic modulation may affect the expression of adipokines and their receptors in human chondrocytes. The expression of leptin, adiponectin and their receptors, as well as cartilage-specific genes was examined in chondrocytes obtained from patients with osteoarthritis either directly after cells harvest or after culture in monolayer or in alginate beads. The results showed major changes in the gene expression pattern after culture in monolayer with a shift from the adipokines to their receptors. Interestingly, this downregulation of adipokines was associated with a loss of chondrocyte phenotype, and chondrocytes recovered a cartilage-like expression profile of leptin and adiponectin when cultured in a tridimensional chondrocyte phenotype-inducing system, but ceased expressing their receptors. Further experiments clearly showed that leptin but not adiponectin promoted the expression of cartilage-specific markers through mitogen-activated protein kinase, Janus kinase and phosphatidylinositol-3 kinase signaling pathways. In conclusion, our data indicate that any phenotypic modulation could affect chondrocyte responsiveness to leptin or adiponectin, and provide evidence for an important role for leptin in regulating the expression of cartilage-specific markers.

Obesity affects the chondrocyte responsiveness to leptin in patients with osteoarthritis.

INTRODUCTION: Increasing evidence support the regulatory role of leptin in osteoarthritis (OA). As high circulating concentrations of leptin disrupt the physiological function of the adipokine in obese individuals, the current study has been undertaken to determine whether the elevated levels of leptin found in the joint from obese OA patients also induce changes in the chondrocyte response to leptin. METHODS: Chondrocytes isolated from OA patients with various body mass index (BMI) were treated with 20, 100 or 500 ng/ml of leptin. The expression of cartilage-specific components (aggrecan, type 2 collagen), as well as regulatory (IGF-1, TGFbeta, MMP-13, TIMP 2) or inflammatory (COX-2, iNOS, IL-1) factors was investigated by real-time PCR to evaluate chondrocyte responsiveness to leptin. Furthermore, the effect of body mass index (BMI) on leptin signalling pathways was analyzed with an enzyme-linked immunosorbent assay for STATs activation. RESULTS: Leptin at 20 ng/ml was unable to modulate gene expression in chondrocytes, except for MMP-13 in obese OA patients. Higher leptin levels induced the expression of IGF-1, type 2 collagen, TIMP-2 and MMP-13. However, the activity of the adipokine was shown to be critically dependent on both the concentration and the BMI of the patients with a negative association between the activation of regulated genes and BMI for 100 ng/ml of adipokine, but a positive association between chondrocyte responsiveness and BMI for the highest leptin dose. In addition, the gene encoding MMP-13 was identified as a target of leptin for chondrocytes originated from obese patients while mRNA level of TIMP-2 was increased in leptin-treated chondrocytes collected from normal or overweight patients. The adipokine at 500 ng/ml triggered signal transduction through a STAT-dependent pathway while 100 ng/ml of leptin failed to activate STAT 3 but induced STAT 1alpha phosphorylation in chondrocytes obtained from obese patients. CONCLUSIONS: The current study clearly showed that characteristics of OA patients and more especially obesity may affect the responsiveness of cultured chondrocytes to leptin. In addition, the BMI-dependent effect of leptin for the expression of TIMP-2 and MMP-13 may explain why obesity is associated with an increased risk for OA.

[Leptin: a link between obesity and osteoarthritis?]. / La leptine: un lien entre obésité et arthrose?

In addition to aging, obesity is one of the most common underlying causes of osteoarthritis (OA). Mechanical loading, together with biochemical and systemic factors linked to altered lipid metabolism, are thought to contribute to the onset of OA. It has been suggested that OA is a systemic metabolic disease associated with lipid disorders affecting joint homeostasis. These gradual changes may be due to the local effect of adipokines, and especially leptin. Indeed, their relative levels in joints differ from that found in plasma. In particular, leptin levels are increased and adiponectin and resistin levels are reduced This hypothesis is supported by--leptin overexpression in OA cartilage and its correlation with the degree of cartilage destruction,--abundant leptin synthesis by osteophytes, and--the high leptin levels found in OA joints from female patients. This link between OA and adipokines provides new leads regarding the prevention of OA and the identification of new drug targets.

Evidence for a key role of leptin in osteoarthritis.

OBJECTIVE: To evaluate the contribution of leptin (an adipose tissue-derived hormone) to the pathophysiology of osteoarthritis (OA), by determining the level of leptin in both synovial fluid (SF) and cartilage specimens obtained from human joints. We also investigated the effect of leptin on cartilage, using intraarticular injections of leptin in rats. METHODS: Leptin levels in SF samples obtained from OA patients undergoing either knee replacement surgery or knee arthroscopy were measured by enzyme-linked immunosorbent assay. In addition, histologic sections of articular cartilage and osteophytes obtained during surgery for total knee replacement were graded using the Mankin score, and were immunostained using antibodies to leptin, transforming growth factor beta (TGFbeta), and insulin-like growth factor 1 (IGF-1). For experimental studies, various doses of leptin (10, 30, 100, and 300 microg) were injected into the knee joints of rats. Tibial plateaus were collected and processed for proteoglycan synthesis by radiolabeled sulfate incorporation, and for expression of leptin, its receptor (Ob-Rb), and growth factors by reverse transcriptase-polymerase chain reaction and immunohistochemical analysis. RESULTS: Leptin was observed in SF obtained from human OA-affected joints, and leptin concentrations correlated with the body mass index. Marked expression of the protein was observed in OA cartilage and in osteophytes, while in normal cartilage, few chondrocytes produced leptin. Furthermore, the pattern and level of leptin expression were related to the grade of cartilage destruction and paralleled those of growth factors (IGF-1 and TGFbeta1). Animal studies showed that leptin strongly stimulated anabolic functions of chondrocytes and induced the synthesis of IGF-1 and TGFbeta1 in cartilage at both the messenger RNA and the protein levels. CONCLUSION: These findings suggest a new peripheral function of leptin as a key regulator of chondrocyte metabolism, and indicate that leptin may play an important role in the pathophysiology of OA.