Objective Retinol binding protein 4 (RBP4) is a member of the lipocalin family and a vitamin A carrier in the blood. More recently, RBP4 has been described as an adipokine that is involved in insulin resistance and metabolic syndrome (MetS). As obesity, MetS and some adipokines contribute to the pathogenesis of osteoarthritis (OA), we investigated RBP4 in patients with OA. Materials and methods Cartilage, synovial fluid and blood samples were collected from 100 OA patients undergoing total knee replacement surgery. Primary chondrocytes and cartilage tissue were cultured to measure the RBP4 expression. The concentrations of RBP4, other adipokines (adipsin, adiponectin, leptin and resistin) and biomarkers of OA (COMP, MMP-1, MMP-3 and YKL-40) were measured by immunoassay, and gene expression was measured by next-generation RNA sequencing. Results The OA cartilage samples released RBP4 into the culture medium, and the levels correlated positively with the expression of the adipokines adipsin, adiponectin, leptin and resistin. RBP4 was the most prominently expressed of these adipokines in the OA chondrocytes, and the expression of the RBP4 receptors STRA6 (stimulated by retinoic acid gene homologue 6) and TLR4 (Toll-like receptor 4) was also detected. Within the cartilage culture medium, RBP4 showed a positive correlation with MMP-1, MMP-3 and YKL-40. RBP4 was also present in the synovial fluid from the OA patients and correlated positively with the concentrations of RBP4 found in the plasma and the cartilage culture medium. Plasma RBP4 concentrations also showed a positive correlation with MMP-3 and adipsin. Conclusions We show here, for the first time, that RBP4 is produced within OA joints and that it is associated with increased levels of adipokines and MMPs. The results suggest a role for RBP4 in the pathogenesis of OA and as a possible target for the disease-modifying drugs for the treatment of OA.

Transient receptor potential ankyrin 1 (TRPA1) is a membrane-bound ion channel found in neurons, where it mediates nociception and neurogenic inflammation. Recently, we have discovered that TRPA1 is also expressed in human osteoarthritic (OA) chondrocytes and downregulated by the anti-inflammatory drugs aurothiomalate and dexamethasone. We have also shown TRPA1 to mediate inflammation, pain, and cartilage degeneration in experimental osteoarthritis. In this study, we investigated the role of TRPA1 in joint inflammation, focusing on the pro-inflammatory cytokine interleukin-6 (IL-6). We utilized cartilage/chondrocytes from wild-type (WT) and TRPA1 knockout (KO) mice, along with primary chondrocytes from OA patients. The results show that TRPA1 regulates the synthesis of the OA-driving inflammatory cytokine IL-6 in chondrocytes. IL-6 was highly expressed in WT chondrocytes, and its expression, along with the expression of IL-6 family cytokines leukemia inhibitory factor (LIF) and IL-11, were significantly downregulated by TRPA1 deficiency. Furthermore, treatment with the TRPA1 antagonist significantly downregulated the expression of IL-6 in chondrocytes from WT mice and OA patients. The results suggest that TRPA1 is involved in the upregulation of IL-6 production in chondrocytes. These findings together with previous results on the expression and functions of TRPA1 in cellular and animal models point to the role of TRPA1 as a potential mediator and novel drug target in osteoarthritis.

Background Intra-articular glucocorticoid (GC) injections are widely used as a symptomatic treatment for osteoarthritis (OA). However, there are also concerns about their potentially harmful effects, and their detailed effects on chondrocyte phenotype remain poorly understood. Methods We studied the effects of dexamethasone on gene expression in OA chondrocytes with RNA-Seq. Chondrocytes were isolated from the cartilage from OA patients undergoing knee replacement surgery and cultured with or without dexamethasone for 24 h. Total RNA was isolated and sequenced, and functional analysis was performed against the Gene Ontology (GO) database. Results for selected genes were confirmed with RT-PCR. We also investigated genes linked to OA in recent genome-wide expression analysis (GWEA) studies. Results Dexamethasone increased the expression of 480 and reduced that of 755 genes with a fold change (FC) 2.0 or greater. Several genes associated with inflammation and cartilage anabolism/catabolism as well as lipid and carbohydrate metabolism were among the most strongly affected genes. In the GO analysis, genes involved in the extracellular matrix organization, cell proliferation and adhesion, inflammation, and collagen synthesis were enriched among the significantly affected genes. In network analysis, NGF, PI3KR1, and VCAM1 were identified as central genes among those most strongly affected by dexamethasone. Conclusions This is the first study investigating the genome-wide effects of GCs on the gene expression in OA chondrocytes. In addition to clear anti-inflammatory and anticatabolic effects, GCs affect lipid and glucose metabolism in chondrocytes, an observation that might be particularly important in the metabolic phenotype of OA.

Macrophage polarization refers to the ability of these cells to adopt different functional phenotypes according to their environment. Mitogen-activated protein kinase phosphatase-1 (MKP-1) is known to regulate the classical lipopolysaccharide (LPS)-induced pro-inflammatory macrophage activation and the inflammatory response. Here, we investigated the effects of MKP-1 on the anti-inflammatory and healing-promoting macrophage phenotype induced by cytokines IL-4 and IL-13 and examined the potential mediator role of MKP-1 in glucocorticoid effects on the two macrophage phenotypes. In MKP-1-deficient macrophages treated with IL-4 and IL-13 to induce the anti-inflammatory phenotype, the expression of phenotypic markers arginase 1, Ym-1 and FGF2 was reduced as compared to wildtype cells. In contrast, LPS-induced expression of the pro-inflammatory factors IL-6 and iNOS was significantly higher in MKP-1-deficient macrophages. Dexamethasone suppressed the pro-inflammatory phenotype and enhanced the anti-inflammatory phenotype. Interestingly, both of these glucocorticoid effects were attenuated in macrophages from MKP-1-deficient mice. Accordingly, dexamethasone increased MKP-1 expression in both LPS-and IL4+13-treated wild-type cells. In conclusion, the findings support MKP-1 as an endogenous mechanism able to shift macrophage activation from the classical pro-inflammatory state towards the anti-inflammatory and healing-promoting phenotype. In addition, MKP-1 was found to mediate the anti-inflammatory effects of dexamethasone in a dualistic manner: by suppressing the pro-inflammatory macrophage activation and by enhancing the healing-promoting macrophage phenotype.