Chemerin induces CCL2 and TLR4 in synovial fibroblasts of patients with rheumatoid arthritis and osteoarthritis

Abstract

Introduction

Chemerin stimulates migration of leukocytes to sites of inflammation and also increases inflammatory signaling in chondrocytes suggesting a function of chemerin in joint inflammation. Synovial fibroblasts (SF) are critically involved in synovitis and subsequent cartilage destruction. Here, we analyzed whether synovial fibroblasts express chemerin and its receptor CMKLR1. Further, the role of chemerin in synovial fibroblast chemotaxis, proliferation, insulin response and release of inflammatory proteins was studied.

Methods

Synovial tissue sections were labeled with chemerin antibody and chemerin was measured in synovial fluid by ELISA. Chemerin mRNA and protein as well as CMKLR1 expression were determined in SFs from patients with osteoarthritis (OA) and rheumatoid arthritis (RA). Effects of chemerin on cytokines, chemokines and matrix metalloproteinases (MMP), and on proliferation, migration and insulin signaling were analyzed appropriately.

Results

SFs expressed CMKLR1 and chemerin mRNA, and chemerin protein was found in cell supernatants of synovial fibroblasts. Immunohistochemistry detected chemerin in synovial tissue predominantly localized within the lining layer. Chemerin was present in synovial fluids of RA, OA and psoriatic arthritis patients in similar concentrations. Chemerin neither increased IL-6 levels nor MMP-2 or − 9 activity in SFs. Also, it did not act as a chemoattractant for these cells. With respect to intracellular signaling, neither basal nor insulin-mediated phosphorylation of Akt was affected. However, chemerin significantly increased TLR4 mRNA and synthesis of CCL2 in SFs while CCL4 and − 5 were not altered. Cell proliferation of SFs, however, was modestly reduced by chemerin.

Conclusions

These data show that human SFs express both chemerin and its receptor. As chemerin enhanced expression of TLR4 and induced release of CCL2 in SFs, a role of this protein in innate immune system-associated joint inflammation is proposed.

Introduction

Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by synovial hyperplasia and progressive joint destruction. Early after disease onset, RA synovial fibroblasts (SFs) become activated and enhance local inflammation by the release of proinflammatory factors. Cytokines and chemokines secreted by RASFs promote the influx of immune cells into the hyperplastic synovium further contributing to synovitis (Neumann et al., 2010).

Chemerin, also known as tazarotene-induced gene 2 (TIG2), is a chemotactic protein which has been attributed to the attraction of immune cells including macrophages and dendritic cells (Meder et al., 2003, Yoshimura and Oppenheim, 2008). Its chemotactic activity is mediated by binding of chemerin to the G-protein coupled receptor CMKLR1 (Meder et al., 2003). More recent studies demonstrate that articular chondrocytes express CMKLR1 and even synthesize chemerin, whose expression is induced by IL1-β, at least in murine chondrocytes (Berg et al., 2010, Conde et al., 2011).

Chemerin increases production of TNF, IL1-β, IL-6, MMP-1 and MMP-8 in human articular chondrocytes. Higher doses of this chemokine also elevate MMP-2, MMP-3, MMP-13, and IL-8 synthesis. These findings suggest a function of chemerin in joint inflammation and cartilage destruction (Berg et al., 2010).

Chemerin seems to be predominantly released by adipocytes and its production and serum levels are increased in obesity (Bauer et al., 2011, Goralski et al., 2007, Sell et al., 2009). Chemerin synthesis in adipocytes is upregulated by proinflammatory cytokines and lipopolysaccharide, and TNF increases circulating chemerin in mice (Cawthorn and Sethi, 2008, Kralisch et al., 2009, Parlee et al., 2010). Systemic chemerin is higher in obese and type 2 diabetic patients, and may contribute to insulin resistance by impairing insulin signaling in adipocytes and skeletal muscle cells (Bauer et al., 2011, Ernst and Sinal, 2010, Lehrke et al., 2009, Sell et al., 2009, Weigert et al., 2010a). Chemerin positively correlates with circulating levels of inflammatory cytokines in obesity indicating that increased chemerin levels are associated with inflammation (Ernst and Sinal, 2010, Lehrke et al., 2009, Weigert et al., 2010a). Of note, high levels of chemerin have also been detected in inflamed tissues and body fluids such as psoriatic skin, ascites and synovial joint fluid of patients with rheumatoid arthritis (Albanesi et al., 2009, Wittamer et al., 2003).

In obesity, proteins released by adipose tissue contribute to systemic inflammation. Levels of several adipokines such as adiponectin were found to be increased in serum and synovial fluid of patients with arthritis independent of their body mass index. Findings indicate that these adipokines may contribute to inflammation and cartilage destruction (Ferraccioli and Gremese, 2011) as has been shown in vitro for the adipokine adiponectin (Frommer et al., 2010).

Murine 3T3-L1 fibroblasts express CMKLR1 whereas chemerin is neither found in the cell lysate nor in the supernatants (Bauer et al., 2011). This suggests that fibroblasts are capable of responding to chemerin. Here, we analyzed the effects of chemerin on SFs from RA and osteoarthritis (OA) patients in order to elucidate the potential role of chemerin in the pathophysiology of these diseases.