Increased apelin serum levels and expression in human chondrocytes in osteoarthritic patients

Abstract

Apelin is a recently discovered hormone secreted by adipocytes. The aim of this study, therefore, was to evaluate the distribution of apelin in paired serum and synovial fluid (SF) of osteoarthritis (OA) patients, as compared to that in healthy controls, and to characterise the expression profile of apelin and its cognate receptor APJ in human chondrocytes. Apelin levels in serum and SF were analysed by enzyme-linked immunosorbent assay (ELISA). Expression of apelin and APJ in human chondrocytes was determined by real-time quantitative polymerase chain reaction (PCR). Apelin was found to be present in OA SF, and concentrations were positively correlated with the severity of OA. OA serum exhibited significantly elevated levels of apelin (2.18±0.22 ng/ml) as compared to normal serum (1.31±0.12 ng/ml) (p<0.05), and serum apelin levels exceeded those in paired SF (p < 0.001). The apelin and APJ transcripts were identified in chondrocytes, and levels were significantly higher in OA cartilage than in healthy donors. These findings suggest that apelin may contribute to the onset and/or progression of OA, and may provide new insights into the pathophysiology of OA.

The apelin receptor APJ: journey from an orphan to a multifaceted regulator of homeostasis

“…Furthermore, apelin and APJ are expressed in osteoblasts where they may induce cell proliferation and promote survival (Xie et al 2006, 2007, Wattanachanya et al 2013; however, an increase in bone mass can be observed in apelin KO mice (Wattanachanya et al 2013). Recently, apelin has been reported to have a potential role in the pathophysiology of osteoarthritis (OA), as apelin is present in synovial fluid, and OA patients have elevated plasma apelin concentrations (Hu et al 2011). Blood plasma levels of apelin are reduced in patients with polycystic ovary syndrome (Chang et al 2011, Olszanecka-Glinianowicz et al 2012, consistent with the role played by apelin/APJ in metabolic disturbances such as insulin resistance.…”

Correlations between serum adipocytokine concentrations, disease stage, radiological status and total body fat content in the patients with primary knee osteoarthritis

“…Adipocytokines were recognised as modulators of the energy homeostasis, as well as an immunological, haemostatic and inflammatory processes [5]. Those substances also play an important role in the degradation processes of cartilage, bone and synovium [6]. Since their effect on the degenerative changes in non-loaded joints were demonstrated [7], they might provide a link between obesity and OA.…”

The role of adipocytokines in the pathogenesis of knee joint osteoarthritis

“…Some adipocytokines might also play a direct role in maintaining an inflammatory state in the joints of obese individuals (the so-called low-grade inflammatory state) [15]. These proteins have been detected in synovial fluid (SF) and the plasma of patients with OA [16][17][18]. A putative source of adipocytokine secretion in the region of the knee joint is the infrapatellar fat pad (IPFP) [19,20].…”