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

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