Down-regulation of adiponectin receptors in osteoarthritic chondrocytes

The objective of this study was to investigate the expression of adiponectin receptors (AdipoR1, R2, and T-cadherin) in both normal subjects and patients with knee osteoarthritis (OA). We used immunofluorescence to assess expression of adiponectin receptors in the chondrocytes of normal subjects (n = 3) and OA patients (n = 3). We also studied mRNA expression of adiponectin receptors in both groups by real-time polymerase chain reaction (real-time PCR). Finally, we utilized Western blotting to confirm the presence of adiponectin receptors. As compared with osteoarthritic chondrocytes, normal chondrocytes showed stronger immunoreactivity for AdipoR1, AdipoR2, and T-cadherin. The expression levels of both AdipoR1 and AdipoR2 mRNA were significantly lower in the osteoarthritic chondrocytes compared with those in the normal chondrocytes, 19 ± 2 and 36 ± 3 % of normal chondrocytes, respectively (P < 0.001). T-cadherin mRNA expression levels of the osteoarthritic chondrocytes were also lower than those in the normal chondrocytes, but not statistical significant (P = 0.072). The expression levels of AdipoR1 and AdipoR2 protein were significantly higher in the normal chondrocytes compared with those in the osteoarthritic chondrocytes (P < 0.001, P < 0.01, respectively). T-cadherin protein expression level of the normal chondrocytes was also higher than those in the osteoarthritic chondrocytes, but the difference is not statistical significant (P = 0.114). Expression of adiponectin receptors protein in normal and osteoarthritic chondrocytes is consistent with its mRNA expression levels. In conclusion, we report for the first time down-regulation of adiponectin receptors (AdipoR1, R2, and T-cadherin) in osteoarthritic chondrocytes. Decreased adiponectin receptors in OA may reduce the tissue sensitivity to adiponectin, thus lost the protection from adiponectin in the progression of OA.