Osteopontin inhibits HIF-2α mRNA expression in osteoarthritic chondrocytes

The aim of the present study was to investigate the in vitro effect of osteopontin (OPN) on the expression of hypoxia-inducible factor-2 alpha (HIF-2 alpha) in chondrocytes and the role of OPN in osteoarthritis (OA). Cartilage was purified from the tibial surfaces of patients with OA of the knee and cultured in vitro to obtain chondrocytes. Recombinant human OPN (rhOPN) and OPN small interfering RNA (siRNA) were used to treat the chondrocytes, and the changes in the expression levels of the HIF-2 alpha gene were measured. An anti-CD44 blocking monoclonal antibody (mAb) was used to determine the probable ligand-receptor interactions. Reverse transcription-quantitative polymerase chain reaction assays were designed and validated with SYBR Green dyes for the simultaneous quantification of the mRNA expression levels of OPN and HIF-2 alpha. The mRNA expression level of HIF-2 alpha was markedly decreased in the rhOPN-treated group compared with that in the control group; by contrast, OPN siRNA increased HIF-2 alpha gene expression. CD44 blocking mAb suppressed the inhibitory effect of OPN on HIF-2 alpha mRNA expression. The results of the present study suggest that OPN may play a protective role in OA by inhibiting HIF-2 alpha gene expression in osteoarthritic chondrocytes through CD44 interaction.