Phosphorylation of osteopontin in osteoarthritis degenerative cartilage and its effect on matrix metalloprotease 13

The purpose of this study is to observe the differences of osteopontin (OPN) phosphorylation in osteoarthritis (OA) cartilage and normal cartilage, and evaluate the possible correlations between the OPN phosphorylation and MMP-13 expression. Degenerative cartilage (n = 29) and normal cartilage (n = 10) were identified by hematoxylin-eosin, safranin-O staining and modified Mankin score. The phosphorylation level of OPN in OA cartilage and normal cartilage was detected by immunoprecipitation. Chondrocytes were treated with phospho-OPN, OPN or buffer. Quantitative reverse transcription polymerase chain reaction (qPCR) and ELISA were used to assess the expression of MMP-13 in different treatments. The OD values of phosphorylation of OPN in normal cartilage and OA cartilage were 137.89 +/- A 10.59 and 153.52 +/- A 8.80, respectively, (P = 0.000). Chondrocytes treated with OPN showed a higher MMP-13 expression at gene and protein level compared with control group. Chondrocytes treated with phospho-OPN showed the highest MMP-13 expression in gene and protein. In conclusion, our results revealed a higher phosphorylation level of OPN in OA cartilage than in normal cartilage. We found OPN leads to elevated expression of MMP-13 (both at gene level and protein level), and phospho-OPN had a more obvious upregulation effect on MMP-13 expression than nonphospho-OPN. Further studies are needed to reveal the mechanism of OPN phosphorylation on cartilage degeneration.