SOX9 exerts a bifunctional effect on type II collagen gene (COL2A1) expression in chondrocytes depending on the differentiation state

As a key transcription factor in cartilage formation, SOX9 is a potent activator of type II collagen expression, a phenotypic marker of articular chondrocytes. This study was designed to determine the potential role of SOX9 on COL2A1 gene transcription during chondrocyte dedifferentiation, a characteristic feature of osteoarthritic cartilage that can be partially mimicked in vitro by subculturing primary chondrocytes. Constructs containing different regions from the promoter and the first intron of human COL2A1 gene were transfected in differentiated (primary) and dedifferentiated (passaged) rabbit articular chondrocytes (RAC), together with an expression vector containing or not the SOX9 cDNA. As expected, low levels of SOX9 overexpression were capable of enhancing COL2A1 gene transcription in both fully differentiated and slightly phenotypically altered chondrocytes, through the specific intronic enhancer. In contrast, when overexpressed at high levels, SOX9 induced an inhibition of COL2A1 gene expression, mediated by the -266 bp promoter region, whatever the differentiation state of chondrocytes. However, in the advanced stages of dedifferentiation, SOX9, independently of its expression level, depressed COL2A1 transcriptional activity through the -63 bp short promoter. Although SOX9 has a crucial role in chondrocyte differentiation, our findings indicate that this factor cannot restore the phenotype of osteoarthritic chondrocytes by itself.