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Trans‐activation of the Mouse Cartilage‐Derived Retinoic Acid‐Sensitive Protein Gene by Sox9

The transcription factor Sox9 is capable of enhancing type II collagen gene expression and may play a crucial role in chondrogenesis. To determine whether Sox9 is an inducer of the chondrocyte phenotype, we investigated the role of Sox9 in transcription of another cartilage gene encoding the cartilage‐derived retinoic acid–sensitive protein (CD‐RAP). CD‐RAP is specifically expressed during chondrogenesis. We show here that Sox9 protein is able to bind to a SOX consensus sequence in the CD‐RAP promoter. Mutation of the SOX motif led to decreased transcription of a CD‐RAP promoter construct in chondrocytes. Overexpression of SOX9 resulted in a dose‐dependent increased activity of CD‐RAP promoter‐driven reporter gene in both chondrocytes and nonchondrogenic cells. A truncated SOX9, which contains a binding domain but no trans‐activation function, inhibited CD‐RAP promoter activity. Overexpression of SOX9 increased the level of endogenous CD‐RAP mRNA in chondrocytes, but was unable to induce endogenous gene expression in 10T1/2 mesenchymal cells or BALB/c‐3T3 fibroblasts. These results suggest that Sox9 is a general transcriptional regulator of cartilage‐specific genes. However, Sox9 does not appear to be able to induce the chondrocyte phenotype in nonchondrogenic cells, implying that other factors are involved in chondrogenesis.