SOX9-dependent and -independent Transcriptional Regulation of Human Cartilage Link Protein

Cartilage link protein is a key component of the cartilage extracellular matrix. The transcriptional regulation of the gene encoding cartilage link protein (CRTL1) is largely unknown, however. Here, we investigated the regulation of CRTL1 by SOX9, a key regulator of cartilage matrix genes and chondrogenesis. Knockdown of SOX9 resulted in decreased CRTL1 expression. SOX9 induced CRTL1 expression effectively in human non-chondrocytic immortalized cell lines as well as in mesenchymal stem cell and adult dermal fibroblast. These results indicate that, like other cartilage matrix genes, SOX9 is a key regulator of CRTL1. Unlike other cartilage matrix genes, however, the activation of CRTL1 by SOX9 and its known transcriptional co-activators L-SOX5 and SOX6 was cell type-dependent. Two cis-acting enhancer elements resided in the 5'-untranslated region of CRTL1. One contained a heptameric SOX binding sequence and showed SOX9-dependent enhancer activity in several cell lines. The other showed cell type-specific SOX9-independent enhancer activity. These findings suggest that the enhancer elements may mediate differential expression of CRTL1 during chondrocyte differentiation and maturation.