Potent inhibition of the master chondrogenic factor Sox9 gene by interleukin-1 and tumor necrosis factor-α

The inflammatory cytokines interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) strongly inhibit the expression of genes for cartilage extracellular matrix proteins. We have recently obtained genetic evidence indicating that the high mobility group domain containing transcription factor Sox9 is required for cartilage formation and for expression of chondrocyte-specific genes including the gene for type II collagen (Col2a1). We show here that IL-1 and TNF-alpha cause a marked and rapid decrease in the levels of Sox9 mRNA and/or protein in chondrocytes. A role for the transcription factor NF kappa B in Sox9 down-regulation was suggested by the ability of pyrrolidine dithiocarbamate, an inhibitor of the NF kappa B pathway, to block the effects of IL-1 and TNF-alpha. This role was further supported by the ability of a dominant-negative mutant of I kappa B alpha to block the IL-1 and TNF-alpha inhibition of Sox9-dependent Col2a1 enhancer elements. Furthermore, forced expression of the NF kappa B subunits p65 or p50 also inhibited Sox9-dependent Col2a1 enhancer. Because Sox9 is essential for chondrogenesis, the marked down-regulation of the Sox9 gene by IL-1 and TNF-alpha in chondrocytes is sufficient to account for the inhibition of the chondrocyte phenotype by these cytokines. The down-regulation of Sox9 may have a crucial role in inhibiting expression of the cartilage phenotype in inflammatory joint diseases.