Conclusion
NF-kappaB and RARs converge to reduce Sox9 activity and cartilage matrix gene expression, probably by limiting the availability of p300. This process may be critical for the loss of cartilage matrix synthesis in inflammatory joint diseases. Therefore, agents that increase p300 levels or activity in chondrocytes may be useful therapeutically.
Methods
Primary cultures of rat chondrocytes were treated with TNF-alpha and/or atRA for 24 hours. Levels of transcripts encoding cartilage matrix proteins were determined by Northern blot analyses and quantitative real-time PCR. Nuclear protein levels, DNA binding and functional activity of transcription factors were assessed by immunoblotting, electrophoretic mobility shift assays and reporter assays, respectively.
Results
Together, TNF-alpha and atRA diminished transcript levels of cartilage matrix proteins and Sox9 activity more than each factor alone. However, neither agent altered nuclear levels of Sox9, and TNF-alpha did not affect protein binding to the Col2a1 48-base-pair minimal enhancer sequence. The effect of TNF-alpha, but not that of atRA, on Sox9 activity was dependent on NF-kappaB activation. Furthermore, atRA reduced NF-kappaB activity and DNA binding. To address the role of p300, we over-expressed constitutively active mitogen-activated protein kinase kinase kinase (caMEKK)1 to increase p300 acetylase activity. caMEKK1 enhanced basal NF-kappaB activity and atRA-induced RAR activity. Over-expression of caMEKK1 also enhanced basal Sox9 activity and suppressed the inhibitory effects of TNF-alpha and atRA on Sox9 function. In addition, over-expression of p300 restored Sox9 activity suppressed by TNF-alpha and atRA to normal levels.