Abstract
Expression of the orphan nuclear receptor NR4A2 is controlled by pro-inflammatory mediators, suggesting that NR4A2 may contribute to pathological processes in the inflammatory lesion. This study identifies the chemoattractant protein, interleukin 8 (IL-8/CXCL8), as a molecular target of NR4A2 in human inflammatory arthritis and examines the mechanism through which NR4A2 modulates IL-8 expression. In TNF-α-activated human synoviocyte cells, enhanced expression of IL-8 mRNA and protein correspond to temporal changes in NR4A2 transcription and nuclear distribution. Ectopic expression of NR4A2 leads to robust changes in endogenous IL-8 mRNA levels and co-treatment with TNF-α results in significant ( p < 0.001) secretion of IL-8 protein. Transcriptional effects of NR4A2 on the human IL-8 promoter are enhanced in the presence of TNF-α, suggesting molecular crosstalk between TNF-α signalling and NR4A2. A dominant negative IκB kinase antagonizes the combined effects of NR4A2 and TNF-α on IL-8 promoter activity. Co-expression of NR4A2 and the p65 subunit of NF-κB enhances IL-8 transcription and functional studies indicate that transactivation occurs independently of NR4A2 binding to DNA or heterodimerization with additional nuclear receptors. The IL-8 minimal promoter region is sufficient to support NR4A2 and NF-κB/p65 co-operative activity and NR4A2 can interact with NF-κB/p65 on a 39 bp sequence within this region. In patients treated with methotrexate for active inflammatory arthritis, a reduction in NR4A2 synovial tissue levels correlate significantly ( n = 10, r = 0.73, p = 0.002) with changes in IL-8 expression. Collectively, these data delineate an important role for NR4A2 in modulating IL-8 expression and reveal novel transcriptional responses to TNF-α in human inflammatory joint disease.
Published Version
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