Differential mechanism of NF‐κB inhibition by two glucocorticoid receptor modulators in rheumatoid arthritis synovial fibroblasts

Abstract Objective To investigate and compare the molecular mechanisms by which 2 glucocorticoid receptor (GR)–activating compounds, dexamethasone (DEX) and Compound A (CpdA), interfere with the NF‐κB activation pathway in rheumatoid arthritis (RA) synovial cells. Methods Quantitative polymerase chain reaction was performed to detect the tumor necrosis factor α (TNFα)–induced cytokine gene expression of interleukin‐1β (IL‐1β) and to investigate the effects of DEX and CpdA in RA fibroblast‐like synoviocytes (FLS) transfected with small interfering RNA (siRNA) against GR (siGR) compared with nontransfected cells. Immunofluorescence analysis was used to detect the subcellular distribution of NF‐κB (p65) under the various treatment conditions, and active DNA‐bound p65 was measured using a TransAM assay and by chromatin immunoprecipitation analysis of IL‐1β. Signaling pathways were studied via Western blotting of siGR‐transfected cells, compared with nontransfected and nontargeting siRNA–transfected control cells, to detect the regulation of phospho‐IKK, IκBα, phospho‐p38, phospho‐ERK, and phospho‐JNK. Results Both DEX and CpdA efficiently inhibited IL‐1β gene expression in a GR‐dependent manner. In addition, CpdA attenuated the TNFα‐induced nuclear translocation and DNA binding of p65 in RA FLS, via the attenuation of IKK phosphorylation and subsequent IκBα degradation. CpdA also displayed profound effects on TNFα‐induced MAPK activation. The effects of CpdA on TNFα‐induced kinase activities occurred independently of the presence of GR. In sharp contrast, DEX did not affect TNFα‐induced IKK phosphorylation, IκBα degradation, p65 nuclear translocation, or MAPK activation in RA FLS. Conclusion DEX and CpdA display a dissimilar molecular mechanism of interaction with the NF‐κB activation pathway ex vivo. A dual pathway, partially dependent and partially independent of GR (nongenomic), may explain the gene‐inhibitory effects of CpdA in RA FLS.