Anti-inflammation: Direct Physical Association and Functional Antagonism Between Transcription Factor NF-KB and the Glucocorticoid Receptor

Glucocorticoids are widely used clinically as anti-inflammatory and immunosuppressive agents. Of all the pharmaceutical approaches to the treatment of asthma, glucocorticoids are undoubtedly the most potent. These effects of glucocorticoids result, to a great extent, from their ability to inhibit cellular release of inflammatory mediators and cytokines. Despite their widespread use, the molecular mechanisms by which glucocorticoids exert their effects are not clearly defined. Glucocorticoids activate genes by interacting with the glucocorticoid receptor (GR) which binds to glucocorticoid response elements (GREs) in the target genes. However, inhibition of gene expression by glucocorticoids cannot always be adequately explained by binding of GR to regulatory sequences in the target genes. Earlier work with the collagenase gene showed that steroids could inhibit gene transcription by interference with the binding of the transcription factors Fos and Jun to the AP-1 site. However, this mechanism does not appear to be the case for inhibition of expression of the interleukin-6 (IL-6) gene and of additional cytokine genes by steroids. We established that activation of the IL-6 gene expression by IL-1 requires both the NF-IL6 and the NF-κB sites in the IL-6 promoter. Since the transcription factor NF-κB has been implicated in the upregulation of multiple inflammation-associated genes, it seemed conceivable that GR might interact with NF-κB to downregulate cytokine gene expression. Our studies indicate that GR physically interacts with the p65 subunit of NF-κB; this interaction may explain the impaired transactivation potential of both proteins in intact cells. In cotransfection experiments, activation of the IL-6 promoter by p65 and NF-IL6 was annulled by the synthetic glucocorticoid dexamethasone, possibly through interactions of GR with p65. Conversely, activation of the mouse mammary tumor virus promoter by GR was inhibited by coexpression of p65. The reciprocal functional antagonism between p65 and GR provides a framework for the opposing actions of inflammatory mediators and steroids.