NF-κB signalling is inhibited by glucocorticoid receptor and STAT6 via distinct mechanisms

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Abstract
NF-κB transcription factors are involved in the cellular response to stress, and are regulated by inhibitor (IκB)proteins, which prevent NF-κB-mediated transcription by maintaining NF-κB in the cytoplasm. Proteins from other pathways are also known to regulate NF-κB negatively, notably the glucocorticoid receptor (GR) and IL-4-responsive STAT6. Both pathways were shown to inhibit NF-κB-mediated transcription, by expressing either STAT6 or GR and activating the respective pathways. Using fluorescent fusion proteins, we show that GR alters the timing of activated p65 NF-κB nuclear occupancy by increasing the export rate of p65 and is independent of whether GR is present as a dimer or monomer. Expression of STAT6 was also shown to alter p65 nuclear occupancy but appeared to affect the import rate and hence the overall maximal level of p65 translocation. Activating STAT6 with IL-4 prior to activating NF-κB significantly increased this inhibition. Investigation of IκBa showed that activated STAT6 inhibited TNFα-mediated IκBa phosphorylation and degradation, whereas GR activation did not alter IκBαkinetics. This demonstrates a clear separation of two distinct mechanisms of inhibition by STAT6 and GR upon the NF-κB pathway.