The glucocorticoid receptor blocks P-TEFb recruitment by NFκB to effect promoter-specific transcriptional repression

To investigate the determinants of promoter-specific gene regulation by the glucocorticoid receptor (GR), we compared the composition and function of regulatory complexes at two NF kappa B-responsive genes that are differentially regulated by GR. Transcription of the IL-8 and I kappa B alpha genes is stimulated by TNF alpha in A549 cells, but GR selectively represses IL-8 mRNA synthesis by inhibiting Ser2 phosphorylation of the RNA polymerase II (pol II) C-terminal domain (CTD). The proximal kappa B elements at these genes differ in sequence by a single base pair, and both recruited RelA and p50. Surprisingly, GR was recruited to both of these elements, despite the fact that GR failed to repress the I kappa B alpha promoter. Rather, the regulatory complexes formed at IL-8 and I kappa B alpha were distinguished by differential recruitment of the Ser2 CTD kinase, P-TEFb. Disruption of P-TEFb function by the Cdk-inhibitor, DRB, or by small interfering RNA selectively blocked TNFa stimulation of IL-8 mRNA production. GR competed with P-TEFb recruitment to the IL-8 promoter. Strikingly, IL-8 mRNA synthesis was repressed by GR at a post-initiation step, demonstrating that promoter proximal regulatory sequences assemble complexes that impact early and late stages of mRNA synthesis. Thus, GR accomplishes selective repression by targeting promoter-specific components of NF kappa B regulatory complexes.