title: Glucocorticoid-independent repression of tumor necrosis factor (TNF) alpha-stimulated interleukin (IL)-6 expression by the glucocorticoid receptor: a potential mechanism for protection against an excessive inflammatory response.
abstract: TNFα signaling and cytokine levels play a crucial role in cervical immunity and the host response to infections. We investigated the role of liganded and unliganded glucocorticoid receptor (GR) in IL-6 and IL-8 gene regulation in response to TNFα in the End1/E6E7 immortalized human endocervical epithelial cell line. In the absence of glucocorticoids, both decreasing GR protein levels by an siRNA strategy and results with the GR antagonist RU486 suggest a role for the unliganded GR in reduction of TNFα-induced IL-6 and IL-8 mRNA levels in End1/E6E7 cells. Moreover, GR-dependent repression of endogenous IL-6 mRNA as well as a minimal IL-6 promoter-reporter gene is also demonstrated in COS-1 cells in the absence of GR ligand, suggesting a transcriptional mechanism that is not cell-specific. TNFα induced recruitment of both the unliganded GR and GR-interacting protein type 1 (GRIP-1) to the IL-6 promoter. This, together with GRIP-1 overexpression studies, suggests a function for GRIP-1 as a GR co-repressor in this context. TNFα was shown to induce phosphorylation of the unliganded human GR at Ser-226 but not Ser-211, unlike dexamethasone, which induced hyperphosphorylation at both serine residues. Ser-226 is shown to be required for the ligand-independent GR-mediated repression of IL-6 in response to TNFα. Taken together, these results support a model whereby the unliganded GR attenuates TNFα-stimulated IL-6 transcription by a mechanism involving selective phosphorylation and recruitment of the unliganded GR and GRIP-1 to the IL-6 promoter. These findings suggest the presence of a novel autoregulatory mechanism that may prevent overproduction of IL-6 in the endocervix, possibly protecting against negative effects of excessive inflammation.
authors.affiliation: Department of Molecular and Cell Biology, University of Cape Town at Rondebosch, Rondebosch, South Africa.
authors.forename: Nicolette J D
authors.initials: NJ
authors.lastname: Verhoog
authors.forename: Andrea
authors.initials: A
authors.lastname: Du Toit
authors.forename: Chanel
authors.initials: C
authors.lastname: Avenant
authors.forename: Janet P
authors.initials: JP
authors.lastname: Hapgood
mesh_terms: D000818:Animals; D019556:COS Cells; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002522:Chlorocebus aethiops; D004847:Epithelial Cells / Q000378:metabolism*; D005786:Gene Expression Regulation; D006727:Hormone Antagonists / Q000494:pharmacology; D006801:Humans; D007249:Inflammation / Q000235:genetics / Q000378:metabolism; D015850:Interleukin-6 / Q000096:biosynthesis* / Q000235:genetics; D015735:Mifepristone / Q000494:pharmacology; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects / Q000235:genetics; D011401:Promoter Regions, Genetic; D011965:Receptors, Glucocorticoid / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic; D014409:Tumor Necrosis Factor-alpha / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; C491310:GRIP1 protein, human; D006727:Hormone Antagonists; C508600:IL6 protein, human; D015850:Interleukin-6; D009419:Nerve Tissue Proteins; D011965:Receptors, Glucocorticoid; D014409:Tumor Necrosis Factor-alpha; D015735:Mifepristone