Repression of TNF-alpha-induced IL-8 expression by the glucocorticoid receptor-beta involves inhibition of histone H4 acetylation

ABSTRACT

Increased expression of a number of proinflammatory genes, including IL-8, is associated with inflammatory conditions such as asthma. Glucocorticoid receptor (GR)beta, one of the GR isoforms, has been suggested to be upregulated in asthma associated with glucocorticoid insensitivity and to work as a dominant negative inhibitor of wild type GRalpha. However, recent data suggest that GRbeta is not a dominant negative inhibitor of GRalpha in the transrepressive process and has its own functional role. We investigated the functional role of GRbeta expression in the suppressive effect of glucocorticoids on tumor necrosis factor (TNF)-alpha-induced IL-8 release in an airway epithelial cell line. GRbeta expression was induced by treatment of epithelial cells with either dexamethasone or TNF-alpha. GRbeta was able to inhibit glucocorticoid-induced transcriptional activation mediated by binding to glucocorticoid response elements (GREs). The suppressive effect of dexamethasone on TNF-alpha-induced IL-8 transcription was not affected by GRbeta overexpression, rather GRbeta had its own weak suppressive activity on TNF-alpha-induced IL-8 expression. Overall histone deacetylase activity and histone acetyltransferase activity were not changed by GRbeta overexpression, but TNF-alpha-induced histone H4 acetylation at the IL-8 promoter was decreased with GRbeta overexpression. This study suggests that GRbeta overexpression does not affect glucocorticoid-induced suppression of IL-8 expression in airway epithelial cells and GRbeta induces its own histone deacetylase activity around IL-8 promoter site.