Inhibition of extracellular signal-regulated kinase (ERK) signaling participates in repression of nuclear factor (NF)-κB activity by glucocorticoids

Glucocorticoid (GC) effects are mediated via the GC-receptor (GR), which either stimulates or represses gene expression. Repression of target genes often involves negative cross-talk between the GR and other transcription factors e.g. NF-kappa B, important for gene activation. Using HEK293 cells we here describe that repression of NF-kappa B requires functions of the GR that are dependent on the signaling pathways employed to activate NF-kappa B. While a GR mutant was able to repress NF-kappa B activity following activation by TNF alpha, it did not so following activation by the phorbol ester TPA. In these cells, TPA stimulation but not TNF alpha, activated extracellular signal-regulated kinase (ERK). We demonstrated that the ability of the dexamethasone activated GR mutant to repress TPA-induced NF-kappa B activity was restored in conjunction with ERK1/2 inhibition. Previous reports have shown GC-mediated inhibition of ERK1/2 phosphorylation to involve GC induction of MAPK phosphatase-1 (MKP-1). Here, we demonstrated that the GR(R488Q) mutant was incapable of inducing gene expression of endogenous MKP-1 following dexamethasone treatment, in contrast to the GR(wt). However, TPA treatment alone resulted in much stronger MKP-1 expression in both GRwt and GRR488Q containing cells than that of dexamethasone suggesting that the inability of GR(R488Q) to inhibit TPA-induced NF-kappa B activity did not involve a lack of MKP-1 expression. In line with this, RNAi targeted towards MKP-1 did not abolish or inhibit the ability of the GR(wt) to repress NF-kappa B activity. Importantly, we observed no difference in activated ERK1/2 (phospho-ERK1/2) expression over time between GR(wt) and GR(R488Q) containing cells following co-treatment with TPA and dexamethasone. Based on these results we suggest that GR(wt) does not directly regulate ERK1/2 but rather alters ERK1/2-mediated effects allowing it to repress NF-kappa B activity, a capacity lacked by the GR(R488Q) mutant. (c) 2008 Elsevier B.V. All rights reserved.