DUSP1 Maintains IRF1 and Leads to Increased Expression of IRF1-dependent Genes

Abstract

Although the mitogen-activated protein kinase (MAPK) phosphatase, DUSP1, mediates dexamethasone-induced repression of MAPKs, 14 of 46 interleukin-1␤ (IL1B)-induced mRNAs were significantly enhanced by DUSP1 overexpression in pulmonary A549 cells. These include the interferon regulatory factor, IRF1, and the chemokine, CXCL10. Of these, DUSP1-enhanced mRNAs, 10 including CXCL10, were IRF1-dependent. MAPK inhibitors and DUSP1 overexpression prolonged IRF1 expression by elevating transcription and increasing IRF1 mRNA and protein stability. Conversely, DUSP1 silencing increased IL1B-induced MAPK phosphorylation while significantly reducing IRF1 protein expression at 4 h. This confirms a regulatory network whereby DUSP1 switches off MAPKs to maintain IRF1 expression. There was no repression of IRF1 expression by dexamethasone in primary human bronchial epithelial cells, and in A549 cells IL1B-induced IRF1 protein was only modestly and transiently repressed. Although dexamethasone did not repress IL1B-induced IRF1 protein expression at 4 -6 h, silencing of IL1B plus dexamethasone-induced DUSP1 significantly reduced IRF1 expression. IL1B-induced expression of CXCL10 was largely insensitive to dexamethasone, whereas other DUSP1-enhanced, IRF1-dependent mRNAs showed various degrees of repression. With IL1B plus dexamethasone, CXCL10 expression was also IRF1-dependent, and expression was reduced by DUSP1 silencing. Thus, IL1B plus dexamethasone-induced DUSP1 maintains expression of IRF1 and the IRF1-dependent gene, CXCL10. This is supported by chromatin immunoprecipitation showing IRF1 recruitment to be essentially unaffected by dexamethasone at the CXCL10 promoter or at the promoters of more highly repressed IRF1-dependent genes. Since IRF1-dependent genes, such as CXCL10, are central to host defense, these data may help explain the reduced effectiveness of glucocorticoids during asthma exacerbations.Inhaled glucocorticoids are referred to clinically as inhaled corticosteroids and bind to the glucocorticoid receptor (GR 2 ; NR3C1). These drugs suppress inflammatory gene expression and are a mainstay in the treatment of asthma (1). However, poor responsiveness to inhaled corticosteroid therapy by asthmatics with neutrophilic inflammation or viral infections and by individuals with chronic obstructive pulmonary disease (COPD) necessitates high, often oral, doses of glucocorticoid (1-3). This, if maintained over prolonged periods, may lead to contraindications, including osteoporosis, muscle wasting, increased blood glucose, weight gain, and suppression of the hypothalamic-pituitary-adrenal axis (4). Thus, an unmet clinical need exists to improve the therapeutic ratio of glucocorticoids in such disease situations.Airway epithelial cells release proinflammatory cytokines, chemokines, adhesion molecules, and inflammatory enzymes and play critical roles in asthma (5). Acting on the airway epithelium, glucocorticoids suppress the production of numerous epithelial-derived mediators, and this helps to reduce inflammatory cell re...

Glucocorticoid and cytokine crosstalk: Feedback, feedforward, and co-regulatory interactions determine repression or resistance

“…4). However, although the rapid induction of IRF1 mRNA and protein by IL1B was largely unaffected by MAPK inhibition, their precipitous loss, after peak expression, was significantly attenuated due to a failure to terminate IRF1 expression (77,80). This constitutes a classical incoherent feedforward circuit in which MAPK, primarily p38, inhibition not only prolonged IRF1 transcription and enhanced IRF1 mRNA stability, but also reduced IRF1 degradation (Fig.…”

“…However, overexpression of DUSP1 also increased, often quite dramatically, the expression of 14 of 46 mRNAs induced by IL1B (77). These included the inflammatory transcription factor, IRF1, as well as IRF1-dependent genes, such as the chemokine, CXCL10 (77). Conversely, in bone marrow-derived macrophage from Dusp1 knock-out mice, IRF1 expression, and some 20% of transcripts that were induced Ն3-fold by LPS, showed lower expression when compared with cells from wild-type animals (66).…”

“…Thus, DUSP1 overexpression reduced expression of 19 out of 46 IL1B-induced mRNAs tested in A549 cells, and in mRNAs tested with DUSP1 silencing, or in cells from Dusp1 Ϫ/Ϫ mice, many of these same genes showed enhanced expression (31,36,77). However, overexpression of DUSP1 also increased, often quite dramatically, the expression of 14 of 46 mRNAs induced by IL1B (77). These included the inflammatory transcription factor, IRF1, as well as IRF1-dependent genes, such as the chemokine, CXCL10 (77).…”

“…Indeed, mice lacking Irf1 are susceptible to death during viral infections (86), whereas ubiquitinmediated proteasomal degradation of IRF1 allows viral sup-pression of the immune response (87). In terms of IRF1 mRNA and protein induced by TLR or IL1 type receptors, these follow spike-like kinetics and are negatively regulated by MAPKs (66,77,80) (Fig. 4).…”

“…Furthermore, expression of late-phase genes, for example, the retinoic acid-induced gene I (DDX58) or CXCL10 (77,83,84), that are central in innate immune and antiviral responses are induced by IRF1 (82,85) (Fig. 4A).…”