In steroid-resistant asthma, inflammatory factors lead to an excess of transcription factors that antagonize steroid signaling via the competition with glucocorticoid receptor (GR)-associated co-activators. We here showed that competition between interferon regulatory factor-1 and GR for GR interacting protein-1 is a novel molecular mechanism promoting steroid dysfunction in inflammatory diseases.
Glucocorticoids (GCs) remain the cornerstone treatment for chronic inflammatory diseases such as asthma (1
). Five to ten percent of patients with asthma, however, develop steroid insensitivity. Therefore, an unmet need requires the exploration of alternative therapeutic targets to be used in conjunction with or separately from GCs (2
). A strong correlation between steroid insensitivity and inflammatory diseases, such as asthma, nasal polyps, and inflammatory bowel disease (3
), prompted investigators to examine whether inflammatory mediators modulate the cellular responses to steroids. Using peripheral blood mononuclear cells (PBMCs), investigators showed that GC effects were dramatically reduced in the presence of cytokines. Kam and colleagues were the first to show that IL-2 and IL-4 reduced the inhibitory effect of methylprednisolone on mitogen-induced T cell proliferation (6
). Other cytokines such as IL-1β, IL-6, IFN-γ, IL-2, IL-7, IL-13, IL-15, or IL-8 also attenuate dexamethasone effects in PBMCs (7
), proliferating T cells, monocytes, and neutrophils. Thus, cytokines may promote the development of steroid insensitivity seen in patients with asthma by reducing cell/tissue sensitivity to GCs.
Despite considerable efforts in immune cells, the alteration of steroid responsiveness in structural cell types of the target tissues remains poorly defined. We have been studying the modulation of steroid responsiveness in airway smooth muscle (ASM), which is increasingly recognized as an important player in the pathogenesis of asthma by driving airway inflammation (11
) and may therefore be a target for inhaled GCs (12
). Accordingly, we and others showed that GCs were effective in abrogating the expression of a number of proinflammatory cytokines, chemokines, and adhesion molecules in ASM cells when exposed to a “single” proinflammatory stimulus (13
). Yet, we recently found that steroid insensitivity can develop in ASM exposed to a “mixture” of proasthmatic cytokines. Treatment of ASM cells with the specific combination of IFNs with TNF-α impairs the ability of GCs to inhibit the expression of calcium regulatory protein CD38, the chemokines RANTES and fractalkine, and cell surface proteins such as intercellular adhesion molecule-1 and Toll-like receptor-2 (8
The mechanisms underlying cytokine-induced steroid insensitivity in ASM cells have not been completely defined, but we recently showed that a “short-term” treatment of ASM cells with IFNs and TNF-α partially inhibits steroid transactivation through the accumulation of interferon regulatory factor (IRF)-1 (14
), an early response gene involved in diverse transcriptional regulatory processes (15
). Interestingly, polymorphism in IRF-1 has been associated with childhood atopic asthma (16
). Because expression of IRF-1 is increased after viral infections (17
) and because IRF-1 suppresses steroid effectiveness in ASM cells (14
), we proposed that IRF-1 may mediate the reduced steroid responsiveness seen in patients with asthma experiencing viral infections (18
The precise transcriptional mechanism by which IRF-1 interferes with GC signaling remains to be determined. Most anti-inflammatory effects of steroids are conferred by the GC receptor (GR), a ligand-dependent transcriptional regulator that suppresses the expression of inflammatory genes (3
). Steroid receptor coactivator (SRC)/p160 family members (SRC1, SRC2/transcriptional intermediary factor 2 [TIF2]/GR interacting protein-1 [GRIP-1], and SRC3/receptor-associated co-activator 3 [RAC3]/p300/CREB binding protein [CBP]-co-integrator protein [pCIP]/amplifie in breast 1 [AIB1]) serve as co-activators for all nuclear receptors including GR. As such, this family of proteins interacts with ligand-bound GR to recruit histone acetyltransferases (CBP/p300) and coactivator-associated arginine methyltransferase 1 (CARM1) that unpack the condensed chromatin, thereby facilitating the access of transcription factors to target genes. Unlike other p160 s, GRIP-1 also possesses a unique GR co-repressor activity facilitating GC-mediated repression of activator protein (AP)-1 and NF-κB activities (19
). Although originally identified as a nuclear receptor co-factor, GRIP-1 was later shown to engage in physical and functional interactions with an IRF family member (IRF-3) and serves as an IRF-3 co-activator in macrophages (21
). However, the role of GRIP-1 in ASM cells has not been assessed. Here we characterized whether GRIP-1 is a component of the IRF-1 transcriptional regulatory complexes and whether such interactions affect the response of ASM cells to GCs.