Cross-regulation of signaling pathways by interferon-gamma: implications for immune responses and autoimmune diseases

Abstract

Interferon-gamma (IFN-gamma) is an important mediator of immunity and inflammation that utilizes the JAK-STAT signaling pathway to activate the STAT1 transcription factor. Many functions of IFN-gamma have been ascribed to direct STAT1-mediated induction of immune effector genes, but recently it has become clear that key IFN-gamma functions are mediated by cross-regulation of cellular responses to other cytokines and inflammatory factors. Here, we review mechanisms by which IFN-gamma and STAT1 regulate signaling by Toll-like receptors, inflammatory factors, tissue-destructive cytokines, anti-inflammatory cytokines, and cytokines that activate opposing STATs. These signaling mechanisms reveal insights about how IFN-gamma regulates macrophage activation, inflammation, tissue remodeling, and helper and regulatory T cell differentiation and how Th1 and Th17 cell responses are integrated in autoimmune diseases.

Figure 1. IFN-γ-induced STAT1 Activation Cycle Mediated by Tyrosine phosphorylation and lysine acetylation

Unphosphorylated STAT1 dimers are present in the cytoplasm in an equilibrium state between a parallel or antiparallel conformation of STAT1 monomers. Upon activation of IFNGR signaling, STAT1 is phosphorylated by Jak kinases on tyrosine 701 and phosphorylation stabilizes a parallel conformational state that exhibits DNA binding activity. Phosphorylated STAT1 translocates to nucleus, binds to GAS DNA sequences and activates transcription of STAT1 target genes. Active STAT1 in the nucleus undergoes acetylation on lysines 410 and 413, a process catalyzed by histone acetyltransferase (HAT) CBP. Acetylation flags STAT1 for dephosphorylation by the STAT1 phosphatase TCP45; an antiparallel structure facilitates efficient dephosphorylation and thus deactivation. Dephosphorylated STAT1 recycles back to cytoplasm, where a histone deacetylase (HDAC), HDAC3, deacetylates STAT1 and completes the phosphorylation/acetylation cycle. Deacetylation of STAT1 results in less efficient TCP45-mediated dephosphorylation and thus primes STAT1 for IFNGR-Jak-mediated tyrosine phosphorylation.

Figure 2. IFN-γ Signaling Disrupts TLR-induced Feedback Inhibitory Loops

(A) IFN-γ enhances TLR-induced TNF production by disrupting an IL-10-mediated inhibitory loop. IFN-γ signaling leads to increased activity of GSK3, which negatively regulates Il10 expression by suppressing activation of transcription factors CREB and AP-1. (B) IFN-γ enhances TLR-induced IL-6 and IL-12 production by disrupting an inhibitory loop mediated by canonical Notch target genes Hes1 and Hey1. IFN-γ signaling downregulates intracellular NICD2 amounts and thus inhibits expression of Hes1 and Hey1. Hes1 and Hey1 are transcription repressors that negatively regulate Il6 and Il12 gene expression.

Figure 3. Signaling Mechanisms Associated with IFN-γ-mediated Attenuation of Tissue Destruction

(A) IFN-γ suppresses inflammatory tissue destruction via regulation of IL-1R and TLR signaling. IFN-γ inhibits IL-1 signaling and subsequent induction of destructive factors in macrophages by downregulating IL-1RI expression. In addition, IFN-γ blocks induction of MMP downstream of TLR signaling by superinducing transcription repressor ATF3 and inhibiting transcription activators CREB and AP-1. IFN-γ inhibits CREB activity by suppressing its serine phosphorylation and inhibits AP-1 by downregulating nuclear protein levels of its subunits. (B) IFN-γ inhibits osteoclastogenesis and bone resorption via regulation of RANK, CSF-1R, and TREM2 signaling. In osteoclast progenitor cells, IFN-γ suppresses expression as well as signal transduction of RANK, CSF-1R, and TREM2, receptors critical for the process of osteoclastogenesis. (C) IFN-γ attenuates fibrosis via inhibition of TGFβR and IL-4R signaling. IFN-γ suppresses TGFβR signaling by induction of inhibitory SMAD (SMAD7) and by direct inhibition of SMAD3 by STAT1. IFN-γ inhibits IL-4R signaling by induction of SOCS1 (see Figure 5A for details).

Figure 4. Signaling Mechanisms Associated with IFN-γ-mediated Regulation of T Cell Differentiation and Function

(A) In Th1 differentiation, IFN-γ-STAT1 signaling is critical for induction of T-bet and thus for sustaining the positive feedback loop that leads to heightened production of IFN-γ. (B) IFN-γ blocks Th2 differentiation by inhibiting IL-4-STAT6 signaling. (C) IFN-γ and STAT1 block Th17 differentiation. IFN-γ-STAT1 signaling can potently inhibit Th17 differentiation but the mechanism of action is not clear. As STAT3 signaling from multiple cytokines including IL-6, IL-23, and IL-21 plays a pivotal role in mediating Th17 differentiation, it is possible that IFN-γ-STAT1 suppresses Th17 by targeting STAT3 (as shown by dotted lines). IFN-γ and STAT1 also inhibit the aryl hydrocarbon nuclear receptor (AHR) important for Th17 differentiation and it is also possible that suppression of TGFβ and IL-1signaling by IFN-γ contributes to inhibition of Th17 differentiation (not depicted). (D) IFN-γ regulates Treg differentiation and function. IFN-γ can block TGFβ-mediated Treg differentiation. Recently, a more complex role of IFN-γ in regulation of Treg differentiation has emerged. In Foxp3⁺ Treg cells, IFN-γ upregulates expression of T-bet, which in turns promotes expression of CXCR3 that regulates homing of T-bet⁺ FoxP3⁺ Tregs to sites of Th1 inflammation. T-bet also increases suppressive function of Tregs, and T-bet⁺ FoxP3⁺ Tregs effectively suppress Th1 inflammation in vivo.

Figure 5. Mechanisms by which STATs can oppose each other

(A) Antagonizing effects between STATs can be mediated by indirect mechanisms such as induction of inhibitory molecules. An example is depicted here as STAT1 cross-inhibits STAT6 via induction of SOCS1. (B) Individual STATs compete for receptor docking sites, DNA binding elements, and/or binding cofactors. (C) A given STAT could bind and sequester other STATs from forming transcriptionally active complexes. Although the function of STAT heterodimers are not clear, it is plausible that heterodimers may be less active than homodimers in gene induction or possess alternative functions distinct from those of homodimers. (D) Although not experimentally proven, it is conceivable that a given STAT could directly bind to negative regulatory element(s) on the promoter of a gene driven by another STAT and suppress transcription.