Therapeutic Targeting of IRFs: Pathway-Dependence or Structure-Based?

Abstract

The interferon regulatory factors (IRFs) are a family of master transcription factors that regulate pathogen-induced innate and acquired immune responses. Aberration(s) in IRF signaling pathways due to infection, genetic predisposition and/or mutation, which can lead to increased expression of type I interferon (IFN) genes, IFN-stimulated genes (ISGs), and other pro-inflammatory cytokines/chemokines, has been linked to the development of numerous diseases, including (but not limited to) autoimmune and cancer. What is currently lacking in the field is an understanding of how best to therapeutically target these transcription factors. Many IRFs are regulated by post-translational modifications downstream of pattern recognition receptors (PRRs) and some of these modifications lead to activation or inhibition. We and others have been able to utilize structural features of the IRFs in order to generate dominant negative mutants that inhibit function. Here, we will review potential therapeutic strategies for targeting all IRFs by using IRF5 as a candidate targeting molecule.

Keywords: IRF5; autoimmunity; inhibition; negative regulation; positive regulation.

Figure 1

A schematic representation of full-length human IRFs showing different functional domains. All IRFs harbor a DNA binding domain that contains a conserved tryptophan pentad (pink) in the N-terminus. They also contain an IRF activation domain termed either IAD1 (orange) or IAD2 (red). Other domains present are a nuclear localization signal (NLS, purple), nuclear export signal (NES, blue-green), an autoinhibitory domain (black), and a regulatory domain (blue). In this scheme, IRF activation (green triangles) is denoted as phosphorylation. The length of each IRF is indicated by the number of amino acids (aa), as found in Uniprot, with each identifier listed. IRF, interferon regulatory factor; C, carboxy terminus; N, amino terminus.

Figure 2

The Serine Rich Region (SRR) is conserved in all IRF5 isoforms. The C-terminus contains conserved serine (S) residues that are targeted for phosphorylation by kinases, such as IKKβ (blue-bolded serine). Red-bolded serines are those originally identified as critical for IRF5 activation (101, 13). Phosphorylation leads to structural changes, including removal of the AID, liberation of the IAD and exposure of the C-terminus for further modification(s) and/or protein interaction. Although IRF5 isoforms range in size, most contain the SRR independent of its numerical amino acid location.

Figure 3

Modified crystal structures of IRF5. (A) Homology model of the inactive IRF5 C-terminal domain (variant 5) generated using the monomeric autoinhibited IRF3 C-terminal domain (PDB: 1QWT) as a template (113). Representative image from docking of an inhibitor (112) to the C-terminal SRR of the inactive IRF5 monomer, which results in maintenance of a closed, non-phosphorylated conformation. Orange balls represent phosphorylation sites at the C-terminal SRR. (B) Representative image generated from IRF5 crystal structure coordinates (13) showing formation of an IRF5 homodimer. Arrows show critical regions that are being therapeutically targeted to inhibit homodimerization between Helix 2 and Helix 5 (111).

Figure 4

The canonical IRF5 signaling pathway and its negative regulation. (A) Upon ligand binding to TLR7/8, MyD88 gets recruited in, along with IRAK1/4 and TRAF6, which leads to the autophosphorylation of IRAK4 and ubiquitination of IRF5 by TRAF6. IRAK4 then activates TAK1, which then phosphorylates IKKβ. The ubiquitinated IRF5 is then phosphorylated by IKKβ (or other kinases). This action results in homodimerization and translocation of the IRF5 homodimer to the nucleus, leading to the production of downstream cytokines. Lyn kinase, IKKα and IRF4, on the other hand, were found to negatively regulate IRF5 activity. TRIM21 is a molecule that targets IRF5 for proteasomal- or lysosomal-mediated degradation. (B) A negative feedback loop may also be involved in the suppression of IRF5-mediated inflammatory gene transcription. TAK1 initiates a series of phosphorylation events on different kinases, including MMK3/MKK6, P38α/MAPK, MSK1/MSK2, and CREB, which leads to the upregulation of IL10. SIK2, on the other hand, inhibits CRTC3 activity by phosphorylation leading to its cytosolic localization and inhibition of IL10 expression. SIK2 also inhibits inflammatory molecules, such as TNF and IL12 by unknown mechanisms that may involve inhibition of IRF5 (shown by ?).