The N-terminal loop of IRAK-4 death domain regulates ordered assembly of the Myddosome signalling scaffold

IL-1 receptor-associated kinase family proteins: An overview of their role in liver disease

The interleukin-1 receptor-associated kinase (IRAK) family is a group of serine-threonine kinases that regulates various cellular processes via toll-like receptor (TLR)/interleukin-1 receptor (IL1R)-mediated signaling. The IRAK family comprises four members, including IRAK1, IRAK2, IRAK3, and IRAK4, which play an important role in the expression of various inflammatory genes, thereby contributing to the inflammatory response. IRAKs are key proteins in chronic and acute liver diseases, and recent evidence has implicated IRAK family proteins (IRAK1, IRAK3, and IRAK4) in the progression of liver-related disorders, including alcoholic liver disease, non-alcoholic steatohepatitis, hepatitis virus infection, acute liver failure, liver ischemia-reperfusion injury, and hepatocellular carcinoma. In this article, we provide a comprehensive review of the role of IRAK family proteins and their associated inflammatory signaling pathways in the pathogenesis of liver diseases. The purpose of this study is to explore whether IRAK family proteins can serve as the main target for the treatment of liver related diseases.

TLR4 TIR domain and nucleolin GAR domain synergistically mediate RSV infection and induce neuronal inflammatory damage in SH-SY5Y cells

Previous research results of our group showed that Toll-like receptor 4 (TLR4) and nucleolin synergistically mediate respiratory syncytial virus (RSV) infection in human central neuron cells, but the specific mechanism remains unclear. Here we designed and synthesized lentiviruses with TIR (674-815 aa), TLR4 (del 674-815 aa), GAR (645-707 aa), and NCL (del 645-707 aa) domains, and obtained stable overexpression cell lines by drug screening, and subsequently infected RSV at different time points. Laser confocal microscopy and coimmunoprecipitation were used for the observation of co-localization and interaction of TIR/GAR domains. Western blot analysis was used for the detection of p-NF-κB and LC3 protein expression. Real-time PCR was used for the detection of TLR4/NCL mRNA expression. ELISA assay was used to measure IL-6, IL-1β, and TNF-α concentrations and flow cytometric analysis was used for the study of apoptosis. Our results suggest that overexpression of TIR and GAR domains can exacerbate apoptosis and autophagy, and that TIR and GAR domains can synergistically mediate RSV infection and activate the NF-κB signaling pathway, which regulates the secretion of downstream inflammatory factors, such as IL-6, IL-1β, and TNF-α, and ultimately leads to neuronal inflammatory injury.

Interleukin-1 receptor accessory protein (IL-1RAP): A magic bullet candidate for immunotherapy of human malignancies

IL-1, plays a role in some pathological inflammatory conditions. This pro-inflammatory cytokine also has a crucial role in tumorigenesis and immune responses in the tumor microenvironment (TME). IL-1 receptor accessory protein (IL-1RAP), combined with IL-1 receptor-1, provides a functional complex for binding and signaling. In addition to the direct role of IL-1, some studies demonstrated that IL1-RAP has essential roles in the progression, angiogenesis, and metastasis of solid tumors such as gastrointestinal tumors, lung carcinoma, glioma, breast and cervical cancers. This molecule also interacts with FLT-3 and c-Kit tyrosine kinases and is involved in the pathogenesis of hematological malignancies such as acute myeloid lymphoma. Additionally, IL-1RAP interacts with solute carrier family 3 member 2 (SLC3A2) and thereby increasing the resistance to anoikis and metastasis in Ewing sarcoma. This review summarizes the role of IL-1RAP in different types of cancers and discusses its targeting as a novel therapeutic approach for malignancies.

Interplay between inflammasomes and PD-1/PD-L1 and their implications in cancer immunotherapy

The inflammasomes play crucial roles in inflammation and cancer development, while the PD-1/PD-L1 pathway is critical for immune suppression in the tumor microenvironment (TME). Recent research indicates a reciprocal regulatory relationship between inflammasomes and PD-1/PD-L1 signaling in cancer development and PD-1 blockade treatment. By activating in diverse cells in tumor tissues, inflammasome upregulates PD-L1 level in the TME. Moreover, the regulation of PD-1/PD-L1 activity by inflammasome activation involves natural killer cells, tumor-associated macrophages and myeloid-derived suppressor cells. Conversely, PD-1 blockade can activate the inflammasome, potentially influencing treatment outcomes. The interplay between inflammasomes and PD-1/PD-L1 has profound and intricate effects on cancer development and treatment. In this review, we discuss the crosstalk between inflammasomes and PD-1/PD-L1 in cancers, exploring their implications for tumorigenesis, metastasis and immune checkpoint inhibitor (ICI) resistance. The combined therapeutic strategies targeting both inflammasomes and checkpoint molecules hold promising potential as treatments for cancer.

IRAK4 degrader in hidradenitis suppurativa and atopic dermatitis: a phase 1 trial

Toll-like receptor-driven and interleukin-1 (IL-1) receptor-driven inflammation mediated by IL-1 receptor-associated kinase 4 (IRAK4) is involved in the pathophysiology of hidradenitis suppurativa (HS) and atopic dermatitis (AD). KT-474 (SAR444656), an IRAK4 degrader, was studied in a randomized, double-blind, placebo-controlled phase 1 trial where the primary objective was safety and tolerability. Secondary objectives included pharmacokinetics, pharmacodynamics and clinical activity in patients with moderate to severe HS and in patients with moderate to severe AD. KT-474 was administered as a single dose and then daily for 14 d in 105 healthy volunteers (HVs), followed by dosing for 28 d in an open-label cohort of 21 patients. Degradation of IRAK4 was observed in HV blood, with mean reductions after a single dose of ≥93% at 600-1,600 mg and after 14 daily doses of ≥95% at 50-200 mg. In patients, similar IRAK4 degradation was achieved in blood, and IRAK4 was normalized in skin lesions where it was overexpressed relative to HVs. Reduction of disease-relevant inflammatory biomarkers was demonstrated in the blood and skin of patients with HS and patients with AD and was associated with improvement in skin lesions and symptoms. There were no drug-related infections. These results, from what, to our knowledge, is the first published clinical trial using a heterobifunctional degrader, provide initial proof of concept for KT-474 in HS and AD to be further confirmed in larger trials. ClinicalTrials.gov identifier: NCT04772885 .

Role of innate immunological/inflammatory pathways in myelodysplastic syndromes and AML: a narrative review

Dysregulation of the innate immune system and inflammatory-related pathways has been implicated in hematopoietic defects in the bone marrow microenvironment and associated with aging, clonal hematopoiesis, myelodysplastic syndromes (MDS), and acute myeloid leukemia (AML). As the innate immune system and its pathway regulators have been implicated in the pathogenesis of MDS/AML, novel approaches targeting these pathways have shown promising results. Variability in expression of Toll like receptors (TLRs), abnormal levels of MyD88 and subsequent activation of NF-κβ, dysregulated IL1-receptor associated kinases (IRAK), alterations in TGF-β and SMAD signaling, high levels of S100A8/A9 have all been implicated in pathogenesis of MDS/AML. In this review we not only discuss the interplay of various innate immune pathways in MDS pathogenesis but also focus on potential therapeutic targets from recent clinical trials including the use of monoclonal antibodies and small molecule inhibitors against these pathways.

Molecular characterization, expression and functional analysis of IRAK1 and IRAK4 in Nile tilapia (Oreochromis niloticus)

Interleukin-1 receptor-associated kinase 1 (IRAK1) and IRAK4 are critical signalling mediators and play pivotal roles in the innate immune and inflammatory responses mediated by TLR/IL-1R. In the present study, two IRAK family members, OnIRAK1 and OnIRAK4, were identified in the Nile tilapia Oreochromis niloticus with a conserved N-terminal death domain and a protein kinase domain, similar to those of other fishes and mammals. The gene structures of OnIRAK1 and OnIRAK4 are organized into fifteen exons split by fourteen introns and ten exons split by nine introns. OnIRAK1 and OnIRAK4 were broadly expressed in all of the tissues tested, with the highest expression levels being observed in the blood and the lowest expression levels being observed in the liver. Both genes could be detected from 2 d post-fertilization (dpf) to 8 dpf during embryonic development. Moreover, the expression levels of OnIRAK1 and OnIRAK4 were clearly altered in all five tissues after Streptococcus agalactiae infection in vivo and could be induced by LPS, Poly I: C, S. agalactiae WC1535 and △CPS in Nile tilapia macrophages. The overexpression of OnIRAK1 and OnIRAK4 in 293T cells showed that they were both distributed in the cytoplasm and could significantly increase NF-κB activation. Interestingly, after transfection, OnIRAK1 significantly upregulated OnMyd88-induced NF-κB activation, while OnIRAK4 had no effect on OnMyd88-induced NF-κB activation. Co-immunoprecipitation (Co-IP) assays showed that OnMyd88 did not interact with either OnIRAK1 or OnIRAK4 and that OnIRAK1 did not interact with OnIRAK4. Taken together, these findings suggest that OnIRAK1 and OnIRAK4 could play important roles in TLR/IL-1R signalling pathways and the immune response to pathogen invasion.

IRAK3 modulates downstream innate immune signalling through its guanylate cyclase activity

Interleukin-1 receptor associated kinase 3 (IRAK3) is a cytoplasmic homeostatic mediator of inflammatory responses and is potentially useful as a prognostic marker in inflammation. IRAK3 inhibits signalling cascades downstream of myddosome complexes associated with toll like receptors. IRAK3 contains a death domain that interacts with other IRAK family members, a pseudokinase domain and a C-terminus domain involved with tumour necrosis factor receptor associated factor 6 (TRAF6). Previous bioinformatic studies revealed that IRAK3 contained a guanylate cyclase centre in its pseudokinase domain but its role in IRAK3 action is unresolved. We demonstrate that wildtype IRAK3 is capable of producing cGMP. Furthermore, we show that a specific point mutation in the guanylate cyclase centre reduced cGMP production. Cells containing toll like receptor 4 and a nuclear factor kappa-light-chain-enhancer of activated B cells (NFĸB) reporter system were transfected with IRAK3 or mutant IRAK3 proteins. Cell-permeable cGMP treatment of untransfected control cells suppresses downstream signalling through modulation of the NFĸB in the presence of lipopolysaccharides. Cells transfected with wildtype IRAK3 also suppress lipopolysaccharide induced NFĸB activity in the absence of exogenous cGMP. Lipopolysaccharide induced NFĸB activity was not suppressed in cells transfected with the IRAK3 mutant with reduced cGMP-generating capacity. Whereas in the presence of exogenously applied cell-permeable cGMP the IRAK3 mutant was able to retain its function by suppressing lipopolysaccharide induced NFĸB activity. Furthermore, increasing the amount of membrane permeable cGMP did not affect IRAK3's ability to reduce NFĸB activity. These results suggest that cGMP generated by IRAK3 may be involved in regulatory function of the protein where the presence of cGMP may selectively affect downstream signalling pathway(s) by modulating binding and/or activity of nearby proteins that interact in the inflammatory signalling cascade.

Putative link between Polo-like kinases (PLKs) and Toll-like receptor (TLR) signaling in transformed and primary human immune cells

Toll-like receptors (TLRs) are important sentinels of bacterial and viral infection and thus fulfil a critical sensory role in innate immunity. Polo-like kinases (PLKs), a five membered family of Ser/Thr protein kinases, have long been studied for their role in mitosis and thus represent attractive therapeutic targets in cancer therapy. Recently, PLKs were implicated in TLR signaling in mice but the role of PLKs in TLR signaling in untransformed primary immune cells has not been addressed, even though PLK inhibitors are in clinical trials. We here identified several phospho-serine and phospho-threonine residues in the known TLR pathway kinases, Interleukin-1 receptor-associated kinase (IRAK) 2 and IRAK4. These sites lie in canonical polo-box motifs (PBM), sequence motifs known to direct recruitment of PLKs to client proteins. Interestingly, PLK1 was phosphorylated and PLK 2 and 3 mRNA induced upon TLR stimulation in primary immune cells, respectively. In whole blood, PLK inhibition disparately affected TLR mediated cytokine responses in a donor- and inhibitor-dependent fashion. Collectively, PLKs may thus potentially interface with TLR signaling in humans. We propose that temporary PLK inhibitor-mediated blockade of TLR-signaling in certain patients receiving such inhibitors during cancer treatment may cause adverse effects such as an increased risk of infections due to a then compromised ability of the TLR recognition system to sense and initiate cytokine responses to invading microbes.

Toll-like receptor 4: A promising therapeutic target for pneumonia caused by Gram-negative bacteria

Gram‐negative bacteria (GNB) emerge as important pathogens causing pulmonary infection, which can develop into sepsis due to bacterial resistance to antibiotics. GNB pneumonia poses a huge social and economic burden all over the world. During GNB infection in the lung, Toll‐like receptor 4 (TLR4) can form a complex with MD2 and CD14 after recognizing lipopolysaccharide of GNB, initiate the MyD88‐ and TRIF‐dependent signalling pathways and stimulate host non‐specific immune response. In this review, we summarize recent progress in our understanding of the role of TLR4 in GNB pneumonia. The latest experimental results, especially in TLR4 knockout animals, suggest a promising potential of targeting TLR4 signalling pathway for the treatment of GNB pneumonia. Furthermore, we highlight the benefits of Traditional Chinese Medicine as novel candidates for the therapy of GNB pneumonia due to the modulation of TLR4 signalling pathway. Finally, we discuss the promise and challenge in the development of TLR4‐based drugs for GNB pneumonia.

U2AF1 mutations induce oncogenic IRAK4 isoforms and activate innate immune pathways in myeloid malignancies

Spliceosome mutations are common in myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), but the oncogenic changes due to these mutations have not been identified. Here a global analysis of exon usage in AML samples revealed distinct molecular subsets containing alternative spliced isoforms of inflammatory and immune genes. Interleukin-1 receptor-associated kinase 4 (IRAK4) was the dominant alternatively spliced isoform in MDS and AML and is characterized by a longer isoform that retains exon 4, which encodes IRAK4-long (IRAK4-L), a protein that assembles with the myddosome, results in maximal activation of nuclear factor kappa-light-chain-enhancer of B cells (NF-κB) and is essential for leukaemic cell function. Expression of IRAK4-L is mediated by mutant U2 small nuclear RNA auxiliary factor 1 (U2AF1) and is associated with oncogenic signalling in MDS and AML. Inhibition of IRAK4-L abrogates leukaemic growth, particularly in AML cells with higher expression of the IRAK4-L isoform. Collectively, mutations in U2AF1 induce expression of therapeutically targetable 'active' IRAK4 isoforms and provide a genetic link to activation of chronic innate immune signalling in MDS and AML.

Toll-like receptor 4 and protease-activated receptor 2 in physiology and pathophysiology of the nervous system: more than just receptor cooperation?

Toll-like receptor 4 (TLR4) and protease-activated receptor 2 (PAR2) play pivotal roles in the mammalian innate immune response. Notably, in addition to their involvement in detection of invading pathogens, PAR2 and TLR4 modulate the levels of cell death-induced sterile inflammation by activating pro- or anti-inflammatory downstream signaling cascades. Within the central nervous system, there is emerging evidence that both receptors are involved in synaptic transmission and brain plasticity. Furthermore, due to their prominent role in mediating neuroinflammation, PAR2 and TLR4 are associated with development and progression of neurodegenerative disorders including but not limited to Alzheimer’s disease, Parkinson’s disease and multiple sclerosis. In this article, we summarise the current knowledge on the cooperation between PAR2 and TLR4, discuss the potential cross-talk levels and highlight the impact of the cross-coupling on neuroinflammation.

Understanding early TLR signaling through the Myddosome

TLRs are expressed on the plasma and endosomal membranes of innate immune cells acting as sensors of foreign and inherent danger signals that threaten the host. Upon activation, TLRs facilitate the assembly of large intracellular oligomeric signaling complexes, termed Myddosomes, which initiate key signal transduction pathways to elicit critical inflammatory immune responses. The formation of the Myddosome is integral for TLR signaling; however, the molecular mechanisms controlling its formation, disassembly, and the subsequent proximal signaling events remain to be clearly defined. In this review, we present a brief overview of TLR signal transduction pathways, summarize the current understanding of the Myddosome and the proteins that comprise its structure, including MyD88 and members of the IL-1 receptor-associated kinase (IRAK) family. Finally, we will discuss recent advances and open questions regarding early TLR signaling in the context of the Myddosome complex.

Mechanism of dysfunction of human variants of the IRAK4 kinase and a role for its kinase activity in interleukin-1 receptor signaling

Interleukin-1 receptor (IL1R)-associated kinase 4 (IRAK4) is a central regulator of innate immune signaling, controlling IL1R and Toll-like receptor (TLR)-mediated responses and containing both scaffolding and kinase activities. Humans deficient in IRAK4 activity have autosomal recessive primary immune deficiency (PID). Here, we characterized the molecular mechanism of dysfunction of two IRAK4 PID variants, G298D and the compound variant R12C (R12C/R391H/T458I). Using these variants and the kinase-inactive D329A variant to delineate the contributions of IRAK4's scaffolding and kinase activities to IL1R signaling, we found that the G298D variant is kinase-inactive and expressed at extremely low levels, acting functionally as a null mutation. The R12C compound variant possessed WT kinase activity, but could not interact with myeloid differentiation primary response 88 (MyD88) and IRAK1, causing impairment of IL-1-induced signaling and cytokine production. Quantitation of IL-1 signaling in IRAK4-deficient cells complemented with either WT or the R12C or D329A variant indicated that the loss of MyD88 interaction had a greater impact on IL-1-induced signaling and cytokine expression than the loss of IRAK4 kinase activity. Importantly, kinase-inactive IRAK4 exhibited a greater association with MyD88 and a weaker association with IRAK1 in IRAK4-deficient cells expressing kinase-inactive IRAK4 and in primary cells treated with a selective IRAK4 inhibitor. Loss of IRAK4 kinase activity only partially inhibited IL-1-induced cytokine and NF-κB signaling. Therefore, the IRAK4-MyD88 scaffolding function is essential for IL-1 signaling, but IRAK4 kinase activity can control IL-1 signal strength by modulating the association of IRAK4, MyD88, and IRAK1.

Interleukin-1 receptor-associated kinase 4 (IRAK4) plays a dual role in myddosome formation and Toll-like receptor signaling

Toll-like receptors (TLRs) form part of the host innate immune system, in which they act as sensors of microbial and endogenous danger signals. Upon TLR activation, the intracellular Toll/interleukin-1 receptor domains of TLR dimers initiate oligomerization of a multiprotein signaling platform comprising myeloid differentiation primary response 88 (MyD88) and members of the interleukin-1 receptor–associated kinase (IRAK) family. Formation of this myddosome complex initiates signal transduction pathways, leading to the activation of transcription factors and the production of inflammatory cytokines. To date, little is known about the assembly and disassembly of the myddosome and about the mechanisms by which these complexes mediate multiple downstream signaling pathways. Here, we isolated myddosome complexes from whole-cell lysates of TLR-activated primary mouse macrophages and from IRAK reporter macrophages to examine the kinetics of myddosome assembly and disassembly. Using a selective inhibitor of IRAK4's kinase activity, we found that whereas TLR cytokine responses were ablated, myddosome formation was stabilized in the absence of IRAK4's kinase activity. Of note, IRAK4 inhibition had only a minimal effect on NF-κB and mitogen-activated protein kinase (MAPK) signaling. In summary, our results indicate that IRAK4 has a critical scaffold function in myddosome formation and that its kinase activity is dispensable for myddosome assembly and activation of the NF-κB and MAPK pathways but is essential for MyD88-dependent production of inflammatory cytokines. Our findings suggest that the scaffold function of IRAK4 may be an attractive target for treating inflammatory and autoimmune diseases.

Interleukin-1 Beta-A Friend or Foe in Malignancies?

Interleukin-1 beta (IL-1β) is induced by inflammatory signals in a broad number of immune cell types. IL-1β (and IL-18) are the only cytokines which are processed by caspase-1 after inflammasome-mediated activation. This review aims to summarize current knowledge about parameters of regulation of IL-1β expression and its multi-facetted role in pathophysiological conditions. IL-1 signaling activates innate immune cells including antigen presenting cells, and drives polarization of CD4+ T cells towards T helper type (Th) 1 and Th17 cells. Therefore, IL-1β has been attributed a largely beneficial role in resolving acute inflammations, and by initiating adaptive anti-tumor responses. However, IL-1β generated in the course of chronic inflammation supports tumor development. Furthermore, IL-1β generated within the tumor microenvironment predominantly by tumor-infiltrating macrophages promotes tumor growth and metastasis via different mechanisms. These include the expression of IL-1 targets which promote neoangiogenesis and of soluble mediators in cancer-associated fibroblasts that evoke antiapoptotic signaling in tumor cells. Moreover, IL-1 promotes the propagation of myeloid-derived suppressor cells. Using genetic mouse models as well as agents for pharmacological inhibition of IL-1 signaling therapeutically applied for treatment of IL-1 associated autoimmune diseases indicate that IL-1β is a driver of tumor induction and development.

The family of the interleukin-1 receptors

The extracellular forms of the IL-1 cytokines are active through binding to specific receptors on the surface of target cells. IL-1 ligands bind to the extracellular portion of their ligand-binding receptor chain. For signaling to take place, a non-binding accessory chain is recruited into a heterotrimeric complex. The intracellular approximation of the Toll-IL-1-receptor (TIR) domains of the 2 receptor chains is the event that initiates signaling. The family of IL-1 receptors (IL-1R) includes 10 structurally related members, and the distantly related soluble protein IL-18BP that acts as inhibitor of the cytokine IL-18. Over the years the receptors of the IL-1 family have been known with many different names, with significant confusion. Thus, we will use here a recently proposed unifying nomenclature. The family includes several ligand-binding chains (IL-1R1, IL-1R2, IL-1R4, IL-1R5, and IL-1R6), 2 types of accessory chains (IL-1R3, IL-1R7), molecules that act as inhibitors of signaling (IL-1R2, IL-1R8, IL-18BP), and 2 orphan receptors (IL-1R9, IL-1R10). In this review, we will examine how the receptors of the IL-1 family regulate the inflammatory and anti-inflammatory functions of the IL-1 cytokines and are, more at large, involved in modulating defensive and pathological innate immunity and inflammation. Regulation of the IL-1/IL-1R system in the brain will be also described, as an example of the peculiarities of organ-specific modulation of inflammation.

NF-κB Activation in Lymphoid Malignancies: Genetics, Signaling, and Targeted Therapy

The NF-κB transcription factor family plays a crucial role in lymphocyte proliferation and survival. Consequently, aberrant NF-κB activation has been described in a variety of lymphoid malignancies, including diffuse large B-cell lymphoma, Hodgkin lymphoma, and adult T-cell leukemia. Several factors, such as persistent infections (e.g., with Helicobacter pylori), the pro-inflammatory microenvironment of the cancer, self-reactive immune receptors as well as genetic lesions altering the function of key signaling effectors, contribute to constitutive NF-κB activity in these malignancies. In this review, we will discuss the molecular consequences of recurrent genetic lesions affecting key regulators of NF-κB signaling. We will particularly focus on the oncogenic mechanisms by which these alterations drive deregulated NF-κB activity and thus promote the growth and survival of the malignant cells. As the concept of a targeted therapy based on the mutational status of the malignancy has been supported by several recent preclinical and clinical studies, further insight in the function of NF-κB modulators and in the molecular mechanisms governing aberrant NF-κB activation observed in lymphoid malignancies might lead to the development of additional treatment strategies and thus improve lymphoma therapy.