Interleukin-1 receptor associated kinase

IRAKs were first identified in 1994 by Michael Martin and colleagues when they successfully co-precipitated a protein kinase with type I interleukin-1 receptors (IL-1RI) from human T cells. They speculated that this kinase was the link between the T cell's transmembrane IL-1 receptor and the cytosolic signalling pathway's downstream components. The name “IRAK” came from Zhaodan Cao and colleagues in 1995. The DNA sequence analysis of IRAK's domains revealed many conserved amino acids with the serine/threonine specific protein kinase Pelle in Drosophila, that functions downstream of a Toll receptor. Cao's lab confirmed the kinase's activity as necessarily associated with the IL-1 receptor by immunoprecipitating the IL-1 receptors from different cell types treated with IL-1 and without IL-1. Even cells without over-expressed IL-1 receptors showed kinase activity when exposed to IL-1, and were able to co-precipitate a protein kinase with endogenous IL-1 receptors. Thus the human IL-1 receptor's accessory protein was named Interleukin-1 Receptor-Associated Kinase. In 1997, MyD88 was identified as the cytosolic protein that recruits IRAKs to the cytosolic domains of IL-1 receptors, mediating IL-1's signal transduction to the cytosolic signal cascade. Subsequent studies associated IRAKs with multiple signalling pathways triggered by interleukin, and specified multiple IRAK types. ==Structure==

Functional domains All IRAK family members are multidomain proteins consisting of a conserved N-terminal Death Domain (DD) and a central kinase domain (KD). The DD is a protein interaction motif that important for interacting with other signaling molecules such as the adaptor protein MyD88 and other IRAK members. The KD is responsible for the kinase activity of IRAK proteins and consists of 12 subdomains. All IRAK KDs have an ATP binding pocket with an invariable lysine residue in subdomain II, however, only IRAK-1 and IRAK-4 have an aspartate residue in the catalytic site of subdomain VI, which is thought to be critical for kinase activity. It is thought that IRAK-2 and IRAK-M are catalytically inactive because they lack this aspartate residue in the KD. IRAK-1 contains a region that is rich in serine, proline, and threonine (proST). It is thought that IRAK-1 undergoes hyperphosphorylation in this region. The proST region also contains two proline (P), glutamic acid (E), serine (S) and threonine (T)-rich (PEST) sequences that are thought to promote the degradation of IRAK-1. ==Role in immune signaling==

Interleukin-1 receptor signaling Interleukin-1 receptors (IL-1Rs) are cytokine receptors that transduce an intracellular signaling cascade in response to the binding of the inflammatory cytokine interleukin-1 (IL-1). This signaling cascade results in the initiation of transcription of certain genes involved in inflammation. Because IL-1Rs do not possess intrinsic kinase activity, they rely on the recruitment of adaptor molecules, such as IRAKs, to transduce their signals. IL-1 binding to IL-1R complex triggers the recruitment of the adaptor molecule MyD88 through interactions with the TIR domain. MyD88 brings IRAK-4 to the receptor complex. Preformed complexes of the adaptor molecule Tollip and IRAK-1 are also recruited to the receptor complex, allowing IRAK-1 to bind MyD88. IRAK-1 binding to MyD88 brings it into close proximity with IRAK-4 so that IRAK-4 can phosphorylate and activate IRAK-1. Once phosphorylated, IRAK-1 recruits the adaptor protein TNF receptor associated factor 6 (TRAF6) and the IRAK-1-TRAF6 complex dissociates from the IL-1R complex. The IRAK-1-TRAF6 complex interacts with a pre-existing complex at the plasma membrane consisting of TGF-β activated kinase 1 (TAK1), and two TAK binding proteins, TAB1 and TAB2. TAK1 is a mitogen-activated protein kinase kinase kinase (MAPKKK). This interaction leads to the phosphorylation of TAB2 and TAK1, which then translocate to the cytosol with TRAF6 and TAB1. IRAK-1 remains at the membrane and is targeted for degradation by ubiquitination. Once the TAK1-TRAF6-TAB1-TAB2 complex is in the cytosol, ubiquitination of TRAF6 in triggers the activation of TAK1 kinase activity. TAK1 can then activate two transcription pathways, the nuclear factor-κB (NF-κB) pathway and the mitogen-activated protein kinase (MAPK) pathway. To activate the NF-κB pathway, TAK1 phosphorylates the IκB kinase (IKK) complex, which subsequently phosphorylates the NF-κB inhibitor, IκB, targeting it for degradation by the proteasome. Once IκB is removed, the NF-κB proteins p65 and p50 are free to translocate into the nucleus and activate transcription of proinflammatory genes. To activate the MAPK pathway, TAK1 phosphorylates MAPK kinase (MKK) 3/4/6, which then phosphorylate members of the MAPK family, c-Jun N-terminal kinase (JNK) and p38. Phosphorylated JNK/p38 can then translocate into the nucleus and phosphorylate and activate transcription factors such as c-Fos and c-Jun. It has been shown that IRAK-1 is essential for TLR7 and TLR9 interferon (IFN) induction. TLR7 and TLR9 in plasmacytoid dendritic cells (pDCs) recognize viral nucleic acids and trigger the production of interferon-α (IFN-α), an important cytokine for inducing an antiviral state in host cells. TLR7 and TLR9 mediated IFN-α induction requires the formation of a complex consisting of MyD88, TRAF6 and the interferon regulatory factor 7 (IRF7). IRF7 is a transcription factor that translocates into the nucleus when activated and initiates transcription of IFN-α. IRAK-1 was shown to directly phosphorylates IRF7 in vitro and the kinase activity of IRAK-1 was shown to be essential for IRF7 transcriptional activation. IRAK-1 has also been shown to play a critical role in TLR4 interleukin-10 (IL-10) induction. TLR4 recognizes bacterial LPS and triggers the transcription of IL-10, a cytokine regulating the inflammatory response. IL-10 transcription is activated by signal transducer and activator of transcription 3 (STAT3). IRAK-1 forms a complex with STAT3 and the IL-10 promoter element in the nucleus and is required for STAT3 phosphorylation and activation of IL-10 transcription. IRAK-2 plays an important role in TLR-mediated NF-κB activation. Knocking down IRAK-2 has been shown to impair NF-κB activation by TLR3, TLR4 and TLR8. The mechanism of how IRAK-2 functions is still unknown, however, IRAK-2 has been shown to interact with a TIR adaptor protein that does not bind to IRAK-1, called Mal/TIRAP. Mal/TIRAP has been specifically implicated in TLR2 and TLR4 mediated NF-κB signaling. In addition, it has been shown that IRAK-2 is recruited to the TLR3 receptor. IRAK-2 is the only IRAK family member that is known to play a role in TLR3 signaling. ==Role in disease==

Interleukin 1 is a cytokine that acts locally and systemically in the innate immune system. IL-1a and IL-1ß are known for causing inflammation, but can also cause induction of other proinflammatory cytokines, and fever. Because IRAKs are a crucial step in the IL-1 receptor signalling pathway, deficiencies or over-expression of IRAKs can cause suboptimal or overactive cellular response to IL-1a and IL-1ß. Thus Interleukin-1 Receptor Associated Kinases are promising therapeutic targets for autoimmune-, immunodeficiency-, and cancer-related disorders. Cancer Inflammation signalling is known to be a major factor in many cancer types, and an inflammatory microclimate is a key aspect of human tumours. IL-1ß, which activates the inflammatory signalling pathway containing IRAKs, is directly involved in tumour cell growth, angiogenesis, invasion, and metastasis. In tumour cells containing the L265P MyD88 mutant, protein-signalling complexes spontaneously assemble, activating IRAK-4's kinase activity and promoting inflammation and growth independent of Interleukin-1 signalling. IRAK-4 inhibiting drugs are thus a potential therapeutic treatment for lymphoid malignancies with the L265P MyD88 mutation, especially in Waldenström's Macroglobulinaemia, in which BTK and IRAK1/4 inhibitors have shown promising but unconfirmed results. In 2013, Garrett Rhyasen and his colleagues at the University of Cincinnati studied the contribution of active IRAK-1 and IRAK-4 in human myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). They found that IRAK1 knockout therapy incited apoptosis and impaired leukemic progenitor activity. They also established that IRAK4, while imperative to proliferation of human hematologic malignancies, is not imperative to the pathogenesis of MDS/AML. Further testing of IRAK-inhibitory therapy could prove essential to cancer therapy development. In most cases, inhibition of IRAK-1 and IRAK-4 are suspected to the most effective targets for knockout drugs, as their functions are integral to the cytokine pathways inducing chronic inflammation. Mutations in the gene for IRAK-M have been identified as contributors to early onset asthma. Compromised IRAK-M leads to overproduction of inflammatory cytokines in the lungs, eventually triggering T cell mediated allergic reactions and exacerbation of asthma symptoms. Researchers have proposed that increasing IRAK-M function in these individuals may moderate asthma symptoms. ==References==