Mutant Cells That Do Not Respond to Interleukin-1 (IL-1) Reveal a Novel Role for IL-1 Receptor-Associated Kinase

Abstract

Interleukin-1 (IL-1), a proinflammatory cytokine produced mainly by macrophages and monocytes in response to inflammation, infection, and other challenges, stimulates a wide spectrum of responses, including fever, lymphocyte activation, and leukocyte infusion to the site of injury or infection (16). IL-1 stimulates the expression of several genes by activating the transcription factors NF-B, ATF, and AP-1 (6, 51, 52).The activation of NF-B has been studied extensively (4, 6, 16). NF-B is kept in the cytoplasm through interaction with B inhibitory proteins. Following stimulation with cytokines (e.g., IL-1 and tumor necrosis factor alpha [TNF-␣]) or other agents (e.g., lipopolysaccharide, phorbol ester, and doublestranded RNA), IB undergoes phosphorylation on specific serine residues and is rapidly ubiquitinated and degraded. The liberated NF-B translocates to the nucleus, where it activates transcription (5,63,66,69). Recent studies have provided a model for how NF-B is activated in response to IL-1 (Fig. 1). First, a complex is formed between the type 1 receptor (IL-1R1) and the receptor accessory protein (IL-1RAcP) (21, 24, 29, 70). The cytosolic myeloid differentiation protein (MyD88) (36) is then recruited to the complex, where it functions as an adaptor, recruiting IL-1R-associated kinase (IRAK) in turn (10,48,71,75). IRAK is phosphorylated and then leaves the receptor complex to interact with TRAF6 (11). IRAK2, an IRAK homolog, was shown to interact with the IL-1R complex, MyD88, and TRAF6 in transfected cells, but how IRAK and IRAK2 function in IL-1 signaling is not understood (48). Six TRAFs (TNF receptor-associated factors) have been described so far (2,17,22,23,25,31,49,58). TRAF2 and TRAF5 have been implicated in activating NF-B in response to the activation of members of the TNF-␣ receptor superfamily (2,17,22,23,25,31,49,58). The TRAFs interact with NF-Binducing kinase (NIK), another serine-threonine kinase believed to be a common downstream component in activating NF-B in response to IL-1, TNF-␣, and other stimuli (41). TRAFs might also activate mitogen-activated protein kinase/ ERK kinase kinase 1 (MEKK1) (30,32,35,64,76). Recently, two IB kinases (IKK␣ and IKK␤) have been implicated in signal-induced phosphorylation of the IB proteins (15,44,57,73,78). Both NIK and MEKK1 activate the IKKs by serine phosphorylation (34, 50). The activated IKKs then phosphorylate IBs on specific serine residues, resulting in the degradation of IB and activation of NF-B. The IKKs are components of a large complex (15,44,78). Two additional components, NEMO (NF-B essential modulator or IKK␥) and IKAP are also part of the IKK complex and are required for its formation (12,59,74).Recent studies provide evidence for a second signaling pathway parallel to the cascade leading to IB degradation and specifically required for NF-B-dependent transcriptional competency (Fig.

Regulation of Interleukin-1α Expression by Integrins and Epidermal Growth Factor Receptor in Keratinocytes from a Mouse Model of Inflammatory Skin Disease

“…Upon stimulation with IL-1 for 7 h, the level of wt IRAK1 decreased slightly, consistent with the degradation that is suggested to occur upon IRAK1 activation and phosphorylation (65). Multiple IRAK1 forms of increased apparent molecular weight were detected in cells expressing kd IRAK, presumably representing multiply phosphorylated or possibly ubiquitinated forms (22,60,65). The reason why kd IRAK1 accumulated in these forms but wt IRAK1 did not is unknown but may be because of a decreased rate of proteolysis of the phosphorylated/ubiquitinated kd IRAK1 compared with wt IRAK1.…”

“…IRAK1 plays a vital role in transducing IL-1-dependent signals, acting just downstream of the IL-1R (58 -61). Overexpression of IRAK1 can constitutively activate and potentiate IL-1-induced NF B activation, an effect not dependent on its kinase activity (22,(55)(56)(57)60). In addition, both wild type (wt) and kinase-dead (kd) IRAK1 overexpression potentiates IL-1-induced Jun N-terminal kinase MAPK activity (56,57).…”

TLR-4, IL-1R and TNF-R signaling to NF-κB: variations on a common theme

“…IRAK-4 becomes activated by intramolecular autophosphorylation of three residues (Thr342, Thr345 and Ser346) within its activation loop, which is required for optimal kinase activity of IRAK-4 [28]. Activation of IRAK-4 leads to phosphorylation of IRAK-1 on Thr209 and Thr387 in its activation loop, leading to full kinase activity [23,29]. Subsequently, IRAK-1 becomes hyperphosphorylated in its ProST region, probably via autophosphorylation, resulting in its dissociation from MyD88 and Tollip, but not from the downstream signaling molecule TRAF6 [21,23].…”

“…Subsequently, IRAK-1 becomes hyperphosphorylated in its ProST region, probably via autophosphorylation, resulting in its dissociation from MyD88 and Tollip, but not from the downstream signaling molecule TRAF6 [21,23]. However, the kinase activity of IRAK-1 is dispensable for IL-1 signaling towards NFkB, as complementation of cells deficient in IRAK-1 with a kinase death mutant can restore IL-1-induced NF-kB activation [29]. In contrast, contradictory data are published about the necessity of the kinase activity of IRAK-4.…”

Regulation of p53 expression by the RAS-MAP kinase pathway

“…We have developed methods for isolating and complementing mammalian cell lines carrying signaling mutations caused by chemical mutagenesis and have applied these methods to interferon-dependent and interleukin-1-dependent pathways Stark and Gudkov, 1999;Li et al, 1999). To bring this approach to bear on p53-dependent signaling, we employed the human ®brosarcoma cell line HT1080, which harbors an N-ras mutation (Plattner et al, 1996) and thus expresses high levels of p53.…”