IL-1-induced NFkappa B and c-Jun N-terminal kinase (JNK) activation diverge at IL-1 receptor-associated kinase (IRAK)

The transmembrane activator TACI triggers immunoglobulin class switching by activating B cells through the adaptor MyD88

BAFF and APRIL are innate immune mediators that trigger immunoglobulin (Ig) G and IgA class switch recombination (CSR) in B cells by engaging the receptor TACI. The mechanism underlying CSR signaling by TACI remains unknown. Here, we found that the cytoplasmic domain of TACI encompasses a conserved motif that bound MyD88, an adaptor protein that activates NF-κB signaling pathways via a Toll-interleukin-1 receptor (TIR) domain. TACI lacks a TIR domain, yet triggered CSR via the DNA-editing enzyme AID by activating NF-κB through a TLR-like MyD88–IRAK-1-IRAK-4–TRAF6–TAK1 pathway. TACI-induced CSR was impaired in mice and humans lacking MyD88 or IRAK-4, indicating that MyD88 controls a B cell-intrinsic, TIR-independent, TACI-dependent pathway for Ig diversification.

Diversity in post-transcriptional control of neutrophil chemoattractant cytokine gene expression

Regulation of neutrophil chemokine gene expression represents an important feature in tissue inflammation. While chemokine gene transcription through the action of NFkappaB is recognized as an essential component of this process, it is now clear that post-transcriptional mechanisms, particularly the rates of decay of mature cytoplasmic mRNA, provides an essential component of this control. Chemokine and other cytokine mRNA half life is known to be controlled via adenine-uridine rich sequence motifs localized within 3' untranslated regions (UTRs), the most common of which contains one or more copies of the pentameric AUUUA sequence. In myeloid cells AUUUA sequences confer instability through the action of RNA binding proteins such as tristetraprolin (TTP). The resulting instability can be regulated in response to extra-cellular stimuli including Toll like receptor ligands that signal to control the function of TTP through pathways involving the activation of p38 MAP kinases. Recent findings indicate that substantial mechanistic diversity is operative in non-myeloid cells in response to alternate pro-inflammatory stimuli such as IL-17. These pathways target distinct instability sequences that do not contain the AUUUA pentamer motif, do not signal through p38 MAPK, and function independently of TTP.

Interleukin-1 Receptor-Associated Kinase-1 (IRAK-1) functionally associates with PKCepsilon and VASP in the regulation of macrophage migration

Macrophage migration is mediated by complex cellular signaling processes and cytoskeleton re-arrangement. In particular, recent advances indicate that the innate immunity signaling process plays a key role in the regulation of macrophage migration. In this report, we have provided evidence demonstrating the involvement of a key innate immunity signaling kinase, Interleukin-1 Receptor-Associated Kinase-1 (IRAK-1) as a critical modulator of macrophage migration. Macrophage migration induced by phorbol 12-myristate 13-acetate (PMA) is significantly attenuated in IRAK-1(-/-) macrophages as compared to wild type macrophages. Mechanistically, we demonstrated that IRAK-1 works downstream of PKCepsilon and upstream of VASP, a member of Ena/VASP family proteins. IRAK-1 forms a close complex with PKCepsilon as well as VASP, and participates in PMA-induced phosphorylation of VASP. Notably, IRAK-1 contains a novel EVH1 domain binding motif (L(167)WPPPP) within its N-terminus, which is responsible for its interaction with VASP. The mutant IRAK-1 (L167A/W168A) fails to associate with VASP. Our findings provide a novel facet regarding the molecular signaling process regulating macrophage migration.

Use of forward genetics to discover novel regulators of NF-kappaB

Forward and reverse genetic experiments have both played important roles in revealing critical aspects of mammalian signal transduction pathways in cell culture experiments. Only recently have we begun to comprehend the depth, breadth, and complexity of these pathways and of their interrelationships. Here, we summarize successful examples in which different forward genetic approaches have led to novel discoveries in NF-kappaB signaling. We believe that forward genetics will continue to play an irreplaceable role in advancing our understanding of the complexities of the pathways that regulate the functions of this key transcription factor.

IRAK1-independent pathways required for the interleukin-1-stimulated activation of the Tpl2 catalytic subunit and its dissociation from ABIN2

The protein kinase Tpl2 (tumour progression locus 2) is activated by LPS (lipopolysaccharide), TNFalpha (tumour necrosis factor alpha) and IL (interleukin)-1. Activation of the native Tpl2 complex by these agonists requires the IKKbeta {IkappaB [inhibitor of NF-kappaB (nuclear factor kappaB)] kinase beta}-catalysed phosphorylation of the p105/NF-kappaB1 subunit and is accompanied by the release of the catalytic subunit from both p105/NF-kappaB1 and another subunit ABIN2 (A20-binding inhibitor of NF-kappaB 2). In the present study we report that IL-1 activates the transfected Tpl2 catalytic subunit in an HEK (human embryonic kidney)-293 cell line that stably expresses the IL-1R (IL-1 receptor), but does not express the protein kinase IRAK1 (IL-1R-associated kinase). In these cells IL-1 does not activate IKKbeta or induce the phosphorylation of p105/NF-kappaB1, and nor does the IKKbeta inhibitor PS1145 prevent the IL-1-induced activation of transfected Tpl2. However, the IL-1-stimulated activation of transfected Tpl2 in IRAK1-null cells or activation of the endogenous Tpl2 complex in IRAK1-expressing cells is suppressed by the protein kinase inhibitor PP2 by a mechanism that does not involve inhibition of Src family protein tyrosine kinases. The IL-1-stimulated activation of transfected Tpl2 is accompanied by its phosphorylation at Thr290 and Ser400 and by enhanced phosphorylation of Ser62, which we demonstrate are autophosphorylation events catalysed by Tpl2 itself. We further show that IL-1 triggers the dissociation of Tpl2 from co-transfected ABIN2 in IRAK1-null IL-1R cells, which is not suppressed by PP2 or by the inhibition of Tpl2 or IKKbeta. These studies identify two new signalling events involved in activation of the native Tpl2 complex by IL-1. First, the IRAK1-, IKKbeta- and PP2-independent dissociation of Tpl2 from ABIN2; secondly, the IRAK1- and IKKbeta-independent, but PP2-sensitive, activation of the Tpl2 catalytic subunit.

Identification of critical residues of the MyD88 death domain involved in the recruitment of downstream kinases

MyD88 couples the activation of the Toll-like receptors and interleukin-1 receptor superfamily with intracellular signaling pathways. Upon ligand binding, activated receptors recruit MyD88 via its Toll-interleukin-1 receptor domain. MyD88 then allows the recruitment of the interleukin-1 receptor-associated kinases (IRAKs). We performed a site-directed mutagenesis of MyD88 residues, conserved in death domains of the homologous FADD and Pelle proteins, and analyzed the effect of the mutations on MyD88 signaling. Our studies revealed that mutation of residues 52 (MyD88(E52A)) and 58 (MyD88(Y58A)) impaired recruitment of both IRAK1 and IRAK4, whereas mutation of residue 95 (MyD88(K95A)) only affected IRAK4 recruitment. Since all MyD88 mutants were defective in signaling, recruitment of both IRAKs appeared necessary for activation of the pathway. Moreover, overexpression of a green fluorescent protein (GFP)-tagged mini-MyD88 protein (GFP-MyD88-(27-72)), comprising the Glu(52) and Tyr(58) residues, interfered with recruitment of both IRAK1 and IRAK4 by MyD88 and suppressed NF-kappaB activation by the interleukin-1 receptor but not by the MyD88-independent TLR3. GFP-MyD88-(27-72) exerted its effect by titrating IRAK1 and suppressing IRAK1-dependent NF-kappaB activation. These experiments identify novel residues of MyD88 that are crucially involved in the recruitment of IRAK1 and IRAK4 and in downstream propagation of MyD88 signaling.

Interleukin-1 stimulates catabolism in C2C12 myotubes

Interleukin-1 (IL-1) is an inflammatory cytokine that has been linked to muscle catabolism, a process regulated by muscle-specific E3 proteins of the ubiquitin-proteasome pathway. To address cellular mechanism, we tested the hypothesis that IL-1 induces myofibrillar protein loss by acting directly on muscle to increase expression of two critical E3 proteins, atrogin1/muscle atrophy F-box (MAFbx) and muscle RING-finger 1 (MuRF1). Experiments were conducted using mature C2C12 myotubes to eliminate systemic cytokine effects and avoid paracrine signaling by nonmuscle cell types. Time-course protocols were used to define the sequence of cellular responses. We found that atrogin1/MAFbx mRNA and MuRF1 mRNA are elevated 60-120 min after myotube exposure to either IL-1alpha or IL-1beta. These responses are preceded by signaling events that promote E3 expression. Both IL-1 isoforms stimulate phosphorylation of p38 mitogen-activated protein kinase and stimulate nuclear factor-kappaB (NF-kappaB) signaling; I-kappaB levels fall and NF-kappaB DNA binding activity increases. Other regulators of E3 expression are unaffected by IL-1 [cytosolic oxidant activity, Forkhead-O (Foxo) activity] or respond paradoxically (AKT). Chronic exposure of C2C12 myotubes over 48 h resulted in reduced myotube width and loss of sarcomeric actin. We conclude that IL-1alpha and IL-1beta act via an oxidant- and AKT/Foxo-independent mechanism to activate p38 MAPK, stimulate NF-kappaB signaling, increase expression of atrogin1/MAFbx and MuRF1, and reduce myofibrillar protein in differentiated myotubes.

Delineation of the chemomechanosensory regulation of gastrin secretion using pure rodent G cells

BACKGROUND & AIMS

Gastrin is a key regulator of gastric acid secretion. We aimed to isolate pure G cells to identify the mechanistic basis of luminal- and strain-mediated regulation.

METHODS

Using gradient centrifugation and fluorescence-activated cell sorting, rat G cells were prepared and luminal, neural, hormonal, and mechanical activation of secretion and signaling pathways studied.

RESULTS

Pure G-cell preparations (>97%) were isolated. Reverse-transcription polymerase chain reaction identified neural, hormonal, bacterial, and luminal G protein-coupled receptors, and immunostaining visualized specific sweet/bitter receptors and the tastant-associated G protein alpha-gustducin. Gastrin release was stimulated by forskolin (adenosine 3',5'-cyclic monophosphate [cAMP] inducer, 10 micromol/L; >3-fold), potentiated by 3-isobutyl-1-methylxanthine (IBMX; phosphodiesterase type 5 inhibitor and adenosine antagonist, 10 micromol/L) and phorbol myristate acetate (phorbol ester, 10 micromol/L), and inhibited by H-89 (protein kinase A inhibitor, 10 micromol/L), PD98059 (MEK1 inhibitor, 0.1 micromol/L), and wortmannin (phosphatidylinositol 3-kinase inhibitor, 1 nmol/L). Gastrin release was stimulated by neuronal G protein-coupled receptor ligands, pituitary adenylate cyclase-activating protein (20 pmol/L, >8-fold) and bombesin (0.1 micromol/L, 8-fold) through cAMP signaling. The tastants sucralose, glucose, caffeine, denatonium, and the vanilloid receptor activator capsaicin all stimulated secretion (>3-fold), as did bacterial lipopolysaccharides Salmonella enteritidis (0.24 nmol/L, 5-fold) greater than Helicobacter pylori (0.57 micromol/L, 3-fold). Secretion was associated with elevated cAMP levels (approximately 2-fold) and could be inhibited by H-89 and PD98059 and potentiated by IBMX and cholera toxin (250 microg/mL). Bacterially mediated secretion also involved activation of nuclear factor kappaB and the c-Jun-N-terminal kinase pathway. Mechanical strain stimulated (2-fold to 8-fold) gastrin release, and decreasing pH from 7.4 to 5.5 inhibited release. The adenosine receptor 2B antagonist MRS1754 inhibited mechanically induced gastrin release.

CONCLUSIONS

G cells are luminal sampling chemomechanosensory cells whose secretion is regulated by neural, hormonal, luminal, and mechanical factors through protein kinase A activation, cAMP signaling, and mitogen-activated protein kinase phosphorylation.

Identification of IRAK1 as a risk gene with critical role in the pathogenesis of systemic lupus erythematosus

A combined forward and reverse genetic approach was undertaken to test the candidacy of IRAK1 (interleukin-1 receptor associated kinase-1) as an X chromosome-encoded risk factor for systemic lupus erythematosus (SLE). In studying approximately 5,000 subjects and healthy controls, 5 SNPs spanning the IRAK1 gene showed disease association (P values reaching 10(-10), odds ratio >1.5) in both adult- and childhood-onset SLE, in 4 different ethnic groups, with a 4 SNP haplotype (GGGG) being strongly associated with the disease. The functional role of IRAK1 was next examined by using congenic mouse models bearing the disease loci: Sle1 or Sle3. IRAK1 deficiency abrogated all lupus-associated phenotypes, including IgM and IgG autoantibodies, lymphocytic activation, and renal disease in both models. In addition, the absence of IRAK1 reversed the dendritic cell "hyperactivity" associated with Sle3. Collectively, the forward genetic studies in human SLE and the mechanistic studies in mouse models establish IRAK1 as a disease gene in lupus, capable of modulating at least 2 key checkpoints in disease development. This demonstration of an X chromosome gene as a disease susceptibility factor in human SLE raises the possibility that the gender difference in SLE may in part be attributed to sex chromosome genes.

Interleukin-1 regulates keratinocyte expression of T cell targeting chemokines through interleukin-1 receptor associated kinase-1 (IRAK1) dependent and independent pathways

IL-1 is a potent pro-inflammatory cytokine that activates intracellular signaling cascades some of which may involve IL-1 receptor associated kinase-1 (IRAK1). Psoriasis is a T cell dependent chronic inflammatory condition of the skin of unknown cause. IL-1 has been implicated in psoriasis pathology, but the mechanism has not been elucidated. Interestingly, expression of IRAK1 is elevated in psoriatic skin. To identify a potential link between IL-1, keratinocytes and T cells in skin inflammation we employed pathway-focused microarrays to evaluate IL-1 dependent gene expression in keratinocytes. Several candidate mRNAs encoding known T cell chemoattractants were identified in primary keratinocytes and the stable keratinocyte cell line HaCaT. CCL5 and CCL20 mRNA and protein levels were confirmed up-regulated by IL-1 in concentration and time-dependent manners. Furthermore IL-1 synergized with IFN-gamma and TNF-alpha. Expression of CXCL9, CXCL10 and CXCL11 mRNAs was also increased in response to IL-1, but protein could only be detected in medium from cells treated with IFN-gamma alone or in combination with IL-1. Over-expression of IRAK1 led to increased constitutive and cytokine induced production of CCL5 and CCL20. Inhibition of IRAK1 activity through RNAi or expression of a dominant negative mutant blocked production of CCL5 and CCL20 but had no effect upon the IL-1 enhancement of IFN-gamma induced CXCL9, CXCL10 and CXCL11 production. In conclusion IL-1 regulates T cell targeting chemokine production in keratinocytes through IRAK1 dependent and independent pathways. These pathways may contribute to acute and chronic skin inflammation.

Sequential control of Toll-like receptor-dependent responses by IRAK1 and IRAK2

Members of the IRAK family of kinases mediate Toll-like receptor (TLR) signaling. Here we show that IRAK2 was essential for sustaining TLR-induced expression of genes encoding cytokines and activation of the transcription factor NF-kappaB, despite the fact that IRAK2 was dispensable for activation of the initial signaling cascades. IRAK2 was activated 'downstream' of IRAK4, like IRAK1, and TLR-induced cytokine production was abrogated in the absence of both IRAK1 and IRAK2. Whereas the kinase activity of IRAK1 decreased within 1 h of TLR2 stimulation, coincident with IRAK1 degradation, the kinase activity of IRAK2 was sustained and peaked at 8 h after stimulation. Thus, IRAK2 is critical in late-phase TLR responses, and IRAK1 and IRAK2 are essential for the initial responses to TLR stimulation.

Lys63-linked polyubiquitination of IRAK-1 is required for interleukin-1 receptor- and toll-like receptor-mediated NF-kappaB activation

Stimulation through the interleukin-1 receptor (IL-1R) and some Toll-like receptors (TLRs) induces ubiquitination of TRAF6 and IRAK-1, signaling components required for NF-kappaB and mitogen-activated protein kinase activation. Here we show that although TRAF6 and IRAK-1 acquired Lys63 (K63)-linked polyubiquitin chains upon IL-1 stimulation, only ubiquitinated IRAK-1 bound NEMO, the regulatory subunit of IkappaB kinase (IKK). The sites of IRAK-1 ubiquitination were mapped to Lys134 and Lys180, and arginine substitution of these residues impaired IL-1R/TLR-mediated IRAK-1 ubiquitination, NEMO binding, and NF-kappaB activation. K63-linked ubiquitination of IRAK-1 required enzymatically active TRAF6, indicating that it is the physiologically relevant E3. Thus, K63-linked polyubiquitination of proximal signaling proteins is a common mechanism used by diverse innate immune receptors for recruiting IKK and activating NF-kappaB.

Interleukin-1 receptor-associated kinase (IRAK) -1-mediated NF-kappaB activation requires cytosolic and nuclear activity

Interleukin-1 receptor-associated kinase (IRAK) -1 plays an essential role in Toll-like receptor/interleukin-1 receptor (TLR/IL-1R) -associated NF-kappaB activation through its involvement in IKK activation, which then leads to subsequent IkappaB degradation and NF-kappaB nuclear translocation. In the present studies, we demonstrate a novel pathway in which IRAK-1 present in the nucleus participates in NF-kappaB-dependent gene expression. Nuclear localization of IRAK-1 is increased on cellular stimulation with IL-1 and LPS, or CRM-1-dependent nuclear export blockade. Induction of IRAK-1 produces enhanced NF-kappaB transcriptional activity that precedes IkappaB-alpha degradation and nuclear translocation of NF-kappaB. IRAK-1 binds to the promoter of NF-kappaB-regulated gene, IkappaB-alpha, and enhances binding of the NF-kappaB p65 subunit to NF-kappaB responsive elements within the IkappaB-alpha promoter. IRAK-1 phosphorylates histone H3 in vitro and is required for IL-1-induced phosphorylation of histone H3 at serine 10 in vivo. These data indicate that both cytosolic and nuclear actions of IRAK-1 participate in the activation of NF-kappaB-dependent transcriptional events.

IRAK1: A critical signaling mediator of innate immunity

The innate immune system is equipped with sensitive and efficient machineries to provide an immediate, first line defense against infections. Toll-like receptors (TLRs) detect pathogens and the IL-1 receptor (IL-1R) family enables cells to quickly respond to inflammatory cytokines by mounting an efficient protective response. Interleukin-1 receptor activated kinases (IRAKs) are key mediators in the signaling pathways of TLRs/IL-1Rs. By means of their kinase and adaptor functions, IRAKs initiate a cascade of signaling events eventually leading to induction of inflammatory target gene expression. Due to this pivotal role, IRAK function is also highly regulated via multiple mechanisms. In this review, we focus on IRAK1, the earliest known and yet the most interesting member of this family. An overview on its structure, function and biology is given, with emphasis on the different novel mechanisms that regulate IRAK1 function. We also highlight several unresolved questions in this field and evaluate the potential of IRAK1 as a target for therapeutic intervention.

The IRAK-catalysed activation of the E3 ligase function of Pellino isoforms induces the Lys63-linked polyubiquitination of IRAK1

The protein kinases IRAK [IL-1 (interleukin 1) receptor-associated kinase] 1 and 4 play key roles in a signalling pathway by which bacterial infection or IL-1 trigger the production of inflammatory mediators. In the present study, we demonstrate that IRAK1 and IRAK4 phosphorylate Pellino isoforms in vitro and that phosphorylation greatly enhances Pellino's E3 ubiquitin ligase activity. We show that, in vitro, Pellino 1 can combine with the E2 conjugating complex Ubc13 (ubiquitin-conjugating enzyme 13)-Uev1a (ubiquitin E2 variant 1a) to catalyse the formation of K63-pUb (Lys63-linked polyubiquitin) chains, with UbcH3 to catalyse the formation of K48-pUb chains and with UbcH4, UbcH5a or UbcH5b to catalyse the formation of pUb-chains linked mainly via Lys11 and Lys48 of ubiquitin. In IRAK1-/- cells, the co-transfection of DNA encoding wild-type IRAK1 and Pellino 2, but not inactive mutants of these proteins, induces the formation of K63-pUb-IRAK1 and its interaction with the NEMO [NF-kappaB (nuclear factor kappaB) essential modifier] regulatory subunit of the IKK (inhibitor of NF-kappaB kinase) complex, a K63-pUb-binding protein. These studies suggest that Pellino isoforms may be the E3 ubiquitin ligases that mediate the IL-1-stimulated formation of K63-pUb-IRAK1 in cells, which may contribute to the activation of IKKbeta and the transcription factor NF-kappaB, as well as other signalling pathways dependent on IRAK1/4.

Interleukin-1 (IL-1) induces the Lys63-linked polyubiquitination of IL-1 receptor-associated kinase 1 to facilitate NEMO binding and the activation of IkappaBalpha kinase

Interleukin 1 (IL-1) has been reported to stimulate the polyubiquitination and disappearance of IL-1 receptor-associated kinase 1 (IRAK1) within minutes. It has been thought that the polyubiquitin chains attached to IRAK1 are linked via Lys48 of ubiquitin, leading to its destruction by the proteasome and explaining the rapid IL-1-induced disappearance of IRAK1. In this paper, we demonstrate that IL-1 stimulates the formation of K63-pUb-IRAK1 and not K48-pUb-IRAK1 and that the IL-1-induced disappearance of IRAK1 is not blocked by inhibition of the proteasome. We also show that IL-1 triggers the interaction of K63-pUb-IRAK1 with NEMO, a regulatory subunit of the IkappaBalpha kinase (IKK) complex, but not with the NEMO[D311N] mutant that cannot bind K63-pUb chains. Moreover, unlike wild-type NEMO, the NEMO[D311N] mutant was unable to restore IL-1-stimulated NF-kappaB-dependent gene transcription to NEMO-deficient cells. Our data suggest a model in which the recruitment of the NEMO-IKK complex to K63-pUb-IRAK1 and the recruitment of the TAK1 complex to TRAF6 facilitate the TAK1-catalyzed activation of IKK by the TRAF6-IRAK1 complex.

TLR-independent induction of human monocyte IL-1 by phosphoglycolipids from thermophilic bacteria

The structures of phosphoglycolipids PGL1 and PGL2 from the thermophilic bacteria Meiothermus taiwanensis, Meiothermus ruber, Thermus thermophilus, and Thermus oshimai are determined recently (Yang et al. in J Lipid Res. 47:1823-1932, 2006). These bacteria belong to Gram-negative bacteria that do not contain lipopolysaccharide, but high amounts of phosphoglycolipids and glycoglycerolipids. Here we show that PGL1/PGL2 mixture (PGL1: PGL2 = 10:1 ~ 10:2) from M. taiwanensis and T. oshimai, but not T. thermophilus and M. ruber, up-regulate interleukin-1beta (IL-1beta) production in human THP-1 monocytes and blood-isolated primary monocytes. PGL2 was purified after phospholipase A2 hydrolysis of PGL1 in the PGL1/PGL2 mixture followed by column chromatography. PGL2 did not induce proIL-1 production, even, partially (35-40%) inhibited PGL1-mediated proIL-1 production, showing that PGL1 is the main inducer of proIL-1 production in PGL1/PGL2 mixture. The production of proIL-1 stimulated by phosphoglycolipids was strongly inhibited by specific PKC-alpha, MEK1/2, and JNK inhibitors, but not by p38-specific inhibitor. The intracellular calcium influx was involved in phosphoglycolipids-mediated proIL-1 production. Using blocking antibody and Toll-like receptor (TLR)-linked NF-kappaB luciferase assays, we found that the cellular receptor(s) for phosphoglycolipids on proIL-1 production was TLR-independent. Further, phosphoglycolipids isolated from T. thermophilus and M. ruber did not induce proIL-1 production, even though T. thermophilus possess more PGL1 than PGL2 (6:4). Specially, the fatty acid composition of phosphoglycolipids from both T. thermophilus and M. ruber consists of a low percentage of C15 (<10%) and a high percentage of C17 (>75%). It suggests, the C15 percentage of PGL may play a critical role in PGL-mediated proIL-1 induction.

TRAF6 distinctively mediates MyD88- and IRAK-1-induced activation of NF-kappaB

MyD88 and IL-1R-associated kinase 1 (IRAK-1) play crucial roles as adaptor molecules in signal transduction of the TLR/IL-1R superfamily, and it is known that expression of these proteins leads to the activation of NF-kappaB in a TNFR-associated factor 6 (TRAF6)-dependent manner. We found in this study, however, that a dominant-negative mutant of TRAF6, lacking the N-terminal RING and zinc-finger domain, did not inhibit IRAK-1-induced activation of NF-kappaB in human embryonic kidney 293 cells, although the TRAF6 mutant strongly suppressed the MyD88-induced activation. The dominant-negative mutant of TRAF6 did not affect the IRAK-1-induced activation, regardless of the expression level of IRAK-1. In contrast, small interfering RNA silencing of TRAF6 expression inhibited MyD88-induced and IRAK-1-induced activation, and supplementation with the TRAF6 dominant-negative mutant did not restore the IRAK-1-induced activation. Expression of IRAK-1, but not MyD88, induced the oligomerization of TRAF6, and IRAK-1 and the TRAF6 dominant-negative mutant were associated with TRAF6. These results indicate that TRAF6 is involved but with different mechanisms in MyD88-induced and IRAK-induced activation of NF-kappaB and suggest that TRAF6 uses a distinctive mechanism to activate NF-kappaB depending on signals.

Variant IL-1 receptor-associated kinase-1 mediates increased NF-kappa B activity

IL-1R-associated kinase (IRAK)-1 is a critical mediator of TLR/IL-1R-induced activation of the transcription factor NF-kappaB. We previously described that a commonly occurring IRAK-1 variant haplotype, containing amino acid changes from serine to phenylalanine at position 196 and from leucine to serine at position 532, is associated with increased activation of NF-kappaB in LPS-stimulated neutrophils from patients with sepsis-induced acute lung injury and also higher mortality and more severe clinical outcomes in such patients. To investigate the underlying molecular mechanisms, we examined the ability of wild-type and variant IRAK-1 to modulate NF-kappaB activation. We found increased NF-kappaB transcriptional activity and expression of NF-kappaB-dependent proinflammatory cytokines in IL-1beta-stimulated IRAK-1-deficient cells transfected with variant IRAK-1 as compared with IRAK-1 wild type. IkappaB-alpha degradation was faster and p65 phosphorylation more prolonged after IL-1beta stimulation in cells expressing the IRAK-1 variant. However, IL-1-induced activation of MAPKs and nuclear translocation of NF-kappaB are comparable in both IRAK-1 variant- and IRAK-1 wild-type-expressing cells. Autophosphorylation of the IRAK-1 variant is greater than that found with wild-type IRAK-1. Additionally, variant IRAK-1 has greater interaction with TNFR-associated factor 6 than does wild-type IRAK-1. The enhanced activity of variant IRAK-1 appeared to be due to the alteration at aa 532, with only minimal effects being associated with change at aa 196. These results demonstrate that variant IRAK-1 is associated with alterations in multiple intracellular events that are likely to contribute to increased NF-kappaB activation and inflammatory responses in individuals with this IRAK-1 haplotype.

MyD88, IRAK1 and TRAF6 knockdown in human chondrocytes inhibits interleukin-1-induced matrix metalloproteinase-13 gene expression and promoter activity by impairing MAP kinase activation

Interleukin-1 (IL-1) is the major prototypic proinflammatory cytokine that stimulates degradation of cartilage in arthritis by inducing prominent collagen II-degrading matrix metalloproteinase-13 (MMP-13). Nothing is known about the involvement of adaptor proteins, MyD88, IRAK1 and TRAF6 in MMP-13 regulation. Here we investigated for the first time the role of these proteins in IL-1-regulated MMP-13 expression in chondrocytes. MyD88 homodimerization inhibitory peptide diminished the expression of MMP-13 gene, promoter activity, phosphorylation of mitogen-activated protein kinases (MAPKs), c-Jun and activating protein 1 (AP-1) activity. Knockdown of MyD88, IRAK1 and TRAF6 by RNA interference (RNAi) drastically down-regulated the expression of IL-1-induced MMP-13 mRNA and protein levels and MMP-13 promoter-driven luciferase activity. Non-specific control siRNA had no effect. Mechanisms of MMP-13 inhibition involved reduced phosphorylation of ERK, p38, JNK and c-Jun as well as AP-1 transcription factor binding activity. The genetic evidence presented here demonstrates that MyD88, IRAK1 and TRAF6 proteins are crucial early mediators for the IL-1-induced MMP-13 regulation through MAPK pathways and AP-1 activity. These proteins could constitute important therapeutic targets for arthritis-associated cartilage loss by MMP-13.

Endothelial dysfunction in insulin resistance and type 2 diabetes

Macrovascular disease is the number one killer in type 2 diabetes patients. The cluster of risk factors in the insulin resistance syndrome (IRS) partly explains this notion. Insulin action in muscle, liver or adipose tissue has been thoroughly described in the literature, whilst this has been less described for the endothelium. Insulin stimulates nitric oxide (NO) production in the endothelium and reduced bioavailability of NO is usually defined as endothelial dysfunction. This impairment might be related to defective insulin signalling in the endothelial cell. Therefore, insulin resistance mechanisms in the endothelial cell will be emphasized in this review. Imbalance between the vasodilating agent NO and the vasoconstrictor endothelin-1 (ET-1) contributes to endothelial dysfunction. Different methods and circulating markers to assess endothelial function will be outlined. Circulating markers of an activated endothelium appear long before type 2 diabetes develops suggesting a unique role of the endothelium in the pathophysiology of the disease. Hampered blood flow in nutritive capillaries due to endothelial dysfunction is coupled with decreased glucose uptake and hyperglycemia. The forearm model combined with muscle microdialysis enables us to measure interstitial glucose and an index for capillary recruitment, the permeability surface area (PS). Available data from this method suggest that capillary recruitment in response of insulin is impaired in insulin resistant human subjects.

Hyperosmolarity-induced apoptosis in human corneal epithelial cells is mediated by cytochrome c and MAPK pathways

PURPOSE

To study whether hyperosmolarity induces apoptosis in human corneal epithelial cells through cytochrome c-mediated death pathways and by activation of mitogen-activated protein kinases (MAPKs).

METHODS

Primary human corneal epithelial cells cultured in normal osmolar media (312 mOsM) were switched to hyperosmolar media (450, 500, and 550 mOsM) by adding 70, 90, and 120 mM NaCl, respectively, with or without the c-jun N-terminal kinase (JNK) inhibitor SB202190 or the extracellular-regulated kinase (ERK) inhibitor PD98059. Apoptosis was assessed by the ApopTag In Situ Oligo Ligation (ISOL) assay. Confocal microscopy was used to detect cytochrome c and active caspase-3. Total RNA was extracted and subjected to reverse transcriptase-polymerase chain reaction for apoptosis-associated genes. Western blots were performed on cell extracts for the apoptogenic molecules cytochrome c and Smac/DIABLO, and phospho-JNK and ERK.

RESULTS

ISOL-positive apoptotic cells significantly increased from 3.3 +/- 1.6% in control medium to 11.4 +/- 5.8%, 18.9 +/- 4.8%, and 43.9 +/- 8.8% in 70, 90, and 120 mM NaCl added media, respectively. The 90 mM NaCl high saline medium notably increased release of cytochrome c and Smac/DIABLO from mitochondria; activated caspase-3, JNK and ERK; stimulated mRNA expression of interleukin-1-converting enzyme and Bax; and reduced Bcl2 expression. SB202190 and PD98059 significantly suppressed hyperosmolarity-induced JNK/ERK activation and ISOL-positive cells. In addition, PD98059 inhibited the release of cytochrome c and Smac/DIABLO from mitochondria.

CONCLUSIONS

These findings show that hyperosmolarity induces apoptosis of human corneal epithelial cells through a cytochrome c-mediated death pathway, which may be mediated by JNK and ERK MAPK signaling pathways.

Effect of a cyclooxygenase-2 inhibitor on interleukin-1beta-stimulated activation of the transcription factor nuclear factor-kappa B in human gingival fibroblasts

BACKGROUND

In previous work, the cyclooxygenase-2 inhibitor NS-398 inhibited interleukin (IL)-1beta-stimulated prostaglandin E(2) (PGE(2)) production almost completely while partially inhibiting IL-6 production in aggressive periodontitis (AgP) human gingival fibroblasts. PGE(2) and the transcription factor nuclear factor-kappa B (NF-kappaB) regulate IL-1beta-stimulated IL-6 production. Cytoplasmic NF-kappaB is bound to inhibitors (IkappaB proteins). IL-1beta initiates a cascade resulting in phosphorylation and degradation of IkappaB, allowing nuclear translocation of NF-kappaB and target gene activation. The purpose of this study was to determine whether NS-398 inhibited phosphorylation of IkappaB and NF-kappaB activation.

METHODS

AgP fibroblasts (1 to 2 x 10(6)) were exposed to IL-1beta (1 x 10(11)M) with or without NS-398 (10 nM) in serum-free medium. The NF-kappaB subunit p65 and phospho-IkappaBalpha were measured in whole cell, cytoplasmic, or nuclear extracts, using colorimetric assays. Enzyme-linked immunosorbent assays were used to measure PGE(2) and IL-6 production by 2.5 x 10(4) cells after exposure to IL-1beta with or without NS-398 in serum-free medium.

RESULTS

Consistent with previous results, NS-398 reduced IL-1beta-stimulated PGE(2) by approximately 98% (P <0.001) and IL-6 by approximately 65% (P <0.001). IL-1beta increased nuclear and cytoplasmic p65 ( approximately 8-fold [P <0.001] and approximately 2.5-fold [P <0.03], respectively) over control levels. NS-398 reduced IL-1beta-stimulated nuclear and cytoplasmic p65 to control levels. IL-1beta increased phospho-IkappaBalpha in whole cell extracts by a maximum of approximately 9.5 times (P = 0.0001), and this was inhibited significantly by NS-398 (P <or=0.008).

CONCLUSIONS

NS-398 inhibited NF-kappaB activation and nuclear p65 levels in human gingival fibroblasts. This seemed to be due to inhibition of the phosphorylation cascade resulting in formation of phospho-IkappaBalpha and free p65. NF-kappaB inhibition may be useful in treating inflammatory diseases such as AgP.

Relationship between Mycobacterium avium subspecies paratuberculosis, IL-1alpha, and TRAF1 in primary bovine monocyte-derived macrophages

Mycobacterium avium subspecies paratuberculosis (MAP) is a facultative intracellular pathogen that resides in host macrophage cells. Presently, little is known about how MAP is able to subvert the normal bacteriocidal functions of infected macrophages. Previously, we reported that ileal tissues from MAP infected cattle contained high levels of interleukin-1 alpha (IL-1alpha) and tumor necrosis factor receptor-associated factor 1 (TRAF1), relative to ileal tissues from uninfected cattle. High-level expression of these two proteins could have profound effects on macrophage function, intracellular signaling, and apoptosis. We now demonstrate that high levels of TRAF1 protein are located primarily within macrophages infiltrating areas of MAP infection. We have also utilized cultured bovine monocyte-derived macrophage cells (MDM) either infected with live MAP or stimulated with recombinant IL-1alpha (rIL-1alpha) to determine if there is a relationship between IL-1alpha and TRAF1 expression. These studies have identified a dose dependent increase in TRAF1 protein levels in bovine MDM in response to infection with live MAP or following treatment with rIL-1alpha. Sustained TRAF1 protein expression was dependent upon interaction of rIL-1alpha with it's receptor and rIL-1beta was also able to enhance TRAF1 gene expression. Our results suggest that MAP may use the IL-1-TRAF1 system to enhance TRAF1 protein expression in infected bovine MDM. These novel results provide evidence for a new avenue of research on the effect of MAP and other intracellular pathogens on macrophage signaling and apoptosis.

IRAK-4 mutation (Q293X): rapid detection and characterization of defective post-transcriptional TLR/IL-1R responses in human myeloid and non-myeloid cells

Innate immunodeficiency has recently been reported as resulting from the Q293X IRAK-4 mutation with consequent defective TLR/IL-1R signaling. In this study we report a method for the rapid allele-specific detection of this mutation and demonstrate both cell type specificity and ligand specificity in defective IL-1R-associated kinase (IRAK)-4-deficient cellular responses, indicating differential roles for this protein in human PBMCs and primary dermal fibroblasts and in LPS, IL-1beta, and TNF-alpha signaling. We demonstrate transcriptional and post-transcriptional defects despite NF-kappaB signaling and intact MyD88-independent signaling and propose that dysfunctional complex 1 (IRAK1/TRAF6/TAK1) signaling, as a consequence of IRAK-4 deficiency, generates specific defects in MAPK activation that could underpin this patient's innate immunodeficiency. These studies demonstrate the importance of studying primary human cells bearing a clinically relevant mutation; they underscore the complexity of innate immune signaling and illuminate novel roles for IRAK-4 and the fundamental importance of accessory proinflammatory signaling to normal human innate immune responses and immunodeficiencies.

Regulation of IRAK-4 kinase activity via autophosphorylation within its activation loop

Interleukin-1 stimulation leads to the recruitment of MyD88, interleukin-1 receptor-associated kinase 1 (IRAK-1) and interleukin-1 receptor-associated kinase 4 (IRAK-4) to the IL-1 receptor. The formation of the IL-1 receptor complex triggers a series of IRAK-1 autophosphorylations, which result in activation. IRAK-4 is upstream of IRAK-1 and may act as IRAK-1 kinase to transmit the signal. To date, there is no upstream kinase reported for IRAK-4; the activation mechanism of IRAK-4 remains poorly understood. Here, for the first time, we report three autophosphorylation sites that are responsible for IRAK-4 kinase activity. LC-MS/MS analysis has identified phosphorylations at T342, T345, and S346, which reside within the activation loop. Site-directed mutants at these positions exhibit significant reductions in the catalytic activity of IRAK-4 (T342A: 57%; T345A: 66%; S346A: 50%). The absence of phosphorylation in kinase-dead IRAK-4 indicates that phosphorylations in the activation loop result from autophosphorylation rather than from phosphorylation by an upstream kinase. Finally, we demonstrate that autophosphorylation is an intramolecular event as wild-type IRAK-4 failed to transphosphorylate kinase-inactive IRAK-4. The present data indicate that the kinase activity of IRAK-4 is dependent on the autophosphorylations at T342, T345, and S346 in the activation loop.

Interleukin-1 (IL-1)-induced TAK1-dependent Versus MEKK3-dependent NFkappaB activation pathways bifurcate at IL-1 receptor-associated kinase modification

Interleukin-1 (IL-1) receptor-associated kinase (IRAK) is phosphorylated after it is recruited to the receptor, subsequently ubiquitinated, and eventually degraded upon IL-1 stimulation. Although a point mutation changing lysine 134 to arginine (K134R) in IRAK abolished IL-1-induced IRAK ubiquitination and degradation, mutations of serines and threonines adjacent to lysine 134 to alanines ((S/T)A (131-144)) reduced IL-1-induced IRAK phosphorylation and abolished IRAK ubiquitination. Through the study of these IRAK modification mutants, we uncovered two parallel IL-1-mediated signaling pathways for NFkappaB activation, TAK1-dependent and MEKK3-dependent, respectively. These two pathways bifurcate at the level of IRAK modification. The TAK1-dependent pathway leads to IKKalpha/beta phosphorylation and IKKbeta activation, resulting in classical NFkappaB activation through IkappaBalpha phosphorylation and degradation. The TAK1-independent MEKK3-dependent pathway involves IKKgamma phosphorylation and IKKalpha activation, resulting in NFkappaB activation through IkappaBalpha phosphorylation and subsequent dissociation from NFkappaB but without IkappaBalpha degradation. These results provide significant insight to our further understanding of NFkappaB activation pathways.

Interleukin-1-dependent sequential chemokine expression and inflammatory cell infiltration in ischemia-reperfusion injury

OBJECTIVE

Ischemia-reperfusion injury is known to cause organ failure, but the mechanisms of pathogenesis remain unclear. Inflammation is a factor in tissue destruction in ischemia reperfusion injury, and interleukin (IL)-1 is a key promoter of inflammation.

DESIGN

Prospective, randomized, and controlled study.

SETTING

University laboratory.

SUBJECTS

Male mice 6-8 wks of age, in which genes for IL-1alpha and IL-1beta (IL-1alpha/beta deficient) and IL-1 receptor antagonist (IL-1RA deficient) are deleted by homologous recombination, and wild-type controls on a Balb/c background.

INTERVENTIONS

In this study, the role of IL-1 on inflammatory cascades, including chemokine expression, inflammatory cell infiltration, and tissue destruction, was investigated in 45 mins of unilateral renal ischemic injury using IL-1alpha/beta-deficient mice and IL-1RA-deficient mice. In addition, the effects of IL-1 on chemokine expression in cultured tubular epithelial cells were investigated.

MEASUREMENTS AND MAIN RESULTS

In vivo study revealed that the number of interstitial infiltrated neutrophils and macrophages, which accompanied the increase of the serum levels of keratinocyte-derived chemokine (KC) and macrophage inflammatory protein (MIP)-1alpha, respectively, significantly increased in IL-1RA-deficient mice. The number of interstitial infiltrated neutrophils correlated well with serum levels of KC at 24 hrs after reperfusion, whereas the number of interstitial infiltrated macrophages correlated well with the serum levels of MIP-1alpha and monocyte chemoattractant protein (MCP)-1 at 24 and 48 hrs after reperfusion, respectively. Likewise, in vitro study revealed that stimulation of tubular epithelial cells by IL-1beta and/or H2O2 sequentially induced KC, MIP-1alpha, and MCP-1 in both protein and messenger RNA levels, which is consistent with in vivo results.

CONCLUSION

IL-1-dependent inflammatory cascades, followed by inflammatory cell infiltration and subsequent tissue destruction, may affect pathogenesis of renal ischemia-reperfusion injury.

Toll-like receptors: from the discovery of NFkappaB to new insights into transcriptional regulations in innate immunity

Toll-like receptors (TLRs) are key components of the innate immune system, functioning as pattern recognition receptors that recognise a wide range of microbial pathogens. TLRs represent a primary line of defence against invading pathogens in mammals, plants and insects. Recognition of microbial components by TLRs triggers a cascade of cellular signals that culminates in the activation of NFkappaB which leads to inflammatory gene expression and clearance of the infectious agent. The history of NFkappaB began with the TLR4 ligand lipopolysaccharide (LPS), a component of the cell wall of Gram-negative bacteria, since this was the stimulus first used to activate NFkappaB in pre-B-cells. However, since those early days it has been a circuitous route, made possible by drawing on information provided by many different fields, that has led us not only to the discovery of TLRs but also to an understanding of the complex pathways that lead from TLR ligation to NFkappaB activation. In this review we will summarize the current knowledge of TLR-mediated NFkappaB activation, and also the recent discoveries that subtle differences in kappaB binding sequences and NFkappaB dimer formation result in specific gene expression profiles.

Intracellular TLR signaling: a structural perspective on human disease

TLRs are crucial sensors of microbial infection. Maintaining structural integrity of TLR signaling components is essential for subsequent immunological protection. Alterations to the structure of these signaling molecules are often associated with profound clinical outcomes and susceptibility to various infectious diseases. These changes in structure are sometimes the result of a single nucleotide polymorphism (SNP). Numerous SNPs have been found in components of the TLR signaling pathway. Recently, the medical consequences and effects on TLR signaling of several of these SNPs have been elucidated. In addition, there have been numerous structures solved that are important to our understanding of the TLR signaling pathway at the molecular level. The scope of this review is to tie together current structural, biochemical, and genetic information of TLR signaling.

TLR8-mediated NF-kappaB and JNK activation are TAK1-independent and MEKK3-dependent

TLR8-mediated NF-kappaB and IRF7 activation are abolished in human IRAK-deficient 293 cells and IRAK4-deficient fibroblast cells. Both wild-type and kinase-inactive mutants of IRAK and IRAK4, respectively, restored TLR8-mediated NF-kappaB and IRF7 activation in the IRAK- and IRAK4-deficient cells, indicating that the kinase activity of IRAK and IRAK4 is probably redundant for TLR8-mediated signaling. We recently found that TLR8 mediates a unique NF-kappaB activation pathway in human 293 cells and mouse embryonic fibroblasts, accompanied only by IkappaBalpha phosphorylation and not IkappaBalpha degradation, whereas interleukin (IL)-1 stimulation causes both IkappaBalpha phosphorylation and degradation. The intermediate signaling events mediated by IL-1 (including IRAK modifications and degradation and TAK1 activation) were not detected in cells stimulated by TLR8 ligands. TLR8 ligands trigger similar levels of IkappaBalpha phosphorylation and NF-kappaB and JNK activation in TAK1(-/-) mouse embryo fibroblasts (MEFs) as compared with wild-type MEFs, whereas lack of TAK1 results in reduced IL-1-mediated NF-kappaB activation and abolished IL-1-induced JNK activation. The above results indicate that although TLR8-mediated NF-kappaB and JNK activation are IRAK-dependent, they do not require IRAK modification and are TAK1-independent. On the other hand, TLR8-mediated IkappaBalpha phosphorylation, NF-kappaB, and JNK activation are completely abolished in MEKK3(-/-) MEFs, whereas IL-1-mediated signaling was only moderately reduced in these deficient MEFs as compared with wild-type cells. The differences between IL-1R- and TLR8-mediated NF-kappaB activation are also reflected at the level of IkappaB kinase (IKK) complex. TLR8 ligands induced IKKgamma phosphorylation, whereas IKKalpha/beta phosphorylation and IKKgamma ubiquitination that can be induced by IL-1 were not detected in cells treated with TLR8 ligands. We postulate that TLR8-mediated MEKK3-dependent IKKgamma phosphorylation might play an important role in the activation of IKK complex, leading to IkappaBalpha phosphorylation.

Reciprocal relationships between insulin resistance and endothelial dysfunction: molecular and pathophysiological mechanisms

Endothelial dysfunction contributes to cardiovascular diseases, including hypertension, atherosclerosis, and coronary artery disease, which are also characterized by insulin resistance. Insulin resistance is a hallmark of metabolic disorders, including type 2 diabetes mellitus and obesity, which are also characterized by endothelial dysfunction. Metabolic actions of insulin to promote glucose disposal are augmented by vascular actions of insulin in endothelium to stimulate production of the vasodilator nitric oxide (NO). Indeed, NO-dependent increases in blood flow to skeletal muscle account for 25% to 40% of the increase in glucose uptake in response to insulin stimulation. Phosphatidylinositol 3-kinase-dependent insulin-signaling pathways in endothelium related to production of NO share striking similarities with metabolic pathways in skeletal muscle that promote glucose uptake. Other distinct nonmetabolic branches of insulin-signaling pathways regulate secretion of the vasoconstrictor endothelin-1 in endothelium. Metabolic insulin resistance is characterized by pathway-specific impairment in phosphatidylinositol 3-kinase-dependent signaling, which in endothelium may cause imbalance between production of NO and secretion of endothelin-1, leading to decreased blood flow, which worsens insulin resistance. Therapeutic interventions in animal models and human studies have demonstrated that improving endothelial function ameliorates insulin resistance, whereas improving insulin sensitivity ameliorates endothelial dysfunction. Taken together, cellular, physiological, clinical, and epidemiological studies strongly support a reciprocal relationship between endothelial dysfunction and insulin resistance that helps to link cardiovascular and metabolic diseases. In the present review, we discuss pathophysiological mechanisms, including inflammatory processes, that couple endothelial dysfunction with insulin resistance and emphasize important therapeutic implications.

MyD88-dependent and -independent signaling by IL-1 in neurons probed by bifunctional Toll/IL-1 receptor domain/BB-loop mimetics

Interleukin (IL)-1beta is a pluripotent proinflammatory cytokine that signals through the type-I IL-1 receptor (IL-1RI), a member of the Toll-like receptor family. In hypothalamic neurons, binding of IL-1beta to IL-1RI mediates transcription-dependent changes that depend on the recruitment of the cytosolic adaptor protein myeloid differentiation primary-response protein 88 (MyD88) to the IL-1RI/IL-1 receptor accessory protein (IL-1RAcP) complex through homomeric Toll/IL-1 receptor (TIR)-TIR interactions. Through design and synthesis of bifunctional TIR mimetics that disrupt the interaction of MyD88 with the IL-1RI/IL-1RAcP complex, we analyzed the involvement of MyD88 in the signaling of IL-1beta in anterior hypothalamic neurons. We show here that IL-1beta-mediated activation of the protein tyrosine kinase Src depended on a MyD88 interaction with the IL-1RI/IL-1RAcP complex. The activation of the protein kinase Akt/PKB depended on the recruitment of the p85 subunit of PI3K to IL-1RI and independent of MyD88 association with the IL-1RI/IL-1RAcP complex. These bifunctional TIR-TIR mimetics represent a class of low-molecular-weight compounds with both an antiinflammatory and neuroprotective potential. These compounds have the potential to inhibit the MyD88-dependent proinflammatory actions of IL-1beta, while permitting the potential neuronal survival supporting actions mediated by the MyD88-independent activation of the protein kinase Akt.

IL-1 receptor-associated kinase 1 is critical for latent membrane protein 1-induced p65/RelA serine 536 phosphorylation and NF-kappaB activation

Epstein-Barr virus latent infection integral membrane protein 1 (LMP1) mimics a constitutively active TNF receptor (TNFR). LMP1 has two C-terminal cytosolic domains, transformation effector sites (TES)1 and -2, that engage TNFR-associated factors (TRAFs) and the TNFR-associated death domain protein, respectively, and activate NF-kappaB. NF-kappaB activation is critical for Epstein-Barr virus-infected lymphoblast survival. TES1- and TES2-mediated NF-kappaB activations are IL-1 receptor-associated kinase 1 (IRAK1)-dependent. Because IRAK1 is upstream of TRAF6 in IL-1 activation of NF-kappaB, the potential role of IRAK1 in LMP1-mediated NF-kappaB activation through TRAF6 and inhibitor of kappaB (IkappaB) kinase (IKK) was initially investigated. Surprisingly, LMP1 expression activated TRAF6 ubiquitination, IKKbeta induction of IkappaB alpha phosphorylation, and p65 nuclear translocation in both WT and IRAK1-deficient I1A 293 cells. LMP1 also induced IKK alpha-mediated p100 processing and p52 nuclear localization in WT and IRAK1-deficient I1A 293 cells. Further, LMP1 TES1 and TES2 induced p65, p50, and p52 NF-kappaB DNA binding in WT and IRAK1-deficient I1A 293 cells. However, LMP1 induced p65/RelA S536 phosphorylation only in WT 293 cells or in IRAK1 kinase point mutant reconstituted I1A 293 cells but not in IRAK1-deficient I1A 293 cells. IRAK1 was also required for LMP1 activation of p38, one of the kinases that can mediate p65/RelA S536 phosphorylation and activate NF-kappaB-dependent transcription. Thus, the critical IRAK1 role in LMP1-induced NF-kappaB activation is in mediating p65/RelA S536 phosphorylation through an effect on p38 or other p65 S536 kinases.

A novel splice variant of interleukin-1 receptor (IL-1R)-associated kinase 1 plays a negative regulatory role in Toll/IL-1R-induced inflammatory signaling

The interleukin-1 (IL-1) receptor-associated kinase 1 (IRAK1) is a member of the IRAK kinase family that plays a pivotal role in the Toll/IL-1 receptor (TIR) family signaling cascade. We have identified a novel splice variant, IRAK1c, which lacks a region encoded by exon 11 of the IRAK1 gene. IRAK1c expression was confirmed by both RNA and protein detection. Although both IRAK1 and IRAK1c are expressed in most tissues tested, IRAK1c is the predominant form of IRAK1 expressed in the brain. Unlike IRAK1, IRAK1c lacks kinase activity and cannot be phosphorylated by IRAK4. However, IRAK1c retains the ability to strongly interact with IRAK2, MyD88, Tollip, and TRAF6. Overexpression of IRAK1c suppressed NF-kappaB activation and blocked IL-1beta-induced IL-6 as well as lipopolysaccharide- and CpG-induced tumor necrosis factor alpha production in multiple cellular systems. Mechanistically, we provide evidence that IRAK1c functions as a dominant negative by failing to be phosphorylated by IRAK4, thus remaining associated with Tollip and blocking NF-kappaB activation. The presence of a regulated, alternative splice variant of IRAK1 that functions as a kinase-dead, dominant-negative protein adds further complexity to the variety of mechanisms that regulate TIR signaling and the subsequent inflammatory response.

Hyperosmolar Saline Is a Proinflammatory Stress on the Mouse Ocular Surface

PURPOSE

To investigate whether hyperosmolar stress stimulates production of inflammatory mediators and activates the mitogen-activated protein kinase (MAPK) signaling pathways, c-jun n-terminal kinases (JNKs), extracellular-regulated kinases (ERKs), and p38 on the mouse ocular surface.

METHODS

129SvEv/CD-1 mixed mice were treated with a balanced salt solution (BSS) (305 mOsM) or a hyperosmotic saline solution (HOSS) (500 mOsM). Untreated age-matched mice were used as controls. The concentrations of interleukin 1beta (IL-1beta) and tumor necrosis factor alpha (TNF-alpha) were measured by enzyme-linked immunosorbent assay. Gelatinase activity was determined by in situ zymography. Corneal and conjunctival epithelia were lysed for Western blot with MAPK antibodies or used for semiquantitative reverse transcription and polymerase chain reaction and gene array.

RESULTS

Compared with age-matched controls and mice treated with BSS, the concentration of IL-1beta in tear fluid washings and the concentrations of IL-1beta and TNF-alpha and gelatinolytic activity in the corneal and conjunctival epithelia were significantly increased in mice treated with HOSS for 2 days. The expressions of IL-1beta, TNF-alpha, and matrix metalloproteinase 9 (MMP-9) messenger RNA by the corneal and conjunctival epithelia were also notably stimulated in mice treated with HOSS. The levels of phosphorylated JNK1/2, ERK1/2, and p38 MAPKs in the corneal and conjunctival epithelia were slightly increased in mice treated with BSS, but markedly increased in mice treated with HOSS.

CONCLUSIONS

These results show that the hyperosmolarity stimulates expression and production of IL-1beta, TNF-alpha, and MMP-9 and activates JNK, ERK, and p38 MAPK signaling pathways on the mouse ocular surface. These findings suggest that hyperosmolar stress, as it may occur in dry eye, promotes ocular surface inflammation.

Cutting edge: expression of IL-1 receptor-associated kinase-4 (IRAK-4) proteins with mutations identified in a patient with recurrent bacterial infections alters normal IRAK-4 interaction with components of the IL-1 receptor complex

In a patient with recurrent bacterial infections and profound hyporesponsiveness to LPS and IL-1, we previously identified two mutations in IL-1R-associated kinase-4 (IRAK-4) that encoded proteins with truncated kinase domains. Overexpression of either of these mutant IRAK-4 variants in HEK293 cells failed to activate endogenous IRAK-1 and suppressed IL-1-induced IRAK-1 kinase activity, in contrast to wild-type (WT) IRAK-4. In this study, interactions of WT and mutant IRAK-4 species with IL-1R, IRAK-1, and MyD88 in HEK293 transfectants were compared. IL-1 induced a strong interaction among the IL-1R, activated IRAK-1, MyD88, and WT, but not mutant, IRAK-4. Truncated IRAK-4 proteins constitutively interacted more strongly with MyD88 and blunted IL-1-induced recruitment of IRAK-1 and MyD88 to the IL-1R. Thus, decreased IL-1-induced association of IRAK-1 and MyD88 with the IL-1RI may result from sequestration of cytoplasmic MyD88 by IRAK-4 mutant proteins. Therefore, mimetics of these truncated IRAK-4 proteins may represent a novel approach to mitigating hyperinflammatory states.

RDP58 is a novel and potentially effective oral therapy for ulcerative colitis

BACKGROUND

RDP58 is a novel anti-inflammatory d-amino acid decapeptide that inhibits synthesis of proinflammatory cytokines by disrupting cell signaling at the pre-MAPK MyD88-IRAK-TRAF6 protein complex. We therefore evaluated its efficacy and safety in parallel multicenter, double-blind, randomized concept studies in ulcerative colitis (UC).

METHODS

In the first trial, 34 patients with mild to moderate active UC were randomized (1:2) to placebo (n = 13) or RDP58 100 mg (n = 21). In the second trial, 93 similar patients were randomized (1:1:1) to placebo (n = 30) RDP58 200 mg (n = 31), or RDP 300 mg (n = 32). In both studies, treatment success was defined as a simple clinical colitis activity index score of no more than 3 at 28 days. Sigmoidoscopy and rectal biopsy (at baseline and 28 days) and safety measures (baseline and 28 and 56 days) were other endpoints.

RESULTS

Treatment success on RDP 100 mg was 29% versus 46% on placebo (P = 0.46). There were no significant differences in sigmoidoscopy or histology score. In the second study, treatment success on the higher doses of RDP58 (200 and 300 mg) was 71% and 72%, respectively, versus 43% on placebo (P = 0.016). Improvements in sigmoidoscopy scores (41% on 200 mg and 46% on 300 mg versus 32% on placebo) did not reach significance, but histology scores improved significantly (P = 0.002) versus placebo. Overall, adverse events were no different between placebo (3.3 +/- 2.4) and RDP58 (2.7 +/- 1.4, 300-mg group).

CONCLUSIONS

RDP58 at a dose of 200 or 300 mg, but not 100 mg, was effective in mild-to-moderate UC. RDP58 was safe and well tolerated, and its novel action makes it an attractive potential therapy.

Cellular trafficking of the IL-1RI-associated kinase-1 requires intact kinase activity

Upon stimulation of cells with interleukin-1 (IL-1) the IL-1 receptor type I (IL-1RI) associated kinase-1 (IRAK-1) transiently associates to and dissociates from the IL-1RI and thereafter translocates into the nucleus. Here we show that nuclear translocation of IRAK-1 depends on its kinase activity since translocation was not observed in EL-4 cells overexpressing a kinase negative IRAK-1 mutant (EL-4(IRAK-1-K239S)). IRAK-1 itself, an endogenous substrate with an apparent molecular weight of 24kDa (p24), and exogenous substrates like histone and myelin basic protein are phosphorylated by nuclear located IRAK-1. Phosphorylation of p24 cannot be detected in EL-4(IRAK-1-K239S) cells. IL-1-dependent recruitment of IRAK-1 to the IL-1RI and subsequent phosphorylation of IRAK-1 is a prerequisite for nuclear translocation of IRAK-1. It is therefore concluded that intracellular localization of IRAK-1 depends on its kinase activity and that IRAK-1 may also function as a kinase in the nucleus as shown by a new putative endogenous substrate.

Phosphorylation of Ser24 in the pleckstrin homology domain of insulin receptor substrate-1 by Mouse Pelle-like kinase/interleukin-1 receptor-associated kinase: cross-talk between inflammatory signaling and insulin signaling that may contribute to insulin resistance

Inflammation contributes to insulin resistance in diabetes and obesity. Mouse Pelle-like kinase (mPLK, homolog of human IL-1 receptor-associated kinase (IRAK)) participates in inflammatory signaling. We evaluated IRS-1 as a novel substrate for mPLK that may contribute to linking inflammation with insulin resistance. Wild-type mPLK, but not a kinase-inactive mutant (mPLK-KD), directly phosphorylated full-length IRS-1 in vitro. This in vitro phosphorylation was increased when mPLK was immunoprecipitated from tumor necrosis factor (TNF)-alpha-treated cells. In NIH-3T3(IR) cells, wild-type mPLK (but not mPLK-KD) co-immunoprecipitated with IRS-1. This association was increased by treatment of cells with TNF-alpha. Using mass spectrometry, we identified Ser(24) in the pleckstrin homology (PH) domain of IRS-1 as a specific phosphorylation site for mPLK. IRS-1 mutants S24D or S24E (mimicking phosphorylation at Ser(24)) had impaired ability to associate with insulin receptors resulting in diminished tyrosine phosphorylation of IRS-1 and impaired ability of IRS-1 to bind and activate PI-3 kinase in response to insulin. IRS-1-S24D also had an impaired ability to mediate insulin-stimulated translocation of GLUT4 in rat adipose cells. Importantly, endogenous mPLK/IRAK was activated in response to TNF-alpha or interleukin 1 treatment of primary adipose cells. In addition, using a phospho-specific antibody against IRS-1 phosphorylated at Ser(24), we found that interleukin-1 or TNF-alpha treatment of Fao cells stimulated increased phosphorylation of endogenous IRS-1 at Ser(24). We conclude that IRS-1 is a novel physiological substrate for mPLK. TNF-alpha-regulated phosphorylation at Ser(24) in the pleckstrin homology domain of IRS-1 by mPLK/IRAK represents an additional mechanism for cross-talk between inflammatory signaling and insulin signaling that may contribute to metabolic insulin resistance.

IRAK1 Serves as a Novel Regulator Essential for Lipopolysaccharide-induced Interleukin-10 Gene Expression

Being one of the key kinases downstream of Toll-like receptors, IRAK1 has initially thought to be responsible for NFkappaB activation. Yet IRAK1 knock-out mice still exhibit NFkappaB activation upon lipopolysaccharide (LPS) challenge, suggesting that IRAK1 may play other un-characterized function. In this report, we show that IRAK1 is mainly involved in Stat3 activation and subsequent interleukin-10 (IL-10) gene expression. Splenocytes from IRAK1-deficient mice fail to exhibit LPS-induced Stat3 serine phosphorylation and IL-10 gene expression yet still maintain normal IL-1beta gene expression upon LPS challenge. Mechanistically, we observe that IRAK1 modification upon LPS challenge leads to its modification, nuclear distribution, and interaction with Stat3. IRAK1 can directly use Stat3 as a substrate and cause Stat3 serine 727 phosphorylation. In addition, nuclear IRAK1 binds directly with IL-10 promoter in vivo upon LPS treatment. Atherosclerosis patients usually have elevated serum IL-10 levels. We document here that IRAK1 is constitutively modified and localized in the nucleus in the peripheral blood mononuclear cells from atherosclerosis patients. These observations reveal the mechanism for the novel role of IRAK1 in the complex Toll-like receptor signaling network and indicate that IRAK1 regulation may be intimately linked with the pathogenesis and/or resolution of atherosclerosis.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Toll IL-1 Receptors Differ in Their Ability to Promote the Stabilization of Adenosine and Uridine-Rich Elements Containing mRNA

Several ligands for Toll IL-1R (TIR) family are known to promote stabilization of a subset of short-lived mRNAs containing AU-rich elements (AREs) in their 3' untranslated regions. It is now evident however, that members of the TIR family may use distinct intracellular signaling pathways to achieve a spectrum of biological end points. Using human embryonic kidney 293 cells transfected to express different TIRs we now report that signals initiated through IL-1R1 or TLR4 but not TLR3 can promote the stabilization of unstable chemokine mRNAs. Similar results were obtained when signaling from endogenous receptors was examined using a mouse endothelial cell line (H5V). The ability of TIR family members to stabilize ARE-containing mRNAs results from their differential use of signaling adaptors MyD88, MyD88 adaptor-like protein, Toll receptor IFN-inducing factor (Trif), and Trif-related adaptor molecule. Overexpression of MyD88 or MyD88 adaptor-like protein was able to promote enhanced stability of ARE-containing mRNA, whereas Trif and Trif-related adaptor molecule exhibited markedly reduced capacity. Hence the ability of TIRs to signal stabilization of mRNA appears to be linked to the MyD88-dependent signaling pathway.

Lipopolysaccharide stimulates nitric oxide synthase-2 expression in murine skeletal muscle and C(2)C(12) myoblasts via Toll-like receptor-4 and c-Jun NH(2)-terminal kinase pathways

The inducible form of nitric oxide synthase (NOS2) catalyzes the synthesis of nitric oxide (NO) from arginine in response to injury and infection. NOS2 is expressed predominantly by macrophages and lymphocytes. However, skeletal muscle also expresses NOS2 in response to inflammatory stimuli. The present study sought to determine whether lipopolysaccharide (LPS) stimulates NOS2 in skeletal muscle via Toll-like receptor-4 (TLR4). Intraperitoneal injection of LPS in wild-type mice (C3H/HeSnJ) increased NOS2 mRNA fourfold in skeletal muscle, while no change in NOS2 mRNA was observed in C3H/HeJ mice that harbored a mutation in the LPS receptor. NOS2 coimmunoprecipitated with the muscle-specific caveolin-3 protein, suggesting that myofibers per se respond to LPS in vivo. LPS stimulated NOS2 mRNA expression in C(2)C(12) myocytes, and the regulation of NOS2 mRNA was comparable in myoblasts and differentiated myotubes. LPS transiently stimulated the phosphorylation of the interleukin-1 receptor-associated kinase (IRAK-1) in C(2)C(12) cells and decreased the total amount of IRAK-1 both in vitro and in vivo over time. LPS stimulated the expression of an NF-kappabeta reporter plasmid, and this was inhibited by the proteasomal inhibitor MG-132. Both myoblasts and myotubes expressed TLR2 and TLR4 mRNA. Expression of a dominant negative form of TLR4 in C(2)C(12) cells blocked LPS-induced NF-kappabeta reporter activity. SP-600125 [a c-Jun NH(2)-terminal kinase (JNK) inhibitor] also prevented LPS stimulation of NOS2 expression. Moreover, the JNK inhibitor prevented the LPS-induced increase in NO synthesis. These data indicate that LPS increases NOS2 mRNA expression in muscle via a TLR4-dependent mechanism.

Prolongation of islet allograft survival following ex vivo transduction with adenovirus encoding a soluble type 1 TNF receptor–Ig fusion decoy

Islet transplantation is a viable long-term therapeutic alternative to daily insulin replacement for type I diabetes. The allogeneic nature of the transplants poses immunological challenges for routine clinical utility. Gene transfer of immunoregulatory molecules and those that improve insulin release kinetics provides rational approaches to facilitate allogeneic islet transplantation as a potential therapy. We have examined the efficacy of a soluble type 1 tumor necrosis factor receptor (TNFR) immunoglobulin-Fc fusion transgene (TNFR-Ig) to protect human islets from cytokine-induced apoptosis in culture, as well as in facilitating allogeneic islet transplants in diabetic mice. Cultured human islets were transduced with an adenoviral vector encoding human TNFR-Ig (Ad-TNFR-Ig). TNFR-Ig protein was secreted by cultured islets, as well as by transduced mouse islet transplants recovered from mouse recipients. Glucose-induced insulin release kinetics were comparable among untransduced, Ad-TNFR-Ig-infected human islets and vector-transduced islets exposed to cytokines. In parallel, Ad-TNFR-Ig-infected islets were protected from cytokine-induced apoptosis activation. Finally, diabetic mice transplanted with allogeneic islets expressing TNFR-Ig returned to and maintained normoglycemia significantly longer than untransduced islet recipients. These data support the potential utility of TNFR-Ig gene transfer to islets as a means of facilitating allogeneic islet transplantation.

The IL-1 receptor accessory protein is essential for PI 3-kinase recruitment and activation

Interleukin-1 (IL-1) binds to its type I receptors (IL-1R), which in complex with IL-1R accessory protein (IL-1R AcP) induces various intracellular signaling events. We report here that IL-1 triggers the recruitment of phosphoinositide 3-kinase (PI 3-kinase) to a signaling complex and induces its lipid kinase activity in a biphasic manner. This IL-1-induced complex consists of IL-1R, IL-1R AcP, PI 3-kinase, and the IL-1-receptor-associated kinase (IRAK). Deletion of the C-terminus 27 amino acids of IL-1R AcP resulted in a mutant, CDelta27, that could not recruit PI 3-kinase to the signalsome nor stimulate PI3-kinase activity. Moreover, CDelta27 functioned as a dominant-negative mutant that inhibited IL-1-induced PI 3-kinase and NFkappaB activation. CDelta27, however, had no effect on IL-1-dependent activation of the Jun N-terminal kinase (JNK), indicating that distinct regions of IL-1R AcP mediate the activation of PI 3-kinase and JNK. Thus, our results identified a functional region in the IL-1R AcP required for the recruitment and activation of PI 3-kinase.

Rat brain vascular distribution of interleukin-1 type-1 receptor immunoreactivity: relationship to patterns of inducible cyclooxygenase expression by peripheral inflammatory stimuli

Interleukin-1 beta (IL-1 beta) is thought to act on the brain to induce fever, neuroendocrine activation, and behavioral changes during disease through induction of prostaglandins at the blood-brain barrier (BBB). However, despite the fact that IL-1 beta induces the prostaglandin-synthesizing enzyme cyclooxygenase-2 (COX-2) in brain vascular cells, no study has established the presence of IL-1 receptor type 1 (IL-1R1) protein in these cells. Furthermore, although COX inhibitors attenuate expression of the activation marker c-Fos in the preoptic and paraventricular hypothalamus after administration of IL-1 beta or bacterial lipopolysaccharide (LPS), they do not alter c-Fos induction in other structures known to express prostaglandin receptors. The present study thus sought to establish whether IL-1R1 protein is present and functional in the rat cerebral vasculature. In addition, the distribution of IL-1R1 protein was compared to IL-1 beta- and LPS-induced COX-2 expression. IL-1R1-immunoreactive perivascular cells were mostly found in choroid plexus and meninges. IL-1R1-immunoreactive vessels were seen throughout the brain, but concentrated in the preoptic area, subfornical organ, supraoptic hypothalamus, and to a lesser extent in the paraventricular hypothalamus, cortex, nucleus of the solitary tract, and ventrolateral medulla. Vascular IL-1R1-ir was associated with an endothelial cell marker, not found in arterioles, and corresponded to the induction patterns of phosphorylated c-Jun and inhibitory-factor kappa B mRNA upon IL-1 beta stimulation, and colocalized with peripheral IL-1 beta- or LPS-induced COX-2 expression. These observations indicate that functional IL-1R1s are expressed in endothelial cells of brain venules and suggest that vascular IL-1R1 distribution is an important factor determining BBB prostaglandin-dependent activation of brain structures during infection.

Analogies between Drosophila and mammalian TRAF pathways

A central event in innate immunity is the activation of the NF-kappaB signaling pathway and up-regulation of NF-kappaB-dependent defense genes. Attack of mammals as well as of insects by microorganisms leads, among other things, to the activation of receptors of the Toll-like receptor group. Various adaptor proteins involving members of the TNF receptor-associated factor (TRAF) family channel these receptor-generated signals to conserved intracellular kinase cascades that finally lead to the activation of NF-kappaB and JNK. In vertebrates, TRAF proteins link these pathways also to IL-1R-related molecules and members of the TNF receptor superfamily, which orchestrate a variety of immunoregulatory processes of the innate but also of the adaptive immune system. In this review, we will focus on the similarities but also the differences in TRAF-dependent signaling pathways of mammals and insects.