Contribution of drugs acting on the TLRs/MyD88 signaling pathway on colitis-associated cancer.

Toll-like receptors play a particularly significant role in colitis-associated cancer (CAC). MyD88 is the mediator in TLRs signal transduction process and it is indispensable for TLRs signaling except for TLR3. The conclusion of studies about the role of TLRs/MyD88 signaling in colon cancer remains contradictory: on one hand, TLRs/MyD88 signaling contributes to colon tumor cell proliferation, invasion and metastasis and inhibition of the expression of TLRs or MyD88 could prevent the growth of colon cancer cells; on the other hand, activation of the TLRs/MyD88 signaling pathway could inhibit the proliferation of colon cancer cells. This article is based on the expression levels of TLRs or MyD88 and the activation degrees of TLRs/MyD88 signaling pathway in different periods of colon cancer and, reviews the roles of TLRs/MyD88 signaling in the tumorigenesis and procession of CAC and the clinical application of agonists and inhibitor of TLRs or MyD88. This article is intended to explore the diverse roles of TLRs/MyD88 signaling pathway in CAC and to reveal the related molecular mechanism.

The mechanism of activation of IRAK1 and IRAK4 by interleukin-1 and Toll-like receptor agonists.

We have developed the first assays that measure the protein kinase activities of interleukin-1 receptor-associated kinase 1 (IRAK1) and IRAK4 reliably in human cell extracts, by employing Pellino1 as a substrate in conjunction with specific pharmacological inhibitors of IRAK1 and IRAK4. We exploited these assays to show that IRAK4 was constitutively active and that its intrinsic activity towards Pellino1 was not increased significantly by stimulation with interleukin-1 (IL-1) in IL-1R-expressing HEK293 cells, Pam3CSK4-stimulated human THP1 monocytes or primary human macrophages. Our results, in conjunction with those of other investigators, suggest that the IL-1-stimulated trans-autophosphorylation of IRAK4 is initiated by the myeloid differentiation primary response gene 88-induced dimerization of IRAK4 and is not caused by an increase in the intrinsic catalytic activity of IRAK4. In contrast with IRAK4, we found that IRAK1 was inactive in unstimulated cells and converted into an active protein kinase in response to IL-1 or Pam3CSK4 in human cells. Surprisingly, the IL-1-stimulated activation of IRAK1 was not affected by pharmacological inhibition of IRAK4 and not reversed by dephosphorylation and/or deubiquitylation, suggesting that IRAK1 catalytic activity is not triggered by a covalent modification but by an allosteric mechanism induced by its interaction with IRAK4.

MAP1S Protein Regulates the Phagocytosis of Bacteria and Toll-like Receptor (TLR) Signaling.

Phagocytosis is a critical cellular process for innate immune defense against microbial infection. The regulation of phagocytosis process is complex and has not been well defined. An intracellular molecule might regulate cell surface-initiated phagocytosis, but the underlying molecular mechanism is poorly understood (1). In this study, we found that microtubule-associated protein 1S (MAP1S), a protein identified recently that is involved in autophagy (2), is expressed primarily in macrophages. MAP1S-deficient macrophages are impaired in the phagocytosis of bacteria. Furthermore, we demonstrate that MAP1S interacts directly with MyD88, a key adaptor of Toll-like receptors (TLRs), upon TLR activation and affects the TLR signaling pathway. Intriguingly, we also observe that, upon TLR activation, MyD88 participates in autophagy processing in a MAP1S-dependent manner by co-localizing with MAP1 light chain 3 (MAP1-LC3 or LC3). Therefore, we reveal that an intracellular autophagy-related molecule of MAP1S controls bacterial phagocytosis through TLR signaling.

FcγRIIIa-Syk Co-signal Modulates CD4+ T-cell Response and Up-regulates Toll-like Receptor (TLR) Expression.

CD4(+) T-cells in systemic lupus erythematosus (SLE) patients show altered T-cell receptor signaling, which utilizes Fc-receptor γ-chain FcRγ-Syk. A role for FcγRIIIa activation from immune complex (IC) ligation and sublytic terminal complement complex (C5b-9) in CD4(+) T-cell responses is not investigated. In this study, we show that the ICs present in SLE patients by ligating to FcγRIIIa on CD4(+) T-cells phosphorylate Syk and provide a co-stimulatory signal to CD4(+) T-cells in the absence of CD28 signal. This led to the development of pathogenic IL-17A(+) and IFN-γ(high) CD4(+) T-cells in vitro. Cytokines IL-1ß, IL-6, TGF-ß1, and IL-23 were the only requirement for the development of both populations. SLE patients CD4(+) T-cells that expressed CD25, CD69, and CD98 bound to ICs showed pSyk and produced IFN-γ and IL-17A. This FcγRIIIa-mediated co-signal differentially up-regulated the expression of IFN pathway genes compared with CD28 co-signal. FcγRIIIa-pSyk up-regulated several toll-like receptor genes as well as the HMGB1 and MyD88 gene transcripts. ICs co-localized with these toll-like receptor pathway proteins. These results suggest a role for the FcγRIIIa-pSyk signal in modulating adaptive immune responses.

Acylcarnitines activate proinflammatory signaling pathways.

Incomplete ß-oxidation of fatty acids in mitochondria is a feature of insulin resistance and type 2 diabetes mellitus (T2DM). Previous studies revealed that plasma concentrations of medium- and long-chain acylcarnitines (by-products of incomplete ß-oxidation) are elevated in T2DM and insulin resistance. In a previous study, we reported that mixed D,L isomers of C12- or C14-carnitine induced an NF-κB-luciferase reporter gene in RAW 264.7 cells, suggesting potential activation of proinflammatory pathways. Here, we determined whether the physiologically relevant L-acylcarnitines activate classical proinflammatory signaling pathways and if these outcomes involve pattern recognition receptor (PRR)-associated pathways. Acylcarnitines induced the expression of cyclooxygenase-2 in a chain length-dependent manner in RAW 264.7 cells. L-C14 carnitine (5-25 µM), used as a representative acylcarnitine, stimulated the expression and secretion of proinflammatory cytokines in a dose-dependent manner. Furthermore, L-C14 carnitine induced phosphorylation of JNK and ERK, common downstream components of many proinflammatory signaling pathways including PRRs. Knockdown of MyD88, a key cofactor in PRR signaling and inflammation, blunted the proinflammatory effects of acylcarnitine. While these results point to potential involvement of PRRs, L-C14 carnitine promoted IL-8 secretion from human epithelial cells (HCT-116) lacking Toll-like receptors (TLR)2 and -4, and did not activate reporter constructs in TLR overexpression cell models. Thus, acylcarnitines have the potential to activate inflammation, but the specific molecular and tissue target(s) involved remain to be identified.

Containment of aerogenic Mycobacterium tuberculosis infection in mice does not require MyD88 adaptor function for TLR2, -4 and -9.

The role of Toll-like receptors (TLR) and MyD88 for immune responses to Mycobacterium tuberculosis (Mtb) infection remains controversial. To address the impact of TLR-mediated pathogen recognition and MyD88-dependent signaling events on anti-mycobacterial host responses, we analyzed the outcome of Mtb infection in TLR2/4/9 triple- and MyD88-deficient mice. After aerosol infection, both TLR2/4/9-deficient and wild-type mice expressed pro-inflammatory cytokines promoting antigen-specific T cells and the production of IFN-gamma to similar extents. Moreover, TLR2/4/9-deficient mice expressed IFN-gamma-dependent inducible nitric oxide synthase and LRG-47 in infected lungs. MyD88-deficient mice expressed pro-inflammatory cytokines and were shown to expand IFN-gamma-producing antigen-specific T cells, albeit in a delayed fashion. Only mice that were deficient for MyD88 rapidly succumbed to unrestrained mycobacterial growth, whereas TLR2/4/9-deficient mice controlled Mtb replication. IFN-gamma-dependent restriction of mycobacterial growth was severely impaired only in Mtb-infected MyD88, but not in TLR2/4/9-deficient bone marrow-derived macrophages. Our results demonstrate that after Mtb infection neither TLR2, -4, and -9, nor MyD88 are required for the induction of adaptive T cell responses. Rather, MyD88, but not TLR2, TLR4 and TLR9, is critical for triggering macrophage effector mechanisms central to anti-mycobacterial defense.

MyDths and un-TOLLed truths: sensor, instructive and effector immunity to tuberculosis.

Controversy exists concerning the role of Toll-like receptors and MyD88 in immunity to tuberculosis (TB). This mini-review argues that (i) Toll-like receptors are not essential for an effective immune response against TB, (ii) MyD88 is essential, but not because it transduces signals from TLRs, (iii) adaptive immunity to TB is largely TLR/MyD88-independent. Some of the discrepancies may be resolved by cogent attribution of distinct immune functions to the individual components of the TLR/MyD88 system. In mice, TLRs and MyD88 are fully dispensable in sensing Mtb infection and instructing T cell-mediated adaptive immunity, and while TLRs are also redundant during macrophage effector immunity, MyD88 is essential for efficient killing of mycobacteria. This distinction should help to molecularly pinpoint the MyD88-dependent, yet TLR-independent critical mechanisms of macrophage activation involved in intracellular growth restriction of Mtb. Disrupted IL-1R and/or IFN-gamma signaling pathways likely play a much more prominent role in explaining the exquisite susceptibility of MyD88-deficient mice to TB than the function of MyD88 as a TLR adaptor.