Tyrosine phosphorylation of MyD88 adapter-like (Mal) is critical for signal transduction and blocked in endotoxin tolerance

Department of Microbiology and Immunology, University of Maryland School of Medicine, Baltimore, Maryland 21201, USA.
Journal of Biological Chemistry (Impact Factor: 4.6). 03/2008; 283(6):3109-19. DOI: 10.1074/jbc.M707400200
Source: PubMed

ABSTRACT Toll-like receptor 4 (TLR4) recognition of lipopolysaccharide triggers signalosome assembly among TLR4, sorting (e.g. MyD88 adapter-like (Mal)) and signaling (e.g. MyD88) adapters, initiating recruitment and activation of kinases, activation of transcription factors, and production of inflammatory mediators. In this study we examined whether tyrosine phosphorylation of Mal regulates its interactions with TLR4, MyD88, interleukin-1 (IL-1) receptor-associated kinase (IRAK)-2, and tumor necrosis factor receptor-associated factor (TRAF)-6 and is important for signaling. Overexpression of wild-type Mal in human embryonic kidney 293T cells induced its constitutive tyrosine phosphorylation and led to activation of p38, NF-kappaB, and IL-8 gene expression. Mutagenesis of Tyr-86, Tyr-106, and Tyr-159 residues within the Toll-IL-1 receptor domain impaired Mal tyrosine phosphorylation, interactions with Bruton tyrosine kinase, phosphorylation of p38, and NF-kappaB activation. Lipopolysaccharide triggered tyrosine phosphorylation of endogenous Mal and initiated Mal-Bruton-tyrosine kinase interactions in 293/TLR4/MD-2 cells and human monocytes that were suppressed in endotoxin-tolerant cells. Compared with wild-type Mal, Y86A-, Y06A-, and Y159A-Mal variants exhibited higher interactions with TLR4 and MyD88, whereas associations with IRAK-2 and TRAF-6 were not affected. Overexpression of Y86A- and Y106A-Mal in 293/TLR4/MD-2 cells exerted dominant-negative effects on TLR4-inducible p38 phosphorylation and NF-kappaB reporter activation to the extent comparable with P125H-Mal-mediated suppression. In contrast, tyrosine-deficient Mal species did not affect NF-kappaB activation when signaling was initiated at the post-receptor level by overexpression of MyD88, IRAK-2, or TRAF-6. Thus, tyrosine phosphorylation of Mal is required for adapter signaling, regulates Mal interactions with TLR4 and receptor signaling, and is inhibited in endotoxin tolerance.

Download full-text


Available from: Wenji Piao, May 16, 2014
1 Follower
  • Source
    • "This appears to be an important aspect of SOCS1—mediated inhibition of TLR4 signaling. This phosphorylation of Mal has also been shown to be impaired in cells rendered tolerant to LPS (Piao et al., 2008). An inhibitory role for proteins with an immunoreceptor tyrosine-based activation motif (ITAM) toward TLR signaling has also been suggested (Hamerman et al., 2005). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Toll-like receptor (TLR) signaling is subjected to crosstalk from other signals, with a resulting positive or negative effect. There is complex crosstalk between the NLR family of immune-regulatory molecules and TLRs, and C-type lectin receptors such as Dectin-1 synergize with TLR2 via the tyrosine kinase Syk. Bruton's tyrosine kinase plays an important positive role in TLR signaling, whereas the TAM family of receptor tyrosine kinases is inhibitory. The tyrosine phosphatase SHP1 has been shown to positively regulate induction of interferon-beta, whereas SHP2 inhibits the kinase TBK1, limiting this response. K63-linked polyubiquination has also been shown to be critical for the initiation of TLR signaling. Finally, glucocorticoids affect TLR signaling by inducing the phosphatase MKP1 and inhibiting TBK1 activation. These recent findings emphasize the importance of considering TLR signaling in the context of other signaling pathways, as is likely to occur in vivo during infection and inflammation.
    Immunity 07/2008; 29(1):12-20. DOI:10.1016/j.immuni.2008.06.004 · 19.75 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The present review summarizes exciting new findings and reports recent advances in the understanding of the role of toll-like receptors in health and disease. It intends to provide a rough survey on topics discussed by researchers in the field and to stimulate discussion on new aspects of the complex processes involved in innate host defence. Novel findings have been reported on the many aspects of toll-like receptors biology, namely the receptor structure and the molecular process of ligand recognition, receptor assembly, cellular localization and trafficking, downstream signaling and the regulatory factors involved, genetic polymorphisms within receptor genes and their linkage to human diseases, and the functional role of toll-like receptors in immune defence and host-microbe homeostasis. Recent advances have allowed a more detailed picture not only of the processes involved in microbial recognition and host defence but also revealed unexpected insights into the cause of inflammatory processes and the close interrelationship between the vertebrate host and the microbially colonized environment.
    Current Opinion in Infectious Diseases 07/2008; 21(3):304-12. DOI:10.1097/QCO.0b013e3282f88ba3 · 5.03 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Research into the five Toll/IL1 receptor (TIR) adaptor proteins involved in innate immunity continues to advance. Here we outline some of the more recent findings. MyD88 has a key role in signalling by the IL1 receptor complex and TLRs. However, a MyD88-independent pathway of IL1beta signalling in neurons has been described which involves the protein kinase Akt, and which has an anti-apoptotic effect. This pathway may also be important for the mechanism whereby Alum exerts its adjuvant effect since this depends on IL1beta but is MyD88-independent. MyD88 is also involved in tumourigenesis in models of hepatocarcinoma and familial associated polyposis (FAP); negative regulation of TLR3 signalling and in PKCepsilon activation. The adaptor Mal is regulated by phosphorylation and caspase-1 cleavage. A variant form of Mal in humans termed S180L confers protection in multiple infectious diseases. TRAM is controlled by myristoylation and phosphorylation and the localisation of TRAM with TLR4 to endosomes is required for activation of IRF3 and induction of IFNbeta. Finally SARM has been shown to regulate TRIF and also appears to be involved in neuronal injury mediated by oxidative stress in mouse neurons. These advances confirm the importance for the TIR domain-containing adapters in host defence and inflammation.
    Cytokine 09/2008; 43(3):342-9. DOI:10.1016/j.cyto.2008.07.010 · 2.87 Impact Factor