Regulation of IL-1 Receptor-Associated Kinases by Lipopolysaccharide

Abstract

IL-1R-associated kinase (IRAK) plays a pivotal role in IL-1R/Toll-like receptor (TLR)-mediated signaling and NF-κB activation. IRAK from leukocytes undergoes rapid activation and inactivation/degradation following IL-1 or LPS stimulation. The rapid degradation of IRAK may serve as a negative feedback mechanism of down-regulating IL-1R/TLR-mediated signaling and cytokine gene transcription. Although IL-1/IL-1R-triggered IRAK degradation has been studied in detail, the mechanism of LPS-induced IRAK activation and degradation is not clearly defined. In this study, we demonstrate that the IRAK N-terminal 186-aa region is required for LPS-induced degradation. The N-terminally truncated IRAK protein expressed in human monocytic THP-1 cells remains stable upon LPS challenge. In comparison, IRAK as well as the IRAK mutant with C-terminal truncation undergo degradation with LPS stimulation. We demonstrate that pretreatment with protein kinase C inhibitor calphostin inhibits LPS-induced IRAK degradation. Furthermore, we observe coimmunoprecipitation of endogenous IRAK and protein kinase C-ζ protein. We show that functional TLR4 is required for LPS-mediated IRAK degradation. IRAK protein in the murine GG2EE cells harboring a mutated TLR4 gene does not undergo degradation upon LPS treatment. In sharp contrast, we observe that the IRAK homolog, IRAK2, does not undergo degradation upon prolonged LPS treatment, suggesting complex regulation of the innate immunity network upon microbial challenge.

The Low Molecular Weight GTPase RhoA and Atypical Protein Kinase Cζ Are Required for TLR2-Mediated Gene Transcription

“…The direct interaction between p62 and the adapter protein TRAF6, which play a crucial role in signaling through IL-1␤ and TLR2, may account for the involvement of aPKCs in different NF-B activation pathways. Hu et al (30) showed direct interaction of IRAK1, a serine/threonine kinase upstream of TRAF6, with PKC when monocytic cells were stimulated with LPS. Using a TLR2-dependent stimulus, we did not detect an association of PKC with IRAK1, IRAK4, or TRAF6.…”

Dual Receptors and Distinct Pathways Mediate Interleukin-1 Receptor-associated Kinase Degradation in Response to Lipopolysaccharide

“…In examining this question, an important lead was provided by the findings that PKC-associated with IRAK in LPS-treated THP-1 cells and the demonstration that PKC activity was required for LPS-induced IRAK degradation (40). These results, indicating a possible role for PKC-in a pathway regulating IRAK degradation, led us to examine whether this may be regulated by PI 3-kinase because LPS has been shown to activate this lipid kinase leading to activation of PKC- (39).…”

“…Previous work from this laboratory (36) and elsewhere (37,38) has demonstrated that the interaction of the complex of both LPS and LPS-binding protein with CD14 brings about activation of PI 3-kinase, which appears to be involved in mediating cellular responses to endotoxin including activation of an atypical isoform of PKC, PKC- (39). These findings, coupled with the observations that PKC-associates with IRAK in LPS-treated cells and that the PKC inhibitor calphostin prevents IRAK degradation (40), provide a basis for a model in which cellular responsiveness to LPS involves a PI 3-kinase signaling pathway that functionally bifurcates. In this model, one arm of a PI 3-kinase pathway leads to cell activation in response to LPS.…”

The immunomodulating neuropeptide alpha-melanocyte-stimulating hormone (α-MSH) suppresses LPS-stimulated TLR4 with IRAK-M in macrophages

“…4C). In addition, the lack of detecting IRAK-M at 30 min following LPS stimulation is expected because of the natural clearance or degradation of IRAK-1 following LPS stimulation (Hu et al, 2002;Jensen and Whitehead, 2001;Yamin and Miller, 1997). Therefore, α-MSH suppresses LPS-stimulated TLR4 activity through a mechanism that promotes IRAK-M binding of IRAK-1.…”