Pellino-1 positively regulates Toll-like Receptor (TLR) 2 and TLR4 signaling and is suppressed upon induction of endotoxin tolerance [Immunology]

June 16th, 2015 by Murphy, M., Xiong, Y., Pattabiraman, G., Qiu, F., Medvedev, A. E.

Endotoxin tolerance reprograms Toll-like receptor (TLR) 4-mediated macrophage responses by attenuating induction of pro-inflammatory cytokines while retaining expression anti-inflammatory and antimicrobial mediators. We previously demonstrated deficient TLR4-induced activation of IL-1 receptor-associated kinase (IRAK) 4, IRAK1, and TANK-binding kinase (TBK) 1 as critical hallmarks of endotoxin tolerance, but mechanisms remain unclear. In this study, we examined the role of the E3 ubiquitin ligase Pellino-1 in endotoxin tolerance and TLR signaling. LPS stimulation increased Pellino-1 mRNA and protein expression in macrophages from mice injected with saline and in medium-pretreated human monocytes, THP-1 and MonoMac-6 cells, while endotoxin tolerization abrogated LPS inducibilty of Pellino-1. Overexpression of Pellino-1 in 293/TLR2 and 293/TLR4/MD2 cells enhanced TLR2- and TLR4-induced nuclear factor κB (NF-κB) and expression of IL-8 mRNA, while Pellino-1 knockdown reduced these responses. Pellino-1 ablation in THP-1 cells impaired induction of myeloid differentiation primary response protein (MyD88) - and Toll-IL-1R domain-containing adapter inducing IFN-β (TRIF)-dependent cytokine genes in response to TLR4 and TLR2 agonists and heat-killed Esherichia coli and Streptococcus aureus, while only weakly affecting phagocytosis of heat-killed bacteria. Co-expressed Pellino-1 potentiated NF-κB activation driven by transfected MyD88, TRIF, IRAK1, TBK1, TGF-β-activated kinase (TAK) 1 and TNFR-associated factor 6, while not affecting p65-induced responses. Mechanistically, Pellino-1 increased LPS-driven K63-linked polyubiquitination of IRAK1, TBK1, TAK1, and phosphorylation of TBK1 and IFN regulatory factor 3. These results reveal a novel mechanism by which endotoxin tolerance re-programs TLR4 signaling via suppression of Pellino-1, a positive regulator of MyD88- and TRIF-dependent signaling that promotes K63-linked polyubiquitination of IRAK1, TBK1, and TAK1.
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