Pellino3 targets the IRF7 pathway and facilitates autoregulation of TLR3- and viral-induced expression of type I interferons

Abstract

0 5 5Recognition of pathogen-associated molecules in microbes by TLRs leads to activation of transcription factors such as NF-κB that promote increased transcription of proinflammatory cytokines and interferons 1 . All mammalian TLRs, with the exception of TLR3, use the adaptor MyD88 as the receptor-proximal signaling molecule to trigger downstream activation of NF-κB 2 . The association of MyD88 with TLRs facilitates recruitment of members of the IRAK family of kinases that in turn activate the E3 ubiquitin ligase TRAF6 (refs. 3-5). The formation of polyubiquitin chains by TRAF6 serves to bring TAK1 into close proximity with its substrates, including IκB kinases (IKKs). The TAK1-induced phosphorylation and activation of IKKα and IKKβ promotes IKK-induced phosphorylation of IκB proteins 6 that normally sequester NF-κB in an inactive form in the cytoplasm. Phosphorylated forms of IκB are subject to polyubiquitination and subsequently proteasome-dependent degradation, thus liberating NF-κB to translocate to the nucleus and transcriptionally upregulate the expression of a plethora of genes 7 . Most TLRs use this MyD88-dependent pathway to activate NF-κB, but TLR4 can additionally deploy another adaptor protein, TRIF, to trigger a MyD88-independent pathway that also activates NF-κB 8 . Among TLRs, TLR3 uses TRIF as its exclusive receptor-proximal adaptor protein. TRIF interacts with RIP1 kinase to trigger downstream IKK-mediated activation of NF-κB 9,10 . TRAF6 has been reported to associate with TRIF and mediate activation of NF-κB 11-13 , but other studies had concluded that TRAF6 is dispensable for TLR3 signaling 14,15 . Such discrepancies in relation to the role of TRAF6 in TRIF signaling may be due to cell-specific roles for TRAF6 and/or functional redundancy of TRAF6 with other members of the TRAF family 11 . In addition to activation of NF-κB, TRIF can also trigger activation of interferon-regulatory factor (IRF) transcription factors. Thus, TRIF forms a complex with the kinases TBK1 and IKKi (also known as IKKε) and both kinases can catalyze phosphorylation and activation of IRF3 and IRF7, leading to their nuclear translocation and induction of type I interferons 1,16 . The latter are key antiviral molecules that block viral replication 17,18 .It is clear from the above that ubiquitination is important in TLR signal transduction. Additionally, there is an emerging appreciation of the roles of the E3 ubiquitin ligase family of Pellino proteins in TLR signaling. The mammalian Pellino family consists of four members: Pellino1, Pellino2 and splice variants of Pellino3 termed Pellino3 long (Pellino3L; also known as Pellino3a) and Pellino3 short (Pellino3S; also known as Pellino3b) 19,20 . Each Pellino family member contains an N-terminal forkhead-associated (FHA) domain that recognizes phosphothreonine residues and mediates association with IRAKs 21 , and a C-terminal RING-like domain that confers E3 ubiquitin ligase activity and an ability to catalyze lysine 63 (Lys63)-linked polyubiquitination of IRAKs [22][23]...

Pellino3 targets RIP1 and regulates the pro-apoptotic effects of TNF-α

“…Pellino3-deficient mice were generated by Taconic Artemis using its proprietary technology as previously described 46 . Briefly, a targeting vector, in which exon 3 of the Pellino3 gene was flanked by loxP sites, was designed to generate homologous recombinant ES clones that were used to generate chimeric mice.…”

“…In order to define the physiological relevance of the role of Pellino3 in TNF signalling, Pellino3-deficient mice were used. Mice were generated as previously described 46 . Murine embryonic fibroblasts (MEFs) from Pellino3 þ / þ and Pellino3 À / À mice were initially used to assess the effects of Pellino3 deficiency on TNF signalling.…”

Pellino3 ubiquitinates RIP2 and mediates Nod2-induced signaling and protective effects in colitis

“…We studied Pellino3-deficient (Peli3 −/− ) mice 39 to assess the role of Pellino3 in activation of the Nod2 pathway. We obtained bone marrow-derived macrophages (BMDMs) from wild-type, Peli3 −/− and Nod2-deficient (Nod2 −/− ) mice and compared their responsiveness to the Nod2 ligand MDP.…”

“…Peli3 −/− mice were generated on a C57BL/6J background as described 39 . Nod2 −/− mice were generated on a C57BL/6J strain background, and wildtype C57BL/6J mice originally from Jackson Laboratories were bred in-house.…”

“…Pellino1 is a key mediator of NF-κB activation in the TLR3 and TLR4 signaling pathways 36 and is a negative regulator of T cell activation 37,38 . Pellino3 negatively regulates the TLR3-induced expression of type I interferons 39 . We identify Pellino3 here as an important mediator in the Nod2 pathway.…”