The pellino e3 ubiquitin ligases recognize specific phosphothreonine motifs and have distinct substrate specificities

PELI2 is a negative regulator of STING signaling that is dynamically repressed during viral infection

The innate immune cGAS-STING pathway is activated by cytosolic double-stranded DNA (dsDNA), a ubiquitous danger signal, to produce interferon, a potent anti-viral and anti-cancer cytokine. However, STING activation must be tightly controlled because aberrant interferon production leads to debilitating interferonopathies. Here, we discover PELI2 as a crucial negative regulator of STING. Mechanistically, PELI2 inhibits the transcription factor IRF3 by binding to phosphorylated Thr354 and Thr356 on the C-terminal tail of STING, leading to ubiquitination and inhibition of the kinase TBK1. PELI2 sets a threshold for STING activation that tolerates low levels of cytosolic dsDNA, such as that caused by silenced TREX1, RNASEH2B, BRCA1, or SETX. When this threshold is reached, such as during viral infection, STING-induced interferon production temporarily downregulates PELI2, creating a positive feedback loop allowing a robust immune response. Lupus patients have insufficient PELI2 levels and high basal interferon production, suggesting that PELI2 dysregulation may drive the onset of lupus and other interferonopathies.

Scaffold Proteins in Autoimmune Disorders

Cells transmit information to the external environment and within themselves through signaling molecules that modulate cellular activities. Aberrant cell signaling disturbs cellular homeostasis causing a number of different diseases, including autoimmunity. Scaffold proteins, as the name suggests, serve as the anchor for binding and stabilizing signaling proteins at a particular locale, allowing both intra and intercellular signal amplification and effective signal transmission. Scaffold proteins play a critical role in the functioning of tight junctions present at the intersection of two cells. In addition, they also participate in cleavage formation during cytokinesis, and in the organization of neural synapses, and modulate receptor management outcomes. In autoimmune settings such as lupus, scaffold proteins can lower the cell activation threshold resulting in uncontrolled signaling and hyperactivity. Scaffold proteins, through their binding domains, mediate protein- protein interaction and play numerous roles in cellular communication and homeostasis. This review presents an overview of scaffold proteins, their influence on the different signaling pathways, and their role in the pathogenesis of autoimmune and auto inflammatory diseases. Since these proteins participate in many roles and interact with several other signaling pathways, it is necessary to gain a thorough understanding of these proteins and their nuances to facilitate effective target identification and therapeutic design for the treatment of autoimmune disorders.

Biology of Pellino1: a potential therapeutic target for inflammation in diseases and cancers

Pellino1 (Peli1) is a highly conserved E3 Ub ligase that exerts its biological functions by mediating target protein ubiquitination. Extensive evidence has demonstrated the crucial role of Peli1 in regulating inflammation by modulating various receptor signaling pathways, including interleukin-1 receptors, Toll-like receptors, nuclear factor-κB, mitogen-activated protein kinase, and phosphoinositide 3-kinase/AKT pathways. Peli1 has been implicated in the development of several diseases by influencing inflammation, apoptosis, necrosis, pyroptosis, autophagy, DNA damage repair, and glycolysis. Peli1 is a risk factor for most cancers, including breast cancer, lung cancer, and lymphoma. Conversely, Peli1 protects against herpes simplex virus infection, systemic lupus erythematosus, esophageal cancer, and toxic epidermolysis bullosa. Therefore, Peli1 is a potential therapeutic target that warrants further investigation. This comprehensive review summarizes the target proteins of Peli1, delineates their involvement in major signaling pathways and biological processes, explores their role in diseases, and discusses the potential clinical applications of Peli1-targeted therapy, highlighting the therapeutic prospects of Peli1 in various diseases.

The Emerging Roles of Pellino Family in Pattern Recognition Receptor Signaling

The Pellino family is a novel and well-conserved E3 ubiquitin ligase family and consists of Pellino1, Pellino2, and Pellino3. Each family member exhibits a highly conserved structure providing ubiquitin ligase activity without abrogating cell and structure-specific function. In this review, we mainly summarized the crucial roles of the Pellino family in pattern recognition receptor-related signaling pathways: IL-1R signaling, Toll-like signaling, NOD-like signaling, T-cell and B-cell signaling, and cell death-related TNFR signaling. We also summarized the current information of the Pellino family in tumorigenesis, microRNAs, and other phenotypes. Finally, we discussed the outstanding questions of the Pellino family in immunity.

Dendritic Cell-Specific Role for Pellino2 as a Mediator of TLR9 Signaling Pathway

Ubiquitination regulates immune signaling, and multiple E3 ubiquitin ligases have been studied in the context of their role in immunity. Despite this progress, the physiological roles of the Pellino E3 ubiquitin ligases, especially Pellino2, in immune regulation remain largely unknown. Accordingly, this study aimed to elucidate the role of Pellino2 in murine dendritic cells (DCs). In this study, we reveal a critical role of Pellino2 in regulation of the proinflammatory response following TLR9 stimulation. Pellino2-deficient murine DCs show impaired secretion of IL-6 and IL-12. Loss of Pellino2 does not affect TLR9-induced activation of NF-κB or MAPKs, pathways that drive expression of IL-6 and IL-12. Furthermore, DCs from Pellino2-deficient mice show impaired production of type I IFN following endosomal TLR9 activation, and it partly mediates a feed-forward loop of IFN-β that promotes IL-12 production in DCs. We also observe that Pellino2 in murine DCs is downregulated following TLR9 stimulation, and its overexpression induces upregulation of both IFN-β and IL-12, demonstrating the sufficiency of Pellino2 in driving these responses. This suggests that Pellino2 is critical for executing TLR9 signaling, with its expression being tightly regulated to prevent excessive inflammatory response. Overall, this study highlights a (to our knowledge) novel role for Pellino2 in regulating DC functions and further supports important roles for Pellino proteins in mediating and controlling immunity.

Pellino2 mediates TLR9-induced cytokine production in dendritic cells.

Pellino2 does not play a role in TLR9 signaling in macrophages.

Pellino2 is a limiting factor for TLR9 signaling in dendritic cells.

Pellino-2 in nonimmune cells: novel interaction partners and intracellular localization

Pellino-2 is an E3 ubiquitin ligase that mediates intracellular signaling in innate immune pathways. Most studies of endogenous Pellino-2 have been performed in macrophages, but none in nonimmune cells. Using yeast two-hybrid screening and co-immunoprecipitation, we identified six novel interaction partners of Pellino-2, with various localizations: insulin receptor substrate 1, NIMA-related kinase 9, tumor necrosis factor receptor-associated factor 7, cyclin-F, roundabout homolog 1, and disheveled homolog 2. Pellino-2 showed cytoplasmic localization in a wide range of nonimmune cells under physiological potassium concentrations. Treatment with the potassium ionophore nigericin resulted in nuclear localization of Pellino-2, which was reversed by the potassium channel blocker tetraethylammonium. Live-cell imaging revealed intracellular migration of GFP-tagged Pellino-2. In summary, Pellino-2 interacts with proteins at different cellular locations, taking part in dynamic processes that change its intracellular localization influenced by potassium efflux.

Tril dampens Nodal signaling through Pellino2- and Traf6-mediated activation of Nedd4l

Toll-like receptor 4 (Tlr) interactor with leucine-rich repeats (Tril) functions as a Tlr coreceptor to mediate innate immunity in adults. In Xenopus embryos, Tril triggers degradation of the transforming growth factor β (Tgf-ß) family inhibitor, Smad7. This enhances bone morphogenetic protein (Bmp) signaling to enable ventral mesoderm to commit to a blood fate. Here, we show that Tril simultaneously dampens Nodal signaling by catalytically activating the ubiquitin ligase NEDD4 Like (Nedd4l). Nedd4l then targets Nodal receptors for degradation. How Tril signals are transduced in a nonimmune context is unknown. We identify the ubiquitin ligase Pellino2 as a protein that binds to the cytoplasmic tail of Tril and subsequently forms a complex with Nedd4l and another E3 ligase, TNF-receptor associated factor 6 (Traf6). Pellino2 and Traf6 are essential for catalytic activation of Nedd4l, both in Xenopus and in mammalian cells. Traf6 ubiquitinates Nedd4l, which is then recruited to membrane compartments where activation occurs. Collectively, our findings reveal that Tril initiates a noncanonical Tlr-like signaling cascade to activate Nedd4l, thereby coordinately regulating the Bmp and Nodal arms of the Tgf-ß superfamily during vertebrate development.

TRAF6 suppresses the apoptosis of hemocytes by activating pellino in Crassostrea hongkongensis

Tumor necrosis factor receptor-associated factor 6 (TRAF6), an E3 ubiquitin ligase, participates in both innate and adaptive immunity and regulates the apoptotic process. In this study, we observed that an ortholog of TRAF6 could inhibit the activity of p53 and suppress the apoptotic process in the Hong Kong oyster, Crassostrea hongkongensis. To investigate the possible molecular mechanism of the ChTRAF6-induced antiapoptotic effect, a GST pull-down screening assay was conducted, and ChPellino was found to physically interact with ChTRAF6. In addition, the interaction between them was confirmed by Co-immunoprecipitation. Furthermore, western blotting revealed that the phosphorylation level of ChPellino was decreased after the RNAi of ChTRAF6, demonstrating that ChTRAF6 may be an upstream regulator of Pellino activation. Furthermore, the apoptosis level of hemocytes increased after ChPellino knockdown, and ChPellino overexpression suppressed ChTRAF6-dependent p53 activation. Taken together, these results indicate that ChPellino plays a critical role in suppressing ChTRAF6-dependent anti-apoptosis in the hemocytes of Crassostrea hongkongensis.

Structural insights into the functional versatility of an FHA domain protein in mycobacterial signaling

Forkhead-associated (FHA) domains are modules that bind to phosphothreonine (pThr) residues in signaling cascades. The FHA-containing mycobacterial protein GarA is a central element of a phosphorylation-dependent signaling pathway that redirects metabolic flux in response to amino acid starvation or cell growth requirements. GarA acts as a phosphorylation-dependent ON/OFF molecular switch. In its nonphosphorylated ON state, the GarA FHA domain engages in phosphorylation-independent interactions with various metabolic enzymes that orchestrate nitrogen flow, such as 2-oxoglutarate decarboxylase (KGD). However, phosphorylation at the GarA N-terminal region by the protein kinase PknB or PknG triggers autoinhibition through the intramolecular association of the N-terminal domain with the FHA domain, thus blocking all downstream interactions. To investigate these different FHA binding modes, we solved the crystal structures of the mycobacterial upstream (phosphorylation-dependent) complex PknB-GarA and the downstream (phosphorylation-independent) complex GarA-KGD. Our results show that the phosphorylated activation loop of PknB serves as a docking site to recruit GarA through canonical FHA-pThr interactions. However, the same GarA FHA-binding pocket targets an allosteric site on nonphosphorylated KGD, where a key element of recognition is a phosphomimetic aspartate. Further enzymatic and mutagenesis studies revealed that GarA acted as a dynamic allosteric inhibitor of KGD by preventing crucial motions in KGD that are necessary for catalysis. Our results provide evidence for physiological phosphomimetics, supporting numerous mutagenesis studies using such approaches, and illustrate how evolution can shape a single FHA-binding pocket to specifically interact with multiple phosphorylated and nonphosphorylated protein partners.

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.

Roles of the TRAF6 and Pellino E3 ligases in MyD88 and RANKL signaling

It is widely accepted that the essential role of TRAF6 in vivo is to generate the Lys63-linked ubiquitin (K63-Ub) chains needed to activate the "master" protein kinase TAK1. Here, we report that TRAF6 E3 ligase activity contributes to but is not essential for the IL-1-dependent formation of K63-Ub chains, TAK1 activation, or IL-8 production in human cells, because Pellino1 and Pellino2 generate the K63-Ub chains required for signaling in cells expressing E3 ligase-inactive TRAF6 mutants. The IL-1-induced formation of K63-Ub chains and ubiquitylation of IRAK1, IRAK4, and MyD88 was abolished in TRAF6/Pellino1/Pellino2 triple-knockout (KO) cells, but not in TRAF6 KO or Pellino1/2 double-KO cells. The reexpression of E3 ligase-inactive TRAF6 mutants partially restored IL-1 signaling in TRAF6 KO cells, but not in TRAF6/Pellino1/Pellino2 triple-KO cells. Pellino1-generated K63-Ub chains activated the TAK1 complex in vitro with similar efficiently to TRAF6-generated K63-Ub chains. The early phase of TLR signaling and the TLR-dependent secretion of IL-10 (controlled by IRAKs 1 and 2) was only reduced modestly in primary macrophages from knockin mice expressing the E3 ligase-inactive TRAF6[L74H] mutant, but the late-phase production of IL-6, IL-12, and TNFα (controlled only by the pseudokinase IRAK2) was abolished. RANKL-induced signaling in macrophages and the differentiation of bone marrow to osteoclasts was similar in TRAF6[L74H] and wild-type cells, explaining why the bone structure and teeth of the TRAF6[L74H] mice was normal, unlike TRAF6 KO mice. We identify two essential roles of TRAF6 that are independent of its E3 ligase activity.

Pellino-1 promotes lung carcinogenesis via the stabilization of Slug and Snail through K63-mediated polyubiquitination

Pellino-1 is an E3 ubiquitin ligase acting as a critical mediator for a variety of immune receptor signaling pathways, including Toll-like receptors, interleukin-1 receptor and T-cell receptors. We recently showed that the Pellino-1-transgenic (Tg) mice developed multiple tumors with different subtypes in hematolymphoid and solid organs. However, the molecular mechanism underlying the oncogenic role of Pellino-1 in solid tumors remains unknown. Pellino-1-Tg mice developed adenocarcinoma in the lungs, and Pellino-1 expression was higher in human lung adenocarcinoma cell lines compared with non-neoplastic bronchial epithelial cell lines. Pellino-1 overexpression increased the cell proliferation, survival, colony formation, invasion and migration of lung adenocarcinoma cells, whereas Pellino-1 knock-down showed the opposite effect. Pellino-1 overexpression activated PI3K/Akt and ERK signaling pathways and elicited an epithelial–mesenchymal transition (EMT) phenotype of lung adenocarcinoma cells. Pellino-1-mediated EMT was demonstrated through morphology, the upregulation of Vimentin, Slug and Snail expression and the downregulation of E-cadherin and β-catenin expression. Notably, Pellino-1 had a direct effect on the overexpression of Snail and Slug through Lys63-mediated polyubiquitination and the subsequent stabilization of these proteins. Pellino-1 expression level was significantly correlated with Snail and Slug expression in human lung adenocarcinoma tissues, and lung tumors from Pellino-1-Tg mice showed Snail and Slug overexpression. The Pellino-1-mediated increase in the migration of lung adenocarcinoma cells was mediated by Snail and Slug expression. Taken together, these results show that Pellino-1 contributes to lung tumorigenesis by inducing overexpression of Snail and Slug and promoting EMT. Pellino-1 might be a potential therapeutic target for lung cancer.

Differential Epigenetic Effects of Atmospheric Cold Plasma on MCF-7 and MDA-MB-231 Breast Cancer Cells

Cold atmospheric plasma (plasma) has emerged as a novel tool for a cancer treatment option, having been successfully applied to a few types of cancer cells, as well as tissues. However, to date, no studies have been performed to examine the effect of plasma on epigenetic alterations, including CpG methylation. In this study, the effects of plasma on DNA methylation changes in breast cancer cells were examined by treating cultured MCF-7 and MDA-MB-231 cells, representing estrogen-positive and estrogen-negative cancer cells, respectively, with plasma. A pyrosequencing analysis of Alu indicated that a specific CpG site was induced to be hypomethylated from 23.4 to 20.3% (p < 0.05) by plasma treatment in the estrogen-negative MDA-MB-231 cells only. A genome-wide methylation analysis identified “cellular movement, connective tissue development and function, tissue development” and “cell-to-cell signaling and interaction, cell death and survival, cellular development” as the top networks. Of the two cell types, the MDA-MB-231 cells underwent a higher rate of apoptosis and a decreased proliferation rate upon plasma treatment. Taken together, these results indicate that plasma induces epigenetic and cellular changes in a cell type-specific manner, suggesting that a careful screening of target cells and tissues is necessary for the potential application of plasma as a cancer treatment option.