Regnase-1 and Roquin Regulate a Common Element in Inflammatory mRNAs by Spatiotemporally Distinct Mechanisms.

Regnase-1 and Roquin are RNA binding proteins essential for degradation of inflammation-related mRNAs and maintenance of immune homeostasis. However, their mechanistic relationship has yet to be clarified. Here, we show that, although Regnase-1 and Roquin regulate an overlapping set of mRNAs via a common stem-loop structure, they function in distinct subcellular locations: ribosome/endoplasmic reticulum and processing-body/stress granules, respectively. Moreover, Regnase-1 specifically cleaves and degrades translationally active mRNAs and requires the helicase activity of UPF1, similar to the decay mechanisms of nonsense mRNAs. In contrast, Roquin controls translationally inactive mRNAs, independent of UPF1. Defects in both Regnase-1 and Roquin lead to large increases in their target mRNAs, although Regnase-1 tends to control the early phase of inflammation when mRNAs are more actively translated. Our findings reveal that differential regulation of mRNAs by Regnase-1 and Roquin depends on their translation status and enables elaborate control of inflammation.

Cutting Edge: Dysregulated CARD9 Signaling in Neutrophils Drives Inflammation in a Mouse Model of Neutrophilic Dermatoses.

Mice homozygous for the Y208N amino acid substitution in the carboxy terminus of SHP-1 (referred to as Ptpn6spin mice) spontaneously develop a severe inflammatory disease resembling neutrophilic dermatosis in humans. Disease in Ptpn6spin mice is characterized by persistent footpad swelling and suppurative inflammation. Recently, in addition to IL-1α and IL-1R signaling, we demonstrated a pivotal role for RIPK1, TAK1, and ASK1 in promoting inflammatory disease in Ptpn6spin mice. In the current study we have identified a previously unknown role for CARD9 signaling as a critical regulator for Ptpn6spin-mediated footpad inflammation. Genetic deletion of CARD9 significantly rescued the Ptpn6spin-mediated footpad inflammation. Mechanistically, enhanced IL-1α-mediated signaling in Ptpn6spin mice neutrophils was dampened in Ptpn6spinCard9-/- mice. Collectively, this study identifies SHP-1 and CARD9 cross-talk as a novel regulator of IL-1α-driven inflammation and opens future avenues for finding novel drug targets to treat neutrophilic dermatosis in humans.

Differential role of the NLRP3 inflammasome in infection and tumorigenesis.

Dysregulated inflammation is one of the hallmarks of cancer initiation and progression. Emerging evidence indicates that inflammasomes play a central role in regulating immune cell functions in various infections and cancer. Inflammasomes are multimeric complexes consisting of nucleotide-binding oligomerization domain (NOD) -like receptors (NLRs). Among the NLRs, NOD1, NOD2 and NLRP3 respond to a variety of endogenous (i.e. damage-associated molecular patterns) and exogenous (i.e. pathogen-associated molecular patterns) stimuli. The NLRP3 inflammasome is associated with the onset and progression of autoinflammatory and autoimmune diseases, including metabolic disorders, multiple sclerosis, inflammatory bowel disease, and cryopyrin-associated periodic fever syndrome. NLRP3 is also associated with a wide variety of infections and tumorigenesis that are closely correlated with chemotherapy response and prognosis. In this review, we explore the rapidly expanding body of research on the expression and functions of NLRP3 in infections and cancers and outline novel inhibitors targeting the NLRP3 inflammasome that could be developed as therapeutic alternatives to current anticancer treatment.

Akirin2 is critical for inducing inflammatory genes by bridging IκB-ζ and the SWI/SNF complex.

Transcription of inflammatory genes in innate immune cells is coordinately regulated by transcription factors, including NF-κB, and chromatin modifiers. However, it remains unclear how microbial sensing initiates chromatin remodeling. Here, we show that Akirin2, an evolutionarily conserved nuclear protein, bridges NF-κB and the chromatin remodeling SWI/SNF complex by interacting with BRG1-Associated Factor 60 (BAF60) proteins as well as IκB-ζ, which forms a complex with the NF-κB p50 subunit. These interactions are essential for Toll-like receptor-, RIG-I-, and Listeria-mediated expression of proinflammatory genes including Il6 and Il12b in macrophages. Consistently, effective clearance of Listeria infection required Akirin2. Furthermore, Akirin2 and IκB-ζ recruitment to the Il6 promoter depend upon the presence of IκB-ζ and Akirin2, respectively, for regulation of chromatin remodeling. BAF60 proteins were also essential for the induction of Il6 in response to LPS stimulation. Collectively, the IκB-ζ-Akirin2-BAF60 complex physically links the NF-κB and SWI/SNF complexes in innate immune cell activation. By recruiting SWI/SNF chromatin remodellers to IκB-ζ, transcriptional coactivator for NF-κB, the conserved nuclear protein Akirin2 stimulates pro-inflammatory gene promoters in mouse macrophages during innate immune responses to viral or bacterial infection.

Akirin2-Mediated Transcriptional Control by Recruiting SWI/SNF Complex in B Cells.

Extensive studies in last decade have demonstrated that dynamic control of gene transcription is key in the regulation of inflammatory responses. Although signaling pathways and transcription factors have a central role, growing evidence for the involvement of chromatin in the regulation of gene expression in immune cells has uncovered an evolutionarily conserved role of pathogen recognition and epigenetic regulation. The substantial potential of these responses to drive pathological inflammation and tissue damage highlights the need for rigorous control of these responses. Recently, an evolutionarily conserved nuclear factor, Akirin2, has been identified as an essential link between nuclear factor-κB and chromatin remodelers for transcriptional regulation in macrophages and B cells. In this review, we discuss current understanding of the molecular mechanisms that have instrumental roles in governing the inflammatory response with special emphasis on Akirin2 in B cells.

Essential Function for the Nuclear Protein Akirin2 in B Cell Activation and Humoral Immune Responses.

Akirin2, an evolutionarily conserved nuclear protein, is an important factor regulating inflammatory gene transcription in mammalian innate immune cells by bridging the NF-κB and SWI/SNF complexes. Although Akirin is critical for Drosophila immune responses, which totally rely on innate immunity, the mammalian NF-κB system is critical not only for the innate but also for the acquired immune system. Therefore, we investigated the role of mouse Akirin2 in acquired immune cells by ablating Akirin2 function in B lymphocytes. B cell-specific Akirin2-deficient (Cd19(Cre/+)Akirin2(fl/fl)) mice showed profound decrease in the splenic follicular (FO) and peritoneal B-1, but not splenic marginal zone (MZ), B cell numbers. However, both Akirin2-deficient FO and MZ B cells showed severe proliferation defect and are prone to undergo apoptosis in response to TLR ligands, CD40, and BCR stimulation. Furthermore, B cell cycling was defective in the absence of Akirin2 owing to impaired expression of genes encoding cyclin D and c-Myc. Additionally, Brg1 recruitment to the Myc and Ccnd2 promoter was severely impaired in Akirin2-deficient B cells. Cd19(Cre/+)Akirin2(fl/fl) mice showed impaired in vivo immune responses to T-dependent and -independent Ags. Collectively, these results demonstrate that Akirin2 is critical for the mitogen-induced B cell cycle progression and humoral immune responses by controlling the SWI/SNF complex, further emphasizing the significant function of Akirin2 not only in the innate, but also in adaptive immune cells.

Essential roles of K63-linked polyubiquitin-binding proteins TAB2 and TAB3 in B cell activation via MAPKs.

Polyubiquitination of proteins plays a critical role in the activation of immune cells. K63-linked polyubiquitin-binding proteins TGF-ß-activated kinase 1 (TAK1)-binding protein (TAB)2 and TAB3 are implicated in NF-κB signaling via TAK1 activation. However, TAB2 alone is dispensable for NF-κB activation in embryonic fibroblasts, and the functional roles of TAB2 and TAB3 in immune cells has yet to be clarified. In this study, we demonstrate that TAB2 and TAB3 are essential for B cell activation leading to Ag-specific Ab responses, as well as B-1 and marginal zone B cell development. TAB2 and TAB3 are critical for the activation of MAPKs, especially ERK, but not NF-κB, in response to TLR and CD40 stimulation in B cells. Surprisingly, TAB2 and TAB3 are dispensable for TAK1 activation in B cells, indicating that TAB2 and TAB3 activate MAPKs via a pathway independent of TAK1. In contrast to B cells, macrophages lacking TAB2 and TAB3 did not show any defects in the cytokine production and the signaling pathway in response to TLR stimulation. Furthermore, TAB2 and TAB3 were dispensable for TNF-induced cytokine production in embryonic fibroblasts. Thus, TAB2- and TAB3-mediated K63-linked polyubiquitin recognition controls B cell activation via MAPKs, but not the TAK1/NF-κB axis.

Critical role of AZI2 in GM-CSF-induced dendritic cell differentiation.

TNFR-associated factor family member-associated NF-κB activator (TANK)-binding kinase 1 (TBK1) is critical for the activation of IFN regulatory factor 3 and type I IFN production upon virus infection. A set of TBK1-binding proteins, 5-azacytidine-induced gene 2 (AZI2; also known as NAP1), TANK, and TBK1-binding protein 1 (TBKBP1), have also been implicated in the production of type I IFNs. Among them, TANK was found to be dispensable for the responses against virus infection. However, physiological roles of AZI2 and TBKBP1 have yet to be clarified. In this study, we found that none of these TBK1-binding proteins is critical for type I IFN production in mice. In contrast, AZI2, but not TBKBP1, is critical for the differentiation of conventional dendritic cells (cDCs) from bone marrow cells in response to GM-CSF. AZI2 controls GM-CSF-induced cell cycling of bone marrow cells via TBK1. GM-CSF-derived DCs from AZI2-deficient mice show severe defects in cytokine production and T cell activation both in vitro and in vivo. Reciprocally, overexpression of AZI2 results in efficient generation of cDCs, and the cells show enhanced T cell activation in response to Ag stimulation. Taken together, AZI2 expression is critical for the generation of cDCs by GM-CSF and can potentially be used to increase the efficiency of immunization by cDCs.

Cyclin J-CDK complexes limit innate immune responses by reducing proinflammatory changes in macrophage metabolism.

Toll-like receptor (TLR) stimulation induces glycolysis and the production of mitochondrial reactive oxygen species (ROS), both of which are critical for inflammatory responses in macrophages. Here, we demonstrated that cyclin J, a TLR-inducible member of the cyclin family, reduced cytokine production in macrophages by coordinately controlling glycolysis and mitochondrial functions. Cyclin J interacted with cyclin-dependent kinases (CDKs), which increased the phosphorylation of a subset of CDK substrates, including the transcription factor FoxK1 and the GTPase Drp1. Cyclin J-dependent phosphorylation of FoxK1 decreased the transcription of glycolytic genes and Hif-1α activation, whereas hyperactivation of Drp1 by cyclin J-dependent phosphorylation promoted mitochondrial fragmentation and impaired the production of mitochondrial ROS. In mice, cyclin J in macrophages limited the growth of tumor xenografts and protected against LPS-induced shock but increased the susceptibility to bacterial infection. Collectively, our findings indicate that cyclin J-CDK signaling promotes antitumor immunity and the resolution of inflammation by opposing the metabolic changes that drive inflammatory responses in macrophages.

A MyD88/IL1R Axis Regulates PD-1 Expression on Tumor-Associated Macrophages and Sustains Their Immunosuppressive Function in Melanoma.

Macrophages are critical mediators of tissue homeostasis, cell proliferation, and tumor metastasis. Tumor-associated macrophages (TAM) are generally associated with tumor-promoting immunosuppressive functions in solid tumors. Here, we examined the transcriptional landscape of adaptor molecules downstream of Toll-like receptors in human cancers and found that higher expression of MYD88 correlated with tumor progression. In murine melanoma, MyD88, but not Trif, was essential for tumor progression, angiogenesis, and maintaining the immunosuppressive phenotype of TAMs. In addition, MyD88 expression in myeloid cells drove melanoma progression. The MyD88/IL1 receptor (IL1R) axis regulated programmed cell death (PD)-1 expression on TAMs by promoting recruitment of NF-κBp65 to the Pdcd1 promoter. Furthermore, a combinatorial immunotherapy approach combining the MyD88 inhibitor with anti-PD-1 blockade elicited strong antitumor effects. Thus, the MyD88/IL1R axis maintains the immunosuppressive function of TAMs and promotes tumor growth by regulating PD-1 expression. SIGNIFICANCE: These findings indicate that MyD88 regulates TAM-immunosuppressive activity, suggesting that macrophage-mediated immunotherapy combining MYD88 inhibitors with PD-1 blockade could result in better treatment outcomes in a wide variety of cancers. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/9/2358/F1.large.jpg.

Ets-2 deletion in myeloid cells attenuates IL-1α-mediated inflammatory disease caused by a Ptpn6 point mutation.

The SHP-1 protein encoded by the Ptpn6 gene has been extensively studied in hematopoietic cells in the context of inflammation. A point mutation in this gene (Ptpn6spin) causes spontaneous inflammation in mice, which has a striking similarity to neutrophilic dermatoses in humans. Recent findings highlighted the role of signaling adapters and kinases in promoting inflammation in Ptpn6spin mice; however, the underlying transcriptional regulation is poorly understood. Here, we report that SYK is important for driving neutrophil infiltration and initiating wound healing responses in Ptpn6spin mice. Moreover, we found that deletion of the transcription factor Ets2 in myeloid cells ameliorates cutaneous inflammatory disease in Ptpn6spin mice through transcriptional regulation of its target inflammatory genes. Furthermore, Ets-2 drives IL-1α-mediated inflammatory signaling in neutrophils of Ptpn6spin mice. Overall, in addition to its well-known role in driving inflammation in cancer, Ets-2 plays a major role in regulating IL-1α-driven Ptpn6spin-mediated neutrophilic dermatoses. Model for the role of ETS-2 in neutrophilic inflammation in Ptpn6spin mice. Mutation of the Ptpn6 gene results in SYK phosphorylation which then sequentially activates MAPK signaling pathways and activation of ETS-2. This leads to activation of ETS-2 target genes that contribute to neutrophil migration and inflammation. When Ets2 is deleted in Ptpn6spin mice, the expression of these target genes is reduced, leading to the reduced pathology in neutrophilic dermatoses.

Inflammasomes in the pathophysiology of autoinflammatory syndromes.

Inflammasomes are a specialized group of intracellular sensors that are key components of the host innate immune system. Autoinflammatory diseases are disorders of the innate immune system that are characterized by recurrent inflammation and serious complications. Dysregulation of the inflammasome is associated with the onset and progression of several autoinflammatory and autoimmune diseases, including cryopyrin-associated periodic fever syndrome, familial Mediterranean fever, rheumatoid arthritis, and systemic lupus erythematosus. In this review, we discuss the involvement of various inflammasome components in the regulation of autoinflammatory disorders and describe the manifestations of these autoinflammatory diseases caused by inflammasome activation.

ASK1/2 signaling promotes inflammation in a mouse model of neutrophilic dermatosis.

Mice homozygous for the Tyr208Asn amino acid substitution in the carboxy terminus of Src homology region 2 (SH2) domain-containing phosphatase 1 (SHP-1) (referred to as Ptpn6spin mice) spontaneously develop a severe inflammatory disease resembling neutrophilic dermatosis in humans. Disease in Ptpn6spin mice is characterized by persistent footpad swelling and suppurative inflammation. Recently, in addition to IL-1α and IL-1R signaling, we demonstrated a pivotal role for several kinases such as SYK, RIPK1, and TAK1 in promoting inflammatory disease in Ptpn6spin mice. In order to identify new kinases involved in SHP-1-mediated inflammation, we took a genetic approach and discovered apoptosis signal-regulating kinases 1 and 2 (ASK1 and ASK2) as novel kinases regulating Ptpn6-mediated footpad inflammation. Double deletion of ASK1 and ASK2 abrogated cutaneous inflammatory disease in Ptpn6spin mice. This double deletion further rescued the splenomegaly and lymphomegaly caused by excessive neutrophil infiltration in Ptpn6spin mice. Mechanistically, ASK regulates Ptpn6spin-mediated disease by controlling proinflammatory signaling in the neutrophils. Collectively, the present study identifies SHP-1 and ASK signaling crosstalk as a critical regulator of IL-1α-driven inflammation and opens future avenues for finding novel drug targets to treat neutrophilic dermatosis in humans.

Pathogen recognition and Toll-like receptor targeted therapeutics in innate immune cells.

The innate immune system deploys a variety of pattern-recognition receptors (PRRs) which include Toll-like receptors (TLRs), RIG-I-like receptors, NOD-like receptors, and C-type lectin receptors to detect the invasion of pathogens and initiate protective responses. The intercellular and intracellular orchestration of signals from different PRRs, their endogenous or microbial ligands and accessory molecules determine the stimulatory or inhibitory responses. Progressing over the last two decades, considerable research on the molecular mechanisms underlying host-pathogen interactions has led to a paradigm shift of our understanding of TLR signaling in the innate immune system. Given that a significant amount of evidence implicates TLRs in the pathogenesis of immune diseases and cancer, and their activation occurs early in the inflammatory cascade, they are attractive targets for novel therapeutic agents. In this review, we discuss the recent advances in TLR signaling cross talks and the mechanism of pathogen recognition with special emphasis on the role of TLRs in tumor immunity and TLR-targeted therapeutics.

Chromatin Remodeling and Transcriptional Control in Innate Immunity: Emergence of Akirin2 as a Novel Player.

Transcriptional regulation of inflammatory gene expression has been at the forefront of studies of innate immunity and is coordinately regulated by transcription factors, including NF-κB, and chromatin modifiers. The growing evidence for involvement of chromatin in the regulation of gene expression in innate immune cells, has uncovered an evolutionarily conserved role of microbial sensing and chromatin remodeling. Toll-like receptors and RIG-I-like receptors trigger these signaling pathways leading to transcriptional expression of a set of genes involved in inflammation. Tightly regulated control of this gene expression is a paramount, and often foremost, goal of most biological endeavors. In this review, we will discuss the recent progress about the molecular mechanisms governing control of pro-inflammatory gene expression by an evolutionarily conserved novel nuclear protein Akirin2 in macrophages and its emergence as an essential link between NF-κB and chromatin remodelers for transcriptional regulation.