Cyclophilin A-regulated ubiquitination is critical for RIG-I-mediated antiviral immune responses

CYPA promotes the progression and metastasis of serous ovarian cancer (SOC) in vitro and in vivo

Ovarian cancer (OC) is a type of gynaecological malignancy with high mortality in females. Serous ovarian cancer (SOC) is a distinct subtype of OC with poor early diagnosis. Given the limitations of traditional therapies, such as chemotherapy, targeted treatment is therefore a promising therapy to improve the survival rate of SOC patients. Cyclophilin A (CYPA) is a member of Cyclophilin family and thought to participates in multiple cellular processes such as cell transduction and immune modulation. Recently, various of studies indicated that CYPA has critical impact on cancer progression. CYPA could regulate cell proliferation, invasion, and chemoresistance of multiple types of cancers. However, it is still unclear whether it could affect ovarian cancer. In this study, we demonstrated that CYPA was highly expressed in SOC tissues compared with adjacent tissues. Further, CYPA was significantly associated with clinical stage and lymphnode metastasis of SOC patients. Additionally, data indicated that knockdown of CYPA by its shRNA dramatically reduces migration and invasion capacity of SOC cells in vitro and blocks tumor metastasis in vivo. Our study investigates the involvement of CYPA in the progression and metastasis of SOC, and therefore provides CYPA as a promising therapeutic target for SOC treatment.

Cyclophilin A contributes to aortopathy induced by postnatal loss of smooth muscle TGFBR1

Recent recognition that TGF-β signaling disruption is involved in the development of aortic aneurysms has led to renewed investigations into the role of TGF-β biology in the aortic wall. We previously found that the type I receptor of TGF-β (TGFBR2) receptor contributes to formation of ascending aortic aneurysms and dissections (AADs) induced by smooth muscle cell (SMC)-specific, postnatal deletion of Tgfbr1 (Tgfbr1iko). Here, we aimed to decipher the mechanistic signaling pathway underlying the pathogenic effects of TGFBR2 in this context. Gene expression profiling demonstrated that Tgfbr1iko triggers an acute inflammatory response in developing AADs, and Tgfbr1iko SMCs express an inflammatory phenotype in culture. Comparative proteomics profiling and mass spectrometry revealed that Tgfbr1iko SMCs respond to TGF-β1 stimulation via robust up-regulation of cyclophilin A (CypA). This up-regulation is abrogated by inhibition of TGFBR2 kinase activity, small interfering RNA silencing of Tgfbr2 expression, or inhibition of SMAD3 activation. In mice, Tgfbr1iko rapidly promotes CypA production in SMCs of developing AADs, whereas treatment with a CypA inhibitor attenuates aortic dilation by 56% (P = 0.003) and ameliorates aneurysmal degeneration (P = 0.016). These protective effects are associated with reduced aneurysm-promoting inflammation. Collectively, these results suggest a novel mechanism, wherein loss of type I receptor of TGF-β triggers promiscuous, proinflammatory TGFBR2 signaling in SMCs, thereby promoting AAD formation.-Zhou, G., Liao, M., Wang, F., Qi, X., Yang, P., Berceli, S. A., Sharma, A. K., Upchurch, G. R., Jr., Jiang, Z. Cyclophilin A contributes to aortopathy induced by postnatal loss of smooth muscle TGFBR1.

Stem Cell-Derived Culture Models of Hepatitis E Virus Infection

Similar to other hepatotropic viruses, hepatitis E virus (HEV) has been notoriously difficult to propagate in cell culture, limiting studies to unravel its biology. Recently, major advances have been made by passaging primary HEV isolates and selecting variants that replicate efficiently in carcinoma cells. These adaptations, however, can alter HEV biology. We have explored human embryonic or induced pluripotent stem cell (hESC/iPSC)-derived hepatocyte-like cells (HLCs) as an alternative to conventional hepatoma and hepatocyte cell culture systems for HEV studies. HLCs are permissive for nonadapted HEV isolate genotypes (gt)1-4 replication and can be readily genetically manipulated. HLCs, therefore, enable studies of pan-genotype HEV biology and will serve as a platform for testing anti-HEV treatments. Finally, we discuss how hepatocyte polarity is likely an important factor in the maturation and spread of infectious HEV particles.

NDR2 promotes the antiviral immune response via facilitating TRIM25-mediated RIG-I activation in macrophages

Retinoic acid-inducible gene I (RIG-I), a pivotal cytosolic sensor, recognizes viral RNAs to initiate antiviral innate immunity. However, posttranslational regulation of RIG-I signaling is not well understood. We report here that nuclear Dbf2-related kinase 2 (NDR2) functions as a crucial positive regulator of the RIG-I-mediated antiviral immune response. Overexpression of NDR2 or its kinase-inactive mutants potentiates RNA virus-induced production of type I interferons and proinflammatory cytokines and dampens viral replication. NDR2 conditional knockout mice (Lysm+NDR2f/f) show an impaired antiviral immune response. Mechanistically, NDR2 directly associates with RIG-I and TRIM25, thus facilitating the RIG-I/TRIM25 complex and enhancing the TRIM25-mediated K63-linked polyubiquitination of RIG-I, which is required for the RIG-I-mediated antiviral immune response. Furthermore, NDR2 expression is notably down-regulated in peripheral blood from respiratory syncytial virus-infected patients and in virus-infected macrophages. Collectively, these findings provide insights into the function of NDR2 in antiviral immunity and its related clinical significance.

Cyclophilin J limits inflammation through the blockage of ubiquitin chain sensing

Maintaining innate immune homeostasis is important for individual health. Npl4 zinc finger (NZF) domain-mediated ubiquitin chain sensing is reported to function in the nuclear factor-kappa B (NF-κB) signal pathway, but the regulatory mechanism remains elusive. Here we show that cyclophilin J (CYPJ), a member of the peptidylprolyl isomerase family, is induced by inflammation. CYPJ interacts with the NZF domain of transform growth factor-β activated kinase 1 binding protein 2 and 3 as well as components of the linear ubiquitin chain assembly complex to block the binding of ubiquitin-chain and negatively regulates NF-κB signaling. Mice with Cypj deficiency are susceptible to lipopolysaccharide and heat-killed Listeria monocytogenes-induced sepsis and dextran sulfate sodium-induced colitis. These findings identify CYPJ as a negative feedback regulator of the NF-κB signaling pathway, and provide insights for understanding the homeostasis of innate immunity.

CypA Regulates AIP4-Mediated M1 Ubiquitination of Influenza A Virus

Cyclophilin A (CypA) is a peptidyl-prolyl cis/trans isomerase that interacts with the matrix protein (M1) of influenza A virus (IAV) and restricts virus replication by regulating the ubiquitin-proteasome-mediated degradation of M1. However, the mechanism by which CypA regulates M1 ubiquitination remains unknown. In this study, we reported that E3 ubiquitin ligase AIP4 promoted K48-linked ubiquitination of M1 at K102 and K104, and accelerated ubiquitin-proteasome-mediated degradation of M1. The recombinant IAV with mutant M1 (K102R/K104R) could not be rescued, suggesting that the ubiquitination of M1 at K102/K104 was essential for IAV replication. Furthermore, CypA inhibited AIP4-mediated M1 ubiquitination by impairing the interaction between AIP4 and M1. More importantly, both the mutations of M1 (K102R/K104R) and CypA inhibited the nuclear export of M1, indicating that CypA regulates the cellular localization of M1 via inhibition of AIP4-mediated M1 ubiquitination at K102 and K104, which results in the reduced replication of IAV. Collectively, our findings reveal a novel ubiquitination-based mechanism by which CypA regulates the replication of IAV.

Latent Membrane Protein 1 of Epstein-Barr Virus Promotes RIG-I Degradation Mediated by Proteasome Pathway

RIG-I signaling is critical to host innate immune response against RNA virus infection, and also can be activated against many kinds of cancer. Oncogene LMP1 of Epstein-Barr virus (EBV) contributes to various tumors progress. In this study, we have provided strong evidence that LMP1 inhibits Sendai virus mediated type I interferon production and downregulates RIG-I signaling pathway by promotion RIG-I degradation dependent on proteasome. Nineteen kinds of E3 ligase are identified by IP-MS as LMP1-interactors, they are candidate E3s, which are possibly recruited by LMP1 to mediate RIG-I degradation. CHIP is among these E3s, which has been reported to lead RIG-I degradation. Notably, we find C666-1, an EBV-positive nasopharyngeal carcinoma cell line, expresses low level of RIG-I, even treated with IFN-α, RIG-I expression could not be induced. This evidence indicates that EBV employs a unique strategy to evade RIG-I mediated immune responses.

Induction of OTUD1 by RNA viruses potently inhibits innate immune responses by promoting degradation of the MAVS/TRAF3/TRAF6 signalosome

During RNA virus infection, the adaptor protein MAVS recruits TRAF3 and TRAF6 to form a signalosome, which is critical to induce the production of type I interferons (IFNs) and proinflammatory cytokines. While activation of the MAVS/TRAF3/TRAF6 signalosome is well studied, the negative regulation of the signalosome remains largely unknown. Here we report that RNA viruses specifically promote the deubiquitinase OTUD1 expression by NF-κB-dependent mechanisms at the early stage of viral infection. Furthermore, OTUD1 upregulates protein levels of intracellular Smurf1 by removing Smurf1 ubiquitination. Importantly, RNA virus infection promotes the binding of Smurf1 to MAVS, TRAF3 and TRAF6, which leads to ubiquitination-dependent degradation of every component of the MAVS/TRAF3/TRAF6 signalosome and subsequent potent inhibition of IFNs production. Consistently, OTUD1-deficient mice produce more antiviral cytokines and are more resistant to RNA virus infection. Our findings reveal a novel immune evasion mechanism exploited by RNA viruses, and elucidate a negative feedback loop of MAVS/TRAF3/TRAF6 signaling mediated by the OTUD1-Smurf1 axis during RNA virus infection.

The Many Roles of Ubiquitin in NF-κB Signaling

The nuclear factor κB (NF-κB) signaling pathway ubiquitously controls cell growth and survival in basic conditions as well as rapid resetting of cellular functions following environment changes or pathogenic insults. Moreover, its deregulation is frequently observed during cell transformation, chronic inflammation or autoimmunity. Understanding how it is properly regulated therefore is a prerequisite to managing these adverse situations. Over the last years evidence has accumulated showing that ubiquitination is a key process in NF-κB activation and its resolution. Here, we examine the various functions of ubiquitin in NF-κB signaling and more specifically, how it controls signal transduction at the molecular level and impacts in vivo on NF-κB regulated cellular processes.

Influence of Novel Highly Pathogenic Avian Influenza A (H5N1) Virus Infection on Migrating Whooper Swans Fecal Microbiota

The migration of wild birds plays an important role in the transmission and spread of H5 highly pathogenic avian influenza (HPAI) virus, posing a severe risk to animal and human health. Substantial evidence suggests that altered gut microbial community is implicated in the infection of respiratory influenza virus. However, the influence of H5N1 infection in gut microbiota of migratory birds remains unknown. In January 2015, a novel recombinant H5N1 virus emerged and killed about 100 migratory birds, mainly including whooper swans in Sanmenxia Reservoir Area of China. Here, we describe the first fecal microbiome diversity study of H5N1-infected migratory birds. By investigating the influence of H5N1 infection on fecal bacterial communities in infected and uninfected individuals, we found that H5N1 infection shaped the gut microbiota composition by a difference in the dominance of some genera, such as Aeromonas and Lactobacillus. We also found a decreased α diversity and increased β diversity in infectious individuals. Our results highlight that increases in changes in pathogen-containing gut communities occur when individuals become infected with H5N1. Our study may provide the first evidence that there are statistical association among H5N1 presence and fecal microbiota compositional shifts, and properties of the fecal microbiota may serve as the risk of gut-linked disease in migrates with H5N1 and further aggravate the disease transmission.

Regulation of Cellular Antiviral Signaling by Modifications of Ubiquitin and Ubiquitin-like Molecules

The initiation of cellular antiviral signaling depends on host pattern-recognition receptors (PRRs)-mediated recognition of viral nucleic acids that are known as classical pathogen-associated molecular patterns (PAMPs). PRRs recruit adaptor proteins and kinases to activate transcription factors and epigenetic modifiers to regulate transcription of hundreds of genes, the products of which collaborate to elicit antiviral responses. In addition, PRRs-triggered signaling induces activation of various inflammasomes which leads to the release of IL-1β and inflammation. Recent studies have demonstrated that PRRs-triggered signaling is critically regulated by ubiquitin and ubiquitin-like molecules. In this review, we first summarize an updated understanding of cellular antiviral signaling and virus-induced activation of inflammasome and then focus on the regulation of key components by ubiquitin and ubiquitin-like molecules.

U-box ubiquitin ligase PPIL2 suppresses breast cancer invasion and metastasis by altering cell morphology and promoting SNAI1 ubiquitination and degradation

Metastasis is the leading cause of breast cancer fatalities. To develop new therapeutic strategies, the mechanisms underlying breast cancer invasion and metastasis need to be further investigated. Peptidylprolyl isomerase (cyclophilin)-like 2 (PPIL2) is a U-box-type E3 ubiquitin ligase belonging to the cyclophilin family. Proteins within this family are the major cytosolic binding proteins of the immunosuppressant drug cyclosporine A (CsA). Although PPIL2 has been reported to potentially be involved in cell migration, its role in breast cancer is still unclear. Herein, we demonstrate that PPIL2 suppressed metastasis in a breast cancer model by altering cell morphology and suppressing the epithelial–mesenchymal transition (EMT) process. Moreover, elevated PPIL2 inhibited EMT and breast cancer invasion by interacting with the classical EMT transcription factor, SNAI1, to enhance its ubiquitin-dependent degradation. Furthermore, PPIL2 protein level and stability was upregulated after CsA treatment, indicating that PPIL2 might be involved in CsA-mediated repression of EMT in breast cancer. Analysis of tissue samples taken from breast cancer patients showed a significant correlation between the expression of PPIL2 and the degree of cancer invasion and metastasis. In summary, these results would shed light on a potential clinical use of CsA in breast cancer patients.

Infectious bursal disease virus protein VP4 suppresses type I interferon expression via inhibiting K48-linked ubiquitylation of glucocorticoid-induced leucine zipper (GILZ)

Viruses have developed a variety of methods to evade host immune response. Our previous study showed that infectious bursal disease virus (IBDV) inhibited type I interferon production via interaction of VP4 with cellular glucocorticoid-induced leucine zipper (GILZ) protein. However, the exact underlying molecular mechanism is still unclear. In this study, we found that IBDV VP4 suppressed GILZ degradation by inhibiting K48-linked ubiquitylation of GILZ. Furthermore, mutation of VP4 (R41G) abolished the inhibitory effect of VP4 on IFN-β expression and GILZ ubiquitylation, indicating that the amino acid 41R of VP4 was required for the suppression of IFN-β expression and GILZ ubiquitylation. Moreover, IBDV infection or VP4 expression markedly inhibited endogenous GILZ ubiquitylation. Thus, IBDV VP4 suppresses type I interferon expression by inhibiting K48-linked ubiquitylation of GILZ, revealing a new mechanism employed by IBDV to suppress host response.

Are Evolution and the Intracellular Innate Immune System Key Determinants in HIV Transmission?

HIV-1 is the single most important sexually transmitted disease in humans from a global health perspective. Among human lentiviruses, HIV-1 M group has uniquely achieved pandemic levels of human-to-human transmission. The requirement to transmit between hosts likely provides the strongest selective forces on a virus, as without transmission, there can be no new infections within a host population. Our perspective is that evolution of all of the virus-host interactions, which are inherited and perpetuated from host-to-host, must be consistent with transmission. For example, CXCR4 use, which often evolves late in infection, does not favor transmission and is therefore lost when a virus transmits to a new host. Thus, transmission inevitably influences all aspects of virus biology, including interactions with the innate immune system, and dictates the biological niche in which the virus exists in the host. A viable viral niche typically does not select features that disfavor transmission. The innate immune response represents a significant selective pressure during the transmission process. In fact, all viruses must antagonize and/or evade the mechanisms of the host innate and adaptive immune systems that they encounter. We believe that viewing host-virus interactions from a transmission perspective helps us understand the mechanistic details of antiviral immunity and viral escape. This is particularly true for the innate immune system, which typically acts from the very earliest stages of the host-virus interaction, and must be bypassed to achieve successful infection. With this in mind, here we review the innate sensing of HIV, the consequent downstream signaling cascades and the viral restriction that results. The centrality of these mechanisms to host defense is illustrated by the array of countermeasures that HIV deploys to escape them, despite the coding constraint of a 10 kb genome. We consider evasion strategies in detail, in particular the role of the HIV capsid and the viral accessory proteins highlighting important unanswered questions and discussing future perspectives.

TRIM25 in the Regulation of the Antiviral Innate Immunity

TRIM25 is an E3 ubiquitin ligase enzyme that is involved in various cellular processes, including regulation of the innate immune response against viruses. TRIM25-mediated ubiquitination of the cytosolic pattern recognition receptor RIG-I is an essential step for initiation of the intracellular antiviral response and has been thoroughly documented. In recent years, however, additional roles of TRIM25 in early innate immunity are emerging, including negative regulation of RIG-I, activation of the melanoma differentiation-associated protein 5–mitochondrial antiviral signaling protein–TRAF6 antiviral axis and modulation of p53 levels and activity. In addition, the ability of TRIM25 to bind RNA may uncover new mechanisms by which this molecule regulates intracellular signaling and/or RNA virus replication.