Viral and cellular FLICE-inhibitory proteins: a comparison of their roles in regulating intrinsic immune responses

Molluscum Contagiosum Virus Protein MC008 Targets NF-κB Activation by Inhibiting Ubiquitination of NEMO

Molluscum contagiosum virus (MCV) is a human-adapted poxvirus that causes a common and persistent yet mild infection characterized by distinct, contagious, papular skin lesions. These lesions are notable for having little or no inflammation associated with them and can persist for long periods without an effective clearance response from the host. Like all poxviruses, MCV encodes potent immunosuppressive proteins that perturb innate immune pathways involved in virus sensing, the interferon response, and inflammation, which collectively orchestrate antiviral immunity and clearance, with several of these pathways converging at common signaling nodes. One such node is the regulator of canonical nuclear factor kappa B (NF-κB) activation, NF-κB essential modulator (NEMO). Here, we report that the MCV protein MC008 specifically inhibits NF-κB through its interaction with NEMO, disrupting its early ubiquitin-mediated activation and subsequent downstream signaling. MC008 is the third NEMO-targeting inhibitor to be described in MCV to date, with each inhibiting NEMO activation in distinct ways, highlighting strong selective pressure to evolve multiple ways of disabling this key signaling protein. IMPORTANCE Inflammation lies at the heart of most human diseases. Understanding the pathways that drive this response is the key to new anti-inflammatory therapies. Viruses evolve to target inflammation; thus, understanding how they do this reveals how inflammation is controlled and, potentially, how to disable it when it drives disease. Molluscum contagiosum virus (MCV) has specifically evolved to infect humans and displays an unprecedented ability to suppress inflammation in our tissue. We have identified a novel inhibitor of human innate signaling from MCV, MC008, which targets NEMO, a core regulator of proinflammatory signaling. Furthermore, MC008 appears to inhibit early ubiquitination, thus interrupting later events in NEMO activation, thereby validating current models of IκB kinase (IKK) complex regulation.

Fatal cytokine release syndrome by an aberrant FLIP/STAT3 axis

Inflammatory responses rapidly detect pathogen invasion and mount a regulated reaction. However, dysregulated anti-pathogen immune responses can provoke life-threatening inflammatory pathologies collectively known as cytokine release syndrome (CRS), exemplified by key clinical phenotypes unearthed during the SARS-CoV-2 pandemic. The underlying pathophysiology of CRS remains elusive. We found that FLIP, a protein that controls caspase-8 death pathways, was highly expressed in myeloid cells of COVID-19 lungs. FLIP controlled CRS by fueling a STAT3-dependent inflammatory program. Indeed, constitutive expression of a viral FLIP homolog in myeloid cells triggered a STAT3-linked, progressive, and fatal inflammatory syndrome in mice, characterized by elevated cytokine output, lymphopenia, lung injury, and multiple organ dysfunctions that mimicked human CRS. As STAT3-targeting approaches relieved inflammation, immune disorders, and organ failures in these mice, targeted intervention towards this pathway could suppress the lethal CRS inflammatory state.

Coronavirus Infection-Associated Cell Death Signaling and Potential Therapeutic Targets

COVID-19 is the name of the disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection that occurred in 2019. The virus-host-specific interactions, molecular targets on host cell deaths, and the involved signaling are crucial issues, which become potential targets for treatment. Spike protein, angiotensin-converting enzyme 2 (ACE2), cathepsin L-cysteine peptidase, transmembrane protease serine 2 (TMPRSS2), nonstructural protein 1 (Nsp1), open reading frame 7a (ORF7a), viral main protease (3C-like protease (3CLpro) or Mpro), RNA dependent RNA polymerase (RdRp) (Nsp12), non-structural protein 13 (Nsp13) helicase, and papain-like proteinase (PLpro) are molecules associated with SARS-CoV infection and propagation. SARS-CoV-2 can induce host cell death via five kinds of regulated cell death, i.e., apoptosis, necroptosis, pyroptosis, autophagy, and PANoptosis. The mechanisms of these cell deaths are well established and can be disrupted by synthetic small molecules or natural products. There are a variety of compounds proven to play roles in the cell death inhibition, such as pan-caspase inhibitor (z-VAD-fmk) for apoptosis, necrostatin-1 for necroptosis, MCC950, a potent and specific inhibitor of the NLRP3 inflammasome in pyroptosis, and chloroquine/hydroxychloroquine, which can mitigate the corresponding cell death pathways. However, NF-κB signaling is another critical anti-apoptotic or survival route mediated by SARS-CoV-2. Such signaling promotes viral survival, proliferation, and inflammation by inducing the expression of apoptosis inhibitors such as Bcl-2 and XIAP, as well as cytokines, e.g., TNF. As a result, tiny natural compounds functioning as proteasome inhibitors such as celastrol and curcumin can be used to modify NF-κB signaling, providing a responsible method for treating SARS-CoV-2-infected patients. The natural constituents that aid in inhibiting viral infection, progression, and amplification of coronaviruses are also emphasized, which are in the groups of alkaloids, flavonoids, terpenoids, diarylheptanoids, and anthraquinones. Natural constituents derived from medicinal herbs have anti-inflammatory and antiviral properties, as well as inhibitory effects, on the viral life cycle, including viral entry, replication, assembly, and release of COVID-19 virions. The phytochemicals contain a high potential for COVID-19 treatment. As a result, SARS-CoV-2-infected cell death processes and signaling might be of high efficacy for therapeutic targeting effects and yielding encouraging outcomes.

Role of p38 mitogen-activated protein kinase signalling in virus replication and potential for developing broad spectrum antiviral drugs

Mitogen-activated protein kinases (MAPKs) play a key role in complex cellular processes such as proliferation, development, differentiation, transformation and apoptosis. Mammals express at least four distinctly regulated groups of MAPKs which include extracellular signal-related kinases (ERK)-1/2, p38 proteins, Jun amino-terminal kinases (JNK1/2/3) and ERK5. p38 MAPK is activated by a wide range of cellular stresses and modulates activity of several downstream kinases and transcription factors which are involved in regulating cytoskeleton remodeling, cell cycle modulation, inflammation, antiviral response and apoptosis. In viral infections, activation of cell signalling pathways is part of the cellular defense mechanism with the basic aim of inducing an antiviral state. However, viruses can exploit enhanced cell signalling activities to support various stages of their replication cycles. Kinase activity can be inhibited by small molecule chemical inhibitors, so one strategy to develop antiviral drugs is to target these cellular signalling pathways. In this review, we provide an overview on the current understanding of various cellular and viral events regulated by the p38 signalling pathway, with a special emphasis on targeting these events for antiviral drug development which might identify candidates with broad spectrum activity.

Revisiting the role of TRAIL/TRAIL-R in cancer biology and therapy

TNF-related apoptosis-inducing ligand (TRAIL), a member of the TNF superfamily, can induce apoptosis in cancer cells, sparing normal cells when bound to its associated death receptors (DR4/DR5). This unique mechanism makes TRAIL a potential anticancer therapeutic agent. However, clinical trials of recombinant TRAIL protein and TRAIL receptor agonist monoclonal antibodies have shown disappointing results due to its short half-life, poor pharmacokinetics and the resistance of the cancer cells. This review summarizes TRAIL-induced apoptotic and survival pathways as well as mechanisms leading to apoptotic resistance. Recent development of methods to overcome cancer cell resistance to TRAIL-induced apoptosis, such as protein modification, combination therapy and TRAIL-based gene therapy, appear promising. We also discuss the challenges and opportunities in the development of TRAIL-based therapies for the treatment of human cancers.

Poxviral Targeting of Interferon Regulatory Factor Activation

As viruses have a capacity to rapidly evolve and continually alter the coding of their protein repertoires, host cells have evolved pathways to sense viruses through the one invariable feature common to all these pathogens-their nucleic acids. These genomic and transcriptional pathogen-associated molecular patterns (PAMPs) trigger the activation of germline-encoded anti-viral pattern recognition receptors (PRRs) that can distinguish viral nucleic acids from host forms by their localization and subtle differences in their chemistry. A wide range of transmembrane and cytosolic PRRs continually probe the intracellular environment for these viral PAMPs, activating pathways leading to the activation of anti-viral gene expression. The activation of Nuclear Factor Kappa B (NFκB) and Interferon (IFN) Regulatory Factor (IRF) family transcription factors are of central importance in driving pro-inflammatory and type-I interferon (TI-IFN) gene expression required to effectively restrict spread and trigger adaptive responses leading to clearance. Poxviruses evolve complex arrays of inhibitors which target these pathways at a variety of levels. This review will focus on how poxviruses target and inhibit PRR pathways leading to the activation of IRF family transcription factors.

FLIP(L): the pseudo-caspase

Possessing structural homology with their active enzyme counterparts but lacking catalytic activity, pseudoenzymes have been identified for all major enzyme groups. Caspases are a family of cysteine‐dependent aspartate‐directed proteases that play essential roles in regulating cell death and inflammation. Here, we discuss the only human pseudo‐caspase, FLIP(L), a paralog of the apoptosis‐initiating caspases, caspase‐8 and caspase‐10. FLIP(L) has been shown to play a key role in regulating the processing and activity of caspase‐8, thereby modulating apoptotic signaling mediated by death receptors (such as TRAIL‐R1/R2), TNF receptor‐1 (TNFR1), and Toll‐like receptors. In this review, these canonical roles of FLIP(L) are discussed. Additionally, a range of nonclassical pseudoenzyme roles are described, in which FLIP(L) functions independently of caspase‐8. These nonclassical pseudoenzyme functions enable FLIP(L) to play key roles in the regulation of a wide range of biological processes beyond its canonical roles as a modulator of cell death.

Chicken toll-like receptors and their significance in immune response and disease resistance

Infectious diseases are a major challenge for the poultry industry that causes widespread production losses. Thus, management and control of poultry health and diseases are essential for the viability of the industry. Toll-like receptors are best characterized as membrane-bound receptors that perform a central role in immune homeostasis and disease resistance by recognition of pathogen-associated molecular patterns. In response to pathogen recognition, TLRs initiate both innate and adaptive immune responses which may help to develop immunomodulatory therapeutics for TLR associated diseases. Vaccination produces specific immunity in the animal's body towards pathogens. However, due to certain disadvantages of vaccines, (inactivation of attenuated pathogens into the virulent strains and weak immunogenicity of inactivated vaccines) there is a crucial need to develop the safe and effective therapeutic intervention. TLR ligands have been classified as a potential adjuvant against the infectious diseases in farm animals. TLR adjuvants induce both specific and nonspecific immune responses in chickens to combat several bacterial, viral and parasitic diseases. Therefore, the aim of this review was to explore the chicken TLR4 and their role in immune responses and disease resistance to develop disease resistance poultry breeds in future.

In Vivo Models of Oncoproteins Encoded by Kaposi's Sarcoma-Associated Herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human oncogenic virus. KSHV utilizes its proteins to modify the cellular environment to promote viral replication and persistence. Some of these proteins are oncogenic, modulating cell proliferation, apoptosis, angiogenesis, genome stability, and immune responses, among other cancer hallmarks. These changes can lead to the development of KSHV-associated malignancies. In this Gem, we focus on animal models of oncogenic KSHV proteins that were developed to enable better understanding of KSHV tumorigenesis.

FLIP as a therapeutic target in cancer

One of the classic hallmarks of cancer is disruption of cell death signalling. Inhibition of cell death promotes tumour growth and metastasis, causes resistance to chemo- and radiotherapies as well as targeted agents, and is frequently due to overexpression of antiapoptotic proteins rather than loss of pro-apoptotic effectors. FLIP is a major apoptosis-regulatory protein frequently overexpressed in solid and haematological cancers, in which its high expression is often correlated with poor prognosis. FLIP, which is expressed as long (FLIP(L)) and short (FLIP(S)) splice forms, achieves its cell death regulatory functions by binding to FADD, a critical adaptor protein which links FLIP to the apical caspase in the extrinsic apoptotic pathway, caspase-8, in a number of cell death regulating complexes, such as the death-inducing signalling complexes (DISCs) formed by death receptors. FLIP also plays a key role (together with caspase-8) in regulating another form of cell death termed programmed necrosis or 'necroptosis', as well as in other key cellular processes that impact cell survival, including autophagy. In addition, FLIP impacts activation of the intrinsic mitochondrial-mediated apoptotic pathway by regulating caspase-8-mediated activation of the pro-apoptotic Bcl-2 family member Bid. It has been demonstrated that FLIP can not only inhibit death receptor-mediated apoptosis, but also cell death induced by a range of clinically relevant chemotherapeutic and targeted agents as well as ionizing radiation. More recently, key roles for FLIP in promoting the survival of immunosuppressive tumour-promoting immune cells have been discovered. Thus, FLIP is of significant interest as an anticancer therapeutic target. In this article, we review FLIP's biology and potential ways of targeting this important tumour and immune cell death regulator.

Kaposi's Sarcoma-Associated Herpesvirus (KSHV) Induces the Oncogenic miR-17-92 Cluster and Down-Regulates TGF-β Signaling

KSHV is a DNA tumor virus that causes Kaposi's sarcoma. Upon KSHV infection, only a limited number of latent genes are expressed. We know that KSHV infection regulates host gene expression, and hypothesized that latent genes also modulate the expression of host miRNAs. Aberrant miRNA expression contributes to the development of many types of cancer. Array-based miRNA profiling revealed that all six miRNAs of the oncogenic miR-17-92 cluster are up-regulated in KSHV infected endothelial cells. Among candidate KSHV latent genes, we found that vFLIP and vCyclin were shown to activate the miR-17-92 promoter, using luciferase assay and western blot analysis. The miR-17-92 cluster was previously shown to target TGF-β signaling. We demonstrate that vFLIP and vCyclin induce the expression of the miR-17-92 cluster to strongly inhibit the TGF-β signaling pathway by down-regulating SMAD2. Moreover, TGF-β activity and SMAD2 expression were fully restored when antagomirs (inhibitors) of miR-17-92 cluster were transfected into cells expressing either vFLIP or vCyclin. In addition, we utilized viral genetics to produce vFLIP or vCyclin knock-out viruses, and studied the effects in infected TIVE cells. Infection with wildtype KSHV abolished expression of SMAD2 protein in these endothelial cells. While single-knockout mutants still showed a marked reduction in SMAD2 expression, TIVE cells infected by a double-knockout mutant virus were fully restored for SMAD2 expression, compared to non-infected TIVE cells. Expression of either vFLIP or vCycIin was sufficient to downregulate SMAD2. In summary, our data demonstrate that vFLIP and vCyclin induce the oncogenic miR-17-92 cluster in endothelial cells and thereby interfere with the TGF-β signaling pathway. Manipulation of the TGF-β pathway via host miRNAs represents a novel mechanism that may be important for KSHV tumorigenesis and angiogenesis, a hallmark of KS.

Immune evasion strategies of molluscum contagiosum virus

Molluscum contagiosum virus (MCV) is the causative agent of molluscum contagiosum (MC), the third most common viral skin infection in children, and one of the five most prevalent skin diseases worldwide. No FDA-approved treatments, vaccines, or commercially available rapid diagnostics for MCV are available. This review discusses several aspects of this medically important virus including: physical properties of MCV, MCV pathogenesis, MCV replication, and immune responses to MCV infection. Sequencing of the MCV genome revealed novel immune evasion molecules which are highlighted here. Special attention is given to the MCV MC159 and MC160 proteins. These proteins are FLIPs with homologs in gamma herpesviruses and in the cell. They are of great interest because each protein regulates apoptosis, NF-κB, and IRF3. However, the mechanism that each protein uses to impart its effects is different. It is important to elucidate how MCV inhibits immune responses; this knowledge contributes to our understanding of viral pathogenesis and also provides new insights into how the immune system neutralizes virus infections.

Innate immune activation of NFκB and its antagonism by poxviruses

In recent years there has been an acceleration of discovery in the field of innate anti-viral immunity to the point that many of the key events in early virus sensing and the discrete anti-viral responses they trigger have been elucidated in detail. In particular, pattern recognition receptors (PRRs) that detect viruses at the plasma membrane, in endosomes, and within the cytosol have been characterized. Upon stimulation by viruses, most of these PRRs trigger signal transduction pathways culminating in NFκB activation. NFκB contributes both to type I interferon induction, and to production of pro-inflammatory cytokines from infected cells. Our understanding of host anti-viral innate immunity has been greatly aided by an appreciation of the ways in which poxviruses have evolved strategies to inhibit both innate sensing and effector responses. A recurring feature of poxviral immunomodulation is the apparent necessity for poxviruses to evolve multiple, non-redundant inhibitors of NFκB activation which often appear to act on the same innate signalling pathway. The reason for such apparent over-targeting of one transcription factor is not clear. Here we describe the current understanding of how host cells sense poxvirus infection to trigger signalling pathways leading to NFκB activation and pro-inflammatory cytokine induction, and the ways in which poxviruses have evolved to concisely antagonize these systems.