HTLV-1 Tax Induces Formation of the Active Macromolecular IKK Complex by Generating Lys63- and Met1-Linked Hybrid Polyubiquitin Chains

Human T-Lymphotropic Virus (HTLV): Epidemiology, Genetic, Pathogenesis, and Future Challenges

Human T-lymphotropic viruses (HTLVs) are deltaretroviruses infecting millions of individuals worldwide, with HTLV-1 and HTLV-2 being the most widespread and clinically relevant types. HTLV-1 is associated with severe diseases such as adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), while HTLV-2 shows a lower pathogenic potential, with occasional links to neurological disorders. HTLV-3 and HTLV-4, identified in Central Africa, remain poorly characterized but are genetically close to their simian counterparts, indicating recent zoonotic transmission events. HTLVs replicate through a complex cycle involving cell-to-cell transmission and clonal expansion of infected lymphocytes. Viral persistence is mediated by regulatory and accessory proteins, notably Tax and HBZ in HTLV-1, which alter host cell signaling, immune responses, and genomic stability. Integration of proviral DNA into transcriptionally active regions of the host genome may contribute to oncogenesis and long-term viral latency. Differences in viral protein function and intracellular localization contribute to the distinct pathogenesis observed between HTLV-1 and HTLV-2. Geographically, HTLV-1 shows endemic clusters in southwestern Japan, sub-Saharan Africa, the Caribbean, South America, and parts of the Middle East and Oceania. HTLV-2 is concentrated among Indigenous populations in the Americas and people who inject drugs in Europe and North America. Transmission occurs primarily via breastfeeding, sexual contact, contaminated blood products, and, in some regions, zoonotic spillover. Diagnostic approaches include serological screening (ELISA, Western blot, LIA) and molecular assays (PCR, qPCR), with novel biosensor and AI-based methods under development. Despite advances in understanding viral biology, therapeutic options remain limited, and preventive strategies focus on transmission control. The long latency period, lack of effective treatments, and global neglect complicate public health responses, underscoring the need for increased awareness, research investment, and targeted interventions.

BSRF1 modulates IFN-β-mediated antiviral responses by inhibiting NF-κB activity via an IKK-dependent mechanism in Epstein-Barr virus infection

The Epstein-Barr virus (EBV) encoded tegument protein BSRF1 plays a significant role in the processes of viral maturation and release, however, it's not clear whether BSRF1 is involved in the modulation of host innate immunity. In this study, we demonstrated that BSRF1 can inhibit interferon β (IFN-β) production by downregulating nuclear factor kappa B (NF-κB) activity and subsequently reducing the yield of inflammatory cytokines, thereby facilitating viral replication. Dual luciferase reporter assays indicated that BSRF1 may inhibit NF-κB signaling at the level of IKK or between IKK and p65, while co-immunoprecipitation experiments revealed its association with multiple critical host adaptor proteins. Mechanistically, BSRF1 hinders the phosphorylation of IκBα at Ser32/36 and K48-linked polyubiquitination, thereby preventing proteasome-mediated degradation of IκBα by disrupting the assembly of the regulatory subunits within the IKK complex. Although BSRF1 interacts with p65 and its N-terminal domain, it does not alter the formation of the p65/p50 heterodimer. Instead, it prevents the nuclear translocation of p65 by inhibiting the dissociation of IκBα from the NF-κB dimer. Collectively, these findings suggested that BSRF1 assists EBV's evasion of host innate immune system by inhibiting the antiviral response to IFN-β through the NF-κB signaling pathway, potentially contributing to the virus's ability to establish persistent infection and its association with tumorigenesis.

TRAF7 determines circadian period through ubiquitination and degradation of DBP

D-site binding protein, DBP, is a clock-controlled transcription factor and drives daily rhythms of physiological processes through the regulation of an array of genes harboring a DNA binding motif, D-box. DBP protein levels show a circadian oscillation with an extremely robust peak/trough ratio, but it is elusive how the temporal pattern is regulated by post-translational regulation. In this study, we show that DBP protein levels are down-regulated by the ubiquitin-proteasome pathway. Analysis using 19 dominant-negative forms of E2 enzymes have revealed that UBE2G1 and UBE2T mediate the degradation of DBP. A proteomic analysis of DBP-interacting proteins and database screening have identified Tumor necrosis factor Receptor-Associated Factor 7 (TRAF7), a RING-type E3 ligase, that forms a complex with UBE2G1 and/or UBE2T. Ubiquitination analysis have revealed that TRAF7 enhances K48-linked polyubiquitination of DBP in cultured cells. Overexpression of TRAF7 down-regulates DBP protein level, while knockdown of TRAF7 up-regulates DBP in cultured cells. Knockout of TRAF7 in NIH3T3 cells have revealed that TRAF7 mediates the time-of-the-day-dependent regulation of DBP levels. Furthermore, TRAF7 has a period-shortening effect on the cellular clock. Together, TRAF7 plays an important role in circadian clock oscillation through destabilization of DBP.

Linear ubiquitination regulates the KSHV replication and transcription activator protein to control infection

Like many other viruses, KSHV has two life cycle modes: the latent phase and the lytic phase. The RTA protein from KSHV is essential for lytic reactivation, but how this protein's activity is regulated is not fully understood. Here, we report that linear ubiquitination regulates the activity of RTA during KSHV lytic reactivation and de novo infection. Overexpressing OTULIN inhibits KSHV lytic reactivation, whereas knocking down OTULIN or overexpressing HOIP enhances it. Intriguingly, we found that RTA is linearly polyubiquitinated by HOIP at K516 and K518, and these modifications control the RTA's nuclear localization. OTULIN removes linear polyubiquitin chains from cytoplasmic RTA, preventing its nuclear import. The RTA orthologs encoded by the EB and MHV68 viruses are also linearly polyubiquitinated and regulated by OTULIN. Our study establishes that linear polyubiquitination plays a critically regulatory role in herpesvirus infection, adding virus infection to the list of biological processes known to be controlled by linear polyubiquitination.

PCBP1 interacts with the HTLV-1 Tax oncoprotein to potentiate NF-κB activation

Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma. The HTLV-1 Tax constitutively activates nuclear factor-κB (NF-κB) to promote the survival and transformation of HTLV-1-infected T cells. Despite extensive study of Tax, how Tax interacts with host factors to regulate NF-κB activation and HTLV-1-driven cell proliferation is not entirely clear. Here, we showed that overexpression of Poly (rC)-binding protein 1 (PCBP1) promoted Tax-mediated IκB kinase (IKK)-NF-κB signaling activation, whereas knockdown of PCBP1 attenuated Tax-dependent IKK-NF-κB activation. However, Tax activation of HTLV-1 long terminal repeat was unaffected by PCBP1. Furthermore, depletion of PCBP1 led to apoptosis and reduced proliferation of HTLV-1-transformed cells. Mechanistically, PCBP1 interacted and co-localized with Tax in the cytoplasm, and PCBP1 KH3 domain was indispensable for the interaction between PCBP1 and Tax. Moreover, PCBP1 facilitated the assembly of Tax/IKK complex. Collectively, our results demonstrated that PCBP1 may exert an essential effect in Tax/IKK complex combination and subsequent NF-κB activation, which provides a novel insight into the pathogenetic mechanisms of HTLV-1.

The helicase-like transcription factor redirects the autophagic flux and restricts human T cell leukemia virus type 1 infection

Retroviruses and their host have coevolved in a delicate balance between viral replication and survival of the infected cell. In this equilibrium, restriction factors expressed by infected cells control different steps of retroviral replication such as entry, uncoating, nuclear import, expression, or budding. Here, we describe a mechanism of restriction against human T cell leukemia virus type 1 (HTLV-1) by the helicase-like transcription factor (HLTF). We show that RNA and protein levels of HLTF are reduced in primary T cells of HTLV-1-infected subjects, suggesting a clinical relevance. We further demonstrate that the viral oncogene Tax represses HLTF transcription via the Enhancer of zeste homolog 2 methyltransferase of the Polycomb repressive complex 2. The Tax protein also directly interacts with HLTF and induces its proteasomal degradation. RNA interference and gene transduction in HTLV-1-infected T cells derived from patients indicate that HLTF is a restriction factor. Restoring the normal levels of HLTF expression induces the dispersal of the Golgi apparatus and overproduction of secretory granules. By synergizing with Tax-mediated NF-κB activation, physiologically relevant levels of HLTF intensify the autophagic flux. Increased vesicular trafficking leads to an enlargement of the lysosomes and the production of large vacuoles containing viral particles. HLTF induction in HTLV-1-infected cells significantly increases the percentage of defective virions. In conclusion, HLTF-mediated activation of the autophagic flux blunts the infectious replication cycle of HTLV-1, revealing an original mode of viral restriction.

STLV-1 Commonly Targets Neurons in the Brain of Asymptomatic Non-Human Primates

The human T-cell leukemia virus (HTLV)-1 is responsible for an aggressive neurodegenerative disease (HAM/TSP) and multiple neurological alterations. The capacity of HTLV-1 to infect central nervous system (CNS) resident cells, together with the neuroimmune-driven response, has not been well-established. Here, we combined the use of human induced pluripotent stem cells (hiPSC) and of naturally STLV-1-infected nonhuman primates (NHP) as models with which to investigate HTLV-1 neurotropism. Hence, neuronal cells obtained after hiPSC differentiation in neural polycultures were the main cell population infected by HTLV-1. Further, we report the infection of neurons with STLV-1 in spinal cord regions as well as in brain cortical and cerebellar sections of postmortem NHP. Additionally, reactive microglial cells were found in infected areas, suggesting an immune antiviral response. These results emphasize the need to develop new efficient models by which to understand HTLV-1 neuroinfection and suggest an alternative mechanism that leads to HAM/TSP.

Viral, genetic, and immune factors in the oncogenesis of adult T-cell leukemia/lymphoma

Adult T-cell leukemia/lymphoma (ATL) is a malignancy of mature CD4 + T cells induced by human T-cell leukemia virus type I (HTLV-1). HTLV-1 maintains life-long infection in the human host by clonal proliferation of infected cells and cell-to-cell spread of the virus. Two viral genes, tax and HTLV-1 bZIP factor (HBZ), promote expansion of infected cells through the important roles they play in acceleration of cell proliferation and protection from cell death. Long-term survival of infected clones in vivo causes genetic mutations and aberrant epigenetic changes to accumulate in host genes, resulting in the emergence of an ATL clone. Recent advances in sequencing technology have revealed the broad picture of genetic and transcriptional abnormalities in ATL cells. ATL cells have hyper-proliferative and anti-apoptotic signatures like those observed in other malignancies, but also notably have traits related to immune evasion. ATL cells exhibit a regulatory T-cell-like immuno-phenotype due to both the function of HBZ and mutation of several host genes, such as CCR4 and CIC. These findings suggest that immune evasion is a critical step in the oncogenesis of ATL, and thus novel therapies that activate anti-ATL/HTLV-1 immunity may be effective in the treatment and prevention of ATL.

NF-κB-Induced R-Loops and Genomic Instability in HTLV-1-Infected and Adult T-Cell Leukemia Cells

Human T-cell leukemia virus type 1 (HTLV-1) is a human delta retrovirus that causes adult T-cell leukemia/lymphoma (ATL) in 3–5% of the infected population after decades of clinical latency. HTLV-1 Tax is a potent activator of IKK/NF-κB and a clastogen. While NF-κB activities are associated with cell survival and proliferation, constitutive NF-κB activation (NF-κB hyperactivation) by Tax leads to senescence and oncogenesis. Until recently, the mechanisms underlying the DNA damage and senescence induced by Tax and NF-κB were unknown. Current data indicate that NF-κB hyperactivation by Tax causes the accumulation of a nucleic acid structure known as an R-loop. R-loop excision by the transcription-coupled nucleotide excision repair (TC-NER) endonucleases, Xeroderma pigmentosum F (XPF), and XPG, in turn, promotes DNA double-strand breaks (DSBs). NF-κB blockade prevents Tax-induced R-loop accumulation, DNA damage, and senescence. In the same vein, the silencing of XPF and XPG mitigates Tax senescence, while deficiency in either or both frequently occurs in ATL of all types. ATL cells maintain constitutively active NF-κB, accumulate R-loops, and resist Tax-induced senescence. These results suggest that ATL cells must have acquired adaptive changes to prevent senescence and benefit from the survival and proliferation advantages conferred by Tax and NF-κB. In this review, the roles of R-loops in Tax- and NF-κB-induced DNA DSBs, senescence, and ATL development, and the epigenetic and genetic alterations that arise in ATL to reduce R-loop-associated DNA damage and avert senescence will be discussed.

New Look of EBV LMP1 Signaling Landscape

Simple Summary

Epstein-Barr Virus (EBV) infection is associated with various lymphomas and carcinomas as well as other diseases in humans. The transmembrane protein LMP1 plays versatile roles in EBV life cycle and pathogenesis, by perturbing, reprograming, and regulating a large range of host cellular mechanisms and functions, which have been increasingly disclosed but not fully understood so far. We summarize recent research progress on LMP1 signaling, including the novel components LIMD1, p62, and LUBAC in LMP1 signalosome and LMP1 novel functions, such as its induction of p62-mediated selective autophagy, regulation of metabolism, induction of extracellular vehicles, and activation of NRF2-mediated antioxidative defense. A comprehensive understanding of LMP1 signal transduction and functions may allow us to leverage these LMP1-regulated cellular mechanisms for clinical purposes.

Abstract

The Epstein–Barr Virus (EBV) principal oncoprotein Latent Membrane Protein 1 (LMP1) is a member of the Tumor Necrosis Factor Receptor (TNFR) superfamily with constitutive activity. LMP1 shares many features with Pathogen Recognition Receptors (PRRs), including the use of TRAFs, adaptors, and kinase cascades, for signal transduction leading to the activation of NFκB, AP1, and Akt, as well as a subset of IRFs and likely the master antioxidative transcription factor NRF2, which we have gradually added to the list. In recent years, we have discovered the Linear UBiquitin Assembly Complex (LUBAC), the adaptor protein LIMD1, and the ubiquitin sensor and signaling hub p62, as novel components of LMP1 signalosome. Functionally, LMP1 is a pleiotropic factor that reprograms, balances, and perturbs a large spectrum of cellular mechanisms, including the ubiquitin machinery, metabolism, epigenetics, DNA damage response, extracellular vehicles, immune defenses, and telomere elongation, to promote oncogenic transformation, cell proliferation and survival, anchorage-independent cell growth, angiogenesis, and metastasis and invasion, as well as the development of the tumor microenvironment. We have recently shown that LMP1 induces p62-mediated selective autophagy in EBV latency, at least by contributing to the induction of p62 expression, and Reactive Oxygen Species (ROS) production. We have also been collecting evidence supporting the hypothesis that LMP1 activates the Keap1-NRF2 pathway, which serves as the key antioxidative defense mechanism. Last but not least, our preliminary data shows that LMP1 is associated with the deregulation of cGAS-STING DNA sensing pathway in EBV latency. A comprehensive understanding of the LMP1 signaling landscape is essential for identifying potential targets for the development of novel strategies towards targeted therapeutic applications.

NF-κB-induced R-loop accumulation and DNA damage select for nucleotide excision repair deficiencies in adult T cell leukemia

Constitutive NF-κB activation (NF-κBCA) confers survival and proliferation advantages to cancer cells and frequently occurs in T/B cell malignancies including adult T cell leukemia (ATL) caused by human T-cell leukemia virus type 1 (HTLV-1). Counterintuitively, NF-κBCA by the HTLV-1 transactivator/oncoprotein Tax induces a senescence response, and HTLV-1 infections in culture mostly result in senescence or cell-cycle arrest due to NF-κBCA How NF-κBCA induces senescence, and how ATL cells maintain NF-κBCA and avert senescence, remain unclear. Here we report that NF-κBCA by Tax increases R-loop accumulation and DNA double-strand breaks, leading to senescence. R-loop reduction via RNase H1 overexpression, and short hairpin RNA silencing of two transcription-coupled nucleotide excision repair (TC-NER) endonucleases that are critical for R-loop excision-Xeroderma pigmentosum F (XPF) and XPG-attenuate Tax senescence, enabling HTLV-1-infected cells to proliferate. Our data indicate that ATL cells are often deficient in XPF, XPG, or both and are hypersensitive to ultraviolet irradiation. This TC-NER deficiency is found in all ATL types. Finally, ATL cells accumulate R-loops in abundance. Thus, TC-NER deficits are positively selected during HTLV-1 infection because they facilitate the outgrowth of infected cells initially and aid the proliferation of ATL cells with NF-κBCA later. We suggest that TC-NER deficits and excess R-loop accumulation represent specific vulnerabilities that may be targeted for ATL treatment.

The Met1-linked ubiquitin machinery in inflammation and infection

Ubiquitination is an essential post-translational modification that regulates most cellular processes. The assembly of ubiquitin into polymeric chains by E3 ubiquitin ligases underlies the pleiotropic functions ubiquitin chains regulate. Ubiquitin chains assembled via the N-terminal methionine, termed Met1-linked ubiquitin chains or linear ubiquitin chains, have emerged as essential signalling scaffolds that regulate pro-inflammatory responses, anti-viral interferon responses, cell death and xenophagy of bacterial pathogens downstream of innate immune receptors. Met1-linked ubiquitin chains are exclusively assembled by the linear ubiquitin chain assembly complex, LUBAC, and are disassembled by the deubiquitinases OTULIN and CYLD. Genetic defects that perturb the regulation of Met1-linked ubiquitin chains causes severe immune-related disorders, illustrating their potent signalling capacity. Here, we review the current knowledge about the cellular machinery that conjugates, recognises, and disassembles Met1-linked ubiquitin chains, and discuss the function of this unique posttranslational modification in regulating inflammation, cell death and immunity to pathogens.

The Role of Ubiquitination in NF-κB Signaling during Virus Infection

The nuclear factor κB (NF-κB) family are the master transcription factors that control cell proliferation, apoptosis, the expression of interferons and proinflammatory factors, and viral infection. During viral infection, host innate immune system senses viral products, such as viral nucleic acids, to activate innate defense pathways, including the NF-κB signaling axis, thereby inhibiting viral infection. In these NF-κB signaling pathways, diverse types of ubiquitination have been shown to participate in different steps of the signal cascades. Recent advances find that viruses also modulate the ubiquitination in NF-κB signaling pathways to activate viral gene expression or inhibit host NF-κB activation and inflammation, thereby facilitating viral infection. Understanding the role of ubiquitination in NF-κB signaling during viral infection will advance our knowledge of regulatory mechanisms of NF-κB signaling and pave the avenue for potential antiviral therapeutics. Thus, here we systematically review the ubiquitination in NF-κB signaling, delineate how viruses modulate the NF-κB signaling via ubiquitination and discuss the potential future directions.

Met1-linked ubiquitin signalling in health and disease: inflammation, immunity, cancer, and beyond

Post-translational modification of proteins with ubiquitin (ubiquitination) provides a rapid and versatile mechanism for regulating cellular signalling systems. Met1-linked (or 'linear') ubiquitin chains have emerged as a key regulatory signal that controls cell death, immune signalling, and other vital cellular functions. The molecular machinery that assembles, senses, and disassembles Met1-linked ubiquitin chains is highly specific. In recent years, the thorough biochemical and genetic characterisation of the enzymes and proteins of the Met1-linked ubiquitin signalling machinery has paved the way for substantial advances in our understanding of how Met1-linked ubiquitin chains control cell signalling and biology. Here, we review current knowledge and recent insights into the role of Met1-linked ubiquitin chains in cell signalling with an emphasis on their role in disease biology. Met1-linked ubiquitin has potent regulatory functions in immune signalling, NF-κB transcription factor activation, and cell death. Importantly, mounting evidence shows that dysregulation of Met1-linked ubiquitin signalling is associated with multiple human diseases, including immune disorders, cancer, and neurodegeneration. We discuss the latest evidence on the cellular function of Met1-linked ubiquitin in the context of its associated diseases and highlight new emerging roles of Met1-linked ubiquitin chains in cell signalling, including regulation of protein quality control and metabolism.

HTLV-1 Replication and Adult T Cell Leukemia Development

Human T-cell leukemia virus type 1 (HTLV-1) was discovered in 1980 as the first, and to date, the only retrovirus that causes human cancer. While HTLV-1 infection is generally asymptomatic, 3-5% of infected individuals develop a T cell neoplasm known as adult T cell leukemia/lymphoma (ATL) decades after infection. Since its discovery, HTLV-1 has served as a model for understanding retroviral oncogenesis, transcriptional regulation, cellular signal transduction, and cell-associated viral infection and spread. Much of the initial research was focused on the viral trans-activator/oncoprotein, Tax. Over the past decade, the study of HTLV-1 has entered the genomic era. With the development of new systems for studying HTLV-1 infection and pathogenesis, the completion of the whole genome, exome and transcriptome sequencing analyses of ATL, and the discovery of HBZ as another HTLV-1 oncogene, many established concepts about how HTLV-1 infects, persists and causes disease have undergone substantial revision. This chapter seeks to integrate our current understanding of the mechanisms of action of Tax and HBZ with the comprehensive genomic information of ATL to provide an overview of how HTLV-1 infects, replicates and causes leukemia.

TNF receptor-associated factor 6 (TRAF6) plays crucial roles in multiple biological systems through polyubiquitination-mediated NF-κB activation

NF-κB was first identified in 1986 as a B cell-specific transcription factor inducing immunoglobulin κ light chain expression. Subsequent studies revealed that NF-κB plays important roles in development, organogenesis, immunity, inflammation, and neurological functions by spatiotemporally regulating cell proliferation, differentiation, and apoptosis in several cell types. Furthermore, studies on the signal pathways that activate NF-κB led to the discovery of TRAF family proteins with E3 ubiquitin ligase activity, which function downstream of the receptor. This discovery led to the proposal of an entirely new signaling mechanism concept, wherein K63-ubiquitin chains act as a scaffold for the signaling complex to activate downstream kinases. This concept has revolutionized ubiquitin studies by revealing the importance of the nonproteolytic functions of ubiquitin not only in NF-κB signaling but also in a variety of other biological systems. TRAF6 is the most diverged among the TRAF family proteins, and our studies uncovered its notable physiological and pathological functions.

The E3/E4 ubiquitin conjugation factor UBE4B interacts with and ubiquitinates the HTLV-1 Tax oncoprotein to promote NF-κB activation

Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma (ATLL), and the neurological disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The HTLV-1 Tax protein persistently activates the NF-κB pathway to enhance the proliferation and survival of HTLV-1 infected T cells. Lysine 63 (K63)-linked polyubiquitination of Tax provides an important regulatory mechanism that promotes Tax-mediated interaction with the IKK complex and activation of NF-κB; however, the host proteins regulating Tax ubiquitination are largely unknown. To identify new Tax interacting proteins that may regulate its ubiquitination we conducted a yeast two-hybrid screen using Tax as bait. This screen yielded the E3/E4 ubiquitin conjugation factor UBE4B as a novel binding partner for Tax. Here, we confirmed the interaction between Tax and UBE4B in mammalian cells by co-immunoprecipitation assays and demonstrated colocalization by proximity ligation assay and confocal microscopy. Overexpression of UBE4B specifically enhanced Tax-induced NF-κB activation, whereas knockdown of UBE4B impaired Tax-induced NF-κB activation and the induction of NF-κB target genes in T cells and ATLL cell lines. Furthermore, depletion of UBE4B with shRNA resulted in apoptotic cell death and diminished the proliferation of ATLL cell lines. Finally, overexpression of UBE4B enhanced Tax polyubiquitination, and knockdown or CRISPR/Cas9-mediated knockout of UBE4B attenuated both K48- and K63-linked polyubiquitination of Tax. Collectively, these results implicate UBE4B in HTLV-1 Tax polyubiquitination and downstream NF-κB activation.

Infection with the retrovirus HTLV-1 leads to the development of either CD4+CD25+ leukemia/lymphoma (ATLL) or a demyelinating neuroinflammatory disease (HAM/TSP) in a subset of infected individuals. The HTLV-1 Tax protein is a regulatory protein which regulates viral gene expression and persistently activates cellular signaling pathways such as NF-κB to drive the clonal expansion and longevity of HTLV-1 infected CD4+ T cells. Polyubiquitination of Tax is a key mechanism of NF-κB activation by assembling and activating IκB kinase (IKK) signaling complexes; however, the host factors regulating Tax ubiquitination have remained elusive. Here, we have identified the E3/E4 ubiquitin conjugation factor UBE4B as a novel Tax binding protein that promotes both K48- and K63-linked polyubiquitination of Tax. Knockdown or knockout of UBE4B impairs Tax-induced NF-κB activation and triggers apoptosis of HTLV-1-transformed cells. Therefore, UBE4B is an integral host factor that supports HTLV-1 Tax polyubiquitination, NF-κB activation and cell survival.

Linear Polyubiquitin Chain Modification of TDP-43-Positive Neuronal Cytoplasmic Inclusions in Amyotrophic Lateral Sclerosis

Neuronal cytoplasmic inclusions (NCIs) containing TAR DNA-binding protein of 43 kDa (TDP-43) are pathological hallmarks of amyotrophic lateral sclerosis (ALS) and are known to be ubiquitinated. Eight linkage types of polyubiquitin chains have been reported, each type of chain exerting different intracellular actions. The linkage type of polyubiquitin chain involved in the formation of NCIs in sporadic ALS (sALS), however, has not yet been elucidated. We performed immunohistochemical study of the spinal cords of 12 patients with sALS and on those of 6 control subjects. Virtually all ubiquitinated NCIs were immunolabeled with lysine 48-linked polyubiquitin chain (K48-Ub). Although the majority of NCIs were triple-immunoreactive for K48-Ub, linear polyubiquitin chain (L-Ub), and lysine 63-linked polyubiquitin chain (K63-Ub), thin parts of K48-Ub-immunopositive NCIs were not labeled for K63-Ub or L-Ub. We also detected HOIP and SHARPIN, components of linear ubiquitin chain assembly complex, colocalizing with L-Ub on NCIs. Moreover, the immunosignal of optineurin, an autophagy receptor working with L-Ub, and that of activated NF-κB p65, were observed to be colocalizing with L-Ub on certain parts of NCIs. The L-Ub modification of TDP-43-positive NCIs may function as an inducer of autophagic clearance of NCIs, neuroinflammation, and neurodegeneration in sALS.

AHR is a tunable knob that controls HTLV-1 latency-reactivation switching

Establishing latent infection but retaining the capability to reactivate in certain circumstance is an ingenious tactic for retroviruses to persist in vivo while evading host immune surveillance. Many evidences indicate that Human T-cell leukemia virus type 1 (HTLV-1) is not completely silent in vivo. However, signals that trigger HTLV-1 latency-reactivation switching remain poorly understood. Here, we show that aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, plays a critical role in HTLV-1 plus-strand expression. Importantly, HTLV-1 reactivation could be tunably manipulated by modulating the level of AHR ligands. Mechanistically, activated AHR binds to HTLV-1 LTR dioxin response element (DRE) site (CACGCATAT) and drives plus-strand transcription. On the other hand, persistent activation of nuclear factor kappa B (NF-κB) pathway constitutes one key prerequisite for AHR overexpression in HTLV-1 infected T-cells, setting the stage for the advent of AHR signaling. Our findings suggest that HTLV-1 might achieve its reactivation in vivo when encountering environmental, dietary, microbial and metabolic cues that induce sufficient AHR signaling.

HTLV-1 is considered largely latent in vivo because viral products were rarely detected in freshly isolated PBMCs of infected individuals. However, the existence of strong HTLV-1-specific immune response in most infected individuals suggests that the virus should not be completely silent in vivo. Since viral gene expression plays a critical role in cell transformation and de novo infection, a novel insight into where and how HTLV-1 achieves its reactivation in vivo is essential for developing new therapeutic approaches. AHR is a ligand-activated transcription factor that regulates intricate transcriptional programs in response to environmental, dietary, microbial and metabolic cues. It has been reported that AHR is constitutively overexpressed in HTLV-1-infected T-cells. Nevertheless, the functional role of AHR in HTLV-1 pathogenesis is still obscure. In this study, we show that activated AHR can directly bind to HTLV-1 LTR DRE site (CACGCATAT) and drive HTLV-1 plus-strand transcription. Importantly, HTLV-1 latency-reactivation-latency switching could be manipulated in MT-1 cells by adding and removing additional kynurenine (a well-known AHR ligand). Moreover, we explicate that the persistent NF-κB activation is critical for AHR overexpression in HTLV-1-infected T-cells. These results imply that constitutive AHR overexpression in infected T-cells endues HTLV-1 the potential to reactivate from latency when the level of AHR ligands reaches a certain threshold. Accordingly, we propose that HTLV-1 might achieve its reactivation in certain parts of the body that are prone to accumulate AHR ligands.

Mechanisms of Oncogenesis by HTLV-1 Tax

The human T-cell lymphotropic virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma (ATLL), a neoplasm of CD4+CD25+ T cells that occurs in 2-5% of infected individuals after decades of asymptomatic latent infection. Multiple HTLV-1-encoded regulatory proteins, including Tax and HTLV-1 basic leucine zipper factor (HBZ), play key roles in viral persistence and latency. The HTLV-1 Tax oncoprotein interacts with a plethora of host cellular proteins to regulate viral gene expression and also promote the aberrant activation of signaling pathways such as NF-κB to drive clonal proliferation and survival of T cells bearing the HTLV-1 provirus. Tax undergoes various post-translational modifications such as phosphorylation and ubiquitination that regulate its function and subcellular localization. Tax shuttles in different subcellular compartments for the activation of anti-apoptotic genes and deregulates the cell cycle with the induction of DNA damage for the accumulation of genomic instability that can result in cellular immortalization and malignant transformation. However, Tax is highly immunogenic and therefore HTLV-1 has evolved numerous strategies to tightly regulate Tax expression while maintaining the pool of anti-apoptotic genes through HBZ. In this review, we summarize the key findings on the oncogenic mechanisms used by Tax that set the stage for the development of ATLL, and the strategies used by HTLV-1 to tightly regulate Tax expression for immune evasion and viral persistence.

Specificity in Ubiquitination Triggered by Virus Infection

Ubiquitination is a prominent posttranslational modification, in which the ubiquitin moiety is covalently attached to a target protein to influence protein stability, interaction partner and biological function. All seven lysine residues of ubiquitin, along with the N-terminal methionine, can each serve as a substrate for further ubiquitination, which effectuates a diverse combination of mono- or poly-ubiquitinated proteins with linear or branched ubiquitin chains. The intricately composed ubiquitin codes are then recognized by a large variety of ubiquitin binding domain (UBD)-containing proteins to participate in the regulation of various pathways to modulate the cell behavior. Viruses, as obligate parasites, involve many aspects of the cell pathways to overcome host defenses and subjugate cellular machineries. In the virus-host interactions, both the virus and the host tap into the rich source of versatile ubiquitination code in order to compete, combat, and co-evolve. Here, we review the recent literature to discuss the role of ubiquitin system as the infection progresses in virus life cycle and the importance of ubiquitin specificity in the regulation of virus-host relation.

RNF8 Dysregulation and Down-regulation During HTLV-1 Infection Promote Genomic Instability in Adult T-Cell Leukemia

The genomic instability associated with adult T cell leukemia/lymphoma (ATL) is causally linked to Tax, the HTLV-1 viral oncoprotein, but the underlying mechanism is not fully understood. We have previously shown that Tax hijacks and aberrantly activates ring finger protein 8 (RNF8) — a lysine 63 (K63)-specific ubiquitin E3 ligase critical for DNA double-strand break (DSB) repair signaling — to assemble K63-linked polyubiquitin chains (K63-pUbs) in the cytosol. Tax and the cytosolic K63-pUbs, in turn, initiate additional recruitment of linear ubiquitin assembly complex (LUBAC) to produce hybrid K63-M1 pUbs, which trigger a kinase cascade that leads to canonical IKK:NF-κB activation. Here we demonstrate that HTLV-1-infected cells are impaired in DNA damage response (DDR). This impairment correlates with the induction of microscopically visible nuclear speckles by Tax known as the Tax-speckle structures (TSS), which act as pseudo DNA damage signaling scaffolds that sequester DDR factors such as BRCA1, DNA-PK, and MDC1. We show that TSS co-localize with Tax, RNF8 and K63-pUbs, and their formation depends on RNF8. Tax mutants defective or attenuated in inducing K63-pUb assembly are deficient or tempered in TSS induction and DDR impairment. Finally, our results indicate that loss of RNF8 expression reduces HTLV-1 viral gene expression and frequently occurs in ATL cells. Thus, during HTLV-1 infection, Tax activates RNF8 to assemble nuclear K63-pUbs that sequester DDR factors in Tax speckles, disrupting DDR signaling and DSB repair. Down-regulation of RNF8 expression is positively selected during infection and progression to disease, and further exacerbates the genomic instability of ATL.

Approximately 3–5% of HTLV-1-infected individuals develop an intractable malignancy called adult T cell leukemia/lymphoma (ATL) decades after infection. Unlike other leukemia, ATL is characterized by extensive genomic instability. Here we show that the genomic instability of ATL is associated with the hijacking and aberrant activation of a molecule known as ring finger protein 8 (RNF8) by HTLV-1 for viral replication. RNF8 is crucial for initiating the cellular DNA damage response (DDR) required for the repair of DNA double-strand breaks (DSBs), the most deleterious DNA damage. Its dysregulation in HTLV-1-infected cells results in the formation of pseudo DNA damage signaling scaffolds known as Tax speckle structures that sequester critical repair factors, causing an inability to repair DSBs efficiently. We have further found that loss of RNF8 expression reduces HTLV-1 viral replication and frequently occurs in ATL of all types. This likely facilitates the immune evasion of virus-infected cells, but degrades their ability to repair DSBs and exacerbates the genomic instability of ATL cells. Since DDR defects impact cancer response to DNA-damaging radiation and chemotherapies, RNF8 deficiency in ATL may be exploited for disease treatment.

Strategies of Human T-Cell Leukemia Virus Type 1 for Persistent Infection: Implications for Leukemogenesis of Adult T-Cell Leukemia-Lymphoma

Human T-cell leukemia virus type 1 (HTLV-1) establishes persistent infection in vivo in two distinct ways: de novo infection and clonal proliferation of infected cells. Two viral genes, Tax and HTLV-1 bZIP factor (HBZ) play critical roles in viral transcription and promotion of T-cell proliferation, respectively. Tax is a potent transactivator not only for viral transcription but also for many cellular oncogenic pathways, such as the NF-κB pathway. HBZ is a suppressor of viral transcription and has the potential to change the immunophenotype of infected cells, conferring an effector regulatory T cell (eTreg)-like signature (CD4+ CD25+ CCR4+ TIGIT+ Foxp3+) and enhancing the proliferation of this subset. Reports that mice transgenic for either gene develop malignant tumors suggest that both Tax and HBZ are involved in leukemogenesis by HTLV-1. However, the immunogenicity of Tax is very high, and its expression is generally suppressed in vivo. Recently, it was found that Tax can be expressed transiently in a small subpopulation of adult T-cell leukemia-lymphoma (ATL) cells and plays a critical role in maintenance of the overall population. HBZ is expressed in almost all infected cells except for the rare Tax-expressing cells, and activates the pathways associated with cell proliferation. These findings indicate that HTLV-1 fine-tunes the expression of viral genes to control the mode of viral propagation. The interplay between Tax and HBZ is the basis of a sophisticated strategy to evade host immune surveillance and increase transmission - and can lead to ATL as a byproduct.

NF-κB and MicroRNA Deregulation Mediated by HTLV-1 Tax and HBZ

The risk of developing adult T-cell leukemia/lymphoma (ATLL) in individuals infected with human T-cell lymphotropic virus 1 (HTLV-1) is about 3-5%. The mechanisms by which the virus triggers this aggressive cancer are still an area of intensive investigation. The viral protein Tax-1, together with additional regulatory proteins, in particular HTLV-1 basic leucine zipper factor (HBZ), are recognized as relevant viral factors required for both viral replication and transformation of infected cells. Tax-1 deregulates several cellular pathways affecting the cell cycle, survival, and proliferation. The effects of Tax-1 on the NF-κB pathway have been thoroughly studied. Recent studies also revealed the impact of Tax-1 and HBZ on microRNA expression. In this review, we summarize the recent progress in understanding the contribution of HTLV-1 Tax- and HBZ-mediated deregulation of NF-κB and the microRNA regulatory network to HTLV-1 pathogenesis.

SQSTM-1/p62 potentiates HTLV-1 Tax-mediated NF-κB activation through its ubiquitin binding function

The NF-κB pathway is constitutively activated in adult T cell leukemia, an aggressive malignancy caused by Human T Leukemia Virus type 1 (HTLV-1). The viral oncoprotein Tax triggers this constitutive activation by interacting with the ubiquitin-rich IKK complex. We previously demonstrated that Optineurin and TAX1BP1, two members of the ubiquitin-binding, Sequestosome-1 (SQSTM-1/p62)-like selective autophagy receptor family, are involved in Tax-mediated NF-κB signaling. Here, using a proximity-dependent biotinylation approach (BioID), we identify p62 as a new candidate partner of Tax and confirm the interaction in infected T cells. We then demonstrate that p62 knock-out in MEF cells as well as p62 knock-down in HEK293T cells significantly reduces Tax-mediated NF-κB activity. We further show that although p62 knock-down does not alter NF-κB activation in Jurkat T cells nor in infected T cells, p62 does potentiate Tax-mediated NF-κB activity upon over-expression in Jurkat T cells. We next show that p62 associates with the Tax/IKK signalosome in cells, and identify the 170–206 domain of p62 as sufficient for the direct, ubiquitin-independent interaction with Tax. However, we observe that this domain is dispensable for modulating Tax activity in cells, and functional analysis of p62 mutants indicates that p62 could potentiate Tax activity in cells by facilitating the association of ubiquitin chains with the Tax/IKK signalosome. Altogether, our results identify p62 as a new ubiquitin-dependent modulator of Tax activity on NF-κB, further highlighting the importance of ubiquitin in the signaling activity of the viral Tax oncoprotein.

SQSTM-1/p62 potentiates HTLV-1 Tax-mediated NF-κB activation through its ubiquitin binding function

The NF-κB pathway is constitutively activated in adult T cell leukemia, an aggressive malignancy caused by Human T Leukemia Virus type 1 (HTLV-1). The viral oncoprotein Tax triggers this constitutive activation by interacting with the ubiquitin-rich IKK complex. We previously demonstrated that Optineurin and TAX1BP1, two members of the ubiquitin-binding, Sequestosome-1 (SQSTM-1/p62)-like selective autophagy receptor family, are involved in Tax-mediated NF-κB signaling. Here, using a proximity-dependent biotinylation approach (BioID), we identify p62 as a new candidate partner of Tax and confirm the interaction in infected T cells. We then demonstrate that p62 knock-out in MEF cells as well as p62 knock-down in HEK293T cells significantly reduces Tax-mediated NF-κB activity. We further show that although p62 knock-down does not alter NF-κB activation in Jurkat T cells nor in infected T cells, p62 does potentiate Tax-mediated NF-κB activity upon over-expression in Jurkat T cells. We next show that p62 associates with the Tax/IKK signalosome in cells, and identify the 170–206 domain of p62 as sufficient for the direct, ubiquitin-independent interaction with Tax. However, we observe that this domain is dispensable for modulating Tax activity in cells, and functional analysis of p62 mutants indicates that p62 could potentiate Tax activity in cells by facilitating the association of ubiquitin chains with the Tax/IKK signalosome. Altogether, our results identify p62 as a new ubiquitin-dependent modulator of Tax activity on NF-κB, further highlighting the importance of ubiquitin in the signaling activity of the viral Tax oncoprotein.

Comparative virology of HTLV-1 and HTLV-2

Human T cell leukemia virus type 1 (HTLV-1) was the first discovered human retrovirus and the etiologic agent of adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. Shortly after the discovery of HTLV-1, human T-cell leukemia virus type 2 (HTLV-2) was isolated from a patient with hairy cell leukemia. Despite possession of similar structural features to HTLV-1, HTLV-2 has not been definitively associated with lymphoproliferative disease. Since their discovery, studies have been performed with the goal of highlighting the differences between HTLV-1 and HTLV-2. A better understanding of these differences will shed light on the specific pathogenic mechanisms of HTLV-1 and lead to novel therapeutic targets. This review will compare and contrast the two oldest human retroviruses with regards to epidemiology, genomic structure, gene products, and pathobiology.

The human T-cell leukemia virus type-1 tax oncoprotein dissociates NF-κB p65RelA-Stathmin complexes and causes catastrophic mitotic spindle damage and genomic instability

Genomic instability is a hallmark of many cancers; however, the molecular etiology of chromosomal dysregulation is not well understood. The human T-cell leukemia virus type-1 (HTLV-1) oncoprotein Tax activates NF-κB-signaling and induces DNA-damage and aberrant chromosomal segregation through diverse mechanisms which contribute to viral carcinogenesis. Intriguingly, Stathmin/oncoprotein-18 (Op-18) depolymerizes tubulin and interacts with the p65^RelA subunit and functions as a cofactor for NF-κB-dependent transactivation. We thus hypothesized that the dissociation of p65^RelA-Stathmin/Op-18 complexes by Tax could lead to the catastrophic destabilization of microtubule (MT) spindle fibers during mitosis and provide a novel mechanistic link between NF-κB-signaling and genomic instability. Here we report that the inhibition of Stathmin expression by the retroviral latency protein, p30^II, or knockdown with siRNA-stathmin, dampens Tax-mediated NF-κB transactivation and counters Tax-induced genomic instability and cytotoxicity. The Tax-G148V mutant, defective for NF-κB activation, exhibited reduced p65^RelA-Stathmin binding and diminished genomic instability and cytotoxicity. Dominant-negative inhibitors of NF-κB also prevented Tax-induced multinucleation and apoptosis. Moreover, cell clones containing the infectious HTLV-1 ACH. p30^II mutant provirus, impaired for p30^II production, exhibited increased multinucleation and the accumulation of cytoplasmic tubulin aggregates following nocodozole-treatment. These findings allude to a mechanism whereby NF-κB-signaling regulates tubulin dynamics and mitotic instability through the modulation of p65^RelA-Stathmin/Op-18 interactions, and support the notion that p30^II enhances the survival of Tax-expressing HTLV-1-transformed cells.

Integrative proteomics reveals an increase in non-degradative ubiquitylation in activated CD4+ T cells

Despite gathering evidence that ubiquitylation can direct non-degradative outcomes, most investigations of ubiquitylation in T cells have focused on degradation. Here, we integrated proteomic and transcriptomic datasets from primary mouse CD4⁺ T cells to establish a framework for predicting degradative or non-degradative outcomes of ubiquitylation. Di-glycine remnant profiling was used to reveal ubiquitylated proteins, which in combination with whole-cell proteomic and transcriptomic data allowed prediction of protein degradation. Analysis of ubiquitylated proteins identified by di-glycine remnant profiling indicated that activation of CD4⁺ T cells led to an increase in non-degradative ubiquitylation. This correlated with an increase in non-proteasome-targeted K29, K33 and K63 polyubiquitin chains. This study revealed over 1,200 proteins that were ubiquitylated in primary mouse CD4⁺ T cells and highlighted the relevance of non-proteasomally targeted ubiquitin chains in T cell signaling.

NF-κB signaling mechanisms in HTLV-1-induced adult T-cell leukemia/lymphoma

The human T-cell leukemia virus type 1 (HTLV-1) is a complex deltaretrovirus linked to adult T-cell leukemia/lymphoma (ATLL), a fatal CD4 +  malignancy in 3-5% of infected individuals. The HTLV-1 Tax regulatory protein plays indispensable roles in regulating viral gene expression and activating cellular signaling pathways that drive the proliferation and clonal expansion of T cells bearing HTLV-1 proviral integrations. Tax is a potent activator of NF-κB, a key signaling pathway that is essential for the survival and proliferation of HTLV-1-infected T cells. However, constitutive NF-κB activation by Tax also triggers a senescence response, suggesting the possibility that only T cells capable of overcoming NF-κB-induced senescence can selectively undergo clonal expansion after HTLV-1 infection. Tax expression is often silenced in the majority of ATLL due to genetic alterations in the tax gene or DNA hypermethylation of the 5'-LTR. Despite the loss of Tax, NF-κB activation remains persistently activated in ATLL due to somatic mutations in genes in the T/B-cell receptor (T/BCR) and NF-κB signaling pathways. In this review, we focus on the key events driving Tax-dependent and -independent mechanisms of NF-κB activation during the multistep process leading to ATLL.

HTLV Deregulation of the NF-κB Pathway: An Update on Tax and Antisense Proteins Role

Human T-cell lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia (ATL), an aggressive CD4+/CD25+ T-cell malignancy and of a severe neurodegenerative disease, HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). The chronic activation or deregulation of the canonical and non-canonical nuclear factor kappa B (NF-κB) pathways play a crucial role in tumorigenesis. The HTLV-1 Tax-1 oncoprotein is a potent activator of the NF-κB transcription factors and the NF-κB response is required for promoting the development of HTLV-1 transformed cell lines. The homologous retrovirus HTLV-2, which also expresses a Tax-2 transforming protein, is not associated with ATL. In this review, we provide an updated synopsis of the role of Tax-1 in the deregulation of the NF-κB pathway, highlighting the differences with the homologous Tax-2. Special emphasis is directed toward the understanding of the molecular mechanisms involved in NF-κB activation resulting from Tax interaction with host factors affecting several cellular processes, such as cell cycle, apoptosis, senescence, cell proliferation, autophagy, and post-translational modifications. We also discuss the current knowledge on the role of the antisense viral protein HBZ in down-regulating the NF-κB activation induced by Tax, and its implication in cellular senescence. In addition, we review the recent studies on the mechanism of HBZ-mediated inhibition of NF-κB activity as compared to that exerted by the HTLV-2 antisense protein, APH-2. Finally, we discuss recent advances aimed at understanding the role exerted in the development of ATL by the perturbation of NF-κB pathway by viral regulatory proteins.

Oncogenic spiral by infectious pathogens: Cooperation of multiple factors in cancer development

Chronic infection is one of the major causes of cancer, and there are several mechanisms for infection-mediated oncogenesis. Some pathogens encode gene products that behave like oncogenic factors, hijacking cellular pathways to promote the survival and proliferation of infected cells in vivo. Some of these viral oncoproteins trigger a cellular damage defense response leading to senescence; however, other viral factors hinder this suppressive effect, suggesting that cooperation of those viral factors is important for malignant transformation. Coinfection with multiple agents is known to accelerate cancer development in certain cases. For example, parasitic or bacterial infection is a risk factor for adult T-cell leukemia-lymphoma induced by human T-cell leukemia virus type 1, and Epstein-Barr virus and malaria are closely associated with endemic Burkitt lymphoma. Human immunodeficiency virus type 1 infection is accompanied by various types of infection-related cancer. These findings indicate that these oncogenic pathogens can cooperate to overcome host barriers against cancer development. In this review, the authors focus on the collaborative strategies of pathogens for oncogenesis from two different points of view: (i) the cooperation of two or more different factors encoded by a single pathogen; and (ii) the acceleration of oncogenesis by coinfection with multiple agents.

Human T Cell Leukemia Virus Type 1: Persistence and Pathogenesis

Human T cell leukemia virus type 1 (HTLV-1), also known as human T lymphotropic virus type 1, was the first exogenous human retrovirus discovered. Unlike the distantly related lentivirus HIV-1, HTLV-1 causes disease in only 5-10% of infected people, depending on their ethnic origin. But whereas HIV-1 infection and the consequent diseases can be efficiently contained in most cases by antiretroviral drug treatment, there is no satisfactory treatment for the malignant or inflammatory diseases caused by HTLV-1. The purpose of the present article is to review recent advances in the understanding of the mechanisms by which the virus persists in vivo and causes disabling or fatal diseases.

Epstein-Barr Virus LMP1-Mediated Oncogenicity

Epstein-Barr virus latent membrane protein 1 (LMP1) is expressed in multiple human malignancies, including nasopharyngeal carcinoma and Hodgkin and immunosuppression-associated lymphomas. LMP1 mimics CD40 signaling to activate multiple growth and survival pathways, in particular, NF-κB. LMP1 has critical roles in Epstein-Barr virus (EBV)-driven B-cell transformation, and its expression causes fatal lymphoproliferative disease in immunosuppressed mice. Here, we review recent developments in studies of LMP1 signaling, LMP1-induced host dependency factors, mouse models of LMP1 lymphomagenesis, and anti-LMP1 immunotherapy approaches.

The Met1-Linked Ubiquitin Machinery: Emerging Themes of (De)regulation

The linear ubiquitin chain assembly complex, LUBAC, is the only known mammalian ubiquitin ligase that makes methionine 1 (Met1)-linked polyubiquitin (also referred to as linear ubiquitin). A decade after LUBAC was discovered as a cellular activity of unknown function, there are now many lines of evidence connecting Met1-linked polyubiquitin to NF-κB signaling, cell death, inflammation, immunity, and cancer. We now know that Met1-linked polyubiquitin has potent signaling functions and that its deregulation is connected to disease. Indeed, mutations and deficiencies in several factors involved in conjugation and deconjugation of Met1-linked polyubiquitin have been implicated in immune-related disorders. Here, we discuss current knowledge and recent insights into the role and regulation of Met1-linked polyubiquitin, with an emphasis on the mechanisms controlling the function of LUBAC.