SHARPIN controls the development of regulatory T cells

Synergistic involvement of the NZF domains of the LUBAC accessory subunits HOIL-1L and SHARPIN in the regulation of LUBAC function

The linear ubiquitin chain assembly complex (LUBAC) plays crucial roles in NF-κB signaling and protection against cell death by generating linear ubiquitin chains. Its accessory subunits, HOIL-1L and SHARPIN, regulate LUBAC function by binding to ubiquitin chains via their Npl4 zinc finger (NZF) domains. However, the synergistic effects of the two NZF domains on LUBAC function remain unclear. Here, we demonstrate that the ubiquitin-binding activity of the two NZF domains cooperatively regulates LUBAC functions. Simultaneous loss of the ubiquitin-binding activity of the NZF domains profoundly impaired both NF-κB activation and cell death protection functions. HOIL-1L NZF robustly binds to linear ubiquitin chains, whereas SHARPIN NZF binds to Lys(K)63-linked ubiquitin chains in addition to linear chains. Binding of both NZF domains to linear ubiquitin chains regulated NF-κB signaling, whereas SHARPIN NZF predominantly regulated the cell death protection function independently of the ubiquitin chain type, K63-linked or linear ubiquitin. However, concomitant loss of linear ubiquitin binding by HOIL-1L NZF drastically impaired cell death protection. A screen of compounds capable of inhibiting binding between HOIL-1L NZF and linear ubiquitin chains identified a small compound that inhibited SHARPIN NZF as well as HOIL-1L NZF binding to linear ubiquitin chains, supporting the synergistic effect of the two NZF domains on cell death protection and suggesting a potential therapeutic strategy for targeting increased LUBAC activity in diseases such as cancer.

Langerhans cells are an essential cellular intermediary in chronic dermatitis

SHARPIN regulates signaling from the tumor necrosis factor (TNF) superfamily and pattern-recognition receptors. An inactivating Sharpin mutation in mice causes TNF-mediated dermatitis. Blocking cell death prevents the phenotype, implicating TNFR1-induced cell death in causing the skin disease. However, the source of TNF that drives dermatitis is unknown. Immune cells are a potent source of TNF in vivo and feature prominently in the skin pathology; however, T cells, B cells, and eosinophils are dispensable for the skin phenotype. We use targeted in vivo cell ablation, immune profiling, and extensive imaging to identify immune populations driving dermatitis. We find that systemic depletion of Langerin⁺ cells significantly reduces disease severity. This is enhanced in mice that lack Langerhans cells (LCs) from soon after birth. Reconstitution of LC-depleted Sharpin mutant mice with TNF-deficient LCs prevents dermatitis, implicating LCs as a potential cellular source of pathogenic TNF and highlighting a T cell-independent role in driving skin inflammation.

Post-Translational Regulations of Foxp3 in Treg Cells and Their Therapeutic Applications

Regulatory T (Treg) cells are indispensable for immune homeostasis due to their roles in peripheral tolerance. As the master transcription factor of Treg cells, Forkhead box P3 (Foxp3) strongly regulates Treg function and plasticity. Because of this, considerable research efforts have been directed at elucidating the mechanisms controlling Foxp3 and its co-regulators. Such work is not only advancing our understanding on Treg cell biology, but also uncovering novel targets for clinical manipulation in autoimmune diseases, organ transplantation, and tumor therapies. Recently, many studies have explored the post-translational regulation of Foxp3, which have shown that acetylation, phosphorylation, glycosylation, methylation, and ubiquitination are important for determining Foxp3 function and plasticity. Additionally, some of these targets have been implicated to have great therapeutic values. In this review, we will discuss emerging evidence of post-translational regulations on Foxp3 in Treg cells and their exciting therapeutic applications.

Downregulation of SHANK-associated RH domain-interacting protein elevates interleukin-33 expression by stimulating the Janus kinase 2/signal transducer and activator of transcription signalling pathway in HaCaT cells

BACKGROUND

Atopic dermatitis (AD) is a chronic inflammatory skin disease in which T-helper type 2 (Th2) immune responses are dominant. SH3 and multiple ankyrin repeat domains (SHANK)-associated RH domain-interacting protein (SHARPIN) is expressed at low levels in AD, resulting in the upregulation of the signal transducer and activator of transcription (STAT)3 protein and the Th2 cytokine, interleukin (IL)-33. However, the roles of SHARPIN in AD are not yet fully elucidated.

AIM

To evaluate the signalling interactions of SHARPIN and IL-33 in order to improve understanding of AD pathogenesis.

METHODS

Western blotting was used to detect the Janus kinase (JAK)/STAT signalling proteins and IL-33 protein in HaCaT cells to determine the key proteins mediating the interaction between SHARPIN and IL-33. The findings were validated by immunofluorescence and immunohistochemical staining. Chromatin immunoprecipitation assays were used to evaluate the activity of STAT3 at the IL-33 promoter.

RESULTS

We found that phosphorylated (p)JAK2 and pSTAT3 were upregulated in SHARPIN-knockdown HaCaT cells. Subsequent chromatin immunoprecipitation assays revealed that STAT3 binds to the IL-33 promoter to mediate IL-33 expression. Moreover, SHARPIN-mediated expression of IL-33 was reduced after treatment of HaCaT cells with the JAK/STAT inhibitor ruxolitinib. STAT3 and IL-33 expression levels were higher in AD skin lesion tissues than in normal skin tissues.

CONCLUSION

These findings suggest that SHARPIN modulates inflammation in HaCaT cells by inhibiting JAK/STAT signalling, supporting the application of SHARPIN as a potential therapeutic target for AD.

Ubiquitin-Dependent Regulation of Treg Function and Plasticity

As an indispensable part of peripheral tolerance, regulatory T (Treg) cells play an important role in immune homeostasis by suppressing other immune cells. Behind this function is a complex network of transcription factors and signaling cascades that regulates the function and plasticity of regulatory T cells. Among these, Forkhead box P3 (Foxp3) is considered as the master transcription factor, and its stability will influence the function and viability of Treg cells. Because of this, understanding the mechanisms that regulate Foxp3 and its co-regulators will provide more understanding to Treg cells and uncover more targets to manipulate Treg cells in treating autoimmune diseases, organ transplantation, and tumor. Interestingly, several recent studies show that ubiquitin-dependent pathways are important regulators of Foxp3, which suggest both great scientific and therapeutic values. In this chapter, we cover emerging evidence of ubiquitin-dependent, posttranslational regulation of Treg function and plasticity.

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.

SHARPIN: Role in Finding NEMO and in Amyloid-Beta Clearance and Degradation (ABCD) Pathway in Alzheimer's Disease?

SHANK- associated RH domain-interacting protein (SHARPIN) is a multifunctional protein associated with numerous physiological functions and many diseases. The primary role of the protein as a LUBAC-dependent component in regulating the activation of the transcription factor NF-κB accounts to its role in inflammation and antiapoptosis. Hence, an alteration of SHARPIN expression or genetic mutations or polymorphisms leads to the alteration of the above-mentioned primary physiological functions contributing to inflammation-associated diseases and cancer, respectively. However, there are complications of targeting SHARPIN as a therapeutic approach, which arises from the wide-range of LUBAC-independent functions and yet unknown roles of SHARPIN including neuronal functions. The identification of SHARPIN as a postsynaptic protein and the emerging studies indicating its role in several neurodegenerative diseases including Alzheimer's disease suggests a strong role of SHARPIN in neuronal functioning. This review summarizes the functional roles of SHARPIN in normal physiology and disease pathogenesis and strongly suggests a need for concentrating more studies on identifying the unknown neuronal functions of SHARPIN and hence its role in neurodegenerative diseases.

Shank-associated RH domain interactor signaling in tumorigenesis

Shank-associated RH domain interactor (SHARPIN) is a component of the linear ubiquitin chain activation complex, which is essential for p53 signaling and inflammation. Previous studies have demonstrated that SHARPIN functions in tumor cell survival, growth, invasion and tumorigenesis. These functions include the regulation of p53 proteins via poly-ubiquitination, interaction with a type II protein arginine methyltransferase 5 in melanoma cells, modulating ras-associated protein-1 through p38 and c-Jun N-terminal kinases/c-Jun signaling, and mediating phosphoinositide 3-kinase/AKT signaling via phosphatase and tensin homologue deleted on chromosome 10. Hence, SHARPIN not only participates in the inflammatory response but also serves a critical role in tumor cells. The present review summarizes the biological functions of the absence or presence of SHARPIN with regard to activating the canonical NF-κB signaling pathway and the effects on p53 and other signaling pathways for the modulation of tumorigenesis. Therefore, this review provides insight into the underlying role and mechanisms of SHARPIN in tumorigenesis, as well as its potential application in cancer therapy.

Modulation of autoimmune pathogenesis by T cell-triggered inflammatory cell death

T cell-mediated autoimmunity encompasses diverse immunopathological outcomes; however, the mechanisms underlying this diversity are largely unknown. Dysfunction of the tripartite linear ubiquitin chain assembly complex (LUBAC) is associated with distinct autonomous immune-related diseases. Cpdm mice lacking Sharpin, an accessory subunit of LUBAC, have innate immune cell-predominant dermatitis triggered by death of LUBAC-compromised keratinocytes. Here we show that specific gene ablation of Sharpin in mouse Treg causes phenotypes mimicking cpdm-like inflammation. Mechanistic analyses find that multiple types of programmed cell death triggered by TNF from tissue-oriented T cells initiate proinflammatory responses to implicate innate immune-mediated pathogenesis in this T cell-mediated inflammation. Moreover, additional disruption of the Hoip locus encoding the catalytic subunit of LUBAC converts cpdm-like dermatitis to T cell-predominant autoimmune lesions; however, innate immune-mediated pathogenesis still remains. These findings show that T cell-mediated killing and sequential autoinflammation are common and crucial for pathogenic diversity during T cell-mediated autoimmune responses.

Many forms of autoimmune disorder involve abnormal T cell functions, but how this versatility is achieved is not fully clear. Here the authors show that Sharpin-deficient Treg cells induce the death of local keratinocytes via multiple programmed cell death and innate inflammation to cause skin inflammation similar to cpdm mice with genetic deletion of Sharpin.

Tumour necrosis factor signalling in health and disease

The master pro-inflammatory cytokine, tumour necrosis factor (TNF), has been shown to modulate multiple signalling pathways, with wide-ranging downstream effects. TNF plays a vital role in the typical immune response through the regulation of a number of pathways encompassing an immediate inflammatory reaction with significant innate immune involvement as well as cellular activation with subsequent proliferation and programmed cell death or necrosis. As might be expected with such a broad spectrum of cellular effects and complex signalling pathways, TNF has also been implicated in a number of disease states, such as rheumatoid arthritis, ankylosing spondylitis, and Crohn’s disease. Since the time of its discovery over 40 years ago, TNF ligand and its receptors, TNF receptor (TNFR) 1 and 2, have been categorised into two complementary superfamilies, namely TNF (TNFSF) and TNFR (TNFRSF), and 19 ligands and 29 receptors have been identified to date. There have been significant advances in our understanding of TNF signalling pathways in the last decade, and this short review aims to elucidate some of the most recent advances involving TNF signalling in health and disease.

Linear ubiquitination at a glance

Ubiquitination (also known as ubiquitylation) is a post-translational modification that creates versatility in cell signalling and regulates a multitude of cellular processes. Its versatility lies in the capacity to form eight different inter-ubiquitin linkages through the seven lysine residues of ubiquitin and through its N-terminal methionine (M1). The latter, referred to as linear or M1 linkage, is created by the linear ubiquitin chain assembly complex (LUBAC), the only E3 ligase known to date that is capable of forming linear ubiquitin chains de novo Linear ubiquitin chains are crucial modulators of innate and adaptive immune responses, and act by regulating inflammatory and cell death signalling. In this Cell Science at a Glance article and the accompanying poster, we review the current knowledge on the role of LUBAC and linear ubiquitination in immune signalling and human physiology. We specifically focus on the role for LUBAC in signalling that is induced by the cytokine tumour necrosis factor (TNF) and its role in inflammation, gene activation and cell death. Furthermore, we highlight the roles of deubiquitinases (DUBs) that cleave M1 linkages and add an additional layer in the control of LUBAC-mediated immune signalling.

Crucial Role of Linear Ubiquitin Chain Assembly Complex-Mediated Inhibition of Programmed Cell Death in TLR4-Mediated B Cell Responses and B1b Cell Development

Linear ubiquitin chain assembly complex (LUBAC)-mediated linear polyubiquitin plays crucial roles in thymus-dependent and -independent type II Ab responses and B1 cell development. In this study, we analyzed the role of LUBAC in TLR-mediated B cell responses. A mouse strain in which LUBAC activity was ablated specifically in B cells (B-HOIP^Δlinear mice) showed defective Ab responses to a type I thymus-independent Ag, NP-LPS. B cells from B-HOIP^Δlinear mice (HOIP^Δlinear B cells) underwent massive cell death in response to stimulation of TLR4, but not TLR9. TLR4 stimulation induced caspase-8 activation in HOIP^Δlinear B cells; this phenomenon, as well as TLR4-induced cell death, was suppressed by ablation of TRIF, a signal inducer specific for TLR4. In addition, LPS-induced survival, proliferation, and differentiation into Ab-producing cells of HOIP^Δlinear B cells were substantially restored by inhibition of caspases together with RIP3 deletion, but not by RIP3 deletion alone, suggesting that LPS stimulation kills HOIP^Δlinear B cells by apoptosis elicited via the TRIF pathway. Further examination of the roles of cell death pathways in B-HOIP^Δlinear mice revealed that deletion of RIP3 increased the number of B1 cells, particularly B1b cells, in B-HOIP^Δlinear mice, indicating that B1b cell homeostasis is controlled via LUBAC-mediated suppression of necroptosis. Taken together, the data show that LUBAC regulates TLR4-mediated B cell responses and B1b cell development and/or maintenance by inhibiting programmed cell death.

Hepatitis B virus X protein influences enrichment profiles of H3K9me3 on promoter regions in human hepatoma cell lines

We previously showed that hepatitis B virus (HBV) X protein (HBx) could promote the trimethylation of histone H3 lysine 9 (H3K9me3) to repress tumor suppressor genes in hepatocellular carcinoma (HCC). In this work, we analyze 23,148 human promoters using ChIP-chip to determine the effects of HBx on H3K9me3 enrichments in hepatoma cells with transfection of HBx-expressing plasmid. Immunohistochemistry for HBx and H3K9me3 was performed in 21 cases of HBV-associated HCC tissues. We identified that H3K9me3 immunoreactivity was significantly correlated with HBx staining in HCC tissues. ChIP-chip data indicated that HBx remarkably altered promoter enrichments of H3K9me3 in hepatoma cells. We identified 25 gene promoters, whose H3K9me3 enrichments are significantly altered in hepatoma cells transfected HBx-expressing plasmid, including 19 gaining H3K9m3, and six losing this mark. Most of these genes have not been previously reported in HCC, and BTBD17, MIR6089, ZNF205-AS1 and ZP1 have not previously been linked to cancer; only two genes (DAB2IP and ZNF185) have been reported in HCC. Genomic analyses suggested that genes with the differential H3K9me3 enrichments function in diverse cellular pathways and many are involved in cancer development and progression.

SHARPIN stabilizes estrogen receptor α and promotes breast cancer cell proliferation

Estrogen receptor α is expressed in the majority of breast cancers and promotes estrogen-dependent cancer progression. In our study, we identified the novel E3 ubiquitin ligase SHARPIN function to facilitate ERα signaling. SHARPIN is highly expressed in human breast cancer and correlates with ERα protein level by immunohistochemistry. SHARPIN expression level correlates with poor prognosis in ERα positive breast cancer patients. SHARPIN depletion based RNA-sequence data shows that ERα signaling is a potential SHARPIN target. SHARPIN depletion significantly decreases ERα protein level, ERα target genes expression and estrogen response element activity in breast cancer cells, while SHARPIN overexpression could reverse these effects. SHARPIN depletion significantly decreases estrogen stimulated cell proliferation in breast cancer cells, which effect could be further rescued by ERα overexpression. Further mechanistic study reveals that SHARPIN mainly localizes in the cytosol and interacts with ERα both in the cytosol and the nuclear. SHARPIN regulates ERα signaling through protein stability, not through gene expression. SHARPIN stabilizes ERα protein via prohibiting ERα protein poly-ubiquitination. Further study shows that SHARPIN could facilitate the mono-ubiquitinaiton of ERα at K302/303 sites and facilitate ERE luciferase activity. Together, our findings propose a novel ERα modulation mechanism in supporting breast cancer cell growth, in which SHARPIN could be one suitable target for development of novel therapy for ERα positive breast cancer.

Immune Regulation by Ubiquitin Tagging as Checkpoint Code

The immune system is equipped with effective machinery to mobilize its activation to defend invading microorganisms, and at the same time, to refrain from attacking its own tissues to maintain immune tolerance. The balance of activation and tolerance is tightly controlled by diverse mechanisms, since breakdown of tolerance could result in disastrous consequences such as the development of autoimmune diseases. One of the mechanisms is by the means of protein ubiquitination, which involves the process of tagging a small peptide ubiquitin to protein substrates. E3 ubiquitin ligases are responsible for catalyzing the final step of ubiquitin-substrate conjugation by specifically recognizing substrates to determine their fates of degradation or functional modification. The ubiquitination process is reversible, which is carried out by deubiquitinating enzymes to release the ubiquitin molecule from the conjugated substrates. Protein ubiquitination and deubiquitination serve as checkpoint codes in many key steps of lymphocyte regulation including the development, activation, differentiation, and tolerance induction. In this chapter, we will discuss a few E3 ligases and deubiquitinating enzymes that are important in controlling immune responses, with emphasis on their roles in T cells.

Distinct role of IL-1β in instigating disease in Sharpincpdm mice

Mice deficient in SHARPIN (Sharpin^cpdm mice), a member of linear ubiquitin chain assembly complex (LUBAC), develop severe dermatitis associated with systemic inflammation. Previous studies have demonstrated that components of the TNF-signaling pathway, NLRP3 inflammasome and IL-1R signaling are required to provoke skin inflammation in Sharpin^cpdm mice. However, whether IL-1α or IL-1β, both of which signals through IL-1R, instigates skin inflammation and systemic disease is not known. Here, we have performed extensive cellular analysis of pre-diseased and diseased Sharpin^cpdm mice and demonstrated that cellular dysregulation precedes skin inflammation. Furthermore, we demonstrate a specific role for IL-1β, but not IL-1α, in instigating dermatitis in Sharpin^cpdm mice. Our results altogether demonstrate distinct roles of SHARPIN in initiating systemic inflammation and dermatitis. Furthermore, skin inflammation in Sharpin^cpdm mice is specifically modulated by IL-1β, highlighting the importance of specific targeted therapies in the IL-1 signaling blockade.

Linear ubiquitin chain assembly complex coordinates late thymic T-cell differentiation and regulatory T-cell homeostasis

The linear ubiquitin chain assembly complex (LUBAC) is essential for innate immunity in mice and humans, yet its role in adaptive immunity is unclear. Here we show that the LUBAC components HOIP, HOIL-1 and SHARPIN have essential roles in late thymocyte differentiation, FOXP3⁺ regulatory T (Treg)-cell development and Treg cell homeostasis. LUBAC activity is not required to prevent TNF-induced apoptosis or necroptosis but is necessary for the transcriptional programme of the penultimate stage of thymocyte differentiation. Treg cell-specific ablation of HOIP causes severe Treg cell deficiency and lethal immune pathology, revealing an ongoing requirement of LUBAC activity for Treg cell homeostasis. These data reveal stage-specific requirements for LUBAC in coordinating the signals required for T-cell differentiation.

LUBAC is a ubiquitin ligase complex of HOIL-1, HOIP and SHARPIN important for signal transduction of a range of stimuli. Here the authors define the function of all three LUBAC components in T cell development and homeostasis.

TNFα in the regulation of Treg and Th17 cells in rheumatoid arthritis and other autoimmune inflammatory diseases

TNFα is a principal pro-inflammatory cytokine vital for immunity to infections. However, its excessive production is involved in chronic inflammation and disease pathology in autoimmune diseases. Evidence for its pathogenic role is validated by the fact that its neutralisation by therapeutic agents in vivo is beneficial in ameliorating disease and controlling symptoms. Paradoxically, however, treatment with TNFα inhibitors can either have no clinical effects, or even exacerbate disease in some patients. The explanation for such contradictory outcomes may lay in how and which downstream signalling pathways are activated and drive disease. TNFα causes its effects by binding to either or both of two membrane-bound receptors, TNFR1 and TNFR2. Engagement of the receptors can induce cell death or cell proliferation. T cells both produce and respond to TNFα and depending on whether the cytokine is membrane-bound or soluble and the level of expression of its two receptors, the biological outcome can be distinct. In addition, polymorphisms in genes encoding TNFα and T cell signalling proteins can significantly impact the outcome of TNFα receptor engagement. Early studies revealed that effector T cells in patients with rheumatoid arthritis (RA) are hyporesponsive due to chronic exposure to TNFα. However, recent evidence indicates that the relationship between TNFα and T cell responses is complex and, at times, can be paradoxical. In addition, there is controversy as to the specific effects of TNFα on different T cell subsets. This review will summarise knowledge on how TNFα modulates T cell responses and the effect of engaging either of its two receptors. Furthermore, we discuss how such interactions can dictate the outcome of treatment with TNFα inhibitors.

The Deubiquitinase OTULIN Is an Essential Negative Regulator of Inflammation and Autoimmunity

Methionine-1 (M1)-linked ubiquitin chains regulate the activity of NF-κB, immune homeostasis, and responses to infection. The importance of negative regulators of M1-linked chains in vivo remains poorly understood. Here, we show that the M1-specific deubiquitinase OTULIN is essential for preventing TNF-associated systemic inflammation in humans and mice. A homozygous hypomorphic mutation in human OTULIN causes a potentially fatal autoinflammatory condition termed OTULIN-related autoinflammatory syndrome (ORAS). Four independent OTULIN mouse models reveal that OTULIN deficiency in immune cells results in cell-type-specific effects, ranging from over-production of inflammatory cytokines and autoimmunity due to accumulation of M1-linked polyubiquitin and spontaneous NF-κB activation in myeloid cells to downregulation of M1-polyubiquitin signaling by degradation of LUBAC in B and T cells. Remarkably, treatment with anti-TNF neutralizing antibodies ameliorates inflammation in ORAS patients and rescues mouse phenotypes. Hence, OTULIN is critical for restraining life-threatening spontaneous inflammation and maintaining immune homeostasis.