The Tyrosine Kinase c-Abl Protects c-Jun from Ubiquitination-mediated Degradation in T Cells

Mitochondria-targeted catalase induced cell malignant transformation by the downregulation of p53 protein stability via USP28/miR-200b/PP2A-Cα axis

Antioxidants exert a paradoxical influence on cancer prevention. The latest explanation for this paradox is the different target sites of antioxidants. However, it remains unclear how mitochondria-targeted antioxidants trigger specific p53-dependent pathways in malignant transformation models. Our study revealed that overexpression of mitochondria-targeted catalase (mCAT) instigated such malignant transformation via mouse double minute 2 homolog (MDM2) -mediated p53 degradation. In mouse epithelial JB6 Cl41 cells, the stable expression of mCAT resulted in MDM2-mediated p53 degradation, unlike in catalase-overexpressed Cl41 cells. Further, we demonstrated that mCAT overexpression upregulated ubiquitin-specific protease 28 (USP28) expression, which in turn stabilized c-Jun protein levels. This alteration initiated the activation of the miR-200b promoter transcription activity and a subsequent increase in miR-200b expression. Furthermore, elevated miR-200b levels then promoted its binding to the 3'-untranslated region of protein phosphatase 2A catalytic subunit (PP2A-C) α-isoform mRNA, consequently resulting in PP2A-C protein downregulation. This cascade of events ultimately contributed to increased MDM2 phosphorylation and p53 protein degradation. Thus, the mCAT overexpression triggers MDM2/p53-dependent malignant transformation through USP28/miR-200b/PP2A-Cα pathway, which may provide a new information for understanding mitochondria-targeted antioxidants facilitate the progression to the tumorigenic state.

CRISPR screening identifies the deubiquitylase ATXN3 as a PD-L1-positive regulator for tumor immune evasion

Regulation of tumoral PD-L1 expression is critical to advancing our understanding of tumor immune evasion and the improvement of existing antitumor immunotherapies. Herein, we describe a CRISPR-based screening platform and identified ATXN3 as a positive regulator for PD-L1 transcription. TCGA database analysis revealed a positive correlation between ATXN3 and CD274 in more than 80% of human cancers. ATXN3-induced Pd-l1 transcription was promoted by tumor microenvironmental factors, including the inflammatory cytokine IFN-γ and hypoxia, through protection of their downstream transcription factors IRF1, STAT3, and HIF-2α. Moreover, ATXN3 functioned as a deubiquitinase of the AP-1 transcription factor JunB, indicating that ATNX3 promotes PD-L1 expression through multiple pathways. Targeted deletion of ATXN3 in cancer cells largely abolished IFN-γ- and hypoxia-induced PD-L1 expression and consequently enhanced antitumor immunity in mice, and these effects were partially reversed by PD-L1 reconstitution. Furthermore, tumoral ATXN3 suppression improved the preclinical efficacy of checkpoint blockade antitumor immunotherapy. Importantly, ATXN3 expression was increased in human lung adenocarcinoma and melanoma, and its levels were positively correlated with PD-L1 as well as its transcription factors IRF1 and HIF-2α. Collectively, our study identifies what we believe to be a previously unknown deubiquitinase, ATXN3, as a positive regulator for PD-L1 transcription and provides a rationale for targeting ATXN3 to sensitize checkpoint blockade antitumor immunotherapy.

Targeting c-Jun Is a Potential Therapy for Luminal Breast Cancer Bone Metastasis

Luminal breast cancer has the highest bone metastasis frequency among all breast cancer subtypes; however, its metastatic mechanism has not been elucidated because of a lack of appropriate models. We have previously developed useful bone metastatic cell lines of luminal breast cancer using MCF7 cells. In this study, we characterized bone metastatic MCF7-BM cell lines and identified c-Jun as a novel bone metastasis marker of luminal breast cancer. The protein level of c-Jun was upregulated in MCF7-BM cells compared with that in parental cells, and its deficiency resulted in the suppression of tumor cell migration, transformation, and reduced osteolytic ability. In vivo, dominant-negative c-Jun exhibited smaller bone metastatic lesions and a lower metastatic frequency. Histologic analysis revealed that c-Jun expression was heterogeneous in bone metastatic lesions, whereas c-Jun overexpression mediated a vicious cycle between MCF7-BM cells and osteoclasts by enhancing calcium-induced migration and releasing the osteoclast activator BMP5. Pharmacological inhibition of c-Jun by the Jun amino-terminal kinase (JNK) inhibitor JNK-IN-8 effectively suppressed tumorigenesis and bone metastasis in MCF7-BM cells. Furthermore, c-Jun downstream signals were specifically correlated with the clinical prognosis of patients with the luminal subtype of breast cancer. Our results illustrate the potential benefits of a therapy that targets c-Jun to prevent bone metastasis in luminal breast cancer.

IMPLICATIONS

c-Jun expression mediates bone metastasis in luminal breast cancer by forming a vicious cycle in the bone microenvironment, which reveals potential strategies for subtype-specific bone metastasis therapy.

c-Abl activates RIPK3 signaling in Gaucher disease

Gaucher disease (GD) is caused by homozygous mutations in the GBA1 gene, which encodes the lysosomal β-glucosidase (GBA) enzyme. GD affects several organs and tissues, including the brain in certain variants of the disease. Heterozygous GBA1 variants are a major genetic risk factor for developing Parkinson's disease. The RIPK3 kinase is relevant in GD and its deficiency improves the neurological and visceral symptoms in a murine GD model. RIPK3 mediates necroptotic-like cell death: it is unknown whether the role of RIPK3 in GD is the direct induction of necroptosis or if it has a more indirect function by mediating necrosis-independent. Also, the mechanisms that activate RIPK3 in GD are currently unknown. In this study, we show that c-Abl tyrosine kinase participates upstream of RIPK3 in GD. We found that the active, phosphorylated form of c-Abl is increased in several GD models, including patient's fibroblasts and GBA null mice. Furthermore, its pharmacological inhibition with the FDA-approved drug Imatinib decreased RIPK3 signaling. We found that c-Abl interacts with RIPK3, that RIPK3 is phosphorylated at a tyrosine site, and that this phosphorylation is reduced when c-Abl is inhibited. Genetic ablation of c-Abl in neuronal GD and GD mice models significantly reduced RIPK3 activation and MLKL downstream signaling. These results showed that c-Abl signaling is a new upstream pathway that activates RIPK3 and that its inhibition is an attractive therapeutic approach for the treatment of GD.

Arginase Inhibition Supports Survival and Differentiation of Neuronal Precursors in Adult Alzheimer's Disease Mice

Adult neurogenesis is a complex physiological process, which plays a central role in maintaining cognitive functions, and consists of progenitor cell proliferation, newborn cell migration, and cell maturation. Adult neurogenesis is susceptible to alterations under various physiological and pathological conditions. A substantial decay of neurogenesis has been documented in Alzheimer’s disease (AD) patients and animal AD models; however, several treatment strategies can halt any further decline and even induce neurogenesis. Our previous results indicated a potential effect of arginase inhibition, with norvaline, on various aspects of neurogenesis in triple-transgenic mice. To better evaluate this effect, we chronically administered an arginase inhibitor, norvaline, to triple-transgenic and wild-type mice, and applied an advanced immunohistochemistry approach with several biomarkers and bright-field microscopy. Remarkably, we evidenced a significant reduction in the density of neuronal progenitors, which demonstrate a different phenotype in the hippocampi of triple-transgenic mice as compared to wild-type animals. However, norvaline showed no significant effect upon the progenitor cell number and constitution. We demonstrated that norvaline treatment leads to an escalation of the polysialylated neuronal cell adhesion molecule immunopositivity, which suggests an improvement in the newborn neuron survival rate. Additionally, we identified a significant increase in the hippocampal microtubule-associated protein 2 stain intensity. We also explore the molecular mechanisms underlying the effects of norvaline on adult mice neurogenesis and provide insights into their machinery.

Regulation of autoimmune disease by the E3 ubiquitin ligase Itch

Itch is a HECT type E3 ubiquitin ligase that is required to prevent the development of autoimmune disease in both mice and humans. Itch is expressed in most mammalian cell types, and, based on published data, it regulates many cellular pathways ranging from T cell differentiation to liver tumorigenesis. Since 1998, when Itch was first discovered, hundreds of publications have described mechanisms through which Itch controls various biologic activities in both immune and non-immune cells. Other studies have provided insight into how Itch catalytic activity is regulated. However, while autoimmunity is the primary clinical feature that occurs in both mice and humans lacking Itch, and Itch control of immune cell function has been well-studied, it remains unclear how Itch prevents the emergence of autoimmune disease. In this review, we explore recent discoveries that advance our understanding of how Itch regulates immune cell biology, and the extent to which these clarify how Itch prevents autoimmune disease. Additionally, we discuss how molecular regulators of Itch impact its ability to control these processes, as this may provide clues on how to therapeutically target Itch to treat patients with autoimmune disease.

Glomerulosclerosis Induced by Deficiency of Membrane-Associated Guanylate Kinase Inverted 2 in Kidney Podocytes

Membrane-associated guanylate kinase inverted 2 (MAGI-2) is a component of the slit diaphragm (SD) of glomerular podocytes. Here, we investigated the podocyte-specific function of MAGI-2 using newly generated podocyte-specific MAGI-2-knockout (MAGI-2-KO) mice. Compared with podocytes from wild-type mice, podocytes from MAGI-2-KO mice exhibited SD disruption, morphologic abnormalities of foot processes, and podocyte apoptosis leading to podocyte loss. These pathologic changes manifested as massive albuminuria by 8 weeks of age and glomerulosclerosis and significantly higher plasma creatinine levels at 12 weeks of age; all MAGI-2-KO mice died by 20 weeks of age. Loss of MAGI-2 in podocytes associated with decreased expression and nuclear translocation of dendrin, which is also a component of the SD complex. Dendrin translocates from the SD to the nucleus of injured podocytes, promoting apoptosis. Our coimmunoprecipitation and in vitro reconstitution studies showed that dendrin is phosphorylated by Fyn and dephosphorylated by PTP1B, and that Fyn-induced phosphorylation prevents Nedd4-2-mediated ubiquitination of dendrin. Under physiologic conditions in vivo, phosphorylated dendrin localized at the SDs; in the absence of MAGI-2, dephosphorylated dendrin accumulated in the nucleus. Furthermore, induction of experimental GN in rats led to the downregulation of MAGI-2 expression and the nuclear accumulation of dendrin in podocytes. In summary, MAGI-2 and Fyn protect dendrin from Nedd4-2-mediated ubiquitination and from nuclear translocation, thereby maintaining the physiologic homeostasis of podocytes, and the lack of MAGI-2 in podocytes results in FSGS.

c-Abl-mediated tyrosine phosphorylation of JunB is required for Adriamycin-induced expression of p21

The non-receptor-type tyrosine kinase c-Abl functions as a cytoplasmic signal transducer upon activation of cell-surface receptors. c-Abl is also involved in DDR (DNA-damage response), which is initiated in the nucleus, whereas its molecular functions in DDR are not fully understood. In the present study, we found that c-Abl phosphorylates JunB, a member of the AP-1 (activator protein 1) transcription factor family. Because JunB was suggested to be involved in DDR, we analysed the role of c-Abl-mediated phosphorylation of JunB in DDR. We first analysed phosphorylation sites of JunB and found that c-Abl majorly phosphorylates JunB at Tyr(173), Tyr(182) and Tyr(188). Because c-Abl promotes expression of the cyclin-dependent kinase inhibitor p21 upon stimulation with the DNA-damaging agent Adriamycin (doxorubicin), we analysed the involvement of JunB in Adriamycin-induced p21 expression. We found that JunB suppresses p21 induction through inhibition of its promoter activity. The phosphomimetic JunB, which was generated by glutamic acid substitutions at the phosphorylation sites, failed to repress p21 induction. Recruitment of JunB to the p21 promoter was promoted by Adriamycin stimulation and was further enhanced by co-treatment with the c-Abl inhibitor imatinib. The phosphomimetic glutamic acid substitutions in JunB or Adriamycin treatment impaired the JunB-c-Fos transcription factor complex formation. Taken together, these results suggest that, although JunB represses p21 promoter activity, c-Abl phosphorylates JunB and conversely inhibits its suppressive role on p21 promoter activity upon Adriamycin stimulation. Therefore JunB is likely to be a key target of c-Abl in expression of p21 in Adriamycin-induced DDR.

Icariin induces synoviolin expression through NFE2L1 to protect neurons from ER stress-induced apoptosis

By suppressing neuronal apoptosis, Icariin is a potential therapeutic drug for neuronal degenerative diseases. The molecular mechanisms of Icariin anti-apoptotic functions are still largely unclear. In this report, we found that Icariin induces the expression of Synoviolin, an endoplasmic reticulum (ER)-anchoring E3 ubiquitin ligase that functions as a suppressor of ER stress-induced apoptosis. The nuclear factor erythroid 2-related factor 1 (NFE2L1) is responsible for Icariin-mediated Synoviolin gene expression. Mutation of the NFE2L1-binding sites in a distal region of the Synoviolin promoter abolished Icariin-induced Synoviolin promoter activity, and knockdown of NFE2L1 expression prevented Icariin-stimulated Synoviolin expression. More importantly, Icariin protected ER stress-induced apoptosis of PC12 cells in a Synoviolin-dependent manner. Therefore, our study reveals Icariin-induced Synoviolin expression through NFE2L1 as a previously unappreciated molecular mechanism underlying the neuronal protective function of Icariin.

Tyrosine phosphorylation of WW proteins

A number of key regulatory proteins contain one or two copies of the WW domain known to mediate protein-protein interaction via proline-rich motifs, such as PPxY. The Hippo pathway components take advantage of this module to transduce tumor suppressor signaling. It is becoming evident that tyrosine phosphorylation is a critical regulator of the WW proteins. Here, we review the current knowledge on the involved tyrosine kinases and their roles in regulating the WW proteins.

c-Abl stabilizes HDAC2 levels by tyrosine phosphorylation repressing neuronal gene expression in Alzheimer's disease

In Alzheimer's disease (AD), there is a decrease in neuronal gene expression induced by HDAC2 increase; however, the mechanisms involved are not fully elucidated. Here, we described how the tyrosine kinase c-Abl increases HDAC2 levels, inducing transcriptional repression of synaptic genes. Our data demonstrate that (1) in neurons, c-Abl inhibition with Imatinib prevents the AβO-induced increase in HDAC2 levels; (2) c-Abl knockdown cells show a decrease in HDAC2 levels, while c-Abl overexpression increases them; (3) c-Abl inhibition reduces HDAC2-dependent repression activity and HDAC2 recruitment to the promoter of several synaptic genes, increasing their expression; (4) c-Abl induces tyrosine phosphorylation of HDAC2, a posttranslational modification, affecting both its stability and repression activity; and (5) treatment with Imatinib decreases HDAC2 levels in a transgenic mice model of AD. Our results support the participation of the c-Abl/HDAC2 signaling pathway in the epigenetic blockade of gene expression in AD pathology.

COP1 and GSK3β cooperate to promote c-Jun degradation and inhibit breast cancer cell tumorigenesis

High abundance of c-Jun is detected in invasive breast cancer cells and aggressive breast tumor malignancies. Here, we demonstrate that a major cause of high c-Jun abundance in invasive breast cancer cells is prolonged c-Jun protein stability owing to poor poly-ubiquitination of c-Jun. Among the known c-Jun-targeting E3 ligases, we identified constitutive photomorphogenesis protein 1 (COP1) as an E3 ligase responsible for c-Jun degradation in less invasive breast cancer cells because depletion of COP1 reduced c-Jun poly-ubiquitination leading to the stabilization of c-Jun protein. In a panel of breast cancer cell lines, we observed an inverse association between the levels of COP1 and c-Jun. However, overexpressing COP1 alone was unable to decrease c-Jun level in invasive breast cancer cells, indicating that efficient c-Jun protein degradation necessitates an additional event. Indeed, we found that glycogen synthase kinase 3 (GSK3) inhibitors elevated c-Jun abundance in less invasive breast cancer cells and that GSK3β nonphosphorylable c-Jun-T239A mutant displayed greater protein stability and poorer poly-ubiquitination compared to the wild-type c-Jun. The ability of simultaneously enforced expression of COP1 and constitutively active GSK3β to decrease c-Jun abundance in invasive breast cancer cells allowed us to conclude that c-Jun is negatively regulated through the coordinated action of COP1 and GSK3β. Importantly, co-expressing COP1 and active GSK3β blocked in vitro cell growth/migration and in vivo metastasis of invasive breast cancer cells. Gene expression profiling of breast tumor specimens further revealed that higher COP1 expression correlated with better recurrence-free survival. Our study supports the notion that COP1 is a suppressor of breast cancer progression.

Hydrogen peroxide sensing, signaling and regulation of transcription factors

The regulatory mechanisms by which hydrogen peroxide (H₂O₂) modulates the activity of transcription factors in bacteria (OxyR and PerR), lower eukaryotes (Yap1, Maf1, Hsf1 and Msn2/4) and mammalian cells (AP-1, NRF2, CREB, HSF1, HIF-1, TP53, NF-κB, NOTCH, SP1 and SCREB-1) are reviewed. The complexity of regulatory networks increases throughout the phylogenetic tree, reaching a high level of complexity in mammalians. Multiple H₂O₂ sensors and pathways are triggered converging in the regulation of transcription factors at several levels: (1) synthesis of the transcription factor by upregulating transcription or increasing both mRNA stability and translation; (ii) stability of the transcription factor by decreasing its association with the ubiquitin E3 ligase complex or by inhibiting this complex; (iii) cytoplasm–nuclear traffic by exposing/masking nuclear localization signals, or by releasing the transcription factor from partners or from membrane anchors; and (iv) DNA binding and nuclear transactivation by modulating transcription factor affinity towards DNA, co-activators or repressors, and by targeting specific regions of chromatin to activate individual genes. We also discuss how H₂O₂ biological specificity results from diverse thiol protein sensors, with different reactivity of their sulfhydryl groups towards H₂O₂, being activated by different concentrations and times of exposure to H₂O₂. The specific regulation of local H₂O₂ concentrations is also crucial and results from H₂O₂ localized production and removal controlled by signals. Finally, we formulate equations to extract from typical experiments quantitative data concerning H₂O₂ reactivity with sensor molecules. Rate constants of 140 M⁻¹ s⁻¹ and ≥1.3 × 10³ M⁻¹ s⁻¹ were estimated, respectively, for the reaction of H₂O₂ with KEAP1 and with an unknown target that mediates NRF2 protein synthesis. In conclusion, the multitude of H₂O₂ targets and mechanisms provides an opportunity for highly specific effects on gene regulation that depend on the cell type and on signals received from the cellular microenvironment.

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Complexity of redox regulation increases along the phylogenetic tree.

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Complex regulatory networks allow for a high degree of H₂O₂ biological plasticity.

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H₂O₂ modulates gene expression at all steps from transcription to protein synthesis.

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Fast response (s) is mediated by sensors with high H₂O₂ reactivity.

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Low reactivity H₂O₂ sensors may mediate slow (h) or localized H₂O₂ responses.

Crosstalk between kinases and Nedd4 family ubiquitin ligases

Understanding the interplay between kinase and E3 ligase signaling pathways will allow better understanding of therapeutically relevant pathways and the design of small molecule therapeutics targeting these pathways.

ATM kinase activity modulates ITCH E3-ubiquitin ligase activity

Ataxia Telangiectasia Mutated (ATM) kinase, a central regulator of the DNA damage response regulates the activity of several E3-ubiquitin ligases and the ubiquitination-proteasome system is a consistent target of ATM. ITCH is an E3-ubiquitin ligase that modulates the ubiquitination of several targets, therefore participating to the regulation of several cellular responses, among which the DNA damage response, TNFα, Notch and Hedgehog signalling and T cell development.

Here we uncover ATM as a novel positive modulator of ITCH E3-ubiquitin ligase activity. A single residue on ITCH protein, S161, which is part of an ATM SQ consensus motif, is required for ATM-dependent activation of ITCH. ATM activity enhances ITCH enzymatic activity, which in turn drives the ubiquitination and degradation of c-FLIP-L and c-Jun, previously identified as ITCH substrates. Importantly, Atm deficient mice show resistance to hepatocyte cell death, similarly to Itch deficient animals, providing in vivo genetic evidence for this circuit.

Our data identify ITCH as a novel component of the ATM-dependent signaling pathway and suggest that the impairment of the correct functionality of ITCH caused by Atm deficiency may contribute to the complex clinical features linked to Ataxia Telangiectasia.

USP22 antagonizes p53 transcriptional activation by deubiquitinating Sirt1 to suppress cell apoptosis and is required for mouse embryonic development

The NAD-dependent histone deacetylase Sirt1 antagonizes p53 transcriptional activity to regulate cell-cycle progression and apoptosis. We have identified a ubiquitin-specific peptidase, USP22, one of the 11 death-from-cancer signature genes that are critical in controlling cell growth and death, as a positive regulator of Sirt1. USP22 interacts with and stabilizes Sirt1 by removing polyubiquitin chains conjugated onto Sirt1. The USP22-mediated stabilization of Sirt1 leads to decreasing levels of p53 acetylation and suppression of p53-mediated functions. In contrast, depletion of endogenous USP22 by RNA interference destabilizes Sirt1, inhibits Sirt1-mediated deacetylation of p53 and elevates p53-dependent apoptosis. Genetic deletion of the usp22 gene results in Sirt1 instability, elevated p53 transcriptional activity and early embryonic lethality in mice. Our study elucidates a molecular mechanism in suppression of cell apoptosis by stabilizing Sirt1 in response to DNA damage and reveals a critical physiological function of USP22 in mouse embryonic development.

c-Abl-mediated tyrosine phosphorylation of the T-bet DNA-binding domain regulates CD4+ T-cell differentiation and allergic lung inflammation

The tyrosine kinase c-Abl is required for full activation of T cells, while its role in T-cell differentiation has not been characterized. We report that c-Abl deficiency skews CD4(+) T cells to type 2 helper T cell (Th2) differentiation, and c-Abl(-/-) mice are more susceptible to allergic lung inflammation. c-Abl interacts with and phosphorylates T-bet, a Th1 lineage transcription factor. c-Abl-mediated phosphorylation enhances the transcriptional activation of T-bet. Interestingly, three tyrosine residues within the T-bet DNA-binding domain are the predominant sites of phosphorylation by c-Abl. Mutation of these tyrosine residues inhibits the promoter DNA-binding activity of T-bet. c-Abl regulates Th cell differentiation in a T-bet-dependent manner because genetic deletion of T-bet in CD4(+) T cells abolishes c-Abl-deficiency-mediated enhancement of Th2 differentiation. Reintroduction of T-bet-null CD4(+) T cells with wild-type T-bet, but not its tyrosine mutant, rescues gamma interferon (IFN-γ) production and inhibits Th2 cytokine production. Therefore, c-Abl catalyzes tyrosine phosphorylation of the DNA-binding domain of T-bet to regulate CD4(+) T cell differentiation.

The type III histone deacetylase Sirt1 protein suppresses p300-mediated histone H3 lysine 56 acetylation at Bclaf1 promoter to inhibit T cell activation

The NAD-dependent histone deacetylase Sirt1 is a negative regulator of T cell activation. Here we report that Sirt1 inhibits T cell activation by suppressing the transcription of Bcl2-associated factor 1 (Bclaf1), a protein required for T cell activation. Sirt1-null T cells have increased acetylation of the histone 3 lysine 56 residue (H3K56) at the bclaf1 promoter, as well as increasing Bclaf1 transcription. Sirt1 binds to bclaf1 promoter upon T cell receptor (TCR)/CD28 stimulation by forming a complex with histone acetyltransferase p300 and NF-κB transcription factor Rel-A. The recruitment of Sirt1, but not p300, requires Rel-A because blocking Rel-A nuclear translocation in T cells and siRNA-mediated knockdown of Rel-A can inhibit Sirt1 binding to bclaf1 promoter. Although knockdown of either p300 or GCN5 partially suppressed global H3K56 acetylation, only p300 knockdown specifically attenuated H3K56 acetylation at the bclaf1 promoter. Lastly, knockdown of Bclaf1 suppresses the hyperactivation observed in Sirt1(-/-) T cells, indicated by less IL-2 production in CD4(+) T cells and reduced proliferation. Therefore, Sirt1 negatively regulates T cell activation via H3K56 deacetylation at the promoter region to inhibit transcription of Bclaf1.

The type III histone deacetylase Sirt1 is essential for maintenance of T cell tolerance in mice

Although many self-reactive T cells are eliminated by negative selection in the thymus, some of these cells escape into the periphery, where they must be controlled by additional mechanisms. However, the molecular mechanisms underlying peripheral T cell tolerance and its maintenance remain largely undefined. In this study, we report that sirtuin 1 (Sirt1), a type III histone deacetylase, negatively regulates T cell activation and plays a major role in clonal T cell anergy in mice. In vivo, we found that loss of Sirt1 function resulted in abnormally increased T cell activation and a breakdown of CD4+ T cell tolerance. Conversely, upregulation of Sirt1 expression led to T cell anergy, in which the activity of the transcription factor AP-1 was substantially diminished.Furthermore, Sirt1 interacted with and deacetylated c-Jun, yielding an inactive AP-1 factor. In addition, Sirt1-deficient mice were unable to maintain T cell tolerance and developed severe experimental allergic encephalomyelitis as well as spontaneous autoimmunity. These findings provide insight into the molecular mechanisms of T cell activation and anergy, and we suggest that activators of Sirt1 may be useful as therapeutic agents for the treatment and/or prevention of autoimmune diseases.

MMR/c-Abl-dependent activation of ING2/p73alpha signaling regulates the cell death response to N-methyl-N'-nitro-N-nitrosoguanidine

Agents inducing O(6)-methylguanine (O(6)MeG) in DNA such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) are cytotoxic and a deficiency in mismatch repair (MMR) results in lack of sensitivity to this genotoxin (termed alkylation tolerance). Here, we show that ING2, a member of the inhibitor of growth family, is required for cell death induced by MNNG. We further observe that MNNG treatment increases cellular protein levels of ING2 that is dependent on intact MMR function and that MNNG-induced ING2 localizes and associates with p73alpha in the nucleus. Suppression of ING2 by short hairpin RNA (shRNA) in MMR-proficient colorectal cancer cells decreased its sensitivity to MNNG and, in addition, abrogated MNNG-induced stabilization and acetylation of p73alpha. Interestingly, suppression of p73alpha had a greater impact on MNNG-induced cell death than ING2 leading us to conclude that ING2 regulates the cell death response, in part, through p73alpha. Inhibition of c-Abl by STI571 or suppression of c-Abl expression by shRNA blocked ING2 induction and p73alpha acetylation induced by this alkylator. Similarly, suppression of MMR (MLH1) by shRNA abrogated ING2 induction/p73alpha acetylation. Taken together, these results demonstrate that MLH1/c-Abl-dependent activation of ING2>p73alpha signaling regulates cell death triggered by MNNG and further suggests that dysregulation of this event may, in part, be responsible for alkylation tolerance observed in MMR compromised cells.

The HECT-type E3 ubiquitin ligase AIP2 inhibits activation-induced T-cell death by catalyzing EGR2 ubiquitination

E3 ubiquitin ligases, which target specific molecules for proteolytic destruction, have emerged as key regulators of immune functions. Several E3 ubiquitin ligases, including c-Cbl, Cbl-b, GRAIL, Itch, and Nedd4, have been shown to negatively regulate T-cell activation. Here, we report that the HECT-type E3 ligase AIP2 positively regulates T-cell activation. Ectopic expression of AIP2 in mouse primary T cells enhances their proliferation and interleukin-2 production by suppressing the apoptosis of T cells. AIP2 interacts with and promotes ubiquitin-mediated degradation of EGR2, a zinc finger transcription factor that has been found to regulate Fas ligand (FasL) expression during activation-induced T-cell death. Suppression of AIP2 expression by small RNA interference upregulates EGR2, inhibits EGR2 ubiquitination and FasL expression, and enhances the apoptosis of T cells. Therefore, AIP2 regulates activation-induced T-cell death by suppressing EGR2-mediated FasL expression via the ubiquitin pathway.

The anti-apoptotic activity of BAG3 is restricted by caspases and the proteasome

Background

Caspase-mediated cleavage and proteasomal degradation of ubiquitinated proteins are two independent mechanisms for the regulation of protein stability and cellular function. We previously reported BAG3 overexpression protected ubiquitinated clients, such as AKT, from proteasomal degradation and conferred cytoprotection against heat shock. We hypothesized that the BAG3 protein is regulated by proteolysis.

Methodology/Principal Findings

Staurosporine (STS) was used as a tool to test for caspase involvement in BAG3 degradation. MDA435 and HeLa human cancer cell lines exposed to STS underwent apoptosis with a concomitant time and dose-dependent loss of BAG3, suggesting the survival role of BAG3 was subject to STS regulation. zVAD-fmk or caspase 3 and 9 inhibitors provided a strong but incomplete protection of both cells and BAG3 protein. Two putative caspase cleavage sites were tested: KEVD (BAG3^E345A/D347A) within the proline-rich center of BAG3 (PXXP) and the C-terminal LEAD site (BAG3^E516A/D518A). PXXP deletion mutant and BAG3^E345A/D347A, or BAG3^E516A/D518A respectively slowed or stalled STS-mediated BAG3 loss. BAG3, ubiquitinated under basal growth conditions, underwent augmented ubiquitination upon STS treatment, while there was no increase in ubiquitination of the BAG3^E516A/D518A caspase-resistant mutant. Caspase and proteasome inhibition resulted in partial and independent protection of BAG3 whereas inhibitors of both blocked BAG3 degradation. STS-induced apoptosis was increased when BAG3 was silenced, and retention of BAG3 was associated with cytoprotection.

Conclusions/Significance

BAG3 is tightly controlled by selective degradation during STS exposure. Loss of BAG3 under STS injury required sequential caspase cleavage followed by polyubiquitination and proteasomal degradation. The need for dual regulation of BAG3 in apoptosis suggests a key role for BAG3 in cancer cell resistance to apoptosis.

Decreased FoxP3 gene expression in the nasal secretions from patients with allergic rhinitis

OBJECTIVE

To determine whether patients with allergic rhinitis have altered FoxP3 gene expression and/or mutations.

STUDY DESIGN AND METHODS

We collected nasal secretions from 14 volunteers (five of whom have allergic rhinitis) and five ENT allergy patients. Total RNA was isolated from these nasal secretions. The gene expression levels of FoxP3 were quantified by both semi-quantitative RT-PCR and real-time PCR using Actin as a housekeeping gene. The cDNA fragments amplified by RT-PCR were analyzed by DNA sequencing.

RESULTS

We found that patients with allergic rhinitis had significantly lower FoxP3 mRNA compared to nonallergic controls (P < 0.01). In addition, we found a point mutation in the FoxP3 gene from a patient who not only has severe allergic rhinitis, but also has asthma. This mutation locates in a highly conserved region of FoxP3 gene and partially impaired FoxP3 functions.

CONCLUSION

Our data indicate that either reduced FoxP3 gene expression or impaired FoxP3 functions are involved in the development of allergic disease in humans.

T cell survival and function requires the c-Abl tyrosine kinase

C-Abl (Abl) regulates multiple cellular processes, including proliferation, survival, shape determination and motility, and participates in cellular responses to genotoxic and oxidative stress stimuli. Mice lacking Abl exhibit retarded growth, osteoporosis and defects in the immune system resulting in lymphopoenia and susceptibility to infections, leading to early death. To define the role of Abl in the regulation of adult T cells we ablated Abl exclusively in T cells by generating mice with floxed abl alleles and expressing an Lck-Cre transgene (Abl-T(-/-)). These mice exhibited thymic atrophy and abnormally reduced T cell numbers in the periphery. The thymic atrophy was caused by increased susceptibility of thymocytes to cell death. Importantly, Abl deficient T cells displayed abnormally reduced response to mitogenic stimulation in vitro. Consequently, Abl-T(-/-) mice exhibited impaired ability to reject syngeneic tumor, to induce T-mediated tumor cell killing, and to generate anti-tumor antibodies. These results demonstrate a cell-autonomous role for Abl in T cell function and survival.

Lysosomal localization of ubiquitinated Jun requires multiple determinants in a lysine-27-linked polyubiquitin conjugate

Ubiquitination regulates many cellular functions, including protein localization and degradation. Each function is specified by unique determinants in the conjugate. Ubiquitinated Jun is localized to lysosomes for degradation. Here, we characterized determinants of Jun ubiquitination and lysosomal localization by using ubiquitin-mediated fluorescence complementation (UbFC) in living cells and analysis of the stoichiometry of ubiquitin linked to Jun extracted from cells. The delta region of Jun and isoleucine-44 in ubiquitin were required for lysosomal localization of the conjugate. Ubiquitin containing only lysine-27, but no other single-lysine ubiquitin, mediated Jun ubiquitination, albeit at lower stoichiometry than wild-type ubiquitin. These conjugates were predominantly nuclear, but coexpression of lysine-27 and lysine-less ubiquitins enhanced the mean stoichiometry of Jun ubiquitination and lysosomal localization of the conjugate. Hepatocyte growth factor-regulated tyrosine kinase substrate (HRS) and tumor susceptibility gene 101 (TSG101) colocalized with ubiquitinated Jun. Knockdown of HRS or TSG101 inhibited lysosomal localization of ubiquitinated Jun and reduced Jun turnover. Ubiquitination of other Fos and Jun family proteins had distinct effects on their localization. Our results indicate that Jun is polyubiquitinated by E3 ligases that produce lysine-27-linked chains. Lysosomal localization of the conjugate requires determinants in Jun and in ubiquitin that are recognized in part by TSG101 and HRS, facilitating selective translocation and degradation of ubiquitinated Jun.

Protein quality control and degradation in cardiomyocytes

The heart is constantly under stress and cardiomyocytes face enormous challenges to correctly fold nascent polypeptides and keep mature proteins from denaturing. To meet the challenge, cardiomyocytes have developed multi-layered protein quality control (PQC) mechanisms which are carried out primarily by chaperones and ubiquitin-proteasome system mediated proteolysis. Autophagy may also participate in PQC in cardiomyocytes, especially under pathological conditions. Cardiac PQC often becomes inadequate in heart disease, which may play an important role in the development of congestive heart failure.

Yap1 phosphorylation by c-Abl is a critical step in selective activation of proapoptotic genes in response to DNA damage

Cells undergo apoptosis upon exposure to severe DNA damage stress. Under this condition, p73 is phosphorylated and activated by c-Abl. The transcription coactivator Yap1 binds p73 to generate a complex that escapes p73 proteasomal degradation and recruits p300 to support transcription of proapoptotic genes. However, the mechanism of selective activation of proapoptotic genes by Yap1 remained unclear. In this study, we show that c-Abl directly phosphorylates Yap1 at position Y357 in response to DNA damage. Tyrosine-phosphorylated Yap1 is a more stable protein that displays higher affinity to p73 and selectively coactivates p73 proapoptotic target genes. Furthermore, we show that Yap1 switches between p73-mediated proapoptotic and growth arrest target genes based on its phosphorylation state. Thus, our data demonstrate that modification of a transcription coactivator, namely the DNA damage-induced phosphorylation of Yap1 by c-Abl, influences the specificity of target gene activation.

The Nedd4-like family of E3 ubiquitin ligases and cancer

Accumulating evidence suggests that E3 ubiquitin ligases play important roles in cancer development. In this article, we provide a comprehensive summary of the roles of the Nedd4-like family of E3 ubiquitin ligases in human cancer. There are nine members of the Nedd4-like E3 family, all of which share a similar structure, including a C2 domain at the N-terminus, two to four WW domains in the middle of the protein, and a homologous to E6-AP COOH terminus domain at the C-terminus. The assertion that Nedd4-like E3s play a role in cancer is supported by the overexpression of Smurf2 in esophageal squamous cell carcinoma, WWP1 in prostate and breast cancer, Nedd4 in prostate and bladder cancer, and Smurf1 in pancreatic cancer. Because Nedd4-like E3s regulate ubiquitin-mediated trafficking, lysosomal or proteasomal degradation, and nuclear translocation of multiple proteins, they modulate important signaling pathways involved in tumorigenesis like TGFbeta, EGF, IGF, VEGF, SDF-1, and TNFalpha. Additionally, several Nedd4-like E3s directly regulate various cancer-related transcription factors from the Smad, p53, KLF, RUNX, and Jun families. Interestingly, multiple Nedd4-like E3s show ligase independent function. Furthermore, Nedd4-like E3s themselves are frequently regulated by phosphorylation, ubiquitination, translocation, and transcription in cancer cells. Because the regulation and biological output of these E3s is such a complex process, study of the role of these E3s in cancer development poses some challenges. However, understanding the oncogenic potential of these E3s may facilitate the identification and development of biomarkers and drug targets in human cancer.

The Involvement of Abl and PTP61F in the Regulation of Abi Protein Localization and Stability and Lamella Formation in Drosophila S2 Cells

Most aspects of cellular events are regulated by a series of protein phosphorylation and dephosphorylation processes. Abi (Abl interactor protein) functions as a substrate adaptor protein for Abl and a core member of the WAVE complex, relaying signals from Rac to Arp2/3 complex and regulating actin dynamics. It is known that the recruitment of Abi into the lamella promotes polymerization of actin, although how it does this is unclear. In this study, we found PTP61F, a Drosophila homolog of mammalian PTP1B, can reverse the Abl phosphorylation of Abi and colocalizes with Abi in Drosophila S2 cells. Abi can be translocalized from the cytosol to the cell membrane by either increasing Abl or reducing endogenous PTP61F. This reciprocal regulation of Abi phosphorylation is also involved in modulating Abi protein level, which is thought to affect the stability of the WAVE complex. Using mass spectrometry, we identified several important tyrosine phosphorylation sites in Abi. We compared the translocalization and protein half-life of wild type (wt) and phosphomutant Abi and their abilities to restore the lamellipodia structure of the Abi-reduced cells. We found the phosphomutant to have reduced ability to translocalize and to have a protein half-life shorter than that of wt Abi. We also found that although the wt Abi could fully restore the lamellipodia structure, the phosphomutant could not. Together, these findings suggest that the reciprocal regulation of Abi phosphorylation by Abl and PTP61F may regulate the localization and stability of Abi and may regulate the formation of lamella.

Expression, mutational analysis and in vitro response of imatinib mesylate and nilotinib target genes in ovarian granulosa cell tumors

OBJECTIVES

Granulosa cell tumors of the ovary (GCT) represent approximately 5% of malignant ovarian tumors. Surgery remains the primary modality of therapy and treatment options for advanced disease are limited. The molecular pathogenesis of GCT is not known but is likely to involve activation of tyrosine kinase-mediated cell signaling pathways. A recent case report of a patient with advanced recurrent GCT responding to the tyrosine kinase inhibitor, imatinib mesylate prompted us to explore a role for these therapies in GCT.

METHODS

The expression of the imatinib-sensitive tyrosine kinases, c-kit, c-Abl, PDGFR-alpha and PDGFR-beta, was determined using RT-PCR in a panel of GCT. Activating mutations of c-kit and PDGFR-alpha were also sought. The functional response was examined in two human-derived GCT cell lines.

RESULTS

All four kinases were expressed but at levels lower than those observed in pre-menopausal ovarian samples. Mutations in c-kit and PDGFR-alpha were not found. Both cell lines responded to imatinib and to the second generation, tyrosine kinase inhibitor, nilotinib, with dose-dependent decreases in cell proliferation and viability. These responses paralleled the imatinib-sensitive, K562 cell line but at approximately 240- and approximately 1000-fold higher concentrations of imatinib and nilotinib, respectively.

CONCLUSIONS

Our study suggests that human GCT, in general, are unlikely to respond to imatinib or nilotinib therapy. The response of the cell lines at high concentrations implies an "off-target" effect, which suggests that a tyrosine kinase inhibitor, of appropriate specificity, may represent a therapeutic option in GCT.

NMR structural studies of the ItchWW3 domain reveal that phosphorylation at T30 inhibits the interaction with PPxY-containing ligands

In this work, we study the role of phosphorylation as a regulatory mechanism for the interaction between the E3 ubiquitin ligase ItchWW3 domain and two PPxY motifs of one of its targets, the Epstein-Barr virus latent membrane protein 2A. Whereas ligand phosphorylation only diminishes binding, domain phosphorylation at residue T30 abrogates it. We show that two ItchWW domains can be phosphorylated at this position, using CK2 and PKA kinases and/or with stimulated T lymphocyte lysates. To better understand the regulation process, we determined the NMR structures of the ItchWW3-PPxY complex and of the phosphoT30-ItchWW3 variant. The peptide binds the domain using both XP and tyrosine grooves. A hydrogen bond from T30 to the ligand is also detected. This hydrogen-bond formation is precluded in the variant, explaining the inhibition upon phosphorylation. Our results suggest that phosphorylation at position 30 in ItchWW domains can be a mechanism to inhibit target recognition in vivo.

RLE-1, an E3 ubiquitin ligase, regulates C. elegans aging by catalyzing DAF-16 polyubiquitination

The forkhead transcription factor, DAF-16, a downstream target of the insulin/IGF-I signaling pathway in C. elegans, is indispensable both for lifespan regulation and stress resistance. The molecular mechanisms involved in regulating DAF-16 transcriptional activation remain undefined. Here, we have identified an E3 ubiquitin ligase, RLE-1 (regulation of longevity by E3), which regulates aging in C. elegans. Disruption of RLE-1 expression in C. elegans increases lifespan; this extension of lifespan is due to elevated DAF-16 protein but not to changes of daf-16 mRNA levels. We have also found that RLE-1 catalyzes DAF-16 ubiquitination, leading to degradation by the proteasome. Elimination of RLE-1 expression in C. elegans causes increased transcriptional activation and sustained nuclear localization of DAF-16. Overexpression of DAF-16 in rle-1 mutants increases worm lifespan, while disruption of DAF-16 expression in rle-1 mutants reverses their longevity. Thus, RLE-1 is an E3 ubiquitin ligase of DAF-16 that regulates C. elegans aging.

The ubiquitination code: a signalling problem

Ubiquitin is a highly versatile post-translational modification that controls virtually all types of cellular events. Over the past ten years we have learned that diverse forms of ubiquitin modifications and of ubiquitin binding modules co-exist in the cell, giving rise to complex networks of protein:protein interactions. A central problem that continues to puzzle ubiquitinologists is how cells translate this myriad of stimuli into highly specific responses. This is a classical signalling problem. Here, we draw parallels with the phosphorylation signalling pathway and we discuss the expanding repertoire of ubiquitin signals, signal tranducers and signalling-regulated E3 enzymes. We examine recent advances in the field, including a new mechanism of regulation of E3 ligases that relies on ubiquitination.

Abi-1 forms an epidermal growth factor-inducible complex with Cbl: role in receptor endocytosis

The Abl-interactor (Abi) proteins are involved in the regulation of actin polymerization and have recently been shown to modulate epidermal growth factor receptor (EGFR) endocytosis. Here we describe the identification of a novel complex between Abi-1 and the Cbl ubiquitin ligase that is induced by stimulation with EGF. Notably, an Abi-1 mutant lacking the SH3 domain (DeltaSH3) fails to interact with Cbl and inhibits EGFR internalization. We show that expression of the Abi-1DeltaSH3 mutant inhibits Cbl accumulation at the plasma membrane after EGF treatment. We have previously shown that the oncogenic Abl tyrosine kinase inhibits EGFR internalization. Here we report that the oncogenic Abl kinase disrupts the EGF-inducible Abi-1/Cbl complex, highlighting the importance of Abl kinases and downstream effectors in the regulation of EGFR internalization. Thus, our work reveals a new role for oncogenic Abl tyrosine kinases in the regulation of the Abi-1/Cbl protein complex and uncovers a role for the Abi-1/Cbl complex in the regulation of EGFR endocytosis.