USP4 inhibits SMAD4 monoubiquitination and promotes activin and BMP signaling

Epidermal Growth Factor-Like Repeats and Discoidin I-Like Domains 3 Deficiency Attenuates Dilated Cardiomyopathy by Inhibiting Ubiquitin Specific Peptidase 10 Dependent Smad4 Deubiquitination

BACKGROUND

Dilated cardiomyopathy (DCM) is the leading cause of heart failure with a poor prognosis. Recent studies suggest that endothelial to mesenchymal transition (EndMT) may be involved in the pathogenesis and cardiac remodeling during DCM development. EDIL3 (epidermal growth factor-like repeats and discoidin I-like domains 3) is an extracellular matrix glycoprotein that has been reported to promote EndMT in various diseases. However, the roles of EDIL3 in DCM still remain unclear.

METHODS AND RESULTS

A mouse model of DCM and human umbilical vein endothelial cells were used to explore the roles and mechanisms of EDIL3 in DCM. The results indicated that EndMT and EDIL3 were activated in DCM mice. EDIL3 deficiency attenuated cardiac dysfunction and remodeling in DCM mice. EDIL3 knockdown alleviated EndMT by inhibiting USP10 (ubiquitin specific peptidase 10) dependent Smad4 deubiquitination in vivo and in vitro. Recombinant human EDIL3 promoted EndMT via reinforcing deubiquitination of Smad4 in human umbilical vein endothelial cells treated with IL-1β (interleukin 1β) and TGF-β (transforming growth factor beta). Inhibiting USP10 abolished EndMT exacerbated by EDIL3. In addition, recombinant EDIL3 also aggravates doxorubicin-induced EndMT by promoting Smad4 deubiquitination in HUVECs.

CONCLUSIONS

Taken together, these results indicate that EDIL3 deficiency attenuated EndMT by inhibiting USP10 dependent Smad4 deubiquitination in DCM mice.

FBXO3 stabilizes USP4 and Twist1 to promote PI3K-mediated breast cancer metastasis

Tumor metastasis is the major cause of breast cancer morbidity and mortality. It has been reported that the F-box protein FBXO3 functions as an E3 ubiquitin ligase in regulating various biological processes, including host autoimmune, antiviral innate immunity, and inflammatory response. However, the role of FBXO3 in tumor metastasis remains elusive. We have previously shown that ΔNp63α is a common inhibitory target in oncogene-induced cell motility and tumor metastasis. In this study, we show that FBXO3 plays a vital role in PI3K-mediated breast cancer metastasis independent of its E3 ligase activity and ΔNp63α in breast cancer cells and in mouse. FBXO3 can bind to and stabilize USP4, leading to Twist1 protein stabilization and increased breast cancer cell migration and tumor metastasis. Mechanistically, FBXO3 disrupts the interaction between USP4 and aspartyl aminopeptidase (DNPEP), thereby protecting USP4 from DNPEP-mediated degradation. Furthermore, p110αH1047R facilitates the phosphorylation and stabilization of FBXO3 in an ERK1-dependent manner. Knockdown of either FBXO3 or USP4 leads to significant inhibition of PI3K-induced breast cancer metastasis. Clinically, elevated expression of p110α/FBXO3/USP4/Twist1 is associated with poor overall survival (OS) and recurrence-free survival (RFS) of breast cancer patients. Taken together, this study reveals that the FBXO3-USP4-Twist1 axis is pivotal in PI3K-mediated breast tumor metastasis and that FBXO3/USP4 may be potential therapeutic targets for breast cancer treatment.

Control of TGFβ signalling by ubiquitination independent function of E3 ubiquitin ligase TRIP12

Transforming growth factor β (TGFβ) pathway is a master regulator of cell proliferation, differentiation, and death. Deregulation of TGFβ signalling is well established in several human diseases including autoimmune disorders and cancer. Thus, understanding molecular pathways governing TGFβ signalling may help better understand the underlying causes of some of those conditions. Here, we show that a HECT domain E3 ubiquitin ligase TRIP12 controls TGFβ signalling in multiple models. Interestingly, TRIP12 control of TGFβ signalling is completely independent of its E3 ubiquitin ligase activity. Instead, TRIP12 recruits SMURF2 to SMAD4, which is most likely responsible for inhibitory monoubiquitination of SMAD4, since SMAD4 monoubiquitination and its interaction with SMURF2 were dramatically downregulated in TRIP12-/- cells. Additionally, genetic inhibition of TRIP12 in human and murine cells leads to robust activation of TGFβ signalling which was rescued by re-introducing wildtype TRIP12 or a catalytically inactive C1959A mutant. Importantly, TRIP12 control of TGFβ signalling is evolutionary conserved. Indeed, genetic inhibition of Drosophila TRIP12 orthologue, ctrip, in gut leads to a reduced number of intestinal stem cells which was compensated by the increase in differentiated enteroendocrine cells. These effects were completely normalised in Drosophila strain where ctrip was co-inhibited together with Drosophila SMAD4 orthologue, Medea. Similarly, in murine 3D intestinal organoids, CRISPR/Cas9 mediated genetic targeting of Trip12 enhances TGFβ mediated proliferation arrest and cell death. Finally, CRISPR/Cas9 mediated genetic targeting of TRIP12 in MDA-MB-231 breast cancer cells enhances the TGFβ induced migratory capacity of these cells which was rescued to the wildtype level by re-introducing wildtype TRIP12. Our work establishes TRIP12 as an evolutionary conserved modulator of TGFβ signalling in health and disease.

Ubiquitin-specific proteases: Vital regulatory molecules in bone and bone-related diseases

Stabilization of bone structure and function involves multiple cell-to-cell and molecular interactions, in which the regulatory functions of post-translational modifications such as ubiquitination and deubiquitination shouldn't be underestimated. As the largest family of deubiquitinating enzymes, the ubiquitin-specific proteases (USPs) participate in the development of bone homeostasis and bone-related diseases through multiple classical osteogenic and osteolytic signaling pathways, such as BMP/TGF-β pathway, NF-κB/p65 pathway, EGFR-MAPK pathway and Wnt/β-catenin pathway. Meanwhile, USPs may also broadly regulate regulate hormone expression level, cell proliferation and differentiation, and may further influence bone homeostasis from gene fusion and nuclear translocation of transcription factors. The number of patients with bone-related diseases is currently enormous, making exploration of their pathogenesis and targeted therapy a hot topic. Pathological increases in the levels of inflammatory mediators such as IL-1β and TNF-α lead to inflammatory bone diseases such as osteoarthritis, rheumatoid arthritis and periodontitis. While impaired body metabolism greatly increases the probability of osteoporosis. Abnormal physiological activity of bone-associated cells results in a variety of bone tumors. The regulatory role of USPs in bone-related disease has received particular attention from academics in recent studies. In this review, we focuse on the roles and mechanisms of USPs in bone homeostasis and bone-related diseases, with the expectation of informing targeted therapies in the clinic.

Ubiquitin-specific peptidases: Players in bone metabolism

Osteoporosis is an ageing-related disease, that has become a major public health problem and its pathogenesis has not yet been fully elucidated. Substantial evidence suggests a strong link between overall age-related disease progression and epigenetic modifications throughout the life cycle. As an important epigenetic modification, ubiquitination is extensively involved in various physiological processes, and its role in bone metabolism has attracted increasing attention. Ubiquitination can be reversed by deubiquitinases, which counteract protein ubiquitination degradation. As the largest and most structurally diverse cysteinase family of deubiquitinating enzymes, ubiquitin-specific proteases (USPs), comprising the largest and most structurally diverse cysteine kinase family of deubiquitinating enzymes, have been found to be important players in maintaining the balance between bone formation and resorption. The aim of this review is to explore recent findings highlighting the regulatory functions of USPs in bone metabolism and provide insight into the molecular mechanisms governing their actions during bone loss. An in-deep understanding of USPs-mediated regulation of bone formation and bone resorption will provide a scientific rationale for the discovery and development of novel USP-targeted therapeutic strategies for osteoporosis.

OTUD3: A Lys6 and Lys63 specific deubiquitinase in early vertebrate development

Ubiquitination and deubiquitylation regulate essential cellular processes and involve hundreds of sequentially acting enzymes, many of which are barely understood. OTUD3 is an evolutionarily highly conserved deubiquitinase involved in many aspects of cellular homeostasis. However, its biochemical properties and physiological role during development are poorly understood. Here, we report on the expression of OTUD3 in human tissue samples where it appears prominently in those of neuronal origin. In cells, OTUD3 is present in the cytoplasm where it can bind to microtubules. Interestingly, we found that OTUD3 cleaves preferentially at K6 and K63, i.e., poly-ubiquitin linkages that are not primarily involved in protein degradation. We employed Xenopus embryos to study the consequences of suppressing otud3 function during early neural development. We found that Otud3 deficiency led to impaired formation of cranial and particularly of cranial neural crest-derived structures as well as movement defects. Thus, OTUD3 appears as a neuronally enriched deubiquitinase that is involved in the proper development of the neural system.

Xiaoaiping improves the general state of rats with malignant ascites secondary to gastric cancer by blocking the TGF-β1 signaling pathway

OBJECTIVE

To investigate the effect of Xiaoaiping on the general state of rats with malignant ascites secondary to gastric cancer by blocking the transforming growth factor-β1 (TGF-β1) signaling pathway.

METHODS

Fifty healthy Wistar rats were randomly divided into 5 groups, with 10 rats in each group. After successful modeling, 0.2 g/(20 g∙d), 0.4 g/(20 g∙d) and 0.8 g/(20 g∙d) of Xiaoaiping injections were administrated at the low dose group (LDG), medium dose group (MDG) and high dose group (HDG), respectively. The model group (MG) was injected intraperitoneally with the same amount of sterile normal saline for 8 d. Rats in the control group (CG) were healthy without any treatment. The general states (mental state, dietary habits, reactions and body shape) of rats in each group were observed. The abdominal circumference, platelet (PLT) count, white blood cell (WBC) count, serum ferritin (SF), ascites volume, cell survival rate, and expression levels of TGF-β1 signaling pathway (TGF-β1, Smad2) were compared among the groups.

RESULTS

PLT and WBC counts in the MG were lower than those in the CG. Ascites volume, tumor cell survival rate, and SF in the MG were higher than those in the LDG, MDG and HDG (P<0.05). Thymus index in the LDG, MDG and HDG were significantly higher than that in the MG (P<0.05). There was no significant difference in the spleen indices among the groups (P>0.05). The kidney index, serum creatinine and urea nitrogen levels in LDG, MDG and HDG were significantly lower than those of MG (P<0.05). The LDG, MDG and HDG exhibited significantly higher peripheral blood CD4+ cells and CD4+/CD8+, and lower CD8+ level, as compared with the MG (P<0.05). The levels of serum interferon γ (IFN-γ), interleukin (IL)-1α, IL-1β, IL-2, IL-4 and tumor necrosis factor-α (TNF-α) in the LDG, MDG and HDG were higher than those in the MG (P<0.05). The ascites volume and tumor cell survival rate were decreased sequentially in the LDG, MDG and HDG (P<0.05). The MG had higher mRNA levels of TGF-β1 and Smad2 than the CG (P<0.05). No statistically significant difference was found in TGF-β1 and Smad2 between the LDG and MG (P>0.05).

CONCLUSION

Xiaoaiping could significantly reduce the ascites volume in rats with gastric cancer, inhibit the production of tumor cells in the abdominal cavity, and improve the general state of rats in a dose-dependent manner. The mechanism may be related to the down-regulation of the mRNA level of TGF-β1 and Smad2.

TGF-beta signal transduction: biology, function and therapy for diseases

The transforming growth factor beta (TGF-β) is a crucial cytokine that get increasing concern in recent years to treat human diseases. This signal controls multiple cellular responses during embryonic development and tissue homeostasis through canonical and/or noncanonical signaling pathways. Dysregulated TGF-β signal plays an essential role in contributing to fibrosis via promoting the extracellular matrix deposition, and tumor progression via inducing the epithelial-to-mesenchymal transition, immunosuppression, and neovascularization at the advanced stage of cancer. Besides, the dysregulation of TGF-beta signal also involves in other human diseases including anemia, inflammatory disease, wound healing and cardiovascular disease et al. Therefore, this signal is proposed to be a promising therapeutic target in these diseases. Recently, multiple strategies targeting TGF-β signals including neutralizing antibodies, ligand traps, small-molecule receptor kinase inhibitors targeting ligand-receptor signaling pathways, antisense oligonucleotides to disrupt the production of TGF-β at the transcriptional level, and vaccine are under evaluation of safety and efficacy for the forementioned diseases in clinical trials. Here, in this review, we firstly summarized the biology and function of TGF-β in physiological and pathological conditions, elaborated TGF-β associated signal transduction. And then, we analyzed the current advances in preclinical studies and clinical strategies targeting TGF-β signal transduction to treat diseases.

Ubiquitin modification in osteogenic differentiation and bone formation: From mechanisms to clinical significance

The ubiquitin-proteasome system is an important pathway for mediating posttranslational modification and protein homeostasis and exerts a wide range of functions in diverse biological processes, including stem cell differentiation, DNA repair, and cell cycle regulation. Many studies have shown that ubiquitination modification plays a critical role in regulating the osteogenic differentiation of stem cells and bone formation through various mechanisms. This review summarizes current progress on the effects and mechanisms of ubiquitin modification on transcription factors and signaling pathways involved in osteogenic differentiation. Moreover, the review highlights the latest advances in the clinical application of drugs in bone tissue engineering. A thorough understanding of ubiquitin modifications may provide promising therapeutic targets for stem cell-based bone tissue engineering.

Cardiac fibroblast heat shock protein 47 aggravates cardiac fibrosis post myocardial ischemia–reperfusion injury by encouraging ubiquitin specific peptidase 10 dependent Smad4 deubiquitination

Despite complications were significantly reduced due to the popularity of percutaneous coronary intervention (PCI) in clinical trials, reperfusion injury and chronic cardiac remodeling significantly contribute to poor prognosis and rehabilitation in AMI patients. We revealed the effects of HSP47 on myocardial ischemia–reperfusion injury (IRI) and shed light on the underlying molecular mechanism. We generated adult mice with lentivirus-mediated or miRNA (mi1/133TS)-aided cardiac fibroblast-selective HSP47 overexpression. Myocardial IRI was induced by 45-min occlusion of the left anterior descending (LAD) artery followed by 24 h reperfusion in mice, while ischemia-mediated cardiac remodeling was induced by four weeks of reperfusion. Also, the role of HSP47 in fibrogenesis was evaluated in cardiac fibroblasts following hypoxia–reoxygenation (HR). Extensive HSP47 was observed in murine infarcted hearts, human ischemic hearts, and cardiac fibroblasts and accelerated oxidative stress and apoptosis after myocardial IRI. Cardiac fibroblast-selective HSP47 overexpression exacerbated cardiac dysfunction caused by chronic myocardial IRI and presented deteriorative fibrosis and cell proliferation. HSP47 upregulation in cardiac fibroblasts promoted TGFβ1–Smad4 pathway activation and Smad4 deubiquitination by recruiting ubiquitin-specific peptidase 10 (USP10) in fibroblasts. However, cardiac fibroblast specific USP10 deficiency abolished HSP47-mediated fibrogenesis in hearts. Moreover, blockage of HSP47 with Col003 disturbed fibrogenesis in fibroblasts following HR. Altogether, cardiac fibroblast HSP47 aggravates fibrosis post-myocardial IRI by enhancing USP10-dependent Smad4 deubiquitination, which provided a potential strategy for myocardial IRI and cardiac remodeling.

Monoubiquitination in Homeostasis and Cancer

Monoubiquitination is a post-translational modification (PTM), through which a single ubiquitin molecule is covalently conjugated to a lysine residue of the target protein. Monoubiquitination regulates the activity, subcellular localization, protein-protein interactions, or endocytosis of the substrate. In doing so, monoubiquitination is implicated in diverse cellular processes, including gene transcription, endocytosis, signal transduction, cell death, and DNA damage repair, which in turn regulate cell-cycle progression, survival, proliferation, and stress response. In this review, we summarize the functions of monoubiquitination and discuss how this PTM modulates homeostasis and cancer.

The E3 ubiquitin ligase SMURF2 stabilizes RNA editase ADAR1p110 and promotes its adenosine-to-inosine (A-to-I) editing function

Epitranscriptomic changes in RNA catalyzed by the RNA-editing enzyme ADAR1 play an essential role in the regulation of diverse molecular and cellular processes, both under physiological conditions and in disease states, including cancer. Yet, despite a growing body of evidence pointing to ADAR1 as a potential therapeutic target, the mechanisms regulating its cellular abundance and activity, particularly of its constitutively expressed and ubiquitous form, ADAR1p110, are poorly understood. Here, we report the HECT-type E3 ubiquitin ligase SMURF2 as a pivotal regulator of ADAR1p110. We show that SMURF2, which is primarily known to promote the ubiquitin-mediated degradation of its protein substrates, protects ADAR1p110 from proteolysis and promotes its A-to-I editase activity in human and mouse cells and tissues. ADAR1p110's interactome analysis performed in human cells also showed a positive influence of SMURF2 on the stability and function of ADAR1p110. Mechanistically, we found that SMURF2 directly binds, ubiquitinates and stabilizes ADAR1p110 in an E3 ubiquitin ligase-dependent manner, through ADAR1p110 ubiquitination at lysine-744 (K744). Mutation of this residue to arginine (K744R), which is also associated with several human disorders, including dyschromatosis symmetrica hereditaria (DSH) and some types of cancer, abolished SMURF2-mediated protection of ADAR1p110 from both proteasomal and lysosomal degradation and inactivated ADAR1p110-mediated RNA editing. Our findings reveal a novel mechanism underlying the regulation of ADAR1 in mammalian cells and suggest SMURF2 as a key cellular factor influencing the protein abundance, interactions and functions of ADAR1p110.

The Emerging Role of E3 Ubiquitin Ligase SMURF2 in the Regulation of Transcriptional Co-Repressor KAP1 in Untransformed and Cancer Cells and Tissues

Simple Summary

KAP1 plays an essential role in different molecular and cellular processes central to carcinogenesis, disease progression, and treatment response, revealing both tumor promoting and anticancer functions. The mechanisms that control the steady-state levels of KAP1 and its protein abundance are not well known. Our findings show that SMURF2, a ubiquitously-expressed HECT-type E3 ubiquitin ligase with suggested anticancer activities, is capable to directly bind, ubiquitinate, and regulate KAP1 expression levels in non-cancerous and tumor cells and tissues. The data further show that SMURF2 has a significant influence on KAP1 interactome, regulating its protein–protein interactions and functions in a catalytically-dependent manner. These findings reveal SMURF2 as a pivotal regulator of KAP1, laying a foundation for the investigation of the role of the SMURF2–KAP1 axis in carcinogenic processes and therapeutic responses to anticancer treatment.

Abstract

KAP1 is an essential nuclear factor acting as a scaffold for protein complexes repressing transcription. KAP1 plays fundamental role in normal and cancer cell biology, affecting cell proliferation, DNA damage response, genome integrity maintenance, migration and invasion, as well as anti-viral and immune response. Despite the foregoing, the mechanisms regulating KAP1 cellular abundance are poorly understood. In this study, we identified the E3 ubiquitin ligase SMURF2 as an important regulator of KAP1. We show that SMURF2 directly interacts with KAP1 and ubiquitinates it in vitro and in the cellular environment in a catalytically-dependent manner. Interestingly, while in the examined untransformed cells, SMURF2 mostly exerted a negative impact on KAP1 expression, a phenomenon that was also monitored in certain Smurf2-ablated mouse tissues, in tumor cells SMURF2 stabilized KAP1. This stabilization relied on the unaltered E3 ubiquitin ligase function of SMURF2. Further investigations showed that SMURF2 regulates KAP1 post-translationally, interfering with its proteasomal degradation. The conducted immunohistochemical studies showed that the reciprocal relationship between the expression of SMURF2 and KAP1 also exists in human normal and breast cancer tissues and suggested that this relationship may be disrupted by the carcinogenic process. Finally, through stratifying KAP1 interactome in cells expressing either SMURF2 wild-type or its E3 ligase-dead form, we demonstrate that SMURF2 has a profound impact on KAP1 protein–protein interactions and the associated functions, adding an additional layer in the SMURF2-mediated regulation of KAP1. Cumulatively, these findings uncover SMURF2 as a novel regulator of KAP1, governing its protein expression, interactions, and functions.

Ubiquitin-specific proteases as therapeutic targets in paediatric primary bone tumours?

In children and young adults, primary malignant bone tumours are mainly composed of osteosarcoma and Ewing's sarcoma. Despite advances in treatments, nearly 40% of patients succumb to these diseases. In particular, the clinical outcome of metastatic osteosarcoma or Ewing's sarcoma remains poor, with less than 30% of patients who develop metastases surviving five years after initial diagnosis. Over the last decade, the cancer research community has shown considerable interest in the processes of protein ubiquitination and deubiquitination. In particular, a growing number of studies show the relevance to target the ubiquitin-specific protease (USP) family in various cancers. This review provides an update on the current knowledge regarding the implication of these USPs in the progression of bone sarcoma: osteosarcoma and Ewing's sarcoma.

Inhibition of ubiquitin-specific protease 13-mediated degradation of Raf1 kinase by Spautin-1 has opposing effects in naïve and primed pluripotent stem cells

Embryonic stem cells (ESCs) are progenitor cells that retain the ability to differentiate into various cell types and are necessary for tissue repair. Improving cell culture conditions to maintain the pluripotency of ESCs in vitro is an urgent problem in the field of regenerative medicine. Here, we reveal that Spautin-1, a specific small-molecule inhibitor of ubiquitin-specific protease (USP) family members USP10 and USP13, promotes the maintenance of self-renewal and pluripotency of mouse ESCs in vitro. Functional studies reveal that only knockdown of USP13, but not USP10, is capable of mimicking the function of Spautin-1. Mechanistically, we demonstrate that USP13 physically interacts with, deubiquitinates, and stabilizes serine/threonine kinase Raf1 and thereby sustains Raf1 protein at the posttranslational level to activate the FGF/MEK/ERK prodifferentiation signaling pathway in naïve mouse ESCs. In contrast, in primed mouse epiblast stem cells and human induced pluripotent stem cells, the addition of Spautin-1 had an inhibitory effect on Raf1 levels, but USP13 overexpression promoted self-renewal. The addition of an MEK inhibitor impaired the effect of USP13 upregulation in these cells. These findings provide new insights into the regulatory network of naïve and primed pluripotency.

UbiBrowser 2.0: a comprehensive resource for proteome-wide known and predicted ubiquitin ligase/deubiquitinase-substrate interactions in eukaryotic species

As an important post-translational modification, ubiquitination mediates ∼80% of protein degradation in eukaryotes. The degree of protein ubiquitination is tightly determined by the delicate balance between specific ubiquitin ligase (E3)-mediated ubiquitination and deubiquitinase-mediated deubiquitination. In 2017, we developed UbiBrowser 1.0, which is an integrated database for predicted human proteome-wide E3-substrate interactions. Here, to meet the urgent requirement of proteome-wide E3/deubiquitinase-substrate interactions (ESIs/DSIs) in multiple organisms, we updated UbiBrowser to version 2.0 (http://ubibrowser.ncpsb.org.cn). Using an improved protocol, we collected 4068/967 known ESIs/DSIs by manual curation, and we predicted about 2.2 million highly confident ESIs/DSIs in 39 organisms, with >210-fold increase in total data volume. In addition, we made several new features in the updated version: (i) it allows exploring proteins' upstream E3 ligases and deubiquitinases simultaneously; (ii) it has significantly increased species coverage; (iii) it presents a uniform confidence scoring system to rank predicted ESIs/DSIs. To facilitate the usage of UbiBrowser 2.0, we also redesigned the web interface for exploring these known and predicted ESIs/DSIs, and added functions of 'Browse', 'Download' and 'Application Programming Interface'. We believe that UbiBrowser 2.0, as a discovery tool, will contribute to the study of protein ubiquitination and the development of drug targets for complex diseases.

Expression and Role of Ubiquitin-Specific Peptidases in Osteoblasts

The ubiquitin-proteasome system regulates biological processes in normal and diseased states. Recent investigations have focused on ubiquitin-dependent modifications and their impacts on cellular function, commitment, and differentiation. Ubiquitination is reversed by deubiquitinases, including ubiquitin-specific peptidases (USPs), whose roles have been widely investigated. In this review, we explore recent findings highlighting the regulatory functions of USPs in osteoblasts and providing insight into the molecular mechanisms governing their actions during bone formation. We also give a brief overview of our work on USP53, a target of PTH in osteoblasts and a regulator of mesenchymal cell lineage fate decisions. Emerging evidence addresses questions pertaining to the complex layers of regulation exerted by USPs on osteoblast signaling. We provide a short overview of our and others' understanding of how USPs modulate osteoblastogenesis. However, further studies using knockout mouse models are needed to fully understand the mechanisms underpinning USPs actions.

Deubiquitinating Enzymes Orchestrate the Cancer Stem Cell-Immunosuppressive Niche Dialogue: New Perspectives and Therapeutic Potential

Cancer stem cells (CSCs) are sparks for igniting tumor recurrence and the instigators of low response to immunotherapy and drug resistance. As one of the important components of tumor microenvironment, the tumor associated immune microenvironment (TAIM) is driving force for the heterogeneity, plasticity and evolution of CSCs. CSCs create the inhibitory TAIM (ITAIM) mainly through four stemness-related signals (SRSs), including Notch-nuclear factor-κB axis, Hedgehog, Wnt and signal transducer and activator of transcription. Ubiquitination and deubiquitination in proteins related to the specific stemness of the CSCs have a profound impact on the regulation of ITAIM. In regulating the balance between ubiquitination and deubiquitination, it is crucial for deubiquitinating enzymes (DUBs) to cleave ubiquitin chains from substrates. Ubiquitin-specific peptidases (USPs) comprise the largest family of DUBs. Growing evidence suggests that they play novel functions in contribution of ITAIM, including regulating tumor immunogenicity, activating stem cell factors, upregulating the SRSs, stabilizing anti-inflammatory receptors, and regulating anti-inflammatory cytokines. These overactive or abnormal signaling may dampen antitumor immune responses. The inhibition of USPs could play a regulatory role in SRSs and reversing ITAIM, and also have great potential in improving immune killing ability against tumor cells, including CSCs. In this review, we focus on the USPs involved in CSCs signaling pathways and regulating ITAIM, which are promising therapeutic targets in antitumor therapy.

The Role of Bone Morphogenetic Protein 4 in Ovarian Function and Diseases

Bone morphogenetic proteins (BMPs) are the largest subfamily of the transforming growth factor-β (TGF-β) superfamily. BMP4 is a secreted protein that was originally identified due to its role in bone and cartilage development. Over the past decades, extensive literature has indicated that BMP4 and its receptors are widely expressed in the ovary. Dysregulation of BMP4 expression may play a vital role in follicular development, polycystic ovary syndrome (PCOS), and ovarian cancer. In this review, we summarized the expression pattern of BMP4 in the ovary, focused on the role of BMP4 in follicular development and steroidogenesis, and discussed the role of BMP4 in ovarian diseases such as polycystic ovary syndrome and ovarian cancer. Some studies have shown that the expression of BMP4 in the ovary is spatiotemporal and species specific, but the effects of BMP4 seem to be similar in follicular development of different species. In addition, BMP4 is involved in the development of hyperandrogenemia in PCOS and drug resistance in ovarian cancer, but further research is still needed to clarify the specific mechanisms.

Spotlight on USP4: Structure, Function, and Regulation

The deubiquitinating enzyme (DUB)–mediated cleavage of ubiquitin plays a critical role in balancing protein synthesis and degradation. Ubiquitin-specific protease 4 (USP4), a member of the largest subfamily of cysteine protease DUBs, removes monoubiquitinated and polyubiquitinated chains from its target proteins. USP4 contains a DUSP (domain in USP)–UBL (ubiquitin-like) domain and a UBL-insert catalytic domain, sharing a common domain organization with its paralogs USP11 and USP15. USP4 plays a critical role in multiple cellular and biological processes and is tightly regulated under normal physiological conditions. When its expression or activity is aberrant, USP4 is implicated in the progression of a wide range of pathologies, especially cancers. In this review, we comprehensively summarize the current knowledge of USP4 structure, biological functions, pathological roles, and cellular regulation, highlighting the importance of exploring effective therapeutic interventions to target USP4.

Deubiquitylases in developmental ubiquitin signaling and congenital diseases

Metazoan development from a one-cell zygote to a fully formed organism requires complex cellular differentiation and communication pathways. To coordinate these processes, embryos frequently encode signaling information with the small protein modifier ubiquitin, which is typically attached to lysine residues within substrates. During ubiquitin signaling, a three-step enzymatic cascade modifies specific substrates with topologically unique ubiquitin modifications, which mediate changes in the substrate's stability, activity, localization, or interacting proteins. Ubiquitin signaling is critically regulated by deubiquitylases (DUBs), a class of ~100 human enzymes that oppose the conjugation of ubiquitin. DUBs control many essential cellular functions and various aspects of human physiology and development. Recent genetic studies have identified mutations in several DUBs that cause developmental disorders. Here we review principles controlling DUB activity and substrate recruitment that allow these enzymes to regulate ubiquitin signaling during development. We summarize key mechanisms of how DUBs control embryonic and postnatal differentiation processes, highlight developmental disorders that are caused by mutations in particular DUB members, and describe our current understanding of how these mutations disrupt development. Finally, we discuss how emerging tools from human disease genetics will enable the identification and study of novel congenital disease-causing DUBs.

Adaptors as the regulators of HECT ubiquitin ligases

The HECT (homologous to E6AP C-terminus) ubiquitin ligases (E3s) are a small family of highly conserved enzymes involved in diverse cellular functions and pathological conditions. Characterised by a C-terminal HECT domain that accepts ubiquitin from E2 ubiquitin conjugating enzymes, these E3s regulate key signalling pathways. The activity and functional regulation of HECT E3s are controlled by several factors including post-translational modifications, inter- and intramolecular interactions and binding of co-activators and adaptor proteins. In this review, we focus on the regulation of HECT E3s by accessory proteins or adaptors and discuss various ways by which adaptors mediate their regulatory roles to affect physiological outcomes. We discuss common features that are conserved from yeast to mammals, regardless of the type of E3s as well as shed light on recent discoveries explaining some existing enigmas in the field.

A novel negative regulatory mechanism of Smurf2 in BMP/Smad signaling in bone

Transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) play important roles in bone metabolism. Smad ubiquitination regulatory factors (Smurfs) regulate TGF-β/BMP signaling via ubiquitination, resulting in degradation of signaling molecules to prevent excessive activation of TGF-β/BMP signaling. Though Smurf2 has been shown to negatively regulate TGF-β/Smad signaling, its involvement in BMP/Smad signaling in bone metabolism has not been thoroughly investigated. In the present study, we sought to evaluate the role of Smurf2 in BMP/Smad signaling in bone metabolism. Absorbable collagen sponges containing 3 μg of recombinant human BMP2 (rhBMP2) were implanted in the dorsal muscle pouches of wild type (WT) and Smurf2^−/− mice. The rhBMP2-induced ectopic bone in Smurf2^−/− mice showed greater bone mass, higher mineral apposition and bone formation rates, and greater osteoblast numbers than the ectopic bone in WT mice. In WT mice, the ectopic bone consisted of a thin discontinuous outer cortical shell and scant inner trabecular bone. In contrast, in Smurf2^−/− mice, the induced bone consisted of a thick, continuous outer cortical shell and abundant inner trabecular bone. Additionally, rhBMP2-stimulated bone marrow stromal cells (BMSCs) from Smurf2^−/− mice showed increased osteogenic differentiation. Smurf2 induced the ubiquitination of Smad1/5. BMP/Smad signaling was enhanced in Smurf2^−/− BMSCs stimulated with rhBMP2, and the inhibition of BMP/Smad signaling suppressed osteogenic differentiation of these BMSCs. These findings demonstrate that Smurf2 negatively regulates BMP/Smad signaling, thereby identifying a new regulatory mechanism in bone metabolism.

SOX2-Upregulated microRNA-30e Promotes the Progression of Esophageal Cancer via Regulation of the USP4/SMAD4/CK2 Axis

Esophageal cancer (EC) is a highly aggressive disease, and its progression involves a complex gene regulation network. Transcription factor SOX2 is amplified in various cancers including EC. A pathway involving SOX2 regulation of microRNAs (miRNAs) and their target genes has been previously revealed. This study aims to delineate the ability of SOX2 to influence the EC progression, with the involvement of miR-30e/USP4/SMAD4/CK2 axis. SOX2 expression was first examined in the clinical tissue samples from 30 EC patients. Effects of SOX2 on proliferation, migration, and invasion alongside tumorigenicity of transfected cells were examined by means of gain- and loss-of-function experiments. EC tissues and cells exhibited high expression of SOX2, miR-30e, and CK2 and poor expression of USP4 and SMAD4. Mechanistically, SOX2 was positively correlated with miR-30e and upregulated the expression of miR-30e. miR-30e specifically targeted USP4, which induced deubiquitination of SMAD4 and promoted its expression. Meanwhile, SMAD4 was enriched in the CK2 promoter region and thus inhibited its expression. SOX2 stimulated EC cell proliferative, invasive, and migratory capacities in vitro and tumor growth in vivo by regulating the miR-30e/USP4/SMAD4/CK2 axis. Collectively, our work reveals a novel SOX2-mediated regulatory network in EC that may be a viable target for EC treatment.

USP4 function and multifaceted roles in cancer: a possible and potential therapeutic target

Cancer remains one of the major culprits causing disease-related deaths and leads to a high morbidity and similar mortality. Insidious onset, difficult early detection and a lack of broad-spectrum and effective multi-cancer therapeutic targets have limited the prolongation of cancer patients’ survival for decades. Therefore, a versatile therapeutic target which is involved in various cancer-related signaling pathways and different cancers may be more effective for cancer targeted therapy. USP4, one of the DUBs members which participates in deubiquitination, an inverse process of ubiquitination, can regulate various classical cancer-related signaling pathways, and thereby plays a vital role in some pathological and physiological processes including tumor initiation and progression. Recently, USP4 has been found to exert versatile influences on cells proliferation, migration and invasion, also apoptosis of various tumors. Moreover, USP4 can also act as a prognostic biomarker in several cancers. This review will give a comprehensive introduction of USP4 about its regulatory mechanisms, related signaling pathways, pathophysiological functions and the roles in various cancers which may help us better understand its biological functions and improve future studies to construct suitable USP4-targeted cancer therapy system.

SMAD4 activates Wnt signaling pathway to inhibit granulosa cell apoptosis

The TGF-β and Wnt signaling pathways are interrelated in many cell types and tissues, and control cell functions in coordination. Here, we report that SMAD4, a downstream effector of the TGF-β signaling pathway, induces FZD4, a receptor of the Wnt signaling pathway, establishing a novel route of communication between these two pathways in granulosa cells (GCs). We found that SMAD4 is a strong inducer of FZD4, not only initiating FZD4 transcription but also activating FZD4-dependent Wnt signaling and GC apoptosis. Furthermore, we identified the direct and indirect mechanisms by which SMAD4 promotes expression of FZD4 in GCs. First, SMAD4 functions as a transcription factor to directly bind to the FZD4 promoter region to increase its transcriptional activity. Second, SMAD4 promotes production of SDNOR, a novel lncRNA that acts as a sponge for miR-29c, providing another mean to block miR-29c from degenerating FZD4 mRNA. Overall, our findings not only reveal a new channel of crosstalk between the TGF-β and Wnt signaling pathways, SMAD4–FZD4 axis, but also provide new insights into the regulatory network of GC apoptosis and follicular atresia. These RNA molecules, such as miR-29c and lnc-SDNOR, represent potential targets for treatment of reproductive diseases and improvement of female fertility.

USP32 promotes tumorigenesis and chemoresistance in gastric carcinoma via upregulation of SMAD2

USP32, a member of the ubiquitin-specific proteases family, has been implicated in the development of breast cancer and small lung cancer. However, its biological functions and clinical significance in gastric cancer (GC) remain unclear. In the present study, we reported that knockdown or depletion of USP32 significantly inhibited GC cell proliferation and migration in vitro and in vivo, indicating that USP32 functions as an oncogene in GC. Importantly, results from immunohistochemical staining in a tissue microarray revealed that USP32 was upregulated in GC tissues compared with paracancerous tissues. Further analyses showed that high expression of USP32 was closely related with high T-staging and poor outcomes of GC patients. Mechanistically, USP32 silencing caused a decrease in the expression of SMAD2, which resulted in the inhibitory effects of GC cells on growth, motility, and chemoresistance to cisplatin. Therefore, our findings strongly suggest the involvement of USP32 in GC progression and provide a potential target for future therapy of GC.

The inhibitory effect of compound ChlA-F on human bladder cancer cell invasion can be attributed to its blockage of SOX2 protein

Sex-determining region Y-box 2 (SOX2), a well-known stemness biomarker, is highly expressed in a variety of cancers, including human highly invasive bladder cancer (BC). However, the role of SOX2 may vary in different kinds of malignancy. In the present study, we discovered that ChlA-F, a novel conformation derivative of isolate Cheliensisin A (Chel A), remarkably inhibits the invasive ability of human invasive BC cells through downregulation of SOX2 protein expression. We found that ChlA-F treatment dramatically decreases SOX2 protein expression in human high-grade invasive BC cells. Ectopic expression of SOX2 reversed ChlA-F inhibition of cell invasion ability in human bladder cancer cells, suggesting that SOX2 is a major target of ChlA-F during its inhibition of human BC invasion. Mechanistic studies revealed that ChlA-F downregulates SOX2 at both the protein degradation and protein translation levels. Further studies revealed that ChlA-F treatment induces HuR protein expression and that the increased HuR interacts with USP8 mRNA, resulting in elevation of USP8 mRNA stability and protein expression. Elevated USP8 subsequently acts as an E3 ligase to promote SOX2 ubiquitination and protein degradation. We also found that ChlA-F treatment substantially increases c-Jun phosphorylation at Ser63 and Ser73, initiating miR-200c transcription. The increased miR-200c directly binds to the 3'-UTR of SOX2 mRNA to suppress SOX2 protein translation. These results present novel mechanistic insight into understanding SOX2 inhibition upon ChlA-F treatment and provide important information for further exploration of ChlA-F as a new therapeutic compound for the treatment of highly invasive/metastatic human BC patients.

Ubiquitin Dynamics in Stem Cell Biology: Current Challenges and Perspectives

Ubiquitination plays a central role in the regulation of stem cell self-renewal, propagation, and differentiation. In this review, the functions of ubiquitin dynamics in a myriad of cellular processes, acting along side the pluripotency network, to regulate embryonic stem cell identity are highlighted. The implication of deubiquitinases (DUBs) and E3 Ubiquitin (Ub) ligases in cellular functions beyond protein degradation is reported, including key functions in the regulation of mRNA stability, protein translation, and intra-cellular trafficking; and how it affects cell metabolism, the micro-environment, and chromatin organization is discussed. Finally, unsolved issues in the field are emphasized and will need to be tackled in order to fully understand the contribution of ubiquitin dynamics to stem cell self-renewal and differentiation.

The functions and regulation of Smurfs in cancers

Smad ubiquitination regulatory factor 1 (Smurf1) and Smurf2 are HECT-type E3 ubiquitin ligases, and both Smurfs were initially identified to regulate Smad protein stability in the TGF-β/BMP signaling pathway. In recent years, Smurfs have exhibited E3 ligase-dependent and -independent activities in various kinds of cells. Smurfs act as either potent tumor promoters or tumor suppressors in different tumors by regulating biological processes, including metastasis, apoptosis, cell cycle, senescence and genomic stability. The regulation of Smurfs activity and expression has therefore emerged as a hot spot in tumor biology research. Further, the Smurf1- or Smurf2-deficient mice provide more in vivo clues for the functional study of Smurfs in tumorigenesis and development. In this review, we summarize these milestone findings and, in turn, reveal new avenues for the prevention and treatment of cancer by regulating Smurfs.

MicroRNA-31/184 is involved in transforming growth factor-β-induced apoptosis in A549 human alveolar adenocarcinoma cells

AIMS

TGF-β-induced alveolar epithelial cells apoptosis were involved in idiopathic pulmonary fibrosis (IPF). This study aimed to explore potential targets and mechanisms of IPF.

MAIN METHODS

mRNA and microRNA arrays were used to analyze differentially expressed genes and miRNAs. Several essential targets of TGF-β-SMADs and TGF-β-PI3K-AKT pathways were detected.

KEY FINDINGS

miR-31 and miR-184 expression levels were positively correlated with smad6 and smad2/akt expression levels in IPF patients. TGF-β could induce miR-31 and suppress miR-184 levels in A549 cells. miR-31 was confirmed to bind to the smad6-3'UTR and functionally suppress its expression. Down-regulated SMAD6 enhanced SMAD2/SMAD4 dimer formation and translocation due to its failure to prevent SMAD2 phosphorylation. In contrast, anti-fibrotic functions of miR-184 were abolished due to TGF-β directly suppressing miR-184 levels in A549 cells. When A549 was stimulated by TGF-β combined with or without miR-31 inhibitor/miR-184 mimic, it was showed that depleted miR-31 and/or increased miR-184 significantly ameliorated TGF-β-induced viability of A549 cells, as well as inhibited the expression of profibrotic factors, MMP7 and RUNX2.

SIGNIFICANCE

Inhibiting miR-31 and/or promoting miR-184 protect against TGF-β-induced fibrogenesis by respectively repressing the TGF-β-SMAD2 and TGF-β-PI3K-AKT signaling pathways, implying that miR-31/184 are potential targets and suggesting a new management strategy for IPF.

Deubiquitinating enzyme USP10 promotes hepatocellular carcinoma metastasis through deubiquitinating and stabilizing Smad4 protein

Hepatocellular carcinoma (HCC) has emerged as one of the most prevalent life-threatening cancers, and the high mortality rate is largely due to the metastasis. The sustained activation of Smad4 and transforming growth factor-β (TGF-β) is closely associated with advanced HCC metastasis. However, the regulatory mechanism underlying the aberrant activation of Smad4 and TGF-β pathway remains elusive. In this study, using a functional screen of USPs siRNA library, we identified ubiquitin-specific proteases USP10 as a deubiquitinating enzyme (DUB) that sustains the protein level of Smad4 and activates TGF-β signaling. Further analysis showed that USP10 directly interacts with Smad4 and stabilizes it through the cleavage of its proteolytic ubiquitination, thus promoting HCC metastasis. The suppression of USP10 by either shRNAs or catalytic inhibitor Spautin-1 significantly inhibited the migration of HCC cells, whereas the reconstitution of Smad4 was able to efficiently rescue this defect. Overall, our study not only uncovers the regulatory effect of USP10 on the protein abundance of Smad4, but also indicates that USP10 could be regarded as a potential intervention target for the metastatic HCC in Smad4-positive patients.

TRIM47 is up-regulated in colorectal cancer, promoting ubiquitination and degradation of SMAD4

Background

Tripartite motif 47 (TRIM47), a member of the TRIM family proteins, plays a key role in many types of cancers including colorectal cancer (CRC). We found that levels of TRIM47 mRNA and protein were increased significantly in colorectal tumors compared with nontumor tissues and the increased levels were associated with advanced tumor stage and poor outcome.

Methods

We used quantitative polymerase chain reaction and western blot to measure levels of TRIM47 mRNA and protein in human colorectal cancer and paired normal tissues. TRIM47 was knocked down and overexpressed in colorectal cancer cells, and the effects on cell proliferation, migration and growth of xenograft tumors in nude mice were assessed. The signaling pathways were examined by western blot and immunoprecipitation assays.

Results

TRIM47 promoted CRC proliferation and metastasis in vitro and in vivo as an oncogene. Mechanistically, TRIM47 interacted physically with SMAD4, increasing its ubiquitination and degradation. Loss of SMAD4 leaded to up-regulation of CCL15 expression and caused growth and invasion in human CRC cells through the CCL15-CCR1 signaling. Moreover, TRIM47 overexpression played a role in CRC chemoresistance in response to 5-FU therapy.

Conclusions

Our study demonstrated a functional role of the TRIM47-SMAD4-CCL15 axis in CRC progression and suggested a potential target for CRC therapy.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1143-x) contains supplementary material, which is available to authorized users.

CD44s is a crucial ATG7 downstream regulator for stem-like property, invasion, and lung metastasis of human bladder cancer (BC) cells

Over half a million US residents are suffering with bladder cancer (BC), which costs a total $4 billion in treatment annually. Although recent studies report that autophagy-related gene 7 (ATG7) is overexpressed in BCs, the regulatory effects of ATG7 on cancer stem-like phenotypes and invasion have not been explored yet. Current studies demonstrated that the deficiency of ATG7 by its shRNA dramatically reduced sphere formation and invasion in vitro, as well as lung metastasis in vivo in human invasive BC cells. Further studies indicated that the knockdown of ATG7 attenuated the expression of CD44 standard (CD44s), while ectopic introduction of CD44s, was capable of completely restoring sphere formation, invasion, and lung metastasis in T24T(shATG7) cells. Mechanistic studies revealed that ATG7 overexpression stabilized CD44s proteins accompanied with upregulating USP28 proteins. Upregulated USP28 was able to bind to CD44s and remove the ubiquitin group from CD44s' protein, resulting in the stabilization of CD44s protein. Moreover, ATG7 inhibition stabilized AUF1 protein and thereby reduced tet1 mRNA stability and expression, which was able to demethylate usp28 promoter, reduced USP28 expression, finally promoting CD44s degradation. In addition, CD44s was defined to inhibit degradation of RhoGDIβ, which in turn promotes BC invasion. Our results demonstrate that CD44s is a key ATG7 downstream regulator of the sphere formation, invasion, and lung metastasis of BCs, providing significant insight into understanding the BC invasions, metastasis, and stem-like properties.

The Deubiquitylase USP4 Interacts with the Water Channel AQP2 to Modulate Its Apical Membrane Accumulation and Cellular Abundance

Aquaporin 2 (AQP2) mediates the osmotic water permeability of the kidney collecting duct in response to arginine vasopressin (VP) and is essential for body water homeostasis. VP effects on AQP2 occur via long-term alterations in AQP2 abundance and short-term changes in AQP2 localization. Several of the effects of VP on AQP2 are dependent on AQP2 phosphorylation and ubiquitylation; post-translational modifications (PTM) that modulate AQP2 subcellular distribution and function. Although several protein kinases, phosphatases, and ubiquitin E3 ligases have been implicated in AQP2 PTM, how AQP2 is deubiquitylated or the role of deubiquitylases (DUBS) in AQP2 function is unknown. Here, we report a novel role of the ubiquitin-specific protease USP4 in modulating AQP2 function. USP4 co-localized with AQP2 in the mouse kidney, and in mpkCCD14 cells USP4 and AQP2 abundance are increased by VP. AQP2 and USP4 co-immunoprecipitated from mpkCCD14 cells and mouse kidney, and in vitro, USP4 can deubiquitylate AQP2. In mpkCCD14 cells, shRNA mediated knockdown of USP4 decreased AQP2 protein abundance, whereas no changes in AQP2 mRNA levels or VP-induced cAMP production were detected. VP-induced AQP2 membrane accumulation in knockdown cells was significantly reduced, which was associated with higher levels of ubiquitylated AQP2. AQP2 protein half-life was also significantly reduced in USP4 knockdown cells. Taken together, the data suggest that USP4 is a key regulator of AQP2 deubiquitylation and that loss of USP4 leads to increased AQP2 ubiquitylation, decreased AQP2 levels, and decreased cell surface AQP2 accumulation upon VP treatment. These studies have implications for understanding body water homeostasis.

Deubiquitinating Enzymes and Bone Remodeling

Bone remodeling, which is essential for bone homeostasis, is controlled by multiple factors and mechanisms. In the past few years, studies have emphasized the role of the ubiquitin-dependent proteolysis system in regulating bone remodeling. Deubiquitinases, which are grouped into five families, remove ubiquitin from target proteins and are involved in several cell functions. Importantly, a number of deubiquitinases mediate bone remodeling through regulating differentiation and/or function of osteoblast and osteoclasts. In this review, we review the functions and mechanisms of deubiquitinases in mediating bone remodeling.

Transcriptional cofactors Ski and SnoN are major regulators of the TGF-β/Smad signaling pathway in health and disease

The transforming growth factor-β (TGF-β) family plays major pleiotropic roles by regulating many physiological processes in development and tissue homeostasis. The TGF-β signaling pathway outcome relies on the control of the spatial and temporal expression of >500 genes, which depend on the functions of the Smad protein along with those of diverse modulators of this signaling pathway, such as transcriptional factors and cofactors. Ski (Sloan-Kettering Institute) and SnoN (Ski novel) are Smad-interacting proteins that negatively regulate the TGF-β signaling pathway by disrupting the formation of R-Smad/Smad4 complexes, as well as by inhibiting Smad association with the p300/CBP coactivators. The Ski and SnoN transcriptional cofactors recruit diverse corepressors and histone deacetylases to repress gene transcription. The TGF-β/Smad pathway and coregulators Ski and SnoN clearly regulate each other through several positive and negative feedback mechanisms. Thus, these cross-regulatory processes finely modify the TGF-β signaling outcome as they control the magnitude and duration of the TGF-β signals. As a result, any alteration in these regulatory mechanisms may lead to disease development. Therefore, the design of targeted therapies to exert tight control of the levels of negative modulators of the TGF-β pathway, such as Ski and SnoN, is critical to restore cell homeostasis under the specific pathological conditions in which these cofactors are deregulated, such as fibrosis and cancer.

Proteins that repress molecular signaling through the transforming growth factor-beta (TGF-β) pathway offer promising targets for treating cancer and fibrosis. Marina Macías-Silva and colleagues from the National Autonomous University of Mexico in Mexico City review the ways in which a pair of proteins, called Ski and SnoN, interact with downstream mediators of TGF-β to inhibit the effects of this master growth factor. Aberrant levels of Ski and SnoN have been linked to diverse range of diseases involving cell proliferation run amok, and therapies that regulate the expression of these proteins could help normalize TGF-β signaling to healthier physiological levels. For decades, drug companies have tried to target the TGF-β pathway, with limited success. Altering the activity of these repressors instead could provide a roundabout way of remedying pathogenic TGF-β activity in fibrosis and oncology.

Generation of Smurf2 Conditional Knockout Mice

Smad ubiquitin regulatory factor 2 (Smurf2) is a HECT domain-containing E3 ubiquitin ligase. Together with its closely related homolog Smurf1, Smurf2 was initially recognized as a negative regulator of transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) signaling, but subsequent studies have expanded its function to regulate many different signaling pathways and play important roles in genomic stability, cell polarity, tissue homeostasis and carcinogenesis. Mice with conventional knockout of Smurf1 or Smurf2 alleles are viable, but conventional Smurf1 and Smurf2 double knockout mice were early embryonic lethal. In order to study the physiological function of Smurfs during late stage of embryonic development or in adult animals, we generated Smurf2^flox/flox mice carrying a targeted mutation for conditional Smurf2 gene inactivation. We demonstrated that Cre-mediated recombination using Alb-Cre, a Cre line expressed in hepatocyte, results in specific deletion of the gene in liver tissue. We also showed that Cre-mediated recombination in mouse embryonic fibroblasts (MEFs) with Smurf2^flox/flox genotype resulted in generation of Smurf2 knockout MEFs, and Smurf2 deficiency affects multiple signaling pathways. Therefore, this animal model will be useful to study the distinct roles of Smurf2 in different tissues at different ages.

BMP Signalling at the Crossroad of Liver Fibrosis and Regeneration

Bone Morphogenetic Proteins (BMPs) belong to the Transforming Growth Factor-β (TGF-β) family. Initially identified due to their ability to induce bone formation, they are now known to have multiple functions in a variety of tissues, being critical not only during development for tissue morphogenesis and organogenesis but also during adult tissue homeostasis. This review focus on the liver as a target tissue for BMPs actions, devoting most efforts to summarize our knowledge on their recently recognized and/or emerging roles on regulation of the liver regenerative response to various insults, either acute or chronic and their effects on development and progression of liver fibrosis in different pathological conditions. In an attempt to provide the basis for guiding research efforts in this field both the more solid and more controversial areas of research were highlighted.

Mechanistic insight into contextual TGF-β signaling

Transforming growth factor-β (TGF-β) controls a wide range of cellular functions by activating both SMADs and non-SMAD pathways. In different tissue or physiological environment, cellular responses to TGF-β can be diverse, even opposite. Complex regulations at the level of ligand mobilization, receptor presentation, and the network of intracellular signal transducers afford the TGF-β pathway with versatile means to induce precise cellular responses in accordance to specific contextual demands. This article summarizes recent development in how cells manage their responses to TGF-β through ligand activation, receptor abundance, as well as SMAD-dependent and SMAD-independent mechanisms.

USP4 inhibits SMAD4 monoubiquitination and promotes activin and BMP signaling

SMAD4 is a common intracellular effector for TGF-β family cytokines, but the mechanism by which its activity is dynamically regulated is unclear. We demonstrated that ubiquitin-specific protease (USP) 4 strongly induces activin/BMP signaling by removing the inhibitory monoubiquitination from SMAD4. This modification was triggered by the recruitment of the E3 ligase, SMURF2, to SMAD4 following ligand-induced regulatory (R)-SMAD-SMAD4 complex formation. Whereas the interaction of the negative regulator c-SKI inhibits SMAD4 monoubiquitination, the ligand stimulates the recruitment of SMURF2 to the c-SKI-SMAD2 complex and triggers c-SKI ubiquitination and degradation. Thus, SMURF2 has a role in termination and initiation of TGF-β family signaling. An increase in monoubiquitinated SMAD4 in USP4-depleted mouse embryonic stem cells (mESCs) decreased both the BMP- and activin-induced changes in the embryonic stem cell fate. USP4 sustained SMAD4 activity during activin- and BMP-mediated morphogenic events in early zebrafish embryos. Moreover, zebrafish depleted of USP4 exhibited defective cell migration and slower coordinated cell movement known as epiboly, both of which could be rescued by SMAD4. Therefore, USP4 is a critical determinant of SMAD4 activity.