Smurf1 facilitates myogenic differentiation and antagonizes the bone morphogenetic protein-2-induced osteoblast conversion by targeting Smad5 for degradation

Unraveling IGFBP3-mediated m6A modification in fracture healing

BACKGROUND

This study investigates the role of IGFBP3-mediated m6A modification in regulating the miR-23a-3p/SMAD5 axis and its impact on fracture healing, aiming to provide insights into potential therapeutic targets.

METHODS

Utilizing fracture-related datasets, we identified m6A modification-related mRNA and predicted miR-23a-3p as a regulator of SMAD5. We established a mouse fracture healing model and conducted experiments, including Micro-CT, RT-qPCR, Alizarin Red staining, and Alkaline phosphatase (ALP) staining, to assess gene expression and osteogenic differentiation.

RESULTS

IGFBP3 emerged as a crucial player in fracture healing, stabilizing miR-23a-3p through m6A modification, leading to SMAD5 downregulation. This, in turn, inhibited osteogenic differentiation and delayed fracture healing. Inhibition of IGFBP3 partially reversed through SMAD5 inhibition, restoring osteogenic differentiation and fracture healing in vivo.

CONCLUSION

The IGFBP3/miR-23a-3p/SMAD5 axis plays a pivotal role in fracture healing, highlighting the relevance of m6A modification. IGFBP3's role in stabilizing miR-23a-3p expression through m6A modification offers a potential therapeutic target for enhancing fracture healing outcomes.

HSP90β chaperoning SMURF1-mediated LATS proteasomal degradation in the regulation of bone formation

Heat shock protein 90 (HSP90) molecular chaperone is responsible for the stabilization and biological activity of a diverse set of client proteins. We have previously demonstrated that inhibition of HSP90 by 17-Demethoxy-17-allyaminogeldanmycin (17-AAG) not only reverses the glucocorticoid-induced bone loss but also enhances the basal level of bone mass in mice. Here, we investigate the potential mechanism underlying HSP90-associated osteoblast differentiation and bone formation. Knockdown of HSP90β but not HSP90α or inhibition of HSP90 by 17-AAG or NVP-BEP800 negates the protein levels of large tumor suppressor (LATS), the core kinases of Hippo signaling, resulting in the inactivation of LATS and activation of Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), in the enhancement of osteoblastic differentiation. In contrast, genetic ablation of Lats1 in mesenchymal stem cells is sufficient to abolish the HSP90 inhibition-induced osteoblastic differentiation and bone formation. Mechanistically, HSP90β but not HSP90α chaperones and prevents the SMAD specific E3 ubiquitin protein ligase 1 (SMURF1)-mediated and ubiquitination-dependent LATS protein proteasomal degradation, whereas 17-AAG abolishes these effects of HSP90β. Thus, these results uncover the HSP90β chaperoning SMURF1-mediated LATS protein proteasomal degradation and the subsequent YAP/TAZ activation as a hitherto uncharacterized mechanism controlling osteoblastic differentiation and bone formation.

Smurf1 regulates ameloblast polarization by ubiquitination-mediated degradation of RhoA

Cell polarity is essential for ameloblast differentiation and enamel formation. Smurf1 can mediate cell polarization through ubiquitination degradation of specific substrates. But it remains unclear whether Smurf1 could regulate ameloblast polarity and the underlying mechanism. Here, immuno-fluorescence staining and RT-qPCR were applied to detect the expression of Smurf1 and F-actin. A mouse lower incisor defect model was constructed. Scanning electron microscope, rat lower incisor culture, western blot, wound healing assay and trans-well migration assay were performed to detect the influence of Smurf1 knockdown on ameloblast. IF double staining, western blot and co-immunoprecipitation were conducted to detect the interaction between Smurf1 and RhoA. The in vivo experiment was also performed. We found that Smurf1 was mainly expressed in the membrane and cell cortex of ameloblast, similar to F-actin. Smurf1 expression increased along ameloblast polarization and differentiation. After knocking down Smurf1, the cytoskeleton and cell morphology changed and the cell polarity was damaged. Smurf1 regulated ameloblast polarity through ubiquitination degradation of activated RhoA in vitro. Local knockdown of Smurf1 in rat lower incisor ameloblast resulted in ameloblast polarity loss, enamel matrix secretion disorder and chalky enamel, but RhoA inhibitor Y-27632 could reverse this effect. Collectively, Smurf1 could regulate the polarization of ameloblast through ubiquitination degradation of activated RhoA, which contributed to the knowledge of tooth development and provided new research ideas for cell polarity.

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.

TRAF4 Inhibits Bladder Cancer Progression by Promoting BMP/SMAD Signaling

Patients with bladder cancer often have a poor prognosis due to the highly invasive and metastatic characteristics of bladder cancer cells. Epithelial-to-mesenchymal transition (EMT) has been causally linked to bladder cancer invasion. The E3 ubiquitin ligase, tumor necrosis factor receptor-associated factor 4 (TRAF4) has been implicated as a tumor promoter in a wide range of cancers. In contrast, here we show that low TRAF4 expression is associated with poor overall survival in patients with bladder cancer. We show that the TRAF4 gene is epigenetically silenced and that ERK mediates TRAF4 phosphorylation, resulting in lower TRAF4 protein levels in bladder cancer cells. In addition, we demonstrate that TRAF4 is inversely correlated with an EMT gene signature/protein marker expression. Functionally, by manipulating TRAF4 expression, we show that TRAF4 regulates EMT genes and epithelial and invasive properties in bladder cancer cells. Transcriptomic analysis of dysregulated TRAF4 expression in bladder cancer cell lines revealed that high TRAF4 expression enhances the bone morphogenetic protein (BMP)/SMAD and inhibits the NF-κB signaling pathway. Mechanistically, we show that TRAF4 targets the E3 ubiquitin ligase SMURF1, a negative regulator of BMP/SMAD signaling, for proteasomal degradation in bladder cancer cells. This was corroborated in patient samples where TRAF4 positively correlates with phospho-SMAD1/5, and negatively correlates with phospho-NFκb-p65. Lastly, we show that genetic and pharmacologic inhibition of SMURF1 inhibits the migration of aggressive mesenchymal bladder cancer cells.

IMPLICATIONS

Our findings identify E3 ubiquitin ligase TRAF4 as a potential therapeutic target or biomarker for bladder cancer progression.

NEDD4 E3 Ligases: Functions and Mechanisms in Bone and Tooth

Protein ubiquitination is a precisely controlled enzymatic cascade reaction belonging to the post-translational modification of proteins. In this process, E3 ligases catalyze the binding of ubiquitin (Ub) to protein substrates and define specificity. The neuronally expressed developmentally down-regulated 4 (NEDD4) subfamily, belonging to the homology to E6APC terminus (HECT) class of E3 ligases, has recently emerged as an essential determinant of multiple cellular processes in different tissues, including bone and tooth. Here, we place special emphasis on the regulatory role of the NEDD4 subfamily in the molecular and cell biology of osteogenesis. We elucidate in detail the specific roles, downstream substrates, and upstream regulatory mechanisms of the NEDD4 subfamily. Further, we provide an overview of the involvement of E3 ligases and deubiquitinases in the development, repair, and regeneration of another mineralized tissue—tooth.

miR-9-5p promotes myogenic differentiation via the Dlx3/Myf5 axis

MicroRNAs play an important role in myogenic differentiation, they bind to target genes and regulate muscle formation. We previously found that miR-9-5p, which is related to bone formation, was increased over time during the process of myogenic differentiation. However, the mechanism by which miR-9-5p regulates myogenic differentiation remains largely unknown. In the present study, we first examined myotube formation and miR-9-5p, myogenesis-related genes including Dlx3, Myod1, Mef2c, Desmin, MyoG and Myf5 expression under myogenic induction. Then, we detected the expression of myogenic transcription factors after overexpression or knockdown of miR-9-5p or Dlx3 in the mouse premyoblast cell line C2C12 by qPCR, western blot and myotube formation under myogenic induction. A luciferase assay was performed to confirm the regulatory relationships between not only miR-9-5p and Dlx3 but also Dlx3 and its downstream gene, Myf5, which is an essential transcription factor of myogenic differentiation. The results showed that miR-9-5p promoted myogenic differentiation by increasing myogenic transcription factor expression and promoting myotube formation, but Dlx3 exerted the opposite effect. Moreover, the luciferase assay showed that miR-9-5p bound to the 3'UTR of Dlx3 and downregulated Dlx3 expression. Dlx3 in turn suppressed Myf5 expression by binding to the Myf5 promoter, ultimately inhibiting the process of myogenic differentiation. In conclusion, the miR-9-5p/Dlx3/Myf5 axis is a novel pathway for the regulation of myogenic differentiation, and can be a potential target to treat the diseases related to muscle dysfunction.

Bortezomib Rescues Ovariectomy-Induced Bone Loss via SMURF-Mediated Ubiquitination Pathway

A balance between bone formation by osteoblasts and bone resorption by osteoclasts is necessary to maintain bone health and homeostasis. As a cancer of plasma cells, multiple myeloma (MM) is accompanied with rapid bone loss and fragility fracture. Bortezomib has been used as a first-line for treating MM for decades. Recently, the potential protection of bortezomib on osteoporosis (OP) is reported; however, the specific mechanism involving bortezomib-mediated antiosteoporotic effect is undetermined. In the present study, we assessed the effects of in vitro bortezomib treatment on osteogenesis and osteoclastogenesis and the protective effect on bone loss in ovariectomized (OVX) mice. Our results indicated that bortezomib treatment increased osteogenic differentiation of MC3T3-E1 cells as evidenced by increased levels of matrix mineralization and osteoblast-specific markers. In bortezomib-treated bone marrow monocytes (BMMs), osteoclast differentiation was suppressed, substantiated by downregulated tartrate-resistant acid phosphatase- (TRAP-) positive multinucleated cells, areas of actin rings, pit formation, and osteoclast-specific genes. Mechanistically, bortezomib exerted a protective effect against OP through the Smad ubiquitination regulatory factor- (SMURF-) mediated ubiquitination pathway. Furthermore, in vivo intraperitoneal injection of bortezomib attenuated the bone microarchitecture in OVX mice. Accordingly, our findings corroborated that bortezomib might have future applications in the treatment of postmenopausal OP.

ASB2 is a novel E3 ligase of SMAD9 required for cardiogenesis

Cardiogenesis requires the orchestrated spatiotemporal tuning of BMP signalling upon the balance between induction and counter-acting suppression of the differentiation of the cardiac tissue. SMADs are key intracellular transducers and the selective degradation of SMADs by the ubiquitin-proteasome system is pivotal in the spatiotemporal tuning of BMP signalling. However, among three SMADs for BMP signalling, SMAD1/5/9, only the specific E3 ligase of SMAD9 remains poorly investigated. Here, we report for the first time that SMAD9, but not the other SMADs, is ubiquitylated by the E3 ligase ASB2 and targeted for proteasomal degradation. ASB2, as well as Smad9, is conserved among vertebrates. ASB2 expression was specific to the cardiac region from the very early stage of cardiac differentiation in embryogenesis of mouse. Knockdown of Asb2 in zebrafish resulted in a thinned ventricular wall and dilated ventricle, which were rescued by simultaneous knockdown of Smad9. Abundant Smad9 protein leads to dysregulated cardiac differentiation through a mechanism involving Tbx2, and the BMP signal conducted by Smad9 was downregulated under quantitative suppression of Smad9 by Asb2. Our findings demonstrate that ASB2 is the E3 ligase of SMAD9 and plays a pivotal role in cardiogenesis through regulating BMP signalling.

Molecular Analysis of Membrane Targeting by the C2 Domain of the E3 Ubiquitin Ligase Smurf1

SMAD ubiquitination regulatory factor 1 (Smurf1) is a Nedd4 family E3 ubiquitin ligase that regulates cell motility, polarity and TGFβ signaling. Smurf1 contains an N-terminal protein kinase C conserved 2 (C2) domain that targets cell membranes and is required for interactions with membrane-localized substrates such as RhoA. Here, we investigated the lipid-binding mechanism of Smurf1 C2, revealing a general affinity for anionic membranes in addition to a selective affinity for phosphoinositides (PIPs). We found that Smurf1 C2 localizes not only to the plasma membrane but also to negatively charged intracellular sites, acting as an anionic charge sensor and selective PIP-binding domain. Site-directed mutagenesis combined with docking/molecular dynamics simulations revealed that the Smurf1 C2 domain loop region primarily interacts with PIPs and cell membranes, as opposed to the β-surface cationic patch employed by other C2 domains. By depleting PIPs from the inner leaflet of the plasma membrane, we found that PIP binding is necessary for plasma membrane localization. Finally, we used a Smurf1 cellular ubiquitination assay to show that the amount of ubiquitin at the plasma membrane interface depends on the lipid-binding properties of Smurf1. This study shows the mechanism by which Smurf1 C2 targets membrane-based substrates and reveals a novel interaction for non-calcium-dependent C2 domains and membrane lipids.

A PINCH-1-Smurf1 signaling axis mediates mechano-regulation of BMPR2 and stem cell differentiation

Mechanical cues from extracellular matrix exert strong effects on stem cell differentiation. This study finds that a signaling axis consisting of PINCH-1, Smurf1, and BMPR2 senses mechanical signals from extracellular matrix and regulates BMP signaling and mesenchymal stem cell differentiation.

Mechano-environment plays multiple critical roles in the control of mesenchymal stem cell (MSC) fate decision, but the underlying signaling mechanisms remain undefined. We report here a signaling axis consisting of PINCH-1, SMAD specific E3 ubiquitin protein ligase 1 (Smurf1), and bone morphogenetic protein type 2 receptor (BMPR2) that links mechano-environment to MSC fate decision. PINCH-1 interacts with Smurf1, which inhibits the latter from interacting with BMPR2 and consequently suppresses BMPR2 degradation, resulting in augmented BMP signaling and MSC osteogenic differentiation (OD). Extracellular matrix (ECM) stiffening increases PINCH-1 level and consequently activates this signaling axis. Depletion of PINCH-1 blocks stiff ECM-induced BMP signaling and OD, whereas overexpression of PINCH-1 overrides signals from soft ECM and promotes OD. Finally, perturbation of either Smurf1 or BMPR2 expression is sufficient to block the effects of PINCH-1 on BMP signaling and MSC fate decision. Our findings delineate a key signaling mechanism through which mechano-environment controls BMPR2 level and MSC fate decision.

MicroRNA-125a inhibits tumorigenesis by targeting Smurf1 in colorectal carcinoma

Aberrant expression of microRNAs (miRNAs) may contribute to the initiation and development of multiple types of human cancer. Several miRNAs have been found to be strongly correlated with the diagnosis, progression, and prognosis of colorectal carcinoma (CRC), but the role of miR‐125a in CRC remains unclear. In the present study, the function of miR‐125a on the expression of Smad ubiquitin regulatory factor 1 (Smurf1) was investigated in vitro and in vivo. We verified that Smurf1 is a downstream target gene of miR‐125a and is involved in miR‐125a‐mediated regulation of CT26 cell (colon cancer cell) proliferation and migration. Overexpression of miR‐125a suppresses CT26 cell growth by inhibiting cell proliferation. Additionally, wound healing assays were performed to show that overexpression of miR‐125a significantly reduced CT26 cell migration, which was reversed by overexpression of Smurf1. In vivo, miR‐125a overexpression downregulated the expression of Ki67 and Smurf1, thus leading to a marked reduction in tumor growth. These results revealed that miR‐125a plays a critical role in CRC by directly targeting Smurf1, a finding that may facilitate the development of improved diagnostic and therapeutic techniques for CRC.

Bortezomib attenuates renal interstitial fibrosis in kidney transplantation via regulating the EMT induced by TNF-α-Smurf1-Akt-mTOR-P70S6K pathway

Allograft interstitial fibrosis was characterized by massive extracellular matrix deposition caused by activated fibroblasts and myofibroblasts. Epithelial‐mesenchymal transition (EMT) is recognized as an important source of myofibroblasts contributing to the pathogenesis of allograft interstitial fibrosis. Smad ubiquitination regulatory factor 1 (Smurf1) has been recently reported to be involved in the progression of EMT. Our study was to detect the effect of Bortezomib and Smurf1 in the EMT and allograft interstitial fibrosis. Biomarkers of EMT, as well as Smurf1, were examined in human proximal tubular epithelial cells (HK‐2) treated with tumour necrosis factor‐alpha (TNF‐α) in various doses or at various time points by Western Blotting or qRT‐PCR. We knockdown or overexpressed Smurf1 in HK‐2 cells. Furthermore, rat renal transplant model was established and intervened by Bortezomib. Allograft tissues from human and rats were also collected and prepared for HE, Masson's trichrome, immunohistochemical staining and western blotting assays. As a result, we found that TNF‐α significantly promoted the development of EMT in a time‐dependent and dose‐dependent manner through Smurf1/Akt/mTOR/P70S6K signalling pathway. More importantly, Bortezomib alleviated the progression of EMT and allograft interstitial fibrosis in vivo and in vitro by inhibiting the production of TNF‐α and expression of Smurf1. In conclusion, Smurf1 plays a critical role in the development of EMT induced by TNF‐α. Bortezomib can attenuate the Sumrf1‐mediated progression of EMT and renal allograft interstitial fibrosis, which could be suggested as a novel choice for the prevention and treatment of renal allograft interstitial fibrosis.

Non-proteolytic ubiquitin modification of PPARγ by Smurf1 protects the liver from steatosis

Nonalcoholic fatty liver disease (NAFLD) is characterized by abnormal accumulation of triglycerides (TG) in the liver and other metabolic syndrome symptoms, but its molecular genetic causes are not completely understood. Here, we show that mice deficient for ubiquitin ligase (E3) Smad ubiquitin regulatory factor 1 (Smurf1) spontaneously develop hepatic steatosis as they age and exhibit the exacerbated phenotype under a high-fat diet (HFD). Our data indicate that loss of Smurf1 up-regulates the expression of peroxisome proliferator-activated receptor γ (PPARγ) and its target genes involved in lipid synthesis and fatty acid uptake. We further show that PPARγ is a direct substrate of Smurf1-mediated non-proteolytic lysine 63 (K63)-linked ubiquitin modification that suppresses its transcriptional activity, and treatment of Smurf1-deficient mice with a PPARγ antagonist, GW9662, completely reversed the lipid accumulation in the liver. Finally, we demonstrate an inverse correlation of low SMURF1 expression to high body mass index (BMI) values in human patients, thus revealing a new role of SMURF1 in NAFLD pathogenesis.

Value of Osteoblast-Derived Exosomes in Bone Diseases

PURPOSE

The authors' purpose is to reveal the value of osteoblast-derived exosomes in bone diseases.

METHODS

Microvesicles from supernatants of mouse Mc3t3 were isolated by ultracentrifugation and then the authors presented the protein profile by proteomics analysis.

RESULTS

The authors detected a total number of 1536 proteins by mass spectrometry and found 172 proteins overlap with bone database. The Ingenuity Pathway Analysis shows network of "Skeletal and Muscular System Development and Function, Developmental Disorder, Hereditary Disorder" and pathway about osteogenesis. EFNB1 and transforming growth factor beta receptor 3 in the network, LRP6, bone morphogenetic protein receptor type-1, and SMURF1 in the pathway seemed to be valuable in the exosome research of related bone disease.

CONCLUSIONS

The authors' study unveiled the content of osteoblast-derived exosome and discussed valuable protein in it which might provide novel prospective in bone diseases research.

ZC4H2 stabilizes Smads to enhance BMP signalling, which is involved in neural development in Xenopus

Bone morphogenetic proteins (BMPs) play vital roles in regulating stem cell maintenance, differentiation and embryonic development. Intracellularly, BMP signalling is mediated by Smad proteins, which are regulated post-transcriptionally through reversible phosphorylation and ubiquitination. ZC4H2 is a small nuclear protein associated with intellectual disability and neural development in humans. Here, we report that ZC4H2 is highly expressed in the developing neural system and is involved in neural patterning and BMP signalling in Xenopus Knockdown of ZC4H2 led to expansion of the expression of the pan neural plate marker Sox2 in Xenopus embryos. In mammalian cells, ZC4H2 promotes BMP signalling and is involved in BMP regulated myogenic and osteogenic differentiation of mouse myoblast cells. Mechanistically, ZC4H2 binds and stabilizes Smad1 and Smad5 proteins through reducing their association with the Smurf ubiquitin ligases and thus their ubiquitination. We also found that a group of ZC4H2 mutations, which have been isolated in patients with intellectual disorders, showed weaker Smad-stabilizing activity, suggesting that the ZC4H2-Smad interaction might contribute to proper neural development in humans.

Uev1A facilitates osteosarcoma differentiation by promoting Smurf1-mediated Smad1 ubiquitination and degradation

Malignant bone tumor osteosarcoma (OS) displays high metastasis incidence and poor prognosis. Its stem cell properties could serve to explain tumor recurrence and resistance to conventional treatments. In this study, we identified UEV1A as a novel suppressor of OS. Elevated UEV1A diminishes stem cell properties of OS cells and drives them to terminal differentiation. Importantly, UEV1A-overexpressed OS cells delay proliferation and are more sensitive to chemotherapeutic agents than control cells. Uev1A appears to be involved in the BMP signaling pathway in which it collaborates with a ubiquitin E3 ligase Smurf1 to promote Smad1 degradation in a Ubc13-independent manner. Indeed, Smad1 is identified as a dominant downstream effector of Uev1A, which unravels the mechanism underlying Uev1A-orchestrated tumor suppression in OS. The above findings identify UEV1A as a potential OS tumor suppression gene, and shed lights to future OS diagnosis and treatment.

MicroRNA expression analysis during FK506-induced osteogenic differentiation in rat bone marrow stromal cells

FK506 (also known as tacrolimus) is a potent immunosuppressive agent that is widely used in the treatment of graft-rejection and autoimmune diseases. FK506 has attracted additional attention owing to its potential role in osteogenic differentiation and bone formation. MicroRNAs (miRNAs) have been demonstrated to serve important roles in the regulation of osteogenic differentiation; however, identification of specific miRNAs and their roles in regulating FK506-induced osteogenic differentiation have been poorly examined. In the present study, osteodifferentiation of rat bone marrow stromal cells (BMSCs) was induced with varying concentrations of FK506 (5–5,000 nM) for 3, 7 and 14 days. Differentially expressed miRNAs were profiled using miRNA array, verified by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and subjected to gene ontology (GO) term and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. Results from the present study identified a subset of miRNAs that were differentially expressed, of which five upregulated miRNAs (miR-106b-5p, miR-101b-3p, miR-193a-3p, miR-485-3p and miR-142-3p) and four downregulated miRNAs (miR-27a-3p, miR-207, miR-218a-2-3p and let-7a-5p) were confirmed by RT-qPCR. GO and KEGG analysis revealed that the predicted target genes of these miRNAs are involved in multiple biological processes and signaling pathways, including cell differentiation and the mitogen-activated protein kinase (MAPK) signaling pathway. Verification of the miRNA-target genes revealed that Smad5, Jagged 1 and MAPK9 were significantly upregulated, whereas Smad7, BMP and activin membrane-bound inhibitor, and dual-specificity phosphatase 2 were significantly downregulated during FK506-induced osteodifferentiation. The present study may provide an experimental basis for further research on miRNA functions during FK506-induced osteogenic differentiation in rat BMSCs.

The function and interrelationship between GDF5 and ERG-010 during chondrogenesis in vitro

Joint formation begins with the establishment of an interzone within the cartilaginous anlagen of the future skeleton. Both GDF5 and ERG are proposed as regulators of chondrocyte differentiation during and post interzone formation. The aim of this study was to examine the relationship between Gdf5 and Erg expression and downstream effects on chondrocyte gene expression. Erg expression was identified in mouse knee joints at E13.5. Expression analyses were performed using micromass cultures of murine C3H10T1/2 mesenchymal cells undergoing induced chondrogenesis in the presence and absence of GDF5 and ERG. At E13.5, Erg expression was found to surround epiphyseal chondrocytes and span the interzone up to the intermediate zone. Erg splice forms were expressed in micromass cultures, and their expression profile was altered by the addition of recombinant GDF5 depending on the stage of differentiation. Overexpression of Erg-010 resulted in a downregulation of Col2a1 and Col10a1. Microarray analysis following Erg-010 overexpression identified two potential downstream targets, Ube2b and Osr2, which were also differentially regulated by GDF5. Erg regulation by GDF5 in induced mesenchymal cells in vitro is dependent on the stage of chondrogenesis, and its expression in vivo demarcates chondrocytes that are not destined to be consumed by endochondral ossification. Functionally, Erg expression causes downregulation of Col2a1 and Col10a1 expression and this effect is potentially mediated by Osr2 and/or Ube2b. Combined, these data suggest a possible pathway linking GDF5, ERG and downstream factors in the processes of chondrocyte differentiation during articular joint formation.

Identification of C-terminal Hsp70-interacting protein as a mediator of tumour necrosis factor action in osteoblast differentiation by targeting osterix for degradation

In patients with inflammatory arthritis, tumour necrosis factor (TNF)-α are overproduced in inflamed joints. This leads to local erosion of cartilage and bone, periarticular osteopenia, as well as osteoporosis. But less is known regarding the molecular mechanisms that mediate the effect of TNF-α on osteoblast function. The purpose of this study was to test that C terminus of Hsc70-interacting protein (CHIP) has a specific role in suppressing the osteogenic activity of osteoblasts under inflammatory conditions. C2C12, MC3T3-E1 and HEK293T cell lines were cultured and cotransfected with related plasmids. After transfection, the cells were cultured further in the presence or absence of murine TNF-α and subjected to real time RT-PCR, Western blot, Ubiquitination assay, Co-immunoprecipitation, Luciferase reporter assay, Small interfering RNAs and Mineralization assay. The expression levels of TNF-α-induced CHIP and Osx were examined by RT-PCR and Western blot analysis. Co-immunoprecipitation and ubiquitination assays revealed ubiquitinated Osx, confirmed that CHIP indeed interacted with Osx and identified K55 and K386 residues as the ubiquitination sites in Osx, Luciferase reporter assay and Small interfering RNAs examined whether TNF-α target the bone morphogenetic protein signalling through CHIP. We established stable cell lines with the overexpression of HA-CHIP, Mineralization assay and CHIP siRNA demonstrated the important roles of CHIP on osteoblast function in conditions in which TNF-α is overexpressed. We found that the K55 and K386 residues are ubiquitination site(s) in Osx, and that TNF-α inhibits osteoblast differentiation by promoting Osx degradation through up-regulation of E3 ubiquitin ligase CHIP in osteoblast. Thus, CHIP targets Osx for ubiquitination and degradation in osteoblasts after chronic exposure to TNF-α, and inhibition of CHIP expression in osteoblasts may be a new mechanism to limit inflammation-mediated osteoporosis by promoting their differentiation into osteoblasts.

Strategies to minimize hypertrophy in cartilage engineering and regeneration

Due to a blood supply shortage, articular cartilage has a limited capacity for self-healing once damaged. Articular chondrocytes, cartilage progenitor cells, embryonic stem cells, and mesenchymal stem cells are candidate cells for cartilage regeneration. Significant current attention is paid to improving chondrogenic differentiation capacity; unfortunately, the potential chondrogenic hypertrophy of differentiated cells is largely overlooked. Consequently, the engineered tissue is actually a transient cartilage rather than a permanent one. The development of hypertrophic cartilage ends with the onset of endochondral bone formation which has inferior mechanical properties. In this review, current strategies for inhibition of chondrogenic hypertrophy are comprehensively summarized; the impact of cell source options is discussed; and potential mechanisms underlying these strategies are also categorized. This paper aims to provide guidelines for the prevention of hypertrophy in the regeneration of cartilage tissue. This knowledge may also facilitate the retardation of osteophytes in the treatment of osteoarthritis.

Cross-talk between redox regulation and the ubiquitin-proteasome system in mammalian cell differentiation

BACKGROUND

Embryogenesis and stem cell differentiation are complex and orchestrated signaling processes. Reactive oxygen species (ROS) act as essential signal transducers in cellular differentiation, as has been shown through recent discoveries. On the other hand, the ubiquitin-proteasome system (UPS) has long been known to play an important role in all cellular regulated processes, including differentiation.

SCOPE OF REVIEW

In the present review, we focus on findings that highlight the interplay between redox signaling and the UPS regarding cell differentiation. Through systems biology analyses, we highlight major routes during cardiomyocyte differentiation based on redox signaling and UPS modulation.

MAJOR CONCLUSION

Oxygen availability and redox signaling are fundamental regulators of cell fate upon differentiation. The UPS plays an important role in the maintenance of pluripotency and the triggering of differentiation.

GENERAL SIGNIFICANCE

Cellular differentiation has been a matter of intense investigation mainly because of its potential therapeutic applications. Understanding regulatory mechanisms underlying cell differentiation is an important issue. Correspondingly, the role of UPS and regulation of redox processes have been emerged as essential factors to control the fate of cells upon differentiation. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Bradykinin regulates osteoblast differentiation by Akt/ERK/NFκB signaling axis

Bradykinin (BK), a well known mediator of pain and inflammation, is also known to be involved in the process of bone resorption. The present study therefore evaluated the role of BK in osteoblast lineage commitment. Our data showed that BK inhibits the migration of bone marrow mesenchymal stem cells, but does not affect their viability. Moreover, BK also inhibits osteoblastic differentiation by significantly downregulating the levels of mRNAs for osteopontin, runX2, col24, osterix, osteocalcin genes and bone mineralization (P < 0.05). Further, BK was found to elicit the BK receptors (BDKR1 and BDKR2) mediated activation of ERK1/2 and Akt pathways, which finally led to the activation of NFκB. BK also promoted the osteoclast differentiation of bone marrow derived preosteoclast cells by upregulating the expression of c-fos, NFATC1, TRAP, clcn7, cathK, and OSCAR genes and increasing TRAP activity through NFκB pathway. In conclusion, our data suggest that BK decreases the differentiation of osteoblasts with concomitant increase in osteoclast formation and thus provides new insight into the mechanism of action of BK in modulating bone resorption.

Significance of expression of Smad ubiquitination regulatory factors in liver fibrosis

AIM: To observe the expression of Smad ubiquitination regulatory factor (Smurf)1, Smurf2, Smad3 and Smad7 proteins in liver fibrosis and to evaluate their interactions.

METHODS: Immunohistochemistry was applied to detect the expression of Smurf1, Smurf2, Smad3 and Smad7 in 9 normal liver tissue samples and 38 chronic HBV infection tissue samples.

RESULTS: Smurf1, Smurf2, Smad3 and Smad7 showed widespread expression in the liver parenchymal cells and nonparenchymal cells. Compared with normal liver tissue, the positive rates of Smad3 and Smurf2 expression increased significantly (66.7% vs 100%, 66.7% vs 92.1%, P < 0.01 for both) and the positive rate of Smad7 expression decreased significantly in liver fibrosis (77.8% vs 39.5%, P < 0.01), although the positive rate of Smurf1 expression had no significant change (77.8% vs 63.2%, P > 0.05). There were significant positive correlations between Smad3 and Smurf2 expression and the degree of fibrosis (P < 0.01 for both). Smad7 expression was negatively correlated with the degree of fibrosis (P < 0.01). There was no significant correlation between Smurf1 expression and liver fibrosis (P > 0.05). Smurf2 expression was positively correlated with Smad3 expression (P < 0.01) and negatively with Smad7 expression (P < 0.01). There was no significant relationship between Smurf1 and Smurf2, Smad3, Smad7 expression (P > 0.05 for all). There was a negative correlation between expression of Smad3 and that of Smad7 (P < 0.01).

CONCLUSION: The increase of Smad3 signal and decrease of Smad7 signal may lead to the development of liver fibrosis, and Smurf2 may play a bidirectionally regulatory role in the progression of liver fibrosis.

Selective small molecule compounds increase BMP-2 responsiveness by inhibiting Smurf1-mediated Smad1/5 degradation

The ubiquitin ligase Smad ubiquitination regulatory factor-1 (Smurf1) negatively regulates bone morphogenetic protein (BMP) pathway by ubiquitinating certain signal components for degradation. Thus, it can be an eligible pharmacological target for increasing BMP signal responsiveness. We established a strategy to discover small molecule compounds that block the WW1 domain of Smurf1 from interacting with Smad1/5 by structure based virtual screening, molecular experimental examination and cytological efficacy evaluation. Our selected hits could reserve the protein level of Smad1/5 from degradation by interrupting Smurf1-Smad1/5 interaction and inhibiting Smurf1 mediated ubiquitination of Smad1/5. Further, these compounds increased BMP-2 signal responsiveness and the expression of certain downstream genes, enhanced the osteoblastic activity of myoblasts and osteoblasts. Our work indicates targeting Smurf1 for inhibition could be an accessible strategy to discover BMP-sensitizers that might be applied in future clinical treatments of bone disorders such as osteopenia.

Regulation of the transforming growth factor β pathway by reversible ubiquitylation

The transforming growth factor β (TGFβ) signalling pathway plays a central role during embryonic development and in adult tissue homeostasis. It regulates gene transcription through a signalling cascade from cell surface receptors to intracellular SMAD transcription factors and their nuclear cofactors. The extent, duration and potency of signalling in response to TGFβ cytokines are intricately regulated by complex biochemical processes. The corruption of these regulatory processes results in aberrant TGFβ signalling and leads to numerous human diseases, including cancer. Reversible ubiquitylation of pathway components is a key regulatory process that plays a critical role in ensuring a balanced response to TGFβ signals. Many studies have investigated the mechanisms by which various E3 ubiquitin ligases regulate the turnover and activity of TGFβ pathway components by ubiquitylation. Moreover, recent studies have shed new light into their regulation by deubiquitylating enzymes. In this report, we provide an overview of current understanding of the regulation of TGFβ signalling by E3 ubiquitin ligases and deubiquitylases.

Ubiquitination of tumor necrosis factor receptor-associated factor 4 (TRAF4) by Smad ubiquitination regulatory factor 1 (Smurf1) regulates motility of breast epithelial and cancer cells

Smad ubiquitin regulatory factors (Smurfs) are HECT-domain ubiquitin E3 ligases that regulate diverse cellular processes, including normal and tumor cell migration. However, the underlying mechanism of the Smurfs' role in cell migration is not fully understood. Here we show that Smurf1 induces ubiquitination of tumor necrosis factor receptor-associated factor 4 (TRAF4) at K190. Using the K190R mutant of TRAF4, we demonstrate that Smurf1-induced ubiquitination is required for proper localization of TRAF4 to tight junctions in confluent epithelial cells. We further show that TRAF4 is essential for the migration of both normal mammary epithelial and breast cancer cells. The ability of TRAF4 to promote cell migration is also dependent on Smurf1-mediated ubiquitination, which is associated with Rac1 activation by TRAF4. These results reveal a new regulatory circuit for cell migration, consisting of Smurf1-mediated ubiquitination of TRAF4 and Rac1 activation.

Tumor necrosis factor-α enhances the transcription of Smad ubiquitination regulatory factor 1 in an activating protein-1- and Runx2-dependent manner

Smad ubiquitination regulatory factor 1 (Smurf1) is an E3 ubiquitin ligase that negatively regulates osteoblast differentiation. Although tumor necrosis factor-α (TNF-α) has been shown to increase Smurf1 expression, the details of the regulatory mechanisms remain unclear. Here, we investigated the molecular mechanism by which TNF-α stimulates Smurf1 expression in C2C12 and primary cultured mouse calvarial cells. TNF-α treatment rapidly induced the activation of NF-κB and MAPKs. Smurf1 induction by TNF-α was blocked by the inhibition of JNK or ERK, while the inhibition of NF-κB and p38 MAPK had no effect on Smurf1 induction. TNF-α treatment or c-Jun overexpression enhanced the activity of a luciferase reporter that contained a 2.7 kb mouse Smurf1 promoter sequence. Site-directed mutagenesis of the Smurf1 reporter and chromatin immunoprecipitation analysis demonstrated that the activating protein-1 (AP-1) binding motif at -922 bp on the mouse Smurf1 promoter mediated TNF-α/JNK/AP-1-stimulated Smurf1 transcription. Interestingly, Smurf1 expression was not observed in Runx2-null mouse calvarial cells. When Runx2 was ectopically expressed in these cells, the basal and TNF-α-induced expression of Smurf1 was restored. Overexpression of Runx2 transactivated the Smurf1 promoter in a dose-dependent manner. Reporter and chromatin immunoprecipitation assays demonstrated that the Runx2-binding motif at -202 bp functioned in Runx2-mediated Smurf1 expression. ERK activation by TNF-α treatment or constitutively active MEK1 overexpression increased Smurf1 expression in a Runx2-dependent manner. These results suggest that the JNK/AP-1 and ERK/Runx2 signaling pathways mediate TNF-α-dependent Smurf1 transcription.

E3 ubiquitin ligases in regulating stress fiber, lamellipodium, and focal adhesion dynamics

Recent discoveries have unveiled the roles of a complicated network of E3 ubiquitin ligases in regulating cell migration machineries. The E3 ubiquitin ligases Smurf1 and Cul/BACURD ubiquitinate RhoA to regulate stress fiber formation and cell polarity, and ASB2α ubiquitinates filamins to modulate cytoskeletal stiffness, thus regulating cell spreading and cell migration. HACE1, XIAP, and Skp1-Cul1-F-box bind to Rac1 and cause its ubiquitination and degradation, thus suppressing lamellipodium protrusions, while PIAS3, a SUMO ligase, activates Rac1 to promote lamellipodium dynamics. Smurf1 also enhances Rac1 activation but it does not ubiquitinate Rac1. Both Smurf1 and HECTD1 regulate focal adhesion (FA) assembly and (or) disassembly through ubiquitinating the talin head domain and phosphatidylinositol 4 phosphate 5-kinase type I γ (PIPKIγ90), respectively. Thus, E3 ubiquitin ligases regulate stress fiber formation, cell polarity, lamellipodium protrusions, and FA dynamics through ubiquitinating the key proteins that control these processes.

Smurf E3 ubiquitin ligases at the cross roads of oncogenesis and tumor suppression

Smad ubiquitin regulatory factors (Smurfs) belong to the HECT- family of E3 ubiquitin ligases and comprise mainly of two members, Smurf1 and Smurf2. Initially, Smurfs have been implicated in determining the competence of cells to respond to TGF-β/BMP signaling pathway. Nevertheless, the intrinsic catalytic activity has extended the repertoire of Smurf substrates beyond the TGF-β/BMP super family expanding its realm further to epigenetic modifications of histones governing the chromatin landscape. Through regulation of a large number of proteins in multiple cellular compartments, Smurfs regulate diverse cellular processes, including cell-cycle progression, cell proliferation, differentiation, DNA damage response, maintenance of genomic stability, and metastasis. As the genomic ablation of Smurfs leads to global changes in histone modifications and predisposition to a wide spectrum of tumors, Smurfs are also considered to have a novel tumor suppressor function. This review focuses on regulation network and biological functions of Smurfs in connection with its role in cancer progression. By providing a portrait of their protein targets, we intend to link the substrate specificity of Smurfs with their contribution to tumorigenesis. Since the regulation and biological functions of Smurfs are quite complex, understanding the oncogenic potential of these E3 ubiquitin ligases may facilitate the development of mechanism-based drugs in cancer treatment.

Functional redundancy of type II BMP receptor and type IIB activin receptor in BMP2-induced osteoblast differentiation

Signaling pathways for bone morphogenetic proteins (BMPs) are important in osteoblast differentiation. Although the precise function of type I BMP receptors in mediating BMP signaling for osteoblast differentiation and bone formation has been characterized previously, the role of type II BMP receptors in osteoblasts is to be well clarified. In this study, we investigated the role of type II BMP receptor (BMPR-II) and type IIB activin receptor (ActR-IIB) in BMP2-induced osteoblast differentiation. While osteoblastic 2T3 cells expressed BMPR-II and ActR-IIB, loss-of-function studies, using dominant negative receptors and siRNAs, showed that BMPR-II and ActR-IIB compensated each other functionally in mediating BMP2 signaling and BMP2-induced osteoblast differentiation. This was evidenced by two findings. First, unless there was loss of function of both type II receptors, isolated disruption of either BMPR-II or ActR-IIB did not remove BMP2 activity. Second, in cells with loss of function of both receptors, restoration of function of either BMPR-II or ActR-IIB by transfection of the wild-type forms, restored BMP2 activity. These findings suggest a functional redundancy between BMPR-II and ActR-IIB in osteoblast differentiation. Results from experiments to test the effects of transforming growth factor β (TGF-β), activin, and fibroblast growth factor (FGF) on osteoblast proliferation and differentiation suggest that inhibition of receptor signaling by double-blockage of BMPR-II and ActR-IIB is BMP-signaling specific. The observed functional redundancy of type II BMP receptors in osteoblasts is novel information about the BMP signaling pathway essential for initiating osteoblast differentiation.

Ablation of Smurf2 reveals an inhibition in TGF-β signalling through multiple mono-ubiquitination of Smad3

TGF-β signalling is regulated by post-translational modifications of Smad proteins to translate quantitative difference in ligand concentration into proportional transcriptional output. Previous studies in cell culture systems suggested that Smad ubiquitination regulatory factors (Smurfs) act in this regulation by targeting Smads for proteasomal degradation, but whether this mechanism operates under physiological conditions is not clear. Here, we generated mice harbouring a target-disrupted Smurf2 allele. Using primary mouse embryonic fibroblasts and dermal fibroblasts, we show that TGF-β-mediated, Smad-dependent transcriptional responses are elevated in the absence of Smurf2. Instead of promoting poly-ubiquitination and degradation, we show that Smurf2 actually induces multiple mono-ubiquitination of Smad3 in vivo. Phosphorylation of T179, immediately upstream of the Smad3 PY motif, enhances Smurf2 and Smad3 interaction and Smad3 ubiquitination. We have mapped Smurf2-induced Smad3 ubiquitination sites to lysine residues at the MH2 domain, and demonstrate that Smad3 ubiquitination inhibits the formation of Smad3 complexes. Thus, our data support a model in which Smurf2 negatively regulates TGF-β signalling by attenuating the activity of Smad3 rather than promoting its degradation.

Pivotal role of the C2 domain of the Smurf1 ubiquitin ligase in substrate selection

The C2-WW-HECT-type ubiquitin ligases Smurf1 and Smurf2 play a critical role in embryogenesis and adult bone homeostasis via regulation of bone morphogenetic protein, Wnt, and RhoA signaling pathways. The intramolecular interaction between C2 and HECT domains autoinhibits the ligase activity of Smurf2. However, the role of the Smurf1 C2 domain remains elusive. Here, we show that the C2-HECT autoinhibition mechanism is not observed in Smurf1, and instead its C2 domain functions in substrate selection. The Smurf1 C2 domain exerts a key role in localization to the plasma membrane and endows Smurf1 with differential activity toward RhoA versus Smad5 and Runx2. Crystal structure analysis reveals that the Smurf1 C2 domain possesses a typical anti-parallel β-sandwich fold. Examination of the sulfate-binding site analysis reveals two key lysine residues, Lys-28 and Lys-85, within the C2 domain that are important for Smurf1 localization at the plasma membrane, regulation on cell migration, and robust ligase activity toward RhoA, which further supports a Ca(2+)-independent localization mechanism for Smurf1. These findings demonstrate a previously unidentified role of the Smurf1 C2 domain in substrate selection and cellular localization.

Involvement of BMPs/Smad signaling pathway in mechanical response in osteoblasts

BACKGROUND/AIMS

Mechanical strain plays an important role in osteoblasts differentiation and bone formation but the underlying mechanism remains unclear. The aim of this study was to determine whether Bone Morphogenetic Proteins (BMPs)/Smad signaling pathway is involved in mechanical response in osteoblasts.

METHODS

MC3T3-E1 cells were exposed to mechanical strain via a four-point bending system. mRNA levels and protein levels of BMP-2, BMP-4, Smad1, Smad5, Smurf1, and Smurf2 were assessed using RT-PCR and immunoblotting. Protein levels of BMP-2 and BMP-4 in the culture medium were also determined using Enzyme-linked Immunosorbent Assay (ELISA). Pretreatment with Noggin and transfection with Smad4 siRNA were carried out to block the BMPs/Smad signaling pathway and MG132 was used to inhibit the proteasome pathway.

RESULTS

We found that mechanical strain enhanced alkaline phosphatase (ALP) expression and activated BMPs/Smad signaling pathway. Mechanical strain induced expression of ALP was attenuated by Noggin and by Smad4 siRNA. The protein levels of Smad1 and Smad5, but not their mRNA levels, were up-regulated by mechanical strain. This finding could be explained by the down-regulation of Smurf1. The protein degradation of Smad might be inhibited by mechanical strain through down-regulation of Smuf1 expression. The addition of MG132 further enhanced the mechanical strain induced activation of Smad proteins and the increased expression of ALP.

CONCLUSIONS

Mechanical strain might promote osteoblasts differentiation through BMPs/Smad signaling pathway. The strain causes a drop in Smurf1 levels, leading to accumulation of Smad proteins and, subsequently, to enhanced BMPs/Smad signaling.

Smurf control in bone cells

The homologous to the E6-associated protein carboxyl terminus (HECT) domain E3 ubiquitin ligase Smurf1 is the first E3 ligase to be implicated in regulating bone cell function. The involvement of Smurf1 in multiple signaling pathways and pathological conditions is presently an area of extensive scientific interest. This review highlights recent works exploring Smurf-regulated biological processes in bone cells and highlights recent discoveries surrounding the regulatory mechanisms modulating its catalytic activity and substrate recognition capability. Moreover, we discuss the relevance of targeting the HECT E3s through the development of small-molecule inhibitors as an anticancer therapeutic strategy.

MicroRNA-214 promotes myogenic differentiation by facilitating exit from mitosis via down-regulation of proto-oncogene N-ras

Vertebrate muscle differentiation is coordinated by an intricate network of transcription factors requiring proliferating myogenic precursors to withdraw irreversibly from the cell cycle. Recent studies have implicated a large number of microRNAs exerting another layer of control in many aspects of muscle differentiation. By annealing to short recognition sequences in the 3'-untranslated region, microRNAs attenuate target gene expression through translation repression or mRNA degradation. Here, we show that miR-214 promotes myogenic differentiation in mouse C2C12 myoblasts at a step preceding the induction of p21 and myogenin. Blocking miR-214 function with a 2'-O-methylated double-stranded inhibitor maintained C2C12 cells in the active cell cycle, thereby inhibiting the myogenic differentiation. By global gene expression profiling, we identified the proto-oncogene N-ras as one of miR-214 targets. Furthermore, manipulating the N-Ras level with small interfering RNA or adenovirus-mediated forced expression either augmented or attenuated the effect of miR-214, respectively. Thus, our data uncovered a novel microRNA-mediated mechanism that controls myogenic differentiation.

Smurf1 inhibits mesenchymal stem cell proliferation and differentiation into osteoblasts through JunB degradation

Ubiquitin ligase Smurf1-deficient mice develop an increased-bone-mass phenotype in an age-dependent manner. It was reported that such a bone-mass increase is related to enhanced activities of differentiated osteoblasts. Although osteoblasts are of mesenchymal stem cell (MSC) origin and MSC proliferation and differentiation can have significant impacts on bone formation, it remains largely unknown whether regulation of MSCs plays a role in the bone-mass increase of Smurf1-deficient mice. In this study we found that bone marrow mesenchymal progenitor cells from Smurf1(-/-) mice form significantly increased alkaline phosphatase-positive colonies, indicating roles of MSC proliferation and differentiation in bone-mass accrual of Smurf1(-/-) mice. Interestingly, Smurf1(-/-) cells have an elevated protein level of AP-1 transcription factor JunB. Biochemical experiments demonstrate that Smurf1 interacts with JunB through the PY motif and targets JunB protein for ubiquitination and proteasomal degradation. Indeed, Smurf1-deficient MSCs have higher proliferation rates, consistent with the facts that cyclin D1 mRNA and protein both are increased in Smurf1(-/-) cells and JunB can induce cyclinD1 promoter. Moreover, JunB overexpression induces osteoblast differentiation, shown by higher expression of osteoblast markers, and JunB knock-down not only decreases osteoblast differentiation but also restores the osteogenic potential to wild-type level in Smurf1(-/-) cells. In conclusion, our results suggest that Smurf1 negatively regulates MSC proliferation and differentiation by controlling JunB turnover through an ubiquitin-proteasome pathway.

Protein phosphatase magnesium-dependent 1A-mediated inhibition of BMP signaling is independent of Smad dephosphorylation

Phosphorylation of Smad1/5/8 at carboxyl-terminal serine residues by type I receptors activates downstream bone morphogenetic protein (BMP) signaling. Protein phosphatase magnesium-dependent 1A (PPM1A) has been shown to suppress BMP activity by dephosphorylating phospho-Smads. We report here that PPM1A suppresses BMP signaling via a novel mechanism. PPM1A inhibited a constitutively activated Smad1 mutant lacking BMP receptor phosphorylation sites. PPM1A reduced the protein levels not only of Smad1 but also of Smad5 and Smad8. A proteasome inhibitor blocked the inhibitory effects of PPM1A on Smad1, but the Smurf-binding motif in the Smad1 linker region was not involved in this inhibition. The phosphatase activity of PPM1A is essential for inhibition. Taken together, these findings suggest that through the dephosphorylation of unidentified substrate(s), PPM1A inhibits BMP signaling by decreasing Smad protein levels via the proteasome pathway. Moreover, knockdown of endogenous PPM1A stimulated osteoblastic differentiation, suggesting that PPM1A may physiologically suppress BMP signaling via Smads.

Development and optimization of a cell-based assay for the selection of synthetic compounds that potentiate bone morphogenetic protein-2 activity

The requirement of large amounts of the recombinant human bone morphogenetic protein-2 (BMP-2) produces a huge translational barrier for its routine clinical use due to high cost. This leads to an urgent need to develop alternative methods to lower costs and/or increase efficacies for using BMP-2. In this study, we describe the development and optimization of a cell-based assay that is sensitive, reproducible, and reliable in identifying reagents that potentiate the effects of BMP-2 in inducing transdifferentiation of C2C12 myoblasts into the osteoblastic phenotype. The assay is based on a BMP-responsive Smad1-driven luciferase reporter gene. LIM mineralization protein-1 (LMP-1) is a novel intracellular LIM domain protein that has been shown by our group to enhance cellular responsiveness to BMP-2. Our previous report elucidated that the binding of LMP-1 with the WW2 domain in Smad ubiquitin regulatory factor-1 (Smurf1) rescues the osteogenic Smads from degradation. Here, using the optimized cell-based assay, we first evaluated the activity of the recombinantly prepared proteins, LMP-1, and its mutant (LMP-1DeltaSmurf1) that lacks the Smurf1-WW2 domain-binding motif. Both the wild type and the mutant proteins were engineered to contain an 11-amino acid HIV-TAT protein derived membrane transduction domain to aid the cellular delivery of recombinant proteins. The cell-based reporter assay confirmed that LMP-1 potentiates the BMP-induced stimulation of C2C12 cells towards the osteoblastic phenotype. The potentiating effect of LMP-1 was significantly reduced when a specific-motif known to interact with Smurf1 was mutated. We validated the results obtained in the reporter assay by also monitoring the expression of mRNA for osteocalcin and alkaline phosphatase (ALP) which is widely accepted osteoblast differentiation marker genes. Finally, we provide further confirmation of our results by measuring the activity of alkaline phosphatase in support of the accuracy and reliability of our cell-based assay. Direct delivery of synthesized protein can be limited by high cost, instability or inadequate post-translational modifications. Thus, there would be a clear benefit for a low cost, cell penetrable chemical compound. We successfully used our gene expression-based assay to choose an active compound from a select group of compounds that were identified by computational screenings as the most likely candidates for mimicking the function of LMP-1. Among them, we selected SVAK-3, a compound that showed a dose-dependent potentiation of BMP-2 activity in inducing osteoblastic differentiation of C2C12 cells. We show that either the full length LMP-1 protein or its potential mimetic compound consistently exhibit similar potentiation of BMP-2 activity even when multiple markers of the osteoblastic phenotype were parallely monitored.

Roles of E3 ubiquitin ligases in cell adhesion and migration

Recent studies have demonstrated that a number of E3 ubiquitin ligases, including Cbl, Smurf1, Smurf2, HDM2, BCA2, SCF(beta-TRCP) and XRNF185, play important roles in cell adhesion and migration. Cbl negatively regulates cell adhesion via alpha integrin and Rap1 and inhibits actin polymerization by ubiquitinating mDab1 and WAVE2. Smurf1 regulates cell migration through ubiquitination of RhoA, talin head domain and hPEM2, while Smurf2 ubiquitinates Smurf1, TGFbeta type I receptor and RaplB to modulate cell migration and adhesion. HDM2 negatively regulates cell migration by targeting NFAT (a transcription factor) for ubiquitination and degradation, while SCF(beta-TRCP) ubiquitinates Snail (a transcriptional repressor of E-cadherin) to inhibit cell migration. TRIM32 promotes cell migration through ubiquitination of Abl interactor 2 (Abi2), a tumor suppressor. RNF5 and XRNF185 modulate cell migration by ubiquitinating paxillin. Thus, these E3 ubiquitin ligases regulate cell adhesion and (or) migration through ubiquitination of their specific substrates.

TGF-beta signaling in chondrocyte terminal differentiation and osteoarthritis: modulation and integration of signaling pathways through receptor-Smads

OBJECTIVE

Chondrocytes and alteration in chondrocyte differentiation play a central role in osteoarthritis. Chondrocyte differentiation is amongst others regulated by members of the transforming growth factor-beta (TGF-beta) superfamily. The major intracellular signaling routes of this family are via the receptor-Smads. This review is focused on the modulation of receptor-Smad signaling and how this modulation can affect chondrocyte differentiation and potentially osteoarthritis development.

METHODS

Peer reviewed publications published prior to April 2009 were searched in the Pubmed database. Articles that were relevant for the role of TGF-beta superfamily/Smad signaling in chondrocyte differentiation and for differential modulation of receptor-Smads were selected.

RESULTS

Chondrocyte terminal differentiation is stimulated by Smad1/5/8 activation and inhibited the by Smad2/3 pathway, most likely by modulation of Runx2 function. Several proteins and signaling pathways differentially affect Smad1/5/8 and Smad2/3 signaling. This will result in an altered Smad1/5/8 and Smad2/3 balance and subsequently have an effect on chondrocyte differentiation and osteoarthritis development.

CONCLUSION

Modulation of receptor-Smads signaling can be expect to play an essential role in both the regulation of chondrocyte differentiation and osteoarthritis development and progression.

Proteomic analysis of sera of asymptomatic, early-stage patients with Wilson's disease

Wilson's disease (WD) is characterized by excessive accumulation of intracellular copper in liver and extrahepatic tissues, leading to significant oxidative stress and tissue damage. To date, several diagnostic biomarkers for WD such as serum ceruloplasmin, serum or urine copper levels and copper content in liver have been identified. However, these biomarkers may not be convincing for the diagnosis in some WD patients. To identify additional novel diagnostic biomarkers, we compared the serum protein profiles of asymptomatic childhood WD patients (n=20), without neurologic manifestation or liver cirrhosis, with normal controls (n=13). Fourteen spots, five up-regulated and nine down-regulated (>2-fold), were differentially expressed in WD patients in comparison to normal control on 2-DE. Among them, three spots were down-regulated in both male and female WD. MS/MS analysis revealed that the three spots were complement component C3, complement factor B and alpha-2 macroglobulin. By comparative proteome analysis, complement component C3, complement factor B and alpha-2 macroglobulin, which are related to oxidative stress and inflammation, turned out to be good candidates for novel diagnostic biomarkers for early stages of WD.

Tumor necrosis factor-receptor-associated factor-4 is a positive regulator of transforming growth factor-beta signaling that affects neural crest formation

The transforming growth factor (TGF)-beta superfamily regulates cell proliferation, apoptosis, differentiation, migration, and development. Canonical TGFbeta signals are transduced to the nucleus via Smads in both major signaling branches, bone morphogenetic protein (BMP) or Activin/Nodal/TGFbeta. Smurf ubiquitin (Ub) ligases attenuate these pathways by targeting Smads and other signaling components for degradation by the 26S proteasome. Here, we identify tumor necrosis factor (TNF)-receptor-associated factor-4 (TRAF4) as a new target of Smurf1, which polyubiquitylates TRAF4 to trigger its proteasomal destruction. Unlike other TRAF family members, which mediate signal transduction by TNF, interleukin, or Toll-like receptors, we find that TRAF4 potentiates BMP and Nodal signaling. In the frog Xenopus laevis, TRAF4 mRNA is stored maternally in the egg animal pole, and in the embryo it is expressed in the gastrula marginal zone, neural plate, and cranial and trunk neural crest. Knockdown of embryonic TRAF4 impairs signaling, neural crest development and neural folding, whereas TRAF4 overexpression boosts signaling and expands the neural crest. In human embryonic kidney 293 cells, small interfering RNA knockdown of Smurf1 elevates TRAF4 levels, indicating endogenous regulation of TRAF4 by Smurf1. Our results uncover new functions for TRAF4 as a Smurf1-regulated mediator of BMP and Nodal signaling that are essential for neural crest development and neural plate morphogenesis.

A negative feedback control of transforming growth factor-beta signaling by glycogen synthase kinase 3-mediated Smad3 linker phosphorylation at Ser-204

Through the action of its membrane-bound type I receptor, transforming growth factor-beta (TGF-beta) elicits a wide range of cellular responses that regulate cell proliferation, differentiation, and apo ptosis. Many of these signaling responses are mediated by Smad proteins. As such, controlling Smad activity is crucial for proper signaling by TGF-beta and its related factors. Here, we show that TGF-beta induces phosphorylation at three sites in the Smad3 linker region in addition to the two C-terminal residues, and glycogen synthase kinase 3 is responsible for phosphorylation at one of these sites, namely Ser-204. Alanine substitution at Ser-204 and/or the neighboring Ser-208, the priming site for glycogen synthase kinase 3 in vivo activity, strengthened the affinity of Smad3 to CREB-binding protein, suggesting that linker phosphorylation may be part of a negative feedback loop that modulates Smad3 transcriptional activity. Thus, our findings reveal a novel aspect of the Smad3 signaling mechanism that controls the final amplitude of cellular responses to TGF-beta.

Smad ubiquitination regulatory factor 2 promotes metastasis of breast cancer cells by enhancing migration and invasiveness

Controlled protein degradation mediated by ubiquitin/proteasome system (UPS) plays a crucial role in modulating a broad range of cellular responses. Dysregulation of the UPS often accompanies tumorigenesis and progression. Here, we report that Smad ubiquitination regulatory factor 2 (Smurf2), a HECT-domain containing E3 ubiquitin ligase, is up-regulated in certain breast cancer tissues and cells. We show that reduction of Smurf2 expression with specific short interfering RNA in metastatic breast cancer cells induces cell rounding and reorganization of the actin cytoskeleton, which are associated with a less motile and invasive phenotype. Overexpression of Smurf2 promotes metastasis in a nude mouse model and increases migration and invasion of breast cancer cells. Moreover, expression of Smurf2CG, an E3 ligase-defective mutant of Smurf2, suppresses the above metastatic behaviors. These results establish an important role for Smurf2 in breast cancer progression and indicate that Smurf2 is a novel regulator of breast cancer cell migration and invasion.