Regulation of TGF-β signal transduction by mono- and deubiquitylation of Smads

Cancer associated autoantibodies in idiopathic inflammatory myopathies: A retrospective cohort from a single center in China

OBJECTIVES

Idiopathic inflammatory myopathies (IIMs) are systemic, heterogeneous diseases, which mainly affect skeletal muscle. Myositis with cancer is often referred to as cancer-associated myositis (CAM), which is associated with poor prognosis. This study aimed to determine the cancer associated myositis-specific autoantibodies (MSAs) and to elucidate their associations with clinical features in Chinese patients with IIMs.

METHODS

This retrospective study enrolled 312 patients with IIMs who were treated at Tianjin Medical University General Hospital, China, from January 2015 to December 2020. Clinical data were collected. Serum MSAs, including anti-Mi-2, anti-TIF1-γ, anti-NXP2, anti-SAE, anti-MDA5, anti-SRP, anti-Jo-1, anti-PL-7, anti-PL-12, anti-OJ, anti-EJ and anti-HMGCR antibodies were detected. Cancer-associated MSAs, their phenotypic and survival features were estimated through SPSS 20.0.

RESULTS

The results revealed that anti-TIF1-γ antibody and anti-SAE antibody were cancer-associated autoantibodies with odds ratios (95% CI) of 8.70 (3.35-22.64) and 22.31 (4.32-115.05), respectively. Skin lesions, proximal weakness, dysphagia and dysarthria were observed more frequently in patients carrying anti-TIF1-γ antibody. By contrast, patients with anti-TIF1-γ antibody had a lower frequencies of fever, arthritis/arthralgia and interstitial lung disease (ILD). Anti-TIF1-γ antibody positive CAM comprised about half of CAM entities and had the characteristic of close temporal association with cancer detection/recurrence. Female-dominant, common reproductive system tumors were other clinical features of this subset. Besides, patients with anti-TIF1-γ antibody positive had significantly lower survival rates than the anti-TIF1-γ antibody negative group.

CONCLUSIONS

Anti-TIF1-γ antibody and anti-SAE antibody were cancer-associated autoantibodies. Anti-TIF1-γ antibody positive CAM was a subset that comprised about half of CAM entities and had the characteristic of poor prognosis.

Characteristics of anti-transcriptional intermediary factor 1 gamma autoantibody-positive dermatomyositis patients in Singapore

Introduction: This study aimed to determine the clinical profile and outcome of anti-transcriptional intermediary factor 1 gamma autoantibody (anti-TIF1-γ Ab)-positive dermatomyositis patients and propose cancer screening programmes based on regional cancer trends. Method: Data on history, physical findings and investigations were collected using chart review on dermatomyositis patients seen at a tertiary hospital in Singapore from 1 January 2015 to 30 June 2021. Comparisons were made between anti-TIF1-γ Ab-positive and anti-TIF1-γ Ab-negative dermatomyositis. Results: Ninety-six dermatomyositis patients were analysed and 36 patients were positive for anti-TIF1-γ Ab. Anti-TIF1-γ Ab-positive patients had more frequent heliotrope rashes, shawl sign, periungual erythema, holster sign, Gottron’s papules, dysphagia and truncal weakness (P<0.05). They had less frequent interstitial lung disease, polyarthritis, cutaneous ulcers, palmar papules and mechanic’s hands (P<0.05). After 48 months of follow-up, a higher proportion of anti-TIF1-γ Ab-positive patients developed cancer compared with Ab-negative patients (63.9% versus 8.5%; odds ratio 19.1, 95% confidence interval 6.1–59.8; P<0.001). Nasopharyngeal carcinoma (NPC) and breast cancer were the most common malignancies, followed by bowel, lung and non-Hodgkin lymphoma. Most malignancies (78.3%) occurred within 13 months prior to, or 4 months after the onset of dermatomyositis. The mortality rate for anti-TIF1-γ Ab-positive patients was significantly higher than Ab-negative patients (36.1% vs 16.7%, P=0.031), and Kaplan-Meier survival estimates at 24 months were 66% and 89%, respectively (P=0.0153). Conclusion: These observational data support periodic screening of NPC and other malignancies in patients with anti-TIF1-γ Ab-positive dermatomyositis in Singapore. Keywords: Autoantibody, cancer, dermatomyositis, nasopharyngeal carcinoma, transcriptional intermediary factor 1 gamma

The Small GTPase RAC1B: A Potent Negative Regulator of-and Useful Tool to Study-TGFβ Signaling

RAC1 and its alternatively spliced isoform, RAC1B, are members of the Rho family of GTPases. Both isoforms are involved in the regulation of actin cytoskeleton remodeling, cell motility, cell proliferation, and epithelial-mesenchymal transition (EMT). Compared to RAC1, RAC1B exhibits a number of distinctive features with respect to tissue distribution, downstream signaling and a role in disease conditions like inflammation and cancer. The subcellular locations and interaction partners of RAC1 and RAC1B vary depending on their activation state, which makes RAC1 and RAC1B ideal candidates to establish cross-talk with cancer-associated signaling pathways-for instance, interactions with signaling by transforming growth factor β (TGFβ), a known tumor promoter. Although RAC1 has been found to promote TGFβ-driven tumor progression, recent observations in pancreatic carcinoma cells surprisingly revealed that RAC1B confers anti-oncogenic properties, i.e., through inhibiting TGFβ-induced EMT. Since then, an unexpected array of mechanisms through which RAC1B cross-talks with TGFβ signaling has been demonstrated. However, rather than being uniformly inhibitory, RAC1B interacts with TGFβ signaling in a way that results in the selective blockade of tumor-promoting pathways, while concomitantly allowing tumor-suppressive pathways to proceed. In this review article, we are going to discuss the specific interactions between RAC1B and TGFβ signaling, which occur at multiple levels and include various components such as ligands, receptors, cytosolic mediators, transcription factors, and extracellular inhibitors of TGFβ ligands.

Anti-TIF1-γ autoantibodies: warning lights of a tumour autoantigen

Anti-transcription intermediary factor 1 (TIF1)-γ autoantibodies are robustly linked with cancer-associated DM in adults. This review aims to give an overview of the physiological context of TIF1-γ and to determine whether there is a pathophysiological link between anti-TIF1-γ autoantibodies and the occurrence of cancer. Detection of anti-TIF1-γ autoantibodies has a high sensitivity and specificity for cancer-associated DM in adults and is therefore useful for both diagnosis and cancer risk stratification. The function of the autoantigen, TIF1-γ, may provide insight into the mechanism behind this association. TIF1-γ is a ubiquitously present protein involved in various biological pathways, including TGF-β signalling. In cancer, it can act either as a tumour suppressor or promoter, depending on the cellular context and cancer stage. Evolving data provide pathophysiological insights, linking anti-TIF1-γ autoantibodies to both the anti-tumour response and to muscle and skin damage. TIF1-γ expression is increased in muscle and skin tissue of patients with DM. Mutations or loss-of-heterozygosity in TIF1-γ alleles in malignant tissue may result in the expression of tumour-specific neo-antigens stimulating autoantibody production. The newly formed autoantibodies are hypothesized to cross-react with antigens in muscle and skin, driving the development of DM. Based on the current evidence, anti-TIF1-γ autoantibodies should be considered warning lights of a potential tumour autoantigen and should alert the physician to the possibility of an underlying cancer.

Mdm2 Promotes Odontoblast-like Differentiation by Ubiquitinating Dlx3 and p53

Dentin is an important structural component of the tooth. Odontoblast differentiation is an essential biological process that guarantees normal dentin formation, which is precisely regulated by various proteins. Murine double minute 2 (Mdm2) is an E3 ubiquitin ligase, and it plays a pivotal role in the differentiation of different cell types, such as osteoblasts and myoblasts. However, whether Mdm2 plays a role in odontoblast differentiation remains unknown. Here, we investigated the spatiotemporal expression of Mdm2 by immunostaining and found that Mdm2 was highly expressed in the odontoblasts and slightly in the dental papilla cells of mouse incisors and molars. Gene knockdown and overexpression experiments verified that Mdm2 promoted the odontoblast-like differentiation of mouse dental papilla cells (mDPCs). Intranuclear colocalization and physical interaction between Mdm2 and distal-less 3 (Dlx3), a transcription factor important for odontoblast differentiation, was found during the odontoblast-like differentiation of mDPCs by double immunofluorescence and immunoprecipitation. Mdm2 was proved to monoubiquitinate Dlx3, which enhanced the expression of Dlx3 target gene Dspp. In addition, p53, the canonical substrate of Mdm2, was validated to be also ubiquitinated but degraded by Mdm2 during the odontoblast-like differentiation of mDPCs. Gene knockdown experiments confirmed that p53 inhibited the odontoblast-like differentiation of mDPCs. p53 and Mdm2 double knockdown partially rescued the reduced odontoblast-like differentiation by knockdown of Mdm2 alone. Taken together, our study revealed that Mdm2 promoted the odontoblast-like differentiation of mDPCs by ubiquitinating both Dlx3 and p53. On one hand, the monoubiquitination of Dlx3 by Mdm2 led to upregulation of Dspp, which is a marker of the odontoblast differentiation. On the other hand, ubiquitination of p53 by Mdm2 resulted in its degradation, which eliminated the inhibitory effect of p53 on the odontoblast-like differentiation of mDPCs.

Overexpression of ubiquitin-specific peptidase 15 in systemic sclerosis fibroblasts increases response to transforming growth factor β

Objective

Ubiquitination of proteins leads to their degradation by the proteasome, and is regulated by ubiquitin ligases and substrate-specific ubiquitin-specific peptidases (USPs). The ubiquitination process also plays important roles in the regulation of cell metabolism and cell cycle. Here, we found that the expression of several USPs is increased in SSc tenosynovial and skin biopsies, and we demonstrated that USP inhibition decreases TGF-β signalling in primary fibroblast cell lines.

Methods

High-density transcriptomic studies were performed using total RNA obtained from SSc tenosynovial samples. Confirmatory immunostaining experiments were performed on tenosynovial and skin samples. In vitro experiments were conducted in order to study the influence of USP modulation on responses to TGF-β stimulation.

Results

Tenosynovial biopsies from SSc patients overexpressed known disease-associated gene pathways: fibrosis, cytokines and chemokines, and Wnt/TGF-β signalling, but also several USPs. Immunohistochemistry experiments confirmed the detection of USPs in the same samples, and in SSc skin biopsies. Exposure of primary fibroblast cell lines to TGF-β induced USP gene expression. The use of a pan-USP inhibitor decreased SMAD3 phosphorylation, and expression of COL1A1, COL3A1 and fibronectin gene expression in TGF-β-stimulated fibroblasts. The effect of the USP inhibitor resulted in increased SMAD3 ubiquitination, and was blocked by a proteasome inhibitor, thereby confirming the specificity of its action.

Conclusion

Overexpression of several USPs, including USP15, amplifies fibrotic responses induced by TGF-β, and is a potential therapeutic target in SSc.

TNAP limits TGF-β-dependent cardiac and skeletal muscle fibrosis by inactivating the SMAD2/3 transcription factors

Fibrosis is associated with almost all forms of chronic cardiac and skeletal muscle diseases. The accumulation of extracellular matrix impairs the contractility of muscle cells contributing to organ failure. Transforming growth factor β (TGF-β) plays a pivotal role in fibrosis, activating pro-fibrotic gene programmes via phosphorylation of SMAD2/3 transcription factors. However, the mechanisms that control de-phosphorylation of SMAD2 and SMAD3 (SMAD2/3) have remained poorly characterized. Here, we show that tissue non-specific alkaline phosphatase (TNAP, also known as ALPL) is highly upregulated in hypertrophic hearts and in dystrophic skeletal muscles, and that the abrogation of TGF-β signalling in TNAP-positive cells reduces vascular and interstitial fibrosis. We show that TNAP colocalizes and interacts with SMAD2. The TNAP inhibitor MLS-0038949 increases SMAD2/3 phosphorylation, while TNAP overexpression reduces SMAD2/3 phosphorylation and the expression of downstream fibrotic genes. Overall our data demonstrate that TNAP negatively regulates TGF-β signalling and likely represents a mechanism to limit fibrosis.

Inhibition of Smurf2 translation by miR-322/503 protects from ischemia-reperfusion injury by modulating EZH2/Akt/GSK3β signaling

Myocardial ischemia-reperfusion (I/R) is a common and lethal disease that threatens people's life worldwide. The underlying mechanisms are under intensive study and yet remain unclear. Here, we explored the function of miR-322/503 in myocardial I/R injury. We used isolated rat perfused heart as an in vivo model and H9c2 cells subjected with the oxygen and glucose deprivation followed by reperfusion as in vitro model to study myocardial I/R injury. 2,3,5-Triphenyltetrazolium chloride (TTC) staining was used to measure the infarct size, and terminal deoxynucleotidyl transferase dUTP-mediated nick-end label (TUNEL) staining was used to examine apoptosis. Quantitative RT-PCR and Western blot were used to determine expression levels of miR-322/503, Smad ubiquitin regulatory factor 2 (Smurf2), enhancer of zeste homolog 2 (EZH2), p-Akt, and p-GSK3β. Overexpression of miR-322/503 decreased infarct size, inhibited cell apoptosis, and promoted cell proliferation through upregualtion of p-Akt and p-GSK3β. Thus the expression of miR-322/503 was reduced during I/R process. On the molecular level, miR-322/503 directly bound Smurf2 mRNA and suppressed its translation. Smurf2 ubiquitinated EZH2 and degraded EZH2, which could activate Akt/GSK3β signaling. Our study demonstrates that miR-322/503 plays a beneficial role in myocardial I/R injury. By inhibition of Smurf2 translation, miR-322/503 induces EZH2 expression and activates Akt/GSK3β pathway, thereby protecting cells from ischemia reperfusion injury.

The role of sentrin-specific protease 2 substrate recognition in TGF-β-induced tumorigenesis

Smad4, a common-mediator of Smads, plays a central role in forming complexes with receptor-phosphorylated Smads, and then transduces transforming growth factor (TGF)-β signals into the nuclei. Although many cellular factors are involved in TGF-β induced epithelial-to-mesenchymal transition (EMT) and cell migration, very little is known with the mechanism of Smad4 regulation on pro-oncogenes response by TGF-β. Herein, we demonstrate the interaction of Sentrin-specific protease 2 (SENP2) with Smad4 through SENP2 residue 363~400. The same segment is also important for desumoylation of Smad4, and able to relieve sumoylation-mediated TGF-β repression. The SENP2363~400 segment is critical for TGF-β-induced cell migration, which is correlated with SENP2363~400 deletion mutant failed to increase matrix metalloproteinase (MMP)-9 and EMT marker gene expression. Moreover, our results suggest that the interaction and desumoylation between SENP2 and Smad4 promote cell migration in triple-negative breast cancer cells. Altogether, our data show how SENP2 regulates its substrate for desumoylation, and also the role of SENP2 in TGF-β induced cancer cell migration.

Contextual determinants of TGFβ action in development, immunity and cancer

Few cell signals match the impact of the transforming growth factor-β (TGFβ) family in metazoan biology. TGFβ cytokines regulate cell fate decisions during development, tissue homeostasis and regeneration, and are major players in tumorigenesis, fibrotic disorders, immune malfunctions and various congenital diseases. The effects of the TGFβ family are mediated by a combinatorial set of ligands and receptors and by a common set of receptor-activated mothers against decapentaplegic homologue (SMAD) transcription factors, yet the effects can differ dramatically depending on the cell type and the conditions. Recent progress has illuminated a model of TGFβ action in which SMADs bind genome-wide in partnership with lineage-determining transcription factors and additionally integrate inputs from other pathways and the chromatin to trigger specific cellular responses. These new insights clarify the operating logic of the TGFβ pathway in physiology and disease.

Influential Mutations in the SMAD4 Trimer Complex Can Be Detected from Disruptions of Electrostatic Complementarity

This article examines three techniques for rapidly assessing the electrostatic contribution of individual amino acids to the stability of protein-protein complexes. Whereas the energetic minimization of modeled oligomers may yield more accurate complexes, we examined the possibility that simple modeling may be sufficient to identify amino acids that add to or detract from electrostatic complementarity. The three methods evaluated were (a) the elimination of entire side chains (e.g., glycine scanning), (b) the elimination of the electrostatic contribution from the atoms of a side chain, called nullification, and (c) side chain structure prediction using SCWRL4. These techniques generate models in seconds, enabling large-scale mutational scanning. We evaluated these techniques on the SMAD2/SMAD4 heterotrimer, whose formation plays a crucial role in antitumor pathways. Many studies have documented the clinical and structural effect of specific mutations on trimer formation. Our results describe how glycine scanning yields more specific predictions, although nullification may be more sensitive, and how side chain structure prediction enables the identification of uncharged-to-charge mutations.

Genetically-defined novel oral squamous cell carcinoma cell lines for the development of molecular therapies

Emerging biological and translational insights from large sequencing efforts underscore the need for genetically-relevant cell lines to study the relationships between genomic alterations of tumors, and therapeutic dependencies. Here, we report a detailed characterization of a novel panel of clinically annotated oral squamous cell carcinoma (OSCC) cell lines, derived from patients with diverse ethnicity and risk habits. Molecular analysis by RNAseq and copy number alterations (CNA) identified that the cell lines harbour CNA that have been previously reported in OSCC, for example focal amplications in 3q, 7p, 8q, 11q, 20q and deletions in 3p, 5q, 8p, 18q. Similarly, our analysis identified the same cohort of frequently mutated genes previously reported in OSCC including TP53, CDKN2A, EPHA2, FAT1, NOTCH1, CASP8 and PIK3CA. Notably, we identified mutations (MLL4, USP9X, ARID2) in cell lines derived from betel quid users that may be associated with this specific risk factor. Gene expression profiles of the ORL lines also aligned with those reported for OSCC. By focusing on those gene expression signatures that are predictive of chemotherapeutic response, we observed that the ORL lines broadly clustered into three groups (cell cycle, xenobiotic metabolism, others). The ORL lines noted to be enriched in cell cycle genes responded preferentially to the CDK1 inhibitor RO3306, by MTT cell viability assay. Overall, our in-depth characterization of clinically annotated ORL lines provides new insight into the molecular alterations synonymous with OSCC, which can facilitate in the identification of biomarkers that can be used to guide diagnosis, prognosis, and treatment of OSCC.

Dermatomyositis and Immune-Mediated Necrotizing Myopathies: A Window on Autoimmunity and Cancer

Autoimmune myopathies (myositides) are strongly associated with malignancy. The link between myositis and cancer, originally noticed by Bohan and Peter in their classification in 1975 (1), has been evidenced by large population-based cohort studies and a recent meta-analysis. The numerous reports of cases in which the clinical course of myositis reflects that of cancer and the short delay between myositis and cancer onset support the notion that myositis may be an authentic paraneoplastic disorder. Thus, cancer-associated myositis raises the question of cancer as a cause rather than a consequence of autoimmunity. Among myositides, dermatomyositis and more recently, although to a lesser extent, immune-mediated necrotizing myopathies are the most documented forms associated with cancer. Interestingly, the current diagnostic approach for myositis is based on the identification of specific antibodies where each antibody determines specific clinical features and outcomes. Recent findings have shown that the autoantibodies anti-TIF1γ, anti-NXP2 and anti-HMGCR are associated with cancers in the course of myositis. Herein, we highlight the fact that the targets of these three autoantibodies involve cellular pathways that intervene in tumor promotion and we discuss the role of cancer mutations as autoimmunity triggers in adult myositis.

SUMO-Specific Cysteine Protease 1 Promotes Epithelial Mesenchymal Transition of Prostate Cancer Cells via Regulating SMAD4 deSUMOylation

In advanced prostate cancer, small ubiquitin-like modifier (SUMO)-specific cysteine protease 1 (SENP1) is up-regulated. However, the role of SENP1 in regulating deSUMOylation of TGF-β/SMADs signaling is unknown. In this study, we developed a lentiviral vector, PLKO.1-shSENP1, to silence SENP1 in prostate cancer cells with high metastatic characteristics (PC3M). Likewise, we also created an adenovirus vector, Ad5/F11p-SENP1 to over-express SENP1 in prostate cancer cells with low metastatic potential (LNCaP). We showed that silencing of SENP1 promoted cellular apoptosis, and inhibited proliferation and migration of PC3M cells. Moreover, SENP1 silencing increased the SMAD4 expression at protein level, up-regulated E-cadherin and down-regulated Vimentin expression, indicating the inhibition of epithelial mesenchymal transition (EMT). Furthermore, SMAD4 interference abolished SENP1-mediated up-regulation of E-cadherin, suggesting that SENP1 regulated E-cadherin expression via SMAD4. SENP1 over-expression in LNCaP cells reduced SMAD4 protein, and promoted EMT via decreasing E-cadherin and increasing Vimentin. Moreover, down-regulation of SMAD4 and E-cadherin were blocked, after transfection with two SUMOylation sites mutated SMAD4, suggesting that SENP1 might reduce SMAD4 levels to regulate E-cadherin expression via deSUMOylation of SMAD4. In conclusion, SENP1 deSUMOylated SMAD4 to promote EMT via up-regulating E-cadherin in prostate cancer cells. Therefore, SENP1 is a potential target for treatment of advanced prostate cancer.

Targeting Deubiquitinating Enzymes in Glioblastoma Multiforme: Expectations and Challenges

Glioblastoma (GBM) is regarded as the most common primary intracranial neoplasm. Despite standard treatment with tumor resection and radiochemotherapy, the outcome remains gloomy. It is evident that a combination of oncogenic gain of function and tumor-suppressive loss of function has been attributed to glioma initiation and progression. The ubiquitin-proteasome system is a well-orchestrated system that controls the fate of most proteins by striking a dynamic balance between ubiquitination and deubiquitination of substrates, having a profound influence on the modulation of oncoproteins, tumor suppressors, and cellular signaling pathways. In recent years, deubiquitinating enzymes (DUBs) have emerged as potential anti-cancer targets due to their targeting several key proteins involved in the regulation of tumorigenesis, apoptosis, senescence, and autophagy. This review attempts to summarize recent studies of GBM-associated DUBs, their roles in various cellular processes, and discuss the relation between DUBs deregulation and gliomagenesis, especially how DUBs regulate glioma stem cells pluripotency, microenvironment, and resistance of radiation and chemotherapy through core stem-cell transcriptional factors. We also review recent achievements and progress in the development of potent and selective reversible inhibitors of DUBs, and attempted to find a potential GBM treatment by DUBs intervention.

Valproic acid (VPA) inhibits the epithelial-mesenchymal transition in prostate carcinoma via the dual suppression of SMAD4

PURPOSES

The epithelial-mesenchymal transition (EMT) plays an important role in cancer metastasis. Previous studies have reported that valproic acid (VPA) suppresses prostate carcinoma (PCa) cell metastasis and down-regulates SMAD4 protein levels, which is the key molecule in TGF-β-induced EMT. However, the correlation between VPA and the EMT in PCa remains uncertain.

METHODS

Markers of the EMT in PCa cells and xenografts were molecularly assessed after VPA treatment. The expression and mono-ubiquitination of SMAD4 were also analyzed. After transfection with plasmids that express SMAD4 or short hairpin RNA for SMAD4 down-regulation, markers of EMT were examined to confirm whether VPA inhibits the EMT of PCa cells through the suppression of SMAD4.

RESULTS

VPA induced the increase in E-cadherin (p < 0.05), and the decrease in N-cadherin (p < 0.05) and Vimentin (p < 0.05), in PCa cells and xenografts. SMAD4 mRNA and protein levels were repressed by VPA (p < 0.05), whereas the level of mono-ubiquitinated SMAD4 was increased (p < 0.05). SMAD4 knockdown significantly increased E-cadherin expression in PC3 cells, but SMAD4 over-expression abolished the VPA-mediated EMT-inhibitory effect.

CONCLUSIONS

VPA inhibits the EMT in PCa cells via the inhibition of SMAD4 expression and the mono-ubiquitination of SMAD4. VPA could serve as a promising agent in PCa treatment, with new strategies based on its diverse effects on posttranscriptional regulation.

Bone morphogenetic protein signaling in bone homeostasis

Bone morphogenetic proteins (BMPs) are cytokines belonging to the transforming growth factor-β (TGF-β) superfamily. They play multiple functions during development and tissue homeostasis, including regulation of the bone homeostasis. The BMP signaling pathway consists in a well-orchestrated manner of ligands, membrane receptors, co-receptors and intracellular mediators, that regulate the expression of genes controlling the normal functioning of the bone tissues. Interestingly, BMP signaling perturbation is associated to a variety of low and high bone mass diseases, including osteoporosis, bone fracture disorders and heterotopic ossification. Consistent with these findings, in vitro and in vivo studies have shown that BMPs have potent effects on the activity of cells regulating bone function, suggesting that manipulation of the BMP signaling pathway may be employed as a therapeutic approach to treat bone diseases. Here we review the recent advances on BMP signaling and bone homeostasis, and how this knowledge may be used towards improved diagnosis and development of novel treatment modalities. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Protein Arginine Methyltransferase 1 Methylates Smurf2

Smurf2, a member of the HECT domain E3 ligase family, is well known for its role as a negative regulator of TGF-β signaling by targeting Smads and TGF-β receptor. However, the regulatory mechanism of Smurf2 has not been elucidated. Arginine methylation is a type of post-translational modification that produces monomethylated or dimethylated arginine residues. In this report, we demonstrated methylation of Smurf2 by PRMT1. In vitro methylation assay showed that Smurf2, not Smurf1, was methylated by PRMT1. Among the type I PRMT family, only PRMT1 showed activity for Smurf2. Transiently expressed Smurf2 was methylated by PRMT1, indicating Smurf2 is a novel substrate of PRMT1. Using deletion constructs, methylation sites were shown to be located within amino acid region 224-298 of Smurf2. In vitro methylation assay following point mutation of putative methylation sites confirmed the presence of Arg232, Arg234, Arg237, and Arg239. Knockdown of PRMT1 resulted in increased Smurf2 expression as well as inhibition of TGF-β-mediated reporter activity. Although it is unclear whether or not increased Smurf2 expression can be directly attributed to lack of methylation of arginine residues, our results suggest that methylation by PRMT1 may regulate Smurf2 stability and control TGF-β signaling.

Ubiquitin-specific proteases as therapeutic targets for the treatment of breast cancer

Key mediators of signaling pathways in breast cancer involve post-translational protein modification, primarily mediated through phosphorylation and ubiquitination. While previous studies focused on phosphorylation events, more recent analysis suggests that ubiquitin plays a parallel and equally important role in several signaling and cell regulatory events in breast cancer. Availability of new tools capable of sensitive detection of gene mutations and aberrant expression of genes and proteins coupled with gene-specific knockdown and silencing protocols have provided insight into the previously unexplored ubiquitin regulatory process within these tumors. Ubiquitin-specific proteases are one class of enzymes with protein deubiquitinating activity, making up the majority of protein deubiquitinating diversity within mammalian cells. Ubiquitin-specific proteases are also emerging as potential therapeutic targets in many diseases, including cancer. In this report, we summarize the involvement of this class of enzymes in breast cancer signaling and cell regulation and illustrate the potential for additional studies to define novel targets and approaches in breast cancer therapy.

Protein monoubiquitylation: targets and diverse functions

Ubiquitin is a 76-amino acid protein whose conjugation to protein targets is a form of post-translational modification. Protein ubiquitylation is characterized by the covalent attachment of the COOH-terminal carboxyl group of ubiquitin to an amino group of the substrate protein. Given that the NH2 -terminal amino group is usually masked, internal lysine residues are most often targeted for ubiquitylation. Polyubiquitylation refers to the formation of a polyubiquitin chain on the substrate as a result of the ubiquitylation of conjugated ubiquitin. The structures of such polyubiquitin chains depend on the specific lysine residues of ubiquitin targeted for ubiquitylation. Most of the polyubiquitin chains other than those linked via lysine-63 and methionine-1 of ubiquitin are recognized by the proteasome and serve as a trigger for substrate degradation. In contrast, polyubiquitin chains linked via lysine-63 and methionine-1 serve as a binding platform for proteins that function in immune signal transduction or DNA repair. With the exception of a few targets such as histones, the functions of protein monoubiquitylation have remained less clear. However, recent proteomics analysis has shown that monoubiquitylation occurs more frequently than polyubiquitylation, and studies are beginning to provide insight into its biologically important functions. Here, we summarize recent findings on protein monoubiquitylation to provide an overview of the targets and molecular functions of this modification.

TIF1γ interferes with TGFβ1/SMAD4 signaling to promote poor outcome in operable breast cancer patients

BACKGROUND

The Transforming growth factor β (TGFβ) signaling has a paradoxical role in cancer development and outcome. Besides, the prognostic significance of the TGFβ1, SMAD4 in breast cancer patients is an area of many contradictions. The transcriptional intermediary factor 1γ (TIF1γ) is thought to interact with the TGFβ/SMAD signaling through different mechanisms. Our study aims to define the prognostic significance of TGFβ1, SMAD4 and TIF1γ expression in breast cancer patients and to detect possible interactions among those markers that might affect the outcome.

METHODS

Immunohistochemistry was performed on tissue microarray (TMA) blocks prepared from samples of 248 operable breast cancer patients who presented at Centre Léon Bérard (CLB) between 1998 and 2001. The intensity and the percentage of stained tumor cells were integrated into a single score (0-6) and a cutoff was defined for high or low expression for each marker. Correlation was done between TGFβ1, SMAD4 and TIF1γ expression with the clinico-pathologic parameters using Pearson's chi-square test. Kaplan-Meier method was used to estimate distant metastasis free survival (DMFS), disease free survival (DFS) and overall survival (OS) and the difference between the groups was evaluated with log-rank test.

RESULTS

223 cases were assessable for TIF1γ, 204 for TGFβ1 and 173 for SMAD4. Median age at diagnosis was 55.8 years (range: 27 to 89 years). Tumors were larger than 20 mm in 49.2% and 45.2% had axillary lymph node (LN) metastasis (N1a to N3). 19.4% of the patients had SBR grade I tumors, 46.8% grade II tumors and 33.9% grade III tumors. ER was positive in 85.4%, PR in 75.5% and Her2-neu was over-expressed in 10% of the cases. Nuclear TIF1γ, cytoplasmic TGFβ1, nuclear and cytoplasmic SMAD4 stainings were high in 35.9%, 30.4%, 27.7% and 52.6% respectively. TIF1γ expression was associated with younger age (p=0.006), higher SBR grade (p<0.001), more ER negativity (p=0.035), and tumors larger than 2 cm (p=0.081), while TGFβ1 was not associated with any of the traditional prognostic factors. TGFβ1 expression in tumor cells was a marker of poor prognosis regarding DMFS (HR=2.28; 95% CI: 1.4 to 3.8; p=0.002), DFS (HR=2.00; 95% CI: 1.25 to 3.5; p=0.005) and OS (HR=1.89; 95 % CI: 1.04 to 3.43; p=0.037). TIF1γ expression carried a tendency towards poorer DMFS (p=0.091), DFS (p=0.143) and OS (p=0.091). In the multivariate analysis TGFβ1 remained an independent predictor of shorter DMFS, DFS and OS. Moreover, the prognostic significance of TGFβ1 was more obvious in the TIF1γ high patient subgroup than in the patients with TIF1γ low expression. The subgroup expressing both markers had the worst DMFS (HR=3.2; 95% CI: 1.7 to 5.9; p<0.0001), DFS (HR=3.02; 95 % CI: 1.6 to 5.6; p<0.0001) and OS (HR=2.7; 95 % CI: 1.4 to 5.4; p=0.005).

CONCLUSION

There is a crosstalk between the TIF1γ and the TGFβ1/SMAD4 signaling that deteriorates the outcome of operable breast cancer patients and when combined together they can serve as an effective prognostic tool for those patients.

MicroRNA-26b functions as a proapoptotic factor in porcine follicular Granulosa cells by targeting Sma-and Mad-related protein 4

Sma- and Mad-related protein 4 (SMAD4) is the central mediator of the transforming growth factor beta signaling pathway and is closely related to mammalian reproductive ability and the development of ovarian follicles. However, little is currently known about the role of SMAD4 in mammalian follicular granulosa cell (GC) apoptosis or its regulation by miRNAs. Here, we found that the porcine SMAD4 protein was expressed at high levels in GCs and oocytes from primary, preantral, and antral follicles, and only slightly expressed in theca cells; its expression level was down-regulated in apoptotic ovarian GCs, suggesting that SMAD4 may be involved in ovary development and selection. Overexpression and knockdown of SMAD4 increased the proliferation and apoptosis of cultured porcine GCs, respectively. In addition, the use of miRNA mimics and luciferase reporter assays revealed that miRNA-26b (miR-26b) functions as a proapoptotic factor in porcine follicular GCs by targeting the 3'-untranslated region of the SMAD4 gene. Overexpression of miR-26b in follicular GCs suppressed SMAD4 mRNA and protein levels, resulting in down-regulation of the antiapoptotic BCL-2 gene and the promotion of GC apoptosis. Furthermore, transforming growth factor beta 1 (TGF-beta1) down-regulates miR-26b expression in porcine GCs. Taken together, these data suggest that SMAD4 plays a critical role in porcine follicular GC apoptosis and follicular atresia and that miR-26b may have a proapoptotic role in GCs by regulating the expression of SMAD4 in the transforming growth factor beta signaling pathway.

The emerging roles of deubiquitylating enzymes (DUBs) in the TGFβ and BMP pathways

The members of the transforming growth factor beta (TGFβ) family of cytokines, including bone morphogenetic proteins (BMP), play fundamental roles in development and tissue homeostasis. Hence, aberrant TGFβ/BMP signalling is associated with several human diseases such as fibrosis, bone and immune disorders, cancer progression and metastasis. Consequently, targeting TGFβ signalling for intervention potentially offers therapeutic opportunities against these diseases. Many investigations have focussed on understanding the molecular mechanisms underpinning the regulation of TGFβ signalling. One of the key areas has been to investigate the regulation of the protein components of the TGFβ/BMP signal transduction pathways by ubiquitylation and deubiquitylation. In the last 15years, extensive research has led to the discovery and characterisation of several E3 ubiquitin ligases that influence the TGFβ pathway. However, the research on DUBs regulating the TGFβ pathway has received prominence only recently and is still an emerging field. This review will provide a concise summary of our current understanding of how DUBs regulate TGFβ signalling.

New gene evolution in the bonus-TIF1-γ/TRIM33 family impacted the architecture of the vertebrate dorsal-ventral patterning network

Uncovering how a new gene acquires its function and understanding how the function of a new gene influences existing genetic networks are important topics in evolutionary biology. Here, we demonstrate nonconservation for the embryonic functions of Drosophila Bonus and its newest vertebrate relative TIF1-γ/TRIM33. We showed previously that TIF1-γ/TRIM33 functions as an ubiquitin ligase for the Smad4 signal transducer and antagonizes the Bone Morphogenetic Protein (BMP) signaling network underlying vertebrate dorsal-ventral axis formation. Here, we show that Bonus functions as an agonist of the Decapentaplegic (Dpp) signaling network underlying dorsal-ventral axis formation in flies. The absence of conservation for the roles of Bonus and TIF1-γ/TRIM33 reveals a shift in the dorsal-ventral patterning networks of flies and mice, systems that were previously considered wholly conserved. The shift occurred when the new gene TIF1-γ/TRIM33 replaced the function of the ubiquitin ligase Nedd4L in the lineage leading to vertebrates. Evidence of this replacement is our demonstration that Nedd4 performs the function of TIF1-γ/TRIM33 in flies during dorsal-ventral axis formation. The replacement allowed vertebrate Nedd4L to acquire novel functions as a ubiquitin ligase of vertebrate-specific Smad proteins. Overall our data reveal that the architecture of the Dpp/BMP dorsal-ventral patterning network continued to evolve in the vertebrate lineage, after separation from flies, via the incorporation of new genes.

USP13 antagonizes gp78 to maintain functionality of a chaperone in ER-associated degradation

Physiological adaptation to proteotoxic stress in the endoplasmic reticulum (ER) requires retrotranslocation of misfolded proteins into the cytoplasm for ubiquitination and elimination by ER-associated degradation (ERAD). A surprising paradox emerging from recent studies is that ubiquitin ligases (E3s) and deubiquitinases (DUBs), enzymes with opposing activities, can both promote ERAD. Here we demonstrate that the ERAD E3 gp78 can ubiquitinate not only ERAD substrates, but also the machinery protein Ubl4A, a key component of the Bag6 chaperone complex. Remarkably, instead of targeting Ubl4A for degradation, polyubiquitination is associated with irreversible proteolytic processing and inactivation of Bag6. Importantly, we identify USP13 as a gp78-associated DUB that eliminates ubiquitin conjugates from Ubl4A to maintain the functionality of Bag6. Our study reveals an unexpected paradigm in which a DUB prevents undesired ubiquitination to sharpen substrate specificity for an associated ubiquitin ligase partner and to promote ER quality control. DOI: http://dx.doi.org/10.7554/eLife.01369.001.

Loss of Usp9x disrupts cortical architecture, hippocampal development and TGFβ-mediated axonogenesis

The deubiquitylating enzyme Usp9x is highly expressed in the developing mouse brain, and increased Usp9x expression enhances the self-renewal of neural progenitors in vitro. USP9X is a candidate gene for human neurodevelopmental disorders, including lissencephaly, epilepsy and X-linked intellectual disability. To determine if Usp9x is critical to mammalian brain development we conditionally deleted the gene from neural progenitors, and their subsequent progeny. Mating Usp9x^loxP/loxP mice with mice expressing Cre recombinase from the Nestin promoter deleted Usp9x throughout the entire brain, and resulted in early postnatal lethality. Although the overall brain architecture was intact, loss of Usp9x disrupted the cellular organization of the ventricular and sub-ventricular zones, and cortical plate. Usp9x absence also led to dramatic reductions in axonal length, in vivo and in vitro, which could in part be explained by a failure in Tgf-β signaling. Deletion of Usp9x from the dorsal telencephalon only, by mating with Emx1-cre mice, was compatible with survival to adulthood but resulted in reduction or loss of the corpus callosum, a dramatic decrease in hippocampal size, and disorganization of the hippocampal CA3 region. This latter phenotypic aspect resembled that observed in Doublecortin knock-out mice, which is an Usp9x interacting protein. This study establishes that Usp9x is critical for several aspects of CNS development, and suggests that its regulation of Tgf-β signaling extends to neurons.

Altered ubiquitin-proteasome signaling in right ventricular hypertrophy and failure

Alterations in the ubiquitin-proteasome system (UPS) have been described in left ventricular hypertrophy and failure, although results have been inconsistent. The role of the UPS in right ventricular (RV) hypertrophy (RVH) and RV failure (RVF) is unknown. Given the greater percent increase in RV mass associated with RV afterload stress, as present in many congenital heart lesions, we hypothesized that alterations in the UPS could play an important role in RVH/RVF. UPS expression and activity were measured in the RV from mice with RVH/RVF secondary to pulmonary artery constriction (PAC). Epoxomicin and MG132 were used to inhibit the proteasome, and overexpression of the 11S PA28α subunit was used to activate the proteasome. PAC mice developed RVH (109.3% increase in RV weight to body weight), RV dilation with septal shift, RV dysfunction, and clinical RVF. Proteasomal function (26S β₅ chymotrypsin-like activity) was decreased 26% (P < 0.05). Protein expression of 19S subunit Rpt5 (P < 0.05), UCHL1 deubiquitinase (P < 0.0001), and Smurf1 E3 ubiquitin ligase (P < 0.01) were increased, as were polyubiquitinated proteins (P < 0.05) and free-ubiquitins (P = 0.05). Pro-apoptotic Bax was increased (P < 0.0001), whereas anti-apoptotic Bcl-2 decreased (P < 0.05), resulting in a sixfold increase in the Bax/Bcl-2 ratio. Proteasomal inhibition did not accelerate RVF. However, proteasome enhancement by cardiac-specific proteasome overexpression partially improved survival. Proteasome activity is decreased in RVH/RVF, associated with upregulation of key UPS regulators and pro-apoptotic signaling. Enhancement of proteasome function partially attenuates RVF, suggesting that UPS dysfunction contributes to RVF.

Mechanisms of prickle1a function in zebrafish epilepsy and retinal neurogenesis

Epilepsy is a complex neurological disorder characterized by unprovoked seizures. The etiology is heterogeneous with both genetic and environmental causes. Genes that regulate neurotransmitters and ion channels in the central nervous system have been associated with epilepsy. However, a recent screening in human epilepsy patients identified mutations in the PRICKLE1 (PK1) locus, highlighting a potentially novel mechanism underlying seizures. PK1 is a core component of the planar cell polarity network that regulates tissue polarity. Zebrafish studies have shown that Pk1 coordinates cell movement, neuronal migration and axonal outgrowth during embryonic development. Yet how dysfunction of Pk1 relates to epilepsy is unknown. To address the mechanism underlying epileptogenesis, we used zebrafish to characterize Pk1a function and epilepsy-related mutant forms. We show that knockdown of pk1a activity sensitizes zebrafish larva to a convulsant drug. To model defects in the central nervous system, we used the retina and found that pk1a knockdown induces neurite outgrowth defects; yet visual function is maintained. Furthermore, we characterized the functional and biochemical properties of the PK1 mutant forms identified in human patients. Functional analyses demonstrate that the wild-type Pk1a partially suppresses the gene knockdown retinal defects but not the mutant forms. Biochemical analysis reveals increased ubiquitylation of one mutant form and decreased translational efficiency of another mutant form compared with the wild-type Pk1a. Taken together, our results indicate that mutation of human PK1 could lead to defects in neurodevelopment and signal processing, providing insight into seizure predisposition in these patients.

OTUB1 enhances TGFβ signalling by inhibiting the ubiquitylation and degradation of active SMAD2/3

SMAD transcription factors are key intracellular transducers of TGFβ cytokines. SMADs are tightly regulated to ensure balanced cellular responses to TGFβ signals. Ubiquitylation has a key role in regulating SMAD stability and activity. Several E3 ubiquitin ligases that regulate the turnover of SMADs are known; however, proteins that prevent the ubiquitylation or cause deubiquitylation of active SMADs remain undefined. Here we demonstrate that OTUB1 is recruited to the active phospho-SMAD2/3 complex only on TGFβ induction. Further, OTUB1 has a crucial role in TGFβ-mediated gene transcription and cellular migration. OTUB1 inhibits the ubiquitylation of phospho-SMAD2/3 by binding to and inhibiting the E2 ubiquitin-conjugating enzymes independent of its catalytic activity. Consequently, depletion of OTUB1 in cells causes a rapid loss in levels of TGFβ-induced phospho-SMAD2/3, which is rescued by the proteasomal inhibitor bortezomib. Our findings uncover a signal-induced phosphorylation-dependent recruitment of OTUB1 to its target in the TGFβ pathway.

Hippo pathway phylogenetics predicts monoubiquitylation of Salvador and Merlin/Nf2

Recently we employed phylogenetics to predict that the cellular interpretation of TGF-β signals is modulated by monoubiquitylation cycles affecting the Smad4 signal transducer/tumor suppressor. This prediction was subsequently validated by experiments in flies, frogs and mammalian cells. Here we apply a phylogenetic approach to the Hippo pathway and predict that two of its signal transducers, Salvador and Merlin/Nf2 (also a tumor suppressor) are regulated by monoubiquitylation. This regulatory mechanism does not lead to protein degradation but instead serves as a highly efficient “off/on” switch when the protein is subsequently deubiquitylated. Overall, our study shows that the creative application of phylogenetics can predict new roles for pathway components and new mechanisms for regulating intercellular signaling pathways.

Drosophila CORL is required for Smad2-mediated activation of Ecdysone Receptor expression in the mushroom body

CORL proteins (FUSSEL/SKOR proteins in humans) are related to Sno/Ski oncogenes but their developmental roles are unknown. We have cloned Drosophila CORL and show that its expression is restricted to distinct subsets of cells in the central nervous system. We generated a deletion of CORL and noted that homozygous individuals rarely survive to adulthood. Df(4)dCORL adult escapers display mushroom body (MB) defects and Df(4)dCORL larvae are lacking Ecdysone Receptor (EcR-B1) expression in MB neurons. This is phenocopied in CORL-RNAi and Smad2-RNAi clones in wild-type larvae. Furthermore, constitutively active Baboon (type I receptor upstream of Smad2) cannot stimulate EcR-B1 MB expression in Df(4)dCORL larvae, which demonstrates a formal requirement for CORL in Smad2 signaling. Studies of mouse Corl1 (Skor1) revealed that it binds specifically to Smad3. Overall, the data suggest that CORL facilitates Smad2 activity upstream of EcR-B1 in the MB. The conservation of neural expression and strong sequence homology of all CORL proteins suggests that this is a new family of Smad co-factors.

Fat facets deubiquitylation of Medea/Smad4 modulates interpretation of a Dpp morphogen gradient

The ability of secreted Transforming Growth Factor β (TGFβ) proteins to act as morphogens dictates that their influence be strictly regulated. Here, we report that maternally contributed fat facets (faf; a homolog of USP9X/FAM) is essential for proper interpretation of the zygotic Decapentaplegic (Dpp) morphogen gradient that patterns the embryonic dorsal-ventral axis. The data suggest that the loss of faf reduces the activity of Medea (a homolog of Smad4) below the minimum necessary for adequate Dpp signaling and that this is likely due to excessive ubiquitylation on a specific lysine. This study supports the hypothesis that the control of cellular responsiveness to TGFβ signals at the level of Smad4 ubiquitylation is a conserved mechanism required for proper implementation of a morphogen gradient.