Axin is a scaffold protein in TGF-beta signaling that promotes degradation of Smad7 by Arkadia

Bioinformatics analysis identifies several intrinsically disordered human E3 ubiquitin-protein ligases

The ubiquitin-proteasome system targets misfolded proteins for degradation. Since the accumulation of such proteins is potentially harmful for the cell, their prompt removal is important. E3 ubiquitin-protein ligases mediate substrate ubiquitination by bringing together the substrate with an E2 ubiquitin-conjugating enzyme, which transfers ubiquitin to the substrate. For misfolded proteins, substrate recognition is generally delegated to molecular chaperones that subsequently interact with specific E3 ligases. An important exception is San1, a yeast E3 ligase. San1 harbors extensive regions of intrinsic disorder, which provide both conformational flexibility and sites for direct recognition of misfolded targets of vastly different conformations. So far, no mammalian ortholog of San1 is known, nor is it clear whether other E3 ligases utilize disordered regions for substrate recognition. Here, we conduct a bioinformatics analysis to examine >600 human and S. cerevisiae E3 ligases to identify enzymes that are similar to San1 in terms of function and/or mechanism of substrate recognition. An initial sequence-based database search was found to detect candidates primarily based on the homology of their ordered regions, and did not capture the unique disorder patterns that encode the functional mechanism of San1. However, by searching specifically for key features of the San1 sequence, such as long regions of intrinsic disorder embedded with short stretches predicted to be suitable for substrate interaction, we identified several E3 ligases with these characteristics. Our initial analysis revealed that another remarkable trait of San1 is shared with several candidate E3 ligases: long stretches of complete lysine suppression, which in San1 limits auto-ubiquitination. We encode these characteristic features into a San1 similarity-score, and present a set of proteins that are plausible candidates as San1 counterparts in humans. In conclusion, our work indicates that San1 is not a unique case, and that several other yeast and human E3 ligases have sequence properties that may allow them to recognize substrates by a similar mechanism as San1.

Regulation of TGF-β Receptors

In cells responding to extracellular polypeptide ligands, regulatory mechanisms at the level of cell surface receptors are increasingly seen to define the nature of the ligand-induced signaling responses. Processes that govern the levels of receptors at the plasma membrane, including posttranslational modifications, are crucial to ensure receptor function and specify the downstream signals. Indeed, extracellular posttranslational modifications of the receptors help define stability and ligand binding, while intracellular modifications mediate interactions with signaling mediators and accessory proteins that help define the nature of the signaling response. The use of various molecular biology and biochemistry techniques, based on chemical crosslinking, e.g., biotin or radioactive labeling, immunofluorescence to label membrane receptors and flow cytometry, allows for quantification of changes of cell surface receptor presentation. Here, we discuss recent progress in our understanding of the regulation of TGF-β receptors, i.e., the type I (TβRI) and type II (TβRII) TGF-β receptors, and describe basic methods to identify and quantify TGF-β cell surface receptors.

Multifaceted signaling regulators of chondrogenesis: Implications in cartilage regeneration and tissue engineering

Defects of articular cartilage present a unique clinical challenge due to its poor self-healing capacity and avascular nature. Current surgical treatment options do not ensure consistent regeneration of hyaline cartilage in favor of fibrous tissue. Here, we review the current understanding of the most important biological regulators of chondrogenesis and their interactions, to provide insight into potential applications for cartilage tissue engineering. These include various signaling pathways, including: fibroblast growth factors (FGFs), transforming growth factor β (TGF-β)/bone morphogenic proteins (BMPs), Wnt/β-catenin, Hedgehog, Notch, hypoxia, and angiogenic signaling pathways. Transcriptional and epigenetic regulation of chondrogenesis will also be discussed. Advances in our understanding of these signaling pathways have led to promising advances in cartilage regeneration and tissue engineering.

Signaling in Fibrosis: TGF-β, WNT, and YAP/TAZ Converge

Chronic organ injury leads to fibrosis and eventually organ failure. Fibrosis is characterized by excessive synthesis, remodeling, and contraction of extracellular matrix produced by myofibroblasts. Myofibroblasts are the key cells in the pathophysiology of fibrotic disorders and their differentiation can be triggered by multiple stimuli. To develop anti-fibrotic therapies, it is of paramount importance to understand the molecular basis of the signaling pathways contributing to the activation and maintenance of myofibroblasts. Several signal transduction pathways, such as transforming growth factor (TGF)-β, Wingless/Int (WNT), and more recently yes-associated protein 1 (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ) signaling, have been linked to the pathophysiology of fibrosis. Activation of the TGF-β1-induced SMAD complex results in the upregulation of genes important for myofibroblast function. Similarly, WNT-stabilized β-catenin translocates to the nucleus and initiates transcription of its target genes. YAP and TAZ are two transcriptional co-activators from the Hippo signaling pathway that also rely on nuclear translocation for their functioning. These three signal transduction pathways have little molecular similarity but do share one principle: the cytosolic/nuclear regulation of its transcriptional activators. Past research on these pathways often focused on the isolated cascades without taking other signaling pathways into account. Recent developments show that parts of these pathways converge into an intricate network that governs the activation and maintenance of the myofibroblast phenotype. In this review, we discuss the current understanding on the signal integration between the TGF-β, WNT, and YAP/TAZ pathways in the development of organ fibrosis. Taking a network-wide view on signal transduction will provide a better understanding on the complex and versatile processes that underlie the pathophysiology of fibrotic disorders.

Signaling in Fibrosis: TGF-β, WNT, and YAP/TAZ Converge

Chronic organ injury leads to fibrosis and eventually organ failure. Fibrosis is characterized by excessive synthesis, remodeling, and contraction of extracellular matrix produced by myofibroblasts. Myofibroblasts are the key cells in the pathophysiology of fibrotic disorders and their differentiation can be triggered by multiple stimuli. To develop anti-fibrotic therapies, it is of paramount importance to understand the molecular basis of the signaling pathways contributing to the activation and maintenance of myofibroblasts. Several signal transduction pathways, such as transforming growth factor (TGF)-β, Wingless/Int (WNT), and more recently yes-associated protein 1 (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ) signaling, have been linked to the pathophysiology of fibrosis. Activation of the TGF-β1-induced SMAD complex results in the upregulation of genes important for myofibroblast function. Similarly, WNT-stabilized β-catenin translocates to the nucleus and initiates transcription of its target genes. YAP and TAZ are two transcriptional co-activators from the Hippo signaling pathway that also rely on nuclear translocation for their functioning. These three signal transduction pathways have little molecular similarity but do share one principle: the cytosolic/nuclear regulation of its transcriptional activators. Past research on these pathways often focused on the isolated cascades without taking other signaling pathways into account. Recent developments show that parts of these pathways converge into an intricate network that governs the activation and maintenance of the myofibroblast phenotype. In this review, we discuss the current understanding on the signal integration between the TGF-β, WNT, and YAP/TAZ pathways in the development of organ fibrosis. Taking a network-wide view on signal transduction will provide a better understanding on the complex and versatile processes that underlie the pathophysiology of fibrotic disorders.

Emerging roles of Axin in cerebral cortical development

Proper functioning of the cerebral cortex depends on the appropriate production and positioning of neurons, establishment of axon–dendrite polarity, and formation of proper neuronal connectivity. Deficits in any of these processes greatly impair neural functions and are associated with various human neurodevelopmental disorders including microcephaly, cortical heterotopias, and autism. The application of in vivo manipulation techniques such as in utero electroporation has resulted in significant advances in our understanding of the cellular and molecular mechanisms that underlie neural development in vivo. Axin is a scaffold protein that regulates neuronal differentiation and morphogenesis in vitro. Recent studies provide novel insights into the emerging roles of Axin in gene expression and cytoskeletal regulation during neurogenesis, neuronal polarization, and axon formation. This review summarizes current knowledge on Axin as a key molecular controller of cerebral cortical development.

SPSB1, a Novel Negative Regulator of the Transforming Growth Factor-β Signaling Pathway Targeting the Type II Receptor

Appropriate cellular signaling is essential to control cell proliferation, differentiation, and cell death. Aberrant signaling can have devastating consequences and lead to disease states, including cancer. The transforming growth factor-β (TGF-β) signaling pathway is a prominent signaling pathway that has been tightly regulated in normal cells, whereas its deregulation strongly correlates with the progression of human cancers. The regulation of the TGF-β signaling pathway involves a variety of physiological regulators. Many of these molecules act to alter the activity of Smad proteins. In contrast, the number of molecules known to affect the TGF-β signaling pathway at the receptor level is relatively low, and there are no known direct modulators for the TGF-β type II receptor (TβRII). Here we identify SPSB1 (a Spry domain-containing Socs box protein) as a novel regulator of the TGF-β signaling pathway. SPSB1 negatively regulates the TGF-β signaling pathway through its interaction with both endogenous and overexpressed TβRII (and not TβRI) via its Spry domain. As such, TβRII and SPSB1 co-localize on the cell membrane. SPSB1 maintains TβRII at a low level by enhancing the ubiquitination levels and degradation rates of TβRII through its Socs box. More importantly, silencing SPSB1 by siRNA results in enhanced TGF-β signaling and migration and invasion of tumor cells.

Arkadia enhances BMP signalling through ubiquitylation and degradation of Smad6

Arkadia, a positive regulator of Smad-dependent signalling via the transforming growth factor-β (TGF-β) family, is an E3 ubiquitin ligase that induces ubiquitylation and proteasome-dependent degradation of TGF-β suppressors such as Smad7, c-Ski and SnoN. In this study, we examined the effects of Arkadia on bone morphogenetic protein (BMP)-induced osteoblast differentiation. Knockdown of Arkadia reduced mineralization and expression of osteoblast differentiation markers. Furthermore, we showed that Smad6, a BMP-specific inhibitory Smad, is a target of Arkadia: wild-type (WT) Arkadia, but not the C937A (CA) mutant lacking E3 ubiquitin-ligase activity, induced ubiquitylation and proteasome-dependent degradation of Smad6. Accordingly, protein levels of Smad6, Smad7 and c-Ski were elevated in MEFs from Arkadia KO mice. Finally, expression of Arkadia attenuated blockade of BMP signalling by Smad6 in a transcriptional reporter assay. These results demonstrate that Smad6 is a novel target of Arkadia, and that Arkadia positively regulates BMP signalling via degradation of Smad6, Smad7 and c-Ski/SnoN.

Zebrafish Rnf111 is encoded by multiple transcripts and is required for epiboly progression and prechordal plate development

Arkadia (also known as RING finger 111) encodes a nuclear E3 ubiquitin ligase that targets intracellular effectors and modulators of TGFβ/Nodal-related signaling for polyubiquitination and proteasome-dependent degradation. In the mouse, loss of Arkadia results in early embryonic lethality, with defects attributed to compromised Nodal signaling. Here, we report the isolation of zebrafish arkadia/rnf111, which is represented by 5 transcript variants. arkadia/rnf111 is broadly expressed during the blastula and gastrula stages, with eventual enrichment in the anterior mesendoderm, including the prechordal plate. Morpholino knockdown experiments reveal an unexpected role for Arkadia/Rnf111 in both early blastula organization and epiboly progression. Using a splice junction morpholino, we present additional evidence that arkadia/rnf111 transcript variants containing a 3' alternative exon are specifically required for epiboly progression in the late gastrula. This result suggests that arkadia/rnf111 transcript variants encode functionally relevant protein isoforms that provide additional intracellular flexibility and regulation to the Nodal signaling pathway.

Regulation of TGF-β Superfamily Signaling by SMAD Mono-Ubiquitination

TGF-β(transforming growth factor-β) superfamily signaling mediators are important regulators of diverse physiological and pathological events. TGF-β signals are transduced by transmembrane type I and type II serine/threonine kinase receptors and their downstream effectors, the SMAD(drosophila mothers against decapentaplegic protein) proteins. Numerous studies have already demonstrated crucial regulatory roles for modification of TGF-β pathway components by poly-ubiquitination. Recently, several studies also uncovered mono-ubiquitination of SMADs as a mechanism for SMAD activation or inactivation. Mono-ubiquitination and subsequent deubiquitination of SMAD proteins accordingly play important roles in the control of TGF-β superfamily signaling. This review highlights the major pathways regulated by SMAD mono-ubiquitination.

SMAD7: a timer of tumor progression targeting TGF-β signaling

In the context of cancer, transforming growth factor β (TGF-β) is a cell growth suppressor; however, it is also a critical inducer of invasion and metastasis. SMAD is the important mediator of TGF-β signaling pathway, which includes receptor-regulated SMADs (R-SMADs), common-mediator SMADs (co-SMADs), and inhibitory SMADs (I-SMADs). I-SMADs block the activation of R-SMADs and co-SMADs and thus play important roles especially in the SMAD-dependent signaling. SMAD7 belongs to the I-SMADs. As an inhibitor of TGF-β signaling, SMAD7 is overexpressed in numerous cancer types and its abundance is positively correlated to the malignancy. Emerging evidence has revealed the switch-in-role of SMAD7 in cancer, from a TGF-β inhibiting protein at the early stages that facilitates proliferation to an enhancer of invasion at the late stages. This role change may be accompanied or elicited by the tumor microenvironment and/or somatic mutation. Hence, current knowledge suggests a tumor-favorable timer nature of SMAD7 in cancer progression. In this review, we summarized the advances and recent findings of SMAD7 and TGF-β signaling in cancer, followed by specific discussion on the possible factors that account for the functional changes of SMAD7.

Multiple Arkadia/RNF111 structures coordinate its Polycomb body association and transcriptional control

The RING domain protein Arkadia/RNF111 is a ubiquitin ligase in the transforming growth factor β (TGFβ) pathway. We previously identified Arkadia as a small ubiquitin-like modifier (SUMO)-binding protein with clustered SUMO-interacting motifs (SIMs) that together form a SUMO-binding domain (SBD). However, precisely how SUMO interaction contributes to the function of Arkadia was not resolved. Through analytical molecular and cell biology, we found that the SIMs share redundant function with Arkadia's M domain, a region distinguishing Arkadia from its paralogs ARKL1/ARKL2 and the prototypical SUMO-targeted ubiquitin ligase (STUbL) RNF4. The SIMs and M domain together promote both Arkadia's colocalization with CBX4/Pc2, a component of Polycomb bodies, and the activation of a TGFβ pathway transcription reporter. Transcriptome profiling through RNA sequencing showed that Arkadia can both promote and inhibit gene expression, indicating that Arkadia's activity in transcriptional control may depend on the epigenetic context, defined by Polycomb repressive complexes and DNA methylation.

Human Sarcoma growth is sensitive to small-molecule mediated AXIN stabilization

Sarcomas are mesenchymal tumors showing high molecular heterogeneity, reflected at the histological level by the existence of more than fifty different subtypes. Genetic and epigenetic evidences link aberrant activation of the Wnt signaling to growth and progression of human sarcomas. This phenomenon, mainly accomplished by autocrine loop activity, is sustained by gene amplification, over-expression of Wnt ligands and co-receptors or epigenetic silencing of endogenous Wnt antagonists. We previously showed that pharmacological inhibition of Wnt signaling mediated by Axin stabilization produced in vitro and in vivo antitumor activity in glioblastoma tumors. Here, we report that targeting different sarcoma cell lines with the Wnt inhibitor/Axin stabilizer SEN461 produces a less transformed phenotype, as supported by modulation of anchorage-independent growth in vitro. At the molecular level, SEN461 treatment enhanced the stability of the scaffold protein Axin1, a key negative regulator of the Wnt signaling with tumor suppressor function, resulting in downstream effects coherent with inhibition of canonical Wnt signaling. Genetic phenocopy of small molecule Axin stabilization, through Axin1 over-expression, coherently resulted in strong impairment of soft-agar growth. Importantly, sarcoma growth inhibition through pharmacological Axin stabilization was also observed in a xenograft model in vivo in female CD-1 nude mice. Our findings suggest the usefulness of Wnt inhibitors with Axin stabilization activity as a potentialyl clinical relevant strategy for certain types of sarcomas.

The regulation of TGF-β/SMAD signaling by protein deubiquitination

Transforming growth factor-β (TGF-β) members are key cytokines that control embryogenesis and tissue homeostasis via transmembrane TGF-β type II (TβR II) and type I (TβRI) and serine/threonine kinases receptors. Aberrant activation of TGF-β signaling leads to diseases, including cancer. In advanced cancer, the TGF-β/SMAD pathway can act as an oncogenic factor driving tumor cell invasion and metastasis, and thus is considered to be a therapeutic target. The activity of TGF-β/SMAD pathway is known to be regulated by ubiquitination at multiple levels. As ubiquitination is reversible, emerging studies have uncovered key roles for ubiquitin-removals on TGF-β signaling components by deubiquitinating enzymes (DUBs). In this paper, we summarize the latest findings on the DUBs that control the activity of the TGF-β signaling pathway. The regulatory roles of these DUBs as a driving force for cancer progression as well as their underlying working mechanisms are also discussed.

The role of targeted protein degradation in early neural development

As neural stem cells differentiate into neurons during neurogenesis, the proteome of the cells is restructured by de novo expression and selective removal of regulatory proteins. The control of neurogenesis at the level of gene regulation is well documented and the regulation of protein abundance through protein degradation via the Ubiquitin/26S proteasome pathway is a rapidly developing field. This review describes our current understanding of the role of the proteasome pathway in neurogenesis. Collectively, the studies show that targeted protein degradation is an important regulatory mechanism in the generation of new neurons.

Nuclear receptor NR4A1 promotes breast cancer invasion and metastasis by activating TGF-β signalling

In advanced cancers, the TGF-β pathway acts as an oncogenic factor and is considered to be a therapeutic target. Here using a genome-wide cDNA screen, we identify nuclear receptor NR4A1 as a strong activator of TGF-β signalling. NR4A1 promotes TGF-β/SMAD signalling by facilitating AXIN2-RNF12/ARKADIA-induced SMAD7 degradation. NR4A1 interacts with SMAD7 and AXIN2, and potently and directly induces AXIN2 expression. Whereas loss of NR4A1 inhibits TGF-β-induced epithelial-to-mesenchymal transition and metastasis, slight NR4A1 ectopic expression stimulates metastasis in a TGF-β-dependent manner. Importantly, inflammatory cytokines potently induce NR4A1 expression, and potentiate TGF-β-mediated breast cancer cell migration, invasion and metastasis in vitro and in vivo. Notably, NR4A1 expression is elevated in breast cancer patients with high immune infiltration and its expression weakly correlates with phosphorylated SMAD2 levels, and is an indicator of poor prognosis. Our results uncover inflammation-induced NR4A1 as an important determinant for hyperactivation of pro-oncogenic TGF-β signalling in breast cancer.

In silico identification of potential therapeutic targets in the TGF-β signal transduction pathway

The transforming growth factor-β (TGF-β) superfamily of cytokines controls fundamental cellular processes, such as proliferation, motility, differentiation, and apoptosis. This fundamental role is emphasized by the widespread presence of mutations of the core components of the TGF-β signal transduction pathway in a number of human diseases. Therefore, there is an increasing interest in the development of therapies to specifically target this pathway. Here we develop a computational approach to identify potential intervention points that are capable of restoring the normal signaling dynamics to the mutated system while maintaining the behavior of normal cells substantially unperturbed. We apply this approach explicitly to the TGF-β pathway to study the signaling dynamics of mutated and normal cells treated with inhibitory drugs and identify the processes in the pathway that are most susceptible to therapeutic intervention.

Regulation of TGF-β family signalling by ubiquitination and deubiquitination

Members of the transforming growth factor-β (TGF-β) family, including TGF-βs, activin and bone morphogenetic proteins (BMPs), are multifunctional proteins that regulate a wide variety of cellular responses, such as proliferation, differentiation, migration and apoptosis. TGF-β family signalling is mainly mediated by membranous serine/threonine kinase receptors and intracellular Smad proteins. This signalling is tightly regulated by various post-translational modifications including ubiquitination. Several E3 ubiquitin ligases play a crucial role in the recognition and ubiquitin-dependent degradation of TGF-β family receptors, Smad proteins and their interacted proteins to regulate positively and negatively TGF-β family signalling. In contrast, non-degradative ubiquitin modifications also regulate TGF-β family signalling. Recently, in addition to protein ubiquitination, deubiquitination by deubiquitinating enzymes has been reported to control TGF-β family signalling pathways. Interestingly, more recent studies suggest that TGF-β signalling is not only regulated via ubiquitination and/or deubiquitination, but also it relies on ubiquitination for its effect on other pathways. Thus, ubiquitin modifications play key roles in TGF-β family signal transduction and cross-talk between TGF-β family signalling and other signalling pathways. Here, we review the current understandings of the positive and negative regulatory mechanisms by ubiquitin modifications that control TGF-β family signalling.

AMP as a low-energy charge signal autonomously initiates assembly of AXIN-AMPK-LKB1 complex for AMPK activation

The AMP-activated protein kinase (AMPK) is a master regulator of metabolic homeostasis by sensing cellular energy status. AMPK is mainly activated via phosphorylation by LKB1 when cellular AMP/ADP levels are increased. However, how AMP/ADP brings about AMPK phosphorylation remains unclear. Here, we show that it is AMP, but not ADP, that drives AXIN to directly tether LKB1 to phosphorylate AMPK. The complex formation of AXIN-AMPK-LKB1 is greatly enhanced in glucose-starved or AICAR-treated cells and in cell-free systems supplemented with exogenous AMP. Depletion of AXIN abrogated starvation-induced AMPK-LKB1 colocalization. Importantly, adenovirus-based knockdown of AXIN in the mouse liver impaired AMPK activation and caused exacerbated fatty liver after starvation, underscoring an essential role of AXIN in AMPK activation. These findings demonstrate an initiating role of AMP and demonstrate that AXIN directly transmits AMP binding of AMPK to its activation by LKB1, uncovering the mechanistic route for AMP to elicit AMPK activation by LKB1.

Clinical implications of Ezrin and CD44 co‑expression in breast cancer

The aim of the present study was to investigate the expression status and clinical implications of the stem cell genes Ezrin and CD44 in breast cancers. Expression of the Ezrin protein in CD44+/CD24-/low tumor cells (CSCs) was detected by western blotting. The resulting expression status and the relationship between Ezrin and CD44 were determined in 726 breast cancers using immunohistochemistry staining and immunofluorescence double staining. Subsequently, the relationship between Ezrin and CD44 protein co-expression and clinicopathological parameters and prognosis was determined. The Ezrin protein was expressed at a higher level in CSCs when compared to that in the control cells and was related to the resistance of CSCs to chemotherapy. The Ezrin and CD44 proteins were co-expressed in the co-immunoprecipitation (Co-IP) test. Ezrin and CD44 co-expression was observed in 235 (32.37%) of the 726 cases examined. After universal analysis and multivariate analysis, histological type, lymph node metastasis, triple-negative breast cancer, TNM stage and distant metastasis were verified as related to Ezrin and CD44 co-expression (P=0.011, 0.006, 0.001, 0.011 and 0.001, respectively). A survival analysis revealed that Ezrin and CD44 co-expression was associated with a poorer prognosis (36.91 vs. 81.54%, P=0.001). After running Cox regression, the factors of age, tumor size, lymph node metastasis, triple-negative tumor status, TNM stage, distant metastasis and Ezrin and CD44 co-expression were shown to be independent prognostic factors of breast cancer. The co-expression of Ezrin and CD44 may be a new biomarker for evaluating the progression and chemotherapy sensitivity of breast cancer.

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.

Computational modelling of Smad-mediated negative feedback and crosstalk in the TGF-β superfamily network

The transforming growth factor-β (TGF-β) signal transduction pathway controls many cellular processes, including differentiation, proliferation and apoptosis. It plays a fundamental role during development and it is dysregulated in many diseases. The factors that control the dynamics of the pathway, however, are not fully elucidated yet and so far computational approaches have been very limited in capturing the distinct types of behaviour observed under different cellular backgrounds and conditions into a single-model description. Here, we develop a detailed computational model for TGF-β signalling that incorporates elements of previous models together with crosstalking between Smad1/5/8 and Smad2/3 channels through a negative feedback loop dependent on Smad7. The resulting model accurately reproduces the diverse behaviour of experimental datasets for human keratinocytes, bovine aortic endothelial cells and mouse mesenchymal cells, capturing the dynamics of activation and nucleocytoplasmic shuttling of both R-Smad channels. The analysis of the model dynamics and its system properties revealed Smad7-mediated crosstalking between Smad1/5/8 and Smad2/3 channels as a major determinant in shaping the distinct responses to single and multiple ligand stimulation for different cell types.

Signal integration in TGF-β, WNT, and Hippo pathways

Complete sequences of animal genomes have revealed a remarkably small and conserved toolbox of signalling pathways, such as TGF-β and WNT that account for all biological diversity. This raises the question as to how such a limited set of cues elaborates so many diverse cell fates and behaviours. It is now clear that components of signalling pathways are physically assembled into higher order networks that ultimately dictate the biological output of pathway activity. Intertwining of pathways is thus emerging as a key feature of a large, integrated and coordinated signalling network that allows cells to read a limited set of extrinsic cues, but mount the diverse responses that underpin successful development and homeostasis. Moreover, this design principle confounds the development of effective therapeutic interventions in complex diseases, such as cancer.

Signal integration in TGF-β, WNT, and Hippo pathways

Complete sequences of animal genomes have revealed a remarkably small and conserved toolbox of signalling pathways, such as TGF-β and WNT that account for all biological diversity. This raises the question as to how such a limited set of cues elaborates so many diverse cell fates and behaviours. It is now clear that components of signalling pathways are physically assembled into higher order networks that ultimately dictate the biological output of pathway activity. Intertwining of pathways is thus emerging as a key feature of a large, integrated and coordinated signalling network that allows cells to read a limited set of extrinsic cues, but mount the diverse responses that underpin successful development and homeostasis. Moreover, this design principle confounds the development of effective therapeutic interventions in complex diseases, such as cancer.

A decade and a half of protein intrinsic disorder: biology still waits for physics

The abundant existence of proteins and regions that possess specific functions without being uniquely folded into unique 3D structures has become accepted by a significant number of protein scientists. Sequences of these intrinsically disordered proteins (IDPs) and IDP regions (IDPRs) are characterized by a number of specific features, such as low overall hydrophobicity and high net charge which makes these proteins predictable. IDPs/IDPRs possess large hydrodynamic volumes, low contents of ordered secondary structure, and are characterized by high structural heterogeneity. They are very flexible, but some may undergo disorder to order transitions in the presence of natural ligands. The degree of these structural rearrangements varies over a very wide range. IDPs/IDPRs are tightly controlled under the normal conditions and have numerous specific functions that complement functions of ordered proteins and domains. When lacking proper control, they have multiple roles in pathogenesis of various human diseases. Gaining structural and functional information about these proteins is a challenge, since they do not typically "freeze" while their "pictures are taken." However, despite or perhaps because of the experimental challenges, these fuzzy objects with fuzzy structures and fuzzy functions are among the most interesting targets for modern protein research. This review briefly summarizes some of the recent advances in this exciting field and considers some of the basic lessons learned from the analysis of physics, chemistry, and biology of IDPs.

FGF, TGFβ and Wnt crosstalk: embryonic to in vitro cartilage development from mesenchymal stem cells

Articular cartilage is easily damaged, yet difficult to repair. Cartilage tissue engineering seems a promising therapeutic solution to restore articular cartilage structure and function, with mesenchymal stem cells (MSCs) receiving increasing attention for their promise to promote cartilage repair. It is known from embryology that members of the fibroblast growth factor (FGF), transforming growth factor-β (TGFβ) and wingless-type (Wnt) protein families are involved in controlling different differentiation stages during chondrogenesis. Individually, these pathways have been extensively studied but so far attempts to recapitulate embryonic development in in vitro MSC chondrogenesis have failed to produce stable and functioning articular cartilage; instead, transient hypertrophic cartilage is obtained. We believe a better understanding of the simultaneous integration of these factors will improve how we relate embryonic chondrogenesis to in vitro MSC chondrogenesis. This narrative review attempts to define current knowledge on the crosstalk between the FGF, TGFβ and Wnt signalling pathways during different stages of mesenchymal chondrogenesis. Connecting embryogenesis and in vitro differentiation of human MSCs might provide insights into how to improve and progress cartilage tissue engineering for the future.

All-trans retinoic acid upregulates the expression of p53 via Axin and inhibits the proliferation of glioma cells

All-trans retinoic acid (ATRA) is a potent chemopreventive and therapeutic agent and exerts its effects by inducing growth arrest. In the present study, we demonstrated that ATRA activated the expression of p53 via Axin and induced cell cycle arrest at the G1/S phase and apoptosis of glioma cells. Briefly, C6 cells were treated with ATRA, and the levels of p53 mRNA and protein were determined by RT-PCR, western blotting and immunohistochemistry. The results showed that ATRA activated the expression of p53. In addition, ectopic expression of Axin by transient transfection of C6 cells with rAxin revealed that overexpression of Axin induced cell cycle arrest and apoptosis with an upregulation of p53. Furthermore, loss-of-function of Axin in glioma cells by RNAi blocked ATRA-induced cell cycle phase arrest and apoptosis via downregulation of p53. The present study revealed a novel function of Axin and identified it as an important regulator of ATRA-activated p53 expression.

Arkadia, a novel SUMO-targeted ubiquitin ligase involved in PML degradation

Arkadia is a RING domain E3 ubiquitin ligase that activates the transforming growth factor β (TGF-β) pathway by inducing degradation of the inhibitor SnoN/Ski. Here we show that Arkadia contains three successive SUMO-interacting motifs (SIMs) that mediate noncovalent interaction with poly-SUMO2. We identify the third SIM (VVDL) of Arkadia to be the most relevant one in this interaction. Furthermore, we provide evidence that Arkadia can function as a SUMO-targeted ubiquitin ligase (STUBL) by ubiquitinating SUMO chains. While the SIMs of Arkadia are not essential for SnoN/Ski degradation in response to TGF-β, we show that they are necessary for the interaction of Arkadia with polysumoylated PML in response to arsenic and its concomitant accumulation into PML nuclear bodies. Moreover, Arkadia depletion leads to accumulation of polysumoylated PML in response to arsenic, highlighting a requirement of Arkadia for arsenic-induced degradation of polysumoylated PML. Interestingly, Arkadia homodimerizes but does not heterodimerize with RNF4, the other STUBL involved in PML degradation, suggesting that these two E3 ligases do not act synergistically but most probably act independently during this process. Altogether, these results identify Arkadia to be a novel STUBL that can trigger degradation of signal-induced polysumoylated proteins.

Wnt/beta-catenin signaling and small molecule inhibitors

Wnt/β-catenin signaling is a branch of a functional network that dates back to the first metazoans and it is involved in a broad range of biological systems including stem cells, embryonic development and adult organs. Deregulation of components involved in Wnt/β-catenin signaling has been implicated in a wide spectrum of diseases including a number of cancers and degenerative diseases. The key mediator of Wnt signaling, β-catenin, serves several cellular functions. It functions in a dynamic mode at multiple cellular locations, including the plasma membrane, where β-catenin contributes to the stabilization of intercellular adhesive complexes, the cytoplasm where β-catenin levels are regulated and the nucleus where β-catenin is involved in transcriptional regulation and chromatin interactions. Central effectors of β-catenin levels are a family of cysteine-rich secreted glycoproteins, known as Wnt morphogens. Through the LRP5/6-Frizzled receptor complex, Wnts regulate the location and activity of the destruction complex and consequently intracellular β- catenin levels. However, β-catenin levels and their effects on transcriptional programs are also influenced by multiple other factors including hypoxia, inflammation, hepatocyte growth factor-mediated signaling, and the cell adhesion molecule E-cadherin. The broad implications of Wnt/β-catenin signaling in development, in the adult body and in disease render the pathway a prime target for pharmacological research and development. The intricate regulation of β-catenin at its various locations provides alternative points for therapeutic interventions.

The four and a half LIM-only protein 2 (FHL2) activates transforming growth factor β (TGF-β) signaling by regulating ubiquitination of the E3 ligase Arkadia

Arkadia is a RING-based ubiquitin ligase that positively regulates TGF-β signaling by targeting several pathway components for ubiquitination and degradation. However, little is known about the mechanisms controlling Arkadia activity. Here we show that the LIM-only protein FHL2 binds and synergistically cooperates with Arkadia to activate Smad3/Smad4-dependent transcription. Knockdown of FHL2 by RNA interference decreases Arkadia level and restricts the amplitude of Arkadia-induced TGF-β target gene responses. We found that Arkadia is ubiquitinated via K63- and K27-linked polyubiquitination. A single mutation at the RING domain that abolishes the E3 activity diminishes Arkadia ubiquitination, indicating that this modification partly involves autocatalytic process. Mutation of seven lysines at the C-terminal region of Arkadia severely impairs ubiquitination through the K27 but not the K63 linkage and slows down the turnover of Arkadia, suggesting that K27-linked polyubiquitination might promote proteolysis-dependent regulation of Arkadia. We show that FHL2 increases the half-life of Arkadia through inhibition of ubiquitin chain assembly on the protein, which provides a molecular basis for functional cooperation between Arkadia and FHL2 in enhancing TGF-β signaling. Our study uncovers a novel regulatory mechanism of Arkadia by ubiquitination and identifies FHL2 as important regulator of Arkadia ubiquitination and TGF-β signal transduction.

Poly-small ubiquitin-like modifier (PolySUMO)-binding proteins identified through a string search

Polysumoylation is a crucial cellular response to stresses against genomic integrity or proteostasis. Like the small ubiquitin-like modifier (SUMO)-targeted ubiquitin ligase RNF4, proteins with clustered SUMO-interacting motifs (SIMs) can be important signal transducers downstream of polysumoylation. To identify novel polySUMO-binding proteins, we conducted a computational string search with a custom Python script. We found clustered SIMs in another RING domain protein Arkadia/RNF111. Detailed biochemical analysis of the Arkadia SIMs revealed that dominant SIMs in a SIM cluster often contain a pentameric VIDLT ((V/I/L/F/Y)(V/I)DLT) core sequence that is also found in the SIMs in PIAS family E3s and is likely the best-fitted structure for SUMO recognition. This idea led to the identification of additional novel SIM clusters in FLASH/CASP8AP2, C5orf25, and SOBP/JXC1. We suggest that the clustered SIMs in these proteins form distinct SUMO binding domains to recognize diverse forms of protein sumoylation.

Lithium Attenuates TGF-β(1)-Induced Fibroblasts to Myofibroblasts Transition in Bronchial Fibroblasts Derived from Asthmatic Patients

Bronchial asthma is a chronic disorder accompanied by phenotypic transitions of bronchial epithelial cells, smooth muscle cells, and fibroblasts. Human bronchial fibroblasts (HBFs) derived from patients with diagnosed asthma display predestination towards TGF-β-induced phenotypic switches. Since the interference between TGF-β and GSK-3β signaling contributes to pathophysiology of chronic lung diseases, we investigated the effect of lithium, a nonspecific GSK-3β inhibitor, on TGF-β₁-induced fibroblast to myofibroblast transition (FMT) in HBF and found that the inhibition of GSK-3β attenuates TGF-β₁-induced FMT in HBF populations derived from asthmatic but not healthy donors. Cytoplasmically sequestrated β-catenin, abundant in TGF-β₁/LiCl-stimulated asthmatic HBFs, most likely interacts with and inhibits the nuclear accumulation and signal transduction of Smad proteins. These data indicate that the specific cellular context determines FMT-related responses of HBFs to factors interfering with the TGF-β signaling pathway. They may also provide a mechanistic explanation for epidemiological data revealing coincidental remission of asthmatic syndromes and their recurrence upon the discontinuation of lithium therapy in certain psychiatric diseases.

Modularity and functional plasticity of scaffold proteins as p(l)acemakers in cell signaling

Cells coordinate and integrate various functional modules that control their dynamics, intracellular trafficking, metabolism and gene expression. Such capacity is mediated by specific scaffold proteins that tether multiple components of signaling pathways at plasma membrane, Golgi apparatus, mitochondria, endoplasmic reticulum, nucleus and in more specialized subcellular structures such as focal adhesions, cell-cell junctions, endosomes, vesicles and synapses. Scaffold proteins act as "pacemakers" as well as "placemakers" that regulate the temporal, spatial and kinetic aspects of protein complex assembly by modulating the local concentrations, proximity, subcellular dispositions and biochemical properties of the target proteins through the intricate use of their modular protein domains. These regulatory mechanisms allow them to gate the specificity, integration and crosstalk of different signaling modules. In addition to acting as physical platforms for protein assembly, many professional scaffold proteins can also directly modify the properties of their targets while they themselves can be regulated by post-translational modifications and/or mechanical forces. Furthermore, multiple scaffold proteins can form alliances of higher-order regulatory networks. Here, we highlight the emerging themes of scaffold proteins by analyzing their common and distinctive mechanisms of action and regulation, which underlie their functional plasticity in cell signaling. Understanding these mechanisms in the context of space, time and force should have ramifications for human physiology and for developing new therapeutic approaches to control pathological states and diseases.

The role of the ubiquitin-proteasome system in kidney diseases

Proteins in mammalian cells are continually being degraded and synthesized. Protein degradation via the ubiquitin-proteasome system (UPS) is the major pathway for non-lysosomal proteolysis of intracellular proteins and plays important roles in a variety of fundamental cellular processes such as regulation of cell cycle progression, differentiation, apoptosis, sodium channel function, and modulation of inflammatory responses. The central element of this system is the covalent linkage of ubiquitins to targeted proteins, which are then recognized by the 26S proteasome composed of adenosine triphosphate-dependent, multi-catalytic proteases. Damaged or misfolded proteins, as well as regulatory proteins that control many critical cellular functions, are among the targets of this degradation process. Consequently, aberration of the system leads to dysregulation of cellular homeostasis and development of many diseases. Based on the findings, it is not surprising that abnormalities of the system are also associated with the pathogenesis of kidney diseases. In this review, I discuss (1) the basic mechanism of the UPS, and (2) the association between the pathogenesis of kidney diseases and the UPS. Diverse roles of the UPS are implicated in the development of kidney diseases, and further studies on this system may reveal new strategies for overcoming kidney diseases.

Post-translational regulation of TGF-β receptor and Smad signaling

TGF-β family signaling through Smads is conceptually a simple and linear signaling pathway, driven by sequential phosphorylation, with type II receptors activating type I receptors, which in turn activate R-Smads. Nevertheless, TGF-β family proteins induce highly complex programs of gene expression responses that are extensively regulated, and depend on the physiological context of the cells. Regulation of TGF-β signaling occurs at multiple levels, including TGF-β activation, formation, activation and destruction of functional TGF-β receptor complexes, activation and degradation of Smads, and formation of Smad transcription complexes at regulatory gene sequences that cooperate with a diverse set of DNA binding transcription factors and coregulators. Here we discuss recent insights into the roles of post-translational modifications and molecular interaction networks in the functions of receptors and Smads in TGF-β signal responses. These layers of regulation demonstrate how a simple signaling system can be coopted to exert exquisitely regulated, complex responses.

Key role for ubiquitin protein modification in TGFβ signal transduction

The transforming growth factor β (TGFβ) superfamily of signal transduction molecules plays crucial roles in the regulation of cell behavior. TGFβ regulates gene transcription through Smad proteins and signals via non-Smad pathways. The TGFβ pathway is strictly regulated, and perturbations lead to tumorigenesis. Several pathway components are known to be targeted for proteasomal degradation via ubiquitination by E3 ligases. Smurfs are well known negative regulators of TGFβ, which function as E3 ligases recruited by adaptors such as I-Smads. TGFβ signaling can also be enhanced by E3 ligases, such as Arkadia, that target repressors for degradation. It is becoming clear that E3 ligases often target multiple pathways, thereby acting as mediators of signaling cross-talk. Regulation via ubiquitination involves a complex network of E3 ligases, adaptor proteins, and deubiquitinating enzymes (DUBs), the last-mentioned acting by removing ubiquitin from its targets. Interestingly, also non-degradative ubiquitin modifications are known to play important roles in TGFβ signaling. Ubiquitin modifications thus play a key role in TGFβ signal transduction, and in this review we provide an overview of known players, focusing on recent advances.

RACK1 suppresses gastric tumorigenesis by stabilizing the β-catenin destruction complex

BACKGROUND & AIMS

Dysregulation of Wnt signaling has been involved in gastric tumorigenesis by mechanisms that are not fully understood. The receptor for activated protein kinase C (RACK1, GNB2L1) is involved in development of different tumor types, but its expression and function have not been investigated in gastric tumors.

METHODS

We analyzed expression of RACK1 in gastric tumor samples and their matched normal tissues from 116 patients using immunohistochemistry. Effects of knockdown with small interfering RNAs or overexpression of RACK1 in gastric cancer cell lines were evaluated in cell growth and tumor xenograft. RACK1 signaling pathways were investigated in cells and zebrafish embryos using immunoblot, immunoprecipitation, microinjection, and in situ hybridization assays.

RESULTS

Expression of RACK1 was reduced in gastric tumor samples and correlated with depth of tumor infiltration and poor differentiation. Knockdown of RACK1 in gastric cancer cells accelerated their anchorage-independent proliferation in soft agar, whereas overexpression of RACK1 reduced their tumorigenicity in nude mice. RACK1 formed a complex with glycogen synthase kinase Gsk3β and Axin to promote the interaction between Gsk3β and β-catenin and thereby stabilized the β-catenin destruction complex. On stimulation of Wnt3a, RACK1 repressed Wnt signaling by inhibiting recruitment of Axin by Dishevelled 2 (Dvl2). Moreover, there was an inverse correlation between expression of RACK1 and localization of β-catenin to the cytoplasm/nucleus in human gastric tumor samples.

CONCLUSIONS

RACK1 negatively regulates Wnt signaling pathway by stabilizing the β-catenin destruction complex and act as a tumor suppressor in gastric cancer cells.

How ubiquitination regulates the TGF-β signalling pathway: new insights and new players: new isoforms of ubiquitin-activating enzymes in the E1-E3 families join the game

Ubiquitination of protein species in regulating signal transduction pathways is universally accepted as of fundamental importance for normal development, and defects in this process have been implicated in the progression of many human diseases. One pathway that has received much attention in this context is transforming growth factor-beta (TGF-β) signalling, particularly during the regulation of epithelial-mesenchymal transition (EMT) and tumour progression. While E3-ubiquitin ligases offer themselves as potential therapeutic targets, much remains to be unveiled regarding mechanisms that culminate in their regulation. With this in mind, the focus of this review highlights the regulation of the ubiquitination pathway and the significance of a recently described group of NEDD4 E3-ubiquitin ligase isoforms in the context of TGF-β pathway regulation. Moreover, we now broaden these observations to incorporate a growing number of protein isoforms within the ubiquitin ligase superfamily as a whole, and discuss their relevance in defining a new 'iso-ubiquitinome'.

Cdk5-mediated phosphorylation of Axin directs axon formation during cerebral cortex development

Axon formation is critical for the establishment of connections between neurons, which is a prerequisite for the development of neural circuitry. Kinases such as cyclin-dependent kinase 5 (Cdk5) and glycogen synthase kinase-3β (GSK-3β), have been implicated to regulate axon outgrowth. Nonetheless, the in vivo roles of these kinases in axon development and the underlying signaling mechanisms remain essentially unknown. We report here that Cdk5 is important for axon formation in mouse cerebral cortex through regulating the functions of axis inhibitor (Axin), a scaffold protein of the canonical Wnt pathway. Knockdown of Axin in utero abolishes the formation and projection of axons. Importantly, Axin is phosphorylated by Cdk5, and this phosphorylation facilitates the interaction of Axin with GSK-3β, resulting in inhibition of GSK-3β activity and dephosphorylation of its substrate collapsin response mediator protein-2 (CRMP-2), a microtubule-associated protein. Specifically, both phosphorylation of Axin and its interaction with GSK-3β are critically required for axon formation in mouse cortex development. Together, our findings reveal a new regulatory mechanism of axon formation through Cdk5-dependent phosphorylation of Axin.

Enhancement of TGF-β signaling responses by the E3 ubiquitin ligase Arkadia provides tumor suppression in colorectal cancer

TGF-β signaling provides tumor protection against colorectal cancer (CRC). Mechanisms that support its tumor-suppressive properties remain unclear. The ubiquitin ligase Arkadia/RNF111 enhances TGF-β signaling responses by targeting repressors of the pathway for degradation. The corepressors SnoN/Ski, critical substrates of Arkadia, complex with the activated TGF-β signaling effectors Smad2/3 (pSmad2/3) on the promoters of target genes and block their transcription. Arkadia degrades this complex including pSmad2/3 and unblocks the promoter. Here, we report that Arkadia is expressed highly in the mouse colonic epithelium. Heterozygous Akd(+/-) mice are normal but express less Arkadia. This leads to reduced expression of several TGF-β target genes, suggesting that normal levels of Arkadia are required for efficient signaling responses. Critically, Akd(+/-) mice exhibit increased susceptibility to azoxymethane/dextran sodium sulfate carcinogen-induced CRC, as they develop four-fold more tumors than wild-type mice. Akd(+/-) tumors also exhibit a more aggressive pathology, higher proliferation index, and reduced cytostasis. Therefore, Arkadia functions as a tumor suppressor whose peak expression is required to suppress CRC development and progression. The accumulation of nuclear SnoN and pSmad2, along with the downregulation of TGF-β target genes observed in Akd(+/-) colon and tumors, suggest that tumor-suppressing properties of Arkadia are mediated by its ability to derepress TGF-β signaling. Consistent with this likelihood, we identified mutations in primary colorectal tumors from human patients that reduce Arkadia function and are associated with the accumulation of nuclear SNON. Collectively, our findings reveal that Arkadia enhances TGF-β signaling responses and supports its tumor-suppressing properties in CRC.

Inhibitory machinery for the TGF-β family signaling pathway

Transforming growth factor-β (TGF-β) family signaling regulates cell growth and differentiation of many different cell types and is widely involved in the regulation of homeostasis during both embryogenesis and adult life. Therefore, aberrant TGF-β family signal transduction is linked to congenital disorders, tumorigenicity, and fibrosis, which can be life-threatening. A specific receptor-ligand complex initiates transduction of TGF-β family signaling to the nucleus via intracellular signal molecules, mainly Smads, whereby a number of bioactivities such as wound healing, immunomodulation, apoptosis, and angiogenesis are controlled. To avoid an excess of TGF-β family signaling in cells, the duration and intensity of the TGF-β family signal appear to be subject to elaborate regulation. In this paper, we describe recent advances in the understanding of how TGF-β family signals are perturbed and terminated to maintain homeostasis in cells.

Mechanistic insight into Myc stabilization in breast cancer involving aberrant Axin1 expression

High expression of the oncoprotein Myc has been linked to poor outcome in human tumors. Although MYC gene amplification and translocations have been observed, this can explain Myc overexpression in only a subset of human tumors. Myc expression is in part controlled by its protein stability, which can be regulated by phosphorylation at threonine 58 (T58) and serine 62 (S62). We now report that Myc protein stability is increased in a number of breast cancer cell lines and this correlates with increased phosphorylation at S62 and decreased phosphorylation at T58. Moreover, we find this same shift in phosphorylation in primary breast cancers. The signaling cascade that controls phosphorylation at T58 and S62 is coordinated by the scaffold protein Axin1. We therefore examined Axin1 in breast cancer and report decreased AXIN1 expression and a shift in the ratio of expression of two naturally occurring AXIN1 splice variants. We demonstrate that this contributes to increased Myc protein stability, altered phosphorylation at S62 and T58, and increased oncogenic activity of Myc in breast cancer. Thus, our results reveal an important mode of Myc activation in human breast cancer and a mechanism contributing to Myc deregulation involving unique insight into inactivation of the Axin1 tumor suppressor in breast cancer.

RB1CC1 protein positively regulates transforming growth factor-beta signaling through the modulation of Arkadia E3 ubiquitin ligase activity

Transforming growth factor-β (TGF-β) signaling is controlled by a variety of regulators, of which Smad7, c-Ski, and SnoN play a pivotal role in its negative regulation. Arkadia is a RING-type E3 ubiquitin ligase that targets these negative regulators for degradation to enhance TGF-β signaling. In the present study we identified a candidate human tumor suppressor gene product RB1CC1/FIP200 as a novel positive regulator of TGF-β signaling that functions as a substrate-selective cofactor of Arkadia. Overexpression of RB1CC1 enhanced TGF-β signaling, and knockdown of endogenous RB1CC1 attenuated TGF-β-induced expression of target genes as well as TGF-β-induced cytostasis. RB1CC1 down-regulated the protein levels of c-Ski but not SnoN by enhancing the activity of Arkadia E3 ligase toward c-Ski. Substrate selectivity is primarily attributable to the physical interaction of RB1CC1 with substrates, suggesting its role as a scaffold protein. RB1CC1 thus appears to play a unique role as a modulator of TGF-β signaling by restricting substrate specificity of Arkadia.

Role of Smads in TGFβ signaling

Transforming growth factor-β (TGFβ) is the prototype for a large family of pleiotropic factors that signal via heterotetrameric complexes of type I and type II serine/threonine kinase receptors. Important intracellular mediators of TGFβ signaling are members of the Smad family. Smad2 and 3 are activated by C-terminal receptor-mediated phosphorylation, whereafter they form complexes with Smad4 and are translocated to the nucleus where they, in cooperation with other transcription factors, co-activators and co-repressors, regulate the transcription of specific genes. Smads have key roles in exerting TGFβ-induced programs leading to cell growth arrest and epithelial-mesenchymal transition. The activity and stability of Smad molecules are carefully regulated by a plethora of post-translational modifications, including phosphorylation, ubiquitination, sumoylation, acetylation and poly(ADP)-ribosylation. The Smad function has been shown to be perturbed in certain diseases such as cancer.

Generation of Axin1 conditional mutant mice

Axin1 is a critical negative regulator of the canonical Wnt-signaling pathway. It is a concentration-limiting factor in the β-catenin degradation complex. Axin1 null mutant mouse embryos died at embryonic day 9.5, precluding direct genetic analysis of the roles of Axin1 in many developmental and physiological processes using these mutant mice. In this study, we have generated mice carrying two directly repeated loxP sites flanking the exon 2 region of the Axin1 gene. We show that floxed-allele-carrying mice (Axin1( fx/fx) ) mice appear normal and fertile. Upon crossing the Axin1( fx/fx) mice to the CMV-Cre transgenic mice, the loxP-flanked exon 2 region that encodes the N-terminus and the conserved regulation of G-protein signaling domain was efficiently deleted by Cre-mediated excision in vivo. Moreover, we show that mouse embryos homozygous for the Cre/loxP-mediated deletion of exon 2 of the Axin1 gene display embryonic lethality and developmental defects similar to those reported for Axin1(-/-) mice. Thus, this Axin1(fx/fx) mouse model will be valuable for systematic tissue-specific dissection of the roles of Axin1 in embryonic and postnatal development and diseases.

Transforming growth factor-β and the hallmarks of cancer

Tumorigenesis is in many respects a process of dysregulated cellular evolution that drives malignant cells to acquire six phenotypic hallmarks of cancer, including their ability to proliferate and replicate autonomously, to resist cytostatic and apoptotic signals, and to induce tissue invasion, metastasis, and angiogenesis. Transforming growth factor-β (TGF-β) is a potent pleiotropic cytokine that functions as a formidable barrier to the development of cancer hallmarks in normal cells and tissues. Paradoxically, tumorigenesis counteracts the tumor suppressing activities of TGF-β, thus enabling TGF-β to stimulate cancer invasion and metastasis. Fundamental gaps exist in our knowledge of how malignant cells overcome the cytostatic actions of TGF-β, and of how TGF-β stimulates the acquisition of cancer hallmarks by developing and progressing human cancers. Here we review the molecular and cellular mechanisms that underlie the ability of TGF-β to mediate tumor suppression in normal cells, and conversely, to facilitate cancer progression and disease dissemination in malignant cells.

TGF-β signal transduction spreading to a wider field: a broad variety of mechanisms for context-dependent effects of TGF-β

Transforming growth factor (TGF)-β signaling is involved in almost all major cell behaviors under physiological and pathological conditions, and its regulatory system has therefore been vigorously investigated. The fundamental elements in TGF-β signaling are TGF-β ligands, their receptors, and intracellular Smad effectors. The TGF-β ligand induces the receptors directly to phosphorylate and activate Smad proteins, which then form transcriptional complexes to control target genes. One of the classical questions in the field of research on TGF-β signaling is how this cytokine induces multiple cell responses depending on cell type and cellular context. Possible answers to this question include cross-interaction with other signaling pathways, different repertoires of Smad-binding transcription factors, and genetic alterations, especially in cancer cells. In addition to these genetic paradigms, recent work has extended TGF-β research into new fields, including epigenetic regulation and non-coding RNAs. In this review, we first describe the basic machinery of TGF-β signaling and discuss several factors that comprise TGF-β signaling networks. We then address mechanisms by which TGF-β induces several responses in a cell-context-dependent fashion. In addition to classical frames, the interaction of TGF-β signaling with epigenetics and microRNA is discussed.