Ligand-independent activation of transforming growth factor (TGF) beta signaling pathways by heteromeric cytoplasmic domains of TGF-beta receptors

Function and therapeutic potential of transient receptor potential ankyrin 1 in fibrosis

The transient receptor potential (TRP) protein superfamily is a special group of cation channels expressed in different cell types and signaling pathways. In this review, we focus on TRPA1 (transient receptor potential ankyrin 1), an ion channel in this family that exists in the cell membrane and shows a different function from other TRP channels. TRPA1 usually has a special activation effect that can induce cation ions, especially calcium ions, to flow into activated cells. In this paper, we review the role of TRPA1 in fibroblasts. To clarify the relationship between fibroblasts and TRPA1, we have also paid special attention to the interactions between TRPA1 and inflammatory factors leading to fibroblast activation. TRPA1 has different functions in the fibrosis process in different organs, and there have also been interesting discussions of the mechanism of TRPA1 in fibroblasts. Therefore, this review aims to describe the function of TRP channels in controlling fibrosis through fibroblasts in different organ inflammatory and immune-mediated diseases. We attempt to prove that TRPA1 is a target for fibrosis. In fact, some clinical trials have already proven that TRPA1 is a potential adjuvant therapy for treating fibrosis.

Curcumin Inhibition of TGFβ signaling in bone metastatic breast cancer cells and the possible role of oxidative metabolites

TGFβ signaling promotes progression of bone-metastatic (BMET) breast cancer (BCa) cells by driving tumor-associated osteolysis, a hallmark of BCa BMETs, thus allowing for tumor expansion within bone. Turmeric-derived bioactive curcumin, enriched in bone via local enzymatic deconjugation of inactive circulating curcumin-glucuronides, inhibits osteolysis and BMET progression in human xenograft BCa BMET models by blocking tumoral TGFβ signaling pathways mediating osteolysis. This is a unique antiosteolytic mechanism in contrast to current osteoclast-targeting therapeutics. Therefore, experiments were undertaken to elucidate the mechanism for curcumin inhibition of BCa TGFβ signaling and the application of this finding across multiple BCa cell lines forming TGFβ-dependent BMETs, including a possible role for bioactive curcumin metabolites in mediating these effects. Immunoblot analysis of TGFβ signaling proteins in bone tropic human (MDA-SA, MDA-1833, MDA-2287) and murine (4T1) BCa cells revealed uniform curcumin blockade of TGFβ-induced Smad activation due to down-regulation of plasma membrane associated TGFβR2 and cellular receptor Smad proteins that propagate Smad-mediated gene expression, resulting in downregulation of PTHrP expression, the osteolytic factor driving in vivo BMET progression. With the exception of early decreases in TGFβR2, inhibitory effects appeared to be mediated by oxidative metabolites of curcumin and involved inhibition of gene expression. Interestingly, while not contributing to changes in Smad-mediated TGFβ signaling, curcumin caused early activation of MAPK signaling in all cell lines, including JNK, an effect possibly involving interactions with TGFβR2 within lipid rafts. Treatment with curcumin or oxidizable analogs of curcumin may have clinical relevancy in the management of TGFβ-dependent BCa BMETs.

TGFβ signalling acts as a molecular brake of myoblast fusion

Fusion of nascent myoblasts to pre-existing myofibres is critical for skeletal muscle growth and repair. The vast majority of molecules known to regulate myoblast fusion are necessary in this process. Here, we uncover, through high-throughput in vitro assays and in vivo studies in the chicken embryo, that TGFβ (SMAD2/3-dependent) signalling acts specifically and uniquely as a molecular brake on muscle fusion. While constitutive activation of the pathway arrests fusion, its inhibition leads to a striking over-fusion phenotype. This dynamic control of TGFβ signalling in the embryonic muscle relies on a receptor complementation mechanism, prompted by the merging of myoblasts with myofibres, each carrying one component of the heterodimer receptor complex. The competence of myofibres to fuse is likely restored through endocytic degradation of activated receptors. Altogether, this study shows that muscle fusion relies on TGFβ signalling to regulate its pace.

Naturally occurring and synthetic constitutive-active cytokine receptors in disease and therapy

Cytokines control immune related events and are critically involved in a plethora of patho-physiological processes including autoimmunity and cancer development. Mutations which cause ligand-independent, constitutive activation of cytokine receptors are quite frequently found in diseases. Many constitutive-active cytokine receptor variants have been directly connected to disease development and mechanistically analyzed. Nature's solutions to generate constitutive cytokine receptors has been recently adopted by synthetic cytokine receptor biology, with the goal to optimize immune therapeutics. Here, CAR T cell immmunotherapy represents the first example to combine synthetic biology with genetic engineering during therapy. Hence, constitutive-active cytokine receptors are therapeutic targets, but also emerging tools to improve or modulate immunotherapeutic strategies. This review gives a comprehensive insight into the field of naturally occurring and synthetic constitutive-active cytokine receptors.

ALK phosphorylates SMAD4 on tyrosine to disable TGF-β tumour suppressor functions

Loss of TGF-β tumour suppressive response is a hallmark of human cancers. As a central player in TGF-β signal transduction, SMAD4 (also known as DPC4) is frequently mutated or deleted in gastrointestinal and pancreatic cancer. However, such genetic alterations are rare in most cancer types and the underlying mechanism for TGF-β resistance is not understood. Here we describe a mechanism of TGF-β resistance in ALK-positive tumours, including lymphoma, lung cancer and neuroblastoma. We demonstrate that, in ALK-positive tumours, ALK directly phosphorylates SMAD4 at Tyr 95. Phosphorylated SMAD4 is unable to bind to DNA and fails to elicit TGF-β gene responses and tumour suppressing responses. Chemical or genetic interference of the oncogenic ALK restores TGF-β responses in ALK-positive tumour cells. These findings reveal that SMAD4 is tyrosine-phosphorylated by an oncogenic tyrosine kinase during tumorigenesis. This suggests a mechanism by which SMAD4 is inactivated in cancers and provides guidance for targeted therapies in ALK-positive cancers.

Molecular cloning and characteristics analysis of Pmtgfbr1 from Pinctada fucata martensii

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This study obtains the full length of Pmtgfbr1 of the pearl oyster P. fucata martensii.

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Pmtgfbr1 possesses the conserved domain of Tgfbr1.

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Pmtgfbr1 holds negatively effect on the growth of P. fucata martensii.

Pinctada fucata martensii is cultured for pearl production. Growth improvement has received considerable research interest. Transforming growth factor β type Ⅰ receptor (TβR-I), which is involved in signals transmission of transforming growth factor beta (TGF-β), participates in cell proliferation and growth. In this study, we characterized a Tgfbr1 gene which encoded TβR-I from P. fucata martensii (Pmtgfbr1). Pmtgfbr1 cDNA contains an open reading frame of 1569 bp and encodes a polypeptide of 522 amino acids (aa). Pmtgfbr1 possesses a typical TβR-I structure (extracellular receptor ligand domain, transmembrane domain, and cytoplasmic tyrosine kinase catalytic domain). Pmtgfbr1 is expressed in all the studied tissues and exhibited the highest expression level in the adductor muscle. Moreover, Pmtgfbr1 exhibited the lower expression level in the larger group (L) than that in the smaller group (S) and is negatively correlated with growth traits (P < 0.01). Our results indicated that Pmtgfbr1 is a candidate functional gene associated with growth traits.

The Role of Transforming Growth Factor β in Cell-to-Cell Contact-Mediated Epstein-Barr Virus Transmission

Infection of Epstein–Barr virus (EBV), a ubiquitous human gamma herpesvirus, is closely linked to various lymphoid and epithelial malignancies. Previous studies demonstrated that the efficiency of EBV infection in epithelial cells is significantly enhanced by coculturing them with latently infected B cells relative to cell-free infection, suggesting that cell-to-cell contact-mediated viral transmission is the dominant mode of infection by EBV in epithelial cells. However, a detailed mechanism underlying this process has not been fully understood. In the present study, we assessed the role of transforming growth factor β (TGF-β), which is known to induce EBV's lytic cycle by upregulation of EBV's latent-lytic switch BZLF1 gene. We have found that 5 days of cocultivation facilitated cell-to-cell contact-mediated EBV transmission. Replication of EBV was induced in cocultured B cells both with and without a direct cell contact in a time-dependent manner. Treatment of a blocking antibody for TGF-β suppressed both induction of the lytic cycle in cocultured B cells and subsequent viral transmission. Cocultivation with epithelial cells facilitated expression of TGF-β receptors in B cells and increased their susceptibility to TGF-β. Finally, we confirmed the spontaneous secretion of TGF-β from epithelial cells, which was not affected by cell-contact. In contrast, the extracellular microvesicles, exosomes derived from cocultured cells partly contributed to cell-to-cell contact-mediated viral transmission. Taken together, our findings support a role for TGF-β derived from epithelial cells in efficient viral transmission, which fosters induction of the viral lytic cycle in the donor B cells.

Frizzled-8 integrates Wnt-11 and transforming growth factor-β signaling in prostate cancer

Wnt-11 promotes cancer cell migration and invasion independently of β-catenin but the receptors involved remain unknown. Here, we provide evidence that FZD₈ is a major Wnt-11 receptor in prostate cancer that integrates Wnt-11 and TGF-β signals to promote EMT. FZD8 mRNA is upregulated in multiple prostate cancer datasets and in metastatic cancer cell lines in vitro and in vivo. Analysis of patient samples reveals increased levels of FZD₈ in cancer, correlating with Wnt-11. FZD₈ co-localizes and co-immunoprecipitates with Wnt-11 and potentiates Wnt-11 activation of ATF2-dependent transcription. FZD8 silencing reduces prostate cancer cell migration, invasion, three-dimensional (3D) organotypic cell growth, expression of EMT-related genes, and TGF-β/Smad-dependent signaling. Mechanistically, FZD₈ forms a TGF-β-regulated complex with TGF-β receptors that is mediated by the extracellular domains of FZD₈ and TGFBR1. Targeting FZD₈ may therefore inhibit aberrant activation of both Wnt and TGF-β signals in prostate cancer.

Wnt11 has been shown to play a role in invasion and metastasis of prostate cancer. Here the authors show that in prostate cancer cells Wnt11 signals through the Fzd8 receptor and report an interaction between Fzd8 and TGF-β receptors regulating the transcription of a subset of TGF-beta genes.

CD4+ TCRγδ+ FoxP3+ cells: An unidentified population of immunosuppressive cells towards disease progression leprosy patients

This study, for the first time, reveals the role of M. leprae-specific CD4⁺ TCRγδ⁺ FoxP3⁺ cells in the progression and pathogenesis of leprosy. Co-culture with CD4⁺ CD25^- cells suggested the immunosuppressive nature of CD4⁺ TCRγδ⁺ cells in dose-dependent manner. Isolation of CD4⁺ TCRγδ⁺ cells from leprosy patients and then culture in presence of M. leprae cell wall antigens (MLCwA) along with TGF β, IPP and IL-2 suggested that these cells are M. leprae specific. TGF-β-mediated SMAD3 signalling was turned out to be major factor towards the expression of FoxP3 in these cells. SMAD3 silencing during induction of these cells barely showed the induction of FoxP3. High density of SMAD3 binding at TGFβRII in CD4⁺ TCRγδ⁺ FoxP3⁺ furthermore suggested the TGF-β-directed SMAD3 signalling in these cells. Taken together the above data, we can conclude that CD4⁺ TCRγδ⁺ FoxP3⁺ cells possess the potential to track the severity of the disease in leprosy patients.

TGF-β triggers rapid fibrillogenesis via a novel TβRII-dependent fibronectin-trafficking mechanism

There is increased recycling of soluble fibronectin from the cell surface for fibrillogenesis. This recycling is regulated by TGF-β in a transcription- and SMAD-independent manner via specific TβRII and integrin α5β1 interactions. The recycling of fibronectin is Rab11 dependent and is required for TGF-β–induced cell migration.

Fibronectin (FN) is a critical regulator of extracellular matrix (ECM) remodeling through its availability and stepwise polymerization for fibrillogenesis. Availability of FN is regulated by its synthesis and turnover, and fibrillogenesis is a multistep, integrin-dependent process essential for cell migration, proliferation, and tissue function. Transforming growth factor β (TGF-β) is an established regulator of ECM remodeling via transcriptional control of ECM proteins. Here we show that TGF-β, through increased FN trafficking in a transcription- and SMAD-independent manner, is a direct and rapid inducer of the fibrillogenesis required for TGF-β–induced cell migration. Whereas TGF-β signaling is dispensable for rapid fibrillogenesis, stable interactions between the cytoplasmic domain of the type II TGF-β receptor (TβRII) and the FN receptor (α5β1 integrin) are required. We find that, in response to TGF-β, cell surface–internalized FN is not degraded by the lysosome but instead undergoes recycling and incorporation into fibrils, a process dependent on TβRII. These findings are the first to show direct use of trafficked and recycled FN for fibrillogenesis, with a striking role for TGF-β in this process. Given the significant physiological consequences associated with FN availability and polymerization, our findings provide new insights into the regulation of fibrillogenesis for cellular homeostasis.

Connective Tissue Growth Factor Domain 4 Amplifies Fibrotic Kidney Disease through Activation of LDL Receptor-Related Protein 6

Connective tissue growth factor (CTGF), a matrix-associated protein with four distinct cytokine binding domains, has roles in vasculogenesis, wound healing responses, and fibrogenesis and is upregulated in fibroblasts and myofibroblasts in disease. Here, we investigated the role of CTGF in fibrogenic cells. In mice, tissue-specific inducible overexpression of CTGF by kidney pericytes and fibroblasts had no bearing on nephrogenesis or kidney homeostasis but exacerbated inflammation and fibrosis after ureteral obstruction. These effects required the WNT receptor LDL receptor-related protein 6 (LRP6). Additionally, pericytes isolated from these mice became hypermigratory and hyperproliferative on overexpression of CTGF. CTGF is cleaved in vivo into distinct domains. Treatment with recombinant domain 1, 1+2 (N terminus), or 4 (C terminus) independently activated myofibroblast differentiation and wound healing responses in cultured pericytes, but domain 4 showed the broadest profibrotic activity. Domain 4 exhibited low-affinity binding to LRP6 in in vitro binding assays, and inhibition of LRP6 or critical signaling cascades downstream of LRP6, including JNK and WNT/β-catenin, inhibited the biologic activity of domain 4. Administration of blocking antibodies specifically against CTGF domain 4 or recombinant Dickkopf-related protein-1, an endogenous inhibitor of LRP6, effectively inhibited inflammation and fibrosis associated with ureteral obstruction in vivo Therefore, domain 4 of CTGF and the WNT signaling pathway are important new targets in fibrosis.

Pericyte MyD88 and IRAK4 control inflammatory and fibrotic responses to tissue injury

Fibrotic disease is associated with matrix deposition that results in the loss of organ function. Pericytes, the precursors of myofibroblasts, are a source of pathological matrix collagens and may be promising targets for treating fibrogenesis. Here, we have shown that pericytes activate a TLR2/4- and MyD88-dependent proinflammatory program in response to tissue injury. Similarly to classic immune cells, pericytes activate the NLRP3 inflammasome, leading to IL-1β and IL-18 secretion. Released IL-1β signals through pericyte MyD88 to amplify this response. Unexpectedly, we found that MyD88 and its downstream effector kinase IRAK4 intrinsically control pericyte migration and conversion to myofibroblasts. Specific ablation of MyD88 in pericytes or pharmacological inhibition of MyD88 signaling by an IRAK4 inhibitor in vivo protected against kidney injury by profoundly attenuating tissue injury, activation, and differentiation of myofibroblasts. Our data show that in pericytes, MyD88 and IRAK4 are key regulators of 2 major injury responses: inflammatory and fibrogenic. Moreover, these findings suggest that disruption of this MyD88-dependent pathway in pericytes might be a potential therapeutic approach to inhibit fibrogenesis and promote regeneration.

A Small Dibromotyrosine Derivative Purified From Pseudoceratina Sp. Suppresses TGF-β Responsiveness by Inhibiting TGF-β Type I Receptor Serine/Threonine Kinase Activity

For clinical application, there is a great need for small-molecule inhibitors (SMIs) that could control pathogenic effects of transforming growth factor (TGF-β) and/or modulate effects of TGF-β in normal responses. Selective SMIs of the TGF-β signaling pathway developed for therapeutics will also be powerful tools in experimentally dissecting this complex pathway, especially its cross-talk with other signaling pathways. In this study, we characterized (1'R,5'S,6'S)-2-(3',5'-dibromo-1',6'-dihydroxy-4'-oxocyclohex-2'-enyl) acetonitrile (DT), a member of a new class of small-molecule inhibitors related to bromotyrosine derivate from Pseudoceratina sp., which inhibits the TGF-β type I receptor serine/threonine kinase known as activin receptor-like kinase (ALK) 5. The inhibitory effects of DT on TGF-β-induced Smad signaling and epithelial-to-mesenchymal transition (EMT) were investigated in epithelial cells using in vitro kinase assay, luciferase reporter assays, immunoblotting, confocal microscopy, and wound healing assays. The novel ALK5 inhibitor, DT, inhibited the TGF-β-stimulated transcriptional activations of 3TP-Lux. In addition, DT decreased phosphorylated Smad2/3 levels and the nuclear translocation of Smad2/3 increased by TGF-β. In addition, DT inhibited TGF-β-induced EMT and wound healing of A549 cells. Our results suggest that DT is a potential therapeutic agent for fibrotic disease and cancer treatment. J. Cell. Biochem. 117: 2800-2814, 2016. © 2016 Wiley Periodicals, Inc.

Discrete spatial organization of TGFβ receptors couples receptor multimerization and signaling to cellular tension

Cell surface receptors are central to the cell's ability to generate coordinated responses to the multitude of biochemical and physical cues in the microenvironment. However, the mechanisms by which receptors enable this concerted cellular response remain unclear. To investigate the effect of cellular tension on cell surface receptors, we combined novel high-resolution imaging and single particle tracking with established biochemical assays to examine TGFβ signaling. We find that TGFβ receptors are discretely organized to segregated spatial domains at the cell surface. Integrin-rich focal adhesions organize TβRII around TβRI, limiting the integration of TβRII while sequestering TβRI at these sites. Disruption of cellular tension leads to a collapse of this spatial organization and drives formation of heteromeric TβRI/TβRII complexes and Smad activation. This work details a novel mechanism by which cellular tension regulates TGFβ receptor organization, multimerization, and function, providing new insight into the mechanisms that integrate biochemical and physical cues.

DOI:http://dx.doi.org/10.7554/eLife.09300.001

Cells constantly encounter diverse physical and biological signals in their surroundings. Information contained in these signals is transmitted from the cell surface to the interior to trigger coordinated changes in the cell’s behavior. Physical signals include the forces generated by cells pulling on one another or on their surroundings. These pulling forces calibrate the cell’s response to biological signals through mechanisms that remain unclear.

The cell surface contains many different proteins that are specialized to sense these signals and guide the cell’s response. In animals, these membrane proteins include the receptors that detect a small signaling protein known as TGFβ. TGFβ first binds to one of these receptors (called TβRII). Next another receptor (called TβRI) is recruited to the complex. Once this complex is formed, the TGFβ receptors activate a complicated signaling pathway that controls how cells grow and divide. Previous work has shown that the TGFβ pathway can also sense and respond to mechanical forces. But it remains poorly understood how pulling forces (or tension) impact TGFβ receptors at the cell surface.

Rys, DuFort et al. have now used cutting-edge microscopy and biochemical techniques to analyze individual TβRI and TβRII receptors and observe how they respond to mechanical forces in real-time. This revealed that TβRI and TβRII exist in discrete regions on the cell surface. Rys, DuFort et al. observed that TβRI is enriched at assemblies of molecules called focal adhesions. Focal adhesions are the sites on cell surfaces that allow cells to adhere to one another and to the molecular scaffolding in their surroundings. Unlike TβRI, TβRII was often excluded from these sites and more commonly appeared to ‘bounce’ around the edges of individual focal adhesions. Therefore, focal adhesions limit the interactions between TβRI and TβRII, by sequestering one away from the other.

Rys, DuFort et al. next treated cells with a chemical that disrupts tension, and saw that the physical separation between TβRI and TβRII collapsed, which permitted these two receptors to interact and form a working signaling complex. Further work is needed to understand how physical control of TGFβ receptor interactions helps cells coordinate their tasks in response to the myriad biological and physical signals in their surroundings.

DOI:http://dx.doi.org/10.7554/eLife.09300.002

TRAF4 promotes TGF-β receptor signaling and drives breast cancer metastasis

TGF-β signaling is a therapeutic target in advanced cancers. We identified tumor necrosis factor receptor-associated factor 4 (TRAF4) as a key component mediating pro-oncogenic TGF-β-induced SMAD and non-SMAD signaling. Upon TGF-β stimulation, TRAF4 is recruited to the active TGF-β receptor complex, where it antagonizes E3 ligase SMURF2 and facilitates the recruitment of deubiquitinase USP15 to the TGF-β type I receptor (TβRI). Both processes contribute to TβRI stabilization on the plasma membrane and thereby enhance TGF-β signaling. In addition, the TGF-β receptor-TRAF4 interaction triggers Lys 63-linked TRAF4 polyubiquitylation and subsequent activation of the TGF-β-activated kinase (TAK)1. TRAF4 is required for efficient TGF-β-induced migration, epithelial-to-mesenchymal transition, and breast cancer metastasis. Elevated TRAF4 expression correlated with increased levels of phosphorylated SMAD2 and phosphorylated TAK1 as well as poor prognosis among breast cancer patients. Our results demonstrate that TRAF4 can regulate the TGF-β pathway and is a key determinant in breast cancer pathogenesis.

IHG-1 must be localised to mitochondria to decrease Smad7 expression and amplify TGF-β1-induced fibrotic responses

TGF-β1 is a prototypic profibrotic cytokine and major driver of fibrosis in the kidney and other organs. Induced in high glucose-1 (IHG-1) is a mitochondrial protein which we have recently reported to be associated with renal disease. IHG-1 amplifies responses to TGF-β1 and regulates mitochondrial biogenesis by stabilising the transcriptional co-activator peroxisome proliferator-activated receptor gamma coactivator-1-alpha. Here we report that the mitochondrial localisation of IHG-1 is pivotal in the amplification of TGF-β1 signalling. We demonstrate that IHG-1 expression is associated with repression of the endogenous TGF-β1 inhibitor Smad7. Intriguingly, expression of a non-mitochondrial deletion mutant of IHG-1 (Δmts-IHG-1) repressed TGF-β1 fibrotic signalling in renal epithelial cells. In cells expressing Δmts-IHG-1 fibrotic responses including CCN2/connective tissue growth factor, fibronectin and jagged-1 expression were reduced following stimulation with TGF-β1. Δmts-IHG-1 modulation of TGF-β1 signalling was associated with increased Smad7 protein expression. Δmts-IHG-1 modulated TGF-β1 activity by increasing Smad7 protein expression as it failed to inhibit TGF-β1 transcriptional responses when endogenous Smad7 expression was knocked down. These data indicate that mitochondria modulate TGF-β1 signal transduction and that IHG-1 is a key player in this modulation.

Phosphorylation of ΔNp63α via a novel TGFβ/ALK5 signaling mechanism mediates the anti-clonogenic effects of TGFβ

Genetic analysis of TP63 implicates ΔNp63 isoforms in preservation of replicative capacity and cellular lifespan within adult stem cells. ΔNp63α is also an oncogene and survival factor that mediates therapeutic resistance in squamous carcinomas. These diverse activities are the result of genetic and functional interactions between TP63 and an array of morphogenic and morphostatic signals that govern tissue and tumor stasis, mitotic polarity, and cell fate; however the cellular signals that account for specific functions of TP63 are incompletely understood. To address this we sought to identify signaling pathways that regulate expression, stability or activity of ΔNp63α. An siRNA-based screen of the human kinome identified the Type 1 TGFβ receptor, ALK5, as the kinase required for phosphorylation of ΔNp63α at Serine 66/68 (S66/68). This activity is TGFβ-dependent and sensitive to either ALK5-directed siRNA or the ALK5 kinase inhibitor A83-01. Mechanistic studies support a model in which ALK5 is proteolytically cleaved at the internal juxtamembrane region resulting in the translocation of the C-terminal ALK5-intracellular kinase domain (ALK5^IKD). In this study, we demonstrate that ALK5-mediated phosphorylation of ΔNp63α is required for the anti-clonogenic effects of TGFΒ and ectopic expression of ALK5^IKD mimics these effects. Finally, we present evidence that ultraviolet irradiation-mediated phosphorylation of ΔNp63α is sensitive to ALK5 inhibitors. These findings identify a non-canonical TGFβ-signaling pathway that mediates the anti-clonogenic effects of TGFβ and the effects of cellular stress via ΔNp63α phosphorylation.

Oligomeric interactions of TGF-β and BMP receptors

Transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) cytokines participate in a multiplicity of ways in the regulation of numerous physiological and pathological processes. Their wide-ranging biological functions are controlled by several mechanisms, including regulation of transcription, complex formation among the signaling receptors (oligomerization) and with co-receptors, binding of the receptors to scaffolding proteins or their targeting to specific membrane domains. Here, we address the generation of TGF-β and BMP receptor homo- and hetero-oligomers and its roles as a mechanism capable of fast regulation of signaling by these crucial cytokines. We examine the available biochemical, biophysical and structural evidence for the ternary structure of these complexes, and the possible roles of homomeric and heteromeric receptor oligomers in signaling.

Heavy ions can enhance TGFβ mediated epithelial to mesenchymal transition

TGFβ is a key modulator of the Epithelial-Mesenchymal Transition (EMT), a process important in cancer progression and metastasis, which leads to the suppression of epithelial genes and expression of mesenchymal proteins. Ionizing radiation was found to specifically induce expression of the TGF-β1 isoform, which can modulate late post-radiation changes and increase the risk of tumor development and metastasis. Interactions between TGFβ induced EMT and DNA damage responses have not been fully elucidated, particularly at low doses and following different radiation quality exposures. Further characterization of the relationship between radiation quality, EMT and cancer development is warranted. We investigated whether space radiation induced TGFβ dependent EMT, using hTERT immortalized human esophageal epithelial cells (EPC2-hTERT) and non-transformed mink lung epithelial cells (Mv1Lu). We have observed morphologic and molecular alterations in EPC2 and Mv1Lu cells consistent with EMT after pre-treatment with TGFβ1. This effect could be efficiently inhibited in both cell lines by the use of a TGFβRI inhibitor. High-energy silicon or iron nuclei were each able to cause a mild induction of EMT, with the inclusion of TGFβ1 inducing a greatly enhanced EMT phenotype even when cells were irradiated with doses as low as 0.1 Gy. A further enhancement of EMT was achieved at a higher dose of 2 Gy. TGFβRI inhibitor was able to reverse the EMT induced by the combination of TGFβ1 and radiation. These studies indicate that heavy ions, even at a low dose, may trigger the process of TGFβ1-induced EMT, and suggest further studies are needed to determine whether the chronic exposures received in space may potentiate this process in astronauts, leading to an increased risk of cancer.

Investigating the role of FGF-2 in stem cell maintenance by global phosphoproteomics profiling

Human embryonic stem cells (hESCs) are of immense interest for regenerative medicine as a source of tissue replacement. Expansion of hESCs as a pluripotent population requires a balance between survival, proliferation and self-renewal signals. One of the key growth factors that maintains this balance is fibroblast growth factor-2 (FGF-2). However, the underlying molecular mechanisms are poorly understood. We recently profiled specifically tyrosine phosphorylation events that occur during FGF-2 stimulation of hESCs (Ding et al., J. Cell. Physiol. 2010, 225, 417-428). Here, we complement this phosphoproteome profiling by analyzing temporal dynamics of mostly serine and threonine protein phosphorylation events. Our multi-dimensional strategy combines strong cation exchange chromatography to reduce the sample complexity followed by titanium dioxide off-line for the enrichment of phosphopeptides and dimethylation-based stable isotope labeling for quantification. This approach allowed us to identify and quantify 3261 unique proteins from which 1064 proteins were found to be phosphorylated in one or more residues (representing 1653 unique phosphopeptides). Approximately 40% of the proteins (553 unique phosphopeptides) showed differential phosphorylation upon FGF-2 treatment. Among those are several members of the canonical pathways involved in pluripotency and self-renewal (e.g. Wnt and PI3K/AKT), hESC-associated proteins such as SOX2, RIF1, SALL4, DPPA4, DNMT3B and 53 proteins that are target genes of the pluripotency transcription factors SOX2, OCT4 and NANOG. These findings complement existing pluripotency analyses and provide new insights into how FGF-2 assists in maintaining the undifferentiated state of hESCs.

Defective myofibroblast formation from mesenchymal stem cells in the aging murine heart rescue by activation of the AMPK pathway

Aged mice in a murine model of myocardial infarction exhibit less effective myocardial repair. We hypothesized that the deficiency arises from altered lineage choice of endogenous mesenchymal stem cells (MSCs) and faulty maturation of myofibroblasts. Examination of cardiac MSCs revealed a substantial reduction in the pluripotency marker Nanog in cells from aged mice. In addition, the aged MSCs demonstrated a redirected lineage choice that favored adipocytic commitment over fibroblast or myofibroblast differentiation. Fibroblasts derived from aged MSCs demonstrated reduced expression of transforming growth factor-β (TGF-β) receptors I and II and diminished SMAD3 phosphorylation, associated with attenuated contractility and migration. Overexpression of constitutively active TGF-β receptor I in aged cardiac fibroblasts ameliorated their defective motility but did not improve their contractility. Culturing of MSCs and fibroblasts with AICAR (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside) to activate adenosine monophosphate-activated kinase resulted in TGF-β-dependent development of myofibroblasts with markedly enhanced contractility despite no reduction in adipocytic commitment or increased expression of TGF-β receptors and SMAD3 phosphorylation. The data suggest an adenosine monophosphate-activated kinase-dependent gain of function as mediated by phosphorylation of TGF-β-activated kinase 1 and p38 mitogen-activated protein kinase, which amplifies the response to TGF-β1 via a non-canonical pathway, thus compensating for the reduced expression of TGF-β receptors.

TSC-22 promotes transforming growth factor β-mediated cardiac myofibroblast differentiation by antagonizing Smad7 activity

Transforming growth factor β (TGF-β) plays a critical role in tissue fibrosis. The duration and intensity of TGF-β signaling are tightly regulated. Here we report that TSC-22 (TGF-β-stimulated clone 22) facilitates TGF-β signaling by antagonizing Smad7 activity to increase receptor stability. TSC-22 enhances TGF-β-induced Smad2/3 phosphorylation and transcriptional responsiveness. The stimulatory effect of TSC-22 is dependent on Smad7, as silencing Smad7 expression abolishes it. TSC-22 interacts with TGF-β type I receptor TβRI and Smad7 in mutually exclusive ways and disrupts the association of Smad7/Smurfs with TβRI, thereby preventing ubiquitination and degradation of the receptor. We also found that TSC-22 can promote the differentiation of cardiac myofibroblasts by increasing expression of the fibrotic genes for α-smooth muscle actin (α-SMA), PAI-1, fibronectin, and collagen I, which is consistent with upregulation of TSC-22, phospho-Smad2/3, and the fibrotic genes in isoproterenol-induced rat myocardial fibrotic hearts. Taken together with the notion that TGF-β induces TSC-22 expression, our findings suggest that TSC-22 regulates TGF-β signaling via a positive-feedback mechanism and may contribute to myocardial fibrosis.

A systematic RNAi synthetic interaction screen reveals a link between p53 and snoRNP assembly

TP53 (tumour protein 53) is one of the most frequently mutated genes in human cancer and its role during cellular transformation has been studied extensively. However, the homeostatic functions of p53 are less well understood. Here, we explore the molecular dependency network of TP53 through an RNAi-mediated synthetic interaction screen employing two HCT116 isogenic cell lines and a genome-scale endoribonuclease-prepared short interfering RNA library. We identify a variety of TP53 synthetic interactions unmasking the complex connections of p53 to cellular physiology and growth control. Molecular dissection of the TP53 synthetic interaction with UNRIP indicates an enhanced dependency of TP53-negative cells on small nucleolar ribonucleoprotein (snoRNP) assembly. This dependency is mediated by the snoRNP chaperone gene NOLC1 (also known as NOPP140), which we identify as a physiological p53 target gene. This unanticipated function of TP53 in snoRNP assembly highlights the potential of RNAi-mediated synthetic interaction screens to dissect molecular pathways of tumour suppressor genes.

Homomeric and heteromeric complexes among TGF-β and BMP receptors and their roles in signaling

Transforming growth factor-β (TGF-β) ligands and bone morphogenetic proteins (BMPs) play myriad roles in many biological processes and diseases. Their pluripotent activities are subject to multiple levels of regulation, including receptor oligomerization, endocytosis, association with co-receptors, cellular scaffolds or membrane domains, as well as transcriptional control. In this review, we focus on TGF-β and BMP receptor homomeric and heteromeric complex formation and their modulation by ligand binding, which regulate signaling on a near-immediate timescale. We discuss the current structural, biochemical and biophysical evidence for the oligomerization of these receptors, and the potential roles of distinct oligomeric interactions in signaling.

The measure of success: constraints, objectives, and tradeoffs in morphogen-mediated patterning

A large, diverse, and growing number of strategies have been proposed to explain how morphogen gradients achieve robustness and precision. We argue that, to be useful, the evaluation of such strategies must take into account the constraints imposed by competing objectives and performance tradeoffs. This point is illustrated through a mathematical and computational analysis of the strategy of self-enhanced morphogen clearance. The results suggest that the usefulness of this strategy comes less from its ability to increase robustness to morphogen source fluctuations per se, than from its ability to overcome specific kinds of noise, and to increase the fraction of a morphogen gradient within which robust threshold positions may be established. This work also provides new insights into the longstanding question of why morphogen gradients show a maximum range in vivo.

TGF-beta receptors, in a Smad-independent manner, are required for terminal skeletal muscle differentiation

Skeletal muscle differentiation is strongly inhibited by transforming growth factor type beta (TGF-beta), although muscle formation as well as regeneration normally occurs in an environment rich in this growth factor. In this study, we evaluated the role of intracellular regulatory Smads proteins as well as TGF-beta-receptors (TGF-beta-Rs) during skeletal muscle differentiation. We found a decrease of TGF-beta signaling during differentiation. This phenomenon is explained by a decline in the levels of the regulatory proteins Smad-2, -3, and -4, a decrease in the phosphorylation of Smad-2 and lost of nuclear translocation of Smad-3 and -4 in response to TGF-beta. No change in the levels and inhibitory function of Smad-7 was observed. In contrast, we found that TGF-beta-R type I (TGF-beta-RI) and type II (TGF-beta-RII) increased on the cell surface during skeletal muscle differentiation. To analyze the direct role of the serine/threonine kinase activities of TGF-beta-Rs, we used the specific inhibitor SB 431542 and the dominant-negative form of TGF-beta-RII lacking the cytoplasmic domain. The TGF-beta-Rs were important for successful muscle formation, determined by the induction of myogenin, creatine kinase activity, and myosin. Silencing of Smad-2/3 expression by specific siRNA treatments accelerated myogenin, myosin expression, and myotube formation; although when SB 431542 was present inhibition in myosin induction and myotube formation was observed, suggesting that these last steps of skeletal muscle differentiation require active TGF-beta-Rs. These results suggest that both down-regulation of Smad regulatory proteins and cell signaling through the TGF-beta receptors independent of Smad proteins are essential for skeletal muscle differentiation.

Coupling of dephosphorylation and nuclear export of Smads in TGF-beta signaling

In eukaryotes, regulation of signaling mediators/effectors in the nucleus is one of the principal mechanisms that govern duration and strength of signaling. Smads are a family of structurally related intracellular proteins that serve as signaling effectors for transforming growth factor beta (TGF-beta) and TGF-beta-related proteins. Accumulating evidence demonstrates that Smads possess intrinsic nucleocytoplasmic shuttling capacity, which enables them to transmit TGF-beta signals from cell membrane to nucleus. We recently identified two important steps in the termination of nuclear Smad signaling. The first step is initiated by a serine/threonine phosphatase PPM1A that dephosphorylates Smad2/3 in the nucleus, thereby shutting down signaling capacity of phosphorylated Smad2/3. The second step involves nuclear export of dephosphorylated Smad2/3 with the aid of nuclear protein RanBP3 to terminate Smad signaling. This chapter introduces methods for examining nuclear export of Smad2/3 in TGF-beta signaling.

Transforming growth factor-beta (TGF-beta1) activates TAK1 via TAB1-mediated autophosphorylation, independent of TGF-beta receptor kinase activity in mesangial cells

Transforming growth factor-beta1 (TGF-beta1) is a multifunctional cytokine that signals through the interaction of type I (TbetaRI) and type II (TbetaRII) receptors to activate distinct intracellular pathways. TAK1 is a serine/threonine kinase that is rapidly activated by TGF-beta1. However, the molecular mechanism of TAK1 activation is incompletely understood. Here, we propose a mechanism whereby TAK1 is activated by TGF-beta1 in primary mouse mesangial cells. Under unstimulated conditions, endogenous TAK1 is stably associated with TbetaRI. TGF-beta1 stimulation causes rapid dissociation from the receptor and induces TAK1 phosphorylation. Deletion mutant analysis indicates that the juxtamembrane region including the GS domain of TbetaRI is crucial for its interaction with TAK1. Both TbetaRI-mediated TAK1 phosphorylation and TGF-beta1-induced TAK1 phosphorylation do not require kinase activity of TbetaRI. Moreover, TbetaRI-mediated TAK1 phosphorylation correlates with the degree of its association with TbetaRI and requires kinase activity of TAK1. TAB1 does not interact with TGF-beta receptors, but TAB1 is indispensable for TGF-beta1-induced TAK1 activation. We also show that TRAF6 and TAB2 are required for the interaction of TAK1 with TbetaRI and TGF-beta1-induced TAK1 activation in mouse mesangial cells. Taken together, our data indicate that TGF-beta1-induced interaction of TbetaRI and TbetaRII triggers dissociation of TAK1 from TbetaRI, and subsequently TAK1 is phosphorylated through TAB1-mediated autophosphorylation and not by the receptor kinase activity of TbetaRI.

Nordihydroguaiaretic acid inhibits transforming growth factor beta type 1 receptor activity and downstream signaling

It has been well documented that nordihydroguaiaretic acid (NDGA), a phenolic lignan isolated from the creosote bush, Larrea tridentate, has anti-cancer activity in vitro and in vivo. Several mechanisms have been identified that could contribute to these actions, as NDGA directly inhibits metabolic enzymes and receptor tyrosine kinases that are established anti-cancer targets. In the present study, we show that NDGA inhibits the transforming growth factor beta (TGF-beta) type I receptor, a serine threonine kinase receptor. In cultured cells, NDGA treatment repressed Smad2 phosphorylation induced by TGF-beta treatment and by a constitutively active mutant of TGF-beta type I receptor (T202D). NDGA also inhibited downstream transcriptional activation mediated by both TGF-beta treatment and the constitutively active mutant receptor. In vitro, NDGA inhibited TGF-beta type I receptor mediated Smad2 phosphorylation in crude cell lysates and in a purified preparation. Importantly, screening select analogs demonstrated that modification of NDGA's structure resulted in altered potency against the receptor. These results indicated that the structure of NDGA can be modified to achieve increased potency. Together our data provide a novel mechanism for NDGA activity which could help explain its anti-cancer activity, and suggest that NDGA could serve as a structural motif for developing serine/threonine kinase inhibitors with selectivity for TGF-beta type I receptor.

The extracellular domain of the TGFβ type II receptor regulates membrane raft partitioning

Cell-surface TGFβ (transforming growth factor β) receptors partition into membrane rafts and the caveolin-positive endocytic compartment by an unknown mechanism. In the present study, we investigated the determinant in the TGFβ type II receptor (TβRII) that is necessary for membrane raft/caveolar targeting. Using subcellular fractionation and immunofluorescence microscopy techniques, we demonstrated that the extracellular domain of TβRII mediates receptor partitioning into raft and caveolin-positive membrane domains. Pharmacological perturbation of glycosylation using tunicamycin or the mutation of Mgat5 [mannosyl(α-1,6)-glycoprotein β-1,6-N-acetylglucosaminyltransferase V] activity interfered with the raft partitioning of TβRII. However, this was not due to the glycosylation state of TβRII, as a non-glycosylated TβRII mutant remained enriched in membrane rafts. This suggested that other cell-surface glycoproteins associate with the extracellular domain of TβRII and direct their partitioning in membrane raft domains. To test this we analysed a GMCSF (granulocyte/macrophage colony-stimulating factor)–TβRII chimaeric receptor, which contains a glycosylated GMCSF extracellular domain fused to the transmembrane and intracellular domains of TβRII. This chimaeric receptor was found to be largely excluded from membrane rafts and caveolin-positive structures. Our results indicate that the extracellular domain of TβRII mediates receptor partitioning into membrane rafts and efficient entrance into caveolin-positive endosomes.

TGFbeta-stimulated Smad1/5 phosphorylation requires the ALK5 L45 loop and mediates the pro-migratory TGFbeta switch

During the course of breast cancer progression, normally dormant tumour-promoting effects of transforming growth factor beta (TGFbeta), including migration, invasion, and metastasis are unmasked. In an effort to identify mechanisms that regulate the pro-migratory TGFbeta 'switch' in mammary epithelial cells in vitro, we found that TGFbeta stimulates the phosphorylation of Smad1 and Smad5, which are typically associated with bone morphogenetic protein signalling. Mechanistically, this phosphorylation event requires the kinase activity and, unexpectedly, the L45 loop motif of the type I TGFbeta receptor, ALK5, as evidenced by studies using short hairpin RNA-resistant ALK5 mutants in ALK5-depleted cells and in vitro kinase assays. Functionally, Smad1/5 co-depletion studies demonstrate that this phosphorylation event is essential to the initiation and promotion of TGFbeta-stimulated migration. Moreover, this phosphorylation event is preferentially detected in permissive environments such as those created by tumorigenic cells or oncogene activation. Taken together, our data provide evidence that TGFbeta-stimulated Smad1/5 phosphorylation, which occurs through a non-canonical mechanism that challenges the notion of selective Smad phosphorylation by ALK5, mediates the pro-migratory TGFbeta switch in mammary epithelial cells.

TRAF6 mediates Smad-independent activation of JNK and p38 by TGF-beta

In many physiological and disease processes, TGF-beta usurps branches of MAP kinase pathways in conjunction with Smads to induce apoptosis and epithelial-to-mesenchymal transition, but the detailed mechanism of how a MAP kinase cascade is activated by TGF-beta receptors is not clear. We report here that TRAF6 is specifically required for the Smad-independent activation of JNK and p38, and its carboxyl TRAF homology domain physically interacts with TGF-beta receptors. TGF-beta induces K63-linked ubiquitination of TRAF6 and promotes association between TRAF6 and TAK1. Our results indicate that TGF-beta activates JNK and p38 through a mechanism similar to that operating in the interleukin-1beta/Toll-like receptor pathway.

IHG-1 amplifies TGF-beta1 signaling and is increased in renal fibrosis

Induced in high glucose-1 (IHG-1) is an evolutionarily conserved gene transcript upregulated by high extracellular glucose concentrations, but its function is unknown. Here, it is reported that the abundance of IHG-1 mRNA is nearly 10-fold higher in microdissected, tubule-rich renal biopsies from patients with diabetic nephropathy compared with control subjects. In the diabetic nephropathy specimens, in situ hybridization localized IHG-1 to tubular epithelial cells along with TGF-beta1 and activated Smad3, suggesting a possible role in the development of tubulointerstitial fibrosis. Supporting this possibility, IHG-1 mRNA and protein expression also increased with unilateral ureteral obstruction. In the HK-2 proximal tubule cell line, overexpression of IHG-1 increased TGF-beta1-stimulated expression of connective tissue growth factor and fibronectin. IHG-1 was found to amplify TGF-beta1-mediated transcriptional activity by increasing and prolonging phosphorylation of Smad3. Conversely, inhibition of endogenous IHG-1 with small interference RNA suppressed transcriptional responses to TGF-beta1. In summary, IHG-1, which increases in diabetic nephropathy, may enhance the actions of TGF-beta1 and contribute to the development of tubulointerstitial fibrosis.

The type I TGF-beta receptor is covalently modified and regulated by sumoylation

Post-translational sumoylation, the covalent attachment of a small ubiquitin-like modifier (SUMO), regulates the functions of proteins engaged in diverse processes. Often associated with nuclear and perinuclear proteins, such as transcription factors, it is not known whether SUMO can conjugate to cell-surface receptors for growth factors to regulate their functions. Here we show that the type I transforming growth factor-beta (TGF-beta) receptor, T beta RI, is sumoylated in response to TGF-beta and that its sumoylation requires the kinase activities of both T beta RI and the type II TGF-beta receptor, T beta RII. Sumoylation of T beta RI enhances receptor function by facilitating the recruitment and phosphorylation of Smad3, consequently regulating TGF-beta-induced transcription and growth inhibition. T beta RI sumoylation modulates the dissemination of transformed cells in a mouse model of T beta RI-stimulated metastasis. T beta RI sumoylation therefore controls responsiveness to TGF-beta, with implications for tumour progression. Sumoylation of cell-surface receptors may regulate other growth factor responses.

A delicate balance: TGF-beta and the tumor microenvironment

The activated form of TGF-beta is a known regulator of epithelial cell autonomous tumor initiation, progression, and metastasis. Recent studies have also indicated that TGF-beta mediates interactions between cancer cells and their local tumor microenvironment. Specifically, the loss of TGF-beta signaling in stromal components including fibroblasts and T-cells can result in an "activated" microenvironment that supports and even initiates transformation of adjacent epithelial cells. TGF-beta signaling in cancer can be regulated through mechanisms involving ligand activation and expression of essential components within the pathway including the receptors and downstream effectors. TGF-beta signaling in the tumor microenvironment significantly impacts carcinoma initiation, progression, and metastasis via epithelial cell autonomous and interdependent stromal-epithelial interactions in vivo.

TGF-beta activates Erk MAP kinase signalling through direct phosphorylation of ShcA

Erk1/Erk2 MAP kinases are key regulators of cell behaviour and their activation is generally associated with tyrosine kinase signalling. However, TGF-beta stimulation also activates Erk MAP kinases through an undefined mechanism, albeit to a much lower level than receptor tyrosine kinase stimulation. We report that upon TGF-beta stimulation, the activated TGF-beta type I receptor (TbetaRI) recruits and directly phosphorylates ShcA proteins on tyrosine and serine. This dual phosphorylation results from an intrinsic TbetaRI tyrosine kinase activity that complements its well-defined serine-threonine kinase function. TGF-beta-induced ShcA phosphorylation induces ShcA association with Grb2 and Sos, thereby initiating the well-characterised pathway linking receptor tyrosine kinases with Erk MAP kinases. We also found that TbetaRI is tyrosine phosphorylated in response to TGF-beta. Thus, TbetaRI, like the TGF-beta type II receptor, is a dual-specificity kinase. Recruitment of tyrosine kinase signalling pathways may account for aspects of TGF-beta biology that are independent of Smad signalling.

Flexible interaction of Drosophila Smad complexes with bipartite binding sites

A subset of BMP-responsive enhancer elements are characterized by pairing of a GC-rich Smad1 binding site and an SBE-type Smad4 binding site. Such paired, or bipartite, sites are in some cases just 5 bp apart and thus might be contacted by a single Smad1-Smad4 complex. Other potential pairings are separated as much as 60 bp but it is not known whether such longer distances can be spanned by a Smad1-Smad4 complex, indeed binding of native Smad1-Smad4 complexes to any of these bipartite elements has yet to be reported. Here we report that a complex of the homologous Drosophila Smad proteins, Mad and Medea, is capable of concerted binding to GC-rich and SBE sites separated by as much as 20 bp. The wider the separation, the more severely binding affinity was reduced by shortening of the linker region that tethers the DNA binding domain of Medea. In contrast, length of the Mad linker did not affect the allowed distance between paired sites, rather it contributes specifically to Mad contact with the GC-rich site. Finally, we show that Smad1 and Smad4 can participate in binding to bipartite sites.

Expression and function of TbetaRII-B, a variant of the type II TGF-beta receptor, in human chondrocytes

OBJECTIVE

Transforming growth factor-beta (TGF-beta) has profound effects on chondrocyte proliferation and matrix production, and dysregulation of TGF-beta action has been implicated in osteoarthritis. The mechanisms by which the diverse actions of TGF-beta are regulated in chondrocytes are unclear. Although it is well documented that TGF-beta signaling is transduced by types I and II receptors, other TGF-beta receptors may play critical roles by regulating signaling receptor activity. Our objective was to examine the expression of TbetaRII-B, a splice variant of the type II TGF-beta receptor, and to analyze its role in regulating TGF-beta signaling in human chondrocytes.

METHODS

TbetaRII-B expression was examined in human cartilage tissue specimens, human chondrocyte cell lines C28/I2 and tsT/AC62, and human primary chondrocytes by Western blot and reverse-transcriptase-polymerase chain reaction. Ligand binding and heteromerization of TbetaRII-B with other TGF-beta receptors on the cell surface were analyzed by affinity labeling, immunoprecipitation, and two-dimensional SDS-PAGE. Regulation of TGF-beta responses by TbetaRII-B was determined by examining Smad2 phosphorylation, Smad3-specific signaling, transcriptional activity, and type II collagen levels.

RESULTS

TbetaRII-B is expressed in normal and osteoarthritic human cartilage. Furthermore, it is a dynamic component of the TGF-beta receptor system in human chondrocytes, forming heteromeric complexes with the types I and II TGF-beta receptors, betaglycan and endoglin. Importantly, overexpression of TbetaRII-B leads to enhanced TGF-beta signaling and responses in chondrocytes.

CONCLUSIONS

These results suggest that TbetaRII-B may play a key role in the regulation of TGF-beta action in human chondrocytes.

Tumour microenvironment: TGFbeta: the molecular Jekyll and Hyde of cancer

Transforming growth factor-beta (TGFbeta) signalling regulates cancer through mechanisms that function either within the tumour cell itself or through host-tumour cell interactions. Studies of tumour-cell-autonomous TGFbeta effects show clearly that TGFbeta signalling has a mechanistic role in tumour suppression and tumour promotion. In addition, factors in the tumour microenvironment, such as fibroblasts, immune cells and the extracellular matrix, influence the ability of TGFbeta to promote or suppress carcinoma progression and metastasis. The complex nature of TGFbeta signalling and crosstalk in the tumour microenvironment presents a unique challenge, and an opportunity to develop therapeutic intervention strategies for targeting cancer.

Repression of Runx2 function by TGF-beta through recruitment of class II histone deacetylases by Smad3

Transforming growth factor-beta (TGF-beta) inhibits osteoblast differentiation through inhibition of the function of Runx2 (Cbfa1) by Smad3. The mechanism through which TGF-beta/Smad3 inhibits Runx2 function has not been characterized. We show that TGF-beta induces histone deacetylation, primarily of histone H4, at the osteocalcin promoter, which is repressed by TGF-beta, and that histone deacetylation is required for repression of Runx2 by TGF-beta. This repression occurs through the action of the class IIa histone deacetylases (HDAC)4 and 5, which are recruited through interaction with Smad3 to the Smad3/Runx2 complex at the Runx2-binding DNA sequence. Accordingly, HDAC4 or 5 is required for efficient TGF-beta-mediated inhibition of Runx2 function and is involved in osteoblast differentiation. Our results indicate that class IIa HDACs act as corepressors for TGF-beta/Smad3-mediated transcriptional repression of Runx2 function in differentiating osteoblasts and are cell-intrinsic regulators of osteoblast differentiation.

Repression of Bone Morphogenetic Protein and Activin-inducible Transcription by Evi-1

Smads, key effectors of transforming growth factor (TGF)-beta, activin, and bone morphogenetic protein (BMP) signaling, regulate gene expression and interact with coactivators and corepressors that modulate Smad activity. The corepressor Evi-1 exerts its oncogenic effects by repressing TGF-beta/Smad3-mediated transcription, thereby blocking TGF-beta-induced growth arrest. Because Evi-1 interacts with the highly conserved MH2 domain of Smad3, we investigated the physical and functional interaction of Evi-1 with Smad1 and Smad2, downstream targets of BMP and activin signaling, respectively. Evi-1 interacted with and repressed the receptor-activated transcription through Smad1 and Smad2, similarly to Smad3. In addition, Evi-1 repressed BMP/Smad1- and activin/Smad2-mediated induction of endogenous Xenopus gene expression, suggesting a role of repression of BMP and activin signals by Evi-1 in vertebrate embryogenesis. Evi-1 also repressed the induction of endogenous Smad7 expression by TGF-beta family ligands. In the course of these studies, we observed Evi-1 repression of Smad transactivation even when Smad binding to DNA was kept constant. We therefore explored the mechanism of Evi-1 repression of TGF-beta family-inducible transcription. Evi-1 repression did not result from displacement of Smad binding to DNA or to CREB-binding protein but from the recruitment of Evi-1 by Smad3 and CREB-binding protein to DNA. Following TGF-beta stimulation, Evi-1 and the associated corepressor CtBP were recruited to the endogenous Smad7 promoter. Evi-1 recruitment to the promoter decreased TGF-beta-induced histone acetylation, coincident with its repression of Smad7 gene expression. In this way, Evi-1 acts as a general Smad corepressor to inhibit TGF-beta-, activin-, and BMP-inducible transcription.

Regulation of the polarity protein Par6 by TGFbeta receptors controls epithelial cell plasticity

The transition of cells from an epithelial to a mesenchymal phenotype is a critical event during morphogenesis in multicellular organisms and underlies the pathology of many diseases, including the invasive phenotype associated with metastatic carcinomas. Transforming growth factor beta (TGFbeta) is a key regulator of epithelial-to-mesenchymal transition (EMT). However, the molecular mechanisms that control the dissolution of tight junctions, an early event in EMT, remain elusive. We demonstrate that Par6, a regulator of epithelial cell polarity and tight-junction assembly, interacts with TGFbeta receptors and is a substrate of the type II receptor, TbetaRII. Phosphorylation of Par6 is required for TGFbeta-dependent EMT in mammary gland epithelial cells and controls the interaction of Par6 with the E3 ubiquitin ligase Smurf1. Smurf1, in turn, targets the guanosine triphosphatase RhoA for degradation, thereby leading to a loss of tight junctions. These studies define how an extracellular cue signals to the polarity machinery to control epithelial cell morphology.

Daxx mediates the small ubiquitin-like modifier-dependent transcriptional repression of Smad4

Daxx has been shown to function as an apoptosis regulator and transcriptional repressor via its interaction with various cytoplasmic and nuclear proteins. Here, we showed that Daxx interacts with Smad4 and represses its transcriptional activity via the C-terminal domain of Daxx. In vitro and in vivo interaction studies indicated that the binding of Smad4 to Daxx depends on Smad4 sumoylation. Substitution of Smad4 SUMO conjugation residue lysine 159, but not 113, to arginine not only disrupted Smad4-Daxx interaction but also relieved Daxx-elicited repression of Smad4 transcriptional activity. Furthermore, chromatin immunoprecipitation analyses revealed the recruitment of Daxx to an endogenous, Smad4-targeted promoter in a Lys(159) sumoylation-dependent manner. Finally, down-regulation of Daxx expression by RNA interference enhanced transforming growth factor beta-induced transcription of reporter and endogenous genes through a Smad4-dependent, but not K159R-Smad4-dependent, manner. Together, these results indicate that Daxx suppresses Smad4-mediated transcriptional activity by direct interaction with the sumoylated Smad4 and identify a novel role of Daxx in regulating transforming growth factor beta signaling.

A molecular recognition paradigm: promiscuity associated with the ligand-receptor interactions of the activin members of the TGF-β superfamily

The structure-function properties of the pleiotropic activins and their relationship to other members of the transforming growth factor-beta superfamily of proteins are described. In order to highlight the molecular promiscuity of these growth factors, emphasis has been placed on molecular features associated with the recognition by activin A and the bone morphogenic proteins of the corresponding extracellular domains of the ActRI and ActRII receptors. The available evidence suggests that the homodimeric activin A in its various functional roles has the propensity to fulfill key tasks in the regulation of mammalian cell behaviour, through coordination of numerous transcriptional and translational processes. Because of these profound effects, under physiologically normal conditions, activin A levels are closely controlled by a variety of binding partners, such as follistatin-288 and follistatin-315, alpha(2)-macroglobulin and other proteins. Moreover, the subunits of other members of the activin subfamily, such as activin B or activin C, are able to form heterodimers with the activin A subunit, thus providing a further avenue to positively or negatively control the physiological concentrations of activin A that are available for interaction with specific receptors and induction of cell signaling events. Based on data from X-ray crystallographic studies and homology modeling experiments, the molecular architecture of the ternary receptor-activin ligand complexes has been dissected, permitting rationalization in structural terms of the pattern of interactions that are the hallmark of this protein family.

Highwire Regulates Presynaptic BMP Signaling Essential for Synaptic Growth

Highwire (Hiw), a putative RING finger E3 ubiquitin ligase, negatively regulates synaptic growth at the neuromuscular junction (NMJ) in Drosophila. hiw mutants have dramatically larger synaptic size and increased numbers of synaptic boutons. Here we show that Hiw binds to the Smad protein Medea (Med). Med is part of a presynaptic bone morphogenetic protein (BMP) signaling cascade consisting of three receptor subunits, Wit, Tkv, and Sax, in addition to the Smad transcription factor Mad. When compared to wild-type, mutants of BMP signaling components have smaller NMJ size, reduced neurotransmitter release, and aberrant synaptic ultrastructure. BMP signaling mutants suppress the excessive synaptic growth in hiw mutants. Activation of BMP signaling, which in wild-type does not cause additional growth, in hiw mutants does lead to further synaptic expansion. These results reveal a balance between positive BMP signaling and negative regulation by Highwire, governing the growth of neuromuscular synapses.