CREB binding protein is a required coactivator for Smad-dependent, transforming growth factor beta transcriptional responses in endothelial cells

Receptor-activated transcription factors and beyond: multiple modes of Smad2/3-dependent transmission of TGF-β signaling

Transforming growth factor β (TGF-β) is a pleiotropic cytokine that is widely distributed throughout the body. Its receptor proteins, TGF-β type I and type II receptors, are also ubiquitously expressed. Therefore, the regulation of various signaling outputs in a context-dependent manner is a critical issue in this field. Smad proteins were originally identified as signal-activated transcription factors similar to signal transducer and activator of transcription proteins. Smads are activated by serine phosphorylation mediated by intrinsic receptor dual specificity kinases of the TGF-β family, indicating that Smads are receptor-restricted effector molecules downstream of ligands of the TGF-β family. Smad proteins have other functions in addition to transcriptional regulation, including post-transcriptional regulation of micro-RNA processing, pre-mRNA splicing, and m6A methylation. Recent technical advances have identified a novel landscape of Smad-dependent signal transduction, including regulation of mitochondrial function without involving regulation of gene expression. Therefore, Smad proteins are receptor-activated transcription factors and also act as intracellular signaling modulators with multiple modes of function. In this review, we discuss the role of Smad proteins as receptor-activated transcription factors and beyond. We also describe the functional differences between Smad2 and Smad3, two receptor-activated Smad proteins downstream of TGF-β, activin, myostatin, growth and differentiation factor (GDF) 11, and Nodal.

SNIP1 and PRC2 coordinate cell fates of neural progenitors during brain development

Stem cell survival versus death is a developmentally programmed process essential for morphogenesis, sizing, and quality control of genome integrity and cell fates. Cell death is pervasive during development, but its programming is little known. Here, we report that Smad nuclear interacting protein 1 (SNIP1) promotes neural progenitor cell survival and neurogenesis and is, therefore, integral to brain development. The SNIP1-depleted brain exhibits dysplasia with robust induction of caspase 9-dependent apoptosis. Mechanistically, SNIP1 regulates target genes that promote cell survival and neurogenesis, and its activities are influenced by TGFβ and NFκB signaling pathways. Further, SNIP1 facilitates the genomic occupancy of Polycomb complex PRC2 and instructs H3K27me3 turnover at target genes. Depletion of PRC2 is sufficient to reduce apoptosis and brain dysplasia and to partially restore genetic programs in the SNIP1-depleted brain in vivo. These findings suggest a loci-specific regulation of PRC2 and H3K27 marks to toggle cell survival and death in the developing brain.

Intercellular coupling between peripheral circadian oscillators by TGF-β signaling

Coupling between cell-autonomous circadian oscillators is crucial to prevent desynchronization of cellular networks and disruption of circadian tissue functions. While neuronal oscillators within the mammalian central clock, the suprachiasmatic nucleus, couple intercellularly, coupling among peripheral oscillators is controversial and the molecular mechanisms are unknown. Using two- and three-dimensional mammalian culture models in vitro (mainly human U-2 OS cells) and ex vivo, we show that peripheral oscillators couple via paracrine pathways. We identify transforming growth factor-β (TGF-β) as peripheral coupling factor that mediates paracrine phase adjustment of molecular clocks through transcriptional regulation of core-clock genes. Disruption of TGF-β signaling causes desynchronization of oscillator networks resulting in reduced amplitude and increased sensitivity toward external zeitgebers. Our findings reveal an unknown mechanism for peripheral clock synchrony with implications for rhythmic organ functions and circadian health.

Recombinant human bone morphogenetic protein 2 and 7 inhibit the degeneration of intervertebral discs by blocking the Puma-dependent apoptotic signaling

Recombinant human bone morphogenetic proteins (rhBMPs) can stimulate bone formation and growth in the treatment of spinal fusions and nonunions. However, it is still unclear whether rhBMPs function in the prevention of intervertebral disc degeneration (IDD). Here, we discovered that BMP levels were decreased in IDD patients, which impaired the BMP/Smad (Mothers against decapentaplegic homologs) signaling. Conducting a microarray assay in Smad4-knockdown cells, we found that expression of PUMA (p53-upregulated modulator of apoptosis) was significantly induced. The molecular analysis revealed that Smad4 recruited HDAC1 (histone deacetylase 1) and the phosphorylated Smad1/5/8 to dock on the promoter of PUMA to repress its expression. The impairment of BMP/Smad signaling in IDD patients caused the significant induction of Puma-dependent apoptosis and resulted in the pathogenesis of IDD. In vitro knockdown of BMP receptors (BMPR1a and BMPR2) in nucleus pulposus (NP) cells could mimic the molecular changes of BMP/Smad signaling and Puma-dependent apoptotic signaling that were observed in IDD patients. Exposing NP cells to RITA (reactivating p53 and inducing tumor apoptosis) small molecule and rhBMP2 (or rhBMP7), we observed that rhBMP2/7 could significantly decrease protein levels of Puma and its downstream proapoptotic molecules, blocking cell apoptosis. Importantly, administration of rhBMPs in aged rats could inhibit the occurrence of IDD. Our results provide a link between BMP/Smad signaling and Puma-dependent apoptotic signaling, revealing a new mechanism of how BMPs contribute to IDD pathogenesis and providing evidence that rhBMPs may decrease apoptosis and improve the outcome of IDD.

Structural basis for transcriptional coactivator recognition by SMAD2 in TGF-β signaling

Transforming growth factor-β (TGF-β) proteins regulate multiple cellular functions, including cell proliferation, apoptosis, and extracellular matrix formation. The dysregulation of TGF-β signaling causes diseases such as cancer and fibrosis, and therefore, understanding the biochemical basis of TGF-β signal transduction is important for elucidating pathogenic mechanisms in these diseases. SMAD proteins are transcription factors that mediate TGF-β signaling-dependent gene expression. The transcriptional coactivator CBP directly interacts with the MH2 domains of SMAD2 to activate SMAD complex-dependent gene expression. Here, we report the structural basis for CBP recognition by SMAD2. The crystal structures of the SMAD2 MH2 domain in complex with the SMAD2-binding region of CBP showed that CBP forms an amphiphilic helix on the hydrophobic surface of SMAD2. The expression of a mutated CBP peptide that showed increased SMAD2 binding repressed SMAD2-dependent gene expression in response to TGF-β signaling in cultured cells. Disrupting the interaction between SMAD2 and CBP may therefore be a promising strategy for suppressing SMAD-dependent gene expression.

CREB acts as a common transcription factor for major epigenetic repressors; DNMT3B, EZH2, CUL4B and E2F6

CREB signaling is known for several decades, but how it regulates both positive and negative regulators of cell proliferation is not well understood. On the other hand functions of major epigenetic repressors such as DNMT3B, EZH2 and CUL4B for their repressive epigenetic modifications on chromatin have also been well studied. However, there is very limited information available on how these repressors are regulated at their transcriptional level. Here, using computational tools and molecular techniques including site directed mutagenesis, promoter reporter assay, chromatin immunoprecipitation (ChIP), we identified that CREB acts as a common transcription factor for DNMT3B, EZH2, CUL4B and E2F6. ChIP assay revealed that pCREB binds to promoters of these repressors at CREs and induce their transcription. As expected, the expression of these repressors and their associated repressive marks particularly H3K27me3 and H2AK119ub are increased and decreased upon CREB overexpression and knock-down conditions respectively in the cancer cells indicating that CREB regulates the functions of these repressors by activating their transcription. Since CREB and these epigenetic repressors are overexpressed in various cancer types, our findings showed the molecular relationship between them and indicate that CREB is an important therapeutic target for cancer therapy.

Repression of Inappropriate Gene Expression in the Vertebrate Embryonic Ectoderm

During vertebrate embryogenesis, precise regulation of gene expression is crucial for proper cell fate determination. Much of what we know about vertebrate development has been gleaned from experiments performed on embryos of the amphibian Xenopus laevis; this review will focus primarily on studies of this model organism. An early critical step during vertebrate development is the formation of the three primary germ layers-ectoderm, mesoderm, and endoderm-which emerge during the process of gastrulation. While much attention has been focused on the induction of mesoderm and endoderm, it has become clear that differentiation of the ectoderm involves more than the simple absence of inductive cues; rather, it additionally requires the inhibition of mesendoderm-promoting genes. This review aims to summarize our current understanding of the various inhibitors of inappropriate gene expression in the presumptive ectoderm.

The Dynamic Roles of TGF-β Signalling in EBV-Associated Cancers

The transforming growth factor-β (TGF-β) signalling pathway plays a critical role in carcinogenesis. It has a biphasic action by initially suppressing tumorigenesis but promoting tumour progression in the later stages of disease. Consequently, the functional outcome of TGF-β signalling is strongly context-dependent and is influenced by various factors including cell, tissue and cancer type. Disruption of this pathway can be caused by various means, including genetic and environmental factors. A number of human viruses have been shown to modulate TGF-β signalling during tumorigenesis. In this review, we describe how this pathway is perturbed in Epstein-Barr virus (EBV)-associated cancers and how EBV interferes with TGF-β signal transduction. The role of TGF-β in regulating the EBV life cycle in tumour cells is also discussed.

DNA Tumor Virus Regulation of Host DNA Methylation and Its Implications for Immune Evasion and Oncogenesis

Viruses have evolved various mechanisms to evade host immunity and ensure efficient viral replication and persistence. Several DNA tumor viruses modulate host DNA methyltransferases for epigenetic dysregulation of immune-related gene expression in host cells. The host immune responses suppressed by virus-induced aberrant DNA methylation are also frequently involved in antitumor immune responses. Here, we describe viral mechanisms and virus–host interactions by which DNA tumor viruses regulate host DNA methylation to evade antiviral immunity, which may contribute to the generation of an immunosuppressive microenvironment during cancer development. Recent trials of immunotherapies have shown promising results to treat multiple cancers; however, a significant number of non-responders necessitate identifying additional targets for cancer immunotherapies. Thus, understanding immune evasion mechanisms of cancer-causing viruses may provide great insights for reversing immune suppression to prevent and treat associated cancers.

Chromatin Modulatory Proteins and Olfactory Receptor Signaling in the Refinement and Maintenance of Fruitless Expression in Olfactory Receptor Neurons

During development, sensory neurons must choose identities that allow them to detect specific signals and connect with appropriate target neurons. Ultimately, these sensory neurons will successfully integrate into appropriate neural circuits to generate defined motor outputs, or behavior. This integration requires a developmental coordination between the identity of the neuron and the identity of the circuit. The mechanisms that underlie this coordination are currently unknown. Here, we describe two modes of regulation that coordinate the sensory identities of Drosophila melanogaster olfactory receptor neurons (ORNs) involved in sex-specific behaviors with the sex-specific behavioral circuit identity marker fruitless (fru). The first mode involves a developmental program that coordinately restricts to appropriate ORNs the expression of fru and two olfactory receptors (Or47b and Ir84a) involved in sex-specific behaviors. This regulation requires the chromatin modulatory protein Alhambra (Alh). The second mode relies on the signaling from the olfactory receptors through CamK and histone acetyl transferase p300/CBP to maintain ORN-specific fru expression. Our results highlight two feed-forward regulatory mechanisms with both developmentally hardwired and olfactory receptor activity-dependent components that establish and maintain fru expression in ORNs. Such a dual mechanism of fru regulation in ORNs might be a trait of neurons driving plastic aspects of sex-specific behaviors.

ELL Protein-associated Factor 2 (EAF2) Inhibits Transforming Growth Factor β Signaling through a Direct Interaction with Smad3

A series of in vitro and in vivo studies has shown that EAF2 can affect multiple signaling pathways involved in cellular processes. However, the molecular mechanisms underlying its effects have remained elusive. Here we report the discovery of a new functional link between EAF2 and TGF-β signaling. Promoter reporter assays indicated that EAF2 suppresses Smad3 transcriptional activity, resulting in inhibition of TGF-β signaling. Coimmunoprecipitation assays showed that EAF2 specifically interacts with Smad3 in vitro and in vivo but not with other Smad proteins. In addition, we observed that EAF2 binding does not alter Smad3 phosphorylation but causes Smad3 cytoplasmic retention, competes with Smad4 for binding to Smad3, and prevents p300-Smad3 complex formation. Furthermore, we demonstrated that EAF2 suppresses both TGF-β-induced G1 cell cycle arrest and TGF-β-induced cell migration. This study identifies and characterizes a novel repressor of TGF-β signaling.

CRH suppressed TGFβ1-induced Epithelial-Mesenchymal Transition via induction of E-cadherin in breast cancer cells

Since its discovery in biopsies from breast cancer patients, the effect of corticotropin-releasing hormone (CRH) on carcinoma progression is still unclear. Transforming growth factorβ1 (TGFβ1) promotes Epithelial-Mesenchymal Transition (EMT) and induces Snail1 and Twist1 expressions. Loss of epithelial cadherin (E-cadherin) mainly repressed by Snail1 and Twist1, has been considered as hallmark of Epithelial-Mesenchymal Transition (EMT). Two breast cancer cell lines, MCF-7 and MDA-MB-231 were used to investigate the effect of CRH on TGFβ1-induced EMT by transwell chamber. And HEK293 cells were transiently transfected with CRHR1 or CRHR2 to explore the definite effects of CRH receptor. We reported that CRH inhibited migration of human breast cancer cells through downregulation of Snail1 and Twist1, and subsequent upregulation of E-cadherin. CRH inhibited TGFβ1-mediated migration of MCF-7 via both CRHR1 and CRHR2 while this inhibition in MDA-MB-231 was mainly via CRHR2. Ectopic re-expression of CRHR1 or CRHR2 respectively in HEK293 cells increased E-cadherin expression after CRH stimulation. Furthermore, CRH repressed expression of mesenchymal marker, N-cadherin and induced expression of Occludin, inhibiting EMT in MCF-7 & MDA-MB-231. Our results suggest that CRH may function as a tumor suppressor, at least partly by regulating TGFβ1-mediated EMT. These results may contribute to uncovering the effect of CRH in breast tumorigenesis and progression.

Molecular basis of organ fibrosis: potential therapeutic approaches

Fibrosis, a non-physiological wound healing in multiple organs, is associated with end-stage pathological symptoms of a wide variety of vascular injury and inflammation related diseases. In response to chemical, immunological and physical insults, the body's defense system and matrix synthetic machinery respond to healing the wound and maintain tissue homeostasis. However, uncontrolled wound healing leads to scarring or fibrosis, a pathological condition characterized by excessive synthesis and accumulation of extracellular matrix proteins, loss of tissue homeostasis and organ failure. Understanding the actual cause of pathological wound healing and identification of igniter(s) of fibrogenesis would be helpful to design novel therapeutic approaches to control pathological wound healing and to prevent fibrosis related morbidity and mortality. In this article, we review the significance of a few key cytokines (TGF-β, IFN-γ, IL-10) transcriptional activators (Sp1, Egr-1, Smad3), repressors (Smad7, Fli-1, PPAR-γ, p53, Klotho) and epigenetic modulators (acetyltransferase, methyltransferases, deacetylases, microRNAs) involved in major matrix protein collagen synthesis under pathological stage of wound healing, and the potentiality of these regulators as therapeutic targets for fibrosis treatment. The significance of endothelial to mesenchymal transition (EndMT) and senescence, two newly emerged fields in fibrosis research, has also been discussed.

Smad2 is essential for maintenance of the human and mouse primed pluripotent stem cell state

Human embryonic stem cells and mouse epiblast stem cells represent a primed pluripotent stem cell state that requires TGF-β/activin signaling. TGF-β and/or activin are commonly thought to regulate transcription through both Smad2 and Smad3. However, the different contributions of these two Smads to primed pluripotency and the downstream events that they may regulate remain poorly understood. We addressed the individual roles of Smad2 and Smad3 in the maintenance of primed pluripotency. We found that Smad2, but not Smad3, is required to maintain the undifferentiated pluripotent state. We defined a Smad2 regulatory circuit in human embryonic stem cells and mouse epiblast stem cells, in which Smad2 acts through binding to regulatory promoter sequences to activate Nanog expression while in parallel repressing autocrine bone morphogenetic protein signaling. Increased autocrine bone morphogenetic protein signaling caused by Smad2 down-regulation leads to cell differentiation toward the trophectoderm, mesoderm, and germ cell lineages. Additionally, induction of Cdx2 expression, as a result of decreased Smad2 expression, leads to repression of Oct4 expression, which, together with the decreased Nanog expression, accelerates the loss of pluripotency. These findings reveal that Smad2 is a unique integrator of transcription and signaling events and is essential for the maintenance of the mouse and human primed pluripotent stem cell state.

Aptamers and their potential to selectively target aspects of EGF, Wnt/β-catenin and TGFβ-smad family signaling

The smooth identification and low-cost production of highly specific agents that interfere with signaling cascades by targeting an active domain in surface receptors, cytoplasmic and nuclear effector proteins, remain important challenges in biomedical research. We propose that peptide aptamers can provide a very useful and new alternative for interfering with protein–protein interactions in intracellular signal transduction cascades, including those emanating from activated receptors for growth factors. By their targeting of short, linear motif type of interactions, peptide aptamers have joined nucleic acid aptamers for use in signaling studies because of their ease of production, their stability, their high specificity and affinity for individual target proteins, and their use in high-throughput screening protocols. Furthermore, they are entering clinical trials for treatment of several complex, pathological conditions. Here, we present a brief survey of the use of aptamers in signaling pathways, in particular of polypeptide growth factors, starting with the published as well as potential applications of aptamers targeting Epidermal Growth Factor Receptor signaling. We then discuss the opportunities for using aptamers in other complex pathways, including Wnt/β-catenin, and focus on Transforming Growth Factor-β/Smad family signaling.

Herbal formula Astragali Radix and Rehmanniae Radix exerted wound healing effect on human skin fibroblast cell line Hs27 via the activation of transformation growth factor (TGF-β) pathway and promoting extracellular matrix (ECM) deposition

Astragali Radix (AR) and Rehmanniae Radix (RR) have long been used in traditional Chinese Medicine and as the principal herbs in treating diabetic foot ulcer. In this study, we investigated the effect of NF3, which comprises of AR and RR in the ratio of 2:1(w/w), on skin fibroblast cell migration and the activation of selected genes and proteins related to wound healing. Human skin fibroblast cell line Hs27 was treated with NF3 at 4 mg/ml for 24h, and in vitro scratch wound healing and quantitative cell migration assays were performed, respectively. The expression of transformation growth factor (TGF-β1) and bone morphogenetic protein 6 (BMP6) in Hs27 cells with or without NF3 treatment was analyzed by western blot analysis. In addition, the expression of a panel of genes involved in human TGF-β signaling pathway was analyzed in Hs27 cells upon NF3 treatment (4 mg/ml, 24 h) by quantitative real-time PCR (qRT-PCR). Furthermore, the expression of several genes and proteins associated with ECM synthesis was investigated by qRT-PCR analysis or/and ELISA techniques. The results suggested that NF3 promoted the migration of human skin fibroblast cells. Western blot analysis demonstrated that NF3 up-regulated TGF-β1 and BMP-6 synthesis. qRT-PCR analysis revealed that the expression of 26 genes in Hs27 cells was changed upon NF3 induction, including TGF-β superfamily ligands and down stream effectors genes, and genes involved in TGF/Smad pathway, and Ras/MAPK (non-Smad) pathway. Among the extracellular matrix (ECM)-related molecules, it was found that NF3 up-regulated the expression of type I and III collagens, fibronectin as well as TIMP-1, and down-regulated the MMP-9 expression in skin fibroblast cells. This study demonstrated that herb formula NF3 could enhance skin fibroblast cell migration and activated genes involved in TGF-β1 pathway. NF3 could regulate gene transcription for extracellular matrix synthesis via the Smad pathway, and gene transcription for cell motility via the Ras/MAPK (non-Smad) pathway.

Diversity in genetic in vivo methods for protein-protein interaction studies: from the yeast two-hybrid system to the mammalian split-luciferase system

The yeast two-hybrid system pioneered the field of in vivo protein-protein interaction methods and undisputedly gave rise to a palette of ingenious techniques that are constantly pushing further the limits of the original method. Sensitivity and selectivity have improved because of various technical tricks and experimental designs. Here we present an exhaustive overview of the genetic approaches available to study in vivo binary protein interactions, based on two-hybrid and protein fragment complementation assays. These methods have been engineered and employed successfully in microorganisms such as Saccharomyces cerevisiae and Escherichia coli, but also in higher eukaryotes. From single binary pairwise interactions to whole-genome interactome mapping, the self-reassembly concept has been employed widely. Innovative studies report the use of proteins such as ubiquitin, dihydrofolate reductase, and adenylate cyclase as reconstituted reporters. Protein fragment complementation assays have extended the possibilities in protein-protein interaction studies, with technologies that enable spatial and temporal analyses of protein complexes. In addition, one-hybrid and three-hybrid systems have broadened the types of interactions that can be studied and the findings that can be obtained. Applications of these technologies are discussed, together with the advantages and limitations of the available assays.

TGF-β - an excellent servant but a bad master

The transforming growth factor (TGF-β) family of growth factors controls an immense number of cellular responses and figures prominently in development and homeostasis of most human tissues. Work over the past decades has revealed significant insight into the TGF-β signal transduction network, such as activation of serine/threonine receptors through ligand binding, activation of SMAD proteins through phosphorylation, regulation of target genes expression in association with DNA-binding partners and regulation of SMAD activity and degradation. Disruption of the TGF-β pathway has been implicated in many human diseases, including solid and hematopoietic tumors. As a potent inhibitor of cell proliferation, TGF-β acts as a tumor suppressor; however in tumor cells, TGF-β looses anti-proliferative response and become an oncogenic factor. This article reviews current understanding of TGF-β signaling and different mechanisms that lead to its impairment in various solid tumors and hematological malignancies.

Structural basis for the versatile interactions of Smad7 with regulator WW domains in TGF-β Pathways

Transforming growth factor (TGF)-β and BMP signaling is mediated by Smads 1-5 (R-Smads and Co-Smads) and inhibited by Smad7, a major hub of regulation of TGF-β and BMP receptors by negative feedback and antagonistic signals. The transcription coactivator YAP and the E3 ubiquitin ligases Smurf1/2 and Nedd4L target R-Smads for activation or degradation, respectively. Pairs of WW domain in these regulators bind PY motifs and adjacent CDK/MAPK and GSK3 phosphorylation sites in R-Smads in a selective and regulated manner. In contrast, here we show that Smad7 binds YAP, Smurf1, Smurf2, and Nedd4L constitutively, the binding involving a PY motif in Smad7 and no phosphorylation. We also provide a structural basis for how regulators that use WW domain pairs for selective interactions with R-Smads, resort to one single versatile WW domain for binding Smad7 to centralize regulation in the TGF-β and BMP pathways.

Down-regulation of CREB-binding protein expression inhibits thrombin-induced proliferation of endothelial cells: possible relevance to PDGF-B

Thrombin acts as a potent mitogenic factor for ECs (endothelial cells) by the release of several growth factors, including PDGF-B (platelet-derived growth factor-B). CBP (CREB-binding protein), which functions as a transcriptional coactivator, links the changes in the extracellular stimuli with alterations in gene expression. Therefore, we hypothesized that CBP could mediate thrombin-induced proliferation of ECs via PDGF-B-dependent way. Short hairpin RNA was used to down-regulate the expression of CBP in ECs. CBP and PDGF-B levels were analysed by real-time RT-PCR and Western blot. To evaluate ECs proliferation, cell cycle and DNA synthesis were analysed by flow cytometry and BrdU (bromodeoxyuridine) incorporation assay, respectively. PDGF-B was involved in the mitogenic effect of thrombin on ECs. Down-regulation of CBP attenuated ECs proliferation and inhibited cell cycle progression induced by thrombin. Silencing CBP expression also suppressed thrombin-induced PDGF-B expression in ECs. Mitogenic activity of thrombin was impaired by silencing CBP expression in ECs. This inhibitory effect was, in part, related to the inability to up-regulate PDGF-B expression in ECs. CBP could be regarded as a potential therapeutic target for vascular injury.

Continuous supply of TGFβ3 via adenoviral vector promotes type I collagen and viability of fibroblasts in alginate hydrogel

In recent years, transforming growth factor-β3 (TGFβ3) has interested more and more researchers with its competence in engineered histogenesis. In the present study we employed recombinant adenoviral vectors to deliver the constitutively active TGFβ3 gene to human dermal fibroblasts, which could maintain the continuous secretion of TGFβ3 from the cells. The expression of type I collagen in the Ad-TGFβ3 group increased significantly in comparison with other three groups: Neg (cells without treatment of the adenovirus), Ad-null (cells with treatment of the adenovirus, without the inserted gene) and Ad-shRNA (cells with treatment of the adenovirus encoding shRNA specific for type I collagen). Additionally, we demonstrated that TGFβ3 enhanced the expression of Smad4 while inhibiting that of MMP-9, thus promoting the collagen transcription via the Smad signal transduction pathway and restraining collagen degradation by MMP-9, which contributed to the increasing type I collagen expression level. As type I collagen mediates cell-material interactions by providing anchorage, the viability of encapsulated fibroblasts in Ad-TGFβ3 group was significantly higher than that in other three groups. Accordingly, this approach forms an effective way to improve the compatibility of non-adhesive hydrogels containing anchorage-dependent cells.

Mammalian two-hybrids come of age

A diverse series of mammalian two-hybrid technologies for the detection of protein-protein interactions have emerged in the past few years, complementing the established yeast two-hybrid approach. Given the mammalian background in which they operate, these assays open new avenues to study the dynamics of mammalian protein interaction networks, i.e. the temporal, spatial and functional modulation of protein-protein associations. In addition, novel assay formats are available that enable high-throughput mammalian two-hybrid applications, facilitating their use in large-scale interactome mapping projects. Finally, as they can be applied in drug discovery and development programs, these techniques also offer exciting new opportunities for biomedical research.

Acetylation of KLF5 alters the assembly of p15 transcription factors in transforming growth factor-beta-mediated induction in epithelial cells

KLF5 plays important roles in a variety of cellular processes including proliferation and differentiation. Recently KLF5 was shown to reverse its function in proliferative and p15 regulation upon transforming growth factor-beta (TGFbeta)-mediated acetylation. To understand how KLF5 acetylation functions in TGFbeta-induced p15 transcription, we characterized the interactions of KLF5 with other transcription factors and promoter DNA elements in the context of TGFbeta. KLF5 interacted with Smad2-4 and Miz-1 in a TGFbeta-independent manner, but interacted with Myc only when TGFbeta was activated, and at least some of the interactions had an additive effect on TGFbeta-induced p15 transcription. Oligo pulldown assays showed that binding of Myc to the Inr element was KLF5-dependent, and TGFbeta could enhance the binding when more KLF5 was available. Furthermore, TGFbeta induced an interaction between KLF5 and the p300 acetylase, and acetylation of KLF5 was necessary for Smad4 to associate with p300. Failure in KLF5 acetylation not only prevented p300-assembled Smad4-KLF5 complex formation on p15 promoter but also affected the binding of Smad4 and FOXO3 on the p15 promoter in vivo. These findings suggest that without TGFbeta, some KLF5 associates with Smads in the nucleus and other KLF5 associates with Miz-1 on the p15 promoter to repress its transcription. Activation of TGFbeta recruits p300 to the KLF5-Smad complex to acetylate KLF5, and the complex with acetylated KLF5 binds to the Smad binding element and alters the binding of other factors to p15 promoter to induce its transcription.

The pericyte: cellular regulator of microvascular blood flow

The vascular system - through its development, response to injury, and remodeling during disease - constitutes one of the key organ systems sustaining normal human physiology; conversely, its dysregulation also underlies multiple pathophysiologic processes. Regulation of vascular endothelial cell function requires the integration of complex signals via multiple cell types, including arterial smooth muscle, capillary and post-capillary pericytes, and other perivascular cells such as glial and immune cells. Here, we focus on the pericyte and its roles in microvascular remodeling, reviewing current concepts in microvascular pathophysiology and offering new insights into the specific roles that pericyte-dependent signaling pathways may play in modulating endothelial growth and microvascular tone during pathologic angiogenesis and essential hypertension.

How the Smads regulate transcription

The primary signalling pathway downstream of ligands of the transforming growth factor beta (TGF-beta) superfamily is the Smad pathway. Activated receptors phosphorylate receptor-regulated Smads, which form homomeric complexes and heteromeric complexes with Smad4. These activated Smad complexes accumulate in the nucleus, where they are directly involved in the regulation of transcription of target genes. This apparently very simple pathway is subject to complex regulation, much of which is at the level of post-translational modifications of pathway components, in particular, the Smads. The enzymes responsible may be constitutively active, may be cell type-specific or may be regulated by other signalling pathways or by the cell cycle. In this way, signals from TGF-beta superfamily ligands are integrated with signals from other growth factors and cytokines, are regulated by the cell cycle and are dependent on cell type. This may go some way to explaining the pleiotropic nature of TGF-beta superfamily responses. In this review we focus on the mechanisms whereby the Smads are modified and regulated. We then go on to discuss how the activated Smad complexes regulate transcription.

A tale of two proteins: differential roles and regulation of Smad2 and Smad3 in TGF-beta signaling

Transforming growth factor-beta (TGF-beta) is an important growth inhibitor of epithelial cells, and insensitivity to this cytokine results in uncontrolled cell proliferation and can contribute to tumorigenesis. Smad2 and Smad3 are direct mediators of TGF-beta signaling, however little is known about the selective activation of Smad2 versus Smad3. The Smad2 and Smad3 knockout mouse phenotypes and studies comparing Smad2 and Smad3 activation of TGF-beta target genes, suggest that Smad2 and Smad3 have distinct roles in TGF-beta signaling. The observation that TGF-beta inhibits proliferation of Smad3-null mammary gland epithelial cells, whereas Smad3 deficient fibroblasts are only partially growth inhibited, suggests that Smad3 has a different role in epithelial cells and fibroblasts. Herein, the current understanding of Smad2 and Smad3-mediated TGF-beta signaling and their relative roles are discussed, in addition to potential mechanisms for the selective activation of Smad2 versus Smad3. Since alterations in the TGF-beta signaling pathway play an important role in promoting tumorigenesis and cancer progression, methods for therapeutic targeting of the TGF-beta signaling pathway are being pursued. Determining how Smad2 or Smad3 differentially regulate the TGF-beta response may translate into developing more effective strategies for cancer therapy.

Competition between Ski and CREB-binding protein for binding to Smad proteins in transforming growth factor-beta signaling

The family of Smad proteins mediates transforming growth factor-beta (TGF-beta) signaling in cell growth and differentiation. Smads repress or activate TGF-beta signaling by interacting with corepressors (e.g. Ski) or coactivators (e.g. CREB-binding protein (CBP)), respectively. Specifically, Ski has been shown to interfere with the interaction between Smad3 and CBP. However, it is unclear whether Ski competes with CBP for binding to Smads and whether they can interact with Smad3 at the same binding surface on Smad3. We investigated the interactions among purified constructs of Smad, Ski, and CBP in vitro by size-exclusion chromatography, isothermal titration calorimetry, and mutational studies. Here, we show that Ski-(16-192) interacted directly with a homotrimer of receptor-regulated Smad protein (R-Smad), e.g. Smad2 or Smad3, to form a hexamer; Ski-(16-192) interacted with an R-Smad.Smad4 heterotrimer to form a pentamer. CBP-(1941-1992) was also found to interact directly with an R-Smad homotrimer to form a hexamer and with an R-Smad.Smad4 heterotrimer to form a pentamer. Moreover, these domains of Ski and CBP competed with each other for binding to Smad3. Our mutational studies revealed that domains of Ski and CBP interacted with Smad3 at a portion of the binding surface of the Smad anchor for receptor activation. Our results suggest that Ski negatively regulates TGF-beta signaling by replacing CBP in R-Smad complexes. Our working model suggests that Smad protein activity is delicately balanced by Ski and CBP in the TGF-beta pathway.

RTK and TGF-beta signaling pathways genes in the sea urchin genome

The Receptor Tyrosine kinase (RTK) and TGF-beta signaling pathways play essential roles during development in many organisms and regulate a plethora of cellular responses. From the genome sequence of Strongylocentrotus purpuratus, we have made an inventory of the genes encoding receptor tyrosine kinases and their ligands, and of the genes encoding cytokines of the TGF-beta superfamily and their downstream components. The sea urchin genome contains at least 20 genes coding for canonical receptor tyrosine kinases. Seventeen of the nineteen vertebrate RTK families are represented in the sea urchin. Fourteen of these RTK among which ALK, CCK4/PTK7, DDR, EGFR, EPH, LMR, MET/RON, MUSK, RET, ROR, ROS, RYK, TIE and TRK are present as single copy genes while pairs of related genes are present for VEGFR, FGFR and INSR. Similarly, nearly all the subfamilies of TGF-beta ligands identified in vertebrates are present in the sea urchin genome including the BMP, ADMP, GDF, Activin, Myostatin, Nodal and Lefty, as well as the TGF-beta sensu stricto that had not been characterized in invertebrates so far. Expression analysis indicates that the early expression of nodal, BMP2/4 and lefty is restricted to the oral ectoderm reflecting their role in providing positional information along the oral-aboral axis of the embryo. The coincidence between the emergence of TGF-beta-related factors such as Nodal and Lefty and the emergence of the deuterostome lineage strongly suggests that the ancestral function of Nodal could have been related to the secondary opening of the mouth which characterizes this clade, a hypothesis supported by functional data in the extant species. The sea urchin genome contains 6 genes encoding TGF-beta receptors and 4 genes encoding prototypical Smad proteins. Furthermore, most of the transcriptional activators and repressors shown to interact with Smads in vertebrates have orthologues in echinoderms. Finally, the sea urchin genome contains an almost complete repertoire of genes encoding extracellular modulators of BMP signaling including Chordin, Noggin, Sclerotin, SFRP, Gremlin, DAN and Twisted gastrulation. Taken together, these findings indicate that the sea urchin complement of genes of the RTK and TGF-beta signaling pathways is qualitatively very similar to the repertoire present in vertebrates, and that these genes are part of the common genetool kit for intercellular signaling of deuterostomes.

E7 proteins from high- and low-risk human papillomaviruses bind to TGF-β-regulated Smad proteins and inhibit their transcriptional activity

Human papillomaviruses (HPV) infect keratinocytes of skin and mucosa. Persistent infection can lead to the formation of benign tumors. In cases of high-risk HPV, such as HPV16 or 18, these may further progress to cancer. In order to support viral replication in suprabasal keratinocytes, the HPV E7 protein employs various strategies to keep keratinocytes in cycle and counteracts anti-proliferative signals from outside. HPV16 E7 can directly interfere with transforming growth factor-beta (TGF-beta) signalling by binding to Smad proteins mediating growth arrest. It has been speculated that this property of HPV16 E7 contributes to HPV-associated carcinogenesis. Here, we show that E7 proteins from different low- and high-risk HPV types bind to Smad 1 to 4. The E7 protein from HPV1, a low-risk HPV causing plantar warts, efficiently inhibited Smad 3-induced transcription. Our data strongly indicate that the Smad-binding capacity of E7 proteins from different HPVs may preserve keratinocyte proliferation required for the productive viral life cycle rather than promoting carcinogenesis.

Cited2 modulates TGF-beta-mediated upregulation of MMP9

Cited (CBP/p300-interacting transactivators with glutamic acid (E)/aspartic acid (D)-rich C-terminal domain) 2, which is a CBP/p300-binding transcription co-activator without typical DNA-binding domains, has been implicated in control of cell growth and malignant transformation in Rat1 cells. In this report, we provide evidence that Cited2 is an important regulator of transforming growth factor (TGF)-beta signaling. Overexpression of Cited2 enhanced TGF-beta-mediated transcription of a Smad-Binding Element-containing luciferase reporter construct, SBE4-Luc. This may occur through a direct physical association of Cited2 with Smads 2 and 3, as supported by co-immunoprecipitation, mammalian two-hybrid and glutathione S-transferase-pull down assays. The transcription factor p300, which binds to Smad3, was shown to further enhance the interaction between Cited2 and Smad3, and the transcriptional responses of Smad3 by Cited2 in reporter assays. Cited2 enhances TGF-beta-mediated upregulation of matrix metalloproteinase 9 (MMP9) in Cited2 inducible mouse embryo fibroblasts. Overexpression of Cited2 enhanced TGF-beta-mediated MMP9 promoter reporter activity. Moreover, knockdown of Cited2 in MDA-MB-231 cells attenuated TGF-beta-mediated upregulation of MMP9 and TGF-beta-mediated cell invasion. Chromatin immunoprecipitation showed that Cited2 and Smad3 were recruited to MMP9 promoter upon TGF-beta stimulation. This is the first demonstration that Cited2 functions as a Smad3/p300-interacting transcriptional co-activator in modulating the expression of MMP9, which could affect tumor cell invasion mediated by TGF-beta.

Conditional knockout mice reveal distinct functions for the global transcriptional coactivators CBP and p300 in T-cell development

The global transcriptional coactivators CREB-binding protein (CBP) and the closely related p300 interact with over 312 proteins, making them among the most heavily connected hubs in the known mammalian protein-protein interactome. It is largely uncertain, however, if these interactions are important in specific cell lineages of adult animals, as homozygous null mutations in either CBP or p300 result in early embryonic lethality in mice. Here we describe a Cre/LoxP conditional p300 null allele (p300flox) that allows for the temporal and tissue-specific inactivation of p300. We used mice carrying p300flox and a CBP conditional knockout allele (CBPflox) in conjunction with an Lck-Cre transgene to delete CBP and p300 starting at the CD4- CD8- double-negative thymocyte stage of T-cell development. Loss of either p300 or CBP led to a decrease in CD4+ CD8+ double-positive thymocytes, but an increase in the percentage of CD8+ single-positive thymocytes seen in CBP mutant mice was not observed in p300 mutants. T cells completely lacking both CBP and p300 did not develop normally and were nonexistent or very rare in the periphery, however. T cells lacking CBP or p300 had reduced tumor necrosis factor alpha gene expression in response to phorbol ester and ionophore, while signal-responsive gene expression in CBP- or p300-deficient macrophages was largely intact. Thus, CBP and p300 each supply a surprising degree of redundant coactivation capacity in T cells and macrophages, although each gene has also unique properties in thymocyte development.

Smad transcription factors

Smad transcription factors lie at the core of one of the most versatile cytokine signaling pathways in metazoan biology-the transforming growth factor-beta (TGFbeta) pathway. Recent progress has shed light into the processes of Smad activation and deactivation, nucleocytoplasmic dynamics, and assembly of transcriptional complexes. A rich repertoire of regulatory devices exerts control over each step of the Smad pathway. This knowledge is enabling work on more complex questions about the organization, integration, and modulation of Smad-dependent transcriptional programs. We are beginning to uncover self-enabled gene response cascades, graded Smad response mechanisms, and Smad-dependent synexpression groups. Our growing understanding of TGFbeta signaling through the Smad pathway provides general principles for how animal cells translate complex inputs into concrete behavior.

A novel biological assay to detect the active form of TGF-beta in urine to monitor renal allograft rejection

BACKGROUND

Transforming growth factor-beta (TGF-beta) plays an important role in renal fibrosis. Measurement of the concentration of the active form of TGF-beta particularly in urine may help our understanding of the mechanism of chronic allograft nephropathy and could be used as a diagnostic tool. However, TGF-beta release and activation are complex and, consequently, there is currently no accurate way to measure TGF-beta activity.

METHODS

TGF-beta-sensitive BL41 cells were stably transfected with a reporter plasmid harboring a synthetic TGF-beta-inducible DNA sequence upstream from the luciferase gene. Cells were incubated with urine samples from normal donors or transplanted recipients with or without patent nephropathy, and the active form of TGF-beta was determined as luciferase activity.

RESULTS

We have established a cell line which expresses luciferase activity in response to active TGF-beta in a dose-dependent manner. Moreover, the use of a histone deacetylase inhibitor greatly increased sensitivity to TGF-beta and also stabilized luciferase inductibility. This test is highly specific to active TGF-beta. Detectable levels of TGF-beta were found in urine from patients with renal dysfunction due to acute or chronic renal allograft rejection (P < 0.001), but not in that from patients with stable, correctly functional kidneys.

CONCLUSION

We describe a highly sensitive and specific assay for active TGF-beta. We also show that, in cases of renal allograft, TGF-beta expression is highly and significantly correlated with acute or chronic rejections.

Smads and chromatin modulation

Smad proteins are critical intracellular effector proteins and regulators of transforming growth factor type beta (TGFbeta) modulated gene transcription. They directly convey signals that initiate at ligand-bound receptor complexes and end in the nucleus with changes in programs of gene expression. Activated Smad proteins seem to recruit chromatin modifying proteins to target genes besides cooperating with DNA-bound transcription factors. We survey here the current and still emerging knowledge on Smad-binding factors, and their different mechanisms of chromatin modification in particular, in Smad-dependent TGFbeta signaling.

Tsc-22 enhances TGF-beta signaling by associating with Smad4 and induces erythroid cell differentiation

Tsc-22 was isolated as a TGF-beta-inducible gene by differential screening of the mouse osteoblastic cell cDNA library [J Biol Chem 267 (1992) 10219]. tsc-22 mRNA is expressed in almost all organs of mice and humans and its expression is induced in a variety of cell lines by many different factors including TGF-beta, phorbol ester, serum, and progestin. tsc-22 encodes a 18-kd protein that contains a leucine zipper motif and a Tsc-box. The leucine zipper motif of the Tsc-22 protein does not have a basic DNA binding motif and when the protein was fused to a heterologous DNA binding domain, it showed various transcription-modulating activities ranging from activation to repression [J Biol Chem 274 (1999) 27439, Biochem Biophys Res Commun 278 (2000) 659]. Although these results suggest that the Tsc-22 protein functions as a transcriptional regulator recruiting various coactivators or repressors, its mechanism is not known. In this study, we examined whether Tsc-22 modulates the TGF-beta-dependant signaling pathway and found that Tsc-22 binds to and modulate the transcriptional activity of Smad3 and Smad4. Its effect on cellular differentiation was also examined.

Selective inhibition of TGF-beta responsive genes by Smad-interacting peptide aptamers from FoxH1, Lef1 and CBP

Transforming growth factor beta (TGF-beta) stimulation results in the assembly of Smad-containing protein complexes that mediate activation or repression of TGF-beta responsive genes. To determine if disruption of specific Smad protein-protein interactions would selectively inhibit responses to TGF-beta or generally interfere with Smad-dependent signaling, we developed three Smad-binding peptide aptamers by introducing Smad interaction motifs from Smad-binding proteins CBP, FoxH1 and Lef1 into the scaffold protein E. coli thioredoxin A (Trx). All three classes of aptamers bound to Smads by GST pulldown assays and co-immunoprecipitation from mammalian cells. Expression of the aptamers in HepG2 cells did not generally inhibit Smad-dependent signaling as evaluated using seven TGF-beta responsive luciferase reporter genes. The Trx-xFoxH1b aptamer inhibited TGF-beta-induced expression from a reporter dependent on the Smad-FoxH1 interaction, A3-lux, by 50%. Trx-xFoxH1b also partially inhibited two reporters not dependent on a Smad-FoxH1 interaction, 3TP-lux and Twntop, and endogenous PAI-1 expression. Trx-Lef1 aptamer only inhibited expression of the Smad-Lef1 responsive reporter gene TwnTop. The Trx-CBP aptamer had no significant effect on reporter gene expression. The results suggest that Smad-binding peptide aptamers can be developed to selectively inhibit TGF-beta-induced gene expression.

Fibroblast expression of the coactivator p300 governs the intensity of profibrotic response to transforming growth factor beta

OBJECTIVE

Transforming growth factor beta (TGFbeta) induces profibrotic responses in normal fibroblasts, and plays a fundamental role in the pathogenesis of fibrosis in scleroderma (systemic sclerosis [SSc]). The intensity of cellular responses elicited by cytokines is modulated by transcriptional coactivators such as the histone acetylase p300. The objective of these studies was to delineate the physiologic role of p300 in Smad-dependent profibrotic responses elicited by TGFbeta.

METHODS

Ectopic p300 was transiently expressed in normal dermal fibroblasts. Cellular p300 levels were suppressed using p300-specific ribozymes. The regulation of gene expression was examined by transient transfection assays, Northern blotting, and immunoblot analysis. The expression of p300 in normal and scleroderma fibroblasts was evaluated by confocal microscopy and immunoblotting, and p300 levels in skin from mice with experimental scleroderma were assessed by immunohistochemistry.

RESULTS

In normal fibroblasts, TGFbeta induced an increase in the levels of p300. Forced expression of ectopic p300 in these cells dramatically enhanced the magnitude of TGFbeta responses, whereas selective depletion of p300 using ribozyme resulted in abrogation of TGFbeta-induced collagen synthesis and promoter activity. Furthermore, TGFbeta lost its ability to induce Smad-dependent transcription in p300-depleted fibroblasts. These responses could be fully rescued with ectopic p300. Abrogation of Smad-mediated TGFbeta signaling was not due to alterations in the levels or the ligand-dependent phosphorylation or intracellular trafficking of endogenous Smads. Immunohistochemical analysis demonstrated substantially increased p300 expression in lesional skin from mice with chronic graft-versus-host disease, an animal model of scleroderma. Furthermore, levels of p300 were 2-3-fold higher in cultured fibroblasts derived from SSc patients than in fibroblasts from matched normal controls.

CONCLUSION

These results establish, for the first time, that the coactivator histone acetylase p300, itself a target of TGFbeta regulation, is an essential component of the cellular TGFbeta signal transduction pathways mediating stimulation of collagen synthesis in fibroblasts. Since the cellular abundance of p300 appears to govern the intensity of profibrotic responses elicited by TGFbeta, elevated p300 expression in lesional tissue may contribute to the progression of skin fibrosis in scleroderma.

The steroid receptor co-activator-1 (SRC-1) potentiates TGF-beta/Smad signaling: role of p300/CBP

The three related 160-kDa proteins, SRC-1, TIF-2 and RAC-3, were initially identified as factors interacting with nuclear receptors. They have also been reported to potentiate the activity of other transcription factors such as AP-1 or NF-kappaB. The aim of this work was to identify whether SRC-1 interferes with the TGF-beta/Smad signaling pathway, and if so, to identify its underlying mechanisms of action. Using transient cell transfection experiments performed in human dermal fibroblasts with the Smad3/4-specific (SBE)4-lux reporter construct, as well as the human PAI-1 promoter, we determined that SRC-1 enhances TGF-beta-induced, Smad-mediated, transcription. Likewise, SRC-1 overexpression potentiated TGF-beta-induced upregulation of PAI-1 steady-state mRNA levels. Using a mammalian two-hybrid system, we demonstrated that SRC-1 interacts with the transcriptional co-activators p300/CBP, but not with Smad3. Overexpression of the adenovirus E1A oncoprotein, an inhibitor of CBP/p300 activity, prevented the enhancing effect of SRC-1 on Smad3/4-mediated transcription, indicating that p300/CBP may be required for SRC-1 effect. Such hypothesis was validated, as expression of a mutant form of SRC-1 lacking the CBP/p300-binding site failed to upregulate Smad3/4-dependent transcription, while full-length SRC-1 potentiated p300.Smad3 interactions. These results identify SRC-1 as a novel Smad3/4 transcriptional partner, facilitating the functional link between Smad3 and p300/CBP.

The Smad3 linker region contains a transcriptional activation domain

Transforming growth factor-beta (TGF-beta)/Smads regulate a wide variety of biological responses through transcriptional regulation of target genes. Smad3 plays a key role in TGF-beta/Smad-mediated transcriptional responses. Here, we show that the proline-rich linker region of Smad3 contains a transcriptional activation domain. When the linker region is fused to a heterologous DNA-binding domain, it activates transcription. We show that the linker region physically interacts with p300. The adenovirus E1a protein, which binds to p300, inhibits the transcriptional activity of the linker region, and overexpression of p300 can rescue the linker-mediated transcriptional activation. In contrast, an adenovirus E1a mutant, which cannot bind to p300, does not inhibit the linker-mediated transcription. The native Smad3 protein lacking the linker region is unable to mediate TGF-beta transcriptional activation responses, although it can be phosphorylated by the TGF-beta receptor at the C-terminal tail and has a significantly increased ability to form a heteromeric complex with Smad4. We show further that the linker region and the C-terminal domain of Smad3 synergize for transcriptional activation in the presence of TGF-beta. Thus our findings uncover an important function of the Smad3 linker region in Smad-mediated transcriptional control.

Epigenetic histone modification and cardiovascular lineage programming in mouse embryonic stem cells exposed to laminar shear stress

Experimental evidence indicates that shear stress (SS) exerts a morphogenetic function during cardiac development of mouse and zebrafish embryos. However, the molecular basis for this effect is still elusive. Our previous work described that in adult endothelial cells, SS regulates gene expression by inducing epigenetic modification of histones and activation of transcription complexes bearing acetyltransferase activity. In this study, we evaluated whether SS treatment could epigenetically modify histones and influence cell differentiation in mouse embryonic stem (ES) cells. Cells were exposed to a laminar SS of 10 dyne per cm2/s(-1), or kept in static conditions in the presence or absence of the histone deacetylase inhibitor trichostatin A (TSA). These experiments revealed that SS enhanced lysine acetylation of histone H3 at position 14 (K14), as well as serine phosphorylation at position 10 (S10) and lysine methylation at position 79 (K79), and cooperated with TSA, inducing acetylation of histone H4 and phosphoacetylation of S10 and K14 of histone H3. In addition, ES cells exposed to SS strongly activated transcription from the vascular endothelial growth factor (VEGF) receptor 2 promoter. This effect was paralleled by an early induction of cardiovascular markers, including smooth muscle actin, smooth muscle protein 22-alpha, platelet-endothelial cell adhesion molecule-1, VEGF receptor 2, myocyte enhancer factor-2C (MEF2C), and alpha-sarcomeric actin. In this condition, transcription factors MEF2C and Sma/MAD homolog protein 4 could be isolated from SS-treated ES cells complexed with the cAMP response element-binding protein acetyltransferase. These results provide molecular basis for the SS-dependent cardiovascular commitment of mouse ES cells and suggest that laminar flow may be successfully applied for the in vitro production of cardiovascular precursors.

TGF-beta-induced SMAD signaling and gene regulation: consequences for extracellular matrix remodeling and wound healing

Members of the transforming growth factor-beta (TGF-beta) superfamily are pleiotropic cytokines that have the ability to regulate numerous cell functions, including proliferation, differentiation, apoptosis, epithelial-mesenchymal transition, and production of extracellular matrix, allowing them to play an important role during embryonic development and for maintenance of tissue homeostasis. Three TGF-beta isoforms have been identified in mammals. They propagate their signal via a signal transduction network involving receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins. Upon phosphorylation and oligomerization, the latter move into the nucleus to regulate transcription of target genes. This review will summarize recent advances in the understanding of the mechanisms underlying SMAD modulation of extracellular matrix gene expression in the context of wound healing and tissue fibrosis.

STAT1-Independent Down-Regulation of Interferon-Gamma-Induced Class II Transactivator and HLA-DR Expression by Transforming Growth Factor Beta-1 in Human Glomerular Endothelial Cells

BACKGROUND

The competition between STAT1 and Smad3 for a limiting amount of the nuclear protein p300, a transcriptional coactivator, was suggested to be a mechanism for the antagonism between interferon-gamma (IFN-gamma) and transforming growth factor-beta1 (TGF-beta1). We investigated the effect of TGF-beta1 on IFN-gamma-induced HLA-DR production in cultured human glomerular endothelial cells (HGECs), and the involvement of p300 in this process.

METHODS

Cell surface expression of HLA-DR and mRNA levels of HLA-DR and class II transactivator (CIITA), the master regulator of HLA-DR gene transcription, were measured by cellular ELISA and Northern blot, respectively. The levels of STAT1 and Smad3 protein were analyzed by Western blot. Nuclear binding activity of STAT1 was assessed by electrophoretic mobility shift assay.

RESULTS

IFN-gamma increased the cell surface expression of HLA-DR along with increases in the mRNA levels of CIITA and HLA-DR, while these stimulatory effects of IFN-gamma were down-regulated by TGF-beta1. IFN-gamma increased phosphorylation of STAT1 and this activation was not inhibited by TGF-beta1. IFN-gamma increased binding of p-STAT1 to p300, while TGF-beta1 increased binding of Smad3 to p300. TGF-beta1-induced Smad3 binding to p300 was inhibited by IFN-gamma, whereas IFN-gamma-induced p-STAT1 binding to p300 was not inhibited by TGF-beta1. IFN-gamma increased DNA binding activity of STAT1. Inhibition of interaction between STAT1 and p300 by addition of anti-p300 antibody to nuclear extract down-regulated DNA binding activity of STAT1. In contrast, TGF-beta1 did not inhibit IFN-gamma-induced STAT1 binding to DNA.

CONCLUSIONS

TGF-beta1 down-regulated IFN-gamma-induced CIITA and HLA-DR expression in HGECs. Though there was an antagonism between IFN-gamma and TGF-beta1, the competition for p300 between p-STAT1 and Smad3 was not the mechanism for it.

The Kaposi's sarcoma-associated herpesvirus K-bZIP protein represses transforming growth factor beta signaling through interaction with CREB-binding protein

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) is involved in the pathogenesis of KS, primary effusion lymphoma, and multicentric Castleman's disease. K-bZIP, the protein encoded by the open reading frame K8 of KSHV, is a member of the basic region-leucine zipper family of transcription factors. We studied the mechanisms that underlie KSHV-induced oncogenesis by investigating whether K-bZIP perturbs signaling through transforming growth factor beta (TGF-beta), which inhibits proliferation of a wide range of cell types. K-bZIP repressed TGF-beta-induced, Smad-mediated transcriptional activity and antagonized the growth-inhibitory effects of TGF-beta. Since both K-bZIP and Smad are known to interact with CREB-binding protein (CBP), the effect of CBP on inhibition of Smad-mediated transcriptional activation by K-bZIP was examined. K-bZIP mutants, which lacked the CBP-binding site, could not repress TGF-beta-induced or Smad3-mediated transcriptional activity. Overexpression of CBP restored K-bZIP-induced inhibition of Smad3-mediated transcriptional activity. Competitive interaction studies showed that K-bZIP inhibited the interaction of Smad3 with CBP. These results suggest that K-bZIP, through its binding to CBP, disrupts TGF-beta signaling by interfering with the recruitment of CBP into transcription initiation complexes on TGF-beta-responsive elements. We propose a possibility that K-bZIP may contribute to oncogenesis through its ability to promote cell survival by repressing TGF-beta signaling.

TGF-beta1 disrupts endotoxin signaling in microglial cells through Smad3 and MAPK pathways

Human formyl peptide receptor-like 1 and its mouse homologue formyl peptide receptor 2 (FPR2) are G protein-coupled receptors used by a number of exogenous and host-derived chemotactic peptides, including the 42 aa form of beta amyloid peptide, a causative factor of Alzheimer's disease. Functional FPR2 was induced by bacterial LPS in murine microglial cells, the resident phagocytic cells that play a pivotal role in inflammatory and immunological diseases in the CNS. To identify agents that may suppress microglial cell activation under proinflammatory conditions, we investigated the effect of TGF-beta1 on the expression of functional FPR2 by microglial cells activated by LPS. TGF-beta1 dose-dependently inhibited the mRNA expression and function of FPR2 in LPS-activated microglial cells. The inhibitory effect of TGF-beta1 was mediated by Smad3, a key signaling molecule coupled to the TGF-beta receptor, and the transcription coactivator, p300. Also, TGF-beta1 activates MAPKs in microglial cells that became refractory to further stimulation by LPS. These effects of TGF-beta1 culminate in the inhibition of LPS-induced activation of NF-kappaB and the up-regulation of FPR2 in microglial cells. Thus, TGF-beta1 may exert a protective role in CNS diseases characterized by microglial cell activation by proinflammatory stimulants.

Transforming growth factor beta/Smad3 signaling regulates IRF-7 function and transcriptional activation of the beta interferon promoter

The rapid induction of alpha interferon (IFN-alpha) and IFN-beta expression plays a critical role in the innate immune response against viral infection. We studied the effects of transforming growth factor beta (TGF-beta) and its intracellular effectors, the Smads, on the function of IRF-7, an essential transcription factor for IFN-alpha and -beta induction. IRF-7 interacted with Smads, and IRF-7, but not IRF-3, cooperated with Smad3 to activate IFN-beta transcription. This transcriptional cooperation occurred at the IRF-binding sequences in the IFN-beta promoter, and dominant-negative interference with TGF-beta receptor signaling and Smad3 function decreased IRF-7-mediated transcription. Furthermore, elimination of Smad3 expression in Smad3(-/-) fibroblasts delayed and decreased double-stranded RNA-induced expression of endogenous IFN-beta, whereas restoration of Smad3 expression enhanced IFN-beta induction. The IRF-7-Smad3 cooperativity resulted from the regulation of the transactivation activity of IRF-7 by Smad3, and dominant-negative interference with Smad3 function decreased IRF-7 activity. Consistent with the regulation by Smad3, the transcriptional activity of IRF-7 depended on and was regulated by TGF-beta signaling. Our studies underscore a role of TGF-beta/Smad3 signaling in IRF-7-mediated induction of IFN-beta expression.

Cyclic adenosine 3',5'-monophosphate-elevating agents inhibit transforming growth factor-beta-induced SMAD3/4-dependent transcription via a protein kinase A-dependent mechanism

Transforming growth factor-beta (TGF-beta) plays complex roles in carcinogenesis, as it may exert both tumor suppressor and pro-oncogenic activities depending on the stage of the tumor. SMAD proteins transduce signals from the TGF-beta receptors to regulate the transcription of specific target genes. Crosstalks with other signaling pathways may contribute to the specificity of TGF-beta effects. In this report, we have investigated the effects of cyclic adenosine 3',5'-monophosphate (cAMP), a key second messenger in the cellular response to various hormones, on SMAD-dependent signaling in human HaCaT keratinocytes. Using either an artificial SMAD3/4-dependent reporter construct or the natural TGF-beta target, plasminogen activator inhibitor-1, we show that membrane-permeable dibutyryl cAMP, and other intracellular cAMP-elevating agents such as the phosphodiesterase inhibitor isobutyl-methylxanthine, the adenylate cyclase activator forskolin, or exogenous prostaglandin E2 (PGE2), interfere with TGF-beta-induced SMAD-specific gene transactivation. Inhibition of protein kinase A (PKA), the main downstream effector of cAMP, with H-89, suppressed cAMP-dependent repression of SMAD-driven gene expression. Inversely, coexpression of either an active PKA catalytic subunit or that of the cAMP response element (CRE)-binding protein (CREB) blocked SMAD-driven gene transactivation. cAMP-elevating agents did not inhibit nuclear translocation and DNA binding of SMAD3/4 complexes, but abolished the interactions of SMAD3 with the transcription coactivators CREB-binding protein (CBP) and p300 in a PKA-dependent manner. These results suggest that suppression of TGF-beta/SMAD signaling and resulting gene transactivation by cAMP-inducing agents occurs via PKA-dependent, CREB-mediated, disruption of SMAD-CBP/p300 complexes.

Activation of Smad transcriptional activity by protein inhibitor of activated STAT3 (PIAS3)

Smad proteins play pivotal roles in mediating the transforming growth factor beta (TGF-beta) transcriptional responses. We show in this report that PIAS3, a member of the protein inhibitor of activated STAT (PIAS) family, activates TGF-beta/Smad transcriptional responses. PIAS3 interacts with Smad proteins, most strongly with Smad3. PIAS3 and Smad3 interact with each other at the endogenous protein level in mammalian cells and also in vitro, and the association occurs through the C-terminal domain of Smad3. We further show that PIAS3 can interact with the general coactivators p300/CBP, the first evidence that a PIAS protein can associate with p300/CBP. In contrast, PIASy, which inhibits Smad transcriptional activity and other transcriptional responses, is unable to interact with p300/CBP. The RING domain of PIAS3 is essential for interaction with p300/CBP, and a RING domain mutant PIAS3, which cannot bind p300/CBP, no longer activates TGF-beta/Smad-dependent transcription. Furthermore, we show that PIAS3, Smad3, and p300 can form a ternary complex, which is markedly increased by TGF-beta treatment. Taken together, our studies indicate that on TGF-beta treatment, PIAS3 can form a complex with Smads and p300/CBP and activate Smad transcriptional activity.

Activation of p38 and Smads mediates BMP-2 effects on human trabecular bone-derived osteoblasts

The bone morphogenetic proteins (BMPs) are potent osteoinductive factors that accelerate osteoblast maturation, accompanied by increased cell-substrate adhesion. BMP-2 treatment of osteoblastic cells increases phosphorylation of the cytoplasmic BMP-2 signaling molecules, Smad1 and Smad5. We have previously reported that BMP-2 treatment increase cytoskeletal organization of human trabecular bone-derived osteoblast-like cells (osteoblasts), which is also accompanied by an activation of the focal adhesion kinase p125(FAK). We report here that activation of p125(FAK) occurs with the same kinetics as the phosphorylation of Smad1, suggesting that BMP-2 initiates cross-talk between Smad signaling and the adhesion-mediated signaling pathway. As an adjunct to these effects, we examined activation of mitogen-activated protein (MAP) kinase family members in response to focal adhesion contact formation. Although phosphorylated forms of all three kinases were apparent, only SAPK2alpha/p38 (p38) was activated in response to BMP-2 treatment. Inhibition of p38 kinase activity suppressed BMP-2 induced Smad1 phosphorylation, as well as its translocation to the nucleus, suggesting the integration of p38 activation with Smad1 signaling. Finally, inhibition of p38 in osteoblasts also led to the complete abrogation of BMP-2 induced osteocalcin gene expression and matrix mineralization. These findings suggest that BMP-2 must activate p38 in order to mediate osteogenic differentiation and maturation.

The CBP coactivator functions both upstream and downstream of Dpp/Screw signaling in the early Drosophila embryo

The CBP histone acetyltransferase plays important roles in development and disease by acting as a transcriptional coregulator. A small reduction in the amount of Drosophila CBP (dCBP) leads to a specific loss of signaling by the TGF-beta molecules Dpp and Screw in the early embryo. We show that the expression of Screw itself, and that of two regulators of Dpp/Screw activity, Twisted-gastrulation and the Tolloid protease, is compromised in dCBP mutant embryos. This prevents Dpp/Screw from initiating a signal transduction event in the receiving cell. Smad proteins, the intracellular transducers of the signal, fail to become activated by phosphorylation in dCBP mutants, leading to diminished Dpp/Screw-target gene expression. At a slightly later stage of development, Dpp/Screw-signaling recovers in dCBP mutants, but without a restoration of Dpp/Screw-target gene expression. In this situation, dCBP acts downstream of Smad protein phosphorylation, presumably via direct interactions with the Drosophila Smad protein Mad. It appears that a major function of dCBP in the embryo is to regulate upstream components of the Dpp/Screw pathway by Smad-independent mechanisms, as well as acting as a Smad coactivator on downstream target genes. These results highlight the exceptional sensitivity of components in the TGF-beta signaling pathway to a decline in CBP concentration.