The transcriptional co-activator P/CAF potentiates TGF-beta/Smad signaling

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.

TGF-β signaling in health, disease, and therapeutics

Transforming growth factor (TGF)-β is a multifunctional cytokine expressed by almost every tissue and cell type. The signal transduction of TGF-β can stimulate diverse cellular responses and is particularly critical to embryonic development, wound healing, tissue homeostasis, and immune homeostasis in health. The dysfunction of TGF-β can play key roles in many diseases, and numerous targeted therapies have been developed to rectify its pathogenic activity. In the past decades, a large number of studies on TGF-β signaling have been carried out, covering a broad spectrum of topics in health, disease, and therapeutics. Thus, a comprehensive overview of TGF-β signaling is required for a general picture of the studies in this field. In this review, we retrace the research history of TGF-β and introduce the molecular mechanisms regarding its biosynthesis, activation, and signal transduction. We also provide deep insights into the functions of TGF-β signaling in physiological conditions as well as in pathological processes. TGF-β-targeting therapies which have brought fresh hope to the treatment of relevant diseases are highlighted. Through the summary of previous knowledge and recent updates, this review aims to provide a systematic understanding of TGF-β signaling and to attract more attention and interest to this research area.

LncRNA-mediated cartilage homeostasis in osteoarthritis: a narrative review

Osteoarthritis (OA) is a degenerative disease of cartilage that affects the quality of life and has increased in morbidity and mortality in recent years. Cartilage homeostasis and dysregulation are thought to be important mechanisms involved in the development of OA. Many studies suggest that lncRNAs are involved in cartilage homeostasis in OA and that lncRNAs can be used to diagnose or treat OA. Among the existing therapeutic regimens, lncRNAs are involved in drug-and nondrug-mediated therapeutic mechanisms and are expected to improve the mechanism of adverse effects or drug resistance. Moreover, targeted lncRNA therapy may also prevent or treat OA. The purpose of this review is to summarize the links between lncRNAs and cartilage homeostasis in OA. In addition, we review the potential applications of lncRNAs at multiple levels of adjuvant and targeted therapies. This review highlights that targeting lncRNAs may be a novel therapeutic strategy for improving and modulating cartilage homeostasis in OA patients.

SET8 is a novel negative regulator of TGF-β signaling in a methylation-independent manner

Transforming growth factor β (TGF-β) is a multifunctional cytokine that induces a diverse set of cellular processes principally through Smad-dependent transcription. Transcriptional responses induced by Smads are tightly regulated by Smad cofactors and histone modifications; however, the underlying mechanisms have not yet been elucidated in detail. We herein report lysine methyltransferase SET8 as a negative regulator of TGF-β signaling. SET8 physically associates with Smad2/3 and negatively affects transcriptional activation by TGF-β in a catalytic activity-independent manner. The depletion of SET8 results in an increase in TGF-β-induced plasminogen activator inhibitor-1 (PAI-1) and p21 expression and enhances the antiproliferative effects of TGF-β. Mechanistically, SET8 occupies the PAI-1 and p21 promoters, and a treatment with TGF-β triggers the replacement of the suppressive binding of SET8 with p300 on these promoters, possibly to promote gene transcription. Collectively, the present results reveal a novel role for SET8 in the negative regulation of TGF-β signaling.

Indole-derived compound SIS3 targets a subset of activated Smad complexes

Smad2 and Smad3 are receptor-regulated Smad proteins that transmit signals from cytokines belonging to the transforming growth factor (TGF)-β family, which are vital for adult tissue homeostasis. The overactivation of such proteins often engenders the development of pathological conditions. Smad3 reportedly mediates TGF-β–induced fibrosis. Although various potential Smad3-specific inhibitors are being developed, their specificity and action mechanisms remain largely unknown. This study aimed to establish a biochemical platform to monitor Smad2- or Smad3-dependent TGF-β signaling using SMAD2, SMAD3 and SMAD2/3 knockout cell lines alongside TGF-β–dependent luciferase reporters and Smad mutant proteins. Using this platform, SIS3, an indole-derived compound widely used as a specific Smad3 inhibitor, was observed to preferentially suppress a subset of activated Smad complexes. However, its inhibition did not favor Smad3 signaling over Smad2 signaling. These findings indicate that SIS3 can be employed as a probe to examine the heterogeneous nature of Smad signaling that induces gene expression. However, its use as a Smad3-specific inhibitor should be avoided.

Tumor microenvironment in non-melanoma skin cancer resistance to photodynamic therapy

Non-melanoma skin cancer has recently seen an increase in prevalence, and it is estimated that this grow will continue in the coming years. In this sense, the importance of therapy effectiveness has increased, especially photodynamic therapy. Photodynamic therapy has attracted much attention as a minimally invasive, selective and repeatable approach for skin cancer treatment and prevention. Although its high efficiency, this strategy has also faced problems related to tumor resistance, where the tumor microenvironment has gained a well-deserved role in recent years. Tumor microenvironment denotes a wide variety of elements, such as cancer-associated fibroblasts, immune cells, endothelial cells or the extracellular matrix, where their interaction and the secretion of a wide diversity of cytokines. Therefore, the need of designing new strategies targeting elements of the tumor microenvironment to overcome the observed resistance has become evident. To this end, in this review we focus on the role of cancer-associated fibroblasts and tumor-associated macrophages in the resistance to photodynamic therapy. We are also exploring new approaches consisting in the combination of new and old drugs targeting these cells with photodynamic therapy to enhance treatment outcomes of non-melanoma skin cancer.

Altered activity-regulated H3K9 acetylation at TGF-beta signaling genes during egocentric memory in Huntington's disease

Molecular mechanisms underlying cognitive deficits in Huntington's disease (HD), a striatal neurodegenerative disorder, are unknown. Here, we generated ChIPseq, 4Cseq and RNAseq data on striatal tissue of HD and control mice during striatum-dependent egocentric memory process. Multi-omics analyses showed altered activity-dependent epigenetic gene reprogramming of neuronal and glial genes regulating striatal plasticity in HD mice, which correlated with memory deficit. First, our data reveal that spatial chromatin re-organization and transcriptional induction of BDNF-related markers, regulating neuronal plasticity, were reduced since memory acquisition in the striatum of HD mice. Second, our data show that epigenetic memory implicating H3K9 acetylation, which established during late phase of memory process (e.g. during consolidation/recall) and contributed to glia-mediated, TGFβ-dependent plasticity, was compromised in HD mouse striatum. Specifically, memory-dependent regulation of H3K9 acetylation was impaired at genes controlling extracellular matrix and myelination. Our study investigating the interplay between epigenetics and memory identifies H3K9 acetylation and TGFβ signaling as new targets of striatal plasticity, which might offer innovative leads to improve HD.

P300/CBP-Associated Factor Activates Cardiac Fibroblasts by SMAD2 Acetylation

Various heart diseases cause cardiac remodeling, which in turn leads to ineffective contraction. Although it is an adaptive response to injury, cardiac fibrosis contributes to this remodeling, for which the reactivation of quiescent myofibroblasts is a key feature. In the present study, we investigated the role of the p300/CBP-associated factor (PCAF), a histone acetyltransferase, in the activation of cardiac fibroblasts. An intraperitoneal (i.p.) injection of a high dose (160 mg/kg) of isoproterenol (ISP) induced cardiac fibrosis and reduced the amount of the PCAF in cardiac fibroblasts in the mouse heart. However, the PCAF activity was significantly increased in cardiac fibroblasts, but not in cardiomyocytes, obtained from ISP-administered mice. An in vitro study using human cardiac fibroblast cells recapitulated the in vivo results; an treatment with transforming growth factor-β1 (TGF-β1) reduced the PCAF, whereas it activated the PCAF in the fibroblasts. PCAF siRNA attenuated the TGF-β1-induced increase in and translocation of fibrosis marker proteins. PCAF siRNA blocked TGF-β1-mediated gel contraction and cell migration. The PCAF directly interacted with and acetylated mothers against decapentaplegic homolog 2 (SMAD2). PCAF siRNA prevented TGF-β1-induced phosphorylation and the nuclear localization of SMAD2. These results suggest that the increase in PCAF activity during cardiac fibrosis may participate in SMAD2 acetylation and thereby in its activation.

TGFB1/INHBA Homodimer/Nodal-SMAD2/3 Signaling Network: A Pivotal Molecular Target in PDAC Treatment

Pancreatic cancer remains a grueling disease that is projected to become the second-deadliest cancer in the next decade. Standard treatment of pancreatic cancer is chemotherapy, which mainly targets the differentiated population of tumor cells; however, it paradoxically sets the roots of tumor relapse by the selective enrichment of intrinsically chemoresistant pancreatic cancer stem cells that are equipped with an indefinite capacity for self-renewal and differentiation, resulting in tumor regeneration and an overall anemic response to chemotherapy. Crosstalk between pancreatic tumor cells and the surrounding stromal microenvironment is also involved in the development of chemoresistance by creating a supportive niche, which enhances the stemness features and tumorigenicity of pancreatic cancer cells. In addition, the desmoplastic nature of the tumor-associated stroma acts as a physical barrier, which limits the intratumoral delivery of chemotherapeutics. In this review, we mainly focus on the transforming growth factor beta 1 (TGFB1)/inhibin subunit beta A (INHBA) homodimer/Nodal-SMAD2/3 signaling network in pancreatic cancer as a pivotal central node that regulates multiple key mechanisms involved in the development of chemoresistance, including enhancement of the stem cell-like properties and tumorigenicity of pancreatic cancer cells, mediating cooperative interactions between pancreatic cancer cells and the surrounding stroma, as well as regulating the deposition of extracellular matrix proteins within the tumor microenvironment.

Narciclasine is a novel YAP inhibitor that disturbs interaction between YAP and TEAD4

Yes-associated protein (YAP) is involved in development, cell growth, cell size, and homeostasis and plays a key role in the progression of various cancers. Among them, constitutive activation of YAP can often be observed in malignant mesothelioma, which arises in the pleura, peritoneum, and pericardium because of inactivation of the Hippo pathway. To date, however, only less-effective treatments such as chemotherapy, radiation therapy, and surgery are available for patients with malignant mesothelioma. In this study, we identified narciclasine as a novel YAP inhibitor that prevents YAP from interacting with TEAD4 because it competes with TEAD4 for binding to YAP. Furthermore, narciclasine could perturb the cell growth and colony formation of malignant mesothelioma NCI-H290 cells in addition to inhibiting their growth in nude mice. Therefore, narciclasine might be a potential seed for a novel antitumor drug against malignant mesothelioma and other cancers in which hyperactivation and/or overexpression of YAP are observed.

Bromodomain and extra-terminal motif (BET) inhibition is synthetic lethal with loss of SMAD4 in colorectal cancer cells via restoring the loss of MYC repression

The tumor suppressor SMAD4 is frequently mutated in colorectal cancer (CRC). However, no effective targeted therapies exist for CRC with SMAD4 loss. Here, we employed a synthetic lethality drug screening in isogenic SMAD4^+/+ and SMAD4^-/- HCT116 CRC cells and found that bromodomain and extra-terminal motif (BET) inhibitors, as selective drugs for the growth of SMAD4^-/- HCT116 cells. BET inhibition selectively induced G1 cell cycle arrest in SMAD4^-/- cells and this effect was accompanied by the reprogramming of the MYC-p21 axis. Mechanistically, SMAD4 is a transcription repressor of MYC, and MYC in turn represses p21 transcription. SMAD4^-/- cells lost MYC repression ability, thereby causing the cells addicted to the MYC oncogenic signaling. BET inhibition significantly reduced MYC level and restored p21 expression in SMAD4^-/- cells, inducing the selective growth arrest. The ectopic overexpression of MYC or the silencing of p21 could rescue the BET inhibitor-induced growth arrest in SMAD4^-/- cells, verifying this model. Tumor xenograft mouse experiments further demonstrated the synthetic lethality interaction between BET and SMAD4 in vivo. Taken together, our data suggest that BET could be a potential drug target for the treatment of SMAD4-deficient CRC.

Dissociation of the AhR/ARNT complex by TGF-β/Smad signaling represses CYP1A1 gene expression and inhibits benze[a]pyrene-mediated cytotoxicity

Cytochrome P450 1A1 (CYP1A1) catalyzes the metabolic activation of polycyclic aromatic hydrocarbons (PAHs) such as benzo[a]pyrene (B[a]P) and is transcriptionally regulated by the aryl hydrocarbon receptor (AhR)/AhR nuclear translocator (ARNT) complex upon exposure to PAHs. Accordingly, inhibition of CYP1A1 expression reduces production of carcinogens from PAHs. Although transcription of the CYP1A1 gene is known to be repressed by transforming growth factor-β (TGF-β), how TGF-β signaling is involved in the suppression of CYP1A1 gene expression has yet to be clarified. In this study, using mammalian cell lines, along with shRNA-mediated gene silencing, CRISPR/Cas9-based genome editing, and reporter gene and quantitative RT-PCR assays, we found that TGF-β signaling dissociates the B[a]P-mediated AhR/ARNT heteromeric complex. Among the examined Smads, Smad family member 3 (Smad3) strongly interacted with both AhR and ARNT via its MH2 domain. Moreover, hypoxia-inducible factor 1α (HIF-1α), which is stabilized upon TGF-β stimulation, also inhibited AhR/ARNT complex formation in the presence of B[a]P. Thus, TGF-β signaling negatively regulated the transcription of the CYP1A1 gene in at least two different ways. Of note, TGF-β abrogated DNA damage in B[a]P-exposed cells. We therefore conclude that TGF-β may protect cells against carcinogenesis because it inhibits CYP1A1-mediated metabolic activation of PAHs as part of its anti-tumorigenic activities.

PMEPA1/TMEPAI isoforms function via its PY and Smad-interaction motifs for tumorigenic activities of breast cancer cells

PMEPA1 (prostate transmembrane protein, androgen-induced 1)/TMEPAI (transmembrane prostate androgen-induced protein) is highly expressed in diverse cancers, including breast, lung and prostate cancers. It consists of four isoforms with distinct extracellular regions (isoforms a-d). The expression and function of these isoforms are still poorly understood. Hence, we aimed to identify the preferentially expressed isoforms in breast cancer cells and analyze possible differences in tumorigenic functions. In this study, we used 5' Rapid Amplification of cDNA Ends (RACE) and Western blot analyses to identify the mRNA variants and protein isoforms of TMEPAI and found that TMEPAI isoform d as the major isoform expressed by TGF-β stimulation in breast cancer cells. We then generated CRISPR/Cas9-mediated TMEPAI knockout (KO) breast cancer cell lines and used a lentiviral expression system to complement each isoform individually. Although there were no clear functional differences between isoforms, double PPxY (PY) motifs and a Smad-interaction motif (SIM) of TMEPAI were both essential for colony and sphere formation. Collectively, our results provide a novel insight into TMEPAI isoforms in breast cancer cells and showed that coordination between double PY motifs and a SIM of TMEPAI are essential for colony and sphere formation but not for monolayer cell proliferation.

TGF-β Signaling

Transforming growth factor-β (TGF-β) represents an evolutionarily conserved family of secreted polypeptide factors that regulate many aspects of physiological embryogenesis and adult tissue homeostasis. The TGF-β family members are also involved in pathophysiological mechanisms that underlie many diseases. Although the family comprises many factors, which exhibit cell type-specific and developmental stage-dependent biological actions, they all signal via conserved signaling pathways. The signaling mechanisms of the TGF-β family are controlled at the extracellular level, where ligand secretion, deposition to the extracellular matrix and activation prior to signaling play important roles. At the plasma membrane level, TGF-βs associate with receptor kinases that mediate phosphorylation-dependent signaling to downstream mediators, mainly the SMAD proteins, and mediate oligomerization-dependent signaling to ubiquitin ligases and intracellular protein kinases. The interplay between SMADs and other signaling proteins mediate regulatory signals that control expression of target genes, RNA processing at multiple levels, mRNA translation and nuclear or cytoplasmic protein regulation. This article emphasizes signaling mechanisms and the importance of biochemical control in executing biological functions by the prototype member of the family, TGF-β.

Epigenetic Regulation of Endothelial-to-Mesenchymal Transition in Chronic Heart Disease

Endothelial-to-mesenchymal transition (EndMT) is a process in which endothelial cells lose their properties and transform into fibroblast-like cells. This transition process contributes to cardiac fibrosis, a common feature of patients with chronic heart failure. To date, no specific therapies to halt or reverse cardiac fibrosis are available, so knowledge of the underlying mechanisms of cardiac fibrosis is urgently needed. In addition, EndMT contributes to other cardiovascular pathologies such as atherosclerosis and pulmonary hypertension, but also to cancer and organ fibrosis. Remarkably, the molecular mechanisms driving EndMT are largely unknown. Epigenetics play an important role in regulating gene transcription and translation and have been implicated in the EndMT process. Therefore, epigenetics might be the missing link in unraveling the underlying mechanisms of EndMT. Here, we review the involvement of epigenetic regulators during EndMT in the context of cardiac fibrosis. The role of DNA methylation, histone modifications (acetylation and methylation), and noncoding RNAs (microRNAs, long noncoding RNAs, and circular RNAs) in the facilitation and inhibition of EndMT are discussed, and potential therapeutic epigenetic targets will be highlighted.

Complementary Roles of GCN5 and PCAF in Foxp3+ T-Regulatory Cells

Functions of the GCN5-related N-acetyltransferase (GNAT) family of histone/protein acetyltransferases (HATs) in Foxp3+ T-regulatory (Treg) cells are unexplored, despite the general importance of these enzymes in cell biology. We now show that two prototypical GNAT family members, GCN5 (general control nonrepressed-protein 5, lysine acetyltransferase (KAT)2a) and p300/CBP-associated factor (p300/CBP-associated factor (PCAF), Kat2b) contribute to Treg functions through partially distinct and partially overlapping mechanisms. Deletion of Gcn5 or PCAF did not affect Treg development or suppressive function in vitro, but did affect inducible Treg (iTreg) development, and in vivo, abrogated Treg-dependent allograft survival. Contrasting effects were seen upon targeting of each HAT in all T cells; mice lacking GCN5 showed prolonged allograft survival, suggesting this HAT might be a target for epigenetic therapy in allograft recipients, whereas transplants in mice lacking PCAF underwent acute allograft rejection. PCAF deletion also enhanced anti-tumor immunity in immunocompetent mice. Dual deletion of GCN5 and PCAF led to decreased Treg stability and numbers in peripheral lymphoid tissues, and mice succumbed to severe autoimmunity by 3–4 weeks of life. These data indicate that HATs of the GNAT family have contributions to Treg function that cannot be replaced by the functions of previously characterized Treg HATs (CBP, p300, and Tip60), and may be useful targets in immuno-oncology.

Specificity, versatility, and control of TGF-β family signaling

Encoded in mammalian cells by 33 genes, the transforming growth factor-β (TGF-β) family of secreted, homodimeric and heterodimeric proteins controls the differentiation of most, if not all, cell lineages and many aspects of cell and tissue physiology in multicellular eukaryotes. Deregulation of TGF-β family signaling leads to developmental anomalies and disease, whereas enhanced TGF-β signaling contributes to cancer and fibrosis. Here, we review the fundamentals of the signaling mechanisms that are initiated upon TGF-β ligand binding to its cell surface receptors and the dependence of the signaling responses on input from and cooperation with other signaling pathways. We discuss how cells exquisitely control the functional presentation and activation of heteromeric receptor complexes of transmembrane, dual-specificity kinases and, thus, define their context-dependent responsiveness to ligands. We also introduce the mechanisms through which proteins called Smads act as intracellular effectors of ligand-induced gene expression responses and show that the specificity and impressive versatility of Smad signaling depend on cross-talk from other pathways. Last, we discuss how non-Smad signaling mechanisms, initiated by distinct ligand-activated receptor complexes, complement Smad signaling and thus contribute to cellular responses.

Variant PRC1 competes with retinoic acid-related signals to repress Meis2 in distal forelimb bud

Suppression of Meis genes in the distal limb bud is required for Proximal-Distal (PD) specification of the forelimb. Polycomb group (PcG) factors play a role in downregulation of retinoic acid (RA)-related signals in the distal forelimb bud, causing Meis repression. It is, however, not known if downregulation of RA-related signals and PcG-mediated proximal genes repression are functionally linked. Here, we reveal that PcG factors and RA-related signals antagonize each other to polarize Meis2 expression along the PD axis. With mathematical modeling and simulation, we propose that PcG factors are required to adjust the threshold for RA-related signaling to regulate Meis2 expression. Finally, we show that a variant Polycomb repressive complex 1 (PRC1), incorporating PCGF3 and PCGF5, represses Meis2 expression in the distal limb bud. Taken together, we reveal a previously unknown link between PcG proteins and downregulation of RA-related signals to mediate the phase transition of Meis2 transcriptional status during forelimb specification.

Downregulation of endometrial mesenchymal marker SUSD2 causes cell senescence and cell death in endometrial carcinoma cells

The cause of death among the majority of endometrial cancer patients involves migration of cancer cells within the peritoneal cavity and subsequent implantation of cancer spheroids into neighbouring organs. It is, thereby, important to identify factors that mediate metastasis. Cell adhesion and migration are modified by the mesenchymal stem cell (MSC) marker Sushi domain containing 2 (SUSD2), a type I transmembrane protein that participates in the orchestration of cell adhesion and migration through interaction with its partner Galactosidase-binding soluble-1 (LGALS1). MSCs have emerged as attractive targets in cancer therapy. Human endometrial adenocarcinoma (Ishikawa) cells were treated with TGFβ (10 ng/ml) for 72h. SUSD2, LGALS1 and MKI67 transcript levels were quantified using qRT-PCR. The proportion of SUSD2 positive (SUSD2+) cells and SMAD2/3 abundance were quantified by FACS and Western blotting, respectively. Senescent cells were identified with β-galactosidase staining; cell cycle and cell death were quantified using Propidium Iodide staining. Treatment of endometrial cancer cells (Ishikawa cells) with TGFβ (10 ng/ml) significantly decreased SUSD2 transcript levels and the proportion of SUSD2 positive cells. Silencing of SUSD2 using siRNA resulted in senescence and cell death of Ishikawa cells via activation of SMAD2/3. These findings suggest that SUSD2 counteracts senescence and cell death and is thus a potential chemotherapeutic target in human endometrial cancer.

Acetylation of MKL1 by PCAF regulates pro-inflammatory transcription

Inflammation is considered a fundamental host defense mechanism and, when aberrantly activated, contributes to a host of human diseases. Previously we have reported that the transcriptional regulator megakaryocytic leukemia 1 (MKL1) plays a role programming cellular inflammatory response by modulating NF-κB activity. Here we report that MKL1 was acetylated in vivo and pro-inflammatory stimuli (TNF-α and LPS) augmented MKL1 acetylation accompanying increased MKL1 binding to NF-κB target promoters. Further analysis revealed that the lysine acetyltransferase PCAF mediated MKL1 acetylation: TNF-α and LPS promoted the interaction between MKL1 and PCAF whereas depletion of PCAF abrogated the induction of MKL1 acetylation by TNF-α and LPS. Acetylation of MKL1 was necessary for MKL1 to activate the transcription of pro-inflammatory genes because mutation of four conserved lysine residues in MKL1 attenuated its capacity as a trans-activator of NF-κB target genes. Mechanistically, MKL1 acetylation served to promote MKL1 nuclear enrichment, to enhance the MKL1-NF-κB interaction, and to stabilize the binding of MKL1 on target promoters. In conclusion, our data unveil an important pathway that contributes to the transcriptional regulation of inflammatory response.

TGF-β induces p53/Smads complex formation in the PAI-1 promoter to activate transcription

Transforming growth factor β (TGF-β) signaling facilitates tumor development during the advanced stages of tumorigenesis, but induces cell-cycle arrest for tumor suppression during the early stages. However, the mechanism of functional switching of TGF-β is still unknown, and it is unclear whether inhibition of TGF-β signaling results amelioration or exacerbation of cancers. Here we show that the tumor suppressor p53 cooperates with Smad proteins, which are TGF-β signal transducers, to selectively activate plasminogen activator inhibitor type-1 (PAI-1) transcription. p53 forms a complex with Smad2/3 in the PAI-1 promoter to recruit histone acetyltransferase CREB-binding protein (CBP) and enhance histone H3 acetylation, resulting in transcriptional activation of the PAI-1 gene. Importantly, p53 is required for TGF-β-induced cytostasis and PAI-1 is involved in the cytostatic activity of TGF-β in several cell lines. Our results suggest that p53 enhances TGF-β-induced cytostatic effects by activating PAI-1 transcription, and the functional switching of TGF-β is partially caused by p53 mutation or p53 inactivation during cancer progression. It is expected that these findings will contribute to optimization of TGF-β-targeting therapies for cancer.

Transcriptional Control by the SMADs

The transforming growth factor-β (TGF-β) family of ligands elicit their biological effects by initiating new programs of gene expression. The best understood signal transducers for these ligands are the SMADs, which essentially act as transcription factors that are activated in the cytoplasm and then accumulate in the nucleus in response to ligand induction where they bind to enhancer/promoter sequences in the regulatory regions of target genes to either activate or repress transcription. This review focuses on the mechanisms whereby the SMADs achieve this and the functional implications. The SMAD complexes have weak affinity for DNA and limited specificity and, thus, they cooperate with other site-specific transcription factors that act either to actively recruit the SMAD complexes or to stabilize their DNA binding. In some situations, these cooperating transcription factors function to integrate the signals from TGF-β family ligands with environmental cues or with information about cell lineage. Activated SMAD complexes regulate transcription via remodeling of the chromatin template. Consistent with this, they recruit a variety of coactivators and corepressors to the chromatin, which either directly or indirectly modify histones and/or modulate chromatin structure.

Histone H3K9 Acetyltransferase PCAF Is Essential for Osteogenic Differentiation Through Bone Morphogenetic Protein Signaling and May Be Involved in Osteoporosis

Human mesenchymal stem cells (MSCs) are multipotent progenitor cells that can differentiate into osteoblasts, chondrocytes, and adipocytes. The importance of epigenetic regulation for osteogenic differentiation of MSCs is widely accepted. However, the molecular mechanisms are poorly understood. Here, we show that histone H3K9 acetyltransferase PCAF plays a critical role in osteogenic differentiation of MSCs. Knockdown of PCAF significantly reduced the bone formation both in vitro and in vivo. Mechanistically, PCAF controls BMP signaling genes expression by increasing H3K9 acetylation. Most importantly, PCAF expression is significantly decreased in bone sections of ovariectomized or aged mice. Histone modification enzyme is chemically modifiable; therefore, PCAF may represent a novel therapeutic target for stem cell-mediated regenerative medicine and the treatment of osteoporosis. Stem Cells 2016;34:2332-2341.

Tumor suppressor bromodomain-containing protein 7 cooperates with Smads to promote transforming growth factor-β responses

Smad proteins are central mediators in the canonical transforming growth factor-β (TGF-β) signaling pathway in mammalian cells. We report here that bromodomain-containing protein 7 (BRD7) functions as a novel transcription coactivator for Smads in TGF-β signaling. BRD7 forms a TGF-β inducible complex with Smad3/4 through its N-terminal Smad-binding domain. BRD7 simultaneously binds to acetylated histones to promote Smad-chromatin association, and associates with histone acetyltransferase p300 to enhance Smad transcriptional activity. Ectopic expression of BRD7, but not its mutants defective in Smad binding, enhances TGF-β transcriptional, tumor suppressing and epithelial-mesenchymal transition (EMT) responses. Conversely, depletion of BRD7 inhibits TGF-β responses. Thus, our study provides compelling evidence for a new function of BRD7 in fine-tuning TGF-β physiological responses.

TGFβ1 regulates Scleraxis expression in primary cardiac myofibroblasts by a Smad-independent mechanism

In cardiac wound healing following myocardial infarction (MI), relatively inactive resident cardiac fibroblasts phenoconvert to hypersynthetic/secretory myofibroblasts that produce large quantities of extracellular matrix (ECM) and fibrillar collagen proteins. Our laboratory and others have identified TGFβ1 as being a persistent stimulus in the chronic and inappropriate wound healing phase that is marked by hypertrophic scarring and eventual stiffening of the entire myocardium, ultimately leading to the pathogenesis of heart failure following MI. Ski is a potent negative regulator of TGFβ/Smad signaling with known antifibrotic effects. Conversely, Scleraxis is a potent profibrotic basic helix-loop-helix transcription factor that stimulates fibrillar collagen expression. We hypothesize that TGFβ1 induces Scleraxis expression by a novel Smad-independent pathway. Our data support the hypothesis that Scleraxis expression is induced by TGFβ1 through a Smad-independent pathway in the cardiac myofibroblast. Specifically, we demonstrate that TGFβ1 stimulates p42/44 (Erk1/2) kinases, which leads to increased Scleraxis expression. Inhibition of MEK1/2 using U0126 led to a sequential temporal reduction of phospho-p42/44 and subsequent Scleraxis expression. We also found that adenoviral Ski expression in primary myofibroblasts caused a significant repression of endogenous Scleraxis expression at both the mRNA and protein levels. Thus we have identified a novel TGFβ1-driven, Smad-independent, signaling cascade that may play an important role in regulating the fibrotic response in activated cardiac myofibroblasts following cardiac injury.

The Retinoblastoma Tumor Suppressor Protein (pRb)/E2 Promoter Binding Factor 1 (E2F1) Pathway as a Novel Mediator of TGFβ-induced Autophagy

TGFβ is a multifunctional cytokine that regulates cell proliferation, cell immortalization, and cell death, acting as a key homeostatic mediator in various cell types and tissues. Autophagy is a programmed mechanism that plays a pivotal role in controlling cell fate and, consequently, many physiological and pathological processes, including carcinogenesis. Although autophagy is often considered a pro-survival mechanism that renders cells viable in stressful conditions and thus might promote tumor growth, emerging evidence suggests that autophagy is also a tumor suppressor pathway. The relationship between TGFβ signaling and autophagy is context-dependent and remains unclear. TGFβ-mediated activation of autophagy has recently been suggested to contribute to the growth inhibitory effect of TGFβ in hepatocarcinoma cells. In the present study, we define a novel process of TGFβ-mediated autophagy in cancer cell lines of various origins. We found that autophagosome initiation and maturation by TGFβ is dependent on the retinoblastoma tumor suppressor protein/E2 promoter binding factor (pRb/E2F1) pathway, which we have previously established as a critical signaling axis leading to various TGFβ tumor suppressive effects. We further determined that TGFβ induces pRb/E2F1-dependent transcriptional activation of several autophagy-related genes. Together, our findings reveal that TGFβ induces autophagy through the pRb/E2F1 pathway and transcriptional activation of autophagy-related genes and further highlight the central relevance of the pRb/E2F1 pathway downstream of TGFβ signaling in tumor suppression.

Regulation of the TMEPAI promoter by TCF7L2: the C-terminal tail of TCF7L2 is essential to activate the TMEPAI gene

We previously found that TCF7L2 could activate the TMEPAI gene efficiently, whereas LEF1 could not nearly augment its transcription. When we comprehended the functional difference(s) between TCF7L2 and LEF1 with respect to the activation of the TMEPAI gene, the C-terminal tail of TCF7L2 was needed to reveal its transcriptional activity as well as its interaction with Smad3. Consistently, both TCF7/TCF7L2 and LEF1/TCF7L2 chimeric proteins exhibited an activity similar to TCF7L2 in transcription and Smad3 binding in contrast with LEF1 and TCF7. Our data elaborated on the diverse activity among TCF/LEF family members with respect to the transcriptional regulation of the TMEPAI gene.

TGFβ Signaling in Tumor Initiation, Epithelial-to-Mesenchymal Transition, and Metastasis

Retaining the delicate balance in cell signaling activity is a prerequisite for the maintenance of physiological tissue homeostasis. Transforming growth factor-beta (TGFβ) signaling is an essential pathway that plays crucial roles during embryonic development as well as in adult tissues. Aberrant TGFβ signaling activity regulates tumor progression in a cancer cell-autonomous or non-cell-autonomous fashion and these effects may be tumor suppressing or tumor promoting depending on the cellular context. The fundamental role of this pathway in promoting cancer progression in multiple stages of the metastatic process, including epithelial-to-mesenchymal transition (EMT), is also becoming increasingly clear. In this review, we discuss the latest advances in the effort to unravel the inherent complexity of TGFβ signaling and its role in cancer progression and metastasis. These findings provide important insights into designing personalized therapeutic strategies against advanced cancers.

Role of TGFβ in regulation of the tumor microenvironment and drug delivery (review)

Deregulation of cell signaling homeostasis is a predominant feature of cancer initiation and progression. Transforming growth factor β (TGFβ) is a pleiotropic cytokine, which regulates numerous biological processes of various tissues in an autocrine and paracrine manner. Aberrant activity of TGFβ signaling is well known to play dual roles in cancer, depending on tumor stage and cellular context. The crucial roles of TGFβ in modulating the tumor microenvironment, its contribution to the accumulation of mechanical forces within the solid constituents of a tumor and its effects on the effective delivery of drugs are also becoming increasingly clear. In this review, we discuss the latest advances in the efforts to unravel the effects of TGFβ signaling in various components of the tumor microenvironment and how these influence the generation of forces and the efficacy of drugs. We also report the implications of tumor mechanics in cancer therapy and the potential usage of anti-TGFβ agents to enhance drug delivery and augment existing therapeutic approaches. These findings provide new insights towards the significance of targeting TGFβ pathway to enhance personalized tumor treatment.

Lysine acetyltransferase PCAF is a key regulator of arteriogenesis

OBJECTIVE

Therapeutic arteriogenesis, that is, expansive remodeling of preexisting collaterals, using single-action factor therapies has not been as successful as anticipated. Modulation of factors that act as a master switch for relevant gene programs may prove more effective. Transcriptional coactivator p300-CBP-associated factor (PCAF) has histone acetylating activity and promotes transcription of multiple inflammatory genes. Because arteriogenesis is an inflammation-driven process, we hypothesized that PCAF acts as multifactorial regulator of arteriogenesis.

APPROACH AND RESULTS

After induction of hindlimb ischemia, blood flow recovery was impaired in both PCAF(-/-) mice and healthy wild-type mice treated with the pharmacological PCAF inhibitor Garcinol, demonstrating an important role for PCAF in arteriogenesis. PCAF deficiency reduced the in vitro inflammatory response in leukocytes and vascular cells involved in arteriogenesis. In vivo gene expression profiling revealed that PCAF deficiency results in differential expression of 3505 genes during arteriogenesis and, more specifically, in impaired induction of multiple proinflammatory genes. Additionally, recruitment from the bone marrow of inflammatory cells, in particular proinflammatory Ly6C(hi) monocytes, was severely impaired in PCAF(-/-) mice.

CONCLUSIONS

These findings indicate that PCAF acts as master switch in the inflammatory processes required for effective arteriogenesis.

Genome-wide microRNA and messenger RNA profiling in rodent liver development implicates mir302b and mir20a in repressing transforming growth factor-beta signaling

MicroRNAs (miRNAs) are recently discovered small RNA molecules that regulate developmental processes, such as proliferation, differentiation, and apoptosis; however, the identity of miRNAs and their functions during liver development are largely unknown. Here we investigated the miRNA and gene expression profiles for embryonic day (E)8.5 endoderm, E14.5 Dlk1(+) liver cells (hepatoblasts), and adult liver by employing Illumina sequencing. We found that miRNAs were abundantly expressed at all three stages. Using K-means clustering analysis, 13 miRNA clusters with distinct temporal expression patterns were identified. mir302b, an endoderm-enriched miRNA, was identified as an miRNA whose predicted targets are expressed highly in E14.5 hepatoblasts but low in the endoderm. We validated the expression of mir302b in the endoderm by whole-mount in situ hybridization. Interestingly, mir20a, the most highly expressed miRNA in the endoderm library, was also predicted to regulate some of the same targets as mir302b. We found that through targeting Tgfbr2, mir302b and mir20a are able to regulate transforming growth factor beta (TGFβ) signal transduction. Moreover, mir302b can repress liver markers in an embryonic stem cell differentiation model. Collectively, we uncovered dynamic patterns of individual miRNAs during liver development, as well as miRNA networks that could be essential for the specification and differentiation of liver progenitors. (HEPATOLOGY 2013).

Mechanisms of activin-stimulated FSH synthesis: the story of a pig and a FOX

Activins were discovered and, in fact, named more than a quarter century ago based on their abilities to stimulate pituitary follicle-stimulating hormone (FSH) synthesis and secretion. However, it is only in the last decade that we have finally come to understand their underlying mechanisms of action in gonadotroph cells. In this minireview, we chronicle the research that led to the recent discovery of forkhead box L2 (FOXL2) as an essential mediator of activin-regulated FSH beta subunit (Fshb) transcription in vitro and in vivo.

A transcriptionally active pRb-E2F1-P/CAF signaling pathway is central to TGFβ-mediated apoptosis

Transforming growth factor-β (TGFβ) modulates the expression of multiple apoptotic target genes; however, a common and central signaling pathway, acting downstream of TGFβ and leading to cell death, has yet to be uncovered. Here, we show that TGFβ-induced apoptosis in cancer cells requires the transcription factor E2F1 (E2 promoter-binding factor 1). Using the E2F1 knockout mouse model, we also found E2F1 to be required for TGFβ-mediated apoptosis in normal cells. Moreover, we found TGFβ to increase E2F1 protein stability, acting at the post-translational level. We further investigated the molecular mechanisms by which E2F1 contributes to TGFβ-mediated apoptosis and found that TGFβ treatment led to the formation of a transcriptionally active E2F1–pRb–P/CAF complex on multiple TGFβ pro-apoptotic target gene promoters, thereby activating their transcription. Together, our findings define a novel process of gene activation by the TGFβ-E2F1 signaling axis and highlight E2F1 as a central mediator of the TGFβ apoptotic program.

A novel function for p21Cip1 and acetyltransferase p/CAF as critical transcriptional regulators of TGFβ-mediated breast cancer cell migration and invasion

Introduction

Tumor cell migration and invasion are critical initiation steps in the process of breast cancer metastasis, the primary cause of breast cancer morbidity and death. Here we investigated the role of p21Cip1 (p21), a member of the core cell cycle machinery, in transforming growth factor-beta (TGFβ)-mediated breast cancer cell migration and invasion.

Methods

A mammary fat pad xenograft mouse model was used to assess the mammary tumor growth and local invasion. The triple negative human breast cancer cell lines MDA-MB231 and its sub-progenies SCP2 and SCP25, SUM159PT, SUM149PT, SUM229PE and SUM1315MO₂ were treated with 5 ng/ml TGFβ and the protein expression levels were measured by Western blot. Cell migration and invasion were examined using the scratch/wound healing and Transwell assay. TGFβ transcriptional activity was measured by a TGFβ/Smad reporter construct (CAGA12-luc) using luciferase assay. q-PCR was used for assessing TGFβ downstream target genes. The interactions among p21, p/CAF and Smad3 were performed by co-immunoprecipitation. In addition, Smad3 on DNA binding ability was measured by DNA immunoprecipitation using biotinylated Smad binding element DNA probes. Finally, the association among active TGFβ/Smad signaling, p21 and p/CAF with lymph node metastasis was examined by immunohistochemistry in tissue microarray containing 50 invasive ductal breast tumors, 25 of which are lymph node positive.

Results

We found p21 expression to correlate with poor overall and distant metastasis free survival in breast cancer patients. Furthermore, using xenograft animal models and in vitro studies, we found p21 to be essential for tumor cell invasion. The invasive effects of p21 were found to correlate with Smad3, and p/CAF interaction downstream of TGFβ. p21 and p/CAF regulates TGFβ-mediated transcription of pro-metastatic genes by controlling Smad3 acetylation, DNA binding and transcriptional activity. In addition, we found that active TGFβ/Smad signaling correlates with high p21 and p/CAF expression levels and lymph node involvement using tissue microarrays from breast cancer patients.

Conclusions

Together these results highlight an important role for p21 and p/CAF in promoting breast cancer cell migration and invasion at the transcriptional level and may open new avenues for breast cancer therapy.

TGF-β control of stem cell differentiation genes

The canonical TGF-β/Smad signaling pathway was delineated in the mid 90s and enriched over the past decade with many findings about its specificity, regulation, networking, and malfunctions in disease. However, a growing understanding of the chromatin status of a critical class of TGF-β target genes - the genes controlling differentiation of embryonic stem cells - recently prompted a reexamination of this pathway and its critical role in the regulation of stem cell differentiation. The new findings reveal master regulators of the pluripotent state set the stage for Smad-mediated activation of master regulators of the next differentiation stage. Furthermore, a novel branch of the TGF-β/Smad pathway has been identified in which a chromatin-reading Smad complex makes the master differentiation genes accessible to canonical Smad complexes for transcriptional activation. These findings provide exciting new insights into the global role of TGF-β signaling in the regulators of stem cell fate.

Identification and functional analysis of Zranb2 as a novel Smad-binding protein that suppresses BMP signaling

Smads 1/5/8 transduce the major intracellular signaling of bone morphogenetic proteins (BMPs). In the present study, we analyzed Smad1-binding proteins in HEK293T cells using a proteomic technique and identified the protein, zinc-finger, RAN-binding domain-containing protein 2 (ZRANB2). Zranb2 interacted strongly with Smad1, Smad5, and Smad8 and weakly with Smad4. The overexpression of Zranb2 inhibited BMP activities in C2C12 myoblasts in vitro, and the injection of Zranb2 mRNA into zebrafish embryos induced weak dorsalization. Deletion analyses of Zranb2 indicated that the serine/arginine-rich (SR) domain and the glutamine-rich domain were required for the inhibition of BMP activity and the interaction with Smad1, respectively. Zranb2 was found to be localized in the nucleus; however, the SR domain-deleted mutant localized to the cytoplasm. The knockdown of endogenous Zranb2 in C2C12 cells enhanced BMP activity. Zranb2 suppressed Smad transcriptional activity without affecting Smad phosphorylation, nuclear localization, or DNA binding. Taken together, these findings suggested that Zranb2 is a novel BMP suppressor that forms a complex with Smads in the nucleus.

The impact of Fli1 deficiency on the pathogenesis of systemic sclerosis

Systemic sclerosis (SSc) is an autoimmune inflammatory disease with unknown etiology characterized by microvascular injury and fibrosis of the skin and internal organs. A growing body of evidence suggests that deficiency of the transcription factor Fli1 (Friend leukemia integration-1) has a pivotal role in the pathogenesis of SSc. Fli1 is expressed in fibroblasts, endothelial cells, and immune cells, and has important roles in the activation, differentiation, development, and survival of these cells. Previous studies demonstrated that Fli1 is downregulated in SSc fibroblasts by an epigenetic mechanism and a series of experiments with Fli1-deficient animal models revealed that Fli1 deficiency in fibroblasts and endothelial cells reproduces the histopathologic features of fibrosis and vasculopathy in SSc, respectively. In this article, we review the impact of Fli1 deficiency on the pathogenesis of SSc and discuss a new therapeutic strategy for SSc by targeting the transcription factor Fli1.

TMEPAI, a transmembrane TGF-beta-inducible protein, sequesters Smad proteins from active participation in TGF-beta signaling

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine of key importance for controlling embryogenesis and tissue homeostasis. How TGF-beta signals are attenuated and terminated is not well understood. Here, we show that TMEPAI, a direct target gene of TGF-beta signaling, antagonizes TGF-beta signaling by interfering with TGF-beta type I receptor (TbetaRI)-induced R-Smad phosphorylation. TMEPAI can directly interact with R-Smads via a Smad interaction motif. TMEPAI competes with Smad anchor for receptor activation for R-Smad binding, thereby sequestering R-Smads from TbetaRI kinase activation. In mammalian cells, ectopic expression of TMEPAI inhibited TGF-beta-dependent regulation of plasminogen activator inhibitor-1, JunB, cyclin-dependent kinase inhibitors, and c-myc expression, whereas specific knockdown of TMEPAI expression prolonged duration of TGF-beta-induced Smad2 and Smad3 phosphorylation and concomitantly potentiated cellular responsiveness to TGF-beta. Consistently, TMEPAI inhibits activin-mediated mesoderm formation in Xenopus embryos. Therefore, TMEPAI participates in a negative feedback loop to control the duration and intensity of TGF-beta/Smad signaling.

Histone acetyltransferases interact with and acetylate p70 ribosomal S6 kinases in vitro and in vivo

The 70kDa ribosomal protein S6 kinases (S6K1 and S6K2) play important roles in the regulation of protein synthesis, cell growth and survival. S6Ks are activated in response to mitogen stimulation and nutrient sufficiency by the phosphorylation of conserved serine and threonine residues. Here we show for the first time, that in addition to phosphorylation, S6Ks are also targeted by lysine acetylation. Following mitogen stimulation, S6Ks interact with the p300 and p300/CBP-associated factor (PCAF) acetyltransferases. S6Ks can be acetylated by p300 and PCAF in vitro and S6K acetylation is detected in cells expressing p300. Furthermore, it appears that the acetylation sites targeted by p300 lie within the divergent C-terminal regulatory domains of both S6K1 and S6K2. Acetylation of S6K1 and 2 is increased upon the inhibition of class I/II histone deacetylases (HDACs) by trichostatin-A, while the enhancement of S6K1 acetylation by nicotinamide suggests the additional involvement of sirtuin deacetylases in S6K deacetylation. Both expression of p300 and HDAC inhibition cause increases in S6K protein levels, and we have shown that S6K2 is stabilized in cells treated with HDAC inhibitors. The finding that S6Ks are targeted by histone acetyltransferases uncovers a novel mode of crosstalk between mitogenic signalling pathways and the transcriptional machinery and reveals additional complexity in the regulation of S6K function.

Loss of steroid receptor co-activator-3 attenuates carbon tetrachloride-induced murine hepatic injury and fibrosis

Hepatic fibrosis, a disease characterized by altered accumulation of extracellular matrix, can cause cirrhosis and liver failure. There is growing interest in the impact of co-activators on hepatic fibrogenesis. Here, we provided genetic evidence that mice lacking steroid receptor co-activator-3 (SRC-3) were protected against carbon tetrachloride (CCl4)-induced acute liver necrosis and chronic hepatic fibrosis. After acute CCl4 treatment, SRC-3(-/-) mice showed attenuated profibrotic response and hepatocyte apoptosis, whereas hepatocyte proliferation was elevated in SRC-3(-/-) mice versus SRC-3+/+ mice. Similarly, chronically CCl4-treated SRC-3(-/-) mice showed significant weakening of inflammatory infiltrates, hepatic stellate cell activation and collagen accumulation in the liver compared with SRC-3+/+ mice. Further investigation revealed that TGFbeta1/Smad signaling pathway was impaired in the absence of SRC-3. Moreover, the expression levels of SRC-3, as assessed in human tissue microarray of liver diseases, correlated positively with degrees of fibrosis. These data revealed that SRC-3(-/-) mice were resistant to CCl4-induced acute and chronic hepatic damage and TGFbeta1/Smad signaling was suppressed in the lack of SRC-3. Our results established an essential involvement of SRC-3 in liver fibrogenesis, which might provide new clues to the future treatment of hepatic fibrosis.

Loss of steroid receptor co-activator-3 attenuates carbon tetrachloride-induced murine hepatic injury and fibrosis

Hepatic fibrosis, a disease characterized by altered accumulation of extracellular matrix, can cause cirrhosis and liver failure. There is growing interest in the impact of co-activators on hepatic fibrogenesis. Here, we provided genetic evidence that mice lacking steroid receptor co-activator-3 (SRC-3) were protected against carbon tetrachloride (CCl4)-induced acute liver necrosis and chronic hepatic fibrosis. After acute CCl4 treatment, SRC-3(-/-) mice showed attenuated profibrotic response and hepatocyte apoptosis, whereas hepatocyte proliferation was elevated in SRC-3(-/-) mice versus SRC-3+/+ mice. Similarly, chronically CCl4-treated SRC-3(-/-) mice showed significant weakening of inflammatory infiltrates, hepatic stellate cell activation and collagen accumulation in the liver compared with SRC-3+/+ mice. Further investigation revealed that TGFbeta1/Smad signaling pathway was impaired in the absence of SRC-3. Moreover, the expression levels of SRC-3, as assessed in human tissue microarray of liver diseases, correlated positively with degrees of fibrosis. These data revealed that SRC-3(-/-) mice were resistant to CCl4-induced acute and chronic hepatic damage and TGFbeta1/Smad signaling was suppressed in the lack of SRC-3. Our results established an essential involvement of SRC-3 in liver fibrogenesis, which might provide new clues to the future treatment of hepatic fibrosis.

Differential regulation of transforming growth factor beta signaling pathways by Notch in human endothelial cells

Notch and transforming growth factor beta (TGFbeta) play critical roles in endothelial-to-mesenchymal transition (EndMT), a process that is essential for heart development. Previously, we have shown that Notch and TGFbeta signaling synergistically induce Snail expression in endothelial cells, which is required for EndMT in cardiac cushion morphogenesis. Here, we report that Notch activation modulates TGFbeta signaling pathways in a receptor-activated Smad (R-Smad)-specific manner. Notch activation inhibits TGFbeta/Smad1 and TGFbeta/Smad2 signaling pathways by decreasing the expression of Smad1 and Smad2 and their target genes. In contrast, Notch increases SMAD3 mRNA expression and protein half-life and regulates the expression of TGFbeta/Smad3 target genes in a gene-specific manner. Inhibition of Notch in the cardiac cushion of mouse embryonic hearts reduces Smad3 expression. Notch and TGFbeta synergistically up-regulate a subset of genes by recruiting Smad3 to both Smad and CSL binding sites and cooperatively inducing histone H4 acetylation. This is the first evidence that Notch activation affects R-Smad expression and that cooperative induction of histone acetylation at specific promoters underlies the selective synergy between Notch and TGFbeta signaling pathways.

TAL1/SCL relieves the E2-2-mediated repression of VEGFR2 promoter activity

The basic helix-loop-helix (bHLH) protein TAL1/SCL is essential for embryonic-vascular development. TAL1/SCL regulates the activation of endothelial cells by binding directly or indirectly to DNA sequences in critical target genes. We recently demonstrated that E-box protein E2-2 blocks endothelial cell activation via perturbation of VEGFR2 promoter activity. Herein, we report that TAL1/SCL interacts with E2-2 and inhibits E2-2-mediated effects on reporter activity. Mutational analysis revealed that the HLH domain of TAL1/SCL, but not its basic region, is required for interaction with E2-2. Importantly, TAL1/SCL relieves the E2-2-mediated repression of VEGFR2 reporter activity in endothelial cells. Our data elaborate on the bHLH protein interactions that regulate endothelial cell activation.

The Kruppel-like factor KLF15 inhibits connective tissue growth factor (CTGF) expression in cardiac fibroblasts

Cardiac fibrosis is a hallmark feature of pathologic remodeling of the heart in response to hemodynamic or neurohormonal stress. Accumulating evidence implicates connective tissue growth factor (CTGF) as a key mediator of this process. Our group has previously identified Kruppel-Like Factor 15 (KLF15) as an important regulator of cardiac remodeling in response to stress; however, the role of this transcription factor in cardiac fibrosis has not been reported. Here we provide evidence that treatment of neonatal rat ventricular fibroblasts (NRVFs) with the potent pro-fibrotic agent Transforming Growth Factor-beta1 (TGFbeta1) strongly reduces KLF15 expression while inducing the pro-fibrotic factor CTGF. Adenoviral overexpression of KLF15 inhibits basal and TGFbeta1-induced CTGF expression in NRVFs. Furthermore, hearts from KLF15-/- mice subjected to aortic banding exhibited increased CTGF levels and fibrosis. From a mechanistic standpoint, KLF15 inhibits basal and TGFbeta1-mediated induction of the CTGF promoter. Chromatin Immunoprecipitation (ChIP) and electrophoretic mobility shift assays demonstrate that KLF15 inhibits recruitment of the co-activator P/CAF to the CTGF promoter with no significant effect on Smad3-DNA binding. Consistent with this observation, KLF15 mediated repression of the CTGF promoter is rescued by P/CAF overexpression. Our result implicates KLF15 as a novel negative regulator of CTGF expression and cardiac fibrosis.

Akt kinase targets association of CBP with SMAD 3 to regulate TGFbeta-induced expression of plasminogen activator inhibitor-1

Transforming growth factor-beta (TGFbeta) controls expression of plasminogen activator inhibitor type 1 (PAI-1), which regulates degradation of extracellular matrix proteins in fibrotic diseases. The TGFbeta receptor-specific Smad 3 has been implicated in the PAI-1 expression. The mechanism by which non-Smad signaling contributes to this process is not known. We studied the cross-talk between Smad 3 and PI 3 kinase/Akt signaling in TGFbeta-induced PAI-1 expression in renal mesangial cells. Inhibition of PI 3 kinase and Akt kinase blocked TGFbeta- and Smad 3-mediated expression of PAI-1. In contrast, constitutively active PI 3 kinase and Akt kinase increased PAI-1 expression, similar to TGFbeta. Inhibition of PI 3 kinase and Akt kinase had no effect on TGFbeta-induced Smad 3 phosphorylation and its translocation to the nucleus. Notably, inhibition of PI 3 kinase-dependent Akt kinase abrogated TGFbeta-induced PAI-1 transcription, without affecting binding of Smad 3 to the PAI-1 Smad binding DNA element. However, PI 3 kinase inhibition and dominant negative Akt kinase antagonized the association of the transcriptional coactivator CBP with Smad 3 in response to TGFbeta, resulting in inhibition of Smad 3 acetylation. Together our findings identify TGFbeta-induced PI 3 kinase/Akt signaling as a critical regulator of Smad 3-CBP interaction and Smad 3 acetylation, which cause increased PAI-1 expression.

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.

Transforming growth factor-beta regulates DNA binding activity of transcription factor Fli1 by p300/CREB-binding protein-associated factor-dependent acetylation

Fli1, a member of Ets transcriptional factors, has been shown to be a negative regulator of collagen gene expression in dermal fibroblasts. Although Fli1 down-regulation is implicated in pathological matrix remodeling such as cutaneous fibrosis in scleroderma, very little is known about the post-translational mechanisms regulating Fli1 function. The aim of this study was to investigate the role of acetylation, one of the main post-translational regulatory mechanisms, in regulating Fli1 activity. We initially demonstrated that Fli1 is acetylated by transforming growth factor (TGF)-beta1 in dermal fibroblasts. An in vivo acetylation assay using 293T cells revealed that Fli1 is mainly acetylated by the histone acetyltransferase activity of p300/CBP-associated factor (PCAF) at lysine 380. Acetylation of Fli1 resulted in a decreased stability of Fli1 protein. More importantly, reduced binding of acetylated Fli1 to the human alpha2(I) collagen (COL1A2) promoter was observed in DNA affinity precipitation and chromatin immunoprecipitation. Conversely, a Fli1 K380R mutant that is resistant to acetylation by PCAF showed increased DNA binding ability. Furthermore, PCAF overexpression reversed the inhibitory effect of Fli1 on TGF-beta1-mediated COL1A2 promoter activity. In contrast, the Fli1 K380R mutant had a greater inhibitory effect on TGF-beta1-induced COL1A2 promoter activity than wild-type Fli1, and PCAF failed to reverse this effect. These results indicate that PCAF-dependent acetylation of lysine 380 abrogates repressor function of Fli1 with respect to collagen gene expression. Furthermore, these data strongly suggest that the TGF-beta-dependent acetylation of Fli1 may represent the principal mechanism responsible for the TGF-beta-induced dissociation of Fli1 from the collagen promoter.

Identification and characterization of an R-Smad ortholog (SmSmad1B) fromSchistosoma mansoni

Smad proteins are the cellular mediators of the transforming growth factor-beta superfamily signals. Herein, we describe the isolation of a fourth Smad gene from the helminth Schistosoma mansoni, a receptor-regulated Smad (R-Smad) gene termed SmSmad1B. The SmSmad1B protein is composed of 380 amino acids, and contains conserved MH1 and MH2 domains separated by a short 42 amino acid linker region. The SmSmad1B gene (> 10.7 kb) is composed of five exons separated by four introns. On the basis of phylogenetic analysis, SmSmad1B demonstrates homology to Smad proteins involved in the bone morphogenetic protein pathway. SmSmad1B transcript is expressed in all stages of schistosome development, and exhibits the highest expression level in the cercariae stage. By immunolocalization experiments, the SmSmad1B protein was detected in the cells of the parenchyma of adult schistosomes as well as in female reproductive tissues. Yeast two-hybrid experiments revealed an interaction between SmSmad1B and the common Smad, SmSmad4. As determined by yeast three-hybrid assays and pull-down assays, the presence of the wild-type or mutated SmTbetaRI receptor resulted in a decreased interaction between SmSmad1B and SmSmad4. These results suggest the presence of a nonfunctional interaction between SmSmad1B and SmTbetaRI that does not give rise to the phosphorylation and the release of SmSmad1B to form a heterodimer with SmSmad4. SmSmad1B, as well as the schistosome bone morphogenetic protein-related Smad SmSmad1 and the transforming growth factor-beta-related SmSmad2, interacted with the schistosome coactivator proteins SmGCN5 and SmCBP1 in pull-down assays. In all, these data suggest the involvement of SmSmad1B in critical biological processes such as schistosome reproductive development.