Smads and chromatin modulation

Transforming growth factor-β in stem cells and tissue homeostasis

TGF-β 1-3 are unique multi-functional growth factors that are only expressed in mammals, and mainly secreted and stored as a latent complex in the extracellular matrix (ECM). The biological functions of TGF-β in adults can only be delivered after ligand activation, mostly in response to environmental perturbations. Although involved in multiple biological and pathological processes of the human body, the exact roles of TGF-β in maintaining stem cells and tissue homeostasis have not been well-documented until recent advances, which delineate their functions in a given context. Our recent findings, along with data reported by others, have clearly shown that temporal and spatial activation of TGF-β is involved in the recruitment of stem/progenitor cell participation in tissue regeneration/remodeling process, whereas sustained abnormalities in TGF-β ligand activation, regardless of genetic or environmental origin, will inevitably disrupt the normal physiology and lead to pathobiology of major diseases. Modulation of TGF-β signaling with different approaches has proven effective pre-clinically in the treatment of multiple pathologies such as sclerosis/fibrosis, tumor metastasis, osteoarthritis, and immune disorders. Thus, further elucidation of the mechanisms by which TGF-β is activated in different tissues/organs and how targeted cells respond in a context-dependent way can likely be translated with clinical benefits in the management of a broad range of diseases with the involvement of TGF-β.

Brachyury and SMAD signalling collaboratively orchestrate distinct mesoderm and endoderm gene regulatory networks in differentiating human embryonic stem cells

The transcription factor brachyury (T, BRA) is one of the first markers of gastrulation and lineage specification in vertebrates. Despite its wide use and importance in stem cell and developmental biology, its functional genomic targets in human cells are largely unknown. Here, we use differentiating human embryonic stem cells to study the role of BRA in activin A-induced endoderm and BMP4-induced mesoderm progenitors. We show that BRA has distinct genome-wide binding landscapes in these two cell populations, and that BRA interacts and collaborates with SMAD1 or SMAD2/3 signalling to regulate the expression of its target genes in a cell-specific manner. Importantly, by manipulating the levels of BRA in cells exposed to different signalling environments, we demonstrate that BRA is essential for mesoderm but not for endoderm formation. Together, our data illuminate the function of BRA in the context of human embryonic development and show that the regulatory role of BRA is context dependent. Our study reinforces the importance of analysing the functions of a transcription factor in different cellular and signalling environments.

Overexpression of the FoxO1 Ameliorates Mesangial Cell Dysfunction in Male Diabetic Rats

The dysfunction of mesangial cells (MCs) in high-glucose (HG) conditions plays pivotal role in inducing glomerular sclerosis by causing the imbalance between generation and degradation of extracellular matrix (ECM) proteins, which ultimately leads to diabetic nephropathy. This study was designed to determine the function of forkhead box protein O1 (FoxO1), an important transcription factors in regulating cell metabolism and oxidative stress, in MCs in HG conditions. Up-regulation of fibronectin, collagen type IV, and plasminogen activator inhibitor (PAI-1) was observed under HG conditions in vivo and in vitro, accompanied with elevation of protein kinase B (Akt) phosphorylation and reduction of FoxO1 bioactivity. After overexpression of constitutively active (CA) FoxO1 in vivo and in vitro by using lentivirus vector, in vivo and in vitro, FoxO1 expression and activity was increased, in accordance with up-regulation of antioxidative genes (catalase and superoxide dismutase, leading to alleviated oxidative stress as well as attenuated Akt activity, whereas overexpression of wild type-FoxO1 only expressed partial effect. Moreover, CA-FoxO1 decreased the expression of fibronectin, collagen type IV, and PAI-1, causing amelioration of renal pathological changes and decrease of ECM protein deposition in glomerulus. Overexpression of CA-FoxO1 in renal cortex also decreased activin type-I receptor-like kinase-5 levels and increased signaling mothers against decapentaplegic (Smad) 7 levels, and simultaneously inhibited Smad3 phosphorylation. Results from in vitro study indicated that increased combination of FoxO1 and Smad3 may interfere with the function of Smad3, including Smad3 phosphorylation and translocation, interaction with cAMP response element binding protein (CREB)-binding protein, and binding with PAI-1 promoter. Together, our findings shed light on the novel function of FoxO1 in inhibiting ECM deposition, which is beneficial to ameliorate MC dysfunction.

The roles of transforming growth factor-β, Wnt, Notch and hypoxia on liver progenitor cells in primary liver tumours (Review)

Primary liver tumours have a high incidence and mortality. The most important forms are hepatocellular carcinoma and intrahepatic cholangiocarcinoma, both can occur together in the mixed phenotype hepatocellular-cholangiocarcinoma. Liver progenitor cells (LPCs) are bipotential stem cells activated in case of severe liver damage and are capable of forming both cholangiocytes and hepatocytes. Possibly, alterations in Wnt, transforming growth factor-β, Notch and hypoxia pathways in these LPCs can cause them to give rise to cancer stem cells, capable of driving tumourigenesis. In this review, we summarize and discuss current knowledge on the role of these pathways in LPC activation and differentiation during hepatocarcinogenesis.

Effect of grafting BMP2-derived peptide to nanoparticles on osteogenic and vasculogenic expression of stromal cells

Bone morphogenetic protein-2 (BMP2) plays a major role in initiating the cascade of osteogenesis. However, high doses of exogenous BMP2 coupled with diffusion away from the intended site cause adverse side-effects. An alternative is to use biodegradable polymeric nanoparticles (NPs) grafted with peptides of the active domains of BMP2. NPs present a multivalent form of the peptide for stronger interaction with cell surface receptors, leading to a stronger activation of osteogenic signalling pathways. The objective of this work was to compare osteogenic activity of the BMP2 peptide (BMP2Pe), corresponding to residues 73-92 of BMP2 protein (BMP2Pr), grafted to biodegradable NPs with that of BMP2 protein (BMP2Pr). BMP2Pe was functionalized with a cysteine residue and grafted to poly(lactide fumarate) and poly(lactide-co-ethylene oxide fumarate) (PLAF/PLEOF) NPs via a thioether link. The calcium content of bone marrow stromal (BMS) cells cultured in osteogenic medium supplemented with BMP2 peptide/protein-grafted NPs (BMP2Pe-gNP and BMP2Pr-gNP) was slightly higher than other BMP2-treated groups, but all osteogenic groups showed similar levels of mineralization after 21 days. The expression pattern of master transcription factors Dlx5 and Runx2 indicated that BMP2 protein induced faster osteogenic signalling than the BMP peptide. The expression level of Osteopontin (OP), Osteocalcin (OC) and PECAM-1 in the NP-grafted BMP2 groups was significantly higher than those of ungrafted BMP2Pr and BMP2Pe groups, which may be due to a more effective presentation of the peptide/protein to cell surface receptors, thus leading to a stronger interaction of the peptide/protein with clustered cell surface receptors.

Smad3 is a key nonredundant mediator of transforming growth factor beta signaling in Nme mouse mammary epithelial cells

Smad2 and Smad3 are intracellular mediators of transforming growth factor beta (TGFbeta) signaling that share various biochemical properties, but data emerging from functional analyses in several cell types indicate that these two Smad proteins may convey distinct cellular responses. Therefore, we have investigated the individual roles of Smad2 and Smad3 in mediating the cytostatic and proapoptotic effects of TGFbeta as well as their function in epithelial-to-mesenchymal transition. For this purpose, we transiently depleted mouse mammary epithelial cells (Nme) of Smad2 and/or Smad3 mainly by a strategy relying on RNaseH-induced degradation of mRNA. The effect of such depletion on hallmark events of TGFbeta-driven epithelial-to-mesenchymal transition was analyzed, including dissolution of epithelial junctions, formation of stress fibers and focal adhesions, activation of metalloproteinases, and transcriptional regulation of acknowledged target genes. Furthermore, we investigated the effect of Smad2 and Smad3 knockdown on the TGFbeta-regulated transcriptome by microarray analysis. Our results identify Smad3 as a key factor to trigger TGFbeta-regulated events and ascribe tumor suppressor as well as oncogenic activities to this protein.

Histone deacetylase inhibition and the regulation of cell growth with particular reference to liver pathobiology

The transcriptional activity of genes largely depends on the accessibility of specific chromatin regions to transcriptional regulators. This process is controlled by diverse post-transcriptional modifications of the histone amino termini of which reversible acetylation plays a vital role. Histone acetyltransferases (HATs) are responsible for the addition of acetyl groups and histone deacetylases (HDACs) catalyse the reverse reaction. In general, though not exclusively, histone acetylation is associated with a positive regulation of transcription, whereas histone deacetylation is correlated with transcriptional silencing. The elucidation of unequivocal links between aberrant action of HDACs and tumorigenesis lies at the base of key scientific importance of these enzymes. In particular, the potential benefit of HDAC inhibition has been confirmed in various tumour cell lines, demonstrating antiproliferative, differentiating and pro-apoptotic effects. Consequently, the dynamic quest for HDAC inhibitors (HDIs) as a new class of anticancer drugs was set off, resulting in a number of compounds that are currently evaluated in clinical trials. Ironically, the knowledge with respect to the expression pattern and function of individual HDAC isoenzymes remains largely elusive. In the present review, we provide an update of the current knowledge on the involvement of HDACs in the regulation of fundamental cellular processes in the liver, being the main site for drug metabolism within the body. Focus lies on the involvement of HDACs in the regulation of growth of normal and transformed hepatocytes and the transdifferentiation process of stellate cells. Furthermore, extrapolation of our present knowledge on HDAC functionality towards innovative treatment of malignant and non-malignant, hyperproliferative and inflammatory disorders is discussed.

Epigenetics of cutaneous melanoma

Tumorigenesis is traditionally thought to be caused by the imbalance between oncogenes and tumor-suppressor genes. Epigenetics is a recently described phenomenon that uses an alternative mechanism to explain the transcriptional inactivation of tumor-suppressor genes predominantly by hypermethylation of the promoter regions. Hypermethylation of these regions has been described extensively in many neoplasms, including cutaneous melanoma. Histone modification, primarily by acetylation and deacetylation, is a current potential target for melanoma therapy, but more research is required to understand the mechanisms involved and the therapeutic effectiveness of regimens involving these agents. These mechanisms not only are important for understanding the origin and progression of neoplasms but also have important potential therapeutic implications. Understanding the epigenetic mechanisms involved in melanoma can provide valuable information with significant implications in diagnosis, treatment, and prevention.

Dynamic equilibrium and heterogeneity of mouse pluripotent stem cells with distinct functional and epigenetic states

Embryonic stem cells (ESCs) are apparently homogeneous self-renewing cells, but we observed heterogeneous expression of Stella in ESCs, which is a marker of pluripotency and germ cells. Here we show that, whereas Stella-positive ESCs were like the inner cell mass (ICM), Stella-negative cells were like the epiblast cells. These states were interchangeable, which reflects the metastability and plasticity of ESCs. The established equilibrium was skewed reversibly in the absence of signals from feeder cells, which caused a marked shift toward an epiblast-like state, while trichostatin A, an inhibitor of histone deactelylase, restored Stella-positive population. The two populations also showed different histone modifications and striking functional differences, as judged by their potential for differentiation. The Stella-negative ESCs were more like the postimplantation epiblast-derived stem cells (EpiSCs), albeit the stella locus was repressed by DNA methylation in the latter, which signifies a robust epigenetic boundary between ESCs and EpiSCs.

Essential role of chromatin remodeling protein Bptf in early mouse embryos and embryonic stem cells

We have characterized the biological functions of the chromatin remodeling protein Bptf (Bromodomain PHD-finger Transcription Factor), the largest subunit of NURF (Nucleosome Remodeling Factor) in a mammal. Bptf mutants manifest growth defects at the post-implantation stage and are reabsorbed by E8.5. Histological analyses of lineage markers show that Bptf^−/− embryos implant but fail to establish a functional distal visceral endoderm. Microarray analysis at early stages of differentiation has identified Bptf-dependent gene targets including homeobox transcriptions factors and genes essential for the development of ectoderm, mesoderm, and both definitive and visceral endoderm. Differentiation of Bptf^−/− embryonic stem cell lines into embryoid bodies revealed its requirement for development of mesoderm, endoderm, and ectoderm tissue lineages, and uncovered many genes whose activation or repression are Bptf-dependent. We also provide functional and physical links between the Bptf-containing NURF complex and the Smad transcription factors. These results suggest that Bptf may co-regulate some gene targets of this pathway, which is essential for establishment of the visceral endoderm. We conclude that Bptf likely regulates genes and signaling pathways essential for the development of key tissues of the early mouse embryo.

While the chromatin of eukaryotes provides an efficient means to compact large amounts of DNA into a small nucleus, it renders the DNA relatively inaccessible. ATP-dependent chromatin remodeling complexes mobilize nucleosomes and provide a means to gain access to DNA in chromatin. While the biochemical functions of chromatin remodeling complexes is well-characterized, less is known of their biological functions. In this manuscript, we elucidate the biological functions of Bptf, a subunit of the NURF chromatin remodeling complex. Our studies show that Bptf is required for the establishment of the anterior–posterior axis of the mouse embryo during the earliest stages of development. To understand its functions in tissue differentiation, we generated and characterized Bptf-mutant ES cells. Mutant embryonic stem cells show significant defects in the differentiation of ectoderm, endoderm, and mesoderm. Genome-wide analysis of gene expression defects during differentiation has identified many Bptf-dependent pathways including key regulators of ectoderm, endoderm, and mesoderm differentiation. Moreover, we have identified critical functions for Bptf during the TGFβ/Smad-induced expression of visceral endoderm and mesoderm markers, an important signaling pathway in the early embryo. These results suggest that chromatin remodeling by Bptf regulates key signaling pathways in the early mouse embryo.

Ttrap is an essential modulator of Smad3-dependent Nodal signaling during zebrafish gastrulation and left-right axis determination

During vertebrate development, signaling by the TGFbeta ligand Nodal is critical for mesoderm formation, correct positioning of the anterior-posterior axis, normal anterior and midline patterning, and left-right asymmetric development of the heart and viscera. Stimulation of Alk4/EGF-CFC receptor complexes by Nodal activates Smad2/3, leading to left-sided expression of target genes that promote asymmetric placement of certain internal organs. We identified Ttrap as a novel Alk4- and Smad3-interacting protein that controls gastrulation movements and left-right axis determination in zebrafish. Morpholino-mediated Ttrap knockdown increases Smad3 activity, leading to ectopic expression of snail1a and apparent repression of e-cadherin, thereby perturbing cell movements during convergent extension, epiboly and node formation. Thus, although the role of Smad proteins in mediating Nodal signaling is well-documented, the functional characterization of Ttrap provides insight into a novel Smad partner that plays an essential role in the fine-tuning of this signal transduction cascade.

Chromatin-bound p53 anchors activated Smads and the mSin3A corepressor to confer transforming-growth-factor-beta-mediated transcription repression

In hepatic cells, Smad and SnoN proteins converge with p53 to repress transcription of AFP, an oncodevelopmental tumor marker aberrantly reactivated in hepatoma cells. Using p53- and SnoN-depleted hepatoma cell clones, we define a mechanism for repression mediated by this novel transcriptional partnership. We find that p53 anchors activated Smads and the corepressor mSin3A to the AFP distal promoter. Sequential chromatin immunoprecipitation analyses and molecular modeling indicate that p53 and Smad proteins simultaneously occupy overlapping p53 and Smad regulatory elements to establish repression of AFP transcription. In addition to its well-known function in antagonizing transforming growth factor beta (TGF-beta) responses, we find that SnoN actively participates in AFP repression by positively regulating mSin3A protein levels. We propose that activation of TGF-beta signaling restores a dynamic interplay between p53 and TGF-beta effectors that cooperate to effectively target mSin3A to tumor marker AFP and reestablish transcription repression.

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.

Erythroleukaemia, diabetes insipidus and hypophyseal damage: Two case reports

We report on two cases of patients who developed diabetes insipidus (DI) before acute erythroleukaemia (EL). A brain MRI showed an empty sella turcica in one case and hypothalamo-hypophyseal peduncle damage in the second case. Reduced levels of TGF-beta1 and Vitamin D3, with associated EVI-1 over-expression and karyotypic abnormalities were documented. These two cases show specific chromosomal/molecular alterations in EL with DI. The hypothesis of pituitary involvement in erythroleukemogenesis is discussed.

Epigenetic events in malignant melanoma

Irreversible changes in the DNA sequence, including chromosomal deletions or amplification, activating or inactivating mutations in genes, have been implicated in the development and progression of melanoma. However, increasing attention is being turned towards the participation of 'epigenetic' events in melanoma progression that do not affect DNA sequence, but which nevertheless may lead to stable inherited changes in gene expression. Epigenetic events including histone modifications and DNA methylation play a key role in normal development and are crucial to establishing the correct program of gene expression. In contrast, mistargeting of such epigenetic modifications can lead to aberrant patterns of gene expression and loss of anti-cancer checkpoints. Thus, to date at least 50 genes have been reported to be dysregulated in melanoma by aberrant DNA methylation and accumulating evidence also suggests that mistargetting of histone modifications and altered chromatin remodeling activities will play a key role in melanoma. This review gives an overview of the many different types of epigenetic modifications and their involvement in cancer and especially in melanoma development and progression.

The expression of CSRP2 encoding the LIM domain protein CRP2 is mediated by TGF-β in smooth muscle and hepatic stellate cells

Transforming growth factor-beta (TGF-beta) is a cytokine implicated in differentiation of smooth muscle cells and other mesenchymal-derived cells. During hepatic fibrogenesis, TGF-beta has a pivotal role in the initiation, promotion, and progression of transdifferentiation of hepatic stellate cells into myofibroblasts that play a central role in the synthesis of extracellular matrix components. Both, smooth muscle and activated hepatic stellate cells, express smooth muscle alpha-actin, the calponin-related protein SM22alpha, and CSRP2 encoding the cysteine- and glycine-rich LIM domain protein 2 (CRP2). The aim of the present study was to determine whether the expression of CSRP2 is influenced by TGF-beta. Stimulation as well as sequestering experiments demonstrated that TGF-beta markedly influences CSRP2 gene activity. Inhibition experiments using the ALK5 inhibitor SB-431542 further reveal that the transcriptional stimulation of the CSRP2 gene is mediated via the ALK5/Smad2/Smad3 signalling pathway. By use of bisulfite genomic analysis of CpG islands within the 5' regulatory regions we could exclude methylation-associated silencing, previously found to be responsible for the transcriptional inactivity of CSRP2 in a variety of human cancer cells and in a multistage carcinogenesis model, as a cause for CSRP2 inactivity in hepatocytes or fully transdifferentiated myofibroblasts.