Interaction between Wnt and TGF-beta signalling pathways during formation of Spemann's organizer

Identification of DNA-dependent protein kinase catalytic subunit as a novel interaction partner of lymphocyte enhancer factor 1

Lymphocyte enhancer factor 1 (LEF1), a member of the LEF/T-cell-specific factor (TCF) family of the high mobility group domain transcription factors, acts downstream in canonical Wnt signaling. Aberrant transactivation of LEF1 contributes to the tumorigenesis of colonic neoplasms, sebaceous skin tumors, and lymphoblastic leukemia. LEF1-associated proteins are crucial for regulating its transcriptional activity. In this study, glutathione-S-transferase pull-down assay and mass spectrometry enabled identification of the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) as a novel interaction partner for LEF1. The interaction between LEF1 and DNA-PKcs was confirmed using in vivo co-immunoprecipitation. Furthermore, double immunofluorescence observations showed that LEF1 and DNA-PKcs colocalized in the nuclei of colon adenocarcinoma cell lines. Identification of the interaction between LEF1 and DNA-PKcs may provide clues for a novel therapy for cancer treatment as well as for understanding LEF1-mediated transcriptional regulation.

Nodal signalling in embryogenesis and tumourigenesis

With few exceptions, most cells in adult organisms have lost the expression of stem cell-associated proteins and are instead characterized by tissue-specific gene expression and function. This cell fate specification is dictated spatially and temporally during embryogenesis. It has become increasingly apparent that the elegant and complicated process of cell specification is "undone" in cancer. This may be because cancer cells respond to their microenvironment and mutations by acquiring a more permissive, plastic epigenome, or because cancer cells arise from mutated stem cells. Regardless, these advanced cancer cells must use stem cell-associated proteins to sustain their phenotype. One such protein is Nodal, an embryonic morphogen belonging to the transforming growth factor-β (TGF-β) superfamily. First described in early developmental models, Nodal orchestrates embryogenesis by regulating a myriad of processes, including mesendoderm induction, left-right asymmetry and embryo implantation. Nodal is relatively restricted to embryonic and reproductive cell types and is thus absent from most normal adult tissues. However, recent studies focusing on a variety of malignancies have demonstrated that Nodal expression re-emerges during cancer progression. Moreover, in almost every cancer studied thus far, the acquisition of Nodal expression is associated with increased tumourigenesis, invasion and metastasis. As the list of cancers that express Nodal grows, it is essential that the scientific and medical communities fully understand how this morphogen is regulated in both normal and neoplastic conditions. Herein, we review the literature relating to normal and pathological Nodal signalling. In particular, we emphasize the role that this secreted protein plays during morphogenic events and how it signals to support stem cell maintenance and tumour progression.

Wnt signaling in vertebrate axis specification

The Wnt pathway is a major embryonic signaling pathway that controls cell proliferation, cell fate, and body-axis determination in vertebrate embryos. Soon after egg fertilization, Wnt pathway components play a role in microtubule-dependent dorsoventral axis specification. Later in embryogenesis, another conserved function of the pathway is to specify the anteroposterior axis. The dual role of Wnt signaling in Xenopus and zebrafish embryos is regulated at different developmental stages by distinct sets of Wnt target genes. This review highlights recent progress in the discrimination of different signaling branches and the identification of specific pathway targets during vertebrate axial development.

Modulation of β-catenin signaling by the inhibitors of MAP kinase, tyrosine kinase, and PI3-kinase pathways

Aberrant activation of β-catenin signaling plays an important role in human tumorigenesis. However, molecular mechanisms behind the β-catenin signaling deregulation are mostly unknown because genetic alterations in this pathway only account for a small fraction of tumors. Here, we investigator if other major pathways can regulate β-catenin signaling activity. By employing a panel of chemical activators and/or inhibitors of several cellular signaling pathways, we assess these modulators' effects on luciferase reporter driven by β-catenin/TCF4-responsive elements. We find that lithium-stimulated β-catenin activity is synergistically enhanced by protein kinase C activator PMA. However, β-catenin-regulated transcriptional (CRT) activity is significantly inhibited by casein kinase II inhibitor DRB, MEK inhibitor PD98059, G-proteins and their receptor uncoupling agent suramin, protein tyrosine kinase inhibitor genistein, and PI-3 kinase inhibitor wortmannin, suggesting that these cellular pathways may participate in regulating β-catenin signaling. Interestingly, the Ca⁺⁺/calmodulin kinase II inhibitor HDBA is shown to activate β-catenin activity at low doses. Furthermore, Wnt3A-stimulated and constitutively activated CRT activities, as well as the intracellular accumulation of β-catenin protein in human colon cancer cells, are effectively suppressed by PD98059, genistein, and wortmannin. We further demonstrate that EGF can activate TCF4/β-catenin activity and induce the tyrosine phosphorylation of β-catenin protein. Thus, our results should provide important insights into the molecular mechanisms underlying Wnt/β-catenin activation. This knowledge should facilitate our efforts to develop efficacious and novel therapeutics by targeting these pathways.

Sustained Wnt/β-catenin signaling rescues high glucose induction of transforming growth factor-β1-mediated renal fibrosis

INTRODUCTION

Although diabetic nephropathy is attributable to transforming growth factor-β1 (TGF-β1) overproduction in glomer-ular mesangial cells, the biological role of Wnt/β-catenin signaling in controlling high glucose-induced TGF-β1 has not yet been elucidated.

METHODS

This study found that sustained Wnt/β-catenin signaling was required to protect glomerular mesangial cells from high glucose induction of TGF-β1-mediated fibrosis using in vitro and in vivo diabetic models.

RESULTS

High glucose down-regulated the Wnt signaling associated with increased TGF-β1 and fibronectin messenger RNA expression in glomerular mesangial cells. Restoring Wnt4, Wnt5a and cytosolic β-catenin levels by transfecting Wnt4, Wnt5a and stable β-catenin alleviated the stimulatory effect of high glucose on c-Jun mediated TGF-β1 fibrosis. Transfection of kinase-active glycogen synthase kinase-3β (GSK-3β) also abrogated high glucose promotion of nuclear c-Jun levels, TGF-β1 and fibronectin messenger RNA expression in mesangial cells. Pharmacological modulation of GSK-3ββ and ββ-catenin signaling by recombinant Wnt5a or GSK-3β inhibitor (BIO or LiCl) suppressed high glucose promotion of TGF-β1-mediated fibrosis. Exogenous BIO and SB216763 alleviated TGF-β1-mediated fibrogenic expression in the kidneys of diabetic rats. Immunohistochemistry showed that GSK-3β inhibitor significantly reversed the diabetic attenuation of TGF-β1 and c-Jun coinciding with fibronectin immunoreactivity within glomeruli. Immunofluorescence demonstrated that cells within the glomeruli restored β-catenin expression after BIO and SB216763 treatment in cells within diabetic glomeruli colocalized with fragmented nuclei by 4',6-diamidino-2-phenylindole staining.

CONCLUSIONS

Sustained Wnt signaling reduced c-Jun-dependent TGF-β1-mediated fibronectin accumulation in mesangial cells. These findings suggest that modulation of Wnt signaling is a viable alternative strategy to rescue the TGF-β1-mediated fibrotic signaling pathway in diabetic renal injury.

TGF-β affects enterocyte turnover in correlation with TGF-β receptor expression after massive small bowel resection

OBJECTIVES

In the present study, we evaluated the effect of transforming growth factor-beta 2 (TGF-β2)-enriched diet on enterocyte turnover and correlated it with TGF-β2 receptor expression along the villus-crypt axis in a rat model of short bowel syndrome (SBS).

METHODS

CaCo-2 cells were incubated with increasing concentrations of TGF-β2. Alamar Blue reduction test was used for investigation of cell viability and evaluation of cell apoptosis was assessed by flow cytometry. Male rats were divided into 4 groups: Sham rats underwent bowel transection, Sham TGF-β rats were treated with diet enriched with TGF-β2, SBS rats underwent a 75% bowel resection, and SBS TGF-β rats were fed a diet enriched with TGF-β2 after bowel resection. Parameters of intestinal adaptation, enterocyte proliferation and apoptosis were determined at sacrifice. TGF-β2r expression in villus tips, lateral villi and crypts was assessed by immunohistochemistry. The effect of TGF-β2 on enterocyte turnover for each compartment was evaluated in correlation with TGF-β2r expression.

RESULTS

Incubation of CaCo-2 cells with TGF-β2 resulted in a significant decrease in cell viability and increased cell apoptosis. TGF-β2r expression in crypts increased in SBS rats (vs sham) and was accompanied by decreased cell proliferation and increased cell apoptosis following TGF-β2 administration. A significant decrease in TGF-β2r expression at villous tips in SBS rats was accompanied by a decreased cell apoptosis in this compartment following exposure to TGF-β2-enriched diet.

CONCLUSIONS

In a rat model of SBS, the inhibiting effect of TGF-β2 on enterocyte turnover correlates with TGF-β2 receptor expression along the villus-crypt axis.

An arrayed RNA interference genome-wide screen identifies candidate genes involved in the MicroRNA 21 biogenesis pathway

MicroRNAs (miRNAs) are evolutionary conserved noncoding molecules that regulate gene expression. They influence a number of diverse biological functions, such as development and differentiation. However, their dysregulation has been shown to be associated with disease states, such as cancer. Genes and pathways regulating their biogenesis remain unknown and are highly sought after. For this purpose, we have validated a multiplexed high-content assay strategy to screen for such modulators. Here, we describe its implementation that makes use of a cell-based gain-of-function reporter assay monitoring enhanced green fluorescent protein expression under the control of miRNA 21 (miR-21); combined with measures of both cell metabolic activities through the use of Alamar Blue and cell death through imaged Hoechst-stained nuclei. The strategy was validated using a panel of known genes and enabled us to successfully progress to and complete an arrayed genome-wide short interfering RNA (siRNA) screen against the Ambion Silencer Select v4.0 library containing 64,755 siRNA duplexes covering 21,565 genes. We applied a high-stringency hit analysis method, referred to as the Bhinder-Djaballah analysis method, leading to the nomination of 1,273 genes as candidate inhibitors of the miR-21 biogenesis pathway; after several iterations eliminating those genes with only one active duplex and those enriched in seed sequence mediated off-target effects. Biological classifications revealed four major control junctions among them vesicular transport via clathrin-mediated endocytosis. Altogether, our screen has uncovered a number of novel candidate regulators that are potentially good druggable targets allowing for the discovery and development of small molecules for regulating miRNA function.

Iron chelators for the treatment of cancer

The study of iron chelators as anti-tumor agents is still in its infancy. Iron is important for cellular proliferation and this is demonstrated by observations that iron-depletion results in cell cycle arrest and also apoptosis. In addition, many iron chelators are known to inhibit ribonucleotide reductase, the iron-containing enzyme that is the rate-limiting step for DNA synthesis. Desferrioxamine is a well known chelator used for the treatment of iron-overload disease, but it has also been shown to possess anti-cancer activity. Another class of chelators, namely the thiosemicarbazones, have been shown to possess anti-cancer activity since the 1950's, although their mechanism(s) of action have only recently been more comprehensively elucidated. In fact, the redox activity of thiosemicarbazone iron complexes is thought to be important in mediating their potent cytotoxicity. Moreover, unlike typical iron chelators which simply act to deplete tumors of iron, several thiosemicarbazones (i.e., Bp44mT and Dp44mT) do not induce this effect, their anti-cancer efficacy being due to other mechanisms e.g., redox activity. Other reports have also shown that some thiosemicarbazones inhibit topoisomerase IIα, demonstrating that this class of agents have multiple molecular targets and act by various mechanisms. The most well characterized thiosemicarbazone iron chelator in terms of its assessment in humans is 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP). Observations from these clinical trials highlight the less than optimal activity of this ligand and several side effects related to its use, including myelo-suppression, hypoxia and methemoglobinemia. The mechanisms responsible for these latter effects must be elucidated and the design of the ligand altered to minimize these problems and increase efficacy. This review discusses the development of chelators as unique agents for cancer treatment.

Intrinsic properties of Tcf1 and Tcf4 splice variants determine cell-type-specific Wnt/β-catenin target gene expression

T-cell factor (Tcf)/lymphoid-enhancer factor (Lef) proteins are a structurally diverse family of deoxyribonucleic acid-binding proteins that have essential nuclear functions in Wnt/β-catenin signalling. Expression of Wnt/β-catenin target genes is highly dependent on context, but the precise role of Tcf/Lef family members in the generation and maintenance of cell-type-specific Wnt/β-catenin responses is unknown. Herein, we show that induction of a subset of Wnt/β-catenin targets in embryonic stem cells depends on Tcf1 and Tcf4, whereas other co-expressed Tcf/Lef family members cannot induce these targets. The Tcf1/Tcf4-dependent gene responses to Wnt are primarily if not exclusively mediated by C-clamp-containing Tcf1E and Tcf4E splice variants. A combined knockdown of Tcf1/Tcf4 abrogates Wnt-inducible transcription but does not affect the active chromatin conformation of their targets. Thus, the transcriptionally poised state of Wnt/β-catenin targets is maintained independent of Tcf/Lef proteins. Conversely, ectopically overexpressed Tcf1E cannot invade silent chromatin and fails to initiate expression of inactive Wnt/β-catenin targets even if repressive chromatin modifications are abolished. The observed non-redundant functions of Tcf1/Tcf4 isoforms in acute transcriptional activation demonstrated that the cell-type-specific complement of Tcf/Lef proteins is a critical determinant of context-dependent Wnt/β-catenin responses. Moreover, the apparent inability to cope with chromatin uncovers an intrinsic property of Tcf/Lef proteins that prevents false ectopic induction and ensures spatiotemporal stability of Wnt/β-catenin target gene expression.

Novel non-canonical TGF-β signaling networks: emerging roles in airway smooth muscle phenotype and function

The airway smooth muscle (ASM) plays an important role in the pathophysiology of asthma and chronic obstructive pulmonary disease (COPD). ASM cells express a wide range of receptors involved in contraction, growth, matrix protein production and the secretion of cytokines and chemokines. Transforming growth factor beta (TGF-β) is one of the major players in determining the structural and functional abnormalities of the ASM in asthma and COPD. It is increasingly evident that TGF-β functions as a master switch, controlling a network of intracellular and autocrine signaling loops that effect ASM phenotype and function. In this review, the various elements that participate in non-canonical TGF-β signaling, including MAPK, PI3K, WNT/β-catenin, and Ca(2+), are discussed, focusing on their effect on ASM phenotype and function. In addition, new aspects of ASM biology and their possible association with non-canonical TGF-β signaling will be discussed.

New insights into the mechanism of Wnt signaling pathway activation

Wnts compromise a large family of secreted, hydrophobic glycoproteins that control a variety of developmental and adult processes in all metazoan organisms. Recent advances in the Wnt-signal studies have revealed that distinct Wnts activate multiple intracellular cascades that regulate cellular proliferation, differentiation, migration, and polarity. Although the mechanism by which Wnts regulate different pathways selectively remains to be clarified, evidence has accumulated that in addition to the formation of ligand-receptor pairs, phosphorylation of receptors, receptor-mediated endocytosis, acidification, and the presence of cofactors, such as heparan sulfate proteoglycans, are also involved in the activation of specific Wnt pathways. Here, we review the mechanism of activation in Wnt signaling initiated on the cell-surface membrane. In addition, the mechanisms for fine-tuning by cross talk between Wnt and other signaling are also discussed.

Regulation of mature T cell responses by the Wnt signaling pathway

The canonical Wnt signaling pathway is evolutionarily conserved and plays key roles during development of many organ systems. This pathway utilizes TCF/LEF transcription factors, β-catenin coactivator, and TLE/GRG corepressors to achieve balanced regulation of its downstream gene expression. It is well established that several Wnt ligands and their effector proteins are crucial for normal T cell development. Recent studies have also revealed critical requirements for TCF-1 in generation and persistence of functional memory CD8(+) T cells, and in promoting Th2-differentiation and suppressing Th17-differentiation of activated CD4(+) T cells. Activation of β-catenin facilitated CD8(+) memory T cell formation, with enhanced protective capacity and extended survival of CD4(+) CD25(+) regulatory T cells. Upregulation of Wnt ligands was observed in Drosophila in response to Toll signaling as well as in mammalian dendritic cells and macrophages upon microbial stimulation. These new findings suggest that modulating the activity of Wnt pathway may be a powerful approach to enhance protective immunity and treat autoimmune diseases.

Interactions between β-catenin and transforming growth factor-β signaling pathways mediate epithelial-mesenchymal transition and are dependent on the transcriptional co-activator cAMP-response element-binding protein (CREB)-binding protein (CBP)

Interactions between transforming growth factor-β (TGF-β) and Wnt are crucial to many biological processes, although specific targets, rationale for divergent outcomes (differentiation versus block of epithelial proliferation versus epithelial-mesenchymal transition (EMT)) and precise mechanisms in many cases remain unknown. We investigated β-catenin-dependent and transforming growth factor-β1 (TGF-β1) interactions in pulmonary alveolar epithelial cells (AEC) in the context of EMT and pulmonary fibrosis. We previously demonstrated that ICG-001, a small molecule specific inhibitor of the β-catenin/CBP (but not β-catenin/p300) interaction, ameliorates and reverses pulmonary fibrosis and inhibits TGF-β1-mediated α-smooth muscle actin (α-SMA) and collagen induction in AEC. We now demonstrate that TGF-β1 induces LEF/TCF TOPFLASH reporter activation and nuclear β-catenin accumulation, while LiCl augments TGF-β-induced α-SMA expression, further confirming co-operation between β-catenin- and TGF-β-dependent signaling pathways. Inhibition and knockdown of Smad3, knockdown of β-catenin and overexpression of ICAT abrogated effects of TGF-β1 on α-SMA transcription/expression, indicating a requirement for β-catenin in these Smad3-dependent effects. Following TGF-β treatment, co-immunoprecipitation demonstrated direct interaction between endogenous Smad3 and β-catenin, while chromatin immunoprecipitation (ChIP)-re-ChIP identified spatial and temporal regulation of α-SMA via complex formation among Smad3, β-catenin, and CBP. ICG-001 inhibited α-SMA expression/transcription in response to TGF-β as well as α-SMA promoter occupancy by β-catenin and CBP, demonstrating a previously unknown requisite TGF-β1/β-catenin/CBP-mediated pro-EMT signaling pathway. Clinical relevance was shown by β-catenin/Smad3 co-localization and CBP expression in AEC of IPF patients. These findings suggest a new therapeutic approach to pulmonary fibrosis by specifically uncoupling CBP/catenin-dependent signaling downstream of TGF-β.

How do they do Wnt they do?: regulation of transcription by the Wnt/β-catenin pathway

Wnt/β-catenin signalling is known to play many roles in metazoan development and tissue homeostasis. Misregulation of the pathway has also been linked to many human diseases. In this review, specific aspects of the pathway's involvement in these processes are discussed, with an emphasis on how Wnt/β-catenin signalling regulates gene expression in a cell and temporally specific manner. The T-cell factor (TCF) family of transcription factors, which mediate a large portion of Wnt/β-catenin signalling, will be discussed in detail. Invertebrates contain a single TCF gene that contains two DNA-binding domains, the high mobility group (HMG) domain and the C-clamp, which increases the specificity of DNA binding. In vertebrates, the situation is more complex, with four TCF genes producing many isoforms that contain the HMG domain, but only some of which possess a C-clamp. Vertebrate TCFs have been reported to act in concert with many other transcription factors, which may explain how they obtain sufficient specificity for specific DNA sequences, as well as how they achieve a wide diversity of transcriptional outputs in different cells.

Wnt/β-catenin pathway regulates bone morphogenetic protein (BMP2)-mediated differentiation of dental follicle cells

BACKGROUND AND OBJECTIVE

Bone morphogenetic protein 2 (BMP2)-induced osteogenic differentiation has been shown to occur through the canonical Wnt/βcatenin pathway, whereas factors promoting canonical Wnt signaling in cementoblasts inhibit cell differentiation and promote cell proliferation in vitro. The aim of this study was to investigate whether putative precursor cells of cementoblasts, dental follicle cells (murine SVF4 cells), when stimulated with BMP2, would exhibit changes in genes/proteins associated with the Wnt/β-catenin pathway.

MATERIAL AND METHODS

SVF4 cells were stimulated with BMP2, and the following assays were carried out: (i) Wnt/β-catenin pathway activation assessed by western blotting, β-catenin/transcription factor (TCF) reporter assays and expression of the lymphoid enhancer-binding factor-1 (Lef1), transcription factor 7 (Tcf7), Wnt inhibitor factor 1 (Wif1) and Axin2 (Axin2) genes; and (ii) cementoblast/osteoblast differentiation assessed by mineralization in vitro, and by the mRNA levels of runt-related transcription factor 2 (Runx2), osterix (Osx), alkaline phosphatase (Alp), osteocalcin (Ocn) and bone sialoprotein (Bsp), determined by quantitative PCR after treatment with wingless-type MMTV integration site family, member 3A (WNT3A) and knockdown of β-catenin.

RESULTS

WNT3A induced β-catenin nuclear translocation and up-regulated the transcriptional activity of a canonical Wnt-responsive reporter, suggesting that the Wnt/β-catenin pathway functions in SVF4 cells. Activation of Wnt signaling with WNT3A suppressed BMP2-mediated induction of cementoblast/osteoblast maturation of SVF4 cells. However, β-catenin knockdown showed that the BMP2-induced expression of cementoblast/osteoblast differentiation markers requires endogenous β-catenin. WNT3A down-regulated transcripts for Runx2, Alp and Ocn in SVF4 cells compared with untreated cells. In contrast, BMP2 induction of Bsp transcripts occurred independently of Wnt/β-catenin signaling.

CONCLUSION

These data suggest that stabilization of β-catenin by WNT3A inhibits BMP2-mediated induction of cementoblast/osteoblast differentiation in SVF4 cells, although BMP2 requires endogenous Wnt/β-catenin signaling to promote cell maturation.

Phosphorylation of Mad controls competition between wingless and BMP signaling

Bone morphogenetic proteins (BMPs) and Wnts are growth factors that provide essential patterning signals for cell proliferation and differentiation. Here, we describe a molecular mechanism by which the phosphorylation state of the Drosophila transcription factor Mad determines its ability to transduce either BMP or Wingless (Wg) signals. Previously, Mad was thought to function in gene transcription only when phosphorylated by BMP receptors. We found that the unphosphorylated form of Mad was required for canonical Wg signaling by interacting with the Pangolin-Armadillo transcriptional complex. Phosphorylation of the carboxyl terminus of Mad by BMP receptor directed Mad toward BMP signaling, thereby preventing Mad from functioning in the Wg pathway. The results show that Mad has distinct signal transduction roles in the BMP and Wnt pathways depending on its phosphorylation state.

Wg signaling via Zw3 and mad restricts self-renewal of sensory organ precursor cells in Drosophila

It is well known that the Dpp signal transducer Mad is activated by phosphorylation at its carboxy-terminus. The role of phosphorylation on other regions of Mad is not as well understood. Here we report that the phosphorylation of Mad in the linker region by the Wg antagonist Zw3 (homolog of vertebrate Gsk3-β) regulates the development of sensory organs in the anterior-dorsal quadrant of the wing. Proneural expression of Mad-RNA interference (RNAi) or a Mad transgene with its Zw3/Gsk3-β phosphorylation sites mutated (MGM) generated wings with ectopic sensilla and chemosensory bristle duplications. Studies with pMad-Gsk (an antibody specific to Zw3/Gsk3-β-phosphorylated Mad) in larval wing disks revealed that this phosphorylation event is Wg dependent (via an unconventional mechanism), is restricted to anterior-dorsal sensory organ precursors (SOP) expressing Senseless (Sens), and is always co-expressed with the mitotic marker phospho-histone3. Quantitative analysis in both Mad-RNAi and MGM larval wing disks revealed a significant increase in the number of Sens SOP. We conclude that the phosphorylation of Mad by Zw3 functions to prevent the self-renewal of Sens SOP, perhaps facilitating their differentiation via asymmetric division. The conservation of Zw3/Gsk3-β phosphorylation sites in vertebrate homologs of Mad (Smads) suggests that this pathway, the first transforming growth factor β-independent role for any Smad protein, may be widely utilized for regulating mitosis during development.

TGF-β regulates β-catenin signaling and osteoblast differentiation in human mesenchymal stem cells

Human adult bone marrow-derived skeletal stem cells a.k.a mesenchymal stem cells (hMSCs) have been shown to be precursors of several different cellular lineages, including osteoblast, chondrocyte, myoblast, adipocyte, and fibroblast. Several studies have shown that cooperation between transforming growth factor β (TGF-β) and Wnt/β-catenin signaling pathways plays a role in controlling certain developmental events and diseases. Our previous data showed that agents like TGF-β, cooperation with Wnt signaling, promote chondrocyte differentiation at the expense of adipocyte differentiation in hMSCs. In this study, we tested mechanisms by which TGF-β activation of β-catenin signaling pathway and whether these pathways interact during osteoblast differentiation of hMSCs. With selective small chemical kinase inhibitors, we demonstrated that TGF-β1 requires TGF-β type I receptor ALK-5, Smad3, phosphoinositide 3-kinases (PI3K), and protein kinase A (PKA) to stabilize β-catenin, and needs ALK-5, PKA, and JNK to inhibit osteoblastogenesis in hMSCs. Knockdown of β-catenin with siRNA stimulated alkaline phosphatase activity and antagonized the inhibitory effects of TGF-β1 on bone sialoprotein (BSP) expression, suggested that TGF-β1 cooperated with β-catenin signaling in inhibitory of osteoblastogenesis in hMSCs. In summary, TGF-β1 activates β-catenin signaling pathway via ALK-5, Smad3, PKA, and PI3K pathways, and modulates osteoblastogenesis via ALK5, PKA, and JNK pathways in hMSCs; the interaction between TGF-β and β-catenin signaling supports the view that β-catenin signaling is a mediator of TGF-β's effects on osteoblast differentiation of hMSCs.

Interferon-α2b and transforming growth factor-β1 treatments on HCC cell lines: Are Wnt/β-catenin pathway and Smads signaling connected in hepatocellular carcinoma?

Wnt/β-catenin pathway is often dysregulated in hepatocellular carcinoma (HCC). Activated β-catenin accumulates in the cytosol and nucleus and forms a nuclear complex with TCF/LEF factors like TCF4. Interferon-α (IFN-α) has recently been recognized to harbor therapeutic potential in prevention and treatment of HCC. Transforming Growth Factor-β1 (TGF-β1) is a mediator of apoptosis, exerting its effects via Smads proteins. One mode of interaction between Wnt/β-catenin and TGF-β1/Smads pathways is the association of Smads with β-catenin/TCF4. In this study we analyzed the effects of IFN-α2b and TGF-β1 treatments on Wnt/β-catenin pathway, Smads proteins levels, β-catenin/TCF4/Smads interaction and proliferation and apoptotic death in HepG2/C3A and Huh7 cell lines. IFN-α2b and TGF-β1 attenuated Wnt/β-catenin signal by decreasing β-catenin and Frizzled7 receptor proteins contents and the interaction of β-catenin with TCF4. Truncated β-catenin form present in C3A cell line also diminished after treatments. Both cytokines declined Smads proteins and their interaction with TCF4. The overall cellular response to cytokines was the decrease in proliferation and increase in apoptotic death. Treatment with Wnt3a, which elevates β-catenin protein levels, also generated the increment of Smads proteins contents when comparing with untreated cells. In conclusion, IFN-α2b and TGF-β1 proved to be effective as modulators of Wnt/β-catenin pathway in HCC cell lines holding both wild-type and truncated β-catenin. Since the inhibition of β-catenin/TCF4/Smads complexes formation may have a critical role in slowing down oncogenesis, IFN-α2b and TGF-β1 could be useful as potential treatments in patients with HCC.

Canonical Wnt signalling requires the BMP pathway to inhibit oligodendrocyte maturation

OLs (oligodendrocytes) are the myelinating cells of the CNS (central nervous system), wrapping axons in conductive sheathes to ensure effective transmission of neural signals. The regulation of OL development, from precursor to mature myelinating cell, is controlled by a variety of inhibitory and inductive signalling factors. The dorsal spinal cord contains signals that inhibit OL development, possibly to prevent premature and ectopic precursor differentiation. The Wnt and BMP (bone morphogenic protein) signalling pathways have been identified as dorsal spinal cord signals with overlapping temporal activity, and both have similar inhibitory effects on OL differentiation. Both these pathways feature prominently in many developmental processes and demyelinating events after injury, and they are known to interact in complex inductive, inhibitive and synergistic manners in many developing systems. The interaction between BMP and Wnt signalling in OL development, however, has not been extensively explored. In the present study, we examine the relationship between the canonical Wnt and BMP pathways. We use pharmacological and genetic paradigms to show that both Wnt3a and BMP4 will inhibit OL differentiation in vitro. We also show that when the canonical BMP signalling pathway is blocked, neither Wnt3a nor BMP4 have inhibitory effects on OL differentiation. In contrast, abrogating the Wnt signalling pathway does not alter the actions of BMP4 treatment. Our results indicate that the BMP signalling pathway is necessary for the canonical Wnt signalling pathway to exert its effects on OL development, but not vice versa, suggesting that Wnt signals upstream of BMP.

The role of Smad signaling in hematopoiesis and translational hematology

Hematopoietic stem cells (HSCs) reside in the bone marrow (BM) of adult individuals and function to produce and regenerate the entire blood and immune system over the course of an individual's lifetime. Historically, HSCs are among the most thoroughly characterized tissue-specific stem cells. Despite this, the regulation of fate options, such as self-renewal and differentiation, has remained elusive, partly because of the expansive plethora of factors and signaling cues that govern HSC behavior in vivo. In the BM, HSCs are housed in specialized niches that dovetail the behavior of HSCs with the need of the organism. The Smad-signaling pathway, which operates downstream of the transforming growth factor-β (TGF-β) superfamily of ligands, regulates a diverse set of biological processes, including proliferation, differentiation and apoptosis, in many different organ systems. Much of the function of Smad signaling in hematopoiesis has remained nebulous due to early embryonic lethality of most knockout mouse models. However, recently new data have been uncovered, suggesting that the Smad-signaling circuitry is intimately linked to HSC regulation. In this review, we bring the Smad-signaling pathway into focus, chronicling key concepts and recent advances with respect to TGF-β-superfamily signaling in normal and leukemic hematopoiesis.

Novelty of Axin 2 and lack of Axin 1 gene mutation in colorectal cancer: a study in Kashmiri population

Colorectal cancer is (CRC) one of the leading causes of mortality and morbidity. Various genetic factors have been reported to be involved in the development of colorectal cancers including Axin gene. Axin, a major scaffold protein, plays an important role in various bio signaling pathways. We aim to study mutational pattern of Axin gene in colorectal cancer patients of Kashmiri population. The paired tumor and adjacent normal tissue specimens of 50 consecutive patients with CRC were used in our study. The DNA preparations were evaluated for the occurrence of Axin 1 and Axin 2 gene mutations by direct DNA sequencing. We analyzed exon 1a, 1b, 1c, 2, 4, 6, and 10 of Axin 1 and exon 7 of Axin 2. In this study, we found a novel mutation of G>T (GCT>TCT) transversion in exon 7 of Axin 2 gene at codon G695T (p.alanine > serine) at a frequency of 6% (3/50). In the same exon of Axin 2 gene a single nucleotide polymorphism (SNP) was detected in codon L688L (CCT>CTT) at a frequency of 36% (18/50). In exon 1c of Axin 1 a SNP was detected at codon D726D (GAT>GAC) at a frequency of 62.5% (31/50). Both the SNPs were synonymous hence do not lead to change of amino acid. Although Axin 1 and Axin 2 gene mutations have been found to be involved in the development of colorectal cancers, it seems to be a relatively rare event in Kashmiri population. However, an interesting finding of this study is the novelty of Axin 2 gene mutations which may be a predisposing factor in ethnic Kashmiri population to CRC.

Jerky/Earthbound facilitates cell-specific Wnt/Wingless signalling by modulating β-catenin-TCF activity

Wnt/Wingless signal transduction directs fundamental developmental processes, and upon hyperactivation triggers colorectal adenoma/carcinoma formation. Responses to Wnt stimulation are cell specific and diverse; yet, how cell context modulates Wnt signalling outcome remains obscure. In a Drosophila genetic screen for components that promote Wingless signalling, we identified Earthbound 1 (Ebd1), a novel member in a protein family containing Centromere Binding Protein B (CENPB)-type DNA binding domains. Ebd1 is expressed in only a subset of Wingless responsive cell types, and is required for only a limited number of Wingless-dependent processes. In addition, Ebd1 shares sequence similarity and can be functionally replaced with the human CENPB domain protein Jerky, previously implicated in juvenile myoclonic epilepsy development. Both Jerky and Ebd1 interact directly with the Wnt/Wingless pathway transcriptional co-activators β-catenin/Armadillo and T-cell factor (TCF). In colon carcinoma cells, Jerky facilitates Wnt signalling by promoting association of β-catenin with TCF and recruitment of β-catenin to chromatin. These findings indicate that tissue-restricted transcriptional co-activators facilitate cell-specific Wnt/Wingless signalling responses by modulating β-catenin-TCF activity.

Siamois and Twin are redundant and essential in formation of the Spemann organizer

The Spemann organizer is an essential signaling center in Xenopus germ layer patterning and axis formation. Organizer formation occurs in dorsal blastomeres receiving both maternal Wnt and zygotic Nodal signals. In response to stabilized βcatenin, dorsal blastomeres express the closely related transcriptional activators, Siamois (Sia) and Twin (Twn), members of the paired homeobox family. Sia and Twn induce organizer formation and expression of organizer-specific genes, including Goosecoid (Gsc). In spite of the similarity of Sia and Twn sequence and expression pattern, it is unclear whether these factors function equivalently in promoter binding and subsequent transcriptional activation, or if Sia and Twn are required for all aspects of organizer function. Here we report that Sia and Twn activate Gsc transcription by directly binding to a conserved P3 site within the Wnt-responsive proximal element of the Gsc promoter. Sia and Twn form homodimers and heterodimers by direct homeodomain interaction and dimer forms are indistinguishable in both DNA-binding and activation functions. Sequential chromatin immunoprecipitation reveals that the endogenous Gsc promoter can be occupied by either Sia or Twn homodimers or Sia-Twn heterodimers. Knockdown of Sia and Twn together, but not individually, results in a failure of organizer gene expression and a disruption of axis formation, consistent with a redundant role for Sia and Twn in organizer formation. Furthermore, simultaneous knockdown of Sia and Twn blocks axis induction in response to ectopic Wnt signaling, demonstrating an essential role for Sia and Twn in mediating the transcriptional response to the maternal Wnt pathway. The results demonstrate the functional redundancy of Sia and Twn and their essential role in direct transcriptional responses necessary for Spemann organizer formation.

Defining the cancer master switch

BACKGROUND

Recent research has focused on signaling cascades and their interactions yielding considerable insight into which genetic pathways are targeted and how they tend to be altered in tumors. Therapeutic interventions now can be designed based on the knowledge of pathways vital to tumor growth and survival. These critical targets for intervention, master switches for cancer, are termed so because the tumor attempts to "flip the switch" in a way that promotes its survival, whereas molecular therapy aims to "switch off" signals important for tumor-related processes.

METHODS

Literature review.

CONCLUSIONS

Defining useful targets for therapy depends on identifying pathways that are crucial for tumor growth, survival, and metastasis. Because not all signaling cascades are created equal, selecting master switches or targets for intervention needs to be done in a systematic fashion. This discussion proposes a set of criteria to define what it means to be a cancer master switch and provides examples to illustrate their application.

Anterior neural development requires Del1, a matrix-associated protein that attenuates canonical Wnt signaling via the Ror2 pathway

During early embryogenesis, the neural plate is specified along the anterior-posterior (AP) axis by the action of graded patterning signals. In particular, the attenuation of canonical Wnt signals plays a central role in the determination of the anterior brain region. Here, we show that the extracellular matrix (ECM) protein Del1, expressed in the anterior neural plate, is essential for forebrain development in the Xenopus embryo. Overexpression of Del1 expands the forebrain domain and promotes the formation of head structures, such as the eye, in a Chordin-induced secondary axis. Conversely, the inhibition of Del1 function by a morpholino oligonucleotide (MO) represses forebrain development. Del1 also augments the expression of forebrain markers in neuralized animal cap cells, whereas Del1-MO suppresses them. We previously reported that Del1 interferes with BMP signaling in the dorsal-ventral patterning of the gastrula marginal zone. By contrast, we demonstrate here that Del1 function in AP neural patterning is mediated mainly by the inhibition of canonical Wnt signaling. Wnt-induced posteriorization of the neural plate is counteracted by Del1, and the Del1-MO phenotype (posteriorization) is reversed by Dkk1. Topflash reporter assays show that Del1 suppresses luciferase activities induced by Wnt1 and beta-catenin. This inhibitory effect of Del1 on canonical Wnt signaling, but not on BMP signaling, requires the Ror2 pathway, which is implicated in non-canonical Wnt signaling. These findings indicate that the ECM protein Del1 promotes forebrain development by creating a local environment that attenuates the cellular response to posteriorizing Wnt signals via a unique pathway.

Requirement of TCF7L2 for TGF-beta-dependent transcriptional activation of the TMEPAI gene

The TGF-β and Wnt pathways are involved in cell fate and tumorigenicity. A recent report indicated that a TGF-β target gene, TMEPAI (transmembrane prostate androgen-induced RNA), is possibly also a downstream target of Wnt signaling. Although TMEPAI was believed to be involved in tumorigenicity because of its blockage of TGF-β signaling, how TGF-β and Wnt signals affect the activation of the TMEPAI gene is not well understood. Herein, we show that the TMEPAI promoter is regulated synergistically by TGF-β/Smad and Wnt/β-catenin/T cell factor (TCF) 7L2. The critical cis-element for dual signals, termed TGF-β-responsive TCF7L2-binding element (TTE), is located in intron 1 of the TMEPAI gene. TCF7L2, but not Smad proteins, bound to TTE, whereas the disruption of TTE by mutagenesis remarkably counteracted both TGF-β and TCF7L2 responses. The introduction of mutations in critical Smad-binding elements blocked the activation of the TMEPAI promoter by TCF7L2. Furthermore, our DNA-protein interaction experiments revealed the indirect binding of TCF7L2 to Smad-binding elements via Smad3 upon TGF-β stimulation as well as its TGF-β-dependent association with TTE. We demonstrate that the Wnt/β-catenin/TCF7L2 pathway is preferentially able to alter the transcriptional regulation of the TGF-β-target gene, TMEPAI.

SERPINB3 induces epithelial-mesenchymal transition

Epithelial-mesenchymal transition is believed to facilitate invasion and metastasis formation of epithelial tumour cells. SERPINB3 is a serine protease inhibitor, physiologically found in normal squamous epithelium but over-expressed in epithelial tumours and known to inhibit apoptosis. We tested the hypothesis that SERPINB3 has a role in invasion by modulating the epithelial-mesenchymal transition programme, using morphological, molecular and cell biology techniques on HepG2 cell clones transfected with the human SERPINB3 gene. The paracrine effect of this serpin was determined by the addition of exogenous recombinant SERPINB3 protein to HepG2 and MDCK cell line. SERPINB3 expression leads to changes in transfected cells morphology, characterized by clusters of loosely connected cells with elongated shape. Ultrastructural analysis confirmed the decrease of desmosomal junctions and widening of intercellular spaces. These alterations were associated with a reduction of E-cadherin and an increase of beta-catenin, with a parallel increase of cell proliferation. SERPINB3 clones, untransfected HepG2 and MDCK cells treated with exogenous SERPINB3 expressed vimentin, undetectable in controls. SERPINB3 induced significant cell scattering, migration and invasiveness in untransfected cells. These effects were not dependent on the anti-protease activity of the protein, as documented by the results obtained with an active loop-deleted recombinant SERPINB3 protein. Scatter activity was inhibited by an anti-SERPINB3 antibody in a dose-dependent manner and SERPINB3-transfected cells formed a significantly higher number of colonies on soft agar than controls. In conclusion, the observed results indicate that SERPINB3 induces deregulation of adhesion processes and increases the invasiveness potential supported by features of epithelial-mesenchymal transition, acting at both the autocrine and the paracrine level.

New insights into the mechanism of fibroblast to myofibroblast transformation and associated pathologies

Myofibroblasts are a differentiated cell type essential for wound healing, participating in tissue remodeling following insult. Myofibroblasts are typically activated fibroblasts, although they can also be derived from other cell types, including epithelial cells, endothelial cells, and mononuclear cells. In most organ systems, cell signals initiated following tissue-specific insult or during the metastatic process lead to differentiation of fibroblasts or other precursor cells to the myofibroblast phenotype. In addition to their beneficial and necessary role in wound healing, myofibroblasts also contribute to a number of pathologies, primarily fibrotic processes and tumor invasiveness. This review explores both traditional and nontraditional concepts of myofibroblast differentiation in the cornea, skin, heart, and other tissues, as well as some of the pathologies associated with myofibroblast activities.

Crosstalk between Wnt and bone morphogenic protein signaling: a turbulent relationship

The Wnt and the bone morphogenic protein (BMP) pathways are evolutionarily conserved and essentially independent signaling mechanisms, which, however, often regulate similar biological processes. Wnt and BMP signaling are functionally integrated in many biological processes, such as embryonic patterning in Drosophila and vertebrates, formation of kidney, limb, teeth and bones, maintenance of stem cells, and cancer progression. Detailed inspection of regulation in these and other tissues reveals that Wnt and BMP signaling are functionally integrated in four fundamentally different ways. The molecular mechanism evolved to mediate this integration can also be summarized in four different ways. However, a fundamental aspect of functional and mechanistic interaction between these pathways relies on tissue-specific mechanisms, which are often not conserved and cannot be extrapolated to other tissues. Integration of the two pathways contributes toward the sophisticated means necessary for creating the complexity of our bodies and the reliable and healthy function of its tissues and organs.

Uncoupled responses of Smad4-deficient cancer cells to TNFalpha result in secretion of monomeric laminin-gamma2

BACKGROUND

Functional loss of the tumor suppressor Smad4 is involved in pancreatic and colorectal carcinogenesis and has been associated with the acquisition of invasiveness. We have previously demonstrated that the heterotrimeric basement membrane protein laminin-332 is a Smad4 target. Namely, Smad4 functions as a positive transcriptional regulator of all three genes encoding laminin-332; its loss is thus implicated in the reduced or discontinuous deposition of the heterotrimeric basement membrane molecule as evident in carcinomas. Uncoupled expression of laminin genes, on the other hand, namely overexpression of the laminin-gamma2 chain is an impressive marker at invasive edges of carcinomas where tumor cells are maximally exposed to signals from stromal cell types like macrophages. As Smad4 is characterized as an integrator of multiple extracellular stimuli in a strongly contextual manner, we asked if loss of Smad4 may also be involved in uncoupled expression of laminin genes in response to altered environmental stimuli. Here, we address Smad4 dependent effects of the prominent inflammatory cytokine TNFalpha on tumor cells.

RESULTS

Smad4-reconstituted colon carcinoma cells like adenoma cells respond to TNFalpha with an increased expression of all three chains encoding laminin-332; coincubation with TGFbeta and TNFalpha leads to synergistic induction and to the secretion of large amounts of the heterotrimer. In contrast, in Smad4-deficient cells TNFalpha can induce expression of the gamma2 and beta3 but not the alpha3 chain. Surprisingly, this uncoupled induction of laminin-332 chains in Smad4-negative cells rather than causing intracellular accumulation is followed by the release of gamma2 into the medium, either in a monomeric form or in complexes with as yet unknown proteins. Soluble gamma2 is associated with increased cell migration.

CONCLUSIONS

Loss of Smad4 may lead to uncoupled induction of laminin-gamma2 in response to TNFalpha and may therefore represent one of the mechanisms which underlie accumulation of laminin-gamma2 at the invasive margin of a tumor. The finding, that gamma2 is secreted from tumor cells in significant amounts and is associated with increased cell migration may pave the way for further investigation to better understand its functional relevance for tumor progression.

Smad signaling in skeletal development and regeneration

Smad proteins are intracellular molecules that mediate the canonical signaling cascade of TGFbeta superfamily growth factors. The TGFbeta superfamily comprises two groups of growth factors, BMPs and TGFbetas. Both groups can be further divided into several sub-groups based on sequence homologies and functional similarities. Ligands of the TGFbeta superfamily bind to cell surface receptors to activate Smad proteins in the cytoplasm; then the activated Smad proteins translocate into the nucleus to activate or repress specific target gene transcription. Both groups of growth factors play important roles in skeletal development and regeneration. However, whether these effects reflect signaling through canonical Smad pathways, or other non-canonical signaling pathways in vivo remains a mystery. Moreover, the mechanisms utilized by Smad proteins to initiate nuclear events and their interactions with cytoplasmic proteins are still under intensive investigation. This review will discuss the most recent progress understanding Smad signaling in the context of skeletal development and regeneration.

Antagonistic effect of the matricellular signaling protein CCN3 on TGF-beta- and Wnt-mediated fibrillinogenesis in systemic sclerosis and Marfan syndrome

Abnormal fibrillinogenesis is associated with connective tissue disorders (CTDs), including Marfan syndrome (MFS), systemic sclerosis (SSc) and Tight-skin (Tsk) mice. We have previously shown that TGF-beta and Wnt stimulate fibrillin-1 assembly and that fibrillin-1 and the developmental regulator CCN3 are both highly increased in Tsk skin. We investigated the role of CCN3 in abnormal fibrillinogenesis in Tsk mice, MFS, and SSc. Smad3 deletion in Tsk mice decreased CCN3 overexpression, suggesting that TGF-beta mediates at least part of the effect of Tsk fibrillin on CCN3 which is consistent with a synergistic effect of TGF-beta and Wnt in vitro on CCN3 expression. Disruption of fibrillin-1 assembly by MFS fibrillin decreased CCN3 expression and skin from patients with early diffuse SSc showed a strong correlation between increased CCN3 and fibrillin-1 expression, suggesting that CCN3 regulation by fibrillin-1 extends to these CTDs. Diffuse SSc skin and sera also showed evidence of increased Wnt activity, implicating a Wnt stimulus behind this correlation. CCN3 overexpression markedly repressed fibrillin-1 assembly and also blocked other TGFbeta- and Wnt-regulated profibrotic gene expression. Together, these data indicate that CCN3 counter-regulates positive signals from TGF-beta and Wnt for fibrillin fibrillogenesis and profibrotic gene expression.

Inactivation of LEF1 in T-cell acute lymphoblastic leukemia

To further unravel the molecular pathogenesis of T-cell acute lymphoblastic leukemia (T-ALL), we performed high-resolution array comparative genomic hybridization on diagnostic specimens from 47 children with T-ALL and identified monoallelic or biallelic LEF1 microdeletions in 11% (5 of 47) of these primary samples. An additional 7% (3 of 44) of the cases harbored nonsynonymous sequence alterations of LEF1, 2 of which produced premature stop codons. Gene expression microarrays showed increased expression of MYC and MYC targets in cases with LEF1 inactivation, as well as differentiation arrest at an early cortical stage of thymocyte development characterized by expression of CD1B, CD1E, and CD8, with absent CD34 expression. LEF1 inactivation was associated with a younger age at the time of T-ALL diagnosis, as well as activating NOTCH1 mutations, biallelic INK4a/ARF deletions, and PTEN loss-of-function mutations or activating mutations of PI3K or AKT genes. These cases generally lacked overexpression of the TAL1, HOX11, HOX11L2, or the HOXA cluster genes, which have been used to define separate molecular pathways leading to T-ALL. Our findings suggest that LEF1 inactivation is an important step in the molecular pathogenesis of T-ALL in a subset of young children.

Lef-1 isoforms regulate different target genes and reduce cellular adhesion

The lymphoid enhancer factor 1 (Lef-1) belongs to the nuclear transducers of canonical Wnt-signalling in embryogenesis and cancer. Lef-1 acts, in cooperation with beta-catenin, as a context-dependent transcriptional activator or repressor, thereby influencing multiple cellular functions such as proliferation, differentiation and migration. Here we report that an increased Lef-1 expression in human pancreatic cancer correlates with advanced tumour stages. In pancreatic tumours, two different transcripts of Lef-1 have been detected in various stages, as demonstrated by RT-PCR analysis. One transcript was identified as the full length Lef-1 (Lef-1 FL), whereas the second, shorter transcript lacked exon VI (Lef-1 Deltaexon VI) compared to the published sequence. Comparative analysis of these two Lef-1 variants revealed that they exhibit different cellular effects after transient expression in pancreatic carcinoma cells. Forced expression of Lef-1 Deltaexon VI inhibited E-cadherin expression in a beta-catenin-independent way. Increased amounts of Lef-1 Deltaexon VI resulted in reduced cellular aggregation and increased cell migration. Expression of Lef-1 FL, but not the newly identified Lef-1 Deltaexon VI, induced the expression of the cell cycle regulating proteins c-myc and cyclin D1 in cooperation with beta-catenin and it enhanced cell proliferation. Our findings indicate that expression of alternatively spliced Lef-1 isoforms is involved in the determination of proliferative or migratory characteristics of pancreatic carcinoma cells.

Smad3 prevents beta-catenin degradation and facilitates beta-catenin nuclear translocation in chondrocytes

Our previous study demonstrated that transforming growth factor (TGF)-beta activates beta-catenin signaling through Smad3 interaction with beta-catenin in chondrocytes. In the present studies, we further investigated the detailed molecular mechanism of the cross-talk between TGF-beta/Smad3 and Wnt/beta-catenin signaling pathways. We found that C-terminal Smad3 interacted with both the N-terminal region and the middle region of beta-catenin protein in a TGF-beta-dependent manner. Both Smad3 and Smad4 were required for the interaction with beta-catenin and protected beta-catenin from an ubiquitin-proteasome-dependent degradation. In addition, the formation of the Smad3-Smad4-beta-catenin protein complex also mediated beta-catenin nuclear translocation. This Smad3-mediated regulatory mechanism of beta-catenin protein stability enhanced the activity of beta-catenin to activate downstream target genes during chondrogenesis. Our findings demonstrate a novel mechanism between TGF-beta and Wnt/beta-catenin signaling pathways during chondrocyte development.

The regulation of TGFbeta signal transduction

Transforming growth factor beta (TGFbeta) pathways are implicated in metazoan development, adult homeostasis and disease. TGFbeta ligands signal via receptor serine/threonine kinases that phosphorylate, and activate, intracellular Smad effectors as well as other signaling proteins. Oligomeric Smad complexes associate with chromatin and regulate transcription, defining the biological response of a cell to TGFbeta family members. Signaling is modulated by negative-feedback regulation via inhibitory Smads. We review here the mechanisms of TGFbeta signal transduction in metazoans and emphasize events crucial for embryonic development.

Gene expression profiling in mouse liver infected with Clonorchis sinensis metacercariae

Clonorchis sinensis, the parasite that causes clonorchiasis, is endemic in many Asian countries, and infection with the organism drives changes in the liver tissues of the host. However, information regarding the molecular events in clonorchiasis remains limited, and little is currently known about host-pathogen interactions in clonorchiasis. In this study, we assessed the gene expression profiles in mice livers via DNA microarray analysis 1, 2, 4, and 6 weeks after induced metacercariae infection. Functional clustering of the gene expression profile showed that the immunity-involved genes were induced in the livers of the mice at the early stage of metacercariae infection, whereas immune responses were reduced in the 6-week liver tissues after infection in which the metacercariae became adult flukes. Many genes involved in fatty acid metabolism, including Peci, Cyp4a10, Acat1, Ehhadh, Gcdh, and Cyp2 family were downregulated in the infected livers. On the other hand, the liver tissues infected with the parasite expressed Wnt signaling molecules such as Wnt7b, Fzd6, and Pdgfrb and cell cycle-regulating genes including cyclin-D1, Cdca3, and Bcl3. These investigations constitute an excellent starting point for increased understanding of the molecular mechanisms underlying host-pathogen interaction during the development of C. sinensis in the host liver.

Complex and context dependent regulation of hematopoiesis by TGF-beta superfamily signaling

The transforming growth factor (TGF)-beta superfamily of growth factors, including the TGF-betas, activins, and bone morphogenetic proteins (BMPs), provide cells with a broad spectrum of regulatory signals through the intracellular Smad pathway. Since loss-of-function studies of a majority of the TGF-beta superfamily members result in embryonic lethality, much of our current knowledge of the TGF-beta superfamily's role in hematopoiesis is generated from studies performed in vitro, or in very early stages of embryonic development. TGF-beta is well documented as a potent inhibitor of hematopoietic stem cell (HSC) proliferation in vitro, while its role in vivo is largely unknown. BMP signaling is crucial for the initiation of hematopoiesis in the developing embryo, although its role in adult hematopoiesis remains elusive. More recently we and others have used conditional knockout models to unravel the role of several components of TGF-beta family signaling in adult hematopoiesis. Here we review the currently known functions for the major factors of this signaling family in embryonic and adult hematopoietic regulation and discuss the context dependency and complexity that permeate this regulation.

Transforming growth factor β inducible apoptotic cascade in epithelial cells during rat molar tooth eruptions

In tooth eruptions, the presence of apoptotic epithelial cells at the eruption site has been reported, but the factors that induce apoptosis in these cells remain to be elucidated, as do the induction pathways. In this study, we focused our attention on transforming growth factor beta (TGF-beta), which is known to induce apoptosis during embryonic development. Oral epithelium and dental lamina of maxillary first molars in 8- and 15-day-old rats were used to investigate the induction pathway of apoptosis by performing the immunohistochemical tests outlined below and assessing the characteristics of cells that undergo apoptosis by transmission electron microscopy in rats 8 and 15 days after birth. We examined TGF-beta-receptor 1, TGF-beta inducible transcription factor 1 (TIEG1), NADPHoxidase 4 (Nox4), cytochrome c, caspase-3 (active form and pro-enzyme), apoptosis-inducing protein Daxx, apoptosis signal-regulating kinase 1 (ASK1), glycogen synthase kinase-3 beta phosphorylated on serine 9 (p-GSK-3beta), and beta-catenin. We also performed periodic acid Schiff (PAS) reaction and terminal deoxynucleotidyl transferase-mediated dUTD nick end labeling (TUNEL) staining. At eruption sites 8 days after birth, reactions to TGF-beta-receptor 1, TIEG1, Nox4, cytochrome c, caspase-3, p-GSK-3beta, and beta-catenin, and PAS-positive cells were observed in areas close to the basal layer of oral epithelium through to the center of the dental lamina, but no reaction to Daxx or ASK1 was noted at these sites. Electron microscopy revealed the accumulation of glycogen granules in the cells that showed reactions to the above-mentioned markers as well as in the spaces among them. In the rats 15 days after birth (immediately before tooth eruption), the PAS-positive cells that showed reactions to the above antibodies remained on the buccal side of the epithelium, and high-electron-density apoptotic bodies and TUNEL-positive bodies were noted. Therefore, during tooth eruption, TGF-beta may induce apoptosis of cells rich in glycogen granules, and cytochrome c and caspase-3 may function to induce apoptosis. In addition, reactive oxygen species may be involved in this induction pathway via TIEG1 and Nox4 without involvement of Daxx and ASK1. Moreover, overexpression of p-GSK-3beta and beta-catenin may also contribute to apoptosis of oral epithelium at the eruption site and dental lamina cells. Glycogen storage mediated by p-GSK-3beta and crosstalk between the TGF-beta and Wnt signaling pathways may participate in the formation of tooth eruption passage.

Cloning of the human GLI2 Promoter: transcriptional activation by transforming growth factor-beta via SMAD3/beta-catenin cooperation

GLI2 (GLI-Kruppel family member 2), a zinc finger transcription factor that mediates Hedgehog signaling, is implicated in the progression of an ever-growing number of human malignancies, including prostate and pancreatic cancer, as well as basal cell carcinoma of the skin. Its expression is up-regulated by transforming growth factor-beta (TGF-beta) in a variety of cell types, both normal and transformed. We report herein that TGF-beta-driven GLI2 expression is transcriptional and does not result from stabilization of GLI2 transcripts. We describe the characterization of the 5'-flanking sequence of human GLI2 mRNA, the identification of a transcription start site, the cloning of approximately 1,600 bp of the regulatory promoter region and the identification and functional analysis of a TGF-beta-responsive region mapped to a 91-bp sequence between nucleotides -119 and -29 of the promoter. This region harbors SMAD and lymphoid enhancer factor/T cell factor binding sites that allow functional cooperation between SMAD3 and beta-catenin, recruited to the promoter in response to TGF-beta to drive GLI2 gene transcription.

Mad is required for wingless signaling in wing development and segment patterning in Drosophila

A key question in developmental biology is how growth factor signals are integrated to generate pattern. In this study we investigated the integration of the Drosophila BMP and Wingless/GSK3 signaling pathways via phosphorylations of the transcription factor Mad. Wingless was found to regulate the phosphorylation of Mad by GSK3 in vivo. In epistatic experiments, the effects of Wingless on wing disc molecular markers (senseless, distalless and vestigial) were suppressed by depletion of Mad with RNAi. Wingless overexpression phenotypes, such as formation of ectopic wing margins, were induced by Mad GSK3 phosphorylation-resistant mutant protein. Unexpectedly, we found that Mad phosphorylation by GSK3 and MAPK occurred in segmental patterns. Mad depletion or overexpression produced Wingless-like embryonic segmentation phenotypes. In Xenopus embryos, segmental border formation was disrupted by Smad8 depletion. The results show that Mad is required for Wingless signaling and for the integration of gradients of positional information.

BMP-9-induced osteogenic differentiation of mesenchymal progenitors requires functional canonical Wnt/beta-catenin signalling

Bone morphogenetic protein 9 (BMP-9) is a member of the transforming growth factor (TGF)-beta/BMP superfamily, and we have demonstrated that it is one of the most potent BMPs to induce osteoblast differentiation of mesenchymal stem cells (MSCs). Here, we sought to investigate if canonical Wnt/beta-catenin signalling plays an important role in BMP-9-induced osteogenic differentiation of MSCs. Wnt3A and BMP-9 enhanced each other's ability to induce alkaline phosphatase (ALP) in MSCs and mouse embryonic fibroblasts (MEFs). Wnt antagonist FrzB was shown to inhibit BMP-9-induced ALP activity more effectively than Dkk1, whereas a secreted form of LPR-5 or low-density lipoprotein receptor-related protein (LRP)-6 exerted no inhibitory effect on BMP-9-induced ALP activity. beta-Catenin knockdown in MSCs and MEFs diminished BMP-9-induced ALP activity, and led to a decrease in BMP-9-induced osteocalcin reporter activity and BMP-9-induced expression of late osteogenic markers. Furthermore, beta-catenin knockdown or FrzB overexpression inhibited BMP-9-induced mineralization in vitro and ectopic bone formation in vivo, resulting in immature osteogenesis and the formation of chondrogenic matrix. Chromatin immunoprecipitation (ChIP) analysis indicated that BMP-9 induced recruitment of both Runx2 and beta-catenin to the osteocalcin promoter. Thus, we have demonstrated that canonical Wnt signalling, possibly through interactions between beta-catenin and Runx2, plays an important role in BMP-9-induced osteogenic differentiation of MSCs.

Beta-catenin hits chromatin: regulation of Wnt target gene activation

The canonical Wnt pathway has gathered much attention in recent years owing to its fundamental contribution to metazoan development, tissue homeostasis and human malignancies. Wnt target gene transcription is regulated by nuclear beta-catenin, and genetic assays have revealed various collaborating protein cofactors. Their daunting number and diverse nature, however, make it difficult to arrange an orderly picture of the nuclear Wnt transduction events. Yet, these findings emphasize that beta-catenin-mediated transcription affects chromatin. How does beta-catenin cope with chromatin regulation to turn on Wnt target genes?

Wnt pathway, an essential role in bone regeneration

Fracture repair is a complex regenerative process initiated in response to injury, resulting in optimal restoration of skeletal function. Although histology characteristics at various phases of fracture repair are clear and well established, much remains to be understood about the process of bone healing, particularly at the molecular signaling level. During the past decade, secreted signaling molecules of the Wnt family have been widely investigated and found to play a central role in controlling embryonic development processes. Wnt signaling pathway also plays a pivotal role in the regulation of bone mass. Recent published data reveal that Wnt signaling pathway is activated during postnatal bone regenerative events, such as ectopic endochondral bone formation and fracture repair. Dysregulation of this pathway greatly inhibits bone formation and healing process. Interestingly, activation of Wnt pathway has potential to improve bone healing, but only utilized after mesenchymal cells have become committed to the osteoblast lineage. These advances suggest an essential role of Wnt pathway in bone regeneration.