TGF-beta signaling by Smad proteins

Mammalian transforming growth factor-betas: Smad signaling and physio-pathological roles

Since its discovery in the early 1980s, transforming growth factor-beta (TGF-beta) has emerged as a family of growth factors involved in essential physiological processes, including embryonic development, differentiation, tissue repair and cell growth control. Knockout experiments for the three mammalian isoforms of TGF-betas in mice have demonstrated their importance in regulating inflammation and tissue repair. Also, TGF-beta has been implicated in the pathogenesis of human diseases, including tissue fibrosis and carcinogenesis where, in the latter case, it may exert both tumor suppressor and pro-oncogenic activities depending on the stage of the tumor. Cellular signaling by TGF-beta family members is initiated by the assembly of specific cell surface serine/threonine kinase type receptors that activate transcription factors of the Smad family.

The oocyte and its role in regulating ovulation rate: a new paradigm in reproductive biology

Ovulation rate in mammals is determined by a complex exchange of hormonal signals between the pituitary gland and the ovary and by a localised exchange of hormones within ovarian follicles between the oocyte and its adjacent somatic cells. From examination of inherited patterns of ovulation rate in sheep, point mutations have been identified in two oocyte-expressed genes, BMP15 (GDF9B) and GDF9. Animals heterozygous for any of these mutations have higher ovulation rates (that is, + 0.8–3) than wild-type contemporaries, whereas those homozygous for each of these mutations are sterile with ovarian follicular development disrupted during the preantral growth stages. Both GDF9 and BMP15 proteins are present in follicular fluid, indicating that they are secreted products.In vitrostudies show that granulosa and/or cumulus cells are an important target for both growth factors. Multiple immunisations of sheep with BMP15 or GDF9 peptide protein conjugates show that both growth factors are essential for normal follicular growth and the maturation of preovulatory follicles. Short-term (that is, primary and booster) immunisation with a GDF9 or BMP15 peptide-protein conjugate has been shown to enhance ovulation rate and lamb production. In summary, recent studies of genetic mutations in sheep highlight the importance of oocyte-secreted factors in regulating ovulation rate, and these discoveries may help to explain why some mammals have a predisposition to produce two or more offspring rather than one.

Characterization of the bone morphogenetic protein (BMP) system in human pulmonary arterial smooth muscle cells isolated from a sporadic case of primary pulmonary hypertension: roles of BMP type IB receptor (activin receptor-like kinase-6) in the mitotic action

The functional involvement of bone morphogenetic protein (BMP) system in primary pulmonary hypertension (PPH) remains unclear. Here we demonstrate a crucial role of the BMP type IB receptor, activin receptor-like kinase (ALK)-6 for pulmonary arterial smooth muscle cell (pphPASMC) mitosis isolated from a sporadic PPH patient bearing no mutations in BMPR2 gene. A striking increase in the levels of ALK-6 mRNA was revealed in pphPASMC compared with control PASMCs, in which ALK-6 transcripts were hardly detectable. BMP-2 and -7 stimulated the mitosis of pphPASMCs, which was opposite to their suppressive effects on the mitosis of the control PASMCs. BMP-4 and -6 and activin inhibited pphPASMC mitosis, whereas these did not affect control PASMCs. The presence of BMP signaling machinery in pphPASMCs was elucidated based on the analysis on Id-1 transcription and Smad-reporter genes. Overexpression of a dominant-negative ALK-6 construct revealed that ALK-6 plays a key role in the mitosis as well as intracellular BMP signaling of pphPASMCs. Gene silencing of ALK-6 using small interfering RNA also reduced DNA synthesis as well as Id-1 transcription in pphPASMCs regardless of BMP-2 stimulation. Although Id-1 response was not stimulated by BMP-2 in control PASMCs, the gene delivery of wild-type ALK-6 caused significant increase in the Id-1 transcripts in response to BMP-2. Additionally, inhibitors of ERK and p38 MAPK pathways suppressed pphPASMC mitosis induced by BMP-2, implying that the mitotic action is in part MAPK dependent. Thus, the BMP system is strongly involved in pphPASMC mitosis through ALK-6, which possibly leads to activation of Smad and MAPK, resulting in the progression of vascular remodeling of pulmonary arteries in PPH.

Immunohistochemical localization of the bone morphogenetic protein receptors in the porcine ovary

The bone morphogenetic protein (BMP) family is emerging as playing a crucial role in regulating normal follicle growth and determining ovulation rate. BMPs exert their effects via BMP receptors (BMPR-IA, -IB and -II). However, there is a paucity of information relating to the expression of the BMPRs within the ovary of large polyovular species such as the pig. Furthermore, there is a lack of information on the expression of BMPRs by fetal ovaries of any species. The purpose of this study was to investigate temporal and spatial expression of the BMPRs in the porcine ovary, at different developmental stages. Immunohistochemistry for BMPR-IA, BMPR-IB and BMPR-II was performed using sections from paraffin wax-embedded ovaries, obtained from fetal (n = 15), prepubertal (n = 3) and cycling postpubertal (n = 4) pigs. Results confirmed the presence of all three receptors in the fetal egg nests and in the granulosa cell layer of follicles ranging from primordial to late antral stages. Immunostaining was also observed in oocytes, theca layer, corpus luteum and ovarian surface epithelium. The expression of BMPRs by fetal ovaries may be related to follicle formation, whereas expression in pre- and post-pubertal animals indicates BMPs are involved in regulating porcine ovarian follicle growth.

BMP signaling initiates a neural crest differentiation program in embryonic rat CNS stem cells

Bone morphogenetic proteins (BMPs) have an important role in neuronal and astrocytic differentiation of embryonic and adult neural stem cells (NSCs). Here, we show that BMP6, BMP7, GDF5, and GDF6 instructively differentiate E12, E14, and E17 rat cortical NSCs into a variety of neural crest lineages. Clonal analysis shows that BMP7-treated NSCs develop mostly into smooth muscle and peripheral glia. We observed a rapid induction of premigratory neural crest markers like p75NTR, and AP-2 alpha followed by Msx1, Msx2, and Slug, transcription factors that participate in neural crest development. These results suggest that NSCs cultured in vitro in the presence of FGF2 display expanded developmental potential.

Kidney-bone, bone-kidney, and cell-cell communications in renal osteodystrophy

The relationship between bone and the kidney in renal osteodystrophy is a complex interplay of kidney to bone connections, bone to kidney connections, and cell to cell connections. In addition, such interactions have a profound effect on the vasculature. In this review, we discuss the role of the bone morphogenetic proteins (BMPs) in the skeleton, kidney, and vasculature. In addition, we propose that deficiencies of these BMPs seen in chronic kidney disease (CKD) result in decreased bone remodeling and a compensatory secondary hyperparathyroidism (high turnover state). Treatment of the hyperparathyroidism blocks this compensatory arm and thus decreased bone remodeling occurs (low turnover). We review animal models of CKD in which treatment with BMP-7 resulted in normalization of both high and low turnover states. Finally, we discuss vascular calcification as it relates to bone metabolism. We discuss the roles of BMP-7 and 2 other bone regulatory proteins, osteoprotegerin (OPG) and alpha2-HS glycoprotein (AHSG, human fetuin), in the human vasculature and their implications for vascular calcification.

Differential effects of the Ski-interacting protein (SKIP) on differentiation induced by transforming growth factor-β1 and bone morphogenetic protein-2 in C2C12 cells

The transforming growth factor-beta (TGF-beta) and bone morphogenetic proteins (BMP) are key regulatory factors that affect many critical cellular events in growth and development. Recently, we have shown that the Ski-interacting protein (SKIP) can augment TGF-beta signals. Here, we extended these studies by examining the biologic consequences of SKIP overexpression on TGF-beta1 and BMP-2 signals in C2C12 cells. C2C12 myoblasts differentiate into myotubes when the media is depleted of mitogenic factors, and TGF-beta1 inhibits this myotube formation. BMP-2 not only inhibits the myotube formation, but also induces C2C12 cells to differentiate into osteoblasts. Here, we show that SKIP-overexpressing C2C12 cells treated with TGF-beta1 or BMP-2 displayed no differences in comparison to vector control cells in their ability to form myotubes or in the expression of the myogenic markers myosin heavy chain-1 and myogenin. Unexpectedly, SKIP-overexpressing C2C12 cells treated with BMP-2 displayed suppressed expression of the induced osteoblast markers alkaline phosphatase, osteocalcin, and the transcription factor Runx2. Lastly, SKIP could repress transcription induced by BMP-2 in luciferase reporter assays done in C2C12 cells. These data show that SKIP has specific inhibitory effects on BMP-2-induced differentiation and implicate SKIP to be a novel regulator of the differentiation programming induced by TGF-beta signals.

[Transforming growth factor-betas: smad signaling and roles in physiopathology]

Transforming growth factor-beta (TGF-beta) family members are multifunctional peptide growth factors that regulate cell growth, differentiation, extracellular matrix production and cell migration and embryonic development. Knock-out experiments for the three mammalian isoforms of TGF-beta in mice have demonstrated their importance in regulating inflammation and tissue repair. Also, TGF-beta has been implicated in the pathogenesis of human diseases, including tissue fibrosis and carcinogenesis. In the latter case, it may exert both tumor suppressor and pro-oncogenic activities depending on the stage of the tumor. Smads proteins constitute the core components of the intracellular signaling cascade initiated by TGF-beta receptors, as they carry signals from the cell surface directly to the nucleus; where they act as transcription factors.

The Nedd4 family of E3 ubiquitin ligases: functional diversity within a common modular architecture

Neuronal precursor cell-expressed developmentally downregulated 4 (Nedd4) is the prototypical protein in a family of E3 ubiquitin ligases that have a common domain architecture. They are comprised of a catalytic C-terminal HECT domain and N-terminal C2 domain and WW domains responsible for cellular localization and substrate recognition. These proteins are found throughout eukaryotes and regulate diverse biological processes through the targeted degradation of proteins that generally have a PPxY motif for WW domain recognition, and are found in the nucleus and at the plasma membrane. Whereas the yeast Saccharomyces cerevisiae uses a single protein, Rsp5p, to carry out these functions, evolution has provided higher eukaryotes with several related Nedd4 proteins that appear to have specialized roles. In this review we discuss how knowledge of individual domain function has provided insight into the physiological roles of the Nedd4 proteins and describe recent results that suggest discrete functions for individual family members.

Transforming growth factor beta-mediated transcriptional repression of c-myc is dependent on direct binding of Smad3 to a novel repressive Smad binding element

Smad proteins are the most well-characterized intracellular effectors of the transforming growth factor beta (TGF-beta) signal. The ability of the Smads to act as transcriptional activators via TGF-beta-induced recruitment to Smad binding elements (SBE) within the promoters of TGF-beta target genes has been firmly established. However, the elucidation of the molecular mechanisms involved in TGF-beta-mediated transcriptional repression are only recently being uncovered. The proto-oncogene c-myc is repressed by TGF-beta, and this repression is required for the manifestation of the TGF-beta cytostatic program in specific cell types. We have shown that Smad3 is required for both TGF-beta-induced repression of c-myc and subsequent growth arrest in keratinocytes. The transcriptional repression of c-myc is dependent on direct Smad3 binding to a novel Smad binding site, termed a repressive Smad binding element (RSBE), within the TGF-beta inhibitory element (TIE) of the c-myc promoter. The c-myc TIE is a composite element, comprised of an overlapping RSBE and a consensus E2F site, that is capable of binding at least Smad3, Smad4, E2F-4, and p107. The RSBE is distinct from the previously defined SBE and may partially dictate, in conjunction with the promoter context of the overlapping E2F site, whether the Smad3-containing complex actively represses, as opposed to transactivates, the c-myc promoter.

Novel action of activin and bone morphogenetic protein in regulating aldosterone production by human adrenocortical cells

We have uncovered a functional bone morphogenetic protein (BMP) and activin system complete with ligands (BMP-6 and activin betaA/betaB), receptors (activin receptor-like kinase receptors 2, 3, and 4; activin type-II receptor; and BMP type-II receptor), and the binding protein follistatin in the human adrenocortical cell line H295R. Administration of activin and BMP-6 to cultures of H295R cells caused concentration-responsive increases in aldosterone production. The mRNA levels of steroidogenic acute regulatory protein or P450 steroid side-chain cleavage enzyme, the rate-limiting steps of adrenocortical steroidogenesis, were enhanced by activin and BMP-6. Activin and BMP-6 also activated the transcription of steroidogenic acute regulatory protein as well as the late-step steriodogenic enzyme CYP11B2. Activin enhanced ACTH-, forskolin-, or dibutyryl-cAMP- but not angiotensin II (Ang II)-induced aldosterone production, whereas BMP-6 specifically augmented Ang II-induced aldosterone production. Activin and ACTH but not BMP-6 increased cAMP production. Follistatin, which inhibits activin actions by binding, suppressed basal and ACTH-induced aldosterone secretion but failed to affect the Ang II-induced aldosterone level. Furthermore, MAPK signaling appeared to be involved in aldosterone production induced by Ang II and BMP-6 because an inhibitor of MAPK activation, U0126, reduced the level of aldosterone synthesis stimulated by Ang II and BMP-6 but not activin. In addition, Ang II reduced the expression levels of BMP-6 but increased that of activin betaB, whereas ACTH had no effect on these levels. Collectively, the present data suggest that activin acts to regulate adrenal aldosterone synthesis predominantly by modulating the ACTH-cAMP-protein kinase A signaling cascade, whereas BMP-6 works primarily by modulating the Ang II-MAPK cascade in human adrenal cortex in an autocrine/paracrine fashion.

The bone morphogenetic protein system in mammalian reproduction

Using molecular, cellular, and genetic approaches, recent studies examining the role of the bone morphogenetic protein (BMP) family of growth factors in the reproductive system have led to significant breakthroughs in our understanding of mammalian reproduction and fertility. Gene expression studies have revealed that key components of the BMP system (ligands, receptors, signaling molecules, and binding proteins) exhibit coordinated spatial and temporal expression patterns in fundamental cell types throughout the reproductive system. Availability of recombinant BMPs has enabled functional studies that have demonstrated important biological activities of BMPs in controlling cellular proliferation, differentiation, and apoptosis in reproductive tissues. The physiological importance of the BMP system for mammalian reproduction has been further highlighted by the elucidation of the aberrant reproductive phenotypes of animals with naturally occurring mutations or targeted deletions of certain BMP family genes. Collectively, these studies have established the concept that the BMP system plays a crucial role in fertility in female and male mammals. The purpose of this article is to review the evidence underpinning the importance of the BMP system in mammalian reproduction.

GADD34-PP1c recruited by Smad7 dephosphorylates TGFbeta type I receptor

The cascade of phosphorylation is a pivotal event in transforming growth factor β (TGFβ) signaling. Reversible phosphorylation regulates fundamental aspects of cell activity. TGFβ-induced Smad7 binds to type I receptor (TGFβ type I receptor; TβRI) functioning as a receptor kinase antagonist. We found Smad7 interacts with growth arrest and DNA damage protein, GADD34, a regulatory subunit of the protein phosphatase 1 (PP1) holoenzyme, which subsequently recruits catalytic subunit of PP1 (PP1c) to dephosphorylate TβRI. Blocking Smad7 expression by RNA interference inhibits association of GADD34–PP1c complex with TβRI, indicating Smad7 acts as an adaptor protein in the formation of the PP1 holoenzyme that targets TβRI for dephosphorylation. SARA (Smad anchor for receptor activation) enhances the recruitment PP1c to the Smad7–GADD34 complex by controlling the specific subcellular localization of PP1c. Importantly, GADD34–PP1c recruited by Smad7 inhibits TGFβ-induced cell cycle arrest and mediates TGFβ resistance in responding to UV light irradiation. The dephosphorylation of TβRI mediated by Smad7 is an effective mechanism for governing negative feedback in TGFβ signaling.

Glycosylphosphatidylinositol-anchored Proteins Regulate Transforming Growth Factor-β Signaling in Human Keratinocytes

Glycosylphosphatidylinositol (GPI)-anchored proteins have been demonstrated to bind transforming growth factor-beta (TGF-beta) in certain cell lines. However, the identity of these GPI-anchored proteins and the role they may play in TGF-beta signaling remain unknown. We have previously reported the presence of GPI-anchored TGF-beta-binding proteins on human skin fibroblasts and keratinocytes (Tam, B. Y. Y., and Philip, A. (1998) J. Cell. Physiol. 176, 553-564; Tam, B. Y. Y., Germain, L., and Philip, A. (1998) J. Cell. Biochem. 70, 573-586). On human keratinocytes, we identified a 150-kDa GPI-anchored TGF-beta1-binding protein (r150) and demonstrated that it can form a heteromeric complex with the type I and II TGF-beta signaling receptors. To explore whether GPI-anchored proteins modulate TGF-beta signaling in keratinocytes, we created keratinocytes defective in GPI anchor biosynthesis (GPI mutant cells) by chemical mutagenesis of HaCaT cells. Mutant clones were selected by fluorescence-activated cell sorting analysis based on the loss of a CD59 marker. In comparison with parental HaCaT cells, GPI mutant cells demonstrated a significant loss of r150 expression. In contrast, the levels of the type I and II TGF-beta receptors and their ligand affinities, cell morphology, and doubling time remained unchanged. Importantly, GPI mutant cells displayed enhanced gene transcriptional activity and Smad2 and Smad3 activation in response to TGF-beta1 treatment in a dose-dependent manner. Taken together, our results indicate that GPI-anchored protein(s) inhibit TGF-beta signaling and implicate r150 as the GPI-anchored protein responsible for this inhibition in human keratinocytes.

Endocytic trafficking during Drosophila development

During the last decade, many of the factors and mechanisms controlling membrane and protein trafficking in general and endocytic trafficking in particular have been uncovered. We have a detailed understanding of the different endocytic trafficking steps: plasma membrane budding, endocytic vesicle motility and fusion with the endosome, recycling, transcytosis and lysosomal degradation. The kinetics and trafficking pathway of many signaling receptors and the relevance of endocytic trafficking during signaling in many mammalian cultured cells are also well understood. However, only in recent years has the role of endocytic trafficking during cell-to-cell communication during development, i.e. during patterning, induction and lateral inhibition, begun to be explored. The contribution of Drosophila developmental genetics and cell biology has been fundamental in elucidating the essential role of endocytosis during these processes. Reviewed here are some of the recent developments on the role of endocytic trafficking during long- and short-range signaling and during lateral inhibition.

From developmental disorder to heritable cancer: it's all in the BMP/TGF-beta family

Transforming growth factor-beta (TGF-beta) regulates many cellular processes through complex signal-transduction pathways that have crucial roles in normal development. Disruption of these pathways can lead to a range of diseases, including cancer. Mutations in the genes that encode members of the TGF-beta pathway are involved in vascular diseases as well as gastrointestinal neoplasia. More recently, they have been implicated in Cowden syndrome, which is normally associated with mutations in the phosphatase and tensin homologue gene PTEN. Molecular studies of TGF-beta signalling are now showing why mutations in genes that encode components of this pathway result in inherited cancer and developmental diseases.

Initiation of Smad-dependent and Smad-independent signaling via distinct BMP-receptor complexes

BACKGROUND

BMP-2 (bone morphogenetic protein-2) signals via two types of transmembrane serine/threonine kinase receptors (BRI and BRII), which form heteromeric complexes prior to and after ligand binding. Within a BMP-bound receptor complex, BRII transphosphorylates and activates BRI-a for further signaling. We investigated which signaling pathway is initiated by BMP-2 via preformed receptor complexes versus BMP-2-induced signaling receptor complexes.

METHODS

Immunofluorescence co-patching was used to study the oligomerization of receptors at the surface of live cells. Binding and chemical cross-linking of iodinated BMP-2 followed by immunoprecipitation was used to show association of receptors in the presence of ligand. Western blots with use of anti-phospho-Smad1 antibodies and reporter gene assays with use of SBE-lux were employed to show activation of the Smad pathway. Phosphorylation of p38-MAPK was shown by Western blots. Induction of alkaline phosphatase was determined by staining the cells. The cluster density of receptors was determined with use of image correlation spectroscopy.

RESULTS AND CONCLUSION

We showed that the Smad pathway is induced by preformed receptor complexes, whereas BMP-2-induced signaling complexes result in the activation of p38-MAPK. We also found evidence that the clustering of BRI-a at the membrane is altered in the presence of BRII, suggesting that it associates with existing clusters of BRII to initiate efficient Smad signaling. These data clearly demonstrate that it is critical to fully understand receptor oligomerization in order to estimate signaling outcome for distinct receptor and ligand mutants.

Smad4 transcriptional pathways and angiogenesis

Smad4/DPC4 is a tumor suppressor gene frequently inactivated in gastrointestinal carcinomas. Smad4 encodes a key intracellular transmitter for signals of the TGF-beta superfamily of cytokines. TGF-beta potently inhibits the growth of normal epithelial cells but tumor cells are frequently resistant; thus, it has been assumed that loss of Smad4 during tumor progression relieves this inhibition. Mediating TGF-beta responses is only one of the many putative functions of Smad4 as a signaling molecule. Smad proteins are versatile transcriptional co-modulators whose activities depend on the genetic makeup of a cell. We have used restoration of Smad4 in deficient cancer cells as an unbiased approach to decipher Smad4's tumor suppressor functions. Stable reexpression of Smad4 in human colon and pancreatic cancer cells potently suppressed tumor growth in vivo in nude mice. Surprisingly, it was not adequate to suppress tumor cell growth in vitro, nor did it restore TGF-beta responsiveness. Rather, Smad4 restoration influenced angiogenesis, decreasing expression of vascular endothelial growth factor and increasing expression of thrombospondin-1. These findings suggest that the acquisition of TGF-beta resistance and loss of Smad4 may be independent consecutive events in the tumorigenic process. They define the control of an angiogenic switch as a novel alternative mechanism of tumor suppression for Smad4.

A novel nuclear export signal in Smad1 is essential for its signaling activity

To investigate the subcellular distributions of Smad proteins, the intracellular mediators of transforming growth factor-beta family cytokines, we examined their sequences for nuclear export signals (NES). We found a leucine-rich NES-like motif (termed NES2) in the central linker region of the receptor-regulated Smads that is absent from the other two classes of Smads (Co-Smads and I-Smads). In microinjection assays, NES2 peptide caused nuclear export of a fused glutathione S-transferase protein. Mutations in NES2 converted Smad1 from an even distribution throughout the cells into an exclusive nuclear localization in both transiently and stably expressing cell lines, and this nuclear enrichment was more pronounced than that induced by mutations in NES1. Furthermore, overexpression of CRM1, the cellular export receptor, transforms Smad1 into a mostly cytoplasmic profile by enhancing its nuclear export. The Smad1 NES2 mutant but not the Smad1 NES1 mutant is mostly resistant to this cytoplasmic targeting, indicating that NES2, not NES1, is the major target for CRM1 in Smad1. We further confirmed the functionality of NES2 by a heterokaryon assay. The Smad1 NES1 mutant displays good ligand responsiveness and moderately lowered transcriptional activity compared with wild type Smad1. In contrast, the Smad1 NES2 mutant shows a severe disruption in reporter gene activation, minimal response to bone morphogenetic protein stimulation, and significantly lowered bone morphogenetic protein-induced phosphorylation, which may be the reason for its deficient transcription activity. Thus, we have defined a major NES in Smad1 that is essential for its ligand-induced coupling with cell surface receptors and hence, transcriptional activity. Our study, along with recent studies of the nucleocytoplasmic shuttling of Smad2 and Smad3 proteins, demonstrate that continued nucleocytoplasmic shuttling is a common requisite for the active signaling of R-Smads. Although conserved in other R-Smads such as Smad3, NES2 is not functional in these R-Smads because CRM1 overexpression fails to target them to cytoplasm. Possible reasons for this discrepancy are discussed.

So-called biological dressing effects of cultured epidermal sheets are mediated by the production of EGF family, TGF-β and VEGF

BACKGROUND

Cultured epidermal sheet (CES) grafts accelerate wound healing as a result of so-called biological dressing effect, which is thought to be mediated by various growth factors. However, the profile of growth factor expression in CESs is unclear.

OBJECTIVE

To investigate whether CESs produce growth factors along with stratification we investigated the production of growth factors and their regulation in CESs.

METHODS

CESs conditioned medium was harvested and the concentration of TGF-alpha, TGF-beta1, TGF-beta2, and VEGF was measured using ELISA. The mRNA of EGF family, TGF-beta family and VEGF was detected by Northern blot or RNase protection assay.

RESULTS

The concentration of TGF-alpha was 100 pg/ml in the monolayer culture, but dramatically increased to 600 pg/ml 2 days after stratification. It decreased to baseline, and then gradually increased to 300 pg/ml in the presence of EGF and remained at that level until day 20. TGF-beta1 increased from 50 to 400 pg/ml after stratification, and remained at that level day 20. TGF-beta2 was undetectable in the monolayer culture, but dramatically increased to 200 pg/ml 2 days after stratification. Unlike TGF-beta1, TGF-beta2 gradually increased over time after stratification. VEGF increased with stratification from 500 to 1500 pg/ml. The addition of EGF upregulated EGF family, TGF-beta, and VEGF production in CESs, as confirmed by ELISA, Northern blot, and RNase protection assay.

CONCLUSION

These results indicate that so-called biological dressing effect of CESs is mediated by production of the EGF family, TGF-beta, and VEGF. Our results also demonstrate the ability of EGF to enhance growth factor production in CESs.

Oncogenic and invasive potentials of human macrophage-stimulating protein receptor, the RON receptor tyrosine kinase

The product of the RON (recepteur d'origine nantais) gene belongs to the MET proto-oncogene family, a distinct subfamily of receptor tyrosine kinases. The ligand of RON was identified as macrophage-stimulating protein (MSP), a member of the plasminogen-related growth factor family. RON is mainly expressed in cells of epithelial origin and is required for embryonic development. In vitro RON activation results in epithelial cell dissociation, migration and matrix invasion, suggesting that RON might be involved in the pathogenesis of certain epithelial cancers in vivo. Indeed, recent studies have shown that RON expression is significantly altered in several primary human cancers, including those of the breast and colon. Truncation of the RON protein has also been found in primary tumors from the gastrointestinal tract. These alterations lead to constitutive activation of RON that causes cell transformation in vitro, induces neoplasm formation in athymic nude mice, and promotes tumor metastasis into the lung. Studies employing transgenic models further demonstrated that over-expression of RON in lung epithelial cells results in multiple tumor formation with features of large cell undifferentiated carcinoma. The oncogenic activities of RON are mediated by RON-transduced signals that promote unbalanced cell growth and transformation leading to tumor development. Thus, abnormal accumulation and activation of RON could play a critical role in vivo in the progression of certain malignant human epithelial cancers.

Nicotine inhibits palatal fusion and modulates nicotinic receptors and the PI-3 kinase pathway in medial edge epithelia

OBJECTIVES

To analyze the effects of nicotine on palatal fusion inhibition in vitro and determine if nicotine modulated transforming growth factor beta3 or phosphatidylinositol-3 kinase signaling. A second objective was to determine the localization and regulation of nicotinic receptors in the medial edge epithelia (MEE) during palatal fusion.

DESIGN

Palatal shelves from embryonic day (E) 13.5 mice were cultured in serum free media and treated with 0, 0.06, 0.6, or 6 mM nicotine, nicotinic receptor antagonist alpha-bungarotoxin, or the combination of nicotine and alpha-bungarotoxin. Tissues harvested at 72 h were analyzed for epithelial-mesenchymal transformation (EMT) and fusion. MEE samples collected at 20 h were analyzed for phosphorylated Akt-Ser473, phosphorylated Smad2, and nicotinic receptors.

RESULTS

Nicotine inhibited palatal fusion in vitro in a dose dependent manner. Activated Akt-Ser473 was greater in control MEE than in nicotine treated tissues; while there was no difference in activated Smad2 between groups. The alpha7 subunit of nicotinic receptor was expressed in MEE during palate fusion and increased in nicotine treated tissues. Alpha-bungarotoxin did not rescue the nicotine treated palates.

CONCLUSION

Nicotine treatment had no effect on Smad2, but caused a down regulation of the PI-3 kinase pathway that may have contributed to inhibiting palatal fusion in vitro.

Keratinocyte growth inhibition by high-dose epidermal growth factor is mediated by transforming growth factor beta autoinduction: a negative feedback mechanism for keratinocyte growth

The epidermal growth factor receptor and its ligands initiate a major signaling pathway that regulates keratinocyte growth in an autocrine manner. It is well known that high doses of epidermal growth factor receptor ligands inhibit keratinocyte growth. Recently, signal transducers and activators of transcription 1-dependent p21Waf1/Cip1 induction were reported to be involved in high-dose epidermal growth factor-dependent cell growth arrest in the A431 squamous cell carcinoma cell line; however, transfection of dominant-negative signal transducers and activators of transcription 1 adenovirus vector did not block epidermal growth factor-induced growth inhibition in normal human keratinocytes. As transforming growth factor beta is a potent inhibitor of keratinocyte proliferation, we hypothesized that transforming growth factor beta contributes to epidermal growth factor-mediated keratinocyte growth inhibition. Epidermal growth factor concentrations of 10 ng per ml enhanced transforming growth factor beta1 mRNA expression from 3 to 6 h poststimulation. Enzyme-linked immunosorbent assay analysis detected 150 pg per ml of transforming growth factor beta1 in the culture medium of keratinocytes incubated with 10 and 100 ng per ml epidermal growth factor, whereas 0.1 and 1.0 ng per ml epidermal growth factor slightly enhance transforming growth factor beta1 production. Epidermal growth factor (100 ng per ml) upregulated luciferase activity of p3TP-lux, which contains three tandem transforming growth factor beta-Smad signaling responsive elements, 6-fold compared with unstimulated cells. The epidermal growth factor-dependent induction of p3TP-lux luciferase activity was disrupted by transfection of the dominant negative form of transforming growth factor beta type I receptor adenovirus vector (AxdnALK5), which suggests that epidermal growth factor-induced transforming growth factor beta acts in an autocrine manner in keratinocytes. Moreover, transfection of AxdnALK5 completely blocked the growth inhibition induced by 100 ng per ml of epidermal growth factor in normal keratinocytes. These data demonstrate that an autocrine transforming growth factor beta1-ALK5 pathway is a negative feedback mechanism for epidermal growth factor-induced normal human keratinocyte growth.

Receptor tyrosine kinases inhibit bone morphogenetic protein-Smad responsive promoter activity and differentiation of murine MC3T3-E1 osteoblast-like cells

Growth factors such as fibroblast growth factor-2 (FGF-2) and epidermal growth factor (EGF) that activate extracellular signal-regulated kinases (ERKs) through receptor tyrosine kinases (RTKs) stimulate proliferation but suppress differentiation of osteoblasts. To study the mechanism of this inhibitory action of these growth factors on osteoblastic differentiation, we evaluated Smad1 transactivity in MC3T3-E1 osteoblast-like cells by reporters of promoter activity of mouse Smad6, an early response gene to bone morphogenetic proteins (BMPs). FGF-2 and EGF inhibited alkaline phosphatase activity and Smad6 promoter activity stimulated by BMP-2. Overexpression of constitutively active MEK by adenovirus mimicked, but that of dominant negative Ras or treatment with a MEK1 inhibitor, PD098059, reversed, the inhibitory effects of these growth factors on both activities. These effects are mediated by BMP-responsive elements (BMPREs) on Smad6 promoter, because an artificial reporter driven by three tandem BMPREs gave similar results, and these effects were all abolished when the BMPREs were mutated. RTK-ERK activation inhibited the promoter activity even when BMP signal was mediated by a mutant Smad1, which lacks phosphorylation sites by ERKs, or by a Smad1 fused to Gal4 DNA binding domain, which constitutively localizes in the nucleus. These results show that the RTK-Ras-ERK pathway suppresses BMP signal by interfering with Smad1 transactivity. Because direct phosphorylation of Smad1 by ERKs is not required for the inhibition, other transcriptional factors that are phosphorylated by ERKs might be involved in the regulation of osteoblastic differentiation by ERKs.

Myofibroblastic conversion of mesothelial cells

BACKGROUND

The continuous chemical, physical, and inflammatory insults of prolonged continuous ambulatory peritoneal dialysis (CAPD) incite mesothelial cell responses, which may result in peritoneal fibrosis. The transforming growth factor-beta (TGF-beta), especially the isoform TGF-beta 1, has long been known to play crucial role in the fibrogenic process. Although several studies have implicated TGF-beta in peritoneal fibrosis, the underlying mechanism has not been completely elucidated.

METHODS

To test the effects of exogenous TGF-beta 1 on mesothelial cells, we assessed cytoarchitectural changes of human peritoneal mesothelial cells (HPMC) in in vitro culture by light, immunofluorescent, electron and immunoelectron microscopy, and differential gene expression analysis using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and cDNA expression array assays.

RESULTS

The TGF-beta 1-induced myofibroblastic conversion was a transdifferentiation process resulting in characteristic myofibroblastic phenotype that included prominent rough endoplasmic reticuli (rER) with dilated cisternas, conspicuous smooth muscle actin (SMA) myofilaments, frequent intercellular intermediate and gap junctions, and active deposition of extracellular matrix (ECM) and formation of fibronexus. The gene expression array analysis revealed complex modulation of gene expression involving cytoskeletal organization, cell adhesion, ECM production, cell proliferation, innate immunity, cytokine/growth factor signaling, cytoprotection, stress response, and many other essential metabolic processes in mesothelial cells.

CONCLUSION

This report describes myofibroblastic conversion of mesothelial cells, a previously undefined, yet frequently speculated, cell adaptive or pathogenic process. Our study helps to elucidate the complex molecular and cellular events involved in myofibroblastic conversion of mesothelial cells. We propose that differentiated epithelial cells of mesothelium convert or transdifferentiate into myofibroblasts, which implies the recruitment of fibrogenic cells from mesothelium during serosal inflammation and wound healing.

N-Acetyl-seryl-aspartyl-lysyl-proline inhibits TGF-beta-mediated plasminogen activator inhibitor-1 expression via inhibition of Smad pathway in human mesangial cells

Recent large clinical trials indicate that angiotensin-converting enzyme inhibitors (ACE-I) attenuate the detrimental outcome of progressive renal disease. The hemoregulatory tetrapeptide N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP, AcSDKP) is hydrolyzed by ACE, and plasma Ac-SDKP level is increased by fivefold after treatment with ACE-I. Ac-SDKP was found to ameliorate cardiac and renal fibrosis in hypertensive animal models. However, the molecular mechanisms by which Ac-SDKP mediates anti-fibrotic effects remain unclear. This study is an examination of the interaction between Ac-SDKP and transforming growth factor-beta (TGF-beta), one of the key cytokines in the progression of renal disease, in human mesangial cells. Ac-SDKP inhibited TGF-beta1-induced plasminogen activator inhibitor-1 (PAI-1) and alpha2 (I) collagen mRNA. Ac-SDKP suppressed not only TGF-beta1-induced Smad2 phosphorylation at Ser-465/467 in a dose-dependent manner, but also the nuclear accumulation of receptor-regulated Smads (R-Smad), Smad2 and Smad3. As expected, Ac-SDKP inhibited TGF-beta-responsive Smad-dependent luciferase reporters, 3TP-luc and 4xSBE-luc. Immunofluorescence analysis revealed that the inhibitory Smad, Smad7, was exported to the cytoplasm from the nucleus by the treatment with Ac-SDKP. These findings provide novel evidence that Ac-SDKP inhibits TGF-beta signal transduction through the suppression of R-Smad activation via nuclear export of Smad7, highlighting an alternative mechanism involved in the reno-protective efficacy of ACE-I.

Pentoxifylline modulates intracellular signalling of TGF-beta in cultured human peritoneal mesothelial cells: implications for prevention of encapsulating peritoneal sclerosis

BACKGROUND

Peritoneal matrix accumulation is a major characteristic of encapsulating peritoneal sclerosis (EPS), which is a serious complication in long-term peritoneal dialysis (PD) patients. We reported previously that TGF-beta stimulates collagen gene expression in cultured HPMC, and is attenuated by pentoxifylline (PTX). The SMAD family and the mitogen-activated protein kinase (MAPK) (ERK1/2, JNK and p38(HOG)) pathways have been shown to participate in TGF-beta signalling. However, how PTX modulates the intracellular signalling downstream to TGF-beta remains undetermined in HPMC. In this study, we explored these signalling pathways in HPMC, and investigated the molecular mechanisms involved in the inhibitory effects of PTX on TGF-beta-induced collagen gene expression in HPMC.

METHODS

HPMC was cultured from human omentum by an enzyme digestion method. The expression of collagen alpha1(I) mRNA was determined by northern blotting, while the SMAD proteins and the MAPK kinase activity were determined by western blotting.

RESULTS

TGF-beta-stimulated collagen alpha1(I) mRNA expression of HPMC was inhibited by PTX. The Smad2, ERK1/2 and p38(HOG) pathways were activated in response to TGF-beta. However, TGF-beta displayed no activation of the JNK pathway in HPMC. The addition of PD98059 and SB203580, which blocked the activation of ERK1/2 and p38(HOG), respectively, suppressed the TGF-beta-induced collagen alpha1(I) mRNA expression. At a concentration (300 micro g/ml) that inhibited the collagen gene expression, PTX suppressed the ERK1/2 and p38(HOG) activation by TGF-beta. In contrast, PTX had no effect on the TGF-beta-induced activation of Smad2, under the same concentration.

CONCLUSION

PTX inhibits the TGF-beta-induced collagen gene expression in HPMC through modulating the ERK1/2 and p38(HOG) pathways. Our study of PTX may provide the therapeutic basis for clinical applications in the prevention of EPS.

Reconstitution of Smad3 restores TGF-beta response of tissue inhibitor of metalloprotease-1 upregulation in human choriocarcinoma cells

Extravillous trophoblast (EVT) cells of the human placenta proliferate, migrate, and invade the pregnant uterus and its vasculature in order to nourish the fetus. However, the normal EVT cell proliferation, migration, and invasiveness are exquisitely controlled in situ by decidua-produced transforming growth factor-beta (TGF-beta), whereas EVT cancer (choriocarcinoma) cells are TGF-beta-resistant. We found that these cells lack in expression of Smad3, a key transcription factor involved in TGF-beta signaling pathway. To test whether Smad3 restitution restores TGF-beta response in choriocarcinoma cells, we produced a Smad3-expressing cell line (JAR-smad3/c). Since anti-invasive effect of TGF-beta in the normal EVT cells was partly mediated by an upregulation of tissue inhibitor of metalloprotease (TIMP)-1, we examined whether Smad3-restituted JAR cells have restored TGF-beta response of TIMP-1 upregulation. The expression of TIMP-1 mRNA was found to be low in JAR and JAR-smad3/c cells. Moreover, the basal level of secreted TIMP-1 protein was very low in these cells as compared to the normal EVT cells. TGF-beta1 upregulated TIMP-1 mRNA and secreted protein in Smad3-restituted JAR cells as well as in the normal EVT cells, whereas no effect was detected in Smad3-deficient (wild-type) JAR cells. We had earlier shown that Smad3-restituted JAR cells had also restored TGF-beta response of plasminogen activator inhibitor-1 upregulation. However, in vitro functional analysis revealed that, in contrast to the normal EVT cells, anti-invasive action of TGF-beta was not restored in Smad3-restituted JAR cells. Thus, additional factors (possibly low expression of Smad4 and/or other unknown factors) may contribute to refractoriness to anti-invasive action of TGF-beta in JAR cells.

Vascularization of the developing chick limb bud: role of the TGFbeta signalling pathway

The developing vertebrate limb has fascinated developmental biologists and theoreticians for decades as a model system for investigating cell fate, cell signalling and tissue interactions. We are beginning to understand the mechanisms and signalling pathways that control and regulate the outgrowth and formation of the limb bud into a differentiated identifiable limb by late embryogenesis. However, the mechanisms underlying the development and maintenance of the vasculature of the developing limb are far from being completely understood. The vasculature supplies oxygen, nutrients and signals to developing tissues, allowing them to develop and grow. Moreover, a lot of evidence recently points to molecules involved in morphological development also controlling vascular development. Thus understanding how the vasculature forms and is patterned in the developing limb may further our understanding of limb development. In this review I outline how blood vessels are formed and maintained and how the developing chick limb is vascularized. I also review the role of the TGFbeta superfamily signalling pathway in the development of the chick limb vasculature: in particular, how antagonizing TGFbeta signalling in the developing chick limb has shed new light on the role vascular smooth muscle cells play in vessel calibre control and how this work has added to our understanding of TGFbeta superfamily signal transduction.

Modulation of thrombomodulin-dependent activation of human protein C through differential expression of endothelial Smads

Protein C is a plasma protease that in its active form plays a central role in the regulation of vascular function by modulating thrombosis, inflammation, and apoptosis. A central player in this pathway is the cytokine-regulated receptor thrombomodulin (TM), which functions as a co-factor for the thrombin-dependent generation of activated protein C. We have found that tumor necrosis factor-beta (TGF-beta)-dependent suppression of TM on endothelial cells is differentially regulated by endothelial Smad6s and Smad7. Overexpression of Smad6s resulted in activation of a TGF-beta reporter alone and enhanced TGF-beta response. Moreover, Smad6s overexpression suppressed TM and subsequently reduced activated protein C generation. Antisense inhibition of Smad6s expression enhanced the TM-dependent activation of protein C, whereas blocking the inhibitory Smad7 by antisense resulted in reduced TM-dependent activation of protein C. The effect of Smad6s appeared to be due, at least in part, to up-regulation of TGF-beta itself. Immunohistochemistry studies in normal versus atherosclerotic vessels showed that TM levels were suppressed in the endothelium over plaque. Consistent with the in vitro data, we found differential expression of Smad6s and Smad7 in normal versus atherosclerotic vessels, with Smad6s expression low in normal vessels but elevated in atherosclerotic vessels. In contrast, the opposite expression pattern was observed for Smad7. Overall, our results suggest that the relative balance of these intracellular Smads modulate the balance of endothelial function with regard to protein C activation.

Identification of mZnf8, a mouse Krüppel-like transcriptional repressor, as a novel nuclear interaction partner of Smad1

To identify novel genes that play critical roles in mediating bone morphogenetic protein (BMP) signal pathways, we performed a yeast two-hybrid screen using Smad1 as bait. A novel mouse Krüppel-type zinc finger protein, mZnf8, was isolated. Interactions between mZnf8 and Smad proteins were further analyzed with various in vitro and in vivo approaches, including mammalian two-hybrid, in vitro glutathione S-transferase pulldown, and copurification assays. Results from functional analysis indicate that mZnf8 is a nuclear transcriptional repressor. Overexpression of mZnf8 represses activity of BMP and transforming growth factor beta (TGF-beta) reporters. Silencing the expression of endogenous mZnf8 with an RNA interference approach caused a significant increase in the expression of one BMP reporter. These results suggest that mZnf8 negatively regulates the TGF-beta/BMP signaling pathway in vivo. Transcription of mZnf8 is ubiquitous in mouse embryos, but high levels are specifically observed in adult mouse testes, with the same cell- and stage-specific transcription pattern as Smad1. Our data support the hypothesis that mZnf8 plays critical roles in mediating BMP signaling during spermatogenesis.

PI-3 kinase activity is required for epithelial-mesenchymal transformation during palate fusion

Epithelial-mesenchymal transformation (EMT) is the primary mechanism for the disappearance of medial edge epithelia (MEE) during palate fusion. This phenotype transition is highly regulated by growth factors, extracellular matrix, cell surface receptors, and a variety of intracellular signaling. Phosphatidylinositol-3 (PI-3) kinase regulates cytoskeleton reorganization, cell migration, and transforming growth factor (TGF) beta-regulated EMT. Therefore, we investigated the role of PI-3 kinase in EMT during palatal fusion in vitro. Palatal shelves from embryonic (E) 13.5 day mouse embryos were collected and cultured for up to 72 hr. A specific PI-3 kinase inhibitor, LY294002, was added to the medium at concentrations of 100 etaM, 1 microM, and 10 microM. The fate of midline epithelia was traced by carboxyfluorescence labeling and analyzed by confocal microscopy. Harvested tissues were also processed for immunohistochemical analysis of a specific marker for basal lamina (laminin). Palatal fusion stages were scored on a scale of 1 to 5, with 1 equal to complete nonfusion and 5 equal to complete fusion. The mean fusion score (MFS) was calculated for each treatment group. Palatal shelves fused after 72 hr of culture in control and 100 etaM LY294002 inhibitor-treated groups, with MFS of 4.67 and 4.5, respectively. Laminin was absent in the midline and epithelia transformed into mesenchyme. However, when cultured palates were treated with 1 and 10 microM LY294002, MEE persisted in the midline and the basal lamina remained intact after 72 hr. The MFS was significantly less in the 1 and 10 microM LY294002-treated tissues at 2.08 and 1.33, respectively. Our results demonstrate that EMT during palatal fusion in vitro is dependent on PI-3 kinase activity.

Content and localization of myostatin in mouse skeletal muscles during aging, mechanical unloading and reloading

Changes in myostatin content and localization in mouse skeletal muscles were investigated during aging, hindlimb suspension (HS) and reloading after HS. During aging, the content of myostatin among solubilized proteins in gastrocnemius and plantaris muscles (Gast/Plant) was initially low and increased until their wet weight/body weight ratio reached a peak. It remained unchanged with further aging, although gradual atrophy of the muscles was seen to occur. Also, the myostatin content did not change significantly during HS (up to 14 days) in both Gast/Plant and soleus muscles, though the muscles showed morphological signs of atrophy. However, reloading for 2 days after a 14-day HS caused significant decreases in the myostatin content in both of these muscles. Immunohistochemical observations showed the sarcoplasmic existence of myostatin, the amount of which appeared to decrease after reloading. The results suggest that myostatin plays a part in the processes of muscular growth and loading-induced hypertrophy, but is not involved in either aging-related or unloading-induced muscular atrophy.

Smad7 mediates transforming growth factor-beta-induced apoptosis in mesangial cells

BACKGROUND

In addition to inhibiting cell growth, transforming growth factor-beta (TGF-beta) has recently been reported to induce apoptosis in various cell lines. Smad proteins are the downstream effectors of TGF-beta signaling. Among them, Smad7 exerts negative feedback control over the action of TGF-beta. However, we do not know how the Smad proteins contribute to TGF-beta-induced apoptosis in mesangial cells. To investigate the function of Smad proteins, we examined the effect of Smad overexpression using adenoviral vector in mesangial cells.

METHODS

Primary cultured rat mesangial cells were transfected with Smad7-promoter-luciferase-plasmid by electroporation. Smad7 promoter activity was investigated by luciferase assay. The apoptotic phenomena elicited by TGF-beta and Smad7 overexpression were investigated using adenoviral vector (AdCMV-Smad7). Apoptosis was detected by the cell death detection ELISA assay, CPP32/caspase-3 assay, and nucleosomal DNA laddering.

RESULTS

TGF-beta significantly increased the protein expression and the promoter activity of Smad7 in rat mesangial cells. Overexpression of Smad7 induced DNA fragmentation and significant increases in cell death ELISA and CPP32/caspase-3 assay. On the other hand, overexpression of Smad2 and Smad3 did not elicit any significant increases in CPP32/caspase-3 activity. Furthermore, the antisense oligonucleotide to Smad7 prevented the TGF-beta-induced apoptosis. Overexpression of Smad7 did not affect nuclear factor-kappaB activity in mesangial cells.

CONCLUSIONS

These data indicate that TGF-beta-induced apoptosis in mesangial cells is mediated through the activation of caspase-3 by Smad7, but not by Smad2 or Smad3. Our results provide new clarification on the function of Smad7 in TGF-beta signaling in mesangial cells.

Transcriptional regulation of Smad2 is required for enhancement of TGF?/Smad signaling by TGF? inducible early gene

TGFbeta inducible early gene (TIEG) is a novel Krüppel-like transcriptional repressor that was recently shown to increase the activity of the TGFbeta/Smad signal transduction pathway by relieving negative feedback through repression of the inhibitory Smad7. Interestingly, while Smad7 is required for maximal enhancement of TGFbeta/Smad signaling, we observe that TIEG is still capable of increasing Smad pathway activity in the absence of Smad7. Furthermore, while Smad7 is known to block both TGFbeta and bone morphogenetic protein (BMP) signaling, we observe that TIEG specifically enhances only the TGFbeta pathway. Similarly, while both TIEG and the related Krüppel-like factor, FKLF2, repress Smad7 transcription, only TIEG is capable of enhancing Smad signaling. In order to identify additional regulatory targets of TIEG important for this enhancement of the Smad pathway activity, we performed microarray analysis and identified Smad2 as a TIEG target gene. We now show evidence that TIEG increases transcription of the Smad2 gene but not the Smad3 or Smad4 genes. Furthermore, while the TGFbeta/Smad pathway remains intact in Smad2 null cells, TIEG enhancement of Smad signaling is dramatically reduced. Thus we propose a new model whereby TIEG enhances Smad signaling by a dual mechanism involving both the repression of the inhibitory Smad7 as well as the activation of Smad2.

Modulation of transforming growth factor beta (TGFbeta)/Smad transcriptional responses through targeted degradation of TGFbeta-inducible early gene-1 by human seven in absentia homologue

Transforming growth factor beta (TGFbeta)-inducible early gene-1 (TIEG1) is a Krüppel-like transcription factor that is rapidly induced upon TGFbeta treatment. TIEG1 promotes TGFbeta/Smad signaling by down-regulating negative feedback through the inhibitory Smad7. In this report, we describe the identification of an E3 ubiquitin ligase, Seven in Absentia homologue-1 (SIAH1), as a TIEG1-interacting protein. We show that TIEG1 and SIAH1 interact through an amino-terminal domain of TIEG1. Co-expression of SIAH1 results in proteasomal degradation of TIEG1 but not of the related factor TIEG2. Importantly, co-expression of SIAH1 completely reverses repression of Smad7 promoter activity by TIEG1. Furthermore, overexpression of a dominant negative SIAH1 stabilizes TIEG1 and synergizes with TIEG1 to enhance TGFbeta/Smad-dependent transcriptional activation. These findings suggest a novel mechanism whereby the ability of TGFbeta to modulate gene transcription may be regulated by proteasomal degradation of the downstream effector TIEG1 through the SIAH pathway. In this manner, turnover of TIEG1 may serve to limit the duration and/or magnitude of TGFbeta responses.

TGFbeta inducible early gene enhances TGFbeta/Smad-dependent transcriptional responses

TGFbeta inducible early gene (TIEG) encodes a three zinc-finger Krüppel-like transcription factor whose overexpression has been shown to mimic the effects of TGFbeta in human osteosarcoma and pancreatic carcinoma cells. In order to investigate a potential role of TIEG in the TGFbeta signal transduction pathway, we studied its impact on a Smad binding element (SBE) reporter which is known to be regulated by TGFbeta through the R-Smad proteins. We demonstrate that TIEG overexpression enhances TGFbeta induction of SBE reporter activity. TIEG overexpression also enhances induction of the endogenous TGFbeta regulated genes p21 and PAI-1. The ability of TIEG to enhance TGFbeta actions is Smad dependent since TIEG has no effect on SBE transcription in the absence of Smad4 expression or when an inhibitory Smad protein, Smad7, is overexpressed. Furthermore, TIEG overexpression enhances TGFbeta induced Smad2 phosphorylation. Lastly, TIEG appears to function by binding to and thereby repressing a specific element in the proximal promoter of the inhibitory Smad7 gene. In conclusion, these results describe a novel mechanism for the potentiation of TGFbeta/Smad signaling via repression of the inhibitory Smad7 gene by TIEG.

A novel cis-element that enhances connective tissue growth factor gene expression in chondrocytic cells

To clarify the chondrocyte-specific regulatory mechanism of connective tissue growth factor (ctgf) gene expression, we analyzed the functionality and DNA-protein interaction of the CTGF promoter. Comparative luciferase assay of the CTGF promoter deletion mutants among HCS-2/8 chondrocytic cells and fibroblastic cells revealed that a 110-bp region in the promoter was crucial for the HCS-2/8-specific transcriptional enhancement. Subsequent competitive gel shift assay revealed that transcription factors in HCS-2/8 nuclei bound to a 60-bp portion in the corresponding region. Relative luciferase activity from a CTGF promoter with mutant TGF-beta response element (TbRE) was 16.9% lower than that from an intact promoter. On the other hand, relative luciferase activity from a CTGF promoter with 4bp point mutations at 30bp upstream of the TbRE was 47.7% lower than that from the intact one. The binding activity of HCS-2/8 nuclear factor(s) to the sequence over the 4-bp was remarkably higher than that of any nuclear extract from other types of cells. Therefore, we entitled the sequence 'TRENDIC', a transcription enhancer dominant in chondrocytes, which stands for a novel enhancer for chondrocyte-specific CTGF gene expression.

Restoration of TGF-beta regulation of plasminogen activator inhibitor-1 in Smad3-restituted human choriocarcinoma cells

Proliferation, migration, and invasiveness of the normal placental extravillous trophoblast (EVT) cells are negatively regulated by transforming growth factor-beta (TGF-beta), whereas malignant EVT (JAR and JEG-3 choriocarcinoma) cells are resistant to TGF-beta. These malignant cells were found to have lost the expression of Smad3. Present study examined whether Smad3 restitution in JAR cells could restore TGF-beta response. We produced a stable Smad3 cDNA-transfected clone (JAR-smad3/c) which exhibited further upregulation of Smad3 in the presence of TGF-beta1. Since anti-invasive effects of TGF-beta in the normal EVT cells were shown to be mediated in part by plasminogen activator inhibitor-1 (PAI-1) and urokinase-type plasminogen activator (uPA), we compared the expression of PAI-1 and uPA in the normal EVT, JAR, and JAR-smad3/c cells in the presence or absence of TGF-beta1. The basal levels of PAI-1 mRNA and secreted PAI-1 and uPA proteins were found to be very low in JAR and JAR-smad3/c cells, as compared to the normal EVT cells. However, TGF-beta1 upregulated PAI-1 and downregulated uPA in JAR-smad3/c cells, but not in JAR cells. Thus, resistance of choriocarcinoma cells to anti-invasive effects of TGF-beta may, at least in part, be due to loss of Smad3 expression.

The mode of bone morphogenetic protein (BMP) receptor oligomerization determines different BMP-2 signaling pathways

Bone morphogenetic proteins (BMPs) are multifunctional proteins regulating cell growth, differentiation, and apoptosis. BMP-2 signals via two types of receptors (BRI and BRII) that are expressed at the cell surface as homomeric as well as heteromeric complexes. Prior to ligand binding, a low but measurable level of BMP-receptors is found in preformed hetero-oligomeric complexes. The major fraction of the receptors is recruited into hetero-oligomeric complexes only after ligand addition. For this, BMP-2 binds first to the high affinity receptor BRI and then recruits BRII into the signaling complex. However, ligand binding to the preformed complex composed of BRII and BRI is still required for signaling, suggesting that it may mediate activating conformational changes. Using several approaches we have addressed the following questions: (i) Are preformed complexes incompetent of signaling in the absence of BMP-2? (ii) Which domains of the BRII receptors are essential for this complex formation? (iii) Are there differences in signals sent from BMP-induced versus preformed receptor complexes? By measuring the activation of Smads, of p38 MAPK and of alkaline phosphatase, we show that the ability of kinase-deficient BRII receptor mutants to inhibit BMP signaling depends on their ability to form heteromeric complexes with BRI. Importantly, a BRII mutant that is incapable in forming preassembled receptor complexes but recruits into a BMP-induced receptor complex does not interfere with the Smad pathway but does inhibit the induction of alkaline phosphatase as well as p38 phosphorylation. These results indicate that signals induced by binding of BMP-2 to preformed receptor complexes activate the Smad pathway, whereas BMP-2-induced recruitment of receptors activates a different, Smad-independent pathway resulting in the induction of alkaline phosphatase activity via p38 MAPK.

TGF-beta1 in breast cancer-estrogen regulation

TGF-beta1 is a pluripotent cytokine with diverse effects in the normal development of mammary glands, and in the development of malignant tumors of the breast. The aim of the study was to determine the levels of TGF-beta1 in the group of advanced breast cancer, in which increased TGF-beta1 levels were most likely to be expected. TGF-beta1 levels were also compared with estradiol levels. Our results suggested that TGF-beta1 synthesis may be regulated by estrogen or anti-estrogen through ER. Finding of increased TGF-beta1 levels, due to its possible role in predicting invasive phenotype in later phases of tumor progression, may indicate the tendency of tumor tissue towards autonomy.

Mining microarray expression data by literature profiling

BACKGROUND

The rapidly expanding fields of genomics and proteomics have prompted the development of computational methods for managing, analyzing and visualizing expression data derived from microarray screening. Nevertheless, the lack of efficient techniques for assessing the biological implications of gene-expression data remains an important obstacle in exploiting this information.

RESULTS

To address this need, we have developed a mining technique based on the analysis of literature profiles generated by extracting the frequencies of certain terms from thousands of abstracts stored in the Medline literature database. Terms are then filtered on the basis of both repetitive occurrence and co-occurrence among multiple gene entries. Finally, clustering analysis is performed on the retained frequency values, shaping a coherent picture of the functional relationship among large and heterogeneous lists of genes. Such data treatment also provides information on the nature and pertinence of the associations that were formed.

CONCLUSIONS

The analysis of patterns of term occurrence in abstracts constitutes a means of exploring the biological significance of large and heterogeneous lists of genes. This approach should contribute to optimizing the exploitation of microarray technologies by providing investigators with an interface between complex expression data and large literature resources.

Smad7 misexpression during embryonic angiogenesis causes vascular dilation and malformations independently of vascular smooth muscle cell function

Numerous in vitro and in vivo studies implicate transforming growth factor-beta (TGFbeta) superfamily signaling in vascular development and maintenance. Mice and humans with mutations in TGFbeta superfamily signaling pathway genes exhibit a range of vascular defects that include dilated, fragile and hemorrhagic vessels, defective angiogenic remodeling, severe vascular malformations including arterio-venous malformations, and disrupted vascular smooth muscle cell recruitment and maintenance. Despite a wealth of data, the functions of TGFbeta superfamily signals during angiogenesis are poorly defined, since early embryonic lethality and difficulty distinguishing between primary and secondary defects frequently confound phenotypic interpretation. To perturb TGFbeta superfamily signaling during angiogenesis, we have misexpressed Smad7, an intracellular antagonist of TGFbeta superfamily signaling, in the developing chick limb and head. We find that the great vessels are strikingly dilated and frequently develop intra and intervascular shunts. Neither noggin nor dominant negative BMP receptor misexpression causes similar vascular phenotypes. However, simultaneous misexpression of constitutively active BMP receptors with Smad7 suppresses the Smad7-induced phenotype, suggesting that a BMP-like intracellular pathway is the target of Smad7 action. Despite the gross morphological defects, further analyses find no evidence of hemorrhage and vessel structure is normal. Furthermore, enlarged vessels and vascular malformations form in either the presence or absence of vascular smooth muscle, and vascular smooth muscle cell recruitment is unperturbed. Our data define the TGFbeta superfamily pathway as an integral regulator of vessel caliber that is also essential for appropriate vessel connectivity. They demonstrate that dilation need not result in vessel rupture or hemorrhage, and dissociate vessel maintenance from the presence of a vascular smooth muscle cell coat. Furthermore they uncouple vascular smooth muscle cell recruitment and differentiation from TGFbeta superfamily signaling.

Reduced Smad3 protein expression and altered transforming growth factor-beta1-mediated signaling in cystic fibrosis epithelial cells

Cystic fibrosis (CF) is a disease characterized by an aggressive inflammatory response in the airways. Given the antiinflammatory properties of transforming growth factor (TGF)-beta1, it was our goal to examine components of TGF-beta1-mediated signaling in both a cultured cell model and a mouse model of CF. A CF-related reduction of protein levels of the TGF-beta1 signaling molecule Smad3 was found in both of these model systems, whereas Smad4 levels were unchanged. Functional effects of reduced Smad3 expression are manifest in our cultured cell model, as reduced basal and TGF-beta1-stimulated levels of luciferase expression using the TGF-beta1-responsive reporter construct 3TP-Lux in the CF-phenotype cells compared with control cells. However, TGF-beta1-stimulated responses using the A3-Luc reporter construct were normal in both cell lines. These results suggest that select TGF-beta1-mediated signaling pathways are impaired in CF epithelial cells. This selective loss of Smad3 protein expression in CF epithelium may also influence inflammatory responses. Our data demonstrate that both CF-phenotype cells lacking Smad3 expression, and A549 cells expressing a dominant-negative Smad3, are unable to support TGF-beta1-mediated inhibition of either the interleukin (IL)-8 or the NOS2 promoter. We conclude that a CF-related reduction in Smad3 protein expression selectively alters TGF- beta1-mediated signaling in CF epithelium, potentially contributing to aggressive inflammatory responses.

Haploinsufficient Bmp4 ocular phenotypes include anterior segment dysgenesis with elevated intraocular pressure

BACKGROUND

Glaucoma is a blinding disease usually associated with high intraocular pressure (IOP). In some families, abnormal anterior segment development contributes to glaucoma. The genes causing anterior segment dysgenesis and glaucoma in most of these families are not identified and the affected developmental processes are poorly understood. Bone morphogenetic proteins (BMPs) participate in various developmental processes. We tested the importance of Bmp4 gene dosage for ocular development and developmental glaucoma.

RESULTS

Bmp4+/- mice have anterior segment abnormalities including malformed, absent or blocked trabecular meshwork and Schlemm's canal drainage structures. Mice with severe drainage structure abnormalities, over 80% or more of their angle's extent, have elevated IOP. The penetrance and severity of abnormalities is strongly influenced by genetic background, being most severe on the C57BL/6J background and absent on some other backgrounds. On the C57BL/6J background there is also persistence of the hyaloid vasculature, diminished numbers of inner retinal cells, and absence of the optic nerve.

CONCLUSIONS

We demonstrate that heterozygous deficiency of BMP4 results in anterior segment dysgenesis and elevated IOP. The abnormalities are similar to those in human patients with developmental glaucoma. Thus, BMP4 is a strong candidate to contribute to Axenfeld-Rieger anomaly and other developmental conditions associated with human glaucoma. BMP4 also participates in posterior segment development and wild-type levels are usually critical for optic nerve development on the C57BL/6J background. Bmp4+/- mice are useful for studying various components of ocular development, and may allow identification of strain specific modifiers affecting a variety of ocular phenotypes.

Two signal transduction pathways involved in the catecholaminergic differentiation of avian neural crest-derived cells in vitro

Molecules derived from the neural tube and found in chick embryo extract (CEE) and bone morphogenetic proteins (BMP) support the differentiation of neural crest-derived catecholaminergic (CA) neurons. We now report that intracellular signaling resulting in the activation of Map kinase (MapK) or translocation of Smad1 mediate the differentiation of CA neurons in response to CEE or BMP 4, respectively. The differentiation of CA neurons was significantly reduced by inhibiting MapK using PD98059 or by pan-specific blockade of tyrosine kinases using Herbimycin A. In the presence of BMP 4 and inhibitors of MapK signaling, differentiation of CA neurons was only moderately reduced. Independent of MapK, BMP 4 induced translocation of Smad1 from the cytosol to the nucleus and induced transcription of dHAND, a DNA binding protein required for the differentiation of CA neurons. The data suggest that CEE-derived factors and BMP4 support the differentiation of CA neurons via independent signaling pathways.