Smad-dependent and Smad-independent pathways in TGF-beta family signalling

Smad3 Knockout Mice Exhibit a Resistance to Skin Chemical Carcinogenesis

It has been shown that Smad3 exerts both tumor-suppressive and -promoting roles. To evaluate the role of Smad3 in skin carcinogenesis in vivo, we applied a chemical skin carcinogenesis protocol to Smad3 knockout mice (Smad3(-/-) and Smad3(+/-)) and wild-type littermates (Smad3(+/+)). Smad3(-/-) mice exhibited reduced papilloma formation in comparison with Smad3(+/+) mice and did not develop any squamous cell carcinomas. Further analysis revealed that Smad3 knockout mice were resistant to 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced epidermal hyperproliferation. Concurrently, increased apoptosis was observed in TPA-treated Smad3(-/-) skin and papillomas when compared with those of wild-type mice. Expression levels of activator protein-1 family members (c-jun, junB, junD, and c-fos) and transforming growth factor (TGF)-alpha were significantly lower in TPA-treated Smad3(-/-) skin, cultured keratinocytes, and papillomas, as compared with Smad3(+/+) controls. Smad3(-/-) papillomas also exhibited reduced leukocyte infiltration, particularly a reduction of tumor-associated macrophage infiltration, in comparison with Smad3(+/+) papillomas. All of these molecular and cellular alterations also occurred to a lesser extent in Smad3(+/-) mice as compared with Smad3(+/+) mice, suggesting a Smad3 gene dosage effect. Given that TGF-beta1 is a well-documented TPA-responsive gene and also has a potent chemotactic effect on macrophages, our study suggests that Smad3 may be required for TPA-mediated tumor promotion through inducing TGF-beta1-responsive genes, which are required for tumor promotion, and through mediating TGF-beta1-induced macrophage infiltration.

Maintenance of mouse trophoblast stem cell proliferation by TGF-β/activin

Mouse trophoblast stem (TS) cells can be grown indefinitely in vitro with FGF4 and embryonic fibroblast conditioned media (EFCM). Here, we report that the active protein components of EFCM include TGF-beta and the related factor activin, and that long-term continuous TS cell proliferation is possible in media supplemented with only serum, FGF4, and TGF-beta. As trophoblasts are an epithelial cell type, the promotion of TS cell proliferation represents an unusual function for TGF-beta and activin since TGF-beta in particular is well known as an inhibitor of nontransformed epithelial cell proliferation. Our data suggest that constitutive FGF signaling in TS cells selectively inhibits the ability of TGF-beta to repress c-myc expression, a central component of the TGF-beta cytostatic transcriptional response previously observed to be lost in other epithelial cell types upon oncogenic Ras transformation.

Runx2/Cbfa1 stimulates transdifferentiation of primary skeletal myoblasts into a mineralizing osteoblastic phenotype

Runx2, a transcriptional activator downstream of bone morphogenetic protein (BMP) signaling, is essential to osteoblastic differentiation and bone formation and maintenance. BMPs activate complex signaling networks, utilizing numerous signaling molecules and transcription factors to induce expression of osteoblastic markers in mesenchymal cell types. However, the role of Runx2 in this process, particularly in an environment independent of the other regulatory elements modulated by BMPs, remains poorly understood. In the present study, we used retroviral gene delivery to examine the effects of sustained Runx2 expression in primary myoblasts. Runx2 inhibited myogenesis, as demonstrated by suppression of MyoD and myogenin mRNA levels and reduced myotube formation. Additionally, Runx2-stimulated osteogenesis including osteoblastic gene expression, alkaline phosphatase activity, and biological mineral deposition. Notably, these osteogenic markers were induced to significantly greater levels than those observed in BMP-2-treated controls. These results demonstrate that direct exogenous expression of the Runx2 transcription factor, only one of numerous downstream targets of BMP signaling, is sufficient to induce transdifferentiation of myogenic cells into a mineralizing osteogenic lineage. This work underscores the potency of Runx2 as a regulator of osteogenesis and cell differentiation and provides new insights into the plasticity of committed mesenchymal cells.

Soluble tumor necrosis factor (TNF) receptor-1 induces apoptosis via reverse TNF signaling and autocrine transforming growth factor-beta1

The pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) plays a central role in inflammatory disorders. Transmembrane TNF-alpha and its two receptors are cleaved by the proteinase TNF-alpha converting enzyme (TACE), resulting in appreciable serum levels of soluble TNF-alpha and soluble TNF-alpha receptors (sTNFR1 and -2). The only known functions of sTNFR1 are to antagonize and buffer circulating TNF-alpha. Here, we present evidence that sTNFR1 exerts immunoregulatory functions by induction of apoptosis in monocytes through reverse signaling via transmembrane TNF-alpha. sTNFR1-induced apoptosis is independent of death receptor pathways but depends on autocrine transforming growth factor (TGF)-beta1 signaling through the mitogen-activated protein kinase p38alpha. This novel mechanism has implications for understanding the physiological role of sTNFR1 and for TNF-alpha-blocking therapies of autoimmune diseases.

Regulation of bone morphogenetic proteins in early embryonic development

Bone morphogenetic proteins (BMPs), a large subgroup of the TGF-beta family of secreted growth factors, control fundamental events in early embryonic development, organogenesis and adult tissue homeostasis. The plethora of dose-dependent cellular processes regulated by BMP signalling demand a tight regulation of BMP activity. Over the last decade, a number of proteins have been identified that bind BMPs in the extracellular space and regulate the interaction of BMPs with their cognate receptors, including the secreted BMP antagonist Chordin. In the early vertebrate embryo, the localized secretion of BMP antagonists from the dorsal blastopore lip establishes a functional BMP signalling gradient that is required for the determination of the dorsoventral - or back to belly - body axis. In particular, inhibition of BMP activity is essential for the formation of neural tissue in the development of vertebrate and invertebrate embryos. Here we review recent studies that have provided new insight into the regulation of BMP signalling in the extracellular space. In particular, we discuss the recently identified Twisted gastrulation protein that modulates, in concert with metalloproteinases of the Tolloid family, the interaction of Chordin with BMP and a family of proteins that share structural similarities with Chordin in the respective BMP binding domains. In addition, genetic and functional studies in zebrafish and frog provide compelling evidence that the secreted protein Sizzled functionally interacts with the Chd-BMP pathway, despite being expressed ventrally in the early gastrula-stage embryo. These intriguing discoveries may have important implications, not only for our current concept of early embryonic patterning, but also for the regulation of BMP activity at later developmental stages and tissue homeostasis in the adult.

Characterization of adrenomedullin in non-human primates

Adrenomedullin (AM) is a 52 amino acid peptide involved in the pathophysiology of several human diseases. Here we show the gene structure, organ distribution, and regulated expression of AM in monkey. The monkey AM (mAM) gene is located on the short arm of chromosome 9 and it codes for a 185 amino acid preprohormone, which contains two amidated peptides identical to the human AM and proadrenomedullin N-terminal 20 peptide. The promoter region of the mAM gene contains a variety of transcription factor binding motifs. mAM is widely expressed throughout many organs as shown by real-time PCR and immunohistochemical techniques, and we have found similar levels of circulating plasma AM in monkeys and humans. A significant upregulation of the mAM mRNA was observed in monkey cells exposed to low oxygen tension conditions, TGF-beta1, all-trans-retinoic acid, and dexamethasone. Our collective data show a high degree of homology between mAM and hAM, which renders the monkey an attractive animal model for future pharmacological and pre-clinical studies targeting AM.

Proteomics-based identification of proteins interacting with Smad3: SREBP-2 forms a complex with Smad3 and inhibits its transcriptional activity

Smad3 is an important component of transforming growth factor-beta (TGFbeta) intracellular signalling. To identify novel interacting proteins of Smad3, we performed pull-down assays with Smad3 constructs fused to glutathione-S-transferase. Proteins which formed complexes with these constructs were analyzed by two-dimensional gel electrophoresis, and were identified by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry. We identified 14 proteins interacting with the Smad3 construct lacking the N-terminal Mad homology domain 1 (MH1), and 12 proteins interacting with the construct lacking the C-terminal MH2 domain. Proteins involved in signalling processes, in metabolism regulation, novel proteins, and components of cytoskeleton form four groups of interacting proteins. Interactions of AGP7, sex-determining region Y protein, actin beta and sterol regulatory element binding protein-2 (SREBP-2) proteins with Smad3 constructs were confirmed by immunoblotting with specific antibodies. Interaction of Smad3 with SREBP-2 was also confirmed by co-immunoprecipitation of myc-Smad3 and Flag-SREBP-2 upon expression in mammalian cells. We found that SREBP-2 inhibited the transcriptional activity of Smad3 in luciferase reporter assays.

IGFBP-3 activates TGF-beta receptors and directly inhibits growth in human intestinal smooth muscle cells

We have shown that human intestinal smooth muscle cells produce IGF-I and IGF binding protein-3 (IGFBP-3). Endogenous IGF-I acts in autocrine fashion to stimulate growth of these cells. IGFBP-3 inhibits the binding of IGF-I to its receptor and thereby inhibits IGF-I-stimulated growth. In several carcinoma cell lines and some normal cells, IGFBP-3 regulates growth independently of IGF-I. Two mechanisms for this effect have been identified: IGFBP-3 can directly activate transforming growth factor-beta (TGF-beta) receptors or it can undergo direct nuclear translocation. The aim of the present study was to determine whether IGFBP-3 acts independently of IGF-I and to characterize the mechanisms mediating this effect in human intestinal smooth muscle cells. The direct effects of IGFBP-3 were determined in the presence of an IGF-I receptor antagonist to eliminate its IGF-I-dependent effects. Affinity labeling of TGF-beta receptors (TGF-betaRI, TGF-betaRII, and TGF-betaRV) with 125I-labeled TGF-beta1 showed that IGFBP-3 displaced binding to TGF-betaRII and TGF-betaRV in a concentration-dependent fashion. IGFBP-3 stimulated TGF-betaRII-dependent serine phosphorylation (activation) of both TGF-betaRI and of its primary substrate, Smad2(Ser465/467). IGFBP-3 also caused IGF-I-independent inhibition of basal [3H]thymidine incorporation. The effects of IGFBP-3 on Smad2 phosphorylation and on smooth muscle cell proliferation were independent of TGF-beta1 and were abolished by transfection of Smad2 siRNA. Immunoneutralization of IGFBP-3 increased basal [3H]thymidine incorporation, implying that endogenous IGFBP-3 inhibits proliferation. We conclude that endogenous IGFBP-3 directly inhibits proliferation of human intestinal smooth muscle cells by activation of TGF-betaRI and Smad2, an effect that is independent of its effect on IGF-I-stimulated growth.

Bone morphogenetic proteins and neurotrophins provide complementary protection of septal cholinergic function during phosphatase inhibitor-induced stress

Cultures of embryonic rat septum were exposed for 24-48 h to 2-5 nm okadaic acid (OA), an inhibitor of pp1A and pp2A phosphatases. This stress killed approximately 75% of neurons. A neurotrophin (NT) combination (nerve growth factor and brain-derived neurotrophic factor, each 100 ng/mL) plus a bone morphogenetic protein (BMP6 or BMP7, 5 nm) reduced the death of both cholinergic and non-cholinergic neurons, and preserved choline acetyltransferase (ChAT) activity assayed 2-6 days post-stress. This NT + BMP combination preserved ChAT activity better than either NTs or BMPs alone, and was effective even if trophic factor addition was delayed until 12 h after stress onset. A general caspase inhibitor (qVD-OPH, 10 micro g/mL) also increased survival of stressed cholinergic neurons, but its protection of ChAT activity was shorter lived than that produced by the NT + BMP combination. Neither the NT + BMP combination nor the caspase inhibitor reduced the OA-induced increase in tau phosphorylation. These findings indicate that NTs and BMPs have synergistic protective effects against an OA stress, and suggest that at least some of these protective effects occur upstream of caspase activation.

A critical role of tropomyosins in TGF-beta regulation of the actin cytoskeleton and cell motility in epithelial cells

We have investigated transforming growth factor beta (TGF-beta)-mediated induction of actin stress fibers in normal and metastatic epithelial cells. We found that stress fiber formation requires de novo protein synthesis, p38Mapk and Smad signaling. We show that TGF-beta via Smad and p38Mapk up-regulates expression of actin-binding proteins including high-molecular-weight tropomyosins, alpha-actinin and calponin h2. We demonstrate that, among these proteins, tropomyosins are both necessary and sufficient for TGF-beta induction of stress fibers. Silencing of tropomyosins with short interfering RNAs (siRNAs) blocks stress fiber assembly, whereas ectopic expression of tropomyosins results in stress fibers. Ectopic-expression and siRNA experiments show that Smads mediate induction of tropomyosins and stress fibers. Interestingly, TGF-beta induction of stress fibers was not accompanied by changes in the levels of cofilin phosphorylation. TGF-beta induction of tropomyosins and stress fibers are significantly inhibited by Ras-ERK signaling in metastatic breast cancer cells. Inhibition of the Ras-ERK pathway restores TGF-beta induction of tropomyosins and stress fibers and thereby reduces cell motility. These results suggest that induction of tropomyosins and stress fibers play an essential role in TGF-beta control of cell motility, and the loss of this TGF-beta response is a critical step in the acquisition of metastatic phenotype by tumor cells.

Altered Expression of Genes of the Bmp/Smad and Wnt/Calcium Signaling Pathways in the Cone-only Nrl-/- Mouse Retina, Revealed by Gene Profiling Using Custom cDNA Microarrays

Many mammalian retinas are rod-dominant, and hence our knowledge of cone photoreceptor biology is relatively limited. To gain insights into the molecular differences between rods and cones, we compared the gene expression profile of the rod-dominated retina of wild type mouse with that of the cone-only retina of Nrl(-/-) (Neural retina leucine zipper knockout) mouse. Our analysis, using custom microarrays of eye-expressed genes, provided equivalent data using either direct or reference-based experimental designs, confirmed differential expression of rod- and cone-specific genes in the Nrl(-/-) retina and identified novel genes that could serve as candidates for retinopathies or for functional studies. In addition, we detected altered expression of several genes that encode cell signaling or structural proteins. Prompted by these findings, additional real-time PCR analysis revealed that genes belonging to the Bmp/Smad and Wnt/Ca(2+) signaling pathways are expressed in the mature wild type retina and that their expression is significantly altered in the Nrl(-/-) retina. Chromatin immunoprecipitation analysis of adult retina identified Bmp4 and Smad4, which are down-regulated in the Nrl(-/-) retina, as possible direct transcriptional targets of Nrl. Consistent with these studies, Bmp4 and Smad4 are expressed in the mature rod photoreceptors of mouse retina. Modulation of Bmp4 and/or Smad4 by Nrl may provide a mechanism for integrating diverse cell signaling networks in rods. We hypothesize that Bmp/Smad and Wnt/Ca(2+) pathways participate in cell-cell communication in the mature retina, and expression changes observed in the Nrl(-/-) retina reflect their biased utilization in rod versus cone homeostasis.

Transient adenoviral gene transfer of Smad7 prevents injury-induced epithelial-mesenchymal transition of lens epithelium in mice

We examined the effect of adenovirus-mediated transient expression of Smad7, an inhibitory Smad in TGFbeta/activin signaling, on injury-induced epithelial-mesenchymal transition (EMT) of lens epithelium in mice. A volume of 3 microl of adenoviral solution was injected into the right lens of adult male C57BL/6 mice (n=56) at the time of capsular injury made using a hypodermic needle under general anesthesia. A mixture of recombinant adenovirus carrying CAG promoter-driven Cre (Cre adv) and mouse Smad7 complementary DNA (Smad7 adv) was administered to induce Smad7 expression, while control lenses were treated with Cre adv alone. After healing intervals of 2, 3, 5, and 10 days, animals were killed 2 h after labeling with bromodeoxyuridine (BrdU) and eyes were processed for histology. During healing, marked expression of Smad7 was observed in lens epithelial cells in the Smad7 adv group with loss of nuclear translocation of Smads2/3, while little Smad7 and abundant nuclear Smads2/3 were seen in cells in the Cre adv group. Lens epithelial cells in the Cre adv control group exhibited a fibroblastic appearance at days 5 and 10 and the capsular break was sealed with fibrous tissue, while Smad7 adv-treated cells around the capsular break retained their epithelial morphology and the break was not sealed. Expression of snail mRNA, and alpha-smooth muscle actin, lumican, and collagen VI proteins, markers of EMT, was observed in control-treated eyes, but not in cells of the Smad7 adv group at day 5 with minimal expression at day 10. Additionally, cell proliferation increased in epithelium infected with Smad7 adv consistent with suppression of injury-induced upregulation of TGFbeta1 in epithelium. We conclude that gene transfer of Smad7 in mice prevents injury-induced EMT of lens epithelial cells and sealing of the capsular break with fibrous tissue.

Liver regeneration: from myth to mechanism

The unusual regenerative properties of the liver are a logical adaptation by organisms, as the liver is the main detoxifying organ of the body and is likely to be injured by ingested toxins. The numerous cytokine- and growth-factor-mediated pathways that are involved in regulating liver regeneration are being successfully dissected using molecular and genetic approaches. So what is known about this process at present and which questions remain?

RLP, a novel Ras-like protein, is an immediate-early transforming growth factor-beta (TGF-beta) target gene that negatively regulates transcriptional activity induced by TGF-beta

We have described previously the use of microarray technology to identify novel target genes of TGF-beta (transforming growth factor-beta) signalling in mouse embryo fibroblasts deficient in Smad2 or Smad3 [Yang, Piek, Zavadil, Liang, Xie, Heyer, Pavlidis, Kucherlapati, Roberts and Böttinger (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 10269-10274]. Among the TGF-beta target genes identified, a novel gene with sequence homology to members of the Ras superfamily was identified, which we have designated as RLP (Ras-like protein). RLP is a Smad3-dependent immediate-early TGF-beta target gene, its expression being induced within 45 min. Bone morphogenetic proteins also induce expression of RLP, whereas epidermal growth factor and phorbol ester PMA suppress TGF-beta-induced expression of RLP. Northern-blot analysis revealed that RLP was strongly expressed in heart, brain and kidney, and below the detection level in spleen and skeletal muscles. At the protein level, RLP is approx. 30% homologous with members of the Ras superfamily, particularly in domains characteristic for small GTPases. However, compared with prototypic Ras, RLP contains a modified P-loop, lacks the consensus G2 loop and the C-terminal prenylation site and harbours amino acid substitutions at positions that render prototypic Ras oncogenic. However, RLP does not have transforming activity, does not affect phosphorylation of mitogen-activated protein kinase and is unable to bind GTP or GDP. RLP was found to associate with certain subtypes of the TGF-beta receptor family, raising the possibility that RLP plays a role in TGF-beta signal transduction. Although RLP did not interact with Smads and did not affect TGF-beta receptor-induced Smad2 phosphorylation, it inhibited TGF-beta-induced transcriptional reporter activation, suggesting that it is a novel negative regulator of TGF-beta signalling.

Ubiquitination and proteolysis of cancer-derived Smad4 mutants by SCFSkp2

Smad4/DPC4, a common signal transducer in transforming growth factor beta (TGF-beta) signaling, is frequently inactivated in human cancer. Although the ubiquitin-proteasome pathway has been established as one mechanism of inactivating Smad4 in cancer, the specific ubiquitin E3 ligase for ubiquitination-mediated proteolysis of Smad4 cancer mutants remains unclear. In this report, we identified the SCFSkp2 complex as candidate Smad4-interacting proteins in an antibody array-based screen and further elucidated the functions of SCFSkp2 in mediating the metabolic instability of cancer-derived Smad4 mutants. We found that Skp2, the F-box component of SCFSkp2, physically interacted with Smad4 at the physiological levels. Several cancer-derived unstable mutants exhibited significantly increased binding to Skp2, which led to their increased ubiquitination and accelerated proteolysis. These results suggest an important role for the SCFSkp2 complex in switching cancer mutants of Smad4 to undergo polyubiquitination-dependent degradation.

A new role for BMP5 during limb development acting through the synergic activation of Smad and MAPK pathways

In an attempt to identify new genes implicated in the control of programmed cell death during limb development, we have generated a cDNA library from the regressing interdigital tissue of chicken embryos. We have analyzed 804 sequences from this library and identified 23 genes involved in apoptosis in different models. One of the genes that came up in the screening was the Bone Morphogenetic Protein family member, Bmp5, that has not been previously involved in the control of apoptosis during limb development. In agreement with a possible role in the control of cell death, Bmp5 exhibited a regulated pattern of expression in the interdigital tissue. Transcripts of Bmp5 and BMP5 protein were abundant within the cytoplasm of the fragmenting apoptotic interdigital cells in a way suggesting that delivery of BMPs into the tissue is potentiated during apoptosis. Gain-of-function experiments demonstrated that BMP5 has the same effect as other interdigital BMPs inducing apoptosis in the undifferentiated mesoderm and growth in the prechondrogenic mesenchyme. We have characterized both Smad proteins and MAPK p38 as intracellular effectors for the action of BMPs in the developing limb autopod. Activation of Smad signaling involves the receptor-regulated genes Smad1 and -8, and the inhibitory Smad6, and results in both the upregulation of gene transcription and protein phosphorylation with subsequent nuclear translocation. MAPK p38 is also quickly phosphorylated after BMP stimulation in the limb mesoderm. Treatment with the inhibitor of p38, SB203580, revealed that there are interdigital genes induced by BMPs in a p38-dependent manner (DKK, Snail and FGFr3), and genes induced in a p38-independent manner (BAMBI, Msx2 and Smads). Together, our results suggest that Smad and MAPK pathways act synergistically in the BMP pathway controlling limb development.

JNK regulates autocrine expression of TGF-beta1

The c-Jun NH2-terminal kinase (JNK) has been implicated in the function of transforming growth factor beta (TGF-beta). To test the role of JNK, we examined the effect of compound disruption of the murine genes that encode the ubiquitously expressed isoforms of JNK (Jnk1 and Jnk2). We report that JNK-deficient fibroblasts isolated from Jnk1-/- Jnk2-/- mice constitutively express TGF-beta1. Complementation studies demonstrate that JNK is a repressor of Tgf-beta1 gene expression. This mechanism of regulation of TGF-beta1 expression by JNK represents an unexpected form of cross-talk between two important signaling pathways. Together, these data demonstrate that the JNK pathway may contribute to the regulation of autocrine TGF-beta1-mediated biological responses in vivo.

JNK regulates expression and autocrine signaling of TGF-beta1

TGF-beta1 can induce its own expression by an autoregulatory mechanism. In this issue of Molecular Cell, Ventura et al. (2004) show, using JNK-deficient fibroblasts, that JNK negatively regulates TGF-beta1 gene expression and, consequently, autocrine TGF-beta signaling. The results highlight the contributions of JNK signaling in the control of TGF-beta expression and signaling, and of autocrine TGF-beta signaling in the JNK-regulated changes in cell behavior.

Targeted disruption of TGF-beta/Smad3 signaling modulates skin fibrosis in a mouse model of scleroderma

Transforming growth factor-beta (TGF-beta) is a potent stimulus of connective tissue accumulation, and is implicated in the pathogenesis of scleroderma and other fibrotic disorders. Smad3 functions as a key intracellular signal transducer for profibrotic TGF-beta responses in normal skin fibroblasts. The potential role of Smad3 in the pathogenesis of scleroderma was investigated in Smad3-null (Smad3(-/-)) mice using a model of skin fibrosis induced by subcutaneous injections of bleomycin. At early time points, bleomycin-induced macrophage infiltration in the dermis and local TGF-beta production were similar in Smad3(-/-) and wild-type mice. In contrast, at day 28, lesional skin from Smad3(-/-) mice showed attenuated fibrosis, lower synthesis and accumulation of collagen, and reduced collagen gene transcription in situ, compared to wild-type mice. Connective tissue growth factor and alpha-smooth muscle actin expression in lesional skin were also significantly attenuated. Electron microscopy revealed an absence of small diameter collagen fibrils in the dermis from bleomycin-treated Smad3(-/-) mice. Compared to fibroblasts derived from wild-type mice, Smad3(-/-) fibroblasts showed reduced in vitro proliferative and profibrotic responses elicited by TGF-beta. Together, these results indicate that ablation of Smad3 is associated with markedly altered fibroblast regulation in vivo and in vitro, and confers partial protection from bleomycin-induced scleroderma in mice. Reduced fibrosis is due to deregulated fibroblast function, as the inflammatory response induced by bleomycin was similar in wild-type and Smad3(-/-) mice.

BMP-2 decreases Mash1 stability by increasing Id1 expression

In neural development, bone morphogenetic proteins (BMPs) restrict neuronal differentiation, thereby promoting the maintenance of progenitor cells or even inducing astrocytogenesis. We report that exposure of neuroendocrine lung carcinoma cells to BMP-2 leads to a rapid decline in steady-state levels of Mash1 protein and some neuron-specific markers. BMP-2 induces a post-transcriptional decrease in Mash1 levels through enhanced degradation. We demonstrate that Mash1 protein stability is tightly regulated by the E47/Id1 expression ratio. Transient induction of Id1 by BMP-2 negatively correlates with Mash1 levels. Furthermore, an ectopic increase in Id1 levels is sufficient to induce degradation of either ectopic or endogenous Mash1, whereas expression of Mash1 in Id1-deficient cells or overexpression of E47 makes Mash1 levels refractory to the addition of BMP-2. Furthermore, we show that the E47/Id1 expression ratio also regulates CK2-mediated phosphorylation of Mash1 on Ser152, which increases interaction of Mash1-E47 heterodimers. We propose a novel mechanism in which the balance between Id and E protein levels regulates not only the transcriptional function but also protein stability of the neurogenic bHLH transcription factor Mash1.

cAMP inhibits TGFbeta1-induced in vitro angiogenesis

Transforming growth factor-beta (TGFbeta1) is a proangiogenic factor both, in vitro and in vivo, that is mainly involved in the later phases of angiogenesis. In an attempt to identify genes that participate in this effect, we found that TGFbeta1 down-regulates expression of adenylate cyclase VI. In addition, cAMP analogs (8-Bromo-cAMP) and forskolin (an adenylate cyclase activator) also reduced TGFbeta1-induced in vitro angiogenesis in mouse endothelial cell lines and in primary cultures of human umbilical vein endothelial cells on collagen gels. Induction of Ets-1 and plasminogen activator inhibitor-1 (PAI-1) by TGFbeta1 was blocked by these cAMP agonists and activators, in the absence of effects on endothelial cell viability. Moreover, the signal transduction pathways stimulated by TGFbeta1 were unaffected. Thus, Smad2 was normally phosphorylated and translocated to the nucleus in the presence of forskolin. In contrast, transfection studies using the PAI-1-promoter indicated that these cAMP analogues inhibit transcriptional stimulation by TGFbeta1. Electrophoretic mobility shift assay showed that Smad2/3 were bound normally to a TGFbeta1-response region in the presence of the cAMP analogs. In all, these data suggest that the cAMP pathway inhibits the transcriptional activity of Smads, that could be responsible for the block of the TGFbeta1-induced in vitro angiogenesis caused by this second messenger.

Critical role of N-cadherin in myofibroblast invasion and migration in vitro stimulated by colon-cancer-cell-derived TGF-beta or wounding

Invasion of stromal host cells, such as myofibroblasts, into the epithelial cancer compartment may precede epithelial cancer invasion into the stroma. We investigated how colon cancer-derived myofibroblasts invade extracellular matrices in vitro in the presence of colon cancer cells. Myofibroblast spheroids invade collagen type I in a stellate pattern to form a dendritic network of extensions upon co-culture with HCT-8/E11 colon cancer cells. Single myofibroblasts also invade Matrigel trade mark when stimulated by HCT-8/E11 colon cancer cells. The confrontation of cancer cells with extracellular matrices and myofibroblasts, showed that cancer-cell-derived transforming growth factor-beta (TGF-beta) is required and sufficient for invasion of myofibroblasts. In myofibroblasts, N-cadherin expressed at the tips of filopodia is upregulated by TGF-beta. Functional N-cadherin activity is implicated in TGF-beta stimulated invasion as evidenced by the neutralizing anti-N-cadherin monoclonal antibody (GC-4 mAb), and specific N-cadherin knock-down by short interference RNA (siRNA). TGF-beta1 stimulates Jun N-terminal kinase (also known as stress-activated protein kinase) (JNK) activity in myofibroblasts. Pharmacological inhibition of JNK alleviates TGF-beta stimulated invasion, N-cadherin expression and wound healing migration. Neutralization of N-cadherin activity by the GC-4 or by a 10-mer N-cadherin peptide or by siRNA reduces directional migration, filopodia formation, polarization and Golgi-complex reorientation during wound healing. Taken together, our study identifies a new mechanism in which cancer cells contribute to the coordination of invasion of stromal myofibroblasts.

Spatio-temporal activation of Smad1 and Smad5 in vivo: monitoring transcriptional activity of Smad proteins

Signaling by bone morphogenetic proteins is essential for a wide variety of developmental processes. Receptor-regulated Smad proteins, Smads 1 and 5, are intracellular mediators of bone morphogenetic protein signaling. Together with Smad4, these proteins translocate to the nucleus and modulate transcription by binding to specific sequences on the promoters of target genes. We sought to map transcriptional Smad1/5 activity in development by generating embryonic stem cell lines carrying a Smad1/5-specific response element derived from the Id1 promoter coupled to beta-galactosidase or luciferase as reporters. Three independent lines (BRE-lac1, BRE-lac2 and BRE-luc) have shown the existence of an autocrine bone morphogenetic protein signaling pathway in mouse embryonic stem cells. Reporter activity was detected in chimeric embryos, suggesting sensitivity to physiological concentrations of bone morphogenetic protein. Reporter activity in embryos from transgenic mouse lines was detected in tissues where an essential role for active bone morphogenetic protein signaling via Smads 1 or 5 had been previously established. We have thus generated, for the first time, an in vivo readout for studying the role of Smad1/5-mediated transcriptional activity in development.

Ig-Reactive CD4+CD25+ T Cells from Tolerized (New Zealand Black × New Zealand White)F1 Mice Suppress In Vitro Production of Antibodies to DNA

We have recently shown that tolerogenic administration of an artificial peptide (pConsensus) that is based on sequences within the V(H) regions of several murine anti-dsDNA Ig delays appearance of autoantibodies in female (New Zealand Black (NZB) x New Zealand White (NZW))F(1) (NZB/W F(1)) mice and significantly prolongs their survival. The aim of this study was to characterize the T cell population(s) involved in pConsensus-induced down-regulation of autoimmune responses in tolerized NZB/W F(1) mice. Using MHC class II dimers loaded with tolerogenic peptide, we found that pCons favored expansion of peptide-reactive CD4(+)CD25(+) regulatory T cells (T(R)) that inhibited in vitro production of anti-dsDNA Ab-forming cells. Suppression by T(R) was abrogated by the presence in culture of Ab to glucocorticoid-induced TNFR family member 18 or to TGFbeta latency-associated protein. These findings suggest possible relevance of Ag specificity in the mechanism of T(R)-mediated immune tolerance to Ig-derived peptides in NZB/W F(1) mice.

Proteomic profiling of bone marrow mesenchymal stem cells upon transforming growth factor beta1 stimulation

Bone marrow mesenchymal stem cells (MSCs) can differentiate into different types of cells and have tremendous potential for cell therapy and tissue engineering. Transforming growth factor beta1 (TGF-beta) plays an important role in cell differentiation and vascular remodeling. We showed that TGF-beta induced cell morphology change and an increase in actin fibers in MSCs. To determine the global effects of TGF-beta on MSCs, we employed a proteomic strategy to analyze the effect of TGF-beta on the human MSC proteome. By using two-dimensional gel electrophoresis and electrospray ionization coupled to quadrupole/time-of-flight tandem mass spectrometers, we have generated a proteome reference map of MSCs, and we identified approximately 30 proteins with an increase or decrease in expression or phosphorylation in response to TGF-beta. The proteins regulated by TGF-beta included cytoskeletal proteins, matrix synthesis proteins, membrane proteins, metabolic enzymes, etc. TGF-beta increased the expression of smooth muscle alpha-actin and decreased the expression of gelsolin. Overexpression of gelsolin inhibited TGF-beta-induced assembly of smooth muscle alpha-actin; on the other hand, knocking down gelsolin expression enhanced the assembly of alpha-actin and actin filaments without significantly affecting alpha-actin expression. These results suggest that TGF-beta coordinates the increase of alpha-actin and the decrease of gelsolin to promote MSC differentiation. This study demonstrates that proteomic tools are valuable in studying stem cell differentiation and elucidating the underlying molecular mechanisms.

Loss of Smad3 in acute T-cell lymphoblastic leukemia

BACKGROUND

The receptors for transforming growth factor beta (TGF-beta) and their signaling intermediates make up an important tumor-suppressor pathway. The role of one of these intermediates--Smad3--in the pathogenesis of lymphoid neoplasia is unknown.

METHODS

We measured Smad3 messenger RNA (mRNA) and protein in leukemia cells obtained at diagnosis from 19 children with acute leukemia, including 10 with T-cell acute lymphoblastic leukemia (ALL), 7 with pre-B-cell ALL, and 2 with acute nonlymphoblastic leukemia (ANLL). All nine exons of the SMAD3 gene (MADH3) were sequenced. Mice in which one or both alleles of Smad3 were inactivated were used to evaluate the role of Smad3 in the response of normal T cells to TGF-beta and in the susceptibility to spontaneous leukemogenesis in mice in which both alleles of the tumor suppressor p27Kip1 were deleted.

RESULTS

Smad3 protein was absent in T-cell ALL but present in pre-B-cell ALL and ANLL. No mutations were found in the MADH3 gene in T-cell ALL, and Smad3 mRNA was present in T-cell ALL and normal T cells at similar levels. In mice, the loss of one allele for Smad3 impairs the inhibitory effect of TGF-beta on the proliferation of normal T cells and works in tandem with the homozygous inactivation of p27Kip1 to promote T-cell leukemogenesis.

CONCLUSIONS

Loss of Smad3 protein is a specific feature of pediatric T-cell ALL. A reduction in Smad3 expression and the loss of p27Kip1 work synergistically to promote T-cell leukemogenesis in mice.

Glypican-1 antisense transfection modulates TGF-β-dependent signaling in Colo-357 pancreatic cancer cells

The heparan sulfate proteoglycan glypican-1 is essential as a co-receptor for heparin binding growth factors, such as HB-EGF and FGF-2, in pancreatic cancer cells. In the present study, the role of glypican-1 in the regulation of TGF-beta signaling was investigated. Colo-357 pancreatic cancer cells were stably transfected with a full-length glypican-1 antisense construct. Cell growth was determined by MTT and soft agar assays. TGF-beta1 induced p21 expression and Smad2 phosphorylation were analyzed by immunoblotting. PAI-1 promoter activity was determined by luciferase assays. Down-regulation of glypican-1 expression by stable transfection of a full-length glypican-1 antisense construct resulted in decreased anchorage-dependent and -independent cell growth in Colo-357 pancreatic cancer cells and attenuated TGF-beta1 induced cell growth inhibition, Smad2 phosphorylation, and PAI-1 promoter activity. There was, however, no significant difference in TGF-beta1 induced p21 expression and Smad2 nuclear translocation. In conclusion, glypican-1 is required for efficient TGF-beta1 signaling in pancreatic cancer cells.

KLF11 mediates a critical mechanism in TGF-beta signaling that is inactivated by Erk-MAPK in pancreatic cancer cells

BACKGROUND & AIMS

Smad-regulated transcription plays a central role in transforming growth factor (TGF)-beta-induced cell growth inhibition and tumor suppression. Like the Smads, KLF11 is an early response transcription factor that mediates TGF-beta-induced growth inhibition in untransformed epithelial cells. Here, we investigated the functional implications of KLF11 in TGF-beta signaling and transcription in normal epithelial as well as pancreatic cancer cells.

METHODS

The effects of KLF11 on TGF-beta signaling and transcription were examined on the levels of reporter transactivation, Smad2 phosphorylation, and expression of endogenous TGF-beta-regulated genes. Promoter analysis, real-time polymerase chain reaction, and coimmunoprecipitation studies were performed to study KLF11-induced and mSin3A corepressor-mediated repression of Smad7. Erk-induced KLF11 phosphorylation was examined in vitro and in vivo, and its impact on KLF11-mSin3A-mediated Smad7 repression was verified in pancreatic cancer cells using site-directed mutagenesis.

RESULTS

KLF11 potentiates TGF-beta signaling by terminating the inhibitory Smad7 loop. Mechanistically, KLF11 represses TGF-beta-induced transcription from the Smad7 promoter by recruiting mSin3a via GC-rich sites. This function is inhibited in pancreatic cancer cells with oncogenic Ras mutations, in which Erk/mitogen-activated protein kinase phosphorylates KLF11, leading to disruption of KLF11-mSin3a interaction. Expression of an Erk-insensitive KLF11 mutant restores both mSin3a binding and Smad7 repression and results in enhanced TGF-beta signaling in pancreatic cancer cells.

CONCLUSIONS

These results define a novel mechanism in TGF-beta-regulated gene expression. KLF11 potentiates Smad-signaling activity in normal epithelial cells through termination of the negative feedback loop imposed by Smad7. The fact that this function of KLF11 is inhibited by oncogenic Erk/mitogen-activated protein kinase in pancreatic cancer cells emphasizes the importance of this mechanism for oncogenesis.

Smad2 Phosphorylation by Type I Receptor

Transforming growth factor-beta (TGFbeta) is a potent regulator of cell proliferation, differentiation, motility, and apoptosis. TGFbeta binds to and activates serine/threonine kinase receptors that phosphorylate Smad2 and Smad3 intracellular signal transducers at two C-terminal serine residues. Here we show that substitutions of Arg-462 and Cys-463 residues, which are in proximity of the C-terminal serine residues, inhibited TGFbeta type I receptor-dependent phosphorylation of the C-terminal Smad2 peptides and full-length GST-Smad2 proteins in vitro. In vivo, mutation of Arg-462 and Cys-463 inhibited TGFbeta1-stimulated phosphorylation of the C-terminal serine residues in Smad2. Moreover, Smad2 with mutated Arg-462 and Cys-463 was less efficient in activation of the Smad2-responsive activin-responsive element-containing luciferase reporter ARE-luc, as compared with the wild-type protein. Thus, Arg-462 and Cys-463, which are in proximity of the C-terminal serine residues, contribute to recognition and phosphorylation of the C terminus of Smad2 by type I TGFbeta receptor.

Autocrine and exogenous transforming growth factor beta control cell cycle inhibition through pathways with different sensitivity

Human colon carcinoma cells HCT116 that lack transforming growth factor beta (TGF-beta) type II receptor (RII) demonstrated restoration of autocrine TGF-beta activity upon reexpression of RII without restoring inhibitory responses to exogenous TGF-beta treatment. RII transfectants (designated RII Cl 37) had a longer lag phase relative to NEO-transfected control cells (designated NEO pool) before entering exponential growth in tissue culture. The prolonged growth arrest of RII Cl 37 cells was associated with markedly reduced cyclin-dependent kinase (CDK)2 activity. Our results demonstrate that p21 induction by autocrine TGF-beta is responsible for reduced CDK2 activity, which at least partially contributes to prolonged growth arrest and reduced cell proliferation in RII Cl 37 cells. In contrast to RII transfectants, HCT116 cells transfected with chromosome 3 (designated HCT116Ch3), which bears the RII gene, restored the response to exogenous TGF-beta as well as autocrine TGF-beta activity. Autocrine TGF-beta activity in HCT116Ch3 cells induced p21 expression as seen in RII Cl 37 cells; however, in addition to autocrine activity, HCT116Ch3 cells responded to exogenous TGF-beta as decreased CDK4 expression and reduced pRb phosphorylation mediated a TGF-beta inhibitory response in these cells. These results indicate that autocrine TGF-beta regulates the cell cycle through a pathway different from exogenous TGF-beta in the sense that p21 is a more sensitive effector of the TGF-beta signaling pathway, which can be induced and saturated by autocrine TGF-beta, whereas CDK4 inhibition is a less sensitive effector, which can only be activated by high levels of exogenous TGF-beta

In vivo convergence of BMP and MAPK signaling pathways: impact of differential Smad1 phosphorylation on development and homeostasis

Integration of diverse signaling pathways is essential in development and homeostasis for cells to interpret context-dependent cues. BMP and MAPK signaling converge on Smads, resulting in differential phosphorylation. To understand the physiological significance of this observation, we have generated Smad1 mutant mice carrying mutations that prevent phosphorylation of either the C-terminal motif required for BMP downstream transcriptional activation (Smad1(C) mutation) or of the MAPK motifs in the linker region (Smad1(L) mutation). Smad1(C/C) mutants recapitulate many Smad1(-/-) phenotypes, including defective allantois formation and the lack of primordial germ cells (PGC), but also show phenotypes that are both more severe (head and branchial arches) and less severe (allantois growth) than the null. Smad1(L/L) mutants survive embryogenesis but exhibit defects in gastric epithelial homeostasis correlated with changes in cell contacts, actin cytoskeleton remodeling, and nuclear beta-catenin accumulation. In addition, formation of PGCs is impaired in Smad1(L/L) mutants, but restored by allelic complementation in Smad1(C/L) compound mutants. These results underscore the need to tightly balance BMP and MAPK signaling pathways through Smad1.

Transforming growth factor Beta1 induction of tissue inhibitor of metalloproteinases 3 in articular chondrocytes is mediated by reactive oxygen species

Transforming growth factor beta1 (TGF-beta1) stimulates cartilage extracellular matrix synthesis but, in excess, evokes synovial inflammation, hyperplasia, and osteophyte formation in arthritic joints. TGF-beta1 induces tissue inhibitor of metalloproteinases 3 (TIMP-3), an inhibitor of cartilage-damaging matrix metalloproteianases and aggrecanases. We investigated the role of reactive oxygen species (ROS) in TIMP-3 induction by TGF-beta1. In primary human and bovine chondrocytes, ROS scavenger and antioxidant N-acetylcysteine (NAC) inhibited TGF-beta1-induced TIMP-3 mRNA and protein increases. Ebselen and ascorbate also reduced this induction. TGF-beta1 time-dependently induced ROS production that was suppressed by NAC. Hydrogen peroxide, a ROS, induced TIMP-3 RNA. The TIMP-3 increase induced by TGF-beta1 was partly Smad2-dependent. TGF-beta1-stimulated Smad2 phosphorylation was inhibited by NAC. Reduced glutathione and L-cysteine also blocked Smad2 and TIMP-3 induction by TGF-beta1, whereas a nonthiol, N-acetylalanine, did not. Smad2 was not activated by H2O2. Smad2 phosphorylation was independent, and TIMP-3 expression was dependent, on new protein synthesis. TGF-beta-stimulated ERK and JNK phosphorylation was also inhibited by NAC. However, inhibitory actions of NAC were not mediated by ERK activation. Thus, ROS mediate TGF-beta1-induced TIMP-3 gene expression. Blocking TGF-beta1-induced gene expression by modulating cellular redox status with thiols can be potentially beneficial for treating arthritic and other disorders caused by excessive TGF-beta1.

Cardiac outflow tract defects in mice lacking ALK2 in neural crest cells

Cardiac neural crest cells are multipotent migratory cells that contribute to the formation of the cardiac outflow tract and pharyngeal arch arteries. Neural crest-related developmental defects account for a large proportion of congenital heart disorders. Recently, the genetic bases for some of these disorders have been elucidated, and signaling pathways required for induction,migration and differentiation of cardiac neural crest have emerged. Bone morphogenetic proteins comprise a family of secreted ligands implicated in numerous aspects of organogenesis, including heart and neural crest development. However, it has remained generally unclear whether BMP ligands act directly on neural crest or cardiac myocytes during cardiac morphogenesis,or function indirectly by activating other cell types. Studies on BMP receptor signaling during organogenesis have been hampered by the fact that receptor knockouts often lead to early embryonic lethality. We have used a Cre/loxP system for neural crest-specific deletion of the type I receptor, ALK2, in mouse embryos. Mutant mice display cardiovascular defects, including persistent truncus arteriosus, and abnormal maturation of the aortic arch reminiscent of common forms of human congenital heart disease. Migration of mutant neural crest cells to the outflow tract is impaired, and differentiation to smooth muscle around aortic arch arteries is deficient. Moreover, in Alk2 mutants, the distal outflow tract fails to express Msx1, one of the major effectors of BMP signaling. Thus, the type I BMP receptor ALK2 plays an essential cell-autonomous role in the development of the cardiac outflow tract and aortic arch derivatives.

Investigation of three new mouse mammary tumor cell lines as models for transforming growth factor (TGF)-beta and Neu pathway signaling studies: identification of a novel model for TGF-beta-induced epithelial-to-mesenchymal transition

Introduction

This report describes the isolation and characterization of three new murine mammary epithelial cell lines derived from mammary tumors from MMTV (mouse mammary tumor virus)/activated Neu + TβRII-AS (transforming growth factor [TGF]-β type II receptor antisense RNA) bigenic mice (BRI-JM01 and BRI-JM05 cell lines) and MMTV/activated Neu transgenic mice (BRI-JM04 cell line).

Methods

The BRI-JM01, BRI-JM04, and BRI-JM05 cell lines were analyzed for transgene expression, their general growth characteristics, and their sensitivities to several growth factors from the epidermal growth factor (EGF) and TGF-β families (recombinant human EGF, heregulin-β₁ and TGF-β₁). The BRI-JM01 cells were observed to undergo a striking morphologic change in response to TGF-β₁, and they were therefore further investigated for their ability to undergo a TGF-β-induced epithelial-to-mesenchymal transition (EMT) using motility assays and immunofluorescence microscopy.

Results

We found that two of the three cell lines (BRI-JM04 and BRI-JM05) express the Neu transgene, whereas, unexpectedly, both of the cell lines that were established from MMTV/activated Neu + TβRII-AS bigenic tumors (BRI-JM01 and BRI-JM05) do not express the TβRII-AS transgene. The cuboidal BRI-JM01 cells exhibit a short doubling time and are able to form confluent monolayers. The BRI-JM04 and BRI-JM05 cell lines are morphologically much less uniform, grow at a much slower rate, and do not form confluent monolayers. Only the BRI-JM05 cells can form colonies in soft agar. In contrast, all three cell lines form colonies in Matrigel, although the BRI-JM04 and BRI-JM05 cell lines do so more efficiently than the BRI-JM01 cell line. All three cell lines express the cell surface marker E-cadherin, confirming their epithelial character. Proliferation assays showed that the three cell lines respond differently to recombinant human EGF and heregulin-β₁, and that all are growth inhibited by TGF-β₁, but that only the BRI-JM01 cell line undergoes an EMT and exhibits increased motility upon TGF-β₁ treatment.

Conclusion

We suggest that the BRI-JM04 and BRI-JM05 cell lines can be used to investigate Neu oncogene driven mammary tumorigenesis, whereas the BRI-JM01 cell line will be useful for studying TGF-β₁-induced EMT.

BMP4: Its Role in Development of the Hematopoietic System and Potential as a Hematopoietic Growth Factor

Blood formation occurs throughout the life of an individual in a process driven by hematopoietic stem cells (HSCs). The ability of bone marrow (BM) and cord blood (CB) HSC to undergo self-renewal and develop into multiple blood lineages has made these cells an important clinical resource. Transplantation with BM- and CB-derived HSCs is now used extensively for treatment of hematological disorders, malignancies, and immunodeficiencies. An understanding of the embryonic origin of HSC and the factors regulating their generation and expansion in vivo will provide important information for the manipulation of these cells ex vivo. This is critical for the further development of CB transplantation, the potential of which is limited by small numbers of HSC in the donor population. Although the origins of HSCs have become clearer and progress has been made in identifying genes that are critical for the formation and maintenance of HSCs, less is known about the signals that commit specific populations of mesodermal precursors to hematopoietic cell fate. Critical signals acting on these precursor cells are likely to be derived from visceral endoderm in yolk sac and from underlying stroma in the aorta-gonad-mesonephros region. Here we summarize briefly the origin of yolk sac and embryonic HSCs before detailing evidence that bone morphogenic protein-4 (BMP4) has a crucial role in Xenopus and mammalian HSC development. We discuss evidence that BMP4 acts as a hematopoietic growth factor and review its potential to modulate HSC in ex vivo expansion cultures from cord blood.

Roles for the MH2 Domain of Smad7 in the Specific Inhibition of Transforming Growth Factor-β Superfamily Signaling

Signals by cytokines of the transforming growth factor-beta (TGF-beta) superfamily are negatively regulated by inhibitory Smads (I-Smads). Smad7 inhibits signaling by both TGF-beta and bone morphogenetic proteins (BMPs), whereas Smad6 inhibits TGF-beta signals less effectively. I-Smads have amino-terminal N domains and carboxyl-terminal Mad homology 2 (MH2) domains. The N domains are essential for specific inhibition of TGF-beta signaling by Smad7, whereas the MH2 domains of I-Smads are involved in the inhibition of TGF-beta superfamily signals through interaction with type I receptors. Here, we have identified four basic amino acid residues (Lys-312, Lys-316, Lys-401, and Arg-409) in the basic surface of the Smad7 MH2 domain that play important roles in interaction with type I receptors. Mutations of the four basic amino acid residues to acidic residues (K312E, K316E, K401E, and R409E) abolished the interaction of Smad7 with TGF-beta type I receptors, inhibition of Smad2 phosphorylation and transcriptional responses induced by TGF-beta, and induction of target genes of endogenous activin/Nodal signals in Xenopus early embryos. The K401E and R409E mutants of Smad7 were also unable to interact with BMP type I receptors (BMPR-I), repress the Smad5 phosphorylation and transcription induced by BMP, and effectively inhibit endogenous BMP signals in Xenopus early embryos. However, the K312E and K316E mutants were able to interact with BMPR-I and retained the ability to inhibit BMP signaling. Thus, the MH2 domain of Smad7 plays important roles in specific inhibition of TGF-beta superfamily signals through differential interaction with type I receptors.

Differential requirements for Smad4 in TGFbeta-dependent patterning of the early mouse embryo

Genetic and biochemical data have identified Smad4 as a key intracellular effector of the transforming growth factor beta (TGFbeta superfamily of secreted ligands. In mouse, Smad4-null embryos do not gastrulate, a phenotype consistent with loss of other TGFbeta-related signaling components. Chimeric analysis reveals a primary requirement for Smad4 in the extra-embryonic lineages; however, within the embryo proper, characterization of the specific roles of Smad4 during gastrulation and lineage specification remains limited. We have employed a Smad4 conditional allele to specifically inactivate the Smad4 gene in the early mouse epiblast. Loss of Smad4 in this tissue results in a profound failure to pattern derivatives of the anterior primitive streak, such as prechordal plate, node, notochord and definitive endoderm. In contrast to these focal defects, many well-characterized TGFbeta- and Bmp-regulated processes involved in mesoderm formation and patterning are surprisingly unaffected. Mutant embryos form abundant extra-embryonic mesoderm, including allantois, a rudimentary heart and middle primitive streak derivatives such as somites and lateral plate mesoderm. Thus, loss of Smad4 in the epiblast results not in global developmental abnormalities but instead in restricted patterning defects. These results suggest that Smad4 potentiates a subset of TGFbeta-related signals during early embryonic development, but is dispensable for others.

Id2 and Id3 define the potency of cell proliferation and differentiation responses to transforming growth factor beta and bone morphogenetic protein

Transforming growth factors beta (TGF-betas) inhibit growth of epithelial cells and induce differentiation changes, such as epithelial-mesenchymal transition (EMT). On the other hand, bone morphogenetic proteins (BMPs) weakly affect epithelial cell growth and do not induce EMT. Smad4 transmits signals from both TGF-beta and BMP pathways. Stimulation of Smad4-deficient epithelial cells with TGF-beta 1 or BMP-7 in the absence or presence of exogenous Smad4, followed by cDNA microarray analysis, revealed 173 mostly Smad4-dependent, TGF-beta-, or BMP-responsive genes. Among 25 genes coregulated by both factors, inhibitors of differentiation Id2 and Id3 showed long-term repression by TGF-beta and sustained induction by BMP. The opposing regulation of Id genes is critical for proliferative and differentiation responses. Hence, ectopic Id2 or Id3 expression renders epithelial cells refractory to growth inhibition and EMT induced by TGF-beta, phenocopying the BMP response. Knockdown of endogenous Id2 or Id3 sensitizes epithelial cells to BMP, leading to robust growth inhibition and induction of transdifferentiation. Thus, Id genes sense Smad signals and create a permissive or refractory nuclear environment that defines decisions of cell fate and proliferation.

Proteomic analysis of the TGF-beta signaling pathway in pancreatic carcinoma cells using stable RNA interference to silence Smad4 expression

Smad4 is a tumor-suppressor gene that is lost or mutated in 50% of pancreatic carcinomas. Smad4 is also an intracellular transmitter of transforming growth factor-beta (TGF-beta) signals. Although its tumor-suppressor function is presumed to reside in its capacity to mediate TGF-beta-induced growth inhibition, there seems to be a Smad4-independent TGF-beta signaling pathway. Here, we succeeded in establishing Smad4 knockdown (S4KD) pancreatic cancer cell lines using stable RNA interference. Smad4 protein expression and TGF-beta-Smad4 signaling were impaired in S4KD cells, and we compared the proteomic changes with TGF-beta stimulation using two-dimensional gel electrophoresis (2-DE) and mass spectrometry. We identified five proteins that were up-regulated and seven proteins that were down-regulated; 10 of them were novel targets for TGF-beta. These proteins function in processes such as cytoskeletal regulation, cell cycle, and oxidative stress. Introducing siRNA-mediated gene silencing into proteomics revealed a novel TGF-beta signal pathway that did not involve Smad4.

Overexpression of HER2 (erbB2) in Human Breast Epithelial Cells Unmasks Transforming Growth Factor β-induced Cell Motility

We have examined overexpression of the human epidermal growth factor receptor 2 (HER2) to determine if it modifies the anti-proliferative effect of transforming growth factor (TGF)-beta against MCF-10A human mammary epithelial cells. Exogenous TGF-beta inhibited cell proliferation and induced Smad-dependent transcriptional reporter activity in both MCF-10A/HER2 and MCF-10A/vector control cells. Ligand-induced reporter activity was 7-fold higher in HER2-overexpressing cells. In wound closure and transwell assays, TGF-beta induced motility of HER2-transduced, but not control cells. The HER2-blocking antibody trastuzumab (Herceptin) prevented TGF-beta-induced cell motility. Expression of a constitutively active TGF-beta type I receptor (ALK5(T204D)) induced motility of MCF-10A/HER2 but not MCF-10A/vector cells. TGF-beta-induced motility was blocked by coincubation with either the phosphatidylinositol 3-kinase inhibitor LY294002, the mitogen-activated protein kinase (MAPK) inhibitor U0126, the p38 MAPK inhibitor SB202190, and an integrin beta(1) blocking antibody. Rac1 activity was higher in HER2-overexpressing cells, where both Rac1 and Pak1 proteins were constitutively associated with HER2. Both exogenous TGF-beta and transduction with constitutively active ALK5 enhanced this association. TGF-beta induced actin stress fibers as well as lamellipodia within the leading edge of wounds. Herceptin blocked basal and TGF-beta-stimulated Rac1 activity but did not repress TGF-beta-stimulated transcriptional reporter activity. These data suggest that 1) overexpression of HER2 in nontumorigenic mammary epithelial is permissive for the ability of TGF-beta to induce cell motility and Rac1 activity, and 2) HER2 and TGF-beta signaling cooperate in the induction of cellular events associated with tumor progression.

Restoration of transforming growth factor-beta signaling through receptor RI induction by histone deacetylase activity inhibition in breast cancer cells

The loss of transforming growth factor-beta (TGF-beta) response due to the dysregulation of TGF-beta receptors type I (RI) and type II (RII) is well known for its contribution to oncogenesis. Estrogen receptor-expressing breast cancer cells are refractory to TGF-beta-mediated growth control because of the reduced expression of TGF-beta receptors. Although RII is required for the binding of TGF-beta to RI, RI is responsible for directly transducing TGF-beta signals through the Smad protein family. Treatment of estrogen receptor-expressing MCF-7L and ZR75 breast cancer cells with the histone deacetylase (HDAC) inhibitor suberoylanilide hydroxamic acid (SAHA) led to a dramatic induction of RI. Accumulation of acetylated histones H3 and H4 was observed in the SAHA-treated cells. Chromatin immunoprecipitation analysis followed by PCR with RI promoter-specific primers indicated an accumulation of acetylated histones in chromatin associated with the RI gene, suggesting that histone deacetylation was involved in the transcriptional inactivation of RI. SAHA treatment stimulated RI promoter activity through the inhibition of Sp1/Sp3-associated HDAC activity. Histone acetyltransferase p300 stimulated RI promoter activity, thus further confirming the involvement of HDAC activity in the transcriptional repression of RI. Significantly, SAHA-mediated RI regeneration restored the TGF-beta response in breast cancer cells.

Nodal and ALK7 inhibit proliferation and induce apoptosis in human trophoblast cells

Nodal, a member of the transforming growth factor-beta superfamily, is known to play critical roles in early vertebrate development, but its functions in extraembryonic tissues are unclear. ALK7 is a type I receptor for Nodal. Recently, we demonstrated that Nodal mRNA and several ALK7 transcripts are expressed in human placenta throughout pregnancy (Roberts, H. J., Hu, S., Qiu, Q., Leung, P. C. K., Cannigia, I., Gruslin, A., Tsang, B., and Peng, C. (2003) Biol. Reprod. 68, 1719-1726). In this study, we determined the role of Nodal and ALK7 in trophoblast cell proliferation and apoptosis. Overexpression of Nodal in normal trophoblast cells (HTR8/SVneo) and several choriocarcinoma cell lines resulted in a significant decrease in the number of metabolically active cells. The effect of Nodal could be mimicked by constitutively active ALK7 (ALK7-ca), but was blocked by kinase-deficient ALK7. The growth inhibitory effect of Nodal was also blocked by dominant-negative Smad2/3. Overexpression of Nodal and ALK7-ca induced apoptosis in trophoblast cells as determined by Hoechst staining, flow cytometry, and caspase-3 Western blotting. In addition, Nodal and ALK7-ca decreased the number of proliferating cells as measured by bromodeoxyuridine assays. Furthermore, overexpression of Nodal or ALK7-ca increased p27 expression, but reduced the levels of Cdk2 and cyclin D(1). Taken together, this study demonstrates for the first time that Nodal, acting through ALK7 and Smad2/3, inhibits proliferation and induces apoptosis in human trophoblast cells. Our findings also suggest that the Nodal-ALK7 pathway inhibits cell proliferation by inducing G(1) cell cycle arrest and that this effect is mediated in part by the p27-cyclin E/Cdk2 pathway.

TGF-beta regulation of human macrophage scavenger receptor CD163 is Smad3-dependent

Tight regulation of the inflammatory response is essential for the maintenance of physiologic homeostasis. A potentially important mediator of this process is CD163, a macrophage-specific member of the scavenger receptor cysteine-rich family. CD163 surface expression is up-regulated by glucocorticoids and the anti-inflammatory cytokine interleukin-10, and CD163 is shed acutely from the cell surface in response to lipopolysaccharide. We now demonstrate that transforming growth factor-beta (TGF-beta) markedly reduces expression of CD163. Treatment of primary human monocytes with TGF-beta inhibited basal as well as dexamethasone-induced CD163 mRNA and protein expression. De novo protein synthesis was not required for this inhibition, suggesting that TGF-beta regulates CD163 expression transcriptionally. To delineate this transcriptional regulation, a 2.5-kb fragment of the CD163 promoter was isolated. This promoter was inhibited by TGF-beta, and suppression was dependent on Smad3 expression. These results define a novel function for TGF-beta and implicate an important role for CD163 in the host response to inflammation.

Status of the DPC4 tumor suppressor gene in sporadic colon adenocarcinoma of Croatian patients: identification of a novel somatic mutation

Loss of heterozygosity (LOH) of loci on chromosome 18q occurs in a majority of colorectal cancers. The DPC4 (Smad4) tumor suppressor gene, located at 18q21.1, may be a predisposing gene for Juvenile Polyposis Syndrome. To investigate alterations of the DPC4 gene in sporadic colon adenocarcinoma, a panel of 60 tumor specimens from Croatian patients was surveyed for evidence of LOH and also for mutations within the entire DPC4 coding region (exons 1-11). Using three pairs of specific primers for the three DPC4 microsatellite repetitive sequences, we investigated the frequency of LOH. The presence of single nucleotide change at restriction sites of specific codons in exons 2, 8, 10, and 11 (which belong to the conserved region of the gene) was examined by RFLP analysis. The investigation was extended to search for any other mutation within the entire coding region of the DPC4 gene by single strand conformation polymorphism (SSCP) analysis. Our results show a high frequency of heterozygosity in 58 of 60 (97%) colon adenocarcinoma samples. LOH at any one of the three flanking markers was observed in 26 (45%) of the 58 informative cases. The loss of one allele of the DPC4 gene was negatively correlated with tumor size; more frequent in smaller tumors (<5 cm) than in larger ones. A mutation was found in exon 11 in only one tumor sample (T18), and the mutation was verified by sequencing. Sequencing demonstrated a novel mutation-a deletion in exon 11 (134-153 del TAGACGAAGTACTTCATACC) of the DPC4 gene in the MH2 domain. These data suggest that inactivation of the DPC4 gene contributes to the genesis of colorectal carcinoma through allelic loss whereas mutation in the coding region of the DPC4 gene is infrequently detected in Croatian patients with A, B or C stages of colorectal cancers.

Smad3 as a mediator of the fibrotic response

Transforming growth factor-beta (TGF-beta) plays a central role in fibrosis, contributing to the influx and activation of inflammatory cells, the epithelial to mesenchymal transdifferentiation (EMT) of cells and the influx of fibroblasts and their subsequent elaboration of extracellular matrix. TGF-beta signals through transmembrane receptor serine/threonine kinases to activate novel signalling intermediates called Smad proteins, which modulate the transcription of target genes. The use of mice with a targeted deletion of Smad3, one of the two homologous proteins which signals from TGF-beta/activin, shows that most of the pro-fibrotic activities of TGF-beta are mediated by Smad3. Smad3 null inflammatory cells and fibroblasts do not respond to the chemotactic effects of TGF-beta and do not autoinduce TGF-beta. The loss of Smad3 also interferes with TGF-beta-mediated induction of EMT and genes for collagens, plasminogen activator inhibitor-1 and the tissue inhibitor of metalloprotease-1. Smad3 null mice are resistant to radiation-induced cutaneous fibrosis, bleomycin-induced pulmonary fibrosis, carbon tetrachloride-induced hepatic fibrosis as well as glomerular fibrosis induced by induction of type 1 diabetes with streptozotocin. In fibrotic conditions that are induced by EMT, such as proliferative vitreoretinopathy, ocular capsule injury and glomerulosclerosis resulting from unilateral ureteral obstruction, Smad3 null mice also show an abrogated fibrotic response. Animal models of scleroderma, cystic fibrosis and cirrhosis implicate involvement of Smad3 in the observed fibrosis. Additionally, inhibition of Smad3 by overexpression of the inhibitory Smad7 protein or by treatment with the small molecule, halofuginone, dramatically reduces responses in animal models of kidney, lung, liver and radiation-induced fibrosis. Small moleucule inhibitors of Smad3 may have tremendous clinical potential in the treatment of pathological fibrotic diseases.

Combinatorial activities of Smad2 and Smad3 regulate mesoderm formation and patterning in the mouse embryo

TGFbeta/activin/Nodal receptors activate both Smad2 and Smad3 intracellular effector proteins. The functional activities of these closely related molecules have been extensively studied in cell lines. We show both are expressed in the early mouse embryo from the blastocyst stage onwards and mediate Foxh1-dependent activation of the Nodal autoregulatory enhancer in vitro. Genetic manipulation of their expression ratios reveals that Smad3 contributes essential signals at early post-implantation stages. Thus, loss of Smad3 in the context of one wild-type copy of Smad2 results in impaired production of anterior axial mesendoderm, while selective removal of both Smad2 and Smad3 from the epiblast additionally disrupts specification of axial and paraxial mesodermal derivatives. Finally, we demonstrate that Smad2;Smad3 double homozygous mutants entirely lack mesoderm and fail to gastrulate. Collectively, these results demonstrate that dose-dependent Smad2 and Smad3 signals cooperatively mediate cell fate decisions in the early mouse embryo.

In the absence of type III receptor, the transforming growth factor (TGF)-beta type II-B receptor requires the type I receptor to bind TGF-beta2

Transforming growth factor beta (TGF-beta) ligands exert their biological effects through type II (TbetaRII) and type I receptors (TbetaRI). Unlike TGF-beta1 and -beta3, TGF-beta2 appears to require the co-receptor betaglycan (type III receptor, TbetaRIII) for high affinity binding and signaling. Recently, the TbetaRIII null mouse was generated and revealed significant non-overlapping phenotypes with the TGF-beta2 null mouse, implying the existence of TbetaRIII independent mechanisms for TGF-beta2 signaling. Because a variant of the type II receptor, the type II-B receptor (TbetaRII-B), has been suggested to mediate TGF-beta2 signaling in the absence of TbetaRIII, we directly tested the ability of TbetaRII-B to bind TGF-beta2. Here we show that the soluble extracellular domain of the type II-B receptor (sTbetaRII-B.Fc) bound TGF-beta1 and TGF-beta3 with high affinity (K(d) values = 31.7 +/- 22.8 and 74.6 +/- 15.8 pm, respectively), but TGF-beta2 binding was undetectable at corresponding doses. Similar results were obtained for the soluble type II receptor (sTbetaRII.Fc). However, sTbetaRII.Fc or sTbetaRII-B.Fc in combination with soluble type I receptor (sTbetaRI.Fc) formed a high affinity complex that bound TGF-beta2, and this complex inhibited TGF-beta2 in a biological inhibition assay. These results show that TGF-beta2 has the potential to signal in the absence of TbetaRIII when sufficient TGF-beta2, TbetaRI, and TbetaRII or TbetaRII-B are present. Our data also support a cooperative model for receptor-ligand interactions, as has been suggested by crystallization studies of TGF-beta receptors and ligands. Our cell-free binding assay system will allow for testing of models of receptor-ligand complexes prior to actual solution of crystal structures.

Arkadia amplifies TGF-beta superfamily signalling through degradation of Smad7

Arkadia was originally identified as a protein that enhances signalling activity of Nodal and induces mammalian nodes during early embryogenesis; however, the mechanisms by which Arkadia affects transforming growth factor-beta (TGF-beta) superfamily signalling have not been determined. Here we show that Arkadia is widely expressed in mammalian tissues, and that it enhances both TGF-beta and bone morphogenetic protein (BMP) signalling. Arkadia physically interacts with inhibitory Smad, Smad7, and induces its poly-ubiquitination and degradation. In contrast to Smurf1, which interacts with TGF-beta receptor complexes through Smad7 and degrades them, Arkadia fails to associate with TGF-beta receptors. In contrast to Smad7, expression of Arkadia is down-regulated by TGF-beta. Silencing of the Arkadia gene resulted in repression of transcriptional activities induced by TGF-beta and BMP, and accumulation of the Smad7 protein. Arkadia may thus play an important role as an amplifier of TGF-beta superfamily signalling under both physiological and pathological conditions.

HIV-Infected Lymphocytes Regulate Fibronectin Synthesis by TGFβ1 Secretion

Alterations in lymph node architecture occur with HIV infection and contribute to immunological derangements. We previously showed that matrix fibronectin stabilized HIV and increased HIV infection of PBL. We showed increased fibronectin deposition in lymph nodes of HIV-infected patients. However, we did not detect a difference in fibronectin synthesis between uninfected and infected PBL. Therefore, we hypothesized that interactions of HIV-infected cells with fibroblasts resulted in increased fibronectin deposition. We detected increased fibronectin deposition by immunofluorescence on fibroblasts cocultured with HIV-infected PBL. We also found a 6-fold increase in fibronectin mRNA levels in fibroblasts cocultured with HIV-infected PBL by real-time PCR. Furthermore, when HIV-infected PBL were added to reporter fibroblasts stably transfected with a fibronectin promoter, we found a 1.5- to 2-fold increase in promoter activity. Since conditioned medium from HIV-infected PBL also increased fibronectin promoter activity, we hypothesized that a soluble factor such as TGFbeta was responsible for increased fibronectin secretion. Pretreatment of supernatant from HIV-infected PBL with a neutralizing Ab to TGFbeta1 abrogated the increased fibronectin promoter activity. We confirmed that HIV-infected PBL produced increased TGFbeta1 by ELISA. Using Mv1Lu reporter cells, we found a 2- to 3-fold increase in biologically active TGFbeta in supernatants of HIV-infected PBL. Finally, we determined that HIV infection did not change the percentage of active TGFbeta. Our data suggest that HIV-infected lymphocytes indirectly contribute to lymph node remodeling by secretion of TGFbeta1, which increases fibronectin synthesis by fibroblasts.