How cells read TGF-beta signals

Transforming growth factor beta--a mediator of immune deviation in seminal plasma

TGFbeta is a potent immune deviating agent, driving active forms of immune tolerance in peripheral tissues through effects on the induction and resolution of inflammatory responses and phenotype skewing in antigen-presenting cells and lymphocytes. The TGFbeta content of seminal plasma from human, rodent and livestock species is amongst the highest measured in biological fluids. The seminal vesicle gland is the principal source of TGFbeta in the semen of mice, where its synthesis is regulated by testosterone. At insemination, seminal TGFbeta is deposited in the female tract and is activated by acidic vaginal pH, enzymes of male or female tract origin, or through cleavage-independent processes involving conformational change after interaction with epithelial cell docking proteins. Seminal TGFbeta has been shown to be a principal stimulating agent in the post-coital inflammatory response, and is likely to be essential for induction of immune tolerance to seminal antigens. As well as preventing aberrant immunity to spermatozoa, these events are implicated in priming an appropriate female immune response to embryo implantation, since many seminal antigens are shared by the conceptus. The cascade of immunological events elicited by seminal TGFbeta may therefore explain epidemiological observations linking acute and cumulative exposure to semen with successful placental development and pregnancy outcome. Depending on whether the female tract has evolved mechanisms to discriminate seminal antigens from opportunistic pathogens, there may be a detrimental cost of seminal TGFbeta in inhibiting protective immunity to agents of sexually transmitted disease including HIV. A better understanding of the significance and role of TGFbeta in semen will facilitate development of novel therapies for immune-based infertility disorders.

Reconstitution of Lethally Irradiated Adult Mice with Dominant Negative TGF-β Type II Receptor-Transduced Bone Marrow Leads to Myeloid Expansion and Inflammatory Disease

TGF-beta regulation of immune homeostasis has been investigated in the context of cytokine knockout (TGF-beta null) mice, in which particular TGF-beta isoforms are disrupted throughout the entire organism, as well as in B and T cell-specific transgenic models, but to date the immunoregulatory effects of TGF-beta have not been addressed in the context of an in vivo mouse model in which multi-isoform TGF-beta signaling is abrogated in multiple leukocyte lineages while leaving nonhemopoietic tissue unaffected. Here we report the development of a murine model of TGF-beta insensitivity limited to the hemopoietic tissue of adult wild-type C57BL/6 mice based on retroviral-mediated gene transfer of a dominant negative TGF-beta type II receptor targeting murine bone marrow. Unlike the lymphoproliferative syndrome observed in TGF-beta1-deficient mice, the disruption of TGF-beta signaling in bone marrow-derived cells leads to dramatic expansion of myeloid cells, primarily monocytes/macrophages, and is associated with cachexia and mortality in lethally irradiated mice reconstituted with dominant negative receptor-transduced bone marrow. Surprisingly, there was a notable absence of T cell expansion in affected animals despite the observed differentiation of most cells in the T cell compartment to a memory phenotype. These results indicate not only that TGF-beta acts as a negative regulator of immune function, but that lack of functional TGF-beta signaling in the myeloid compartment of adult mice may trigger suppression of lymphocytes, which would otherwise proliferate when rendered insensitive to TGF-beta.

A high-throughput induction gene trap approach defines C4ST as a target of BMP signaling

Here we describe a novel gene trap protocol to screen for target genes that are regulated during inductive events in undifferentiated and differentiated mouse embryonic stem cells. This approach integrates several features that allows in vitro screening of large numbers of gene trap clones prior to generating lines of mutant mice. Moreover, targets of spatially and temporally restricted signaling pathways can be analyzed by screening undifferentiated ES cells versus ES cells differentiated into embryoid bodies. We employed this protocol to screen 1920 gene trap lines to identify targets and mediators of signaling through three growth factors of the TGFbeta superfamily--BMP2, activin and nodal. We identified two genes that are induced by BMP2 in a differentiation-dependent manner. One of the genes encodes for Chondroitin-4-sulfotransferase and displays a highly specific temporal and spatial expression pattern during mouse embryogenesis. These results demonstrate the feasibility of a high-throughput gene trap approach as a means to identify mediators and targets of multiple growth factor signaling pathways that function during different stages of development.

TGF-beta signaling in renal disease

Since discovery over a decade ago of a role for the cytokine TGF-beta as key mediator of glomerular and tubulointerstitial pathobiology in chronic kidney diseases, studies of TGF-beta signaling in the kidney have focused on the molecular biology of fibrogenesis. In recent years, glomerular and tubular epithelial cell apoptosis and cellular transdifferentiation have been proposed as putative primary pathomechanisms that may underlie progression of renal disease. This review describes evidence in support of nonlinear models and functional roles of TGF-beta signaling in mediating apoptosis and epithelial-to-mesenchymal transdifferentiation (EMT) in chronic progressive renal disease. Emphasis is placed on cell context-dependent models of TGF-beta signaling providing a conceptual framework to consolidate seemingly distinct pathomechanisms of progression of glomerular and tubulointerstitial disease.

Activins, inhibins, and follistatins: from endocrinology to signaling. A paradigm for the new millennium

It has been 70 years since the name inhibin was used to describe a gonadal factor that negatively regulated pituitary hormone secretion. The majority of this period was required to achieve purification and definitive characterization of inhibin, an event closely followed by identification and characterization of activin and follistatin (FS). In contrast, the last 15-20 years saw a virtual explosion of information regarding the biochemistry, physiology, and biosynthesis of these proteins, as well as identification of activin receptors, and a unique mechanism for FS action-the nearly irreversible binding and neutralization of activin. Many of these discoveries have been previously summarized; therefore, this review will cover the period from the mid 1990s to present, with particular emphasis on emerging themes and recent advances. As the field has matured, recent efforts have focused more on human studies, so the endocrinology of inhibin, activin, and FS in the human is summarized first. Another area receiving significant recent attention is local actions of activin and its regulation by both FS and inhibin. Because activin and FS are produced in many tissues, we chose to focus on a few particular examples with the most extensive experimental support, the pituitary and the developing follicle, although nonreproductive actions of activin and FS are also discussed. At the cellular level, it now seems that activin acts largely as an autocrine and/or paracrine growth factor, similar to other members of the transforming growh factor beta superfamily. As we discuss in the next section, its actions are regulated extracellularly by both inhibin and FS. In the final section, intracellular mediators and modulators of activin signaling are reviewed in detail. Many of these are shared with other transforming growh factor beta superfamily members as well as unrelated molecules, and in a number of cases, their physiological relevance to activin signal propagation remains to be elucidated. Nevertheless, taken together, recent findings suggest that it may be more appropriate to consider a new paradigm for inhibin, activin, and FS in which activin signaling is regulated extracellularly by both inhibin and FS whereas a number of intracellular proteins act to modulate cellular responses to these activin signals. It is therefore the balance between activin and all of its modulators, rather than the actions of any one component, that determines the final biological outcome. As technology and model systems become more sophisticated in the next few years, it should become possible to test this concept directly to more clearly define the role of activin, inhibin, and FS in reproductive physiology.

Identification of a BMP-responsive element inId1, the gene for inhibition of myogenesis

BACKGROUND

Bone morphogenetic protein-2 (BMP-2) stimulates osteoblast differentiation, but inhibits myogenic differentiation in C2C12 myoblasts. BMP-2 induces transcription of Id1, an inhibitor for myogenesis, within 1 h in the cells. To examine the molecular mechanism of the action of BMP-2, we analysed a BMP-2-responsive element (BRE) in the 5' flanking region of the human Id1 gene.

RESULTS

A GC-rich region between -985 bp and -957 bp of the human Id1 gene was identified as a BRE. The BRE containing promoter activity was stimulated by BMP-2 or by constitutively active BMP receptors (BMPR-IA and BMPR-IB). The stimulation was blocked by co-transfecting with dominant negative BMPR-IA or Smad7. A unique DNA-protein complex was induced in response to BMP-2 on the BRE. The complex induced by BMP-2 contained Smad1 and Smad4, possibly as a complex of both Smads. BMP-2 failed to stimulate the expression of Id1 mRNA in Smad4-deficient cells. Over-expression of Smad4, but not Smad1, stimulated the Id1 reporter activity and the expression of endogenous Id1 mRNA in Smad4-deficient cells.

CONCLUSION

Signalling of BMP-2 to stimulate the expression of Id1 would be transduced by BMPR-IA and mediated by Smad1 and Smad4, both of which form a complex on the 29 bp GC-rich element.

Regulation of TGF-beta ligand and receptor expression in neonatal rat lungs exposed to chronic hypoxia

Long-term effects of hypoxia are largely due to its modulatory effects on proliferation and differentiation of epithelial and endothelial cells, processes also regulated by the transforming growth factor (TGF)-beta system. We investigated the effects of hypoxia on the TGF-beta system in rat lungs from different developmental stages. Sprague-Dawley rats were exposed to 9.5% oxygen during either the first 2 wk of life or adulthood. Analysis revealed an arrest of alveolarization in hypoxic postnatal day 14 rats. Bioactive TGF-beta levels in bronchoalveolar lavage fluid were increased in these animals, and Western blot analysis revealed upregulation of TGF-beta receptor (TbetaR) I and II. None of these changes was observed in hypoxic adults. Hypoxia did, however, lead to decreased expression of TbetaRIII in both postnatal day 14 and adult rats. Immunohistochemical analysis localized TbetaRI-III predominantly to bronchiolar and alveolar epithelium; these patterns did not change with hypoxia. Thus we observed changes in TGF-beta activity and TbetaR isotype expression in rat lung that parallel the arrest in alveolarization seen with chronic hypoxia in early development. These alterations may partly explain the morphological changes observed in hypoxia.

Transforming growth factor-beta1-modulated cerebral gene expression

Transforming growth factor-beta1 (TGF-beta1) plays a central role in the response of the brain to different types of injury. Increased TGF-beta1 has been found in the central nervous system of patients with acute or chronic disorders such as stroke or Alzheimer disease. To further define the molecular targets of TGF-beta1 in cerebral tissues, a selection of high-density cDNA arrays was used to characterize the mRNA expression profile of 7,000 genes in transgenic mice overexpressing TGF-beta1 from astrocytes as compared with the wild-type line. Selected findings were further evaluated by reverse transcription-polymerase chain reactions from independent transgenic and wild-type mice. Furthermore, the expression pattern of seven selected genes such as Delta-1, CRADD, PRSC-1, PAI-1, Apo-1/Fas, CTS-B, and TbetaR-II were confirmed in either cultured cortical neurons or astrocytes following TGF-beta1 treatment. The authors' observations enlarge the repertoire of known TGF-beta1-modulated genes and their possible involvement in neurodegenerative processes.

Interaction of FGF, Ihh/Pthlh, and BMP signaling integrates chondrocyte proliferation and hypertrophic differentiation

Mutations in fibroblast growth factor (FGF) receptor 3 lead to the human dwarfism syndrome achondroplasia. Using a limb culture system, we have analyzed the role of FGF signaling and its interaction with the Ihh/Pthlh and BMP pathways in regulating chondrocyte differentiation. In contrast to previous suggestions, we demonstrate that FGF signaling accelerates both the onset and the pace of hypertrophic differentiation. We furthermore found that FGF and BMP signaling act in an antagonistic relationship regulating chondrocyte proliferation, Ihh expression, and the process of hypertrophic differentiation. Importantly, BMP signaling rescues the reduced domains of proliferating and hypertrophic chondrocytes in a mouse model for achondroplasia. We propose a model in which the balance of BMP and FGF signaling adjusts the pace of the differentiation process to the proliferation rate.

Extracellular and cytoplasmic domains of endoglin interact with the transforming growth factor-beta receptors I and II

Endoglin is an auxiliary component of the transforming growth factor-beta (TGF-beta) receptor system, able to associate with the signaling receptor types I (TbetaRI) and II (TbetaRII) in the presence of ligand and to modulate the cellular responses to TGF-beta1. Endoglin cannot bind ligand on its own but requires the presence of the signaling receptors, supporting a critical role for the interaction between endoglin and TbetaRI or TbetaRII. This study shows that full-length endoglin interacts with both TbetaRI and TbetaRII, independently of their kinase activation state or the presence of exogenous TGF-beta1. Truncated constructs encoding either the extracellular or the cytoplasmic domains of endoglin demonstrated that the association with the signaling receptors occurs through both extracellular and cytoplasmic domains. However, a more specific mapping revealed that the endoglin/TbetaRI interaction was different from that of endoglin/TbetaRII. TbetaRII interacts with the amino acid region 437-558 of the extracellular domain of endoglin, whereas TbetaRI interacts not only with the region 437-558 but also with the protein region located between amino acid 437 and the N terminus. Both TbetaRI and TbetaRII interact with the cytoplasmic domain of endoglin, but TbetaRI only interacts when the kinase domain is inactive, whereas TbetaRII remains associated in its active and inactive forms. Upon association, TbetaRI and TbetaRII phosphorylate the endoglin cytoplasmic domain, and then TbetaRI, but not TbetaRII, kinase dissociates from the complex. Conversely, endoglin expression results in an altered phosphorylation state of TbetaRII, TbetaRI, and downstream Smad proteins as well as a modulation of TGF-beta signaling, as measured by the reporter gene expression. These results suggest that by interacting through its extracellular and cytoplasmic domains with the signaling receptors, endoglin might affect TGF-beta responses.

Up-regulated transcriptional repressors SnoN and Ski bind Smad proteins to antagonize transforming growth factor-beta signals during liver regeneration

Transforming growth factor-beta (TGF-beta) functions as an antiproliferative factor for hepatocytes. However, for unexplained reasons, hepatocytes become resistant to TGF-beta signals and can proliferate despite the presence of TGF-beta during liver regeneration. TGF-beta is up-regulated during liver regeneration, although it is not known whether it is active or latent. TGF-beta activity may be examined by assessing Smad activation, a downstream signaling pathway. Smad pathway activation during liver regeneration induced by partial hepatectomy or CC4 injury was examined by assessing the levels of phospho-Smad2 and Smad2-Smad4 complexes. We found that Smad proteins were slightly activated in quiescent liver, but that their activation was further enhanced in regenerating liver. Interestingly, TGF-beta/Smad pathway inhibitors (SnoN and Ski) were up-regulated during regeneration, and notably, SnoN was induced mainly in hepatocytes. SnoN and Ski are transcriptional repressors that may render some cells resistant to TGF-beta via binding Smad proteins. Complexes between SnoN, Ski, and the activated Smad proteins were detected from 2 to 120 h during the major proliferative phase in regenerating liver. Inhibitory complexes decreased after liver mass restitution (5-15 days), suggesting that persistently activated Smad proteins might participate in returning the liver to a quiescent state. Our data show that active TGF-beta/Smad signals are present during regeneration and suggest that SnoN/Ski induction might explain hepatocyte resistance to TGF-beta during the proliferative phase.

A component of the ARC/Mediator complex required for TGF beta/Nodal signalling

The transforming growth factor beta (TGF beta) family of cytokines, including Nodal, Activin and bone morphogenetic protein (BMP), have essential roles in development and tumorigenesis. TGF beta molecules activate the Smad family of signal transducers, which form complexes with specific DNA-binding proteins to regulate gene expression. Two discrete Smad-dependent signalling pathways have been identified: TGF beta, Activin and Nodal signal via the Smad2 (or Smad3)-Smad4 complex, whereas BMP signals via the Smad1-Smad4 complex. How distinct Smad complexes regulate specific gene expression is not fully understood. Here we show that ARC105, a component of the activator-recruited co-factor (ARC) complex or the metazoan Mediator complex, is essential for TGF beta/Activin/Nodal/Smad2/3 signal transduction. Expression of ARC105 stimulates Activin/Nodal/Smad2 signalling in Xenopus laevis embryos, inducing axis duplication and mesendoderm differentiation, and enhances TGF beta response in human cells. Depletion of ARC105 inhibits TGF beta/Activin/Nodal/Smad2/3 signalling and Xenopus axis formation, but not BMP/Smad1 signalling. ARC105 protein binds to Smad2/3-Smad4 in response to TGF beta and is recruited to Activin/Nodal-responsive promoters in chromatin in a Smad2-dependent fashion. Thus ARC105 is a specific and key ARC/Mediator component linking TGF beta/Activin/Nodal/Smad2/3 signalling to transcriptional activation.

Smad7: a new key player in TGF-beta-associated disease

Smad7 is a major inhibitory regulator of transforming growth factor (TGF)-beta signaling. Smad7 expression is induced by TGF-beta itself and other signaling pathways, indicating a key role for Smad7 in feedback or cross-talk control of TGF-beta signaling. Recent reports have implicated Smad7 as a crucial regulator of TGF-beta activity in human disease; aberrant expression of Smad7 is involved in inflammatory bowel disease and scleroderma. Thus, modulation of Smad7 expression could provide a novel therapeutic basis for TGF-beta-associated disorders.

Resistance to transforming growth factor-beta occurs in the presence of normal Smad activation

BACKGROUND

Resistance to the growth inhibitory actions of transforming growth factor-beta (TGF-beta) is common in human cancers. This resistance can be a result of decreased expression of TGF-beta receptors. Downregulation of c-Myc by TGF-beta is critical for TGF-beta-mediated growth inhibition. In this study we hypothesized that decreased TGF-beta receptor expression leads to reduced Smad signaling and overexpression of c-Myc in intestinal epithelial (RIE) and transformed intestinal epithelial cells (RIE-Tr) cells.

METHODS

RIE (TGF-beta-sensitive) and RIE-Tr (TGF-beta-resistant) cells were treated with and without fetal bovine serum and TGF-beta. Western blot analysis was performed to detect levels of c-Myc, Smad2, Smad4, and phosphorylated Smad2 in RIE and RIE-Tr cells. Smad complex formation was analyzed by immunoprecipitation-coupled Western blotting.

RESULTS

c-Myc is overexpressed in RIE-Tr cells. TGF-beta-mediated downregulation of c-Myc is abrogated in RIE-Tr cells. Smad expression and activation is normal in RIE-Tr cells. We found that Smad2, Smad4, and Smad6 expression remained constant in RIE and RIE-Tr cells with or without serum or TGF-beta treatment. In addition, TGF-beta induced similar Smad2 phosphorylation and Smad complex formation in both RIE and RIE-Tr cells.

CONCLUSIONS

Our data demonstrate that Smad signaling is preserved in the face of decreased TGF-beta receptor levels. We also demonstrate that Smad signaling is not sufficient for TGF-beta-mediated c-Myc repression.

TGF-beta receptor-activated p38 MAP kinase mediates Smad-independent TGF-beta responses

Through the action of its membrane-bound type I receptors, transforming growth factor-beta (TGF-beta) elicits a wide range of cellular responses that regulate cell proliferation, differentiation and apoptosis. Many of the signaling responses induced by TGF-beta are mediated by Smad proteins, but certain evidence has suggested that TGF-beta can also signal independently of Smads. We found in mouse mammary epithelial (NMuMG) cells, which respond to TGF-beta treatment in multiple ways, that TGF-beta-induced activation of p38 MAP kinase is required for TGF-beta-induced apoptosis, epithelial-to-mesenchymal transition (EMT), but not growth arrest. We further demonstrated that activation of p38 is independent of Smads using a mutant type I receptor, which is incapable of activating Smads but still retains the kinase activity. This mutant receptor is sufficient to activate p38 and cause NMuMG cells to undergo apoptosis. However, it is not sufficient to induce EMT. These results indicate that TGF-beta receptor signals through multiple intracellular pathways and provide first-hand biochemical evidence for the existence of Smad-independent TGF-beta receptor signaling.

Transforming growth factor-beta stimulates parathyroid hormone-related protein and osteolytic metastases via Smad and mitogen-activated protein kinase signaling pathways

Transforming growth factor (TGF)-beta promotes breast cancer metastasis to bone. To determine whether the osteolytic factor parathyroid hormone-related protein (PTHrP) is the primary mediator of the tumor response to TGF-beta, mice were inoculated with MDA-MB-231 breast cancer cells expressing a constitutively active TGF-beta type I receptor. Treatment of the mice with a PTHrP-neutralizing antibody greatly decreased osteolytic bone metastases. There were fewer osteoclasts and significantly decreased tumor area in the antibody-treated mice. TGF-beta can signal through both Smad and mitogen-activated protein (MAP) kinase pathways. Stable transfection of wild-type Smad2, Smad3, or Smad4 increased TGF-beta-stimulated PTHrP secretion, whereas dominant-negative Smad2, Smad3, or Smad4 only partially reduced TGF-beta-stimulated PTHrP secretion. When the cells were treated with a variety of protein kinases inhibitors, only specific inhibitors of the p38 MAP kinase pathway significantly reduced both basal and TGF-beta-stimulated PTHrP production. The combination of Smad dominant-negative blockade and p38 MAP kinase inhibition resulted in complete inhibition of TGF-beta-stimulated PTHrP production. Furthermore, TGF-beta treatment of MDA-MB-231 cells resulted in a rapid phosphorylation of p38 MAP kinase. Thus, the p38 MAP kinase pathway appears to be a major component of Smad-independent signaling by TGF-beta and may provide a new molecular target for anti-osteolytic therapy.

Single cysteine to tyrosine transition inactivates the growth inhibitory function of Piedmontese myostatin

Myostatin, a member of the transforming growth factor-beta superfamily, is a secreted growth factor that is proteolytically processed to give COOH-terminal mature myostatin and NH2-terminal latency-associated peptide in myoblasts. Piedmontese cattle are a heavy-muscled breed that express a mutated form of myostatin in which cysteine (313) is substituted with tyrosine. Here we have characterized the biology of this mutated Piedmontese myostatin. Northern and Western analyses indicate that there is increased expression of myostatin mRNA and precursor myostatin protein in the skeletal muscle of Piedmontese cattle. In contrast, a decrease in mature myostatin was observed in Piedmontese skeletal muscle. However, there is no detectable change in the circulatory levels of mature myostatin in Piedmontese cattle. Myoblast proliferation assay performed with normal and Piedmontese myostatin indicated that mature wild-type myostatin protein inhibited the proliferation of C2C12 myoblasts. Piedmontese myostatin, by contrast, failed to inhibit myoblast proliferation. In addition, when added in molar excess, Piedmontese myostatin acted as a potent "competitive inhibitor" molecule. These results indicate that, in Piedmontese myostatin, substitution of cysteine with tyrosine results in the distortion of the "cystine knot" structure and a loss of biological activity of the myostatin. This mutation also appears to affect either processing or stability of mature myostatin without altering the secretion of myostatin.

Mechanisms of TGF-beta signaling in regulation of cell growth and differentiation

Transforming growth factor beta (TGF-beta) is a secreted protein that regulates proliferation, differentiation and death of various cell types. All immune cell lineages, including B, T and dendritic cells as well as macrophages, secrete TGF-beta, which negatively regulates their proliferation, differentiation and activation by other cytokines. Thus, TGF-beta is a potent immunosuppressor and perturbation of TGF-beta signaling is linked to autoimmunity, inflammation and cancer. Regulation of cell proliferation and differentiation by TGF-beta is a topic of great basic and clinical importance. We summarize our work on the growth inhibitory pathway downstream of TGF-beta, which is triggered by receptor serine/threonine kinases at the cell surface and downstream effectors of the Smad family. Activated Smads regulate transcription of target genes, including cell cycle inhibitors such as p21, which mediate the anti-proliferative response and partially explain the tumor suppressive action of the TGF-beta pathway. We have described a molecular mechanism of regulation of the p21 gene by Smads and transcription factor Sp1. At late stages of tumor progression, TGF-beta promotes tumorigenesis via suppression of the immune system and changes in cell differentiation of epithelial tumor cells, a phenomenon termed epithelial to mesenchymal transdifferentiation (EMT). We review our work on the role of the Smad pathway in controlling EMT. In conclusion, the molecular pathways that describe the anti-proliferative and transdifferentiating effects of TGF-beta in epithelial cells have been uncovered to great molecular detail; a future challenge will be to test their generality in other systems, including the immune system.

Molecular pathogenesis of granulosa cell tumours

Granulosa cell tumours (GCT) of the ovary arise from granulosa cells of the ovary on morphological, biochemical and molecular criteria. In order to understand the molecular pathogenesis of these tumours better we have sought to define their molecular phenotype, to identify activating mutations of the FSH-signalling pathway, to characterise their estrogen receptor expression and to explore the hypothesis that GCT may be resistant to inhibin. The pattern of gene expression observed in GCT suggests a phenotype which is similar to that of late preovulatory granulosa cells which would be consistent with activation of the FSH receptor signalling pathway, however, there is no evidence for activating mutations of either the FSH receptor or the associated trimeric G-proteins. Estrogen receptor beta is abundantly expressed in GCT. The various subunits and isoforms of the activin-inhibin receptor are expressed in GCT. These observations provide a basis for future studies of GCT, including further characterisation of signalling pathways known to be important in the regulation of granulosa cell growth and differentiation.

Transforming growth factor-beta 1 increases bad phosphorylation and protects neurons against damage

Despite the characterization of neuroprotection by transforming growth factor-beta1 (TGF-beta1), the signaling pathway mediating its protective effect is unclear. Bad is a proapoptotic member of the Bcl-2 family and is inactivated on phosphorylation via mitogen-activated protein kinase (MAPK). This study attempted to address whether MAPK signaling and Bad phosphorylation were influenced by TGF-beta1 and, furthermore, whether these two events were involved in the antiapoptotic effect of TGF-beta1. We found a gradual activation of extracellular signal-regulated kinase 1/2 (Erk1/2) and MAPK-activated protein kinase-1 (also called Rsk1) and a concomitant increase in Bad phosphorylation at Ser(112) in mouse brains after adenovirus-mediated TGF-beta1 transduction under nonischemic and ischemic conditions induced by transient middle cerebral artery occlusion. Consistent with these effects, the ischemia-induced increase in Bad protein level and caspase-3 activation were suppressed in TGF-beta1-transduced brain. Consequently, DNA fragmentation, ischemic lesions, and neurological deficiency were significantly reduced. In cultured rat hippocampal cells, TGF-beta1 inhibited the increase in Bad expression caused by staurosporine. TGF-beta1 concentration- and time-dependently activated Erk1/2 and Rsk1 accompanied by an increase in Bad phosphorylation. These effects were blocked by U0126, a mitogen-activated protein kinase/Erk kinase 1/2 inhibitor, suggesting an association between Bad phosphorylation and MAPK activation. Notably, U0126 and a Rsk1 inhibitor (Ro318220) abolished the neuroprotective activity of TGF-beta1 in staurosporine-induced apoptosis, indicating that activation of MAPK is necessary for the antiapoptotic effect of TGF-beta1 in cultured hippocampal cells. Together, we demonstrate that TGF-beta1 suppresses Bad expression under lesion conditions, increases Bad phosphorylation, and activates the MAPK/Erk pathway, which may contribute to its neuroprotective activity.

Disruption of transforming growth factor beta signaling by a novel ligand-dependent mechanism

Transforming growth factor (TGF)-beta is the prototype in a family of secreted proteins that act in autocrine and paracrine pathways to regulate cell development and function. Normal cells typically coexpress TGF-beta receptors and one or more isoforms of TGF-beta, thus the synthesis and secretion of TGF-beta as an inactive latent complex is considered an essential step in regula-ting the activity of this pathway. To determine whether intracellular activation of TGF-beta results in TGF-beta ligand-receptor interactions within the cell, we studied pristane-induced plasma cell tumors (PCTs). We now demonstrate that active TGF-beta1 in the PCT binds to intracellular TGF-beta type II receptor (TbetaRII). Disruption of the expression of TGF-beta1 by antisense TGF-beta1 mRNA restores localization of TbetaRII at the PCT cell surface, indicating a ligand-induced impediment in receptor trafficking. We also show that retroviral expression of a truncated, dominant-negative TbetaRII (dnTbetaRII) effectively competes for intracellular binding of active ligand in the PCT and restores cell surface expression of the endogenous TbetaRII. Analysis of TGF-beta receptor-activated Smad2 suggests the intracellular ligand-receptor complex is not capable of signaling. These data are the first to demonstrate the formation of an intracellular TGF-beta-receptor complex, and define a novel mechanism for modulating the TGF-beta signaling pathway.

The effect of TGF-beta receptor binding peptides on smooth muscle cells

TGF-beta1 is a potent regulator of vascular smooth muscle cell (VSMC) proliferation, migration, and extracellular matrix (ECM) synthesis. In this study, we selected two peptides, IM-1 and IM-2, that bind to the TGF-beta type II receptor (TGF-beta RII) using phage display. IM-1 and IM-2 bind to the TGF-beta RII, with a K(d) of 1 microM. Like TGF-beta, IM-1 induced VSMC chemotaxis and PAI-1 mRNA expression, as determined using Boyden chambers and real time quantitative PCR. In contrast, IM-2 had no effect on VSMC chemotaxis or PAI-1 induction. Induction of ECM synthesis, involving proteins such as osteopontin and alpha-smooth muscle actin, was determined by ELISA. Osteopontin expression was inhibited by both peptides, but TGF-beta-induced alpha-smooth muscle actin expression could only be inhibited by IM-1. In conclusion, IM-1 activity on VSMC is agonistic with TGF-beta, except for ECM synthesis, whereas the IM-2 peptide is antagonistic for some examined TGF-beta functions.

Regulatory mechanisms of osteoblast and osteoclast differentiation

Bone is continuously destroyed and reformed to maintain constant bone volume and calcium homeostasis in vertebrates throughout their lives. Osteoblasts and osteoclasts are specialized cells responsible for bone formation and resorption, respectively. Recent developments in bone cell biology have greatly changed our conceptions of the regulatory mechanisms of the differentiation of osteoblasts and osteoclasts. Bone morphogenetic proteins (BMPs) play critical roles in osteoblast differentiation. The discovery of Smad-mediated signals revealed the precise functions of BMPs in osteoblast differentiation. Transcription factors, Runx2 and Osterix, are found to be essential molecules for inducing osteoblast differentiation, as indicated by the fact that both Runx2-null mice and Osterix-null mice have neither bone tissue nor osteoblasts. Smad transcriptional factors are shown to interact with other transcription regulators, including Runx2. Also, the recent discovery of receptor activator of NF-kappaB ligand (RANKL)-RANK interaction confirms the well-known hypothesis that osteoblasts play an essential role in osteoclast differentiation. Osteoblasts express RANKL as a membrane-associated factor. Osteoclast precursors that express RANK, a receptor for RANKL, recognize RANKL through the cell-cell interaction and differentiate into osteoclasts. Recent studies have shown that lipopolysaccharide and inflammatory cytokines such as tumor necrosis factor receptor-alpha and interleukin I directly regulate osteoclast differentiation and function through a mechanism independent of the RANKL-RANK interaction. Transforming growth factor-beta super family members and interferon-gamma are also shown to be important regulators in osteoclastogenesis. These findings have opened new areas for exploring the molecular mechanisms of osteoblast and osteoclast differentiation.

A changing morphogen gradient is interpreted by continuous transduction flow

In vertebrate development, most signalling factors behave as morphogens, eliciting divergent cell fates according to their concentration. We ask how cells interpret morphogen concentration as it changes during the establishment of a gradient. Using dissociated blastula cells of Xenopus exposed to activin for only 10 minutes, we have followed the phosphorylation of tagged Smad2, the principal activin transducer, from a cytoplasmic pool to the nucleus in real time. We show that a changing concentration of extracellular activin is rapidly and continuously transduced to provide a corresponding nuclear concentration of Smad2, even though gene response may be delayed for several hours. Nuclear Smad2 concentration changes up as the extracellular concentration of activin increases. We conclude that cells interpret a changing extracellular concentration by maintaining a continuous flow of activated transducer from a large cytoplasmic pool to the nucleus where it is degraded. The volume of this flow determines the steady state concentration of Smad2 in the nucleus and this is used by cells to interpret extracellular morphogen concentration.

Approaches to studying cellular signaling: a primer for morphologists

Many research projects will lead to understanding tissue and/or cell responses to extracellular influences either from soluble factors or the surrounding extracellular matrix. These types of investigations will require the understanding of signal transduction. This particular cell biological field has literally exploded with information and new technical approaches in the past 10 years. This article is directed toward investigators interested in using these new approaches to study their systems. An overview of the general principles of signal transduction events including the types of receptors and intracellular signaling events is followed by an introduction to methods for visualizing signal transduction. This is followed by an introduction to biochemical analysis and an example of combining several approaches to understanding a tissue response to extracellular matrix stimulus.

Cardiotrophin-like cytokine induces astrocyte differentiation of fetal neuroepithelial cells via activation of STAT3

Cardiotrophin-like cytokine (CLC), also known as novel neurotrophin-1/B cell stimulating factor-3 (NNT-1/BSF-3), is a recently identified member of the interleukin (IL)-6 family of cytokines that share gp130 as a signal-transducing receptor component. In this study, we demonstrate that CLC is expressed in fetal mouse neuroepithelial cells and has a potential to induce their astrocyte differentiation in a synergistic manner with bone-morphogenetic protein (BMP)-2, which is also expressed in the fetal mouse brain. CLC-stimulation led to promoter activation of the gene for an astrocyte marker, glial fibrillary acidic protein (GFAP), which was clearly inhibited by expression of a dominant negative form of a transcription factor, STAT3, or by introduction of a mutation in a single STAT3-binding site in the promoter, suggesting a critical role of STAT3 in the CLC-induced GFAP transcription. These results suggest that CLC plays a role in astrocyte differentiation via STAT3 activation within the developing brain.

Transforming growth factor-beta and insulin-like signalling pathways in parasitic helminths

The signal transduction pathways involved in regulating developmental arrest in the free-living nematode, Caenorhabditis elegans, are fairly well characterised. However, much less is known about how these processes may influence the developmental timing and maturation in helminth parasites. Here, we provide an overview of two signalling pathways implicated in the regulation of dauer larva formation in C. elegans, the insulin-like signalling pathway and the transforming growth factor-beta pathway, and explore what is known about these signalling pathways in a variety of parasitic helminths. Understanding the differences about how these pathways are affected by environmental cues in free-living versus parasitic species of helminths may provide insights into novel mechanisms for the control or prevention of helminth-induced disease.

Regulation of cell proliferation by Smad proteins

Transforming growth factor-beta (TGF-beta) family members which include TGF-betas, activins, and bone morphogenetic proteins (BMPs) regulate a broad spectrum of biological responses on a large variety of cell types. TGF-beta family members initiate their cellular responses by binding to distinct receptors with intrinsic serine/threonine kinase activity and activation of specific downstream intracellular effectors termed Smad proteins. Smads relay the signal from the cell membrane to the nucleus, where they affect the transcription of target genes. Smad activation, subcellular distribution, and stability have been found to be intricately regulated and a broad array of transcription factors have been identified as Smad partners. Important activities of TGF-beta are its potent anti-mitogenic and pro-apoptotic effects that, at least in part, are mediated via Smad proteins. Escape from TGF-beta/Smad-induced growth inhibition and apoptosis is frequently observed in tumors. Certain Smads have been found to be mutated in specific types of cancer and gene ablation of particular Smads in mice has revealed increased rate of tumorigenesis. In late stage tumors, TGF-beta has been shown to function as a tumor promoter. TGF-beta can stimulate the de-differentiation of epithelial cells to malignant invasive and metastatic fibroblastic cells. Interestingly, TGF-beta may mediate these effects directly on tumor cells via subverted Smad-dependent and/or Smad-independent pathways.

Loss of transgelin in breast and colon tumors and in RIE-1 cells by Ras deregulation of gene expression through Raf-independent pathways

Activated Ras but not Raf can transform RIE-1 and other epithelial cells, indicating the critical importance of Raf-independent effector function in Ras transformation of epithelial cells. To elucidate the nature of these Raf-independent activities, we utilized representational difference analysis to identify genes aberrantly expressed by Ras through Raf-independent mechanisms in RIE-1 cells. We identified a total of 22 genes, both known and novel, whose expression was either activated or abolished by Ras but not Raf. The genes up-regulated encode proteins involved in protein or DNA synthesis, regulation of protease activity, or ligand binding, whereas those genes down-regulated encode actin cytoskeletal-, extracellular matrix-, and gap junction-associated proteins, and transmembrane receptor- or cytokine-like proteins. These results suggest that a key function of Raf-independent signaling involves deregulation of gene expression. We further characterized transgelin as a gene whose expression was abolished by Ras. Transgelin was identified previously as a protein whose expression was lost in virally transformed cell lines. We show that this loss is regulated at the level of gene expression and that both Raf-dependent and Raf-independent pathways are required to cause Ras down-regulation of transgelin in RIE-1 cells, whereas Raf alone is sufficient to cause its loss in NIH 3T3 fibroblasts. We also found that Ras-dependent and Ras-independent mechanisms can cause the down-regulation of transgelin in human breast and colon carcinoma cells lines and patient-derived tumor samples. We conclude that loss of transgelin gene expression may be an important early event in tumor progression and a diagnostic marker for breast and colon cancer development.

Inhibition of transforming growth factor beta signaling and Smad-dependent activation of transcription by the Latent Membrane Protein 1 of Epstein-Barr virus

Inhibition of transforming growth factor beta (TGFbeta) signaling by the Epstein-Barr virus Latent Membrane Protein 1 (LMP1) may account, at least in part, for the oncogenic activity of LMP1. We found that LMP1 is a potent inhibitor of TGFbeta signaling and Smad-dependent activation of transcription in 293 epithelial cells and COS-7 fibroblasts. LMP1 strongly inhibited the uninduced and the Smad-inducible activity of the promoters of the human p21/WAF1/Cip1 gene and the mouse Smad7 gene. Inhibition of TGFbeta signaling and Smad-dependent activation of transcription by LMP1 was greatly reduced by deletion of both C-terminal activating regions 1 and 2 of LMP1 as well as by overexpression of a non-degradable form of IkappaB. In contrast, specific inhibitors of p38 kinase or MEK kinase did not reverse the inhibitory activity of LMP1. TGFbeta signaling was enhanced by overexpression of dominant negative forms of the LMP1 effectors TRAF2, NIK, and IKKbeta and was abolished by overexpression of p65/RelA or a p50/p65 fusion protein. Deletion of the transactivation domain of p65 abolished its inhibitory activity. Immunoblotting and immunofluorescence microscopy indicated that suppression of TGFbeta signaling and Smad transcriptional activity by LMP1 was not due to Smad degradation or cytoplasmic retention suggesting that LMP1 affects the nuclear function of Smad proteins. Our data are consistent with an essential role of NF-kappaB activation by LMP1 in the inhibition of TGFbeta signaling and Smad-mediated transcriptional responses.

DEC1 is a downstream target of TGF-beta with sequence-specific transcriptional repressor activities

To identify genes that mediate transforming growth factor-beta (TGF-beta) signaling, a colorectal cancer cell line that was sensitive to the growth inhibitory effects of this cytokine was created. We then determined the global gene expression profiles of these cells, and those of HaCaT human keratinocytes, in the presence and absence of TGF-beta. Of the several genes identified in this screen, DEC1 was of particular note in light of the rapidity and consistency of its induction and its potential biochemical activities. We identified a consensus DNA-binding site for DEC1 and showed that DEC1 could repress the transcription of a reporter containing this binding site in its promoter. Finally, both alleles of the DEC1 locus in HaCaT cells were inactivated through targeted homologous recombination. This approach revealed that DEC1 induction was not required for the growth inhibition mediated by TGF-beta in this line. However, DEC1 may function in concert with other signaling components to mediate certain biologic effects of TGF-beta.

Mice lacking Smad3 are protected against cutaneous injury induced by ionizing radiation

Transforming growth factor-beta (TGF-beta) plays a central role in the pathogenesis of inflammatory and fibrotic diseases, including radiation-induced fibrosis. We previously reported that mice null for Smad3, a key downstream mediator of TGF-beta, show accelerated healing of cutaneous incisional wounds with reduced inflammation and accumulation of matrix. To determine if loss of Smad3 decreases radiation-induced injury, skin of Smad3+/+ [wild-type (WT)] and -/- [knockout (KO)] mice was exposed to a single dose of 30 to 50 Gy of gamma-irradiation. Six weeks later, skin from KO mice showed significantly less epidermal acanthosis and dermal influx of mast cells, macrophages, and neutrophils than skin from WT littermates. Skin from irradiated KO mice exhibited less immunoreactive TGF-beta and fewer myofibroblasts, suggesting that these mice will have a significantly reduced fibrotic response. Although irradiation induced no change in the immunohistochemical expression of the TGF-beta type I receptor, the epidermal expression of the type II receptor was lost after irradiation whereas its dermal expression remained high. Primary keratinocytes and dermal fibroblasts prepared from WT and KO mice showed similar survival when irradiated, as did mice exposed to whole-body irradiation. These results suggest that inhibition of Smad3 might decrease tissue damage and reduce fibrosis after exposure to ionizing irradiation.

Identification of novel inhibitors of the transforming growth factor beta1 (TGF-beta1) type 1 receptor (ALK5)

Screening of our internal compound collection for inhibitors of the transforming growth factor beta1 (TGF-beta1) type I receptor (ALK5) identified several hits. Optimization of the dihydropyrroloimidazole hit 2 by introduction of a 2-pyridine and 3,4-methylenedioxyphenyl group gave 7, a selective ALK5 inhibitor. With this information, optimization of the triarylimidazole hit 8 gave the selective inhibitor 14, which inhibits TGF-beta1-induced fibronectin mRNA formation while displaying no measurable cytotoxicity in the 48 h XTT assay.

Identification of a candidate tumor-suppressor gene specifically activated during Ras-induced senescence

Normal cells display protective responses against oncogenes. Notably, oncogenic Ras triggers an irreversible proliferation arrest that is reminiscent of replicative senescence and that is considered a relevant tumor-suppressor mechanism. Here, we have used microarrayed filters to identify genes specifically upregulated in Ras-senescent human fibroblasts. Among the initial set of genes selected from the microarrays, we found the cell-cycle inhibitor p21(Cip1/Waf1), thus validating the potency of the screening to identify markers and mediators of Ras-senescence. A group of six genes, formed by those more highly upregulated during Ras-senescence, was analyzed in further detail to evaluate their specificity. In particular, we examined their expression in cells overexpressing Ras but rendered resistant to Ras-senescence by the viral oncoprotein E1a; also, we have studied their expression during replicative senescence, organismal aging, H(2)O(2)-induced senescence, and DNA damage. In this manner, we have identified a novel gene, RIS1 (for Ras-induced senescence 1), which is not upregulated in association to any of the above-mentioned processes, but exclusively during Ras-senescence. Furthermore, RIS1 is also upregulated by the transcriptional factor Ets2, which is a known mediator of Ras-induced senescence. Interestingly, RIS1 is located at chromosomal position 3p21.3 and, more specifically, it is included in a short segment of just 1 Mb previously defined by other investigators for its tumor-suppressor activity. In summary, we report the identification of a novel gene, RIS1, as a highly specific marker of Ras-induced senescence and a candidate tumor-suppressor gene.

Signal transduction and the control of gene expression

More than 2000 transcription factors are encoded in the human genome. Such proteins have often been classified according to common structural elements. But because transcription factors evolved in the service of biologic function, we propose an alternative grouping of eukaryotic transcription factors on the basis of characteristics that describe their roles within cellular regulatory circuits.

Molecular integration of casanova in the Nodal signalling pathway controlling endoderm formation

Endoderm originates from a large endomesodermal field requiring Nodal signalling. The mechanisms that ensure segregation of endoderm from mesoderm are not fully understood. We first show that the timing and dose of Nodal activation are crucial for endoderm formation and the endoderm versus mesoderm fate choice, because sustained Nodal signalling is required to ensure endoderm formation but transient signalling is sufficient for mesoderm formation. In zebrafish, downstream of Nodal signals, three genes encoding transcription factors (faust, bonnie and clyde and the recently identified gene casanova) are required for endoderm formation and differentiation. However their positions within the pathway are not completely established. In the present work, we show that casanova is the earliest specification marker for endodermal cells and that its expression requires bonnie and clyde. Furthermore, we have analysed the molecular activities of casanova on endoderm formation and found that it can induce endodermal markers and repress mesodermal markers during gastrulation, as well as change the fate of marginal blastomeres to endoderm. Overexpression of casanova also restores endoderm markers in the absence of Nodal signalling. In addition, casanova efficiently restores later endodermal differentiation in these mutants, but this process requires, in addition, a partial activation of Nodal signalling.

Regulation of nodal signalling and mesendoderm formation by TARAM-A, a TGFbeta-related type I receptor

Nodal signalling is essential for many developmental events during vertebrate development, including the establishment of left-right asymmetry, of dorsoventral axis of the central nervous system, and endoderm and mesoderm formation. The zebrafish TGFbeta-related type I receptor, TARAM-A (Tar), is expressed in the prospective mesendodermal territory and, when activated, can transfate early blastomeres into endoderm, suggesting that Nodal and Tar may represent similar signalling pathways. We have analysed the functional relationships between those two pathways in zebrafish. We first demonstrate that tar and the zebrafish nodal genes cyc and sqt functionally interact. We also show that a dominant-negative isoform of Tar, TarMR, interferes specifically with the function of Cyc and Sqt in vitro, but does not interfere with the function of BMP2, another TGFbeta-related molecule. TarMR interferes also with Nodal signalling in vivo since it enhances the phenotype of embryos with weakened Nodal signalling. Overexpression of tarMR in wild-type embryos interfered with the formation of endoderm-derived structures. Conversely, overexpression of tar enlarged the presumptive mesendodermal region at the onset of gastrulation. Together, our results point to Tar as an essential factor for endoderm formation and an important modulator of Nodal signalling, potentially representing one of the Nodal receptors. (c)2001 Elsevier Science.

Transforming growth factor-beta in T-cell biology

Strict control of T-cell homeostasis is required to permit normal immune responses and prevent undesirable self-targeted responses. Transforming growth factor-beta (TGF-beta) has been shown to have an essential role in that regulation. Owing to its broad expression, and inhibitory effects on multiple cell types of the immune system, TGF-beta regulation is complex. Through advances in cell-specific targeting of TGF-beta signalling in vivo, the role of TGF-beta in T-cell regulation has become clearer. Recent in vitro studies provide a better understanding of how TGF-beta regulates T-cell homeostasis, through multiple mechanisms involving numerous cell types.

Targeting growth factors to the kidney: myth or reality?

Growth factors and cytokines play a crucial role in the progression of renal diseases. A growing body of evidence has been obtained from experimental studies, suggesting that manipulation of the activity of growth factors and cytokines is a potential form of therapy for renal diseases. To preserve the renal function structure in progressive renal diseases, this approach is achieved by inhibition of apoptosis of renal intrinsic cells and by decrease in the fibrotic signal. Inhibition of transforming growth factor beta, platelet-derived growth factor, interleukin-1 and tumor necrosis factor alpha, and supplementation of hepatocyte growth factor, vascular endothelial growth factor and bone morphogenic protein-7 may be beneficial. Recent progress in therapeutic implements including humanized antibodies, chimeric soluble receptors, aptamers, antisense oligonucleotides, and gene therapy allow us to target the causal molecules. Administration of a combination of growth factors and cytokines is a potential therapeutic approach. Targeting signal transduction molecules and their co-factors and regulators is another possibility because the signals from various growth factors use a common pathway. Thus, targeting growth factors and cytokines in renal diseases could be a promising therapeutic approach.

Curcumin differentially regulates TGF-beta1, its receptors and nitric oxide synthase during impaired wound healing

Wound healing is a highly ordered process, requiring complex and coordinated interactions involving peptide growth factors of which transforming growth factor-beta (TGF-beta) is one of the most important. Nitric oxide is also an important factor in healing and its production is regulated by inducible nitric oxide synthase (iNOS). We have earlier shown that curcumin (diferuloylmethane), a natural product obtained from the plant Curcuma longa, enhances cutaneous wound healing in normal and diabetic rats. In this study, we have investigated the effect of curcumin treatment by topical application in dexamethasone-impaired cutaneous healing in a full thickness punch wound model in rats. We assessed healing in terms of histology, morphometry, and collagenization on the fourth and seventh days post-wounding and analyzed the regulation of TGF-beta1, its receptors type I (tIrc) and type II (tIIrc) and iNOS. Curcumin significantly accelerated healing of wounds with or without dexamethasone treatment as revealed by a reduction in the wound width and gap length compared to controls. Curcumin treatment resulted in the enhanced expression of TGF-beta1 and TGF-beta tIIrc in both normal and impaired healing wounds as revealed by immunohistochemistry. Macrophages in the wound bed showed an enhanced expression of TGF-beta1 mRNA in curcumin treated wounds as evidenced by in situ hybridization. However, enhanced expression of TGF-beta tIrc by curcumin treatment observed only in dexamethasone-impaired wounds at the 7th day post-wounding. iNOS levels were increased following curcumin treatment in unimpaired wounds, but not so in the dexamethasone-impaired wounds. The study indicates an enhancement in dexamethasone impaired wound repair by topical curcumin and its differential regulatory effect on TGF-beta1, it's receptors and iNOS in this cutaneous wound-healing model.

TGF-beta induces apoptosis through Smad-mediated expression of DAP-kinase

Transforming growth factor-beta (TGF-beta) and TGF-beta-related factors induce apoptosis in a variety of tissues; however, the mechanism underlying this induction is largely unknown. Here, we demonstrate that TGF-beta induces the expression of the death-associated protein kinase (DAP-kinase) as an immediate early response in cells that undergo apoptosis in response to TGF-beta. DAP-kinase is a positive mediator of apoptosis induced by certain cytokines and oncogenes. We show that the DAP-kinase promoter is activated by TGF-beta through the action of Smad2, Smad3 and Smad4. Overexpression of DAP-kinase triggers apoptosis in the absence of TGF-beta, whereas inhibition of DAP-kinase activity protects cells from TGF-beta-induced apoptosis, blocks TGF-beta-induced release of cytochrome c from mitochondria and prevents TGF-beta-induced dissipation of the mitochondrial membrane potential. Our findings indicate that DAP-kinase mediates TGF-beta-dependent apoptosis by linking Smads to mitochondrial-based pro-apoptotic events.

Meeting report: signaling schemes for TGF-beta

The transforming growth factor-beta (TGF-beta) superfamily of signaling molecules regulates many developmental processes in a range of organisms from worms to humans. Understanding the mechanisms by which they exert their repertoire of effects has required identification of the components of signaling pathways that they control. Roberts and Derynck focus on this aspect of TGF-beta biology in their review of a recent Federation of American Societies for Experimental Biology (FASEB) meeting on TGF-beta signaling and development and summarize current signaling paradigms and future prospects in TGF-beta signaling from the cell surface to the nucleus.