Nucleocytoplasmic shuttling of Smads 2, 3, and 4 permits sensing of TGF-beta receptor activity

Smad Signaling in the Rat Model of Monocrotaline Pulmonary Hypertension

Mutations in the bone morphogenetic protein receptor type II (BMPrII) gene have been implicated in the development of familial pulmonary artery hypertension (PAH). The function of BMP signal transduction within the pulmonary vasculature and the role BMPrII mutations have in the development of PAH are incompletely understood. We used the monocrotaline (MCT) model of PAH to examine alterations in Smad signal transduction pathways in vivo. Lungs harvested from Sprague-Dawley rats treated with a single 60-mg/kg intraperitoneal (IP) injection of MCT were compared to saline-treated controls 2 weeks following treatment. Smad 4 was localized by immunohistochemistry to endothelial nuclei of the intra-acinar vessels undergoing remodeling. Smad 4, common to both BMP and transforming growth factor β (TGFβ) signaling, and BMP-specific Smad 1 were significantly decreased in western blot from whole lungs of treated animals, while no change was found for TGFβ-specific Smad 2. MCT-treated rats also had increased expression of phosphorylated Smad 1 (P-Smad 1) but not phosphorylated Smad 2 (P-Smad 2). There was a decrease in the expression of the full BMPrII protein but not its short form variant in MCT-treated rat lungs. The type I receptor Alk1 had increased expression. Collectively, our data indicate that vascular remodeling in the MCT model is associated with alterations in BMP receptors and persistent endothelial Smad 1 signaling.

Cytokine-induced nuclear translocation of signaling proteins and their analysis using the inducible translocation trap system

Binding of cytokines to their specific receptors induces activation of signal transduction pathways, many of which involve nuclear translocation of signaling proteins. In this review, an overview of cytokine-induced nuclear translocation of signaling proteins is provided. In addition, inducible translocation trap (ITT), a novel reporter-based system to detect nuclear translocation, and its application for identification of nuclear translocating proteins are elaborated. Finally, analysis of "nuclear translocatome", the entire set of proteins that translocate into or out of the nucleus in response to extracellular stimuli, by ITT is discussed.

Retinoic acid induces TGFbeta-dependent autocrine fibroblast growth

To evaluate the role of murine TFIID subunit TAF4 in activation of cellular genes by all-trans retinoic acid (T-RA), we have characterized the T-RA response of taf4(lox/-) and taf4(-/-) embryonic fibroblasts. T-RA regulates almost 1000 genes in taf4(lox/-) cells, but less than 300 in taf4(-/-) cells showing that TAF4 is required for T-RA regulation of most, but not all cellular genes. We further show that T-RA-treated taf4(lox/-) cells exhibit transforming growth factor (TGF)beta-dependent autocrine growth and identify a set of genes regulated by loss of TAF4 and by T-RA corresponding to key mediators of the TGFbeta signalling pathway. T-RA rapidly and potently induces expression of connective tissue growth factor (CTGF) via a conserved DR2 type response element in its proximal promoter leading to serum-free autocrine growth. These results highlight the role of TAF4 as a cofactor in the cellular response to T-RA and identify the genetic programme of a novel cross talk between the T-RA and TGFbeta pathways that leads to deregulated cell growth.

TGFbeta-SMAD signal transduction: molecular specificity and functional flexibility

Ligands of the transforming growth factor-beta (TGFbeta) superfamily of growth factors initiate signal transduction through a bewildering complexity of ligand-receptor interactions. Signalling then converges to nuclear accumulation of transcriptionally active SMAD complexes and gives rise to a plethora of specific functional responses in both embryos and adult organisms. Current research is focused on the mechanisms that regulate SMAD activity to evoke cell-type-specific and context-dependent transcriptional programmes. An equally important challenge is understanding the functional role of signal strength and duration. How are these quantitative aspects of the extracellular signal regulated? How are they then sensed and interpreted, and how do they affect responses?

'Yin-Yang' functions of transforming growth factor-beta and T regulatory cells in immune regulation

Transforming growth factor-beta (TGF-beta) and forkhead box p3-expressing T-regulatory (Treg) cells are critical in maintaining self-tolerance and immune homeostasis. The immune suppressive functions of TGF-beta and Treg cells are widely acknowledged and extensively studied. Nonetheless, recent studies revealed the positive roles of TGF-beta and Treg cells in shaping the immune system and the inflammatory responses. This review discusses our and other's efforts in understanding the negative (Yin) as well as the positive (Yang) roles for TGF-beta and Treg cells in immune regulation.

Bone morphogenetic protein (BMP) type II receptor is required for BMP-mediated growth arrest and differentiation in pulmonary artery smooth muscle cells

Bone morphogenetic protein (BMP) signals regulate the growth and differentiation of diverse lineages. The association of mutations in the BMP type II receptor (BMPRII) with idiopathic pulmonary arterial hypertension suggests an important role of this receptor in vascular remodeling. Pulmonary artery smooth muscle cells lacking BMPRII can transduce BMP signals using ActRIIa (Activin type II receptor). We investigated whether or not BMP signaling via the two receptors leads to differential effects on vascular smooth muscle cells. BMP4, but not BMP7, inhibited platelet-derived growth factor-activated proliferation in wild-type pulmonary artery smooth muscle cells, whereas neither ligand inhibited the growth of BMPRII-deficient cells. Adenoviral gene transfer of BMPRII enabled BMP4, as well as BMP7, to inhibit proliferation in BMPRII-deficient cells. BMP-mediated growth inhibition was also reconstituted by the BMPRII short isoform, lacking the C-terminal domain present in the long form. BMP4, but not BMP7, induced the expression of osteoblast markers in wild-type cells, whereas neither ligand induced these markers in BMPRII-deficient cells. Overexpression of short or long forms of BMPRII in BMPRII-deficient cells enabled BMP4 and BMP7 to induce osteogenic differentiation. Although signaling via BMPRII or ActRIIa transiently activated SMAD1/5/8, only BMPRII signaling led to persistent SMAD1/5/8 activation and sustained increases in Id1 mRNA and protein expression. Pharmacologic blockade of BMP type I receptor function within 24 h after BMP stimulation abrogated differentiation. These data suggest that sustained BMP pathway activation, such as that mediated by BMPRII, is necessary for growth and differentiation control in vascular smooth muscle.

Arkadia enhances Nodal/TGF-beta signaling by coupling phospho-Smad2/3 activity and turnover

Regulation of transforming growth factor-β (TGF-β) signaling is critical in vertebrate development, as several members of the TGF-β family have been shown to act as morphogens, controlling a variety of cell fate decisions depending on concentration. Little is known about the role of intracellular regulation of the TGF-β pathway in development. E3 ubiquitin ligases target specific protein substrates for proteasome-mediated degradation, and several are implicated in signaling. We have shown that Arkadia, a nuclear RING-domain E3 ubiquitin ligase, is essential for a subset of Nodal functions in the embryo, but the molecular mechanism of its action in embryonic cells had not been addressed. Here, we find that Arkadia facilitates Nodal signaling broadly in the embryo, and that it is indispensable for cell fates that depend on maximum signaling. Loss of Arkadia in embryonic cells causes nuclear accumulation of phospho-Smad2/3 (P-Smad2/3), the effectors of Nodal signaling; however, these must be repressed or hypoactive as the expression of their direct target genes is reduced or lost. Molecular and functional analysis shows that Arkadia interacts with and ubiquitinates P-Smad2/3 causing their degradation, and that this is via the same domains required for enhancing their activity. Consistent with this dual function, introduction of Arkadia in homozygous null (−/−) embryonic stem cells activates the accumulated and hypoactive P-Smad2/3 at the expense of their abundance. Arkadia−/− cells, like Smad2−/− cells, cannot form foregut and prechordal plate in chimeras, confirming this functional interaction in vivo. As Arkadia overexpression never represses, and in some cells enhances signaling, the degradation of P-Smad2/3 by Arkadia cannot occur prior to their activation in the nucleus. Therefore, Arkadia provides a mechanism for signaling termination at the end of the cascade by coupling degradation of P-Smad2/3 with the activation of target gene transcription. This mechanism can account for achieving efficient and maximum Nodal signaling during embryogenesis and for rapid resetting of target gene promoters allowing cells to respond to dynamic changes in extracellular signals.

In development, cells respond to secreted signals (called morphogens) by turning on or off sets of target genes. How does gene activity adjust quickly in response to rapidly changing extracellular signals? This should require efficient removal of old/used signaling effectors (signal-activated transcription factors) from the promoters of target genes to allow new ones to assume control. We previously discovered Arkadia, an E3 ubiquitin ligase, and showed that it is an essential factor for normal development. (Ubiquitin ligases trigger the addition of ubiquitin residues to proteins, typically marking them for degradation.) Here, we show that Arkadia is required for high activity of the major signaling pathway, TGF-β/Nodal. Arkadia has a dual role to degrade Smads, the TGF-β signaling effectors, and enhance their transcriptional activity. This coupling of degradation with activation provides a mechanism to ensure that only effectors “in use” are degraded, allowing the new ones to proceed. It is possible that very similar mechanisms operate in other pathways to establish dynamic regulation and efficient signaling, while their failure may be associated with developmental abnormalities and disease, including cancer.

Arkadia enhances TGF-β family activity by degrading its inhibitory Smads but also stimulating transcription of phospho-Smads.

Visualizing Smad1/4 signaling response to bone morphogenetic protein-4 activation by FRET biosensors

Smad proteins are the major signal transducers for the Transforming Growth Factor superfamily of cytokines and their serine/threonine kinase receptors. Smads mediate the signal from the membrane into the nucleus. Bone Morphogenetic Protein-4 stimulates phosphorylation of Smad1, which interacts with Smad4. This complex translocates into the nucleus and regulates transcription of target genes. Here, we report our development of cellular fluorescence biosensors for direct visualization of Smad signaling in live mammalian cells. Fluorescence resonance energy transfer between cyan and yellow fluorescent proteins fused to the Smad1 and Smad4 proteins was used to unravel the temporal aspects of BMP/Smad signaling. A rate-limiting delay of 2-5 min occurred between BMP activation and Smad1 activity. A similar delay was observed in the Smad1/Smad4 complexation. Further experimentation indicated that the delay is dependent on the MH1 domain and linker of Smad1. These results give new insights into the dynamics of the BMP receptor -Smad1/4 signaling process and provide a new tool for studying Smads.

How the Smads regulate transcription

The primary signalling pathway downstream of ligands of the transforming growth factor beta (TGF-beta) superfamily is the Smad pathway. Activated receptors phosphorylate receptor-regulated Smads, which form homomeric complexes and heteromeric complexes with Smad4. These activated Smad complexes accumulate in the nucleus, where they are directly involved in the regulation of transcription of target genes. This apparently very simple pathway is subject to complex regulation, much of which is at the level of post-translational modifications of pathway components, in particular, the Smads. The enzymes responsible may be constitutively active, may be cell type-specific or may be regulated by other signalling pathways or by the cell cycle. In this way, signals from TGF-beta superfamily ligands are integrated with signals from other growth factors and cytokines, are regulated by the cell cycle and are dependent on cell type. This may go some way to explaining the pleiotropic nature of TGF-beta superfamily responses. In this review we focus on the mechanisms whereby the Smads are modified and regulated. We then go on to discuss how the activated Smad complexes regulate transcription.

Constraint-based modeling and kinetic analysis of the Smad dependent TGF-beta signaling pathway

BACKGROUND

Investigation of dynamics and regulation of the TGF-beta signaling pathway is central to the understanding of complex cellular processes such as growth, apoptosis, and differentiation. In this study, we aim at using systems biology approach to provide dynamic analysis on this pathway.

METHODOLOGY/PRINCIPAL FINDINGS

We proposed a constraint-based modeling method to build a comprehensive mathematical model for the Smad dependent TGF-beta signaling pathway by fitting the experimental data and incorporating the qualitative constraints from the experimental analysis. The performance of the model generated by constraint-based modeling method is significantly improved compared to the model obtained by only fitting the quantitative data. The model agrees well with the experimental analysis of TGF-beta pathway, such as the time course of nuclear phosphorylated Smad, the subcellular location of Smad and signal response of Smad phosphorylation to different doses of TGF-beta.

CONCLUSIONS/SIGNIFICANCE

The simulation results indicate that the signal response to TGF-beta is regulated by the balance between clathrin dependent endocytosis and non-clathrin mediated endocytosis. This model is useful to be built upon as new precise experimental data are emerging. The constraint-based modeling method can also be applied to quantitative modeling of other signaling pathways.

Imaging Diffusion in Living Cells Using Time-Correlated Single-Photon Counting

Current efforts to monitor the diffusion of proteins in living cells are based on either fluorescence correlation spectroscopy (FCS), fluorescence recovery after photobleaching, or image correlation spectroscopy. However, these methods cannot generate a map of diffusion times. Here, we introduce a new method termed diffusion imaging microscopy that combines scanning confocal microscopy, time-correlated single-photon counting, and FCS and thus allows us to measure spatially resolved diffusion times. In our approach, we record scan images with time-resolved photon streams within each individual pixel. By extending the pixel dwell time to 25-100 ms, a software correlation of individual photons within each pixel yields the average diffusion time. Additionally, information on fluorescence intensity (number of photons) and fluorescence lifetime is available and can be used to sort fluorescence photons and to discriminate from autofluorescence. We evaluated our method by measuring diffusion times of dT20-TMR in solutions of different viscosity. We further demonstrate the applicability of the method to living cells and recorded a diffusion map of a living 3T3 mouse fibroblast incubated with dT20-ATTO488.

Msk is required for nuclear import of TGF-{beta}/BMP-activated Smads

Nuclear translocation of Smad proteins is a critical step in signal transduction of transforming growth factor β (TGF-β) and bone morphogenetic proteins (BMPs). Using nuclear accumulation of the Drosophila Smad Mothers against Decapentaplegic (Mad) as the readout, we carried out a whole-genome RNAi screening in Drosophila cells. The screen identified moleskin (msk) as important for the nuclear import of phosphorylated Mad. Genetic evidence in the developing eye imaginal discs also demonstrates the critical functions of msk in regulating phospho-Mad. Moreover, knockdown of importin 7 and 8 (Imp7 and 8), the mammalian orthologues of Msk, markedly impaired nuclear accumulation of Smad1 in response to BMP2 and of Smad2/3 in response to TGF-β. Biochemical studies further suggest that Smads are novel nuclear import substrates of Imp7 and 8. We have thus identified new evolutionarily conserved proteins that are important in the signal transduction of TGF-β and BMP into the nucleus.

A role for TGF-beta1-induced cellular responses during wound healing of the non-scarring early human fetus?

Early human fetuses regenerate cutaneous wounds perfectly without scarring. However, transforming growth factor-beta1 (TGF-beta1), the cytokine linked with scarring in mature tissue, is also present during fetal wound repair, albeit transiently. We present a comparison of response to TGF-beta1 by fibroblasts derived from early human fetal skin (non-scarring) and their mature (scarring) postnatal counterparts, which revealed that although fetal fibroblasts do indeed differentiate into myofibroblasts, this response is altogether more rapid and short-lived. Fetal fibroblasts also failed to exhibit the TGF-beta1-induced increase in collagen (mRNA and protein) demonstrated by their postnatal counterparts. Fetal cells exhibited a comparatively short-lived or rapid phosphorylation of several components of the TGF-beta1 signaling pathways: Smad2/3 and c-Jun N-terminal kinase. Unlike quiescent postnatal fibroblasts, quiescent fetal fibroblasts also phosphorylated extracellular signal-regulated kinases in response to TGF-beta1. These altered responses to TGF-beta1 may well contribute to the transition between perfect regeneration and scar formation seen during development.

TGFbeta signalling in control of T-cell-mediated self-reactivity

In the immune system, transforming growth factor-beta (TGFbeta) affects multiple cell lineages by either promoting or opposing their differentiation, survival and proliferation. Understanding the cellular mechanisms of TGFbeta-mediated regulation is complicated due to a broad distribution of TGFbeta receptors on the surface of different immune-cell types. Recent studies using in vivo genetic approaches revealed a critical role for TGFbeta signalling in T cells in restraining fatal autoimmune lesions. Here, we review recent advances in our understanding of a role for TGFbeta signalling in the regulation of T-cell differentiation in the thymus and in the periphery, with a particular emphasis on TGFbeta-mediated control of self-reactive T cells.

Erbin inhibits transforming growth factor beta signaling through a novel Smad-interacting domain

Smad proteins are critical intracellular signaling mediators for the transforming growth factor beta (TGFbeta) superfamily. Here, we report that Erbin (for "ErbB2/Her2-interacting protein"), which contains leucine-rich repeats and a PDZ (PSD-95/DLG/ZO-1) domain, interacts specifically with Smad3 and, to a lesser extent, with Smad2 through a novel Smad-interacting domain (SID) adjacent to its PDZ domain. Increased expression of Erbin does not affect the level of TGFbeta-induced phosphorylation of Smad2/Smad3, but it physically sequesters Smad2/Smad3 from their association with Smad4 and hence negatively modulates TGFbeta-dependent transcriptional responses and cell growth inhibition. An isoform of Erbin encoded by an alternatively spliced transcript in human tissues lacks this SID and fails to inhibit TGFbeta responses. Consistently, knockdown of the endogenous Erbin gene with short hairpin RNA enhances TGFbeta-induced antiproliferative and transcriptional responses. In addition, Erbin suppresses activin/Smad2-dependent, but not BMP/Smad1-mediated, induction of endogenous gene expression in Xenopus embryos. Therefore, these results define Erbin as a novel negative modulator of Smad2/Smad3 functions and expand the physiological role of Erbin to the regulation of TGFbeta signaling.

A tale of two proteins: differential roles and regulation of Smad2 and Smad3 in TGF-beta signaling

Transforming growth factor-beta (TGF-beta) is an important growth inhibitor of epithelial cells, and insensitivity to this cytokine results in uncontrolled cell proliferation and can contribute to tumorigenesis. Smad2 and Smad3 are direct mediators of TGF-beta signaling, however little is known about the selective activation of Smad2 versus Smad3. The Smad2 and Smad3 knockout mouse phenotypes and studies comparing Smad2 and Smad3 activation of TGF-beta target genes, suggest that Smad2 and Smad3 have distinct roles in TGF-beta signaling. The observation that TGF-beta inhibits proliferation of Smad3-null mammary gland epithelial cells, whereas Smad3 deficient fibroblasts are only partially growth inhibited, suggests that Smad3 has a different role in epithelial cells and fibroblasts. Herein, the current understanding of Smad2 and Smad3-mediated TGF-beta signaling and their relative roles are discussed, in addition to potential mechanisms for the selective activation of Smad2 versus Smad3. Since alterations in the TGF-beta signaling pathway play an important role in promoting tumorigenesis and cancer progression, methods for therapeutic targeting of the TGF-beta signaling pathway are being pursued. Determining how Smad2 or Smad3 differentially regulate the TGF-beta response may translate into developing more effective strategies for cancer therapy.

Activation of Mps1 Promotes Transforming Growth Factor-β-independent Smad Signaling

The primary intracellular mediators of TGF-beta signaling are the Smad proteins. Phosphorylation of R-Smad at the C-terminal SSXS motif by the activated TGF-beta type I receptor kinase triggers a conformation change in R-Smad and facilitates complex formation between R-Smad and Smad4, which shuttle into the nucleus where they interact with DNA and other transcription factors to regulate gene expression. In an attempt to identify proteins interacting with activated Smad signaling complex, we discovered that Mps1, a protein kinase that plays important roles in normal mitotic progression and mitotic checkpoint signaling, co-purifies with this complex. We demonstrated that Smad2 and Smad3 but not Smad4 are substrates of Mps1 in vitro and in vivo. Mps1 phosphorylates Smad2 and Smad3 at the SSXS motif in their C-terminal regions in vitro and in vivo. Disruption of microtubule networks by nocodazole activates Mps1 and promotes TGF-beta-independent activation of Smad signaling. We found that Mps1 is involved in turning on Smad signaling by phosphorylating R-Smads. Our results reveal a novel functional link between Mps1 and Smads in a non-canonical Smad signaling pathway.

DrosophilaNemo antagonizes BMP signaling by phosphorylation of Mad and inhibition of its nuclear accumulation

Drosophila Nemo is the founding member of the Nemo-like kinase(Nlk) family of serine/threonine protein kinases that are involved in several Wnt signal transduction pathways. Here we report a novel function for Nemo in the inhibition of bone morphogenetic protein (BMP) signaling. Genetic interaction studies demonstrate that nemo can antagonize BMP signaling and can inhibit the expression of BMP target genes during wing development. Nemo can bind to and phosphorylate the BMP effector Mad. In cell culture, phosphorylation by Nemo blocks the nuclear accumulation of Mad by promoting export of Mad from the nucleus in a kinase-dependent manner. This is the first example of the inhibition of Drosophila BMP signaling by a MAPK and represents a novel mechanism of Smad inhibition through the phosphorylation of a conserved serine residue within the MH1 domain of Mad.

Monocrotaline pyrrole induces Smad nuclear accumulation and altered signaling expression in human pulmonary arterial endothelial cells

The mechanistic relationship between the widely used monocrotaline model of primary pulmonary hypertension and altered TGFbeta family signaling due to genetic defects in the Bone Morphogenetic Protein type II receptor in affected humans has not been investigated. In this study we use fluorescent microscopy to demonstrate nuclear translocation of Smad 4 in human pulmonary arterial endothelial cell (HPAEC) cultures treated with monocrotaline pyrrole (MCTP), Bone Morphogenetic Protein (BMP) and TGFbeta. While MCTP induced transient nuclear accumulation of phosphorylated Smad 1 (P-Smad 1) and phosphorylated Smad 2 (P-Smad 2), only expression of P-Smad 1 was significantly altered in western blots. P-Smad 1 expression significantly increased 30 min following treatment with MCTP correlating with P-Smad 1 and Smad 4 nuclear translocation. Although a modest, but significant decrease in P-Smad 1 expression occurred 1 h after treatment, expression was significantly increased at 72 h. Evaluation of components of the signal and response pathway at 72 h showed decreased expression of the BMP type II receptor (BMPrII), no change in TGFbeta Activin Receptor-like Kinase 1 (Alk 1), no change in Smad 4 but increase in the inhibitory Smad 6, decrease in the alternate BMP signaling pathway p38(MAPK) but no change in the psmad1 response element ID 1. Our results suggest transient activation of Smad signaling pathways in initial MCTP endothelial cell toxicity, and a persistent dysregulation of BMP signaling. Electron microscopy of cell membrane caveoli revealed a dramatic decrease in these structures after 72 h. Loss of these structural elements, noted for their sequestration and inhibition of receptor activity, may contribute to prolonged alterations in BMP signaling.

Mice develop normally in the absence of Smad4 nucleocytoplasmic shuttling

Smad4 in partnership with R-Smads (receptor-regulated Smads) activates TGF-beta (transforming growth factor-beta)-dependent signalling pathways essential for early mouse development. Smad4 null embryos die shortly after implantation due to severe defects in cell proliferation and visceral endoderm differentiation. In the basal state, Smad4 undergoes continuous shuttling between the cytoplasm and the nucleus due to the combined activities of an N-terminal NLS (nuclear localization signal) and an NES (nuclear export signal) located in its linker region. Cell culture experiments suggest that Smad4 nucleocytoplasmic shuttling plays an important role in TGF-beta signalling. In the present study we have investigated the role of Smad4 shuttling in vivo using gene targeting to engineer two independent mutations designed to eliminate Smad4 nuclear export. As predicted this results in increased levels of Smad4 in the nucleus of homozygous ES cells (embryonic stem cells) and primary keratinocytes, in the presence or absence of ligand. Neither mutation affects Smad4 expression levels nor its ability to mediate transcriptional activation in homozygous cell lines. Remarkably mouse mutants lacking the Smad4 NES develop normally. Smad4 NES mutants carrying one copy of a Smad4 null allele also fail to display developmental defects. The present study clearly demonstrates that Smad4 nucleocytoplasmic shuttling is not required for embryonic development or tissue homoeostasis in normal, healthy adult mice.

Acetylation of Smad2 by the co-activator p300 regulates activin and transforming growth factor beta response

Transforming growth factor beta (TGFbeta) signals primarily through the Smad proteins to regulate cell growth, differentiation, and extracellular matrix production. Post-translational modifications, such as phosphorylation, play an important role in the regulation of the Smad proteins. TGFbeta signaling results in the phosphorylation of Smad2 and Smad3 that then oligomerize with Smad4 and translocate into the nucleus to initiate transcription of TGFbeta target genes. The initiation of transcription is significantly enhanced by the direct interaction of the Smad complex with p300/CBP (CREB-binding protein), a co-activator with intrinsic acetyltransferase activity. However, how p300/CBP enhances transcription through this interaction is not entirely understood. In this report, we show that Smad2, but not the highly homologous Smad3, can be acetylated by p300/CBP in a ligand-dependent manner. At least three lysine residues, Lys(19), Lys(20), and Lys(39), are required for efficient acetylation of Smad2, as mutations altering these lysines abolished Smad2 acetylation in vivo. This acetylation event is required for the ability of Smad2 to mediate activin and TGFbeta signaling. Mutation of the three key lysine residues did not alter the stability of Smad2 or the ability of Smad2 to form a complex with Smad4 on promoter DNA, but it prevented nuclear accumulation of Smad2 and subsequent TGFbeta and activin responses. Thus, our studies reveal a novel mechanism of modulating Smad2 activity and localization through protein acetylation.

Cx43 mediates TGF-beta signaling through competitive Smads binding to microtubules

Transforming growth factor-beta (TGF-beta) superfamily members play an important role in growth, differentiation, adhesion, apoptosis, and development in many species from insects and worms to vertebrates. Recently, TGF-beta signaling has been demonstrated to be negatively regulated by microtubules (MTs), which anchor endogenous Smad2/3 to cytosol and also directly interact with connexin43 (Cx43), and the activity of TGF-beta is mediated by Cx43. However, the mechanism underlying the intracellular regulation of TGF-beta activity by Cx43 remains unknown. Here, we found that the functional link between TGF-beta activation and Cx43 is mediated by interactions among Smad2/3, MTs, and Cx43. We confirmed that Cx43 competes with Smad2/3 for binding to MTs, which Cx43 specifically induces release of Smad2/3 from MTs and increases phospho-Smad2 and which, as a result, Smad2/3 and Smad4 are accumulated in the nucleus, leading to activation of the transcription of target genes. Consistently, knockdown of the endogenous Cx43 activity with double-strand RNA (dsRNA) in HL1 cardiomyocytes and Cx43 knockout mice cardiomyocytes consistently show the opposite effect. Our findings demonstrate a novel mechanism for Cx43 positive regulation of TGF-beta function.

Kinesin-mediated transport of Smad2 is required for signaling in response to TGF-beta ligands

During vertebrate development, Activin/Nodal-related ligands signal through Smad2, leading to its activation and accumulation in the nucleus. Here, we demonstrate that Smad2 constantly shuttles between the cytoplasm and nucleus both in early Xenopus embryo explants and in living zebrafish embryos, providing a mechanism whereby the intracellular components of the pathway constantly monitor receptor activity. We have gone on to demonstrate that an intact microtubule network and kinesin ATPase activity are required for Smad2 phosphorylation and nuclear accumulation in response to Activin/Nodal in early vertebrate embryos and TGF-beta in mammalian cells. The kinesin involved is kinesin-1, and Smad2 interacts with the kinesin-1 light chain subunit. Interfering with kinesin activity in Xenopus and zebrafish embryos phenocopies loss of Nodal signaling. Our results reveal that kinesin-mediated transport of Smad2 along microtubules to the receptors is an essential step in ligand-induced Smad2 activation.

Functional role of Meox2 during the epithelial cytostatic response to TGF-beta

Transforming growth factor beta (TGF-beta) suppresses epithelial cell growth. We have identified a new target gene of the TGF-beta/Smad pathway, Meox2, encoding the homeodomain transcription factor that is known to regulate endothelial cell proliferation and muscle development. Knockdown of endogenous Meox2 by RNA interference prevented the TGF-beta1-induced cytostatic response. Moreover, ectopic Meox2 suppressed epithelial cell proliferation in cooperation with TGF-beta1, and mediated induction of the cell cycle inhibitor gene p21. Transcriptional induction of p21 by Meox2 required a distal region of the p21 promoter that spans the p53-binding site. We show that Meox2 can form protein complexes with Smads leading to cooperative regulation of p21 gene expression. Finally, we found that in cell models that undergo both cell cycle arrest and epithelial-mesenchymal transition (EMT), ectopic Meox2 failed to induce EMT and inhibited the proper EMT response to TGF-beta. Thus, Meox2 is primarily involved in the TGF-beta tumor suppressor pathway.

Endofin acts as a Smad anchor for receptor activation in BMP signaling

Signaling through receptors of the transforming growth factor beta (TGFbeta) superfamily is mediated by cytoplasmic Smad proteins. It has been demonstrated that Smad anchor for receptor activation (SARA) facilitates TGFbeta and activin/nodal signaling by recruiting and presenting Smad2/3 to the receptor complex. SARA does not bind Smad1 and hence does not enhance bone morphogenetic protein (BMP) signaling. Here we report for the first time that the endosome-associated FYVE-domain protein endofin acts as a Smad anchor for receptor activation in BMP signaling. We demonstrate that endofin binds Smad1 preferentially and enhances Smad1 phosphorylation and nuclear localization upon BMP stimulation. Silencing of endofin by RNAi resulted in a reduction in BMP-dependent Smad1 phosphorylation. Moreover, disruption of the membrane-anchoring FYVE motif by point mutation led to a reduction of BMP-responsive gene expression in cell culture and Xenopus ectodermal explants. Furthermore, we demonstrate that endofin contains a protein-phosphatase-binding motif, which functions to negatively modulate BMP signals through receptor dephosphorylation. Taken together, our results suggest that endofin plays an important role in both positive and negative feedback regulation of the BMP signaling pathway.

Role of transforming growth factor-beta in cancer progression

Invasion and metastasis are the most lethal characteristics of cancer and the leading causes of cancer-related death. Transforming growth factor (TGF)-beta is a multifunctional cytokine that normally functions to prevent the uncontrolled proliferation of epithelial, endothelial and hematopoietic cells. Quite dichotomously, however, aberrant genetic or epigenetic events often negate the cytostatic function of TGF-beta in these cells, leading to tumor formation. Once freed from the growth-inhibitory effects of TGF-beta, cancer cells acquire the ability to proliferate, invade and metastasize when stimulated by TGF-beta. A thorough understanding of the molecular mechanisms underlying these paradoxical functions of TGF-beta remains elusive. Here, the authors review the tumor-suppressing and -promoting activities of TGF-beta and discuss the potential use and targeting of the TGF-beta-signaling system to prevent the progression and acquisition of metastatic phenotypes by human malignancies.

Negative regulation of TGF-beta receptor/Smad signal transduction

Members of the transforming growth factor-beta (TGF-beta) family are highly conserved multifunctional cell-cell signaling proteins that are of key importance for controlling embryogenesis and tissue homeostasis. At first glance, signaling through TGF-beta family members appears to be a simple process: ligands bind to specific serine/threonine kinase transmembrane receptors, which activate intracellular Smad effector proteins, which in turn relay the signal to the nucleus to control gene transcription. However, recent research has revealed that additional layers of complexity exist at each step in the TGF-beta/Smad pathway. The expression, activation and inactivation, subcellular localization, and stability of TGF-beta signaling components are tightly regulated and subject to input from other signaling pathways. A broad array of Smad interacting partners and diverse post-translational modifications of Smads have been identified. Recently, important advances have been made in our understanding of how TGF-beta family signals are attenuated and terminated to maintain control over this versatile pathway.

Identification of a Smad phosphatase

Activation of Smad signaling pathways downstream of TGF-beta superfamily ligands via receptor-mediated Smad phosphorylation is well understood, but little is known about the phosphatases that turn off Smad activity. Now in Cell, Feng and colleagues (Lin, X., et al. (2006) Cell 125 , 915-928) report their discovery that PPM1A acts as a Smad phosphatase to terminate TGF-beta signaling.

Nucleocytoplasmic shuttling of the retinoblastoma tumor suppressor protein via Cdk phosphorylation-dependent nuclear export

The retinoblastoma (RB) tumor suppressor protein is a negative regulator of cell proliferation that is functionally inactivated in the majority of human tumors. Elevated Cdk activity via RB pathway mutations is observed in virtually every human cancer. Thus, Cdk inhibitors have tremendous promise as anticancer agents although detailed mechanistic knowledge of their effects on RB function is needed to harness their full potential. Here, we illustrate a novel function for Cdks in regulating the subcellular localization of RB. We present evidence of significant cytoplasmic mislocalization of ordinarily nuclear RB in cells harboring Cdk4 mutations. Our findings uncover a novel mechanism to circumvent RB-mediated growth suppression by altered nucleocytoplasmic trafficking via the Exportin1 pathway. Cytoplasmically mislocalized RB could be efficiently confined to the nucleus by inhibiting the Exportin1 pathway, reducing Cdk activity, or mutating the Cdk-dependent phosphorylation sites in RB that result in loss of RB-Exportin1 association. Thus RB-mediated tumor suppression can be subverted by phosphorylation-dependent enhancement of nuclear export. These results support the notion that tumor cells can modulate the protein transport machinery thereby making the protein transport process a viable therapeutic target.

Protein Serine/Threonine Phosphatase PPM1A Dephosphorylates Smad1 in the Bone Morphogenetic Protein Signaling Pathway

Bone morphogenetic proteins (BMPs) are secreted polypeptides belonging to the transforming growth factor-beta (TGF-beta) superfamily that activates a broad range of biological responses in the metazoan organism. The BMP-initiated signaling pathway is under tight control by processes including regulation of the ligands, the receptors, and the key downstream intracellular effector Smads. A critical point of control in BMP signaling is the phosphorylation of Smad1, Smad5, and Smad8 in their C-terminal SXS motif. Although such phosphorylation, which is mediated by the type I BMP receptor kinases in response to BMP stimulation, is well characterized, biochemical mechanisms underlying Smad dephosphorylation remain to be elucidated. In this study, we have found that PPM1A, a metal ion-dependent protein serine/threonine phosphatase, physically interacts with and dephosphorylates Smad1 both in vitro and in vivo. Functionally, overexpression of PPM1A abolishes BMP-induced transcriptional responses, whereas RNA interference-mediated knockdown of PPM1A enhances BMP signaling. Collectively, our study suggests that PPM1A plays an important role in controlling BMP signaling through catalyzing Smad dephosphorylation.

Regulation of Smad activities

TGF-beta (Transforming Growth Factor-beta) cytokines employ Smad proteins as the intracellular mediator of signaling. Upon TGF-beta stimulation, the cytoplasmic Smads become phosphorylated and consequently accumulate in the nucleus to regulate target gene expression. The cytoplasm-to-nucleus redistribution of Smads, as well as the ability of Smads to activate or repress gene transcription, is under multiple layers of regulation by factors not limited to TGF-beta. With recent advance in the knowledge of regulatory factors impinged on Smads, we are beginning to understand the complexity in cellular responses to TGF-beta.

The roles for cytokines in the generation and maintenance of regulatory T cells

As an essential mechanism for self-tolerance, immune suppression has attracted much attention since the discovery of suppressor T cells, now called regulatory T cells (Tregs), in the 1990s. Different types of Tregs have been described based on distinct expression patterns of surface markers and cytokines. Cytokines are not only essential for function but also important for the generation of Tregs. Interleukin-2 (IL-2), transforming growth factor-beta, IL-10, and other immunoregulatory molecules have been shown to control the generation of Tregs. The presence of other types of cells, in particular antigen-presenting cells (APCs), is critical for the generation of Tregs. Cytokines can serve as either initiators or intermediates for the interactions between APCs and Tregs. This review discusses our current knowledge of how cytokines regulate the generation and maintenance of Tregs.

A rate equation approach to elucidate the kinetics and robustness of the TGF-beta pathway

We present a rate equation model for the TGF-beta pathway in endothelial cells together with novel measurements. This pathway plays a prominent role in inter- and intracellular communication and subversion can lead to cancer, fibrosis vascular disorders, and immune diseases. The model successfully describes the kinetics of experimental data and also correctly predicts the behavior in experiments where the system is perturbed. A novel method in this context, simulated tempering, is used to fit the model parameters to the data. It provides an ensemble of high quality solutions, which are analyzed with clustering methods and display a hierarchical structure highlighting distinct parameter subspaces with biological interpretations. This analysis discriminates between different biological mechanisms to achieve a transient signal from a sustained TGF-beta input, where one mechanism is to use a negative feedback to turn the signal off. Further analysis in terms of parameter sensitivity reveals that this negative feedback loop in TGF-beta signaling renders the system global robustness. This sheds light upon the role of the Smad7 protein in this system.

Unique players in the BMP pathway: small C-terminal domain phosphatases dephosphorylate Smad1 to attenuate BMP signaling

Smad transcription factors are key signal transducers for the TGF-beta/bone morphogenetic protein (BMP) family of cytokines and morphogens. C-terminal serine phosphorylation by TGF-beta and BMP membrane receptors drives Smads into the nucleus as transcriptional regulators. Dephosphorylation and recycling of activated Smads is an integral part of this process, which is critical for agonist sensing by the cell. However, the nuclear phosphatases involved have remained unknown. Here we provide functional, biochemical, and embryological evidence identifying the SCP (small C-terminal domain phosphatase) family of nuclear phosphatases as mediators of Smad1 dephosphorylation in the BMP signaling pathway in vertebrates. Xenopus SCP2/Os4 inhibits BMP activity in the presumptive ectoderm and leads to neuralization. In Xenopus embryos, SCP2/Os4 and human SCP1, 2, and 3 cause selective dephosphorylation of Smad1 compared with Smad2, inhibiting BMP- and Smad1-dependent transcription and leading to the induction of the secondary dorsal axis. In human cells, RNAi-mediated depletion of SCP1 and SCP2 increases the extent and duration of Smad1 phosphorylation in response to BMP, the transcriptional action of Smad1, and the strength of endogenous BMP gene responses. The present identification of the SCP family as Smad C-terminal phosphatases sheds light on the events that attenuate Smad signaling and reveals unexpected links to the essential phosphatases that control RNA polymerase II in eukaryotes.

A phosphatase controls the fate of receptor-regulated Smads

In this issue of Cell, Lin et al. (2006) answer one of the long-standing questions in the TGFbeta field by identifying a phosphatase, PPM1A, that directly dephosphorylates Smad2 and Smad3 to limit their activation.

Identification of a Smad phosphatase

Activation of Smad signaling pathways downstream of TGF-beta superfamily ligands via receptor-mediated Smad phosphorylation is well understood, but little is known about the phosphatases that turn off Smad activity. Now in Cell, Feng and colleagues (Lin, X., et al. (2006) Cell 125 , 915-928) report their discovery that PPM1A acts as a Smad phosphatase to terminate TGF-beta signaling.

Genetic alterations of the TGF-beta signaling pathway in colorectal cancer cell lines: a novel mutation in Smad3 associated with the inactivation of TGF-beta-induced transcriptional activation

To investigate genetic alterations involved in the TGF-beta signaling pathway in colorectal cancer, we assayed DNA synthesis rates after treating TGF-beta and checked for genetic alterations in TGF-betaRII, TGF-betaRI, Smad2, Smad3, and Smad4 in 12 colorectal cancer cell lines. Eleven lines, except SNU-61, show no significant change in DNA synthesis rate after TGF-beta treatment. In these 11 lines, several mutations were found in genes involved in the TGF-beta signaling pathway: (i) frameshift deletions in the poly(A)(10) tract of the TGF-betaRII gene in SNU-407, SNU-769A, SNU-769B, and SNU-1047 cell lines, (ii) a missense mutation of Smad2 (R321Q) in SNU-81, (iii) two missense mutations in TGF-betaRI (R487W in SNU-175 and A202V in SNU-1040), and (iv) a monoallelic loss at the Smad4 locus in three cell lines. Interestingly, a missense mutation (R373H) in Smad3 gene was found in SNU-769A. To our knowledge, this is the first report of Smad3 mutation in human malignancy. This mutation was found to result in the inhibition of translocation of Smad3 protein to the nucleus and a reduction in the activity of Smad3 during TGF-beta-induced transcriptional activation. These results indicate that the majority of cell lines, which are insensitive to TGF-beta, have alterations in genes involved in the TGF-beta signaling pathway in colorectal cancer cell lines.

Akt down-regulates ERK1/2 nuclear localization and angiotensin II-induced cell proliferation through PEA-15

Angiotensin II (AngII) type 1 receptors (AT1) regulate cell growth through the extracellular signal-regulated kinase (ERK)1/2 and phosphatidylinositol 3-kinase (PI3K) pathways. ERK1/2 and Akt/protein kinase B, downstream of PI3K, are independently activated but both required for mediating AngII-induced proliferation when expressed at endogenous levels. We investigate the effect of an increase in the expression of wild-type Akt1 by using Chinese hamster ovary (CHO)-AT1 cells. Unexpectedly, Akt overexpression inhibits the AT1-mediated proliferation. This effect could be generated by a cross-talk between the PI3K and ERK1/2 pathways. A functional partner is the phosphoprotein enriched in astrocytes of 15 kDa (PEA-15), an Akt substrate known to bind ERK1/2 and to regulate their nuclear translocation. We report that Akt binds to PEA-15 and that Akt activation leads to PEA-15 stabilization, independently of PEA-15 interaction with ERK1/2. Akt cross-talk with PEA-15 does not affect ERK1/2 activation but decreases their nuclear activity as a result of the blockade of ERK1/2 nuclear accumulation. In response to AngII, PEA-15 overexpression displays the same functional consequences on ERK1/2 signaling as Akt overactivation. Thus, Akt overactivation prevents the nuclear translocation of ERK1/2 and the AngII-induced proliferation through interaction with and stabilization of endogenous PEA-15.

Transforming growth factor-beta regulation of immune responses

Transforming growth factor-beta (TGF-beta) is a potent regulatory cytokine with diverse effects on hemopoietic cells. The pivotal function of TGF-beta in the immune system is to maintain tolerance via the regulation of lymphocyte proliferation, differentiation, and survival. In addition, TGF-beta controls the initiation and resolution of inflammatory responses through the regulation of chemotaxis, activation, and survival of lymphocytes, natural killer cells, dendritic cells, macrophages, mast cells, and granulocytes. The regulatory activity of TGF-beta is modulated by the cell differentiation state and by the presence of inflammatory cytokines and costimulatory molecules. Collectively, TGF-beta inhibits the development of immunopathology to self or nonharmful antigens without compromising immune responses to pathogens. This review highlights the findings that have advanced our understanding of TGF-beta in the immune system and in disease.

Inhibition of transforming growth factor-beta1-induced signaling and epithelial-to-mesenchymal transition by the Smad-binding peptide aptamer Trx-SARA

Overexpression of the inhibitory Smad, Smad7, is used frequently to implicate the Smad pathway in cellular responses to transforming growth factor beta (TGF-beta) signaling; however, Smad7 regulates several other proteins, including Cdc42, p38MAPK, and beta-catenin. We report an alternative approach for more specifically disrupting Smad-dependent signaling using a peptide aptamer, Trx-SARA, which comprises a rigid scaffold, the Escherichia coli thioredoxin A protein (Trx), displaying a constrained 56-amino acid Smad-binding motif from the Smad anchor for receptor activation (SARA) protein. Trx-SARA bound specifically to Smad2 and Smad3 and inhibited both TGF-beta-induced reporter gene expression and epithelial-to-mesenchymal transition in NMuMG murine mammary epithelial cells. In contrast to Smad7, Trx-SARA had no effect on the Smad2 or 3 phosphorylation levels induced by TGF-beta1. Trx-SARA was primarily localized to the nucleus and perturbed the normal cytoplasmic localization of Smad2 and 3 to a nuclear localization in the absence of TGF-beta1, consistent with reduced Smad nuclear export. The key mode of action of Trx-SARA was to reduce the level of Smad2 and Smad3 in complex with Smad4 after TGF-beta1 stimulation, a mechanism of action consistent with the preferential binding of SARA to monomeric Smad protein and Trx-SARA-mediated disruption of active Smad complexes.

Activin signaling and its role in regulation of cell proliferation, apoptosis, and carcinogenesis

Activins, cytokine members of the transforming growth factor-beta superfamily, have various effects on many physiological processes, including cell proliferation, cell death, metabolism, homeostasis, differentiation, immune responses endocrine function, etc. Activins interact with two structurally related serine/threonine kinase receptors, type I and type II, and initiate downstream signaling via Smads to regulate gene expression. Understanding how activin signaling is controlled extracellularly and intracellularly would not only lead to more complete understanding of cell growth and apoptosis, but would also provide the basis for therapeutic strategies to treat cancer and other related diseases. This review focuses on the recent progress on activin-receptor interactions, regulations of activin signaling by ligand-binding proteins, receptor-binding proteins, and nucleocytoplasmic shuttling of Smad proteins.

PPM1A functions as a Smad phosphatase to terminate TGFbeta signaling

TGFbeta signaling controls diverse normal developmental processes and pathogenesis of diseases including cancer and autoimmune and fibrotic diseases. TGFbeta responses are generally mediated through transcriptional functions of Smads. A key step in TGFbeta signaling is ligand-induced phosphorylation of receptor-activated Smads (R-Smads) catalyzed by the TGFbeta type I receptor kinase. However, the potential of Smad dephosphorylation as a regulatory mechanism of TGFbeta signaling and the identity of Smad-specific phosphatases remain elusive. Using a functional genomic approach, we have identified PPM1A/PP2Calpha as a bona fide Smad phosphatase. PPM1A dephosphorylates and promotes nuclear export of TGFbeta-activated Smad2/3. Ectopic expression of PPM1A abolishes TGFbeta-induced antiproliferative and transcriptional responses, whereas depletion of PPM1A enhances TGFbeta signaling in mammalian cells. Smad-antagonizing activity of PPM1A is also observed during Nodal-dependent early embryogenesis in zebrafish. This work demonstrates that PPM1A/PP2Calpha, through dephosphorylation of Smad2/3, plays a critical role in terminating TGFbeta signaling.

Specification of ectoderm restricts the size of the animal plate and patterns neurogenesis in sea urchin embryos

The animal plate of the sea urchin embryo becomes the apical organ, a sensory structure of the larva. In the absence of vegetal signaling, an expanded and unpatterned apical organ forms. To investigate the signaling that restricts the size of the animal plate and patterns neurogenesis, we have expressed molecules that regulate specification of ectoderm in embryos and chimeras. Enhancing oral ectoderm suppresses serotonergic neuron differentiation, whereas enhancing aboral or ciliary band ectoderm increases differentiation of serotonergic neurons. In embryos in which vegetal signaling is blocked, Nodal expression does not reduce the size of the thickened animal plate; however, almost no neurons form. Expression of BMP in the absence of vegetal signaling also does not restrict the size of the animal plate, but abundant serotonergic neurons form. In chimeras in which vegetal signaling is blocked in the entire embryo, and one half of the embryo expresses Nodal, serotonergic neuron formation is suppressed in both halves. In similar chimeras in which vegetal signaling is blocked and one half of the embryo expresses Goosecoid (Gsc), serotonergic neurons form only in the half of the embryo not expressing Gsc. We propose that neurogenesis is specified by a maternal program that is restricted to the animal pole by signaling that is dependent on nuclearization of beta-catenin and specifies ciliary band ectoderm. Subsequently, neurogenesis in the animal plate is patterned by suppression of serotonergic neuron formation by Nodal. Like other metazoans, echinoderms appear to have a phase of neural development during which the specification of ectoderm restricts and patterns neurogenesis.

The logic of TGFbeta signaling

The identification of the TGFbeta cytokine signaling pathway, including membrane receptor serine/threonine kinases and Smad transcription factors as their substrates, has allowed the delineation of a process for conversion of these signals into programs of gene activation and repression that underlie critical cell fate and developmental decisions. The deconstruction of one of these responses - the cell cycle arrest response - into its elemental molecular parts has shed light into the mechanisms used by tumors to evade surveillance and cause metastasis.

The mechanism of nuclear export of Smad3 involves exportin 4 and Ran

Transforming growth factor beta (TGF-beta) receptors phosphorylate Smad3 and induce its nuclear import so it can regulate gene transcription. Smad3 can return to the cytoplasm to propagate further cycles of signal transduction or to be degraded. We demonstrate that Smad3 is exported by a constitutive mechanism that is insensitive to leptomycin B. The Mad homology 2 (MH2) domain is responsible for Smad3 export, which requires the GTPase Ran. Inactive, GDP-locked RanT24N or nuclear microinjection of Ran GTPase activating protein 1 blocked Smad3 export. Inactivation of the Ran guanine nucleotide exchange factor RCC1 inhibited Smad3 export and led to nuclear accumulation of phosphorylated Smad3. A screen for importin/exportin family members that associate with Smad3 identified exportin 4, which binds a conserved peptide sequence in the MH2 domain of Smad3 in a Ran-dependent manner. Exportin 4 is sufficient for carrying the in vitro nuclear export of Smad3 in cooperation with Ran. Knockdown of endogenous exportin 4 completely abrogates the export of endogenous Smad3. A short peptide representing the minimal interaction domain in Smad3 effectively competes with Smad3 association to exportin 4 and blocks nuclear export of Smad3 in vivo. We thus delineate a novel nuclear export pathway for Smad3.

IFN-type-I-mediated signaling is regulated by modulation of STAT2 nuclear export

Signaling through the IFN type I receptor is mediated by assembly of the ISGF3 complex consisting of STAT1, STAT2 and IRF9. Whereas STAT1 is instrumentalized by many cytokines, STAT2 is specifically used by type I IFNs. Here, we report that the main regulatory mechanism of nuclear accumulation of STAT2 is nuclear export. We determined the kinetics of nucleocytoplasmic shuttling of STAT2 in living cells. In the absence of IFN, a virtually exclusive cytoplasmic localisation of STAT2 can be detected. Nevertheless, STAT2 is permanently and rapidly shuttling between the cytoplasm and the nucleus. The steady-state localization is explained by a very efficient nuclear export. Our studies indicate that at least two pathways (one of which is CRM1-dependent, the other not yet identified) are responsible for clearing the nucleus from STAT2. The constitutive nucleocytoplasmic shuttling of STAT2 does neither depend on the presence of IRF9 or STAT1, nor does it require tyrosine phosphorylation. Upon treatment with IFN type I, nuclear export of STAT2 is completely abolished in cells used within this study, whereas nuclear import is functioning. This explains the observed nuclear accumulation of STAT2. We have identified a region in the C-terminus of STAT2 that is essential for its almost exclusively cytoplasmic localization in the absence of IFN and responsible for CRM1-specific export. In comparative studies we show that nucleocytoplasmic shuttling of STAT2 is significantly different from that of STAT1. STAT1 is also shuttling in the absence of IFN, but the exchange rate in unstimulated cells is more than ten times lower. We further show that the latent STAT2 protein has stronger intrinsic nuclear-export activity than STAT1. Together, these observations lead to a model for IFN-type-I-induction in which the receptor-mediated heterodimerization overcomes the slow nuclear import of STAT1 and blocks the strong STAT2 export activity that leads to the accumulation of both signal transducers in the nucleus.

Identification of phosphatases for Smad in the BMP/DPP pathway

Phosphorylation of the SSXS motif of Smads is critical in activating the transforming growth factor beta (TGF-beta) and bone morphogenetic protein (BMP) pathways. However, the phosphatase(s) involved in dephosphorylating and hence inactivating Smads remained elusive. Through RNA interference (RNAi)-based screening of serine/threonine phosphatases in Drosophila S2 cells, we identified pyruvate dehydrogenase phosphatase (PDP) to be required for dephosphorylation of Mothers against Decapentaplegic (MAD), a Drosophila Smad. Biochemical and genetic evidence suggest that PDP directly dephosphorylates MAD and inhibits signal transduction of Decapentaplegic (DPP). We show that the mammalian PDPs are important in dephosphorylation of BMP-activated Smad1 but not TGF-beta-activated Smad2 or Smad3. Thus, PDPs specifically inactivate Smads in the BMP/DPP pathway.

Role of transforming growth factor-beta in hematologic malignancies

The transforming growth factor-beta (TGF-beta) signaling pathway is an essential regulator of cellular processes, including proliferation, differentiation, migration, and cell survival. During hematopoiesis, the TGF-beta signaling pathway is a potent negative regulator of proliferation while stimulating differentiation and apoptosis when appropriate. In hematologic malignancies, including leukemias, myeloproliferative disorders, lymphomas, and multiple myeloma, resistance to these homeostatic effects of TGF-beta develops. Mechanisms for this resistance include mutation or deletion of members of the TGF-beta signaling pathway and disruption of the pathway by oncoproteins. These alterations define a tumor suppressor role for the TGF-beta pathway in human hematologic malignancies. On the other hand, elevated levels of TGF-beta can promote myelofibrosis and the pathogenesis of some hematologic malignancies through their effects on the stroma and immune system. Advances in the TGF-beta signaling field should enable targeting of the TGF-beta signaling pathway for the treatment of hematologic malignancies.

Signal processing in the TGF-beta superfamily ligand-receptor network

The TGF-β pathway plays a central role in tissue homeostasis and morphogenesis. It transduces a variety of extracellular signals into intracellular transcriptional responses that control a plethora of cellular processes, including cell growth, apoptosis, and differentiation. We use computational modeling to show that coupling of signaling with receptor trafficking results in a highly versatile signal-processing unit, able to sense by itself absolute levels of ligand, temporal changes in ligand concentration, and ratios of multiple ligands. This coupling controls whether the response of the receptor module is transient or permanent and whether or not different signaling channels behave independently of each other. Our computational approach unifies seemingly disparate experimental observations and suggests specific changes in receptor trafficking patterns that can lead to phenotypes that favor tumor progression.

Sensing of the environment by cells relies extensively on receptors that bind extracellular molecules and trigger intracellular responses. The TGF-β pathway transduces a broad range of extracellular signals into transcriptional responses that affect many cellular processes, including cell growth, apoptosis, differentiation, homeostasis, and morphogenesis. It is used, for instance, to control the precise patterns and forms that arise during development, and its malfunction contributes to a wide variety of diseases and developmental disorders. Here the authors develop a concise computational model of the TGF-β pathway and show that the first layer of communication with the environment, the ligand-receptor network, is not merely a passive transducer of signals but rather embeds properties that makes it a signal processing unit. Receptors traffic between different cellular compartments from which they signal distinctly, leading to an unexpected richness of types of behavior that is not apparent from the simplicity of the typical cartoon representations of this pathway. At the receptor level, the system can select among different functioning modes to sense absolute levels of ligand, temporal changes in ligand concentration, and ratios of multiple ligands. This extra level of regulation can explain a wide variety of phenomena and leads to a unified interpretation of seemingly disparate experimental observations.