Elsevier

Cytokine & Growth Factor Reviews

Volume 10, Issues 3–4, September–December 1999, Pages 187-199
Cytokine & Growth Factor Reviews

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Remarkable versatility of Smad proteins in the nucleus of transforming growth factor-β activated cells

https://doi.org/10.1016/S1359-6101(99)00012-XGet rights and content

Abstract

Smad proteins were identified three years ago as intracellular mediators of signaling by Transforming Growth Factor-β (TGF-β) family members. Two subclasses of the Smad proteins, the receptor-regulated Smads and common mediator Smads, transduce signals from the cell surface to the nucleus, where they participate in the regulation of gene expression. Meanwhile, it has become evident that Smads should be envisaged as very versatile proteins, which integrate multiple signaling pathways and can directly affect target gene expression in many ways. Indeed, their direct binding to DNA and their interaction in the nucleus with non-Smad proteins, many of which are DNA-binding activators or repressors of transcription uncover a unique but complex mode of action. We summarize some of the most recent data with regard to this aspect in this rapidly advancing field.

Section snippets

Structure-function aspects and introduction to nuclear function of Smads

Ligands of the Transforming Growth Factor type β (TGF-β) family mediate cell–cell interaction and elicit a wide variety of biological responses in different cell types. They have been documented extensively to regulate cell proliferation and differentiation, apoptosis and extracellular matrix production. These activities render them essential for many biological processes including mesoderm induction, axis formation, patterning and organogenesis during embryonic development, and tumor

Different signaling pathways converge on Smads in a multistimulatory environment

There are many examples of TGF-β ligands acting synergistically or antagonistically with other ligands some of which signal via tyrosine kinase receptors [24]. The few published studies may represent the basis for an emerging picture that some of these pathways can integrate on Smads. Epidermal Growth Factor, through stimulation of the Ras/MAPK (mitogen activated protein kinase) pathway, induces in cell lines phosphorylation of Smad1 at four ERK (extracellular signal regulated kinase) consensus

Smads bind DNA and act as transcription factors

When fused to a well-defined heterologous DNA binding domain (like the one from the yeast transcription activator GAL4), the MH2 domain of R-Smads displays a transcriptional activation activity in mammalian cells [1], [2], [3], [4], [5], [6]. This, together with the observed ligand induced nuclear accumulation of Smad complexes, led to the idea that Smads might bind to DNA and act as transcription factors. A sequence-specific binding function of Smads became first apparent in Drosophila, where

Smads synergize with transcription factors to induce gene transcription

Although Smad4 and some R-Smads can bind DNA and activate transcription from multimerized Smad binding elements, in the context of natural promoters they need to co-operate with transcription factors to mediate their effect. An important contribution in this respect came from the cloning of FAST-1 (Forkhead activin signal transducer), a winged-helix protein able to bind to the activin response element (ARE) of the Mix.2 promoter in Xenopus [43]. Binding of FAST-1 to the ARE is in itself not

Smads act as transcriptional repressors

Smads usually participate in the induction of gene transcription, but it is important to note that in some cases they have been shown to act as transcriptional repressors. Like Smad2, Smad3 can support the formation of a DNA binding complex on the goosecoid promoter that contains FAST-2 and Smad4. However, unlike Smad2, Smad3 inhibits ligand-dependent transcriptional activation of the goosecoid promoter. This inhibition is dependent on the Smad3 MH1 domain which can bind to the GC-rich Smad4

Smads interact with transcriptional co-repressors or co-activators

The MH2 domains of Smad1, Smad2 and Smad3 interact in a ligand-dependent manner with the transcriptional co-activators CREB Binding Protein (CBP) and P300 [56], [57], [58], [59], [60], [61]. Since overexpression of the adenoviral oncoprotein E1A, which binds and sequesters P300/CBP, abolishes the transcriptional activity of Smads, it was implied that Smad-P300/CBP interaction in general is critical for Smad-dependent transcriptional induction [56], [57], [58], [59], [60], [61]. P300 and CBP

Relief of trancriptional repression by TGF-β signaling

Two transcriptional repressors have been identified in yeast two-hybrid screens, using Smad1 as bait [65], [66]. The homeodomain protein Hoxc-8 was isolated as a protein that interacted with full-length Smad1 in yeast [65]. Hoxc-8 binds to and acts as a transcriptional repressor of the osteopontin promoter, which is activated by BMP. Mutations in the promoter that interfere with binding of Hoxc-8 to its DNA target abolish this repression. Interaction of the Smad1 MH2 domain with Hoxc-8 prevents

Smad-dependent and other important signaling pathways often converge on the same promoters

Whereas Smads themselves may integrate unrelated signaling pathways, there is ample evidence that cross talk occurs at another level as well, i.e. through interaction of Smads with transcription factors which may be modulated by other signaling cascades [24]. For example, TGF-β but not BMP enhances Vitamin D receptor (VDR) mediated transcriptional activation [70]. This nuclear receptor interacts with Smad3 in a ligand (i.e. VitD or 1,25(OH)2-D3)-dependent manner. Its association with this

Synergistic action of Smad and TAK1 pathways in the regulation of gene expression

TAK1 (TGF-β activated kinase-1) a member of the MAP kinase kinase kinase (MAPKKK) family, and its upstream activator TAB1 have been implicated in mediating responses to TGF-β family members [74]. The kinase activity of TAK1 is stimulated in response to TGF-β or BMP4. The physical link of the Xenopus XTAK1 pathway with the activated BMP type I receptor can be mediated by XIAP, a human protein that is member of the emerging IAP (inhibitor of apoptosis) family [75]. XIAP has been proposed to serve

Smad-interacting proteins and disease

Very recent data indicate that mutations which disrupt interaction of Smads with transcription factors or which lead to inappropriate expression of Smad-interacting proteins could contribute to certain pathologies.

Gli proteins are zinc finger-rich transcription factors implicated in Hedgehog signaling. Smad1 and Smad2, and to a lesser extent Smad3 and Smad4, have been found to specifically interact with C-terminally truncated Gli3, but not full-length Gli3 [80]. These C-terminally truncated Gli

Concluding remarks

The past fifteen years have witnessed a tremendous progress in our knowledge on the TGF-β family, their receptors, the signal transduction and a limited number of direct target genes in the embryo and the adult organism. Compared with growth factors acting via tyrosine kinase receptors and with cytokines, the TGF-β field is catching up rapidly, and the challenge is now to meet the obligatory requirement of a multidisciplinary approach to understand the structure-function, biochemical, cell

Acknowledgments

The authors are indebted to many members of the Huylebroeck laboratory who have either joined us during the last years in our studies in this field or — when working in other projects — always participated in animated discussions, and continuously formulated appropriate criticisms and suggestions. We also want to thank P. De Waele, J.C. Smith, C.-H. Heldin, P. ten Dijke, K. Miyazono, J. van den Eijnden-van Raaij, C. Mummery, G. Gross and M. Matzuk for the enthusiastic and productive

References (95)

  • K. Yagi et al.

    Alternatively spliced variant of Smad2 lacking exon3: comparison with wild-type Smad2 and Smad3

    J. Biol. Chem.

    (1999)
  • C.Z. Song et al.

    Smad4/DPC4 and Smad3 mediate transforming growth factor-beta (TGF-beta) signaling through direct binding to a novel TGF-beta-responsive element in the human plasminogen activator inhibitor-1 promoter

    J. Biol. Chem.

    (1998)
  • S.L. Stroschein et al.

    Cooperative binding of Smad proteins to two adjacent DNA elements in the plasminogen activator inhibitor-1 promoter mediates transforming growth factor beta-induced smad-dependent transcriptional activation

    J. Biol. Chem.

    (1999)
  • L.J. Jonk et al.

    Identification and functional characterization of a Smad binding element (SBE) in the JunB promoter that acts as a transforming growth factor-beta, activin, and bone morphogenetic protein-inducible enhancer

    J. Biol. Chem.

    (1998)
  • E. Labbé et al.

    Smad2 and Smad3 positively and negatively regulate TGF-β dependent transcription through the forkhead DNA-binding protein FAST2

    Mol. Cell.

    (1998)
  • S. Zhou et al.

    Characterization of human FAST-1, a TGF beta and activin signal transducer

    Mol. Cell

    (1998)
  • E. Weisberg et al.

    A mouse homologue of FAST-1 transduces TGF-β superfamily signals and is expressed during early embryogenesis

    Mech. Dev.

    (1998)
  • C. Pouponnot et al.

    Physical and functional interaction of SMADs and p300/CBP

    J. Biol. Chem.

    (1998)
  • D. Wotton et al.

    A Smad transcriptional corepressor

    Cell

    (1999)
  • X. Shi et al.

    Smad1 interacts with homeobox DNA-binding proteins in bone morphogenetic signalling

    J. Biol. Chem.

    (1999)
  • K. Verschueren et al.

    SIP1, a novel zinc finger/homeodomain repressor, interacts with Smad proteins and binds to 5′-CACCT sequences in candidate target genes

    J. Biol. Chem.

    (1999)
  • A. Jazwinska et al.

    The Drosophila gene brinker reveals a novel mechanism of Dpp target gene regulation

    Cell

    (1999)
  • G. Campbell et al.

    Transducing the Dpp morphogradient in the wing of Drosophila: regulation of Dpp targets by brinker

    Cell

    (1999)
  • Y. Sano et al.

    ATF-2 is a common nuclear target of Smad and TAK1 pathways in transforming growth factor-beta signaling

    J. Biol. Chem.

    (1999)
  • E. Oeda et al.

    Interaction of Drosophila inhibitors of apoptosis with thick veins, a type I serine/threonine kinase receptor for decapentaplegic

    J. Biol. Chem.

    (1998)
  • M. Kurokawa et al.

    The t(3;21) fusion product, AML1/Evi-1, interacts with Smad3 and blocks transforming growth factor-beta-mediated growth inhibition of myeloid cells

    Blood

    (1998)
  • S. Piccolo et al.

    Dorsoventral patterning in Xenopus: inhibition of ventral signals by direct binding of chordin to BMP-4

    Cell

    (1996)
  • S. Piccolo et al.

    Cleavage of chordin by xolloid metalloprotease suggests a role for proteolytic processing in the regulation of Spemann organizer activity

    Cell

    (1997)
  • W.R. Waldrip et al.

    Smad2 signaling in extraembryonic tissues determines anterior-posterior polarity of the early mouse embryo

    Cell

    (1998)
  • Y. Zhu et al.

    Smad3 mutant mice develop metastatic colorectal cancer

    Cell

    (1998)
  • C.H. Heldin et al.

    TGF-beta signalling from cell membrane to nucleus through SMAD proteins

    Nature

    (1997)
  • M. Kawabata et al.

    Signal transduction of the TGF-beta superfamily by Smad proteins

    J. Biochem. (Tokyo)

    (1999)
  • J. Massagué

    TGF-beta signal transduction

    Annu. Rev. Biochem.

    (1998)
  • R.W. Padgett

    Intracellular signaling: Fleshing out the TGF-β pathway

    Current Biology

    (1999)
  • M. Whitman

    Smads and early developmental signaling by the TGF-β superfamily

    Genes & Development

    (1998)
  • Y.G. Chen et al.

    Determinants of specificity in TGF-β signal transduction

    Genes Dev.

    (1998)
  • R.S. Lo et al.

    The L3 loop, a structural motif determining specific interactions between Smad proteins and TGF-β receptors

    EMBO J.

    (1998)
  • J.A. LeSueur et al.

    Spemann organizer activity of Smad10

    Development

    (1999)
  • J.L. Christian et al.

    Can’t get no SMADisfaction: Smad proteins as positive and negative regulators of TGF-beta family signals

    Bioessays

    (1999)
  • P. ten Dijke et al.

    Signaling via hetero-oligomeric complexes of typeI and type II serine/threonine kinase receptors

    Curr. Opin. Cell. Biol.

    (1996)
  • H. Inoue et al.

    Interplay of signal mediators of decapentaplegic (Dpp): molecular characterization of mothers against dpp, Medea, and daughters against dpp

    Mol. Biol. Cell

    (1998)
  • T. Brummel et al.

    The Drosophila activin receptor baboon signals through dSmad2 and controls cell proliferation but not patterning during larval development

    Genes Dev.

    (1999)
  • P. Das et al.

    Drosophila dSmad2 and Atr-I transmit activin/TGFbeta signals

    Genes to Cells

    (1999)
  • P. Das et al.

    The Drosophila gene Medea demonstrates the requirement for different classes of Smads in dpp signaling

    Development

    (1998)
  • S. Krishna et al.

    Specificity of TGFbeta signaling is conferred by distinct type I receptors and their associated SMAD proteins in Caenorhabditis elegans

    Development

    (1999)
  • G.I. Patterson et al.

    The DAF-3 Smad protein antagonizes TGF-beta-related receptor signaling in the Caenorhabditis elegans dauer pathway

    Genes Dev.

    (1997)
  • M. Kawabata et al.

    Smad proteins exist as monomers in vivo and undergo homo- and hetero-oligomerization upon activation by serine/threonine kinase receptors

    EMBO J.

    (1998)
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