SurveyRemarkable versatility of Smad proteins in the nucleus of transforming growth factor-β activated cells
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 (orkhead ctivin ignal ransducer), 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
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