Review
Signaling inputs converge on nuclear effectors in TGF-β signaling

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Abstract

Recent studies have consolidated the pivotal role of Smads as intracellular effectors of TGF-β family members. Upon binding to their specific type I and type II serine/threonine kinase receptors, each family member activates a particular subset of Smad proteins. Activated, receptor-regulated Smads form hetero-oligomeric complexes with common-partner Smads that translocate into the nucleus, where they control the expression of target genes in a cell-type-specific manner. Smads appear to function not only as nuclear effectors for TGF-β family members, but as signal integrators within an extensive intracellular network.

Section snippets

Smad family: R-Smads, Co-Smads and I-Smads

The TGF-β/Smad pathway has been conserved throughout evolution. Smad-related genes were first discovered through genetic screens in Drosophila and Caenorhabditis elegans. The name Smad is a fusion of two gene names, Drosophila mothers against dpp (Mad) and C. elegans Sma, the gene products of which were found to perform critical roles downstream of a BMP homolog, Dpp, and Daf-4 serine/threonine kinase receptor, respectively1. Nine distinct vertebrate Smad family members have now been

Activation of R- and Co-Smads

The first intracellular step in the TGF-β/Smad pathway, the recruitment of Smad2 and Smad3 to the TGF-β receptor complex, is controlled by a membrane-associated FYVE-domain-containing protein, termed Smad anchor for activation (SARA)6. SARA presents Smad2 and Smad3 to the activated type I receptor by binding cooperatively to both non-phosphorylated Smads and the TGF-β receptor complex. SARA does not interact with Smad1, Smad4 or I-Smads. Ectopic expression of SARA mutants that lack the

Intrinsic sequence-specific DNA binding of Smads

In the nucleus, R-Smad–Co-Smad complexes are involved in transcriptional regulation of target genes1. The MH1 domains of Smad3 and Smad4 were found to possess an intrinsic property to bind to specific DNA sequences that contain 5′-AGAC-3′ sequences, termed Smad-binding elements (SBEs)10, 11, 12. Whereas full-length Smad4 can bind to SBE, a prerequisite for Smad3–SBE interaction is the relief of the MH1–MH2 auto-inhibitory interaction through type I receptor phosphorylation8 or artificially by

Smad-interacting transcription factors

Although SBE is critical for TGF-β-mediated induction of certain genes, target gene selection by TGF-β cannot be explained solely by the presence or absence of SBEs in their promoters. SBE is a short sequence found in many promoters, even in some that are not responsive to TGF-β family members. The affinity of Smads for DNA is relatively low13, and Smads have been found to require other sequence-specific-binding factors (Table 1) to bind efficiently to the promoters of certain responsive genes.

Smad co-activators and co-repressors

Transactivation properties of MH2 domains for R- and Co-Smads were initially revealed by fusion of these domains to heterologous GAL4-DNA-binding domains1. Subsequent studies have provided a mechanistic explanation; R-Smads were found to interact directly though their MH2 domains in a type-I-receptor-phosphorylation-dependent manner with CBP/p300 co-activators (Fig. 3)25, 26. CBP/p300 has an intrinsic histone acetyltransferase (HAT) activity, which facilitates transcription by decreasing the

Negative regulation of the TGF-β/Smad pathway

The duration and intensity of the TGF-β/Smad response are important determinants for signaling specificity and need to be tightly regulated. For example, different sets of genes have been found to be activated at different Smad levels, and R-Smads have been found to compete for complex formation with Smad4 and even to antagonize each others functions15. To achieve a tight regulation of these processes, negative regulatory mechanisms exist that switch off the activated Smads and mitigate Smad

Conclusions and perspectives

To dissect further the role of Smads within the intracellular wiring plan, it will be important to identify the partners with which the Smads physically interact or functionally cooperate in different cell types to mediate their wide spectrum of biological responses. Smad activity is controlled by a plethora of negative inputs that could regulate the intensity, duration or selectivity of the TGF-β/Smad responses. Other questions that remain to be answered are whether Smads have functions

Acknowledgements

We thank our colleagues for valuable discussions and apologize to those authors whose work we were not able to cite because of space constraints.

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    *

    P. ten Dijke is currently at the Division of Cellular Biochemistry, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands. Email: [email protected]

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