The transforming growth factor beta (TGF-beta) family of growth factors control the development and homeostasis of most tissues in metazoan organisms. Work over the past few years has led to the elucidation of a TGF-beta signal transduction network. This network involves receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins, which move into the nucleus, where they activate target gene transcription in association with DNA-binding partners. Distinct repertoires of receptors, SMAD proteins, and DNA-binding partners seemingly underlie, in a cell-specific manner, the multifunctional nature of TGF-beta and related factors. Mutations in these pathways are the cause of various forms of human cancer and developmental disorders.

TGF-beta signal transduction.

The recent identification of the SMAD family of signal transducer proteins has unravelled the mechanisms by which transforming growth factor-beta (TGF-beta) signals from the cell membrane to the nucleus. Pathway-restricted SMADs are phosphorylated by specific cell-surface receptors that have serine/threonine kinase activity, then they oligomerize with the common mediator Smad4 and translocate to the nucleus where they direct transcription to effect the cell's response to TGF-beta. Inhibitory SMADs have been identified that block the activation of these pathway-restricted SMADs.

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

Smad transcription factors lie at the core of one of the most versatile cytokine signaling pathways in metazoan biology-the transforming growth factor-beta (TGFbeta) pathway. Recent progress has shed light into the processes of Smad activation and deactivation, nucleocytoplasmic dynamics, and assembly of transcriptional complexes. A rich repertoire of regulatory devices exerts control over each step of the Smad pathway. This knowledge is enabling work on more complex questions about the organization, integration, and modulation of Smad-dependent transcriptional programs. We are beginning to uncover self-enabled gene response cascades, graded Smad response mechanisms, and Smad-dependent synexpression groups. Our growing understanding of TGFbeta signaling through the Smad pathway provides general principles for how animal cells translate complex inputs into concrete behavior.

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Smad transcription factors

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Controlling TGF-β signaling

Cell proliferation, differentiation and death are controlled by a multitude of cell-cell signals, and loss of this control has devastating consequences Prominent among these regulatory signals is the transforming growth factor-beta (TGF-beta) family of cytokines, which can trigger a bewildering diversity of responses, depending on the genetic makeup and environment of the target cell What are the networks of cell-specific molecules that mould the TGF-beta response to each cell's needs?

How cells read TGF-beta signals.

SMAD proteins1 have been identified as signalling mediators of the TGF-β superfamily, which is involved in a range of biological activities including cell growth, morphogenesis, development and immune responses2,3. Smad1, Smad2, Smad3 and Smad5 are ligand-specific: Smad1 and Smad5 transduce signals from bone morphogenetic proteins4,5,6,7, and Smad2 and Smad3 mediate signalling by TGF-β and activin8,9, whereas Smad4 acts as a common signalling component10. For example, Smad2 is phosphorylated by the TGF-β type I receptor upon ligand binding, forms a heteromer with Smad4, and then translocates into the nucleus where it activates transcription10,11. Here we report the isolation of Smad6 in the mouse. Smad6 is quite different in structure from the other SMAD proteins, and forms stable associations with type I receptors. Smad6 interferes with the phosphorylation of Smad2 and the subsequent heteromerization with Smad4, but does not inhibit the activity of Smad3. Smad6 also inhibits the phosphorylation of Smad1 that is induced by the bone morphogenetic protein type IB receptor. These data indicate that signals of the TGF-β superfamily are regulated both positively and negatively by members of the SMAD family.

Smad6 inhibits signalling by the TGF-Beta superfamily

Background

Smad proteins are novel transcriptional regulators mediating the signalling of the transforming growth factor-β (TGF-β) superfamily. Coactivators such as p300/CBP promote transactivation by various transcription factors through a direct interaction with them. Adenoviral oncoprotein E1A, which binds p300, was shown to inhibit the signalling of TGF-β. These findings raise the possibility that p300 may be involved in TGF-β signalling.



Results

We investigated whether p300 is involved in transactivation by Smads. p300 enhanced the Smad-induced transactivation of p3TP-Lux, a TGF-β responsive reporter. E1A inhibited this enhancement, and the inhibition required its ability to bind p300/CBP. p300 and Smad3, as well as Smad2, interacted in vivo in a ligand-dependent manner. The binding region in Smad3 was its C-terminal half that was previously shown to possess an intrinsic transactivation activity. The binding region in p300 was mapped to its C-terminal 678 amino acids. The minimal Smad2/3-interacting region, as well as the rest of the p300, inhibited the transactivation of p3TP-Lux in a dominant-negative fashion.



Conclusion

p300 interacted with Smad2 and Smad3 in a ligand-dependent manner, and enhanced the transactivation by Smads. Our results present the molecular basis of the transactivation by Smad proteins.

Role of p300, a transcriptional coactivator, in signalling of TGF-beta.

The differentiation, growth, and survival of endothelial cells (ECs) are regulated by multiple signalling pathways, such as vascular endothelial growth factors (VEGFs) and angiopoietins through their receptor tyrosine kinases, VEGF receptor (VEGFR) 2 and Tie2, respectively. Bone morphogenetic proteins (BMPs), members of the transforming growth factor (TGF)-β family, have been implicated in the development and maintenance of vascular systems. However, their effects on EC proliferation remain to be elucidated. In the present study, we show that BMPs induce the proliferation and migration of mouse embryonic stem cell (ESC)-derived endothelial cells (MESECs) and human microvascular endothelial cells (HMECs). Addition of BMP-4 to culture induced significant proliferation and migration of both types of ECs. BMP-4 also increased the expression and phosphorylation of VEGFR2 and Tie2. These findings suggest that BMP signalling activates endothelium via activation of VEGF/VEGFR2 and Angiopoietin/Tie2 signalling.

BMPs Promote Proliferation and Migration of Endothelial Cells via Stimulation of VEGF-A/VEGFR2 and Angiopoietin-1/Tie2 Signalling

http://www.jbc.org/content/274/40/28716.full.pdf

Ayako Nishihara

Papers

Smad6 inhibits signalling by the TGF-Beta superfamily

Role of p300, a transcriptional coactivator, in signalling of TGF-beta.

BMPs Promote Proliferation and Migration of Endothelial Cells via Stimulation of VEGF-A/VEGFR2 and Angiopoietin-1/Tie2 Signalling

E1A Inhibits Transforming Growth Factor-β Signaling through Binding to Smad Proteins

Effects of Lysophosphatidic Acid on Proliferation of Stellate Cells and Hepatocytes in Culture