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Transcriptional regulation by Smads: crosstalk between the TGF-beta and Wnt pathways
A Letamendia1, E Labbé, L Attisano
1Department of Anatomy and Cell Biology, University of Toronto, Ontario, Canada.
Background:
Several studies have shown that cooperation between transforming growth factor beta (TGF-beta) and Wnt/wingless signaling pathways plays a role in controlling certain developmental events. These factors elicit their biological effects through distinct pathways in which TGF-beta and Wnt signaling induce activation of the transcriptional regulators Smads and lymphoid enhancer binding factor/T-cell-specific factor (LEF/TCF), respectively. To understand the mechanism for cooperativity between these pathways, we have investigated the molecular mechanism for this synergistic effect.
Methods:
Transcriptional assays were conducted by transient transfection of HepG2 cells with use of luciferase reporter constructs. Protein/protein interaction studies were conducted in vitro with the use of glutathione-S-transferase pull-down assays and in intact cells by immunoprecipitation and immunoblotting.
Results:
We show that Smads physically interact with LEF1/TCF transcription factors and that specific DNA binding sites in the Xenopus twin promoter are required for synergistic activation by TGF-beta and Wnt pathways. In addition, we demonstrate that TGF-beta-dependent activation of LEF1/TCF target genes can occur independently of beta-catenin, an essential component of the Wnt signaling pathway.
Conclusions:
TGF-beta and Wnt signaling pathways can independently or cooperatively regulate LEF1/TCF target genes. This suggests that the cooperation between these pathways may be important for the specification of cell fates during development.
Insights
Transforming growth factor beta (TGF-beta) and Wnt signaling pathways cooperate to regulate gene expression. Smads physically interact with LEF1/TCF transcription factors, influencing cell fate during development.
Area of Science:
- Molecular Biology
- Developmental Biology
- Cell Signaling
Background:
- Transforming growth factor beta (TGF-beta) and Wnt/wingless signaling pathways are crucial for developmental events.
- These pathways activate distinct transcriptional regulators: Smads for TGF-beta and lymphoid enhancer binding factor/T-cell-specific factor (LEF/TCF) for Wnt.
Purpose of the Study:
- To elucidate the molecular mechanisms underlying the synergistic interaction between TGF-beta and Wnt signaling pathways.
- To understand how these pathways cooperate in regulating gene expression and developmental processes.
Main Methods:
- Transcriptional assays using luciferase reporter constructs in HepG2 cells.
- In vitro and in vivo protein-protein interaction studies, including glutathione-S-transferase pull-down assays, immunoprecipitation, and immunoblotting.
Main Results:
- Demonstrated physical interaction between Smads and LEF1/TCF transcription factors.
- Identified specific DNA binding sites in the Xenopus twin promoter essential for synergistic activation by TGF-beta and Wnt pathways.
- Showed that TGF-beta can activate LEF1/TCF target genes independently of beta-catenin.
Conclusions:
- TGF-beta and Wnt signaling pathways exhibit independent and cooperative regulation of LEF1/TCF target genes.
- The cooperation between these signaling pathways is likely vital for cell fate specification during embryonic development.