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Association of Smads with lymphoid enhancer binding factor 1/T cell-specific factor mediates cooperative signaling by
E Labbé1, A Letamendia, L Attisano
1Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada, M5S 1A8.
Abstract:
The transforming growth factor-beta (TGFbeta) and Wnt/wingless pathways play pivotal roles in tissue specification during development. Activation of Smads, the effectors of TGFbeta superfamily signals, results in Smad translocation from the cytoplasm into the nucleus where they act as transcriptional comodulators to regulate target gene expression. Wnt/wingless signals are mediated by the DNA-binding HMG box transcription factors lymphoid enhancer binding factor 1/T cell-specific factor (LEF1/TCF) and their coactivator beta-catenin. Herein, we show that Smad3 physically interacts with the HMG box domain of LEF1 and that TGFbeta and Wnt pathways synergize to activate transcription of the Xenopus homeobox gene twin (Xtwn). Disruption of specific Smad and LEF1/TCF DNA-binding sites in the promoter abrogates synergistic activation of the promoter. Consistent with this observation, introduction of Smad sites into a TGFbeta-insensitive LEF1/TCF target gene confers cooperative TGFbeta and Wnt responsiveness to the promoter. Furthermore, we demonstrate that TGFbeta-dependent activation of LEF1/TCF target genes can occur in the absence of beta-catenin binding to LEF1/TCF and requires both Smad and LEF1/TCF DNA-binding sites in the Xtwn promoter. Thus, our results show that TGFbeta and Wnt signaling pathways can independently or cooperatively regulate LEF1/TCF target genes and suggest a model for how these pathways can synergistically activate target genes.
Insights
Transforming growth factor-beta (TGFbeta) and Wnt signaling pathways cooperate to regulate gene expression during development. Smad3 physically interacts with LEF1, synergistically activating the Xenopus twin (Xtwn) gene transcription.
Area of Science:
- Developmental Biology
- Molecular Biology
- Signaling Pathways
Background:
- Transforming growth factor-beta (TGFbeta) and Wnt/wingless signaling pathways are crucial for embryonic development.
- Smads mediate TGFbeta signals, translocating to the nucleus to regulate gene expression.
- Wnt signals involve lymphoid enhancer binding factor 1/T cell-specific factor (LEF1/TCF) and beta-catenin.
Purpose of the Study:
- To investigate the interaction between TGFbeta and Wnt signaling pathways.
- To elucidate the mechanism of synergistic gene activation by these pathways.
Main Methods:
- Physical interaction assays between Smad3 and LEF1.
- Reporter gene assays to measure promoter activity.
- Site-directed mutagenesis to disrupt DNA-binding sites.
Main Results:
- Smad3 physically interacts with the HMG box domain of LEF1.
- TGFbeta and Wnt pathways synergistically activate the Xenopus homeobox gene twin (Xtwn) promoter.
- Both Smad and LEF1/TCF DNA-binding sites are essential for synergistic activation.
- TGFbeta can activate LEF1/TCF target genes independently of beta-catenin.
Conclusions:
- TGFbeta and Wnt signaling pathways can independently or cooperatively regulate LEF1/TCF target genes.
- A model is proposed for synergistic gene activation involving Smad and LEF1/TCF interactions.