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Transforming growth factor-beta 2 is a transcriptional target for Akt/protein kinase B via forkhead transcription
Ahmed A Samatar1, Luquan Wang, Asra Mirza
1Department of Tumor Biology, Schering-Plough Research Institute, Kenilworth, New Jersey 07033, USA.
Abstract:
Tumors evade cell death by constitutively activating cell survival pathways and suppressing intrinsic death machinery. Activation of cell survival pathways leads to transcriptional repression of genes associated with cell death and activation of ones promoting anti-apoptosis. Akt/protein kinase B phosphorylates forkhead transcription factors and prevents their nuclear localization, leading to repression of genes involved in apoptosis, such as Fas ligand (FasL). Using bioinformatic approaches, we have identified three consensus sequences for forkhead transcription factor binding in transforming growth factor beta2 (TGF-beta2) promoter. TGF-beta inhibits cell proliferation and induces apoptosis in many cell types, and acquisition of TGF-beta resistance is linked to tumorigenesis. In this study, we show that activated Akt down-regulates TGF-beta2 promoter, and sequences within the promoter that are related to consensus forkhead binding sites are necessary for repression. Forkhead factor FKHRL1 binds in vitro to the three consensus sequences and can activate TGF-beta2 promoter in normal and Akt-transformed cell lines. In human breast and pancreatic tumors, activated Akt expression correlated with down-regulation of TGF-beta 2 mRNA levels. A number of tumor cells expressing activated Akt were responsive to TGF-beta addition, indicating the presence of an intact TGF-beta-signaling pathway. These results suggest that repression of TGF-beta 2 promoter activity in cells expressing activated Akt may play a role in promoting tumorigenesis and escape from the growth-inhibitory and/or apoptotic effects of TGF-beta.
Insights
Tumors resist cell death by activating survival pathways, like Akt, which represses the TGF-beta2 gene. This repression, driven by forkhead transcription factors, may contribute to tumor growth and resistance to TGF-beta
Area of Science:
- Molecular Biology
- Cancer Biology
- Cell Signaling
Background:
- Tumors evade apoptosis by activating survival pathways, suppressing cell death machinery.
- The Akt pathway phosphorylates forkhead transcription factors, repressing apoptosis-related genes like Fas ligand.
- Transforming growth factor beta2 (TGF-beta2) normally inhibits proliferation and induces apoptosis, and resistance to it is linked to tumorigenesis.
Purpose of the Study:
- To investigate the role of activated Akt in regulating TGF-beta2 expression.
- To identify the mechanisms by which Akt influences TGF-beta2 promoter activity.
- To explore the implications of TGF-beta2 repression in tumorigenesis.
Main Methods:
- Bioinformatic analysis to identify forkhead transcription factor binding sites in the TGF-beta2 promoter.
- In vitro binding assays using forkhead factor FKHRL1.
- Analysis of TGF-beta2 mRNA levels and TGF-beta responsiveness in tumor cells with varying Akt activation status.
Main Results:
- Activated Akt was found to down-regulate the TGF-beta2 promoter.
- Specific sequences within the TGF-beta2 promoter, recognized by forkhead binding sites, were essential for Akt-mediated repression.
- FKHRL1 binds to these sequences and can activate the TGF-beta2 promoter.
- Activated Akt expression in human breast and pancreatic tumors correlated with reduced TGF-beta2 mRNA levels.
- Tumor cells with activated Akt often retained responsiveness to TGF-beta, suggesting an intact signaling pathway.
Conclusions:
- Repression of TGF-beta2 promoter activity by activated Akt, mediated by forkhead transcription factors, may be a mechanism tumors use to escape growth inhibition and apoptosis.
- This pathway contributes to tumorigenesis by promoting resistance to TGF-beta's anti-cancer effects.