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Two serum response elements mediate transcriptional repression of human smooth muscle alpha-actin promoter in
1Department of Tumor Biology, Schering-Plough Research Institute, Kenilworth, New Jersey 07033, USA.
Abstract:
The mechanism by which activated ras oncogene expression leads to repression of genes encoding specific actin filament proteins is not understood. However, these changes associated with loss of organized actin filaments, are necessary to maintain the transformed phenotype. The human smooth muscle (sm) alpha-actin promoter is repressed in ras-transformed fibroblast cells and derepressed in revertant cell lines. In this study, we demonstrate that two serum response elements (SREs) present in the alpha-actin promoter are required for transcriptional repression in ras-transformed cells and the two SREs act synergistically to repress heterologous promoters in a ras-transformation dependent manner. Serum response factor (SRF), which can bind to the sm alpha-actin SREs, restores alpha-actin promoter activity in ras-transformed cells. c-Fos, c-Jun and YY1 also repress alpha-actin promoter through SREs, suggesting that these transcription factors may play a role in repressing alpha-actin promoter in ras-transformed cells.
Insights
Ras oncogene activation represses actin gene expression through specific DNA elements called serum response elements (SREs). This repression is crucial for maintaining the transformed cell phenotype, with transcription factors potentially mediating the effect.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncology
Background:
- Activated ras oncogene expression causes repression of genes encoding actin filament proteins.
- This repression is essential for maintaining the transformed cellular phenotype.
- The human smooth muscle (sm) alpha-actin promoter is specifically repressed in ras-transformed cells.
Purpose of the Study:
- To investigate the mechanism of transcriptional repression of actin genes by activated ras.
- To identify the specific DNA elements and transcription factors involved in ras-mediated repression of the sm alpha-actin promoter.
Main Methods:
- Analysis of the human smooth muscle alpha-actin promoter in ras-transformed and revertant fibroblast cells.
- Deletion and mutation analysis of serum response elements (SREs) within the alpha-actin promoter.
- Reporter gene assays to assess promoter activity and transcriptional regulation.
- Electrophoretic mobility shift assays (EMSAs) to study transcription factor binding.
Main Results:
- Two SREs in the sm alpha-actin promoter are critical for transcriptional repression in ras-transformed cells.
- These SREs act synergistically to repress heterologous promoters in a manner dependent on ras transformation.
- Reintroduction of serum response factor (SRF) restores alpha-actin promoter activity in ras-transformed cells.
- Transcription factors c-Fos, c-Jun, and YY1 were identified as potential repressors of the alpha-actin promoter via SREs.
Conclusions:
- Transcriptional repression of sm alpha-actin by activated ras oncogene is mediated through specific SREs.
- Synergistic action of SREs and involvement of transcription factors like c-Fos, c-Jun, and YY1 are implicated in maintaining the transformed phenotype.
- SRF can counteract the ras-induced repression, highlighting its role in regulating actin gene expression.