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Antagonistic regulation of a proline-rich transcription factor by transforming growth factor beta and tumor necrosis
1Institute of Animal Biology, University of Lausanne, CH-1015 Lausanne, Switzerland. nicolas.mermod@iba.unil.ch
Abstract:
Transforming growth factor beta (TGF-beta) and tumor necrosis factor alpha (TNF-alpha) often exhibit antagonistic actions on the regulation of various activities such as immune responses, cell growth, and gene expression. However, the molecular mechanisms involved in the mutually opposing effects of TGF-beta and TNF-alpha are unknown. Here, we report that binding sites for the transcription factor CTF/NF-I mediate antagonistic TGF-beta and TNF-alpha transcriptional regulation in NIH3T3 fibroblasts. TGF-beta induces the proline-rich transactivation domain of specific CTF/NF-I family members, such as CTF-1, whereas TNF-alpha represses both the uninduced as well as the TGF-beta-induced CTF-1 transcriptional activity. CTF-1 is thus the first transcription factor reported to be repressed by TNF-alpha. The previously identified TGF-beta-responsive domain in the proline-rich transcriptional activation sequence of CTF-1 mediates both transcriptional induction and repression by the two growth factors. Analysis of potential signal transduction intermediates does not support a role for known mediators of TNF-alpha action, such as arachidonic acid, in CTF-1 regulation. However, overexpression of oncogenic forms of the small GTPase Ras or of the Raf-1 kinase represses CTF-1 transcriptional activity, as does TNF-alpha. Furthermore, TNF-alpha is unable to repress CTF-1 activity in NIH3T3 cells overexpressing ras or raf, suggesting that TNF-alpha regulates CTF-1 by a Ras-Raf kinase-dependent pathway. Mutagenesis studies demonstrated that the CTF-1 TGF-beta-responsive domain is not the primary target of regulatory phosphorylations. Interestingly, however, the domain mediating TGF-beta and TNF-alpha antagonistic regulation overlapped precisely the previously identified histone H3 interaction domain of CTF-1. These results identify CTF-1 as a molecular target of mutually antagonistic TGF-beta and TNF-alpha regulation, and they further suggest a molecular mechanism for the opposing effects of these growth factors on gene expression.
Insights
Transforming growth factor beta (TGF-beta) and tumor necrosis factor alpha (TNF-alpha) have opposing effects on gene expression. This study reveals that the transcription factor CTF-1 mediates these antagonistic actions through a Ras-Raf kinase-dependent pathway.
Area of Science:
- Cellular and Molecular Biology
- Signal Transduction
- Gene Regulation
Background:
- Transforming growth factor beta (TGF-beta) and tumor necrosis factor alpha (TNF-alpha) are key cytokines with often opposing roles in cellular processes.
- The molecular mechanisms underlying the antagonistic transcriptional regulation by TGF-beta and TNF-alpha remain largely unknown.
Purpose of the Study:
- To elucidate the molecular mechanisms by which TGF-beta and TNF-alpha exert their opposing transcriptional effects.
- To identify the specific transcription factors and signaling pathways involved in mediating this antagonism.
Main Methods:
- Utilized NIH3T3 fibroblasts to study transcriptional regulation.
- Employed reporter gene assays to assess the activity of the CTF-1 promoter.
- Investigated the roles of Ras and Raf-1 kinases in mediating TNF-alpha signaling.
- Performed mutagenesis studies to map regulatory domains within CTF-1.
Main Results:
- Identified binding sites for the transcription factor CTF/NF-I as crucial mediators of antagonistic TGF-beta and TNF-alpha regulation.
- Demonstrated that TGF-beta induces CTF-1 activity, while TNF-alpha represses it.
- Showed that TNF-alpha represses CTF-1 activity via a Ras-Raf kinase-dependent pathway.
- Found that the TGF-beta-responsive domain of CTF-1, which interacts with histone H3, is critical for mediating the antagonistic effects of both growth factors.
Conclusions:
- CTF-1 is a direct molecular target of mutually antagonistic TGF-beta and TNF-alpha regulation.
- A novel mechanism involving a Ras-Raf kinase pathway explains TNF-alpha's repression of CTF-1.
- The findings provide insight into how these opposing growth factors coordinately regulate gene expression.