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The transcription factor E2F-1 modulates TGF-beta1 RNA expression in glial cells
P Thatikunta1, G V Raj, M Kundu
1Center for Neurovirology, Allegheny University of the Health Sciences, Philadelphia, Pennsylvania 19107, USA.
Abstract:
The cell type specificity of the regulation of expression of the potent growth inhibitory cytokine transforming growth factor-beta (TGF-beta), prompted our analyses of the regulation of TGF-beta1 gene expression in glial cells by viral and cellular oncoproteins. We have shown that SV40 T-antigen diminished TGF-beta1 expression in glial cells and this repression was dependent on the ability of T-antigen to interact with the tumor suppressor protein, pRb, and two structurally related proteins, p107 and p130. The cellular transcription factor E2F-1, which is a downstream effector of T-antigen, was unable to influence expression from the TGF-beta1 promoter by itself. Interestingly, E2F-1 could overcome viral T-antigen-mediated repression of the TGF-beta1 promoter, suggesting potential feedback loop between TGF-beta and E2F in virally transformed glial cells. Using deletion analyses, we have mapped two E2F-1-responsive regions on the TGF-beta1 promoter: a T-antigen-dependent negative regulatory sequence (TdNRS) between -323 and -175, and a T-antigen-independent positive regulatory sequence (TiPRS) between -34 and +10 on the TGF-beta1 promoter. Further examination of TiPRS revealed the presence of a functional E2F binding site. Interestingly, the amino terminus of E2F-1 was required for its activation of TGF-beta1 expression, as mutations in that domain abolished the ability of E2F-1 to increase TGF-beta1 expression. These data suggest that yet-uncharacterized interaction between the amino terminus of E2F-1 and cellular proteins regulates TGF-beta1 expression. The mechanism for E2F-1-mediated T-antigen-dependent regulation of TGF-beta1 expression from TdNRS awaits further characterization.
Insights
SV40 T-antigen represses TGF-beta1 expression in glial cells via pRb family proteins. E2F-1 can overcome this repression, suggesting a feedback loop involving TGF-beta and E2F in viral glial transformation.
Area of Science:
- Molecular Biology
- Cell Biology
- Virology
Background:
- Transforming growth factor-beta (TGF-beta) is a potent cytokine with cell-type-specific regulatory functions.
- Viral and cellular oncoproteins can influence the expression of growth-regulatory genes like TGF-beta1.
- Glial cells play crucial roles in the central nervous system and are susceptible to viral transformation.
Purpose of the Study:
- To investigate the regulation of TGF-beta1 gene expression in glial cells by viral oncoproteins, specifically SV40 T-antigen.
- To elucidate the role of the cellular transcription factor E2F-1 in mediating TGF-beta1 expression and its interaction with viral oncoproteins.
Main Methods:
- Deletion analyses of the TGF-beta1 promoter to identify regulatory regions.
- Assays to assess the interaction between SV40 T-antigen, pRb family proteins, and E2F-1.
- Mutational analysis of E2F-1 to determine functional domains involved in TGF-beta1 regulation.
Main Results:
- SV40 T-antigen repressed TGF-beta1 expression in glial cells, dependent on its interaction with pRb, p107, and p130.
- E2F-1 alone did not affect TGF-beta1 promoter activity but could overcome T-antigen-mediated repression.
- Two E2F-1-responsive regions were mapped: a T-antigen-dependent negative regulatory sequence (TdNRS) and a T-antigen-independent positive regulatory sequence (TiPRS) containing an E2F binding site.
- The amino terminus of E2F-1 was essential for its ability to activate TGF-beta1 expression.
Conclusions:
- SV40 T-antigen-mediated repression of TGF-beta1 in glial cells involves interactions with pRb family proteins.
- A potential feedback loop exists between TGF-beta and E2F in virally transformed glial cells.
- Regulation of TGF-beta1 expression by E2F-1 involves its amino terminus and interactions with unknown cellular proteins, with specific regulatory elements on the TGF-beta1 promoter.