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Mechanism of E2F1-induced apoptosis in primary vascular smooth muscle cells
Jens Stanelle1, Thorsten Stiewe, Florian Rödicker
1Center for Cancer Research and Cancer Therapy, Institute of Molecular Biology, University of Essen Medical School, Hufelandstrasse 55, 45122 Essen, Germany.
Objective:
The transcription factor E2F1 serves as a major regulator of the cell-cycle by controlling G1-S phase transition. However, apart from its proliferative function high levels of deregulated E2F1 are capable of inducing apoptosis depending on the cellular context. In particular the tumor suppressor p53 and its homologue p73 are implicated in this proapoptotic function.
Methods:
Here, we investigated the mechanistic basis for E2F1-mediated apoptosis in vascular smooth muscle cells (VSMCs) which have previously been shown to be E2F1-responsive.
Results:
Interestingly, E2F1-expression in these cells induced clear signs of apoptosis in the absence of any proliferative activity. Although cell-cycle regulated genes such as CCNE1 and CDC25A were activated, BrdU-staining revealed no S-phase entry. Instead, a rapid loss of cell viability by induction of apoptosis was observed. Using a transactivation-defective E2F1-mutant, we show that apoptosis induction is independent of the transactivation function and therefore independent of ARF and p73. However, this mutant retains its ability to stabilize and phosphorylate p53, suggesting that p53 is sufficient for the effect of E2F1.
Conclusion:
VSMCs therefore represent a cellular system in which the transactivation-independent, proapoptotic activity of E2F1 is the primary cellular function. Ectopic expression of E2F1 might therefore be a suitable therapy to prevent VSMC hyperproliferation.
Insights
Transcription factor E2F1 induces apoptosis in vascular smooth muscle cells (VSMCs) independently of cell-cycle regulation. This suggests E2F1
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- The transcription factor E2F1 regulates cell-cycle progression, particularly the G1-S phase transition.
- Deregulation of E2F1 can induce apoptosis, with p53 and p73 implicated in this process.
- Vascular smooth muscle cells (VSMCs) are known to be responsive to E2F1.
Purpose of the Study:
- To investigate the mechanism of E2F1-mediated apoptosis in VSMCs.
- To determine if E2F1 induces proliferation or apoptosis in VSMCs.
- To elucidate the role of E2F1's transactivation function in apoptosis induction.
Main Methods:
- Studied E2F1 expression in VSMCs.
- Assessed cell proliferation using BrdU staining.
- Utilized a transactivation-defective E2F1 mutant to analyze apoptosis pathways.
- Examined p53 stabilization and phosphorylation.
Main Results:
- E2F1 expression induced apoptosis in VSMCs without promoting cell-cycle progression (S-phase entry).
- Cell-cycle genes CCNE1 and CDC25A were activated, but proliferation did not occur.
- Apoptosis induction was independent of E2F1's transactivation function, ARF, and p73.
- A transactivation-defective E2F1 mutant stabilized and phosphorylated p53, indicating p53 sufficiency.
Conclusions:
- VSMCs exhibit transactivation-independent E2F1 proapoptotic activity as a primary function.
- Ectopic E2F1 expression could be a therapeutic strategy against VSMC hyperproliferation.
- p53 plays a sufficient role in E2F1-mediated apoptosis in VSMCs.