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Suppression of the p53- or pRB-mediated G1 checkpoint is required for E2F-induced S-phase entry
Marina Lomazzi1, M Cristina Moroni, Michael R Jensen
1Department of Experimental Oncology, European Institute of Oncology, Via Ripamonti 435, 20141 Milan, Italy.
Abstract:
Deregulation of the retinoblastoma protein (pRB) pathway is a hallmark of cancer. In the absence of other genetic alterations, this deregulation results in lack of differentiation, hyperproliferation and apoptosis. The pRB protein acts as a transcriptional repressor by targeting the E2F transcription factors, whose functions are required for entry into S phase. Increased E2F activity can induce S phase in quiescent cells--this is a central element of most models for the development of cancer. We show that although E2F1 alone is not sufficient to induce S phase in diploid mouse and human fibroblasts, increased E2F1 activity can result in S-phase entry in diploid fibroblasts in which the p53-mediated G1 checkpoint is suppressed. In addition, we show that E2F1 can induce S phase in primary mouse fibroblasts lacking pRB. These results indicate that, in addition to acting as an E2F-dependent transcriptional repressor, pRB is also required for the cells to retain the G1 checkpoint in response to unprogrammed proliferative signals.
Insights
The retinoblastoma protein (pRB) pathway is crucial in cancer. Suppressing the p53 checkpoint or removing pRB allows E2F1 to drive cell cycle progression, highlighting pRB
Area of Science:
- Molecular Biology
- Cell Cycle Regulation
- Cancer Biology
Background:
- Deregulation of the retinoblastoma protein (pRB) pathway is a key event in cancer development.
- The pRB pathway controls cell cycle progression by repressing E2F transcription factors, which are essential for S phase entry.
- Dysregulation of pRB leads to uncontrolled cell proliferation, lack of differentiation, and apoptosis resistance.
Purpose of the Study:
- To investigate the role of E2F1 in cell cycle progression in the context of pRB and p53 pathways.
- To determine if E2F1 can induce S phase entry in diploid fibroblasts under specific genetic conditions.
- To elucidate the contribution of pRB to maintaining the G1 checkpoint against aberrant proliferative signals.
Main Methods:
- Experiments were conducted using diploid mouse and human fibroblasts.
- The study involved manipulating the p53-mediated G1 checkpoint.
- Primary mouse fibroblasts lacking pRB were utilized to assess E2F1's function.
Main Results:
- Increased E2F1 activity alone was insufficient to induce S phase in normal diploid fibroblasts.
- However, elevated E2F1 activity promoted S-phase entry in diploid fibroblasts with a suppressed p53-mediated G1 checkpoint.
- E2F1 was capable of inducing S phase in primary mouse fibroblasts that lacked pRB.
Conclusions:
- pRB functions not only as an E2F-dependent transcriptional repressor but also plays a critical role in maintaining the G1 checkpoint.
- pRB is essential for preventing cell cycle progression in response to inappropriate proliferative stimuli.
- These findings underscore the multifaceted role of pRB in cell cycle control and cancer prevention.