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Regulation of a senescence checkpoint response by the E2F1 transcription factor and p14(ARF) tumor suppressor
G P Dimri1, K Itahana, M Acosta
1Department of Cell Biology, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Abstract:
Normal cells do not divide indefinitely due to a process known as replicative senescence. Human cells arrest growth with a senescent phenotype when they acquire one or more critically short telomeres as a consequence of cell division. Recent evidence suggests that certain types of DNA damage, chromatin remodeling, and oncogenic forms of Ras or Raf can also elicit a senescence response. We show here that E2F1, a multifunctional transcription factor that binds the retinoblastoma (pRb) tumor suppressor and that can either promote or suppress tumorigenesis, induces a senescent phenotype when overexpressed in normal human fibroblasts. Normal human cells stably arrested proliferation and expressed several markers of replicative senescence in response to E2F1. This activity of E2F1 was independent of its pRb binding activity but dependent on its ability to stimulate gene expression. The E2F1 target gene critical for the senescence response appeared to be the p14(ARF) tumor suppressor. Replicatively senescent human fibroblasts overexpressed p14(ARF), and ectopic expression of p14(ARF) in presenescent cells induced a phenotype similar to that induced by E2F1. Consistent with a critical role for p14(ARF), cells with compromised p53 function were immune to senescence induction by E2F1, as were cells deficient in p14(ARF). Our findings support the idea that the senescence response is a critical tumor-suppressive mechanism, provide an explanation for the apparently paradoxical roles of E2F1 in oncogenesis, and identify p14(ARF) as a potentially important mediator of the senescent phenotype.
Insights
Normal human cells stop dividing and enter senescence when the E2F1 transcription factor is overexpressed. This process, mediated by p14ARF, acts as a tumor suppressor mechanism.
Area of Science:
- Cell Biology
- Molecular Biology
- Oncology
Background:
- Normal cells have a limit to their division, known as replicative senescence, triggered by short telomeres.
- Other factors like DNA damage, chromatin changes, and oncogenes can also induce senescence.
- The transcription factor E2F1 has complex roles in tumor development.
Purpose of the Study:
- To investigate if E2F1 can induce a senescent phenotype in normal human fibroblasts.
- To elucidate the molecular mechanisms by which E2F1 might induce senescence.
- To understand the role of E2F1 and its target genes in tumor suppression.
Main Methods:
- Overexpression of E2F1 in normal human fibroblasts.
- Assessment of cell proliferation and senescence markers.
- Analysis of E2F1's interaction with pRb and its gene-regulatory activity.
- Investigation of the role of p14ARF and p53 in E2F1-induced senescence.
Main Results:
- E2F1 overexpression induced stable growth arrest and senescence markers in human fibroblasts.
- E2F1's senescence-inducing activity was independent of pRb binding but required gene expression.
- The tumor suppressor p14ARF was identified as a critical E2F1 target gene mediating senescence.
- Cells lacking functional p53 or p14ARF were resistant to E2F1-induced senescence.
Conclusions:
- E2F1 can induce a senescent phenotype in normal human cells, acting as a tumor suppressor.
- p14ARF is a key mediator of E2F1-induced senescence, highlighting its role in tumor suppression.
- These findings clarify E2F1's dual role in oncogenesis and emphasize senescence as a critical anti-cancer mechanism.