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Molecular and Cellular Biology, September 1999, p. 6379-6395, Vol. 19, No. 9
Department of Experimental Oncology, European
Institute of Oncology, 20141 Milan, Italy
Received 20 January 1999/Returned for modification 28 May
1999/Accepted 14 June 1999
Functional inactivation of the pRB pathway is a very frequent event
in human cancer, resulting in deregulated activity of the E2F
transcription factors. To understand the functional role of the E2Fs in
cell proliferation, we have developed cell lines expressing E2F-1,
E2F-2, and E2F-3 fused to the estrogen receptor ligand binding domain
(ER). In this study, we demonstrated that activation of all three E2Fs
could relieve the mitogen requirement for entry into S phase in Rat1
fibroblasts and that E2F activity leads to a shortening of the
G0-G1 phase of the cell cycle by 6 to 7 h.
In contrast to the current assumption that E2F-1 is the only E2F
capable of inducing apoptosis, we showed that deregulated E2F-2 and
E2F-3 activities also result in apoptosis. Using the ERE2F-expressing
cell lines, we demonstrated that several genes containing E2F DNA
binding sites are efficiently induced by the E2Fs in the absence of
protein synthesis. Furthermore, CDC25A is defined as a
novel E2F target whose expression can be directly regulated by E2F-1.
Data showing that CDC25A is an essential target for E2F-1,
since its activity is required for efficient induction of S phase by
E2F-1, are provided. Finally, our results show that expression of two
E2F target genes, namely CDC25A and cyclin E, is sufficient
to induce entry into S phase in quiescent fibroblasts. Taken together,
our results provide an important step in defining how E2F activity
leads to deregulated proliferation.
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
CDC25A Phosphatase Is a Target of E2F and Is
Required for Efficient E2F-Induced S Phase
*
Corresponding author. Mailing address: Department of
Experimental Oncology, European Institute of Oncology, Via Ripamonti 435, 20141 Milan, Italy. Phone: 39 02 5748 9860. Fax: 39 02 5748 9851. E-mail: khelin{at}ieo.cilea.it.
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