E 2 FI - specific Induction of Apoptosis and p 53 Accumulation , Which Is Blocked by Mdm 21


Previous work has demonstrated a role for E2F transcription factor activity in the regulation of cell growth during the G jG1-S phase transition. Indeed, overexpression of E2F proteins, including the E2FI and E2F2 products, induces DNA synthesis in quiescent fibroblasts. Other experiments have shown that E2FI expression also induces apoptosis, dependent on p53. Although this could represent a response to aberrant cell cycle progression, we show that only E2FI induces apoptosis and that this coincides with an ability of E2FI to induce accumulation of p53 protein. We also find that coexpression of Mdm2, which is known to regulate p53 activity, blocks the E2FI-mediated induction of apoptosis and also blocks the E2FImediated accumulation of p53. We propose that E2FI acts as a specific signal for the induction of apoptosis by affecting the accumulation of p53, which under normal proliferative conditions may be controlled by Mdm2. Introduction A large body of work has demonstrated a critical role for E2F transcription activity in the regulation of cell growth, particularly during the G1JG1-S phase transition (1-3). E2F activity is controlled by the action of Rb family member proteins whose own function is regulated by the GI cyclin-dependent kinases. Thus, the control of E2F accumulation can be seen as an end point in a cascade involving a variety of cell cycle-regulatory activities. It is also clear that the disruption of various components of this control pathway, which indude Rb and Rb family members, Gi cyclins and associated Received 7/9/97; revised 1 1/23/97; accepted 11/24/97. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to mdicate this fact. 1 T. F. K. is a Leukemia Society of America Special Fellow. G. L is supported by fellowships from the Alberta Heritage Fund for Medical Research and the Medical Research Council of Canada. J. D. was supported by a fellowship from the NIH. 2 Present address: Department of Molecular Genetics and Microbiology, University of Massachusetts Medical Center, Worcester, MA 01655. 3 Present address: Department of Biochemistry and Molecular Genetics, University of Colorado Health Sciences Center, Denver, CO 80262. 4To whom requests for reprints should be addressed. kinases, and various activities that regulate these kinases, is a regular event in the development of human cancer (4, 5). Expression of the E2F1 product can block cells from entering quiescence and can induce already quiescent cells to enter S phase, dependent upon its ability to bind DNA and activate transcription (6, 7). Moreover, deregulated expression of E2F1 can lead to transformation of an established rat embryo fibroblast cell line (8), or in cooperation with an activated ras oncogene, can lead to oncogenic transformation of primary rat embryo cells (9). As originally described by Wu and Levine (1 1), the induction of S phase by E2F1 is also accompanied by a p53-dependent induction of apoptosis (7, 10-12). The mechanism by which the E2F1 product might specifically trigger apoptosis is not clear but could involve an inappropriate induction of cell cycle progression. For instance, p53 may monitor the E2F pathway as a means to assess the initiation of a cell growth signal transduction pathway that may or may not be accompanied by the other necessary signal transduction pathways essential for a normal proliferative process. Several recent observations suggest the possibility that E2F1 may provide a signal for apoptosis during a normal growth response: (a) our recent work has shown that the ability of E2F1 to induce apoptosis is an activity unique to this E2F family member, despite the fact that other E2F family members also induce S phase (1 3); (b) previous work has shown that E2F1 , together with the heterodimeric partner DPi , can induce an accumulation of p53 protein (14), consistent with earlier work showing that adenovirus El A induces p53 accumulation (1 5). In light of our experiments demonstrating a unique role for E2F1 in the induction of apoptosis, we have explored the possibility that this may reflect a specificity in the action of E2F1 toward p53. Indeed, we find that E2F1 , but not E2F2, does induce p53 accumulation, coincident with the ability of E2F1 to induce apopto515. We also find that coexpression of the Mdm2 protein blocks both the E2F1 -mediated apoptosis as well as the E2F1 -mediated induction of p53 accumulation. Results p53 Is Limiting for E2FI-mediated Apoptosis. Previous experiments have shown that E2F1 overexpression in quiescent fibroblast cultures leads to an induction of cellular DNA synthesis and apoptosis (7, 10-12). Although this result might be explained as an artificial induction of cell cycle proliferation in the absence of normal growth-regulatory signals, our recent experiments have shown that the induction of apoptosis is a property unique to the E2F1 protein (13). This specificity is illustrated by the result shown in Fig. 1 in which E2F1 efficiently induces apoptosis, as indicated by the appearance of a cell population with a sub = G1 DNA content, whereas expression of the E2F2 protein does not lead to cell death. The differential ability of E2F1 and E2F2 to 114 E2F1 -specific Induction of Apoptosis Con E2F1 E2F2 p53 E2F1 + p53 E2F2 + p53 48hrs LU

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@inproceedings{Kowalik2005E2F, title={E 2 FI - specific Induction of Apoptosis and p 53 Accumulation , Which Is Blocked by Mdm 21}, author={Timothy F. Kowalik and Joseph Nevins}, year={2005} }