E2F – at the Crossroads of Life and Death

Review E2F – at the crossroads of life and death Shirley Polager and Doron Ginsberg The Mina and Everard Goodman Faculty of Life Science, Bar Ilan University, Ramat Gan 52900, Israel The retinoblastoma tumor suppressor, pRb, restricts is cell-cycle regulated, with dissociation leading to dere- cell-cycle progression mainly by regulating members pression and activation of E2F-regulated genes and S- of the E2F-transcription-factor family. The Rb pathway phase entry. is often inactivated in human tumors, resulting in Deregulated-E2F activity occurs in the vast majority of deregulated-E2F activity that promotes proliferation or human tumors through several different mechanisms. cell death, depending on the cellular context. Specifi- These include functional loss of pRB; amplification of cyclin cally, the outcome of deregulated-E2F activity is deter- D, which promotes phosphorylation of pRB; loss of p16, a mined by integration of signals coming from the cellular cyclin-dependent kinase inhibitor that inhibits the phos- DNA and the external environment. Alterations in cell phorylation of pRB; and expression of the human papillo- proliferation and cell-death pathways are key features of mavirus (HPV) oncoprotein E7, which disrupts pRB–E2F transformed cells and, therefore, an understanding of complexes [6]. the variables that determine the outcome of E2F acti- In mammals, the E2F family comprises eight genes vation is pivotal for cancer research and treatment. In (E2F1–8), which give rise to nine distinct proteins. These this review, we discuss recent studies that have eluci- include E2F3a and E2F3b, which are generated by the use dated some of the signals affecting E2F activity and that of alternative promoters [5]. All family members contain a have revealed additional E2F targets and functions, DNA-binding domain and E2F1–5 have a transactivation thereby enriching the understanding of this versatile domain that enables activation of gene expression. A short transcription-factor family. amino-acid stretch that mediates binding of pRB is embedded within the transactivation domain and, thus, Introduction this domain also functions in repression of gene expression. Members of the E2F family of transcription factors are E2F1–6 contain, in addition, a dimerization domain that is downstream effectors of the tumor suppressor pRB and are required for their interaction with a member of the dimer- considered to have a pivotal role in controlling cell-cycle ization-partner family (DP1–DP4). This interaction progression. Initially, studies revealed that E2Fs deter- enables them to bind DNA and function as transcriptional mine the timely expression of many genes required for regulators. E2F7 and E2F8 do not heterodimerize with DP- entry into and progression through the S phase of the cell family members and can bind DNA as homodimers or as cycle. However, it has become clear that transcriptional E2F7–E2F8 heterodimers [7,8]. E2F-family members have activation of S-phase-related genes is only one facet of E2F been categorized into subfamilies based on their transcrip- activity; it is now known that E2Fs both transactivate and tional activity, structure and interaction with pRB-family repress gene expression. Furthermore, E2Fs function in a members, p107 and p130. E2F1, E2F2 and E2F3a, which wide range of biological processes, including DNA replica- interact only with pRB, constitute one subfamily and are tion, mitosis, the mitotic checkpoint, DNA-damage check- often referred to as the ‘activator E2Fs’, because they are points, DNA repair, differentiation, development and believed to function mainly in activating gene expression. apoptosis [1–4]. Considering this extensive list, it is per- However, this classification is probably an over-simplifica- haps not surprising that the best-studied member of the tion because DNA microarray studies show that activation family, E2F1, exhibits both oncogenic and tumor-suppres- of the so-called activator E2Fs leads to repression of almost sive activities. as many genes as they activate. E2F4–8 function mainly in E2F transcriptional activity is modulated by multiple repression of gene expression and are often referred to as mechanisms, the best known being interaction with the the ‘repressor E2Fs’. product of the retinoblastoma (Rb) tumor-suppressor gene, A somewhat puzzling feature of activator E2Fs, and in pRB [5]. This association not only inhibits the ability of particular E2F1, is the ability to induce the two seemingly E2F to transactivate but, also, actively represses transcrip- contradictory processes of cell proliferation and apoptosis. tion through the recruitment of various chromatin modi- Recent studies shed some light on a long-lasting question: fiers and remodeling factors to the promoters of E2F- how does E2F1 ‘decide’ whether to induce cell proliferation responsive genes. These co-repressors include histone dea- or cell death? These studies show that signals from cetylases (HDACs), histone methyltransferases and DNA damaged DNA direct E2F to apoptosis, whereas signal- methyltransferases [4]. Formation of pRB–E2F complexes transduction pathways, such as the phosphatidylinositol 3 kinase (PI3K)–protein kinase B (Akt) and epidermal- Corresponding author: Ginsberg, D. ([email protected]). growth-factor receptor (EGFR)–ras pathways, inhibit 528 0962-8924/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2008.08.003 Available online 18 September 2008 Review Trends in Cell Biology Vol.18 No.11 E2F-induced apoptosis. These findings are of clinical proliferation. Moreover, these data imply that non-immor- relevance to the treatment and prognosis of cancer tal cells might respond to ectopic-E2F expression by indu- patients. In addition, recent studies identify novel func- cing senescence (or apoptosis, as discussed later) to inhibit tions of E2F, demonstrating that it regulates signal-trans- tumorigenesis. duction pathways and autophagy, a pathway that has a Different activator E2Fs seem to have partially over- dual role in cell survival and cell death. The relevance of lapping, but distinct, roles in the regulation of cell prolifer- these novel activities of E2F to cancer development is ation. For instance, both E2F1 and E2F3 are required for currently being studied. In this review, these recent devel- cell-cycle entry, but only E2F3 is required for continued cell opments are discussed, focusing on the activator E2Fs. proliferation [11]. Combined loss of E2F1, E2F2 and E2F3 completely abolishes the ability of cells to progress through E2F and cell proliferation the cell cycle and proliferate [12]. Cells lacking E2F1, E2F2 In quiescent cells, the activator E2Fs are bound to and and E2F3 exhibit impaired expression of E2F target genes repressed by pRB. Mitogenic stimuli that elevate cyclin-D that are essential for cell-cycle progression [12], thereby levels promote phosphorylation of pRB by cyclin D–cyclin- indicating that lack of activation of crucial E2F targets dependent kinase (CDK) 4 or cyclin D–CDK6 complexes, leads to cell-cycle defects. However, cells deficient in E2F3 which prevents pRB from binding E2Fs (Figure 1). Con- exhibit derepression of the tumor suppressor Arf [13]. The sequently, genes repressed by pRB–E2F complexes are Arf gene encodes a protein that stabilizes and activates p53 derepressed and the now-free E2Fs also activate gene by negating the effects of the E3 ubiquitin ligase Mdm2, expression. Many of the upregulated genes encode proteins which otherwise promotes p53 degradation. Therefore, involved in DNA replication and cell-cycle progression, derepression of Arf in cells lacking E2F3 leads to activation such as DNA polymerases, minichromosome maintenance of p53 and cell-cycle defects, in particular a defect in re- complex components (MCMs), cdc6 and cyclin E. In line entering the cell cycle from quiescence. These abnormal- with the regulation of many proliferation-related genes by ities of E2F3-deficient cells are rescued by losing Arf [13]. E2F, overexpression of E2F1, E2F2 or E2F3a induces Similarly, a recent study demonstrates that cells lacking quiescent immortalized cells to enter S phase [5]. These E2F1, E2F2 and E2F3 exhibit p53 activation; the respon- findings underlie the well-established role of E2Fs as pro- siveness of these cells to normal growth signals and expres- proliferative factors. However, E2F1 overexpression in sion of E2F target genes can both be restored by p53 primary fibroblasts does not lead to S-phase entry, but loss [14]. E2F transcriptional-repressor complexes are cru- instead promotes senescence [9,10]. This indicates cial downstream targets of ARF (alternative reading that immortalization is required for deregulated E2F1 frame of cyclin-dependent-kinase inhibitor 2A) and p53- to promote ectopic S-phase entry and uncontrolled cell induced proliferative arrest [15]. Based on these data, it is Figure 1. Upstream signals direct E2F1 to proliferation or apoptosis. Mitogenic signals and DNA damage activate different signaling pathways that determine whether E2F1 activity will induce transcription of proliferative or apoptotic genes. Mitogenic stimuli elevate cyclin-D levels, leading to repression of pRB activity via phosphorylation by cyclin D–CDK4 or cyclin D–CDK6 complexes. Subsequently, E2F1 is free to activate proliferative or apoptotic genes. When mitogenic signals also switch on the PI3K–Akt pathway or, in some situations EGFR–Ras–Raf signaling, the apoptotic activity of E2F1 is inhibited and it induces mainly proliferation.

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