The retinoblastoma tumour suppressor (Rb) pathway is believed to have a critical role in the control of cellular proliferation by regulating E2F activities. E2F1, E2F2 and E2F3 belong to a subclass of E2F factors thought to act as transcriptional activators important for progression through the G1/S transition. Here we show, by taking a conditional gene targeting approach, that the combined loss of these three E2F factors severely affects E2F target expression and completely abolishes the ability of mouse embryonic fibroblasts to enter S phase, progress through mitosis and proliferate. Loss of E2F function results in an elevation of p21Cip1 protein, leading to a decrease in cyclin-dependent kinase activity and Rb phosphorylation. These findings suggest a function for this subclass of E2F transcriptional activators in a positive feedback loop, through down-modulation of p21Cip1, that leads to the inactivation of Rb-dependent repression and S phase entry. By targeting the entire subclass of E2F transcriptional activators we provide direct genetic evidence for their essential role in cell cycle progression, proliferation and development.
The E2F transcription factor family plays a crucial and well established role in cell cycle progression. Deregulation of E2F activities in vivo leads to developmental defects and cancer. Based on current evidence in the field, mammalian E2Fs can be functionally categorized into either transcriptional activators (E2F1, E2F2, and E2F3a) or repressors (E2F3b, E2F4, E2F5, E2F6, and E2F7). We have identified a novel E2F family member, E2F8, which is conserved in mice and humans and has its counterpart in Arabidopsis thaliana (E2Ls). Interestingly, E2F7 and E2F8 share unique structural features that distinguish them from other mammalian E2F repressor members, including the presence of two distinct DNA-binding domains and the absence of DP-dimerization, retinoblastoma-binding, and transcriptional activation domains. Similar to E2F7, overexpression of E2F8 significantly slows down the proliferation of primary mouse embryonic fibroblasts. These observations, together with the fact that E2F7 and E2F8 can homodimerize and are expressed in the same adult tissues, suggest that they may have overlapping and perhaps synergistic roles in the control of cellular proliferation.
The mammalian E2F family of transcription factors plays a crucial role in the regulation of cellular proliferation, apoptosis, and differentiation. Consistent with its biological role in a number of important cellular processes, E2F regulates the expression of genes involved in cell cycle, DNA replication, DNA repair, and mitosis. It has proven difficult, however, to determine the specific roles played by the various known family members in these cellular processes. The work presented here now extends the complexity of this family even further by the identification of a novel E2F family member, which we now term E2F7. Like the expression of the known E2F activators, E2F1, E2F2, and E2F3, the expression of E2F7 is growth-regulated, at least in part, through E2F binding elements on its promoter, and its protein product is localized to the nucleus and associates with DNA E2F recognition sites with high affinity. A number of salient features, however, make this member unique among the E2F family. First, the E2F7 gene encodes a protein that possesses two distinct DNA-binding domains and that lacks a dimerization domain as well as a transcriptional activation and a retinoblastoma-binding domain. In contrast to the E2F activators, E2F7 can block the E2F-dependent activation of a subset of E2F target genes as well as mitigate cellular proliferation of mouse embryo fibroblasts. These findings identify E2F7 as a novel member of the mammalian E2F transcription factor family that has properties of a transcriptional repressor capable of negatively influencing cellular proliferation.
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