Principal Investigator Jacqueline Lees
We have cloned a family of transcription factors, called E2F, that control the expression of genes that are essential for cellular proliferation. The activity of the E2Fs is regulated by their interaction with a second family of proteins called the pocket proteins. This includes the retinoblastoma protein (pRB), a tumor suppressor that is functionally inactivated in all human tumors, and two related proteins, p107 and p130. Pocket protein binding is sufficient to block the transcriptional activity of the E2Fs. Moreover, the resulting complexes can actively repress E2F-responsive genes by recruiting histone deactylases. Thus, the E2Fs have the potential to mediate either the repression or activation of E2F-responsive genes. Depending on the balance of these two activities, the E2F/pocket protein complexes control a mammalian cell's decision to proliferate or to exit the cell cycle. Significantly, the tumor-derived forms of pRB are all disrupted in their ability to interact with E2F. This strongly suggests that E2F plays a key role in the development of human tumors.
The current goal is to understand how the interplay between the various E2F and pocket proteins mediates the appropriate control of cell cycle entry and exit that is required for normal development and tumor suppression. We are addressing this question using mutant mouse models and the resulting E2F/pocket protein deficient cells. These studies show that the E2F proteins can be divided into three distinct subgroups that have unique biological properties.
E2F1, 2 and 3 are potent transcriptional activators that are specifically regulated by pRB, and not p107 or p130, in vivo. We have shown that E2F3 plays a key role in activating almost all E2F-responsive genes and thereby determines the rate of proliferation of both normal and tumor cells. E2F3 is essential for full viability and E2f3 homozygous and heterozygous mutant mice display a variety of developmental abnormalities that are consistent with a dose-dependent requirement for E2F3 in the induction of proliferation. Importantly, through the generation and analysis of Rb:E2f compound mutant mice, we have shown that the tumor suppressive properties of pRB are primarily dependent upon its ability to inhibit the activating E2Fs, E2F1, 2 and 3. We are continuing to study how the interplay between pRB and the activating E2Fs controls both normal proliferation and tumorigenesis.
In contrast to the activating E2Fs, E2F4 and 5 lack nuclear localization signals and can only exist in the nucleus when bound to the pocket protein/histone deactylase complexes. Our analysis of compound E2f4:pocket protein mutant mice and cells indicates that the E2F4/pocket protein complexes play a key role in the repression of E2F-responsive genes. Consistent with this observation, these complexes are required for cells to exit the cell cycle in response to normal growth arrest signals and they are also essential for the terminal differentiation of a wide variety of cell lineages. Significantly, using in vitro differentiation assays, we have shown that E2F4 plays a key role in the differentiation process that is independent of any effect on cell cycle regulation. We are continuing to investigate the underlying basis for this activity.
E2F6 lacks the sequences necessary for either pocket protein binding or transcriptional activation and it participates in the repression of E2F-responsive genes through recruitment of a mammalian polycomb complex. At least one component of this polycomb complex, Bmi-1, has been shown to promote cell cycle progression and tumorigenesis. We continue to study the role of E2F6 and its associated polycomb complex in cell cycle control, normal development and tumorigenicity.