E2f1–3 switch from activators in progenitor cells to repressors in differentiating cells

Abstract

In the classic paradigm of mammalian cell cycle control, Rb functions to restrict cells from entering S phase by sequestering E2F activators (E2f1, E2f2 and E2f3), which are invariably portrayed as the ultimate effectors of a transcriptional program that commit cells to enter and progress through S phase1, 2. Using a panel of tissue-specific cre-transgenic mice and conditional E2f alleles we examine the effects of E2f1, E2f2 and E2f3 triple deficiency in murine ES cells, embryos and small intestines. We show that in normal dividing progenitor cells E2F1-3 function as transcriptional activators, but contrary to current dogma, are dispensable for cell division and instead are necessary for cell survival. In differentiating cells they function in complex with Rb as repressors to silence E2F targets and facilitate exit from the cell cycle. The inactivation of Rb in differentiating cells resulted in a switch of E2F1-3 from repressors to activators, leading to the superactivation of E2F responsive targets and ectopic cell divisions, and loss of E2f1-3 completely suppressed these phenotypes. This work contextualizes the activator versus repressor functions of E2F1-3 in vivo, revealing distinct roles in dividing versus differentiating cells and in normal versus cancer-like cell cycles in vivo.

E2F1 and E2F2 prevent replicative stress and subsequent p53-dependent organ involution

“…Similar findings have been reported for differentiating monocyte and intestinal epithelial cells. 13,17 However, the cell fates in these cases are different from those observed in pancreatic tissue, as monocyte progenitor cells lacking E2F1/2 result in senescence, and intestinal progenitor cells lacking E2F1-3 lead to p53-independent apoptosis. The mechanism that leads to replication stress in DKO cells is still unknown, and it would be interesting to determine whether it involves illegitimate replication origin firing and fork stalling, as shown for replication stress caused by overexpression of Cdt1 33 or by aberrant CDK activity 34 in some cell types.…”

“…[8][9][10] E2F1 and E2F2 are known to function both as transcriptional activators and as repressors of their target genes, including those coding for microRNAs. [11][12][13][14][15] Moreover, E2F1 and E2F2 perform positive as well as negative regulatory roles in cellular proliferation, which appear to be cell specific and context dependent. They have been reported to promote cellcycle entry and progression upon their ectopic expression.…”

“…16 By contrast, targeted inactivation of these E2Fs revealed their essential role in sustaining quiescence in hematopoietic cells, 11 in supporting survival of progenitor cells 16 or in facilitating exit from the cell cycle in differentiating cells. 13,17 However, the mechanisms by which E2F1 and E2F2 accomplish such diverse functions remain unresolved.…”

Accelerated DNA replication in E2F1- and E2F2-deficient macrophages leads to induction of the DNA damage response and p21CIP1-dependent senescence

“…For instance, it has been recently shown that E2F1-3 are dispensable for proliferation, but necessary for cell survival in normally dividing progenitor cells. By contrast, they function as repressors in non-proliferating cells that are undergoing differentiation (Chong et al, 2009). However, the relative contribution of E2F1-3 in cell survival or in repression has not been determined for cellular contexts where terminal differentiation is not coupled to cell cycle exit, but is instead concurrent with continued proliferation.…”

pVHL acts as a downstream target of E2F1 to suppress E2F1 activity

“…Meanwhile, apoptotic induction of E2F1 separates it from other E2F family members [17]. In response to various DNA-damage conditions, including ionizing radiation, UV radiation and chemotherapeutic drugs, E2F1 is induced and activated, resulting in expression of its downstream genes for induction of cell apoptosis [18,19]. Notably, upon DNA damage, E2F1 undergoes post-translational modifications, including phosphorylation, acetylation and deacetylation, resulting in modulation of its transactivation [20][21][22][23].…”