E2f1, E2f2, and E2f3 Control E2F Target Expression and Cellular Proliferation via a p53-Dependent Negative Feedback Loop

Abstract

E2F-mediated control of gene expression is believed to have an essential role in the control of cellular proliferation. Using a conditional gene-targeting approach, we show that the targeted disruption of the entire E2F activator subclass composed of E2f1, E2f2, and E2f3 in mouse embryonic fibroblasts leads to the activation of p53 and the induction of p53 target genes, including p21 CIP1 . Consequently, cyclin-dependent kinase activity and retinoblastoma (Rb) phosphorylation are dramatically inhibited, leading to Rb/E2F-mediated repression of E2F target gene expression and a severe block in cellular proliferation. Inactivation of p53 in E2f1-, E2f2-, and E2f3-deficient cells, either by spontaneous mutation or by conditional gene ablation, prevented the induction of p21 CIP1 and many other p53 target genes. As a result, cyclin-dependent kinase activity, Rb phosphorylation, and E2F target gene expression were restored to nearly normal levels, rendering cells responsive to normal growth signals. These findings suggest that a critical function of the E2F1, E2F2, and E2F3 activators is in the control of a p53-dependent axis that indirectly regulates E2F-mediated transcriptional repression and cellular proliferation.

Tumor-suppressive miR-34a induces senescence-like growth arrest through modulation of the E2F pathway in human colon cancer cells

“…In contrast, fibroblasts with triple knockout of E2f1-E2f3 demonstrate strong inhibition of cell proliferation throughout the cell cycle. Interestingly, induction of p53 and its downstream target p21 was observed in the triple knockout cells (23)(24)(25). This phenotype resembles the results obtained in our current study with miR-34a introduction in colon cancer cells.…”

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

“…A similar block in the G2 or S phases of the cell cycle has been described in cells that undergo senescence upon oncogenic activation (Olsen et al, 2002;Di Micco et al, 2006). Embryonic fibroblasts carrying inactivating mutations for E2F1-3 also arrest at various phases of the cell cycle, concomitant with induction of p21 and p53 (Timmers et al, 2007). In these cultures, however, no early proliferative burst or overexpression of E2F target genes was reported.…”

E2F2 represses cell cycle regulators to maintain quiescence

“…Instead, the mechanism appears to involve a loss of E2F3-dependent repression of the p19 ARF protein 12 or E2F3-dependent repression of p53-regulated genes. 37,38 In fact, activation of the p53 pathway has been proposed to be a primary consequence of E2F1-3 ablation. 37,38 By contrast, deregulation of the p53 pathway does not appear to contribute to E2F2-mediated cell cycle gene derepression, that a large set of E2F target genes involved in cell cycle regulation is aberrantly activated by loss of E2F2 in resting T cells.…”

“…37,38 In fact, activation of the p53 pathway has been proposed to be a primary consequence of E2F1-3 ablation. 37,38 By contrast, deregulation of the p53 pathway does not appear to contribute to E2F2-mediated cell cycle gene derepression, that a large set of E2F target genes involved in cell cycle regulation is aberrantly activated by loss of E2F2 in resting T cells. Regulation of gene expression by E2F2 has been proposed to be restricted mainly to G 1 /S, coinciding with an increase in E2F2 protein levels.…”