Control of the p53-p21CIP1 Axis by E2f1, E2f2, and E2f3 Is Essential for G1/S Progression and Cellular Transformation

Abstract

The E2F family of transcription factors is believed to have an essential role in the control of cellular proliferation by regulating the transcription of genes involved in cell cycle progression. Previous work has demonstrated that the targeted inactivation of E2f1, E2f2, and E2f3 results in elevated p21 CIP1 protein levels, loss of E2F target gene expression, and cell cycle arrest at G 1 /S and G 2 /M, suggesting a strict requirement for these E2Fs in the control of normal cellular proliferation. We now demonstrate that E2f1, E2f2, and E2f3 are also required for oncogene-mediated transformation of mouse embryonic fibroblasts. Analysis of synchronized populations of mouse embryonic fibroblasts revealed that the inactivation of p21 CIP1 restores the ability of E2f1-3-deficient cells to enter and transit through G 1 /S (but not G 2 /M). In contrast, loss of p53 restored the ability of these cells to progress through both G 1 /S and mitosis, leading to their continued proliferation. The inactivation of p53 (but not p21 CIP1 ) rendered E2f1-3-deficient cells sensitive to transformation and tumorigenesis. These results suggest that the negative regulation of the p53-p21 CIP1 axis by the E2F1-3 factors is critical for cell cycle progression and cellular transformation.The timely entry and proper progression through the cell division cycle requires an ordered series of cyclin-dependent phosphorylation events and the execution of a well orchestrated transcriptional program. Growth factor-dependent activation of cyclin-dependent kinases (cdks) 3 cdk4/6 and cdk2 results in the sequential phosphorylation of the Retinoblastoma (Rb) family of pocket proteins. This phosphorylation results in the release of E2F transcription factors from Rb-E2F complexes and activation of E2F target gene expression, leading to cell cycle progression (1, 2). In addition to the temporal association with the Rb family of pocket proteins, E2F activity is also regulated through cell cycle-dependent expression, post-translational modifications, interactions with cofactors, and protein degradation (1-3).Several decades of work support the view that E2F represents an important activity that controls cellular proliferation (4 -6). More recent studies suggest a particularly important role for the E2F3 family member in the control of gene expression during the G 1 /S transition. Inhibitory antibodies against E2F3 (but not E2F1) blocked the entry of cells into S phase (7). Consistent with these results, gene-targeting strategies in mice demonstrated a significant role for E2F3 in the proliferation of MEFs (8, 9). Some functional redundancy among family members is apparent, however, as the combined inactivation of E2f1, E2f2, and E2f3 led to a much more profound proliferation arrest that was manifested at multiple stages of the cell cycle (9). Interestingly, this block was accompanied by the induction of p53 transcriptional activity and the elevation of its target genes including p21 CIP1 (9, 10). These observations raise the possibility that E2F1-3 activa...

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

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Pregnane X receptor suppresses proliferation and tumourigenicity of colon cancer cells

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

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