title: Expression level is a key determinant of E2F1-mediated cell fate.
abstract: The Rb/E2F network has a critical role in regulating cell cycle progression and cell fate decisions. It is dysfunctional in virtually all human cancers, because of genetic lesions that cause overexpression of activators, inactivation of repressors, or both. Paradoxically, the downstream target of this network, E2F1, is rarely strongly overexpressed in cancer. E2F1 can induce both proliferation and apoptosis but the factors governing these critical cell fate decisions remain unclear. Previous studies have focused on qualitative mechanisms such as differential cofactors, posttranslational modification or state of other signaling pathways as modifiers of the cell fate decisions downstream of E2F1 activation. In contrast, the importance of the expression levels of E2F1 itself in dictating the downstream phenotypes has not been rigorously studied, partly due to the limited resolution of traditional population-level measurements. Here, through single-cell quantitative analysis, we demonstrate that E2F1 expression levels have a critical role in determining the fate of individual cells. Low levels of exogenous E2F1 promote proliferation, moderate levels induce G1, G2 and mitotic cell cycle arrest, and very high levels promote apoptosis. These multiple anti-proliferative mechanisms result in a strong selection pressure leading to rapid elimination of E2F1-overexpressing cells from the population. RNA-sequencing and RT-PCR revealed that low levels of E2F1 are sufficient to induce numerous cell cycle-promoting genes, intermediate levels induce growth arrest genes (i.e., p18, p19 and p27), whereas higher levels are necessary to induce key apoptotic E2F1 targets APAF1, PUMA, HRK and BIM. Finally, treatment of a lung cancer cell line with a proteasome inhibitor, MLN2238, resulted in an E2F1-dependent mitotic arrest and apoptosis, confirming the role of endogenous E2F1 levels in these phenotypes. The strong anti-proliferative activity of moderately overexpressed E2F1 in multiple cancer types suggests that targeting E2F1 for upregulation may represent an attractive therapeutic strategy in cancer.
authors.affiliation: Department of Biomedical Engineering, Duke University, Durham, NC, USA.
authors.forename: Igor
authors.initials: I
authors.lastname: Shats
authors.affiliation: Department of Biomedical Engineering, Duke University, Durham, NC, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Deng
authors.affiliation: Department of Biomedical Engineering, Duke University, Durham, NC, USA.
authors.forename: Adam
authors.initials: A
authors.lastname: Davidovich
authors.affiliation: Department of Biomedical Engineering, Duke University, Durham, NC, USA.
authors.forename: Carolyn
authors.initials: C
authors.lastname: Zhang
authors.affiliation: Department of Molecular and Cellular Biology, University of Arizona, Tucson, AZ, USA.
authors.forename: Jungeun S
authors.initials: JS
authors.lastname: Kwon
authors.affiliation: Department of Biostatistics and Bioinformatics, Center for Genomic and Computational Biology, Duke University, Durham, NC, USA.
authors.forename: Dinesh
authors.initials: D
authors.lastname: Manandhar
authors.affiliation: Department of Biostatistics and Bioinformatics, Center for Genomic and Computational Biology, Duke University, Durham, NC, USA.
authors.forename: Raluca
authors.initials: R
authors.lastname: Gordân
authors.affiliation: Department of Molecular and Cellular Biology, University of Arizona, Tucson, AZ, USA.
authors.forename: Guang
authors.initials: G
authors.lastname: Yao
authors.affiliation: Department of Biomedical Engineering, Duke University, Durham, NC, USA.|Department of Biostatistics and Bioinformatics, Center for Genomic and Computational Biology, Duke University, Durham, NC, USA.|Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, NC, USA.
authors.forename: Lingchong
authors.initials: L
authors.lastname: You
mesh_terms: D017209:Apoptosis / Q000187:drug effects; D051017:Apoptosis Regulatory Proteins / Q000737:chemistry / Q000378:metabolism; D053477:Apoptotic Protease-Activating Factor 1 / Q000737:chemistry / Q000378:metabolism; D000072224:Bcl-2-Like Protein 11 / Q000737:chemistry / Q000378:metabolism; D001896:Boron Compounds / Q000494:pharmacology; D059447:Cell Cycle Checkpoints / Q000187:drug effects; D045744:Cell Line, Tumor; D050687:E2F1 Transcription Factor / Q000235:genetics / Q000378:metabolism*; D005998:Glycine / Q000031:analogs & derivatives / Q000494:pharmacology; D045325:HCT116 Cells; D006657:Histones / Q000378:metabolism; D006801:Humans; D011518:Proto-Oncogene Proteins / Q000737:chemistry / Q000378:metabolism; D013629:Tamoxifen / Q000633:toxicity; D059008:Time-Lapse Imaging
chemical_list: C506592:APAF1 protein, human; D051017:Apoptosis Regulatory Proteins; D053477:Apoptotic Protease-Activating Factor 1; C438378:BBC3 protein, human; D000072224:Bcl-2-Like Protein 11; D001896:Boron Compounds; D050687:E2F1 Transcription Factor; D006657:Histones; C000595706:MLN2238; D011518:Proto-Oncogene Proteins; D013629:Tamoxifen; D005998:Glycine