Regulation of E2F1-induced Apoptosis by the Nucleolar Protein RRP1B

Abstract

Regulation of the E2F family of transcription factors is important in control of cellular proliferation; dysregulation of the E2Fs is a hallmark of many cancers. One member of the E2F family, E2F1, also has the paradoxical ability to induce apoptosis; however, the mechanisms underlying this selectivity are not fully understood. We now identify a nucleolar protein, RRP1B, as an E2F1-specific transcriptional target. We characterize the RRP1B promoter and demonstrate its selective response to E2F1. Consistent with the activation of E2F1 activity upon DNA damage, RRP1B is induced by several DNA-damaging agents. Importantly, RRP1B is required for the expression of certain E2F1 proapoptotic target genes and the induction of apoptosis by DNA-damaging agents. This activity is mediated in part by complex formation between RRP1B and E2F1 on selective E2F1 target gene promoters. Interaction between RRP1B and E2F1 can be found inside the nucleolus and diffuse nucleoplasmic punctates. Thus, E2F1 makes use of its transcriptional target RRP1B to activate other genes directly involved in apoptosis. Our data also suggest an underappreciated role for nucleolar proteins in transcriptional regulation.E2F1 is a critical regulator of DNA damage response and apoptosis. As part of the E2F family of transcription factors, E2F1 is also involved in regulation of a wide array of genes important for cell cycle progression and other functions (1). Paradoxically, E2F1 has the unique ability to induce apoptosis (2). Overexpression of E2F1 ex vivo leads to apoptosis of breast cancer and other cells (3-5). Deletion of E2F1 in vivo shows a defect in thymocyte apoptosis and increased tumor incidence (6, 7). An endogenous role for E2F1 apoptosis is illustrated by its activation and stabilization by genotoxic stimuli. Overexpression of E2F1 sensitizes cells to radiation and chemotherapy (8, 9). DNA damage activates E2F1 expression and induces E2F1 stabilization through phosphorylation by DNA damage-responsive kinases ATM (ataxia telangiectasia mutated) (10) and Chk2 (checkpoint kinase 2) (11) and through acetylation (12, 13). E2F1 transactivates proapopotic genes, such as p73 (14, 15), Apaf-1 (16), and caspases (16) independently of p53 and cooperates with p53 through transactivation of p19 ARF (17). Investigation of how E2F1 specifically regulates apoptosis through selective transcriptional regulation vis-à-vis other E2F family members may reveal targets for future study that might improve the sensitivity of cancer to radiotherapy and chemotherapy. We therefore attempted to identify genes specifically regulated by E2F1 that potentially regulate E2F1-induced apoptosis. Previously, the Helin group published a microarray data set in which expression profiles were compared between cells that overexpressed E2F1, E2F2, and E2F3 (18). We screened their data set to include only those genes that were significantly induced by E2F1 but whose expression did not change more than 1-fold either positively or negatively upon E2F2 or E2F3 overexpression. The list o...

Regulation of E2 Promoter Binding Factor 1 (E2F1) Transcriptional Activity through a Deubiquitinating Enzyme, UCH37

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E2F1 impairs all-trans retinoic acid-induced osteogenic differentiation of osteosarcoma via promoting ubiquitination-mediated degradation of RARα

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