Canonical and Atypical E2Fs Regulate the Mammalian Endocycle

SUMMARY The endocycle is a variant cell cycle consisting of successive DNA synthesis and Gap phases that yield highly polyploid cells. Although essential for metazoan development, relatively little is known about its control or physiologic role in mammals. Using novel lineage-specific cre mice we identified two opposing arms of the E2F program, one driven by canonical transcription activation (E2F1, E2F2 and E2F3) and the other by atypical repression (E2F7 and E2F8), that converge on the regulation of endocycles in vivo. Ablation of canonical activators in the two endocycling tissues of mammals, trophoblast giant cells in the placenta and hepatocytes in the liver, augmented genome ploidy, whereas ablation of atypical repressors diminished ploidy. These two antagonistic arms coordinate the expression of a unique G2/M transcriptional program that is critical for mitosis, karyokinesis and cytokinesis. These results provide in vivo evidence for a direct role of E2F family members in regulating non-traditional cell cycles in mammals

Inhibition of lytic reactivation of Kaposi's sarcoma-associated herpesvirus by alloferon

Background: Alloferon, an immunomodulatory peptide, has antiviral capability against herpesvirus. In this research, we aimed to investigate the effect of alloferon on the regulation of the life cycle of Kaposi's sarcoma-associated herpesvirus (KSHV), and its mechanisms. We also assessed the antiviral activity of alloferon on natural killer (NK) cells as an early antiviral immune responder. Methods: We first examined the change in cell proliferation and the expression of the viral genes in a KSHV-infected cell line, body-cavity-based B lymphoma (BCBL)-1, under the lytic cycle by 12-O-tetradecanoyl-phorbol-13-acctate (TPA) treatment. To elucidate the antiviral mechanism of alloferon, we tested calcium influx and the activation of the extracellular signal-regulated kinase (ERK) pathway. Furthermore, we evaluated the cytotoxicity of NK cells against BCBL-1 by alloferon. Results: Alloferon effectively recovered the suppressed proliferation of BCBL-1 by TPA, which was achieved by the down-regulation of lytic-cycle-related viral genes, RTA, K8 and vIRF2. To clarify the signal transduction pathways related to the regulation of the viral genes by alloferon, we confirmed that the calcium influx into BCBL-1 was apparently inhibited by alloferon, which preceded the suppression of the phosphorylation of ERK and the activation of AP-1 by TPA. Moreover, when NK cells were exposed to alloferon, their cytolytic activity was improved, and this was mediated by the enhancement of perforin/granzyme secretion. Conclusions: The results of this study suggest that alloferon can be used as an effective antiviral agent for the regulation of the KSHV life cycle by the down-regulation of AP-1 activity and for the the enhancement of antiviral immunity by up-regulation of NK cell cytotoxicity.N

Dosage-dependent copy number gains in E2f1 and E2f3 drive hepatocellular carcinoma

Disruption of the retinoblastoma (RB) tumor suppressor pathway, either through genetic mutation of upstream regulatory components or mutation of RB1 itself, is believed to be a required event in cancer. However, genetic alterations in the RB-regulated E2F family of transcription factors are infrequent, casting doubt on a direct role for E2Fs in driving cancer. In this work, a mutation analysis of human cancer revealed subtle but impactful copy number gains in E2F1 and E2F3 in hepatocellular carcinoma (HCC). Using a series of loss-and gain-of-function alleles to dial E2F transcriptional output, we have shown that copy number gains in E2f1 or E2f3b resulted in dosage-dependent spontaneous HCC in mice without the involvement of additional organs. Conversely, germ-line loss of E2f1 or E2f3b, but not E2f3a, protected mice against HCC. Combinatorial mapping of chromatin occupancy and transcriptome profiling identified an E2F1- and E2F3B-driven transcriptional program that was associated with development and progression of HCC. These findings demonstrate a direct and cell-autonomous role for E2F activators in human cancer

E2f8 mediates tumor suppression in postnatal liver development

E2F-mediated transcriptional repression of cell cycle–dependent gene expression is critical for the control of cellular proliferation, survival, and development. E2F signaling also interacts with transcriptional programs that are downstream of genetic predictors for cancer development, including hepatocellular carcinoma (HCC). Here, we evaluated the function of the atypical repressor genes E2f7 and E2f8 in adult liver physiology. Using several loss-of-function alleles in mice, we determined that combined deletion of E2f7 and E2f8 in hepatocytes leads to HCC. Temporal-specific ablation strategies revealed that E2f8’s tumor suppressor role is critical during the first 2 weeks of life, which correspond to a highly proliferative stage of postnatal liver development. Disruption of E2F8’s DNA binding activity phenocopied the effects of an E2f8 null allele and led to HCC. Finally, a profile of chromatin occupancy and gene expression in young and tumor-bearing mice identified a set of shared targets for E2F7 and E2F8 whose increased expression during early postnatal liver development is associated with HCC progression in mice. Increased expression of E2F8-specific target genes was also observed in human liver biopsies from HCC patients compared to healthy patients. In summary, these studies suggest that E2F8-mediated transcriptional repression is a critical tumor suppressor mechanism during postnatal liver development