Feedback regulation between atypical E2Fs and APC/CCdh1 coordinates cell cycle progression

Posttranslational modifications of E2F family members in the physiological state and in cancer: Roles, mechanisms and therapeutic targets

The E2F transcription factor family, whose members are encoded by the E2F1-E2F8 genes, plays pivotal roles in the cell cycle, apoptosis, metabolism, stemness, metastasis, aging, angiogenesis, tumor promotion or suppression, and other biological processes. The activity of E2Fs is regulated at multiple levels, with posttranslational modifications being an important regulatory mechanism. There are numerous types of posttranslational modifications, among which phosphorylation, acetylation, methylation, ubiquitination, SUMOylation, neddylation, and poly(ADP-ribosyl)ation are the most commonly studied in the context of the E2F family. Posttranslational modifications of E2F family proteins regulate their biological activity, stability, localization, and interactions with other biomolecules, affecting cell proliferation, apoptosis, DNA damage, etc., and thereby playing roles in physiological and pathological processes. Notably, these modifications do not always act alone but rather form an interactive regulatory network. Currently, several drugs targeting posttranslational modifications are being studied or clinically applied, in which the proteolysis-targeting chimera and molecular glue can target E2Fs. This review aims to summarize the roles and regulatory mechanisms of different PTMs of E2F family members in the physiological state and in cancer and to briefly discuss their clinical significance and potential therapeutic use.

Decoding the ubiquitin language: Orchestrating transcription initiation and gene expression through chromatin remodelers and histones

Eukaryotic transcription is a finely orchestrated process and it is controlled by transcription factors as well as epigenetic regulators. Transcription factors and epigenetic regulators undergo different types of posttranslational modifications including ubiquitination to control transcription process. Ubiquitination, traditionally associated with protein degradation, has emerged as a crucial contributor to the regulation of chromatin structure through ubiquitination of histone and chromatin remodelers. Ubiquitination introduces new layers of intricacy to the regulation of transcription initiation through controlling the equilibrium between euchromatin and heterochromatin states. Nucleosome, the fundamental units of chromatin, spacing in euchromatin and heterochromatin states are regulated by histone modification and chromatin remodeling complexes. Chromatin remodeling complexes actively sculpt the chromatin architecture and thereby influence the transcriptional states of genes. Therefore, understanding the dynamic behavior of nucleosome spacing is critical as it impacts various cellular functions through controlling gene expression profiles. In this comprehensive review, we discussed the intricate interplay between ubiquitination and transcription initiation, and illuminated the underlying molecular mechanisms that occur in a variety of biological contexts. This exploration sheds light on the complex regulatory networks that govern eukaryotic transcription, providing important insights into the fine orchestration of gene expression and chromatin dynamics.

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

E2F8 exerts cancer-promoting effects by transcriptionally activating RRM2 and E2F8 knockdown synergizes with WEE1 inhibition in suppressing lung adenocarcinoma

Ribonucleotide reductase (RR) is a rate-limiting enzyme that facilitates DNA replication and repair by reducing nucleotide diphosphates (NDPs) to deoxyribonucleotide diphosphates (dNDPs) and is thereby crucial for cell proliferation and cancer development. The E2F family of transcription factors includes key regulators of gene expression involved in cell cycle control. In this study, E2F8 expression was significantly increased in most cancer tissues of lung adenocarcinoma (LUAD) patients and was correlated with the expression of RRM2 through database and clinical samples analysis. The protein expression of E2F8 and RRM2 were positively correlated with tumor-node-metastasis (TNM) pathological stage, and high expression of E2F8 and RRM2 predicted a low 5-year overall survival rate in LUAD patients. Overexpression and knockdown experiments showed that E2F8 was essential for LUAD cell proliferation, DNA synthesis, and cell cycle progression, which were RRM2-dependent. Reporter gene, ChIP-qPCR, and DNA pulldown-Western blot assays indicated that E2F8 activated the transcription of the RRM2 gene by directly binding with the RRM2 promoter in LUAD cells. Previous studies indicated that inhibition of WEE1 kinase can suppress the phosphorylation of CDK1/2 and promote the degradation of RRM2. We further showed here that the combination of E2F8 knockdown with MK-1775, an inhibitor of WEE1 being evaluated in clinical trials, synergistically suppressed proliferation and promoted apoptosis of LUAD cells in vitro and in vivo. Thus, this study reveals a novel role of E2F8 as a proto-oncogenic transcription activator by activating RRM2 expression in LUAD, and targeting both the transcription and degradation mechanisms of RRM2 could produce a synergistic inhibitory effect for LUAD treatment in addition to conventional inhibition of RR enzyme activity.

Emerging role of E2F8 in human cancer

E2F8 is a multifaceted transcription factor that plays a crucial role in mediating the hallmarks of cancer, including sustaining proliferative signaling, resisting cell death, and activating invasion and metastasis. Aberrant E2F8 expression is associated with poor clinical outcomes in most human cancers. However, E2F8 also exhibits tumor-suppressing activity; thus, the role of E2F8 in cell-fate determination is unclear. In this review, we highlight the recent progress in understanding the role of E2F8 in human cancers, which will contribute to building a conceptual framework and broadening our knowledge pertaining to E2F8. This review provides insight into future challenges and perspectives regarding the translation of biological knowledge into therapeutic strategies for the treatment of cancer.

Role of E2F transcription factor in Oral cancer: Recent Insight and Advancements

The family of mammalian E2F transcription factors (E2Fs) comprise of 8 members (E2F1-E2F8) classified as activators (E2F1-E2F3) and repressors (E2F4-E2F8) primarily regulating the expression of several genes related to cell proliferation, apoptosis and differentiation, mainly in a cell cycle-dependent manner. E2F activity is frequently controlled via the retinoblastoma protein (pRb), cyclins, p53 and the ubiquitin-proteasome pathway. Additionally, genetic or epigenetic changes result in the deregulation of E2F family genes expression altering S phase entry and apoptosis, an important hallmark for the onset and development of cancer. Although studies reveal E2Fs to be involved in several human malignancies, the mechanisms underlying the role of E2Fs in oral cancer lies nascent and needs further investigations. This review focuses on the role of E2Fs in oral cancer and the etiological factors regulating E2Fs activity, which in turn transcriptionally control the expression of their target genes, thus contributing to cell proliferation, metastasis, and drug/therapy resistance. Further, we will discuss therapeutic strategies for E2Fs, which may prevent oral tumor growth, metastasis, and drug resistance.

MEKs/ERKs-mediated FBXO1/E2Fs interaction interference modulates G1/S cell cycle transition and cancer cell proliferation

E2F 1, 2, and 3a, (refer to as E2Fs) are a subfamily of E2F transcription factor family that play essential roles in cell-cycle progression, DNA replication, DNA repair, apoptosis, and differentiation. Although the transcriptional regulation of E2Fs has focused on pocket protein retinoblastoma protein complex, recent studies indicate that post-translational modification and stability regulation of E2Fs play key roles in diverse cellular processes. In this study, we found that FBXO1, a component of S-phase kinase-associated protein 1 (SKP1)-cullin 1-F-box protein (SCF) complex, is an E2Fs binding partner. Furthermore, FBXO1 to E2Fs binding induced K48 ubiquitination and subsequent proteasomal degradation of E2Fs. Binding domain analysis indicated that the Arg (R)/Ile (I) and R/Val (V) motifs, which are located in the dimerization domain of E2Fs, of E2F 1 and 3a and E2F2, respectively, acted as degron motifs (DMs) for FBXO1. Notably, RI/AA or RV/AA mutation in the DMs reduced FBXO1-mediated ubiquitination and prolonged the half-lives of E2Fs. Importantly, the stabilities of E2Fs were affected by phosphorylation of threonine residues located near RI and RV residues of DMs. Phosphorylation prediction database analysis and specific inhibitor analysis revealed that MEK/ERK signaling molecules play key roles in FBXO1/E2Fs' interaction and modulate E2F protein turnover. Moreover, both elevated E2Fs protein levels by knockdown of FBXO1 and decreased E2Fs protein levels by sh-E2F3a delayed G₁/S cell cycle transition, resulting in inhibition of cancer cell proliferation. These results demonstrated that FBXO1-E2Fs axis-mediated precise E2Fs stability regulation plays a key role in cell proliferation via G₁/S cell cycle transition.

Efficient terminal erythroid differentiation requires the APC/C cofactor Cdh1 to limit replicative stress in erythroblasts

The APC/C-Cdh1 ubiquitin ligase complex drives proteosomal degradation of cell cycle regulators and other cellular proteins during the G1 phase of the cycle. The complex serves as an important modulator of the G1/S transition and prevents premature entry into S phase, genomic instability, and tumor development. Additionally, mounting evidence supports a role for this complex in cell differentiation, but its relevance in erythropoiesis has not been addressed so far. Here we show, using mouse models of Cdh1 deletion, that APC/C-Cdh1 activity is required for efficient terminal erythroid differentiation during fetal development as well as postnatally. Consistently, Cdh1 ablation leads to mild but persistent anemia from birth to adulthood. Interestingly, loss of Cdh1 seems to affect both, steady-state and stress erythropoiesis. Detailed analysis of Cdh1-deficient erythroid populations revealed accumulation of DNA damage in maturing erythroblasts and signs of delayed G2/M transition. Moreover, through direct assessment of replication dynamics in fetal liver cells, we uncovered slow fork movement and increased origin usage in the absence of Cdh1, strongly suggesting replicative stress to be the underlying cause of DNA lesions and cell cycle delays in erythroblasts devoid of Cdh1. In turn, these alterations would restrain full maturation of erythroblasts into reticulocytes and reduce the output of functional erythrocytes, leading to anemia. Our results further highlight the relevance of APC/C-Cdh1 activity for terminal differentiation and underscore the need for precise control of replication dynamics for efficient supply of red blood cells.

Collection of cells for single-cell RNA sequencing using high-resolution fluorescence microscopy

FACS sorting followed by single-cell RNA-sequencing (SORT-Seq) is a popular procedure to select cells of interest for single-cell transcriptomics. However, FACS is not suitable for measurement of subcellular distribution of fluorescence or for small samples (<1,000 cells). The VYCAP puncher system overcomes these limitations. Here, we describe a workflow to capture, image, and collect fluorescent human retina pigment epithelium cells for SORT-Seq using this system. The workflow can be used for any cell type with a diameter of ∼5-50 μm. For complete details on the use and execution of this protocol, please refer to Segeren et al. (2020).

Cell cycle regulation by complex nanomachines

The cell cycle is the essential biological process where one cell replicates its genome and segregates the resulting two copies into the daughter cells during mitosis. Several aspects of this process have fascinated humans since the nineteenth century. Today, the cell cycle is exhaustively investigated because of its profound connections with human diseases and cancer. At the heart of the molecular network controlling the cell cycle, we find the cyclin-dependent kinases (CDKs) acting as an oscillator to impose an orderly and highly regulated progression through the different cell cycle phases. This oscillator integrates both internal and external signals via a multitude of signalling pathways involving posttranslational modifications including phosphorylation, protein ubiquitination and mechanisms of transcriptional regulation. These tasks are specifically performed by multi-subunit complexes, which are intensively studied both biochemically and structurally with the aim to unveil mechanistic insights into their molecular function. The scope of this review is to summarise the structural biology of the cell cycle machinery, with specific focus on the core cell cycle machinery involving the CDK-cyclin oscillator. We highlight the contribution of cryo-electron microscopy, which has started to revolutionise our understanding of the molecular function and dynamics of the key players of the cell cycle.

Intricate Regulatory Mechanisms of the Anaphase-Promoting Complex/Cyclosome and Its Role in Chromatin Regulation

The ubiquitin (Ub)-proteasome system is vital to nearly every biological process in eukaryotes. Specifically, the conjugation of Ub to target proteins by Ub ligases, such as the Anaphase-Promoting Complex/Cyclosome (APC/C), is paramount for cell cycle transitions as it leads to the irreversible destruction of cell cycle regulators by the proteasome. Through this activity, the RING Ub ligase APC/C governs mitosis, G1, and numerous aspects of neurobiology. Pioneering cryo-EM, biochemical reconstitution, and cell-based studies have illuminated many aspects of the conformational dynamics of this large, multi-subunit complex and the sophisticated regulation of APC/C function. More recent studies have revealed new mechanisms that selectively dictate APC/C activity and explore additional pathways that are controlled by APC/C-mediated ubiquitination, including an intimate relationship with chromatin regulation. These tasks go beyond the traditional cell cycle role historically ascribed to the APC/C. Here, we review these novel findings, examine the mechanistic implications of APC/C regulation, and discuss the role of the APC/C in previously unappreciated signaling pathways.

Atypical E2Fs either Counteract or Cooperate with RB during Tumorigenesis Depending on Tissue Context

Simple Summary

In virtually all human malignancies, the CDK-RB-E2F pathway is dysregulated resulting in the activation of the E2F transcriptional network. Rb and atypical E2Fs are the most important negative regulators of E2F-dependent transcription during tumorigenesis. However, it is unknown whether they cooporate or act independently in tumor development. Here we show that combined loss of RB and atypical E2Fs in mice enhances tumorigenesis in the liver, while in the pituitary gland, we observe inhibition of tumorigenesis. These findings suggest that the interaction between RB and atypical E2Fs in controlling tumorigenesis occurs in a tissue cell-type specific manner.

Abstract

E2F-transcription factors activate many genes involved in cell cycle progression, DNA repair, and apoptosis. Hence, E2F-dependent transcription must be tightly regulated to prevent tumorigenesis, and therefore metazoan cells possess multiple E2F regulation mechanisms. The best-known is the Retinoblastoma protein (RB), which is mutated in many cancers. Atypical E2Fs (E2F7 and −8) can repress E2F-target gene expression independently of RB and are rarely mutated in cancer. Therefore, they may act as emergency brakes in RB-mutated cells to suppress tumor growth. Currently, it is unknown if and how RB and atypical E2Fs functionally interact in vivo. Here, we demonstrate that mice with liver-specific combinatorial deletion of Rb and E2f7/8 have reduced life-spans compared to E2f7/8 or Rb deletion alone. This was associated with increased proliferation and enhanced malignant progression of liver tumors. Hence, atypical repressor E2Fs and RB cooperatively act as tumor suppressors in hepatocytes. In contrast, loss of either E2f7 or E2f8 largely prevented the formation of pituitary tumors in Rb^+/− mice. To test whether atypical E2Fs can also function as oncogenes independent of RB loss, we induced long-term overexpression of E2f7 or E2f8 in mice. E2F7 and −8 overexpression increased the incidence of tumors in the lungs, but not in other tissues. Collectively, these data show that atypical E2Fs can promote but also inhibit tumorigenesis depending on tissue type and RB status. We propose that the complex interactions between atypical E2Fs and RB on maintenance of genetic stability underlie this context-dependency.

E2F7 Is a Potent Inhibitor of Liver Tumor Growth in Adult Mice

BACKGROUND AND AIMS

Up-regulation of the E2F-dependent transcriptional network has been identified in nearly every human malignancy and is an important driver of tumorigenesis. Two members of the E2F family, E2F7 and E2F8, are potent repressors of E2F-dependent transcription. They are atypical in that they do not bind to dimerization partner proteins and are not controlled by retinoblastoma protein. The physiological relevance of E2F7 and E2F8 remains incompletely understood, largely because tools to manipulate their activity in vivo have been lacking.

APPROACH AND RESULTS

Here, we generated transgenic mice with doxycycline-controlled transcriptional activation of E2f7 and E2f8 and induced their expression during postnatal development, in adulthood, and in the context of cancer. Systemic induction of E2f7 and, to lesser extent, E2f8 transgenes in juvenile mice impaired cell proliferation, caused replication stress, DNA damage, and apoptosis, and inhibited animal growth. In adult mice, however, E2F7 and E2F8 induction was well tolerated, yet profoundly interfered with DNA replication, DNA integrity, and cell proliferation in diethylnitrosamine-induced liver tumors.

CONCLUSION

Collectively, our findings demonstrate that atypical E2Fs can override cell-cycle entry and progression governed by other E2F family members and suggest that this property can be exploited to inhibit proliferation of neoplastic hepatocytes when growth and development have subsided during adulthood.

Excessive E2F Transcription in Single Cancer Cells Precludes Transient Cell-Cycle Exit after DNA Damage

E2F transcription factors control the expression of cell-cycle genes. Cancers often demonstrate enhanced E2F target gene expression, which can be explained by increased percentages of replicating cells. However, we demonstrate in human cancer biopsy specimens that individual neoplastic cells display abnormally high levels of E2F-dependent transcription. To mimic this situation, we delete the atypical E2F repressors (E2F7/8) or overexpress the E2F3 activator in untransformed cells. Cells with elevated E2F activity during S/G2 phase fail to exit the cell cycle after DNA damage and undergo mitosis. In contrast, wild-type cells complete S phase and then exit the cell cycle by activating the APC/C^Cdh1 via repression of the E2F target Emi1. Many arrested wild-type cells eventually inactivate APC/C^Cdh1 to execute a second round of DNA replication and mitosis, thereby becoming tetraploid. Cells with elevated E2F transcription fail to exit the cell cycle after DNA damage, which potentially causes genomic instability, promotes malignant progression, and reduces drug sensitivity.

E2F7-EZH2 axis regulates PTEN/AKT/mTOR signalling and glioblastoma progression

BACKGROUND

E2F transcription factors are considered to be important drivers of tumour growth. E2F7 is an atypical E2F factor, and its role in glioblastoma remains undefined.

METHODS

E2F7 expression was examined in patients by IHC and qRT-PCR. The overall survival probability was determined by statistical analyses. MTT assay, colony formation, cell-cycle assay, cell metastasis and the in vivo model were employed to determine the functional role of E2F7 in glioblastoma. Chromatin immunoprecipitation, luciferase assay and western blot were used to explore the underlying mechanisms.

RESULTS

E2F7 was found to be up-regulated in glioblastoma patients, and high E2F7 expression was associated with poor overall survival in glioblastoma patients. Functional studies showed that E2F7 promoted cell proliferation, cell-cycle progression, cell metastasis and tumorigenicity abilities in vitro and in vivo. E2F7 promoted the transcription of EZH2 by binding to its promoter and increased H3K27me3 level. EZH2 recruited H3K27me3 to the promoter of PTEN and inhibited PTEN expression, and then activated the AKT/mTOR signalling pathway. In addition, restored expression of EZH2 recovered the abilities of cell proliferation and metastasis in E2F7-silencing cells.

CONCLUSION

Collectively, our findings indicate that E2F7 promotes cell proliferation, cell metastasis and tumorigenesis via EZH2-mediated PTEN/AKT/mTOR pathway in glioblastoma.

Complex Cartography: Regulation of E2F Transcription Factors by Cyclin F and Ubiquitin

The E2F family of transcriptional regulators sits at the center of cell cycle gene expression and plays vital roles in normal and cancer cell cycles. Whereas control of E2Fs by the retinoblastoma family of proteins is well established, much less is known about their regulation by ubiquitin pathways. Recent studies placed the Skp1-Cul1-F-box-protein (SCF) family of E3 ubiquitin ligases with the F-box protein Cyclin F at the center of E2F regulation, demonstrating temporal proteolysis of both activator and atypical repressor E2Fs. Importantly, these E2F members, in particular activator E2F1 and repressors E2F7 and E2F8, form a feedback circuit at the crossroads of cell cycle and cell death. Moreover, Cyclin F functions in a reciprocal circuit with the cell cycle E3 ligase anaphase-promoting complex/cyclosome (APC/C), which also controls E2F7 and E2F8. This review focuses on the complex contours of feedback within this circuit, highlighting the deep crosstalk between E2F, SCF-Cyclin F, and APC/C in regulating the oscillator underlying human cell cycles.

Cell cycle oscillators underlying orderly proteolysis of E2F8

E2F8 is a transcriptional repressor that antagonizes E2F1 at the crossroads of the cell cycle, apoptosis, and cancer. Previously, we discovered that E2F8 is a direct target of the APC/C ubiquitin ligase. Nevertheless, it remains unknown how E2F8 is dynamically controlled throughout the entirety of the cell cycle. Here, using newly developed human cell-free systems that recapitulate distinct inter-mitotic and G1 phases and a continuous transition from prometaphase to G1, we reveal an interlocking dephosphorylation switch coordinating E2F8 degradation with mitotic exit and the activation of APC/C^Cdh1. Further, we uncover differential proteolysis rates for E2F8 at different points within G1 phase, accounting for its accumulation in late G1 while APC/C^Cdh1 is still active. Finally, we demonstrate that the F-box protein Cyclin F regulates E2F8 in G2-phase. Altogether, our data define E2F8 regulation throughout the cell cycle, illuminating an extensive coordination between phosphorylation, ubiquitination and transcription in mammalian cell cycle.

Cyclin F-dependent degradation of E2F7 is critical for DNA repair and G2-phase progression

E2F7 and E2F8 act as tumor suppressors via transcriptional repression of genes involved in S-phase entry and progression. Previously, we demonstrated that these atypical E2Fs are degraded by APC/CCdh1 during G1 phase of the cell cycle. However, the mechanism driving the downregulation of atypical E2Fs during G2 phase is unknown. Here, we show that E2F7 is targeted for degradation by the E3 ubiquitin ligase SCFcyclin F during G2. Cyclin F binds via its cyclin domain to a conserved C-terminal CY motif on E2F7. An E2F7 mutant unable to interact with SCFcyclin F remains stable during G2. Furthermore, SCFcyclin F can also interact and induce degradation of E2F8. However, this does not require the cyclin domain of SCFcyclin F nor the CY motifs in the C-terminus of E2F8, implying a different regulatory mechanism than for E2F7. Importantly, depletion of cyclin F causes an atypical-E2F-dependent delay of the G2/M transition, accompanied by reduced expression of E2F target genes involved in DNA repair. Live cell imaging of DNA damage revealed that cyclin F-dependent regulation of atypical E2Fs is critical for efficient DNA repair and cell cycle progression.

Two Distinct E2F Transcriptional Modules Drive Cell Cycles and Differentiation

Orchestrating cell-cycle-dependent mRNA oscillations is critical to cell proliferation in multicellular organisms. Even though our understanding of cell-cycle-regulated transcription has improved significantly over the last three decades, the mechanisms remain untested in vivo. Unbiased transcriptomic profiling of G0, G1-S, and S-G2-M sorted cells from FUCCI mouse embryos suggested a central role for E2Fs in the control of cell-cycle-dependent gene expression. The analysis of gene expression and E2F-tagged knockin mice with tissue imaging and deep-learning tools suggested that post-transcriptional mechanisms universally coordinate the nuclear accumulation of E2F activators (E2F3A) and canonical (E2F4) and atypical (E2F8) repressors during the cell cycle in vivo. In summary, we mapped the spatiotemporal expression of sentinel E2F activators and canonical and atypical repressors at the single-cell level in vivo and propose that two distinct E2F modules relay the control of gene expression in cells actively cycling (E2F3A-8-4) and exiting the cycle (E2F3A-4) during mammalian development.

The broken cycle: E2F dysfunction in cancer

The cyclin-dependent kinase (CDK)-RB-E2F axis forms the core transcriptional machinery driving cell cycle progression, dictating the timing and fidelity of genome replication and ensuring genetic material is accurately passed through each cell division cycle. The ultimate effectors of this axis are members of a family of eight distinct E2F genes encoding transcriptional activators and repressors. E2F transcriptional activity is tightly regulated throughout the cell cycle via transcriptional and translational regulation, post-translational modifications, protein degradation, binding to cofactors and subcellular localization. Alterations in one or more key components of this axis (CDKs, cyclins, CDK inhibitors and the RB family of proteins) occur in virtually all cancers and result in heightened oncogenic E2F activity, leading to uncontrolled proliferation. In this Review, we discuss the activities of E2F proteins with an emphasis on the newest atypical E2F family members, the specific and redundant functions of E2F proteins, how misexpression of E2F transcriptional targets promotes cancer and both current and developing therapeutic strategies being used to target this oncogenic pathway.

Functional interplay between E2F7 and ribosomal rRNA gene transcription regulates protein synthesis

A prerequisite for protein synthesis is the transcription of ribosomal rRNA genes by RNA polymerase I (Pol I), which controls ribosome biogenesis. UBF (upstream binding factor) is one of the main Pol I transcription factors located in the nucleolus that activates rRNA gene transcription. E2F7 is an atypical E2F family member that acts as a transcriptional repressor of E2F target genes, and thereby contributes to cell cycle arrest. Here, we describe an unexpected role for E2F7 in regulating rRNA gene transcription. We have found that E2F7 localises to the perinucleolar region, and further that E2F7 is able to exert repressive effects on Pol I transcription. At the mechanistic level, this is achieved in part by E2F7 hindering UBF recruitment to the rRNA gene promoter region, and thereby reducing rRNA gene transcription, which in turn compromises global protein synthesis. Our results expand the target gene repertoire influenced by E2F7 to include Pol I-regulated genes, and more generally suggest a mechanism mediated by effects on Pol I transcription where E2F7 links cell cycle arrest with protein synthesis.

CDH1 regulates E2F1 degradation in response to differentiation signals in keratinocytes

The E2F1 transcription factor plays key roles in skin homeostasis. In the epidermis, E2F1 expression is essential for normal proliferation of undifferentiated keratinocytes, regeneration after injury and DNA repair following UV radiation-induced photodamage. Abnormal E2F1 expression promotes nonmelanoma skin carcinoma. In addition, E2F1 must be downregulated for proper keratinocyte differentiation, but the relevant mechanisms involved remain poorly understood. We show that differentiation signals induce a series of post-translational modifications in E2F1 that are jointly required for its downregulation. Analysis of the structural determinants that govern these processes revealed a central role for S403 and T433. In particular, substitution of these two amino acid residues with non-phosphorylatable alanine (E2F1 ST/A) interferes with E2F1 nuclear export, K11- and K48-linked polyubiquitylation and degradation in differentiated keratinocytes. In contrast, replacement of S403 and T433 with phosphomimetic aspartic acid to generate a pseudophosphorylated E2F1 mutant protein (E2F1 ST/D) generates a protein that is regulated in a manner indistinguishable from that of wild type E2F1. Cdh1 is an activating cofactor that interacts with the anaphase-promoting complex/cyclosome (APC/C) ubiquitin E3 ligase, promoting proteasomal degradation of various substrates. We found that Cdh1 associates with E2F1 in keratinocytes. Inhibition or RNAi-mediated silencing of Cdh1 prevents E2F1 degradation in response to differentiation signals. Our results reveal novel regulatory mechanisms that jointly modulate post-translational modifications and downregulation of E2F1, which are necessary for proper epidermal keratinocyte differentiation.

Chk1 and 14-3-3 proteins inhibit atypical E2Fs to prevent a permanent cell cycle arrest

The atypical E2Fs, E2F7 and E2F8, act as potent transcriptional repressors of DNA replication genes providing them with the ability to induce a permanent S-phase arrest and suppress tumorigenesis. Surprisingly in human cancer, transcript levels of atypical E2Fs are frequently elevated in proliferating cancer cells, suggesting that the tumor suppressor functions of atypical E2Fs might be inhibited through unknown post-translational mechanisms. Here, we show that atypical E2Fs can be directly phosphorylated by checkpoint kinase 1 (Chk1) to prevent a permanent cell cycle arrest. We found that 14-3-3 protein isoforms interact with both E2Fs in a Chk1-dependent manner. Strikingly, Chk1 phosphorylation and 14-3-3-binding did not relocate or degrade atypical E2Fs, but instead, 14-3-3 is recruited to E2F7/8 target gene promoters to possibly interfere with transcription. We observed that high levels of 14-3-3 strongly correlate with upregulated transcription of atypical E2F target genes in human cancer. Thus, we reveal that Chk1 and 14-3-3 proteins cooperate to inactivate the transcriptional repressor functions of atypical E2Fs. This mechanism might be of particular importance to cancer cells, since they are exposed frequently to DNA-damaging therapeutic reagents.

Tissue-Specific Control of the Endocycle by the Anaphase Promoting Complex/Cyclosome Inhibitors UVI4 and DEL1

The endocycle represents a modified mitotic cell cycle that in plants is often coupled to cell enlargement and differentiation. Endocycle onset is controlled by activity of the Anaphase Promoting Complex/Cyclosome (APC/C), a multisubunit E3 ubiquitin ligase targeting cell-cycle factors for destruction. CELL CYCLE SWITCH52 (CCS52) proteins represent rate-limiting activator subunits of the APC/C. In Arabidopsis (Arabidopsis thaliana), mutations in either CCS52A1 or CCS52A2 activators result in a delayed endocycle onset, whereas their overexpression triggers increased DNA ploidy levels. Here, the relative contribution of the APC/C^CCS52A1 and APC/C^CCS52A2 complexes to different developmental processes was studied through analysis of their negative regulators, being the ULTRAVIOLET-B-INSENSITIVE4 protein and the DP-E2F-Like1 transcriptional repressor, respectively. Our data illustrate cooperative activity of the APC/C^CCS52A1 and APC/C^CCS52A2 complexes during root and trichome development, but functional interdependency during leaf development. Furthermore, we found APC/C^CCS52A1 activity to control CCS52A2 expression. We conclude that interdependency of CCS52A-controlled APC/C activity is controlled in a tissue-specific manner.

Synergistic functions of E2F7 and E2F8 are critical to suppress stress-induced skin cancer

E2F transcription factors are important regulators of the cell cycle, and unrestrained activation of E2F-dependent transcription is considered to be an important driver of tumor formation and progression. Although highly expressed in normal skin and skin cancer, the role of the atypical E2Fs, E2F7 and E2F8, in keratinocyte homeostasis, regeneration and tumorigenesis is unknown. Surprisingly, keratinocyte-specific deletion of E2F7 and E2F8 in mice did not interfere with skin development and wound healing. However, the rate for successful isolation and establishment of E2f7/8-deficient primary keratinocyte cultures was much higher than for wild-type keratinocytes. Moreover, E2f7/8-deficient primary keratinocytes proliferate more efficiently under stress conditions, such as low/high confluence or DNA damage. Application of in vivo stress using the DMBA/TPA skin carcinogenesis protocol revealed that combined inactivation of E2f7/8 enhanced tumorigenesis and accelerated malignant progression. Loss of atypical E2Fs resulted in increased expression of E2F target genes, including E2f1. Additional loss of E2f1 did not rescue, but worsened skin tumorigenesis. We show that loss of E2F7/8 triggers apoptosis via induction of E2F1 in response to stress, indicating that the tumor-promoting effect of E2F7/8 inactivation can be partially compensated via E2F1-dependent apoptosis. Importantly, E2F7/8 repressed a large set of E2F target genes that are highly expressed in human patients with skin cancer. Together, our studies demonstrate that atypical E2Fs act as tumor suppressors, most likely via transcriptional repression of cell cycle genes in response to stress.

Feedback regulation between atypical E2Fs and APC/CCdh1 coordinates cell cycle progression

E2F transcription factors control the oscillating expression pattern of multiple target genes during the cell cycle. Activator E2Fs, E2F1-3, induce an upswing of E2F targets, which is essential for the G1-to-S phase transition, whereas atypical E2Fs, E2F7 and E2F8, mediate a downswing of the same targets during late S, G2, and M phases. Expression of atypical E2Fs is induced by E2F1-3, but it is unknown how atypical E2Fs are inactivated in a timely manner. Here, we demonstrate that E2F7 and E2F8 are substrates of the anaphase-promoting complex/cyclosome (APC/C). Removal of CDH1, or mutating the CDH1-interacting KEN boxes, stabilized E2F7/8 from anaphase onwards and during G1. Expressing KEN mutant E2F7 during G1 impairs S phase entry and eventually results in cell death. Furthermore, we show that E2F8, but not E2F7, interacts also with APC/C(C) (dc20). Importantly, atypical E2Fs can activate APC/C(C) (dh1) by repressing its inhibitors cyclin A, cyclin E, and Emi1. In conclusion, we discovered a feedback loop between atypical E2Fs and APC/C(C) (dh1), which ensures balanced expression of cell cycle genes and normal cell cycle progression.