E2F1 overexpression in quiescent fibroblasts leads to induction of cellular DNA synthesis and apoptosis

DLGAP5 triggers proliferation and metastasis of bladder cancer by stabilizing E2F1 via USP11

Bladder cancer (BLCA) is one of the most widespread malignancies worldwide, and displays significant tumor heterogeneity. Understanding the molecular mechanisms exploitable for treating aggressive BLCA represents a crucial objective. Despite the involvement of DLGAP5 in tumors, its precise molecular role in BLCA remains unclear. BLCA tissues exhibit a substantial increase in DLGAP5 expression compared with normal bladder tissues. This heightened DLGAP5 expression positively correlated with the tumor's clinical stage and significantly affected prognosis negatively. Additionally, experiments conducted in vitro and in vivo revealed that alterations in DLGAP5 expression notably influence cell proliferation and migration. Mechanistically, the findings demonstrated that DLGAP5 was a direct binding partner of E2F1 and that DLGAP5 stabilized E2F1 by preventing the ubiquitination of E2F1 through USP11. Furthermore, as a pivotal transcription factor, E2F1 fosters the transcription of DLGAP5, establishing a positive feedback loop between DLGAP5 and E2F1 that accelerates BLCA development. In summary, this study identified DLGAP5 as an oncogene in BLCA. Our research unveils a novel oncogenic mechanism in BLCA and offers a potential target for both diagnosing and treating BLCA.

P97/VCP ATPase inhibitors can rescue p97 mutation-linked motor neuron degeneration

Mutations in p97/VCP cause two motor neuron diseases: inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia and familial amyotrophic lateral sclerosis. How p97 mutations lead to motor neuron degeneration is, however, unknown. Here we used patient-derived induced pluripotent stem cells to generate p97 mutant motor neurons. We reduced the genetic background variation by comparing mutant motor neurons to its isogenic wild type lines. Proteomic analysis reveals that p97^R155H/+ motor neurons upregulate several cell cycle proteins at Day 14, but this effect diminishes by Day 20. Molecular changes linked to delayed cell cycle exit are observed in p97 mutant motor neurons. We also find that two p97 inhibitors, CB-5083 and NMS-873, restore some dysregulated protein levels. In addition, two p97 inhibitors and a food and drug administration-approved cyclin-dependent kinase 4/6 inhibitor, Abemaciclib, can rescue motor neuron death. Overall, we successfully used iPSC-derived motor neurons, identified dysregulated proteome and transcriptome and showed that p97 inhibitors rescue phenotypes in this disease model.

PRR11 promotes ccRCC tumorigenesis by regulating E2F1 stability

Proline rich 11 (PRR11), a novel tumor-related gene, has been identified in different tumors. However, the relevant biological functions of PRR11 in human clear cell renal cell carcinoma (ccRCC) have not been studied. In this study, we first identified PRR11 as a biomarker of ccRCC and predictor of poor prognosis by bioinformatics. Then, we showed that PRR11 silencing substantially reduced ccRCC cell proliferation and migration in vitro and in vivo. Importantly, we found that PRR11 induced the degradation of the E2F1 protein through its interaction with E2F1, and PRR11 reduced the stability of the E2F1 protein in ccRCC cells, thereby affecting cell cycle progression. Further results indicated that the downregulation of E2F1 expression partially reversed the changes in ccRCC cell biology caused by PRR11 deletion. In addition, we showed that PRR11 was a target gene of c-Myc. The transcription factor c-Myc may have promoted the expression of PRR11 in ccRCC cells by binding to the PRR11 promoter region, thereby accelerating the progression of ccRCC. In summary, we found that PRR11 served as an oncogene in ccRCC, and PRR11 reduced the protein stability of E2F1 and could be activated by c-Myc.

The Role and Clinical Implications of the Retinoblastoma (RB)-E2F Pathway in Gastric Cancer

Gastric cancer is the most common malignant tumor in the digestive tract, with very high morbidity and mortality in developing countries. The pathogenesis of gastric cancer is a complex biological process mediated by abnormal regulation of proto-oncogenes and tumor suppressor genes. Although there have been some in-depth studies on gastric cancer at the molecular level, the specific mechanism has not been fully elucidated. RB family proteins (including RB, p130, and p107) are involved in cell cycle regulation, a process that largely depends on members of the E2F gene family that encode transcriptional activators and repressors. In gastric cancer, inactivation of the RB-E2F pathway serves as a core transcriptional mechanism that drives cell cycle progression, and is regulated by cyclins, cyclin-dependent kinases, cyclin-dependent kinase inhibitors, p53, Helicobacter pylori and some other upstream molecules. The E2F proteins are encoded by eight genes (i.e. E2F1 to E2F8), each of which may play a specific role in gastric cancer. Interestingly, a single E2F such as E2F1 can activate or repress transcription, and enhance or inhibit cell proliferation, depending on the cell environment. Thus, the function of the E2F transcription factor family is very complex and needs further exploration. Importantly, the presence of H. pylori in stomach mucosa may affect the RB and p53 tumor suppressor systems, thereby promoting the occurrence of gastric cancer. This review aims to summarize recent research progress on important roles of the complex RB-E2F signaling network in the development and effective treatment of gastric cancer.

Unraveling the Big Sleep: Molecular Aspects of Stem Cell Dormancy and Hibernation

Tissue-resident stem cells may enter a dormant state, also known as quiescence, which allows them to withstand metabolic stress and unfavorable conditions. Similarly, hibernating mammals can also enter a state of dormancy used to evade hostile circumstances, such as food shortage and low ambient temperatures. In hibernation, the dormant state of the individual and its cells is commonly known as torpor, and is characterized by metabolic suppression in individual cells. Given that both conditions represent cell survival strategies, we here compare the molecular aspects of cellular quiescence, particularly of well-studied hematopoietic stem cells, and torpor at the cellular level. Critical processes of dormancy are reviewed, including the suppression of the cell cycle, changes in metabolic characteristics, and cellular mechanisms of dealing with damage. Key factors shared by hematopoietic stem cell quiescence and torpor include a reversible activation of factors inhibiting the cell cycle, a shift in metabolism from glucose to fatty acid oxidation, downregulation of mitochondrial activity, key changes in hypoxia-inducible factor one alpha (HIF-1α), mTOR, reversible protein phosphorylation and autophagy, and increased radiation resistance. This similarity is remarkable in view of the difference in cell populations, as stem cell quiescence regards proliferating cells, while torpor mainly involves terminally differentiated cells. A future perspective is provided how to advance our understanding of the crucial pathways that allow stem cells and hibernating animals to engage in their 'great slumbers.'

RB, p130 and p107 differentially repress G1/S and G2/M genes after p53 activation

Cell cycle gene expression occurs in two waves. The G1/S genes encode factors required for DNA synthesis and the G2/M genes contribute to mitosis. The Retinoblastoma protein (RB) and DREAM complex (DP, RB-like, E2F4 and MuvB) cooperate to repress all cell cycle genes during G1 and inhibit entry into the cell cycle. DNA damage activates p53 leading to increased levels of p21 and inhibition of cell cycle progression. Whether the G1/S and G2/M genes are differentially repressed by RB and the RB-like proteins p130 and p107 in response to DNA damage is not known. We performed gene expression profiling of primary human fibroblasts upon DNA damage and assessed the effects on G1/S and G2/M genes. Upon p53 activation, p130 and RB cooperated to repress the G1/S genes. In addition, in the absence of RB and p130, p107 contributed to repression of G1/S genes. In contrast, G2/M genes were repressed by p130 and p107 after p53 activation. Furthermore, repression of G2/M genes by p107 and p130 led to reduced entry into mitosis. Our data demonstrates specific roles for RB, p130-DREAM, and p107-DREAM in p53 and p21 mediated repression of cell cycle genes.

The Nonsynonymous Polymorphisms Val276Met and Gly393Ser of E2F1 Gene are Strongly Associated with Lung, and Head and Neck Cancers

AIM

The early gene factor-2 (E2F), a family of transcription factors, is involved in cell cycle regulation. Deregulated expression of most of the members of the E2F family is associated with various human cancers. In this study, we investigated the association between the E2F1 genetic variants rs3213173 (C/T) (Val276Met) and rs3213176 (G/A) (Gly393Ser) with the risk of lung cancer (LC) and head and neck cancer (HNC) in 190 patients and 230 control samples.

MATERIALS AND METHODS

We used polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and mutagenic primer-based PCR-RFLP methods to genotype all target polymorphisms.

RESULTS

The rs3213173 (C/T) polymorphism was associated with LC risk in the homozygous model (odds ratio [OR] = 2.954, 95% confidence interval [CI] 1.366-6.386; p = 0.004) as well as in heterozygous model (OR = 2.314; 95% CI = 1.369-3.912; p = 0.001). A significant association was also observed for the rs3213176 (G/A) polymorphism with LC risk in homozygous model, GG versus AA (OR = 2.750; 95% CI = 1.236-6.118; p = 0.01); in heterozygous model, GG versus GA (OR = 2.111; 95% CI = 1.256-3.549; p = 0.004); and in combined mutant GG versus GA+AA (OR = 2.214; 95% CI = 1.343-3.650; p = 0.001). The rs3213176 (G/A) marker was also associated with HNC risk.

CONCLUSIONS

Our findings reveal that the rs3213173 (C/T) and rs3213176 (G/A) polymorphisms of the E2F1 gene are genetic risk factors for susceptibility to LC and HNC in the North Indian Population.

Metformin induces cell cycle arrest at the G1 phase through E2F8 suppression in lung cancer cells

A target molecule responsible for cell cycle arrest by metformin was discovered using a gene chip array in lung cancer cells and the effect of metformin on E2F8 was assessed. The siRNA-mediated knockdown of E2F8 significantly suppressed G1-S progression while ectopic expression of E2F8 relieved metformin-induced G1 arrest. The mRNA levels of p21 were found to be inversely related to those of E2F8 in lung cancer cells while siRNA-mediated knockdown of p21 partly rescued siE2F8-induced arrest of the cell cycle. Metformin had no effect on degradation of E2F8 mRNA. Activation and inhibition of AMPK by AICAR and Dorsomorphin, respectively, did not affect E2F8 suppression by metformin. The clinical significance of E2F8 was analyzed in The Cancer Genome Atlas (TCGA) data. One hundred six (13%) of 848 TCGA lung cancers overexpressed E2F8 mRNA. The overexpression of E2F8 was associated with poor overall survival (adjusted hazard ratio = 1.58, 95% confidence interval = 1.13-2.22; P = 0.008). The present study suggests that metformin may induce cell cycle arrest at the G1 phase by suppressing E2F8 expression in lung cancer cells. In addition, E2F8 may be associated with poor overall survival in lung cancer patients irrespective of histology.

Elimination of quiescent slow-cycling cells via reducing quiescence depth by natural compounds purified from Ganoderma lucidum

The medical mushroom Ganoderma lucidum has long been used in traditional Chinese medicine and shown effective in the treatment of many diseases including cancer. Here we studied the cytotoxic effects of two natural compounds purified from Ganoderma lucidum, ergosterol peroxide and ganodermanondiol. We found that these two compounds exhibited cytotoxicity not only against fast proliferating cells, but on quiescent, slow-cycling cells. Using a fibroblast cell-quiescence model, we found that the cytotoxicity on quiescent cells was due to induced apoptosis, and was associated with a shallower quiescent state in compound-treated cells, resultant from the increased basal activity of an Rb-E2F bistable switch that controls quiescence exit. Accordingly, we showed that quiescent breast cancer cells (MCF7), compared to its non-transformed counterpart (MCF10A), were preferentially killed by ergosterol peroxide and ganodermanondiol treatment presumably due to their already less stable quiescent state. The cytotoxic effect of natural Ganoderma lucidum compounds against quiescent cells, preferentially on quiescent cancer cells vs. non-cancer cells, may help future antitumor development against the slow-cycling cancer cell subpopulations including cancer stem and progenitor cells.

Expression level is a key determinant of E2F1-mediated cell fate

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.

Ablation of the transcription factors E2F1-2 limits neuroinflammation and associated neurological deficits after contusive spinal cord injury

Traumatic spinal cord injury (SCI) induces cell cycle activation (CCA) that contributes to secondary injury and related functional impairments such as motor deficits and hyperpathia. E2F1 and E2F2 are members of the activator sub-family of E2F transcription factors that play an important role in proliferating cells and in cell cycle-related neuronal death, but no comprehensive study have been performed in SCI to determine the relative importance of these factors. Here we examined the temporal distribution and cell-type specificity of E2F1 and E2F2 expression following mouse SCI, as well as the effects of genetic deletion of E2F1-2 on neuronal cell death, neuroinflammation and associated neurological dysfunction. SCI significantly increased E2F1 and E2F2 expression in active caspase-3(+) neurons/oligodendrocytes as well as in activated microglia/astrocytes. Injury-induced up-regulation of cell cycle-related genes and protein was significantly reduced by intrathecal injection of high specificity E2F decoy oligodeoxynucleotides against the E2F-binding site or in E2F1-2 null mice. Combined E2F1+2 siRNA treatment show greater neuroprotection in vivo than E2F1 or E2F2 single siRNA treatment. Knockout of both E2F1 and E2F2 genes (E2Fdko) significantly reduced neuronal death, neuroinflammation, and tissue damage, as well as limiting motor dysfunction and hyperpathia after SCI. Both CCA reduction and functional improvement in E2Fdko mice were greater than those in E2F2ko model. These studies demonstrate that SCI-induced activation of E2F1-2 mediates CCA, contributing to gliopathy and neuronal/tissue loss associated with motor impairments and post-traumatic hyperesthesia. Thus, E2F1-2 provide a therapeutic target for decreasing secondary tissue damage and promoting recovery of function after SCI.

Systems-level effects of ectopic galectin-7 reconstitution in cervical cancer and its microenvironment

Background

Galectin-7 (Gal-7) is negatively regulated in cervical cancer, and appears to be a link between the apoptotic response triggered by cancer and the anti-tumoral activity of the immune system. Our understanding of how cervical cancer cells and their molecular networks adapt in response to the expression of Gal-7 remains limited.

Methods

Meta-analysis of Gal-7 expression was conducted in three cervical cancer cohort studies and TCGA. In silico prediction and bisulfite sequencing were performed to inquire epigenetic alterations. To study the effect of Gal-7 on cervical cancer, we ectopically re-expressed it in the HeLa and SiHa cervical cancer cell lines, and analyzed their transcriptome and SILAC-based proteome. We also examined the tumor and microenvironment host cell transcriptomes after xenotransplantation into immunocompromised mice. Differences between samples were assessed with the Kruskall-Wallis, Dunn’s Multiple Comparison and T tests. Kaplan–Meier and log-rank tests were used to determine overall survival.

Results

Gal-7 was constantly downregulated in our meta-analysis (p < 0.0001). Tumors with combined high Gal-7 and low galectin-1 expression (p = 0.0001) presented significantly better prognoses (p = 0.005). In silico and bisulfite sequencing assays showed de novo methylation in the Gal-7 promoter and first intron. Cells re-expressing Gal-7 showed a high apoptosis ratio (p < 0.05) and their xenografts displayed strong growth retardation (p < 0.001). Multiple gene modules and transcriptional regulators were modulated in response to Gal-7 reconstitution, both in cervical cancer cells and their microenvironments (FDR < 0.05 %). Most of these genes and modules were associated with tissue morphogenesis, metabolism, transport, chemokine activity, and immune response. These functional modules could exert the same effects in vitro and in vivo, even despite different compositions between HeLa and SiHa samples.

Conclusions

Gal-7 re-expression affects the regulation of molecular networks in cervical cancer that are involved in diverse cancer hallmarks, such as metabolism, growth control, invasion and evasion of apoptosis. The effect of Gal-7 extends to the microenvironment, where networks involved in its configuration and in immune surveillance are particularly affected.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2700-8) contains supplementary material, which is available to authorized users.

Calcium, a Cell Cycle Commander, Drives Colon Cancer Cell Diffpoptosis

The story of the cell commonder, calcium, reaches into all corners of the cell and controls cell proliferation, differentiation, function, and even death. The calcium-driven eukaryotic revolution is one of the great turning points in the life history, happened about two billion years later when it was converted from a dangerous killer that had to be kept out of cell into the cell master which drives the cell. This review article will take the readers to a tour of tissues chosen to best show the calcium's many faces (proliferator, differentiator, and killer). The reader will first see calcium and its many helpers, such as the calcium-binding signaler protein calmodulin, directing the key events of the cell cycle. Then the tour will move onto the colon to show calcium driving the proliferation of progenitor cells, then the differentiation and ultimately the programmed death of their progeny. Moreover, the reader will learn of the striking disabling and bypassing of calcium-dependent control mechanisms during carcinogenesis. Finally, recommendations should be taken from the underlying mechanisms through which calcium masters the presistance, progression, and even apoptosis of colorectal cancer cells. Thus, this could be of great interest for designing of chemoprevention protocols.

CDK9 and its repressor LARP7 modulate cardiomyocyte proliferation and response to injury in the zebrafish heart

Cyclin dependent kinase (Cdk)9 acts through the positive transcription elongation factor-b (P-TEFb) complex to activate and expand transcription through RNA polymerase II. It has also been shown to regulate cardiomyocyte hypertrophy, with recent evidence linking it to cardiomyocyte proliferation. We hypothesised that modification of CDK9 activity could both impair and enhance the cardiac response to injury by modifying cardiomyocyte proliferation. Cdk9 expression and activity were inhibited in the zebrafish (Danio rerio) embryo. We show that dephosphorylation of residue Ser2 on the C-terminal domain of RNA polymerase II is associated with impaired cardiac structure and function, and cardiomyocyte proliferation and also results in impaired functional recovery following cardiac laser injury. In contrast, de-repression of Cdk9 activity, through knockdown of La-related protein (Larp7) increases phosphorylation of Ser2 in RNA polymerase II and increases cardiomyocyte proliferation. Larp7 knockdown rescued the structural and functional phenotype associated with knockdown of Cdk9. The balance of Cdk9 and Larp7 plays a key role in cardiomyocyte proliferation and response to injury. Larp7 represents a potentially novel therapeutic target to promote cardiomyocyte proliferation and recovery from injury.

Summary: The balance of CDK9 and LARP7 plays a key role in cardiomyocyte proliferation and response to injury. LARP7 represents a potentially novel therapeutic target in promoting recovery from injury.

Protease Omi facilitates neurite outgrowth in mouse neuroblastoma N2a cells by cleaving transcription factor E2F1

AIM

Omi is an ATP-independent serine protease that is necessary for neuronal function and survival. The aim of this study was to investigate the role of protease Omi in regulating differentiation of mouse neuroblastoma cells and to identify the substrate of Omi involved in this process.

METHODS

Mouse neuroblastoma N2a cells and Omi protease-deficient mnd2 mice were used in this study. To modulate Omi and E2F1 expression, N2a cells were transfected with expression plasmids, shRNA plasmids or siRNA. Protein levels were detected using immunoblot assays. The interaction between Omi and E2F1 was studied using immunoprecipitation, GST pulldown and in vitro cleavage assays. N2a cells were treated with 20 μmol/L retinoic acid (RA) and 1% fetal bovine serum to induce neurite outgrowth, which was measured using Image J software.

RESULTS

E2F1 was significantly increased in Omi knockdown cells and in brain lysates of mnd2 mice, and was decreased in cells overexpressing wild-type Omi, but not inactive Omi S276C. In brain lysates of mnd2 mice, endogenous E2F1 was co-immunoprecipitated with endogenous Omi. In vitro cleavage assay demonstrated that Omi directly cleaved E2F1. Treatment of N2a cells with RA induced marked differentiation and neurite outgrowth accompanied by significantly increased Omi and decreased E2F1 levels, which were suppressed by pretreatment with the specific Omi inhibitor UCF-101. Knockdown of Omi in N2a cells suppressed RA-induced neurite outgrowth, which was partially restored by knockdown of E2F1.

CONCLUSION

Protease Omi facilitates neurite outgrowth by cleaving the transcription factor E2F1 in differentiated neuroblastoma cells; E2F1 is a substrate of Omi.

E2F1-Mediated Induction of NFYB Attenuates Apoptosis via Joint Regulation of a Pro-Survival Transcriptional Program

The E2F1 transcription factor regulates cell proliferation and apoptosis through the control of a considerable variety of target genes. Previous work has detailed the role of other transcription factors in mediating the specificity of E2F function. Here we identify the NF-YB transcription factor as a novel direct E2F1 target. Genome-wide expression analysis of the effects of NFYB knockdown on E2F1-mediated transcription identified a large group of genes that are co-regulated by E2F1 and NFYB. We also provide evidence that knockdown of NFYB enhances E2F1-induced apoptosis, suggesting a pro-survival function of the NFYB/E2F1 joint transcriptional program. Bioinformatic analysis suggests that deregulation of these NFY-dependent E2F1 target genes might play a role in sarcomagenesis as well as drug resistance.

p57 regulates T-cell development and prevents lymphomagenesis by balancing p53 activity and pre-TCR signaling

Ablation of p57 in T cells blocks differentiation at an early developmental stage as a result of excessive activation of E2F. Additional ablation of E2F1 or p53 normalizes p57-deficiency phenotypes, but loss of both p57 and p53 eventually results in thymic lymphoma.

FOXO transcription factors control E2F1 transcriptional specificity and apoptotic function

The transcription factor E2F1 is a key regulator of proliferation and apoptosis but the molecular mechanisms that mediate these cell fate decisions remain unclear. Here, we identify FOXO transcription factors as E2F1 target genes that act in a feed-forward regulatory loop to reinforce gene induction of multiple apoptotic genes. We found that E2F1 forms a complex with FOXO1 and FOXO3. RNAi-mediated silencing of FOXO impaired E2F1 binding to the promoters of cooperative target genes. A FOXO3 mutant insensitive to inactivation by survival kinases rescued the inhibitory effect of growth factor signaling on E2F1-mediated transcription and apoptosis. The E2F1/FOXO axis is frequently blocked in cancer, as evidenced by the specific downregulation of the FOXO-dependent E2F1 transcriptional program in multiple cancer types and by the association of a reduced E2F1/FOXO transcriptional program with poor prognosis. HDAC and phosphoinositide 3-kinase (PI3K) inhibitors were identified as specific activators of E2F1/FOXO transcription, acting to enhance E2F1-induced apoptosis in a FOXO3-dependent manner. Notably, combining the histone deacetylase inhibitor vorinostat with a PI3K inhibitor led to enhanced FOXO-dependent apoptosis. Collectively, our results identify E2F1/FOXO cooperation as a regulatory mechanism that places E2F1 apoptotic activity under the control of survival signaling. Therapeutic reactivation of this tumor suppressive mechanism may offer a novel broad-acting therapy for cancer.

Api5 contributes to E2F1 control of the G1/S cell cycle phase transition

Background

The E2f transcription factor family has a pivotal role in controlling the cell fate in general, and in particular cancer development, by regulating the expression of several genes required for S phase entry and progression through the cell cycle. It has become clear that the transcriptional activation of at least one member of the family, E2F1, can also induce apoptosis. An appropriate balance of positive and negative regulators appears to be necessary to modulate E2F1 transcriptional activity, and thus cell fate.

Methodology/Principal Findings

In this report, we show that Api5, already known as a regulator of E2F1 induced-apoptosis, is required for the E2F1 transcriptional activation of G1/S transition genes, and consequently, for cell cycle progression and cell proliferation. Api5 appears to be a cell cycle regulated protein. Removal of Api5 reduces cyclin E, cyclin A, cyclin D1 and Cdk2 levels, causing G1 cell cycle arrest and cell cycle delay. Luciferase assays established that Api5 directly regulates the expression of several G1/S genes under E2F1 control. Using protein/protein and protein/DNA immunoprecipitation studies, we demonstrate that Api5, even if not physically interacting with E2F1, contributes positively to E2F1 transcriptional activity by increasing E2F1 binding to its target promoters, through an indirect mechanism.

Conclusion/Significance

The results described here support the pivotal role of cell cycle related proteins, that like E2F1, may act as tumor suppressors or as proto-oncogenes during cancer development, depending on the behavior of their positive and negative regulators. According to our findings, Api5 contributes to E2F1 transcriptional activation of cell cycle-associated genes by facilitating E2F1 recruitment onto its target promoters and thus E2F1 target gene transcription.

Flipping the switch from g1 to s phase with e3 ubiquitin ligases

The cell cycle ensures genome maintenance by coordinating the processes of DNA replication and chromosome segregation. Of particular importance is the irreversible transition from the G1 phase of the cell cycle to S phase. This transition marks the switch from preparing chromosomes for replication ("origin licensing") to active DNA synthesis ("origin firing"). Ubiquitin-mediated proteolysis is essential for restricting DNA replication to only once per cell cycle and is the major mechanism regulating the G1 to S phase transition. Although some changes in protein levels are attributable to regulated mRNA abundance, protein degradation elicits very rapid changes in protein abundance and is critical for the sharp and irreversible transition from one cell cycle stage to the next. Not surprisingly, regulation of the G1-to-S phase transition is perturbed in most cancer cells, and deregulation of key molecular events in G1 and S phase drives not only cell proliferation but also genome instability. In this review we focus on the mechanisms by which E3 ubiquitin ligases control the irreversible transition from G1 to S phase in mammalian cells.

E2f2 induces cone photoreceptor apoptosis independent of E2f1 and E2f3

The 'activating' E2fs (E2f1-3) are transcription factors that potently induce quiescent cells to divide. Work on cultured fibroblasts suggested they were essential for division, but in vivo analysis in the developing retina and other tissues disproved this notion. The retina, therefore, is an ideal location to assess other in vivo adenovirus E2 promoter binding factor (E2f) functions. It is thought that E2f1 directly induces apoptosis, whereas other activating E2fs only induce death indirectly by upregulating E2f1 expression. Indeed, mouse retinoblastoma (Rb)-null retinal neuron death requires E2f1, but not E2f2 or E2f3. However, we report an entirely distinct mechanism in dying cone photoreceptors. These neurons survive Rb loss, but undergo apoptosis in the cancer-prone retina lacking both Rb and its relative p107. We show that while E2f1 killed Rb/p107 null rod, bipolar and ganglion neurons, E2f2 was required and sufficient for cone death, independent of E2f1 and E2f3. Moreover, whereas E2f1-dependent apoptosis was p53 and p73-independent, E2f2 caused p53-dependent cone death. Our in vivo analysis of cone photoreceptors provides unequivocal proof that E2f-induces apoptosis independent of E2f1, and reveals distinct E2f1- and E2f2-activated death pathways in response to a single tumorigenic insult.

Regulation of the MDR1 promoter by E2F1 and EAPP

Multidrug resistance (MDR), one of the main reasons for diminishing efficacy of prolonged chemotherapy, is frequently caused by the elevated expression of the ABCB1/MDR1 gene encoding PGP (P-glycoprotein). EAPP (E2F Associated PhosphoProtein) is a frequently overexpressed protein in human tumor cells. It inhibits apoptosis in a p21-dependent manner. We show here that EAPP stimulates the MDR1 promoter resulting in higher PGP levels. Independently of EAPP, E2F1 also increases the activity of the MDR1 promoter. Co-expression of pRb inhibits E2F1-, but not EAPP-dependent promoter activation. The upregulation of PGP might contribute to the survival of tumor cells during chemotherapy and worsen the prognosis for the patient.

The role of the DNA damage response kinase ataxia telangiectasia mutated in neuroprotection

It has been estimated that a human cell is confronted with 1 million DNA lesions every day, one fifth of which may originate from the activity of Reactive Oxygen Species (ROS) alone [1,2]. Terminally differentiated neurons are highly active cells with, if any, very restricted regeneration potential [3]. In addition, genome integrity and maintenance during neuronal development is crucial for the organism. Therefore, highly accurate and robust mechanisms for DNA repair are vital for neuronal cells. This requirement is emphasized by the long list of human diseases with neurodegenerative phenotypes, which are either caused by or associated with impaired function of proteins involved in the cellular response to genotoxic stress [4-8]. Ataxia Telangiectasia Mutated (ATM), one of the major kinases of the DNA Damage Response (DDR), is a node that links DDR, neuronal development, and neurodegeneration [2,9-12]. In humans, inactivating mutations of ATM lead to Ataxia-Telangiectasia (A-T) disease [11,13], which is characterized by severe cerebellar neurodegeneration, indicating an important protective function of ATM in the nervous system [14]. Despite the large number of studies on the molecular cause of A-T, the neuroprotective role of ATM is not well established and is contradictory to its general proapoptotic function. This review discusses the putative functions of ATM in neuronal cells and how they might contribute to neuroprotection.

Tripartite motif containing 28 (Trim28) can regulate cell proliferation by bridging HDAC1/E2F interactions

BACKGROUND

Trim28 appears up-regulated in many cancers.

RESULTS

In early stage lung tumors high Trim28 correlates with increased overall survival and Trim28 reduces cell proliferation in model lung cancer cell lines through E2F interactions.

CONCLUSION

Trim28 may have a tumor suppressing role in the early stages of lung cancer.

SIGNIFICANCE

These results suggest a complex role for Trim28 in lung cancer. Trim28 is a poorly understood transcriptional co-factor with pleiotropic biological activities. Although Trim28 mRNA is found in many studies to be up-regulated in both lung and breast cancer tissues relative to normal adjacent tissue, we found that within a panel of early-stage lung adenocarcinomas high levels of Trim28 protein correlate with better overall survival. This surprising observation suggests that Trim 28 may have anti-proliferative activity within tumors. To test this hypothesis, we used shRNAi to generate Trim28-knockdown breast and lung cancer cell lines and found that Trim28 depletion led to increased cell proliferation. Likewise, overexpression of Trim28 led to decreased cell proliferation. Confocal microscopy indicated co-localization of E2F3 and E2F4 with Trim28 within the cell nucleus, and co-immunoprecipitation assays demonstrated that Trim28 can bind both E2F3 and E2F4. Trim28 overexpression inhibited the transcriptional activity of E2F3 and E2F4, whereas Trim28 deficiency enhanced their activity. Co-immunoprecipitations further indicated that Trim28 bridges an interaction between E2Fs 3 and 4 and HDAC1. Promoter-reporter assays demonstrated that the ability of HDAC1 to repress E2F3 and E2F4-driven transcription is dependent on Trim28. Trim28 depletion increased E2F3 and E2F4 DNA binding activity, as measured by chromatin-immunoprecipitation (ChIP) assays while simultaneously reducing HDAC1 binding. Finally, ChIP-ReChIP experiments demonstrated that Trim/E2F complexes exist on several E2F-regulated promoters. Taken together, these results suggest that Trim28 has anti-proliferative activity in lung cancers via repression of members of the E2F family that are critical for cell proliferation.

Regulation of the g1/s transition in hepatocytes: involvement of the cyclin-dependent kinase cdk1 in the DNA replication

A singular feature of adult differentiated hepatocytes is their capacity to proliferate allowing liver regeneration. This review emphasizes the literature published over the last 20 years that established the most important pathways regulating the hepatocyte cell cycle. Our article also aimed at illustrating that many discoveries in this field benefited from the combined use of in vivo models of liver regeneration and in vitro models of primary cultures of human and rodent hepatocytes. Using these models, our laboratory has contributed to decipher the different steps of the progression into the G1 phase and the commitment to S phase of proliferating hepatocytes. We identified the mitogen dependent restriction point located at the two-thirds of the G1 phase and the concomitant expression and activation of both Cdk1 and Cdk2 at the G1/S transition. Furthermore, we demonstrated that these two Cdks contribute to the DNA replication. Finally, we provided strong evidences that Cdk1 expression and activation is correlated to extracellular matrix degradation upon stimulation by the pro-inflammatory cytokine TNFα leading to the identification of a new signaling pathway regulating Cdk1 expression at the G1/S transition. It also further confirms the well-orchestrated regulation of liver regeneration via multiple extracellular signals and pathways.

Non-apoptotic programmed cell death with paraptotic-like features in bleomycin-treated plant cells is suppressed by inhibition of ATM/ATR pathways or NtE2F overexpression

In plants, different forms of programmed cell death (PCD) have been identified, but they only partially correspond to those described for animals, which is most probably due to structural differences between animal and plant cells. Here, the results show that in tobacco BY-2 cells, bleomycin (BLM), an inducer of double-strand breaks (DSBs), triggers a novel type of non-apoptotic PCD with paraptotic-like features. Analysis of numerous PCD markers revealed an extensive vacuolization, vacuolar rupture, and chromatin condensation, but no apoptotic DNA fragmentation, fragmentation of the nuclei, or sensitivity to caspase inhibitors. BLM-induced PCD was cell cycle regulated, occurring predominantly upon G(2)/M cell cycle checkpoint activation. In addition, this paraptotic-like PCD was at least partially inhibited by caffeine, a known inhibitor of DNA damage sensor kinases ATM and ATR. Interestingly, overexpression of one NtE2F transcriptional factor, whose homologues play a dual role in animal apoptosis and DNA repair, reduced PCD induction and modulated G(2)/M checkpoint activation in BY-2 cells. These observations provide a solid ground for further investigations into the paraptotic-like PCD in plants, which might represent an ancestral non-apoptotic form of PCD conserved among animals, protists, and plants.

Rad18 is a transcriptional target of E2F3

The E2F family of transcription factors responds to a variety of intracellular and extracellular signals and, as such, are key regulators of cell growth, differentiation and cell death. The cellular response to DNA damage is a multistep process generally involving the initial detection of DNA damage, propagation of signals via posttranslational modifications (e.g., phosphorylation and ubiquitination) and, finally, the implementation of a response. We have previously reported that E2F3 can be induced by DNA damage, and that it plays an important role in DNA damage-induced apoptosis. Here, we demonstrate that E2F3 knockdown compromises two canonical DNA damage modification events, the ubiquitination of H2AX and PCNA. We find that the defect in these posttranscriptional modifications after E2F3 knockdown is due to reduced expression of important DNA damage responsive ubiquitin ligases. We characterized the regulation of one of these ligases, Rad18, and we demonstrated that E2F3 associates with the Rad18 promoter and directly controls its activity. Furthermore, we find that ectopic expression of Rad18 is sufficient to rescue the PCNA ubiquitination defect resulting from E2F3 knockdown. Our study reveals a novel facet of E2F3's control of the DNA damage response.

E2F1 mediated apoptosis induced by the DNA damage response is blocked by EBV nuclear antigen 3C in lymphoblastoid cells

EBV latent antigen EBNA3C is indispensible for in vitro B-cell immortalization resulting in continuously proliferating lymphoblastoid cell lines (LCLs). EBNA3C was previously shown to target pRb for ubiquitin-proteasome mediated degradation, which facilitates G1 to S transition controlled by the major transcriptional activator E2F1. E2F1 also plays a pivotal role in regulating DNA damage induced apoptosis through both p53-dependent and -independent pathways. In this study, we demonstrate that in response to DNA damage LCLs knocked down for EBNA3C undergo a drastic induction of apoptosis, as a possible consequence of both p53- and E2F1-mediated activities. Importantly, EBNA3C was previously shown to suppress p53-induced apoptosis. Now, we also show that EBNA3C efficiently blocks E2F1-mediated apoptosis, as well as its anti-proliferative effects in a p53-independent manner, in response to DNA damage. The N- and C-terminal domains of EBNA3C form a stable pRb independent complex with the N-terminal DNA-binding region of E2F1 responsible for inducing apoptosis. Mechanistically, we show that EBNA3C represses E2F1 transcriptional activity via blocking its DNA-binding activity at the responsive promoters of p73 and Apaf-1 apoptosis induced genes, and also facilitates E2F1 degradation in an ubiquitin-proteasome dependent fashion. Moreover, in response to DNA damage, E2F1 knockdown LCLs exhibited a significant reduction in apoptosis with higher cell-viability. In the presence of normal mitogenic stimuli the growth rate of LCLs knockdown for E2F1 was markedly impaired; indicating that E2F1 plays a dual role in EBV positive cells and that active engagement of the EBNA3C-E2F1 complex is crucial for inhibition of DNA damage induced E2F1-mediated apoptosis. This study offers novel insights into our current understanding of EBV biology and enhances the potential for development of effective therapies against EBV associated B-cell lymphomas.

Aberrant cellular proliferation due to deregulation of E2F1 transcriptional activity as a result of either genetic or functional alterations of its upstream components is a hallmark of human cancer. Interestingly, E2F1 can also promote cellular apoptosis regardless of p53 status by activating a number of pro-apoptotic genes in response to DNA damage stimuli. Epstein-Barr virus (EBV) encoded essential latent antigen EBNA3C can suppress p53-mediated apoptotic activities. This study now demonstrates that EBNA3C can further impede E2F1 mediated apoptosis by inhibiting its transcriptional ability, as well as by facilitating its degradation in an ubiquitin-proteasome dependent manner. This is the first evidence, which shows through targeting EBNA3C function linked to the E2F1-mediated apoptotic pathway, an additional therapeutic platform could be implemented against EBV-associated human B-cell lymphomas.

Deriving transcriptional programs and functional processes from gene expression databases

MOTIVATION

A system-wide approach to revealing the underlying molecular state of a cell is a long-standing biological challenge. Developed over the last decade, gene expression profiles possess the characteristics of such an assay. They have the capacity to reveal both underlying molecular events as well as broader phenotypes such as clinical outcomes. To interpret these profiles, many gene sets have been developed that characterize biological processes. However, the full potential of these gene sets has not yet been achieved. Since the advent of gene expression databases, many have posited that they can reveal properties of activities that are not evident from individual datasets, analogous to how the expression of a single gene generally cannot reveal the activation of a biological process.

RESULTS

To address this issue, we have developed a high-throughput method to mine gene expression databases for the regulation of gene sets. Given a set of genes, we scored it against each gene expression dataset by looking for enrichment of co-regulated genes relative to an empirical null distribution. After validating the method, we applied it to address two biological problems. First, we deciphered the E2F transcriptional network. We confirmed that true transcriptional targets exhibit a distinct regulatory profile across a database. Second, we leveraged the patterns of regulation across a database of gene sets to produce an automatically generated catalog of biological processes. These demonstrations revealed the power of a global analysis of the data contained within gene expression databases, and the potential for using them to address biological questions.

The cooperating mutation or “second hit” determines the immunologic visibility toward MYC-induced murine lymphomas

In Eμ-myc transgenic animals lymphoma formation requires additional genetic alterations, which frequently comprise loss of p53 or overexpression of BCL-2. We describe that the nature of the “second hit” affects the ability of the immune system to contain lymphoma development. Tumors with disrupted p53 signaling killed the host more rapidly than BCL-2 overexpressing ones. Relaxing immunologic control, using Tyk2−/− mice or by Ab-mediated depletion of CD8+ T or natural killer (NK) cells accelerated formation of BCL-2–overexpressing lymphomas but not of those lacking p53. Most strikingly, enforced expression of BCL-2 prolonged disease latency in the absence of p53, whereas blocking p53 function in BCL-2–overexpressing tumors failed to accelerate disease. This shows that blocking apoptosis in p53-deficient cells by enforcing BCL-2 expression can mitigate disease progression increasing the “immunologic visibility.” In vitro cytotoxicity assays confirmed that high expression of BCL-2 protein facilitates NK and T cell–mediated killing. Moreover, we found that high BCL-2 expression is accompanied by significantly increased levels of the NKG2D ligand MULT1, which may account for the enhanced killing. Our findings provide first evidence that the nature of the second hit affects tumor immunosurveillance in c-MYC–driven lymphomas and define a potential shortcoming of antitumor therapies targeting BCL-2.

Network calisthenics: control of E2F dynamics in cell cycle entry

Stimulation of quiescent mammalian cells with mitogens induces an abrupt increase in E2F1-3 expression just prior to the onset of DNA synthesis, followed by a rapid decline as replication ceases. This temporal adaptation in E2F facilitates a transient pattern of gene expression that reflects the ordered nature of DNA replication. The challenge to understand how E2F dynamics coordinate molecular events required for high-fidelity DNA replication has great biological implications. Indeed, precocious, prolonged, elevated or reduced accumulation of E2F can generate replication stress that culminates in either arrest or death. Accordingly, temporal characteristics of E2F are regulated by several network modules that include feedforward and autoregulatory loops. In this review, we discuss how these network modules contribute to "shaping" E2F dynamics in the context of mammalian cell cycle entry.

mir-11 limits the proapoptotic function of its host gene, dE2f1

The E2F family of transcription factors regulates the expression of both genes associated with cell proliferation and genes that regulate cell death. The net outcome is dependent on cellular context and tissue environment. The mir-11 gene is located in the last intron of the Drosophila E2F1 homolog gene dE2f1, and its expression parallels that of dE2f1. Here, we investigated the role of miR-11 and found that miR-11 specifically modulated the proapoptotic function of its host gene, dE2f1. A mir-11 mutant was highly sensitive to dE2F1-dependent, DNA damage-induced apoptosis. Consistently, coexpression of miR-11 in transgenic animals suppressed dE2F1-induced apoptosis in multiple tissues, while exerting no effect on dE2F1-driven cell proliferation. Importantly, miR-11 repressed the expression of the proapoptotic genes reaper (rpr) and head involution defective (hid), which are directly regulated by dE2F1 upon DNA damage. In addition to rpr and hid, we identified a novel set of cell death genes that was also directly regulated by dE2F1 and miR-11. Thus, our data support a model in which the coexpression of miR-11 limits the proapoptotic function of its host gene, dE2f1, upon DNA damage by directly modulating a dE2F1-dependent apoptotic transcriptional program.

Deregulation of the p57-E2F1-p53 axis results in nonobstructive hydrocephalus and cerebellar malformation in mice

The cyclin-dependent kinase inhibitor (CKI) p57(Kip2) plays a pivotal role in cell cycle arrest during development, in particular, in the regulation of the entry of proliferating progenitors into quiescence. The gene encoding p57 undergoes genomic imprinting, and impairment of the regulation of p57 expression results in various developmental anomalies in humans and mice. We now show that p57 is expressed predominantly in the subcommissural organ and cerebellar interneurons in the mouse brain and that mice with brain-specific deletion of the p57 gene (Kip2) manifest prominent nonobstructive hydrocephalus as well as cerebellar malformation associated with the loss of Pax2-positive interneuron precursors and their descendants, including Golgi cells and γ-aminobutyric acid-containing neurons of the deep cerebellar nuclei. These abnormalities were found to be attributable to massive apoptosis of precursor cells in the developing brain. The morphological defects of the p57-deficient mice were corrected by knock-in of the gene for the related CKI p27(Kip1) at the Kip2 locus. The abnormalities were also prevented by additional genetic ablation of p53 or E2F1. Our results thus implicate p57 in cell cycle arrest in the subcommissural organ and Pax2-positive interneuron precursors, with the lack of p57 resulting in induction of p53-dependent apoptosis due to hyperactivation of E2F1.

MicroRNA: Biogenesis, Function and Role in Cancer

MicroRNAs are small, highly conserved non-coding RNA molecules involved in the regulation of gene expression. MicroRNAs are transcribed by RNA polymerases II and III, generating precursors that undergo a series of cleavage events to form mature microRNA. The conventional biogenesis pathway consists of two cleavage events, one nuclear and one cytoplasmic. However, alternative biogenesis pathways exist that differ in the number of cleavage events and enzymes responsible. How microRNA precursors are sorted to the different pathways is unclear but appears to be determined by the site of origin of the microRNA, its sequence and thermodynamic stability. The regulatory functions of microRNAs are accomplished through the RNA-induced silencing complex (RISC). MicroRNA assembles into RISC, activating the complex to target messenger RNA (mRNA) specified by the microRNA. Various RISC assembly models have been proposed and research continues to explore the mechanism(s) of RISC loading and activation. The degree and nature of the complementarity between the microRNA and target determine the gene silencing mechanism, slicer-dependent mRNA degradation or slicer-independent translation inhibition. Recent evidence indicates that P-bodies are essential for microRNA-mediated gene silencing and that RISC assembly and silencing occurs primarily within P-bodies. The P-body model outlines microRNA sorting and shuttling between specialized P-body compartments that house enzymes required for slicer –dependent and –independent silencing, addressing the reversibility of these silencing mechanisms. Detailed knowledge of the microRNA pathways is essential for understanding their physiological role and the implications associated with dysfunction and dysregulation.

Transcriptional activation of the senescence regulator Lsh by E2F1

Lsh, a protein related to the SNF2 family of chromatin-remodeling ATPases, is a major epigenetic regulator that is essential for DNA methylation and histone acetylation at repetitive elements. Lsh represses endogenous p16(INK4a) expression by recruiting HDAC to the p16(INK4a) promoter, which in turn delays cell senescence. However, the molecular mechanisms that govern loss of Lsh expression during cellular senescence have yet to be elucidated. Here we investigate the transcriptional regulation of the human Lsh promoter. We find that the minimal Lsh promoter is located between positions -216 and -119 relative to the transcription start site, and contains two putative E2F binding sites. Ectopic E2F1 increases expression of Lsh at both transcriptional and translational levels. E2F1 physically interacts with the Lsh promoter by binding to each of the two putative binding sites and transactivates the Lsh promoter. E2F1 also induces Lsh protein expression and transactivates the Lsh promoter in 2BS cells. At the same time, E2F1-induced Lsh promoter activity is reduced in senescent cells compared to young cells. These results indicate that E2F1 plays a crucial role in transcriptional control of the human Lsh gene and the decrease of Lsh expression in senescent cells is related to the repression of E2F1.

Prediction and genetic demonstration of a role for activator E2Fs in Myc-induced tumors

Advances in genomic signatures have begun to dissect breast cancer heterogeneity and application of these signatures will allow the prediction of which pathways are important in tumor development. Here we used genomic signatures to predict involvement of specific E2F transcription factors in Myc-induced tumors. We genetically tested this prediction by interbreeding Myc transgenics with mice lacking various activator E2F alleles. Tumor latency decreased in the E2F1 mutant background and significantly increased in both the E2F2 and E2F3 mutants. Investigating the mechanism behind these changes revealed a reduction in apoptosis in the E2F1 knockout strain. E2F2 and E2F3 mutant backgrounds alleviated Myc proliferative effects on the pregnant mammary gland, reducing the susceptible tumor target population. Gene expression data from tumors revealed that the E2F2 knockout background resulted in fewer tumors with EMT, corresponding with a reduction in probability of Ras activation. In human breast cancer we found that a low probability of E2F2 pathway activation was associated with increased relapse-free survival time. Together these data illustrate the predictive utility of genomic signatures in deciphering the heterogeneity within breast cancer and illustrate the unique genetic requirements for individual E2Fs in mediating tumorigenesis in both mouse models and human breast cancer.

Transcription factor NF-Y induces apoptosis in cells expressing wild-type p53 through E2F1 upregulation and p53 activation

The CCAAT-binding transcription factor NF-Y plays a central role in regulating cellular proliferation by controlling the expression of genes required for cell-cycle progression such as cyclin A, cyclin B1, cyclin B2, cdc25A, cdc25C, and cdk1. Here we show that unrestricted NF-Y activity leads to apoptosis in an E2F1- and wild-type p53 (wtp53)-dependent manner. Unrestricted NF-Y activity induced an increase in E2F1 mRNA and protein levels. Furthermore, NF-Y directly bound the E2F1 promoter and this correlated with the appearance of open chromatin marks. The ability of NF-Y to induce apoptosis was impaired in cells lacking E2F1 and wtp53. Moreover, NF-Y overexpression elicited phosphorylation of wt p53Ser18 in an E2F1-dependent manner. Our findings establish that NF-Y acts upstream of E2F1 in p53-mediated apoptosis.

E2F1 represses beta-catenin/TCF activity by direct up-regulation of Siah1

Transcription factor E2F1 is a key regulator of cell proliferation and apoptosis. Its activity is strictly controlled by the pRB/E2F pathway. In the majority of cancer cells, however, this pathway is frequently found deregulated, and the underlying mechanism involving transcriptional control by E2F1 has not yet been fully elucidated. Here we report the identification of two putative E2F1-binding sites located upstream from Siah1 transcription start site (+1). Chromatin immunoprecipitation assay reveals that transcription factor E2F1 is capable of binding to the putative sites, and luciferase reporter assay shows that E2F1 can activate transcription from the Siah1 promoter. Ectopic expression of E2F1 elevates the Siah1 level, hence suppressing the beta-catenin/TCF activity. Consistently, knock-down of endogenous E2F1 by a shRNA strategy results in reduced expression of Siah1. Moreover, repression of beta-catenin/TCF activity by E2F1 can be attenuated by shRNA-based repression of endogenous Siah1, implying that Siah1 is a bona fide E2F1 target gene, which at least partly, mediates the suppression of beta-catenin/TCF signalling pathway.

Ectopic expression of E2F1 stimulates beta-cell proliferation and function

OBJECTIVE

Generating functional beta-cells by inducing their proliferation may provide new perspectives for cell therapy in diabetes. Transcription factor E2F1 controls G(1)- to S-phase transition during the cycling of many cell types and is required for pancreatic beta-cell growth and function. However, the consequences of overexpression of E2F1 in beta-cells are unknown.

RESEARCH DESIGN AND METHODS

The effects of E2F1 overexpression on beta-cell proliferation and function were analyzed in isolated rat beta-cells and in transgenic mice.

RESULTS

Adenovirus AdE2F1-mediated overexpression of E2F1 increased the proliferation of isolated primary rat beta-cells 20-fold but also enhanced beta-cell death. Coinfection with adenovirus AdAkt expressing a constitutively active form of Akt (protein kinase B) suppressed beta-cell death to control levels. At 48 h after infection, the total beta-cell number and insulin content were, respectively, 46 and 79% higher in AdE2F1+AdAkt-infected cultures compared with untreated. Conditional overexpression of E2F1 in mice resulted in a twofold increase of beta-cell proliferation and a 70% increase of pancreatic insulin content, but did not increase beta-cell mass. Glucose-challenged insulin release was increased, and the mice showed protection against toxin-induced diabetes.

CONCLUSIONS

Overexpression of E2F1, either in vitro or in vivo, can stimulate beta-cell proliferation activity. In vivo E2F1 expression significantly increases the insulin content and function of adult beta-cells, making it a strategic target for therapeutic manipulation of beta-cell function.

Estrogen receptor regulates E2F1 expression to mediate tamoxifen resistance

Antiestrogen resistance often develops with prolonged exposure to hormone therapies, including tamoxifen, and is a major problem in the treatment of breast cancer. Understanding the mechanisms involved in the development of antiestrogen resistance is an important step in the development of new targeted therapies. Two forms of antiestrogen resistance exist: de novo resistance and acquired resistance. To mimic acquired resistance, we have established a tamoxifen-resistant breast cancer cell line (MCF-7TamR) by treating parental MCF-7 cells with tamoxifen over a period of 6 months to select for cells with the resistant phenotype. Characterization of the MCF-7TamR cells under normal, hormone-deprived, and tamoxifen-treated conditions suggests that these cells continue to grow in the presence of tamoxifen. Additionally, a greater percentage of resistant cells enter the S phase under tamoxifen conditions, compared with parental MCF-7 cells. Consistent with these growth results, molecular analysis indicates that tamoxifen-resistant cells express higher levels of cyclin E1, cdk2, ACTR, and E2F1. Faslodex or ICI 182, 780 (ICI)-mediated degradation of estrogen receptor (ER) reduced the proliferation of these cells, as well as the level of E2F1 expression in tamoxifen-resistant cells, suggesting that tamoxifen resistance and E2F1 expression are in part dependent on ER. We further showed that tamoxifen enhances the ERalpha/Sp-1 interaction and promotes the recruitment of ERalpha and Sp-1 to the proximal promoter of E2F1 in resistant cells. Collectively, our findings suggest that tamoxifen resistance is a result of increased ERalpha/Sp-1 interaction, which enhances the expression of E2F1 to promote tamoxifen resistance.

A combinatorial mechanism for determining the specificity of E2F activation and repression

Various studies have detailed the role of E2F proteins in both transcription activation and repression. Further work has shown that distinct promoter elements, but comprising the same E2F recognition motif, confer positive or negative E2F control and that this reflects binding of either activator or repressor E2F proteins respectively. We now show that the specificity of binding of an activator or repressor E2F protein is determined by adjacent sequences that bind a cooperating transcription factor. We propose that the functional E2F element is a module comprising not only the E2F binding site but also the adjacent site for the cooperating transcription factor.

DDB2 decides cell fate following DNA damage

The xeroderma pigmentosum complementation group E (XP-E) gene product damaged-DNA binding protein 2 (DDB2) plays important roles in nucleotide excision repair (NER). Previously, we showed that DDB2 participates in NER by regulating the level of p21(Waf1/Cip1). Here we show that the p21(Waf1/Cip1) -regulatory function of DDB2 plays a central role in defining the response (apoptosis or arrest) to DNA damage. The DDB2-deficient cells are resistant to apoptosis in response to a variety of DNA-damaging agents, despite activation of p53 and the pro-apoptotic genes. Instead, these cells undergo cell cycle arrest. Also, the DDB2-deficient cells are resistant to E2F1-induced apoptosis. The resistance to apoptosis of the DDB2-deficient cells is caused by an increased accumulation of p21(Waf1/Cip1) after DNA damage. We provide evidence that DDB2 targets p21(Waf1/Cip1) for proteolysis. The resistance to apoptosis in DDB2-deficient cells also involves Mdm2 in a manner that is distinct from the p53-regulatory activity of Mdm2. Our results provide evidence for a new regulatory loop involving the NER protein DDB2, Mdm2, and p21(Waf1/Cip1) that is critical in deciding cell fate (apoptosis or arrest) upon DNA damage.

Overexpression of E2F-5 correlates with a pathological basal phenotype and a worse clinical outcome

The purpose of the present study is to identify genes that contribute to cell proliferation or differentiation of breast cancers independent of signalling through the oestrogen receptor (ER) or human epidermal growth factor receptor 2 (HER2). An oligonucleotide microarray assayed 40 tumour samples from ER(+)/HER2(−), ER(+)/HER2(+), ER(−)/HER2(+), and ER(−)/HER2(−) breast cancer tissues. Quantitative reverse transcriptase PCR detected overexpression of a cell cycle-related transcription factor, E2F-5, in ER-negative breast cancers, and fluorescence in situ hybridisation detected gene amplification of E2F-5 in 5 out of 57 (8.8%) breast cancer samples. No point mutations were found in the DNA-binding or DNA-dimerisation domain of E2F-5. Immunohistochemically, E2F-5-positive cancers correlated with a higher Ki-67 labelling index (59.5%, P=0.001) and higher histological grades (P=0.049). E2F-5-positive cancers were found more frequently in ER(−)/progesterone receptor (PgR)(−)/HER2(−) cancer samples (51.9%, P=0.0049) and in breast cancer samples exhibiting a basal phenotype (56.0%, P=0.0012). Disease-free survival in node-negative patients with E2F-5-positive cancers was shorter than for patients with E2F-5-negative cancers. In conclusion, we identify, for the first time, a population of breast cancer cells that overexpress the cell cycle-related transcription factor, E2F-5. This E2F-5-positive breast cancer subtype was associated with an ER(−)/PgR(−)/HER2(−) status, a basal phenotype, and a worse clinical outcome.

Antisense MDM2 enhances E2F1-induced apoptosis and the combination sensitizes androgen-sensitive [corrected] and androgen-insensitive [corrected] prostate cancer cells to radiation

We have previously shown in separate studies that MDM2 knockdown via antisense MDM2 (AS-MDM2) and E2F1 overexpression via adenoviral-mediated E2F1 (Ad-E2F1) sensitized prostate cancer cells to radiation. Because E2F1 and MDM2 affect apoptosis through both common and independent pathways, we hypothesized that coupling these two treatments would result in increased killing of prostate cancer cells. In this study, the effect of Ad-E2F1 and AS-MDM2 in combination with radiation was investigated in three prostate cancer cell lines: LNCaP cells, LNCaP-Res cells [androgen insensitive with functional p53 and androgen receptor (AR)], and PC3 cells (androgen insensitive, p53(null), and AR(null)). A supra-additive radiosensitizing effect was observed in terms of clonogenic inhibition and induction of apoptosis (caspase-3 + caspase-7 activity) in response to Ad-E2F1 plus AS-MDM2 treatments in all three cell lines. In LNCaP and LNCaP-Res, these combination treatments elevated the levels of phospho-Ser(15) p53 with significant induction of p21(waf1/cip1), phospho-gammaH2AX, PUMA, and Bax levels and reduction of AR and bcl-2 expression. Similarly, AR(null) and p53(null) PC-3 cells showed elevated levels of Bax and phospho-gammaH2AX expression. These findings show that the combination of Ad-E2F1 and AS-MDM2 significantly increases cell death in prostate cancer cells exposed to radiation and that this effect occurs in the presence or absence of AR and p53.

Regulation of the retinoblastoma proteins by the human herpesviruses

Viruses are obligate intracellular parasites that alter the environment of infected cells in order to replicate more efficiently. One way viruses achieve this is by modulating cell cycle progression. The main regulators of progression out of G0, through G1, and into S phase are the members of the retinoblastoma (Rb) family of tumor suppressors. Rb proteins repress the transcription of genes controlled by the E2F transcription factors. Because the expression of E2F-responsive genes is required for cell cycle progression into the S phase, Rb arrests the cell cycle in G0/G1. A number of viral proteins directly target Rb family members for inactivation, presumably to create an environment more hospitable for viral replication. Such viral proteins include the extensively studied oncoproteins E7 (from human papillomavirus), E1A (from adenovirus), and the large T (tumor) antigen (from simian virus 40). Elucidating how these three viral proteins target and inactivate Rb has proven to be an invaluable approach to augment our understanding of both normal cell cycle progression and carcinogenesis. In addition to these proteins, a number of other virally-encoded inactivators of the Rb family have subsequently been identified including a surprising number encoded by human herpesviruses. Here we review how the human herpesviruses modulate Rb function during infection, introduce the individual viral proteins that directly or indirectly target Rb, and speculate about what roles Rb modulation by these proteins may play in viral replication, pathogenesis, and oncogenesis.

Suppression of the p53-dependent replicative senescence response by lysophosphatidic acid signaling

Lysophosphatidic acid (LPA) is a lipid mediator of a large number of biological processes, including wound healing, brain development, vascular remodeling, and tumor progression. Its role in tumor progression is probably linked to its ability to induce cell proliferation, migration, and survival. In particular, the ascites of ovarian cancers is rich in LPA and has been implicated in growth and invasion of ovarian tumor cells. LPA binds to specific G protein-coupled receptors and thereby activates multiple signal transduction pathways, including those initiated by the small GTPases Ras, Rho, and Rac. We report here a genetic screen with retroviral cDNA expression libraries to identify genes that allow bypass of the p53-dependent replicative senescence response in mouse neuronal cells, conditionally immortalized by a temperature-sensitive mutant of SV40 large T antigen. Using this approach, we identified the LPA receptor type 2 (LPA(2)) and the Rho-specific guanine nucleotide exchange factor Dbs as potent inducers of senescence bypass. Enhanced expression of LPA(2) or Dbs also results in senescence bypass in primary mouse embryo fibroblasts in the presence of wild-type p53, in a Rho GTPase-dependent manner. Our results reveal a novel and unexpected link between LPA signaling and the p53 tumor-suppressive pathway.

E2F and p53 induce apoptosis independently during Drosophila development but intersect in the context of DNA damage

In mammalian cells, RB/E2F and p53 are intimately connected, and crosstalk between these pathways is critical for the induction of cell cycle arrest or cell death in response to cellular stresses. Here we have investigated the genetic interactions between RBF/E2F and p53 pathways during Drosophila development. Unexpectedly, we find that the pro-apoptotic activities of E2F and p53 are independent of one another when examined in the context of Drosophila development: apoptosis induced by the deregulation of dE2F1, or by the overexpression of dE2F1, is unaffected by the elimination of dp53; conversely, dp53-induced phenotypes are unaffected by the elimination of dE2F activity. However, dE2F and dp53 converge in the context of a DNA damage response. Both dE2F1/dDP and dp53 are required for DNA damage-induced cell death, and the analysis of rbf1 mutant eye discs indicates that dE2F1/dDP and dp53 cooperatively promote cell death in irradiated discs. In this context, the further deregulation in the expression of pro-apoptotic genes generates an additional sensitivity to apoptosis that requires both dE2F/dDP and dp53 activity. This sensitivity differs from DNA damage-induced apoptosis in wild-type discs (and from dE2F/dDP-induced apoptosis in un-irradiated rbf1 mutant eye discs) by being dependent on both hid and reaper. These results show that pro-apoptotic activities of dE2F1 and dp53 are surprisingly separable: dp53 is required for dE2F-dependent apoptosis in the response to DNA damage, but it is not required for dE2F-dependent apoptosis caused simply by the inactivation of rbf1.

Patterns of cell signaling pathway activation that characterize mammary development

Previous work has detailed the histological and biochemical changes associated with mammary development and remodeling. We have now made use of gene expression profiling, and in particular of the previously described signatures of cell signaling pathway activation, to explore the events associated with mammary gland development. We find that there is elevated E2F-specific pathway activity prior to lactation and relatively low levels of other important signaling pathways, such as RAS, MYC and SRC. Upon lactation and continuing into the involution phase, these patterns reverse with a dramatic increase in RAS, SRC and MYC pathway activity and a decline in E2F activity. At the end of involution, these patterns return to that of the adult non-lactating mammary gland. The importance of the changes in E2F pathway activity, particularly during the proliferative phase of mammary development, was confirmed through the analysis of mice deficient for various E2F proteins. Taken together, these results reveal a complex pattern of pathway activity in relation to the various phases of mammary gland development.