E2F4 and E2F1 have similar proliferative properties but different apoptotic and oncogenic properties in vivo

Chromatin accessibility and microRNA expression in nephron progenitor cells during kidney development

Mammalian nephrons originate from a population of nephron progenitor cells, and changes in these cells' transcriptomes contribute to the cessation of nephrogenesis, an important determinant of nephron number. To characterize microRNA (miRNA) expression and identify putative cis-regulatory regions, we collected nephron progenitor cells from mouse kidneys at embryonic day 14.5 and postnatal day zero and assayed small RNA expression and transposase-accessible chromatin. We detect expression of 1104 miRNA (114 with expression changes), and 46,374 chromatin accessible regions (2103 with changes in accessibility). Genome-wide, our data highlight processes like cellular differentiation, cell migration, extracellular matrix interactions, and developmental signaling pathways. Furthermore, they identify new candidate cis-regulatory elements for Eya1 and Pax8, both genes with a role in nephron progenitor cell differentiation. Finally, we associate expression-changing miRNAs, including let-7-5p, miR-125b-5p, miR-181a-2-3p, and miR-9-3p, with candidate cis-regulatory elements and target genes. These analyses highlight new putative cis-regulatory loci for miRNA in nephron progenitors.

E2F4 Promotes the Proliferation of Hepatocellular Carcinoma Cells through Upregulation of CDCA3

Liver cancer, the second most commonly diagnosed cancer, is associated with high mortality rates. E2F4 is a member of the E2F transcription factor family. There are limited studies on the role of E2F4 in hepatocellular carcinoma (HCC). In this study, the expression of E2F4 in HCC tissue samples and cell lines was analyzed using quantitative real-time polymerase chain reaction. E2F4 expression positively correlated with tumor size in patients with HCC. Additionally, E2F4 expression was greater in HCC cells than in normal LO2 cells. Furthermore, overexpression of E2F4 significantly enhanced the proliferation, migration, and invasion of HCC cells. The results of a luciferase assay revealed that E2F4 upregulated the expression of CDCA3 by binding to its promoter region (1863'-ACGCGCGAGAATG-1875') and consequently promoted proliferation and cell cycle progression of HCC cells. Taken together, these results demonstrated that E2F4 might play a vital role in HCC progression and could serve as a potential biomarker for the diagnosis and as a therapeutic target of HCC.

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.

Gene co-expression in the interactome: moving from correlation toward causation via an integrated approach to disease module discovery

In this study, we integrate the outcomes of co-expression network analysis with the human interactome network to predict novel putative disease genes and modules. We first apply the SWItch Miner (SWIM) methodology, which predicts important (switch) genes within the co-expression network that regulate disease state transitions, then map them to the human protein-protein interaction network (PPI, or interactome) to predict novel disease-disease relationships (i.e., a SWIM-informed diseasome). Although the relevance of switch genes to an observed phenotype has been recently assessed, their performance at the system or network level constitutes a new, potentially fascinating territory yet to be explored. Quantifying the interplay between switch genes and human diseases in the interactome network, we found that switch genes associated with specific disorders are closer to each other than to other nodes in the network, and tend to form localized connected subnetworks. These subnetworks overlap between similar diseases and are situated in different neighborhoods for pathologically distinct phenotypes, consistent with the well-known topological proximity property of disease genes. These findings allow us to demonstrate how SWIM-based correlation network analysis can serve as a useful tool for efficient screening of potentially new disease gene associations. When integrated with an interactome-based network analysis, it not only identifies novel candidate disease genes, but also may offer testable hypotheses by which to elucidate the molecular underpinnings of human disease and reveal commonalities between seemingly unrelated diseases.

IRF2 is a master regulator of human keratinocyte stem cell fate

Resident adult epithelial stem cells maintain tissue homeostasis by balancing self-renewal and differentiation. The stem cell potential of human epidermal keratinocytes is retained in vitro but lost over time suggesting extrinsic and intrinsic regulation. Transcription factor-controlled regulatory circuitries govern cell identity, are sufficient to induce pluripotency and transdifferentiate cells. We investigate whether transcriptional circuitry also governs phenotypic changes within a given cell type by comparing human primary keratinocytes with intrinsically high versus low stem cell potential. Using integrated chromatin and transcriptional profiling, we implicate IRF2 as antagonistic to stemness and show that it binds and regulates active cis-regulatory elements at interferon response and antigen presentation genes. CRISPR-KD of IRF2 in keratinocytes with low stem cell potential increases self-renewal, migration and epidermis formation. These data demonstrate that transcription factor regulatory circuitries, in addition to maintaining cell identity, control plasticity within cell types and offer potential for therapeutic modulation of cell function.

Epidermal homeostasis requires long term stem cell function. Here, the authors apply transcriptional circuitry analysis based on integrated epigenomic profiling of primary human keratinocytes with high and low stem cell function to identify IRF2 as a negative regulator of stemness.

TREX1 suppression imparts cancer-stem-cell-like characteristics to CD133- osteosarcoma cells through the activation of E2F4 signaling

There is ongoing debate whether cancer stem cells (CSCs) could arise from the transformation of non-CSCs under specific conditions. In the present study, the role of the three prime repair exonuclease 1 (TREX1) in regulating CSC generation form human osteosarcoma cells was investigated. High, intermediate and low levels of TREX1 expression were respectively observed in low-grade, high-grade and metastatic human osteosarcoma samples, while the opposite tendency was observed for E2F4, a transcription factor associated with G2 arrest. Luciferase assay proved that TREX1 had a negative impact on the activity of E2F4 promoter. TREX1 was highly expressed in CD133^- HOS cells (non-CSC osteosarcoma cells) compared to CD133⁺ ones; whereas TREX1 knockdown endowed the CD133^- non-CSCs with CSC-like characteristics in vitro relying on E2F4 activation, as demonstrated by enlarged proportion of the subset expressing CSC markers in flow cytometry analysis, enhanced self-renewal ability in osteosphere formation assay, increased metastasis capacity in migration and invasion assays, together with improved chemoresistance to cisplatin. Furthermore, TREX1 knockdown and subsequent E2F4 activation could promote the tumorigenicity of CD133^- non-CSCs in vivo. With respect to underlying mechanisms, it was found that in CD133^- HOS cells, TREX1 suppression would allow the activation of β-catenin signaling in the dependence of E2F4, thus possibly leading to the up-regulation of the transcription factor OCT4. These findings suggested that TREX1 was probably a negative regulator of CSC formation and hence worth to be further studied for developing new treatments in cancer therapies targeting CSCs.

Aberrant promoter methylation status is associated with upregulation of the E2F4 gene in breast cancer

E2F4 is an important basal transcription factor with the potential to promote tumor growth. Its upregulation in various types of cancer has been linked to numerous genetic factors; however, the nature of the involvement of epigenetic mechanisms, including DNA methylation, remains elusive. In the present study, E2F4 expression profiles were determined in 100 paired breast tumor and control samples, through RT-qPCR using the SYBR^® green method. Furthermore, the E2F4 promoter methylation status in each of these samples was assessed using methylation specific PCR, in order to evaluate its impact on gene expression. A two-fold increase in E2F4 gene expression was observed in the breast tumors compared with in their respective controls (P=0.022); of these tumors, ~72% were under-methylated. The change in methylation status was also significantly higher (P<0.001) in the tumor samples. Methylation status was negatively correlated (r=-30) with E2F4 expression profiles, indicating that a decrease in methylation may promote higher expression of E2F4. The two study cohorts (>45 and ≤45 years) had comparable methylation profiles, though they had significantly decreased methylation status compared with controls. Various histo-pathological types also have different methylation profiles, indicating the presence of a tissue specific methylation signature. The results of the present study demonstrated that E2F4 methylation status can have a notable influence on its expression, and that it may have prognostic value in breast carcinogenesis.

Incorporating genomic, transcriptomic and clinical data: a prognostic and stem cell-like MYC and PRC imbalance in high-risk neuroblastoma

BACKGROUND

Previous studies suggested that cancer cells possess traits reminiscent of the biological mechanisms ascribed to normal embryonic stem cells (ESCs) regulated by MYC and Polycomb repressive complex 2 (PRC2). Several poorly differentiated adult tumors showed preferentially high expression levels in targets of MYC, coincident with low expression levels in targets of PRC2. This paper will reveal this ESC-like cancer signature in high-risk neuroblastoma (HR-NB), the most common extracranial solid tumor in children.

METHODS

We systematically assembled genomic variants, gene expression changes, priori knowledge of gene functions, and clinical outcomes to identify prognostic multigene signatures. First, we assigned a new, individualized prognostic index using the relative expressions between the poor- and good-outcome signature genes. We then characterized HR-NB aggressiveness beyond these prognostic multigene signatures through the imbalanced effects of MYC and PRC2 signaling. We further analyzed Retinoic acid (RA)-induced HR-NB cells to model tumor cell differentiation. Finally, we performed in vitro validation on ZFHX3, a cell differentiation marker silenced by PRC2, and compared cell morphology changes before and after blocking PRC2 in HR-NB cells.

RESULTS

A significant concurrence existed between exons with verified variants and genes showing MYCN-dependent expression in HR-NB. From these biomarker candidates, we identified two novel prognostic gene-set pairs with multi-scale oncogenic defects. Intriguingly, MYC targets over-represented an unfavorable component of the identified prognostic signatures while PRC2 targets over-represented a favorable component. The cell cycle arrest and neuronal differentiation marker ZFHX3 was identified as one of PRC2-silenced tumor suppressor candidates. Blocking PRC2 reduced tumor cell growth and increased the mRNA expression levels of ZFHX3 in an early treatment stage. This hypothesis-driven systems bioinformatics work offered novel insights into the PRC2-mediated tumor cell growth and differentiation in neuroblastoma, which may exert oncogenic effects together with MYC regulation.

CONCLUSION

Our results propose a prognostic effect of imbalanced MYC and PRC2 moderations in pediatric HR-NB for the first time. This study demonstrates an incorporation of genomic landscapes and transcriptomic profiles into the hypothesis-driven precision prognosis and biomarker discovery. The application of this approach to neuroblastoma, as well as other cancer more broadly, could contribute to reduced relapse and mortality rates in the long term.

PAD2 overexpression in transgenic mice augments malignancy and tumor-associated inflammation in chemically initiated skin tumors

We previously found that transgenic mice overexpressing MMTV-FLAG-hPAD2 (PAD2OE) developed spontaneous skin lesions, with a subset of these lesions progressing to invasive squamous cell carcinoma (SCC). The goal of this report was to better understand the potential mechanisms by which PAD2 overexpression promotes skin cancer. Here, PAD2OE mice were treated with the carcinogen, 9,10-dimethyl-1,2-benzanthracene and with O-tetradecanoylphorbol-13-acetate and then scored for papilloma formation. Additionally, tumor sections were evaluated for evidence of tumor cell invasion and inflammation. We found that the total number of papillomas was significantly increased in PAD2OE mice compared to controls. Histopathologic analysis of the lesions found that in PAD2OE skin tumors progressed to invasive SCC more frequently than controls. Additionally, we found that PAD2OE lesions were highly inflamed, with a dense inflammatory cell infiltrate and an associated increase in nuclear phospho-STAT3 (signal transducer and activator of transcription 3) in the transgenic tumors. These data suggest that overexpression of the hPAD2 transgene in the epidermis increases the malignant conversion rate of benign tumors by promoting an inflammatory microenvironment.

SWIM: a computational tool to unveiling crucial nodes in complex biological networks

SWItchMiner (SWIM) is a wizard-like software implementation of a procedure, previously described, able to extract information contained in complex networks. Specifically, SWIM allows unearthing the existence of a new class of hubs, called “fight-club hubs”, characterized by a marked negative correlation with their first nearest neighbors. Among them, a special subset of genes, called “switch genes”, appears to be characterized by an unusual pattern of intra- and inter-module connections that confers them a crucial topological role, interestingly mirrored by the evidence of their clinic-biological relevance. Here, we applied SWIM to a large panel of cancer datasets from The Cancer Genome Atlas, in order to highlight switch genes that could be critically associated with the drastic changes in the physiological state of cells or tissues induced by the cancer development. We discovered that switch genes are found in all cancers we studied and they encompass protein coding genes and non-coding RNAs, recovering many known key cancer players but also many new potential biomarkers not yet characterized in cancer context. Furthermore, SWIM is amenable to detect switch genes in different organisms and cell conditions, with the potential to uncover important players in biologically relevant scenarios, including but not limited to human cancer.

Novel functions for the transcription factor E2F4 in development and disease

The E2F family of transcription factors is a key determinant of cell proliferation in response to extra- and intra-cellular signals. Within this family, E2F4 is a transcriptional repressor whose activity is critical to engage and maintain cell cycle arrest in G0/G1 in conjunction with members of the retinoblastoma (RB) family. However, recent observations challenge this paradigm and indicate that E2F4 has a multitude of functions in cells besides this cell cycle regulatory role, including in embryonic and adult stem cells, during regenerative processes, and in cancer. Some of these new functions are independent of the RB family and involve direct activation of target genes. Here we review the canonical functions of E2F4 and discuss recent evidence expanding the role of this transcription factor, with a focus on cell fate decisions in tissue homeostasis and regeneration.

The Histone Demethylase PHF8 Is Essential for Endothelial Cell Migration

Epigenetic marks critically control gene expression and thus the cellular activity state. The functions of many epigenetic modifiers in the vascular system have not yet been studied. We screened for histone modifiers in endothelial cells and observed a fairly high expression of the histone plant homeodomain finger protein 8 (PHF8). Given its high expression, we hypothesize that this histone demethylase is important for endothelial cell function. Overexpression of PHF8 catalyzed the removal of methyl-groups from histone 3 lysine 9 (H3K9) and H4K20, whereas knockdown of the enzyme increased H3K9 methylation. Knockdown of PHF8 by RNAi also attenuated endothelial proliferation and survival. As a functional readout endothelial migration and tube formation was studied. PHF8 siRNA attenuated the capacity for migration and developing of capillary-like structures. Given the impact of PHF8 on cell cycle genes, endothelial E2F transcription factors were screened, which led to the identification of the gene repressor E2F4 to be controlled by PHF8. Importantly, PHF8 maintains E2F4 but not E2F1 expression in endothelial cells. Consistently, chromatin immunoprecipitation revealed that PHF8 reduces the H3K9me2 level at the E2F4 transcriptional start site, demonstrating a direct function of PHF8 in endothelial E2F4 gene regulation. Conclusion: PHF8 by controlling E2F4 expression maintains endothelial function.

E2F4 Program Is Predictive of Progression and Intravesical Immunotherapy Efficacy in Bladder Cancer

Bladder cancer is a common malignant disease, with non-muscle-invasive bladder cancer (NMIBC) representing the majority of tumors. This cancer subtype is typically treated by transurethral resection. In spite of treatment, up to 70% of patients show local recurrences. Intravesical BCG (Bacillus Calmette-Guerin) immunotherapy has been widely used to treat NMIBC, but it fails to suppress recurrence of bladder tumors in up to 40% of patients. Therefore, the development of prognostic markers is needed to predict the progression of bladder cancer and the efficacy of intravesical BCG treatment. This study demonstrates the effectiveness of an E2F4 signature for prognostic prediction of bladder cancer. E2F4 scores for each sample in a bladder cancer expression dataset were calculated by summarizing the relative expression levels of E2F4 target genes identified by ChIP-seq, and then the scores were used to stratify patients into good- and poor-outcome groups. The molecular signature was investigated in a single bladder cancer dataset and then its effectiveness was confirmed in two meta-bladder datasets consisting of specimens from multiple independent studies. These results were consistent in different datasets and demonstrate that the E2F4 score is predictive of clinical outcomes in bladder cancer, with patients whose tumors exhibit an E2F4 score >0 having significantly shorter survival times than those with an E2F4 score <0, in both non-muscle-invasive, and muscle-invasive bladder cancer. Furthermore, although intravesical BCG immunotherapy can significantly improve the clinical outcome of NMIBC patients with positive E2F4 scores (E2F4>0 group), it does not show significant treatment effect for those with negative scores (E2F4<0 group).

IMPLICATIONS

The E2F4 signature can be applied to predict the progression/recurrence and the responsiveness of patients to intravesical BCG immunotherapy in bladder cancer.

Regulation of the human ABCB10 gene by E2F transcription factors

Here, we report for the first time a functional study of transcriptional regulation of the human ABCB10 gene. We cloned a functional promoter sequence, and then showed that E2F2, E2F3 and E2F4 can activate transcription from the ABCB10 promoter. We identified sites responsible for this activation and confirmed direct binding of E2F4 to these sites in EMSA and ChIP assays. Finally, by silencing the expression of E2F factors we demonstrated their importance in maintenance of the basal ABCB10 expression. This study provides important atypical examples of E2F4 being a transcriptional activator rather than repressor as well as directly binding to a promoter and regulating it through an alternative and classical DNA consensus response element sequences. It also provides a mechanistic link between E2F4 and ABCB10, both of which are involved in the same physiological phenomena: erythroid lineage differentiation and maturation as well as protection against cardiomyocyte cell death.

ERK-associated changes in E2F4 phosphorylation, localization and transcriptional activity during mitogenic stimulation in human intestinal epithelial crypt cells

Background

The transcription factor E2F4 controls proliferation of normal and cancerous intestinal epithelial cells. E2F4 localization in normal human intestinal epithelial cells (HIEC) is cell cycle-dependent, being cytoplasmic in quiescent differentiated cells but nuclear in proliferative cells. However, the intracellular signaling mechanisms regulating such E2F4 localization remain unknown.

Results

Treatment of quiescent HIEC with serum induced ERK1/2 activation, E2F4 phosphorylation, E2F4 nuclear translocation and G1/S phase transition while inhibition of MEK/ERK signaling by U0126 prevented these events. Stimulation of HIEC with epidermal growth factor (EGF) also led to the activation of ERK1/2 but, in contrast to serum or lysophosphatidic acid (LPA), EGF failed to induce E2F4 phosphorylation, E2F4 nuclear translocation and G1/S phase transition. Furthermore, Akt and GSK3β phosphorylation levels were markedly enhanced in serum- or LPA-stimulated HIEC but not by EGF. Importantly, E2F4 phosphorylation, E2F4 nuclear translocation and G1/S phase transition were all observed in response to EGF when GSK3 activity was concomitantly inhibited by SB216763. Finally, E2F4 was found to be overexpressed, phosphorylated and nuclear localized in epithelial cells from human colorectal adenomas exhibiting mutations in APC and KRAS or BRAF genes, known to deregulate GSK3/β-catenin and MEK/ERK signaling, respectively.

Conclusions

The present results indicate that MEK/ERK activation and GSK3 inhibition are both required for E2F4 phosphorylation as well as its nuclear translocation and S phase entry in HIEC. This finding suggests that dysregulated E2F4 nuclear localization may be an instigating event leading to hyperproliferation and hence, of tumor initiation and promotion in the colon and rectum.

A transcriptionally active pRb-E2F1-P/CAF signaling pathway is central to TGFβ-mediated apoptosis

Transforming growth factor-β (TGFβ) modulates the expression of multiple apoptotic target genes; however, a common and central signaling pathway, acting downstream of TGFβ and leading to cell death, has yet to be uncovered. Here, we show that TGFβ-induced apoptosis in cancer cells requires the transcription factor E2F1 (E2 promoter-binding factor 1). Using the E2F1 knockout mouse model, we also found E2F1 to be required for TGFβ-mediated apoptosis in normal cells. Moreover, we found TGFβ to increase E2F1 protein stability, acting at the post-translational level. We further investigated the molecular mechanisms by which E2F1 contributes to TGFβ-mediated apoptosis and found that TGFβ treatment led to the formation of a transcriptionally active E2F1–pRb–P/CAF complex on multiple TGFβ pro-apoptotic target gene promoters, thereby activating their transcription. Together, our findings define a novel process of gene activation by the TGFβ-E2F1 signaling axis and highlight E2F1 as a central mediator of the TGFβ apoptotic program.

The role of the E2F transcription factor family in UV-induced apoptosis

The E2F transcription factor family is traditionally associated with cell cycle control. However, recent data has shown that activating E2Fs (E2F1-3a) are potent activators of apoptosis. In contrast, the recently cloned inhibitory E2Fs (E2F7 and 8) appear to antagonize E2F-induced cell death. In this review we will discuss (i) the potential role of E2Fs in UV-induced cell death and (ii) the implications of this to the development of UV-induced cutaneous malignancies.

Predicting and preventing melanoma invasiveness: advances in clarifying E2F1 function

Malignant melanoma of the skin is one of the most aggressive human cancers with increasing incidence, despite efforts to improve primary prevention. In particular, the prognosis of patients at late stages of the disease has not significantly improved in the last three decades, because systemic therapies have proven disappointing. Thus, metastatic melanoma continues to be a daunting clinical problem. The increasingly high rates of lethal outcome associated with advanced melanoma rely on the acquisition of invasiveness, early metastatic dissemination of tumor cells from their primary sites, and generation of chemoresistance as a consequence of alteration of key molecules involved in the regulation of cell survival. Thus far, extensive studies have been conducted to understand the molecular mechanisms that drive tumor progression, but the specific requirements underlying the aggressive behavior are still widely unknown. Understanding the determinants of this process is key to unveiling its dynamics, especially those that promote invasiveness, and may open new routes for the development of therapeutic strategies that control metastatic spread, and eventually the prevention of life-threatening metastases. Here, we review recent advances on molecular aspects, particularly of E2F1 transcription factor function, in the context of patient data, and discuss the implications for targeting melanoma cells when they begin to invade and metastasize.

Wide-ranging functions of E2F4 in transcriptional activation and repression revealed by genome-wide analysis

The E2F family of transcription factors has important roles in cell cycle progression. E2F4 is an E2F family member that has been proposed to be primarily a repressor of transcription, but the scope of its binding activity and functions in transcriptional regulation is not fully known. We used ChIP sequencing (ChIP-seq) to identify around 16 000 E2F4 binding sites which potentially regulate 7346 downstream target genes with wide-ranging functions in DNA repair, cell cycle regulation, apoptosis, and other processes. While half of all E2F4 binding sites (56%) occurred near transcription start sites (TSSs), ∼20% of sites occurred more than 20 kb away from any annotated TSS. These distal sites showed histone modifications suggesting that E2F4 may function as a long-range regulator, which we confirmed by functional experimental assays on a subset. Overexpression of E2F4 and its transcriptional cofactors of the retinoblastoma (Rb) family and its binding partner DP-1 revealed that E2F4 acts as an activator as well as a repressor. E2F4 binding sites also occurred near regulatory elements for miRNAs such as let-7a and mir-17, suggestive of regulation of miRNAs by E2F4. Taken together, our genome-wide analysis provided evidence of versatile roles of E2F4 and insights into its functions.

A functional role of RB-dependent pathway in the control of quiescence in adult epidermal stem cells revealed by genomic profiling

Continuous cell renewal in mouse epidermis is at the expense of a pool of pluripotent cells that lie in a well defined niche in the hair follicle known as the bulge. To identify mechanisms controlling hair follicle stem cell homeostasis, we developed a strategy to isolate adult bulge stem cells in mice and to define their transcriptional profile. We observed that a large number of transcripts are underexpressed in hair follicle stem cells when compared to non-stem cells. Importantly, the majority of these downregulated genes are involved in cell cycle. Using bioinformatics tools, we identified the E2F transcription factor family as a potential element involved in the regulation of these transcripts. To determine their functional role, we used engineered mice lacking Rb gene in epidermis, which showed increased expression of most E2F family members and increased E2F transcriptional activity. Experiments designed to analyze epidermal stem cell functionality (i.e.: hair regrowth and wound healing) imply a role of the Rb-E2F axis in the control of stem cell quiescence in epidermis.

Pro-oncogenic and anti-oncogenic pathways: opportunities and challenges of cancer therapy

Carcinogenesis is the uncontrolled growth of cells gaining the potential to invade and disrupt vital tissue functions. This malignant process includes the occurrence of 'unwanted' gene mutations that induce the transformation of normal cells, for example, by overactivation of pro-oncogenic pathways and inactivation of tumor-suppressive or anti-oncogenic pathways. It is now recognized that the number of major signaling pathways that control oncogenesis is not unlimited; therefore, suppressing these pathways can conceivably lead to a cancer cure. However, the clinical application of cancer intervention has not matched up to scientific expectations. Increasing numbers of studies have revealed that many oncogenic-signaling elements show double faces, in which they can promote or suppress cancer pathogenesis depending on tissue type, cancer stage, gene dosage and their interaction with other players in carcinogenesis. This complexity of oncogenic signaling poses challenges to traditional cancer therapy and calls for considerable caution when designing an anticancer drug strategy. We propose future oncology interventions with the concept of integrative cancer therapy.

E2F1 in melanoma progression and metastasis

Metastases are responsible for cancer deaths, but the molecular alterations leading to tumor progression are unclear. Overexpression of the E2F1 transcription factor is common in high-grade tumors that are associated with poor patient survival. To investigate the association of enhanced E2F1 activity with aggressive phenotype, we performed a gene-specific silencing approach in a metastatic melanoma model. Knockdown of endogenous E2F1 via E2F1 small hairpin RNA (shRNA) expression increased E-cadherin expression of metastatic SK-Mel-147 melanoma cells and reduced their invasive potential but not their proliferative activity. Although growth rates of SK-Mel-147 and SK-Mel-103 xenograft tumors expressing E2F1 shRNA or control shRNA were similar, mice implanted with cells expressing E2F1 shRNA had a smaller area of metastases per lung than control mice (n = 3 mice per group; 5% vs 46%, difference = 41%, 95% confidence interval = 15% to 67%; P = .01; one-way analysis of variance). We identified epidermal growth factor receptor as a direct target of E2F1 and demonstrated that inhibition of receptor signaling abrogates E2F1-induced invasiveness, emphasizing the importance of the E2F1-epidermal growth factor receptor interaction as a driving force in melanoma progression that may serve as a paradigm for E2F1-induced metastasis in other human cancers.

Emerging roles of E2Fs in cancer: an exit from cell cycle control

Mutations of the retinoblastoma tumour suppressor gene (RB1) or components regulating the RB pathway have been identified in almost every human malignancy. The E2F transcription factors function in cell cycle control and are intimately regulated by RB. Studies of model organisms have revealed conserved functions for E2Fs during development, suggesting that the cancer-related proliferative roles of E2F family members represent a recent evolutionary adaptation. However, given that some human tumours have concurrent RB1 inactivation and E2F amplification and overexpression, we propose that there are alternative tumour-promoting activities for the E2F family, which are independent of cell cycle regulation.

E2F4 expression is required for cell cycle progression of normal intestinal crypt cells and colorectal cancer cells

The generation of knock-out mice for E2F4 gene expression has suggested a role for this transcription factor in establishing and/or maintaining the intestinal crypt compartment. Having previously demonstrated that E2F4 is cytoplasmic in quiescent-differentiated cells but nuclear in growth factor-stimulated proliferative cells, the present study was aimed at determining the role of E2F4 in the control of human intestinal epithelial proliferation. Results herein demonstrate that lentiviral infection of an shRNA which specifically knocked-down E2F4 expression slowed down G1/S phase transition and the proliferation rate of normal human intestinal epithelial cells (HIEC) and of colon cancer cells. Protein expression of Cdk2, cyclins D1 and A, Cdc25A and c-myc was markedly down-regulated in shE2F4-expressing cells; by contrast, expression of the cell cycle inhibitors p21(Cip/Waf) and p27(Kip1) was increased. In addition, the expression of many genes involved in DNA synthesis was down-regulated in shE2F4-expressing cells, whereas no modulation in E2F1 expression was observed. A decrease in E2F4 in colon cancer cell lines also resulted in a reduction in soft-agar growth capacity. Immunofluorescence experiments in human fetal intestine revealed that cells expressing high nuclear levels of E2F4 also expressed cyclin A protein. Lastly, E2F4 and its target cyclin A were up-regulated and mostly nuclear in human colorectal tumor cells in comparison to the corresponding benign epithelium. These results indicate that nuclear E2F4 may be determinant in the promotion of proliferation of human intestinal epithelial crypt cells and colorectal cancer cells.

Gene profiling approaches help to define the specific functions of retinoblastoma family in epidermis

The epidermal-specific ablation of Rb gene leads to increased proliferation, aberrant differentiation, and the disengagement of these processes in vivo and in vitro. These differences in phenotype are more severe with the loss of p107, demonstrating the functional compensation between pRb and p107. As p107 and p130 also exert overlapping functions in epidermis, we have generated Rb(F19/F19)K14cre;Rbl2-/- (pRb-;p130-) mice to analyze possible functional redundancies between pRb and p130. The epidermal phenotype was very similar between pRb- and pRb-;p130- mice, suggesting that pRb and p130 activities are not redundant in epidermis. Importantly, we can correlate the proliferation differences with specific changes in gene expression between pRb-, pRb-;p107- and pRb-;p130- primary keratinocytes using microarray analysis, and explain the phenotypes in the context of altered E2F expression and functionality. Our findings support a model in which the distinct retinoblastoma family members, in conjunction with E2F members, play a central role in regulating epidermal homeostasis through specific or overlapping activities.

Identification of E2F1 as a positive transcriptional regulator for delta-catenin

delta-Catenin is upregulated in human carcinomas. However, little is known about the potential transcriptional factors that regulate delta-catenin expression in cancer. Using a human delta-catenin reporter system, we have screened several nuclear signaling modulators to test whether they can affect delta-catenin transcription. Among beta-catenin/LEF-1, Notch1, and E2F1, E2F1 dramatically increased delta-catenin-luciferase activities while beta-catenin/LEF-1 induced only a marginal increase. Rb suppressed the upregulation of delta-catenin-luciferase activities induced by E2F1 but did not interact with delta-catenin. RT-PCR and Western blot analyses in 4 different prostate cancer cell lines revealed that regulation of delta-catenin expression is controlled mainly at the transcriptional level. Interestingly, the effects of E2F1 on delta-catenin expression were observed only in human cancer cells expressing abundant endogenous delta-catenin. These studies identify E2F1 as a positive transcriptional regulator for delta-catenin, but further suggest the presence of strong negative regulator(s) for delta-catenin in prostate cancer cells with minimal endogenous delta-catenin expression.

Transforming growth factor-beta inhibits telomerase through SMAD3 and E2F transcription factors

Cancer arises from multiple genetic changes within the cell, among which constitutive telomerase activity and attainment of immortality are central. Expression of hTERT, the protein component of telomerase, is increased in most cancer cells. Transforming growth factor-beta (TGFbeta), a potent tumor suppressor, has been reported to regulate hTERT expression. We found that TGFbeta represses hTERT expression in normal and cancer cells and that this effect is mediated through Smad3 but also requires Erk1/2, p38 kinase and histone deacetylase activity. Furthermore, we identified four critical E2F transcription factor binding sites within the hTERT gene promoter that confer the TGFbeta response. Finally, using the E2F-1 knockout model, we showed that loss of E2F-1 abolishes TGFbeta inhibition of telomerase expression. These findings highlight the prominent role of TGFbeta in regulating telomerase expression and identify Smad3 and E2F-1 as critical mediators of TGFbeta effects in both normal and cancer cells.

DNA damage response involves modulation of Ku70 and Rb functions by cyclin A1 in leukemia cells

Cyclin A1 plays a critical role in hematopoietic malignancies, notably, acute myeloid leukemia. The molecular mechanisms of cyclin A1 action are incompletely understood. Here, we show that cyclin A1 functions are mediated by the retinoblastoma and the Ku70 pathway. High levels of cyclin A1 and the associated CDK2 kinase activity were associated with increasing levels of phosphorylated retinoblastoma in vivo. UV irradiation induced a switch of the CDK2 towards cyclin A1, with accordance to changes in CDK2 kinase activity. The C-terminus of cyclin A1 directly interacted with Ku70, and DNA binding activity of Ku70 was modulated by cyclin A1/CDK2 and phosphatase treatment. Cyclin A1-deficiency induced by shRNA increased apoptosis that is induced by DNA damage and death receptor ligands. Taken together, these analyses demonstrate that cyclin A1 exerts antiapoptotic functions by interacting with retinoblastoma and Ku proteins in leukemia cells.

Human lactoferrin controls the level of retinoblastoma protein and its activityThis paper is one of a selection of papers published in this Special Issue, entitled 7th International Conference on Lactoferrin: Structure, Function, and Applications, and has undergone the Journal's usual peer review process

Lactoferrin (Lf) has been implicated in the regulation of cell growth. However, the molecular mechanism underlying this effect remains to be elucidated. In this study, we show that Lf is involved in the cell cycle control system in a variety of cell lines, through retinoblastoma protein (Rb) - mediated growth arrest. We observed that Lf induces the expression of Rb, a signal mediator of cell cycle control, and that a majority of this Lf-induced Rb persists in a hypophosphorylated form. In addition, we determined that Lf specifically augments the level of a cyclin-dependent kinase inhibitor, p21, but not p27. Upon treatment with Lf, H1299 cells expressing defective p53 effected an augmentation of endogenous p21 levels, which may contribute to the accumulation of hypophosphorylated Rb. A substantial quantity of active Rb binds more efficiently to E2F1 in cells that express Lf and consequently blocks the expression of an E2F1-responsive gene, thereby suggesting that Lf plays a crucial role in the inhibition of tumor cell growth. Therefore, we conclude that the antiproliferative effects of Lf can likely be attributed to the elevated levels of hypophosphorylated Rb.

Host Cell Factor-1 and E2F4 Interact Via Multiple Determinants in Each Protein

Host Cell Factor (HCF-1) is a conserved, essential protein initially identified as a co-regulator for the Herpes Simplex Virus transactivator VP16. HCF-1 is variously involved in regulating transcription, splicing, cell proliferation and cytokinesis; however, its mechanisms of action remain unknown. HCF-1 function is manifested through an increasing assortment of cellular factors that target different regions of the protein. Several HCF-1 partners target the amino-terminal kelch domain of HCF-1 (residues 1-380) via a consensus HCF-binding motif (HBM) comprising the tetrapeptide (D/E)HXY. Searches of sequence databases indicated that this motif is present in E2F1 and E2F4, two members of the E2F family of cell cycle regulators. We show here that E2F4 specifically and directly interacts with HCF-1. Mutational analysis showed E2F4 independently targets the kelch domain and the basic domain (residues 450-902) of HCF-1, both of which are required for normal cell-cycle progression via separate determinants. The HBM-containing domain of E2F4 was necessary for interaction with the kelch domain of HCF-1 but not for interaction with the basic domain. Mutations in the HCF-1 kelch domain known to block cell growth abrogated E2F4 binding to the kelch domain in the absence but not in the presence of the juxtaposed basic region. Functionally, HCF-1 co-activated E2F4/DP-1 in transient transfection assays, while E2F4 blocked HCF-1-dependent rescue of a cell line that harbors a temperature sensitive mutant of HCF-1 that causes growth arrest. Our findings show that HCF-1 and E2F4 interact via multiple determinants and suggest a linkage between E2F4 and HCF-1 cell growth pathways.

E2F1 suppresses skin carcinogenesis via the ARF-p53 pathway

The E2F1 transcription factor, which is deregulated in most human cancers by mutations in the p16-cyclin D-Rb pathway, has both oncogenic and tumor-suppressive properties. This is dramatically illustrated by the phenotype of an E2F1 transgenic mouse model that spontaneously develops tumors in the skin and other epithelial tissues but is resistant to papilloma formation when subjected to a two-stage carcinogenesis protocol. Here, this E2F1 transgenic model was used to further explore the tumor-suppressive property of E2F1. Transgenic expression of E2F1 was found to inhibit ras-driven skin carcinogenesis at the promotion stage independent of the type of promoting agent used. E2F1 transgenic epidermis displayed increased expression of p19(ARF), p53, and p21(Cip1). Inactivation of either p53 or Arf in E2F1 transgenic mice restored sensitivity to two-stage skin carcinogenesis. While Arf inactivation impaired tumor suppression and p21 induction by E2F1, it did not reduce the level of apoptosis observed in E2F1 transgenic mice. Based on these findings, we propose that E2F1 suppresses ras-driven skin carcinogenesis through a nonapoptotic mechanism involving ARF and p53.

Unexpected roles for pRb in mouse skin carcinogenesis

The mouse skin carcinogenesis represents one of the best models for the understanding of malignant transformation, including the multistage nature of tumor development. The retinoblastoma gene product (pRb) plays a critical role in cell cycle regulation, differentiation, and inhibition of oncogenic transformation. In epidermis, Rb-/- deletion leads to proliferation and differentiation defects. Numerous evidences showed the involvement of the retinoblastoma pathway in this model. However, the actual role of pRb is still unknown. To study the possible involvement of pRb in keratinocyte malignant transformation, we have carried out two-stage chemical skin carcinogenesis on Rb(F19/F19) (thereafter Rb+/+) and Rb(F19/F19);K14Cre (thereafter Rb-/-) animals. Unexpectedly, we found that Rb-/- mice developed fewer and smaller papillomas than the Rb+/+ counterparts. Moreover, the small size of the pRb-deficient tumors is associated with an increase in the apoptotic index. Despite this, pRb-deficient tumors display an increased conversion rate to squamous cell carcinomas. Biochemical analyses revealed that these characteristics correlate with the differential expression and activity of different pathways, including E2F/p19arf/p53, PTEN/Akt, c-jun NH2-terminal kinase/p38, and nuclear factor-kappaB. Collectively, our findings show unexpected and hitherto nondescribed roles of pRb during the process of epidermal carcinogenesis.

Signalling in the epidermis: the E2F cell cycle regulatory pathway in epidermal morphogenesis, regeneration and transformation

The epidermis is the outermost layer in the skin, and it is the first line of defence against the environment. The epidermis also provides a barrier against loss of fluids and electrolytes, which is crucial for life. Essential in the maintenance of this tissue is its ability to continually self-renew and regenerate after injury. These two characteristics are critically dependent on the ability of the principal epidermal cell type, the keratinocyte, to proliferate and to respond to differentiation cues. Indeed, the epidermis is a multilayered tissue composed of keratinocyte stem cells and their differentiated progeny. Central for the control of cell proliferation is the E2F transcription factor regulatory network. This signaling network also includes cyclins, cdk, cdk inhibitors and the retinoblastoma (pRb) family of proteins. The biological importance of the E2F/pRb pathway is emphasized by the fact that a majority of human tumours exhibit alterations that disrupt the ability of pRb proteins to inhibit E2F, leading to permanent activation of the latter. Further, E2F is essential for normal epidermal regeneration after injury. Other member of the E2F signaling pathway are also involved in epidermal development and pathophysiology. Thus, whereas the pRb family of proteins is essential for epidermal morphogenesis, abnormal regulation of cyclins and E2F proteins results in tumorgenesis in this tissue. In this review, we discuss the role of each member of this important growth regulatory network in epidermal formation, homeostasis and carcinogenesis.

Divergent Siblings

Proliferation of mammalian cardiomyocytes ceases around birth when a transition from hyperplastic to hypertrophic myocardial growth occurs. Previous studies demonstrated that directed expression of the transcription factor E2F1 induces S-phase entry in cardiomyocytes along with stimulation of programmed cell death. Here, we show that directed expression of E2F2 and E2F4 by adenovirus mediated gene transfer in neonatal cardiomyocytes induced S-phase entry but did not result in an onset of apoptosis whereas directed expression of E2F1 and E2F3 strongly evoked programmed cell death concomitant with cell cycle progression. Although both E2F2 and E2F4 induced S-phase entry only directed expression of E2F2 resulted in mitotic cell division of cardiomyocytes. Expression of E2F5 or a control LacZ-Adenovirus had no effects on cell cycle progression. Quantitative real time PCR revealed that E2F1, E2F2, E2F3, and E2F4 alleviate G0 arrest by induction of cyclinA and E cyclins. Furthermore, directed expression of E2F1, E2F3, and E2F5 led to a transcriptional activation of several proapoptotic genes, which were mitigated by E2F2 and E2F4. Our finding that expression of E2F2 induces cell division of cardiomyocytes along with a suppression of proapoptotic genes might open a new access to improve the regenerative capacity of cardiomyocytes.

Regulation of epidermal apoptosis and DNA repair by E2F1 in response to ultraviolet B radiation

The E2F1 transcription factor regulates the expression of genes involved in cell proliferation, apoptosis and DNA repair. Following DNA damage, E2F1 is phosphorylated and stabilized, but the physiological role of E2F1 in the response to DNA damage is unclear. We find that mice lacking E2F1 have increased levels of epidermal apoptosis compared to wild-type mice following exposure to ultraviolet B (UVB) radiation. Moreover, transgenic overexpression of E2F1 in basal layer keratinocytes suppresses apoptosis induced by UVB. Inhibition of UVB-induced apoptosis by E2F1 is unexpected given that most studies have demonstrated a proapoptotic function for E2F1. E2F1-mediated suppression of apoptosis does not involve alterations in mitogen-activated protein kinase activation or Bcl-2 downregulation in response to UVB and is independent of p53. Instead, inhibition of UVB-induced apoptosis by E2F1 correlates with a stimulation of DNA repair. Mice lacking E2F1 are impaired for the removal of DNA photoproducts, while E2F1 transgenic mice repair UVB-induced DNA damage at an accelerated rate compared to wild-type mice. These findings suggest that E2F1 participates in the response to UVB by promoting DNA repair and suppressing apoptosis.

The DP-1 transcription factor is required for keratinocyte growth and epidermal stratification

The epidermis is a stratified epithelium constantly replenished through the ability of keratinocytes in its basal layer to proliferate and self-renew. The epidermis arises from a single-cell layer ectoderm during embryogenesis. Large proliferative capacity is central to ectodermal cell and basal keratinocyte function. DP-1, a heterodimeric partner of E2F transcription factors, is highly expressed in the ectoderm and all epidermal layers during embryogenesis. To investigate the role of DP-1 in epidermal morphogenesis, we inhibited DP-1 activity through exogenous expression of a dominant-negative mutant (dnDP-1). Expression of the dnDP-1 mutant interferes with binding of E2F/DP-1 heterodimers to DNA and inhibits DNA replication, as well as cyclin A mRNA and protein expression. Chromatin immunoprecipitation analysis demonstrated that the cyclin A promoter is predominantly bound in proliferating keratinocytes by complexes containing E2F-3 and E2F-4. Thus, the mechanisms of decreased expression of cyclin A in the presence of dnDP-1 seem to involve inactivation of DP-1 complexes containing E2F-3 and E2F-4. To assess the consequences on epidermal morphogenesis of inhibiting DP-1 activity, we expressed dnDP-1 in rat epithelial keratinocytes in organotypic culture and observed that DP-1 inhibition negatively affected stratification of these cells. Likewise, expression of dnDP-1 in embryonic ectoderm explants produced extensive disorganization of subsequently formed epidermal basal and suprabasal layers, interfering with normal epidermal formation. We conclude that DP-1 activity is required for normal epidermal morphogenesis and ectoderm-to-epidermis transition.

Genetically Targeted Cancer Therapy

The is a double-stranded RNA-activated protein kinase (PKR) has been largely investigated for its key role in viral host defense. Although best characterized by its function in mediating the antiviral and antiproliferative effects of interferon (IFN), PKR is also implicated in transcriptional regulation, cell differentiation, signal transduction, and tumor suppression. However, recent findings identifying PKR as an important effector of apoptosis have led to an increased interest in PKR modulation as an antitumor strategy. PKR can either be up-regulated through direct induction by the transcription factor E2F-1, or it can be activated through direct protein-protein interactions with the melanoma differentiation-associated gene-7 (MDA7, IL-24). Additionally, the intracellular formation of double-stranded RNA by transfection with antisense RNA complementary to tumor-specific RNA sequences can induce PKR activation and apoptosis selective to these tumor cells. The growing application of viral vector-based gene therapies and oncolytic, replicating viruses that must elude viral defense in order to be effective, has also drawn attention to PKR. Oncolytic viruses, like the attenuated herpes simplex virus R3616, the vesicular stomatitis virus, or reovirus, specifically replicate in tumor cells only because the viral host defense in the permissive cells is suppressed. In this article we review the role of PKR as an effector of apoptosis and a target for tumor treatment strategies and discuss the potential of PKR-modifying agents to treat patients with cancer. Targeted gene therapy against cancer can be approached by activation of PKR with the down-regulation of protein synthesis and induction of apoptosis, or by suppression of PKR with the propagation of oncolytic virus. Since the PKR pathway can be modified by many routes, antitumor therapies combining oncolytic virus, gene therapies, and chemotherapy with PKR modifiers are likely to emerge in the near future as therapeutic options in the treatment of patients with cancer.

Expression of cyclin E renders cyclin D-CDK4 dispensable for inactivation of the retinoblastoma tumor suppressor protein, activation of E2F, and G1-S phase progression

The activation of CDK2-cyclin E in late G1 phase has been shown to play a critical role in retinoblastoma protein (pRb) inactivation and G1-S phase progression of the cell cycle. The phosphatidylinositol 3-OH-kinase inhibitor LY294002 has been shown to block cyclin D1 accumulation, CDK4 activity and, thus, G1 progression in alpha-thrombin-stimulated IIC9 cells (Chinese hamster embryonic fibroblasts). Our previous results show that expression of cyclin E rescues S phase progression in alpha-thrombin-stimulated IIC9 cells treated with LY294002, arguing that cyclin E renders CDK4 activity dispensable for G1 progression. In this work we investigate the ability of alpha-thrombin-induced CDK2-cyclin E activity to inactivate pRb in the absence of prior CDK4-cyclin D1 activity. We report that in the absence of CDK4-cyclin D1 activity, CDK2-cyclin E phosphorylates pRb in vivo on at least one residue and abolishes pRb binding to E2F response elements. We also find that expression of cyclin E rescues E2F activation and cyclin A expression in cyclin D kinase-inhibited, alpha-thrombin-stimulated cells. Furthermore, the rescue of E2F activity, cyclin A expression, and DNA synthesis by expression of E can be blocked by the expression of either CDK2(D145N) or RbDeltaCDK, a constitutively active mutant of pRb. However, restoring four known cyclin E-CDK2 phosphorylation sites to RbDeltaCDK renders it susceptible to inactivation in late G1, as assayed by E2F activation, cyclin A expression, and S phase progression. These data indicate that CDK2-cyclin E, without prior CDK4-cyclin D activity, can phosphorylate and inactivate pRb, activate E2F, and induce DNA synthesis.

HCF-1 functions as a coactivator for the zinc finger protein Krox20

HCF-1 is a transcriptional cofactor required for activation of herpes simplex virus immediate-early genes by VP16 as well as less clearly defined roles in cell proliferation, cytokinesis, and spliceosome formation. It is expressed as a large precursor that undergoes proteolysis to yield two subunits that remain stably associated. VP16 uses a degenerate 4-amino acid sequence, known as the HCF-binding motif, to bind to a six-bladed beta-propeller domain at the N terminus of HCF-1. Functional HCF-binding motifs are also found in LZIP and Zhangfei, two cellular bZIP transcription factors of unknown function. Here we show that the HCF-binding motif occurs in a wide spectrum of DNA-binding proteins and transcriptional cofactors. Three well characterized examples were further analyzed for their ability to use HCF-1 as a coactivator. Krox20, a zinc finger transcription factor required for Schwann cell differentiation, and E2F4, a cell cycle regulator, showed a strong requirement for functional HCF-1 to activate transcription. In contrast, activation by estrogen receptor-alpha did not display HCF dependence. In Krox20, the HCF-binding motif lies within the N-terminal activation domain and mutation of this sequence diminishes both transactivation and association with the HCF-1 beta-propeller. The activation domain in the C-terminal subunit of HCF-1 contributes to activation by Krox20, possibly through recruitment of p300. These results suggest that HCF-1 is recruited by many different classes of cellular transcription factors and is therefore likely to be required for a variety of cellular processes including cell cycle progression and development.

E2F modulates keratinocyte squamous differentiation: implications for E2F inhibition in squamous cell carcinoma

E2F regulation is essential for normal cell cycle progression. Therefore, it is not surprising that squamous cell carcinoma cell lines (SCC) overexpress E2F1 and exhibit deregulated E2F activity when compared with normal keratinocytes. Indeed, deliberate E2F1 deregulation has been shown to induce hyperplasia and skin tumor formation. In this study, we report on a dual role for E2F as a mediator of keratinocyte proliferation and modulator of squamous differentiation. Overexpression of E2F isoforms in confluent primary keratinocyte cultures resulted in suppression of differentiation-associated markers. Moreover, we found that the DNA binding domain and the trans-activation domain of E2F1 are important in mediating suppression of differentiation. Use of a dominant/negative form of E2F1 (E2F d/n) found that E2F inhibition alone is sufficient to suppress the activity of proliferation-associated markers but is not capable of inducing differentiation markers. However, if the E2F d/n is expressed in differentiated keratinocytes, differentiation marker activity is further induced, suggesting that E2F may act as a modulator of squamous differentiation. We therefore examined the effects of E2F d/n in a differentiation-insensitive SCC cell line. We found that treatment with the differentiating agent, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), or expression of E2F d/n alone had no effect on differentiation markers. However, a combination of E2F d/n + TPA induced the expression of differentiation markers. Combined, these data indicate that E2F may play a key role in keratinocyte differentiation. These data also illustrate the unique potential of anti-E2F therapies in arresting proliferation and inducing differentiation of SCCs.

Constitutive E2F expression in tobacco plants exhibits altered cell cycle control and morphological change in a cell type-specific manner

The E2F family plays a pivotal role in cell cycle control and is conserved among plants and animals, but not in fungi. This provides for the possibility that the E2F family was integrated during the development of higher organisms, but little is known about this. We examined the effect of E2F ectopically expressed in transgenic tobacco (Nicotiana tabacum) plants on growth and development using E2Fa (AtE2F3) and DPa from Arabidopsis. E2Fa-DPa double transgenic lines exhibited altered phenotypes with curled leaves, round shaped petals, and shortened pistils. In mature but not immature leaves of the double transgenic lines, there were enlarged nuclei with increasing ploidy levels accompanied by the ectopic expression of S phase- but not M phase-specific genes. This indicates that a high expression of E2F promotes endoreduplication by accelerating S phase entry in terminally differentiated cells with limited mitotic activity. Furthermore, mature leaves of the transgenic plants contained increased numbers of small cells, especially on the palisade (adaxial) side of the outer region toward the edge, and the leaf strips exhibited hormone-independent callus formation when cultured in vitro. These observations suggest that an enhanced E2F activity modulates cell cycle in a cell type-specific manner and affects plant morphology depending on a balance between activities for committing to S phase and M phase, which likely differ between organs or tissues.

Abnormal epidermal differentiation and impaired epithelial-mesenchymal tissue interactions in mice lacking the retinoblastoma relatives p107 and p130

The functions of p107 and p130, members of the retinoblastoma family, include the control of cell cycle progression and differentiation in several tissues. Our previous studies suggested a role for p107 and p130 in keratinocyte differentiation in vitro. We now extend these data using knockout animal models. We found impaired terminal differentiation in the interfollicular keratinocytes of p107/p130-double-null mice epidermis. In addition, we observed a decreased number of hair follicles and a clear developmental delay in hair, whiskers and tooth germs. Skin grafts of p107/p130-deficient epidermis onto NOD/scid mice showed altered differentiation and hyperproliferation of the interfollicular keratinocytes, thus demonstrating that the absence of p107 and p130 results in the deficient control of differentiation in keratinocytes in a cell-autonomous manner. Besides normal hair formation, follicular cysts, misoriented and dysplastic follicles, together with aberrant hair cycling, were also observed in the p107/p130 skin transplants. Finally, the hair abnormalities in p107/p130-null skin were associated with altered Bmp4-dependent signaling including decreased DeltaNp63 expression. These results indicate an essential role for p107 and p130 in the epithelial-mesenchimal interactions.

N-Acetylcysteine enhances UV-mediated caspase-3 activation, fragmentation of E2F-4, and apoptosis in human C8161 melanoma: inhibition by ectopic Bcl-2 expression

Redox imbalance due to oxidative stress or excessive antioxidant levels can alter apoptotic responses. Recently, antioxidants like N-acetylcysteine (NAC) were reported to inhibit H(2)O(2)-mediated necrotic cell death, although they were inactive against apoptosis induced by other agents like etoposide. NAC was also found to kill preferentially tumor cells compared to normal fibroblasts at 20-50mM, but these concentrations are lethal to normal splenocytes. We now demonstrate that 10mM NAC, a non-toxic concentration, can enhance the UV radiation-mediated apoptosis of human C8161 melanoma cells. Compared to treatment with UV radiation alone, combination treatment with NAC doubled the ratio of activated caspase-3 to pro-caspase-3 and produced greater fragmentation of the retinoblastoma protein and the E2F-4 transcription factor without affecting the E2F-1 protein. These effects of joint NAC-UV radiation treatment were counteracted by the overexpression of the bcl-2 gene. To our knowledge, this report is the first to: (i) demonstrate a synergy between DNA-damaging agents, like UV radiation, and antioxidants, like NAC, and (ii) show that a Bcl-2-inhibitable E2F-4 fragmentation occurs concurrently with caspase-3 activation and apoptosis.

Constitutive E2F1 overexpression delays endochondral bone formation by inhibiting chondrocyte differentiation

Longitudinal bone growth results from endochondral ossification, a process that requires proliferation and differentiation of chondrocytes. It has been shown that proper endochondral bone formation is critically dependent on the retinoblastoma family members p107 and p130. However, the precise functional roles played by individual E2F proteins remain poorly understood. Using both constitutive and conditional E2F1 transgenic mice, we show that ubiquitous transgene-driven expression of E2F1 during embryonic development results in a dwarf phenotype and significantly reduced postnatal viability. Overexpression of E2F1 disturbs chondrocyte maturation, resulting in delayed endochondral ossification, which is characterized by reduced hypertrophic zones and disorganized growth plates. Employing the chondrogenic cell line ATDC5, we investigated the effects of enforced E2F expression on the different phases of chondrocyte maturation that are normally required for endochondral ossification. Ectopic E2F1 expression strongly inhibits early- and late-phase differentiation of ATDC5 cells, accompanied by diminished cartilage nodule formation as well as decreased type II collagen, type X collagen, and aggrecan gene expression. In contrast, overexpression of E2F2 or E2F3a results in only a marginal delay of chondrocyte maturation, and increased E2F4 levels have no effect. These data are consistent with the notion that E2F1 is a regulator of chondrocyte differentiation.

Evolving intricacies and implications of E2F1 regulation

E2F transcription factors may play a pivotal role in the transcriptional regulation of several cellular processes far beyond the originally described cell cycle and proliferation. Among the six E2F family members, only E2F1 is noted for its role in apoptosis. The pocket protein family members Rb, p107, and p130 act as the main regulators of E2F activity. Nonetheless, in recent years other protein-protein interactions have been described for E2Fs. The post-translational modifications resulting from such protein interactions may have significant implications in the stability, half-life, and functional activity of E2Fs. In human diseases the significance of E2Fs is still under appreciated and is primarily recognized only as a consequence of the impairment in retinoblastoma gene product (Rb). However, with increasing knowledge of other protein interactions, the derailment of E2F activity could be anticipated to stem from an abnormality of any node in the complex network governing their availability and activity. The present review is intended to provide a perspective on the diversity of biochemical mechanisms underlying abnormal E2F expression and activity, understanding of which may have significant clinical implications.

The mitochondrial apoptosis-inducing factor plays a role in E2F-1-induced apoptosis in human colon cancer cells

BACKGROUND

Overexpression of the transcription factor E2F-1 provokes apoptosis in cancer cells; the mechanism, however, is not completely understood. We sought to evaluate E2F-1 gene therapy in human colon cancer and to investigate the apoptotic pathway involved.

METHODS

Adenoviral vectors were used to transfect the E2F-1 gene (Ad5E2F-1) or the control gene luciferase (Ad5Luc) into four human colon carcinoma cell lines. Apoptosis was confirmed by flow cytometry and poly (ADP-ribose) polymerase cleavage. Expression of apoptotic factors was determined with Western blot analysis. Inhibitory assays were used to determine the involvement of caspases in the apoptotic pathway.

RESULTS

Overexpression of E2F-1 was evident in all cells treated with Ad5E2F-1; upregulation of Bcl-2, and activation of caspases were noted. The apoptosis-inducing factor in the cytosolic fraction was markedly upregulated after Ad5E2F-1 treatment. E2F-1 overexpression inhibited proliferation and induced significant apoptosis in all cell lines (P <.005). This apoptotic response could be only partially blocked by caspase inhibitors.

CONCLUSIONS

These findings demonstrate that E2F-1 induces apoptosis and inhibits proliferation in human colon cancer cell lines. The marked upregulation of apoptosis-inducing factor and the fact that E2F-1-induced apoptosis is incompletely blocked by caspase inhibitors suggest a caspase-independent pathway of E2F-1-mediated apoptosis, reported here for the first time.

Role of cell cycle regulator p19ARF in regulating T cell responses

Although it is well established that the processes of cellular proliferation and apoptosis are linked, the role of cell cycle regulators in T cell responses in vivo is not well understood. In recent years, tumor suppressor molecule p19(ARF) has emerged as a key cell cycle regulator important in cellular apoptosis against strong mitogenic stimuli. In this study, we compared the antigen-specific T cell responses between wild type (+/+) and p19(ARF)-deficient (p19-/-) mice following an acute infection with lymphocytic choriomeningitis virus (LCMV). p19-/- mice mounted a potent CD8 T cell response and the magnitude of expansion of LCMV-specific CD8 T cells was comparable to that of +/+ mice. Further, the clonal downsizing of the expanded virus-specific CD8 T cells and establishment of long-term T cell memory were minimally affected by p19(ARF) deficiency. Therefore, p19(ARF) function is not essential to regulate T cell responses following an acute viral infection.

Role for the double-stranded RNA activated protein kinase PKR in E2F-1-induced apoptosis

The transcription factor E2F-1 induces cell cycle progression at the G1/S checkpoint, and deregulation of E2F-1 provokes apoptosis in a wide variety of malignant cells. To date only p14(ARF) and p73, a p53 homologue, have been identified as E2F-1-inducible genes capable of mediating an apoptotic response. Here we show that adenovirus-mediated E2F-1 overexpression in cancer cells induces expression and autophosphorylation of the double-stranded RNA-dependent protein kinase PKR leading to phosphorylation of its downstream target, the alpha-subunit of the eukaryotic translation initiation factor 2 (eIF-2alpha) and to apoptotic cell death. This PKR-dependent apoptosis occurs in cell lines with mutated p53 and in cell lines with mutated p53 and p73, and is significantly reduced by the chemical inhibition of PKR activation. Further, PKR(-/-) mouse embryo fibroblasts, but not PKR(+/+) mouse embryo fibroblasts, demonstrate significant resistance to E2F-1-induced apoptosis. We conclude that an important pathway of E2F-1-mediated apoptosis is dependent on PKR activation and does not require p53 or p73.