Cyclin A-kinase regulation of E2F-1 DNA binding function underlies suppression of an S phase checkpoint

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

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

Cyclin A/B RxL Macrocyclic Inhibitors to Treat Cancers with High E2F Activity

Cancer cell proliferation requires precise control of E2F1 activity; excess activity promotes apoptosis. Here, we developed cell-permeable and bioavailable macrocycles that selectively kill small cell lung cancer (SCLC) cells with inherent high E2F1 activity by blocking RxL-mediated interactions of cyclin A and cyclin B with select substrates. Genome-wide CRISPR/Cas9 knockout and random mutagenesis screens found that cyclin A/B RxL macrocyclic inhibitors (cyclin A/Bi) induced apoptosis paradoxically by cyclin B- and Cdk2-dependent spindle assembly checkpoint activation (SAC). Mechanistically, cyclin A/Bi hyperactivate E2F1 and cyclin B by blocking their RxL-interactions with cyclin A and Myt1, respectively, ultimately leading to SAC activation and mitotic cell death. Base editor screens identified cyclin B variants that confer cyclin A/Bi resistance including several variants that disrupted cyclin B:Cdk interactions. Unexpectedly but consistent with our base editor and knockout screens, cyclin A/Bi induced the formation of neo-morphic Cdk2-cyclin B complexes that promote SAC activation and apoptosis. Finally, orally-bioavailable cyclin A/Bi robustly inhibited tumor growth in chemotherapy-resistant patient-derived xenograft models of SCLC. This work uncovers gain-of-function mechanisms by which cyclin A/Bi induce apoptosis in cancers with high E2F activity, and suggests cyclin A/Bi as a therapeutic strategy for SCLC and other cancers driven by high E2F activity.

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

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

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

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

Deregulated E2F Activity as a Cancer-Cell Specific Therapeutic Tool

The transcription factor E2F, the principal target of the tumor suppressor pRB, plays crucial roles in cell proliferation and tumor suppression. In almost all cancers, pRB function is disabled, and E2F activity is enhanced. To specifically target cancer cells, trials have been undertaken to suppress enhanced E2F activity to restrain cell proliferation or selectively kill cancer cells, utilizing enhanced E2F activity. However, these approaches may also impact normal growing cells, since growth stimulation also inactivates pRB and enhances E2F activity. E2F activated upon the loss of pRB control (deregulated E2F) activates tumor suppressor genes, which are not activated by E2F induced by growth stimulation, inducing cellular senescence or apoptosis to protect cells from tumorigenesis. Deregulated E2F activity is tolerated in cancer cells due to inactivation of the ARF-p53 pathway, thus representing a feature unique to cancer cells. Deregulated E2F activity, which activates tumor suppressor genes, is distinct from enhanced E2F activity, which activates growth-related genes, in that deregulated E2F activity does not depend on the heterodimeric partner DP. Indeed, the ARF promoter, which is specifically activated by deregulated E2F, showed higher cancer-cell specific activity, compared to the E2F1 promoter, which is also activated by E2F induced by growth stimulation. Thus, deregulated E2F activity is an attractive potential therapeutic tool to specifically target cancer cells.

Cell cycle control by the insulin-like growth factor signal: at the crossroad between cell growth and mitotic regulation

In proliferating cells and tissues a number of checkpoints (G1/S and G2/M) preceding cell division (M-phase) require the signal provided by growth factors present in serum. IGFs (I and II) have been demonstrated to constitute key intrinsic components of the peptidic active fraction of mammalian serum. In vivo genetic ablation studies have shown that the cellular signal triggered by the IGFs through their cellular receptors represents a non-replaceable requirement for cell growth and cell cycle progression. Retroactive and current evaluation of published literature sheds light on the intracellular circuitry activated by these factors providing us with a better picture of the pleiotropic mechanistic actions by which IGFs regulate both cell size and mitogenesis under developmental growth as well as in malignant proliferation. The present work aims to summarize the cumulative knowledge learned from the IGF ligands/receptors and their intracellular signaling transducers towards control of cell size and cell-cycle with particular focus to their actionable circuits in human cancer. Furthermore, we bring novel perspectives on key functional discriminants of the IGF growth-mitogenic pathway allowing re-evaluation on some of its signal components based upon established evidences.

Cytotoxic Marine Alkaloid 3,10-Dibromofascaplysin Induces Apoptosis and Synergizes with Cytarabine Resulting in Leukemia Cell Death

Myeloid leukemia is a hematologic neoplasia characterized by a clonal proliferation of hematopoietic stem cell progenitors. Patient prognosis varies depending on the subtype of leukemia as well as eligibility for intensive treatment regimens and allogeneic stem cell transplantation. Although significant progress has been made in the therapy of patients including novel targeted treatment approaches, there is still an urgent need to optimize treatment outcome. The most common therapy is based on the use of chemotherapeutics cytarabine and anthrayclines. Here, we studied the effect of the recently synthesized marine alkaloid 3,10-dibromofascaplysin (DBF) in myeloid leukemia cells. Unsubstituted fascaplysin was early found to affect cell cycle via inhibiting CDK4/6, thus we compared the activity of DBF and other brominated derivatives with known CDK4/6 inhibitor palbociclib, which was earlier shown to be a promising candidate to treat leukemia. Unexpectedly, the effect DBF on cell cycle differs from palbociclib. In fact, DBF induced leukemic cells apoptosis and decreased the expression of genes responsible for cancer cell survival. Simultaneously, DBF was found to activate the E2F1 transcription factor. Using bioinformatical approaches we evaluated the possible molecular mechanisms, which may be associated with DBF-induced activation of E2F1. Finally, we found that DBF synergistically increase the cytotoxic effect of cytarabine in different myeloid leukemia cell lines. In conclusion, DBF is a promising drug candidate, which may be used in combinational therapeutics approaches to reduce leukemia cell growth.

A TIMM17A Regulatory Network Contributing to Breast Cancer

Background

Translocase of inner mitochondrial membrane 17A (TIMM17A) is overexpressed in breast cancer (BRCA), and upregulation can increase the aggressiveness of BRCA cells. This study examined the influence of the TIMM17A gene network on BRCA outcome.

Methods

Expression levels of TIMM17A were compared between normal and tumor tissues from the Oncomine^TM database, and the association with patient survival was analyzed using Kaplan–Meier Plotter. Clinical factors influencing TIMM17A expression were studied by UALCAN. cBioPotal was then used to identify genes interacting with TIMM17A, and network relationships were assessed using the R clusterProfiler package. The association between TIMM17A mutation and mRNA expression in BRCA was examined using the LinkFinder application in LinkedOmics, and coexpressed genes were assessed for functional enrichment using the LinkInterpreter application. Furthermore, TIMM17A expression correlation with cell cycle phase distribution was performed by flow cytometry. Finally, the target networks of kinases, microRNAs (miRNAs), and transcription factors were identified using GeneMANIA. The expression and correlation of potential miRNAs and targets were further validated in BRCA cell lines by qRT-PCR.

Results

Expression of TIMM17A was significantly elevated in BRCA compared with normal tissue (p < 0.05), and overexpression was associated with both poor overall survival (OS) and shorter distant metastasis-free survival (DMFS) (p < 0.05). Expression of TIMM17A was not associated with age, sex, BRCA subclass, clinical stage, or patient ethnicity. The coexpressed TIMM17A network was enriched in genes targeted by cell cycle regulators such as CDK1, miR-331, and E2F family transcription factors (FDR < 0.001). Furthermore, flow cytometry revealed a strong association between higher TIMM17A expression and faster cell cycle progression in these BRCA cell lines. In addition, expression of TIMM17A protein was correlated with CDK1 protein expression in BRCA cell lines as measured by western blotting.

Conclusion

Elevated TIMM17A expression accelerates the progression of BRCA, thereby reducing OS and DMFS. The TIMM17A-associated networks identified here provide clues to the molecular pathogenesis of BRCA and potential targets for BRCA treatment.

Polysaccharides derived from Balanophora polyandra significantly suppressed the proliferation of ovarian cancer cells through P53-mediated pathway

Ovarian cancer (OC) is ranked the first among the cancers threatening women's health. It attracts tremendous attention of cancer researchers because of its extremely high mortality rate. Recent studies have indicated that traditional herbal medicines (THMs) can play a pivotal role in cancer prevention and treatment. THMs are gaining popularity as a source of anti-cancer agents. The plant of Balanophora polyandra, which has been used as a traditional herbal medicine, has been known for exhibiting potential haemostatic, analgesic, anti-inflammatory and anti-cancer properties. However, few studies on inhibitory effect of B. polyandra on OC have been performed. In the present study, we found that B. polyandra polysaccharides (BPP) induced cell cycle arrest at S phase, triggered apoptosis and inhibited migration and invasion of OC cells. Furthermore, we also found that there was a potential and close relationship between BPP and P53-mediated pathway. Overall, these findings suggest that BPP can be a potential therapeutic agent for the treatment of OC.

Mammalian cell cycle cyclins

Proper progression throughout the cell division cycle depends on the expression level of a family of proteins known as cyclins, and the subsequent activation of cyclin-dependent kinases (Cdks). Among the numerous members of the mammalian cyclin family, only a few of them, cyclins A, B, C, D and E, are known to display critical roles in the cell cycle. These functions will be reviewed here with a special focus on their relevance in different cell types in vivo and their implications in human disease.

Foeniculum Vulgare and Pelargonium Graveolens Essential Oil Mixture Triggers the Cell Cycle Arrest and Apoptosis in MCF-7 Cells

Background:

Fennel (Foeniculum vulgare) and rose geranium (Pelargonium graveolens) oils are known for their various biological effects including anticancer properties.

Objective:

This study aimed to evaluate the anticancer mechanism of fennel and geranium oils combined treatment on MCF-7 cells.

Methods:

The GC-MS method for essential oil characterization as well as the in vitro cytotoxicity, morphological changes, real-time PCR and immunocytochemical investigation for apoptosis-related markers, in addition, to flow cytometric cell cycle distribution analysis were done.

Results:

The major constituents of both essential oils were anethole (55.33 %) and estragole (11.57 %) for fennel essential oil. However, cintronellol (34.40 %) and geraniol (8.67 %) were identified in geranium oil. The results revealed an IC50 of 220±5.7 and 60±2.1µg/ml for fennel and geranium oils, respectively. The mechanistic anticancer properties were investigated throughout the 70, 50, and 25µg/ml of oils mixture. The marked apoptotic morphology and the flow cytometric cell cycle distribution analysis in addition to the levels of apoptosisrelated makers such as p53, caspase-3, mir-21, mir-92a, Bcl-2, and ki-67 confirmed that fennel and geranium oils combination induced cell cycle arrest and apoptosis in MCF-7 cells. Moreover, the oils mixture did not exert any significant (P<0.01) toxicity on normal human peripheral blood lymphocytes in vitro.

Conclusion:

The findings showed that the mixture of oils exerted selective cytotoxicity towards MCF-7 cells through induction of cell cycle arrest and apoptosis which may be triggered by the synergistic effect between the active ingredients of fennel and geranium oils.

Targeting Kinase Interaction Networks: A New Paradigm in PPI Based Design of Kinase Inhibitors

BACKGROUND

Kinases are key modulators in regulating diverse range of cellular activities and are an essential part of the protein-protein interactome. Understanding the interaction of kinases with different substrates and other proteins is vital to decode the cell signaling machinery as well as causative mechanism for disease onset and progression.

OBJECTIVE

The objective of this review is to present all studies on the structure and function of few important kinases and highlight the protein-protein interaction (PPI) mechanism of kinases and the kinase specific interactome databases and how such studies could be utilized to develop anticancer drugs.

METHODS

The article is a review of the detailed description of the various domains in kinases that are involved in protein-protein interactions and specific inhibitors developed targeting these PPI domains.

RESULTS

The review has surfaced in depth the interacting domains in key kinases and their features and the roles of PPI in the human kinome and the various signaling cascades that are involved in certain types of cancer.

CONCLUSION

The insight availed into the mechanism of existing peptide inhibitors and peptidomimetics against kinases will pave way for the design and generation of domain specific peptide inhibitors with better productivity and efficiency and the various software and servers available can be of great use for the identification and analysis of protein-protein interactions.

Downregulation of miR-144 by triptolide enhanced p85α-PTEN complex formation causing S phase arrest of human nasopharyngeal carcinoma cells

Selective pharmacologic targeting of cell cycle regulators is a potent anti-cancer therapeutic strategy. Here, we show that caspase-3-mediated p21 cleavage involves p53 independent of triptolide (TPL)-induced S phase arrest in human type 1 nasopharyngeal carcinoma (NPC) cells. Coimmunoprecipitation studies demonstrated that TPL causes S phase cell cycle arrest by suppressing the formation of cyclin A-phosphor (p)-cyclin-dependent kinas 2 (CDK2) (Thr 39) complexes. Ectopic expression of constitutively active protein kinase B1 (Akt1) blocks the induction of S phase arrest and the suppression of cyclin A expression and CDK2 Thr 39 phosphorylation by TPL. Expression of the phosphomimetic mutant CDK2 (T39E) rescues the cells from TPL-induced S phase arrest, whereas phosphorylation-deficient CDK2 (T39A) expression regulates cell growth with significant S phase arrest and enhances TPL-triggered S phase arrest. Treatment with TPL induces an increase in the formation of complexes between unphosphorylated phosphatase and tensin homolog deleted from chromosome 10 (PTEN) and p85α in the plasma membrane. Decreased microRNA (miR)-144 expression and increased PTEN expression after TPL treatment were demonstrated, and TPL-enhanced p85α-PTEN complexes and inhibitory effects on Akt (Ser 473) phosphorylation and S phase arrest were suppressed by ectopic PTEN short hairpin RNA or miR-144 expression. Knockdown of endogenous miR-144 by miR-144 Trap upregulated PTEN expression and accordingly enhanced p85α-PTEN complex formation and S phase arrest. Collectively, the effect of TPL on S phase arrest in human NPC cells is likely to enhance the p85α-PTEN interaction in the plasma membrane by suppressing miR-144 expression, resulting in the attenuation of cyclin A-p-CDK2 (Thr 39) complex formation via Akt inactivation.

SiSTL1, encoding a large subunit of ribonucleotide reductase, is crucial for plant growth, chloroplast biogenesis, and cell cycle progression in Setaria italica

The activity of ribonucleotide reductase (RNR), which catalyses the transformation of four ribonucleoside diphosphates (NDPs) to their corresponding deoxyribonucleoside diphosphates (dNDPs), is the main determiner of the cellular concentration of dNTP pools and should be tightly coordinated with DNA synthesis and cell-cycle progression. Constitutively increased or decreased RNR activity interferes with DNA replication and leads to arrested cell cycle progression; however, the mechanisms underlying these disruptive effects in higher plants remain to be uncovered. In this study, we identified a RNR large subunit mutant, sistl1, in Setaria italica (foxtail millet), which exhibited growth retardation as well as striped leaf phenotype, i.e. irregularly reduced leaf vein distances and decreased chloroplast biogenesis. We determined that a Gly737 to Glu substitution occurring in the C-terminus of the SiSTL1 protein slightly affected its optimal function, leading in turn to the reduced expression of genes variously involved in the assembly and activation of the DNA pre-replicative complex, elongation of replication forks and S phase entry. Our study provides new insights into how SiSTL1 regulates plant growth, chloroplast biogenesis, and cell cycle progression in Poaceae crops.

Regulation of E2F1 Transcription Factor by Ubiquitin Conjugation

Ubiquitination is a post-translational modification that defines the cellular fate of intracellular proteins. It can modify their stability, their activity, their subcellular location, and even their interacting pattern. This modification is a reversible event whose implementation is easy and fast. It contributes to the rapid adaptation of the cells to physiological intracellular variations and to intracellular or environmental stresses. E2F1 (E2 promoter binding factor 1) transcription factor is a potent cell cycle regulator. It displays contradictory functions able to regulate both cell proliferation and cell death. Its expression and activity are tightly regulated over the course of the cell cycle progression and in response to genotoxic stress. I discuss here the most recent evidence demonstrating the role of ubiquitination in E2F1’s regulation.

Melanoma antigen-A11 regulates substrate-specificity of Skp2-mediated protein degradation

Melanoma antigen-A11 (MAGE-A11) is a proto-oncogene involved in androgen receptor signaling and androgen-dependent cell growth. In this report we provide evidence that MAGE-A11 interacts with Skp2 (S phase kinase-associated protein), the substrate recognition protein of the Skp1-Cullin1-F-box E3 ubiquitin ligase, and with Skp2 binding protein, cyclin A. A similar cyclin A binding motif in MAGE-A11 and Skp2 was consistent with a competitive relationship between MAGE-A11 and Skp2 in binding cyclin A. Skp2 inhibited MAGE-A11 interaction with cyclin A. Differential effects of MAGE-A11 on Skp2-mediated protein degradation were also revealed. MAGE-A11 increased Skp2-mediated degradation of cyclin A and retinoblastoma-related protein p130. In contrast, MAGE-A11 decreased Skp2-mediated degradation of E2F1 and Skp2 self-ubiquitination. Stabilization of E2F1 by MAGE-A11 was associated with sequestration and inactivation of Skp2 through the formation of an E2F1-MAGE-A11-Skp2 complex. We conclude that direct interactions of MAGE-A11 with Skp2 and cyclin A regulate the substrate-specificity of Skp2-mediated protein degradation.

Increase of Intracellular Cyclic AMP by PDE4 Inhibitors Affects HepG2 Cell Cycle Progression and Survival

Type 4 cyclic nucleotide phosphodiesterases (PDE4) are major members of a superfamily of enzymes (PDE) involved in modulation of intracellular signaling mediated by cAMP. Broadly expressed in most human tissues and present in large amounts in the liver, PDEs have in the last decade been key therapeutic targets for several inflammatory diseases. Recently, a significant body of work has underscored their involvement in different kinds of cancer, but with no attention paid to liver cancer. The present study investigated the effects of two PDE4 inhibitors, rolipram and DC-TA-46, on the growth of human hepatoma HepG2 cells. Treatment with these inhibitors caused a marked increase of intracellular cAMP level and a dose- and time-dependent effect on cell growth. The concentrations of inhibitors that halved cell proliferation to about 50% were used for cell cycle experiments. Rolipram (10 μM) and DC-TA-46 (0.5 μM) produced a decrease of cyclin expression, in particular of cyclin A, as well as an increase in p21, p27 and p53, as evaluated by Western blot analysis. Changes in the intracellular localization of cyclin D1 were also observed after treatments. In addition, both inhibitors caused apoptosis, as demonstrated by an Annexin-V cytofluorimetric assay and analysis of caspase-3/7 activity. Results demonstrated that treatment with PDE4 inhibitors affected HepG2 cell cycle and survival, suggesting that they might be useful as potential adjuvant, chemotherapeutic or chemopreventive agents in hepatocellular carcinoma. J. Cell. Biochem. 118: 1401-1411, 2017. © 2016 Wiley Periodicals, Inc.

Evaluation of the cytotoxicity, cell-cycle arrest, and apoptotic induction by Euphorbia hirta in MCF-7 breast cancer cells

CONTEXT

Euphorbia hirta L. (Euphorbiaceae) has been used as a folk remedy in Southeast Asia for the treatment of various ailments.

OBJECTIVE

The current study evaluates the cytotoxicity, cell-cycle arrest, and apoptotic induction by E. hirta in MCF-7 breast cancer cells.

MATERIALS AND METHODS

Cytotoxic activity of methanol extract of whole part of E. hirta was determined by the MTT assay at various concentrations ranging from 1.96 to 250.00 µg/mL in MCF-7 cells. Cell morphology was assessed by light and fluorescence microscopy. Apoptosis and cell-cycle distribution were determined by annexin V staining and flow cytometry. DNA fragmentation, caspase activity, and reactive oxygen species (ROS) assays were performed using the commercially available kits. To identify the cytotoxic fraction, E. hirta extract was subjected to bioassay-guided fractionation.

RESULTS

Euphorbia hirta exhibited significant inhibition of the survival of MCF-7 cells and the half inhibitory concentration (IC50) values was 25.26 µg/mL at 24 h. Microscopic studies showed that E. hirta-treated cells exhibited marked morphological features characteristic of apoptosis. Euphorbia hirta extract also had an ignorable influence on the LDH leakage and generating intracellular ROS. The flow cytometry study confirmed that E. hirta extract induced apoptosis in MCF-7 cells. Euphorbia hirta also resulted in DNA fragmentation in MCF-7 cells. Moreover, E. hirta treatment resulted in the accumulation of cells at the S and G2/M phases as well as apoptosis. The caspase activity study revealed that E. hirta extract induced apoptosis through the caspase-3-independent pathway by the activation of caspase-2, 6, 8, and 9. Euphorbia hirta hexane fraction, namely HFsub4 fraction, demonstrated highest activity among all the fractions tested with an IC50 value of 10.01 µg/mL at 24 h.

DISCUSSION AND CONCLUSION

This study revealed that E. hirta induced apoptotic cell death and suggests that E. hirta could be used as an apoptosis-inducing anticancer agent for breast cancer treatment with further detailed studies.

Cell Cycle Control: A System of Interlinking Oscillators

The cell cycle is the sequence of events through which a cell duplicates its genome, grows, and divides. Key cell cycle transitions are driven by oscillators comprising cyclin-dependent kinases and other kinases. Different cell cycle oscillators are inextricably linked to ensure orderly activation of oscillators. A recurring theme in their regulation is the abundance of auto-amplifying loops that ensure switch-like and unidirectional cell cycle transitions. The periodicity of many cell cycle oscillators is choreographed by inherent mechanisms that promote automatic inactivation, often involving dephosphorylation and ubiquitin-mediated protein degradation. These inhibitory signals are subsequently suppressed to enable the next cell cycle to occur. Although the activation and inactivation of cell cycle oscillators are in essence autonomous during the unperturbed cell cycle, a number of checkpoint mechanisms are able to halt the cell cycle until defects are addressed. Together, these mechanisms orchestrate orderly progression of the cell cycle to produce more cells and to safeguard genome integrity.

Elevated cyclin A associated kinase activity promotes sensitivity of metastatic human cancer cells to DNA antimetabolite drug

Drug resistance is a major obstacle in successful systemic therapy of metastatic cancer. We analyzed the involvement of cell cycle regulatory proteins in eliciting response to N (phosphonoacetyl)-L-aspartate (PALA), an inhibitor of de novo pyrimidine synthesis, in two metastatic variants of human cancer cell line MDA-MB-435 isolated from lung (L-2) and brain (Br-1) in nude mouse, respectively. L-2 and Br-l cells markedly differed in their sensitivity to PALA. While both cell types displayed an initial S phase delay/arrest, Br-l cells proliferated but most L-2 cells underwent apoptosis. There was distinct elevation in cyclin A, and phosphorylated Rb proteins concomitant with decreased expression of bcl-2 protein in the PALA treated L-2 cells undergoing apoptosis. Markedly elevated cyclin A associated and cdk2 kinase activities together with increased E2F1-DNA binding were detected in these L-2 cells. Induced ectopic cyclin A expression sensitized Br-l cells to PALA by activating an apoptotic pathway. Our findings demonstrate that elevated expression of cyclin A and associated kinase can activate an apoptotic pathway in cells exposed to DNA antimetabolites. Abrogation of this pathway can lead to resistance against these drugs in metastatic variants of human carcinoma cells.

Genomic and genetic variation in E2F transcription factor-1 in men with nonobstructive azoospermia

OBJECTIVE

To identify gene dosage changes associated with nonobstructive azoospermia (NOA) using array comparative genomic hybridization (aCGH).

DESIGN

Prospective study.

SETTING

Medical school.

PATIENT(S)

One hundred ten men with NOA and 78 fertile controls.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

The study has four distinct analytic components: aCGH, a molecular karyotype that detects copy number variations (CNVs); Taqman CNV assays to validate CNVs; mutation identification by Sanger sequencing; and histological analyses of testicular tissues.

RESULT(S)

A microduplication at 20q11.22 encompassing E2F transcription factor-1 (E2F1) was identified in one of eight men with NOA analyzed using aCGH. CNVs were confirmed and in an additional 102 men with NOA screened using Taqman CNV assays, for a total of 110 NOA men analyzed for CNVs in E2F1. Eight of 110 (7.3%) NOA men had microduplications or microdeletions of E2F1 that were absent in fertile controls.

CONCLUSION(S)

E2F1 microduplications or microdeletions are present in men with NOA (7.3%). Duplications or deletions of E2F1 occur very rarely in the general population (0.011%), but E2F1 gene dosage changes, previously reported only in cancers, are present in a subset of NOA men. These results recapitulate the infertility phenotype seen in mice lacking or overexpressing E2f1.

Antitumor mechanisms when pRb and p53 are genetically inactivated

pRb and p53 are the two major tumor suppressors. Their inactivation is frequent when cancers develop and their reactivation is rationale of most cancer therapeutics. When pRb and p53 are genetically inactivated, cells irreparably lose the antitumor mechanisms afforded by them. Cancer genome studies document recurrent genetic inactivation of RB1 and TP53, and the inactivation becomes more frequent in more advanced cancers. These findings may explain why more advanced cancers are more likely to resist current therapies. Finding successful treatments for more advanced and multi-therapy resistant cancers will depend on finding antitumor mechanisms that remain effective when pRb and p53 are genetically inactivated. Here, we review studies that have begun to make progress in this direction.

Division of labour between Myc and G1 cyclins in cell cycle commitment and pace control

A body of evidence has shown that the control of E2F transcription factor activity is critical for determining cell cycle entry and cell proliferation. However, an understanding of the precise determinants of this control, including the role of other cell-cycle regulatory activities, has not been clearly defined. Here, recognizing that the contributions of individual regulatory components could be masked by heterogeneity in populations of cells, we model the potential roles of individual components together with the use of an integrated system to follow E2F dynamics at the single-cell level and in real time. These analyses reveal that crossing a threshold amplitude of E2F accumulation determines cell cycle commitment. Importantly, we find that Myc is critical in modulating the amplitude, whereas cyclin D/E activities have little effect on amplitude but do contribute to the modulation of duration of E2F activation, thereby affecting the pace of cell cycle progression.

Skp2 suppresses apoptosis in Rb1-deficient tumours by limiting E2F1 activity

One mechanism of tumor suppression by pRb is repressing E2F1. Hence, E2f1 deletion diminishes tumorigenesis following Rb1 loss. However, E2F1 promotes both proliferation and apoptosis. It therefore remains unclear how de-repressed E2F1 promotes tumorigenesis. Another mechanism of pRb function is repressing Skp2 to elevate p27 to arrest proliferation. However, Skp2 deletion induced apoptosis, not proliferation arrest, in Rb1 deficient pituitary tumorigenesis. Here, we show that Rb1 deletion induces higher expression of E2F1 target genes in the absence of Skp2. E2F1 binds less cyclin A but more target promoters when Rb1 is deleted with Skp2 knockout or p27T187A knockin, suggesting that stabilized p27 prevents cyclin A from binding and inhibiting E2F1. In Rb1 deficient pituitary tumorigenesis, Skp2 deletion or p27T187A mutation converts E2F1’s role from proliferative to apoptotic. These findings delineate a pRb-Skp2-p27-cyclin A-E2F1 pathway that determines whether E2F1 is proliferative or apoptotic in Rb1 deficient tumorigenesis.

Systematic identification of transcriptional regulatory modules from protein-protein interaction networks

Transcription factors (TFs) combine with co-factors to form transcriptional regulatory modules (TRMs) that regulate gene expression programs with spatiotemporal specificity. Here we present a novel and generic method (rTRM) for the reconstruction of TRMs that integrates genomic information from TF binding, cell type-specific gene expression and protein–protein interactions. rTRM was applied to reconstruct the TRMs specific for embryonic stem cells (ESC) and hematopoietic stem cells (HSC), neural progenitor cells, trophoblast stem cells and distinct types of terminally differentiated CD4⁺ T cells. The ESC and HSC TRM predictions were highly precise, yielding 77 and 96 proteins, of which ∼75% have been independently shown to be involved in the regulation of these cell types. Furthermore, rTRM successfully identified a large number of bridging proteins with known roles in ESCs and HSCs, which could not have been identified using genomic approaches alone, as they lack the ability to bind specific DNA sequences. This highlights the advantage of rTRM over other methods that ignore PPI information, as proteins need to interact with other proteins to form complexes and perform specific functions. The prediction and experimental validation of the co-factors that endow master regulatory TFs with the capacity to select specific genomic sites, modulate the local epigenetic profile and integrate multiple signals will provide important mechanistic insights not only into how such TFs operate, but also into abnormal transcriptional states leading to disease.

Transcriptional networks controlling the cell cycle

In this work, we map the transcriptional targets of 107 previously identified Drosophila genes whose loss caused the strongest cell-cycle phenotypes in a genome-wide RNA interference screen and mine the resulting data computationally. Besides confirming existing knowledge, the analysis revealed several regulatory systems, among which were two highly-specific and interconnected feedback circuits, one between the ribosome and the proteasome that controls overall protein homeostasis, and the other between the ribosome and Myc/Max that regulates the protein synthesis capacity of cells. We also identified a set of genes that alter the timing of mitosis without affecting gene expression, indicating that the cyclic transcriptional program that produces the components required for cell division can be partially uncoupled from the cell division process itself. These genes all have a function in a pathway that regulates the phosphorylation state of Cdk1. We provide evidence showing that this pathway is involved in regulation of cell size, indicating that a Cdk1-regulated cell size checkpoint exists in metazoans.

Canonical and atypical E2Fs regulate the mammalian endocycle

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

S phase-coupled E2f1 destruction ensures homeostasis in proliferating tissues

Precise control of cell cycle regulators is critical for normal development and tissue homeostasis. E2F transcription factors are activated during G1 to drive the G1-S transition and are then inhibited during S phase by a variety of mechanisms. Here, we genetically manipulate the single Drosophila activator E2F (E2f1) to explore the developmental requirement for S phase–coupled E2F down-regulation. Expression of an E2f1 mutant that is not destroyed during S phase drives cell cycle progression and causes apoptosis. Interestingly, this apoptosis is not exclusively the result of inappropriate cell cycle progression, because a stable E2f1 mutant that cannot function as a transcription factor or drive cell cycle progression also triggers apoptosis. This observation suggests that the inappropriate presence of E2f1 protein during S phase can trigger apoptosis by mechanisms that are independent of E2F acting directly at target genes. The ability of S phase-stabilized E2f1 to trigger apoptosis requires an interaction between E2f1 and the Drosophila pRb homolog, Rbf1, and involves induction of the pro-apoptotic gene, hid. Simultaneously blocking E2f1 destruction during S phase and inhibiting the induction of apoptosis results in tissue overgrowth and lethality. We propose that inappropriate accumulation of E2f1 protein during S phase triggers the elimination of potentially hyperplastic cells via apoptosis in order to ensure normal development of rapidly proliferating tissues.

Rapidly growing tissues provide an excellent opportunity to study the careful balance between cell proliferation and apoptosis needed for normal organ structure and function in developing organisms. We present evidence that a transcription factor critical for regulating progression of the Drosophila melanogaster cell cycle, E2f1, serves also as an indicator of normal tissue development. E2f1 activation during G1 phase of the cell cycle triggers entry into S phase. E2f1 activity is then rapidly inhibited during S phase by a mechanism that couples E2f1 proteolysis directly to DNA synthesis. Expression during larval development of an S phase-stabilized form of E2f1 results in apoptosis in rapidly proliferating adult wing precursor cells, even when this stabilized E2f1 protein is mutated such that it cannot induce transcription or cell cycle progression. Preventing the ability of S phase-stabilized E2f1 to induce apoptosis results in massive tissue overgrowth. We propose that aberrant E2f1 accumulation during S phase triggers apoptosis in order to remove potentially hyper-proliferative cells and to maintain homeostasis during tissue growth.

Killing of Kras-mutant colon cancer cells via Rac-independent actin remodeling by the βGBP cytokine, a physiological PI3K inhibitor therapeutically effective in vivo

Activating mutations in Kras are the most frequent mutations in human cancer. They define a subset of patients who do not respond to current therapies and for whom prognosis is poor. Oncogenic Kras has been shown to deregulate numerous signaling pathways of which the most intensively studied are the Ras/extracellular signal-regulated kinase cascade and the phosphoinositide 3-kinase (PI3K)/Akt cascade. However, to date, there are no effective targeted therapies in the clinic against Kras-mutant cancers. Here, we report that the β-galactoside-binding protein (βGBP) cytokine, a physiologic inhibitor of class I PI3Ks, is a potent activator of apoptosis in Kras-mutant colorectal cancer cells, even when coharboring mutant-activated PIK3CA. Our study unveils an elective route to intrinsic and extrinsic apoptosis, which involves the cytoskeleton. Early events are inhibition of PI3K activity and Rac-independent actin rearrangement assignable to phosphoinositide changes at the plasma membrane. Cyclin E deregulation, arrest of DNA synthesis, and checkpoint kinase 2 activation underscore events critical to the activation of an intrinsic apoptotic program. Clustering of CD95/Fas death receptors underscore events critical to the activation of extrinsic apoptosis. In nude mice, we present the first evidence that xenograft tumor development is strongly inhibited by Hu-r-βGBP. Taken together, our results open a new therapeutic opportunity to a subset of patients refractive to current treatments. This first demonstration of therapeutic efficacy against Kras-mutant colon cancer suggests that Hu-r-βGBP may also be therapeutically effective against other cancers harboring activating Ras mutations as well as PIK3CA mutations.

Tension and Robustness in Multitasking Cellular Networks

Cellular networks multitask by exhibiting distinct, context-dependent dynamics. However, network states (parameters) that generate a particular dynamic are often sub-optimal for others, defining a source of “tension” between them. Though multitasking is pervasive, it is not clear where tension arises, what consequences it has, and how it is resolved. We developed a generic computational framework to examine the source and consequences of tension between pairs of dynamics exhibited by the well-studied RB-E2F switch regulating cell cycle entry. We found that tension arose from task-dependent shifts in parameters associated with network modules. Although parameter sets common to distinct dynamics did exist, tension reduced both their accessibility and resilience to perturbation, indicating a trade-off between “one-size-fits-all” solutions and robustness. With high tension, robustness can be preserved by dynamic shifting of modules, enabling the network to toggle between tasks, and by increasing network complexity, in this case by gene duplication. We propose that tension is a general constraint on the architecture and operation of multitasking biological networks. To this end, our work provides a framework to quantify the extent of tension between any network dynamics and how it affects network robustness. Such analysis would suggest new ways to interfere with network elements to elucidate the design principles of cellular networks.

Multitasking pervades our daily lives. For example, the technological devices that we increasingly rely upon are now engineered with such multifunctionality or “integration” in mind. Similarly, cellular networks also multitask in that they generate multiple, distinct dynamics according to their operating context. Here we show that differences in parameter spaces that underlie different dynamics thus cause a “tension”, which ultimately constrains network operation. In particular, our analysis reveals that tension negatively impacts robustness by reducing accessibility of parameters able to accomplish two tasks and reduces their ability to withstand perturbations. The presence of tension and its negative impact on network robustness represents a fundamental, generic constraint on the operation of different multitasking networks.

UHRF1 phosphorylation by cyclin A2/cyclin-dependent kinase 2 is required for zebrafish embryogenesis

Although UHRF1 is essential for many epigenetic marks, the mechanism that regulates UHRF1 is not understood. This study shows that a key component of the cell cycle machinery—cyclin-dependent kinase 2/cyclin A2—phosphorylates UHRF1 and that this phosphorylation is essential for early zebrafish development.

Ubiquitin-like, containing PHD and RING finger domains 1 (uhrf1) is regulated at the transcriptional level during the cell cycle and in developing zebrafish embryos. We identify phosphorylation as a novel means of regulating UHRF1 and demonstrate that Uhrf1 phosphorylation is required for gastrulation in zebrafish. Human UHRF1 contains a conserved cyclin-dependent kinase 2 (CDK2) phosphorylation site at Ser-661 that is phosphorylated in vitro by CDK2 partnered with cyclin A2 (CCNA2), but not cyclin E. An antibody specific for phospho-Ser-661 recognizes UHRF1 in both mammalian cancer cells and in nontransformed zebrafish cells, but not in zebrafish bearing a mutation in ccna2. Depleting Uhrf1 from zebrafish embryos by morpholino injection causes arrest before gastrulation and early embryonic death. This phenotype is rescued by wild-type UHRF1, but not by UHRF1 in which the phospho-acceptor site is mutated, demonstrating that UHRF1 phosphorylation is essential for embryogenesis. UHRF1 was detected in the nucleus and cytoplasm, whereas nonphosphorylatable UHRF1 is unable to localize to the cytoplasm, suggesting the importance of localization in UHRF1 function. Together, these data point to an essential role for UHRF1 phosphorylation by CDK/CCNA2 during early vertebrate development.

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.

S-phase-coupled apoptosis in tumor suppression

DNA replication is essential for accurate transmission of genomic information from parental to daughter cells. DNA replication is licensed once per cell division cycle. This process is highly regulated by both positive and negative regulators. Over-replication, under-replication, as well as DNA damage in a cell all induce the activation of checkpoint control pathways such as ATM/ATR, CHK kinases, and the tumor suppressor protein p53, which provide "damage controls" via either DNA repairs or apoptosis. This review focuses on accumulating evidence, with the emphasis on recently discovered Killin, that S-phase checkpoint control is crucial for a mammalian cell to make a life and death decision in order to safeguard genome integrity.

The cyclin-dependent kinase inhibitor SCH 727965 (dinacliclib) induces the apoptosis of osteosarcoma cells

Although rare, osteosarcoma is an aggressive cancer that often metastasizes to the lungs. Toward the goal of developing new treatment options for osteosarcoma, we show that the cyclin-dependent kinase (CDK) inhibitor SCH 727965 (SCH) induces the apoptosis of several osteosarcoma cell lines including those resistant to doxorubicin and dasatinib. Cell lines prepared in our laboratory from patients who had received adjuvant chemotherapy and explants derived from a human osteosarcoma xenograft in mice were also responsive to SCH. Apoptosis occurred at low nanomolar concentrations of SCH, as did CDK inhibition, and was p53-independent. SCH activated the mitochondrial pathway of apoptosis as evidenced by caspase-9 cleavage and accumulation of cytoplasmic cytochrome c. Amounts of the apoptotic proteins Bax and Bim increased in mitochondria, whereas amounts of the antiapoptotic proteins Mcl-1 and Bcl-x(L) declined. Osteosarcoma cells apoptosed when codepleted of CDK1 and CDK2 but not when depleted of other CDK combinations. We suggest that SCH triggers the apoptosis of osteosarcoma cells by inactivating CDK1 and CDK2 and that SCH may be useful for treatment of drug-resistant osteosarcomas. SCH also induced the apoptosis of other sarcoma types but not of normal quiescent osteoblasts or fibroblasts.

E2F-1 regulation by an unusual DNA damage-responsive DP partner subunit

E2F activity is negatively regulated by retinoblastoma protein (pRb) through binding to the E2F-1 subunit. Within the E2F heterodimer, DP proteins are E2F partner subunits that allow proper cell cycle progression. In contrast to the other DP proteins, the newest member of the family, DP-4, downregulates E2F activity. In this study we report an unexpected role for DP-4 in regulating E2F-1 activity during the DNA damage response. Specifically, DP-4 is induced in DNA-damaged cells, upon which it binds to E2F-1 as a non-DNA-binding E2F-1/DP-4 complex. Consequently, depleting DP-4 in cells re-instates E2F-1 activity that coincides with increased levels of chromatin-bound E2F-1, E2F-1 target gene expression and associated apoptosis. Mutational analysis of DP-4 highlighted a C-terminal region, outside the DNA-binding domain, required for the negative control of E2F-1 activity. Our results define a new pathway, which acts independently of pRb and through a biochemically distinct mechanism, involved in negative regulation of E2F-1 activity.

Interferon-inducible IFI16, a negative regulator of cell growth, down-regulates expression of human telomerase reverse transcriptase (hTERT) gene

Background

Increased levels of interferon (IFN)-inducible IFI16 protein (encoded by the IFI16 gene located at 1q22) in human normal prostate epithelial cells and diploid fibroblasts (HDFs) are associated with the onset of cellular senescence. However, the molecular mechanisms by which the IFI16 protein contributes to cellular senescence-associated cell growth arrest remain to be elucidated. Here, we report that increased levels of IFI16 protein in normal HDFs and in HeLa cells negatively regulate the expression of human telomerase reverse transcriptase (hTERT) gene.

Methodology/Principal Findings

We optimized conditions for real-time PCR, immunoblotting, and telomere repeat amplification protocol (TRAP) assays to detect relatively low levels of hTERT mRNA, protein, and telomerase activity that are found in HDFs. Using the optimized conditions, we report that treatment of HDFs with inhibitors of cell cycle progression, such as aphidicolin or CGK1026, which resulted in reduced steady-state levels of IFI16 mRNA and protein, was associated with increases in hTERT mRNA and protein levels and telomerase activity. In contrast, knockdown of IFI16 expression in cells increased the expression of c-Myc, a positive regulator of hTERT expression. Additionally, over-expression of IFI16 protein in cells inhibited the c-Myc-mediated stimulation of the activity of hTERT-luc-reporter and reduced the steady-state levels of c-Myc and hTERT.

Conclusions/Significance

These data demonstrated that increased levels of IFI16 protein in HDFs down-regulate the expression of hTERT gene. Our observations will serve basis to understand how increased cellular levels of the IFI16 protein may contribute to certain aging-dependent diseases.

Synthetic lethality: a framework for the development of wiser cancer therapeutics

The challenge in medical oncology has always been to identify compounds that will kill, or at least tame, cancer cells while leaving normal cells unscathed. Most chemotherapeutic agents in use today were selected primarily for their ability to kill rapidly dividing cancer cells grown in cell culture and in mice, with their selectivity determined empirically during subsequent animal and human testing. Unfortunately, most of the drugs developed in this way have relatively low therapeutic indices (low toxic dose relative to the therapeutic dose). Recent advances in genomics are leading to a more complete picture of the range of mutations, both driver and passenger, present in human cancers. Synthetic lethality provides a conceptual framework for using this information to arrive at drugs that will preferentially kill cancer cells relative to normal cells. It also provides a possible way to tackle 'undruggable' targets. Two genes are synthetically lethal if mutation of either gene alone is compatible with viability but simultaneous mutation of both genes leads to death. If one is a cancer-relevant gene, the task is to discover its synthetic lethal interactors, because targeting these would theoretically kill cancer cells mutant in the cancer-relevant gene while sparing cells with a normal copy of that gene. All cancer drugs in use today, including conventional cytotoxic agents and newer 'targeted' agents, target molecules that are present in both normal cells and cancer cells. Their therapeutic indices almost certainly relate to synthetic lethal interactions, even if those interactions are often poorly understood. Recent technical advances enable unbiased screens for synthetic lethal interactors to be undertaken in human cancer cells. These approaches will hopefully facilitate the discovery of safer, more efficacious anticancer drugs that exploit vulnerabilities that are unique to cancer cells by virtue of the mutations they have accrued during tumor progression.

Mechanisms of resistance to interferon-gamma-mediated cell growth arrest in human oral squamous carcinoma cells

Interferon-gamma (IFNgamma) has an antiproliferative effect on a variety of tumor cells. However, many tumor cells resist treatment with IFNs. Here, we show that IFNgamma fails to inhibit the growth of some types of oral squamous cell carcinoma (OSCC) cells that possess a fully functional IFNgamma/STAT1 (signal transducer and activator of transcription-1) signaling pathway. IFNgamma inhibited the growth of the HSC-2, HSC-3, and HSC-4 OSCC cell lines. However, Ca9-22 cells were resistant to IFNgamma despite having intact STAT1-dependent signaling, such as normal tyrosine phosphorylation, DNA binding activity, and transcriptional activity of STAT1. The growth inhibition of HSC-2 cells resulted from S-phase arrest of the cell cycle. IFNgamma inhibited cyclin A2 (CcnA2)-associated kinase activity, which correlated with the IFNgamma-mediated down-regulation of CcnA2 and Cdk2 expression at both the transcriptional and post-transcriptional level in HSC-2 cells but not in Ca9-22 cells. RNAi-mediated knockdown of CcnA2 and Cdk2 resulted in growth inhibition in both cell lines. These results indicate that the resistance of OSCC to IFNgamma is not due simply to the deficiency in STAT1-dependent signaling but results from a defect in the signaling component that mediates this IFNgamma-induced down-regulation of CcnA2 and Cdk2 expression at the transcriptional and post-transcriptional levels.

The E2F family and the role of E2F1 in apoptosis

The E2F family of transcription factors plays a pivotal role in the regulation of cellular proliferation and differentiation. Although the deregulation of E2Fs is considered an oncogenic event that predisposes immortalized cells to transformation, paradoxically, E2F1 is also equipped with an ability to induce apoptosis under certain cellular contexts. It has become evident that E2Fs, in particular E2F1, participate in many aspects of the apoptotic process, either by acting alone or in cooperation with other factors, such as p53, to protect organisms from tumor development in the face of oncogenic lesions. Given the frequent inactivation of p53 in human cancers, the E2F1-induced apoptosis pathway is rapidly gaining attention as a key mechanism to compensate the loss of p53 in human tumors. In this review, we will focus on the recent progress in our understanding of E2F1-mediated apoptosis and discuss how these discoveries can be translated into potential therapeutic intervention.

Intrinsic negative cell cycle regulation provided by PIP box- and Cul4Cdt2-mediated destruction of E2f1 during S phase

E2F transcription factors are key regulators of cell proliferation that are inhibited by pRb family tumor suppressors. pRb-independent modes of E2F inhibition have also been described, but their contribution to animal development and tumor suppression is unclear. Here, we show that S phase-specific destruction of Drosophila E2f1 provides a novel mechanism for cell cycle regulation. E2f1 destruction is mediated by a PCNA-interacting-protein (PIP) motif in E2f1 and the Cul4(Cdt2) E3 ubiquitin ligase and requires the Dp dimerization partner but not direct Cdk phosphorylation or Rbf1 binding. E2f1 lacking a functional PIP motif accumulates inappropriately during S phase and is more potent than wild-type E2f1 at accelerating cell cycle progression and inducing apoptosis. Thus, S phase-coupled destruction is a key negative regulator of E2f1 activity. We propose that pRb-independent inhibition of E2F during S phase is an evolutionarily conserved feature of the metazoan cell cycle that is necessary for development.

Cyclin C and cyclin dependent kinases 1, 2 and 3 in thrombin-induced neuronal cell cycle progression and apoptosis

The extent to which neurons proceed into the cell cycle and the mechanisms whereby cell cycle re-entry leads to apoptosis vary in response to agonists. We previously showed upregulation of early G1 regulators in thrombin-treated neurons yet neurons did not proceed to S phase but to apoptosis. The objective of this study is to explore mechanisms which might prevent S phase entry and promote apoptosis in thrombin-treated neurons. Cultured rat brain neurons are exposed to thrombin (200nM) for 30min to 4.5h and the expression of cyclin C, cyclin dependent kinases (cdk1, cdk2, cdk3, cdk8) and the cell cycle inhibitor p27 assessed. Our data show a simultaneous decrease of both cyclin C and cdk3 proteins soon after thrombin treatment. The decrease in cyclin C also correlates with decreases in cdk1 and cdk2, at both mRNA and protein levels. There is no change in expression of cdk8 or the cell cycle inhibitor p27 in response to thrombin treatment. These results suggest that decreases in G1-S regulators cyclin C and cdks 3, cdk2 and cdk1 in response to thrombin could make conditions unfavorable for S phase entry and favor neuronal apoptosis.

Sustained NF-kappaB activation produces a short-term cell proliferation block in conjunction with repressing effectors of cell cycle progression controlled by E2F or FoxM1

NF-kappaB transcription factors induce a host of genes involved in pro-inflammatory/stress-like responses; but the collateral effects and consequences of sustained NF-kappaB activation on other cellular gene expression programming remain less well understood. Here enforced expression of a constitutively active IKKbeta T-loop mutant (IKKbetaca) drove murine fibroblasts into transient growth arrest that subsided within 2-3 weeks of continuous culture. Proliferation arrest was associated with a G1/S phase block in immortalized and primary early passage MEFs. Molecular analysis in immortalized MEFs revealed that inhibition of cell proliferation in the initial 1-2 weeks after their IKKbetaca retroviral infection was linked to the transient, concerted repression of essential cell cycle effectors that are known targets of either E2F or FoxM1. Co-expression of a phosphorylation resistant IkappaBalpha super repressor and IKKbetaca abrogated growth arrest and cell cycle effector repression, thereby linking IKKbetaca's effects to canonical NF-kappaB activation. Transient growth arrest of IKKbetaca cells was associated with enhanced p21 (cyclin-dependent kinase inhibitor 1A) protein expression, due in part to transcriptional activation by NF-kappaB and also likely due to strong repression of Skp2 and Csk1, both of which are FoxM1 direct targets mediating proteasomal dependent p21 turnover. Ablation of p21 in immortalized MEFs reduced their IKKbetaca mediated growth suppression. Moreover, trichostatin A inhibition of HDACs alleviated the repression of E2F and FoxM1 targets induced by IKKbetaca, suggesting chromatin mediated gene silencing in IKKbetaca's short term repressive effects on E2F and FoxM1 target gene expression.

Comparative proteomic analysis reveals differentially expressed proteins regulated by a potential tumor promoter, BRE, in human esophageal carcinoma cells

Esophageal tumorigenesis is a complex and cascading process, involving the interaction of many genes and proteins. In this study, we have used the comparative proteomic approach to identify tumor-associated proteins and explore the carcinogenic mechanisms. Two-dimensional electrophoresis (2-DE) and MALDI-TOF MS analysis of esophageal carcinoma and control cells revealed 10 proteins that were upregulated. A further 10 proteins were downregulated. Among these 20 differentially expressed proteins, brain and reproductive organ-expressed (BRE) protein was identified as a potential tumor promoter. It was high expressed by the esophageal carcinoma cells, as confirmed by RT-PCR and immunoblotting. BRE has been reported to be a stress-responsive protein. To gain further insight into its function, BRE expression was silenced in esophageal carcinoma cells using BRE-specific small interference RNA. It was discovered that silencing BRE expression downregulated prohibitin expression, but upregulated tumor-suppressor p53 expression. Furthermore, cyclin A and CDK2 expressions were suppressed suggesting that BRE inhibited cell proliferation. These results implied that BRE plays a significant role in mediating antiapoptotic and proliferative responses in esophageal carcinoma cells.

CRM1-mediated nuclear export is required for 26 S proteasome-dependent degradation of the TRIP-Br2 proto-oncoprotein

Overexpression of the proto-oncogene TRIP-Br2 (SERTAD2) has been shown to induce E2F activity and promote tumorigenesis, whereas ablation of TRIP-Br2 arrests cell proliferation. Timely degradation of many cell cycle regulators is fundamental to the maintenance of proper cell cycle progression. Here we report novel mechanism(s) that govern the tight regulation of TRIP-Br2 levels during cell cycle progression. TRIP-Br2 was observed to be a short-lived protein in which the expression level peaks at the G(1)/S boundary. TRIP-Br2 accumulated in cells treated with 26 S proteasome inhibitors. Co-immunoprecipitation studies revealed that TRIP-Br2 forms ubiquitin conjugates. In silico analysis identified a putative leucine-rich nuclear export signal (NES) motif that overlaps with the PHD-Bromo interaction domain in the acidic C-terminal transactivation domain (TAD) of TRIP-Br2. This NES motif is highly conserved in widely divergent species and in all TRIP-Br family members. TRIP-Br2 was shown to be stabilized in G(2)/M phase cells through nuclear entrapment, either by deletion of the acidic C-terminal TAD, which includes the NES motif, or by leptomycin B-mediated inhibition of the CRM1-dependent nuclear export machinery. Mutation of leucine residue 238 of this NES motif abolished the interaction between CRM1 and TRIP-Br2, as well as the nuclear export of TRIP-Br2 and its subsequent 26 S proteasome-dependent degradation. These data suggest that CRM1-mediated nuclear export may be required for the proper execution of ubiquitin-proteasome-dependent degradation of TRIP-Br2.

Disrupting Skp2-cyclin A interaction with a blocking peptide induces selective cancer cell killing

Skp2 fulfills the definition of an oncoprotein with its frequent overexpression in cancer cells and oncogenic activity in various laboratory assays and therefore is a potential cancer therapy target. The best-known function of Skp2 is that of an F-box protein of the SCF(Skp2)-Roc1 E3 ubiquitin ligase targeting the cyclin-dependent kinase inhibitor p27(Kip1). Knockdown of Skp2 generally leads to accumulation of p27 but its effects on cancer cells are less certain. Another function of Skp2 is its stable interaction with cyclin A, which directly protects cyclin A from inhibition by p27 in in vitro kinase assays. Here, we report that an 18-residue blocking peptide of Skp2-cyclin A interaction can indirectly inhibit cyclin A/Cdk2 kinase activity dependent on the presence of p27 in in vitro kinase assays. Transmembrane delivery of this blocking peptide can induce cell death in a panel of four cancer cell lines in which Skp2 knockdown only have mild inhibitory effects. This Skp2-cyclin A interaction blocking peptide can synergize with a previously identified E2F1-derived LDL peptide, which blocks its access to cyclin A, in killing cancer cells. IC(50) of the Skp2-cyclin A blocking peptide correlated with abundance of Skp2, its intended target, in cancer cells. These results suggest that Skp2-cyclin A interaction plays an important role in cancer cell survival and is an attractive target for cancer drug discovery.

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.

Cell cycle dysregulation in oral cancer

The dysregulation of the molecular events governing cell cycle control is emerging as a central theme of oral carcinogenesis. Regulatory pathways responding to extracellular signaling or intracellular stress and DNA damage converge on the cell cycle apparatus. Abrogation of mitogenic and anti-mitogenic response regulatory proteins, such as the retinoblastoma tumor suppressor protein (pRB), cyclin D1, cyclin-dependent kinase (CDK) 6, and CDK inhibitors (p21(WAF1/CIP1), p27(KIP1), and p16(INK4a)), occur frequently in human oral cancers. Cellular responses to metabolic stress or genomic damage through p53 and related pathways that block cell cycle progression are also altered during oral carcinogenesis. In addition, new pathways and cell cycle regulatory proteins, such as p12(DOC-1), are being discovered. The multistep process of oral carcinogenesis likely involves functional alteration of cell cycle regulatory members combined with escape from cellular senescence and apoptotic signaling pathways. Detailing the molecular alterations and understanding the functional consequences of the dysregulation of the cell cycle apparatus in the malignant oral keratinocyte will uncover novel diagnostic and therapeutic approaches.