The differential binding of E2F and CDF repressor complexes contributes to the timing of cell cycle-regulated transcription

Ang-1 and VEGF: central regulators of angiogenesis

Angiopoietin-1 (Ang-1) and Vascular Endothelial Growth Factor (VEGF) are central regulators of angiogenesis and are often inactivated in various cardiovascular diseases. VEGF forms complexes with ETS transcription factor family and exerts its action by downregulating multiple genes. Among the target genes of the VEGF-ETS complex, there are a significant number encoding key angiogenic regulators. Phosphorylation of the VEGF-ETS complex releases transcriptional repression on these angiogenic regulators, thereby promoting their expression. Ang-1 interacts with TEK, and this phosphorylation release can be modulated by the Ang-1-TEK signaling pathway. The Ang-1-TEK pathway participates in the transcriptional activation of VEGF genes. In summary, these elements constitute the Ang-1-TEK-VEGF signaling pathway. Additionally, Ang-1 is activated under hypoxic and inflammatory conditions, leading to an upregulation in the expression of TEK. Elevated TEK levels result in the formation of the VEGF-ETS complex, which, in turn, downregulates the expression of numerous angiogenic genes. Hence, the Ang-1-dependent transcriptional repression is indirect. Reduced expression of many target genes can lead to aberrant angiogenesis. A significant overlap exists between the target genes regulated by Ang-1-TEK-VEGF and those under the control of the Ang-1-TEK-TSP-1 signaling pathway. Mechanistically, this can be explained by the replacement of the VEGF-ETS complex with the TSP-1 transcriptional repression complex at the ETS sites on target gene promoters. Furthermore, VEGF possesses non-classical functions unrelated to ETS and DNA binding. Its supportive role in TSP-1 formation may be exerted through the VEGF-CRL5-VHL-HIF-1α-VH032-TGF-β-TSP-1 axis. This review assesses the regulatory mechanisms of the Ang-1-TEK-VEGF signaling pathway and explores its significant overlap with the Ang-1-TEK-TSP-1 signaling pathway.

Insulin Protects Cardiac Myocytes from Doxorubicin Toxicity by Sp1-Mediated Transactivation of Survivin

Insulin inhibits ischemia/reperfusion-induced myocardial apoptosis through the PI3K/Akt/mTOR pathway. Survivin is a key regulator of anti-apoptosis against doxorubicin-induced cardiotoxicity. Insulin increases survivin expression in cardiac myocytes to mediate cytoprotection. However, the mechanism by which survivin mediates the protective effect of insulin against doxorubicin-associated injury remains to be determined. In this study, we demonstrated that pretreatment of H9c2 cardiac myocytes with insulin resulted in a significant decrease in doxorubicin-induced apoptotic cell death by reducing cytochrome c release and caspase-3 activation. Doxorubicin-induced reduction of survivin mRNA and protein levels was also significantly perturbed by insulin pretreatment. Reducing survivin expression with survivin siRNA abrogated insulin-mediated inhibition of caspase-3 activation, suggesting that insulin signals to survivin inhibited caspase-3 activation. Interestingly, pretreatment of H9c2 cells with insulin or MG132, a proteasome inhibitor, inhibited doxorubicin-induced degradation of the transcription factor Sp1. ChIP assay showed that pretreatment with insulin inhibited doxorubicin-stimulated Sp1 dissociation from the survivin promoter. Finally using pharmacological inhibitors of the PI3K pathway, we showed that insulin-mediated activation of the PI3K/Akt/mTORC1 pathway prevented doxorubicin-induced proteasome-mediated degradation of Sp1. Taken together, insulin pretreatment confers a protective effect against doxorubicin-induced cardiotoxicity by promoting Sp1-mediated transactivation of survivin to inhibit apoptosis. Our study is the first to define a role for survivin in cellular protection by insulin against doxorubicin-associated injury and show that Sp1 is a critical factor in the transcriptional regulation of survivin.

Critical role of a survivin/TGF-β/mTORC1 axis in IGF-I-mediated growth of prostate epithelial cells

Survivin is a unique member of the inhibitor of apoptosis (IAP) proteins that is overexpressed in numerous cancers through poorly defined mechanisms. One such mechanism may be through constitutive activation of the insulin-like growth factor-I (IGF-I) signaling pathway, implicated in the development and progression of prostate cancer. Using the pre-neoplastic NRP-152 rat prostate cell line as a model, we showed that IGF-I induces Survivin expression, and that silencing Survivin by lentiviral-mediated small hairpin RNA (shRNA) represses IGF-I-stimulated cell growth, implicating Survivin as a mediator of this growth response. Moreover, our data support that the induction of Survivin by IGF-I occurs through a transcriptional mechanism that is mediated in part by the PI3K/Akt/mTORC1 pathway. Use of various Survivin promoter-luciferase constructs revealed that the CDE and CHR response elements in the proximal region of the Survivin promoter are involved in this IGF-I response. Transforming growth factor (TGF-β) signaling antagonists similarly activated the Surivin promoter and rendered cells refractory to further promoter activation by IGF-I. IGF-I suppressed levels of phospho-Smads 2 and 3 with kinetics similar to that of Survivin induction. Suppression of TGF-β signaling, either by TGF-β receptor kinase inhibitors or by silencing Smads 2 and 3, induced Survivin expression and promoted cell growth similar to that induced by IGF-I. TGF-β receptor antagonists also rescued cells from down-regulation of Survivin expression and growth suppression by pharmacological inhibitors of PI3K, Akt, MEK and mTOR. Sh-RNA gene silencing studies suggest that mTORC1 induces while mTORC2 represses the expression of Survivin by IGF-I. Taken together, these results suggest that IGF-I signaling through a PI3K/Akt/mTORC1 mechanism elevates expression of Survivin and promotes growth of prostate epithelial cells by suppressing Smad-dependent autocrine TGF-β signaling.

LIN54 is an essential core subunit of the DREAM/LINC complex that binds to the cdc2 promoter in a sequence-specific manner

Recently, the conserved human LINC/DREAM complex has been described as an important regulator of cell cycle genes. LINC consists of a core module that dynamically associates with E2F transcription factors, p130 and the B-MYB transcription factor in a cell cycle-dependent manner. In this study, we analyzed the evolutionary conserved LIN54 subunit of LINC. We found that LIN54 is required for cell cycle progression. Protein interaction studies demonstrated that a predicted helix-coil-helix motif is required for the interaction of LIN54 with p130 and B-MYB. In addition, we found that the cysteine-rich CXC domain of LIN54 is a novel DNA-binding domain that binds to the cdc2 promoter in a sequence-specific manner. We identified two binding sites for LIN54 in the cdc2 promoter, one of which overlaps with the cell cycle homology region at the transcriptional start site. Gel shift assays suggested that, in quiescent cells, the binding of LIN54 at the cell cycle homology region is stabilized by the binding of E2F4 to the adjacent cell cycle-dependent element. Our data demonstrate that LIN54 is an important and integral subunit of LINC.

Rb/E2F4 and Smad2/3 link survivin to TGF-beta-induced apoptosis and tumor progression

Survivin is a prosurvival protein overexpressed in many cancers through mechanisms that remain poorly explored, and is implicated in control of tumor progression and resistance to cancer chemotherapeutics. Here, we report a critical role for survivin in the induction of apoptosis by transforming growth factor-beta (TGF-beta). We show that TGF-beta rapidly downregulates survivin expression in prostate epithelial cells, through a unique mechanism of transcriptional suppression involving Smads 2 and 3, Rb/E2F4, and the cell-cycle repressor elements CDE and CHR. This TGF-beta response is triggered through a Smad2/3-dependent hypophosphorylation of Rb and the subsequent association of the Rb/E2F4 repressive complex to CDE/CHR elements in the proximal region of the survivin promoter. Viral-mediated gene delivery experiments, involving overexpressing or silencing survivin, reveal critical roles of survivin in apoptosis induced by TGF-beta alone or in cooperation with cancer therapeutic agents. We propose a novel TGF-beta/Rb/survivin axis with a putative role in the functional switch of TGF-beta from tumor suppressor to tumor promoter.

Cell cycle-dependent regulation of the human aurora B promoter

Aurora B is an important regulator of mitosis, and its mRNA and protein levels are tightly regulated during the cell cycle. In this study, we cloned the 5' flanking region of the human aurora B gene and characterized its promoter activity. Two major transcription initiation sites were identified by primer extension. aurora B promoter activity was upregulated during M phase, and its cell cycle-dependent element (CDE) and cell cycle-gene homology region (CHR) upstream of the transcription initiation sites regulated the cell cycle-dependent promoter activity. Several CDE-binding protein complexes were identified using the electrophoretic mobility shift assay. Using the biotin-streptavidin pull-down assay, binding of E2F-1, E2F-4, and DP-2, but not of DP-1, to the CDE was detected. These results demonstrate that aurora B mRNA level is regulated by CDE-CHR and that a subset of E2F/DP family proteins binds to the CDE.

A B-myb promoter corepressor site facilitates in vivo occupation of the adjacent E2F site by p107 x E2F and p130 x E2F complexes

Transcription from the B-myb (MybL2 gene) promoter is strictly cell cycle-regulated by repression mediated through an E2F site during G(0)/early G(1). We report here the characterization of a corepressor site (downstream repression site (DRS)) required for this activity that is closely linked to the E2F site. Systematic mutagenesis of the DRS enabled a consensus to be derived, and it is notable that this sequence is compatible with cell cycle gene homology region sequences associated with cell cycle-dependent elements in the cyclin A, cdc2, and CDC25C promoters. The B-myb promoter is inappropriately active during G(0) in mouse embryo fibroblasts lacking the p107 and p130 pocket proteins, and we show that the ability of transfected p107 and p130 to re-impose repression on the promoter is dependent on the DRS. In contrast, transfected Rb was unable to repress the B-myb promoter. Consistent with the notion that Rb.E2F complexes are unable to bind the B-myb promoter E2F site in vivo, footprinting showed that this site is unoccupied in cells lacking p107 and p130. Chromatin immunoprecipitation assays showed a requirement for the DRS in recruiting p107 and p130 complexes to the B-myb promoter, indicating that in vivo the DRS governs the occupancy of the adjacent E2F site by transcriptional repressors.

E2F sites that can interact with E2F proteins cloned from rice are required for meristematic tissue-specific expression of rice and tobacco proliferating cell nuclear antigen promoters

Plants have recently been found to have E2F-like and Rb-like proteins, regulators responsible for the G1(G0)-S phase transition of the cell cycle in animals. Here we show that E2F is involved in transcription of plant genes for proliferating cell nuclear antigen (PCNA), which is required for DNA replication. Potential E2F binding sites found in the rice PCNA promoters mediated transcriptional activation in actively dividing cells and tissues of tobacco, but not transcriptional repression in terminally differentiated tissues, as also observed for the PCF binding sites previously found in the rice promoter. Similar results were obtained from analyses for a PCNA promoter isolated from tobacco, which contained two E2F-like sites, each with a different degree of contribution to the promoter activation. These E2F-like sites except for a rice site were indeed bound specifically by recombinant proteins of rice E2F, OsE2F1 and OsE2F2, and complexes of OsE2F1 with Arabidopsis DP proteins. Furthermore, OsE2F1 had the ability to transactivate an E2F-reporter gene containing the tobacco E2F site on co-expression with an Arabidopsis DP, and the transactivation was greatly enhanced by tagging a canonical nuclear localization signal to OsE2F1, suggesting a nuclear import-mediated regulation of the OsE2F1 function. In addition, we found that a large number of replication- and mismatch repair-associated genes in Arabidopsis contain E2F binding sequences conserved in their predicted promoter regions.

Cyclin A2 transcriptional regulation: modulation of cell cycle control at the G1/S transition by peripheral cues

Several types of cyclins have been identified and among these, cyclin A2 is synthesized in somatic cells at the onset of DNA synthesis as well as during the G2/M transition associated with cyclin-dependent protein kinases 1 and 2. Modulation of cyclin A transcription is due to the interplay between a cell cycle-dependent periodic relief of a transcriptional repression and signals transduced through adenosine 3',5'-cyclic monophosphate, transforming growth factor-beta, and the integrin-mediated pathways. Using primary mouse embryonic fibroblasts from embryos where the genes coding for the protein responsible for susceptibility to retinoblastoma (pRB) and the related p107 and p130 proteins had been individually inactivated, we showed that cyclin A is a functional target of pRB-mediated cell cycle arrest. The factors involved are discussed.

Cell cycle regulation of the tobacco ribonucleotide reductase small subunit gene is mediated by E2F-like elements

Ribonucleotide reductase (RNR) is a key enzyme involved in the DNA synthesis pathway. The RNR-encoded genes are cell cycle regulated and specifically expressed in S phase. The promoter of the RNR2 gene encoding for the small subunit was isolated from tobacco. Both in vivo and in vitro studies of the DNA-protein interactions in synchronized BY2 tobacco cells showed that two E2F-like motifs were involved in multiple specific complexes, some of which displayed cell cycle-regulated binding activities. Moreover, these two elements could specifically interact with a purified tobacco E2F protein. Involvement of the E2F elements in regulating the RNR2 promoter was checked by functional analyses in synchronized transgenic BY2 cells transformed with various RNR2 promoter constructs fused to the luciferase reporter gene. The two E2F elements were involved in upregulation of the promoter at the G1/S transition and mutation of both elements prevented any significant induction of the RNR promoter. In addition, one of the E2F elements sharing homology with the animal E2F/cell cycle-dependent element motif behaved like a repressor when outside of the S phase. These data provide evidence that E2F elements play a crucial role in cell cycle regulation of gene transcription in plants.

Cell cycle regulation of the tobacco ribonucleotide reductase small subunit gene is mediated by E2F-like elements

Ribonucleotide reductase (RNR) is a key enzyme involved in the DNA synthesis pathway. The RNR-encoded genes are cell cycle regulated and specifically expressed in S phase. The promoter of the RNR2 gene encoding for the small subunit was isolated from tobacco. Both in vivo and in vitro studies of the DNA-protein interactions in synchronized BY2 tobacco cells showed that two E2F-like motifs were involved in multiple specific complexes, some of which displayed cell cycle-regulated binding activities. Moreover, these two elements could specifically interact with a purified tobacco E2F protein. Involvement of the E2F elements in regulating the RNR2 promoter was checked by functional analyses in synchronized transgenic BY2 cells transformed with various RNR2 promoter constructs fused to the luciferase reporter gene. The two E2F elements were involved in upregulation of the promoter at the G1/S transition and mutation of both elements prevented any significant induction of the RNR promoter. In addition, one of the E2F elements sharing homology with the animal E2F/cell cycle-dependent element motif behaved like a repressor when outside of the S phase. These data provide evidence that E2F elements play a crucial role in cell cycle regulation of gene transcription in plants.

Activity of the human cytochrome c1 promoter is modulated by E2F

The human cytochrome c(1) promoter is strongly activated in transfected Drosophila SL2 cells expressing exogenous human E2F1. Transfection-deletion experiments, DNase I protection by E2F1 and gel mobility-shift experiments locate E2F1 activation sites to two regions on either side of the transcription start site. Deletion of either region prevents E2F1 activation in transfected SL2 cells, suggesting a co-operative interaction between them. E2F6, a member of the E2F family that lacks transactivation domains but contains specific suppressor domains, inhibits cytochrome c(1) promoter activity when co-transfected into HeLa cells, indicating that the E2F proteins modulate the cytochrome c(1) promoter in mammalian cells. However, E2F is not a general regulator of oxidative phosphorylation genes since three additional nuclear-encoded mitochondrial genes were unaffected by E2F1 or E2F6.

Cell cycle-dependent regulation of TIAP/m-survivin expression

TIAP, a murine homologue of human survivin, is a member of the inhibitor of apoptosis (IAP) family and is specifically expressed at G2/M phase of the cell cycle. To elucidate regulatory mechanisms of the cycle-dependent expression, we have analyzed the promoter region of TIAP/mouse survivin (m-survivin). The 5'-flanking region of the TIAP/m-survivin gene contained a TATA-less promoter, two AP2 sites, three NF-kB sites, one Sp1 site, many cell cycle-dependent elements (CDEs) and one cell cycle gene homology region (CHR). Primer extension and 5'-rapid amplification of cDNA ends identified one transcription start site at position -100 upstream of the ATG start site (+1). TIAP/m-survivin promoter-luciferase analysis identified a minimal promoter region within the most proximal -271 bp upstream of the ATG start site, and the region between -410 and -272 was critical for the enhancer activity. The combination between the CHR at -51 and the CDE at -57 is also essential for the cell cycle-dependent expression. Mutation of the CDE/CHR element and the enhancer elements may cause disordered expression of TIAP/m-survivin to affect cell survival and oncogenesis.

Transcription of the catalytic 180-kDa subunit gene of mouse DNA polymerase alpha is controlled by E2F, an Ets-related transcription factor, and Sp1

We have isolated a genomic DNA fragment spanning the 5'-end of the gene encoding the catalytic subunit of mouse DNA polymerase alpha. The nucleotide sequence of the upstream region was G/C-rich and lacked a TATA box. Transient expression assays in cycling NIH 3T3 cells demonstrated that the GC box of 20 bp (at nucleotides -112/-93 with respect to the transcription initiation site) and the palindromic sequence of 14 bp (at nucleotides -71/-58) were essential for basal promoter activity. Electrophoretic mobility shift assays showed that Sp1 binds to the GC box. We also purified a protein capable of binding to the palindrome and identified it as GA-binding protein (GABP), an Ets- and Notch-related transcription factor. Transient expression assays in synchronized NIH 3T3 cells revealed that three variant E2F sites near the transcription initiation site (at nucleotides -23/-16, -1/+7 and +17/+29) had no basal promoter activity by themselves, but were essential for growth-dependent stimulation of the gene expression. These data indicate that E2F, GABP and Sp1 regulate the gene expression of this principal replication enzyme.

CHF: a novel factor binding to cyclin A CHR corepressor element

Cell cycle modulation of cyclin A expression is due to the periodic relief of a transcriptional repression mediated by a bipartite negative DNA regulatory region. The 5' element (Cell Cycle Responsive Element: CCRE; cell Cycle Dependent Element: CDE) is clearly occupied in a cyclic manner in vivo, whereas the 3' element, whose sequence is shared by B-myb, cdc25C and cdc2 genes (cell Cycle gene Homology Region: CHR), is involved in more subtle interactions. Mutation of either element results in complete deregulation of cyclin A promoter activity. Whereas some reports claim that E2F/DP can bind to the CCRE/CDE, the nature of the protein(s) interacting with the CHR is unknown. In the present work we have characterized an activity present in quiescent cells and absent in cells blocked in S phase, which binds specifically to cyclin A CHR, but not to B-myb, or to cdc25C, or to cdc2 CHRs. A 90 kD protein, named CHF (cyclin A CHR binding factor), has been identified through preparative electrophoresis and UV crosslinking experiments. In order to address in more functional terms the binding of CHF to cyclin A CHR, we developed in vitro and in vivo oligonucleotide competition assays. Both in vitro transcription and in vivo microinjection experiments demonstrate that a functional difference exists between the composite CCRE/CDE-CHR repressor regions of cell cycle regulated genes such as cyclin A and cdc25C.

E2F target genes and cell-cycle checkpoint control

In this review, we will focus on the role played by transcription factors of the E2F/DP family in controlling the expression of genes that carry out important cell-cycle control functions, thereby ensuring ordered progression through the mammalian cell division cycle. The emerging picture is that cell-cycle progression depends on the execution of a regulatory cascade of gene expression, driven by E2F/DP transcription factors, which are in turn regulated by the products of some of these genes. That E2F factors are potent regulators of cell-cycle checkpoints in mammalian cells is supported by experiments demonstrating that ectopic expression of individual E2F family members is sufficient to modulate cell proliferation and apoptosis. It is also clear that deregulation of E2F activity will result in the loss of particular checkpoint controls, thereby predisposing cells to malignant conversion.

Sequence of murine CDC10 cDNA, gene organization and expression analysis

The sequence of the 2391 bp murine CDC10 cDNA is reported. The gene transcription unit, composed of 13 exons, encodes a potential 417/446 amino acid GTP-binding protein, highly similar to human CDC10. The ubiquitous expression of the gene, as judged by Northern analysis, is consistent with its putative promoter structure.