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K Helin

Publications and source records attributed to K Helin.

At least 19 recordsLinked to original sources
PubMed2002

The regulation of APAF1 expression during development and tumourigenesis.

Apoptosis Protease-Activating Factor 1, APAF1, was originally isolated four years ago and shown to be the mammalian homologue of the C. elegans pro-apoptotic ced4 gene. Since then, the expression of APAF1 has been demonstrated to be involved in several cell death pathways, including the induction of apoptosis by the p53 tumour suppressor protein and neuronal apoptosis. In this review we will focus on the regulation of APAF1 expression, in particular with regard to recent developments in our understanding of the role of APAF1 in both tumourigenesis and mammalian development.

Apoptosis↗
PubMed2001

Mad2 binding to Mad1 and Cdc20, rather than oligomerization, is required for the spindle checkpoint.

Mad2 is a key component of the spindle checkpoint, a device that controls the fidelity of chromosome segregation in mitosis. The ability of Mad2 to form oligomers in vitro has been correlated with its ability to block the cell cycle upon injection into Xenopus embryos. Here we show that Mad2 forms incompatible complexes with Mad1 and Cdc20, neither of which requires Mad2 oligomerization. A monomeric point mutant of Mad2 can sustain a cell cycle arrest of comparable strength to that of the wild-type protein. We show that the interaction of Mad2 with Mad1 is crucial for the localization of Mad2 to kinetochores, where Mad2 interacts with Cdc20. We propose a model that features the kinetochore as a 'folding factory' for the formation of a Mad2-Cdc20 complex endowed with inhibitory activity on the anaphase promoting complex.

Anaphase↗
PubMed2001

APAF1 is a key transcriptional target for p53 in the regulation of neuronal cell death.

p53 is a transcriptional activator which has been implicated as a key regulator of neuronal cell death after acute injury. We have shown previously that p53-mediated neuronal cell death involves a Bax-dependent activation of caspase 3; however, the transcriptional targets involved in the regulation of this process have not been identified. In the present study, we demonstrate that p53 directly upregulates Apaf1 transcription as a critical step in the induction of neuronal cell death. Using DNA microarray analysis of total RNA isolated from neurons undergoing p53-induced apoptosis a 5-6-fold upregulation of Apaf1 mRNA was detected. Induction of neuronal cell death by camptothecin, a DNA-damaging agent that functions through a p53-dependent mechanism, resulted in increased Apaf1 mRNA in p53-positive, but not p53-deficient neurons. In both in vitro and in vivo neuronal cell death processes of p53-induced cell death, Apaf1 protein levels were increased. We addressed whether p53 directly regulates Apaf1 transcription via the two p53 consensus binding sites in the Apaf1 promoter. Electrophoretic mobility shift assays demonstrated p53-DNA binding activity at both p53 consensus binding sequences in extracts obtained from neurons undergoing p53-induced cell death, but not in healthy control cultures or when p53 or the p53 binding sites were inactivated by mutation. In transient transfections in a neuronal cell line with p53 and Apaf1 promoter-luciferase constructs, p53 directly activated the Apaf1 promoter via both p53 sites. The importance of Apaf1 as a p53 target gene in neuronal cell death was evaluated by examining p53-induced apoptotic pathways in primary cultures of Apaf1-deficient neurons. Neurons treated with camptothecin were significantly protected in the absence of Apaf1 relative to those derived from wild-type littermates. Together, these results demonstrate that Apaf1 is a key transcriptional target for p53 that plays a pivotal role in the regulation of apoptosis after neuronal injury.

Animals↗
PubMed2001

E2Fs regulate the expression of genes involved in differentiation, development, proliferation, and apoptosis.

The retinoblastoma protein (pRB) and its two relatives, p107 and p130, regulate development and cell proliferation in part by inhibiting the activity of E2F-regulated promoters. We have used high-density oligonucleotide arrays to identify genes in which expression changed in response to activation of E2F1, E2F2, and E2F3. We show that the E2Fs control the expression of several genes that are involved in cell proliferation. We also show that the E2Fs regulate a number of genes involved in apoptosis, differentiation, and development. These results provide possible genetic explanations to the variety of phenotypes observed as a consequence of a deregulated pRB/E2F pathway.

Apoptosis↗
PubMed2001

Apaf-1 is a transcriptional target for E2F and p53.

Loss of function of the retinoblastoma protein, pRB, leads to lack of differentiation, hyperproliferation and apoptosis. Inactivation of pRB results in deregulated E2F activity, which in turn induces entry to S-phase and apoptosis. Induction of apoptosis by either the loss of pRB or the deregulation of E2F activity occurs via both p53-dependent and p53-independent mechanisms. The mechanism by which E2F induces apoptosis is still unclear. Here we show that E2F1 directly regulates the expression of Apaf-1, the gene for apoptosis protease-activating factor 1. These results provide a direct link between the deregulation of the pRB pathway and apoptosis. Furthermore, because the pRB pathway is functionally inactivated in most cancers, the identification of Apaf-1 as a transcriptional target for E2F might explain the increased sensitivity of tumour cells to chemotherapy. We also show that, independently of the pRB pathway, Apaf-1 is a direct transcriptional target of p53, suggesting that p53 might sensitize cells to apoptosis by increasing Apaf-1 levels.

Animals↗
PubMed2001

E2F1-mediated transcriptional inhibition of the plasminogen activator inhibitor type 1 gene.

Gene expression of the plasminogen activation system is cell-cycle dependent. Previously, we showed that ectopic expression of E2F1 repressed the plasminogen activator inhibitor type 1 (PAI-1) promoter in a manner dependent on the presence of DNA-binding and transactivation domains of E2F1 but independent of binding to pocket-binding proteins, suggesting a novel mechanism for E2F-mediated negative gene regulation [Koziczak, M., Krek, W. & Nagamine, Y. (2000) Mol. Cell. Biol. 20, 2014-2022]. However, it remains to be seen whether endogenous E2F can exert a similar effect. We report here that down-regulation of PAI-1 gene expression correlates with an increase in endogenous E2F activity. When cells were treated with a cdk2/4-specific inhibitor, which maintains E2F in an inactive state, the decline of serum-induced PAI-1 mRNA levels was suppressed. In mutant U2OS cells expressing a temperature-sensitive retinoblastoma protein (pRB), a shift to a permissive temperature induced PAI-1 mRNA expression. In U2OS cells stably expressing an E2F1-estrogen receptor chimeric protein that could be activated by tamoxifen, PAI-1 gene transcription was markedly reduced by tamoxifen even in the presence of cycloheximide. These results all indicate that endogenous E2F can directly repress the PAI-1 gene. DNase I hypersensitive-site analysis of the PAI-1 promoter suggested the involvement of conformation changes in chromatin structure of the PAI-1 promoter. 5' deletion analysis of the PAI-1 promoter showed that multiple sites were responsible for the E2F negative regulation, some of which were promoter dependent. Interestingly, one of these sites is a p53-binding element.

Animals↗
PubMed2000

Cell cycle- and cell growth-regulated proteolysis of mammalian CDC6 is dependent on APC-CDH1.

CDC6 is conserved during evolution and is essential and limiting for the initiation of eukaryotic DNA replication. Human CDC6 activity is regulated by periodic transcription and CDK-regulated subcellular localization. Here, we show that, in addition to being absent from nonproliferating cells, CDC6 is targeted for ubiquitin-mediated proteolysis by the anaphase promoting complex (APC)/cyclosome in G(1). A combination of point mutations in the destruction box and KEN-box motifs in CDC6 stabilizes the protein in G(1) and in quiescent cells. Furthermore, APC, in association with CDH1, ubiquitinates CDC6 in vitro, and both APC and CDH1 are required and limiting for CDC6 proteolysis in vivo. Although a stable mutant of CDC6 is biologically active, overexpression of this mutant or wild-type CDC6 is not sufficient to induce multiple rounds of DNA replication in the same cell cycle. The APC-CDH1-dependent proteolysis of CDC6 in early G(1) and in quiescent cells suggests that this process is part of a mechanism that ensures the timely licensing of replication origins during G(1).

Amino Acid Sequence↗
PubMed2000

DcE2F, a functional plant E2F-like transcriptional activator from Daucus carota.

In animal cells the progression of the cell cycle through G(1)/S transition and S phase is under the control of the pRB/E2F regulatory pathway. The E2F transcription factors are key activators of genes coding for several regulatory proteins and for enzymes involved in nucleotide and DNA synthesis. In this report we have detected the presence of E2F-like DNA binding activities in carrot nuclear extracts, and we have isolated a carrot cDNA (DcE2F) encoding a plant E2F homologue. The DcE2F gene is expressed in proliferating cells and is induced during the G(1)/S transition of the cell cycle. Supershift experiments using anti-DcE2F antiserum have confirmed that the DcE2F protein is a component of the carrot E2F-like nuclear activities. DNA binding assays have demonstrated that the DcE2F protein can recognize a canonical E2F cis-element in association with a mammalian DP protein. Furthermore, transactivation assays have revealed that DcE2F is a functional transcription factor that can transactivate, together with a DP partner, an E2F-responsive reporter gene in both plant and mammalian cells.

Amino Acid Sequence↗
PubMed2000

E2F family members are differentially regulated by reversible acetylation.

The six members of the E2F family of transcription factors play a key role in the control of cell cycle progression by regulating the expression of genes involved in DNA replication and cell proliferation. E2F-1, -2, and -3 belong to a structural and functional subfamily distinct from those of the other E2F family members. Here we report that E2F-1, -2, and -3, but not E2F-4, -5, and -6, associate with and are acetylated by p300 and cAMP-response element-binding protein acetyltransferases. Acetylation occurs at three conserved lysine residues located at the N-terminal boundary of their DNA binding domains. Acetylation of E2F-1 in vitro and in vivo markedly increases its binding affinity for a consensus E2F DNA-binding site, which is paralleled by enhanced transactivation of an E2F-responsive promoter. Acetylation of E2F-1 can be reversed by histone deacetylase-1, indicating that reversible acetylation is a mechanism for regulation also of non-histone proteins.

Acetylation↗
PubMed2000

The E2F transcription factors: key regulators of cell proliferation.

Ever since its discovery, the RB-1 gene and the corresponding protein, pRB, have been a focal point of cancer research. The isolation of E2F transcription factors provided the key to our current understanding of RB-1 function in the regulation of the cell cycle and in tumor suppression. It is becoming more and more evident that the regulatory circuits governing the cell cycle are very complex and highly interlinked. Certain aspects of RB-1 function, for instance its role in differentiation, cannot be easily explained by the current models of pRB-E2F interaction. One reason is that pRB has targets different from E2F, molecules like MyoD for instance. Another reason may be that we have not completely understood the full complexity of E2F function, itself. In this review, we will try to illuminate the role of E2F in pRB- and p53-mediated tumor suppression pathways with particular emphasis on the aspect of E2F-mediated transcriptional regulation. We conclude that E2F can mediate transcriptional activation as well as transcriptional repression of E2F target genes. The net effect of E2F on the transcriptional activity of a particular gene may be the result of as yet poorly understood protein-protein interactions of E2F with other components of the transcriptional machinery, as well as it may reflect the readout of the different ways of regulating E2F activity, itself. We will discuss the relevance of a thorough understanding of E2F function for cancer therapy.

Animals↗
PubMed2000

Nucleocytoplasmic shuttling of transcription factors.

To elicit the transcriptional response following intra- or extracellular stimuli, the signals need to be transmitted to their site of action within the nucleus. The nucleocytoplasmic shuttling of transcription factors is a mechanism mediating this process. The activation and inactivation of the transcriptional response is essential for cells to progress through the cell cycle in a normal manner. The involvement of cytoplasmic and nuclear accessory molecules, and the general nuclear membrane transport components, are essential for this process. Although nuclear import and export for different transcription factor families are regulated by similar mechanisms, there are several differences that allow for the specific activation of each transcription factor. This review discusses the general import and export pathways found to be common amongst many different transcription factors, and highlights a select group of transcription factors that demonstrate the diversity displayed in their mode of activation and inactivation.

Animals↗
PubMed2000

E2F-Rb complexes assemble and inhibit cdc25A transcription in cervical carcinoma cells following repression of human papillomavirus oncogene expression.

Expression of the bovine papillomavirus E2 protein in cervical carcinoma cells represses expression of integrated human papillomavirus (HPV) E6/E7 oncogenes, followed by repression of the cdc25A gene and other cellular genes required for cell cycle progression, resulting in dramatic growth arrest. To explore the mechanism of repression of cell cycle genes in cervical carcinoma cells following E6/E7 repression, we analyzed regulation of the cdc25A promoter, which contains two consensus E2F binding sites and a consensus E2 binding site. The wild-type E2 protein inhibited expression of a luciferase gene linked to the cdc25A promoter in HT-3 cervical carcinoma cells. Mutation of the distal E2F binding site in the cdc25A promoter abolished E2-induced repression, whereas mutation of the proximal E2F site or the E2 site had no effect. None of these mutations affected the activity of the promoter in the absence of E2 expression. Expression of the E2 protein also led to posttranscriptional increase in the level of E2F4, p105(Rb), and p130 and induced the formation of nuclear E2F4-p130 and E2F4-p105(Rb) complexes. This resulted in marked rearrangement of the protein complexes that formed at the distal E2F site in the cdc25A promoter, including the replacement of free E2F complexes with E2F4-p105(Rb) complexes. These experiments indicated that repression of E2F-responsive promoters following HPV E6/E7 repression was mediated by activation of the Rb tumor suppressor pathway and the assembly of repressing E2F4-Rb DNA binding complexes. Importantly, these experiments revealed that HPV-induced alterations in E2F transcription complexes that occur during cervical carcinogenesis are reversed by repression of HPV E6/E7 expression.

Binding Sites↗
PubMed2000

Differential cytotoxic pathways of topoisomerase I and II anticancer agents after overexpression of the E2F-1/DP-1 transcription factor complex.

The transcription factor complex E2F-1/DP-1 regulates the G1-to-S-phase transition and has been associated with sensitivity to the S-phase-specific anticancer agents camptothecin and etoposide, which poison DNA topoisomerase I and II, respectively. To investigate the relationship between E2F-1 and drug sensitivity in detail, we established human osteosarcoma U-20S-TA cells expressing full-length E2F-1/ DP-1 under the control of a tetracycline-responsive promoter, designated UE1DP-1 cells. Topoisomerase I levels and activity as well as the number of camptothecin-induced DNA single- and double-strand breaks were unchanged in UEIDP-1/tc- cells with >10-fold E2F-1/DP-1 overexpression. However, UE1DP-1/tc- cells were hypersensitive to camptothecin in both a clonogenic assay and four different apoptotic assays. This indicates that camptothecin-induced toxicity in this model is due to the activation of an E2F-1/ DP-1-induced post-DNA damage pathway rather than an increase in the number of replication forks caused by the S-phase initiation. In contrast, topoisomerase IIalpha levels (but not topoisomerase IIbeta levels), together with topoisomerase IIalpha promoter activity, increased 2--3-fold in UE1DP-1/tc-cells. Furthermore, the number of etoposide-induced DNA single- and double-strand breaks increased in UE1DP-1/tc-cells together with a rise in clonogenic sensitivity to etoposide, but an equal apoptotic sensitivity to etoposide. The increase in topoisomerase IIalpha promoter activity in UE1DP-1/tc--cells was shown to be due to S-phase initiation per se because it was blocked by ectopic expression of dominant negative cyclin-dependent kinase 2. In conclusion, overexpression of E2F-1/DP-1 in U-20S-TA cells is sufficient to increase clonogenic sensitivity to both topoisomerase I- and II-targeted anticancer drugs. However, the mechanism by which this occurs appears to be qualitatively different. The UE1DP-1 cell model may be used to elucidate post-DNA damage mechanisms of cell death induced by topoisomerase I-directed anticancer agents.

Antineoplastic Agents↗
PubMed1999

E2F activates late-G1 events but cannot replace E1A in inducing S phase in terminally differentiated skeletal muscle cells.

We have previously shown that the adenovirus E1A oncogene can reactivate the cell cycle in terminally differentiated cells. Current models imply that much or all of this E1A activity is mediated by the release of the E2F transcription factors from pocket-protein control. In contrast, we show here that overexpression of E2F-1, E2F-2 and E2F-4, or a chimeric E2F-4 tethered to a nuclear localization signal cannot reactivate postmitotic skeletal muscle cells (myotubes). This is not due to lack of transcriptional activity, as demonstrated on both a reporter construct and a number of endogenous target genes. Although cyclin E was strongly overexpressed in E2F-transduced myotubes, it lacked associated kinase activity, possibly explaining the inability of the myotubes to enter S phase and accumulate cyclin A. Although E2F is not sufficient to trigger DNA synthesis in myotubes, its activity is necessary even in the presence of E1A, as dominant-negative DP-1 mutants inhibit E1A-mediated cell cycle reentry. Our data show that, to reactivate myotubes, E1A must exert other functions, in addition to releasing E2F. They also establish mouse myotubes as an experimental system uniquely suited to study the most direct E2F functions in the absence of downstream cell cycle effects.

Adenovirus E1A Proteins↗
PubMed1999

Adenovirus E4 open reading frame 4-induced dephosphorylation inhibits E1A activation of the E2 promoter and E2F-1-mediated transactivation independently of the retinoblastoma tumor suppressor protein.

Previous studies have shown that the cell cycle-regulated E2F transcription factor is subjected to both positive and negative control by phosphorylation. Here we show that in transient transfection experiments, adenovirus E1A activation of the viral E2 promoter is abrogated by coexpression of the viral E4 open reading frame 4 (E4-ORF4) protein. This effect does not to require the retinoblastoma protein that previously has been shown to regulate E2F activity. The inhibitory activity of E4-ORF4 appears to be specific because E4-ORF4 had little effect on, for example, E4-ORF6/7 transactivation of the E2 promoter. We further show that the repressive effect of E4-ORF4 on E2 transcription works mainly through the E2F DNA-binding sites in the E2 promoter. In agreement with this, we find that E4-ORF4 inhibits E2F-1/DP-1-mediated transactivation. We also show that E4-ORF4 inhibits E2 mRNA expression during virus growth. E4-ORF4 has previously been shown to bind to and activate the cellular protein phosphatase 2A. The inhibitory effect of E4-ORF4 was relieved by okadaic acid, which inhibits protein phosphatase 2A activity, suggesting that E4-ORF4 represses E2 transcription by inducing transcription factor dephosphorylation. Interestingly, E4-ORF4 did not inhibit the transactivation capacity of a Gal4-E2F hybrid protein. Instead, E4-ORF4 expression appears to result in reduced stability of E2F/DNA complexes.

Adenovirus E1A Proteins↗
PubMed1999

Phosphorylation of mammalian CDC6 by cyclin A/CDK2 regulates its subcellular localization.

Cyclin-dependent kinases (CDKs) are essential for regulating key transitions in the cell cycle, including initiation of DNA replication, mitosis and prevention of re-replication. Here we demonstrate that mammalian CDC6, an essential regulator of initiation of DNA replication, is phosphorylated by CDKs. CDC6 interacts specifically with the active Cyclin A/CDK2 complex in vitro and in vivo, but not with Cyclin E or Cyclin B kinase complexes. The cyclin binding domain of CDC6 was mapped to an N-terminal Cy-motif that is similar to the cyclin binding regions in p21(WAF1/SDI1) and E2F-1. The in vivo phosphorylation of CDC6 was dependent on three N-terminal CDK consensus sites, and the phosphorylation of these sites was shown to regulate the subcellular localization of CDC6. Consistent with this notion, we found that the subcellular localization of CDC6 is cell cycle regulated. In G1, CDC6 is nuclear and it relocalizes to the cytoplasm when Cyclin A/CDK2 is activated. In agreement with CDC6 phosphorylation being specifically mediated by Cyclin A/CDK2, we show that ectopic expression of Cyclin A, but not of Cyclin E, leads to rapid relocalization of CDC6 from the nucleus to the cytoplasm. Based on our data we suggest that the phosphorylation of CDC6 by Cyclin A/CDK2 is a negative regulatory event that could be implicated in preventing re-replication during S phase and G2.

Animals↗
PubMed1999

CDC25A phosphatase is a target of E2F and is required for efficient E2F-induced S phase.

Functional inactivation of the pRB pathway is a very frequent event in human cancer, resulting in deregulated activity of the E2F transcription factors. To understand the functional role of the E2Fs in cell proliferation, we have developed cell lines expressing E2F-1, E2F-2, and E2F-3 fused to the estrogen receptor ligand binding domain (ER). In this study, we demonstrated that activation of all three E2Fs could relieve the mitogen requirement for entry into S phase in Rat1 fibroblasts and that E2F activity leads to a shortening of the G(0)-G(1) phase of the cell cycle by 6 to 7 h. In contrast to the current assumption that E2F-1 is the only E2F capable of inducing apoptosis, we showed that deregulated E2F-2 and E2F-3 activities also result in apoptosis. Using the ERE2F-expressing cell lines, we demonstrated that several genes containing E2F DNA binding sites are efficiently induced by the E2Fs in the absence of protein synthesis. Furthermore, CDC25A is defined as a novel E2F target whose expression can be directly regulated by E2F-1. Data showing that CDC25A is an essential target for E2F-1, since its activity is required for efficient induction of S phase by E2F-1, are provided. Finally, our results show that expression of two E2F target genes, namely CDC25A and cyclin E, is sufficient to induce entry into S phase in quiescent fibroblasts. Taken together, our results provide an important step in defining how E2F activity leads to deregulated proliferation.

Animals↗