Novel function of the cyclin A binding site of E2F in regulating p53-induced apoptosis in response to DNA damage

EAPP, a novel E2F binding protein that modulates E2F-dependent transcription

E2F transcription factors play an essential role in cell proliferation and apoptosis and their activity is frequently deregulated in human cancers. In a yeast two-hybrid screen we identified a novel E2F-binding protein. Due to its strong phosphorylation we named it EAPP (e2F-associated phosphoprotein). EAPP is localized in the nucleus and interacts with E2F-1, E2F-2, and E2F-3, but not with E2F-4. Examination of a number of human cell lines revealed that EAPP levels are elevated in most transformed cells. Moreover, EAPP mRNA was detected in all investigated human tissues in varying amounts. EAPP is present throughout the cell cycle but disappears during mitosis. In transfection assays with reporters controlled by either an artificial E2F-dependent promoter or the murine thymidine kinase promoter, EAPP increased the activation caused by E2F-1 but not by E2F-4. Surprisingly, the promoter of the p14(ARF) gene, which was also activated by E2F-1, became repressed by EAPP. Overexpression of EAPP in U2OS cells resulted in a significant increase of cells in S-phase, whereas RNAi-mediated knock down of EAPP reduced the fraction of cells in S-phase. Taken together, these data suggest that EAPP modulates E2F-regulated transcription, stimulates proliferation, and may be involved in the malignant transformation of cells.

Splicing DNA-damage responses to tumour cell death

The ability of a tumour cell to evade programmed cell death (apoptosis) is crucial in the development of cancer. The process of apoptosis is complex and involves the careful interplay of a host of signalling molecules. Cellular stresses, such as DNA-damage, can initiate apoptosis through multiple pathways, all of which eventually lead to eradication of damaged cells that may otherwise go on to form a tumour. Moreover, the relevance of this to combating cancer is very strong since several therapeutic agents used to treat malignant disease utilize the cells' apoptotic machinery. The purpose of this review is to provide an insight into what we know about how apoptosis is initiated by DNA-damaging agents, how pro- and anti-apoptotic signals converge in the execution of cell death, and how such mechanisms can be perturbed in cancer.

Human MUC1 oncoprotein regulates p53-responsive gene transcription in the genotoxic stress response

The MUC1 oncoprotein is aberrantly overexpressed by most human carcinomas. The present work demonstrates that MUC1 associates with the p53 tumor suppressor, and that this interaction is increased by genotoxic stress. The MUC1 cytoplasmic domain binds directly to p53 regulatory domain. Chromatin immunoprecipitation assays demonstrate that MUC1 coprecipitates with p53 on the p53-responsive elements of the p21 gene promoter and coactivates p21 gene transcription. Conversely, MUC1 attenuates activation of Bax transcription. In concert with these results, MUC1 promotes selection of the p53-dependent growth arrest response and suppresses the p53-dependent apoptotic response to DNA damage. These findings indicate that MUC1 regulates p53-responsive genes and thereby cell fate in the genotoxic stress response.

Opposing roles of E2Fs in cell proliferation and death

Progression through the cell cycle is dependent upon the temporal and spatial regulation of the various members of the E2F family of transcription factors. Two of these members, E2F1 and E2F4 have opposing roles in cell cycle progression, which were defined over a decade ago. While E2F1 is an activator of cell cycle progression, E2F4 functions as a transcriptional repressor. Recent data indicate that these transcription factors also play a role in the cellular response to DNA damage. In the case of E2F1, its overexpression leads to apoptosis. In contrast, the decreased expression of E2F4, in response to siRNA-mediated knockdown or to certain therapeutic agents, induces apoptosis. Conversely, increased levels of E2F4 may confer resistance to apoptosis-inducing therapies used in the clinic. The balance between the activities of these two proteins in tumor cells is of great interest. Directed control of E2F1 and E2F4 action may lead to better diagnosis of disease and improved therapeutic modalities.

The NH2 terminus of influenza virus hemagglutinin-2 subunit peptides enhances the antitumor potency of polyarginine-mediated p53 protein transduction

Protein transduction therapy is a newly developing method that allows proteins, peptides, and biologically active compounds to penetrate across the plasma membrane by being fused with cell-penetrating peptides such as polyarginine. Polyarginine-fused p53 protein penetrates across the plasma membrane of cancer cells and inhibits the growth of the cells. However, the protein is often entrapped inside macropinosomes in the cytoplasm. Therefore, high dose concentrations of the protein are needed for it to function effectively. To overcome this problem, in the present study, polyarginine-fused p53 was linked with the NH(2)-terminal domain of influenza virus hemagglutinin-2 subunit (HA2), which is a pH-dependent fusogenic peptide that induces the lysis of membranes at low pH levels. The protein was capable of efficiently translocating into the nucleus of glioma cells and induced p21(WAF1) transcriptional activity more effectively than did polyarginine-fused p53 protein. Moreover, low concentrations of the protein significantly inhibited the growth of cancer cells. These results suggest that protein transduction therapy using polyarginine and HA2 may be useful as a method for cancer therapy.

Apoptosis-stimulating protein of p53-2 (ASPP2/53BP2L) is an E2F target gene

The p53 pathway is a central apoptotic regulator. Deregulation of the Rb/E2F pathway occurs in a majority of tumors, resulting in both unrestrained proliferation and enhanced apoptosis sensitivity via p53-dependent and independent mechanisms. However, the mechanisms coupling the p53 and Rb/E2F pathways remain incompletely understood. We report that ASPP2/53BP2L, a p53/p73-binding protein that promotes p53/p73-dependent apoptosis, is an E2F target gene. The ASPP2/53BP2L promoter was identified and ectopic expression of transcription-competent E2F-1 (E2F-2 and E2F-3) stimulated an ASPP2/53BP2L promoter-luciferase reporter. Mutational analysis of the ASPP2/53BP2L promoter identified E2F-binding sites that cooperate for E2F-1 induction and basal repression of ASPP2/53BP2L. Moreover, endogenous ASPP2/53BP2L levels increased after E2F-1 expression, and E2F-1 bound the endogenous ASPP2/53BP2L promoter after chromatin immunoprecipitation. Typical for an E2F target, ASPP2/53BP2L expression was maximal in early S-phase. Thus, ASPP2/53BP2L is downstream of E2F, suggesting that it functions as a common link between the p53/p73 and Rb/E2F apoptotic pathways.

Human immunodeficiency virus type 1 Vpr-mediated G2 arrest requires Rad17 and Hus1 and induces nuclear BRCA1 and gamma-H2AX focus formation

Eukaryotic cells have evolved a complex mechanism for sensing DNA damage during genome replication. Activation of this pathway prevents entry into mitosis to allow for either DNA repair or, in the event of irreparable damage, commitment to apoptosis. Under conditions of replication stress, the damage signal is initiated by the ataxia-telangiectasia-mutated and Rad3-related kinase ATR. We recently demonstrated that the human immunodeficiency virus type 1 (HIV-1) gene product viral protein R (Vpr) arrests infected cells in the G(2) phase via the activation of ATR. In the present study, we show that the activation of ATR by Vpr is analogous to activation by certain genotoxic agents, both mechanistically and in its downstream consequences. Specifically, we show a requirement for Rad17 and Hus1 to induce G(2) arrest as well as Vpr-induced phosphorylation of histone 2A variant X (H2AX) and formation of nuclear foci containing H2AX and breast cancer susceptibility protein 1. These results demonstrate that G(2) arrest mediated by the HIV-1 gene product Vpr utilizes the cellular signaling pathway whose physiological function is to recognize replication stress. These findings should contribute to a greater understanding of how HIV-1 manipulates the CD4(+)-lymphocyte cell cycle and apoptosis induction in the progressive CD4(+)-lymphocyte depletion characteristic of HIV-1 pathogenesis.

The Proapoptotic Gene SIVA Is a Direct Transcriptional Target for the Tumor Suppressors p53 and E2F1

The p53 tumor suppressor gene is believed to play an important role in neuronal cell death in acute neurological disease and in neurodegeneration. The p53 signaling cascade is complex, and the mechanism by which p53 induces apoptosis is cell type-dependent. Using DNA microarray analysis, we have found a striking induction of the proapoptotic gene, SIVA. SIVA is a proapoptotic protein containing a death domain and interacts with members of the tumor necrosis factor receptor family as well as anti-apoptotic Bcl-2 family proteins. SIVA is induced following direct p53 gene delivery, treatment with a DNA-damaging agent camptothecin, and stroke injury in vivo. SIVA up-regulation is sufficient to initiate the apoptotic cascade in neurons. Through isolation and analysis of the SIVA promoter, we have identified response elements for both p53 and E2F1. Like p53, E2F1 is another tumor suppressor gene involved in the regulation of apoptosis, including neuronal injury models. We have identified E2F consensus sites in the promoter region, whereas p53 recognition sequences were found in intron1. Sequence analysis has shown that these consensus sites are also conserved between mouse and human SIVA genes. Electrophoretic mobility shift assays reveal that both transcription factors are capable of binding to putative consensus sites, and luciferase reporter assays reveal that E2F1 and p53 can activate transcription from the SIVA promoter. Here, we report that the proapoptotic gene, SIVA, which functions in a broad spectrum of cell types, is a direct transcriptional target for both tumor suppressors, p53 and E2F1.

To die or not to die: how does p53 decide?

p53 is frequently mutated in cancer and as a result is one of the most intensely studied tumour suppressors. Analysis of the primitive forms of p53 found in Caenorhabditis elegans and Drosophila, alongside studies using transgenic mouse models, indicate that the induction of apoptosis is both the most conserved function of p53 and vital for tumour suppression. p53-mediated apoptosis occurs through a combination of mechanisms which include pathways that are both dependent and independent of alterations in gene expression. In response to genotoxic insult, these pathways probably act together, thereby amplifying the apoptotic signal. However, the picture is complicated because the p53 activity is determined by stress type and individual cellular characteristics. The numerous p53 responsive genes that have been identified also provide further means of controlling the actions of p53. The recent discoveries of proteins that interact with p53 and specifically regulate the ability of p53 to trigger apoptosis have provided further mechanistic insights into the role of p53 in inducing cell death. Understanding the molecular basis of the proapoptotic action of p53 can assist in our quest to reintroduce or reactivate p53 in human tumours.

Specific role for p300/CREB-binding protein-associated factor activity in E2F1 stabilization in response to DNA damage

E2F1, a member of the E2F family of transcription factors, plays a pivotal role in controlling both physiological cell-cycle progression and apoptotic cell death in response to DNA damage and oncogene activation. In response to genotoxic stresses, E2F1 is stabilized by signals that include ATM-dependent phosphorylation. We recently demonstrated that DNA damage induces also E2F1 acetylation, which is required for its recruitment onto apoptotic gene promoters. Here we show that E2F1 is stabilized in response to doxorubicin and cisplatin treatments even in the absence of either ATM-dependent phosphorylation or p53 and cAbl, two major transducers of DNA damage signaling. We found that acetylation of E2F1 is, instead, required to stabilize the protein in response to doxorubicin. Finally, we report that the formation of E2F1-p300/CREB-binding protein-associated factor (P/CAF) complexes is preferentially induced in doxorubicin-treated cells, and that P/CAF acetyltransferase (HAT), but not p300 HAT activity, is required for a significant E2F1 stabilization and accumulation. Our results unveil a differential role of P/CAF and p300 in acetylation-induced stabilization of E2F1, thus supporting a specific role for P/CAF HAT activity in E2F1-dependent apoptosis in response to DNA damage.

DNA damage responses triggered by a highly cytotoxic monofunctional DNA alkylator, hedamycin, a pluramycin antitumor antibiotic

Long-term exposure (72 h) to hedamycin, a monofunctional DNA alkylator of the pluramycin class of antitumor antibiotics, decreased growth of mammalian cells by 50% at subnanomolar concentrations. Short-term treatment (4 h) rapidly reduced DNA synthesis by 50% also at subnanomolar concentrations, but substantially higher levels were needed to block RNA synthesis while protein synthesis even at very high hedamycin concentrations remained unaffected. Hedamycin treatment at concentrations below its growth IC50 induced only a transient and temporary accumulation of cells in G2. Somewhat higher concentrations resulted in substantial S-phase arrest, and at increasing concentrations, complete cell cycle arrest in G1 was observed without the appearance of a sub-G1 cell population. Neither inhibition of cell growth nor cell cycle arrest appeared to be dependent on ataxia and Rad-related kinase expression. DNA damage checkpoint proteins including p53, chk1, and chk2 were differentially activated by hedamycin depending on the concentration and duration of treatment. The level of downstream cell cycle regulators such as cdc25A, E2F1, cyclin E, and p21 were also altered under conditions that induced cell cycle arrest, but atypically, p21 overexpression was observed only in S-phase-arrested cells. Apoptotic indicators were only observed at moderate hedamycin concentrations associated with S-phase arrest, while increasing concentrations, when cells were arrested in G1, resulted in a reduction of these signals. Taken together, the responses of cells to hedamycin are distinct with regard to its effect on cell cycle but also in the unusual concentration-dependent manner of activation of DNA damage and cell cycle checkpoint proteins as well as the induction of apoptotic-associated events.

Immortalization in a normal foreskin fibroblast culture following transduction of cyclin A2 or cdk1 genes in retroviral vectors

Human diploid fibroblasts (HDF) rarely, if ever, undergo spontaneous transformation to an immortalized cell type. Here we report the immortalization of an HDF cell line following transduction with cyclin A2 or cdk1 human genes via retroviral vectors. Fluorescence in situ hybridization (FISH) studies using the retroviral vector as a probe indicate that these cell lines are monoclonal. No telomerase activity could be detected in these cell lines, and the telomere length in the immortalized cells was observed to be 10-20 kb longer than that in low-passage cells from the parental fibroblast line. Cytogenetic studies revealed that the immortal lines share common chromosomal aberrations. FISH studies with a probe for p53 revealed loss of one copy of this gene which was associated with reduced steady-state levels of both p53 and p53-regulated p21(WAF1/Sdi1/CIP1) messages in both quiescent and proliferating immortalized cultures relative to the parental cells. Additional FISH studies with probes for p16(INK4a) and Rb, carried out after the immortalized cells proliferated in excess of 100 population doublings, also revealed loss of one copy of these genes in both cell lines. These cell lines, together with the well-characterized parental cells, could provide useful research material for the study of the mechanisms of immortalization and of regulation of proliferative senescence in HDF.

The extracellular matrix, p53 and estrogen compete to regulate cell-surface Fas/Apo-1 suicide receptor expression in proliferating embryonic cerebral cortical precursors, and reciprocally, Fas-ligand modifies estrogen control of cell-cycle proteins

BACKGROUND

Apoptosis is important for normal cerebral cortical development. We previously showed that the Fas suicide receptor was expressed within the developing cerebral cortex, and that in vitro Fas activation resulted in caspase-dependent death. Alterations in cell-surface Fas expression may significantly influence cortical development. Therefore, in the following studies, we sought to identify developmentally relevant cell biological processes that regulate cell-surface Fas expression and reciprocal consequences of Fas receptor activation.

RESULTS

Flow-cytometric analyses identified two distinct neural sub-populations that expressed Fas on their cell surface at high (FasHi) or moderate (FasMod) levels. The anti-apoptotic protein FLIP further delineated a subset of Fas-expressing cells with potential apoptosis-resistance. FasMod precursors were mainly in G0, while FasHi precursors were largely apoptotic. However, birth-date analysis indicated that neuroblasts express the highest levels of cell-surface Fas at the end of S-phase, or after their final round of mitosis, suggesting that Fas expression is induced at cell cycle checkpoints or during interkinetic nuclear movements. FasHi expression was associated with loss of cell-matrix adhesion and anoikis. Activation of the transcription factor p53 was associated with induction of Fas expression, while the gonadal hormone estrogen antagonistically suppressed cell-surface Fas expression. Estrogen also induced entry into S-phase and decreased the number of Fas-expressing neuroblasts that were apoptotic. Concurrent exposure to estrogen and to soluble Fas-ligand (sFasL) suppressed p21/waf-1 and PCNA. In contrast, estrogen and sFasL, individually and together, induced cyclin-A expression, suggesting activation of compensatory survival mechanisms.

CONCLUSIONS

Embryonic cortical neuronal precursors are intrinsically heterogeneous with respect to Fas suicide-sensitivity. Competing intrinsic (p53, cell cycle, FLIP expression), proximal (extra-cellular matrix) and extrinsic factors (gonadal hormones) collectively regulate Fas suicide-sensitivity either during neurogenesis, or possibly during neuronal migration, and may ultimately determine which neuroblasts successfully contribute neurons to the differentiating cortical plate.

pRB contains an E2F1-specific binding domain that allows E2F1-induced apoptosis to be regulated separately from other E2F activities

The interaction between pRB and E2F is critical for control of the cell cycle and apoptosis. Here we report that pRB contains two distinct E2F binding sites. The previously identified E2F binding site on pRB is necessary for stable association with E2Fs on DNA. A second E2F interaction site is located entirely within the C-terminal domain of pRB and is specific for E2F1. E2F1/pRB complexes formed through this site have low affinity for DNA, but the interaction is sufficient for pRB to regulate E2F1-induced apoptosis, and E2F1 loses the ability to interact with this site following DNA damage. These results show that pRB interacts with individual E2F proteins in different ways and suggest that pRB's regulation of E2F1-induced apoptosis is physically separable from its transcriptional control of other E2F proteins.

Antisense therapy targeting MDM2 oncogene in prostate cancer: Effects on proliferation, apoptosis, multiple gene expression, and chemotherapy

This study was undertaken to investigate the role of mouse double minute 2 (MDM2) oncogene in prostate cancer growth and the potential of MDM2 as a target for prostate cancer therapy. An antisense anti-human-MDM2 mixed-backbone oligonucleotide was tested in human prostate cancer models with various p53 statuses, LNCaP (p53wt/wt), DU145 (p53mt/mt), and PC3 (p53null). In a dose- and time-dependent manner, it specifically inhibited MDM2 expression and modified expression of several genes, at both mRNA and protein levels. In LNCaP cells, p53, p21, Bax, and hypophosphorylated retinoblastoma tumor suppressor protein (pRb) levels increased, whereas Bcl2, pRb protein, and E2F transcription factor 1 (E2F1) levels decreased. In DU145 cells, p21 levels were elevated and E2F1 levels decreased, although mutant p53, Rb, and Bax levels remained unchanged. In PC3 cells, MDM2 inhibition resulted in elevated p21, Bax, and pRb levels and decreased ppRb and E2F1 levels. In all three cell lines, MDM2 inhibition reduced cell proliferation, induced apoptosis, and potentiated the effects of the chemotherapeutic agents 10-hydroxycamptothecin and paclitaxel. The anti-MDM2 oligonucleotide showed antitumor activity and increased therapeutic effectiveness of paclitaxel in both LNCaP and PC3 xenografts, causing changes in gene expression similar to those seen in vitro. In summary, this study demonstrates that MDM2 has a role in prostate cancer growth via p53-dependent and p53-independent mechanisms and that multiple genes are involved in the process. MDM2 inhibitors such as second-generation antisense oligonucleotides have a broad spectrum of antitumor activities in human cancers regardless of p53 status, providing novel approaches to therapy of human prostate cancer.

Overexpression of cyclin A overrides the effect of p53 alterations in breast cancer patients with long follow-up time

The tumour suppressor gene p53 and its protein controls critical cellular functions in cell cycle regulation as well as in apoptosis. Recently, in an in vitro study on breast cancer cell line MCF-7, the apoptotic function of p53 has been shown to be altered by overexpression of cyclin A. In this study we have demonstrated a similar association in a consecutive series of 166 breast cancer patients operated for invasive breast carcinomas. We detected mutations (exon 5-8) in the tumour tissue from 28 (16.0%) of the patients, and positive immunoreactivity of p53 protein was detected in tumour tissue samples from 32 (18.8%) patients. A statistically significant correlation between TP53 gene mutations and positive immunohistochemistry of p53 protein was observed (p = 0.0038). Mutations of the TP53 gene, as well as positive immunoreactivity to p53, were associated with poor prognosis (mutations p = 0.053, HR = 1.8, 95% CI 0.99-3.4; positive immunoreactivity p = 0.029, HR 1.33, 95% CI 1.0-1.7; mutation and/or positive immunoreactivity p = 0.015, HR 2.1, 95% CI 1.2-3.7) when cyclin A was not included in the survival analysis. However, when cyclin A overexpression was included, alteration of the p53 protein (mutations and/or positive immunoreactivity) lost its statistical power (p = 0.088). In a stratified survival analysis the OR fell from 3.0 (95% CI 1.2-8.3, p = 0.03) in the low-expression cyclin A stratum to 1.3 (95% CI 0.42-4.1, p = 0.77) in the overexpression cyclin A stratum.

Inactivating E2f1 reverts apoptosis resistance and cancer sensitivity in Trp53-deficient mice

The E2f1 transcription factor, which regulates genes required for S-phase entry, also induces apoptosis by transcriptional and post-translational mechanisms. As E2f1 is inducible by DNA damage we investigated its importance in vivo in ultraviolet (UV)-induced apoptosis, a protective mechanism that prevents the epidermis from accumulating UV-induced mutations. Contrary to expectation, E2f1-/- mice demonstrated enhanced keratinocyte apoptosis after UVB exposure, whereas apoptosis was suppressed by epidermis-specific overexpression of human E2F1. Apoptosis induced by -radiation was also repressed by E2f1. E2f1-/-;Trp53-/- double knockout mice exhibited the elevated UVB-induced apoptosis of E2f1-/- alone, rather than the profound apoptosis defect seen in Trp53-/- mice, indicating that Trp53 (p53) lies functionally upstream of E2f1. Transfecting E2F1 into E2f1-/-;Trp53-/- primary fibroblasts suppressed UVB-induced apoptosis and this suppression was relieved by Trp53. The double knockout also reverted the abnormal sex ratio and early-onset tumours of Trp53-/- mice. These results imply that E2f1 functions as a suppressor of an apoptosis pathway that is initiated by DNA photoproducts and perhaps genetic abnormalities; p53 relieves this suppression.

Association between histology grade, expression of HsMCM2, and cyclin A in human invasive breast carcinomas

AIM

Increased proliferation of tumour cells has prognostic value in human invasive breast carcinomas (IBCs), and high histology grade and cyclin A expression, which may reflect high proliferation rate, are associated with poor prognosis. Expression of HsMCM2 is related to cell proliferation. This study evaluates the correlation between the expression of cyclins A, D1, D3, and E, Ki-67, proliferating cell nuclear antigen (PCNA), histology grade, and HsMCM2 expression, in addition to the independent prognostic value of HsMCM2 expression in human IBCs.

METHODS

Immunohistochemistry to evaluate HsMCM2, Ki-67, and PCNA expression in tumours from 147 patients with IBC.

RESULTS

Nuclear staining for HsMCM2 was seen in 10-30% of the tumour cells in 30 samples, in 30-70% in 40 samples, in > 70% in 44 samples, and in < 10% in 33 samples. One way ANOVA showed a significant association between expression of HsMCM2 and cyclin A, D3, E, histology grade, and Ki-67. A borderline correlation was seen between HsMCM2 and PCNA. In multivariate analysis, the only association was with cyclin A, in addition to a borderline association with histology grade. In a Cox regression hazards model, expression of HsMCM2 was associated with poor patient survival, although it lost its independent prognostic value when cyclin A expression was included. Ki-67 and PCNA expression were not associated with patient survival.

CONCLUSION

Cyclin A expression is independently associated with HsMCM2 expression, histology grade, and Ki-67. HsMCM2 expression is associated with poor patient survival, although it loses prognostic value when adjusted for cyclin A.

Immunohistochemical expression of cyclins, cyclin-dependent kinases, tumor-suppressor gene products, Ki-67, and sex steroid receptors in endometrial carcinoma: positive staining for cyclin A as a poor prognostic indicator

Although aberrant expression of several cell-cycle regulators has been reported in endometrial carcinoma, correlations among these factors and their prognostic significance have not fully been elucidated. In the present study, expression of cyclins (D1, E, A, and B1), cyclin-dependent kinases (cdk2, cdk4, and cdc2), and tumor-suppressor gene products (p53, p21, and p27) were systematically examined by immunohistochemistry in 82 cases of endometrial carcinoma and 20 normal endometria. Results were compared with the expression of Ki-67, sex steroid receptor status, clinicopathological parameters, and patient outcomes. Positive staining for cyclin D1, cyclin E, cyclin A, cyclin B1, cdk2, cdk4, cdc2, p53, p21, and p27 was observed in 63%, 66%, 31%, 32%, 51%, 77%, 71%, 43%, 35%, and 60% of the 82 carcinomas, respectively. Among these factors, positive staining for cyclin D1, cdk4, and p53 was significantly frequent in advanced-stage tumors, and that for cyclin D1, cyclin A, cdk4, p21, and p53 was more frequent in higher-grade tumors. High correlation was found between cyclin A and p53 expression, between cyclin D1 and cdk4 expression, between cdk4 and Ki-67 expression, and between p21 and Ki-67 expression. Multivariate analysis showed that the factors for poor prognosis were advanced stage and cyclin A positivity. These findings suggest that various cell-cycle regulators are involved in activated cell growth of endometrial carcinoma, and that positive staining for cyclin A could be a useful marker for unfavorable patient prognosis.

Emerging roles for E2F: beyond the G1/S transition and DNA replication

E2F activity is crucial for the G1/S transition and DNA replication in mammalian cells. The retinoblastoma (pRB) family of proteins is the primary negative regulator of E2F. Recent findings have begun to clarify distinct roles for E2F family members during cell cycle progression and have considerably broadened our understanding of E2F transcriptional control beyond S phase. In this review, we examine the relative contribution of two distinct subclasses of E2F to repression and activation and how this division of labor could explain the role of E2F in DNA damage and repair checkpoints as well as tumorigenesis.

Decision making by p53: life, death and cancer

The p53 tumor-suppressor plays a critical role in the prevention of human cancer. In the absence of cellular stress, the p53 protein is maintained at low steady-state levels and exerts very little, if any, effect on cell fate. However, in response to various types of stress, p53 becomes activated; this is reflected in elevated protein levels, as well as augmented biochemical capabilities. As a consequence of p53 activation, cells can undergo marked phenotypic changes, ranging from increased DNA repair to senescence and apoptosis. This review deals with the mechanisms that underlie the apoptotic activities of p53, as well as the complex interactions between p53 and central regulatory signaling networks. In p53-mediated apoptosis, the major role is played by the ability of p53 to transactivate specific target genes. The choice of particular subsets of target genes, dictated by covalent p53 modifications and protein-protein interactions, can make the difference between life and apoptotic death of a cell. In addition, transcriptional repression of antiapoptotic genes, as well as transcription-independent activities of p53, can also contribute to the apoptotic effects of p53. Regarding the crosstalk between p53 and signaling networks, this review focuses on the interplay between p53 and two pivotal regulatory proteins: beta-catenin and Akt/PKB. Both proteins can regulate p53 as well as be regulated by it. In addition, p53 interacts with the GSK-3beta kinase, which serves as a link between Akt and beta-catenin. This review discusses how the functional balance between these different interactions might dictate the likelihood of a given cell to become cancerous or be eliminated from the replicative pool, resulting in suppression of cancer.

Apoptosis and brain ischaemia

There is increasing evidence that some neuronal death after brain ischaemia is mediated by the action of cysteine-requiring aspartate-directed proteases (caspases), the proteases responsible for apoptosis in mammals, although this form of neuronal death is not always accompanied by the morphological changes that are typical of apoptosis in other tissues. Caspase-mediated neuronal death is more extensive after transient than permanent focal brain ischaemia and may contribute to delayed loss of neurons from the penumbral region of infarcts. The activation of caspases after brain ischaemia is largely consequent on the translocation of Bax, Bak, and other BH3-only members of the Bcl-2 family to the mitochondrial outer membrane and the release of cytochrome c, procaspase-9, and apoptosis activating factor-1 (Apaf-1) from the mitochondrial intermembrane space. How exactly ischaemia induces this translocation is still poorly understood. NF-kappaB, the c-jun N-terminal kinase-c-Jun pathway, p53, E2F1, and other transcription factors are probably all involved in regulating the expression of BH3-only proteins after brain ischaemia, and mitochondrial translocation of Bad from sequestering cytosolic proteins is promoted by inactivation of the serine-threonine kinase, Akt. Other processes that are probably involved in the activation of caspases after brain ischaemia include the mitochondrial release of the second mitochondrial activator of caspases (Smac) or direct inhibitor-of-apoptosis-binding protein with low pI (DIABLO), the accumulation of products of lipid peroxidation, a marked reduction in protein synthesis, and the aberrant reentry of neurons into the cell cycle. Non-caspase-mediated neuronal apoptosis may also occur, but there is little evidence to date that this makes a significant contribution to brain damage after ischaemia. The intracellular processes that contribute to caspase-mediated neuronal death after ischaemia are all potential targets for therapy. However, anti-apoptotic interventions in stroke patients will require detailed evaluation using a range of outcome measures, as some such interventions seem simply to delay neuronal death and others to preserve neurons but not neuronal function.

Macrophage migration inhibitory factor deficiency is associated with altered cell growth and reduced susceptibility to Ras-mediated transformation

Macrophage migration inhibitory factor (MIF) has been shown to functionally inactivate the p53 tumor suppressor and to inhibit p53-responsive gene expression and apoptosis. To better understand the role of MIF in cell growth and tumor biology, we evaluated MIF-null embryonic fibroblasts with respect to their immortalization and transformation properties. Although minor deviations in the growth characteristics of MIF(-/-) fibroblasts were observed under normal culture conditions, MIF-deficient cells were growth-impaired following the introduction of immortalizing oncogenes. The growth retardation by the immortalized MIF(-/-) cultures correlated with their reduced susceptibility to Ras-mediated transformation. Our results identify E2F as part of the restraining mechanism that is activated in response to oncogenic signaling and show that the biological consequences of E2F induction in MIF(-/-) fibroblasts vary depending on the p53 status, inducing predominantly G(1) arrest or apoptosis in p53-positive cells. This E2F activity is independent of Rb binding, but contingent on binding DNA. Resistance to oncogenic transformation by MIF(-/-) cells could be overcome by concomitant interference with p53- and E2F-responsive transcriptional control. Our results demonstrate that MIF plays a role in an E2F/p53 pathway that operates downstream of Rb regulation and implicate MIF as a mediator of normal and malignant cell growth.

Heat shock treatment decreases E2F1-DNA binding and E2F1 levels in human A549 cells

The transcription factor E2F1 plays a decisive role in the G1/S and G2/M checkpoint transitions of proliferating cells. Because cells are arrested at these checkpoints after heat shock it was of interest to test heat shock effects on E2F1 activity. In human A549 cells, heat shock (44 degrees C, 30 min) caused an immediate reduction of E2F1-DNA binding as determined by electrophoretic mobility shift assay (EMSA). The complex of E2F1-DNA with the retinoblastoma protein (pRB) was also reduced after heat shock. This indicates that the former effect is not caused by a lower phosphorylation and therefore a higher binding capacity of pRB. Western blot analyses showed that the lower E2F1-DNA binding is probably due to a decrease of the E2F1 level (40% of the controls) induced by heat shock. This result was confirmed by an experiment with HeLa cells in which heat shock decreased the level to 60% of the controls. In order to test whether this decrease resulted from inhibition of transcription, RT-PCR measurements were conducted and showed only a slight reduction of the E2F1 mRNA (89% of controls). This indicates that the heat shock effect is not predominantly caused by transcriptional inhibition. Six hours after heat shock the E2F1-DNA binding capacity recovered to control levels. These results provide evidence for E2F1 involvement in heat shock-induced cell cycle arrests at the G1/S and G2/M checkpoints, which also may be relevant for hyperthermic cancer therapy.

E2F1 pathways to apoptosis

The E2F family of transcription factors plays a pivotal role in the regulation of cell proliferation and their activity is often deregulated in human tumors. Recent studies demonstrate that E2F1 can induce both proliferation and apoptosis. E2F1-induced apoptosis occurs via multiple pathways, some of which induce stabilization and activation of the tumor suppressor p53. The pro-apoptotic activity of E2F1 suggests that its deregulation constitutes an oncogenic stress that may target pre-malignant cells to undergo apoptosis, thus preventing tumor development.

Activation of ARF by oncogenic stress in mouse fibroblasts is independent of E2F1 and E2F2

The ARF tumour suppressor protein (p14(ARF) in human and p19(ARF) in mouse) is a major mediator of the activation of p53 in response to oncogenic stress. Little is known about the signalling pathways connecting oncogenic stimuli to the activation of ARF. Regulation of ARF occurs primarily at the transcriptional level and several modulators of ARF transcription have been identified. Notably, ectopic expression of E2F1 upregulates ARF transcriptionally, and both E2F1 and ARF have been implicated in apoptosis and cell-cycle arrest. We have used primary mouse fibroblasts deficient for E2F1, E2F2, or both to determine the possible role of these E2F proteins as upstream regulators of ARF in response to oncogenic stimuli and other stresses. In particular, we have studied the effects of oncogenic Ras and the viral oncoprotein E1A on ARF levels, neoplastic transformation, and sensitization to apoptosis. We have also examined the behaviour of the E2F-deficient MEFs with respect to immortalization and sensitivity to DNA damage. None of the ARF-mediated responses that we have analysed is significantly affected in E2F1(-/-), E2F2(-/-) or E2F1/2(-/-) MEFs, and ARF is upregulated normally in all cases. Taken together, our results indicate that the activation of ARF in response to oncogenic stress can occur by E2F1- and E2F2-independent mechanisms. This challenges previous suggestions implicating E2F factors as key mediators in the activation of ARF by oncogenic stress.