Role for the p53 homologue p73 in E2F-1-induced apoptosis

Induction of p73beta by a naphthoquinone analog is mediated by E2F-1 and triggers apoptosis in HeLa cells

Recently, p73 was identified as a structural and functional homolog of p53. The p73 protein activates the transcription of genes downstream of p53 and induces apoptosis when overexpressed in several cell lines, similar to the tumor suppressor p53. However, the extracellular stimuli and molecular mechanisms regulating p73 activity remain to be elucidated. In this paper, we present evidence that the naphthoquinone analog, 2,3-dichloro-5,8-dihydroxy-1,4-naphthoquinone (NA), is a novel apoptotic stimulus that induces p73beta expression. Treatment with NA induced the expression of p73beta mRNA and protein and its downstream genes, p21 and bax, in HeLa cells. Similar results were obtained in MCF7 cells (p53(+/+), p73(+/+)). In the MCF7 cells, p53 protein level was rather decreased by NA treatment. Overexpression of p73beta led to the apoptosis of HeLa cells and enhancement of NA-induced cell death. Expression of p73beta was mediated by E2F-1, which was activated via release from pRB after exposure of cells to NA. We additionally observed that overexpression of pRB inhibited NA-induced apoptosis. These results imply that p53-independent p73beta-dependent p21 expression is involved in NA-induced apoptosis of HeLa cells.

Differences in E2F subunit expression in quiescent and proliferating vascular smooth muscle cells

E2F is a family of transcriptional factors that control G(1)/S transition. We investigated how the E2F family participates in the biological responses of vascular smooth muscle cells (VSMC) to vasoconstrictive hormones compared with fetal bovine serum (FBS). FBS induced upregulation of E2F-1 and E2F-5 at both mRNA and protein levels and slightly reduced E2F-3 protein. Angiotensin II (ANG II) and arginine vasopressin increased E2F-3 protein, but not E2F-1 and E2F-5, without upregulating its mRNA level. FBS transactivated the E2F-1 gene through the induction of free E2F-1 binding onto its promoter, whereas ANG II-induced binding of E2F-3 did not result in activation of the E2F-1 promoter. These changes are responsible for hypertrophic or hyperplastic response of VSMC to different growth factors or stimulants. In contrast, both FBS and vasoconstrictive hormones drove transcription of the cdc6 gene by downregulating p130 and recruiting free E2F-3 in the latter, which underlies the progression of VSMC into S phase.

Mdm2 inhibition of p53 induces E2F1 transactivation via p21

The transcription factor E2F1 functions as a key regulator for both cell-cycle progression and apoptosis. Mdm2, a major cellular regulator of the p53 tumor suppressor protein, is also closely involved in cell cycle and apoptosis. In addition to regulation of p53, Mdm2 has been reported to stimulate E2F1 transactivation by a mechanism that remains unclear. Here we examined how overexpression of Mdm2 alters E2F1/DP1 transactivation. Using a set of cell lines with differing p53 and Rb status we determined that Mdm2 induction of E2F1 transactivation was p53-dependent, resulting from release of repression by p53. While Mdm2 association with p53 was required to increase E2F1 transactivation, Mdm2 mediated degradation of p53 was not. p53 repression of E2F1 transactivation required a functional DNA binding and transactivation domain. Consistent with Mdm2 activation of E2F1 via an inhibition of p53 transactivation we demonstrate a concomitant reduction in p21 protein levels with Mdm2 overexpression. Furthermore, E2F1 repression by an Rb-phosphorylation mutant could not be reversed by Mdm2 overexpression. Mdm2 was also unable to enhance E2F1 transactivation in Mouse embryo fibroblasts lacking p21. Taken together, these results suggest that Mdm2 activation of E2F1 occurs through the repression of p53-dependent transcription of p21, a p53-target gene and cyclin dependent kinase inhibitor.

p14(ARF) regulates E2F activity

The ARF protein product of the ink4a/arf locus is induced by a variety of oncogenic signals. ARF facilitates growth arrest through the p53 pathway by hindering the down-regulation of p53 activity mediated by MDM2, through the formation of a protein complex with MDM2. Here we have explored the possibility that human p14(ARF) activity is integrated with growth regulating pathways other than p53, and report our results that p14(ARF) can control the activity of the E2F transcription factor. p14(ARF) regulates E2F activity in different cell-types, including p53(-/-)/mdm(-/-) MEFs, thus excluding that the effects of p14(ARF) are indirectly caused through MDM2 modulation. p14(ARF) down-regulates E2F-dependent transcription, and in cells undergoing E2F-dependent apoptosis prompts cell cycle arrest. p14(ARF) possesses multiple binding domains for E2F-1, one of which resides within the N-terminal region and coincides with the regulation of E2F activity. A mutational analysis of p14(ARF) indicates that the E2F-1 and MDM2 binding domains can be distinguished. These results highlight the potential interplay between p14(ARF) and E2F, and establish p14(ARF) as a pleiotrophic regulator of cell growth that acts by targetting at least two key pathways in the control of proliferation, namely E2F and p53.

Induction of apoptosis by telomere 3' overhang-specific DNA

Telomeres are tandem repeats of a specific TTAGGG nucleotide sequence at the ends of chromosomes. Telomere shortening is proposed to act as a biological clock and cancer prevention mechanism by inducing a nonproliferative, senescent phenotype after a limited number of cellular divisions. Recent evidence also suggests that telomere disruption can trigger apoptosis in certain cell types, mimicking a major cellular response to DNA damage. Here, we show that addition of DNA oligonucleotides homologous to the telomere 3' overhang sequence causes lymphocytic (Jurkat) cells to undergo apoptosis, as described for lymphocytes following telomere loop disruption. We further implicate the p53 tumor suppressor and transcription factor, as well as the p53 homolog p73 and the E2F1 transcription factor, in mediating the apoptotic response. We propose that exposure of the telomere 3' overhang due to opening of the normal telomere loop structure is a physiologic signal for these DNA damage-like responses in vivo and that oligonucleotides partially or completely homologous to the telomere overhang mimic this signal in the absence of DNA damage or telomere disruption.

TP53 polymorphism of exon 4 at codon 72 in cutaneous squamous cell carcinoma and benign epithelial lesions of renal transplant recipients and immunocompetent individuals: lack of correlation with human papillomavirus status

A common polymorphism at codon 72 of exon 4 encoding either arginine or proline has been shown to confer a susceptibility to the development of skin tumor in renal transplant recipients. Moreover, this polymorphism may affect proteolytic degradation of p53 promoted by E6 protein from mucosal human papillomaviruses and represent a risk factor for human-papillomavirus-induced carcinogenesis. In this study, we analyzed the human papillomavirus presence and the TP53 allele distribution in cutaneous squamous cell carcinoma of renal transplant recipients and immunocompetent patients. Fifty-three squamous cell carcinomas from 40 renal transplant recipients, 50 benign epithelial skin lesions from 50 renal transplant recipients with no history of skin cancer, 51 squamous cell carcinomas from immunocompetent patients, and 29 blood samples from immunocompetent individuals without skin cancer were investigated. Human papillomavirus DNA was detected using polymerase chain reaction performed with two pairs of primers (MY09-MY11 and FAP59-FAP64). TP53 allele distribution was studied by denaturing gradient gel electrophoresis assay, followed by sequencing analysis. Human papillomavirus DNA was detected in 64% of squamous cell carcinoma and 79% of benign epithelial lesions from renal transplant recipients (NS) and only in 37% of squamous cell carcinoma from immunocompetent patients (p < 0.05). Mucosal oncogenic human papillomavirus types were predominant in squamous cell carcinoma from both renal transplant recipients and immunocompetent patients. Rate of arginine homozygosity in squamous cell carcinoma from renal transplant recipients was significantly higher (83%) than in immunocompetent patients with or without squamous cell carcinoma (60% and 59%, respectively). Our results suggest that TP53 arginine/arginine genotype could represent a potential risk factor for the development of squamous cell carcinoma in renal transplant recipients compared to immunocompetent patients. No association between TP53 arginine/arginine genotype and human papillomavirus status could be determined, however.

DN-p73 is activated after DNA damage in a p53-dependent manner to regulate p53-induced cell cycle arrest

p53 and p73 genes are both activated in response to DNA damage to induce either cell cycle arrest or apoptosis, depending on the strength and the quality of the damaging stimulus. p53/p73 transcriptional activity must be tightly regulated to ensure that the appropriate biological response is achieved and to allow the cell to re-enter into the cell cycle after the damage has been repaired. In addition to multiple transcriptionally active (TA) isoforms, dominant negative (DN) variants, that lack the amino-terminal transactivation domain and function as trans-repressors of p53, p63 and p73, are expressed from a second internal promoter (P2-p73Pr). Here we show that, in response to a non apoptotic DNA damage induced by low doses of doxorubicin, p53 binds in vivo, as detected by a p53-specific chromatin immunoprecipitation assay, and activates the P2-p73 promoter. DN-p73alpha protein accumulates under the same conditions and exogenously expressed DN-p73alpha is able to counteract the p53-induced activation of the P2-p73Pr. These results suggest that DN-p73 may contribute to the autoregulatory loops responsible for the termination of p53/p73 responses in cells that do not undergo apoptosis. Accordingly, the activation of the P2-p73Pr is markedly enhanced in both p73-/- murine fibroblasts and in human cells in which p73 transcripts are selectively knocked-out by p73-specific small interfering RNAs.

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.

C-terminal deletion mutant p21(WAF1/CIP1) enhances E2F-1-mediated apoptosis in colon adenocarcinoma cells

The present study was designed to investigate the efficacy of combination gene therapy using adenoviral vectors expressing gene products shown to possess apoptotic activity: E2F-1 (Ad-E2F-1) and a C-terminal deletion mutant of p21(WAF1/cIP1) (Ad-p21(-PCNA)), on growth inhibition and apoptosis of human colon cancer cells in vitro and in vivo. Marked E2F-1 and p21(-PCNA) overexpression in response to adenovirus infection was evident by Western blot analysis. IC(25) concentrations of each virus were used for each treatment in vitro to detect cooperative effects on cell death. Coexpression of E2F-1 and p21(-PCNA) resulted in an additive effect on cell death compared to infection with either virus alone. Cell cycle analysis, poly(ADP-ribose) polymerase (PARP) cleavage and analysis of cell morphology also revealed that coinfection with both Ad-E2F-1 and Ad-p21(-PCNA) enhanced cellular apoptosis compared to either virus alone. Interestingly, E2F-1 protein expression was markedly enhanced in the E2F-1/p21(-PCNA) adenovirus combination compared to Ad-E2F-1 infection alone. However, these same effects were not evident in cells coinfected with Ad-E2F-1 and an adenovirus expressing wild-type human p21(WAF1/CIP1) (Ad-p21(WT)). The increase in E2F-1 expression with coexpression of E2F-1 and p21(-PCNA) was not a result of increased E2F-1 protein stability, but was related to increased transcriptional activity from the CMV promoter. Cell cycle analysis revealed G1 arrest 72 hours following single-gene therapy with either the wild-type or mutant p21, whereas increased accumulation of cells in G2/M phase was demonstrated in the E2F-1-overexpressing cells. In the combined therapies, E2F-1/p21(-PCNA) treatment still resulted in G1 arrest, but E2F-1 was able to counteract the G1 arrest when coinfected with p21(WT). These results provide further evidence of the importance of the p21:PCNA-binding domain in mediating the complex cell cycle interaction between E2F-1 and p21. Simultaneous intratumoral injection of Ad-E2F-1 and Ad-p21(-PCNA) dramatically reduced tumor burden of SW620 xenografts compared to either treatment alone in our in vivo model but not in HT-29 colon cancer xenografts. When combined with Ad-p21(-PCNA), E2F-1 adenovirus therapy resulted in approximately 95% decrease in tumor volume of SW620 tumor xenografts compared with controls (P<.05). In conclusion, although simultaneous delivery of E2F-1 and p21(-PCNA) transgenes results in increased E2F-1 expression and enhanced apoptosis of both SW620 and HT-29 colon cancer cells in vitro, this combination was only effective in the treatment of SW620 metastatic colon cancer in vivo. This may represent a potentially useful combination gene therapy strategy for metastatic colon cancer.

The human p73 promoter: characterization and identification of functional E2F binding sites

p73, a member of the p53 family, is overexpressed in many cancers. To understand the mechanism(s) underlying this overexpression, we have undertaken a detailed characterization of the human p73 promoter. The promoter is strongly activated in cells expressing exogenous E2F1 and suppressed by exogenous Rb. At least three functional E2F binding sites, located immediately upstream of exon 1 (at -284, -155 and -132) mediate this induction. 5' serially deleted promoter constructs and constructs harboring mutated E2F sites were analyzed for their response to exogenously expressed E2F1 or Rb to establish functionality of these sites. Authenticity of E2F sites was further confirmed by electrophoretic mobility shift assay (EMSA) using E2F1/DP1 heterodimers synthesized in vitro, followed by competition assays with unlabeled wild-type or mutant oligonucleotides and supershift analysis using anti-E2F1 antibodies. In vivo binding of E2F1 to the p73 promoter was demonstrated using nuclear extracts prepared from E2F1-inducible Saos2 cells. The region conferring the highest promoter activity was found to reside between -113 to -217 of the p73 gene. Two of the three functional E2F sites (at -155 and -132) reside within this region. Our results suggest that regulation of p73 expression is primarily mediated through binding of E2F1 to target sites at -155 and -132.

E2F1 and c-Myc potentiate apoptosis through inhibition of NF-kappaB activity that facilitates MnSOD-mediated ROS elimination

Overexpression of c-Myc or E2F1 sensitizes host cells to various types of apoptosis. Here, we found that overexpressed c-Myc or E2F1 induces accumulation of reactive oxygen species (ROS) and thereby enhances serum-deprived apoptosis in NIH3T3 and Saos-2. During serum deprivation, MnSOD mRNA was induced by NF-kappaB in mock-transfected NIH3T3, while this induction was inhibited in NIH3T3 overexpressing c-Myc or E2F1. In these clones, E2F1 inhibited NF-kappaB activity by binding to its subunit p65 in competition with a heterodimeric partner p50. In addition to overexpressed E2F1, endogenous E2F1 released from Rb was also found to inhibit NF-kappaB activity in a cell cycle-dependent manner by using E2F1(+/+) and E2F1(-/-) murine embryonic fibroblasts. These results indicate that E2F1 promotes apoptosis by inhibiting NF-kappaB activity.

Physical interaction of p73 with c-Myc and MM1, a c-Myc-binding protein, and modulation of the p73 function

p73 shares high sequence homology with the tumor suppressor p53. Like p53, ectopic overexpression of p73 induces cell cycle arrest and/or apoptosis, and these biological activities are linked to its sequence-specific transactivation function. The COOH-terminal region of p73 is unique and has a function to modulate DNA-binding ability and transactivation activity. To identify and characterize cellular proteins that interact with the COOH-terminal region of p73 alpha and regulate its activity, we employed a yeast-based two-hybrid screen with a human fetal brain cDNA library. We found MM1, a nuclear c-Myc-binding protein, was associated with p73 alpha in both yeast two-hybrid and in vitro pull-down assays. In mammalian cells, MM1 co-immunoprecipitated with p73 alpha, whereas p73 beta and tumor suppressor p53 did not interact with MM1. Overexpression of MM1 in p53-deficient osteosarcoma SAOS-2 cells enhanced the p73 alpha-dependent transcription from the p53/p73-responsive Bax and PG13 promoters, whereas p73 beta- and p53-mediated transcriptional activation was unaffected in the presence of MM1. MM1 also stimulated the p73 alpha-mediated growth suppression in SAOS-2 cells. More importantly, we found that c-Myc was physically associated with p73 alpha and significantly impaired the transcriptional activity of p73 alpha on Bax and p21(waf1) promoters. Expression of MM1 strongly reduced the c-Myc-mediated inhibitory activity on p73 alpha. These results suggest that MM1 may act as a molecular partner for p73 to prevent the c-Myc-mediated inhibitory effect on its activity.

Transactivation-deficient Delta TA-p73 inhibits p53 by direct competition for DNA binding: implications for tumorigenesis

The p53 family member p73 displays significant structural and functional homology to p53. However, instead of mutational inactivation, overexpression of wild-type p73 has been reported in various tumor types compared with normal tissues, arguing against a classical tumor suppressor function. Recently, N-terminally truncated, transactivation-deficient p73 isoforms (DeltaTA-p73) have been identified as a second class of p73 proteins. Because overexpression of p73 in tumors includes DeltaTA-p73, we further characterized these novel p73 isoforms. We show that DeltaTA-p73 retains DNA-binding competence but lacks transactivation functions, resulting in an inability to induce growth arrest and apoptosis. Importantly, DeltaTA-p73 acts as a dominant-negative inhibitor of p53 and full-length p73 (TA-p73). We demonstrate that inhibition of p53 involves competition for DNA binding, whereas TA-p73 can be inhibited by direct protein-protein interaction. Further, we show that up-regulation of endogenous p73 just like ectopic overexpression of DeltaTA-p73 confers resistance to p53-mediated apoptosis induced by the chemotherapeutic agent H-7. Because inhibition of p53 is a common theme in human cancer, our data strongly support a role of DeltaTA-p73 expression for tumor formation.

Gene expression changes in response to E2F1 activation

The p16/RB/E2F regulatory pathway, which controls transit through the G1 restriction point of the cell cycle, is one of the most frequent targets of genetic alterations in human cancer. Any of these alterations results in the deregulated expression of the transcription factor E2F, one of the key mediators of cell cycle progression. Under these conditions, E2F1 also participates in the induction of apoptosis by a p53-dependent pathway, and independently of p53. Recently, we identified the p53-homolog p73 as a first direct target of p53-independent apoptosis. Here, we used a cDNA microarray to screen an inducible E2F1-expressing Saos-2 cell line for E2F1 target genes. Expression analysis by cDNA microarray and RT-PCR revealed novel E2F1 target genes involved in E2F1-regulated cellular functions such as cell cycle control, DNA replication and apoptosis. In addition, the identification of novel E2F1 target genes participating in the processes of angiogenesis, invasion and metastasis supports the view that E2F1 plays a central role in many aspects of cancer development. These results provide new insight into the role of E2F1 in tumorigenesis as a basis for the development of novel anti-cancer therapeutics.

Autoinhibitory regulation of p73 by Delta Np73 to modulate cell survival and death through a p73-specific target element within the Delta Np73 promoter

p73 is a p53-related tumor suppressor but is also induced by oncogene products such as E2F-1, raising a question as to whether p73 is a tumor suppressor gene or oncogene. Unlike p53, p73 has several variants, including Delta Np73, which lacks the NH(2)-terminal transactivation domain. Although, in developing neurons, Delta Np73 is expressed abundantly and seems to inhibit the proapoptotic function of p53, the role of p73 and Delta Np73 and their regulatory mechanism in cell growth and differentiation are poorly understood. Here we report that p73, but not p53, directly activates the transcription of endogenous Delta Np73 by binding to the p73-specific target element located at positions -76 to -57 within the Delta Np73 promoter region. The activation of Delta Np73 promoter by p63 was marginal. Delta Np73 was associated with p73 alpha, p73 beta, and p53, as demonstrated by immunoprecipitation assays, and inhibited their transactivation activities when we used reporters of Mdm2, Bax, or Delta Np73 itself in SAOS-2 cells. Furthermore, induction or overexpression of Delta Np73 promoted cell survival by competing with p53 and p73 itself. Thus, our results suggest that the negative feedback regulation of p73 by its target Delta Np73 is a novel autoregulatory system for modulating cell survival and death.

E2F-1 is essential for normal epidermal wound repair

E2F factors are involved in proliferation and apoptosis. To understand the role of E2F-1 in the epidermis, we screened wild type and E2F-1(-/-) keratinocyte mRNA for genes differentially expressed in the two cell populations. We demonstrate the reduced expression of integrins alpha(5), alpha(6), beta(1), and beta(4) in E2F-1(-/-) keratinocytes associated with reduced activation of Jun terminal kinase and Erk upon integrin stimulation. As a consequence of altered integrin expression and function, E2F-1(-/-) keratinocytes also show impaired migration, adhesion to extracellular matrix proteins, and a blunted chemotactic response to transforming growth factor-gamma1. E2F-1(-/-) keratinocytes, but not dermal fibroblasts, exhibit altered patterns of proliferation, including significant delays in transit through both G(1) and S phases of the cell cycle. Recognizing that proliferation and migration are key for proper wound healing in vivo, we postulated that E2F-1(-/-) mice may exhibit abnormal epidermal repair upon injury. Consistent with our hypothesis, E2F-1(-/-) mice exhibited impaired cutaneous wound healing. This defect is associated with substantially reduced local inflammatory responses and rates of re-epithelialization. Thus, we demonstrate that E2F-1 is indispensable for a hitherto unidentified cell type-specific and unique role in keratinocyte proliferation, adhesion, and migration as well as in proper wound repair and epidermal regeneration in vivo.

Induction of p57(KIP2) expression by p73beta

The p53-related protein p73 has many functions similar to that of p53 including the ability to induce cell-cycle arrest and apoptosis. Both p53 and p73 function as transcription factors, and p73 activates expression of many genes that also are regulated by p53. Despite their similarities, it is evident that p53 and p73 are not interchangeable functionally, with p73 playing a role in normal growth and development that is not shared by p53. In this paper we describe the ability of p73beta but not p53 to activate expression of the cyclin-dependent kinase inhibitor p57(KIP) and KvLQT1, two genes that are coregulated in an imprinted region of the genome. Our results suggest that p73 may regulate expression of genes through mechanisms that are not shared by p53, potentially explaining the different contributions of p53 and p73 to normal development.

Role of p73 in malignancy: tumor suppressor or oncogene?

The recently identified p53 family member, p73, shows substantial structural and functional homology with p53. However, despite the established role of p53 as a proto-type tumor suppressor, a similar function of p73 in malignancy is questionable. Overexpression of p73 can activate typical p53-responsive genes, and activation of p73 has been implicated in apoptotic cell death induced by aberrant cell proliferation and some forms of DNA-damage. These data together with the localization of TP73 on chromosome 1p36, a region frequently deleted in a variety of human tumors, led to the hypothesis that p73 has tumor suppressor activity just like p53. However, unlike p53-/- mice, p73 knockout mice do not develop tumors. Extensive studies on primary tumor tissues have revealed overexpression of wild-type p73 in the absence of p73 mutations instead, suggesting that p73 may augment, rather than inhibit tumor development. In contrast to p53, differential splicing of the TP73 gene locus gives rise to a complex pattern of interacting p73 isoforms with antagonistic functions. In fact, induction of apoptosis by increased levels of p73 can be blocked by both p53 mutants and the N-terminally truncated p73 isoforms, which were recently shown to possess oncogenic potential. In the light of these new findings the contradictory role of p73 in malignancy will be discussed.

Expression of DeltaNp73 is a molecular marker for adverse outcome in neuroblastoma patients

The p73 gene is a p53 homologue which induces apoptosis and inhibits cell proliferation. Although p73 maps at 1p36.3 and is frequently deleted in neuroblastoma (NB), it does not act as a classic oncosuppressor gene. In developing sympathetic neurons of mice, p73 is predominantly expressed as a truncated anti-apoptotic isoform (DeltaNp73), which antagonizes both p53 and the full-length p73 protein (TAp73). This suggests that p73 may be part of a complex tumor-control mechanism. To determine the role of DeltaNp73 in NB we analyzed the pattern of expression of this gene in vivo and evaluated the prognostic significance of its expression. Our results indicate that DeltaNp73 expression is associated with reduced apoptosis in a NB tumor tissue. Expression of this variant in NB patients significantly correlates with age at diagnosis and VMA urinary excretion. Moreover it is strongly associated with reduced survival (HR=7.93; P<0.001) and progression-free survival (HR=5.3; P<0.001) and its role in predicting a poorer outcome is independent from age, primary tumor site, stage and MYCN amplification (OS: HR=5.24, P=0.012; PFS: HR=4.36, P=0.005). In conclusion our data seem to indicate that DeltaNp73 is a crucial gene in neuroblastoma pathogenesis.

ARF differentially modulates apoptosis induced by E2F1 and Myc

The ARF tumor suppressor participates in a p53-dependent apoptotic pathway that is stimulated in response to some oncogenic stimuli. The E2F1 transcription factor is a critical downstream target of the Rb tumor suppressor and, when active, can promote proliferation as well as apoptosis. The finding that E2F1 transcriptionally regulates the ARF gene has led to the suggestion that ARF contributes to E2F1-induced apoptosis. Counter to this hypothesis, this study demonstrates not only that ARF is unnecessary for E2F1 to induce apoptosis but also that inactivation of ARF actually enhances the ability of E2F1 to promote apoptosis. Inactivation of ARF also cooperates with E2F1 activity to promote entry into the S phase of the cell cycle. This relationship between ARF and E2F1 is demonstrated in transgenic epidermis in vivo and in mouse embryo fibroblast cultures in vitro. In contrast, the ability of Myc to induce apoptosis is diminished in the absence of ARF. E2F1 induces the accumulation of p53 in the absence of ARF, and this is associated with the phosphorylation of p53 on several residues. These findings demonstrate that ARF is a negative regulator of E2F1 activity and is not required for E2F1-induced apoptosis.

Direct repression of the Mcl-1 promoter by E2F1

E2F1 induces apoptosis via both p53-dependent and p53-independent mechanisms. The direct targets in the p53-independent pathway remain enigmatic; however, the induction of this pathway does not require the transactivation domain of E2F1. Using cells that are defective in p53 activation, we show that E2F1 potently represses the expression of Mcl-1--an anti-apoptotic Bcl-2 family member whose depletion results in apoptosis. We also show that this transcriptional repression is direct and dependent upon E2F1's DNA-binding domain, but does not require the transactivation domain of E2F1. Consistent with this DNA binding requirement of E2F1 to repress Mcl-1, we show that E2F1 binds to the Mcl-1 promoter both in vitro and in vivo, and have identified the DNA element (-143/-117) within this promoter that is required for E2F1 binding and repression. Additionally, cell lines constitutively expressing Mcl-1 are resistant to E2F1-mediated apoptosis--suggesting that Mcl-1 downregulation is a necessary event in the p53-independent apoptotic process. Thus, we identify a p53 family-independent mechanism of E2F1-induced apoptosis in which E2F1 directly represses Mcl-1 expression.

Genetic analysis of Pten and Ink4a/Arf interactions in the suppression of tumorigenesis in mice

Dual inactivation of PTEN and INK4a/ARF tumor suppressor genes is a common feature observed in a broad spectrum of human cancer types. To validate functional collaboration between these genes in tumor suppression, we examined the biological consequences of Pten and/or Ink4a/Arf deficiency in cells and mice. Relative to single mutant controls, Ink4a/Arf-/-Pten+/- mouse embryonic fibroblast cultures exhibited faster rates of growth in reduced serum, grew to higher saturation densities, produced more colonies upon low density seeding, and showed increased susceptibility to transformation by oncogenic H-Ras. Ink4a/Arf deficiency reduced tumor-free survival and shortened the latency of neoplasias associated with Pten heterozygosity, specifically pheochromocytoma, prostatic intraepithelial neoplasia, and endometrial hyperplasia. Compound mutant mice also exhibited an expanded spectrum of tumor types including melanoma and squamous cell carcinoma. Functional synergy between Ink4a/Arf and Pten manifested most prominently in the development of pheochromocytoma, prompting an analysis of genes and loci implicated in this rare human neoplasm. The classical pheochromocytoma genes Ret, Vhl, and Nf-1 remained intact, a finding consistent with the intersection of these genes with pathways engaged by Pten and Ink4a/Arf. Notably, conventional and array-comparative genomic hybridization revealed frequent loss of distal mouse chromosome 4 in a region syntenic to human chromosome 1p that is implicated in human pheochromocytoma. This study provides genetic evidence of collaboration between Pten and Ink4a/Arf in constraining the growth and oncogenic transformation of cultured cells and in suppressing a wide spectrum of tumors in vivo.

On the shoulders of giants: p63, p73 and the rise of p53

The discoveries of the p53 homologs, p63 and p73, have both fueled new insights and exposed enigmas in our understanding of the iconic p53 tumor suppressor. Although the pivotal role of p53 in cancer pathways remains unchallenged, because p63 and p73 are now implicated in stem cell identity, neurogenesis, natural immunity and homeostatic control. Despite their seemingly separate tasks, there are hints that the p53 family members both collaborate and interfere with one another. The question remains, therefore, as to whether these genes evolved to function independently or whether their familial ties still bind them in pathways of cell proliferation, death and tumorigenesis.

Regulation of p73 by c-Abl through the p38 MAP kinase pathway

p73 is a novel member of the p53 family of tumor suppressor proteins which is involved in cellular differentiation, tumor suppression, and the response to genotoxic stress. The molecular mechanisms regulating p73 activity are still poorly understood. Recently, p73 was found to be a target of the enzymatic activity of c-Abl, a non-receptor tyrosine kinase that potently activated in response to DNA damage. Here, we present evidence that c-Abl induces the phosphorylation of p73 in threonine residues adjacent to prolines, and that the p38 MAP kinase pathway mediates this response. Furthermore, we found that activation of p38 is sufficient to enhance the stability of p73, and that the transcriptional activation of p73 by c-Abl requires the activity of p38. These findings indicate that members of the MAP kinases superfamily of signaling molecules can regulate p73, and support a role for the p38 MAP kinase in a novel biochemical pathway by which c-Abl regulates this p53-related molecule.

E7 proteins from oncogenic human papillomavirus types transactivate p73: role in cervical intraepithelial neoplasia

In common with other E2F1 responsive genes such as p14(ARF) and B-myb, the promoter of p73 is shown to be positively regulated in cell lines and primary human keratinocytes by E7 proteins from oncogenic human papillomavirus (HPV) types 16, 18, 31 and 33, but not HPV 6. Mutational analysis revealed that transactivation of the p73 promoter by HPV 16E7 requires association with pRb. Expression of p73 in normal cervical epithelium is confined to the basal and supra-basal layers. In contrast, expression in neoplastic lesions is detected throughout the epithelium and increases with grade of neoplasia, being maximal in squamous cell cancers (SCC). Deregulation of expression of the N-terminal splice variant p73Delta2 was observed in a significant proportion of cancers, but not in normal epithelium. The frequent over-expression of p73Delta2, which has recognized transdominant properties, in malignant and pre-malignant lesions suggests a role in the oncogenic process in cervical epithelium.

Animal models of tumor-suppressor genes

The development of cancer requires multiple genetic alterations perturbing distinct cellular pathways. In human cancers, these alterations often arise owing to mutations in tumor-suppressor genes whose normal function is to either inhibit the proliferation, apoptosis, or differentiation of cells, or maintain their genomic integrity. Mouse models for tumor suppressors frequently provide definitive evidence for the antitumorigenic functions of these genes. In addition, animal models permit the identification of previously unsuspected roles of these genes in development and differentiation. The availability of null and tissue-specific mouse mutants for tumor-suppressor genes has greatly facilitated our understanding of the mechanisms leading to cancer. In this review, we describe mouse models for tumor-suppressor genes.

Macrophage migration inhibitory factor (MIF) sustains macrophage proinflammatory function by inhibiting p53: regulatory role in the innate immune response

The importance of the macrophage in innate immunity is underscored by its secretion of an array of powerful immunoregulatory and effector molecules. We report herein that macrophage migration inhibitory factor (MIF), a product of activated macrophages, sustains macrophage survival and function by suppressing activation-induced, p53-dependent apoptosis. Endotoxin administration to MIF(-/-) mice results in decreased macrophage viability, decreased proinflammatory function, and increased apoptosis when compared with wild-type controls. Moreover, inhibition of p53 in endotoxin-treated, MIF-deficient macrophages suppresses enhanced apoptosis and restores proinflammatory function. MIF inhibits p53 activity in macrophages via an autocrine regulatory pathway, resulting in a decrease in cellular p53 accumulation and subsequent function. Inhibition of p53 by MIF coincides with the induction of arachidonic acid metabolism and cyclooxygenase-2 (Cox-2) expression, which is required for MIF regulation of p53. MIF's effect on macrophage viability and survival provides a previously unrecognized mechanism to explain its critical proinflammatory action in conditions such as sepsis, and suggests new approaches for the modulation of innate immune responses.

Activation of retinoblastoma protein in mammary gland leads to ductal growth suppression, precocious differentiation, and adenocarcinoma

The retinoblastoma (Rb) tumor suppressor controls cellular proliferation, survival, and differentiation and is functionally inactivated by mutations or hyperphosphorylation in most human cancers. Although activation of endogenous Rb is thought to provide an effective approach to suppress cell proliferation, long-term inhibition of apoptosis by active Rb may have detrimental consequences in vivo. To directly test these paradigms, we targeted phosphorylation-resistant constitutively active Rb alleles, Rb Delta Ks, to the mouse mammary gland. Pubescent transgenic females displayed reduced ductal elongation and cell proliferation at the endbuds. Post-puberty transgenic mice exhibited precocious cellular differentiation and beta-casein expression and extended survival of the mammary epithelium with a moderate but specific effect on the expression of E2F1, IGF1R alpha, and phospho-protein kinase B/AKT. Remarkably, approximately 30% Rb Delta K transgenic females developed focal hyperplastic nodules, and approximately 7% exhibited full-blown mammary adenocarcinomas within 15 mo. Expression of the Rb Delta K transgene in these mammary tumors was reduced greatly. Our results suggest that transient activation of Rb induces cancer by extending cell survival and that the dual effects of Rb on cell proliferation and apoptosis impose an inherent caveat to the use of the Rb pathway for long-term cancer therapy.

Induction of apoptosis in cancer: new therapeutic opportunities

Autonomous cell proliferation is one of the hallmarks of cancer cells, driven by activated growth-promoting oncogenes. However, deregulated activation of these oncogenes also triggers apoptosis via multiple pathways. Among them, the ARF-p53 pathway appears to play a major role in mediating oncogene-induced apoptosis. Consequently, suppression of apoptosis by inactivation of p53 and other tumor suppressors is central to tumor development. These findings have broad implications in understanding cancer genetics and therapy. They help define the roles for oncogenes and tumor suppressor genes in tumorigenesis. Furthermore, the notion that cancer cells often carry specific defects in apoptotic pathways but are inherently sensitive to apoptosis as a result of deregulated proliferation, offers numerous opportunities for manipulating apoptosis in directions of clinical application.

Sibling rivalry in the E2F family

The E2F transcription factor family determines whether or not a cell will divide by controlling the expression of key cell-cycle regulators. The individual E2Fs can be divided into distinct subgroups that act in direct opposition to one another to promote either cellular proliferation or cell-cycle exit and terminal differentiation. What is the underlying molecular basis of this 'push-me-pull-you' regulation, and what are its biological consequences?

p53, p63 and p73--solos, alliances and feuds among family members

p53 controls crucial stress responses that play a major role in preventing malignant transformation. Hence, inactivation of p53 is the single most common genetic defect in human cancer. With the recent discovery of two close structural homologs, p63 en p73, we are getting a broader view of a fascinating gene family that links developmental biology with tumor biology. While unique roles are apparent for each of these genes, intimate biochemical cross-talk among family members suggests a functional network that might influence many different aspects of individual gene action. The most interesting part of this family network derives from the fact that the p63 and p73 genes are based on the "two-genes-in-one" idea, encoding both agonist and antagonist in the same open reading frame. In this review, we attempt to present an overview of the current status of this fast moving field.

Activation and activities of the p53 tumour suppressor protein

The p53 tumour suppressor protein inhibits malignant progression by mediating cell cycle arrest, apoptosis or repair following cellular stress. One of the major regulators of p53 function is the MDM2 protein, and multiple forms of cellular stress activate p53 by inhibiting the MDM2-mediated degradation of p53. Mutations in p53, or disruption of the pathways that allow activation of p53, seem to be a general feature of all cancers. Here we review recent advances in our understanding of the pathways that regulate p53 and the pathways that are induced by p53, as well as their implications for cancer therapy.

A transcriptionally inactive E2F-1 targets the MDM family of proteins for proteolytic degradation

E2F-1-activated transcription promotes cell cycle progression and apoptosis. These functions are regulated by several factors including the E2F-1-binding protein MDM2 and the retinoblastoma protein pRb. Using a yeast two-hybrid screen we have identified the MDM2-related protein, MDMX, as an E2F-1-binding protein. In these studies we find that coexpression of MDMX with E2F-1 results in degradation of the MDMX protein. Although this proteolytic degradation can be blocked by the protease inhibitors bafilomycin A(1), N-acetyl-Leu-Leu-Norleu-AL, and N-acetyl-Leu-Leu-Met-AL, MDMX degradation is not inhibited by lactacystin, suggesting that degradation occurs by a proteasome-independent mechanism. Using an E2F-1 deletion mutant (E2F-1(180-437)) we show that E2F-1-targeted degradation of MDMX does not require the E2F-1 DNA binding domain and therefore is independent of E2F-1-driven transcription. We also find that this transcriptionally inactive E2F-1 mutant is capable of degrading the MDMX-related protein MDM2 and the MDMX isoform MDMX-S. Mapping of the E2F-1 C terminus reveals that neither a previously characterized C-terminal MDM2 binding domain nor the pRb binding domain on E2F-1 is required for MDMX and MDM2 degradation.

E2F1 and E2F2 determine thresholds for antigen-induced T-cell proliferation and suppress tumorigenesis

E2F activity is critical for the control of the G(1) to S phase transition. We show that the combined loss of E2F1 and E2F2 results in profound effects on hematopoietic cell proliferation and differentiation, as well as increased tumorigenesis and decreased lymphocyte tolerance. The loss of E2F1 and E2F2 impedes B-cell differentiation, and hematopoietic progenitor cells in the bone marrow of mice lacking E2F1 and E2F2 exhibit increased cell cycling. Importantly, we show that E2F1 and E2F2 double-knockout T cells exhibit more rapid entry into S phase following antigenic stimulation. Furthermore, T cells lacking E2F1 and E2F2 proliferate much more extensively in response to subthreshold antigenic stimulation. Consistent with these observations, E2F1/E2F2 mutant mice are highly predisposed to the development of tumors, and some mice exhibit signs of autoimmunity.

The transcriptional repressor ZEB regulates p73 expression at the crossroad between proliferation and differentiation

The newly discovered p73 gene encodes a nuclear protein that has high homology with p53. Furthermore, ectopic expression of p73 in p53(+/+) and p53(-/-) cancer cells recapitulates some of the biological activities of p53 such as growth arrest, apoptosis, and differentiation. p73(-/-)-deficient mice exhibit severe defects in proper development of the central nervous system and pheromone sensory pathway. They also suffer from inflammation and infections. Here we studied the transcriptional regulation of p73 at the crossroad between proliferation and differentiation. p73 mRNA is undetectable in proliferating C2C12 cells and is expressed at very low levels in undifferentiated P19 and HL60 cells. Conversely, it is upregulated during muscle and neuronal differentiation as well as in response to tetradecanoyl phorbol acetate-induced monocytic differentiation of HL60 cells. We identified a 1-kb regulatory fragment located within the first intron of p73, which is positioned immediately upstream to the ATG codon of the second exon. This fragment exerts silencer activity on p73 as well as on heterologous promoters. The p73 intronic fragment contains six consensus binding sites for transcriptional repressor ZEB, which binds these sites in vitro and in vivo. Ectopic expression of dominant-negative ZEB (ZEB-DB) restores p73 expression in proliferating C2C12 and P19 cells. Thus, transcriptional repression of p73 expression by ZEB binding may contribute to the modulation of p73 expression during differentiation.

p73 is over-expressed in vulval cancer principally as the Delta 2 isoform

p73 was studied in squamous cancers and precursor lesions of the vulva. Over-expression of p73 occurred commonly in both human papillomavirus (HPV)-positive and -negative squamous cell cancers (SCC) and high-grade premalignant lesions. Whereas expression in normal vulval epithelium was detected only in the basal and supra-basal layers, expression in neoplastic epithelium increased with grade of neoplasia, being maximal at both protein and RNA levels in SCC. p73 Delta 2 was the principal over-expressed isoform in the majority of cases of vulval SCC and often the sole form expressed in SCC. Over-expression of p73 was associated with expression of HPV-encoded E7 or with hypermethylation or mutation of p16(INK4a) in HPV-negative cases. There was a close correlation between expression of p73 and p14(ARF) in cancers with loss of p53 function. The frequent over-expression of p73 Delta 2 in neoplastic but not normal vulval epithelium, and its co-ordinate deregulation with other E2F-1 responsive genes suggests a role in the oncogenic process.

Insights into cancer therapeutic design based on p53 and TRAIL receptor signaling

Knowledge of the emerging pathways of cell death downstream of the p53 tumor suppressor and the TRAIL death-inducing ligand is suggesting ways to improve therapeutic design in cancer. In contrast to its unique G1 cell cycle arresting mechanism that is maintained by p21(WAF1), there are signals transduced by p53 to multiple apoptotic effectors perhaps due to the importance of apoptosis in suppressing tumors. There is evidence for cytoplasmic as well as mitochondrial activation of caspases downstream of p53, although in some cell lineages the signal ultimately involves the mitochondria. The TRAIL signaling pathway appears promising for therapeutic development despite sharing some similarities with the toxic Fas and TNF pathways, in terms of effector molecules and downstream signals. One of the key findings is the tissue specificity of cell death responses, a feature that could be exploited in strategies to widen the therapeutic window of combination cancer therapies. Efforts continue to develop p53-targeted cancer therapy, and novel clues to enhance or block specific effectors may improve therapeutic design.

The functional genomic response of developing embryonic submandibular glands to NF-kappa B inhibition

BACKGROUND

The proper balance between epithelial cell proliferation, quiescence, and apoptosis during development is mediated by the specific temporal and spatial appearance of transcription factors, growth factors, cytokines, caspases, etc. Since our prior studies suggest the importance of transcription factor NF-kappaB during embryonic submandibular salivary gland (SMG) development, we attempted to delineate the emergent dynamics of a cognate signaling network by studying the molecular patterns and phenotypic outcomes of interrupted NF-kappaB signaling in embryonic SMG explants.

RESULTS

SN50-mediated inhibition of NF-kappaB nuclear translocation in E15 SMG explants cultured for 2 days results in a highly significant increase in apoptosis and decrease in cell proliferation. Probabilistic Neural Network (PNN) analyses of transcriptomic and proteomic assays identify specific transcripts and proteins with altered expression that best discriminate control from SN50-treated SMGs. These include PCNA, GR, BMP1, BMP3b, Chk1, Caspase 6, E2F1, c-Raf, ERK1/2 and JNK-1, as well as several others of lesser importance. Increased expression of signaling pathway components is not necessarily probative of pathway activity; however, as confirmation we found a significant increase in activated (phosphorylated/cleaved) ERK 1/2, Caspase 3, and PARP in SN50-treated explants. This increased activity of proapoptotic (caspase3/PARP) and compensatory antiapoptotic (ERK1/2) pathways is consistent with the dramatic cell death seen in SN50-treated SMGs.

CONCLUSIONS

Our morphological and functional genomic analyses indicate that the primary and secondary effects of NF-kappaB-mediated transcription are critical to embryonic SMG developmental homeostasis. Relative to understanding complex genetic networks and organogenesis, our results illustrate the importance of evaluating the gene, protein, and activated protein expression of multiple components from multiple pathways within broad functional categories.

Apoptosis regulators and their role in tumorigenesis

It has become clear that, together with deregulated growth, inhibition of programmed cell death (PCD) plays a pivotal role in tumorigenesis. In this review, we present an overview of the genes and mechanisms involved in PCD. We then summarize the evidence that impaired PCD is a prerequisite for tumorigenesis, as indicated by the fact that more and more neoplastic mutations appear to act by interfering with PCD. This has made the idea of restoration of corrupted 'death programs' an intriguing new area for potential cancer therapy.

The INK4a/ARF network in tumour suppression

The retinoblastoma protein (RB) and p53 transcription factor are regulated by two distinct proteins that are encoded by the INK4a/ARF locus. Genes encoding these four tumour suppressors are disabled, either in whole or in part, in most human cancers. A complex signalling network that interconnects the activities of RB and p53 monitors oncogenic stimuli to provide a cell-autonomous mode of tumour surveillance.

Acquired expression of transcriptionally active p73 in hepatocellular carcinoma cells

p53 and p73 proteins activate similar target genes and induce apoptosis and cell cycle arrest. However, p53, but not p73 is considered a tumour-suppressor gene. Unlike p53, p73 deficiency in mice does not lead to a cancer-prone phenotype, and p73 gene is not mutated in human cancers, including hepatocellular carcinoma. Here we report that normal liver cells express only DeltaN-p73 transcript forms giving rise to the synthesis of N-terminally truncated, transcriptionally inactive and dominant negative p73 proteins. In contrast, most hepatocellular carcinoma cells express TA-p73 transcript forms encoding full-length and transcriptionally active p73 proteins, in addition to DeltaN-p73. We also show that together with the acquired expression of TA-p73, the 'retinoblastoma pathway' is inactivated, and E2F1-target genes including cyclin E and p14(ARF) are activated in hepatocellular carcinoma. However, there was no full correlation between 'retinoblastoma pathway' inactivation and TA-p73 expression. Most TA-p73-expressing hepatocellular carcinoma cells have also lost p53 function either by lack of expression or missense mutations. The p73 gene, encoding only DeltaN-p73 protein, may function as a tumour promoter rather than a tumour suppressor in liver tissue. This may be one reason why p73 is not a mutation target in hepatocellular carcinoma.

Role for c-Abl and p73 in the radiation response of male germ cells

p53 plays a central role in the induction of apoptosis of spermatogonia in response to ionizing radiation. In p53(-/-) testes, however, spermatogonial apoptosis still can be induced by ionizing radiation, so p53 independent apoptotic pathways must exist in spermatogonia. Here we show that the p53 homologues p63 and p73 are present in the testis and that p73, but not p63, is localized in the cytoplasm of spermatogonia. Unlike p53, neither p63 nor p73 protein levels were found to increase after a dose of 4 Gy of X-rays. Although p73 protein levels did not increase, its interaction with the non-receptor tyrosine kinase c-Abl and its phosphorylation on tyrosine residues did. c-Abl and p73 co-localize in the cytoplasm of spermatogonia and spermatocytes and in the residual bodies. Furthermore, c-Abl protein levels increase after irradiation. p63 was not found to co-localize or interact with c-Abl neither before nor after irradiation. In conclusion, in the testis ionizing radiation elevates cytoplasmic c-Abl that in turn interacts with p73. This may represent an additional, cytoplasmic, apoptotic pathway. Although less efficient than the p53 route, this pathway may cause spermatogonial apoptosis as observed in p53 deficient mice.

The transcription factor E2F1 promotes dopamine-evoked neuronal apoptosis by a mechanism independent of transcriptional activation

The E2F1 transcription factor plays an important role in promoting neuronal apoptosis; however, it is not clear how E2F1 does this. Here we show that E2F1 is involved in dopamine (DA)-evoked apoptosis in cerebellar granule neurons (CGNs). E2F1 -/- CGNs and CGNs expressing an antisense E2F1 cDNA were significantly protected from DA-toxicity relative to controls. The neuronal protection was accompanied by significantly reduced caspase 3 activity. E2F1-mediated neuronal apoptosis did not require activation of gene transcription because: (1) ectopic expression of E2F1 or its mutants lacking the transactivation domain induced neuronal apoptosis, whereas an E2F1 mutant lacking the DNA-binding domain did not; (2) under all of these conditions, known E2F1 target genes including cyclin A, cdc2 and p19(ARF) were not induced; and (3) DA-evoked neuronal apoptosis was associated with up-regulated E2F1, but not transcription of its target genes. Finally, E2F1-mediated neuronal apoptosis was associated with reduced nuclear factor (NF)-kappaB DNA-binding activity. Taken together, these data suggest that E2F1 promotes DA-evoked caspase 3-dependent neuronal apoptosis by a mechanism independent of gene transactivation, and this may possibly occur through inhibition of anti-apoptotic genes including NF-kappaB.

Up-regulation of E2F-1 in Down's syndrome brain exhibiting neuropathological features of Alzheimer-type dementia

We studied the expression of the apoptosis-related protein, E2F-1, in Down's syndrome (DS) brains. The immunoreactivity for E2F-1 was detected in the pyramidal neurons of the cerebral cortex from DS brains exhibiting the neuropathological features of dementia of Alzheimer type (DAT), in accordance with the amyloid beta protein (A beta) deposition in the neuron. Therefore, the implication is that A beta deposition may trigger E2F-1-mediated neuronal apoptosis in DS brains with DAT.

Transcriptional activities of p73 splicing variants are regulated by inter-variant association

p73 has been identified as a gene that encodes a protein with significant identity with the tumour suppressor p53. The main structural difference between p73 and p53 is the additional C-terminal region of p73. Six isoforms of p73 with differing C-terminal structures, alpha, beta, gamma, delta, epsilon and xi, have been reported. These variants differ in transcriptional activity on p53-responsive promoters. Here we report a possible mechanism of transcriptional activation by p73 splicing variants. C-terminal deletion mutants of p73 alpha showed a significantly higher level of transcriptional activity than wild-type p73 alpha, suggesting that the C-terminal structure of p73 alpha functions to repress the transcriptional activity of p73 alpha. The results of immunoprecipitation assays and two-hybrid assays in mammalian cells showed that the p73 variants interacted with each other, but not with p53. The transcriptional activity of p73 beta was reduced by co-expression with either p73 alpha or p73 epsilon, which bears an identical C-terminal structure to p73 alpha. Co-expression of the C-terminal portion of p73 alpha or p73 epsilon with p73 beta also resulted in reduced transcriptional activity. Moreover, we observed that the level of endogenous p21 protein induced by p73 beta was decreased by co-expression of full-length p73 epsilon or the C-terminal region of p73 alpha or p73 epsilon. These observations suggest that p73-mediated gene expression is regulated by the interactions of p73 splicing variants in the cell.

Activation-induced cell death

The death of T lymphocytes following their activation involves several signal pathways that converge on a series of proteases, known as caspases, that degrade cellular proteins and activate a DNAse. Caspases are activated through ligation of cell surface death receptors as well as via direct activation of downstream caspases, often through metabolic stress such as cytokine withdrawal or generation of oxygen radicals, that culminates in mitochondrial dysfunction and release of the pro-apoptotic molecules, cytochrome c and Smac/DIABLO. The Bcl-2 family members serve to regulate the mitochondrial membrane integrity. Recent studies are now revealing the significant contribution to the activation-induced cell death of T cells by downstream caspases such as caspase-3 and Bcl-2-homology domain 3 (BH3)-only members of the Bcl-2 family.

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.