A common E2F-1 and p73 pathway mediates cell death induced by TCR activation

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.

Mutant mouse models reveal the relative roles of E2F1 and E2F3 in vivo

The E2F1, -2, and -3 transcription factors are key downstream targets of the retinoblastoma protein (pRB) tumor suppressor that drive expression of proliferation-associated genes. Here we use mutant mouse strains to investigate E2F3's role in vivo. We show that E2F3 is essential for embryonic viability in the pure 129/Sv background but the presence of C57BL/6 alleles yields some adult survivors. Although growth retarded, surviving E2f3(-/-) animals are initially healthy. However, they die prematurely, exhibiting no obvious tumor phenotype but with the typical signs of congestive heart failure. The defects are completely distinct from those arising in E2f1 mutant mice (S. J. Field et al., Cell 85:549-561; 1996; L. Yamasaki et al., Cell 85:537-548, 1996), supporting the prevailing view that these E2Fs must have some unique biological functions in vivo. To test this model, we examined the phenotypes of E2f1 E2f3 compound mutant mice. Almost all of the developmental and age-related defects arising in the individual E2f1 or E2f3 mice were exacerbated by the mutation of the other E2f. Thus, E2F1 and E2F3 appear to play critical, overlapping roles in the development and maintenance of a variety of tissues. Importantly, this study did identify one major difference in the properties of E2F1 and E2F3: either alone or in combination with E2F1 loss, E2f3 mutation did not increase the incidence of tumor formation. These data strongly suggest that tumor suppression is a specific property of E2F1 and not E2F3.

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.

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.

Protein transduction technology

Intracellular delivery of macromolecules remains problematic because of the bioavailability restriction imposed by the cell membrane. Recent studies on protein transduction domains have circumvented this barrier, however, and have resulted in the delivery of peptides, full-length proteins, iron beads, liposomes, and radioactive isotopes into cells in culture and animal models 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.

Human papillomavirus type 16 E7 binds to E2F1 and activates E2F1-driven transcription in a retinoblastoma protein-independent manner

The human papillomavirus (HPV) E7 oncoprotein can immortalize primary human cells and induce tumor formation. These properties of E7 depend on its ability to inhibit the activity of retinoblastoma protein (pRB), which in turn affects E2F function. E2F proteins control the expression of genes involved in differentiation, development, cell proliferation, and apoptosis. By using genetic and biochemical approaches, the present study shows that E7 binds to E2F1 in vivo and in vitro and that both proteins co-localize in the nucleus. Importantly, the binding of the high risk group HPV E7 to E2F1 is tighter than the binding of the low risk group HPV E7 to E2F1. Although E7 of the high risk group HPVs activates E2F1-dependent transcription strongly in C33A or 293T cells, E7 of the low risk group HPVs activates transcription only weakly. By using electrophoretic mobility shift assay, we also showed that E7 binds to E2F1-DNA complexes. Furthermore, we show that these activities of E7 are independent of pRB by using E7 and E2F1 mutants that cannot bind to pRB. Taken together, these data suggest that E7 contributes to the deregulation of pRB-dependent E2F1 repression and to the further activation of E2F1 independently of pRB.

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.

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.

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.

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.

The role of p53 in regulating antiviral T cell responses

It is now well established that viral infections can induce large expansions of Ag-specific CD8(+) T cells. These cells divide very rapidly with an estimated doubling time of approximately 6 h. When virus is cleared, the vast majority of these effector CD8 T cells undergo apoptosis. The remaining memory cells persist at constant levels and provide the basis for the accelerated recall response upon rechallenge. The molecular mechanisms that control the rapid proliferation and death of Ag-specific T cells are poorly understood. Because of its important role in controlling cell proliferation and death, we examined antiviral immune responses in p53(-/-) mice using lymphocytic choriomeningitis virus. We found that effector CD8 and CD4 responses were comparable but that memory levels were slightly higher in -/- mice compared with +/+ mice. The lack of a major difference in virus-specific T cell responses between +/+ and -/- mice suggests that p53 only plays a minor role in regulating the proliferation, apoptosis, and maintenance of Ag-specific T cells. Thus, it appears that the primary function of p53 is in controlling "illegitimate" proliferation and tumor development and not in regulating Ag-specific T cell responses.

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.

p73 is not mutated in meningiomas as determined with a functional yeast assay but p73 expression increases with tumor grade

The p53 gene is normally wild type in meningiomas. Since all three members of the p53 gene family recognize the same DNA sequence, tumors containing wild type p53 could decrease transactivation of p53 target genes by mutating either p63 or p73. In meningiomas the most likely target is p73, because loss of heterozygosity of the chromosomal band containing p73 is the commonest genetic lesion in these tumors. To screen p73 for mutations we have developed a functional assay which tests the ability of p73 to activate transcription from a p53-responsive promoter in yeast. The assay correctly identified p73 mutants with mutations equivalent to hotspot mutations in p53, demonstrating that the assay can detect transcriptionally inactive p73. No mutations in p73 were identified in meningiomas. p73 RNA level was higher in more advanced tumors, but there was no correlation between the expression level of p73 and p21, a known p53 target gene. The yeast assay was also used to measure the intrinsic sensitivity of the p73 protein to mutagenesis. Like p53, p73 is exceptionally easy to inactivate as a transcription factor by point mutation. Taken together, these results indicate that p53 and p73 serve very different functions in tumors.

Differential regulation of retinoblastoma tumor suppressor protein by G(1) cyclin-dependent kinase complexes in vivo

The retinoblastoma tumor suppressor protein (pRB) negatively regulates early-G(1) cell cycle progression, in part, by sequestering E2F transcription factors and repressing E2F-responsive genes. Although pRB is phosphorylated on up to 16 cyclin-dependent kinase (Cdk) sites by multiple G(1) cyclin-Cdk complexes, the active form(s) of pRB in vivo remains unknown. pRB is present as an unphosphorylated protein in G(0) quiescent cells and becomes hypophosphorylated (approximately 2 mol of PO(4) to 1 mol of pRB) in early G(1) and hyperphosphorylated (approximately 10 mol of PO(4) to 1 mol of pRB) in late G(1) phase. Here, we report that hypophosphorylated pRB, present in early G(1), represents the biologically active form of pRB in vivo that is assembled with E2Fs and E1A but that both unphosphorylated pRB in G(0) and hyperphosphorylated pRB in late G(1) fail to become assembled with E2Fs and E1A. Furthermore, using transducible dominant-negative TAT fusion proteins that differentially target cyclin D-Cdk4 or cyclin D-Cdk6 (cyclin D-Cdk4/6) and cyclin E-Cdk2 complexes, namely, TAT-p16 and TAT-dominant-negative Cdk2, respectively, we found that, in vivo, cyclin D-Cdk4/6 complexes hypophosphorylate pRB in early G(1) and that cyclin E-Cdk2 complexes inactivate pRB by hyperphosphorylation in late G(1). Moreover, we found that cycling human tumor cells expressing deregulated cyclin D-Cdk4/6 complexes, due to deletion of the p16(INK4a) gene, contained hypophosphorylated pRB that was bound to E2Fs in early G(1) and that E2F-responsive genes, including those for dihydrofolate reductase and cyclin E, were transcriptionally repressed. Thus, we conclude that, physiologically, pRB is differentially regulated by G(1) cyclin-Cdk complexes.

Mutation analysis of the p73 gene in nonastrocytic brain tumours

Loss of heterozygosity (LOH) involving the distal chromosome 1 p36 region occurs frequently in nonastrocytic brain tumours, but the tumour suppressor gene targeted by this deletion is unknown. p73 is a novel gene that has high sequence homology and similar gene structure to the p53 gene; it has been mapped to 1 p36, and may thus represent a candidate for this tumour suppressor gene. To determine whether p73 is involved in nonastrocytic brain tumour development, we analysed 65 tumour samples including 26 oligodendrogliomas, 4 ependymomas, 5 medulloblastomas, 10 meningiomas, 2 meningeal haemangiopericytomas, 2 neurofibrosarcomas, 3 primary lymphomas, 8 schwannomas and 5 metastatic tumours to the brain, for p73 alterations. Characterization of allelic loss at 1 p36-p35 showed LOH in about 50% of cases, primarily involving oligodendroglial tumours (22 of 26 cases analysed; 85%) and meningiomas (4 of 10; 40%). PCR-SSCP and direct DNA sequencing of exons 2 to 14 of p73 revealed a missense mutation in one primary lymphoma: a G-to-A transition, with Glu291Lys change. 8 additional cases displayed no tumour-specific alterations, as 3 distinct polymorphic changes were identified: a double polymorphic change of exon 5 was found in one ependymoma and both samples derived from an oligodendroglioma, as follows: a G-to-A transition with no change in Pro 146, and a C-to-T variation with no change in Asn 204: a delG at exon 3/+12 position was identified in 4 samples corresponding to 2 oligodendrogliomas, 1 ependymoma and 1 meningioma, and a C-to-T change at exon 2/+10 position was present in a metastatic tumour. Although both LOH at 1 p36 and p73 sequence changes were evidenced in 4 cases, it is difficult to establish a causal role of the p73 variations and nonastrocytic brain tumours development.

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.

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.

Human T-cell leukemia virus type I tax repression of p73beta is mediated through competition for the C/H1 domain of CBP

The Tax protein, encoded by the human T-cell leukemia virus type I (HTLV-I), is required for high level viral transcription and HTLV-I-associated malignant transformation. Although the precise mechanism of malignant transformation by Tax is unclear, it is well established that Tax represses the transcription function of the tumor suppressor p53, possibly accelerating the accumulation of genetic mutations that are critical in HTLV-I-mediated malignant transformation. Tax repression of p53 transcription function appears to occur, at least in part, through competition for the cellular coactivator CBP/p300. In this study, we characterize the effect of Tax on the p53 family member, p73. We demonstrate that Tax also represses the transcription function of p73beta and that the repression is reciprocal in vivo, consistent with the idea that both transcription factors may compete for CBP/p300 in vivo. We provide evidence showing that both Tax and p73 interact strongly with the C/H1 domain of CBP and that their binding to this region is mutually exclusive in vitro. This finding provides evidence supporting the idea that reciprocal transcriptional repression between Tax and p73 is mediated through coactivator competition.

p73beta, a variant of p73, enhances Wnt/beta-catenin signaling in Saos-2 cells

The Wnt/beta-catenin pathway and p53 are very common targets for genetic alterations in colorectal cancer, and relationships between them have been reported. Here, we describe the relation between Wnt/beta-catenin signaling and the p53-related gene p73. p73, but not p53, activated a promoter containing the Tcf-binding sequence in Saos-2 cells, and the degree of activation was positively correlated with that on a p53-responsive promoter. Moreover, p73beta enhanced Wnt/beta-catenin signaling synergistically with Wnt-3a or exogenously expressed beta-catenin, unlike p53, and the enhancement was not caused by the accumulation of beta-catenin. These results show that the effects of p73 on Wnt/beta-catenin signaling differ from those of p53.

Oncogenes induce and activate endogenous p73 protein

The identification of upstream pathways that signal to TP73 is crucial for understanding the biological role of this gene. Since some evidence suggests that TP73 might play a role in tumorigenesis, we asked whether oncogenes can induce and activate endogenous TP73. Here, we show that endogenous p73 alpha and beta proteins are up-regulated in p53-deficient tumor cells in response to overexpressed E2F1, c-Myc, and E1A. E2F1, c-Myc, and E1A-mediated p73 up-regulation leads to activation of the p73 transcription function, as shown by p73-responsive reporter activity and by induction of known endogenous p73 target gene products such as p21 and HDM2. Importantly, E2F1-, c-Myc-, and E1A-mediated activation of endogenous p73 induces apoptosis in SaOs-2 cells. Conversely, inactivation of p73 by a dominant negative p73 inhibitor (p73DD), but not by a mutant p73DD, inhibits oncogene-induced apoptosis. These data show that oncogenes can signal to TP73 in vivo. Moreover, in the absence of p53, oncogenes may enlist p73 to induce apoptosis in tumor cells.

From p63 to p53 across p73

Most genes are members of a family. It is generally believed that a gene family derives from an ancestral gene by duplication and divergence. The tumor suppressor p53 was a striking exception to this established rule. However, two new p53 homologs, p63 and p73, have recently been described [1-6]. At the sequence level, p63 and p73 are more similar to each other than each is to p53, suggesting the possibility that the ancestral gene is a gene resembling p63/p73, while p53 is phylogenetically younger [1,2].The complexity of the family has also been enriched by the alternatively spliced forms of p63 and p73, which give rise to a complex network of proteins involved in the control of cell proliferation, apoptosis and development [1,2,4,7-9]. In this review we will mainly focus on similarities and differences as well as relationships among p63, p73 and p53.

E2F3 contributes both to the inappropriate proliferation and to the apoptosis arising in Rb mutant embryos

The E2F transcription factors are thought to be key downstream targets of the retinoblastoma protein (pRB) tumor suppressor. It is widely believed that E2F1, E2F2, and E2F3 can all activate cellular proliferation but that E2F1 is the specific inducer of apoptosis. Here we show that the E2f3 mutation completely suppresses both the inappropriate proliferation and the p53-dependent apoptosis arising in the Rb mutant embryos. Through the analysis of Rb(-/-);E2f3(+/-) embryos, we have been able to separate E2F3's role in the induction of apoptosis from its ability to induce proliferation. Thus, contrary to the prevailing view of E2F action, E2F3 makes a major contribution to the apoptosis resulting from pRB loss.

p21cip1 is required for the differentiation of oligodendrocytes independently of cell cycle withdrawal

Differentiation of most cell types requires both establishment of G1 arrest and the induction of a program related to achieving quiescence. We have chosen to study the differentiation of oligodendrocyte cells to determine the role of p27 and p21 in this process. Here we report that both p27 and p21 are required for the appropriate differentiation of these cells. p27 is required for proper withdrawal from the cell cycle, p21 is not. Instead, p21 is required for the establishment of the differentiation program following growth arrest. Similar observations were made in vivo. We show that p21-/- cells withdraw from the cell cycle similar to wild-type cells; however, early in animal life, the brain is hypomyelinated, inferring that the loss of p21 delayed myelination in the cerebellum. We found that we could complement or bypass the differentiation failure in p21-/- cells with either PD98059, an inhibitor of Mek1, or by transducing them with a tat-p16ink4a protein. We concluded that the two cdk inhibitors serve non-redundant roles in this program of differentiation, with p27 being responsible for arrest and p21 having a function in differentiation independent of its ability to control exit from the cell cycle.

Temperature-sensitive mutants of p53 homologs

Two homologs of the p53 tumor suppressor, p63 and p73 have recently been discovered. These proteins have activities similar to p53 in cell culture but have distinct developmental functions in vivo. We found that temperature-sensitive mutants of certain p63 and p73 isoforms can be created by single amino acid substitutions of an alanine residue corresponding to alanine 135 of murine p53. The mutants (p63gamma-Pro167, p73alpha-Leu156 and p73beta-Ile156) can be controlled by temperature shift between 32 degrees C and 39 degrees C. They can be stably expressed in p53-null H1299 cells at 39 degrees C, become transcriptionally activated at 32 degrees C, and induce expression of p53-responsive genes MDM2 and p21WAF1. Activation of p73beta-Ile156 in H1299 cells inhibits cell division but induces significant increase in cell size (hypertrophy), whereas activation of p73alpha-Leu156 and p63gamma-Pro167 induces apoptosis. These mutants may be useful tools for gaining further insight to the functions of p53 homologs.

P63 and P73: P53 mimics, menaces and more

Inactivation of the tumour suppressor p53 is the most common defect in cancer cells. The discovery of its two close relatives, p63 and p73, was therefore both provocative and confounding. Were these new genes tumour suppressors, p53 regulators, or evolutionary spin-offs? Both oncogenic and tumour-suppressor properties have now been attributed to the p53 homologues, perhaps reflecting the complex, often contradictory, protein products encoded by these genes. p63 and p73 are further implicated in many p53-independent pathways, including stem-cell regeneration, neurogenesis and sensory processes.

Defying death after DNA damage

DNA damage frequently triggers death by apoptosis. The irreversible decision to die can be facilitated or forestalled through integration of a wide variety of stimuli from within and around the cell. Here we address some fundamental questions that arise from this model. Why should DNA damage initiate apoptosis in the first place? In damaged cells, what are the alternatives to death and why should they be selected in some circumstances but not others? What signals register DNA damage and how do they impinge on the effector pathways of apoptosis? Is there a suborganellar apoptosome complex effecting the integration of death signals within the nucleus, just as there is in the cytoplasm? And what are the consequences of failure to initiate apoptosis in response to DNA damage?

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

The transcription factor E2F-1 induces both cell-cycle progression and, in certain settings, apoptosis. E2F-1 uses both p53-dependent and p53-independent pathways to kill cells. The p53-dependent pathway involves the induction by E2F-1 of the human tumour-suppressor protein p14ARF, which neutralizes HDM2 (human homologue of MDM2) and thereby stabilizes the p53 protein. Here we show that E2F-1 induces the transcription of the p53 homologue p73. Disruption of p73 function inhibited E2F-1-induced apoptosis in p53-defective tumour cells and in p53-/- mouse embryo fibroblasts. We conclude that activation of p73 provides a means for E2F-1 to induce death in the absence of p53.