p53 induces the expression of its antagonist p73 Delta N, establishing an autoregulatory feedback loop

p53 and p73 display common and distinct requirements for sequence specific binding to DNA

Although p53 and p73 share considerable homology in their DNA-binding domains, there have been few studies examining their relative interactions with DNA as purified proteins. Comparing p53 and p73beta proteins, our data show that zinc chelation by EDTA is significantly more detrimental to the ability of p73beta than of p53 to bind DNA, most likely due to the greater effect that the loss of zinc has on the conformation of the DNA-binding domain of p73. Furthermore, prebinding to DNA strongly protects p73beta but not p53 from chelation by EDTA suggesting that DNA renders the core domain of p73 less accessible to its environment. Further exploring these biochemical differences, a five-base sub-sequence was identified in the p53 consensus binding site that confers a greater DNA-binding stability on p73beta than on full-length p53 in vitro. Surprisingly, p53 lacking its C-terminal non-specific DNA-binding domain (p53Delta30) demonstrates the same sequence discrimination as does p73beta. In vivo, both p53 and p73beta exhibit higher transactivation of a reporter with a binding site containing this sub-sequence, suggesting that lower in vitro dissociation translates to higher in vivo transactivation of sub-sequence-containing sites.

p53 family members in myogenic differentiation and rhabdomyosarcoma development

The p53 family comprises the tumor suppressor p53 and the structural homologs p63 and p73. How the three family members cooperate in tumor suppression remains unclear. Here, we report different but complementary functions of the individual members for regulating retinoblastoma protein (RB) function during myogenic differentiation. Whereas p53 transactivates the retinoblastoma gene, p63 and p73 induce the cyclin-dependent kinase inhibitor p57 to maintain RB in an active, hypophosphorylated state. DeltaNp73 inhibits these functions of the p53 family in differentiation control, prevents myogenic differentiation, and enables cooperating oncogenes to transform myoblasts to tumorigenicity. DeltaNp73 is frequently overexpressed in rhabdomyosarcoma and essential for tumor progression in vivo. These findings establish differentiation control as a key tumor suppressor activity of the p53 family.

Potential role for p53 in the permissive life cycle of human cytomegalovirus

Infection of primary fibroblasts with human cytomegalovirus (HCMV) causes a rapid stabilization of the cellular protein p53. p53 is a major effector of the cellular damage response, and activation of this transcription factor can lead either to cell cycle arrest or to apoptosis. Viruses employ many tactics to avoid p53-mediated effects. One method HCMV uses to counteract p53 is sequestration into its viral replication centers. In order to determine whether or not HCMV benefits from this sequestration, we infected a p53(-/-) fibroblast line. We find that although these cells are permissive for viral infection, several parameters are substantially altered compared to wild-type (wt) fibroblasts. p53(-/-) cells show delayed and decreased accumulation of infectious viral particles compared to control fibroblasts, with the largest difference of 100-fold at 72 h post infection (p.i.) and peak titers decreased by approximately 10- to 20-fold at 144 h p.i. Viral DNA accumulation is also delayed and somewhat decreased in p53(-/-) cells; however, on average, levels of DNA are not more than fivefold lower than wt at any time p.i. and thus cannot account entirely for the observed differences in titers. In addition, there are delays in the expression of several key viral proteins, including the early replication protein UL44 and some of the late structural proteins, pp28 (UL99) and MCP (UL86). UL44 localization also indicates delayed formation and maturation of the replication centers throughout the course of infection. Localization of the major tegument protein pp65 (UL83) is also altered in these p53(-/-) cells. Partial reconstitution of the p53(-/-) cells with a wt copy of p53 returns all parameters toward wt, while reconstitution with mutant p53 does not. Taken together, our data suggest that wt p53 enhances the ability of HCMV to replicate and produce high concentrations of infectious virions in permissive cells.

Differential expression of p73 isoforms in relation to drug resistance in childhood T-lineage acute lymphoblastic leukaemia

The T-lineage phenotype of childhood acute lymphoblastic leukaemia (ALL) is associated with an increased relapse-risk and in vitro resistance to drugs as compared to a precursor B phenotype. Antiapoptotic isoforms of p73 that lack part of the transactivation (TA) domain (DeltaTA-p73, i.e. p73Deltaex2, p73Deltaex3, p73Deltaex2/3 and DeltaN-p73) may cause resistance to anticancer agents through inhibition of p53 and/or proapoptotic p73 family members (TA-p73). We demonstrate in our study that the expression of total p73 mRNA was higher in childhood T-ALL compared to controls (P=0.004). In T-ALL, the relative contribution of antiapoptotic DeltaTA-p73 (88%) was larger than of proapoptotic TA-p73 (12%). Leukaemic cells of T-ALL patients expressing higher levels of antiapoptotic p73 were more resistant to the DNA-damaging drug daunorubicin compared to cells of patients with low or negative expression or these isoforms (P(trend)=0.045). Interestingly, p73Deltaex2 was the most abundantly expressed antiapoptotic isoform in daunorubicin-resistant patient cells (44% of total p73). No association was found between high expression of proapoptotic TA-p73 or antiapoptotic DeltaTA-p73 and relapse-risk. Our results suggest that childhood T-ALL is associated with a high expression of DeltaTA-p73. These isoforms may play a role in cellular resistance to DNA-damaging drugs in children at initial diagnosis of T-ALL.

DNp73α protects myogenic cells from apoptosis

The P73 gene is transcribed from two promoters, P1 and P2, that direct the expression of multiple transactivation competent (TA) and dominant negative (DN) isoforms. TAp73 transcription factors mediate cell cycle arrest and/or apoptosis in response to DNA damage and are involved in developmental processes. P73 mRNA levels increase and the P1p73 promoter is upregulated during myogenic differentiation of C2C12 skeletal muscle satellite cells. The DNp73 proteins act as trans-repressors of p53- and p73-dependent transcription, and possess both antiapoptotic and pro-proliferative potential. Here, we show that DNp73alpha is expressed in proliferating C2C12 myoblasts, rapidly accumulates in differentiating myocytes and remains elevated in C2C12 myotubes. By combining transactivation assays and chromatin immunoprecipitation analysis, we could show that the upregulation of the P2p73 promoter during myogenic differentiation is mediated by the coordinated recruitment and activity of MyoD and p53/p73. Abrogation of DNp73 expression by specific siRNA led to a strong potentiation of the spontaneous apoptosis of C2C12 myoblasts induced to differentiate. Finally, unlike TAp73 that contributes to DNA damage-induced apoptosis of myotubes, endogenous DNp73 mediates the relative resistance of differentiated myotubes to DNA damage. Altogether, our findings identify DNp73alpha as an important target in designing strategies aimed at the potentiation of the regenerative potential of skeletal satellite cells.

Up-regulation of NFκB-responsive gene expression by ΔNp73α in p53 null cells

Transactivation domain (TAD)-truncated p73, DeltaNp73, associates with p53, resulting in suppression of p53's functions. Using p53 null cell lines, we examined whether or not DeltaNp73 can regulate gene expression in a p53-independent manner. When DeltaNp73alpha was co-transfected with a luciferase reporter plasmid with various enhancer elements, NFkappaB-responsive luciferase gene expression was selectively up-regulated by DeltaNp73alpha, but not by other p73-isoforms with TAD and DeltaNp73beta. Deletion of the TAD endowed p73alpha with the ability to enhance the responsive gene's expression, but deletion of the N-terminal proline-rich domain (PRD) rendered the TAD-deleted p73alpha inactive. Considering the inability of DeltaNp73beta, which is the C-terminus-truncated form of DeltaNp73alpha, to function, these results indicate that both the PRD and C-terminus are necessary for DeltaNp73alpha to can activate NFkappaB-responsive luciferase expression. Over-expression of p53 suppressed the TAD-truncated p73alpha-mediated luciferase expression, suggesting that p53 interferes with the TAD-truncated p73alpha-mediated activation of NFkappaB. Inhibitors for NFkappaB activation reduced the TAD-truncated p73alpha-dependent NFkappaB-responsive gene expression, indicating that TAD-truncated p73alpha activates NFkappaB as does TNFalpha. In addition to the results obtained in the reporter gene assay, TAD-truncated p73alpha stimulated the translocation of NFkappaB to the nucleus and the expression of an endogenous NFkappaB-responsive gene, Bcl-XL. Taken together, these results demonstrate that TAD-truncated p73alpha can activate NFkappaB.

Increased DNA damage sensitivity and apoptosis in cells lacking the Snf5/Ini1 subunit of the SWI/SNF chromatin remodeling complex

The gene encoding the SNF5/Ini1 core subunit of the SWI/SNF chromatin remodeling complex is a tumor suppressor in humans and mice, with an essential role in early embryonic development. To investigate further the function of this gene, we have generated a Cre/lox-conditional mouse line. We demonstrate that Snf5 deletion in primary fibroblasts impairs cell proliferation and survival without the expected derepression of most retinoblastoma protein-controlled, E2F-responsive genes. Furthermore, Snf5-deficient cells are hypersensitive to genotoxic stress, display increased aberrant mitotic features, and accumulate phosphorylated p53, leading to elevated expression of a specific subset of p53 target genes, suggesting a role for Snf5 in the DNA damage response. p53 inactivation does not rescue the proliferation defect caused by Snf5 deficiency but reduces apoptosis and strongly accelerates tumor formation in Snf5-heterozygous mice.

Regulation of telomerase activity by the p53 family member p73

The terminal ends of eukaryotic chromosomes, termed telomeres, progressively shorten during each round of cell division eventually leading cells into senescence. Tumor cells typically overcome this barrier to unlimited proliferation by activation of the human telomerase reverse transcriptase (hTERT) gene. In contrast, in most human somatic cells hTERT expression is tightly repressed by multiple tumor suppressors. Here, we studied the regulation of hTERT by the p53 family member p73. We show that forced expression of p73 or activation of endogenous p73 by E2F1 results in the downregulation of telomerase activity. Vice versa, siRNA-mediated knockdown of p73 induces hTERT expression. Responsiveness to p73 is conferred by Sp1 binding sites within the hTERT core promoter. In tumor cells, p73 isoforms lacking the transactivation domain (DeltaNp73) are frequently overexpressed and believed to function as oncogenes. We show that DeltaNp73 antagonizes the repressive effect of the proapoptotic p53 family members on hTERT expression and, in addition, induces hTERT expression in telomerase-negative cells by interfering with E2F-RB-mediated repression of the hTERT core promoter. These data provide evidence that the p73 gene functions as an important regulator of telomerase activity with implications for embryonic development, cellular differentiation and tumorigenesis.

ΔTAp73 Upregulation Correlates With Poor Prognosis in Human Tumors: Putative In Vivo Network Involving p73 Isoforms, p53, and E2F-1

Purpose

Although full-length TAp73 variants largely mimic p53 suppressor activities, the transactivation-deficient transcripts ΔTAp73 exert an oncogenic effect by inactivating p53 and TAp73 suppressor properties. Additionally, ΔTAp73 may cooperate with oncogenic RAS to induce cell transformation, confer drug resistance, and induce the phosphorylation of phosphorylated Rb. Here, we study the expression of TAp73 and ΔTAp73 variants and assess possible associations with E2F-1, p53 and K-ras status. We address the possible clinical relevance of alterations in these genes.

Patients and Methods

We determine in 113 colon and 60 breast cancer patients (a) the expression levels of TAp73, ΔTAp73 (ΔEx2p73, ΔEx2/3p73, and ΔNp73), and E2F-1 transcripts by quantitative real-time reverse transcriptase polymerase chain reaction (PCR); (b) mutations in the first exon of K-ras by PCR–single-stranded confirmational polymorphism; and (c) p53 status by immunohistochemistry. Tumor characteristics were examined in each patient.

Results

Both suppressor and oncogenic isoforms of TP73 were significantly coupregulated in tumor tissues. Associations were observed between (a) p53 wild type status and upregulation of some TP73 variants; (b) overexpression of E2F-1 and some TP73 forms; and (c) upregulation of ΔTAp73 variants and advanced pathologic stage, lymph node metastasis, vascular invasion, presence of polyps, and tumor localization.

Conclusion

Overexpression of TP73 variants in tumor tissues indicates that they may be involved in colon and breast carcinogenesis. The association between upregulation of ΔTAp73 isoforms and poor prognosis features, specifically advanced tumor stage, suggests that they may be of practical clinical prognostic value. Interestingly, the in vivo associations identified here may indicate a functional network involving p73 variants, p53, and E2F-1.

Allelic expression and quantitative RT-PCR study of TAp73 and ΔNp73 in non-Hodgkin's lymphomas

p73 shares structural and functional homology to p53. p73 generates different proteins using alternative promoters and splicing which have different biological characteristics. We investigated the pattern (monoallelic or biallelic) of expression of TAp73 and DeltaNp73 in normal lymphocytes and lymphomas using two p73 polymorphisms. We found monoallelic expression of TAp73 in normal lymphocytes and tumors, and a selective expression of AT allele in all cases. Moreover, the quantitative expression analysis revealed DeltaNp73 over-expression in both B- and T-cell lymphomas comparing with normal lymphoid cells, suggesting a role in tumorigenesis. Finally, we have confirmed that although DeltaNp73 over-expression could be an alternative mechanism of p53 inactivation, both alterations may appear together.

Aberrant expression of DeltaNp73 in benign and malignant tumours of the prostate: correlation with Gleason score

BACKGROUND

The p73 gene is a p53 homologue that induces apoptosis and inhibits cell proliferation. N-terminal truncated isoforms of p73 (DeltaNp73) act as dominant-negative inhibitors of wild-type p53 and TAp73 and result in tumour growth in nude mice.

AIMS

To detect DeltaNp73 expression in 24 benign prostatic hyperplasia samples, 33 prostate carcinomas, and five normal samples and to evaluate the relation between DeltaNp73, TAp73 concentrations, and the clinicopathological characteristics of patients with prostate cancer.

METHODS

TAp73 was determined by real time polymerase chain reaction (PCR); DeltaNp73 and DeltaN'p73 were assessed using reverse transcription PCR. western blotting was used to analyse protein expression. p53 mutation was determined by immunohistochemistry.

RESULTS

A significant increase of DeltaNp73 was seen in 20 of 33 carcinomas and 17 of 24 benign prostate hyperplasia tissues, but in none of the normal samples. None of the specimens expressed DeltaN'p73. No significant relation was found between TAp73 expression and clinical parameters. The incidence of positive expression of DeltaNp73 correlated with the Gleason score in prostate carcinomas. Cancer samples with wild-type p53 had significantly higher expression of DeltaNp73 than p53 mutant cancers.

CONCLUSION

These data suggest a potential role for DeltaNp73 in prostate cancer progression.

RNA interference: from gene silencing to gene-specific therapeutics

In the past 4 years, RNA interference (RNAi) has become widely used as an experimental tool to analyse the function of mammalian genes, both in vitro and in vivo. By harnessing an evolutionary conserved endogenous biological pathway, first identified in plants and lower organisms, double-stranded RNA (dsRNA) reagents are used to bind to and promote the degradation of target RNAs, resulting in knockdown of the expression of specific genes. RNAi can be induced in mammalian cells by the introduction of synthetic double-stranded small interfering RNAs (siRNAs) 21–23 base pairs (bp) in length or by plasmid and viral vector systems that express double-stranded short hairpin RNAs (shRNAs) that are subsequently processed to siRNAs by the cellular machinery. RNAi has been widely used in mammalian cells to define the functional roles of individual genes, particularly in disease. In addition, siRNA and shRNA libraries have been developed to allow the systematic analysis of genes required for disease processes such as cancer using high throughput RNAi screens. RNAi has been used for the knockdown of gene expression in experimental animals, with the development of shRNA systems that allow tissue-specific and inducible knockdown of genes promising to provide a quicker and cheaper way to generate transgenic animals than conventional approaches. Finally, because of the ability of RNAi to silence disease-associated genes in tissue culture and animal models, the development of RNAi-based reagents for clinical applications is gathering pace, as technological enhancements that improve siRNA stability and delivery in vivo, while minimising off-target and nonspecific effects, are developed.

Effects of inducible overexpression of DNp73alpha on cancer cell growth and response to treatment in vitro and in vivo

The p73 gene has a complex regulation, which leads to the expression of different isoforms, often with opposite biological effects. We have generated in the human colocarcinoma cell line HCT116, expressing a wild-type p53, an inducible DNp73alpha expressing system. Two clones (HCT116/DN3 and HCT116/DN14), upon doxycycline addition, show a strong expression of DNp73alpha. In vitro the two DNp73alpha overexpressing clones grow at similar rate of the control transfected clone (HCT116/8a) and similarly respond to DNA damage. When injected in mice, HCT116/DN3, HCT116/DN14, and HCT116/8a cells grew similarly in the absence or presence of tetracycline. In HCT116/DN3 and HCT116/DN14 tumors, tetracycline induced a strong expression of DNp73alpha both as mRNA and protein. These results indicate that in this system the overexpression of the DNp73alpha does not induce a more aggressive phenotype and does not seem to be associated with a reduced response of the cells to treatment with anticancer agents.

Functional genomics using high-throughput RNA interference

RNA interference (RNAi) describes the post-transcriptional silencing of gene expression that occurs in response to the introduction of double-stranded RNA into cells. Application of RNAi in experimental systems has provided a great leap forward in the elucidation of gene function. To facilitate large-scale functional genomics studies using RNAi, several high throughput approaches have been developed based on microarray or microwell assays. Recent establishment of large libraries of RNAi reagents combined with a variety of detection assays further opens the door for genome-wide screens of gene function in mammalian cells.

Variation in transcriptional regulation of cyclin dependent kinase inhibitor p21waf1/cip1 among human bronchogenic carcinomas

Background

Cell proliferation control depends in part on the carefully ordered regulation of transcription factors. The p53 homolog p73, contributes to this control by directly upregulating the cyclin dependent kinase inhibitor, p21^waf1/cip1. E2F1, an inducer of cell proliferation, directly upregulates p73 and in some systems upregulates p21 directly. Because of its central role in controlling cell proliferation, upregulation of p21 has been explored as a modality for treating bronchogenic carcinoma (BC). Improved understanding of p21 transcriptional regulation will facilitate identification of BC tissues that are responsive to p21-directed therapies. Toward this goal, we investigated the role that E2F1 and p73 each play in the transcriptional regulation of p21.

Results

Among BC samples (N = 21) p21 transcript abundance (TA) levels varied over two orders of magnitude with values ranging from 400 to 120,000 (in units of molecules/10⁶ molecules β-actin). The p21 values in many BC were high compared to those observed in normal bronchial epithelial cells (BEC) (N = 18). Among all BC samples, there was no correlation between E2F1 and p21 TA but there was positive correlation between E2F1 and p73α (p < 0.001) TA. Among BC cell lines with inactivated p53 and wild type p73 (N = 7) there was positive correlation between p73α and p21 TA (p < 0.05). Additionally, in a BC cell line in which both p53 and p73 were inactivated (H1155), E2F1 TA level was high (50,000), but p21 TA level was low (470). Transiently expressed exogenous p73α in the BC cell line Calu-1, was associated with a significant (p < 0.05) 90% increase in p21 TA and a 20% reduction in E2F1 TA. siRNA mediated reduction of p73 TA in the N417 BC cell line was associated with a significant reduction in p21 TA level (p < 0.01).

Conclusion

p21 TA levels vary considerably among BC patients which may be attributable to 1) genetic alterations in Rb and p53 and 2) variation in TA levels of upstream transcription factors E2F1 and p73. Here we provide evidence that p73 upregulates p21 TA in BC tissues and upregulated p21 TA may result from E2F1 upregulation of p73 but not from E2F1 directly.

TNF-α-mediated apoptosis in vascular smooth muscle cells requires p73

Atherosclerosis, now considered an inflammatory process, is the leading cause of death in the Western world and is manifested by a variety of diseases in multiple organ systems. Because of its prevalence and associated morbidity, novel therapies directed at arresting this progressive process are urgently needed. The inflammatory mediator TNF-α, which is known to contribute to apoptosis in vascular smooth muscle cells, has been shown to be intimately involved in the atherosclerotic process, being present at elevated levels in human atheroma as well as possibly being responsible for plaque rupture, a clinically devastating event. In light of our earlier finding that p73 is a proapoptotic protein in vascular smooth muscle cells, which are involved in plaque progression as well as rupture, we asked whether TNF-α mediates apoptosis in these cells through p73. We now show that p73 is present in spindle-shaped cells within human atheroma, and p73β, an isoform that is pivotal in both apoptosis and growth suppression, is induced in vascular smooth muscle cells in vitro by serum but not by PDGF-BB. In addition, TNF-α, when added to these cells in the presence of serum-containing media, increases p73β expression and causes apoptosis in both rat and human vascular smooth muscle cells. Inhibition of p73 activity with a dominant inhibitory NH2-terminally deleted p73 plasmid results in markedly decreased TNF-α-induced apoptosis. Thus p73β is likely a mediator of the apoptotic effect of TNF-α in the vasculature, such that future targeting of the p73 isoforms may ultimately prove useful in novel atherosclerosis therapies.

Transactivation-dependent and -independent regulation of p73 stability

The tumor suppressor p53 regulates its own stability by transcriptionally activating Mdm2, Pirh2, and COP1, which target p53 for degradation. However, whether such a negative feedback mechanism exists to regulate the stability of p73, the structural and functional homologue of p53, is unclear. Unlike p53, p73 is not mutated in cancers, but its expression is significantly elevated. Thus, we have investigated the regulation of p73 turnover. Our data suggest the existence of a negative feedback mechanism for p73 degradation. p73 mutants with compromised transactivation activity are generally more stable than the full-length TAp73 form. TAp73 appears to promote its own turnover as well as that of other p73 forms, including the DeltaNp73 that lacks the amino-terminal transactivation domain, in a transactivation-dependent manner. This degradation-inducing property of TAp73 was inhibited only by p73 mutants that also inhibit the transactivation activity TAp73 but not by mutant p53, highlighting the specificity in the regulation of p73 stability. Moreover, regions in the amino and carboxyl termini of p73 confer both stabilizing and destabilizing effects on the protein, independent of its transactivation ability. Finally, we have identified the regions between amino acids 56 and 248 of p73 as being the region required for p73-mediated and for ubiquitin-mediated degradation. Taken together, the data suggest that p73 turnover is tightly regulated in a transactivation-dependent and -independent manner, resulting in the controlled expression of the various p73 forms.

The p53 pathway: positive and negative feedback loops

The p53 pathway responds to stresses that can disrupt the fidelity of DNA replication and cell division. A stress signal is transmitted to the p53 protein by post-translational modifications. This results in the activation of the p53 protein as a transcription factor that initiates a program of cell cycle arrest, cellular senescence or apoptosis. The transcriptional network of p53-responsive genes produces proteins that interact with a large number of other signal transduction pathways in the cell and a number of positive and negative autoregulatory feedback loops act upon the p53 response. There are at least seven negative and three positive feedback loops described here, and of these, six act through the MDM-2 protein to regulate p53 activity. The p53 circuit communicates with the Wnt-beta-catenin, IGF-1-AKT, Rb-E2F, p38 MAP kinase, cyclin-cdk, p14/19 ARF pathways and the cyclin G-PP2A, and p73 gene products. There are at least three different ubiquitin ligases that can regulate p53 in an autoregulatory manner: MDM-2, Cop-1 and Pirh-2. The meaning of this redundancy and the relative activity of each of these feedback loops in different cell types or stages of development remains to be elucidated. The interconnections between signal transduction pathways will play a central role in our understanding of cancer.

Differential Regulation of MDR1 Transcription by the p53 Family Members

Although the p53 family members share a similar structure and function, it has become clear that they differ with respect to their role in development and tumor progression. Because of the high degree of homology in their DNA binding domains (DBDs), it is not surprising that both p63 and p73 activate the majority of p53 target genes. However, recent studies have revealed some differences in a subset of the target genes affected, and the mechanism underlying this diversity has only recently come under investigation. Our laboratory has demonstrated previously that p53 represses transcription of the P-glycoprotein-encoding MDR1 gene via direct DNA binding through a novel p53 DNA-binding site (the HT site). By transient transfection analyses, we now show that p63 and p73 activate rather than repress MDR1 transcription, and they do so through an upstream promoter element (the alternative p63/p73 element (APE)) independent of the HT site. This activation is dependent on an intact DNA binding domain, because mutations within the p63DBD or p73DBD are sufficient to prevent APE-mediated activation. However, neither p63 nor p73 directly interact with the APE, suggesting an indirect mechanism of activation through this site. Most interestingly, when the p53DBD is replaced by the p63DBD, p53 is converted from a repressor working through the HT site to an activator working through the APE. Taken together, these data indicate that, despite considerable homology, the DBD of the p53 family members have unique properties and can differentially regulate gene targeting and transcriptional output by both DNA binding-dependent and -independent mechanisms.

p73 induction after DNA damage is regulated by checkpoint kinases Chk1 and Chk2

The checkpoint kinases Chk1 and Chk2 are central to the induction of cell cycle arrest, DNA repair, and apoptosis as elements in the DNA-damage checkpoint. We report here that in several human tumor cell lines, Chk1 and Chk2 control the induction of the p53 related transcription factor p73 in response to DNA damage. Multiple experimental systems were used to show that interference with or augmentation of Chk1 or Chk2 signaling strongly impacts p73 accumulation. Furthermore, Chk1 and Chk2 control p73 mRNA accumulation after DNA damage. We demonstrate as well that E2F1 directs p73 expression in the presence and absence of DNA damage. Chk1 and Chk2, in turn, are vital to E2F1 stabilization and activity after genotoxic stress. Thus, Chk1, Chk2, E2F1, and p73 function in a pathway mediating p53-independent cell death produced by cytotoxic drugs. Since p53 is often obviated through mutation as a cellular port for anticancer intervention, this pathway controlling p53 autonomous pro-apoptotic signaling is of potential therapeutic importance.

The expression of TA and DeltaNp63 are regulated by different mechanisms in liver cells

The TP63 gene, a member of the TP53 gene family, encodes several isoforms with (TAp63) or without (DeltaNp63) transactivating properties. Whereas the role of p63 in the normal development of squamous epithelia is well established, its function in other cell types remains to be elucidated. Here, we have analysed the expression of TA and DeltaNp63 isoforms in liver cells, by using both primary hepatocytes from wild type and p53-null mice and three human hepatocellular carcinoma (HCC) cell lines, according to the transformation state and the TP53 status of the cells. We observed the expression of DeltaNp63 isoforms only in a p53-null context. On the other hand, the expression of TAp63 isoforms was restricted to the HCC cell lines, whatever the TP53 status. We then studied the expression of TP63 upon genotoxic treatment. When treated with UVB or H(2)O(2), hepatocytes did not exhibit any change in p63 mRNA level. At the opposite, upon treatment with topoisomerase II inhibitors (doxorubicin or etoposide), the expression of TAp63 isoforms was clearly induced, independently of the TP53 status of cells. The same treatment did not induce any variation in the expression of DeltaNp63 isoforms, both at mRNA and protein levels. In HCC cell lines, doxorubicin or etoposide treatment also resulted in an increase of TAp63 transcripts only. This increase was accompanied by an increase in the intracellular level of TAp63 alpha protein. In parallel, we observed an upregulation of some p53-target genes related to cell cycle regulation, such as WAF1/CIP1, PIG3, 14-3-3sigma or GADD45, independently of the TP53 status of cells. In conclusion, we report for the first time that TA and DeltaNp63 alpha proteins are present in liver cells. Furthermore, our results suggest that p63 may partially substitute for wild-type p53, in counteracting uncontrolled liver cell proliferation in response to certain forms of DNA-damage.

The neurogene BTG2TIS21/PC3 is transactivated by DeltaNp73alpha via p53 specifically in neuroblastoma cells

The p53 gene and its homologue p73 are rarely mutated in neuroblastoma. In recent studies, we showed that overexpression of DeltaNp73alpha, an isoform lacking the N-terminal transactivation (TA) domain, surprisingly induces p53 protein accumulation in the wild-type (wt) p53 human neuroblastoma line SH-SY5Y. As can be expected owing to its dominant-negative effect, DeltaNp73alpha inhibits Waf1/p21 gene expression, but equally importantly, it upregulates BTG2TIS21/PC3, another p53 target gene. This effect is not observed in neuroblastoma cells that express a mutated p53. To better understand the DeltaNp73-mediated transactivation of the BTG2TIS21/PC3 gene we performed luciferase assays with two reporter plasmids harboring long and short BTG2 promoter sequences in three human neuroblastoma cell lines and one breast cancer cell line. Our results demonstrate that BTG2TIS21/PC3 transactivation by DeltaNp73alpha depends on both p53 status (as it is not observed in a p53-/- neuroblastoma cell line) and cellular context (as it occurs in a p53+/+ neuroblastoma cell line but not in a p53+/+ breast tumor cell line). The fact that DeltaNp73alpha may either inhibit or stimulate wt-p53 transcriptional activity, depending on both the p53 target gene and the cellular context, was confirmed by real-time quantitative PCR. Moreover, transactivation of the BTG2TIS21/PC3 promoter requires a complete DeltaNp73alpha C-terminus sequence as it is not observed with DeltaNp73beta, which lacks most of the C-terminal domain. We have previously shown that DeltaNp73alpha is the only p73 isoform expressed in undifferentiated neuroblastoma tumors. In light of all these findings, we propose that DeltaNp73alpha not only acts as an inhibitor of p53/TAp73 functions in neuroblastoma tumors, but also cooperates with wt-p53 in playing a physiological role through the activation of BTG2TIS21/PC3 gene expression.

Evidence that DeltaNp73 promotes neuronal survival by p53-dependent and p53-independent mechanisms

The p53 family member, p73, is essential for the survival of sympathetic neurons during the developmental period of naturally occurring neuronal death. Here, we have asked whether DeltaNp73, which is the only p73 isoform expressed in sympathetic neurons, mediates this survival by p53-dependent and/or p53-independent mechanisms. Initially, we used a genetic approach and crossed p53+/- and p73+/- mice. Quantitation of neurons in the sympathetic superior cervical ganglion during the period of naturally occurring cell death revealed that the loss of p53 partially rescued the death of neurons seen in p73-/- animals. Moreover, exogenous expression of DeltaNp73 in cultured p53-/- sympathetic neurons rescued these neurons from apoptosis after NGF withdrawal. Biochemical studies asking how DeltaNp73 inhibited NGF withdrawal-induced apoptosis in wild-type neurons demonstrated that it prevented the upregulation of the direct p53 targets p21 and Apaf-1 as well as cleavage of caspase-3. It also inhibited events at the mitochondrial apoptotic checkpoint, suppressing the induction of BimEL and the release of mitochondrial cytochrome c. Interestingly, DeltaNp73 expression also inhibited one very early event in the apoptotic cascade, the activation of c-Jun N-terminal protein kinase (JNK), likely by binding directly to JNK. Finally, we show that neuronal cell size is decreased in p73-/- mice, and that this decrease is not rescued by the lack of p53, suggesting a role for p73 in regulating cell size that does not involve interactions with p53. Thus, DeltaNp73 promotes neuronal survival via p53-dependent and -independent mechanisms, and it does so at multiple points, including some of the most proximal events that occur after NGF withdrawal.

Analysis of p73 expression pattern in acute myeloid leukemias: lack of DeltaN-p73 expression is a frequent feature of acute promyelocytic leukemia

p73, the homologue of p53, is a nuclear protein whose ectopic expression, in p53+/+ and p53-/- cells, recapitulates the most well-characterized p53 effects, such as growth arrest, apoptosis and differentiation. Unlike p53, which is mutated in half of human cancers, p73 is rarely mutated. However, altered expression of the p73 gene has been reported in neuroblastoma, lung cancer, prostate cancer and renal cell carcinoma. To investigate the potential involvement of p73 in acute myeloid leukemias (AMLs), we analyzed 71 samples from AML patients for the expression pattern of N-terminal transactivation-p73alpha (TA-p73alpha), its spliced isoforms and N-terminal-deleted-p73 transcripts (DeltaN-p73). We detected p73 gene expression in AML irrespective of FAB (French-American-British) subtypes. Notably, the analysis of DeltaN-p73 expression, which has been reported to inactivate both p53 and p73 antitumor effects, revealed a rather peculiar pattern. In fact, DeltaN-p73 transcript and protein were detectable in 27/28 (96.4%) cases of M0, M1, M2, M4, M5 and M6 AML and in 13/41 (31.7%) cases of PML-RARalpha-positive M3 AML (P<0.01). Thus, the distinct gene expression profile of p73 further supports the notion that acute promyelocytic leukemia is a biologically different subset of AML.

Developmental roles of p73 in Cajal-Retzius cells and cortical patterning

To examine the role of the p53 homolog p73 in brain development, we studied p73-/-, p73+/-, E2F1-/-, and reeler mutant mice. p73 in developing brain is expressed in Cajal-Retzius (CR) cells, the cortical hem, and the choroid plexus. p73-expressing CR cells are lost in p73-/- embryos, although Reelin is faintly expressed in the marginal zone. Ectopic neurons in the p73-/- preplate and cortical hem at embryonic day 12 implicate p73 in the early developmental program of the cortex; however, preplate partition and early cortical plate formation are not disturbed. Postnatal p73-/- mice show a mild hypoplasia of the rostral cortex and a severely disrupted architecture of the posterior telencephalon. In the developing p73-/- hippocampus, the most striking abnormality is the absence of the hippocampal fissure, suggesting a role of p73 in cortical folding. p73+/- mice have a less severe cortical phenotype; they display a dorsal shift of the entorhinal cortex and a reduced size of occipital and posterior temporal areas, which acquire entorhinal-like features such as Reelin-positive cells in layer II. CR cells appear unaffected by heterozygosity. We relate the malformations of the posterior pole in p73 mutant mice to alterations of p73 expression in the cortical hem and suggest that p73 forms part of an early signaling network that controls neocortical and archicortical regionalization. In mice deficient for the transcription factor E2F1, a main activator of the TAp73 (transactivating p73) isoform, we find a defect of the caudal cortical architecture resembling the p73+/- phenotype along with reduced TAp73 protein levels and propose that an E2F1-TAp73 dependent pathway is involved in cortical patterning.

Quantitative TP73 transcript analysis in hepatocellular carcinomas

PURPOSE

The p53 family member p73 displays significant homology to p53, but data from primary tumors demonstrating increased expression levels of p73 in the absence of any gene mutations argue against a classical tumor suppressor function. A detailed analysis of the p73 protein in tumor tissues has revealed expression of two classes of p73 isoforms. Whereas the proapoptotic, full-length, transactivation-competent p73 protein (TA-p73) has a putative tumor suppressor activity similar to p53, the antiapoptotic, NH(2)-terminally truncated, transactivation-deficient p73 protein (DeltaTA-p73) has been shown to possess oncogenic activity. The oncogenic proteins can be generated by the following two different mechanisms: (a) aberrant splicing (p73Deltaex2, p73Deltaex2/3, DeltaN'-p73) and (b) alternative promoter usage of a second intronic promoter (DeltaN-p73). The purpose of our study was to elucidate the origin of DeltaTA-p73 isoforms in hepatocellular carcinomas.

EXPERIMENTAL DESIGN

We analyzed the underlying mechanisms of p73 overexpression in cancer cells by quantification of p73 transcripts from 10 hepatocellular carcinoma patients using isoform-specific real-time reverse transcription-PCR.

RESULTS

Our data demonstrate that only aberrantly spliced DeltaTA-p73 transcripts from the TA promoter show significantly increased expression levels in the tumor whereas the DeltaN-p73 transcript generated from the second promoter is not significantly up-regulated.

CONCLUSIONS

Although we only analyzed 10 patient samples the results strongly suggest that the elevated activity of the first promoter (TA promoter) accounts for high-level expression of both full-length TA-p73 and aberrantly spliced DeltaTA-p73 isoforms in hepatocellular carcinoma tissues.

p73 competes with co-activators and recruits histone deacetylase to NF-Y in the repression of PDGF beta-receptor

We investigated mechanisms of the p73alpha-mediated repression of the platelet-derived growth factor beta-receptor (PDGFRB) promoter caused by its interaction with NF-Y. Treatment of cells with the histone deacetylase (HDAC) inhibitor, Trichostatin A, increases PDGFRB promoter activity through the CCAAT motif and counteracts the repression caused by p73alpha. Activation of the PDGFRB promoter by the co-activator p300 also occurs through the CCAAT motif. Expression of p73alpha counteracts both p300- and P/CAF-mediated activation of the PDGFRB promoter, and expression of p300 or P/CAF attenuates the p73alpha-mediated repression of the promoter activity. In concordance, p73alpha decreases the p300-mediated acetylation of NF-YC, p300 competes with p73alpha for binding NF-YB, and P/CAF competes with p73alpha for binding NF-YB and NF-YC. Furthermore, p73alpha, but not the oncogenic DeltaNp73alpha, binds directly to HDAC1. We performed chromatin immunoprecipitation with antibodies against p73, DeltaNp73, NFYB, p300 and HDAC1 at different periods after serum stimulation in serum-starved NIH3T3 cells. A marked decrease of DeltaNp73, NF-YB and p300 was detected 6 hours after serum stimulation when the expression of PDGFRB decreased. Conversely, HDAC1 was found bound at its maximum and the anti-p73 detecting both TAp73 and DeltaNp73 was found at all time points, indicating that p73, but not DeltaNp73, remains bound at this time. Double immunofluorescence staining of TAp73 and HDAC1 revealed that both of these molecules exist in the nucleus at this time point, supporting the presence of endogenous interaction. These results suggest that p73 and DeltaNp73 behave as physiological regulators for the transcription of the PDGFRB promoter.

E1B-55-kilodalton protein is not required to block p53-induced transcription during adenovirus infection

The adenovirus E1B-55-kDa protein binds and inactivates the tumor suppressor protein p53. However, the role of this interaction during infection is still poorly understood and was therefore examined here. Infection with a virus carrying the E1B-55-kDa mutation R239A, preventing the interaction with p53, led to the accumulation of p53. However, p53 target genes were not activated in the infected cells, although p53 phosphorylation did occur and the p53 antagonists Mdm2 and deltaNp73 did not accumulate. Deletion of E4orf6, alone or in combination with E1B-55-kDa, did not allow the induction of p53-responsive genes either. In transient reporter assays, the viral E1A-13S protein antagonized p53 activity; mutational analysis suggested that this depends partially on p300 binding, but it depends even more strongly on the interaction of E1A with the p400/TRRAP protein complex. However, viruses expressing E1A mutants lacking these binding activities, in combination with E1B-55-kDa R239A, still abolished p53 activity. In contrast, when the mutation of E1B-55-kDa at R239A was combined with a deletion of the apoptosis inhibitor E1B-19-kDa, infected cells showed more extensive apoptosis than after infection with single mutants, suggesting that accumulated p53, albeit transcriptionally inactive, might nonetheless enhance apoptosis. Despite extensive apoptosis of the infected cells, the deletion of E1B-19-kDa, in combination with the E1B-55-kDa mutation or in the presence of the constitutively active p53 mutant p53mt24-28, reduced virus replication less than fivefold. In conclusion, adenovirus does not need direct binding of E1B-55-kDa to inactivate p53, and forced p53 activity with consecutive apoptosis does not severely impair virus replication.

p63 and p73: Roles in Development and Tumor Formation

The tumor suppressor p53 is critically important in the cellular damage response and is the founding member of a family of proteins. All three genes regulate cell cycle and apoptosis after DNA damage. However, despite a remarkable structural and partly functional similarity among p53, p63, and p73, mouse knockout studies revealed an unexpected functional diversity among them. p63 and p73 knockouts exhibit severe developmental abnormalities but no increased cancer susceptibility, whereas this picture is reversed for p53 knockouts. Neither p63 nor p73 is the target of inactivating mutations in human cancers. Genomic organization is more complex in p63 and p73, largely the result of an alternative internal promoter generating NH2-terminally deleted dominant-negative proteins that engage in inhibitory circuits within the family. Deregulated dominant-negative p73 isoforms might play an active oncogenic role in some human cancers. Moreover, COOH-terminal extensions specific for p63 and p73 enable further unique protein-protein interactions with regulatory pathways involved in development, differentiation, proliferation, and damage response. Thus, p53 family proteins take on functions within a wide biological spectrum stretching from development (p63 and p73), DNA damage response via apoptosis and cell cycle arrest (p53, TAp63, and TAp73), chemosensitivity of tumors (p53 and TAp73), and immortalization and oncogenesis (ΔNp73).

Differential response of p53 target genes to p73 overexpression in SH-SY5Y neuroblastoma cell line

p73, the first p53 gene homologue, encodes an array of p73 proteins including p73 alpha full-length (TAp73 alpha) and amino-truncated isoforms (Delta Np73 alpha), two proteins with opposite biological functions. TAp73 alpha can induce tumor suppressive properties, while Delta Np73 alpha antagonizes p53 as well as TAp73 in a dominant-negative manner. In human malignant neuroblasts, p53 protein is wild-type but known to be excluded from the nucleus, therefore disabling its function as a tumor suppressor. The present study investigates whether there is a functional link between p73 isoforms and p53 in neuroblastoma. Experiments were performed on two neuroblastoma cell lines differing in their p53 status, e.g. wild-type p53 SH-5Y5Y cells and mutated p53 IGR-N-91 cells. Data indicate that (i) both TA- and Delta N-p73 alpha enhance p53 protein level in SH-SY5Y cells, whereas level remains unchanged in IGR-N-91 cells; (ii) only in SH-SY5Y cells does forced TAp73 alpha overexpression markedly induce nuclear accumulation of p53 protein; (iii) p21 protein expression is increased in both cell lines infected with TAp73, suggesting that, in IGR-N-91 cells, p21 is induced by p73 through a p53-independent pathway; (iv) in the SHSY5Y cell line, Btg2 expression is strongly enhanced in cells overexpressing TA, and to a lesser extent in cells overexpressing Delta N. Taken together our results suggest that TAp73 may restore p53 function in NB with wild-type nonfunctional p53, but not in NB with mutated p53.

Transdominant DeltaTAp73 isoforms are frequently up-regulated in ovarian cancer. Evidence for their role as epigenetic p53 inhibitors in vivo

Despite strong homology, the roles of TP53 and TP73 in tumorigenesis seem to be fundamentally different. In contrast to TP53, tumor-associated overexpression of TP73 in many different cancers, combined with virtual absence of inactivating mutations and lack of a cancer phenotype in the TP73 null mouse are inconsistent with a suppressor function but instead support an oncogenic function. The discovery of NH(2)-terminally truncated p73 isoforms, collectively called DeltaTAp73, is now the focus of intense interest because they act as potent transdominant inihibitors of wild-type p53 and transactivation-competent TAp73. Therefore, establishing deregulated DeltaTAp73 expression in tumors could be the crucial link to decipher which of the two opposing roles of this bipolar gene is the biologically relevant one. This study is the largest to date and encompasses 100 ovarian carcinomas with complete expression profile of all NH(2)-terminal isoforms, discriminating between TAp73 and DeltaTAp73 (DeltaNp73, DeltaN'p73, Ex2p73, and Ex2/3p73) by isoform-specific real-time reverse transcription-PCR. We find that the set of NH(2)-terminal p73 isoforms distinguishes ovarian cancer patients from healthy controls and thus is a molecular marker for this diagnosis. Ovarian cancers strongly and almost universally overexpress DeltaN'p73 compared with normal tissues (95% of cancers). About one-third of tumors also exhibit concomitant up-regulation of the antagonistic TAp73, whereas only a small subgroup of tumors overexpress DeltaNp73. Thus, deregulation of the E2F1-responsive P1 promoter, rather than the alternate P2 promoter, is mainly responsible for the production of transdominant p53/TAp73 antagonists in ovarian cancer. Tumor stage, grade, presence of metastases, p53 status, and residual disease after resection are significant prognostic markers for overall and recurrence-free survival. A trend is found for better overall survival in patients with low expression of DeltaN'p73/DeltaNp73, compared with patients with high expression. A strong correlation between deregulated DeltaTAp73 and p53 status exists. p53 wild-type cancers exhibit significantly higher deregulation of DeltaN'p73, DeltaNp73, and Ex2/3p73 than p53 mutant cancers. This data strongly supports the hypothesis that overexpression of transdominant p73 isoforms can function as epigenetic inhibitors of p53 in vivo, thereby alleviating selection pressure for p53 mutations in tumors.

Regulation of HSF1-responsive gene expression by N-terminal truncated form of p73alpha

DNp73 is a transactivation domain (TAD)-truncated form of p73. The ability of DNp73alpha to regulate gene expression was examined using reporter assays with luciferase gene constructs. Among various promoter-regulated reporter genes tested, heat shock factor (HSF)-responsive gene expression was selectively activated by DNp73alpha, but not by other p73-isoforms with TAD and DNp73beta. Deletion of TAD endowed p73alpha with the ability to activate HSF-responsive gene expression, but deletion of N-terminal proline-rich domain (PRD) rendered both DNp73alpha and the TAD-deleted p73alpha inactive. Considering the inability of DNp73beta, which is the C-terminus-truncated form of DNp73alpha, to function, these results indicate that both the PRD and C-terminus are necessary for DNp73alpha to be able to activate the HSF-dependent gene expression. In addition to the reporter gene expression, both DNp73alpha and TAD-deleted p73alpha activated the expression of an endogenous gene, hsp70, corresponding with an increase in the active form of HSF1. Taken together, these results demonstrate that TAD-truncated p73alpha can activate HSF-dependent gene expression via induction of active HSF1.

[Interrelations between p73 and p53: a model, neuroblastoma]

Homologies in sequence and gene organization of p53 and their relatives, p73 and p63, suggest similar biological functions. However differences exist between the p53 family members. Indeed in human tumors p53 is often mutated while p63 and p73 are very rarely mutated. In addition, in contrast to p53 which is transcribed in a unique mRNA species spanning all gene exons, each homologue expresses two types of isoforms: some with transactivation domain (TAD) showing tumor suppressive properties, the others deprived of TAD, with oncogenic properties. If p53 responds to immediate genotoxic stress, its homologues participate to the cell homeostasis of specific tissues along their development and differentiation, neuronal tissue for p73, epithelial for p63. However a collaboration between the three p53 family members has been shown to occur in response to cell genotoxic damages. Neuroblastic tumors characterized by a large spectrum of neuronal differentiation constitute a good model to study relationship between p73 and p53 as well as the regulation of their respective expression.

Treatment with arsenic trioxide (ATO) and MEK1 inhibitor activates the p73-p53AIP1 apoptotic pathway in leukemia cells

Arsenic trioxide (ATO) induces differentiation and apoptosis of malignant cells in vitro and in vivo and has been used in the treatment of a variety of hematologic malignancies. We found that in NB4 acute promyelocytic and in K562 erythroleukemia cell lines treatment with the MEK1 inhibitors PD98059 and PD184352 greatly enhances apoptotic cell death induced by ATO alone. Combined treatment results in the induction of the p53AIP1 (p53-regulated apoptosis-inducing protein 1) gene in both cell lines. Because NB4 and K562 cell lines carry an inactive p53, we investigated the possible role of p73, a p53 paralogue that has been shown to regulate several p53 target genes including p21, Bax, and p53AIP1. We found that MEK1 inhibitors reduce the levels of dominant-negative (DeltaN) p73 proteins and promote the accumulation of endogenous p73alpha through its transcriptional activation and its tyrosine phosphorylation, resulting in p21 up-regulation and significant inhibition of cell growth. ATO reduces DeltaNp73 levels and promotes a p300-mediated acetylation of endogenous p73, thus favoring cell cycle arrest and apoptosis. Finally, the combined treatment with MEK1 inhibitors and ATO enhances the affinity of phosphoacetylated p73 for the p53AIP1 promoter in vivo, as determined by chromatin immunoprecipitation experiments, leading to p53AIP1 up-regulation and increased apoptosis.

The p53 homologue p73 accumulates in the nucleus and localizes to neurites and neurofibrillary tangles in Alzheimer disease brain

The molecular mechanisms that regulate neuronal survival vs. death during Alzheimer disease (AD) remain unclear. Nonetheless, a number of recent studies indicate that increased expression or altered subcellular distribution of numerous cell cycle proteins during AD may contribute to disease pathogenesis. Because homologues of p53, a key regulatory protein in the cell cycle, such as p73, have been identified and shown to participate in cellular differentiation and death pathways, we examined the expression and distribution of p73 in the hippocampus of eight control and 16 AD subjects. In control subjects, hippocampal pyramidal neurones exhibit p73 immunoreactivity that is distributed predominately in the cytoplasm. In AD hippocampus, increased levels of p73 are located in the nucleus of pyramidal neurones and p73 is located in dystrophic neurites and cytoskeletal pathology. Immunoblot analysis confirmed the presence of p73 in the hippocampus. These data indicate that p73 is expressed within hippocampal pyramidal neurones and exhibits altered subcellular distribution in AD.

DeltaNp73beta is active in transactivation and growth suppression

p73, a p53 family protein, shares significant sequence homolog and functional similarity with p53. However, unlike p53, p73 has at least seven alternatively spliced isoforms with different carboxyl termini (p73alpha-eta). Moreover, the p73 gene can be transcribed from a cryptic promoter located in intron 3, producing seven more proteins (DeltaNp73alpha-eta). DeltaNp73, which does not contain the N-terminal activation domain in p73, has been thought to be transcriptionally inactive and dominant negative over p53 or p73. To systemically analyze the activity of the DeltaN variant, we generated stable cell lines, which inducibly express DeltaNp73alpha, DeltaNp73beta, and various DeltaNp73beta mutants by using the tetracycline-inducible expression system. Surprisingly, we found that DeltaNp73beta is indeed active in inducing cell cycle arrest and apoptosis. Importantly, we found that, when DeltaNp73beta is expressed at a physiologically relevant level, it is capable of suppressing cell growth. We then demonstrated that these DeltaNp73beta activities are not cell type specific. We showed that the 13 unique residues at the N terminus are required for DeltaNp73beta to suppress cell growth. We also found that, among the 13 residues, residues 6 to 10 are critical to DeltaNp73beta function. Furthermore, we found that DeltaNp73beta is capable of inducing some p53 target genes, albeit to a lesser extent than does p73beta. Finally, we found that the 13 unique residues, together with the N-terminal PXXP motifs, constitute a novel activation domain. Like DeltaNp73beta, DeltaNp73gamma is active in transactivation. However, unlike DeltaNp73beta, DeltaNp73alpha is inactive in suppressing cell growth. Our data, together with others' previous findings, suggest that DeltaNp73beta may have distinct functions under certain cellular circumstances.

p73 Induces Apoptosis via PUMA Transactivation and Bax Mitochondrial Translocation

p73, an important developmental gene, shares a high sequence homology with p53 and induces both G(1) cell cycle arrest and apoptosis. However, the molecular mechanisms through which p73 induces apoptosis are unclear. We found that p73-induced apoptosis is mediated by PUMA (p53 up-regulated modulator of apoptosis) induction, which, in turn, causes Bax mitochondrial translocation and cytochrome c release. Overexpression of p73 isoforms promotes cell death and bax promoter transactivation in a time-dependent manner. However, the kinetics of apoptosis do not correlate with the increase of Bax protein levels. Instead, p73-induced mitochondrial translocation of Bax is kinetically compatible with the induction of cell death. p73 is localized in the nucleus and remains nuclear during the induction of cell death, indicating that the effect of p73 on Bax translocation is indirect. The ability of p73 to directly transactivate PUMA and the direct effect of PUMA on Bax conformation and mitochondrial relocalization suggest a molecular link between p73 and the mitochondrial apoptotic pathway. Our data therefore indicate that PUMA-mediated Bax mitochondrial translocation, rather than its direct transactivation, correlates with cell death. Finally, human DeltaNp73, an isoform lacking the amino-terminal transactivation domain, inhibits TAp73-induced as well as p53-induced apoptosis. The DeltaNp73 isoforms seem therefore to act as dominant negatives, repressing the PUMA/Bax system and, thus, finely tuning p73-induced apoptosis. Our findings demonstrate that p73 elicits apoptosis via the mitochondrial pathway using PUMA and Bax as mediators.

DeltaNp73 can modulate the expression of various genes in a p53-independent fashion

DeltaNp73alpha is an isoform of the p53 homologue p73 that lacks an amino-terminal transactivation domain and antagonizes the induction of gene expression by p53. Here, we examined whether DeltaNp73alpha might also modulate cellular transcription in the absence of p53. The expression of DeltaNp73alpha in the p53-/- cell line H1299 reduced the mRNA levels of p21/CDKN1A, but did not affect other p53-responsive genes. Correspondingly, the p21/CDKN1A promoter was downregulated by DeltaNp73alpha in reporter assays, whereas other p53-responsive promoters were not inhibited. To identify additional genes that respond to DeltaNp73alpha in the absence of p53, microarrays carrying 4600 cDNA clones were hybridized. The expression of 30 genes was found to be altered more than threefold by overexpressed DeltaNp73alpha. For instance, DeltaNp73alpha increased the expression of EGR1 and CDC6, whereas it decreased the mRNA levels of c-MYC, cyclin A2/CCNA2, NF-kappaB1, ODC1, and RET finger protein/RFP. Semiquantitative reverse transcription-PCR confirmed these results and further revealed that the influence of DeltaNp73alpha on the regulation of these genes differs from other p73 isoforms and p53. We conclude that the impact of DeltaNp73alpha on gene expression is not limited to p53-responsive genes. Rather, DeltaNp73alpha can regulate the expression of a variety of genes independently of p53.

MDM2 and Prognosis

The cellular stress response pathway regulated by the p53 tumor suppressor is critical to the maintenance of genomic integrity and to the prevention of oncogenic transformation. Intracellular levels of p53 are tightly regulated by an autoregulatory feedback loop comprised of p53 and MDM2. It might be predicted that disruption of this loop, either through p53 mutation or overexpression of MDM2, would be a negative prognostic marker for cancer development, likelihood of relapse, or response to therapy. In fact, although MDM2 overexpression is common in cancer, it can be both a positive and a negative predictor of outcome in different tumors, and its significance as a biomarker remains controversial. Data from a number of different tumor types are reviewed for the predictive significance of MDM2 expression, along with evidence for different mechanisms of MDM2 overexpression in these different tumors.

In light of the biological complexities underlying the p53-MDM2 loop, it is, perhaps, not surprising that no simple paradigm exists that is generally applicable. Much work remains to be done to elucidate the basic mechanisms underlying the physical interactions between the two proteins, the role of protein modifications in altering those interactions, and also the genetic and transcriptional deregulations by which protein levels are altered in human cancers. Only in this way will truly biologically relevant predictive factors emerge.

Alterations of DeltaTA-p 73 splice transcripts during melanoma development and progression

In the last 2 years, it has become apparent that the p53-family members p53 and p73 play fundamentally different roles in human malignancies. In contrast to p53, many studies on cancer patients failed to detect mutational inactivation of p73 and reported overexpression of wild-type p73 instead. A possible explanation was provided by the recent discovery of N-terminal truncated isoforms of p73 (DeltaTA-p73) that act as dominant-negative inhibitors of wild-type p53 and TA-p73 and result in tumor growth in nude mice. We investigated the role of DeltaTA-p73 in the development and progression of human melanomas, which lack p53 mutations. We analyzed 8 benign melanocytic nevi, 8 primary melanomas and 19 melanoma metastases for alterations of TA-p73 and DeltaTA-p73 expression using isoform-specific real-time RT-PCR. Based on our results, p73Deltaex2 and Deltaex2/3 spliced transcripts derived from the first promoter were significantly up-regulated in melanoma metastases, whereas DeltaN-p73 generated from the second promoter was the predominant isoform in benign nevi. Moreover, increased expression of p73Deltaex2 and p73Deltaex2/3 correlated with high-levels of both TA-p73 and E2F1. Our data suggest a potential function of DeltaTA-p73 splice isoforms in melanoma progression.

RNA Interference in Biology and Medicine

First discovered in plants the nematode Caenorhabditis elegans, the production of small interfering RNAs (siRNAs) that bind to and induce the degradation of specific endogenous mRNAs is now recognized as a mechanism that is widely employed by eukaryotic cells to inhibit protein production at a post-transcriptional level. The endogenous siRNAs are typically 19- to 23-base double-stranded RNA oligonucleotides, produced from much larger RNAs that upon binding to target mRNAs recruit RNases to a protein complex that degrades the targeted mRNA. Methods for expressing siRNAs in cells in culture and in vivo using viral vectors, and for transfecting cells with synthetic siRNAs, have been developed and are being used to establish the functions of specific proteins in various cell types and organisms. RNA interference methods provide several major advantages over prior methods (antisense DNA or antibody-based techniques) for suppressing gene expression. Recent preclinical studies suggest that RNA interference technology holds promise for the treatment of various diseases. Pharmacologists have long dreamed of the ability to selectively antagonize or eliminate the function of individual proteins--RNAi technology may eventually make that dream a reality.

DNA damage induces transcriptional activation of p73 by removing C-EBPalpha repression on E2F1

p73 is a member of the p53 family often overexpressed in human cancer. Its regulation, particularly following DNA damage, is different from that of p53. Following DNA damage, we found induction of p73 at both the protein and mRNA levels. Furthermore, by using different p73 promoter fragments, we found a role for E2F1 in mediating transcription of p73. However, this observation alone does not account for the observed DNA damage-induced activation of p73 in the cells used in these experiments. By analyzing the p73 promoter sequence, we revealed a new mechanism of p73 induction associated with the removal of transcriptional repression from the p73 promoter. We found, in fact, that treatment of cells with DNA damaging agents induced nuclear export of the transcription factor C-EBPalpha and blockage of this export abolished drug-induced p73 activation. We also show that C-EBPalpha has a direct repressive activity on transfactor E2F1, and for this repression the binding of C-EBPalpha to its consensus sequence in the DNA is required. These data suggest that in normal conditions a repressor complex involving C-EBPalpha, E2F1 and perhaps other proteins is present on the p73 promoter. This repressor complex is destroyed following damage by removal of C-EBPalpha from nuclei.

p21/CDKN1A mediates negative regulation of transcription by p53

The tumor suppressor p53 regulates transcription positively and negatively, depending on the target gene. Whereas p53 induces transcription through direct interaction with promoter DNA, the mechanism of p53-mediated transcriptional repression is less well understood. Early reports described the alleviation of p53-mediated repression by inhibitors of apoptosis, suggesting that negative regulation of transcription might occur only in conjunction with programmed cell death. More recently, it has been proposed that certain genes, such as survivin, are repressed by direct association of p53 with their promoters, followed by recruitment of a repressor complex. We show here that p53-mediated negative regulation of transcription could occur independently of apoptosis. In contrast, the amino-terminal transactivation domain of p53 was required for negative regulation of transcription. Similarly, the p53 homologue p73 diminished the expression of survivin and stathmin, depending on its transactivation domain. Mutation of the putative p53 binding site within the survivin promoter did not impair its repression. These observations raised the hypothesis that activation of an effector gene might be required for repression by p53. Strikingly, when the p53-inducible p21/CDKN1A gene was deleted, p53 no longer repressed any one among 11 genes that it down-regulates otherwise. Most of these genes were also repressed by ectopic p21 in the absence of p53. Overexpressed c-Myc reduced the transcription of p21/CDKN1A and impaired p53-mediated repression but did not abolish repression by ectopic p21. Taken together, these results strongly suggest that increased expression of p21/CDKN1A is necessary and sufficient for the negative regulation of gene expression by p53.

Differential regulation of p63 and p73 expression

Two homologs of the tumor suppressor p53, named p63 and p73, are each expressed from at least two start sites of mRNA synthesis, yielding full-length, transactivating (TA) isoforms, and also aminoterminally truncated (DeltaN) isoforms that act as antagonists to p53. The expression of TAp73-transcripts is induced by E2F and negatively regulated by transforming growth factor beta (TGFbeta). The DeltaNp73 promoter is induced by p53, resulting in negative feedback to control p53 activity. Here, we have analysed the expression of p63 in comparison with p73. In contrast to the induction of DeltaNp73, the expression of DeltaNp63 was reduced by p53 particularly in human keratinocytes, at the mRNA and protein levels. Accordingly, the 3' promoter of p73, but not that of p63, was activated by p53 in reporter assays. DeltaNp73 mRNA and DeltaNp73 protein, but not the p63 gene products, also accumulated when HaCat cells (lacking functional p53) were grown to high density. TAp73, but not TAp63, expression was suppressed by TGFbeta in these cells, and the TAp73, but not the TAp63, promoter was induced by E2F-1. Thus, in contrast to the functional similarities of their respective products, the expression levels of p63 and p73 are regulated by different mechanisms. This might be responsible for the discordant biological roles of p63 and p73 in development, as well as their deviant expression characteristics in cancer.

p73alpha expression induces both accumulation and activation of wt-p53 independent of the p73alpha transcriptional activity

The p53 tumor suppressor gene belongs to a multigene family that includes two paralogues, p63 and p73. p73alpha has common activities with p53, such as DNA binding and transactivation, and can thus activate the transcription of p53-responsive genes. Using the adenoviral system, we report that an overexpression of either wt-p73alpha or one of the two transcriptional inactive mutants, deltaNp73alpha or p73alphaR292H, induces an accumulation of the endogenous wt-p53 expressed in the three transformed cell lines, SK-N-SH, MCF-7 and U-2OS, without stimulating the p53 gene transcription. p73-mediated accumulation of p53 protein coincides with an increase of p53-target gene expression in cells expressing either wt-p73alpha or the transcriptional inactive mutant p73alphaR292H, but not deltaNp73alpha that encodes a dominant-negative mutant of both p73 and p53. The fact that an ectopic expression of p73alphaR292H leads to both accumulation of p53 and stimulation of p53 target gene expression strongly suggests that p73alpha is able to induce activation of p53. This was confirmed by showing that p73alphaR292H no longer stimulated Waf1/p21 expression in MCF7/R-A1 cells that expressed a transcriptional inactive mutant of p53. We thus conclude that p73alpha protein was able to both stabilize and activate wt-p53 protein, independent of the p73alpha transcriptional activity.

Data mining the p53 pathway in the Fugu genome: evidence for strong conservation of the apoptotic pathway

The p53 tumour suppressor gene belongs to a small family of related proteins that includes two other members, p63 and p73. Phylogenetic and functional studies suggest that p63 and p73 are ancient genes that have essential roles in normal development, whereas p53 seems to have evolved more recently to prevent cell transformation. In mammalian cells, a plethora of proteins have been found to specifically regulate p53 activity. The genome of the fish Fugu rubripes has been recently published. It is the second vertebrate genome for which the entire sequence is now available. Phylogenetic studies are essential in order to analyse and define signalling pathways important for cell cycle regulation. The presence or absence of a critical member in any pathway can shed light about the evolution of these pathways. The Fugu genome databank has been analysed for several members of the p53 network, including p53, p63 and p73. A good conservation of the network that regulates p53 stability and apoptosis has been found. We also discovered that some cofactors that cooperate with p53 for apoptosis are also well conserved and belong to multigene families not detected in the human genome.

A naturally occurring p73 mutation in a p73-p53 double-mutant lung cancer cell line encodes p73 alpha protein with a dominant-negative function

p73, a close homolog of p53 tumor suppressor, induces growth arrest and apoptosis. However, its role in cancers is controversial because of the rarity of p73 mutations, lack of tumors in p73-knockout mice, and the presence of multiple isotypes, among which Delta N isotypes inhibit the function of TA isotypes. We analyzed three naturally occurring p73 mutants found in lung cancer cell lines, NCI-H1155, DMS 92 and A427. NCI-H1155 is a cell line that has a p73 mutation [p73(G264W)] in the DNA-binding domain, as well as a p53 mutation [p53(R273H)], which is frequently found in human cancers and has a "gain-of-function" characteristic. p73 alpha(G264W) not only lacks transactivation activity itself, but also suppressed the transactivation activity of the wild-type p73 alpha in a dose-dependent manner, indicating that p73 alpha(G264W) is a dominant-negative mutant. p73 alpha(G264W) failed to suppress colony formation. We tested two other mutations, p73(Del418) in DMS 92 and p73(Del603) in A427. Both mutants retained similar levels of transactivation activity and suppression of colony formation to those of wild-type p73. The biological significance of these two mutations is unclear. In NCI-H1155 cells the coexistence of mutations that abrogate the normal functions of p73 and p53 may indicate that each mutation confers an additive growth advantage upon the cells.

RNA interference: gene silencing in the fast lane

Sequencing of whole genomes has provided new perspectives into the blueprints of diverse organisms. Knowing the sequences, however, does not always tell us much about the function of the genes that regulate development and homeostasis. RNA interference (RNAi) is becoming the method of choice for gene function analysis in cells and whole organisms. Here we review the approaches available to perform RNAi experiments in mammalian cells and in mice. We discuss usage of RNAi in cancer research and as a possible therapeutic tool for cancer treatment.

Tumor suppressor p53 and its homologue p73alpha affect cell migration

The p53 tumor suppressor plays a central role in the negative control of growth and survival of abnormal cells. Previously we demonstrated that in addition to these functions, p53 expression affects cell morphology and lamellar activity of the cell edge (Alexandrova, A., Ivanov, A., Chumakov, P. M., Kopnin, P. B., and Vasiliev, J. M. (2000) Oncogene 19, 5826-5830). In the present work we studied the effects of p53 and its homologue p73alpha on cell migration. We found that loss of p53 function correlated with decreased cell migration that was analyzed by in vitro wound closure test and Boyden chamber assay. The decreased motility of p53-deficient cells was observed in different cell contexts: human foreskin fibroblasts (BJ), human colon and lung carcinoma cell lines (HCT116 and H1299, respectively), as well as mouse normal fibroblasts from lung and spleen, peritoneal macrophages, and keratinocytes. On the other hand, overexpression of the p53 family member p73alpha stimulated cell migration. Changes in cell migration correlated directly with transcription activation induced by p53 or p73alpha. Noteworthy, p53 modulated cell motility in the absence of stress. The effect of p53 and p73alpha on cell migration was mediated through the activity of the phosphatidylinositol 3-kinase/Rac1 pathway. This p53/p73 function was mainly associated with some modulation of intracellular signaling rather than with stimulation of production of secreted motogenic factors. The identified novel activity of the p53 family members might be involved in regulation of embryogenesis, wound healing, or inflammatory response.