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

Differential recognition of response elements determines target gene specificity for p53 and p63

p63 is a member of the p53 tumor suppressor gene family, which regulates downstream target gene expression by binding to sequence-specific response elements similar to those of p53. By using oligonucleotide expression microarray analysis and analyzing the promoters of p63-induced genes, we have identified novel p63-specific response elements (p63-REs) in the promoter regions of EVPL and SMARCD3. These p63-REs exhibit characteristic differences from the canonical p53-RE (RRRCWWGYYY) in both the core-binding element (CWWG) as well as the RRR and/or YYY stretches. Luciferase assays on mutagenized promoter constructs followed by electromobility shift analysis showed that p53 preferentially activates and binds to the RRRCATGYYY sequence, whereas p63 preferentially activates RRRCGTGYYY. Whereas EVPL protein is highly expressed in epithelial cells of the skin and pharynx in the p63+/+ mouse, it is undetectable in these tissues in the p63-/- mouse. Our results indicate that p63 can regulate expression of specific target genes such as those involved in skin, limb, and craniofacial development by preferentially activating distinct p63-specific response elements.

The p53 Gene Family: Control of Cell Proliferation and Developmental Programs

For a quarter of a century the gene p53 has attracted close attention of scientists who deal with problems of carcinogenesis and maintenance of genetic stability. Multicellular organisms on our planet owe their rich evolution in many respects to the ability of this gene to protect cells from oncogenic transformation and harmful changes in DNA. A relatively recent discovery of structural p53 homologs, the genes p63 and p73, which seem to have more ancient roots, has roused keen interest in their function. Do they carry out oncosuppressor functions in partnership with p53 or do they possess their own specific functions? This review analyzes data on p53, p63, and p73 functional activity at the levels of the molecule, cell, and whole organism with the accent on examination of specific p63/p73 targets indicating a unique role of these genes in control of developmental processes.

DeltaNp73 antisense activates PUMA and induces apoptosis in neuroblastoma cells

The p73 gene codes for various different protein isoforms. They include proteins expressed under the control of the P1 promoter that contain a transactivation domain and are similar in function to p53 (TAp73 isoforms), as well as proteins regulated by the P2 promoter that lack this domain and function as dominant negative inhibitors of TAp73 and p53 (DeltaNp73 isoforms). Whereas TAp73 functions as a tumor suppressor with pro-apoptotic function, DeltaNp73 is likely to prevent the induction of apoptosis in tumor cells and to participate in oncogenesis. Here we used a loss-of-function strategy to assess the role of DeltaNp73 in SH-SY5Y neuroblastoma cells. An antisense oligonucleotide designed to target DeltaNp73 mRNA, but not TAp73, was used to effectively downregulate this transcript. DeltaNp73 downregulation was accompanied by increased levels of the pro-apoptotic BH3 family member PUMA at the mRNA and protein level, and by conformational activation of BAX which translocated to mitochondria. These DeltaNp73 antisense-mediated alterations led to the induction of apoptosis as detected by decreased cell viability, augmented DNA fragmentation and increased caspase-3 activity in cell lysates. Our results demonstrate the cytoprotective role of DeltaNp73 in neuroblastoma and suggest its use as a target for molecular intervention therapy.

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.

Spatiotemporal Switch from ΔNp73 to TAp73 Isoforms during Nephrogenesis

p73 is a member of the p53 gene family, which also includes p53 and p63. These proteins share sequence similarity and target genes but also have divergent roles in cancer and development. Unlike p53, transcription of the p73 gene yields multiple full-length (transactivation (TA) domain) and amino terminus-truncated (DeltaN) isoforms. DeltaNp73 acts in a dominant negative fashion to inhibit the actions of TAp73 and p53 on their target genes, promoting cell survival and proliferation and suppressing apoptosis. The balance between TAp73 and its negative regulator, DeltaNp73, may therefore represent an important determinant of developmental cell fate. There is little if anything known regarding the developmental regulation of the p73 gene. In this study, we showed that TAp73 and DeltaNp73 exhibit reciprocal spatiotemporal expression and functions during nephrogenesis. TAp73 was predominantly expressed in the differentiation domain of the renal cortex in an overlapping manner with the vasopressin-sensitive water channel aquaporin-2 (AQP-2). Chromatin immunoprecipitation assays demonstrated that the endogenous AQP-2 promoter was occupied by TAp73 in a developmentally regulated manner. Furthermore TAp73 stimulated AQP-2 promoter-driven reporter expression. TAp73 also activated the bradykinin B2 receptor (B2R) promoter, a developmentally regulated gene involved in regulation of sodium excretion. The transcriptional effects of TAp73 on AQP-2 and B2R were independent of p53. In marked contrast to TAp73, DeltaNp73 isoforms were induced early in development and were preferentially expressed in proliferating nephron precursors. Moreover DeltaNp73 was a potent repressor of B2R gene transcription. We conclude that the p73 gene is developmentally regulated during kidney organogenesis. The spatiotemporal switch from DeltaNp73 to TAp73 may play an important role in the terminal differentiation program of the developing nephron.

The metabotropic P2Y4 receptor participates in the commitment to differentiation and cell death of human neuroblastoma SH-SY5Y cells

Extracellular nucleotides exert a variety of biological actions through different subtypes of P2 receptors. Here we characterized in the human neuroblastoma SH-SY5Y cells the simultaneous presence of various P2 receptors, belonging to the P2X ionotropic and P2Y metabotropic families. Western blot analysis detected the P2X1,2,4,5,6,7 and P2Y1,2,4,6, but not the P2X3 and P2Y12 receptors. We then investigated which biological effects were mediated by the P2Y4 subtype and its physiological pyrimidine agonist UTP. We found that neuronal differentiation of the SH-SY5Y cells with dibutiryl-cAMP increased the expression of the P2Y4 protein and that UTP itself was able to positively interfere with neuritogenesis. Moreover, transient transfection and activation of P2Y4 also facilitated neuritogenesis in SH-SY5Y cells, as detected by morphological phase contrast analysis and confocal examination of neurofilament proteins NFL. This was concurrent with increased transcription of immediate-early genes linked to differentiation such as cdk-5 and NeuroD6, and activity of AP-1 transcription family members such as c-fos, fos-B, and jun-D. Nevertheless, a prolonged activation of the P2Y4 receptor by UTP also induced cell death, both in naive, differentiated, and P2Y4-transfected SH-SY5Y cells, as measured by direct count of intact nuclei and cytofluorimetric analysis of damaged DNA. Taken together, our data indicate that the high expression and activation of the P2Y4 receptor participates in the neuronal differentiation and commitment to death of SH-SY5Y cells.

Protein stability and function of p73 are modulated by a physical interaction with RanBPM in mammalian cultured cells

Upon a certain DNA damage including cisplatin treatment, p73 is stabilized and exerts its growth-suppressive and/or proapoptotic function. However, the precise molecular basis by which the intracellular levels of p73 are regulated remains unclear. In the present study, we have identified RanBPM as a novel binding partner of p73alpha by yeast-based two-hybrid screening, and also found that RanBPM has an ability to stabilize p73alpha. GST pull-down assays and co-immunoprecipitation experiments revealed that RanBPM directly bound to the extreme COOH-terminal region of p73alpha, whereas it failed to interact with p53. Co-expression of RanBPM with p73alpha resulted in the nuclear translocation of RanBPM, and both proteins co-localized in cell nucleus as examined by indirect immunofluorescent staining. It is worth noting that the expression of RanBPM inhibited the ubiquitination of p73alpha, and thereby prolonged its half-life. Subsequent studies demonstrated that the proapoptotic activity of p73alpha was significantly enhanced in the presence of RanBPM. Taken together, our present findings implicate a novel role for RanBPM in the regulation of p73 stability and function.

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.

Identification of protein kinase A catalytic subunit beta as a novel binding partner of p73 and regulation of p73 function

Post-translational modifications play a crucial role in regulation of the protein stability and pro-apoptotic function of p53 as well as its close relative p73. Using a yeast two-hybrid screening based on the Sos recruitment system, we identified protein kinase A catalytic subunit beta (PKA-Cbeta) as a novel binding partner of p73. Co-immunoprecipitation and glutathione S-transferase pull-down assays revealed that p73alpha associated with PKA-Cbeta in mammalian cells and that their interaction was mediated by both the N- and C-terminal regions of p73alpha. In contrast, p53 failed to bind to PKA-Cbeta. In vitro phosphorylation assay demonstrated that glutathione S-transferase-p73alpha-(1-130), which has one putative PKA phosphorylation site, was phosphorylated by PKA. Enforced expression of PKA-Cbeta resulted in significant inhibition of the transactivation function and pro-apoptotic activity of p73alpha, whereas a kinase-deficient mutant of PKA-Cbeta had no detectable effect. Consistent with this notion, treatment with H-89 (an ATP analog that functions as a PKA inhibitor) reversed the dibutyryl cAMP-mediated inhibition of p73alpha. Of particular interest, PKA-Cbeta facilitated the intramolecular interaction of p73alpha, thereby masking the N-terminal transactivation domain with the C-terminal inhibitory domain. Thus, our findings indicate a PKA-Cbeta-mediated inhibitory mechanism of p73 function.

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.

Identification of two contiguous minimally deleted regions on chromosome 1p36.31-p36.32 in oligodendroglial tumours

Loss of the short arm of chromosome 1 is a hallmark of oligodendroglial tumours (OTs). Deletion mapping studies in OTs have revealed multiple commonly deleted regions on chromosome 1p, suggesting that there are more than one tumour suppressor gene. To map critical deletion regions on 1p, a series of 25 OTs were examined for loss of heterozygosity (LOH) on 19 polymorphic markers across the 1p arm using microsatellite analysis. Our study revealed that 60% of tumours had LOH of all informative markers on 1p and identified one tumour showing LOH at telomeric markers only. Since this deletion region lies in one of the critical deletion intervals defined previously, we then screened another series of 27 OTs specifically at 1p36.3 for LOH using nine polymorphic markers. A total of 12% (six out of 52) of tumours were found to carry interstitial deletions. The allelic status and the deletion breakpoints in these tumours with interstitial deletion were further verified by fluorescent in situ hybridisation. The small overlapping intervals facilitated the delineation of two contiguous minimally deleted regions of 0.76 Mb, defined by D1S468 and D1S2845, and of 0.41 Mb, bound by D1S2893 and D1S1608, on 1p36.31–36.32. Based on current reference human genome sequence these deletion regions have been sequenced almost to entirety and contain eight annotated genes. TP73, DFFB and SHREW1 are the only known genes located in these deletion regions, while the others are uncharacterised novel genes. In conclusion, our study has narrowed down the critical tumour suppressor loci on 1p36.3, in which two minimally deleted regions are mapped, and markedly reduced the number of candidate genes to be screened for their involvement in OT development.

p73 isoforms can induce T-cell factor-dependent transcription in gastrointestinal cells

A new p53 family member, p73, and its isoform DeltaNp73 are increasingly recognized in cancer research as important players in tumorigenesis, as well as in chemotherapeutic drug sensitivity. Despite substantial structural similarities to p53, accumulating evidence suggests that p53 and p73 may play different roles in human tumorigenesis. In this study, we have investigated the role of p73 and DeltaNp73 in upper gastrointestinal tumorigenesis. Our results indicate that p73 and DeltaNp73 are frequently overexpressed in >60% of primary adenocarcinomas of the stomach and esophagus. We have demonstrated that this overexpression can lead to the suppression of p73 transcriptional and apoptotic activity in gastrointestinal cells. Moreover, it induces beta-catenin up-regulation and T-cell factor/lymphocyte enhancement factor-dependent transcription. Wild-type p53, but not mutant p53, can inhibit this effect. Our results demonstrate a novel mechanism for activation of beta-catenin in gastrointestinal tumors and support the concept that overexpression of p73 isoforms can play an important role in tumorigenesis.

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).

Polo-like kinase 1 (Plk1) inhibits p53 function by physical interaction and phosphorylation

Polo-like kinase 1 (Plk1) has an important role in the regulation of M phase of the cell cycle. In addition to its cell cycle-regulatory function, Plk1 has a potential role in tumorigenesis. Here we found for the first time that Plk1 physically binds to the tumor suppressor p53 in mammalian cultured cells, and inhibits its transactivation activity as well as its pro-apoptotic function. During the cisplatin-induced apoptosis in human neuroblastoma SH-SY5Y cells, the expression level of Plk1 was significantly decreased both at mRNA and protein levels, whereas cisplatin treatment caused a remarkable stabilization of p53. Systematic immunoprecipitation analyses using a series of deletion mutants of p53 revealed that a sequence-specific DNA-binding region of p53 is required and sufficient for the physical interaction with Plk1. The ectopically overexpressed Plk1 was co-localized with the endogenous p53 in mammalian cell nucleus, as shown by confocal laser microscopy. Expression of exogenous Plk1 and p53 in p53-deficient lung carcinoma H1299 cells greatly decreased the p53-mediated transcription from the p53-responsive p21(WAF1), MDM2, and BAX promoters, whereas the kinase-deficient mutant form of Plk1 failed to reduce the transcriptional activity of p53. Consistent with the luciferase reporter analysis, Plk1 had an ability to block the p53-dependent induction of the endogenous p21(WAF1). In addition, Plk1 inhibited the pro-apoptotic function of p53 in H1299 cells. Intriguingly, Plk1-mediated repression of p53 was attenuated with ATM. Thus, our present findings strongly suggest that p53 is a critical target of Plk1, and its function is abrogated through the physical interaction with Plk1.

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.

How many mutant p53 molecules are needed to inactivate a tetramer?

The tumor suppressor p53 is transcription factor composed of four identical subunits. The majority of the mutations in p53 are missense mutations that impair DNA binding. On the other hand, the p53-related p63 and p73 genes are rarely mutated, but many cell types express natural variants lacking the N-terminal transactivation domain (NDelta). Compelling evidence indicates that both the DNA binding-defective and NDelta mutants can impair the function of wild-type p53 in a dominant-negative manner. Interestingly, it is uncertain how many mutant subunit(s) a p53 tetramer can tolerate. In this study, we first made theoretical predictions based on the number of mutant p53 monomers needed to inactivate a tetramer and then tested how well the experimental data fit the predicted values. Surprisingly, these experiments reveal that DNA binding-defective p53 mutants (R249S and R273H) are very ineffective in impairing the transcriptional activity of p53: at least three mutants are required to inactivate a tetramer. In marked contrast, p53NDelta is a very potent inhibitor of p53: one NDelta subunit per tetramer is sufficient to abolish the transcriptional activity. DNA binding is not necessary for the NDelta proteins to inactivate p53. Similarly, NDelta variants of p63 and p73 are also powerful inhibitors of members of the p53 family. These results have important implications for our thinking about the mechanism of tumorigenesis involving missense p53 mutants or the N-terminally truncated isoforms.

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.

Regulation of the MiTF/TFE bHLH-LZ transcription factors through restricted spatial expression and alternative splicing of functional domains

The MiTF/TFE (MiT) family of basic helix-loop-helix leucine zipper transcription factors is composed of four closely related members, MiTF, TFE3, TFEB and TFEC, which can bind target DNA both as homo- or heterodimers. Using real-time RT-PCR, we have analyzed the relative expression levels of the four members in a broad range of human tissues, and found that their ratio of expression is tissue-dependent. We found that, similar to the MiTF gene, the genes for TFEB and TFEC contain multiple alternative first exons with restricted and differential tissue distributions. Seven alternative 5' exons were identified in the TFEB gene, of which three displayed specific expression in placenta and brain, respectively. A novel TFEC transcript (TFEC-C) encodes an N-terminally truncated TFEC isoform lacking the acidic activation domain (AAD), and is exclusively expressed in kidney and small intestine. Furthermore, we observed that a considerable proportion of the TFEC transcripts splice out protein-coding exons, resulting in transcription factor isoforms lacking one or more functional domains, primarily the basic region and/or the AAD. These isoforms were always co-expressed with the intact transcription factors and may act as negative regulators of MiTF/TFE proteins. Our data reveal that multiple levels of regulation exist for the MiTF/TFE family of transcription factors, which indicates how these transcription factors may participate in various cellular processes in different tissues.

[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.

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.

Neural crest development and neuroblastoma: the genetic and biological link

Neuroblastoma is one of the most common pediatric solid tumors originating from the sympathoadrenal lineage of neural crest. The tumor shows extremely different clinical phenotypes such as spontaneous regression on one hand and aggressive growth on the other hand. The different biological behavior of neuroblastoma appears to be determined by the genetic abnormalities including amplification of MYCN oncogene, DNA ploidy and some allelic imbalances. However, the spontaneous regression of neuroblastoma mimics the programmed cell death normally occurring in developing sympathetic cells expressing both TrkA tyrosine kinase A and p75NTR neurotrophin receptor. Indeed, TrkA expression is the most important factor related to the induction of tumor cell differentiation and/or programmed cell death because without its expression spontaneous regression of neuroblastoma never occurs. Thus, the enigmatic clinical behaviors of neuroblastoma are strictly linked to the molecular mechanism of neural crest development.

Positive and negative regulation of ΔN-p63 promoter activity by p53 and ΔN-p63-α contributes to differential regulation of p53 target genes

Mammary epithelial regeneration implies the existence of cellular progenitors with retained replicative capacity, prolonged lifespan and developmental potency. Evidence exists that deltaN-p63 isoforms preserve these features by modulating p53 activity in basal epithelia. deltaN-p63 mRNA levels decline at the onset of differentiation suggesting that its transcriptional regulation may contribute to the initiation of differentiation. To study transcriptional regulation of deltaN-p63, a 10.3 kbp fragment containing the deltaN-p63 promoter was isolated. We report here that deltaN-p63 is a positive and negative transcriptional target of p53 and deltaN-p63-alpha, respectively. Disruption of p53 activity or expression abolishes the expression of deltaN-p63-alpha. This regulation is mediated by a p53-binding element sufficient to confer these activities to a heterologous promoter. Chromatin immune-precipitation indicates that, in asynchronously growing cells, p53 occupies this element. In response to DNA damage, deltaN-p63-alpha is recruited to this element as transcription of deltaN-p63 declines. Disruption of deltaN-p63-alpha expression had differential effects on the transcriptional regulation of several p53-target genes. These findings indicate that p53 contributes to the preservation of basal epithelia by driving the expression of deltaN-p63 isoforms. These studies also suggest that in response to genotoxic stress, deltaN-p63-alpha mediates the silencing of its own promoter thereby altering the pattern of p53-target gene expression.

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.

A novel HECT-type E3 ubiquitin ligase, NEDL2, stabilizes p73 and enhances its transcriptional activity

Expression of p73, a p53 family member regulating cell growth and apoptosis, is maintained at low levels in mammalian cells, and cellular activation of p73 is usually controlled at the protein level. However, the precise molecular mechanisms by which p73 stability is regulated are unclear. During the search for interacting molecules with the COOH-terminal proline-rich region of p73, we identified a novel NEDD4-related protein (termed as NEDL2) which contains a C2 domain at its NH(2)-terminus, two WW domains, and a HECT domain at its COOH-terminus. As expected, NEDL2 catalyzed the ubiquitination of bacterial cellular proteins in vitro. Reciprocal co-immunoprecipitation experiments and in vitro pull-down assays revealed that NEDL2 bound to p73, which carries two putative PY motifs. p73 was efficiently ubiquitinated but stabilized in a NEDL2-dependent manner. Accordingly, p73 decayed at faster rates in the absence of NEDL2 than in its presence. Consistent with the NEDL2-mediated stabilization of p73, NEDL2 enhanced the p73-dependent transcriptional activation. Thus, our results suggest that NEDL2 activates the function of p73 by increasing its stability.

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.

deltaNp73 facilitates cell immortalization and cooperates with oncogenic Ras in cellular transformation in vivo

TP73, despite significant homology to TP53, is not a classic tumor suppressor gene, since it exhibits upregulation of nonmutated products in human tumors and lacks a tumor phenotype in p73-deficient mice. We recently reported that an N-terminally truncated isoform, DeltaNp73, is upregulated in breast and gynecological cancers. We further showed that DeltaNp73 is a potent transdominant inhibitor of wild-type p53 and TAp73 in cultured human tumor cells by efficiently counteracting their target gene transactivations, apoptosis, and growth suppression functions (A. I. Zaika et al., J. Exp. Med. 6:765-780, 2002). Although these data strongly suggest oncogenic properties of DeltaNp73, this can only be directly shown in primary cells. We report here that DeltaNp73 confers resistance to spontaneous replicative senescence of primary mouse embryo fibroblasts (MEFs) and immortalizes MEFs at a 1,000-fold-higher frequency than occurs spontaneously. DeltaNp73 cooperates with cMyc and E1A in promoting primary cell proliferation and colony formation and compromises p53-dependent MEF apoptosis. Importantly, DeltaNp73 rescues Ras-induced senescence. Moreover, DeltaNp73 cooperates with oncogenic Ras in transforming primary fibroblasts in vitro and in inducing MEF-derived fibrosarcomas in vivo in nude mice. Wild-type p53 is likely a major target of DeltaNp73 inhibition in primary fibroblasts since deletion of p53 or its requisite upstream activator ARF abrogates the growth-promoting effect of DeltaNp73. Taken together, DeltaNp73 behaves as an oncogene that targets p53 that might explain why DeltaNp73 upregulation may be selected for during tumorigenesis of human cancers.

When p53 needs p73 to be functional - forced p73 expression induces nuclear accumulation of endogenous p53 protein

In human neuroblastoma (NB), wild type p53 protein does not elicit its archetypal human tumor suppressive activity so far described. To elucidate this alteration, substantial investigations using NB cell lines have underscored p53 protein nuclear localization defect and/or inappropriate conformation, but no definitive evidence has been provided so far. p73, the first homologue of the p53 gene, locates at the 1p36.3 locus, which is known to be deleted in various human tumors including NB. Unlike p53 mRNA, which specifies a single protein, p73alpha mRNAs encode two types of isoform (TAp73alpha and DeltaNp73alpha) resulting from the use of two different promoters, and eliciting or lacking NH(2)-terminal transactivation domain, respectively. DeltaNp73alpha inhibits p53 pro-apoptotic function in murine developing neurons and is abundantly expressed in human undifferentiated NB tumors. However, critical issues have been raised regarding p73alpha isoform roles, and their possible link to p53 are yet to be clarified in human NB using adenoviral infection approach.

Negative autoregulation of p73 and p53 by DeltaNp73 in regulating differentiation and survival of human neuroblastoma cells

p73, mapped to 1p36.2-3, is a p53-related tumor suppressor but is also induced by the oncogene products such as E2F1, raising a question whether p73 is a tumor suppressor gene or oncogene. p73 has several splicing variants including DeltaNp73 which lacks the NH(2)-terminal transactivation domain. In developing neurons, DeltaNp73 is expressed abundantly and seems to inhibit the pro-apoptotic function of p53. However, the role of TAp73 and DeltaNp73 as well as their regulatory mechanism in cell growth and differentiation of neuroblastoma cells are poorly understood. We have found that TAp73 directly activates the transcription of endogenous DeltaNp73 by binding to the TAp73-specific target element located at position-76 to 57 within the DeltaNp73 promoter region. DeltaNp73 was physically associated with TAp73alpha, TAp73beta and p53, and inhibited their transactivation activities when used reporters of Mdm2, Bax or DeltaNp73 itself in SAOS-2 cells. Overexpression of DeltaNp73 in SH-SY5Y neuroblastoma cells promoted cell survival by competing with p53 and TAp73 itself. Thus, our results suggest that the negative feedback regulation of TAp73 by its target DeltaNp73 is a novel autoregulatory system for modulating cell survival and death, that is also functioning in neuroblastoma cells.

Characterization of p73 functional domains necessary for transactivation and growth suppression

p73, a p53 family member, is highly similar to p53 in both structure and function. Like p53, the p73 protein contains an N-terminal activation domain, a DNA-binding domain, a tetramerization domain, and several PXXP motifs. Previously, we and others have shown that some functional domains in p53, such as the DNA-binding and tetramerization domains, are required for inducing both cell cycle arrest and apoptosis whereas others, such as the second activation domain, the proline-rich domain, and the C-terminal basic domain, are only required for inducing apoptosis. To determine the activity of p73 functional domains, we have generated stable inducible cell lines that express p73beta and various mutants deficient in one or more functional domains. We found that in addition to the DNA-binding domain, p73-mediated growth suppression requires the N-terminal activation domain and the tetramerization domain. However, unlike p53, p73-mediated apoptosis does not require the region adjacent to the activation domain or the entire C-terminal region. Interestingly, while the N- or the C-terminal PXXP motifs are dispensable for p73 function, deletion of both the N- and the C-terminal PXXP motifs renders p73 inactive in transactivation. In addition, we found that substitution of two conserved tandem hydrophobic residues with two hydrophilic ones, which can abrogate the activity of the first activation domain in p53, has no effect on p73 transcriptional activity. Together, we showed that the p73 protein has its own unique determinants for transactivation and growth suppression.

Function of p73, not of p53, is inhibited by the physical interaction with RACK1 and its inhibitory effect is counteracted by pRB

The newly identified p53-related gene, p73, encodes a nuclear transcription factor. Unlike p53, p73 has various isoforms with different NH(2)- and COOH-terminal tails. p73alpha with the longest COOH-terminal extension is most abundantly expressed in many tissues and cells among those splicing isoforms of p73 and the COOH-terminal region appears to have an autoregulatory function. To isolate and characterize the cellular protein(s) that interacts with the unique COOH-terminal region of p73alpha, we employed a yeast two-hybrid screen with a human fetal brain and 293 cell cDNA libraries. We identified the receptor for activated C kinase (RACK1) as a new member of p73alpha-binding proteins. The interaction was confirmed by coimmunoprecipitation experiments, whereas RACK1 did not interact with p53 or p73beta. Ectopic overexpression of RACK1 in SAOS-2 cells reduced the p73alpha-mediated transcription from the p53/p73-responsive promoters, and inhibited the p73alpha-dependent apoptosis. On the other hand, the p53-dependent transcriptional activation as well as apoptosis was unaffected in the presence of RACK1. Furthermore, we found that pRB physically bound to RACK1, and repressed the RACK1-dependent inhibition of p73alpha. Taken together, our observations suggest that pRB diminishes the RACK1-mediated inhibition of p73alpha activity through the interaction with RACK1.

Inactivation of retinoblastoma (RB) tumor suppressor by oncogenic isoforms of the p53 family member p73

The p53 family includes three members that share significant sequence homology, yet exhibit fundamentally different functions in tumorigenesis. Whereas p53 displays all characteristics of a classical tumor suppressor, its homologues p63 and p73 do not. We have previously shown, that NH(2)-terminally truncated isoforms of p73 (Delta TA-p73), which act as dominant-negative inhibitors of p53 are frequently overexpressed in cancer cells. Here we provide evidence that Delta TA-p73 isoforms also affect the retinoblastoma protein (RB) tumor suppressor pathway independent of p53. Delta TA-p73 isoforms inactivate RB by increased phosphorylation, resulting in enhanced E2F activity and proliferation of fibroblasts. By inactivating the two major tumor suppressor pathways in human cells they act functionally analogous to several viral oncoproteins. These findings provide an explanation for the fundamentally different functions of p53 and p73 in tumorigenesis.

TP53 family members and human cancers

Based on gene sequence homologies, a p53 (TP53) gene family become apparent with the addition of the most recently identified p63 (TP73L; formerly TP63) and p73 (TP73) genes to the already known p53. The p53 gene encodes for a unique protein eliciting well-known tumor suppressor gene (TSG) properties that mediate cellular response to DNA damage, e.g., cell cycle arrest or apoptosis. In contrast, both homologues specify an array of isoforms different in their N- and C-terminal domains. Transactivating isoforms, such as TAp63/p73, show TSG properties similar to p53, while isoforms lacking N-terminal transactivating domain such as DeltaNp63/p73, induce a functional block against p53 as well as TAp63/p73 activities. Both p63/p73 types of isoforms are involved in development: p63 is critical for epithelial stem cell renewal and epithelial homeostasis, and p73 is involved in neurogenesis and natural immune response. These facts support interdependent functions for the p53 family members, which appear linked together in a complex and tight regulation network to fulfill cellular functions related to DNA damage and tissue homeostasis maintenance. The lack of p63/p73 mutations in human cancers rule out a typical TSG role for either of the p53 homologues. Nonetheless, p63 and p73 genes seem strongly involved in malignancy acquisition and maintenance process because of: 1) their tissue identities, and 2) their close interplay activities within the p53 family members, and primarily through the negative regulatory role played by DeltaNp63/p73 isoforms for cell death control and differentiation.

Novel cell-specific and dominant negative anti-apoptotic roles of p73 in transformed leukemia cells

Although extensive homology exists between related genes p53 and p73, recent data suggest that the family members have divergent roles. We demonstrate that the differential regulatory roles of p53 family member p73 are highly cell-context and promoter-specific. Full-length p73 expressed in the transformed leukemia cell line Jurkat behaves as a specific dominant negative transcriptional repressor of the cell cycle inhibitor gene p21 and blocks p53-mediated apoptosis. These findings provide evidence for a new mechanism in oncogenesis through which the functional properties of p73 can be altered in an inheritable and cell-specific fashion independent of transcriptional coding.

From neurotrophins to immunotrophins. NGF 2002: The 7th international conference on NGF and related molecules

NGF 2002: The 7th international conference on NGF and related molecules

DeltaNp73, a dominant-negative inhibitor of wild-type p53 and TAp73, is up-regulated in human tumors

p73 has significant homology to p53. However, tumor-associated up-regulation of p73 and genetic data from human tumors and p73-deficient mice exclude a classical Knudson-type tumor suppressor role. We report that the human TP73 gene generates an NH(2) terminally truncated isoform. DeltaNp73 derives from an alternative promoter in intron 3 and lacks the transactivation domain of full-length TAp73. DeltaNp73 is frequently overexpressed in a variety of human cancers, but not in normal tissues. DeltaNp73 acts as a potent transdominant inhibitor of wild-type p53 and transactivation-competent TAp73. DeltaNp73 efficiently counteracts transactivation function, apoptosis, and growth suppression mediated by wild-type p53 and TAp73, and confers drug resistance to wild-type p53 harboring tumor cells. Conversely, down-regulation of endogenous DeltaNp73 levels by antisense methods alleviates its suppressive action and enhances p53- and TAp73-mediated apoptosis. DeltaNp73 is complexed with wild-type p53, as demonstrated by coimmunoprecipitation from cultured cells and primary tumors. Thus, DeltaNp73 mediates a novel inactivation mechanism of p53 and TAp73 via a dominant-negative family network. Deregulated expression of DeltaNp73 can bestow oncogenic activity upon the TP73 gene by functionally inactivating the suppressor action of p53 and TAp73. This trait might be selected for in human cancers.

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

The p53 tumor suppressor protein activates transcription and induces cell death. A close homologue of p53, termed p73, is expressed in transactivating (TA) forms that induce growth arrest and apoptosis much like p53. However, the p73 gene contains a second promoter, giving rise to the expression of p73 Delta N, a species of p73 proteins that lack the N-terminal transactivation domain. We show here that the expression of p73 Delta N is induced by p53 on the mRNA and protein level. The promoter that regulates p73 Delta N expression in human cells was cloned and found to be activated by p53, as well as by p73TA, directly through a specific DNA element. The p73 Delta N proteins, that are thereby expressed, bound to p53-responsive promoter DNA, competed with p53 for DNA binding, antagonized the activation of transcription by p53, and prevented p53-induced cell death. In addition, a transcriptional repressor domain was identified within the splicing variant p73 Delta Nalpha. The combination of p73DeltaNalpha and mdm2 antagonized p53 more strongly than either p73Nalpha or mdm2 alone. Blocking endogenous p73 Delta N by a trans dominant fragment, or its removal by siRNA, increased the activity of a p53-responsive promoter in cells that contain a wild type p53 gene. Thus, the induction of p73 Delta N expression by p53 establishes an autoregulatory feedback loop that keeps the trigger of cell death under tight control.

p73-dependent expression of DAN during cisplatin-induced cell death and osteoblast differentiation

We previously reported that DAN, a founding member of the DAN family of secreted proteins, acts as an inhibitor of cell cycle progression and is closely involved in retinoic acid-induced neuroblastoma differentiation. In this study, we found that DAN as well as p73, the recently identified p53 family member, was up-regulated during osteoblast differentiation. Additionally, the expression of DAN was increased in response to cisplatin-induced cell death of neuroblastoma SH-SY5Y cells. Consistent with the previous reports, p73 was accumulated after the treatment with cisplatin. Intriguingly, we found a putative p53/p73-binding site in the 5'-upstream region of the human DAN gene. A luciferase reporter assay and an in vitro DNA-binding experiment revealed that this canonical p53/p73-binding site was a functional responsive element and was specific for p73. Our results suggest that there exists a functional association between DAN and p73 during osteoblast differentiation as well as cisplatin-induced cell death.