Increased DeltaN-p73 expression in tumors by upregulation of the E2F1-regulated, TA-promoter-derived DeltaN'-p73 transcript

p53/p73 Protein Network in Colorectal Cancer and Other Human Malignancies

Simple Summary

The p53 family of proteins comprises p53, p63, and p73, which share high structural and functional similarity. The two distinct promoters of each locus, the alternative splicing, and the alternative translation initiation sites enable the generation of numerous isoforms with different protein-interacting domains and distinct activities. The co-expressed p53/p73 isoforms have significant but distinct roles in carcinogenesis. Their activity is frequently impaired in human tumors including colorectal carcinoma due to dysregulated expression and a dominant-negative effect accomplished by some isoforms and p53 mutants. The interactions between isoforms are particularly important to understand the onset of tumor formation, progression, and therapeutic response. The understanding of the p53/p73 network can contribute to the development of new targeted therapies.

Abstract

The p53 tumor suppressor protein is crucial for cell growth control and the maintenance of genomic stability. Later discovered, p63 and p73 share structural and functional similarity with p53. To understand the p53 pathways more profoundly, all family members should be considered. Each family member possesses two promoters and alternative translation initiation sites, and they undergo alternative splicing, generating multiple isoforms. The resulting isoforms have important roles in carcinogenesis, while their expression is dysregulated in several human tumors including colorectal carcinoma, which makes them potential targets in cancer treatment. Their activities arise, at least in part, from the ability to form tetramers that bind to specific DNA sequences and activate the transcription of target genes. In this review, we summarize the current understanding of the biological activities and regulation of the p53/p73 isoforms, highlighting their role in colorectal tumorigenesis. The analysis of the expression patterns of the p53/p73 isoforms in human cancers provides an important step in the improvement of cancer therapy. Furthermore, the interactions among the p53 family members which could modulate normal functions of the canonical p53 in tumor tissue are described. Lastly, we emphasize the importance of clinical studies to assess the significance of combining the deregulation of different members of the p53 family to define the outcome of the disease.

Synergistic activation of the NEU4 promoter by p73 and AP2 in colon cancer cells

More than 50% of colon cancers bear mutations in p53, one of the most important tumor suppressors, and its family members p63 or p73 are expected to contribute to inhibiting the progression of colon cancers. The AP2 family also acts as a tumor suppressor. Here we found that p73 and AP2 are able to activate NEU4, a neuraminidase gene, which removes the terminal sialic acid residues from cancer-associated glycans. Under serum starvation, NEU4 was up-regulated and one of the NEU4 target glycans, sialyl Lewis X, was decreased, whereas p73 and AP2 were up-regulated. Sialyl Lewis X levels were not, however, decreased under starvation conditions in p73- or AP2-knockdown cells. p53 and AP2 underwent protein-protein interactions, exerting synergistic effects to activate p21, and interaction of p53 with AP2 was lost in cells expressing the L350P mutation of p53. The homologous residues in p63 and p73 are L423 and L377, respectively. The synergistic effect of p53/p63 with AP2 to activate genes was lost with the L350P/L423P mutation in p53/p63, but p73 bearing the L377P mutation was able to interact with AP2 and exerted its normal synergistic effects. We propose that p73 and AP2 synergistically activate the NEU4 promoter in colon cancer cells.

p73 gene in dopaminergic neurons is highly susceptible to manganese neurotoxicity

Chronic exposure to elevated levels of manganese (Mn) has been linked to a Parkinsonian-like movement disorder, resulting from dysfunction of the extrapyramidal motor system within the basal ganglia. However, the exact cellular and molecular mechanisms of Mn-induced neurotoxicity remain elusive. In this study, we treated C57BL/6J mice with 30mg/kg Mn via oral gavage for 30 days. Interestingly, in nigral tissues of Mn-exposed mice, we found a significant downregulation of the truncated isoform of p73 protein at the N-terminus (ΔNp73). To further determine the functional role of Mn-induced p73 downregulation in Mn neurotoxicity, we examined the interrelationship between the effect of Mn on p73 gene expression and apoptotic cell death in an N27 dopaminergic neuronal model. Consistent with our animal study, 300μM Mn treatment significantly suppressed p73 mRNA expression in N27 dopaminergic cells. We further determined that protein levels of the ΔNp73 isoform was also reduced in Mn-treated N27 cells and primary striatal cultures. Furthermore, overexpression of ΔNp73 conferred modest cellular protection against Mn-induced neurotoxicity. Taken together, our results demonstrate that Mn exposure downregulates p73 gene expression resulting in enhanced susceptibility to apoptotic cell death. Thus, further characterization of the cellular mechanism underlying p73 gene downregulation will improve our understanding of the molecular underpinnings of Mn neurotoxicity.

Transcriptional regulation of the p73 gene by Nrf-2 and promoter CpG methylation in human breast cancer

To understand the transcriptional regulation of p73 by promoter methylation and Nrf-2 in breast carcinogenesis, ChIP assay indicated that Nrf-2 can bind to both promoters and can activate the transcription of TAp73 and ΔΝp73 in MCF-7 cell line, knockdown of Nrf-2 gene resulted in an abrogation of TAp73 and ΔΝp73 expression in the cells transfected with sh-Nrf-2 as well as Nrf-2 knock out mouse model. However, we found Nrf-2 induced ΔΝp73 expression was abolished with 5-aza-dC treatment, thus lead to a down-regulated ΔΝp73 and an up-regulated TAp73 expression in breast cancer cells lines. Consistent with this model, we detected decreased TAp73 and increased ΔNp73 expression in breast cancer tissue, along with increased TAp73 but decreased ΔNp73 expression in corresponding surrounding noncancerous tissues (NCTs) in a breast cancer tissue assay. A significant inverse correlation was found between TAp73 and ΔNp73 expression in the above tissue-array (P = 0.047) and validated in another set consisting of 128 breast cancer tumor tissue (P = 0.034). Taken together, our findings suggest that Nrf-2 and promoter methylation cooperatively govern the transcriptional regulation of p73, and unbalanced expression of TAp73 and ΔNp73 expression plays a critical role in breast cancer development.

Regulation of p38, PKC/Foxo3a/p73 signaling network by GTP during erythroid differentiation in chronic myelogenous leukemia

It is well established that Foxo3a is a fundamental module of signal transduction pathways regulating erythropoiesis; however, precise mechanism which regulates its physiological function still remains unclear. Here, our results revealed that the nuclear localization and stability of Foxo3a were modulated by the physical interaction of PKC and p38 signaling elements and that direct interactions led to phosphorylation of threonine residue(s) in Foxo3a. In addition, our findings revealed that the sequential activity of Foxo3a by guanosine 5'-triphosphate can impede cellular proliferation and suppress p73 expression as oncoprotein in K562 cells; thus identifying Foxo3a as a tumor suppressor in these p53 null cells. However, down-regulation of Foxo3a-dependent p73 expression causes cell differentiation along the erythroid lineage. Collectively, our findings suggest that restoration of Foxo3a function by pharmacological agents under the influence of specific activated protein kinases might constitute a potential therapeutic strategy for combating the CML disease.

Analysis of the oligomeric state and transactivation potential of TAp73α

The proteins p73 and p63 are members of the p53 protein family and are involved in important developmental processes. Their high sequence identity with the tumor suppressor p53 has suggested that they act as tumor suppressors as well. While p63 has a crucial role in the maintenance of epithelial stem cells and in the quality control of oocytes without a clear role as a tumor suppressor, p73's tumor suppressor activity is well documented. In a recent study we have shown that the transcriptional activity of TAp63α, the isoform responsible for the quality control in oocytes, is regulated by its oligomeric state. The protein forms an inactive, dimeric and compact conformation in resting oocytes, while the detection of DNA damage leads to the formation of an active, tetrameric and open conformation. p73 shows a high sequence identity to p63, including those domains that are crucial in stabilizing its inactive state, thus suggesting that p73's activity might be regulated by its oligomeric state as well. Here, we have investigated the oligomeric state of TAp73α by size exclusion chromatography and detailed domain interaction mapping, and show that in contrast to p63, TAp73α is a constitutive open tetramer. However, its transactivation potential depends on the cellular background and the promoter context. These results imply that the regulation of p73's transcriptional activity might be more closely related to p53 than to p63.

Kinetic modeling-based detection of genetic signatures that provide chemoresistance via the E2F1-p73/DNp73-miR-205 network

Drug resistance is a major cause of deaths from cancer. E2F1 is a transcription factor involved in cell proliferation, apoptosis. and metastasis through an intricate regulatory network, which includes other transcription factors like p73 and cancer-related microRNAs like miR-205. To investigate the emergence of drug resistance, we developed a methodology that integrates experimental data with a network biology and kinetic modeling. Using a regulatory map developed to summarize knowledge on E2F1 and its interplay with p73/DNp73 and miR-205 in cancer drug responses, we derived a kinetic model that represents the network response to certain genotoxic and cytostatic anticancer drugs. By perturbing the model parameters, we simulated heterogeneous cell configurations referred to as in silico cell lines. These were used to detect genetic signatures characteristic for single or double drug resistance. We identified a signature composed of high E2F1 and low miR-205 expression that promotes resistance to genotoxic drugs. In this signature, downregulation of miR-205, can be mediated by an imbalance in the p73/DNp73 ratio or by dysregulation of other cancer-related regulators of miR-205 expression such as TGFβ-1 or TWIST1. In addition, we found that a genetic signature composed of high E2F1, low miR-205, and high ERBB3 can render tumor cells insensitive to both cytostatic and genotoxic drugs. Our model simulations also suggested that conventional genotoxic drug treatment favors selection of chemoresistant cells in genetically heterogeneous tumors, in a manner requiring dysregulation of incoherent feedforward loops that involve E2F1, p73/DNp73, and miR-205.

E2F1 confers anticancer drug resistance by targeting ABC transporter family members and Bcl-2 via the p73/DNp73-miR-205 circuitry

Resistance to anti-neoplastic agents is the major cause of therapy failure, leading to disease recurrence and metastasis. E2F1 is a strong inducer of apoptosis in response to DNA damage through its capacity to activate p53/p73 death pathways. Recent evidence, however, showed that E2F1, which is aberrantly expressed in advanced malignant melanomas together with antagonistic p73 family members, drives cancer progression. Investigating mechanisms responsible for dysregulated E2F1 losing its apoptotic function, we searched for genomic signatures in primary and late clinical tumor stages to allow the prediction of downstream effectors associated with apoptosis resistance and survival of aggressive melanoma cells. We identified miR-205 as specific target of p73 and found that upon genotoxic stress, its expression is sufficiently abrogated by endogenous DNp73. Significantly, metastatic cells can be rescued from drug resistance by selective knockdown of DNp73 or overexpression of miR-205 in p73-depleted cells, leading to increased apoptosis and the reduction of tumor growth in vivo. Our data delineate an autoregulatory circuit, involving high levels of E2F1 and DNp73 to downregulate miR-205, which, in turn, controls E2F1 accumulation. Finally, drug resistance associated to this genetic signature is mediated by removing the inhibitory effect of miR-205 on the expression of Bcl-2 and the ATP-binding cassette transporters A2 (ABCA2) and A5 (ABCA5) related to multi-drug resistance and malignant progression. These results define the E2F1-p73/DNp73-miR-205 axis as a crucial mechanism for chemoresistance and, thus, as a target for metastasis prevention.

Regulation of p73 activity by post-translational modifications

The transcription factor p73 is a member of the p53 family that can be expressed as at least 24 different isoforms with pro- or anti-apoptotic attributes. The TAp73 isoforms are expressed from an upstream promoter and are regarded as bona fide tumor suppressors; they can induce cell cycle arrest/apoptosis and protect against genomic instability. On the other hand, ΔNp73 isoforms lack the N-terminus transactivation domain; hence, cannot induce the expression of pro-apoptotic genes, but still can oligomerize with TAp73 or p53 to block their transcriptional activities. Therefore, the ratio of TAp73 isoforms to ΔNp73 isoforms is critical for the quality of the response to a genomic insult and needs to be delicately regulated at both transcriptional and post-translational level. In this review, we will summarize the current knowledge on the post-translational regulatory pathways involved to keep p73 protein under control. A comprehensive understanding of p73 post-translational modifications will be extremely useful for the development of new strategies for treating and preventing cancer.

Hierarchies of transcriptional regulation during liver regeneration

The remarkable capacity of the liver to regenerate after severe injury or disease has excited interest for centuries. The goal of harnessing this process in treatment of liver disease, and the appreciation of the parallels between regeneration and tumor development in the liver, remain a major driver for research in this area. Studies of liver regeneration as a model system offer a view of intricate and precisely timed regulatory pathways that drive the process toward completion. Successful regeneration of the liver mass demands a hierarchal and well-controlled balance between proliferative and metabolic functions, which is orchestrated by signaling and regulation of transcription factors. Control and regulation of these cascades of transcriptional activities, necessary for induction, renewal, and cessation of liver growth, are the focus of this chapter.

Differential control of TAp73 and DeltaNp73 protein stability by the ring finger ubiquitin ligase PIR2

p73 is a p53-related transcription factor with fundamental roles in development and tumor suppression. Transcription from two different promoters on the p73 gene results in generation of transcriptionally active TAp73 isoforms and dominant negative DeltaNp73 isoforms with opposing pro- and anti-apoptotic functions. Therefore, the relative ratio of each isoform is an important determinant of the cell fate. Proteasomal degradation of p73 is mediated by polyubiquitination-dependent and -independent processes both of which appear, thus far, to lack selectivity for the TAp73 and DeltaNp73 isoforms. Here, we describe the characterization of another transcriptional target of TAp73; a ring finger domain ubiquitin ligase p73 Induced RING 2 protein (PIR2). Although PIR2 was initially identified a p53-induced gene (p53RFP), low abundance of PIR2 transcript in mouse embryonic fibroblasts of TAp73 KO mice compared with WT mice and comparison of PIR2 mRNA and protein levels following TAp73 or p53 overexpression substantiate TAp73 isoforms as strong inducers of PIR2. Although PIR2 expression was induced by DNA damage, its expression did not alter apoptotic response or cell cycle profile per se. However, coexpression of PIR2 with TAp73 or DeltaNp73 resulted in an increase of the TA/DeltaNp73 ratio, due to preferential degradation of DeltaNp73. Finally, PIR2 was able to relieve the inhibitory effect of DeltaNp73 on TAp73 induced apoptosis following DNA damage. These results suggest that PIR2, by being induced by TAp73 and degrading DeltaNp73, differentially regulates TAp73/DeltaNp73 stability, and, hence, it may offer a therapeutic approach to enhance the chemosensitivity of tumor cells.

Cascades of transcription regulation during liver regeneration

An increasing demand for new strategies in cancer prevention and regenerative medicine requires a better understanding of molecular mechanisms that control cell proliferation in tissue-specific manner. Regenerating liver is a unique model allowing use of biochemical, genetic, and engineering tools to uncover molecular mechanisms and improve treatment of hepatic cancers, liver failure, and fibrotic disease. Molecular mechanisms of liver regeneration involve extra- and intracellular factors to activate transcription of genes normally silenced in quiescent liver. While many upstream signaling pathways of the regenerating liver have been extensively studied, our knowledge of the downstream effectors, transcription factors (TFs), remains limited. This review describes consecutive engagement of pre-existing and de novo synthesized TFs, as cascades that regulate expression of growth-related and metabolic genes during liver regeneration after partial hepatectomy in mice. Several previously recognized regulators of regenerating liver are described in the light of recently identified co-activator and co-repressor complexes that interact with primary DNA-binding TFs. Published results of gene expression and chromatin immunoprecipitation analyses, as well as studies of transgenic mouse models, are used to emphasize new potential regulators of transcription during liver regeneration. Finally, a more detailed description of newly identified transcriptional regulators of liver regeneration illustrates the tightly regulated balance of proliferative and metabolic responses to partial hepatectomy.

Cyclooxygenase inhibitors differentially modulate p73 isoforms in neuroblastoma

p73 encodes multiple functionally distinct isoforms. Proapoptotic TAp73 isoforms contain a transactivation (TA) domain, and like p53, have tumor suppressor properties and are activated by chemotherapies to induce cell death. In contrast, antiapoptotic DeltaNp73 isoforms lack the TA domain and are dominant-negative inhibitors of p53 and TAp73. DeltaNp73 proteins are overexpressed in a variety of tumors including neuroblastoma. Thus, identification of drugs that upregulate TAp73 and/or downregulate DeltaNp73 represents a potential therapeutic strategy. Here, we report that cyclooxygenase (COX) inhibitors induce apoptosis independent of p53, and differentially modulate endogenous p73 isoforms in neuroblastoma and other tumors. COX inhibitor-mediated apoptosis is associated with the induction of TAp73beta and its target genes. COX inhibitors also downregulate the alternative-spliced DeltaNp73(AS) isoforms, Deltaexon2 and Deltaexon2/3. Furthermore, forced expression of DeltaNp73(AS) results in diminished apoptosis in response to the selective COX-2 inhibitor celecoxib. Celecoxib-mediated downregulation of DeltaNp73(AS) is associated with decreased E2F1 levels and diminished E2F1 activation of the p73 promoter. These results provide the first evidence that COX inhibitors differentially modulate p73 isoforms leading to enhanced apoptosis, and support the potential use of COX inhibitors as novel regulators of p73 to enhance chemosensitivity in tumors with deregulated E2F1 and in those with wild-type (wt) or mutant p53.

TAp73 knockout shows genomic instability with infertility and tumor suppressor functions

The Trp53 gene family member Trp73 encodes two major groups of protein isoforms, TAp73 and DeltaNp73, with opposing pro- and anti-apoptotic functions; consequently, their relative ratio regulates cell fate. However, the precise roles of p73 isoforms in cellular events such as tumor initiation, embryonic development, and cell death remain unclear. To determine which aspects of p73 function are attributable to the TAp73 isoforms, we generated and characterized mice in which exons encoding the TAp73 isoforms were specifically deleted to create a TAp73-deficient (TAp73(-/-)) mouse. Here we show that mice specifically lacking in TAp73 isoforms develop a phenotype intermediate between the phenotypes of Trp73(-/-) and Trp53(-/-) mice with respect to incidence of spontaneous and carcinogen-induced tumors, infertility, and aging, as well as hippocampal dysgenesis. In addition, cells from TAp73(-/-) mice exhibit genomic instability associated with enhanced aneuploidy, which may account for the increased incidence of spontaneous tumors observed in these mutants. Hence, TAp73 isoforms exert tumor-suppressive functions and indicate an emerging role for Trp73 in the maintenance of genomic stability.

DNp73 a matter of cancer: mechanisms and clinical implications

The p53 family proteins carry on a wide spectrum of biological functions from differentiation, cell cycle arrest, apoptosis, and chemosensitivity of tumors. NH2-terminally truncated p73 (referred to as DNp73) acts as a potent inhibitor of all these tumor suppressor properties, implying that it has oncogenic functions in human tumorigenesis. This was favored by the observation that high DNp73 expression levels in a variety of cancers are associated with adverse clinico-pathological characteristics and the response failure to chemotherapy. The actual challenge is the deciphering of the molecular mechanisms by which DNp73 promotes malignancy and to unravel the regulatory pathways for controlling TP73 isoform expression. This review is focused on recent findings leaving no doubt that N-terminally truncated p73 proteins are operative during oncogenesis, thus underscoring its significance as a marker for disease severity in patients and as target for cancer therapy.

Expression of deltaNp73 and TAp73alpha independently associated with radiosensitivities and prognoses in cervical squamous cell carcinoma

PURPOSE

The p73 gene produces different protein isoforms using alternative promoters and splicing, which have different biological characteristics. This study was to investigate the expression patterns of two distinct p73 isoforms (deltaNp73 and TAp73alpha) in cervical squamous cell carcinomas (SCC) and the relationship between their expressions and prognostic significance in cervical SCC patients.

EXPERIMENTAL DESIGN

We investigated the protein expressions of deltaNp73 and TAp73alpha in 117 cervical SCC and 113 normal cervical tissues using immunohistochemistry. The expression levels were analyzed with clinical variables and patients' survival.

RESULTS

DeltaNp73and TAp73alpha were significantly overexpressed in cervical SCC compared with those in normal cervical epithelium (P < 0.001). However, their expressions were inversely correlated (P < 0.001, R = -0.368) and associated with differential tumor radiosensitivity. Overexpression of deltaNp73 was significantly found in SCC resistant to irradiation (P < 0.001), whereas increase of TAp73alpha expression was observed in the majority of SCC sensitive to irradiation (P < 0.001). Multivariate and survival analyses indicated that the expressions of deltaNp73 and TAp73alpha were independently associated with prognosis: deltaNp73 was associated with recurrence of the disease [P = 0.001; odds ratio (OR), 4.857] and an adverse outcome (P = 0.012; OR, 4.676), whereas TAp73alpha predicted a better survival of cervical SCC patients (P = 0.018; OR, 0.065).

CONCLUSIONS

The p73 gene might be an important determinant of cellular response to irradiation. The expressions of the two main isoforms (deltaNp73 and TAp73alpha) might be potential markers for predicting the prognosis and sensitivity to radiotherapy in patients with cervical SCC.

Implication of the transcription factor E2F-1 in the modulation of neuronal apoptosis

Neurodegenerative diseases as Alzheimer's disease, Parkinson's disease and other neurological disorders remain major problem worldwide since is currently no effective treatment. Thus, studying the mechanisms involved in neuronal apoptotic pathways is imperative if drugs that might stop or delay these disease processes are to be synthesized. In recent years it has become evident that mitochondria are key component of the neuronal apoptotic route. In addition to mitochondria, other intracellular components have been implicated in this process. Thus, DNA damage and re-entry into the cell cycle may constitute a common pathway in apoptosis in neurological diseases. The implication of cell cycle in neurodegenerative disorders is supported by data on the brain of patients who showed an increase in cell cycle protein expression. Indeed, studies performed in neuronal cell preparations indicate that re-entry into the cell cycle and, more specifically, an increase in the expression of E2F-1 transcription role of DNA damage/repair as a potential mechanism in cell cycle re-entry. In this context, ataxia telangiectasia mutated protein could be the enzyme responsible for neuronal apoptosis activation. Furthermore, the potential routes involved in E2F-1 induced apoptosis, p53-dependent and p53-independent, are similarly reviewed. Under this hypothesis, multiple pathways have been suggested, including the route of caspases. Finally, given the increasing experimental data on the neuroprotective and antiapoptotic effects of cyclin dependent kinase CDK inhibitory drugs, including flavopiridol, their application for the treatment of neurological disorders is proposed.

Regulating p73 isoforms in human tumours

Although mutations in the TP73 gene are extremely rare in human tumours, altered expression is common. In some tumours, most notably leukaemias and lymphomas, expression of TP73 is reduced, suggesting a tumour suppressor role. In contrast, TP73 is over-expressed in many other tumour types, implying that it has oncogenic functions in human tumourigenesis. These conflicting scenarios can be reconciled by the observations that the TP73 gene produces p53-like isoforms (TAp73) and anti-p53 isoforms (DeltaTAp73). Thus, loss of TAp73 or over-expression of DeltaTAp73 should each promote oncogenic transformation, and the balance of expression of the opposing isoforms is the crucial factor. The mechanisms that regulate expression of TP73 isoforms are therefore of great interest. Recent data provide evidence for interacting roles of ZEB1, p300, and a polymorphic 73 bp deletion in intron 1 of the human TP73 gene in this process. Importantly, alterations to the proposed regulatory pathway for controlling TP73 isoform expression in colorectal cancer are associated with adverse clinico-pathological characteristics. Because p73 is also associated with tumour chemosensitivity, these new findings should provide prognostic information and have the potential to guide future therapeutic decisions.

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.

Ubiquitin and ubiquitin-like modifications of the p53 family

Regulation of p53 by the ubiquitin-proteasomal pathway has been studied considerably. Studies have also demonstrated that the ubiquitin-like proteins SUMO-1 and NEDD8 modify p53. Similarly, p63 and p73 are subject to regulation by ubiquitin and ubiquitin-like modifications, and perturbations of these pathways in the regulation of the p53 family have been implicated in tumorigenesis and developmental abnormalities. Here, we provide an overview of the current understanding of the regulation of the p53 family by covalent modification by ubiquitin, SUMO-1, and NEDD8.

Mdm2-mediated NEDD8 modification of TAp73 regulates its transactivation function

Mutations in p73 are rare in cancer. Emerging evidence suggests that the relative expression of various p73 isoforms may contribute to tumorigenesis. Alternative promoters and N-terminal splicing result in the transcription and processing of either full-length (TA) or N-terminally truncated (deltaN) p73 isoforms. TAp73 possesses pro-apoptotic functions, while deltaNp73 has anti-apoptotic properties via functional inhibition of TAp73 and p53. Here, we report that TAp73, but not deltaNp73, is covalently modified by NEDD8 under physiologic conditions in an Mdm2-dependent manner. Co-expression of NEDP1, a cysteine protease that specifically cleaves NEDD8 conjugates, was shown to deneddylate TAp73. In addition, blockage of the endogenous NEDD8 pathway increased TAp73-mediated transactivation of p53- and p73-responsive promoter-driven reporter activity, and in conjunction, neddylated TAp73 species were found preferentially in the cytoplasm. These results suggest that Mdm2 attenuates TAp73 transactivation function, at least in part, by promoting NEDD8-dependent TAp73 cytoplasmic localization and provide the first evidence of a covalent post-translational modification exclusively targeting the TA isoforms of p73.

Viral hepatitis and liver cancer: the case of hepatitis C

Chronic infection with the hepatitis C virus (HCV) is a major risk factor for the development of hepatocellular carcinoma (HCC) worldwide. The pathogenesis of HCC in HCV infection has extensively been analysed. Hepatitis C virus-induced chronic inflammation and the effects of cytokines in the development of fibrosis and liver cell proliferation are considered as one of the major pathogenic mechanisms. Increasing experimental evidence suggests that HCV contributes to HCC by directly modulating pathways that promote the malignant transformation of hepatocytes. Hepatitis C virus is an RNA virus that does not integrate into the host genome but HCV proteins interact with many host-cell factors well beyond their roles in the viral life cycle and are involved in a wide range of activities, including cell signaling, transcription, cell proliferation, apoptosis, membrane rearrangements, vesicular trafficking and translational regulation. At least four of the HCV gene products, namely HCV core, NS3, NS4B and NS5A, have been shown to exhibit transformation potential in tissue culture and several potentially oncogenic pathways have been shown to be altered by the expression of HCV proteins. Both HCV core and NS5A induce the accumulation of wild-type beta-catenin and the Wnt-beta-catenin pathway emerges as a common target for HCV (and HBV) in human HCCs, also independently from axin/beta-catenin gene mutations. Induction of both endoplasmic reticulum stress and oxidative stress by HCV proteins might also contribute to HCV transformation. Most of the putative transforming functions of the HCV proteins have been defined in artificial cellular systems, which may not be applicable to HCV infection in vivo, and still need to be established in relevant infection and disease models.

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.

Novel link between E2F1 and Smac/DIABLO: proapoptotic Smac/DIABLO is transcriptionally upregulated by E2F1

Deregulated expression of E2F1 not only promotes S-phase entry but also induces apoptosis. Although it has been well documented that E2F1 is able to induce p53-dependent apoptosis via raising ARF activity, the mechanism by which E2F induces p53-independent apoptosis remains unclear. Here we report that E2F1 can directly bind to and activate the promoter of Smac/DIABLO, a mitochondrial proapoptotic gene, through the E2F1-binding sites BS2 (−542 ∼ −535 bp) and BS3 (−200 ∼ −193 bp). BS2 and BS3 appear to be utilized in combination rather than singly by E2F1 in activation of Smac/DIABLO. Activation of BS2 and BS3 are E2F1-specific, since neither E2F2 nor E2F3 is able to activate BS2 or BS3. Using the H1299 ER-E2F1 cell line where E2F1 activity can be conditionally induced, E2F1 has been shown to upregulate the Smac/DIABLO expression at both mRNA and protein levels upon 4-hydroxytamoxifen treatment, resulting in an enhanced mitochondria-mediated apoptosis. Reversely, reducing the Smac/DIABLO expression by RNA interference significantly diminishes apoptosis induced by E2F1. These results may suggest a novel mechanism by which E2F1 promotes p53-independent apoptosis through directly regulating its downstream mitochondrial apoptosis-inducing factors, such as Smac/DIABLO.

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.

Regulation of the p73 protein stability and degradation

p73, a homologue to the tumor suppressor gene p53, is involved in tumorigenesis, though its specific role remains unclear. The gene has two distinct promoters which allow the formation of two protein isoforms with opposite effects: full-length transactivating (TA) p73 shows pro-apoptotic effects, while the shorter DeltaNp73, which lacks the N-terminal transactivating domain, has an evident anti-apoptotic function. Unlike p53, the p73 gene is rarely mutated in human cancers. However, alterations in the relative levels of TA and DeltaNp73 have been shown to correlate with prognosis in several human cancers, suggesting that the fine regulation of these two isoforms is of pivotal importance in controlling proliferation and cell death. Much effort is currently focused on the elucidation of the mechanisms that differentially control TA and DeltaNp73 activity and protein stability, a process complicated by the finding that both proteins are regulated by a similar suite of complex post-translational modifications that include ubiquitination, sequential phosphorylation, prolyl-isomerization, recruitment into the PML-nuclear body (PML-NB), and acetylation. Here we shall consider the main regulatory partners of p73, with particular attention to the recently discovered Itch- and Nedd8-mediated degradation pathways, along with the emerging roles of PML, p38 MAP kinase, Pin1, and p300 in p73 transcriptional activation, and possible mechanisms for the differential regulation of the TAp73 and DeltaNp73 isoforms.

The ubiquitin-protein ligase Itch regulates p73 stability

p73, a member of the p53 family of transcription factors, is upregulated in response to DNA damage, inducing cell cycle arrest and apoptosis. Besides indications that this p73 response is post-transcriptional, little is known about the underlying molecular mechanisms of p73 protein degradation. Ubiquitination and proteasomal-dependent degradation of p53 are regulated by its transcriptional target MDM2. However, unlike p53, p73 binds to, but is not degraded by, MDM2. Here we describe the binding of p73 to Itch, a Hect ubiquitin-protein ligase. Itch selectively binds and ubiquitinates p73 but not p53; this results in the rapid proteasome-dependent degradation of p73. Upon DNA damage Itch itself is downregulated, allowing p73 protein levels to rise and thus interfere with p73 function. In conclusion, we have identified a key mechanism in the control of p73 protein levels both in normal as well as in stress conditions.

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).

p73: in silico evidence for a putative third promoter region

The TP73 gene, first identified in 1997, encodes the p73 protein, a p53 tumour suppressor homologue. The lack of mutations in the TP73 gene in cancers and the developmentally abnormal phenotype of the TP73 knockout mouse suggest a different function for TP73 gene derived proteins. An alternative promoter in the third intron of the TP73 gene, which produces a transcription deficient and dominant negative protein (DeltaNp73), produces increased complexity in the function of p73 proteins. Functional studies of transcriptionally active (TAp73; regulated by the first promoter) and DeltaNp73 (regulated by the second promoter) show that the TP73 gene encodes both a candidate tumour suppressor (TAp73) and an oncogene (DeltaNp73), with pro-apoptotic and anti-apoptotic properties, respectively. This "two in one" gene architecture leads us to make an in silico search for other probable promoter regions and transcription start sites in different introns of the TP73 gene. To identify such regulatory regions, we have analysed the genomic structure of human and mouse TP73 genes. We have found introns 1 and 4 to be extremely large and relatively conserved in size in mouse and human, in addition to intron 3, which includes the DeltaNp73 promoter. We have further characterized these introns by transcription factor binding motifs, transcription initiation sites, open reading frames, and splicing using six programs that can successfully identify the already characterized promoters. Our results suggest the presence of a candidate 2 kbp genomic DNA in the first intron of human TP73 gene, harbouring 2 putative promoter regions, together with a similar region within intron 1 of the mouse gene.