Identification of direct p73 target genes combining DNA microarray and chromatin immunoprecipitation analyses

Chlorophyllides repress gain-of-function p53 mutated HNSCC cell proliferation via activation of p73 and repression of p53 aggregation in vitro and in vivo

Head and neck squamous cell carcinoma (HNSCC) cells have a high p53 mutation rate, but there were rare reported about the p53 gain of function through the prion-like aggregated form in p53 mutated HNSCC cells. Thioflavin T (ThT) is used to stain prion-like proteins in cells. Previously, we found that ThT and p53 staining were co-localized in HNSCC cells (Detroit 562 cells) with homozygous p53 R175H mutation. NAMPT inhibitor can repress ThT staining in Detroit 562 cells. In our previous study, co-treatment with p73 activator NSC59984 and NAMPT inhibitor FK886 synergistically repressed Detroit 562 cell proliferation. In this study, we found that two heterozygous p53-R280T mutation HNSCC cell lines, TW01 and HONE-1, also have the ThT staining signal. Treatment with chlorophyllides and p73 activator or NAMPT inhibitor did not synergistically repress cell proliferation in either Detroit 562 or HONE-1 cells. Chlorophyllides reduced the ThT aggregation signal in both Detroit 562 and HONE-1 cells. Chlorophyllides also induced p73 and caspase 3/7 expression and repressed NAMPT expression in both Detroit 562 and HONE-1 cells. Chlorophyllides reduced tumor size in vivo in Detroit 562 cells injected into a xenograft nude mice model, but this in vivo tumor repression effect was not found in p73 knockdown Detroit 562 cells. Moreover, NAMPT was repressed by chlorophyllides independent of p73 status in vivo. We thus concluded that chlorophyllides have a dual anticancer function when applied to HNSCC cells with p53 gain-of-function mutation, via activation of p73 and repression of p53 aggregation.

Progress in the Study of TAp73 and Sperm Apoptosis

The study of the mechanism of oligoasthenospermia, which is a major cause of male infertility, has been the focus of research in the field of male reproduction. TAp73, a member of the p53 family of oncogenes, is endowed with tumor-suppressing activity due to its structural and functional homology with p53. It has been found that TAp73, plays a key role in spermatogenesis and maintaining male reproduction. When TAp73 is low-expressed or absent, the process of spermatogenesis is severely impaired, and mice deficient in TAp73 exhibit spermatogonial DNA damage, disturbed apical cytoplasmic specialization, and spermatocyte malformations resulting in reduced male fertility. Nevertheless, when TAp73 is overexpressed, it not only drives exogenous death receptors to regulate germ cell apoptosis, but also interacts with its various substrate proteins to promote the translocation of cytoplasmic Bax proteins to the mitochondria, resulting in the upregulation of the Bax/Bcl-2 ratio on the mitochondrial membrane and triggering a series of mitochondrial apoptotic effects. In this article, we will analyze the mechanism of TAp73 and sperm apoptosis, and elaborate the mechanism of TAp73 upregulation, exogenous apoptosis pathway and mitochondrial apoptosis pathway to systematically explain that the process of apoptosis induced by high expression of TAp73 is not fixed and single, but is interconnected, so as to provide a basis for the treatment of oligoasthenospermia and the research and development of new drugs using TAp73 as a target.

Identification and clinicopathological analysis of potential p73-regulated biomarkers in colorectal cancer via integrative bioinformatics

This study aims to decipher crucial biomarkers regulated by p73 for the early detection of colorectal cancer (CRC) by employing a combination of integrative bioinformatics and expression profiling techniques. The transcriptome profile of HCT116 cell line p53- / - p73+ / + and p53- / - p73 knockdown was performed to identify differentially expressed genes (DEGs). This was corroborated with three CRC tissue expression datasets available in Gene Expression Omnibus. Further analysis involved KEGG and Gene ontology to elucidate the functional roles of DEGs. The protein-protein interaction (PPI) network was constructed using Cytoscape to identify hub genes. Kaplan-Meier (KM) plots along with GEPIA and UALCAN database analysis provided the insights into the prognostic and diagnostic significance of these hub genes. Machine/deep learning algorithms were employed to perform TNM-stage classification. Transcriptome profiling revealed 1289 upregulated and 1897 downregulated genes. When intersected with employed CRC datasets, 284 DEGs were obtained. Comprehensive analysis using gene ontology and KEGG revealed enrichment of the DEGs in metabolic process, fatty acid biosynthesis, etc. The PPI network constructed using these 284 genes assisted in identifying 20 hub genes. Kaplan-Meier, GEPIA, and UALCAN analyses uncovered the clinicopathological relevance of these hub genes. Conclusively, the deep learning model achieved TNM-stage classification accuracy of 0.78 and 0.75 using 284 DEGs and 20 hub genes, respectively. The study represents a pioneer endeavor amalgamating transcriptomics, publicly available tissue datasets, and machine learning to unveil key CRC-associated genes. These genes are found relevant regarding the patients' prognosis and diagnosis. The unveiled biomarkers exhibit robustness in TNM-stage prediction, thereby laying the foundation for future clinical applications and therapeutic interventions in CRC management.

The E2F4/p130 Repressor Complex Cooperates with Oncogenic ΔNp73α To Inhibit Gene Expression in Human Papillomavirus 38 E6/E7-Transformed Keratinocytes and in Cancer Cells

Tumor suppressor p53 and its related proteins, p63 and p73, can be synthesized as multiple isoforms lacking part of the N- or C-terminal regions. Specifically, high expression of the ΔNp73α isoform is notoriously associated with various human malignancies characterized by poor prognosis. This isoform is also accumulated by oncogenic viruses, such as Epstein-Barr virus (EBV), as well as genus beta human papillomaviruses (HPV) that appear to be involved in carcinogenesis. To gain additional insight into ΔNp73α mechanisms, we have performed proteomics analyses using human keratinocytes transformed by the E6 and E7 proteins of the beta-HPV type 38 virus as an experimental model (38HK). We find that ΔNp73α associates with the E2F4/p130 repressor complex through a direct interaction with E2F4. This interaction is favored by the N-terminal truncation of p73 characteristic of ΔNp73 isoforms. Moreover, it is independent of the C-terminal splicing status, suggesting that it could represent a general feature of ΔNp73 isoforms (α, β, γ, δ, ε, ζ, θ, η, and η1). We show that the ΔNp73α-E2F4/p130 complex inhibits the expression of specific genes, including genes encoding for negative regulators of proliferation, both in 38HK and in HPV-negative cancer-derived cell lines. Such genes are not inhibited by E2F4/p130 in primary keratinocytes lacking ΔNp73α, indicating that the interaction with ΔNp73α rewires the E2F4 transcriptional program. In conclusion, we have identified and characterized a novel transcriptional regulatory complex with potential implications in oncogenesis. IMPORTANCE The TP53 gene is mutated in about 50% of human cancers. In contrast, the TP63 and TP73 genes are rarely mutated but rather expressed as ΔNp63 and ΔNp73 isoforms in a wide range of malignancies, where they act as p53 antagonists. Accumulation of ΔNp63 and ΔNp73, which is associated with chemoresistance, can result from infection by oncogenic viruses such as EBV or HPV. Our study focuses on the highly carcinogenic ΔNp73α isoform and uses a viral model of cellular transformation. We unveil a physical interaction between ΔNp73α and the E2F4/p130 complex involved in cell cycle control, which rewires the E2F4/p130 transcriptional program. Our work shows that ΔNp73 isoforms can establish interactions with proteins that do not bind to the TAp73α tumor suppressor. This situation is analogous to the gain-of-function interactions of p53 mutants supporting cellular proliferation.

Inhibition of multiple CDKs potentiates colon cancer chemotherapy via p73-mediated DR5 induction

Targeting cyclin-dependent kinases (CDKs) has recently emerged as a promising therapeutic approach against cancer. However, the anticancer mechanisms of different CDK inhibitors (CDKIs) are not well understood. Our recent study revealed that selective CDK4/6 inhibitors sensitize colorectal cancer (CRC) cells to therapy-induced apoptosis by inducing Death Receptor 5 (DR5) via the p53 family member p73. In this study, we investigated if this pathway is involved in anticancer effects of different CDKIs. We found that less-selective CDKIs, including flavopiridol, roscovitine, dinaciclib, and SNS-032, induced DR5 via p73-mediated transcriptional activation. The induction of DR5 by these CDKIs was mediated by dephosphorylation of p73 at Threonine 86 and p73 nuclear translocation. Knockdown of a common target of these CDKIs, including CDK1, 2, or 9, recapitulated p73-mediated DR5 induction. CDKIs strongly synergized with 5-fluorouracil (5-FU), the most commonly used CRC chemotherapy agent, in vitro and in vivo to promote growth suppression and apoptosis, which required DR5 and p73. Together, these findings indicate p73-mediated DR5 induction as a potential tumor suppressive mechanism and a critical target engaged by different CDKIs in potentiating therapy-induced apoptosis in CRC cells. These findings help better understand the anticancer mechanisms of CDKIs and may help facilitate their clinical development and applications in CRC.

Targeting mutant p53-R248W reactivates WT p53 function and alters the onco-metabolic profile

TP53 is the most commonly mutated gene in cancer, and gain-of-function mutations have wide-ranging effects. Efforts to reactivate wild-type p53 function and inhibit mutant functions have been complicated by the variety of TP53 mutations. Identified from a screen, the NSC59984 compound has been shown to restore activity to mutant p53 in colorectal cancer cells. Here, we investigated its effects on esophageal adenocarcinoma cells with specific p53 hot-spot mutations. NSC59984 treatment of cells reactivated p53 transcriptional regulation, inducing mitochondrial intrinsic apoptosis. Analysis of its effects on cellular metabolism demonstrated increased utilization of the pentose phosphate pathway and inhibition of glycolysis at the fructose-1,6-bisphosphate to fructose 6-phosphate junction. Furthermore, treatment of cells with NSC59984 increased reactive oxygen species production and decreased glutathione levels; these effects were enhanced by the addition of buthionine sulfoximine and inhibited by N-acetyl cysteine. We found that the effects of NSC59984 were substantially greater in cells harboring the p53 R248W mutation. Overall, these findings demonstrate p53-dependent effects of NSC59984 on cellular metabolism, with increased activity in cells harboring the p53 R248W mutation. This research highlights the importance of defining the mutational status of a particular cancer to create a patient-centric strategy for the treatment of p53-driven cancers.

Structural diversity of p63 and p73 isoforms

Abstract

The p53 protein family is the most studied protein family of all. Sequence analysis and structure determination have revealed a high similarity of crucial domains between p53, p63 and p73. Functional studies, however, have shown a wide variety of different tasks in tumor suppression, quality control and development. Here we review the structure and organization of the individual domains of p63 and p73, the interaction of these domains in the context of full-length proteins and discuss the evolutionary origin of this protein family.

Facts

Open questions

CDK4/6 Inhibition Suppresses p73 Phosphorylation and Activates DR5 to Potentiate Chemotherapy and Immune Checkpoint Blockade

Targeting cyclin-dependent kinases 4 and 6 (CDK4/6) is a successful therapeutic approach against breast and other solid tumors. Inhibition of CDK4/6 halts cell cycle progression and promotes antitumor immunity. However, the mechanisms underlying the antitumor activity of CDK4/6 inhibitors are not fully understood. We found that CDK4/6 bind and phosphorylate the p53 family member p73 at threonine 86, which sequesters p73 in the cytoplasm. Inhibition of CDK4/6 led to dephosphorylation and nuclear translocation of p73, which transcriptionally activated death receptor 5 (DR5), a cytokine receptor and key component of the extrinsic apoptotic pathway. p73-mediated induction of DR5 by CDK4/6 inhibitors promoted immunogenic cell death of cancer cells. Deletion of DR5 in cancer cells in vitro and in vivo abrogated the potentiating effects of CDK4/6 inhibitors on immune cytokine TRAIL, 5-fluorouracil chemotherapy, and anti-PD-1 immunotherapy. Together, these results reveal a previously unrecognized consequence of CDK4/6 inhibition, which may be critical for potentiating the killing and immunogenic effects on cancer cells.

SIGNIFICANCE

This work demonstrates how inhibition of CDK4/6 sensitizes cancer cells to chemotherapy and immune checkpoint blockade and may provide a new molecular marker for improving CDK4/6-targeted cancer therapies. See related commentary by Frank, p. 1170.

NAMPT Inhibitor and P73 Activator Represses P53 R175H Mutated HNSCC Cell Proliferation in a Synergistic Manner

The p53 family has the following three members: p53, p63 and p73. p53 is a tumor suppressor gene that frequently exhibits mutation in head and neck cancer. Most p53 mutants are loss-of-function (LoF) mutants, but some acquire some oncogenic function, such as gain of function (GoF). It is known that the aggregation of mutant p53 can induce p53 GoF. The p73 activators RETRA and NSC59984 have an anti-cancer effect in p53 mutation cells, but we found that p73 activators were not effective in all head and neck squamous cell carcinoma (HNSCC) cell lines, with different p53 mutants. A comparison of the gene expression profiles of several regulator(s) in mutant HNSCC cells with or without aggregation of p53 revealed that nicotinamide phosphoribosyltransferase (NAMPT) is a key regulator of mutant p53 aggregation. An NAMPT inhibitor, to reduce abnormal aggregation of mutant p53, used in combination with a p73 activator, was able to effectively repress growth in HNSCC cells with p53 GoF mutants. This study, therefore, suggests a potential combination therapy approach for HNSCC with a p53 GoF mutation.

The p53 family member p73 in the regulation of cell stress response

During oncogenesis, cells become unrestrictedly proliferative thereby altering the tissue homeostasis and resulting in subsequent hyperplasia. This process is paralleled by resumption of cell cycle, aberrant DNA repair and blunting the apoptotic program in response to DNA damage. In most human cancers these processes are associated with malfunctioning of tumor suppressor p53. Intriguingly, in some cases two other members of the p53 family of proteins, transcription factors p63 and p73, can compensate for loss of p53. Although both p63 and p73 can bind the same DNA sequences as p53 and their transcriptionally active isoforms are able to regulate the expression of p53-dependent genes, the strongest overlap with p53 functions was detected for p73. Surprisingly, unlike p53, the p73 is rarely lost or mutated in cancers. On the contrary, its inactive isoforms are often overexpressed in cancer. In this review, we discuss several lines of evidence that cancer cells develop various mechanisms to repress p73-mediated cell death. Moreover, p73 isoforms may promote cancer growth by enhancing an anti-oxidative response, the Warburg effect and by repressing senescence. Thus, we speculate that the role of p73 in tumorigenesis can be ambivalent and hence, requires new therapeutic strategies that would specifically repress the oncogenic functions of p73, while keeping its tumor suppressive properties intact.

Tumor suppressor p53: Biology, signaling pathways, and therapeutic targeting

TP53 is the most commonly mutated gene in human cancer with over 100,000 literature citations in PubMed. This is a heavily studied pathway in cancer biology and oncology with a history that dates back to 1979 when p53 was discovered. The p53 pathway is a complex cellular stress response network with multiple diverse inputs and downstream outputs relevant to its role as a tumor suppressor pathway. While inroads have been made in understanding the biology and signaling in the p53 pathway, the p53 family, transcriptional readouts, and effects of an array of mutants, the pathway remains challenging in the realm of clinical translation. While the role of mutant p53 as a prognostic factor is recognized, the therapeutic modulation of its wild-type or mutant activities remain a work-in-progress. This review covers current knowledge about the biology, signaling mechanisms in the p53 pathway and summarizes advances in therapeutic development.

Deciphering the Nature of Trp73 Isoforms in Mouse Embryonic Stem Cell Models: Generation of Isoform-Specific Deficient Cell Lines Using the CRISPR/Cas9 Gene Editing System

Simple Summary

The Trp73 gene is involved in the regulation of multiple biological processes such as response to stress, differentiation and tissue architecture. This gene gives rise to structurally different N and C-terminal isoforms which lead to differences in its biological activity in a cell type dependent manner. However, there is a current lack of physiological models to study these isoforms. The aim of this study was to generate specific p73-isoform-deficient mouse embryonic stem cell lines using the CRISPR/Cas9 system. Their special features, self-renewal and pluripotency, make embryonic stem cells a useful research tool that allows the generation of cells from any of the three germ layers carrying specific inactivation of p73-isoforms. Characterization of the generated cell lines indicates that while the individual elimination of TA- or DN-p73 isoform is compatible with pluripotency, it results in alterations of the transcriptional profiles and the pluripotent state of the embryonic stem cells in an isoform-specific manner.

Abstract

The p53 family has been widely studied for its role in various physiological and pathological processes. Imbalance of p53 family proteins may contribute to developmental abnormalities and pathologies in humans. This family exerts its functions through a profusion of isoforms that are generated by different promoter usage and alternative splicing in a cell type dependent manner. In particular, the Trp73 gene gives rise to TA and DN-p73 isoforms that confer p73 a dual nature. The biological relevance of p73 does not only rely on its tumor suppression effects, but on its pivotal role in several developmental processes. Therefore, the generation of cellular models that allow the study of the individual isoforms in a physiological context is of great biomedical relevance. We generated specific TA and DN-p73-deficient mouse embryonic stem cell lines using the CRISPR/Cas9 gene editing system and validated them as physiological bona fide p73-isoform knockout models. Global gene expression analysis revealed isoform-specific alterations of distinctive transcriptional networks. Elimination of TA or DN-p73 is compatible with pluripotency but prompts naïve pluripotent stem cell transition into the primed state, compromising adequate lineage differentiation, thus suggesting that differential expression of p73 isoforms acts as a rheostat during early cell fate determination.

TAp73β Can Promote Hepatocellular Carcinoma Dedifferentiation

Simple Summary

Hepatocellular carcinoma (HCC) is a highly complex and heterogeneous type of cancer. Hepatocyte dedifferentiation is one of the important steps in the development of HCC. However, its molecular mechanisms are not well known. In this study, we report that transcriptionally active TAp73 isoforms are overexpressed in HCC. We also show that TAp73β suppresses the expression of the hepatocyte markers including CYP3A4, AFP, ALB, HNF4α, while increasing the expression of several cholangiocyte markers in HCC cell lines. In conclusion, this report reveals a pro-oncogenic role for TAp73β in liver cancer.

Abstract

Hepatocyte dedifferentiation is a major source of hepatocellular carcinoma (HCC), but its mechanisms are unknown. We explored the p73 expression in HCC tumors and studied the effects of transcriptionally active p73β (TAp73β) in HCC cells. Expression profiles of p73 and patient clinical data were collected from the Genomic Data Commons (GDC) data portal and the TSVdb database, respectively. Global gene expression profiles were determined by pan-genomic 54K microarrays. The Gene Set Enrichment Analysis method was used to identify TAp73β-regulated gene sets. The effects of TAp73 isoforms were analyzed in monolayer cell culture, 3D-cell culture and xenograft models in zebrafish using western blot, flow cytometry, fluorescence imaging, real-time polymerase chain reaction (RT-PCR), immunohistochemistry and morphological examination. TAp73 isoforms were significantly upregulated in HCC, and high p73 expression correlated with poor patient survival. The induced expression of TAp73β caused landscape expression changes in genes involved in growth signaling, cell cycle, stress response, immunity, metabolism and development. Hep3B cells overexpressing TAp73β had lost hepatocyte lineage biomarkers including ALB, CYP3A4, AFP, HNF4α. In contrast, TAp73β upregulated genes promoting cholangiocyte lineage such as YAP, JAG1 and ZO-1, accompanied with an increase in metastatic ability. Our findings suggest that TAp73β may promote malignant dedifferentiation of HCC cells.

MCL1 binds and negatively regulates the transcriptional function of tumor suppressor p73

MCL1, an anti-apoptotic protein that controls chemosensitivity and cell fate through its regulation of intrinsic apoptosis, has been identified as a high-impact target in anti-cancer therapeutic development. With MCL1-specific inhibitors currently in clinical trials, it is imperative that we understand the roles that MCL1 plays in cells, especially when targeting the Bcl-2 homology 3 (BH3) pocket, the central region of MCL1 that mediates apoptotic regulation. Here, we establish that MCL1 has a direct role in controlling p73 transcriptional activity, which modulates target genes associated with DNA damage response, apoptosis, and cell cycle progression. This interaction is mediated through the reverse BH3 (rBH3) motif in the p73 tetramerization domain, which restricts p73 assembly on DNA. Here, we provide a novel mechanism for protein-level regulation of p73 transcriptional activity by MCL1, while also framing a foundation for studying MCL1 inhibitors in combination with platinum-based chemotherapeutics. More broadly, this work expands the role of Bcl-2 family signaling beyond cell fate regulation.

BCL11B suppresses tumor progression and stem cell traits in hepatocellular carcinoma by restoring p53 signaling activity

Accumulating evidence indicates that hepatocellular carcinoma (HCC) tumorigenesis, recurrence, metastasis, and therapeutic resistance are strongly associated with liver cancer stem cells (CSCs), a rare subpopulation of highly tumorigenic cells with self-renewal capacity and differentiation potential. Previous studies identified B cell leukemia/lymphoma-11b (BCL11B) as a novel tumor suppressor with impressive capacity to restrain CSC traits. However, the implications of BCL11B in HCC remain unclear. In this study, we found that low BCL11B expression was an independent indicator for shorter overall survival (OS) and time to recurrence (TTR) for HCC patients with surgical resection. In vitro and in vivo experiments confirmed BCL11B as a tumor suppressor in HCC with inhibitory effects on proliferation, cell cycle progression, apoptosis, and mobility. Furthermore, BCL11B could suppress CSC traits, as evidenced by dramatically decreased tumor spheroid formation, self-renewal potential and drug resistance. A Cignal Finder Array and dual-luciferase activity reporter assays revealed that BCL11B could activate the transcription of P73 via an E2F1-dependent manner. Thus, we concluded that BCL11B is a strong suppressor of retaining CSC traits in HCC. Ectopic expression of BCL11B might be a promising strategy for anti-HCC treatment with the potential to cure HBV-related HCC regardless of P53 mutation status.

p73 - NAV3 axis plays a critical role in suppression of colon cancer metastasis

p73 is a member of the p53 tumor suppressor family, which transactivates p53-responsive genes and mediates DNA damage response. Recent evidences suggest that p73 exerts its tumor suppressor functions by suppressing metastasis, but the exact mechanism remains unknown. Here, we identify Navigator-3 (NAV3), a microtubule-binding protein, as a novel transcriptional target of p73, which gets upregulated by DNA damage in a p73-dependent manner and plays a vital role in p73-mediated inhibition of cancer cell invasion, migration, and metastasis. Induction of p73 in response to DNA damage leads to rapid increase in endogenous NAV3 mRNA and protein levels. Through bioinformatic analysis, we identified two p73-binding sites in NAV3 promoter. Consistent with this, p73 binding to NAV3 promoter was confirmed through luciferase, Chromatin Immunoprecipitation, and site-directed mutagenesis assays. Abrogation of NAV3 and p73 expression significantly increased the invasion and migration rate of colorectal cancer cells as confirmed by wound-healing, cell invasion, and cell migration assays. Also, knockdown of NAV3 decreased the expression of E-cadherin and increased the expression of other prominent mesenchymal markers such as N-cadherin, Snail, Vimentin, and Fibronectin. Immunohistochemistry analysis revealed the downregulation of both NAV3 and p73 expression in metastatic colon cancer tissues as compared to non-metastatic cancer tissues. Additionally, the expression pattern of NAV3 and p73 showed extensively significant correlation in both non-metastatic and metastatic human colon cancer tissue samples. Taken together, our study provide conclusive evidence that Navigator-3 is a direct transcriptional target of p73 and plays crucial role in response to genotoxic stress in p73-mediated inhibition of cancer cell invasion, migration, and metastasis.

Mechanistic understanding of β-cryptoxanthin and lycopene in cancer prevention in animal models

To better understand the potential function of carotenoids in the chemoprevention of cancers, mechanistic understanding of carotenoid action on genetic and epigenetic signaling pathways is critically needed for human studies. The use of appropriate animal models is the most justifiable approach to resolve mechanistic issues regarding protective effects of carotenoids at specific organs and tissue sites. While the initial impetus for studying the benefits of carotenoids in cancer prevention was their antioxidant capacity and pro-vitamin A activity, significant advances have been made in the understanding of the action of carotenoids with regards to other mechanisms. This review will focus on two common carotenoids, provitamin A carotenoid β-cryptoxanthin and non-provitamin A carotenoid lycopene, as promising chemopreventive agents or chemotherapeutic compounds against cancer development and progression. We reviewed animal studies demonstrating that β-cryptoxanthin and lycopene effectively prevent the development or progression of various cancers and the potential mechanisms involved. We highlight recent research that the biological functions of β-cryptoxanthin and lycopene are mediated, partially via their oxidative metabolites, through their effects on key molecular targeting events, such as NF-κB signaling pathway, RAR/PPARs signaling, SIRT1 signaling pathway, and p53 tumor suppressor pathways. The molecular targets by β-cryptoxanthin and lycopene, offer new opportunities to further our understanding of common and distinct mechanisms that involve carotenoids in cancer prevention. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

The Roles of P53 and Its Family Proteins, P63 and P73, in the DNA Damage Stress Response in Organogenesis-Stage Mouse Embryos

Members of the P53 transcription factor family, P53, P63, and P73, play important roles in normal development and in regulating the expression of genes that control apoptosis and cell cycle progression in response to genotoxic stress. P53 is involved in the DNA damage response pathway that is activated by hydroxyurea in organogenesis-stage murine embryos. The extent to which P63 and P73 contribute to this stress response is not known. To address this question, we examined the roles of P53, P63, and P73 in mediating the response of Trp53-positive and Trp53-deficient murine embryos to a single dose of hydroxyurea (400 mg/kg) on gestational day 9. Hydroxyurea treatment downregulated the expression of Trp63 and upregulated Trp73 in the absence of effects on the levels of Trp53 transcripts; Trp73 upregulation was P53-dependent. At the protein level, hydroxyurea treatment increased the levels and phosphorylation of P53 in the absence of effects on P63 and P73. Upregulation of the expression of genes that regulate cell cycle arrest and apoptosis, Cdkn1a, Rb1, Fas, Trp53inp1, and Pmaip1, was P53-dependent in hydroxyurea-treated embryos. The increase in cleaved caspase-3 and cleaved mammalian sterile-20-like-1 kinase levels induced by hydroxyurea was also P53-dependent; in contrast, the increase in phosphorylated H2AX, a marker of DNA double-strand breaks, in response to hydroxyurea treatment was only partially P53-dependent. Together, our data show that P53 is the principal P53 family member that is activated in the embryonic DNA damage response.

Topoisomerase IIβ-binding protein 1 activates expression of E2F1 and p73 in HPV-positive cells for genome amplification upon epithelial differentiation

High-risk human papillomaviruses (HPVs) constitutively activate the ataxia telangiectasia mutated (ATM) and the ataxia telangiectasia and Rad3-related (ATR) DNA damage repair pathways for viral genome amplification. HPVs activate these pathways through the immune regulator STAT-5. For the ATR pathway, STAT-5 increases expression of the topoisomerase IIβ-binding protein 1 (TopBP1), a scaffold protein that binds ATR and recruits it to sites of DNA damage. TopBP1 also acts as a transcriptional regulator and we investigated how this activity influenced the HPV life cycle. We determined that TopBP1 levels are increased in cervical intraepithelial neoplasias as well as cervical carcinomas, consistent with studies in HPV-positive cell lines. Suppression of TopBP1 by shRNAs impairs HPV genome amplification and activation of the ATR pathway but does not affect the total levels of ATR and CHK1. In contrast, knockdown reduces the expression of other DNA damage factors such as RAD51 and Mre11 but not BRCA2 or NBS1. Interestingly, TopBP1 positively regulates the expression of E2F1, a TopBP1 binding partner, and p73, in HPV positive cells in contrast to effects in other cell types. TopBP1 transcriptional activity is regulated by AKT and treatment with AKT inhibitors suppresses expression of E2F1 and p73 without interfering with ATR signaling. Importantly, the levels of p73 are elevated in HPV-positive cells and knockdown impairs HPV genome amplification. This demonstrates that p73, like p63 and p53, is an important regulator of the HPV life cycle that is controlled by the transcriptional activating properties of the multifunctional TopBP1 protein.

miRNA-193a-5p repression of p73 controls Cisplatin chemoresistance in primary bone tumors

Osteosarcoma and Ewing Sarcoma are the two most common types of Bone Sarcomas, principally localized at the long bones of the extremities and mainly affecting adolescents and young adults. Cisplatin is one of the current options in the therapeutic arsenal of drugs available to cure these aggressive cancers. Unfortunately, chemoresistance against this agent is still a major cause of patient relapse. Thus, a better understanding of the molecular pathways by which these drugs induce cancer cell death, together with a better delineation of the origins of chemoresistance are required to improve the success rate of current treatments. Furthermore, as p53 is frequently mutated in Bone Sarcomas, other pathways in these cancers must mediate drug-induced cell death. Here, we demonstrate for the first time that TAp73β, a p53-family protein, is implicated in Cisplatin-induced apoptosis of Bone Sarcomas'. Furthermore, while acquired resistance developed by cancer cells against such drugs can have multiple origins, it is now well accepted that epigenetic mechanisms involving microRNAs (miRNAs) are one of them. We show that miRNA-193a-5p modulates the viability, the clonogenic capacity and the Cisplatin-induced apoptosis of the Bone Sarcoma cells through inhibition of TAp73β. Collectively, these results shed light on the involvement of miR-193a-5p in Cisplatin chemoresistance of Bone Sarcomas', and they open the road to new therapeutic opportunities provided by targeting the miR-193a-5p/TAp73β axis in the context of these malignancies.

RASSF1A uncouples Wnt from Hippo signalling and promotes YAP mediated differentiation via p73

Transition from pluripotency to differentiation is a pivotal yet poorly understood developmental step. Here, we show that the tumour suppressor RASSF1A is a key player driving the early specification of cell fate. RASSF1A acts as a natural barrier to stem cell self-renewal and iPS cell generation, by switching YAP from an integral component in the β-catenin-TCF pluripotency network to a key factor that promotes differentiation. We demonstrate that epigenetic regulation of the Rassf1A promoter maintains stemness by allowing a quaternary association of YAP-TEAD and β-catenin-TCF3 complexes on the Oct4 distal enhancer. However, during differentiation, promoter demethylation allows GATA1-mediated RASSF1A expression which prevents YAP from contributing to the TEAD/β-catenin-TCF3 complex. Simultaneously, we find that RASSF1A promotes a YAP-p73 transcriptional programme that enables differentiation. Together, our findings demonstrate that RASSF1A mediates transcription factor selection of YAP in stem cells, thereby acting as a functional "switch" between pluripotency and initiation of differentiation.

Comparative transcriptomes of adenocarcinomas and squamous cell carcinomas reveal molecular similarities that span classical anatomic boundaries

Advances in genomics in recent years have provided key insights into defining cancer subtypes “within-a-tissue”—that is, respecting traditional anatomically driven divisions of medicine. However, there remains a dearth of data regarding molecular profiles that are shared across tissues, an understanding of which could lead to the development of highly versatile, broadly applicable therapies. Using data acquired from The Cancer Genome Atlas (TCGA), we performed a transcriptomics-centered analysis on 1494 patient samples, comparing the two major histological subtypes of solid tumors (adenocarcinomas and squamous cell carcinomas) across organs, with a focus on tissues in which both subtypes arise: esophagus, lung, and uterine cervix. Via principal component and hierarchical clustering analysis, we discovered that histology-driven differences accounted for a greater degree of inherent molecular variation in the tumors than did tissue of origin. We then analyzed differential gene expression, DNA methylation, and non-coding RNA expression between adenocarcinomas and squamous cell carcinomas and found 1733 genes, 346 CpG sites, and 42 microRNAs in common between organ sites, indicating specific adenocarcinoma-associated and squamous cell carcinoma-associated molecular patterns that were conserved across tissues. We then identified specific pathways that may be critical to the development of adenocarcinomas and squamous cell carcinomas, including Liver X receptor activation, which was upregulated in adenocarcinomas but downregulated in squamous cell carcinomas, possibly indicating important differences in cancer cell metabolism between these two histological subtypes of cancer. In addition, we highlighted genes that may be common drivers of adenocarcinomas specifically, such as IGF2BP1, which suggests a possible link between embryonic development and tumor subtype. Altogether, we demonstrate the need to consider biological similarities that transcend anatomical boundaries to inform the development of novel therapeutic strategies. All data sets from our analysis are available as a resource for further investigation.

In clinical practice, the organ in which a cancer arises typically classifies it. However, developments in our understanding of cancer have revealed that this method overlooks key aspects of cancer biology relevant to both disease prevention and treatment. In fact, work characterizing the genetic make-up of cancers arising in a single organ has revealed that a shared organ of origin does not necessarily imply biological similarity (i.e. not all lung cancers share similar biological and molecular properties). While this approach, known as “within-a-tissue subtyping,” identifies key differences between cancers that arise in a single organ, a broader perspective may highlight important biological similarities between cancers across organs. Here we utilize this second approach, or “across-tissue subtyping,” to gain insight into similarities between cancers (of different organs) that share the same histology—or appear similarly under a microscope. Using publicly available data from The Cancer Genome Atlas (TCGA), we compare gene expression of two major classes of solid tumors—adenocarcinomas (which arise from cells that form glands) and squamous cell carcinomas (which arise from flattened cells that form physical barriers). We identify several genes and biological pathways that may be common to adenocarcinomas and serve as targets for highly versatile therapies.

p53 pathway determines the cellular response to alcohol-induced DNA damage in MCF-7 breast cancer cells

Alcohol consumption is associated with increased breast cancer risk; however, the underlying mechanisms that contribute to mammary tumor initiation and progression are unclear. Alcohol is known to induce oxidative stress and DNA damage; likewise, p53 is a critical modulator of the DNA repair pathway and ensures genomic integrity. p53 mutations are frequently detected in breast and other tumors. The impact of alcohol on p53 is recognized, yet the role of p53 in alcohol-induced mammary carcinogenesis remains poorly defined. In our study, we measured alcohol-mediated oxidative DNA damage in MCF-7 cells using 8-OHdG and p-H2AX foci formation assays. p53 activity and target gene expression after alcohol exposure were determined using p53 luciferase reporter assay, qPCR, and Western blotting. A mechanistic study delineating the role of p53 in DNA damage response and cell cycle arrest was based on isogenic MCF-7 cells stably transfected with control (MCF-7/Con) or p53-targeting siRNA (MCF-7/sip53), and MCF-7 cells that were pretreated with Nutlin-3 (Mdm2 inhibitor) to stabilize p53. Alcohol treatment resulted in significant DNA damage in MCF-7 cells, as indicated by increased levels of 8-OHdG and p-H2AX foci number. A p53-dependent signaling cascade was stimulated by alcohol-induced DNA damage. Moderate to high concentrations of alcohol (0.1-0.8% v/v) induced p53 activation, as indicated by increased p53 phosphorylation, reporter gene activity, and p21/Bax gene expression, which led to G0/G1 cell cycle arrest. Importantly, compared to MCF-7/Con cells, alcohol-induced DNA damage was significantly enhanced, while alcohol-induced p21/Bax expression and cell cycle arrest were attenuated in MCF-7/sip53 cells. In contrast, inhibition of p53 degradation via Nutlin-3 reinforced G0/G1 cell cycle arrest in MCF-7 control cells. Our study suggests that functional p53 plays a critical role in cellular responses to alcohol-induced DNA damage, which protects the cells from DNA damage associated with breast cancer risk.

Aggregation and Prion-Like Properties of Misfolded Tumor Suppressors: Is Cancer a Prion Disease?

Prion diseases are disorders that share several characteristics that are typical of many neurodegenerative diseases. Recently, several studies have extended the prion concept to pathological aggregation in malignant tumors involving misfolded p53, a tumor-suppressor protein. The aggregation of p53 and its coaggregation with p53 family members, p63 and p73, have been shown. Certain p53 mutants exert a dominant-negative regulatory effect on wild-type (WT) p53. The basis for this dominant-negative effect is that amyloid-like mutant p53 converts WT p53 into an aggregated species, leading to a gain-of-function (GoF) phenotype and the loss of its tumor-suppressor function. Recently, it was shown that p53 aggregates can be internalized by cells and can coaggregate with endogenous p53, corroborating the prion-like properties of p53 aggregates. The prion-like behavior of oncogenic p53 mutants provides an explanation for its dominant-negative and GoF properties, including the high metastatic potential of cancer cells carrying p53 mutations. The inhibition of p53 aggregation appears to represent a promising target for therapeutic intervention in patients with malignant tumors.

Integrated analysis of microRNAs, transcription factors and target genes expression discloses a specific molecular architecture of hyperdiploid multiple myeloma

Multiple Myeloma (MM) is a malignancy characterized by the hyperdiploid (HD-MM) and the non-hyperdiploid (nHD-MM) subtypes. To shed light within the molecular architecture of these subtypes, we used a novel integromics approach. By annotated MM patient mRNA/microRNA (miRNA) datasets, we investigated mRNAs and miRNAs profiles with relation to changes in transcriptional regulators expression. We found that HD-MM displays specific gene and miRNA expression profiles, involving the Signal Transducer and Activator of Transcription (STAT)3 pathway as well as the Transforming Growth Factor–beta (TGFβ) and the transcription regulator Nuclear Protein-1 (NUPR1). Our data define specific molecular features of HD-MM that may translate in the identification of novel relevant druggable targets.

Oncogenic Intra-p53 Family Member Interactions in Human Cancers

The p53 gene family members p53, p73, and p63 display several isoforms derived from the presence of internal promoters and alternative splicing events. They are structural homologs but hold peculiar functional properties. p53, p73, and p63 are tumor suppressor genes that promote differentiation, senescence, and apoptosis. p53, unlike p73 and p63, is frequently mutated in cancer often displaying oncogenic “gain of function” activities correlated with the induction of proliferation, invasion, chemoresistance, and genomic instability in cancer cells. These oncogenic functions are promoted either by the aberrant transcriptional cooperation of mutant p53 (mutp53) with transcription cofactors (e.g., NF-Y, E2F1, Vitamin D Receptor, Ets-1, NF-kB and YAP) or by the interaction with the p53 family members, p73 and p63, determining their functional inactivation. The instauration of these aberrant transcriptional networks leads to increased cell growth, low activation of DNA damage response pathways (DNA damage response and DNA double-strand breaks response), enhanced invasion, and high chemoresistance to different conventional chemotherapeutic treatments. Several studies have clearly shown that different cancers harboring mutant p53 proteins exhibit a poor prognosis when compared to those carrying wild-type p53 (wt-p53) protein. The interference of mutantp53/p73 and/or mutantp53/p63 interactions, thereby restoring p53, p73, and p63 tumor suppression functions, could be among the potential therapeutic strategies for the treatment of mutant p53 human cancers.

Small-Molecule Prodigiosin Restores p53 Tumor Suppressor Activity in Chemoresistant Colorectal Cancer Stem Cells via c-Jun-Mediated ΔNp73 Inhibition and p73 Activation

Tumor suppressor p53 is frequently mutated or inactivated in colorectal cancer. In contrast, p53 family member p73 is rarely mutated in colorectal cancer and p73 activation elicits p53-like tumor suppression. Colorectal cancer stem cells (CRCSC) comprise a rare self-renewing subpopulation that contributes to tumor maintenance and chemoresistance. p53 restoration is known to target CRCSCs, but p73 restoration in CRCSCs has not been examined. In this study, we investigated the effects of the small-molecule prodigiosin, which restores the p53 pathway in tumor cells via p73 activation, on CRCSCs in vitro and in vivo Prodigiosin prevented colonosphere formation independent of p53 status and reduced the viability of self-renewing, 5-fluorouracil-resistant Aldefluor positive [Aldefluor(+)] CRCSCs in vitro Furthermore, prodigiosin inhibited the growth of xenograft tumors initiated with Aldefluor+ cells without toxic effects and limited the tumorigenic potential of these cells. Consistently, prodigiosin induced activation of a p53-responsive luciferase reporter in colonospheres, Aldefluor(+) cells, and tumor xenografts. Mechanistic studies revealed that prodigiosin increased the levels of p73 and reduced levels of the oncogenic N-terminally truncated isoform ΔNp73 in Aldefluor(+) cells. Accordingly, p73 knockdown or ΔNp73 overexpression suppressed prodigiosin-mediated inhibition of colonosphere formation. Moreover, prodigiosin increased levels of the transcription factor c-Jun, a regulator of p73 and ΔNp73, in both the cytoplasm and nucleus. c-Jun knockdown attenuated prodigiosin-mediated p53-reporter activation, ΔNp73 downregulation, p73 activation, and cell death. Collectively, our findings highlight the previously uncharacterized use of p73-activating therapeutics to target CRCSCs. Cancer Res; 76(7); 1989-99. ©2016 AACR.

p53 Activity Dominates That of p73 upon Mdm4 Loss in Development and Tumorigenesis

Mdm4 negatively regulates the p53 tumor suppressor. Mdm4 loss in mice leads to an embryonic lethal phenotype that is p53-dependent. Biochemical studies indicate that Mdm4 also binds p73, a member of the p53 family, with higher affinity than p53. In this study, the significance of the Mdm4 and p73 interaction in vivo during embryogenesis and tumorigenesis was examined. The data revealed that p73 loss did not rescue either the early Mdm4-deficient embryonic lethality or the runted phenotype of Mdm4(Δ2/Δ2) p53(+/-) embryos. Furthermore, studies in the developing central nervous system wherein both genes have prominent roles indicated that loss of p73 also did not rescue the Mdm4-null brain phenotype as did p53 loss. This p53 dependency occurred despite evidence for p73-specific transcriptional activity. In tumor studies, the combination of Mdm4 overexpression and p73 loss did not alter survival of mice or the tumor spectrum as compared with Mdm4 overexpression alone. In summary, these data demonstrate that the Mdm4-p73 axis cannot override the dominant role of p53 in development and tumorigenesis.

IMPLICATIONS

Genetic characterization of the Mdm4 and p73 interaction during development and tumorigenesis suggests new insight into the role of p53 family members, which may influence treatment options for patients.

Genetic analysis of genes causing hypertension and stroke in spontaneously hypertensive rats: Gene expression profiles in the kidneys

Spontaneously hypertensive rats (SHRs) and stroke-prone SHRs (SHRSP) are frequently used as models not only of essential hypertension and stroke, but also of attention-deficit hyperactivity disorder (ADHD). Normotensive Wistar-Kyoto (WKY) rats are normally used as controls in these studies. In the present study, we aimed to identify the genes causing hypertension and stroke, as well as the genes involved in ADHD using these rats. We previously analyzed gene expression profiles in the adrenal glands and brain. Since the kidneys can directly influence the functions of the cardiovascular, endocrine and sympathetic nervous systems, gene expression profiles in the kidneys of the 3 rat strains were examined using genome-wide microarray technology when the rats were 3 and 6 weeks old, a period in which rats are considered to be in a pre-hypertensive state. Gene expression profiles were compared between the SHRs and WKY rats and also between the SHRSP and SHRs. A total of 232 unique genes showing more than a 4-fold increase or less than a 4-fold decrease in expression were isolated as SHR- and SHRSP-specific genes. Candidate genes were then selected using two different web tools: the 1st tool was the Database for Annotation, Visualization and Integrated Discovery (DAVID), which was used to search for significantly enriched genes and categorized them using Gene Ontology (GO) terms, and the 2nd was Ingenuity Pathway Analysis (IPA), which was used to search for interactions among SHR- and also SHRSP‑specific genes. The analyses of SHR-specific genes using IPA revealed that B-cell CLL/lymphoma 6 (Bcl6) and SRY (sex determining region Y)-box 2 (Sox2) were possible candidate genes responsible for causing hypertension in SHRs. Similar analyses of SHRSP-specific genes revealed that angiotensinogen (Agt), angiotensin II receptor-associated protein (Agtrap) and apolipoprotein H (Apoh) were possible candidate genes responsible for triggering strokes. Since our results revealed that SHRSP-specific genes isolated from the kidneys of rats at 6 weeks of age, included 6 genes related to Huntington's disease, we discussed the genetic association between ADHD and Huntington's disease.

A pro-apoptotic function of iASPP by stabilizing p300 and CBP through inhibition of BRMS1 E3 ubiquitin ligase activity

The p53 family and its cofactors are potent inducers of apoptosis and form a barrier to cancer. Here, we investigated the impact of the supposedly inhibitory member of the apoptosis-stimulating protein of p53, iASPP, on the activity of the p53 homolog TAp73, and its cofactors p300 and CBP. We found that iASPP interacted with and stabilized the histone acetyltransferase p300 and its homolog CBP upon cisplatin treatment. Vice versa, iASPP depletion by shRNA resulted in decreased amounts of p300 and CBP, impaired binding of p300 and TAp73 to target site promoters, reduced induction of pro-apoptotic TAp73 target genes, and impaired apoptosis. Mechanistically, we observed that the p300-regulatory E3 ubiquitin ligase BRMS1 could rescue the degradation of p300 and CBP in cisplatin-treated, iASPP-depleted cells. This argues that iASPP stabilizes p300 and CBP by interfering with their BRMS1-mediated ubiquitination, thereby contributing to apoptotic susceptibility. In line, iASPP overexpression partially abolished the interaction of BRMS1 and CBP upon DNA damage. Reduced levels of iASPP mRNA and protein as well as CBP protein were observed in human melanoma compared with normal skin tissue and benign melanocytic nevi. In line with our findings, iASPP overexpression or knockdown of BRMS1 each augmented p300/CBP levels in melanoma cell lines, thereby enhancing apoptosis upon DNA damage. Taken together, destabilization of p300/CBP by downregulation of iASPP expression levels appears to represent a molecular mechanism that contributes to chemoresistance in melanoma cells.

TP63 and TP73 in cancer, an unresolved "family" puzzle of complexity, redundancy and hierarchy

TP53 belongs to a small gene family that includes, in mammals, two additional paralogs, TP63 and TP73. The p63 and p73 proteins are structurally and functionally similar to p53 and their activity as transcription factors is regulated by a wide repertoire of shared and unique post-translational modifications and interactions with regulatory cofactors. p63 and p73 have important functions in embryonic development and differentiation but are also involved in tumor suppression. The biology of p63 and p73 is complex since both TP63 and TP73 genes are transcribed into a variety of different isoforms that give rise to proteins with antagonistic properties, the TA-isoforms that act as tumor-suppressors and DN-isoforms that behave as proto-oncogenes. The p53 family as a whole behaves as a signaling "network" that integrates developmental, metabolic and stress signals to control cell metabolism, differentiation, longevity, proliferation and death. Despite the progress of our knowledge, the unresolved puzzle of complexity, redundancy and hierarchy in the p53 family continues to represent a formidable challenge.

ShaPINg Cell Fate Upon DNA Damage: Role of Pin1 Isomerase in DNA Damage-Induced Cell Death and Repair

The peptidyl-prolyl cis/trans isomerase Pin1 acts as a molecular timer in proline-directed Ser/Thr kinase signaling and shapes cellular responses based on recognition of phosphorylation marks and implementing conformational changes in its substrates. Accordingly, Pin1 has been linked to numerous phosphorylation-controlled signaling pathways and cellular processes such as cell cycle progression, proliferation, and differentiation. In addition, Pin1 plays a pivotal role in DNA damage-triggered cell fate decisions. Whereas moderate DNA damage is balanced by DNA repair, cells confronted with massive genotoxic stress are eliminated by the induction of programed cell death or cellular senescence. In this review, we summarize and discuss the current knowledge on how Pin1 specifies cell fate through regulating key players of the apoptotic and the repair branch of the DNA-damage response.

DEC1 coordinates with HDAC8 to differentially regulate TAp73 and ΔNp73 expression

P73, a member of the p53 family, plays a critical role in neural development and tumorigenesis. Due to the usage of two different promoters, p73 is expressed as two major isoforms, TAp73 and ΔNp73, often with opposing functions. Here, we reported that transcriptional factor DEC1, a target of the p53 family, exerts a distinct control of TAp73 and ΔNp73 expression. In particular, we showed that DEC1 was able to increase TAp73 expression via transcriptional activation of the TAp73 promoter. By contrast, Np73 transcription was inhibited by DEC1 via transcriptional repression of the ΔNp73 promoter. To further explore the underlying mechanism, we showed that DEC1 was unable to increase TAp73 expression in the absence of HDAC8, suggesting that HDAC8 is required for DEC1 to enhance TAp73 expression. Furthermore, we found that DEC1 was able to interact with HDAC8 and recruit HDAC8 to the TAp73, but not the ΔNp73, promoter. Together, our data provide evidence that DEC1 and HDAC8 in differentially regulate TAp73 and ΔNp73 expression, suggesting that this regulation may lay a foundation for a therapeutic strategy to enhance the chemosensitivity of tumor cells.

TAp73 protein stability is controlled by histone deacetylase 1 via regulation of Hsp90 chaperone function

Histone deacetylases (HDACs) play important roles in fundamental cellular processes, and HDAC inhibitors are emerging as promising cancer therapeutics. p73, a member of the p53 family, plays a critical role in tumor suppression and neural development. Interestingly, p73 produces two classes of proteins with opposing functions: the full-length TAp73 and the N-terminally truncated ΔNp73. In the current study, we sought to characterize the potential regulation of p73 by HDACs and found that histone deacetylase 1 (HDAC1) is a key regulator of TAp73 protein stability. Specifically, we showed that HDAC1 inhibition by HDAC inhibitors or by siRNA shortened the half-life of TAp73 protein and subsequently decreased TAp73 expression under normal and DNA damage-induced conditions. Mechanistically, we found that HDAC1 knockdown resulted in hyperacetylation and inactivation of heat shock protein 90, which disrupted the interaction between heat shock protein 90 and TAp73 and thus promoted the proteasomal degradation of TAp73. Functionally, we found that down-regulation of TAp73 was required for the enhanced cell migration mediated by HDAC1 knockdown. Together, we uncover a novel regulation of TAp73 protein stability by HDAC1-heat shock protein 90 chaperone complex, and our data suggest that TAp73 is a critical downstream mediator of HDAC1-regulated cell migration.

Differentiation-induced skin cancer suppression by FOS, p53, and TACE/ADAM17

Squamous cell carcinomas (SCCs) are heterogeneous and aggressive skin tumors for which innovative, targeted therapies are needed. Here, we identify a p53/TACE pathway that is negatively regulated by FOS and show that the FOS/p53/TACE axis suppresses SCC by inducing differentiation. We found that epidermal Fos deletion in mouse tumor models or pharmacological FOS/AP-1 inhibition in human SCC cell lines induced p53 expression. Epidermal cell differentiation and skin tumor suppression were caused by a p53-dependent transcriptional activation of the metalloprotease TACE/ADAM17 (TNF-α-converting enzyme), a previously unknown p53 target gene that was required for NOTCH1 activation. Although half of cutaneous human SCCs display p53-inactivating mutations, restoring p53/TACE activity in mouse and human skin SCCs induced tumor cell differentiation independently of the p53 status. We propose FOS/AP-1 inhibition or p53/TACE reactivating strategies as differentiation-inducing therapies for SCCs.

Mammary epithelial cell polarity is regulated differentially by p73 isoforms via epithelial-to-mesenchymal transition

p73 is expressed as TA and ΔN isoforms, both of which are implicated in tumor suppression and/or promotion. To address how p73 possesses these opposing functions, we developed three-dimensional culture of MCF10A cells, which undergo cell morphogenesis to form polarized spheroids with hollow lumen similar to normal mammary acini in vivo. Here, we showed that upon knockdown of p73, particularly TAp73 but not ΔNp73, MCF10A cells formed irregular and near-normal acini without hollow lumen in three-dimensional culture. We also found that upon knockdown of p73 or TAp73, but not ΔNp73, MCF10A cells underwent epithelial-to-mesenchymal transition (EMT) via down-regulation of E-cadherin coupled with up-regulation of β-catenin and laminin V. In addition, we found that Snail-1, Slug, and Twist, all of which are known to act as EMT inducers by repressing E-cadherin expression, were increased markedly upon knockdown of p73 and TAp73 but little if any by ΔNp73. Furthermore, we showed that knockdown of p73 or TAp73 in MCF10A cells led to a marked increase in cell proliferation and migration. Together, our data suggest that TAp73 is necessary for maintaining normal cell polarity by suppressing EMT.

Structure of p73 DNA-binding domain tetramer modulates p73 transactivation

The transcription factor p73 triggers developmental pathways and overlaps stress-induced p53 transcriptional pathways. How p53-family response elements determine and regulate transcriptional specificity remains an unsolved problem. In this work, we have determined the first crystal structures of p73 DNA-binding domain tetramer bound to response elements with spacers of different length. The structure and function of the adaptable tetramer are determined by the distance between two half-sites. The structures with zero and one base-pair spacers show compact p73 DNA-binding domain tetramers with large tetramerization interfaces; a two base-pair spacer results in DNA unwinding and a smaller tetramerization interface, whereas a four base-pair spacer hinders tetramerization. Functionally, p73 is more sensitive to spacer length than p53, with one base-pair spacer reducing 90% of transactivation activity and longer spacers reducing transactivation to basal levels. Our results establish the quaternary structure of the p73 DNA-binding domain required as a scaffold to promote transactivation.

The transcriptome of the adenovirus infected cell

Alternations of cellular gene expression following an adenovirus type 2 infection of human primary cells were studied by using superior sensitive cDNA sequencing. In total, 3791 cellular genes were identified as differentially expressed more than 2-fold. Genes involved in DNA replication, RNA transcription and cell cycle regulation were very abundant among the up-regulated genes. On the other hand, genes involved in various signaling pathways including TGF-β, Rho, G-protein, Map kinase, STAT and NF-κB stood out among the down-regulated genes. Binding sites for E2F, ATF/CREB and AP2 were prevalent in the up-regulated genes, whereas binding sites for SRF and NF-κB were dominant among the down-regulated genes. It is evident that the adenovirus has gained a control of the host cell cycle, growth, immune response and apoptosis at 24 h after infection. However, efforts from host cell to block the cell cycle progression and activate an antiviral response were also observed.

p73 as a pharmaceutical target for cancer therapy

About half of all human tumors contain an inactivating mutation of p53, while in the remaining tumors, the p53 pathway is frequently abrogated by alterations of other components of its signaling pathway. In humans, the p53 tumor suppressor is part of a small gene family that includes two other members, p73 and p63, structurally and functionally related to p53. Accumulating evidences indicate that all p53-family proteins function as molecular hubs of a highly interconnected signaling network that coordinates cell proliferation, differentiation and death in response to physiological inputs and oncogenic stress. Therefore, not only the p53-pathway but the entire “p53-family pathway” is a primary target for cancer drug development. In particular, the p53-related protein p73 has a crucial role in determining cellular responses to chemotherapy, and can vicariate p53 functions in triggering cell death after DNA damage in multiple experimental models. The biology and regulation of p73 is complex, since the TP73 gene incorporates both tumor-suppressive and proto-oncogenic functions. However, the p73 gene is rarely mutated in tumors, so appropriate pharmacological manipulation of the p73 pathway is a very promising approach for cancer therapy. Here we provide an overview of the principal mechanism of p73 regulation, and describe several examples of pharmacological tools that can induce p73 accumulation and function by acting on upstream p73 modulators or displacing inhibitory p73 interactors. A better understanding of how the p73 pathway works is mandatory to discover additional players intervening in this pathway and has important implications for the improvement of cancer treatment with the development of new molecules or with the reposition of currently available drugs.

p73 protein regulates DNA damage repair

Although the p53 tumor suppressor is relatively well characterized, much less is known about the functions of other members of the p53 family, p73 and p63. Here, we present evidence that in specific pathological conditions caused by exposure of normal cells to bile acids in acidic conditions, p73 protein plays the predominant role in the DNA damage response. These pathological conditions frequently occur during gastric reflux in the human esophagus and are associated with progression to esophageal adenocarcinoma. We found that despite strong DNA damage induced by bile acid exposure, only p73 (but not p53 and p63) is selectively activated in a c-Abl kinase-dependent manner. The activated p73 protein induces DNA damage repair. Using a human DNA repair PCR array, we identified multiple DNA repair genes affected by p73. Two glycosylases involved in base excision repair, SMUG1 and MUTYH, were characterized and found to be transcriptionally regulated by p73 in DNA damage conditions. Using a surgical procedure in mice, which recapitulates bile acid exposure, we found that p73 deficiency is associated with increased DNA damage. These findings were further investigated with organotypic and traditional cell cultures. Collectively our studies demonstrate that p73 plays an important role in the regulation of DNA damage repair.

Mdm2 inhibition induces apoptosis in p53 deficient human colon cancer cells by activating p73- and E2F1-mediated expression of PUMA and Siva-1

Camptothecin (CPT) and Nutlin-3 caused apoptosis by increasing p53 protein and its activation in intestinal epithelial cells (IEC-6). We studied the effectiveness of these inducers on apoptosis in human colon cancer cells (Caco2) lacking p53 expression. CPT failed to activate caspase-3 and cause apoptosis in these cells. The absence of p53 expression, higher basal Bcl-xL and lower Bax proteins prevented CPT-induced apoptosis. However, the Mdm2 antagonist Nutlin-3 induced apoptosis in a dose dependent manner by activating caspases-9 and -3. Nutlin-3 prevented the activation of AKT via PTEN-mediated inhibition of the PI3K pathway. Nutlin-3 increased the phosphorylation of retinoblastoma protein causing E2F1 release leading to induction of Siva-1. Nutlin-3-mediated degradation of Mdm2 caused the accumulation of p73, which induced the expression of p53 up-regulated modulator of apoptosis (PUMA). E2F1 and p73 knockdown decreased the expression of Siva and PUMA, respectively and abolished Nutlin-3-induced caspase-3 activation. Cycloheximide (CHX) inhibited Nutlin-3-induced Siva, Noxa, and PUMA expression and inhibited apoptosis in IEC-6 and Caco2 cells. These results indicate that translation of mRNAs induced by Nutlin-3 is critical for apoptosis. In summary, apoptosis in Caco2 cells lacking functional p53 occurred following the disruption of Mdm2 binding with p73 and Rb leading to the expression of pro-apoptotic proteins, PUMA, Noxa, and Siva-1.

P73 regulates cisplatin-induced apoptosis in ovarian cancer cells via a calcium/calpain-dependent mechanism

P73 is important in drug-induced apoptosis in some cancer cells, yet its role in the regulation of chemosensitivity in ovarian cancer (OVCA) is poorly understood. Furthermore, if and how the deregulation of p73-mediated apoptosis confers resistance to cisplatin (CDDP) treatment is unclear. Here we demonstrate that TAp73α over-expression enhanced CDDP-induced PARP cleavage and apoptosis in both chemosensitive (OV2008 and A2780s) and their resistant counterparts (C13* and A2780cp) and another chemoresistant OVCA cells (Hey); in contrast, the effect of ΔNp73α over-expression was variable. P73α downregulation attenuated CDDP-induced PUMA and NOXA upregulation and apoptosis in OV2008 cells. CDDP decreased p73α steady-state protein levels in OV2008, but not in C13*, although the mRNA expression was identical. CDDP-induced p73α downregulation was mediated by a calpain-dependent pathway. CDDP induced calpain activation and enhanced its cytoplasmic interaction and co-localization with p73α in OV2008, but not C13* cells. CDDP increased the intracellular calcium concentration ([Ca(2+)](i)) in OV2008 but not C13* whereas cyclopiazonic acid (CPA), a Ca(2+)-ATPase inhibitor, caused this response and calpain activation, p73α processing and apoptosis in both cell types. CDDP-induced [Ca(2+)](i) increase in OV2008 cells was not effected by the elimination of extracellular Ca(2+), but this was attenuated by the depletion of internal Ca(2+) store, indicating that mobilization of intracellular Ca(2+]) stores was potentially involved. These findings demonstrate that p73α and its regulation by the Ca(2+)-mediated calpain pathway are involved in CDDP-induced apoptosis in OVCA cells and that dysregulation of Ca(2+)/calpain/p73 signaling may in part be the pathophysiology of CDDP resistance. Understanding the cellular and molecular mechanisms of chemoresistance will direct the development of effective strategies for the treatment of chemoresistant OVCA.

Crosstalk between c-Jun and TAp73alpha/beta contributes to the apoptosis-survival balance

The p53-family member p73 plays a role in various cellular signaling pathways during development and growth control and it can have tumor suppressor properties. Several isoforms of p73 exist with considerable differences in their function. Whereas the functions of the N-terminal isoforms (TA and ΔNp73) and their opposing pro- and antiapoptotic roles have become evident, the functional differences of the distinct C-terminal splice forms of TAp73 have remained unclear. Here, we characterized the global genomic binding sites for TAp73α and TAp73β by chromatin immunoprecipitation sequencing as well as the transcriptional responses by performing RNA sequencing. We identified a specific p73 consensus binding motif and found a strong enrichment of AP1 motifs in close proximity to binding sites for TAp73α. These AP1 motif-containing target genes are selectively upregulated by TAp73α, while their mRNA expression is repressed upon TAp73β induction. We show that their expression is dependent on endogenous c-Jun and that recruitment of c-Jun to the respective AP1 sites was impaired upon TAp73β expression, in part due to downregulation of c-Jun. Several of these AP1-site containing TAp73α-induced genes impinge on apoptosis induction, suggesting an underlying molecular mechanism for the observed functional differences between TAp73α and TAp73β.

TAp73 induction by nitric oxide: regulation by checkpoint kinase 1 (CHK1) and protection against apoptosis

Nitric oxide (NO) is a potent activator of the p53 tumor suppressor protein, thereby inducing cell cycle arrest and apoptosis. However, little is known about the regulation of the two other p53-family members, p63 and p73, by nitrogen oxides. We report here an up-regulation of p73 by NO in p53-null K-562 leukemia cells. Chemical NO prodrugs or macrophage iNOS activity induced an accumulation of the TAp73α isoform in these cells, whereas macrophages from iNOS(-/-) mice did not. NO also up-regulated TAp73 mRNA expression, suggesting a transcriptional regulation. The checkpoint kinases Chk1 and Chk2 can regulate TAp73 induction after DNA damage. We show that these kinases were rapidly phosphorylated upon NO treatment. Genetic silencing or pharmacological inhibition of Chk1 impaired NO-mediated accumulation of TAp73α. Because NO is known to block DNA synthesis through ribonucleotide reductase inhibition, the up-regulation of TAp73α might be caused by DNA damage induced by an arrest of DNA replication forks. In support of this hypothesis, DNA replication inhibitors such as hydroxyurea and aphidicolin similarly enhanced TAp73α expression and Chk1 phosphorylation. Moreover, inhibition of Chk1 also prevented TAp73α accumulation in response to replication inhibitors. The knockdown of TAp73 with siRNA sensitized K-562 cells to apoptosis induced by a nitrosative (NO) or oxidative (H(2)O(2)) injury. Therefore, TAp73α has an unusual cytoprotective role in K-562 cells, contrasting with its pro-apoptotic functions in many other cell models. In conclusion, NO up-regulates several p53 family members displaying pro- and anti-apoptotic effects, suggesting a complex network of interactions and cross-regulations between NO production and p53-related proteins.

Unraveling the genetics of otitis media: from mouse to human and back again

Otitis media (OM) is among the most common illnesses of early childhood, characterised by the presence of inflammation in the middle ear cavity. Acute OM and chronic OM with effusion (COME) affect the majority of children by school age and have heritability estimates of 40-70%. However, the majority of genes underlying this susceptibility are, as yet, unidentified. One method of identifying genes and pathways that may contribute to OM susceptibility is to look at mouse mutants displaying a comparable phenotype. Single-gene mouse mutants with OM have identified a number of genes, namely, Eya4, Tlr4, p73, MyD88, Fas, E2f4, Plg, Fbxo11, and Evi1, as potential and biologically relevant candidates for human disease. Recent studies suggest that this "mouse-to-human" approach is likely to yield relevant data, with significant associations reported between polymorphisms at the FBXO11, TLR4, and PAI1 genes and disease in humans. An association between TP73 and chronic rhinosinusitis has also been reported. In addition, the biobanks of available mouse mutants provide a powerful resource for functional studies of loci identified by future genome-wide association studies of OM in humans. Mouse models of OM therefore are an important component of current approaches attempting to understand the complex genetic susceptibility to OM in humans, and which aim to facilitate the development of preventative and therapeutic interventions for this important and common disease.

NF-kappaB inhibits T-cell activation-induced, p73-dependent cell death by induction of MDM2

NF-κB is a key transcription factor involved in the regulation of T-cell activation and proliferation upon engagement of the T-cell receptor (TCR). T cells that lack the IκB kinase (IKKβ) are unable to activate NF-κB, and rapidly undergo apoptosis upon activation. NF-κB activation following T-cell receptor engagement induces the expression of Mdm2 through interaction with NF-κB sites in its P1 promoter, and enforced expression of Mdm2 protected T cells deficient for NF-κB activation from activation-induced cell death. In T cells with intact NF-κB signaling, ablation or pharmacologic inhibition of Mdm2 resulted in activation-induced apoptosis. Mdm2 coprecipitates with p73 in activated T cells, and apoptosis induced by inhibition of Mdm2 was p73-dependent. Further, Bim was identified as a p73 target gene required for cell death induced by Mdm2 inhibition, and a p73-responsive element in intron 1 of Bim was characterized. Our results demonstrate a pathway for survival of activated T cells through NF-κB-induced Mdm2, which blocks Bim-dependent apoptosis through binding and inhibition of p73.

Direct activation of forkhead box O3 by tumor suppressors p53 and p73 is disrupted during liver regeneration in mice

The p53 family of proteins regulates the expression of target genes that promote cell cycle arrest and apoptosis, which may be linked to cellular growth control as well as tumor suppression. Within the p53 family, p53 and the transactivating p73 isoform (TA-p73) have hepatic-specific functions in development and tumor suppression. Here, we determined TA-p73 interactions with chromatin in the adult mouse liver and found forkhead box O3 (Foxo3) to be one of 158 gene targets. Global profiling of hepatic gene expression in the regenerating liver versus the quiescent liver revealed specific, functional categories of genes regulated over the time of regeneration. Foxo3 is the most responsive gene among transcription factors with altered expression during regenerative cellular proliferation. p53 and TA-p73 bind a Foxo3 p53 response element (p53RE) and maintain active expression in the quiescent liver. During regeneration of the liver, the binding of p53 and TA-p73, the recruitment of acetyltransferase p300, and the active chromatin structure of Foxo3 are disrupted along with a loss of Foxo3 expression. In agreement with the loss of Foxo3 transcriptional activation, a decrease in histone activation marks (dimethylated histone H3 at lysine 4, acetylated histone H3 at lysine 14, and acetylated H4) at the Foxo3 p53RE was detected after partial hepatectomy in mice. These parameters of Foxo3 regulation are reestablished with the completion of liver growth and regeneration and support a temporary suspension of p53 and TA-p73 regulatory functions in normal cells during tissue regeneration. p53-dependent and TA-p73-dependent activation of Foxo3 was also observed in mouse embryonic fibroblasts and in mouse hepatoma cells overexpressing p53, TA-p73alpha, and TA-p73beta isoforms.

CONCLUSION

p53 and p73 directly bind and activate the expression of the Foxo3 gene in the adult mouse liver and murine cell lines. p53, TA-p73, and p300 binding and Foxo3 expression decrease during liver regeneration, and this suggests a critical growth control mechanism mediated by these transcription factors in vivo.