Mechanisms of transcriptional repression of cell-cycle G2/M promoters by p63

ΔNp63α facilitates proliferation and migration, and modulates the chromatin landscape in intrahepatic cholangiocarcinoma cells

p63 plays a crucial role in epithelia-originating tumours; however, its role in intrahepatic cholangiocarcinoma (iCCA) has not been completely explored. Our study revealed the oncogenic properties of p63 in iCCA and identified the major expressed isoform as ΔNp63α. We collected iCCA clinical data from The Cancer Genome Atlas database and analyzed p63 expression in iCCA tissue samples. We further established genetically modified iCCA cell lines in which p63 was overexpressed or knocked down to study the protein function/function of p63 in iCCA. We found that cells overexpressing p63, but not p63 knockdown counterparts, displayed increased proliferation, migration, and invasion. Transcriptome analysis showed that p63 altered the iCCA transcriptome, particularly by affecting cell adhesion-related genes. Moreover, chromatin accessibility decreased at p63 target sites when p63 binding was lost and increased when p63 binding was gained. The majority of the p63 bound sites were located in the distal intergenic regions and showed strong enhancer marks; however, active histone modifications around the Transcription Start Site changed as p63 expression changed. We also detected an interaction between p63 and the chromatin structural protein YY1. Taken together, our results suggest an oncogenic role for p63 in iCCA.

The dual role of p63 in cancer

The p53 family is made up of three transcription factors: p53, p63, and p73. These proteins are well-known regulators of cell function and play a crucial role in controlling various processes related to cancer progression, including cell division, proliferation, genomic stability, cell cycle arrest, senescence, and apoptosis. In response to extra- or intracellular stress or oncogenic stimulation, all members of the p53 family are mutated in structure or altered in expression levels to affect the signaling network, coordinating many other pivotal cellular processes. P63 exists as two main isoforms (TAp63 and ΔNp63) that have been contrastingly discovered; the TA and ΔN isoforms exhibit distinguished properties by promoting or inhibiting cancer progression. As such, p63 isoforms comprise a fully mysterious and challenging regulatory pathway. Recent studies have revealed the intricate role of p63 in regulating the DNA damage response (DDR) and its impact on diverse cellular processes. In this review, we will highlight the significance of how p63 isoforms respond to DNA damage and cancer stem cells, as well as the dual role of TAp63 and ΔNp63 in cancer.

C-terminal α Domain of p63 Binds to p300 to Coactivate β-Catenin

TP63 (p63), a member of the tumor suppressor TP53 (p53) gene family, is essential for ectodermal tissue development and suppresses malignant progression of carcinomas. The most abundant isoform, ΔNp63α (referred to as p63), lacks the N-terminal transactivation (TA) domain, and was originally characterized as a dominant-negative type suppressor against p53 family proteins. It also binds to TCF/LEF to inhibit β-catenin. Nevertheless, transcriptional activation by p63 has also been observed in varied systems. To understand the puzzling results, we analyzed the structure–function relationship of p63 in the control of β-catenin-dependent transcription. p63 acted as a suppressor of moderately induced β-catenin. However, when nuclear targeted S33Y β-catenin was applied to cause the maximum enhancer activation, p63 displayed a β-catenin-coactivating function. The DNA-binding domain of p63 and the target sequence facilitated it. Importantly, we newly found that, despite the absence of TA domain, p63 was associated with p300, a general adaptor protein and chromatin modifier causing transcriptional activation. C-terminal α domain of p63 was essential for p300-binding and for the coactivator function. These results were related to endogenous p63-p300 complex formation and Wnt/β-catenin-responsive gene regulation by p63 in squamous cell carcinoma lines. The novel p63-p300 interaction may be involved in positive regulation of gene expression in tissue development and carcinogenesis.

Human papillomavirus E7 induces p63 expression to modulate DNA damage response

Cervical cancer is the third most common malignancy diagnosed in women worldwide. The major aetiological factor underlying the malignant transformation of cervical cells is the persistent infection with high-risk human papillomaviruses (HR-HPV), with more than 99% of cases expressing viral sequences. Here, we report a previously unknown mechanism driven by high-risk human papillomavirus E7 protein to modulate response to DNA damage in cervical cancer cells. Our data shows that HR-HPV E7 oncoprotein induces the transcription of the p53-family member p63, which modulates DNA damage response pathways, to facilitate repair of DNA damage. Based on our findings, we proposed a model, where HR-HPV could interfere with the sensitivity of transformed cells to radiation therapy by modulating DNA damage repair efficiency. Importantly, we have shown for the first time a critical role for p63 in response to DNA damage in cervical cancer cells.

p63 exerts spatio-temporal control of palatal epithelial cell fate to prevent cleft palate

Cleft palate is a common congenital disorder that affects up to 1 in 2500 live births and results in considerable morbidity to affected individuals and their families. The aetiology of cleft palate is complex with both genetic and environmental factors implicated. Mutations in the transcription factor p63 are one of the major individual causes of cleft palate; however, the gene regulatory networks in which p63 functions remain only partially characterized. Our findings demonstrate that p63 functions as an essential regulatory molecule in the spatio-temporal control of palatal epithelial cell fate to ensure appropriate fusion of the palatal shelves. Initially, p63 induces periderm formation and controls its subsequent maintenance to prevent premature adhesion between adhesion-competent, intra-oral epithelia. Subsequently, TGFβ3-induced down-regulation of p63 in the medial edge epithelia of the palatal shelves is a pre-requisite for palatal fusion by facilitating periderm migration from, and reducing the proliferative potential of, the midline epithelial seam thereby preventing cleft palate.

Cleft palate is a serious congenital condition which affects approximately 1 in every 2500 births. Cleft palate occurs when the palatal shelves fail to grow, adhere or fuse during development. Mutations in the gene encoding the transcription factor p63 result in cleft palate in humans and mice. However, the role of p63 and how it controls the network of genes to regulate palate development is not well understood.In this study, we demonstrate that p63 controls the spatio-temporal regulation of palatal epithelial cell fate to ensure appropriate palatal adhesion: p63 induces the formation of a flattened layer of epithelial (periderm) cells and controls its subsequent maintenance. We also demonstrate that TGFβ3-induced, down-regulation of p63 in the medial edge epithelial cells, through which the palatal shelves adhere and fuse, controls Jag2-induced periderm migration to the oral and nasal epithelial triangles. In addition, p63 plays a central role in maintaining the proliferative potential of the basal layer of the medial edge epithelia. Our study provides significant new insights into the mechanisms that regulate development of the palate by establishing the role of p63 in governing the fate of the midline epithelial cells.

Molecular and cytoskeletal regulations in epidermal development

At the surface of the body, the epidermis covers great depth in its developmental regulation. While many genes have been shown to be important for skin development through their associations with disease phenotypes in mice and human, it is in the past decade that the intricate interplay between various molecules become gradually revealed through sophisticated genetic models and imaging analyses. In particular, there is increasing evidence suggesting that cytoskeleton-associated proteins, including adhesion proteins and the crosslinker proteins may play critical roles in regulating epidermis development. We here provide a broad overview of the various molecules involved in epidermal development with special emphasis on the cytoskeletal components.

Investigating the potential of Oxymatrine as a psoriasis therapy

Psoriasis vulgaris is a chronic inflammatory skin disease, stubbornly intractable, with substantial consequences for patient physical and mental welfare. Approaches currently available to treat psoriasis are not satisfactory due to undesirable side-effects or expense. Psoriasis is characterized by hyperproliferation and inflammation. Oxymatrine, an active component extracted from Sophora flavescens, has been demonstrated to possess anti-proliferation, anti-inflammatory, anti-tumorigenic, immune regulation and pro-apoptotic properties. This investigation presents a detailed retrospective review examining the effect of Oxymatrine on psoriasis and investigates the mechanisms underlying patient responses to Oxymatrine. We confirm that Oxymatrine administration significantly reduced the Psoriasis Area Severity Index score, with high efficacy compared to the control group. In addition, we have found that Oxymatrine significantly inhibits the viability, proliferation and differentiation of human keratinocyte in vitro. Immunohistochemical analysis indicates Oxymatrine significantly suppresses the expression of Pan-Cytokeratin, p63 and keratin 10. The results indicate that the suppression of p63 expression may lead to the anti-proliferation effect of Oxymatrine on human skin keratinocytes. Oxymatrine does not affect the formation of basement membrane, which is very important to maintain the normal function of human skin keratinocytes. In summary, Oxymatrine offers an effective, economical, and safe treatment for patients presenting with intractable psoriasis vulgaris.

Transcription factor p63 controls the reserve status but not the stemness of horizontal basal cells in the olfactory epithelium

Adult tissue stem cells can serve two broad functions: to participate actively in the maintenance and regeneration of a tissue or to wait in reserve and participate only when activated from a dormant state. The adult olfactory epithelium, a site for ongoing, life-long, robust neurogenesis, contains both of these functional stem cell types. Globose basal cells (GBCs) act as the active stem cell population and can give rise to all the differentiated cells found in the normal tissue. Horizontal basal cells (HBCs) act as reserve stem cells and remain dormant unless activated by tissue injury. Here we show that HBC activation following injury by the olfactotoxic gas methyl bromide is coincident with the down-regulation of protein 63 (p63) but anticipates HBC proliferation. Gain- and loss-of-function studies show that this down-regulation of p63 is necessary and sufficient for HBC activation. Moreover, activated HBCs give rise to GBCs that persist for months and continue to act as bona fide stem cells by participating in tissue maintenance and regeneration over the long term. Our analysis provides mechanistic insight into the dynamics between tissue stem cell subtypes and demonstrates that p63 regulates the reserve state but not the stem cell status of HBCs.

Cell type-dependent modulation of the gene encoding heat shock protein HSPA2 by hypoxia-inducible factor HIF-1: Down-regulation in keratinocytes and up-regulation in HeLa cells

HSPA2 belongs to the multigene HSPA family, whose members encode chaperone proteins. Although expression and function of HSPA2 is mainly associated with spermatogenesis, recent studies demonstrated that in humans, the gene is active in various cancers, as well as in normal tissues, albeit in a cell type-specific manner. In the epidermis, HSPA2 is expressed in keratinocytes in the basal layer. Currently, the mechanisms underlying the regulation of HSPA2 expression remain unknown. This study was aimed at determining whether HIF-1 and its binding site, the hypoxia-response element (HRE) located in the HSPA2 promoter, are involved in HSPA2 regulation. As a model system, we used an immortal human keratinocyte line (HaCaT) and cervical cancer cells (HeLa) grown under control or hypoxic conditions. Using an in vitro gene reporter assay, we demonstrated that in keratinocytes HSPA2 promoter activity is reduced under conditions that facilitate stabilization of HIF-1α, whereas HIF-1 inhibitors abrogated the suppressive effect of hypoxia on promoter activity. Chromatin immunoprecipitation revealed that HIF-1α binds to the HSPA2 promoter. In keratinocytes, hypoxia or overexpression of a stable form of HIF-1α attenuated the expression of endogenous HSPA2, whereas targeted repression of HIF-1α by RNAi increased transcription of HSPA2 under hypoxia. Conversely, in HeLa cells, HSPA2 expression increased under conditions that stimulated HIF-1α activity, whereas inhibition of HIF-1α abrogated hypoxia-induced up-regulation of HSPA2 expression. Taken together, our results demonstrate that HIF-1 can exert differential, cell context-dependent regulatory control of the HSPA2 gene. Additionally, we also showed that HSPA2 expression can be stimulated during hypoxia/reoxygenation stress.

Integrating animal models and in vitro tissue models to elucidate the role of desmosomal proteins in diseases

Desmosomes are intercellular junctions that provide tissues with structural stability. These junctions might also act as signaling centers that transmit environmental clues to the cell, thereby affecting cell differentiation, migration, and proliferation. The importance of desmosomes is underscored by devastating skin and heart diseases caused by mutations in desmosomal genes. Recent observations suggest that abnormal desmosomal protein expression might indirectly contribute to skin disorders previously not linked to these proteins. For example, it has been postulated that reduced desmosomal protein expression occurs in patients affected by Ankyloblepharon-ectodermal defects-cleft lip/palate syndrome (AEC), a skin fragility disorder caused by mutations in the transcription factor TP63. Currently, it is not clear how these changes in desmosomal gene expression contribute to AEC. We will discuss new approaches that combine in vitro and in vivo models to elucidate the role of desmosomal gene deregulation in human skin diseases such as AEC.

Modeling AEC-New approaches to study rare genetic disorders

Ankyloblepharon-ectodermal defects-cleft lip/palate (AEC) syndrome is a rare monogenetic disorder that is characterized by severe abnormalities in ectoderm-derived tissues, such as skin and its appendages. A major cause of morbidity among affected infants is severe and chronic skin erosions. Currently, supportive care is the only available treatment option for AEC patients. Mutations in TP63, a gene that encodes key regulators of epidermal development, are the genetic cause of AEC. However, it is currently not clear how mutations in TP63 lead to the various defects seen in the patients' skin. In this review, we will discuss current knowledge of the AEC disease mechanism obtained by studying patient tissue and genetically engineered mouse models designed to mimic aspects of the disorder. We will then focus on new approaches to model AEC, including the use of patient cells and stem cell technology to replicate the disease in a human tissue culture model. The latter approach will advance our understanding of the disease and will allow for the development of new in vitro systems to identify drugs for the treatment of skin erosions in AEC patients. Further, the use of stem cell technology, in particular induced pluripotent stem cells (iPSC), will enable researchers to develop new therapeutic approaches to treat the disease using the patient's own cells (autologous keratinocyte transplantation) after correction of the disease-causing mutations.

Mutations in AEC syndrome skin reveal a role for p63 in basement membrane adhesion, skin barrier integrity and hair follicle biology

BACKGROUND

AEC (ankyloblepharon-ectodermal defects-clefting) syndrome is an autosomal dominant ectodermal dysplasia disorder caused by mutations in the transcription factor p63. Clinically, the skin is dry and often fragile; other features can include partial eyelid fusion (ankyloblepharon), hypodontia, orofacial clefting, sparse hair or alopecia, and nail dystrophy.

OBJECTIVES

To investigate how p63 gene mutations affect gene and protein expression in AEC syndrome skin.

METHODS

We performed microarray analysis on samples of intact and eroded AEC syndrome skin compared with control skin. Changes were verified by quantitative real-time reverse transcription-polymerase chain reaction and, for basal keratinocyte-associated genes, by immunohistochemistry and analysis of microdissected skin.

RESULTS

We identified significant upregulation of six genes and downregulation of 69 genes in AEC syndrome skin, with the main changes in genes implicated in epidermal adhesion, skin barrier formation and hair follicle biology. There was reduced expression of genes encoding the basement membrane proteins FRAS1 and collagen VII, as well as the skin barrier-associated small proline-rich proteins 1A and 4, late cornified envelope protein 5A, hornerin, and lipid transporters including ALOX15B. Reduced expression of the hair-associated keratins 25, 27, 31, 33B, 34, 35, 81 and 85 was also noted. We also confirmed similar alterations in gene expression for 26 of the 75 genes in eroded AEC scalp skin.

CONCLUSIONS

This study identifies specific changes in skin structural biology and signalling pathways that result from mutant p63 and provides new molecular insight into the AEC syndrome phenotype.

p63 regulates human keratinocyte proliferation via MYC-regulated gene network and differentiation commitment through cell adhesion-related gene network

Although p63 and MYC are important in the control of epidermal homeostasis, the underlying molecular mechanisms governing keratinocyte proliferation or differentiation downstream of these two genes are not completely understood. By analyzing the transcriptional changes and phenotypic consequences of the loss of either p63 or MYC in human developmentally mature keratinocytes, we have characterized the networks acting downstream of these two genes to control epidermal homeostasis. We show that p63 is required to maintain growth and to commit to differentiation by two distinct mechanisms. Knockdown of p63 led to down-regulation of MYC via the Wnt/β-catenin and Notch signaling pathways and in turn reduced keratinocyte proliferation. We demonstrate that a p63-controlled keratinocyte cell fate network is essential to induce the onset of keratinocyte differentiation. This network contains several secreted proteins involved in cell migration/adhesion, including fibronectin 1 (FN1), interleukin-1β (IL1B), cysteine-rich protein 61 (CYR61), and jagged-1 (JAG1), that act downstream of p63 as key effectors to trigger differentiation. Our results characterized for the first time a connection between p63 and MYC and a cell adhesion-related network that controls differentiation. Furthermore, we show that the balance between the MYC-controlled cell cycle progression network and the p63-controlled cell adhesion-related network could dictate skin cell fate.

The NF-Y/p53 liaison: well beyond repression

NF-Y is a sequence-specific transcription factor - TF - targeting the common CCAAT promoter element. p53 is a master TF controlling the response to stress signals endangering genome integrity, often mutated in human cancers. The NF-Y/p53 - and p63, p73 - interaction results in transcriptional repression of a subset of genes within the vast NF-Y regulome under DNA-damage conditions. Recent data shows that NF-Y is also involved in pro-apoptotic activities, either directly, by mediating p53 transcriptional activation, or indirectly, by being targeted by a non coding RNA, PANDA. The picture is subverted in cells carrying Gain-of-function mutant p53, through interactions with TopBP1, a protein also involved in DNA repair and replication. In summary, the connection between p53 and NF-Y is crucial in determining cell survival or death.

Diverse p63 and p73 isoforms regulate Δ133p53 expression through modulation of the internal TP53 promoter activity

In response to stress, p53 binds and transactivates the internal TP53 promoter, thus regulating the expression of its own isoform, Δ133p53α. Here, we report that, in addition to p53, at least four p63/p73 isoforms regulate Δ133p53 expression at transcriptional level: p63β, ΔNp63α, ΔNp63β and ΔNp73γ. This regulation occurs through direct DNA-binding to the internal TP53 promoter as demonstrated by chromatin immunoprecipitation and the use of DNA-binding mutant p63. The promoter regions involved in the p63/p73-mediated transactivation were identified using deleted, mutant and polymorphic luciferase reporter constructs. In addition, we observed that transient expression of p53 family members modulates endogenous Δ133p53α expression at both mRNA and protein levels. We also report concomitant variation of p63 and Δ133p53 expression during keratinocyte differentiation of HaCat cells and induced pluripotent stem cells derived from mutated p63 ectodermal dysplasia patients. Finally, proliferation assays indicated that Δ133p53α isoform regulates the anti-proliferative activities of p63β, ΔNp63α, ΔNp63β and ΔNp73γ. Overall, this study shows a strong interplay between p53, p63 and p73 isoforms to orchestrate cell fate outcome.

NF-Y and the transcriptional activation of CCAAT promoters

The CCAAT box promoter element and NF-Y, the transcription factor (TF) that binds to it, were among the first cis-elements and trans-acting factors identified; their interplay is required for transcriptional activation of a sizeable number of eukaryotic genes. NF-Y consists of three evolutionarily conserved subunits: a dimer of NF-YB and NF-YC which closely resembles a histone, and the "innovative" NF-YA. In this review, we will provide an update on the functional and biological features that make NF-Y a fundamental link between chromatin and transcription. The last 25 years have witnessed a spectacular increase in our knowledge of how genes are regulated: from the identification of cis-acting sequences in promoters and enhancers, and the biochemical characterization of the corresponding TFs, to the merging of chromatin studies with the investigation of enzymatic machines that regulate epigenetic states. Originally identified and studied in yeast and mammals, NF-Y - also termed CBF and CP1 - is composed of three subunits, NF-YA, NF-YB and NF-YC. The complex recognizes the CCAAT pentanucleotide and specific flanking nucleotides with high specificity (Dorn et al., 1997; Hatamochi et al., 1988; Hooft van Huijsduijnen et al, 1987; Kim & Sheffery, 1990). A compelling set of bioinformatics studies clarified that the NF-Y preferred binding site is one of the most frequent promoter elements (Suzuki et al., 2001, 2004; Elkon et al., 2003; Mariño-Ramírez et al., 2004; FitzGerald et al., 2004; Linhart et al., 2005; Zhu et al., 2005; Lee et al., 2007; Abnizova et al., 2007; Grskovic et al., 2007; Halperin et al., 2009; Häkkinen et al., 2011). The same consensus, as determined by mutagenesis and SELEX studies (Bi et al., 1997), was also retrieved in ChIP-on-chip analysis (Testa et al., 2005; Ceribelli et al., 2006; Ceribelli et al., 2008; Reed et al., 2008). Additional structural features of the CCAAT box - position, orientation, presence of multiple Transcriptional Start Sites - were previously reviewed (Dolfini et al., 2009) and will not be considered in detail here.

Chromatin dynamics of gene activation and repression in response to interferon alpha (IFN(alpha)) reveal new roles for phosphorylated and unphosphorylated forms of the transcription factor STAT2

Signal transducer and activator of transcription 2 (STAT2), the critical component of type I interferons signaling, is a prototype latent cytoplasmic signal-dependent transcription factor. Activated tyrosine-phosphorylated STAT2 associates with STAT1 and IRF9 to bind the ISRE elements in the promoters of a subset of IFN-inducible genes (ISGs). In addition to activate hundreds of ISGs, IFNα also represses numerous target genes but the mechanistic basis for this dual effect and transcriptional repression is largely unknown. We investigated by ChIP-chip the binding dynamics of STAT2 and "active" phospho(P)-STAT2 on 113 putative IFNα direct target promoters before and after IFNα induction in Huh7 cells and primary human hepatocytes (PHH). STAT2 is already bound to 62% of our target promoters, including most "classical" ISGs, before IFNα treatment. 31% of STAT2 basally bound promoters also show P-STAT2 positivity. By correlating in vivo promoter occupancy with gene expression and changes in histone methylation marks we found that: 1) STAT2 plays a role in regulating ISGs expression, independently from its phosphorylation; 2) P-STAT2 is involved in ISGs repression; 3) "activated" ISGs are marked by H3K4me1 and H3K4me3 before IFNα; 4) "repressed" genes are marked by H3K27me3 and histone methylation plays a dominant role in driving IFNα-mediated ISGs repression.

Phospho-ΔNp63α/Rpn13-dependent regulation of LKB1 degradation modulates autophagy in cancer cells

Oxidative stress was shown to promote the translocation of Ataxia-telangiectasia mutated (ATM) to cytoplasm and trigger the LKB1-AMPK-tuberin pathway leading to a down-regulation of mTOR and subsequently inducing the programmed cell death II (autophagy). Cisplatin was previously found to induce the ATM-dependent phosphorylation of ΔNp63α in squamous cell carcinoma (SCC) cells. In this study, phosphorylated (p)-ΔNp63α was shown to bind the ATM promoter, to increase the ATM promoter activity and to enhance the ATM cytoplasmic accumulation. P-ΔNp63α protein was further shown to interact with the Rpn13 protein leading to a proteasome-dependent degradation of p-ΔNp63α and thereby protecting LKB1 from the degradation. In SCC cells (with an altered ability to support the ATM-dependent ΔNp63α phosphorylation), the non-phosphorylated ΔNp63α protein failed to form protein complexes with the Rpn13 protein and thereby allowing the latter to bind and target LKB1 into a proteasome-dependent degradation pathway thereby modulating a cisplatin-induced autophagy. We thus suggest that SCC cells sensitive to cisplatin-induced cell death are likely to display a greater ratio of p-ΔNp63α/non-phosphorylated ΔNp63α than cells with the innate resistant/impaired response to a cisplatin-induced cell death. Our data also suggest that the choice made by Rpn13 between p-ΔNp63α or LKB1 to be targeted for degradation is critical for cell death decision made by cancer cells in response to chemotherapy.

DeltaNp63 transcriptionally regulates ATM to control p53 Serine-15 phosphorylation

BACKGROUND

DeltaNp63alpha is an epithelial progenitor cell marker that maintains epidermal stem cell self-renewal capacity. Previous studies revealed that UV-damage induced p53 phosphorylation is confined to DeltaNp63alpha-positive cells in the basal layer of human epithelium.

RESULTS

We now report that phosphorylation of the p53 tumour suppressor is positively regulated by DeltaNp63alpha in immortalised human keratinocytes. DeltaNp63alpha depletion by RNAi reduces steady-state ATM mRNA and protein levels, and attenuates p53 Serine-15 phosphorylation. Conversely, ectopic expression of DeltaNp63alpha in p63-null tumour cells stimulates ATM transcription and p53 Serine-15 phosphorylation. We show that ATM is a direct DeltaNp63alpha transcriptional target and that the DeltaNp63alpha response element localizes to the ATM promoter CCAAT sequence. Structure-function analysis revealed that the DeltaNp63-specific TA2 transactivation domain mediates ATM transcription in coordination with the DNA binding and SAM domains.

CONCLUSIONS

Germline p63 point mutations are associated with a range of ectodermal developmental disorders, and targeted p63 deletion in the skin causes premature ageing. The DeltaNp63alpha-ATM-p53 damage-response pathway may therefore function in epithelial development, carcinogenesis and the ageing processes.

Epstein-Barr virus latent membrane protein-2A-induced DeltaNp63alpha expression is associated with impaired epithelial-cell differentiation

Epstein-Barr virus (EBV) is an oncogenic γ-herpes virus associated with malignancies that develop in both lymphoid and epithelial cells including nasopharyngeal carcinoma (NPC). The EBV protein latent membrane protein 2A (LMP2A) is expressed in NPC and can modulate epithelial proliferation, transformation, and differentiation, and as such, may promote malignancy. A key regulator of epithelial cell differentiation is the transcription factor p63, a member of the p53 family. This study examines the potential contribution of p63 to LMP2A-mediated inhibition of epithelial differentiation. Stable expression of LMP2A increased the protein level and stability of the ΔNp63α isoform, and in two epithelial cell lines, LMP2A interacted with ΔNp63α under stable and transient expression systems. LMP2A and ΔNp63α were localized to the cytoplasm and nuclear membrane and co-immunoprecipitated in the same fractions. Following induction of epithelial cell differentiation by calcium, expression of differentiation markers was impaired in both ΔNp63α and LMP2 expressing cells. Induction of p63α, association of p63α with LMP2A, and impairment of differentiation required the PY and ITAM signaling motif of LMP2A. By associating with and being regulated by LMP2A,ΔNp63α may function as a unique regulator of LMP2A effects on epithelial differentiation and contribute to EBV-associated epithelial cancers.

p63 in skin development and ectodermal dysplasias

The transcription factor p63 is critically important for skin development and maintenance. Processes that require p63 include epidermal lineage commitment, epidermal differentiation, cell adhesion, and basement membrane formation. Not surprisingly, alterations in the p63 pathway underlie a subset of ectodermal dysplasias, developmental syndromes in which the skin and skin appendages do not develop normally. This review summarizes the current understanding of the role of p63 in normal development and ectodermal dysplasias.

Embryonic stem cells as an ectodermal cellular model of human p63-related dysplasia syndromes

Heterozygous mutations in the TP63 transcription factor underlie the molecular basis of several similar autosomal dominant ectodermal dysplasia (ED) syndromes. Here we provide a novel cellular model derived from embryonic stem (ES) cells that recapitulates in vitro the main steps of embryonic skin development. We show that ES cells carrying AEC or EEC mutations are unable to differentiate into the epidermal fate. Comparative transcriptome analysis strongly reveals an embryonic epidermal signature and suggests that mutations in the SAM domain (AEC) provide activating properties while mutations in the DBD domain (EEC) induce strong inhibitory capabilities. Our model uncovers the effect of relevant ED mutations that otherwise are difficult to evaluate on the ectodermal embryonic stage, an embryonic event critical for proper skin formation.

DeltaNp63 knockdown mice: A mouse model for AEC syndrome

Dominant mutations in TP63 cause ankyloblepharon ectodermal dysplasia and clefting (AEC), an ectodermal dysplasia characterized by skin fragility. Since DeltaNp63alpha is the predominantly expressed TP63 isoform in postnatal skin, we hypothesized that mutant DeltaNp63alpha proteins are primarily responsible for skin fragility in AEC patients. We found that mutant DeltaNp63alpha proteins expressed in AEC patients function as dominant-negative molecules, suggesting that the human AEC skin phenotype could be mimicked in mouse skin by downregulating DeltaNp63alpha. Indeed, downregulating DeltaNp63 expression in mouse epidermis caused severe skin erosions, which resembled lesions that develop in AEC patients. In both cases, lesions were characterized by suprabasal epidermal proliferation, delayed terminal differentiation, and basement membrane abnormalities. By failing to provide structural stability to the epidermis, these defects likely contribute to the observed skin fragility. The development of a mouse model for AEC will allow us to further unravel the genetic pathways that are normally regulated by DeltaNp63 and that may be perturbed in AEC patients. Ultimately, these studies will not only contribute to our understanding of the molecular mechanisms that cause skin fragility in AEC patients, but may also result in the identification of targets for novel therapeutic approaches aimed at treating skin erosions. (c) 2009 Wiley-Liss, Inc.

p73 and p63 sustain cellular growth by transcriptional activation of cell cycle progression genes

Despite extensive studies on the role of tumor suppressor p53 protein and its homologues, p73 and p63, following their overexpression or cellular stress, very little is known about the regulation of the three proteins in cells during physiologic cell cycle progression. We report a role for p73 and p63 in supporting cellular proliferation through the transcriptional activation of the genes involved in G(1)-S and G(2)-M progression. We found that in MCF-7 cells, p73 and p63, but not p53, are modulated during the cell cycle with a peak in S phase, and their silencing determines a significant suppression of proliferation compared with the control. Chromatin immunoprecipitation analysis shows that in cycling cells, p73 and p63 are bound to the p53-responsive elements (RE) present in the regulatory region of cell cycle progression genes. On the contrary, when the cells are arrested in G(0)-G(1), p73 detaches from the REs and it is replaced by p53, which represses the expression of these genes. When the cells move in S phase, p73 is recruited again and p53 is displaced or is weakly bound to the REs. These data open new possibilities for understanding the involvement of p73 and p63 in cancer. The elevated concentrations of p73 and p63 found in many cancers could cause the aberrant activation of cell growth progression genes and therefore contribute to cancer initiation or progression under certain conditions.

The microRNA miR-92 increases proliferation of myeloid cells and by targeting p63 modulates the abundance of its isoforms

MicroRNAs (miRs) are 21- to 23-nucleotide RNA molecules that regulate the stability or translational efficiency of target messenger RNAs of proteins involved in cell growth and apoptosis. miR-92 is part of the mir-17-92 cluster, which comprises members with an effect on cell proliferation. However, the role of miR-92 is unknown, and its targets have not been identified. Here, we describe a mechanism through which miR-92 contributes to regulate cell proliferation. Using a miR-92 synthetic double-strand oligonucleotide, we demonstrate that miR-92 increases 32D myeloid cell proliferation and 5-bromo-2-deoxyuridine (BrdU) incorporation and inhibits cell death. The effect is miR-92 specific since the miR-92 antagomir inhibits cell proliferation. Moreover, we show that miR-92 acts by modulating p63-isoform abundance through down-regulatation of endogenous DeltaNp63beta. Using luciferase reporters containing p63 3'UTR fragments with wild-type or mutant miR-92 complementary sites, we demonstrate that the wild-type 3'UTR is a direct target of miR-92. Finally, we observed that a miR-92-resistant DeltaNp63beta isoform (without 3'UTR) inhibits cell proliferation and parallels the effect of the antagomir. We conclude that one of the molecular mechanisms through which miR-92 increases cell proliferation is by negative regulation of an isoform of the cell-cycle regulator p63.

Peroxisome proliferator-activated receptor-alpha is a functional target of p63 in adult human keratinocytes

p63 is a master switch in the complex network of signaling pathways controlling the establishment and maintenance of stratified epithelia. We provide evidence that peroxisome proliferator-activated receptor-alpha (PPARalpha), a ligand-activated nuclear receptor that participates in the skin wound healing process, is a target of p63 in human keratinocytes. Silencing of p63 by RNA interference and transient transfections showed that p63 represses PPARalpha through a functional region of promoter B. Chromatin immunoprecipitation analyses indicate that p63 is bound to this region, in the absence of a recognizable p63-binding motif, suggesting that it acts through interactions with other transcription factors (TFs). Distinct PPARalpha transcripts are differentially regulated by p63, indicating a bimodal action in promoter and/or transcription start specification. PPARalpha repression is consistent with lack of expression in the interfollicular epidermis under physiological conditions. Furthermore, we show that PPARalpha is a negative regulator of DeltaNp63alpha levels and that it also binds to a functional region of the DeltaNp63 promoter that lacks PPRE motifs. Therefore, the reciprocal regulation is exerted either through binding to non-consensus sites or through interactions with other DNA-bound TFs. In conclusion, our data establish a link between two TFs intimately involved in the maintenance of skin homeostatic conditions.

Identification of a novel transcriptional repressor (HEPIS) that interacts with nsp-10 of SARS coronavirus

A novel gene was previously isolated from a cDNA library of human embryo lung tissue by its encoded protein, which interacts with non-structural protein 10 (nsp-10) of the severe acute respiratory syndrome coronavirus (SARS-CoV). The protein was named human embryo lung cellular protein interacting with SARS-CoV nsp-10 (HEPIS), and it is composed of 147 amino acids with several CK II phosphorylation sites. In the present study, we demonstrated that HEPIS was capable of suppressing chloramphenicol acetyltransferase (CAT) gene expression controlled by different enhancerelements in a transcription assay. HEPIS interacted specifically with the HSP70 TATA sequence, and not with various other enhancer elements in a binding test. Furthermore, we co-immunoprecipitated HEPIS with BTF3, a component of the RNA pol II initiation complex, and observed reduced proliferation of HeLa cells transfected with the HEPIS gene. Taken together, our results suggest that HEPIS may function as a potential transcriptional repressor.

Claudin-1 is a p63 target gene with a crucial role in epithelial development

The epidermis of the skin is a self-renewing, stratified epithelium that functions as the interface between the human body and the outer environment, and acts as a barrier to water loss. Components of intercellular junctions, such as Claudins, are critical to maintain tissue integrity and water retention. p63 is a transcription factor essential for proliferation of stem cells and for stratification in epithelia, mutated in human hereditary syndromes characterized by ectodermal dysplasia. Both p63 and Claudin-1 null mice die within few hours from birth due to dehydration from severe skin abnormalities. These observations suggested the possibility that these two genes might be linked in one regulatory pathway with p63 possibly regulating Claudin-1 expression. Here we show that silencing of ΔNp63 in primary mouse keratinocytes results in a marked down-regulation of Claudin-1 expression (−80%). ΔNp63α binds in vivo to the Claudin-1 promoter and activates both the endogenous Claudin-1 gene and a reporter vector containing a –1.4 Kb promoter fragment of the Claudin-1 gene. Accordingly, Claudin-1 expression was absent in the skin of E15.5 p63 null mice and natural p63 mutant proteins, specifically those found in Ankyloblepharon–Ectodermal dysplasia–Clefting (AEC) patients, were indeed altered in their capacity to regulate Claudin-1 transcription. This correlates with deficient Claudin-1 expression in the epidermis of an AEC patient carrying the I537T p63 mutation. Notably, AEC patients display skin fragility similar to what observed in the epidermis of Claudin-1 and p63 null mice. These findings reinforce the hypothesis that these two genes might be linked in a common regulatory pathway and that Claudin-1 may is an important p63 target gene involved in the pathogenesis of ectodermal dysplasias.

Regulation of Dlx5 and Dlx6 gene expression by p63 is involved in EEC and SHFM congenital limb defects

The congenital malformation Split Hand-Foot Malformation (SHFM, or ectrodactyly) is characterized by a medial cleft of hands and feet, and missing central fingers. Five genetically distinct forms are known in humans; the most common (type-I) is linked to deletions of DSS1 and the distalless-related homeogenes DLX5 and DLX6. As Dlx5;Dlx6 double-knockout mice show a SHFM-like phenotype, the human orthologs are believed to be the disease genes. SHFM-IV and Ectrodactyly-Ectodermal dysplasia-Cleft lip (EEC) are caused by mutations in p63, an ectoderm-specific p53-related transcription factor. The similarity in the limb phenotype of different forms of SHFM may underlie the existence of a regulatory cascade involving the disease genes. Here, we show that p63 and Dlx proteins colocalize in the nuclei of the apical ectodermal ridge (AER). In homozygous p63- (null) and p63EEC (R279H) mutant limbs, the AER fails to stratify and the expression of four Dlx genes is strongly reduced; interestingly, the p63+/EEC and p63+/- hindlimbs, which develop normally and have a normally stratified AER, show reduced Dlx gene expression. The p63+/EEC mutation combined with an incomplete loss of Dlx5 and Dlx6 alleles leads to severe limb phenotypes, which are not observed in mice with either mutation alone. In vitro, DeltaNp63alpha induces transcription from the Dlx5 and Dlx6 promoters, an activity abolished by EEC and SHFM-IV mutations, but not by Ankyloblepharon-Ectodermal defects-Cleft lip/palate (AEC) mutations. ChIP analysis shows that p63 is directly associated with the Dlx5 and Dlx6 promoters. Thus, our data strongly implicate p63 and the Dlx5-Dlx6 locus in a pathway relevant in the aetio-pathogenesis of SHFM.

Discovery of novel tumor suppressor p53 response elements using information theory

An accurate method for locating genes under tumor suppressor p53 control that is based on a well-established mathematical theory and built using naturally occurring, experimentally proven p53 sites is essential in understanding the complete p53 network. We used a molecular information theory approach to create a flexible model for p53 binding. By searching around transcription start sites in human chromosomes 1 and 2, we predicted 16 novel p53 binding sites and experimentally demonstrated that 15 of the 16 (94%) sites were bound by p53. Some were also bound by the related proteins p63 and p73. Thirteen of the adjacent genes were controlled by at least one of the proteins. Eleven of the 16 sites (69%) had not been identified previously. This molecular information theory approach can be extended to any genetic system to predict new sites for DNA-binding proteins.

Characterization of DeltaNp63 isoforms in normal cornea and telomerase-immortalized human corneal epithelial cells

Previous reports have suggested that specific isoforms of the potential stem cell marker p63 may regulate corneal epithelial homeostatic renewal through control of cell proliferation. In this study, we characterized the presence of DeltaNp63 isoforms in telomerase-immortalized human corneal epithelial cells (hTCEpi) in comparison to normal human corneal epithelium to validate the hTCEpi cell line as a viable model for the study of p63 isoforms. We further examined roles for DeltaNp63 in proliferation and differentiation. For in vitro studies, hTCEpi cells were cultured in serum-free culture media and grown under 0.15 mM calcium or sequential 1.15 mM calcium/air-lifted culture. Fresh donor human corneal tissue was used to assess expression and localization in situ. mRNA and protein levels were assessed by real-time PCR, Immunofluorescence (IF) and Western blotting (WB). DeltaNp63 expression levels throughout the cell cycle were assessed by double-labeling with DeltaNp63 and Ki-67. In situ, DeltaNp63 localized to nuclei throughout the human corneal epithelium and was lost only in superficial cells. WB confirmed the presence of all three DeltaNp63 isoforms in the central corneal epithelium and in hTCEpi cells. DeltaNp63 mRNA levels decreased when grown on collagen substrate and under increased calcium/air-lifted culture. mRNA and protein levels increased as cells approached confluence, with a significant decrease in post-confluent culture. DeltaNp63 expression levels did not vary with the cell cycle, as assessed by Ki-67 labeling. Collectively, the presence of all three DeltaNp63 isoforms in hTCEpi cells and in intact cornea validates the use of this cell line for the study of individual isoforms in the corneal epithelium; and these data suggest that expression of DeltaNp63 isoforms are not altered as a function of the cell cycle or cell division in subconfluent hTCEpi cells cultured in serum-free media, but demonstrate reduced expression upon contact-inhibited growth down-regulation and differentiation. Significantly, the localization of DeltaNp63 in central corneal epithelial cells with a loss of expression in superficial cells suggests that DeltaNp63 may play a role in mediating desquamative events at the ocular surface.

A novel mutation of p63 in a Chinese family with inherited syndactyly and adactylism

p63 is a transcription factor homologous to p53 and p73; mutations in this gene have been identified in individuals with several types of developmental abnormalities, including EEC (ectrodactyly, ectodermal dysplasia, facial clefts) syndrome and split-hand/split-foot malformation (SHFM). Several mutations in the p63 gene have previously been shown to be related to SHFM. In this study, we report on a Chinese family with intrafamilial clinical variability of SHFM that have a novel heterozygous mutation in all four affected individuals. The mutation is in exon 8 of p63, 1046G --> A, which predicts an amino acid substitution G310E. SSCP analysis of the segregation pattern of the mutation strongly suggests a causal relationship to the SHFM phenotype in p63. This mutation has not been observed in other countries in the world.

Cleft lip and palate repair in Hay-Wells/ankyloblepharon-ectodermal dysplasia-clefting syndrome

Hay-Wells/ankyloblepharon-ectodermal dysplasia-clefting syndrome is a rare autosomal dominant disorder characterized by ankyloblepharon, ectodermal dysplasia, and cleft lip and/or cleft palate. Mutations in the p63 gene recently have been shown to be etiologic in the majority of cases of ankyloblepharon-ectodermal dysplasia-clefting syndrome. To date, there have been no reports to document wound healing after cleft lip and/or palate repair in ankyloblepharon-ectodermal dysplasia-clefting patients. We describe two patients with ankyloblepharon-ectodermal dysplasia-clefting syndrome and provide a review of the literature. There have been no reported instances of wound healing complications in affected patients. Seventeen percent (3/18) of reported patients required revisions or repair of oronasal fistulae. Cleft lip and palate repair can be performed safely in patients with Hay-Wells syndrome.

A new E6/P63 pathway, together with a strong E7/E2F mitotic pathway, modulates the transcriptome in cervical cancer cells

Cervical carcinoma is associated with certain types of human papillomaviruses expressing the E6 and E7 oncogenes, which are involved in carcinogenesis through their interactions with the p53 and pRB pathways, respectively. A critical event on the path to malignant transformation is often manifested by the loss of expression of the viral E2 transcription factor due to the integration into the host genome of the viral DNA. Using microarrays, we have previously shown that reintroduction of a functional E2 in the HeLa cervical carcinoma cell line activates a cluster of p53 target genes while at the same time severely repressing a group of E2F target genes. In the present study, using new high-density microarrays containing more than 22,000 human cDNA sequences, we identified a novel p63 pathway among E2-activated genes and 38 new mitotic genes repressed by E2. We then sought to determine the pathways through which these genes were modulated and used an approach that relies on small interfering RNA to demonstrate that the p63 target genes were activated through silencing of the E6/E6AP pathway while the mitotic genes were mainly repressed through E7 silencing. Importantly, a subset of the mitotic genes was shown to be significantly induced in biopsies of stage IV cervical cancers, which points to a prominent E7 pathway in cervical carcinoma.

Mechanisms regulating epithelial stratification

The epidermis is a stratified epithelium that functions as a barrier protecting the organism from dehydration, mechanical trauma, and microbial insults. This barrier function is established during embryogenesis through a complex and tightly controlled stratification program. Whereas the morphological changes that occur during epidermal development have been extensively studied, the molecular mechanisms that govern this process remain poorly understood. In this review we summarize the current advances that have been made in understanding the molecular mechanisms that regulate epidermal morphogenesis.

p63 induces key target genes required for epidermal morphogenesis

Mice lacking p63, a single gene that encodes a group of transcription factors that either contain (TA) or lack (DeltaN) a transactivation domain, fail to develop stratified epithelia as well as epithelial appendages and limbs. DeltaNp63 isoforms are predominantly expressed during late embryonic and postnatal epidermal development, however, the function of these proteins remains elusive. Using an epidermal-specific inducible knockdown mouse model, we demonstrate that DeltaNp63 proteins are essential for maintaining basement membrane integrity and terminal differentiation of keratinocytes. Furthermore, we have identified two DeltaNp63alpha target genes that mediate these processes. We propose that DeltaNp63alpha initially induces expression of the extracellular matrix component Fras1, which is required for maintaining the integrity of the epidermal-dermal interface at the basement membrane. Subsequently, induction of IkappaB kinase-alpha by DeltaNp63alpha initiates epidermal terminal differentiation resulting in the formation of the spinous layer. Our data provide insights into the role of DeltaNp63alpha in epidermal morphogenesis and homeostasis, and may contribute to our understanding of the pathogenic mechanisms underlying disorders caused by p63 mutations.

The THAP–zinc finger protein THAP1 regulates endothelial cell proliferation through modulation of pRB/E2F cell-cycle target genes

We recently cloned a novel human nuclear factor (designated THAP1) from postcapillary venule endothelial cells (ECs) that contains a DNA-binding THAP domain, shared with zebrafish E2F6 and several Caenorhabditis elegans proteins interacting genetically with retinoblastoma gene product (pRB). Here, we show that THAP1 is a physiologic regulator of EC proliferation and cell-cycle progression, 2 essential processes for angiogenesis. Retroviral-mediated gene transfer of THAP1 into primary human ECs inhibited proliferation, and large-scale expression profiling with microarrays revealed that THAP1-mediated growth inhibition is due to coordinated repression of pRB/E2F cell-cycle target genes. Silencing of endogenous THAP1 through RNA interference similarly inhibited EC proliferation and G1/S cell-cycle progression, and resulted in down-regulation of several pRB/E2F cell-cycle target genes, including RRM1, a gene required for S-phase DNA synthesis. Chromatin immunoprecipitation assays in proliferating ECs showed that endogenous THAP1 associates in vivo with a consensus THAP1-binding site found in the RRM1 promoter, indicating that RRM1 is a direct transcriptional target of THAP1. The similar phenotypes observed after THAP1 overexpression and silencing suggest that an optimal range of THAP1 expression is essential for EC proliferation. Together, these data provide the first links in mammals among THAP proteins, cell proliferation, and pRB/E2F cell-cycle pathways.