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

Nucleosome binding by TP53, TP63, and TP73 is determined by the composition, accessibility, and helical orientation of their binding sites

The TP53 family of transcription factors plays key roles in driving development and combating cancer by regulating gene expression. TP53, TP63, and TP73-the three members of the TP53 family-regulate gene expression by binding to their DNA binding sites, many of which are situated within nucleosomes. To thoroughly examine the nucleosome-binding abilities of the TP53 family, we used Pioneer-seq, a technique that assesses a transcription factor's binding affinity to its DNA-binding sites at all possible positions within the nucleosome core particle. Using Pioneer-seq, we analyzed the binding affinities of TP53, TP63, and TP73 to 10 TP53 family binding sites across the nucleosome core particle. We find that the affinities of TP53, TP63, and TP73 for nucleosomes are primarily determined by the positioning of TP53 family binding sites within nucleosomes; TP53 family members bind strongly to the more accessible edges of nucleosomes but weakly to the less accessible centers of nucleosomes. Our results further show that the DNA-helical orientation of TP53 family binding sites within nucleosomal DNA impacts the nucleosome-binding affinities of TP53 family members, with binding-site composition impacting the affinity of each TP53 family member only when the binding-site location is accessible. Taken together, our results show that the accessibility, composition, and helical orientation of TP53 family binding sites collectively determine the nucleosome-binding affinities of TP53, TP63, and TP73. These findings help explain the rules underlying TP53 family-nucleosome binding and thus provide requisite insight into how we may better control gene expression changes involved in development and tumor suppression.

Nucleosome-binding by TP53, TP63, and TP73 is determined by the composition, accessibility, and helical orientation of their binding sites

The p53 family of transcription factors plays key roles in driving development and combating cancer by regulating gene expression. TP53, TP63, and TP73-the three members of the p53 family-regulate gene expression by binding to their DNA binding sites, many of which are situated within nucleosomes. To thoroughly examine the nucleosome-binding abilities of the p53 family, we used Pioneer-seq, a technique that assesses a transcription factor's binding affinity to its DNA binding sites at all possible positions within the nucleosome core particle. Using Pioneer-seq, we analyzed the binding affinity of TP53, TP63, and TP73 to 10 p53-family binding sites across the nucleosome core particle. We found that the affinity of TP53, TP63, and TP73 for nucleosomes was largely determined by the positioning of p53-family binding sites within nucleosomes; p53-family members bind strongly to the more accessible edges of nucleosomes but weakly to the less accessible centers of nucleosomes. We also found that the DNA-helical orientation of p53-family binding sites within nucleosomal DNA impacted the nucleosome-binding affinity of p53-family members. The composition of their binding sites also impacted each p53-family member's nucleosome-binding affinities only when the binding site was located in an accessible location. Taken together, our results show that the accessibility, composition, and helical orientation of p53-family binding sites collectively determine the nucleosome-binding affinities of TP53, TP63, and TP73. These findings help explain the rules underlying p53-family-nucleosome binding and thus provide requisite insight into how we may better control gene-expression changes involved in development and tumor suppression.

Genome-wide p63-Target Gene Analyses Reveal TAp63/NRF2-Dependent Oxidative Stress Responses

The p53 family member TP63 encodes two sets of N-terminal isoforms, TAp63 and ΔNp63 isoforms. They each regulate diverse biological functions in epidermal morphogenesis and in cancer. In the skin, where their activities have been extensively characterized, TAp63 prevents premature aging by regulating the quiescence and genomic stability of stem cells required for wound healing and hair regeneration, while ΔNp63 controls maintenance and terminal differentiation of epidermal basal cells. This functional diversity is surprising given that these isoforms share a high degree of similarity, including an identical sequence for a DNA-binding domain. To understand the mechanisms of the transcriptional programs regulated by each p63 isoform and leading to diverse biological functions, we performed genome-wide analyses using p63 isoform-specific chromatin immunoprecipitation, RNA sequencing, and metabolomics of TAp63-/- and ΔNp63-/- mouse epidermal cells. Our data indicate that TAp63 and ΔNp63 physically and functionally interact with distinct transcription factors for the downstream regulation of their target genes, thus ultimately leading to the regulation of unique transcriptional programs and biological processes. Our findings unveil novel transcriptomes regulated by the p63 isoforms to control diverse biological functions, including the cooperation between TAp63 and NRF2 in the modulation of metabolic pathways and response to oxidative stress providing a mechanistic explanation for the TAp63 knock out phenotypes.

SIGNIFICANCE

The p63 isoforms, TAp63 and ΔNp63, control epithelial morphogenesis and tumorigenesis through the interaction with distinct transcription factors and the subsequent regulation of unique transcriptional programs.

Synonymous alterations of cancer-associated Trp53 CpG mutational hotspots cause fatal developmental jaw malocclusions but no tumors in knock-in mice

Intragenic CpG dinucleotides are tightly conserved in evolution yet are also vulnerable to methylation-dependent mutation, raising the question as to why these functionally critical sites have not been deselected by more stable coding sequences. We previously showed in cell lines that altered exonic CpG methylation can modify promoter start sites, and hence protein isoform expression, for the human TP53 tumor suppressor gene. Here we extend this work to the in vivo setting by testing whether synonymous germline modifications of exonic CpG sites affect murine development, fertility, longevity, or cancer incidence. We substituted the DNA-binding exons 5-8 of Trp53, the mouse ortholog of human TP53, with variant-CpG (either CpG-depleted or -enriched) sequences predicted to encode the normal p53 amino acid sequence; a control construct was also created in which all non-CpG sites were synonymously substituted. Homozygous Trp53-null mice were the only genotype to develop tumors. Mice with variant-CpG Trp53 sequences remained tumor-free, but were uniquely prone to dental anomalies causing jaw malocclusion (p < .0001). Since the latter phenotype also characterises murine Rett syndrome due to dysfunction of the trans-repressive MeCP2 methyl-CpG-binding protein, we hypothesise that CpG sites may exert non-coding phenotypic effects via pre-translational cis-interactions of 5-methylcytosine with methyl-binding proteins which regulate mRNA transcript initiation, expression or splicing, although direct effects on mRNA structure or translation are also possible.

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.

Mutant Ras and inflammation-driven skin tumorigenesis is suppressed via a JNK-iASPP-AP1 axis

Concurrent mutation of a RAS oncogene and the tumor suppressor p53 is common in tumorigenesis, and inflammation can promote RAS-driven tumorigenesis without the need to mutate p53. Here, we show, using a well-established mutant RAS and an inflammation-driven mouse skin tumor model, that loss of the p53 inhibitor iASPP facilitates tumorigenesis. Specifically, iASPP regulates expression of a subset of p63 and AP1 targets, including genes involved in skin differentiation and inflammation, suggesting that loss of iASPP in keratinocytes supports a tumor-promoting inflammatory microenvironment. Mechanistically, JNK-mediated phosphorylation regulates iASPP function and inhibits iASPP binding with AP1 components, such as JUND, via PXXP/SH3 domain-mediated interaction. Our results uncover a JNK-iASPP-AP1 regulatory axis that is crucial for tissue homeostasis. We show that iASPP is a tumor suppressor and an AP1 coregulator.

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

p63, a key regulator of Ago2, links to the microRNA-144 cluster

Abstract

As a key component of the RNA-induced silencing complex (RISC), Argonaute2 (Ago2) exhibits a dual function regulatory role in tumor progression. However, the mechanistic basis of differential regulation remains elusive. p63 is a homolog of the tumor suppressor p53. p63 isoforms play a critical role in tumorigenesis and metastasis. Herein, we show that p63 isoforms physically interact with and stabilize Ago2. Expression of p63 isoforms increases the levels of Ago2 protein, while depletion of p63 isoforms by shRNA decreases Ago2 protein levels. p63 strongly guides Ago2 dual functions in vitro and in vivo. Ectopic expression of the miR-144/451 cluster increases p63 protein levels; TAp63 transactivates the miR-144/451 cluster, forming a positive feedback loop. Notably, miR-144 activates p63 by directly targeting Itch, an E3 ligase of p63. Ectopic expression of miR-144 induces apoptosis in H1299 cells. miR-144 enhances TAp63 tumor suppressor function and inhibits cell invasion. Our findings uncover a novel function of p63 linking the miRNA-144 cluster and the Ago2 pathway.

Facts and questions

Identification of Ago2 as a p63 target.

Ago2 exhibits a dual function regulatory role in tumor progression; however, the molecular mechanism of Ago2 regulation remains unknown.

p63 strongly guides Ago2 dual functions in vitro and in vivo.

Unraveling a novel function of p63 links the miRNA-144 cluster and the Ago2 pathway.

Puf-A promotes cancer progression by interacting with nucleophosmin in nucleolus

Previously, we identified Puf-A as a novel member of Puf-family RNA-binding proteins; however, its biological functions remain obscure. Analysis of tumor samples of non-small cell lung cancer (NSCLC) showed that high Puf-A expression correlated with high histology grade and abnormal p53 status. Kaplan-Meier curve for overall survival revealed high expression of Puf-A to predict poor prognosis in stage I NSCLC. Among patients with colorectal cancer, high Puf-A expression also showed an adverse impact on overall survival. In lung cancer cell lines, downregulation of p53 increased Puf-A expression, and upregulation of p53 dampened its expression. However, luciferase reporter assays indicated that PUF-A locus harbored the p53-response element, but regulated Puf-A transcription indirectly. In vivo suppression of p53 in CCSP-rtTA/TetO-Cre/LSL-KrasG12D/p53flox/flox conditional mutant mice accelerated the progression of the KrasG12D-driven lung cancer, along with enhanced expression of Puf-A. Importantly, intranasal delivery of shPuf-A to the inducible KrasG12D/p53flox/flox mice suppressed tumor progression. Puf-A silencing led to marked decreases in the 80S ribosomes, along with decrease in S6 and L5 in the cytoplasm and accumulation in the nucleolus. Based on immunofluorescence staining and immunoprecipitation studies, Puf-A interacted with NPM1 in nucleolus. Puf-A silencing resulted in NPM1 translocation from nucleolus to nucleoplasm and this disruption of NPM1 localization was reversed by a rescue experiment. Mechanistically, Puf-A silencing altered NPM1 localization, leading to the retention of ribosomal proteins in nucleolus and diminished ribosome biogenesis, followed by cell-cycle arrest/cell death. Puf-A is a potential theranostic target for cancer therapy and an important player in cancer progression.

Tissue-specific expression of p73 and p63 isoforms in human tissues

p73 and p63 are members of the p53 family that exhibit overlapping and distinct functions in development and homeostasis. The evaluation of p73 and p63 isoform expression across human tissue can provide greater insight to the functional interactions between family members. We determined the mRNA isoform expression patterns of TP73 and TP63 across a panel of 36 human tissues and protein expression within the highest-expressing tissues. TP73 and TP63 expression significantly correlated across tissues. In tissues with concurrent mRNA expression, nuclear co-expression of both proteins was observed in a majority of cells. Using GTEx data, we quantified p73 and p63 isoform expression in human tissue and identified that the α-isoforms of TP73 and TP63 were the predominant isoform expressed in nearly all tissues. Further, we identified a previously unreported p73 mRNA product encoded by exons 4 to 14. In sum, these data provide the most comprehensive tissue-specific atlas of p73 and p63 protein and mRNA expression patterns in human and murine samples, indicating coordinate expression of these transcription factors in the majority of tissues in which they are expressed.

The attributes of plakins in cancer and disease: perspectives on ovarian cancer progression, chemoresistance and recurrence

The plakin family of cytoskeletal proteins play an important role in cancer progression yet are under-studied in cancer, especially ovarian cancer. These large cytoskeletal proteins have primary roles in the maintenance of cytoskeletal integrity but are also associated with scaffolds of intermediate filaments and hemidesmosomal adhesion complexes mediating signalling pathways that regulate cellular growth, migration, invasion and differentiation as well as stress response. Abnormalities of plakins, and the closely related spectraplakins, result in diseases of the skin, striated muscle and nervous tissue. Their prevalence in epithelial cells suggests that plakins may play a role in epithelial ovarian cancer progression and recurrence. In this review article, we explore the roles of plakins, particularly plectin, periplakin and envoplakin in disease-states and cancers with emphasis on ovarian cancer. We discuss the potential role the plakin family of proteins play in regulating cancer cell growth, survival, migration, invasion and drug resistance. We highlight potential relationships between plakins, epithelial-mesenchymal transition (EMT) and cancer stem cells (CSCs) and discuss how interaction of these processes may affect ovarian cancer progression, chemoresistance and ultimately recurrence. We propose that molecular changes in the expression of plakins leads to the transition of benign ovarian tumours to carcinomas, as well as floating cellular aggregates (commonly known as spheroids) in the ascites microenvironment, which may contribute to the sustenance and progression of the disease. In this review, attempts have been made to understand the crucial changes in plakin expression in relation to progression and recurrence of ovarian cancer.

Evaluating the Influence of a G-Quadruplex Prone Sequence on the Transactivation Potential by Wild-Type and/or Mutant P53 Family Proteins through a Yeast-Based Functional Assay

P53, P63, and P73 proteins belong to the P53 family of transcription factors, sharing a common gene organization that, from the P1 and P2 promoters, produces two groups of mRNAs encoding proteins with different N-terminal regions; moreover, alternative splicing events at C-terminus further contribute to the generation of multiple isoforms. P53 family proteins can influence a plethora of cellular pathways mainly through the direct binding to specific DNA sequences known as response elements (REs), and the transactivation of the corresponding target genes. However, the transcriptional activation by P53 family members can be regulated at multiple levels, including the DNA topology at responsive promoters. Here, by using a yeast-based functional assay, we evaluated the influence that a G-quadruplex (G4) prone sequence adjacent to the p53 RE derived from the apoptotic PUMA target gene can exert on the transactivation potential of full-length and N-terminal truncated P53 family α isoforms (wild-type and mutant). Our results show that the presence of a G4 prone sequence upstream or downstream of the P53 RE leads to significant changes in the relative activity of P53 family proteins, emphasizing the potential role of structural DNA features as modifiers of P53 family functions at target promoter sites.

GDF-15: Diagnostic, prognostic, and therapeutic significance in glioblastoma multiforme

Glioblastoma multiforme (GBM) is the commonest primary malignant brain tumor and has a remarkably weak prognosis. According to the aggressive form of GBM, understanding the accurate molecular mechanism associated with GBM pathogenesis is essential. Growth differentiation factor 15 (GDF-15) belongs to transforming growth factor-β superfamily with important roles to control biological processes. It affects cancer growth and progression, drug resistance, and metastasis. It also can promote stemness in many cancers, and also can stress reactions control, bone generation, hematopoietic growth, adipose tissue performance, and body growth, and contributes to cardiovascular disorders. The role GDF-15 to develop and progress cancer is complicated and remains unclear. GDF-15 possesses tumor suppressor properties, as well as an oncogenic effect. GDF-15 antitumorigenic and protumorigenic impacts on tumor development are linked to the cancer type and stage. However, the GDF-15 signaling and mechanism have not yet been completely identified because of no recognized cognate receptor.

The p63 C-terminus is essential for murine oocyte integrity

The transcription factor p63 mediates distinct cellular responses, primarily regulating epithelial and oocyte biology. In addition to the two amino terminal isoforms, TAp63 and ΔNp63, the 3'-end of p63 mRNA undergoes tissue-specific alternative splicing that leads to several isoforms, including p63α, p63β and p63γ. To investigate in vivo how the different isoforms fulfil distinct functions at the cellular and developmental levels, we developed a mouse model replacing the p63α with p63β by deletion of exon 13 in the Trp63 gene. Here, we report that whereas in two organs physiologically expressing p63α, such as thymus and skin, no abnormalities are detected, total infertility is evident in heterozygous female mice. A sharp reduction in the number of primary oocytes during the first week after birth occurs as a consequence of the enhanced expression of the pro-apoptotic transcriptional targets Puma and Noxa by the tetrameric, constitutively active, TAp63β isoform. Hence, these mice show a condition of ovary dysfunction, resembling human primary ovary insufficiency. Our results show that the p63 C-terminus is essential in TAp63α-expressing primary oocytes to control cell death in vivo, expanding the current understanding of human primary ovarian insufficiency.

P63 modulates the expression of the WDFY2 gene which is implicated in cancer regulation and limb development

TP63 is a member of the TP53 gene family, sharing a common gene structure that produces two groups of mRNAs' encoding proteins with different N-terminal regions (ΔN and TA isoforms); both transcripts are also subjected to alternative splicing mechanisms at C-terminus, generating a variety of isoforms. p63 is a master regulator of epidermal development and homoeostasis as well as an important player in tumorigenesis and cancer progression with both oncogenic and tumour suppressive roles. A number of studies have aimed at the identification of p63 target genes, allowing the dissection of the molecular pathways orchestrated by the different isoforms. In the present study we investigated in more detail the p63 responsiveness of the WDFY2 (WD repeat and FYVE domain containing 2) gene, encoding for an endosomal protein identified as a binding partner of the PI-3K/AKT signalling pathway. We showed that overexpression of different p63 isoforms was able to induce WDFY2 expression in TP53-null cells. The p63-dependent transcriptional activation was associated with specific response elements (REs) that have been identified by a bioinformatics tool and validated by yeast- and mammal-based assays. Interestingly, to confirm that WDFY2 belongs to the p63 network of cancer regulation, we analysed the impact of WDFY2 alterations, by showing its frequent deletion in different types of tumours and suggesting its expression level as a prognostic biomarker. Lastly, we identified a chromosomal translocation involving the WDFY2 locus in a patient affected by a rare congenital limb anomaly, indicating WDFY2 as a possible susceptibility gene placed downstream p63 in the network of limb development.

Rapid and Efficient Synthetic Assembly of Multiplex Luciferase Reporter Plasmids for the Simultaneous Monitoring of Up to Six Cellular Signaling Pathways

High-throughput cell-based screening assays are valuable tools in the discovery of chemical probes and therapeutic agents. Such assays are designed to examine the effects of small compounds on targets, pathways, or phenotypes participating in normal and disease processes. While most cell-based assays measure single quantities, multiplexed assays seek to address these limitations by obtaining multiple simultaneous measurements. The signals from such measurements should be independently detectable and cover large dynamic ranges. Luciferases are good candidates for generation of such signals. They are genetically encoded, versatile, and cost-effective, and their output signals can be sensitively detected. We recently developed a multiplex luciferase assay that allows monitoring the activity of five experimental pathways against one control simultaneously. We used synthetic assembly cloning to assemble all six luciferase reporter units into a single vector over eight stitching rounds. Because all six reporters are on a single piece of DNA, a single vector ensures stoichiometric ratios of each transcriptional unit in each transfected cell, resulting in lower experimental variation. Our proof-of-concept multiplex hextuple luciferase assay was designed to simultaneously monitor the p53, TGF-β, NF-κβ, c-Myc, and MAPK/JNK signaling pathways. The same synthetic assembly cloning pipeline allows the stitching of numerous other cellular pathway luciferase reporters. Here we present an improved three-step synthetic assembly protocol to quickly and efficiently generate multiplex hextuple luciferase reporter plasmids for other signaling pathways of interest. This improved assembly protocol provides the opportunity to analyze any five desired pathways at once much more quickly. Protocols are provided on how to prepare DNA components and destination vector plasmids, design synthetic DNA, perform assembly cloning of new transcriptional reporter elements, implement multipartite synthetic assembly cloning of single-pathway luciferase reporters, and carry out one-step assembly of final multiplex hextuple luciferase vectors. We present protocols on how to perform multiplex hextuple luciferase in an accompanying Current Protocols in Molecular Biology article. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Preparation of DNA parts and destination vectors for synthetic assembly cloning Basic Protocol 2: DNA synthesis and assembly cloning of a typical transcriptional reporter element Alternate Protocol: DNA synthesis and assembly cloning of a challenging transcriptional reporter element Basic Protocol 3: Multipartite synthetic assembly cloning of individual pathway luciferase reporters Basic Protocol 4: One step assembly into final multiplex hextuple luciferase vectors Support Protocol: Generation of home-made chemocompetent E. coli DH10B-T1R cells.

Crosstalk Between Vitamin D and p53 Signaling in Cancer: An Update

It has now been convincingly shown that vitamin D and p53 signaling protect against spontaneous or carcinogen-induced malignant transformation of cells. The vitamin D receptor (VDR) and the p53/p63/p73 proteins (the p53 family hereafter) exert their effects as receptors/sensors that turn into transcriptional regulators upon stimulus. While the p53 clan, mostly in the nucleoplasm, responds to a large and still growing number of alterations in cellular homeostasis commonly referred to as stress, the nuclear VDR is transcriptionally activated after binding its naturally occurring biologically active ligand 1,25-dihydroxyvitamin D with high affinity. Interestingly, a crosstalk between vitamin D and p53 signaling has been demonstrated that occurs at different levels, has genome-wide implications, and is of high importance for many malignancies, including non-melanoma skin cancer. These interactions include the ability of p53 to upregulate skin pigmentation via POMC derivatives including alpha-MSH and ACTH. Increased pigmentation protects the skin against UV-induced DNA damage and skin photocarcinogenesis, but also inhibits cutaneous synthesis of vitamin D. A second level of interaction is characterized by binding of VDR and p53 protein, an observation that may be of relevance for the ability of 1,25-dihydroxyvitamin D to increase the survival of skin cells after UV irradiation. UV irradiation-surviving cells show significant reductions in thymine dimers in the presence of 1,25-dihydroxyvitamin D that are associated with increased nuclear p53 protein expression and significantly reduced NO products. A third level of interaction is documented by the ability of vitamin D compounds to regulate the expression of the murine double minute (MDM2) gene in dependence of the presence of wild-type p53. MDM2 has a well-established role as a key negative regulator of p53 activity. Finally, p53 and its family members have been implicated in the direct regulation of the VDR. This review gives an update on some of the implications of the crosstalk between vitamin D and p53 signaling for carcinogenesis in the skin and other tissues, focusing on a genome-wide perspective.

p53 regulates katanin-p60 promoter in HCT 116 cells

Tumor suppressor protein p53, which functions in the cell cycle, apoptosis and neuronal differentiation via transcriptional regulations of target genes or interactions with several proteins, has been associated with neurite outgrowth through microtubule re-organization. We previously demonstrated in neurons that upon p53 induction, the level of microtubule severing protein Katanin-p60 increases, indicating that p53 might be a transcriptional regulator of the KATNA1 gene encoding Katanin-p60. In this context, we firstly elucidated the activity of KATNA1 regulatory regions and endogenous KATNA1 mRNA levels in the presence or absence of p53 using HCT 116 WT and HCT 116 p53 (-/-) cells. Next, we demonstrated the binding of p53 to the KATNA1 promoter and then investigated the role of p53 on KATNA1 gene expression by ascertaining KATNA1 mRNA and Katanin-p60 protein levels upon p53 overexpression and activation in both cells. Moreover, we showed changes in microtubule network upon increased Katanin-p60 level due to p53 overexpression. Also, the changes in KATNA1 mRNA and Katanin-p60 protein levels upon p53 knockdown were investigated. Our results indicate that p53 is an activator of KATNA1 gene expression and we show that both p53 and Katanin-p60 expression have strict regulations and are maintained at balanced levels as they are vital proteins to orchestrate either survival and apoptosis or differentiation.

Comprehensive Exploration to Identify Predictive DNA Markers of ΔNp63/SOX2 in Drug Resistance in Human Esophageal Squamous Cell Carcinoma

BACKGROUND

OBP-801 is a novel histone deacetylase inhibitor being developed as an anticancer drug. In this study, we explored genes to predict drug resistance in human cancer.

METHODS

OBP-801 resistance was assessed in 37 strains of human cancer cell lines. Expression microarrays harboring 54,675 genes were used to focus on candidate genes, which were validated for both functional and clinical relevance in esophageal squamous cell carcinoma (ESCC).

RESULTS

OBP-801 is sensitive to esophageal, gastric, and thyroid cancer, and resistant to some esophageal and colorectal cancers. We therefore used ESCC to explore genes. Comprehensive exploration focused on ΔNp63/SOX2, which were both genetically and epigenetically overexpressed in ESCC. Genomic amplifications of ΔNp63/SOX2 were tightly correlated each other (r = 0.81). Importantly, genomic amplification of ΔNp63/SOX2 in the resected tumors after neoadjuvant chemotherapy was significantly associated with histological grade of response (G1). Forced expression of either of these two genes did not induce each other, suggesting that their functional relevances were independent and showed robust drug resistance in OBP-801, as well as 5-fluorouracil. Furthermore, ΔNp63 could exert a potent oncogenic potential. RNA interference of ΔNp63 supported its oncological properties, as well as drug resistance.

CONCLUSION

Comprehensive exploration of genes involved in anticancer drug residence could identify critical oncogenes of ΔNp63/SOX2 that would predict chemotherapy response in ESCC.

Roles of p53 Family Structure and Function in Non-Canonical Response Element Binding and Activation

The p53 canonical consensus sequence is a 10-bp repeat of PuPuPuC(A/T)(A/T)GPyPyPy, separated by a spacer with up to 13 bases. C(A/T)(A/T)G is the core sequence and purine (Pu) and pyrimidine (Py) bases comprise the flanking sequence. However, in the p53 noncanonical sequences, there are many variations, such as length of consensus sequence, variance of core sequence or flanking sequence, and variance in number of bases making up the spacer or AT gap composition. In comparison to p53, the p53 family members p63 and p73 have been found to have more tolerance to bind and activate several of these noncanonical sequences. The p53 protein forms monomers, dimers, and tetramers, and its nonspecific binding domain is well-defined; however, those for p63 or p73 are still not fully understood. Study of p63 and p73 structure to determine the monomers, dimers or tetramers to bind and regulate noncanonical sequence is a new challenge which is crucial to obtaining a complete picture of structure and function in order to understand how p63 and p73 regulate genes differently from p53. In this review, we will summarize the rules of p53 family non-canonical sequences, especially focusing on the structure of p53 family members in the regulation of specific target genes. In addition, we will compare different software programs for prediction of p53 family responsive elements containing parameters with canonical or non-canonical sequences.

Molecular Mechanisms of p63-Mediated Squamous Cancer Pathogenesis

The p63 gene is a member of the p53/p63/p73 family of transcription factors and plays a critical role in development and homeostasis of squamous epithelium. p63 is transcribed as multiple isoforms; ΔNp63α, the predominant p63 isoform in stratified squamous epithelium, is localized to the basal cells and is overexpressed in squamous cell cancers of multiple organ sites, including skin, head and neck, and lung. Further, p63 is considered a stem cell marker, and within the epidermis, ΔNp63α directs lineage commitment. ΔNp63α has been implicated in numerous processes of skin biology that impact normal epidermal homeostasis and can contribute to squamous cancer pathogenesis by supporting proliferation and survival with roles in blocking terminal differentiation, apoptosis, and senescence, and influencing adhesion and migration. ΔNp63α overexpression may also influence the tissue microenvironment through remodeling of the extracellular matrix and vasculature, as well as by enhancing cytokine and chemokine secretion to recruit pro-inflammatory infiltrate. This review focuses on the role of ΔNp63α in normal epidermal biology and how dysregulation can contribute to cutaneous squamous cancer development, drawing from knowledge also gained by squamous cancers from other organ sites that share p63 overexpression as a defining feature.

Intersection of the p63 and NF-κB pathways in epithelial homeostasis and disease

Overexpression of ΔNp63α, a member of the p53/p63/p73 family of transcription factors, is a molecular attribute of human squamous cancers of the head and neck, lung and skin. The TP63 gene plays important roles in epidermal morphogenesis and homeostasis, regulating diverse biological processes including epidermal fate decisions and keratinocyte proliferation and survival. When overexpressed experimentally in primary mouse keratinocytes, ΔNp63α maintains a basal cell phenotype including the loss of normal calcium-mediated growth arrest, at least in part through the activation and enhanced nuclear accumulation of the c-rel subunit of NF-κB (Nuclear Factor-kappa B). Initially identified for its role in the immune system and hematopoietic cancers, c-Rel has increasingly been associated with solid tumors and other pathologies. ΔNp63α and c-Rel have been shown to be associated in the nuclei of ΔNp63α overexpressing human squamous carcinoma cells. Together, these transcription factors cooperate in the transcription of genes regulating intrinsic keratinocyte functions, as well as the elaboration of factors that influence the tumor microenvironment (TME). This review provides an overview of the roles of ΔNp63α and c-Rel in normal epidermal homeostasis and elaborates on how these pathways may intersect in pathological conditions such as cancer and the associated TME.

ZNF185 is a p63 target gene critical for epidermal differentiation and squamous cell carcinoma development

Development and maintenance of healthy stratified epithelia require the coordination of complex transcriptional programmes. The transcription factor p63, a member of the p53 family, plays a crucial role in epithelial development and homeostasis. Analysis of the p63-dependent transcriptome indicated that one important aspect of p63 functions in epithelial development is the regulation of cell–cell and cell–matrix adhesion programmes. However, limited knowledge exists on the relevant cell–cell adhesion molecules involved in physiological epithelial formation. Similarly, limited data are available to understand if deregulation of the cell–cell adhesion programme is important in tumour formation. Here, using the epidermis as an experimental model with the RNA sequencing approach, we identify a novel p63-regulated gene induced during differentiation, ZNF185. ZNF185 is an actin-cytoskeleton-associated Lin-l 1, Isl-1 and Mec-3 (LIM) domain-containing protein, whose function is poorly known. We found that p63 binds to a specific enhancer region, promoting its expression to sustain epithelial differentiation. ZNF185 silencing strongly impaired keratinocyte differentiation according to gene array analysis. ZNF185 is detected at the cell–cell periphery where it physically interacts with E-cadherin, indicating that it is important to maintain epithelial integrity beyond its pro-differentiation role. Interestingly, poorly differentiated, including head and neck, cervical and oesophageal, squamous cell carcinomas display loss of ZNF185 expression. Together, these studies reinforce that p63 is a crucial gene for maintaining epithelial tissue integrity and support the deregulation of the cell-cell adhesion programme,which plays a critical role in carcinoma development.

Epstein-Barr Virus Gene BARF1 Expression is Regulated by the Epithelial Differentiation Factor ΔNp63α in Undifferentiated Nasopharyngeal Carcinoma

Epstein-Barr Virus (EBV) BamHI-A rightward frame 1 (BARF1) protein is considered a viral oncogene in epithelial cells and has immune-modulating properties. During viral lytic replication BARF1 is expressed as an early gene, regulated by the immediate early EBV protein R. However, in viral latency BARF1 is exclusively expressed in epithelial tumors such as nasopharyngeal (NPC) and gastric carcinoma (GC) but not in lymphomas, indicating that activation of the BARF1 promoter is cell type specific. Undifferentiated NPC is characterized by high expression of ΔNp63 isoforms of the epithelial differentiation marker p63, a member of the p53 family of transcription factors. Transcription factor binding site analysis indicated potential p53 family binding sites within the BARF1 promoter region. This study investigated ability of various p53 family members to transactivate the BARF1 promoter. Using BARF1 promoter luciferase reporter constructs we demonstrate that only p63 isoform ΔNp63α is capable of transactivating the BARF1 promoter, but not the TAp63 isoforms, p53 or p73. Direct promoter binding of ΔNp63α was confirmed by Chromatin Immune Precipitation (ChIP) analysis. Deletion mutants of the BARF1 promoter revealed multiple ΔNp63 response elements to be responsible for BARF1 promoter transactivation. However, ΔNp63α alone was not sufficient to induce BARF1 in tumor cells harboring full EBV genomes, indicating that additional cofactors might be required for full BARF1 regulation. In conclusion, in EBV positive NPC and GC, BARF1 expression might be induced by the epithelial differentiation marker ΔNp63α, explaining BARF1 expression in the absence of lytic reactivation.

A double dealing tale of p63: an oncogene or a tumor suppressor

As a member of tumor suppressor p53 family, p63, a gene encoding versatile protein variant, has been documented to correlate with cancer formation and progression, though it is rarely mutated in cancer patients. However, it has long been controversial on whether p63 is an oncogene or a tumor suppressor. Here, we comprehensively reviewed reports on roles of p63 in development, tumorigenesis and tumor progression. According to data from molecular cell biology, genetic models and clinic research, we conclude that p63 may act as either an oncogene or a tumor suppressor gene in different scenarios: TA isoforms of p63 gene are generally tumor-suppressive through repressing cell proliferation, survival and metastasis; ΔN isoforms, however, may initiate tumorigenesis via promoting cell proliferation and survival, but inhibit tumor metastasis and progression; effects of p63 on tumor formation and progression depend on the context of the whole p53 family, and either amplification or loss of p63 gene locus can break the balance to cause tumorigenesis.

The epidermal polarity protein Par3 is a non-cell autonomous suppressor of malignant melanoma

Mescher et al. uncover a novel tissue-borne tumor suppression mechanism, engaging polarity proteins in the epithelial microenvironment that prevent malignant outgrowth of neighboring cell types through control of heterologous cell–cell contacts. Moreover, their data support an emerging role of P-cadherin, which is frequently amplified in human carcinoma, as a protumorigenic and proinvasive adhesion molecule, thus placing it as a promising druggable target to disrupt tumor–microenvironment interactions for anticancer therapy.

Melanoma, an aggressive skin malignancy with increasing lifetime risk, originates from melanocytes (MCs) that are in close contact with surrounding epidermal keratinocytes (KCs). How the epidermal microenvironment controls melanomagenesis remains poorly understood. In this study, we identify an unexpected non–cell autonomous role of epidermal polarity proteins, molecular determinants of cytoarchitecture, in malignant melanoma. Epidermal Par3 inactivation in mice promotes MC dedifferentiation, motility, and hyperplasia and, in an autochthonous melanoma model, results in increased tumor formation and lung metastasis. KC-specific Par3 loss up-regulates surface P-cadherin that is essential to promote MC proliferation and phenotypic switch toward dedifferentiation. In agreement, low epidermal PAR3 and high P-cadherin expression correlate with human melanoma progression, whereas elevated P-cadherin levels are associated with reduced survival of melanoma patients, implying that this mechanism also drives human disease. Collectively, our data show that reduced KC Par3 function fosters a permissive P-cadherin–dependent niche for MC transformation, invasion, and metastasis. This reveals a previously unrecognized extrinsic tumor-suppressive mechanism, whereby epithelial polarity proteins dictate the cytoarchitecture and fate of other tissue-resident cells to suppress their malignant outgrowth.

Repression of Wnt/β-catenin response elements by p63 (TP63)

Submitted: TP63 (p63), a member of the tumor suppressor TP53 (p53) gene family, is expressed in keratinocyte stem cells and well-differentiated squamous cell carcinomas to maintain cellular potential for growth and differentiation. Controversially, activation of the Wnt/β-catenin signaling by p63 (Patturajan M. et al., 2002, Cancer Cells) and inhibition of the target gene expression (Drewelus I. et al., 2010, Cell Cycle) have been reported. Upon p63 RNA-silencing in squamous cell carcinoma (SCC) lines, a few Wnt target gene expression substantially increased, while several target genes moderately decreased. Although ΔNp63α, the most abundant isoform of p63, appeared to interact with protein phosphatase PP2A, neither GSK-3β phosphorylation nor β-catenin nuclear localization was altered by the loss of p63. As reported earlier, ΔNp63α enhanced β-catenin-dependent luc gene expression from pGL3-OT having 3 artificial Wnt response elements (WREs). However, this activation was detectable only in HEK293 cells examined so far, and involved a p53 family-related sequence 5' to the WREs. In Wnt3-expressing SAOS-2 cells, ΔNp63α rather strongly inhibited transcription of pGL3-OT. Importantly, ΔNp63α repressed WREs isolated from the regulatory regions of MMP7. ΔNp63α-TCF4 association occurred in their soluble forms in the nucleus. Furthermore, p63 and TCF4 coexisted at a WRE of MMP7 on the chromatin, where β-catenin recruitment was attenuated. The combined results indicate that ΔNp63α serves as a repressor that regulates β-catenin-mediated gene expression.

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.

Identification of a Functional Risk Variant for Pemphigus Vulgaris in the ST18 Gene

Pemphigus vulgaris (PV) is a life-threatening autoimmune mucocutaneous blistering disease caused by disruption of intercellular adhesion due to auto-antibodies directed against epithelial components. Treatment is limited to immunosuppressive agents, which are associated with serious adverse effects. The propensity to develop the disease is in part genetically determined. We therefore reasoned that the delineation of PV genetic basis may point to novel therapeutic strategies. Using a genome-wide association approach, we recently found that genetic variants in the vicinity of the ST18 gene confer a significant risk for the disease. Here, using targeted deep sequencing, we identified a PV-associated variant residing within the ST18 promoter region (p<0.0002; odds ratio = 2.03). This variant was found to drive increased gene transcription in a p53/p63-dependent manner, which may explain the fact that ST18 is up-regulated in the skin of PV patients. We then discovered that when overexpressed, ST18 stimulates PV serum-induced secretion of key inflammatory molecules and contributes to PV serum-induced disruption of keratinocyte cell-cell adhesion, two processes previously implicated in the pathogenesis of PV. Thus, the present findings indicate that ST18 may play a direct role in PV and consequently represents a potential target for the treatment of this disease.

A/T gap tolerance in the core sequence and flanking sequence requirements of non-canonical p53 response elements

The canonical core sequence of the p53 response element, CATG, has a two-base A/T gap. Previously, we found that p53 can also activate a non-canonical four-base A/T gap CATATG core sequence. In this study, we investigated the possible number of A/T bases used by p53 and showed that a six-base A/T gap CATATATG core sequence was the maximum A/T gap in the p53 response element that could be upregulated by p53 and p63. Canonical and non-canonical p53 response elements also have three-base flanking sequences. A/T bases could be substituted by G/C bases, including CACACG and CGTGTG, but not CGCGCG. We found that the SV40 promoter with functional six- and two-base A/T gap core sequences could be activated by TAp63γ and that TAp63γ could upregulate SV40 small and large T antigens expression in COS7 cells. We also found that the distal region of PUMA promoter with functional two six-base A/T gap core sequences could be activated by TAp63γ in 293T cells. These new findings could provide novel rules for the non-canonical p53 family response element and could extend the entire p53 family regulation network.

Amino-terminal residues of ΔNp63, mutated in ectodermal dysplasia, are required for its transcriptional activity

p63, a member of the p53 family, is a crucial transcription factor for epithelial development and skin homeostasis. Heterozygous mutations in TP63 gene have been associated with human ectodermal dysplasia disorders. Most of these TP63 mutations are missense mutations causing amino acidic substitutions at p63 DNA binding or SAM domains that reduce or abolish the transcriptional activity of mutants p63. A significant number of mutants, however, resides in part of the p63 protein that apparently do not affect DNA binding and/or transcriptional activity, such as the N-terminal domain. Here, we characterize five p63 mutations at the 5' end of TP63 gene aiming to understand the pathogenesis of the diseases and to uncover the role of ΔNp63α N-terminus residues in determining its transactivation potential.

Maintaining epithelial stemness with p63

In stratified epithelial and glandular tissues, homeostasis relies on the self-renewing capacity of stem cells, which are within the basal layer. The p53 family member p63 is an indispensable transcription factor for epithelial morphogenesis and stemness. A splice variant of the transcription factor p63 that lacks an amino-terminal domain, ΔNp63, is selectively found in the basal compartments of several ectoderm-derived tissues such as stratified and glandular epithelia, in which it is required for the replenishment of stem cells. Thus far, the transcriptional programs downstream of p63 in stemness regulation remain incompletely defined. Unveiling the molecular basis of stem cell self-renewal may be relevant in understanding how this process may contribute to cancer development. In this review, we specifically highlight experimental investigations, which suggest that p63 is a marker of normal epithelial stem cells and describe p63 transcriptional targets that may be involved in stemness regulation. Finally, we discuss relevant findings implicating p63 in epithelial cancer stem cell biology.

The microRNA feedback regulation of p63 in cancer progression

The transcription factor p63 is a member of the p53 gene family that plays a complex role in cancer due to its involvement in epithelial differentiation, cell cycle arrest and apoptosis. MicroRNAs are a class of small, non-coding RNAs with an important regulatory role in various cellular processes, as well as in the development and progression of cancer. A number of microRNAs have been shown to function as transcriptional targets of p63. Conversely, microRNAs also can modulate the expression and activity of p63. However, the p63-microRNA regulatory circuit has not been addressed in depth so far. Here, computational genomic analysis was performed using miRtarBase, Targetscan, microRNA.ORG, DIANA-MICROT, RNA22-HSA and miRDB to analyze miRNA binding to the 3'UTR of p63. JASPAR (profile score threshold 80%) and TFSEARCH datasets were used to search transcriptional start sites for p53/p63 response elements. Remarkably, these data revealed 63 microRNAs that targeted p63. Furthermore, there were 39 microRNAs targeting p63 that were predicted to be regulated by p63. These analyses suggest a crosstalk between p63 and microRNAs. Here, we discuss the crosstalk between p63 and the microRNA network, and the role of their interactions in cancer.

The role of P-cadherin in skin biology and skin pathology: lessons from the hair follicle

Adherens junctions (AJs) are one of the major intercellular junctions in various epithelia including the epidermis and the follicular epithelium. AJs connect the cell surface to the actin cytoskeleton and comprise classic transmembrane cadherins, such as P-cadherin, armadillo family proteins, and actin microfilaments. Loss-of-function mutations in CDH3, which encodes P-cadherin, result in two allelic autosomal recessive disorders: hypotrichosis with juvenile macular dystrophy (HJMD) and ectodermal dysplasia, ectrodactyly, and macular dystrophy (EEM) syndromes. Both syndromes feature sparse hair heralding progressive macular dystrophy. EEM syndrome is characterized in addition by ectodermal and limb defects. Recent studies have demonstrated that, together with its involvement in cell-cell adhesion, P-cadherin plays a crucial role in regulating cell signaling, malignant transformation, and other major intercellular processes. Here, we review the roles of P-cadherin in skin and hair biology, with emphasize on human hair growth, cycling and pigmentation.

∆N-P63α and TA-P63α exhibit intrinsic differences in transactivation specificities that depend on distinct features of DNA target sites

TP63 is a member of the TP53 gene family that encodes for up to ten different TA and ΔN isoforms through alternative promoter usage and alternative splicing. Besides being a master regulator of gene expression for squamous epithelial proliferation, differentiation and maintenance, P63, through differential expression of its isoforms, plays important roles in tumorigenesis. All P63 isoforms share an immunoglobulin-like folded DNA binding domain responsible for binding to sequence-specific response elements (REs), whose overall consensus sequence is similar to that of the canonical p53 RE. Using a defined assay in yeast, where P63 isoforms and RE sequences are the only variables, and gene expression assays in human cell lines, we demonstrated that human TA- and ΔN-P63α proteins exhibited differences in transactivation specificity not observed with the corresponding P73 or P53 protein isoforms. These differences 1) were dependent on specific features of the RE sequence, 2) could be related to intrinsic differences in their oligomeric state and cooperative DNA binding, and 3) appeared to be conserved in evolution. Since genotoxic stress can change relative ratio of TA- and ΔN-P63α protein levels, the different transactivation specificity of each P63 isoform could potentially influence cellular responses to specific stresses.

Efficient one-pot synthesis of trans-Pt(ii)(salicylaldimine)(4-picoline)Cl complexes: effective agents for enhanced expression of p53 tumor suppressor genes

One-pot synthesizedtrans-Pt(ii)(salicylaldimine)(4-picoline)Cl complexes showed promisingin vitrocytotoxicity in MCF-7 and A549 cancer cell lines.

Studying p53 family proteins in yeast: induction of autophagic cell death and modulation by interactors and small molecules

In this work, the yeast Saccharomyces cerevisiae was used to individually study human p53, p63 (full length and truncated forms) and p73. Using this cell system, the effect of these proteins on cell proliferation and death, and the influence of MDM2 and MDMX on their activities were analyzed. When expressed in yeast, wild-type p53, TAp63, ΔNp63 and TAp73 induced growth inhibition associated with S-phase cell cycle arrest. This growth inhibition was accompanied by reactive oxygen species production and autophagic cell death. Furthermore, they stimulated rapamycin-induced autophagy. On the contrary, none of the tested p53 family members induced apoptosis either per se or after apoptotic stimuli. As previously reported for p53, also TAp63, ΔNp63 and TAp73 increased actin expression levels and its depolarization, suggesting that ACT1 is also a p63 and p73 putative yeast target gene. Additionally, MDM2 and MDMX inhibited the activity of all tested p53 family members in yeast, although the effect was weaker on TAp63. Moreover, Nutlin-3a and SJ-172550 were identified as potential inhibitors of the p73 interaction with MDM2 and MDMX, respectively. Altogether, the yeast-based assays herein developed can be envisaged as a simplified cell system to study the involvement of p53 family members in autophagy, the modulation of their activities by specific interactors (MDM2 and MDMX), and the potential of new small molecules to modulate these interactions.

Tumor suppression in skin and other tissues via cross-talk between vitamin D- and p53-signaling

P53 and its family members have been implicated in the direct regulation of the vitamin D receptor (VDR). Vitamin D- and p53-signaling pathways have a significant impact on spontaneous or carcinogen-induced malignant transformation of cells, with VDR and p53 representing important tumor suppressors. VDR and the p53/p63/p73 proteins all function typically as receptors or sensors that turn into transcriptional regulators upon stimulus, with the main difference being that the nuclear VDR is activated as a transcription factor after binding its naturally occurring ligand 1,25-dihydroxyvitamin D with high affinity while the p53 family of transcription factors, mostly in the nucleoplasm, responds to a large number of alterations in cell homeostasis commonly referred to as stress. An increasing body of evidence now convincingly demonstrates a cross-talk between vitamin D- and p53-signaling that occurs at different levels, has genome-wide implications and that should be of high importance for many malignancies, including non-melanoma skin cancer. One interaction involves the ability of p53 to increase skin pigmentation via POMC derivatives including alpha-MSH and ACTH. Pigmentation protects the skin against UV-induced DNA damage and skin carcinogenesis, yet on the other hand reduces cutaneous synthesis of vitamin D. A second level of interaction may be through the ability of 1,25-dihydroxyvitamin D to increase the survival of skin cells after UV irradiation. UV irradiation-surviving cells show significant reductions in thymine dimers in the presence of 1,25-dihydroxyvitamin D that are associated with increased nuclear p53 protein expression, and significantly reduced NO products. A third level of interaction is documented by the ability of vitamin D compounds to regulate the expression of the murine double minute 2 (MDM2) gene in dependence of the presence of wild-type p53. MDM2 has a well-established role as a key negative regulator of p53 activity. Finally, p53 and family members have been implicated in the direct regulation of VDR. This overview summarizes some of the implications of the cross-talk between vitamin D- and p53-signaling for carcinogenesis in the skin and other tissues.

Alterations of p63 and p73 in human cancers

p53 and its related genes, p63 and p73 constitute the p53 gene family. While p53 is the most frequently mutated gene in human tumors, p63 and p73 are rarely mutated or deleted in cancers. Many studies have reported p63/p73 overexpression in human cancers while others showed that a loss of p63/p73 is associated with tumor progression and metastasis. Thus, whether p63 or p73 is a tumor suppressor gene or an oncogene has been a matter of debate. This controversy has been attributed to the existence of multiple splicing isoforms with distinct functions; the full-length TA isoform of p63 has structural and functional similarity to wild-type p53, whereas the ΔNp63 acts primarily in dominant-negative fashion against all family members of p53. Differential activities of TA and ΔN isoforms have been shown in vivo by creating isform-specific gene knockout mice. All p53, p63, p73 proteins bind to and activate target genes with p53-response elements; p63 also binds to distinct p63-response elements and regulate expression of specific target genes involved in skin, limb, and craniofacial development. Interestingly, several studies have shown that both p63 and p73 are involved in cellular response to cancer therapy and others have indicated that both of these molecules are required for p53-induced apoptosis, suggesting functional interplay among p53 family proteins. Consistent with these findings, aberrant splicing that result in ΔNp63 or ΔNp73 overexpression are frequently found in human cancers, and is associated with poor clinical outcomes of patients in the latter. Thus immunohistochemical staining of tumor specimen with ΔNp73-specific antibody might have diagnostic values in cancer clinics.

p53: its mutations and their impact on transcription

p53 is a tumor suppressor protein whose key function is to maintain the integrity of the cell. Mutations in p53 have been found in up to 50 % of all human cancers and cause an increase in oncogenic phenotypes such as proliferation and tumorigenicity. Both wild-type and mutant p53 have been shown to transactivate their target genes, either through directly binding to DNA, or indirectly through protein-protein interactions. This review discusses possible mechanisms behind both wild-type and mutant p53-mediated transactivation and touches on the concept of addiction to mutant p53 of cancer cells and how that may be used for future therapies.

Growth differentiating factor 15 enhances the tumor-initiating and self-renewal potential of multiple myeloma cells

Disease relapse remains a major factor limiting the survival of cancer patients. In the plasma cell malignancy multiple myeloma (MM), nearly all patients ultimately succumb to disease relapse and progression despite new therapies that have improved remission rates. Tumor regrowth indicates that clonogenic growth potential is continually maintained, but the determinants of self-renewal in MM are not well understood. Normal stem cells are regulated by extrinsic niche factors, and the tumor microenvironment (TME) may similarly influence tumor cell clonogenic growth and self-renewal. Growth differentiation factor 15 (GDF15) is aberrantly secreted by bone marrow stromal cells (BMSCs) in MM. We found that GDF15 is produced by BMSCs after direct contact with plasma cells and enhances the tumor-initiating potential and self-renewal of MM cells in a protein kinase B- and SRY (sex-determining region Y)-box-dependent manner. Moreover, GDF15 induces the expansion of MM tumor-initiating cells (TICs), and changes in the serum levels of GDF15 were associated with changes in the frequency of clonogenic MM cells and the progression-free survival of MM patients. These findings demonstrate that GDF15 plays a critical role in mediating the interaction among mature tumor cells, the TME, and TICs, and strategies targeting GDF15 may affect long-term clinical outcomes in MM.

Human GGT2 does not autocleave into a functional enzyme: A cautionary tale for interpretation of microarray data on redox signaling

AIMS

Human γ-glutamyltranspeptidase 1 (hGGT1) is a cell-surface enzyme that is a regulator of redox adaptation and drug resistance due to its glutathionase activity. The human GGT2 gene encodes a protein that is 94% identical to the amino-acid sequence of hGGT1. Transcriptional profiling analyses in a series of recent publications have implicated the hGGT2 enzyme as a modulator of disease processes. However, hGGT2 has never been shown to encode a protein with enzymatic activity. The aim of this study was to express the protein encoded by hGGT2 and each of its known variants and to assess their stability, cellular localization, and enzymatic activity.

RESULTS

We discovered that the proteins encoded by hGGT2 and its variants are inactive propeptides. We show that hGGT2 cDNAs are transcribed with a similar efficiency to hGGT1, and the expressed propeptides are N-glycosylated. However, they do not autocleave into heterodimers, fail to localize to the plasma membrane, and do not metabolize γ-glutamyl substrates. Substituting the coding sequence of hGGT1 to conform to alterations in a CX3C motif encoded by hGGT2 mRNAs disrupted autocleavage of the hGGT1 propeptide into a heterodimer, resulting in loss of plasma membrane localization and catalytic activity.

INNOVATION AND CONCLUSIONS

This is the first study to evaluate hGGT2 protein. The data show that hGGT2 does not encode a functional enzyme. Microarray data which have reported induction of hGGT2 mRNA should not be interpreted as induction of a protein that has a role in the metabolism of extracellular glutathione and in maintaining the redox status of the cell.

Transactivation specificity is conserved among p53 family proteins and depends on a response element sequence code

Structural and biochemical studies have demonstrated that p73, p63 and p53 recognize DNA with identical amino acids and similar binding affinity. Here, measuring transactivation activity for a large number of response elements (REs) in yeast and human cell lines, we show that p53 family proteins also have overlapping transactivation profiles. We identified mutations at conserved amino acids of loops L1 and L3 in the DNA-binding domain that tune the transactivation potential nearly equally in p73, p63 and p53. For example, the mutant S139F in p73 has higher transactivation potential towards selected REs, enhanced DNA-binding cooperativity in vitro and a flexible loop L1 as seen in the crystal structure of the protein–DNA complex. By studying, how variations in the RE sequence affect transactivation specificity, we discovered a RE-transactivation code that predicts enhanced transactivation; this correlation is stronger for promoters of genes associated with apoptosis.

P53 family members modulate the expression of PRODH, but not PRODH2, via intronic p53 response elements

The tumor suppressor p53 was previously shown to markedly up-regulate the expression of the PRODH gene, encoding the proline dehydrogenase (PRODH) enzyme, which catalyzes the first step in proline degradation. Also PRODH2, which degrades 4-hydroxy-L-proline, a product of protein (e.g. collagen) catabolism, was recently described as a p53 target. Here, we confirmed p53-dependent induction of endogenous PRODH in response to genotoxic damage in cell lines of different histological origin. We established that over-expression of TAp73β or TAp63β is sufficient to induce PRODH expression in p53-null cells and that PRODH expression parallels the modulation of endogenous p73 by genotoxic drugs in several cell lines. The p53, p63, and p73-dependent transcriptional activation was linked to specific intronic response elements (REs), among those predicted by bioinformatics tools and experimentally validated by a yeast-based transactivation assay. p53 occupancy measurements were validated in HCT116 and MCF7 human cell lines. Conversely, PRODH2 was not responsive to p63 nor p73 and, at best, could be considered a weak p53 target. In fact, minimal levels of PRODH2 transcript induction by genotoxic stress was observed exclusively in one of four p53 wild-type cell lines tested. Consistently, all predicted p53 REs in PRODH2 were poor matches to the p53 RE consensus and showed very weak responsiveness, only to p53, in the functional assay. Taken together, our results highlight that PRODH, but not PRODH2, expression is under the control of p53 family members, specifically p53 and p73. This supports a deeper link between proteins of the p53-family and metabolic pathways, as PRODH modulates the balance of proline and glutamate levels and those of their derivative alpha-keto-glutarate (α-KG) under normal and pathological (tumor) conditions.

Identification of human guanylate-binding protein 1 gene (hGBP1) as a direct transcriptional target gene of p53

Human guanylate-binding protein 1 (hGBP1) plays an important role in antitumor and antiviral immune responses. Here, we show that tumor suppressor p53 positively regulated hGBP1 transcription via binding to the p53 response element (p53RE) present in the hGBP1 promoter region. p53 activation by 5-fluorouracil significantly increased hGBP1 expression in wild-type p53 cells, but not in p53-null cells. Knockdown of p53 expression remarkably impaired hGBP1 expression induced by 5-fluorouracil, type I interferon treatment, or influenza A virus infection. Among three deductive p53REs present in the hGBP1 promoter region, two p53REs were found to be transactivated by p53.

Delineating Molecular Mechanisms of Squamous Tissue Homeostasis and Neoplasia: Focus on p63

Mouse models have informed us that p63 is critical for normal epidermal development and homeostasis. The p53/p63/p73 family is expressed as multiple protein isoforms due to a combination of alternative promoter usage and C-terminal alternative splicing. These isoforms can mimic or interfere with one another, and their balance ultimately determines biological outcome in a context-dependent manner. While not frequently mutated, p63, and in particular the ΔNp63 subclass, is commonly overexpressed in human squamous cell cancers. In vitro keratinocytes and murine transgenic and transplantation models have been invaluable in elucidating the contribution of altered p63 levels to cancer development, and studies have identified the roles for ΔNp63 isoforms in keratinocyte survival and malignant progression, likely due in part to their transcriptional regulatory function. These findings can be extended to human cancers; for example, the novel recognition of NFκB/c-Rel as a downstream effector of p63 has identified a role for NFκB/c-Rel in human squamous cell cancers. These models will be critical in enhancing the understanding of the specific molecular mechanisms of cancer development and progression.

GJB6, of which mutations underlie Clouston syndrome, is a potential direct target gene of p63

BACKGROUND

Clouston syndrome is a rare autosomal dominant condition characterized by hypotrichosis, nail dystrophy, and occasionally palmoplantar keratoderma. The disease is caused by mutations in GJB6 gene, which encodes a gap junction protein connexin 30 (Cx30).

OBJECTIVE

To disclose the molecular basis of Clouston syndrome in a Lebanese-German family, and also to determine precise expression of Cx30 in normal skin of humans and mice, as well as transcriptional regulation for the GJB6 expression.

METHODS

We searched for mutations in the GJB6 gene using DNA of the family members with Clouston syndrome. We performed immunostaining to localize the Cx30 expression in normal human skin and mouse embryos. In addition, we did a series of in vitro studies to investigate if the GJB6 could be a direct transcriptional target gene of p63.

RESULTS

We identified a recurrent heterozygous mutation c.31G>C (p.Gly11Arg) in the GJB6 gene in the Lebanese-German family with Clouston syndrome. Immunostaining in normal human skin sections demonstrated predominant expression of Cx30 in hair follicles, nails, and palmoplantar epidermis, which partially overlapped with p63 expression. We also showed co-expression of Cx30 and p63 in developing mouse hair follicles and nail units. In cultured cells, the GJB6 expression was significantly upregulated by ΔNp63α isoform. Further in vitro analyses suggested that ΔNp63α was potentially involved in the GJB6 expression via binding to the sequences in intron 1 of the GJB6 gene.

CONCLUSION

Our data further underscore the crucial roles of Cx30 in morphogenesis and development of skin and its appendages.

Chemosensitivity is controlled by p63 modification with ubiquitin-like protein ISG15

Identification of the cellular mechanisms that mediate cancer cell chemosensitivity is important for developing new cancer treatment strategies. Several chemotherapeutic drugs increase levels of the posttranslational modifier ISG15, which suggests that ISGylation could suppress oncogenesis. However, how ISGylation of specific target proteins controls tumorigenesis is unknown. Here, we identified proteins that are ISGylated in response to chemotherapy. Treatment of a human mammary epithelial cell line with doxorubicin resulted in ISGylation of the p53 family protein p63. An alternative splice variant of p63, ΔNp63α, suppressed the transactivity of other p53 family members, and its expression was abnormally elevated in various human epithelial tumors, suggestive of an oncogenic role for this variant. We showed that ISGylation played an essential role in the downregulation of ΔNp63α. Anticancer drugs, including doxorubicin, induced ΔNp63α ISGylation and caspase-2 activation, leading to cleavage of ISGylated ΔNp63α in the nucleus and subsequent release of its inhibitory domain to the cytoplasm. ISGylation ablated the ability of ΔNp63α to promote anchorage-independent cell growth and tumor formation in vivo as well to suppress the transactivities of proapoptotic p53 family members. These findings establish ISG15 as a tumor suppressor via its conjugation to ΔNp63α and provide a molecular rationale for therapeutic use of doxorubicin against ΔNp63α-mediated cancers.