Structural basis for gene activation by p53 family members

Dichotomous transactivation domains contribute to growth inhibitory and promotion functions of TAp73

The transcription factor p73, a member of the p53 tumor-suppressor family, regulates cell death and also supports tumorigenesis, although the mechanistic basis for the dichotomous functions is poorly understood. We report here the identification of an alternate transactivation domain (TAD) located at the extreme carboxyl (C) terminus of TAp73β, a commonly expressed p73 isoform. Mutational disruption of this TAD significantly reduced TAp73β's transactivation activity, to a level observed when the amino (N)-TAD that is similar to p53's TAD, is mutated. Mutation of both TADs almost completely abolished TAp73β's transactivation activity. Expression profiling highlighted a unique set of targets involved in extracellular matrix-receptor interaction and focal adhesion regulated by the C-TAD, resulting in FAK phosphorylation, distinct from the N-TAD targets that are common to p53 and are involved in growth inhibition. Interestingly, the C-TAD targets are also regulated by the oncogenic, amino-terminal-deficient DNp73β isoform. Consistently, mutation of C-TAD reduces cellular migration and proliferation. Mechanistically, selective binding of TAp73β to DNAJA1 is required for the transactivation of C-TAD target genes, and silencing DNAJA1 expression abrogated all C-TAD-mediated effects. Taken together, our results provide a mechanistic basis for the dichotomous functions of TAp73 in the regulation of cellular growth through its distinct TADs.

p53/MDM2 signaling pathway in aging, senescence and tumorigenesis

A wealth of evidence has emerged that there is an association between aging, senescence and tumorigenesis. Senescence, a biological process by which cells cease to divide and enter a status of permanent cell cycle arrest, contributes to aging and aging-related diseases, including cancer. Aging populations have the higher incidence of cancer due to a lifetime of exposure to cancer-causing agents, reduction of repairing DNA damage, accumulated genetic mutations, and decreased immune system efficiency. Cancer patients undergoing cytotoxic therapies, such as chemotherapy and radiotherapy, accelerate aging. There is growing evidence that p53/MDM2 (murine double minute 2) axis is critically involved in regulation of aging, senescence and oncogenesis. Therefore, in this review, we describe the functions and mechanisms of p53/MDM2-mediated senescence, aging and carcinogenesis. Moreover, we highlight the small molecular inhibitors, natural compounds and PROTACs (proteolysis targeting chimeras) that target p53/MDM2 pathway to influence aging and cancer. Modification of p53/MDM2 could be a potential strategy for treatment of aging, senescence and tumorigenesis.

Extracellular and intracellular functions of coiled-coil domain containing 3

Coiled-coil domain containing 3 (CCDC3, also called Favine) is a highly conserved protein initially identified as a protein secreted from adipocytes and endothelial cells in the vascular system with endocrine-like functions. Recently, CCDC3 was also found to function as a nuclear tumor suppressor in breast cancers. Although it is still understudied, CCDC3, since its discovery, has been shown to play multiple roles in lipid metabolism, fatty liver, abdominal obesity, anti-inflammation, atherosclerosis, and cancer. This essay is thus composed to offer an overview of these extracellular endocrine-like and intracellular (nuclear) functions of CCDC3. We also discuss the possible underlying cellular and molecular mechanisms of CCDC3, the implications for clinical translation, and the remaining puzzles about this special molecule.

Phylogenetic analysis of the MCL1 BH3 binding groove and rBH3 sequence motifs in the p53 and INK4 protein families

B-cell lymphoma 2 (Bcl-2) proteins are central, conserved regulators of apoptosis. Bcl-2 family function is regulated by binding interactions between the Bcl-2 homology 3 (BH3) motif in pro-apoptotic family members and the BH3 binding groove found in both the pro-apoptotic effector and anti-apoptotic Bcl-2 family members. A novel motif, the reverse BH3 (rBH3), has been shown to interact with the anti-apoptotic Bcl-2 homolog MCL1 (Myeloid cell leukemia 1) and have been identified in the p53 homolog p73, and the CDK4/6 (cyclin dependent kinase 4/6) inhibitor p18INK4c, (p18, cyclin-dependent kinase 4 inhibitor c). To determine the conservation of rBH3 motif, we first assessed conservation of MCL1's BH3 binding groove, where the motif binds. We then constructed neighbor-joining phylogenetic trees of the INK4 and p53 protein families and analyzed sequence conservation using sequence logos of the rBH3 locus. This showed the rBH3 motif is conserved throughout jawed vertebrates p63 and p73 sequences and in chondrichthyans, amphibians, mammals, and some reptiles in p18. Finally, a potential rBH3 motif was identified in mammalian and osteichthyan p19INK4d (p19, cyclin dependent kinase 4 inhibitor d). These findings demonstrate that the interaction between MCL1 and other cellular proteins mediated by the rBH3 motif may be conserved throughout jawed vertebrates.

P63 and P73 Activation in Cancers with p53 Mutation

The members of the p53 family comprise p53, p63, and p73, and full-length isoforms of the p53 family have a tumor suppressor function. However, p53, but not p63 or p73, has a high mutation rate in cancers causing it to lose its tumor suppressor function. The top and second-most prevalent p53 mutations are missense and nonsense mutations, respectively. In this review, we discuss possible drug therapies for nonsense mutation and a missense mutation in p53. p63 and p73 activators may be able to replace mutant p53 and act as anti-cancer drugs. Herein, these p63 and p73 activators are summarized and how to improve these activator responses, particularly focusing on p53 gain-of-function mutants, is discussed.

Context-dependent activation of p53 target genes and induction of apoptosis by actinomycin D in aerodigestive tract cancers

Actinomycin D (ActD) was the first anticancer antibiotic approved for the management of human cancers. However, the notorious toxicity profile limits its widespread application in cancers, including cancers of the aerodigestive tract. Recent studies show that combining low-dose ActD with existing chemotherapies could potentially protect normal cells from the toxicity of chemotherapy drugs through p53 activation (cyclotherapy). An understanding of ActD's effect on p53 signaling is critical for the meaningful application of ActD in cyclotherapy-based combinations. This study evaluated the anti-tumor efficacy and mechanism of action of ActD in aerodigestive tract cancers. We found that ActD strongly inhibited the growth of a panel of aerodigestive tract cancer cell lines and induced efficient apoptosis, although the sensitivity varies among cell lines. The IC₅₀ values of ActD spanned between 0.021 and 2.96 nM. Mechanistic studies revealed that ActD increased the expression of total and phosphorylated p53 (ser15) in a time- and dose-dependent manner. Moreover, ActD-induced apoptosis is dependent on p53 in cells expressing wild-type p53 and that ActD induced context-dependent differential expression of downstream targets p21 and PUMA without significant effects on p27. In the final analysis, this study revealed that p53-p21 is the predominant pathway activated by low-dose ActD, supporting further development of ActD in cyclotherapy.

Deciphering the mechanism of p73 recognition of p53 response elements using the crystal structure of p73-DNA complexes and computational studies

P73 belongs to p53 family transcription factor activating more than 50% of cell fate p53 target genes involved in cell cycle, apoptosis, DNA damage response alongside neuronal system development and differentiation by binding to 20-bp response elements (REs) having sequence motif (PPPCA/T-T/AGYYY) where P-purines and Y-pyrimidines with each 10-bp separated by minimum 0 to 13-bp spacer. The promiscuous nature of recognizing both cell fate and development genes and the underlying RE selectivity mechanism by p73 is not well understood. Here, we report the molecular details of p73 recognizing the REs using the crystal structure of p73 DNA binding domain (DBD) in complex with 12 base pair DNA sequence 5'-cAGGCATGCCTg-3' and molecular dynamics simulations with six different p53 natural promoter sequences. Each 20-base pair natural promoter forms a different major/minor groove due to the presence of nucleotides A/T, A/C, G/G, T/T and G/T at positions 3, 8, 13, 18 uniquely recognized by p73 key residues Lys138 and Arg268. The loops L1 and L3 bearing these residues influence inter-and intra-dimer interfaces interactions and hence p73 forms a unique tetramer with each natural promoter sequence. Structural features of the DNA and the spacing between half-sites influence p73 tetramerization and its transactivation function.

Inhibition of tumor suppressor p73 by nerve growth factor receptor via chaperone-mediated autophagy

The tumor suppressr p73 is a homolog of p53 and is capable of inducing cell cycle arrest and apoptosis. Here, we identify nerve growth factor receptor (NGFR, p75NTR, or CD271) as a novel negative p73 regulator. p73 activates NGFR transcription, which, in turn, promotes p73 degradation in a negative feedback loop. NGFR directly binds to p73 central DNA-binding domain and suppresses p73 transcriptional activity as well as p73-mediated apoptosis in cancer cells. Surprisingly, we uncover a previously unknown mechanism of NGFR-facilitated p73 degradation through the chaperone-mediated autophagy (CMA) pathway. Collectively, our studies demonstrate a new oncogenic function for NGFR in inactivating p73 activity by promoting its degradation through the CMA.

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

Simple Summary

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

Abstract

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

Distinct p63 and p73 Protein Interactions Predict Specific Functions in mRNA Splicing and Polyploidy Control in Epithelia

Epithelial organs are the first barrier against microorganisms and genotoxic stress, in which the p53 family members p63 and p73 have both overlapping and distinct functions. Intriguingly, p73 displays a very specific localization to basal epithelial cells in human tissues, while p63 is expressed in both basal and differentiated cells. Here, we analyse systematically the literature describing p63 and p73 protein-protein interactions to reveal distinct functions underlying the aforementioned distribution. We have found that p73 and p63 cooperate in the genome stability surveillance in proliferating cells; p73 specific interactors contribute to the transcriptional repression, anaphase promoting complex and spindle assembly checkpoint, whereas p63 specific interactors play roles in the regulation of mRNA processing and splicing in both proliferating and differentiated cells. Our analysis reveals the diversification of the RNA and DNA specific functions within the p53 family.

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

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

The Emerging Landscape of p53 Isoforms in Physiology, Cancer and Degenerative Diseases

p53, first described four decades ago, is now established as a master regulator of cellular stress response, the “guardian of the genome”. p53 contributes to biological robustness by behaving in a cellular-context dependent manner, influenced by several factors (e.g., cell type, active signalling pathways, the type, extent and intensity of cellular damage, cell cycle stage, nutrient availability, immune function). The p53 isoforms regulate gene transcription and protein expression in response to the stimuli so that the cell response is precisely tuned to the cell signals and cell context. Twelve isoforms of p53 have been described in humans. In this review, we explore the interactions between p53 isoforms and other proteins contributing to their established cellular functions, which can be both tumour-suppressive and oncogenic in nature. Evidence of p53 isoform in human cancers is largely based on RT-qPCR expression studies, usually investigating a particular type of isoform. Beyond p53 isoform functions in cancer, it is implicated in neurodegeneration, embryological development, progeroid phenotype, inflammatory pathology, infections and tissue regeneration, which are described in this review.

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.

Anti-cancer targets of formononetin and molecular mechanisms in osteosarcoma: Findings of bioinformatic and experimental assays

In current study, a bioinformatic‐based network pharmacology was used to identify the osteosarcoma (OGS)‐pathological targets and formononetin (FN)‐treated targets before the main core predictive biotargets were screened. In addition, all core targets were selected through a number of bioinformatic databases, followed by identification of predominant biological processes and signalling pathways of FN anti‐OGS. Further, top three core targets of FN anti‐OGS were determined as oestrogen receptor 1 (ESR1), tumour protein p53 (TP53), receptor tyrosine‐protein kinase erbB‐2 (ERBB2) respectively. In clinical biochemical data, the plasma samples of OGS showed the increased trends of alkaline phosphatase, triglyceride, blood glucose, lactate dehydrogenase, high‐sensitive C‐reactive protein and some immune cell counts when referenced to medical criteria. In clinicopathological examination, histological OGS sections resulted in increased positive cell counts of neoplastic ESR1, TP53, ERBB2. To further validate these corn proteins in experimental study in vivo, FN‐treated tumour‐bearing nude mice showed intracellular reductions of ESR1, TP53, ERBB2 positive expressions, accompanied with visibly reduced tumour weights. Collectively, our bioinformatic and experimental findings disclosed main core targets, biological processes and signalling pathways of FN anti‐OGS. Interestingly, the top core targets were representatively validated following FN treatment in vivo. Therefore, we reasoned that these predictive targets might be the potential biomarkers for screening and treating osteosarcoma.

p53 Phosphomimetics Preserve Transient Secondary Structure but Reduce Binding to Mdm2 and MdmX

The disordered p53 transactivation domain (p53TAD) contains specific levels of transient helical secondary structure that are necessary for its binding to the negative regulators, mouse double minute 2 (Mdm2) and MdmX. The interactions of p53 with Mdm2 and MdmX are also modulated by posttranslational modifications (PTMs) of p53TAD including phosphorylation at S15, T18 and S20 that inhibits p53-Mdm2 binding. It is unclear whether the levels of transient secondary structure in p53TAD are changed by phosphorylation or other PTMs. We used phosphomimetic mutants to determine if adding a negative charge at positions 15 and 18 has any effect on the transient secondary structure of p53TAD and protein-protein binding. Using a combination of biophysical and structural methods, we investigated the effects of single and multisite phosphomimetics on the transient secondary structure of p53TAD and its interaction with Mdm2, MdmX, and the KIX domain. The phosphomimetics reduced Mdm2 and MdmX binding affinity by 3–5-fold, but resulted in minimal changes in transient secondary structure, suggesting that the destabilizing effect of phosphorylation on the p53TAD-Mdm2 interaction is primarily electrostatic. Phosphomimetics had no effect on the p53-KIX interaction, suggesting that increased binding of phosphorylated p53 to KIX may be influenced by decreased competition with its negative regulators.

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.

Differential effects of diverse p53 isoforms on TAp73 transcriptional activity and apoptosis

The p53 activities are due, at least in part, to its ability to form oligomers that bind to specific DNA sequences and activate transcription. Since some mutant p53 proteins and ΔNp73 isoforms form heterocomplexes with TAp73, we asked whether p53 isoforms can do the same and potentially act as dominant-negative inhibitors of TAp73. Moreover, it has already been found that some isoforms form complex with wtp53 and some of them inhibit p53 tumor-suppressor functions. Therefore, we studied the complex formation and co-immunoprecipitation assays show that all six p53 isoforms examined can form complexes with TAp73β, whereas only Δ133p53α/β/γ isoforms form complex with TAp73α. All p53 isoforms counteract TAp73β transactivation function but with different efficiency and in a promoter-dependent manner. Furthermore, apoptotic activity of TAp73β was augmented by coexpression of p53β, whereas Δ133p53α and β inhibit its apoptotic activity most efficiently. We have determined the half-life of different p53 isoforms: p53γ isoform has the shortest half-life, whereas Δ133p53γ has the longest half-life. Inhibitory interactions of two proteins in complex often lead to their stabilization. However, only three isoforms (Δ133p53α, Δ133p53β and Δ40p53α) stabilize TAp73β. We are convinced that defining the interactions between p53/p73 would give a new insight into how the p53 isoforms modulate the p73 functions in tumorigenesis.

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

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

The evolution of MDM2 family genes

MDM2 and MDM4 are proto-oncoproteins that bind to and inhibit members of the p53 protein family, p53, p73 and possibly p63. p53 is a mammalian tumor suppressor and p63 and p73 are critical for development. With the sequencing of genomes from multiple organisms there is mounting evidence for a consensus scenario of p53 gene family evolution. A single p53/p63/p73 gene is in invertebrates and required for maintenance of germline DNA. Gene duplication occurred in an ancestor in common with cartilaginous fishes, giving rise to a separate p53 gene and at least one ancestral p63/p73 gene. In bony vertebrates, all three p53 gene family paralogs, p53, p63, and p73 are distinct genes. This raises the question of how MDM2 and MDM4 genes evolved. We show evidence that MDM2 and MDM4 arose from a gene duplication event prior to the emergence of bony vertebrates more than 440 millionyears ago. Comparative genome studies indicate that invertebrate organisms have only one MDM homolog. In jawed vertebrates, the p53-binding domains of MDM2 and MDM4 proteins evolved at a high rate, approaching the evolution rate of the MDM2-binding domain of p53. However, the MDM2-binding domain of p73 exhibits markedly stronger conservation suggesting novel p53-independent functions. The most conserved domain within all MDM2 family members is the RING domain of the MDM2 ortholog which is responsible for ubiquitination of p53 and heterodimerization with MDM4. We suggest a model where oligomerization is an ancient function of MDM and ubiquitination activity was acquired later near the MDM gene duplication event coinciding with the time of the emergence of p53 as a distinct gene.

The p53 family: guardians of maternal reproduction

The p53 family of proteins consists of p53, p63 and p73, which are transcription factors that affect both cancer and development. It is now emerging that these proteins also regulate maternal reproduction. Whereas p63 is important for maturation of the egg, p73 ensures normal mitosis in the developing blastocyst. p53 subsequently regulates implantation of the embryo through transcriptional control of leukaemia inhibitory factor. Elucidating the cell biological basis of how these factors regulate female fertility may lead to new approaches to the control of human maternal reproduction.

Interaction of TAp73 and breast cancer-associated gene 3 enhances the sensitivity of cervical cancer cells in response to irradiation-induced apoptosis

Identification of proteins that are involved in the sensitivity of radiotherapy of cancers is important to enhance the response to cancer treatment. Expression of TAp73 is associated with the sensitivity of radiotherapy in cervical cancer patients, suggesting it plays an important role in controlling radiosensitivity. Here, by using yeast two-hybrid system, we identify breast cancer-associated gene 3 (BCA3) as the first and novel protein interacting partner of TAp73. By coimmunoprecipitation and Western blot analysis, we confirm that TAp73 binds with and stabilizes BCA3 in cervical cancer cell line HeLa. Immunofluorescence staining indicates that BCA3 is localized in the cytoplasm and nucleus. Interestingly, when coexpressed with TAp73, BCA3 interacts and colocalizes with TAp73 at the mitochondria. Mutagenesis reveals that the oligomerization domain of TAp73 is responsible for the interaction with BCA3. Furthermore, BCA3 augments the transactivation activity of TAp73 on bax promoter and protein expression. In addition, the expression of BCA3 also increases the sensitivity of TAp73-transfected cells in response to gamma-irradiation-induced apoptosis. Western blot analysis also shows that TAp73 and BCA3 induce activation of caspase-7 and caspase-9. In summary, these findings suggested that BCA3 is a novel protein partner of TAp73, and they cooperate with each other to exert tumor-suppressive functions and sensitize the response of cervical cancer cells to radiotherapy.

Regulating the genome surveillance system: miRNAs and the p53 super family

The p53 gene super family consists of three members; TP53, TP63 and TP73, encoding proteins p53, p63 and p73. Whilst p63 appears to have an essential role in embryonic development with a less clear role in carcinogenesis, irregularities in p53 and p73 signalling are implicated in tumour formation. As such, p53 is a tumour suppressor which is mutated in over 50% cancers and p73 was recently formally classified as a tumour suppressor based on data showing p73 deficient mice generate spontaneous tumours similar to those observed in p53 null mice. Dysregulation of both p53 and p73 has been correlated with cancer progression in many cell types and although mutation of these genes is often observed, some form of p53/p73 deregulation likely occurs in all tumour cells. The discovery that complementary micro RNAs (miRNAs) are able to target both of these genes provides a potential new means of perturbing p53/p73 signalling networks in cancer cells. Here we summarise the current literature regarding the involvement of miRNAs in the modulation of p53 family proteins and cancer development and detail the use of in silico methods to reveal key miRNA targets.

Molecular basis of S100 proteins interacting with the p53 homologs p63 and p73

S100 proteins modulate p53 activity by interacting with its tetramerization (p53TET, residues 325-355) and transactivation (residues 1-57) domains. In this study, we characterized biophysically the binding of S100A1, S100A2, S100A4, S100A6 and S100B to homologous domains of p63 and p73 in vitro by fluorescence anisotropy, analytical ultracentrifugation and analytical gel filtration. We found that S100A1, S100A2, S100A4, S100A6 and S100B proteins bound different p63 and p73 tetramerization domain variants and naturally occurring isoforms with varying affinities in a calcium-dependent manner. Additional interactions were observed with peptides derived from the p63 and p73 N-terminal transactivation domains. Importantly, S100 proteins bound p63 and p73 with different affinities in their different oligomeric states, similarly to the differential modes of binding to p53. On the basis of our data, we hypothesize that S100 proteins regulate the oligomerization state of all three p53 family members and their isoforms, with a potential physiological relevance in developmental and disease-related processes. The regulation of the p53 family by S100 is complicated and depends on the target preference of each individual S100 protein, the concentration of the proteins and calcium, as well as the splicing variation of p63 or p73. Our results outlining the complexity of the interaction should be considered when studying the functional effects of S100 proteins in their biological context.

Conservation of DNA-binding specificity and oligomerisation properties within the p53 family

Background

Transcription factors activate their target genes by binding to specific response elements. Many transcription factor families evolved from a common ancestor by gene duplication and subsequent divergent evolution. Members of the p53 family, which play key roles in cell-cycle control and development, share conserved DNA binding and oligomerisation domains but exhibit distinct functions. In this study, the molecular basis of the functional divergence of related transcription factors was investigated.

Results

We characterised the DNA-binding specificity and oligomerisation properties of human p53, p63 and p73, as well as p53 from other organisms using novel biophysical approaches. All p53 family members bound DNA cooperatively as tetramers with high affinity. Despite structural differences in the oligomerisation domain, the dissociation constants of the tetramers was in the low nanomolar range for all family members, indicating that the strength of tetramerisation was evolutionarily conserved. However, small differences in the oligomerisation properties were observed, which may play a regulatory role. Intriguingly, the DNA-binding specificity of p53 family members was highly conserved even for evolutionarily distant species. Additionally, DNA recognition was only weakly affected by CpG methylation. Prediction of p53/p63/p73 binding sites in the genome showed almost complete overlap between the different homologs.

Conclusion

Diversity of biological function of p53 family members is not reflected in differences in sequence-specific DNA binding. Hence, additional specificity factors must exist, which allowed the acquisition of novel functions during evolution while preserving original roles.

Structural evolution of p53, p63, and p73: implication for heterotetramer formation

Oligomerization of members of the p53 family of transcription factors (p53, p63, and p73) is essential for their distinct functions in cell-cycle control and development. To elucidate the molecular basis for tetramer formation of the various family members, we solved the crystal structure of the human p73 tetramerization domain (residues 351-399). Similarly to the canonical p53 tetramer, p73 forms a tetramer with D(2) symmetry that can be described as a dimer of dimers. The most striking difference between the p53 and p73 tetramerization domain is the presence of an additional C-terminal helix in p73. This helix, which is conserved in p63, is essential for stabilizing the overall architecture of the tetramer, as evidenced by the different oligomeric structures observed for a shortened variant lacking this helix. The helices act as clamps, wrapping around the neighboring dimer and holding it in place. In addition, we show by mass spectrometry that the tetramerization domains of p63 and p73, but not p53, fully exchange, with different mixed tetramers present at equilibrium, albeit at a relatively slow rate. Taken together, these data provide intriguing insights into the divergent evolution of the oligomerization domain within the p53 family, from the ancestral p63/p73-like protein toward smaller, less promiscuous monomeric building blocks in human p53, allowing functional separation of the p53 pathway from that of its family members.

Nuclear-cytosolic transport of COMMD1 regulates NF-kappaB and HIF-1 activity

Copper metabolism MURR1 domain1 (COMMD1) is a novel inhibitor of the transcription factors NF-kappaB and HIF-1, which play important roles in inflammation and tumor growth, respectively. In this study, we identified two highly conserved nuclear export signals (NESs) in COMMD1 and revealed that these NESs were essential and sufficient to induce maximal nuclear export of COMMD1. Inhibition of CRM1-mediated nuclear export by Leptomycin B resulted in nuclear accumulation of COMMD1. In addition, low oxygen concentrations induced the active export of COMMD1 from the nucleus in a CRM1-dependent manner. Disruption of the NESs in COMMD1 increased the repression of COMMD1 in transcriptional activity of NF-kappaB and HIF-1. In conclusion, these data indicate that COMMD1 undergoes constitutive nucleocytoplasmic transport as a novel mechanism to regulate NF-kappaB and HIF-1 signaling.

Phosphorylation of p53 is regulated by TPX2-Aurora A in xenopus oocytes

p53 is an important tumor suppressor regulating the cell cycle at multiple stages in higher vertebrates. The p53 gene is frequently deleted or mutated in human cancers, resulting in loss of p53 activity. This leads to centrosome amplification, aneuploidy, and tumorigenesis, three phenotypes also observed after overexpression of the oncogenic kinase Aurora A. Accordingly, recent studies have focused on the relationship between these two proteins. p53 and Aurora A have been reported to interact in mammalian cells, but the function of this interaction remains unclear. We recently reported that Xenopus p53 can inhibit Aurora A activity in vitro but only in the absence of TPX2. Here we investigate the interplay between Xenopus Aurora A, TPX2, and p53 and show that newly synthesized TPX2 is required for nearly all Aurora A activation and for full p53 synthesis and phosphorylation in vivo during oocyte maturation. In vitro, phosphorylation mediated by Aurora A targets serines 129 and 190 within the DNA binding domain of p53. Glutathione S-transferase pull-down studies indicate that the interaction occurs via the p53 transactivation domain and the Aurora A catalytic domain around the T-loop. Our studies suggest that targeting of TPX2 might be an effective strategy for specifically inhibiting the phosphorylation of Aurora A substrates, including p53.

Dynamic compartmentalization of base excision repair proteins in response to nuclear and mitochondrial oxidative stress

DNAs harbored in both nuclei and mitochondria of eukaryotic cells are subject to continuous oxidative damage resulting from normal metabolic activities or environmental insults. Oxidative DNA damage is primarily reversed by the base excision repair (BER) pathway, initiated by N-glycosylase apurinic/apyrimidinic (AP) lyase proteins. To execute an appropriate repair response, BER components must be distributed to accommodate levels of genotoxic stress that may vary considerably between nuclei and mitochondria, depending on the growth state and stress environment of the cell. Numerous examples exist where cells respond to signals, resulting in relocalization of proteins involved in key biological transactions. To address whether such dynamic localization contributes to efficient organelle-specific DNA repair, we determined the intracellular localization of the Saccharomyces cerevisiae N-glycosylase/AP lyases, Ntg1 and Ntg2, in response to nuclear and mitochondrial oxidative stress. Fluorescence microscopy revealed that Ntg1 is differentially localized to nuclei and mitochondria, likely in response to the oxidative DNA damage status of the organelle. Sumoylation is associated with targeting of Ntg1 to nuclei containing oxidative DNA damage. These studies demonstrate that trafficking of DNA repair proteins to organelles containing high levels of oxidative DNA damage may be a central point for regulating BER in response to oxidative stress.

The ethanol extract from Artemisia princeps Pampanini induces p53-mediated G1 phase arrest in A172 human neuroblastoma cells

In the present study, the antiproliferative effects of the ethanol extract of Artemisia princeps Pampanini (EAPP) and the mechanism involved were investigated. Of the various cancer cells examined, human neuroblastoma A172 cells were most sensitive to EAPP, and their proliferation was dose- and time-dependently inhibited by EAPP. DNA flow cytometry analysis indicated that EAPP notably induced the G(1) phase arrest in A172 cells. Of the G(1) phase cycle-related proteins examined, the expressions of cyclin-dependent kinase (CDK) 2, CDK4, and CDK6 and of cyclin D(1), D(2), and D(3) were found to be markedly reduced by EAPP, whereas cyclin E was unaffected. Moreover, the protein and mRNA levels of the CDK inhibitors p16(INK4a), p21(CIP1/WAF1), and p27(KIP1) were increased, and the activities of CDK2, CDK4, and CDK6 were reduced. Furthermore, the expressions of E2F-1 and of phosphorylated pRb were also decreased, and the protein levels of p53 and pp53 (Ser15) were increased. Up-regulation of p21(CIP1/WAF1) was found to be mediated by a p53-dependent pathway in EAPP-induced G(1)-arrested A172 cells. When these data are taken together, the EAPP was found to potently inhibit the proliferation of human neuroblastoma A172 cells via G(1) phase cell cycle arrest.

Protein methylation: a new mechanism of p53 tumor suppressor regulation

The tumor suppressor p53 is the most frequently inactivated gene in human cancers. The p53 protein functions as a sequence-specific transcription factor to regulate key cellular processes, including cell-cycle arrest, DNA repair, apoptosis, and senescence in response to stress signals. P53 is maintained at a low level in the cell, but becomes rapidly stabilized and activated in response to DNA damage, hypoxia, hyperproliferation, and other types of cellular stresses. The stability and transcriptional activity of p53 are tightly regulated through multiple post-translational modifications, such as phosphorylation, acetylation, and ubiquitination. Within the past few years, several studies have established that protein methylation is a novel mechanism by which p53 is regulated. Indeed, histone lysine methyltransferases KMT5 (Set9), KMT3C (Smyd2), and KMT5A (Set8) methylate p53 at specific C-terminal lysines. Lysine methylation enhances or suppresses p53 transcriptional activity depending on the methylation site. Furthermore, the lysine-specific demethylase KDM1 (LSD1) mediates p53 demethylation, which prevents p53 interaction with its co-activator 53BP1 to induce apoptosis. Finally, protein arginine methyltransferases CARM1 and PRMT1 are co-activators of p53 involved in the methylation of histones H3 and H4 to facilitate p53-mediated transcription. In response to cellular stresses, the interplay between p53 methylation, demethylation, and other post-translational modifications fine-tunes the activity of p53 to ultimately prevent tumor formation.

Cataloging and organizing p73 interactions in cell cycle arrest and apoptosis

We have compiled the p73-mediated cell cycle arrest and apoptosis pathways. p73 is a member of the p53 family, consisting of p53, p63 and p73. p73 exists in several isoforms, presenting different domain structures. p73 functions not only as a tumor suppressor in apoptosis but also as differentiator in embryo development. p53 mutations are responsible for half of the human cancers; p73 can partially substitute mutant p53 as tumor suppressor. The pathways we assembled create a p73-centered network consisting of 53 proteins and 176 interactions. We clustered our network into five functional categories: Upregulation, Activation, Suppression, Transcriptional Activity and Degradation. Our literature searches led to discovering proteins (c-Jun and pRb) with apparent opposing functional effects; these indicate either currently missing proteins and interactions or experimental misidentification or functional annotation. For convenience, here we present the p73 network using the molecular interaction map (MIM) notation. The p73 MIM is unique amongst MIMs, since it further implements detailed domain features. We highlight shared pathways between p53 and p73. We expect that the compiled and organized network would be useful to p53 family-based studies.

U-box-type ubiquitin E4 ligase, UFD2a attenuates cisplatin mediated degradation of DeltaNp63alpha

DeltaNp63alpha, the dominant negative isoform of the p63 family is an essential survival factor in head and neck squamous cell carcinoma. This isoform has been shown to be down regulated in response to several DNA damaging agents, including cisplatin. But little is understood about the post-translational protein stability of DeltaNp63alpha. In this present study we demonstrate for the first time that DeltaNp63alpha physically interacts with U-box-type E4 ubiquitin ligase UFD2a. UFD2a stabilizes DeltaNp63alpha, and ubiquitylation of DeltaNp63alpha is attenuated by UFD2a both in the presence and absence of cisplatin. Ectopic expression of UFD2a increased the half-life of DeltaNp63alpha in association with a significant enhancement of the repressive transcriptional activity of DeltaNp63alpha. Downregulation of endogenous UFD2a by RNAi resulted in degradation of DeltaNp63alpha. Taken together, our current study provides an insight onto the regulation of DeltaNp63alpha protein levels in response to cisplatin and also suggests that UFD2a might play an important role in the regulation of cisplatin mediated cell death mediated by p63.

A role for caspase 2 and PIDD in the process of p53-mediated apoptosis

When treated with some DNA-damaging agents, human tumor-derived H1299 cells expressing inducible versions of wild-type or mutant p53 with inactive transactivation domain I (p53(Q22/S23)) undergo apoptosis as evidenced by cytochrome c release, nuclear fragmentation, and sub-G1 DNA content. Apoptosis induced by p53(Q22/S23) is relatively slow, however, and key downstream effector caspases are not activated. Nevertheless, with either version of p53, caspase 2 activation is required for release of cytochrome c and cell death. Remarkably, although p53(Q22/S23) is known to be defective in transcriptional activation of numerous p53 target genes, it can induce expression of proapoptotic targets including PIDD and AIP1 at least to the same extent as wild-type p53. Furthermore, RNAi silencing of PIDD, previously shown to be required for caspase 2 activation, suppresses apoptosis by both wild-type p53 and p53(Q22/S23). Thus, the initial stage of DNA damage-facilitated, p53-mediated apoptosis occurs by a PIDD- and caspase 2-dependent mechanism, and p53's full transcriptional regulatory functions may be required only for events that are downstream of cytochrome c release.

The Rb family connects with the Tp53 family in skin carcinogenesis

In contrast with the low frequency of alterations found in the Rb gene, the pRb pathway is inactivated in the vast majority of human tumors. A similar situation takes place in mouse models of cancer, including two-stage skin tumorigenesis. This might be explained if the Rb functions are carried out, in its absence, by other proteins that are also controlled by the same upstream regulators and display similar effectors. The other Rb family members, p107 and or p130, are plausible candidates. The embryonic lethality of pRb-deficient animals, which precludes the analysis of the roles of Rb gene in mouse models, has been avoided using tissue-specific deletion of pRb. In epidermis, pRb deletion leads to altered proliferation and differentiation. However, these deficient mice do not develop spontaneous tumors, and chemical carcinogenesis experiments revealed that the absence of pRb renders fewer and smaller tumors than control animals, but showing increased malignant conversion to squamous cell carcinomas (SCC). Detailed biochemical analyses have indicated that, in the absence of pRb, multiple pathways, including the aberrant p53 activation mediated by E2F/p19(ARF), are activated leading to increased tumor apoptosis. As Rb loss in epidermis is functionally compensated by Rbl1 (p107), this might also suggest that p107 could behave as a tumor suppressor. We summarize here our findings in support of this hypothesis. The pRb-;p107-/- epidermis form spontaneous tumors, and the reduction of p107 levels restores the susceptibility of pRb-mice to chemical skin carcinogenesis experiments. Moreover, Rb-deficient keratinocytes are highly susceptible to Ha-ras-induced transformation, and this susceptibility is enhanced by p107 loss. Further functional studies have indicated that the loss of p107 in the absence of pRb produces the reduction of p53-dependent proapoptotic signals through the modulation of p63 and p73 isoforms. In addition, expression profiling analysis has revealed multiple oncogenic alterations that can contribute to tumor susceptibility in epidermis in the absence of pRb and p107.

Filamin A negatively regulates the transcriptional activity of p73alpha in the cytoplasm

The transcription regulator p73alpha is structurally different from p53 in that it possesses a unique C-terminal domain, which has been implicated in transcriptional repression. To dissect the mechanism of repression by this domain, we performed a yeast two-hybrid screen of a HeLa cDNA library using residues 487-636 of p73alpha as bait and isolated a cDNA clone encoding the C-terminal portion (residues 2210-2647) of filamin A, a 280-kDa actin-binding protein. Additional yeast two-hybrid assays indicated that filamin A specifically interacts with the p73alpha C-terminus, which is lacking in p53 and p73beta. The interaction was confirmed by GST pull-down assays in vitro and by immunoprecipitation analysis in vivo. Immunofluorescence microscopy revealed that p73alpha remained in the cytoplasm in A7 melanoma cells stably expressing filamin A, whereas it was localized in the nucleus of filamin A-deficient M2 cells. Deletion of the C-terminus of p73alpha (residues 487-636) resulted in nuclear localization in both cell types. Consistent with our interaction data, transient co-expression of filamin A resulted in the down-regulation of p73alpha, but not of p53, transcriptional activity on various p53-responsive promoters. Taken together, our data suggest that p73alpha is sequestered in the cytoplasm by filamin A, thereby inhibiting its transcriptional activity.

Differential regulation of vitamin D receptor (VDR) by the p53 Family: p73-dependent induction of VDR upon DNA damage

p63 and p73, members of the p53 family, have been shown to be functionally distinct from p53. Vitamin D receptor (VDR) is a ligand (vitamin D(3))-dependent transcription factor, which is shown to play a major role in calcium homeostasis and keratinocyte differentiation. Vitamin D and its analogues in combination with DNA-damaging agents are extensively used for cancer chemotherapy. In this report, we examined whether p53 affects p63-mediated induction of VDR and studied the effect of DNA damage on VDR induction in p53 null cell lines. Our results demonstrate that p53 itself does not induce VDR expression, nor does it affect p63-mediated VDR induction in the cell lines tested in this study. Furthermore, we observed p53-independent activation of VDR upon DNA damage and associated the induction of VDR to p73. We have demonstrated that ectopic expression of various p73 isoforms can induce VDR expression. Inhibition of p73 in cells treated with DNA-damaging agents exhibited decreased VDR expression. Finally, we show that upon DNA damage, induction of VDR sensitizes the cells to vitamin D treatment. In conclusion, our results indicate that VDR is regulated by p63 and p73 and that the induction of VDR expression upon DNA damage is p73-dependent.

p53 and p73 display common and distinct requirements for sequence specific binding to DNA

Although p53 and p73 share considerable homology in their DNA-binding domains, there have been few studies examining their relative interactions with DNA as purified proteins. Comparing p53 and p73beta proteins, our data show that zinc chelation by EDTA is significantly more detrimental to the ability of p73beta than of p53 to bind DNA, most likely due to the greater effect that the loss of zinc has on the conformation of the DNA-binding domain of p73. Furthermore, prebinding to DNA strongly protects p73beta but not p53 from chelation by EDTA suggesting that DNA renders the core domain of p73 less accessible to its environment. Further exploring these biochemical differences, a five-base sub-sequence was identified in the p53 consensus binding site that confers a greater DNA-binding stability on p73beta than on full-length p53 in vitro. Surprisingly, p53 lacking its C-terminal non-specific DNA-binding domain (p53Delta30) demonstrates the same sequence discrimination as does p73beta. In vivo, both p53 and p73beta exhibit higher transactivation of a reporter with a binding site containing this sub-sequence, suggesting that lower in vitro dissociation translates to higher in vivo transactivation of sub-sequence-containing sites.

P53 and p73 differ in their ability to inhibit glucocorticoid receptor (GR) transcriptional activity

Background

p53 is a tumor suppressor and potent inhibitor of cell growth. P73 is highly similar to p53 at both the amino acid sequence and structural levels. Given their similarities, it is important to determine whether p53 and p73 function in similar or distinct pathways. There is abundant evidence for negative cross-talk between glucocorticoid receptor (GR) and p53. Neither physical nor functional interactions between GR and p73 have been reported. In this study, we examined the ability of p53 and p73 to interact with and inhibit GR transcriptional activity.

Results

We show that both p53 and p73 can bind GR, and that p53 and p73-mediated transcriptional activity is inhibited by GR co-expression. Wild-type p53 efficiently inhibited GR transcriptional activity in cells expressing both proteins. Surprisingly, however, p73 was either unable to efficiently inhibit GR, or increased GR activity slightly. To examine the basis for this difference, a series of p53:p73 chimeric proteins were generated in which corresponding regions of either protein have been swapped. Replacing N- and C-terminal sequences in p53 with the corresponding sequences from p73 prevented it from inhibiting GR. In contrast, replacing p73 N- and C-terminal sequences with the corresponding sequences from p53 allowed it to efficiently inhibit GR. Differences in GR inhibition were not related to differences in transcriptional activity of the p53:p73 chimeras or their ability to bind GR.

Conclusion

Our results indicate that both N- and C-terminal regions of p53 and p73 contribute to their regulation of GR. The differential ability of p53 and p73 to inhibit GR is due, in part, to differences in their N-terminal and C-terminal sequences.

The epithelial cell transforming sequence 2, a guanine nucleotide exchange factor for Rho GTPases, is repressed by p53 via protein methyltransferases and is required for G1-S transition

The epithelial cell transforming sequence 2 (ECT2), a member of the Dbl family of guanine nucleotide exchange factor for Rho GTPases, is required for cytokinesis. The tumor suppressor p53 plays a crucial role in coordinating cellular processes, such as cell cycle arrest and apoptosis, in response to stress signals. Here, we showed that ECT2 is negatively regulated by wild-type p53 but not tumor-derived mutant p53 or other p53 family members. In addition, ECT2 is down-regulated in multiple cell lines by DNA damage agents and Nutlin-3, an MDM2 antagonist, in a p53-dependent manner. We also showed that the activity of the ECT2 promoter is repressed by wild-type p53, and to a lesser extent, by p21. In addition, the second activation domain in p53 is necessary for the efficient repression of ECT2. Importantly, we found that the ECT2 gene is bound by p53 in vivo in response to DNA damage and Nutlin-3 treatment. Furthermore, we provided evidence that inhibition of protein methyltransferases, especially arginine methyltransferases, relieve the repression of ECT2 induced by DNA damage or Nutlin-3 in a p53-dependent manner. Finally, we generated multiple cell lines in which ECT2 is inducibly knocked down and found that ECT2 knockdown triggers cell cycle arrest in G1. Taken together, we uncovered a novel function for ECT2 and provided a novel mechanism by which p53 represses gene expression via protein methyltransferases.

The Role of Trp53 in the Transcriptional Response to Ionizing Radiation in the Developing Brain

Brain formation results from a series of well-timed consecutive waves of cellular proliferation, migration and differentiation. Acute irradiation during pregnancy selectively interferes with these events to result in malformations such as microcephaly, reduced cortical thickness and mental retardation. In the present study we performed a straight-through cDNA-microarray analysis of the developing mouse brain at embryonic day E13, 3 h after in utero exposure to 50 cGy X-radiation. This dataset was used as an indication of genes involved in different pathways that are activated upon early radiation exposure, and for further evaluation using quantitative PCR (qPCR). Microarray and qPCR data revealed that the main activated pathways in irradiated wild-type embryos are involved in the regulation of a p53-mediated pathway that may lead to cell cycle delay/arrest and increased levels of apoptosis. To define whether the transcriptional radiation response was solely p53 mediated, we analysed the expression of cell cycle regulating genes in a Trp53 null mutant. The modulated expression of cell cycle regulating genes such as cyclins and Cdk genes indicated the induction of a cell cycle arrest, without evidence for the onset of apoptosis. Additional gene-expression studies have shown that various E2F transcription factors may be involved in this event. Together, these results provide a detailed view of the different p53-related mechanisms that are triggered in response to ionizing radiation in the developing brain.