p53 negatively regulates cdc2 transcription via the CCAAT-binding NF-Y transcription factor

Exosome-mediated viral nucleic acid presentation in a crustacean expounds innate immunity from a novel perspective

As an enduring hot topic in the field of innate immunity, apoptosis is widely considered an effective approach to eliminate pathogenic microbes and plays a crucial role during host-pathogen interactions. Recently, researchers have found that the virus-containing host cells could transmit apoptotic signals to the surrounding uninfected cells during infection, but the mechanism remains unclear. Here, we found that exosomes secreted by WSSV-infected mud crab hemocytes contain viral nucleic acid wsv277, which could be transported to the recipient cells and further expressed viral protein with phosphokinase activity. Besides, by using transcriptome, proteome, ChIP-seq, and coIP techniques, the results revealed that wsv277 could activate the transcription and translation of apoptotic genes via interacting with CBF and EF-1α so as to suppress the spread of virus infection by inducing apoptosis of the surrounding cells. Therefore, for the first time, our study proved that the components of DNA virus could be encapsulated into exosomes and elucidated the mechanism of apoptotic signal transduction between cells from the perspective of exosomes.

IMPORTANCE

Our study revealed that the components of DNA virus could be packaged and transmitted through the exosomes of lower invertebrates, which strongly demonstrated the diversity of exosome-mediated viral immunity and its universality in animals. Furthermore, we elucidated the mechanism of apoptotic signal transduction between cells from the perspective of exosomes and revealed a novel strategy for the host to cope with viral infection.

Proteasome regulators in pancreatic cancer

Pancreatic adenocarcinoma is one of the most lethal cancers with rising incidence. Despite progress in its treatment, with the introduction of more effective chemotherapy regimens in the last decade, prognosis of metastatic disease remains inferior to other cancers with long term survival being the exception. Molecular characterization of pancreatic cancer has elucidated the landscape of the disease and has revealed common lesions that contribute to pancreatic carcinogenesis. Regulation of proteostasis is critical in cancers due to increased protein turnover required to support the intense metabolism of cancer cells. The proteasome is an integral part of this regulation and is regulated, in its turn, by key transcription factors, which induce transcription of proteasome structural units. These include FOXO family transcription factors, NFE2L2, hHSF1 and hHSF2, and NF-Y. Networks that encompass proteasome regulators and transduction pathways dysregulated in pancreatic cancer such as the KRAS/ BRAF/MAPK and the Transforming growth factor beta/SMAD pathway contribute to pancreatic cancer progression. This review discusses the proteasome and its transcription factors within the pancreatic cancer cellular micro-environment. We also consider the role of stemness in carcinogenesis and the use of proteasome inhibitors as therapeutic agents.

Programmed cell death, redox imbalance, and cancer therapeutics

Cancer cells are disordered by nature and thus featured by higher internal redox level than healthy cells. Redox imbalance could trigger programmed cell death if exceeded a certain threshold, rendering therapeutic strategies relying on redox control a possible cancer management solution. Yet, various programmed cell death events have been consecutively discovered, complicating our understandings on their associations with redox imbalance and clinical implications especially therapeutic design. Thus, it is imperative to understand differences and similarities among programmed cell death events regarding their associations with redox imbalance for improved control over these events in malignant cells as well as appropriate design on therapeutic approaches relying on redox control. This review addresses these issues and concludes by bringing affront cold atmospheric plasma as an emerging redox controller with translational potential in clinics.

p53-mediated regulation of mitochondrial dynamics plays a pivotal role in the senescence of various normal cells as well as cancer cells

The tumor suppressor p53 is known as a critical mediator of many cellular processes, including cellular senescence, but its role in mitochondrial dynamics is not fully understood. We have previously shown that p53 regulates mitochondrial dynamics via the PKA-Drp1 pathway to induce cellular senescence. In this study, to further understand the role of p53-dependent regulation of mitochondrial dynamics, the effect of p53 expression on mitochondrial morphology was examined in various cancer cell lines and normal human cells. We found that p53 induced remarkable mitochondrial elongation and cellular senescence in various cancer cells regardless of their p53 status. p53 also induced mitochondrial elongation in various human primary normal cells, suggesting that p53-mediated mitochondrial elongation is a general phenomenon. Moreover, we found that p53 plays an essential role in mitochondrial elongation in H-Ras-induced cellular senescence and in the replicative senescence of normal human cells. Treatment with the MDM-2 antagonist Nutlin-3a also induced mitochondrial elongation through the PKA-Drp1 pathway in IMR90 normal human cells. Furthermore, the inhibition of PKA activity in late-passage normal cells significantly reduced both mitochondrial elongation and cellular senescence, suggesting that the p53-PKA pathway is essential for maintaining the senescence phenotype in normal cells. Together, these results further confirm the direct regulation of mitochondrial dynamics by p53 and the important role of p53-mediated mitochondrial elongation in cellular senescence.

Comprehensive gene and pathway analysis of cervical cancer progression

Cervical Cancer is one of the leading causes of cancer-associated mortality in women. The present study aimed to identify key genes and pathways involved in cervical cancer (CC) progression, via a comprehensive bioinformatics analysis. The GSE63514 dataset from the Gene Expression Omnibus database was analyzed for hub genes and cancer progression was divided into four phases (phases I-IV). Pathway enrichment, protein-protein interaction (PPI) and pathway crosstalk analyses were performed, to identify key genes and pathways using a criterion nodal degree ≥5. Gene pathway analysis was determined by mapping the key genes into the key pathways. Co-expression between key genes and their effect on overall survival (OS) time was assessed using The Cancer Genome Atlas database. A total of 3,446 differentially expressed genes with 107 hub genes were identified within the four phases. A total of 14 key genes with 11 key pathways were obtained, following extraction of ≥5 degree nodes from the PPI and pathway crosstalk networks. Gene pathway analysis revealed that CDK1 and CCNB1 regulated the cell cycle and were activated in phase I. Notably, the following terms, 'pathways in cancer', 'focal adhesion' and the 'PI3K-Akt signaling pathway' ranked the highest in phases II-IV. Furthermore, FN1, ITGB1 and MMP9 may be associated with metastasis of tumor cells. STAT1 was indicated to predominantly function at the phase IV via cancer-associated signaling pathways, including 'pathways in cancer' and 'Toll-like receptor signaling pathway'. Survival analysis revealed that high ITGB1 and FN1 expression levels resulted in significantly worse OS. CDK1 and CCNB1 were revealed to regulate proliferation and differentiation through the cell cycle and viral tumorigenesis, while FN1 and ITGB1, which may be developed as novel prognostic factors, were co-expressed to induce metastasis via cancer-associated signaling pathways, including PI3K-Art signaling pathway, and focal adhesion in CC; however, the underlying molecular mechanisms require further research.

Screening and Identification of Molecular Targets Involved in Preventing Gastric Precancerous Lesions in Chronic Atrophic Gastritis by Qilianshupi Decoction

Chronic atrophic gastritis (CAG) is a common and possibly precancerous digestive tract disease. Development of drugs with effect of preventing precancerous lesions draws the eyes of global researchers. Qilianshupi decoction (QLSP) is a Traditional Chinese Medicine (TCM) that is commonly used to treat CAG, but few studies have explored the mechanism of QLSP on treating CAG. This study investigated the molecular targets of the component herbs of QLSP in preventing precancerous lesions based on network pharmacology. Network pharmacology analysis revealed that the 6 herbs regulated multiple CAG-related genes, among which the most important were cancer-related pathway (apoptosis, p53, and VEGF) and epithelial cell signaling in Helicobacter pylori infection. Further animal experiments showed that the expression of survivin and p53 in precancerous lesions of CAG rats was significantly increased which was suppressed by QLSP. Moreover, telomerase activity was inhibited in precancerous lesions of CAG rats, and telomere length of gastric mucosa was increased, which was reversed by QLSP. Our results suggest that the components of QLSP prevents gastric precancerous lesions through decreasing the expression of survivin and p53 and regulating telomerase activity and telomere length in CAG.

p53 regulates mitochondrial dynamics by inhibiting Drp1 translocation into mitochondria during cellular senescence

Cellular senescence acts as an important barrier to tumorigenesis by eliminating precancerous cells. Previous studies have shown an essential role of the tumor suppressor p53 in cellular senescence, but how p53 induces cellular senescence is not fully understood. We found that p53 promoted the formation of highly interconnected and elongated mitochondria prior to the onset of cellular senescence. The inhibition of mitochondrial elongation upon p53 expression suppressed cellular senescence, suggesting that mitochondrial elongation is required for the induction of p53-dependent senescence. p53-induced mitochondrial elongation resulted in mitochondrial dysfunction and subsequent increases in intracellular reactive oxygen species (ROS) levels, an important mediator of cellular senescence. Mechanistically, the inhibitory phosphorylation of Drp1 Ser637 increased upon p53 expression, suppressing the translocation of Drp1 into mitochondria. The transcriptional function of p53 was crucial for controlling the inhibitory phosphorylation of Drp1, whereas p21 was nonessential. Protein kinase A (PKA) activity was responsible for p53-mediated Drp1 Ser637 phosphorylation and mitochondrial dysfunction. Taken together, these results suggest that p53 regulates mitochondrial dynamics through the PKA-Drp1 pathway to induce cellular senescence.

Inhibition of RNA polymerase I transcription initiation by CX-5461 activates non-canonical ATM/ATR signaling

RNA polymerase I (Pol I)-mediated transcription of the ribosomal RNA genes (rDNA) is confined to the nucleolus and is a rate-limiting step for cell growth and proliferation. Inhibition of Pol I by CX-5461 can selectively induce p53-mediated apoptosis of tumour cells in vivo. Currently, CX-5461 is in clinical trial for patients with advanced haematological malignancies (Peter Mac, Melbourne). Here we demonstrate that CX-5461 also induces p53-independent cell cycle checkpoints mediated by ATM/ATR signaling in the absence of DNA damage. Further, our data demonstrate that the combination of drugs targeting ATM/ATR signaling and CX-5461 leads to enhanced therapeutic benefit in treating p53-null tumours in vivo, which are normally refractory to each drug alone. Mechanistically, we show that CX-5461 induces an unusual chromatin structure in which transcriptionally competent relaxed rDNA repeats are devoid of transcribing Pol I leading to activation of ATM signaling within the nucleoli. Thus, we propose that acute inhibition of Pol transcription initiation by CX-5461 induces a novel nucleolar stress response that can be targeted to improve therapeutic efficacy.

Complex integrated analysis of lncRNAs-miRNAs-mRNAs in oral squamous cell carcinoma

OBJECTIVES

This study aims to reveal regulatory network of lncRNAs-miRNAs-mRNAs in oral squamous cell carcinoma (OSCC) through gene expression data.

MATERIAL AND METHODS

Differentially expressed lncRNAs, miRNAs and mRNAs (cut-off: False discovery rate (FDR)<0.05 and |fold change|>1.5) were unveiled by package edgeR of R. Cox regression analysis was performed to screen prognostic factors in OSCC related with overall survival (OS) and relapse-free survival (RFS). Protein-protein interaction (PPI) network was constructed for differentially expressed mRNAs using BioGRID, HPRD and DIP. Key hub genes were identified from top 100 differentially expressed mRNAs ranked by betweenness centrality using recursive feature elimination. LncRNA-miRNA and miRNA-mRNA regulatory network were constructed and combined into ceRNAs regulatory network. Gene ontology biological terms and Kyoto Encyclopedia of Genes and Genomes pathways were identified using Fisher's exact test.

RESULTS

A total of 929 differentially expressed mRNAs, 23 differentially expressed lncRNAs and 29 differentially expressed miRNAs were identified. 59 mRNAs, 6 miRNAs (hsa-mir-133a-1, hsa-mir-1-2, hsa-mir-486, hsa-mir-135b, hsa-mir-196b, hsa-mir-193b) and 6 lncRNAs (C10orf91, C2orf48, SFTA1P, FLJ41941,PART1,TTTY14) were related with OS; and 52 mRNAs, 4 miRNAs (hsa-mir-133a-1, hsa-mir-135b, hsa-mir-196b, hsa-mir-193b) and 2 lncRNAs (PART1, TTTY14) were associated with RFS. A support vector machine (SVM) classifier containing 37 key hub genes was obtained. A ceRNA regulatory network containing 417 nodes and 696 edges was constructed. ECM-receptor interaction, cytokine-cytokine receptor interaction, focal adhesion, arachidonic acid metabolism, and p53 signaling pathway were significantly enriched in the network.

CONCLUSION

These findings uncover the pathogenesis of OSCC and might provide potential therapeutic targets.

Microtubule inhibitor, SP-6-27 inhibits angiogenesis and induces apoptosis in ovarian cancer cells

In ovarian cancer (OVCA), treatment failure due to chemo-resistance is a serious challenge. It is therefore critical to identify new therapies that are effective against resistant tumors and have reduced side effects. We recently identified 4-H-chromenes as tubulin depolymerizing agents that bind to colchicine site of beta-tubulin. Here, we screened a chemical library of substituted 4-H-chromenes and identified SP-6-27 to exhibit most potent anti-proliferative activity towards a panel of human cisplatin sensitive and resistant OVCA cell lines with 50% inhibitory concentration (IC₅₀; mean ± SD) ranging from 0.10 ± 0.01 to 0.84 ± 0.20 μM. SP-6-27 exhibited minimum cytotoxicity to normal ovarian epithelia. A pronounced decrease in microtubule density as well as G2/M cell cycle arrest was observed in SP-6-27 treated cisplatin sensitive/resistant OVCA cells. The molecular mechanism of SP-6-27 induced cell death revealed modulation in cell-cycle regulation by upregulation of growth arrest and DNA damage inducible alpha transcripts (GADD45). An enhanced intrinsic apoptosis was observed in OVCA cells through upregulation of Bax, Apaf-1, caspase-6, -9, and caspase-3. In vitro wound healing assay revealed reduced OVCA cell migration upon SP-6-27 treatment. Additionally, SP-6-27 and cisplatin combinatorial treatment showed enhanced cytotoxicity in chemo-sensitive/resistant OVCA cells. Besides effect on cancer cells, SP-6-27 further restrained angiogenesis by inhibiting capillary tube formation by human umbilical vein endothelial cells (HUVEC). Together, these findings show that the chromene analog SP-6-27 is a novel chemotherapeutic agent that offers important advantages for the treatment of ovarian cancer.

Oridonin induces G2/M cell cycle arrest and apoptosis via the PI3K/Akt signaling pathway in hormone-independent prostate cancer cells

Oridonin is an active constituent isolated from the traditional Chinese herb Rabdosia rubescens, which exerts antitumor effects in experimental and clinical settings. However, its antitumor effects and underlying mechanisms on prostate cancer cells have not yet been clearly identified. In the present study, the androgen-independent prostate cancer PC3 and DU145 cell lines were used as models to investigate the effects and possible mechanisms of oridonin on cellular proliferation and apoptosis. Results demonstrated that oridonin inhibited cellular proliferation, and was able to significantly induce G₂/M cell cycle arrest and apoptosis. Detailed signaling pathway analysis by western blotting demonstrated that the expression levels of p53 and p21 were upregulated, whereas the expression of cyclin-dependent kinase 1 was downregulated following oridonin treatment, which led to cell cycle arrest in the G₂/M phase. Oridonin also upregulated the proteolytic cleaved forms of caspase-3, caspase-9 and poly (ADP-ribose) polymerase. Furthermore, the protein expression levels of B-cell lymphoma 2 were decreased and those of Bcl-2-associated X protein were increased following oridonin treatment. In addition, oridonin treatment significantly inhibited the expression of phosphoiniositide-3 kinase (PI3K) p85 subunit and the phosphorylation of Akt. The downstream gene murine double minute 2 was also downregulated, which may contribute to the elevated expression of p53 following oridonin treatment. In conclusion, the results of the present study suggested that oridonin is able to inactivate the PI3K/Akt pathway and activate p53 pathways in prostate cancer cells, resulting in the suppression of proliferation and the induction of caspase-mediated apoptosis.

Non-Canonical Cell Death Induced by p53

Programmed cell death is a vital biological process for multicellular organisms to maintain cellular homeostasis, which is regulated in a complex manner. Over the past several years, apart from apoptosis, which is the principal mechanism of caspase-dependent cell death, research on non-apoptotic forms of programmed cell death has gained momentum. p53 is a well characterized tumor suppressor that controls cell proliferation and apoptosis and has also been linked to non-apoptotic, non-canonical cell death mechanisms. p53 impacts these non-canonical forms of cell death through transcriptional regulation of its downstream targets, as well as direct interactions with key players involved in these mechanisms, in a cell type- or tissue context-dependent manner. In this review article, we summarize and discuss the involvement of p53 in several non-canonical modes of cell death, including caspase-independent apoptosis (CIA), ferroptosis, necroptosis, autophagic cell death, mitotic catastrophe, paraptosis, and pyroptosis, as well as its role in efferocytosis which is the process of clearing dead or dying cells.

Gene expression in Catla catla (Hamilton) subjected to acute and protracted doses of gamma radiation

Studies on transcriptional modulation after gamma radiation exposure in fish are limited. Cell cycle perturbations and expression of apoptotic genes were investigated in the fish, Catla catla after acute and protracted exposures to gamma radiation over a 90day period. Significant changes in gene expression were observed between day 1 and 90 post-exposure. Gamma radiation induced a significant down-regulation of target genes gadd45α, cdk1 and bcl-2 from day 1 to day 3 after protracted exposure, whereas it persists till day 6 upon acute exposure. From day 12 onwards, Gadd45α, cdk1 and bcl-2 genes were up-regulated following protracted exposure, indicating DNA repair, cell-cycle arrest and apoptosis. There exists a linear correlation between these genes (gadd45α - r=0.85, p=0.0073; cdk1 - r=0.86, p=0.0053; bcl-2 - r=0.89, p=0.0026) at protracted exposures. This is the first report on the dual role of bcl-2 gene in fish exposed to acute and protracted radiation and correlation among the aforementioned genes that work in concert to promote 'repair' and 'death' circuitries in fish blood cells.

Ca2+/calmodulin-dependent protein kinase II-γ (CaMKIIγ) negatively regulates vascular smooth muscle cell proliferation and vascular remodeling

Vascular smooth muscle (VSM) expresses calcium/calmodulin-dependent protein kinase II (CaMKII)-δ and -γ isoforms. CaMKIIδ promotes VSM proliferation and vascular remodeling. We tested CaMKIIγ function in vascular remodeling after injury. CaMKIIγ protein decreased 90% 14 d after balloon injury in rat carotid artery. Intraluminal transduction of adenovirus encoding CaMKIIγC rescued expression to 35% of uninjured controls, inhibited neointima formation (>70%), inhibited VSM proliferation (>60%), and increased expression of the cell-cycle inhibitor p21 (>2-fold). Comparable doses of CaMKIIδ2 adenovirus had no effect. Similar dynamics in CaMKIIγ mRNA and protein expression were observed in ligated mouse carotid arteries, correlating closely with expression of VSM differentiation markers. Targeted deletion of CaMKIIγ in smooth muscle resulted in a 20-fold increase in neointimal area, with a 3-fold increase in the cell proliferation index, no change in apoptosis, and a 60% decrease in p21 expression. In cultured VSM, CaMKIIγ overexpression induced p53 mRNA (1.7 fold) and protein (1.8-fold) expression; induced the p53 target gene p21 (3-fold); decreased VSM cell proliferation (>50%); and had no effect on expression of apoptosis markers. We conclude that regulated CaMKII isoform composition is an important determinant of the injury-induced vasculoproliferative response and that CaMKIIγ and -δ isoforms have nonequivalent, opposing functions.

Systematic analysis to identify a key role of CDK1 in mediating gene interaction networks in cervical cancer development

OBJECTIVE

This study aims to identify corresponding differentially expressed genes in cervical cancer by comparing gene expression profiles between normal and cervical cancer samples.

METHOD

To identify differentially expressed genes in cervical cancer, two groups of Affymetrix microarray data available online were analyzed. One group consisted of 43 carcinomatous cervical epithelial cell samples, and the other was composed of 17 healthy cervical epithelial cell samples, both from the Amerindian. R packages-GO.db, KEGG.db and KEGGREST were used to detect GO categories and KEGG pathways with significant overrepresentation in differentially expressed genes comparing with the whole genome. Cytoscape was utilized to construct biological networks.

RESULTS

By comparing gene expression profile of normal and cervical cancer samples, 122 differentially expressed genes were identified including 46 up-regulated genes and 76 down-regulated genes. Using the identified differentially expressed genes, a large and a small biological network was constructed. In addition, 402 GO biological processes and 9 KEGG pathways were over-represented. Top significant biological processes included cell cycle and cell proliferation. Moreover, top significant KEGG pathways were oocyte meiosis, cell cycle and progesterone-mediated oocyte maturation. Most importantly, CDK1 frequently appeared in these processes and pathways, which indicated its significant role in the progression of cervical cancer.

CONCLUSION

CDK1 plays a comprehensive role in mediating genetic networks implicated in the progression of cervical cancer. Novel therapeutics targeting CDK1 or its related pathways might help improve prognosis of advanced stage cervical cancer.

Oridonin inhibits BxPC-3 cell growth through cell apoptosis

Oridonin, an ent-kaurene diterpenoid extracted from the traditional Chinese herb Rabdosia rubescens, has multiple biological and pharmaceutical functions and has been used clinically for many years. While the antitumor function of oridonin has been corroborated by numerous lines of evidence, its anticancer mechanism has not been well documented. In this study, the pancreatic cancer cell line BxPC-3 was used as a model to investigate a possible anticancer mechanism of oridonin through examining its effects on cell viability. The results showed that oridonin affected cell viability in a time- and dose-dependent manner. After exposure to different oridonin concentrations, growth rates and cell cycle arrest of BxPC-3 cells were significantly reduced compared with untreated cells, suggesting its effects on proliferation inhibition. Detailed signaling pathway analysis by western blot analysis revealed that low-dose oridonin treatment inhibited BxPC-3 cell proliferation by up-regulating p53 and down-regulating cyclin-dependent kinase 1 (CDK1), which led to cell cycle arrest in the G2/M phase. A high-dose oridonin not only arrested BxPC-3 cells in the G2/M phase but also induced cell accumulation in the S phase, presumably through γH2AX up-regulation and DNA damage. In addition, our results showed that a cell subpopulation was stained with propidium iodide after oridonin treatment. Protein quantification showed that cleaved poly(ADP-ribose) polymerase (PARP) expression was increased after a high-dose oridonin treatment, especially after long-term exposure. Accompanied by the increased level of deactivated PARP in BxPC-3 cells, the apoptosis initiators caspase-3 and caspase-7 expressions were also significantly increased, suggesting that caspase-mediated apoptosis contributed to cell death.

Histopathological and in vivo evidence of regucalcin as a protective molecule in mammary gland carcinogenesis

Regucalcin (RGN) is a calcium-binding protein, which has been shown to be underexpressed in cancer cases. This study aimed to determine the association of RGN expression with clinicopathological parameters of human breast cancer. In addition, the role of RGN in malignancy of mammary gland using transgenic rats overexpressing the protein (Tg-RGN) was investigated. Wild-type (Wt) and Tg-RGN rats were treated with 7,12-dimethylbenz[α]anthracene (DMBA). Carcinogen-induced tumors were histologically classified and the Ki67 proliferation index was estimated. Immunohistochemistry analysis showed that RGN immunoreactivity was negatively correlated with the histological grade of breast infiltrating ductal carcinoma suggesting that progression of breast cancer is associated with loss of RGN. Tg-RGN rats displayed lower incidence of carcinogen-induced mammary gland tumors, as well as lower incidence of invasive forms. Moreover, higher proliferation was observed in non-invasive tumors of Wt animals comparatively with Tg-RGN. Overexpression of RGN was associated with diminished expression of cell-cycle inhibitors and increased expression of apoptosis inducers. Augmented activity of apoptosis effector caspase-3 was found in the mammary gland of Tg-RGN. RGN overexpression protected from carcinogen-induced mammary gland tumor development and was linked with reduced proliferation and increased apoptosis. These findings indicated the protective role of RGN in the carcinogenesis of mammary gland.

Effects of TP53 mutational status on gene expression patterns across 10 human cancer types

Mutations in the TP53 tumour suppressor gene occur in half of all human cancers, indicating its critical importance in inhibiting cancer development. Despite extensive studies, the mechanisms by which mutant p53 enhances tumour progression remain only partially understood. Here, using data from the Cancer Genome Atlas (TCGA), genomic and transcriptomic analyses were performed on 2256 tumours from 10 human cancer types. We show that tumours with TP53 mutations have altered gene expression profiles compared to tumours retaining two wild-type TP53 alleles. Among 113 known p53-up-regulated target genes identified from cell culture assays, 10 were consistently up-regulated in at least eight of 10 cancer types that retain both copies of wild-type TP53. RPS27L, CDKN1A (p21(CIP1)) and ZMAT3 were significantly up-regulated in all 10 cancer types retaining wild-type TP53. Using this p53-based expression analysis as a discovery tool, we used cell-based assays to identify five novel p53 target genes from genes consistently up-regulated in wild-type p53 cancers. Global gene expression analyses revealed that cell cycle regulatory genes and transcription factors E2F1, MYBL2 and FOXM1 were disproportionately up-regulated in many TP53 mutant cancer types. Finally, > 93% of tumours with a TP53 mutation exhibited greatly reduced wild-type p53 messenger expression, due to loss of heterozygosity or copy neutral loss of heterozygosity, supporting the concept of p53 as a recessive tumour suppressor. The data indicate that tumours with wild-type TP53 retain some aspects of p53-mediated growth inhibitory signalling through activation of p53 target genes and suppression of cell cycle regulatory genes.

Association of p21 with NF-YA suppresses the expression of Polo-like kinase 1 and prevents mitotic death in response to DNA damage

Polo-like kinase 1 (PLK1) is an important mitotic kinase and its expression is tightly regulated in the cell cycle and in the DNA damage response. PLK1 expression is previously shown to be suppressed by p53 and/or p21. Here, we demonstrate that the CCAAT box in the PLK1 promoter is pivotal for p53/p21-mediated PLK1 repression. Chromatin immunoprecipitation showed that cyclin-dependent kinase 2 (CDK2) associated with the CCAAT box-containing region of PLK1 promoter in unstressed cells, whereas adriamycin (ADR) induced the recruitment of p21 with a concomitant reduction in the occupancy of CDK2 in this region. Expression of p21 inhibited the interaction between CDK2 and the nuclear factor YA (NF-YA) subunit of the CCAAT box-binding transcription factor NF-Y. A mutant p21 that is defective in CDK2 binding was unable to disrupt the CDK2-NF-YA interaction or suppress PLK1 transcription. Co-immunoprecipitation experiments demonstrated the interaction between NF-YA and p21, and in vitro assays showed that p21 could directly bind to NF-YA. Knockdown of NF-YA decreased the amount of PLK1 promoter-associated p21 and abolished p21-mediated PLK1 repression in cells treated with ADR. Depletion of NF-YA diminished the p53-regulated transcriptional activation and suppressed the p53-mediated protection from mitotic death after DNA damage, and these effects of NF-YA deletion were alleviated by PLK1 depletion. Our findings have uncovered a novel p21/NF-YA/PLK1 axis critical for maintaining the checkpoint function of p53 to prevent mitotic death in the DNA damage-induced response.

The transcription factor p53: not a repressor, solely an activator

The predominant function of the tumor suppressor p53 is transcriptional regulation. It is generally accepted that p53-dependent transcriptional activation occurs by binding to a specific recognition site in promoters of target genes. Additionally, several models for p53-dependent transcriptional repression have been postulated. Here, we evaluate these models based on a computational meta-analysis of genome-wide data. Surprisingly, several major models of p53-dependent gene regulation are implausible. Meta-analysis of large-scale data is unable to confirm reports on directly repressed p53 target genes and falsifies models of direct repression. This notion is supported by experimental re-analysis of representative genes reported as directly repressed by p53. Therefore, p53 is not a direct repressor of transcription, but solely activates its target genes. Moreover, models based on interference of p53 with activating transcription factors as well as models based on the function of ncRNAs are also not supported by the meta-analysis. As an alternative to models of direct repression, the meta-analysis leads to the conclusion that p53 represses transcription indirectly by activation of the p53-p21-DREAM/RB pathway.

Deacetylation of p53 induces autophagy by suppressing Bmf expression

Interferon γ (IFN-γ)-induced cell death is mediated by the BH3-only domain protein, Bik, in a p53-independent manner. However, the effect of IFN-γ on p53 and how this affects autophagy have not been reported. The present study demonstrates that IFN-γ down-regulated expression of the BH3 domain-only protein, Bmf, in human and mouse airway epithelial cells in a p53-dependent manner. p53 also suppressed Bmf expression in response to other cell death-stimulating agents, including ultraviolet radiation and histone deacetylase inhibitors. IFN-γ did not affect Bmf messenger RNA half-life but increased nuclear p53 levels and the interaction of p53 with the Bmf promoter. IFN-γ-induced interaction of HDAC1 and p53 resulted in the deacetylation of p53 and suppression of Bmf expression independent of p53's proline-rich domain. Suppression of Bmf facilitated IFN-γ-induced autophagy by reducing the interaction of Beclin-1 and Bcl-2. Furthermore, autophagy was prominent in cultured bmf(-/-) but not in bmf(+/+) cells. Collectively, these observations show that deacetylation of p53 suppresses Bmf expression and facilitates autophagy.

Nutlin-3a, an MDM2 antagonist and p53 activator, helps to preserve the replicative potential of cancer cells treated with a genotoxic dose of resveratrol

Resveratrol is a natural compound that has been intensely studied due to its role in cancer prevention and potential as an anti-cancer therapy. Its effects include induction of apoptosis and senescence-like growth inhibition. Here, we report that two cancer cell lines (U-2 OS and A549) differ significantly in their molecular responses to resveratrol. Specifically, in U-2 OS cells, the activation of the p53 pathway is attenuated when compared to the activation in A549 cells. This attenuation is accompanied by a point mutation (458: CGA→TGA) in the PPM1D gene and overexpression of the encoded protein, which is a negative regulator of p53. Experimentally induced knockdown of PPM1D in U-2 OS cells resulted in slightly increased activation of the p53 pathway, most clearly visible as stronger phosphorylation of p53 Ser37. When treated with nutlin-3a, a non-genotoxic activator of p53, U-2 OS and A549 cells both responded with substantial activation of the p53 pathway. Nutlin-3a improved the clonogenic survival of both cell lines treated with resveratrol. This improvement was associated with lower activation of DNA-damage signaling (phosphorylation of ATM, CHK2, and histone H2AX) and higher accumulation of cells in the G1 phase of the cell cycle. Thus, the hyperactivation of p53 by nutlin-3a helps to preserve the replicative potential of cells exposed to resveratrol.

CMV promoter is repressed by p53 and activated by JNK pathway

Viral promoters are widely utilized in commercial and customized vectors to drive expression of genes of interest including reporter, effector and transfection control, because of their high transcription efficiency in a variety of primary and transformed cell lines. However, we observed altered rate of transcription for these promoters under conditions such as presence of an effector protein. These variations in viral promoter driven expressions can potentially lead to incorrect conclusion, especially in comparative and quantitative experiments. We found significantly reduced viral promoter activity in cells overexpressing tumor suppressor protein p53, whereas markedly induced transcription in cells overexpressing MAP/ERK kinase kinase 1 (Mekk 1). Using deletion constructs generated from the CMV promoter, we found the transcription reduction by p53 is possibly mediated through the TATA motif present in proximal CMV promoter. The activation of the CMV promoter by Mekk 1, on the other hand, is attributed to the proximal CRE binding site in the promoter. These findings may be of interest to investigators who use CMV (or other viral) promoter driven vectors for either comparative or quantitative gene expression, or effect on promoter activity.

NF-Y binds to both G1- and G2-specific cyclin promoters; a possible role in linking CDK2/Cyclin A to CDK1/Cyclin B

We previously reported that CDK2/Cyclin A can phosphorylate and activate the transcription factor NF-Y. In this study, we investigated a potential regulatory role for NF-Y in the transcription of Cyclin A and other cell cycle regulatory genes. Gel-shift assays demonstrate that NF-Y binds to CCAAT sequences in the Cyclin A promoter, as well as to those in the promoters of cell cycle G2 regulators such as CDC2, Cyclin B and CDC25C. Furthermore, expression of Cyclin A increases NF-Y's affinity for CCAAT sequences in the CDC2 promoter; however, Cyclin A's induction of CDC2 transcription is antagonized by p21, an inhibitor of CDK2/Cyclin A. These results suggest a model wherein NF-Y binds to and activates transcription from the Cyclin A promoter, increasing cellular levels of Cyclin A/CDK2 and potentiating NF-Y's capacity for transcriptional transactivation, and imply a positive feedback loop between NF-Y and Cyclin A/CDK2. Our findings are additionally indicative of a role for Cyclin A in activating Cyclin B/CDK1 through promoting NF-Y dependent transcription of Cyclin B and CDC2; NF-Y mediated crosstalk may therefore help to orchestrate cell-cycle progression.

p41-Arc, a regulatory subunit of Arp2/3 complex, can induce premature senescence in the absence of p53 and Rb

Cellular senescence is a tumor-suppressive process instigated by proliferation in the absence of telomere replication, by cellular stresses such as oncogene activation, or by activation of the tumor suppressor proteins, such as Rb or p53. This process is characterized by an irreversible cell cycle exit, a unique morphology, and expression of senescence-associated-β-galactosidase (SA-β-gal). Despite the potential biological importance of cellular senescence, little is known of the mechanisms leading to the senescent phenotype. p41-Arc has been known to be a putative regulatory component of the mammalian Arp2/3 complex, which is required for the formation of branched networks of actin filaments at the cell cortex. In this study, we demonstrate that p41-Arc can induce senescent phenotypes when it is overexpressed in human tumor cell line, SaOs-2, which is deficient in p53 and Rb tumor suppressor genes, implying that p41 can induce senescence in a p53-independent way. p41-Arc overexpression causes a change in actin filaments, accumulating actin filaments in nuclei. Therefore, these results imply that a change in actin filament can trigger an intrinsic senescence program in the absence of p53 and Rb tumor suppressor genes.

The NF-Y/p53 liaison: well beyond repression

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

Suppression of Polo like kinase 1 (PLK1) by p21(Waf1) mediates the p53-dependent prevention of caspase-independent mitotic death

Polo-like kinase 1 (Plk1) plays key roles in many aspects of mitosis. We have previously shown that induction of p21(Waf1) by p53 is responsible for protection of cells against adriamycin-induced polyploidy formation and mitotic catastrophe. Here we show that adriamycin treatment suppressed Plk1 expression in a p53- and p21(Waf1)-dependent manner. Ablation of p21(Waf1) inhibited the adriamycin-induced p53 activation, and this inhibition was alleviated by knockdown of Plk1, suggesting that p21(Waf1)-dependent suppression of Plk1 expression is responsible for maintaining p53 activation during stress response. Plk1 associated with p53 and disrupted its interaction with target gene promoters in cells treated with adriamycin. Overexpression of Plk1 inhibited the p53-mediated prevention of caspase-independent mitotic death, but not polyploidy formation, in adriamycin-treated cells. Together our results indicate that suppression of Plk1 by p21(Waf1) is responsible for p53-dependent protection against adriamycin-induced caspase-independent mitotic death.

The p53-p21WAF1 checkpoint pathway plays a protective role in preventing DNA rereplication induced by abrogation of FOXF1 function

We previously identified FOXF1 as a potential tumor suppressor gene with an essential role in preventing DNA rereplication to maintain genomic stability, which is frequently inactivated in breast cancer through the epigenetic mechanism. Here we further addressed the role of the p53-p21(WAF1) checkpoint pathway in DNA rereplication induced by silencing of FOXF1. Knockdown of FOXF1 by small interference RNA (siRNA) rendered colorectal p53-null and p21(WAF1)-null HCT116 cancer cells more susceptible to rereplication and apoptosis than the wild-type parental cells. In parental HCT116 cells with a functional p53 checkpoint, the p53-p21(WAF1) checkpoint pathway was activated upon FOXF1 knockdown, which was concurrent with suppression of the CDK2-Rb cascade and induction of G(1) arrest. In contrast, these events were not observed in FOXF1-depleted HCT116-p53-/- and HCT116-p21-/- cells, indicating that the p53-dependent checkpoint function is vital for inhibiting CDK2 to induce G(1) arrest and protect cells from rereplication. The pharmacologic inhibitor (caffeine) of ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3 related (ATR) protein kinases abolished activation of the p53-p21(WAF1) pathway upon FOXF1 knockdown, suggesting that suppression of FOXF1 function triggered the ATM/ATR-mediated DNA damage response. Cosilencing of p53 by siRNA synergistically enhanced the effect of FOXF1 depletion on the stimulation of DNA rereplication and apoptosis in wild-type HCT116. Finally, we show that FOXF1 expression is predominantly silenced in breast and colorectal cancer cell lines with inactive p53. Our study demonstrated that the p53-p21(WAF1) checkpoint pathway is an intrinsically protective mechanism to prevent DNA rereplication induced by silencing of FOXF1.

MDM2 promotes SUMO-2/3 modification of p53 to modulate transcriptional activity

The tumor suppressor p53 is extensively regulated by post-translational modification, including modification by the small ubiquitin-related modifier SUMO. We show here that MDM2, previously shown to promote ubiquitin, Nedd8 and SUMO-1 modification of p53, can also enhance conjugation of endogenous SUMO-2/3 to p53. Sumoylation activity requires p53-MDM2 binding but does not depend on an intact RING finger. Both ARF and L11 can promote SUMO-2/3 conjugation of p53. However, unlike the previously described SUMO-1 conjugation of p53 by an MDM2-ARF complex, this activity does not depend on the ability of MDM2 to relocalize to the nucleolus. Interestingly, the SUMO consensus is not conserved in mouse p53, which is therefore not modified by SUMO-2/3. Finally, we show that conjugation of SUMO-2/3 to p53 correlates with a reduction of both activation and repression of a subset of p53-target genes.

Transcriptional modulation induced by ionizing radiation: p53 remains a central player

The cellular response to DNA damage is vital for maintaining genomic stability and preventing undue cell death or cancer formation. The DNA damage response (DDR), most robustly mobilized by double-strand breaks (DSBs), rapidly activates an extensive signaling network that affects numerous cellular systems, leading to cell survival or programmed cell death. A major component of the DDR is the widespread modulation of gene expression. We analyzed together six datasets that probed transcriptional responses to ionizing radiation (IR) - our novel experimental data and 5 published datasets - to elucidate the scope of this response and identify its gene targets. According to the mRNA expression profiles we recorded from 5 cancerous and non-cancerous human cell lines after exposure to 5 Gy of IR, most of the responses were cell line-specific. Computational analysis identified significant enrichment for p53 target genes and cell cycle-related pathways among groups of up-regulated and down-regulated genes, respectively. Computational promoter analysis of the six datasets disclosed that a statistically significant number of the induced genes contained p53 binding site signatures. p53-mediated regulation had previously been documented for subsets of these gene groups, making our lists a source of novel potential p53 targets. Real-time qPCR and chromatin immunoprecipitation (ChIP) assays validated the IR-induced p53-dependent induction and p53 binding to the respective promoters of 11 selected genes. Our results demonstrate the power of a combined computational and experimental approach to identify new transcriptional targets in the DNA damage response network.

The nuclear exporter, Crm1, is regulated by NFY and Sp1 in cancer cells and repressed by p53 in response to DNA damage

The nuclear exporter protein, Crm1, plays a key role in normal cell functioning, mediating the nucleo-cytoplasmic transport of cargo proteins. Elevated Crm1 expression has recently been identified in various tumours; however, the mechanisms driving its expression have not been investigated to date. In this study we identified the Crm1 promoter and factors associated with its elevated expression and with its repression under conditions of DNA damage. The -1405 to +99 Crm1 promoter region was found to be significantly more active in cancer and transformed cells compared to normal, and the -175 to +99 region identified as responsible for the differential activity. Mutation of two CCAAT boxes and a GC box within this region significantly diminished Crm1 promoter activity and ChIP analysis revealed binding of NFY and Sp1 to these sites, with increased binding in transformed and cancer cells. In addition, p53 was found to repress Crm1 promoter activity, after induction with doxorubicin, with p53 siRNA blocking the effect. Crm1 promoter constructs with mutated CCAAT boxes were significantly less responsive to p53 repression, and in vivo binding of NFY to the CCAAT boxes was diminished upon p53 binding, suggesting that p53 mediates repression of the Crm1 promoter via interfering with NFY. This was confirmed using NFY knock-down cells, in which Crm1 promoter activity was significantly less responsive to p53. In vitro EMSAs revealed that NFY and p53 bind the CCAAT boxes as a single complex under conditions of DNA damage. In summary, this study is a first to analyse Crm1 promoter regulation and reveals NFY and Sp1 as contributors to Crm1 overexpression in cancer. In addition, this study reveals that Crm1 transcription is inhibited by DNA damage and that the mechanism of inhibition involves p53 interfering with NFY function.

To repress or not to repress: this is the guardian's question

p53 is possibly the most central tumor suppressor gene of our cells, integrating stress signals to activate a transcriptional program responsible for maintaining cellular homeostasis. Many of the downstream effects of p53 are a consequence of its activity as a transcription factor, resulting in the induction of multiple target genes. In addition to gene activation, however, gene repression is an essential part of the p53 cellular response. Despite extensive research efforts towards the elucidation of p53 functions, the molecular mechanisms and biological consequences of gene repression by p53 have not been studied extensively. We review our current knowledge of the mechanisms and biological consequences of p53 repression, with special attention to recently discovered mechanisms of repression that involve non-coding RNA molecules, an emerging aspect of regulation in the p53 cellular network.

ATM-dependent IGF-1 induction regulates secretory clusterin expression after DNA damage and in genetic instability

Secretory clusterin (sCLU) is a stress-induced, pro-survival glycoprotein elevated in early-stage cancers, in particular in APC/Min-defective colon cancers. sCLU is upregulated after exposure to various cytotoxic agents, including ionizing radiation (IR), leading to a survival advantage. We found that stimulation of insulin-like growth factor-1 (IGF-1) and IGF-1R protein kinase signaling was required for sCLU induction after IR exposure. Here, we show that activation of Ataxia telangiectasia-mutated kinase (ATM) by endogenous or exogenous forms of DNA damage was required to relieve basal repression of IGF-1 transcription by the p53/NF-YA complex, leading to sCLU expression. Although p53 levels were stabilized and elevated after DNA damage, dissociation of NF-YA, and thereby p53, from the IGF-1 promoter resulted in IGF-1 induction, indicating that NF-YA was rate limiting. Cells with elevated endogenous DNA damage (deficient in H2AX, MDC1, NBS1, mTR or hMLH1) or cells exposed to DNA-damaging agents had elevated IGF-1 expression, resulting in activation of IGF-1R signaling and sCLU induction. In contrast, ATM-deficient cells were unable to induce sCLU after DNA damage. Our results integrate DNA damage resulting from genetic instability, IR, or chemotherapeutic agents, to ATM activation and abrogation of p53/NF-YA-mediated IGF-1 transcriptional repression, that induces IGF-1-sCLU expression. Elucidation of this pathway should uncover new mechanisms for cancer progression and reveal new targets for drug development to overcome resistance to therapy.

One function--multiple mechanisms: the manifold activities of p53 as a transcriptional repressor

Maintenance of genome integrity is a dynamic process involving complex regulation systems. Defects in one or more of these pathways could result in cancer. The most important tumor-suppressor is the transcription factor p53, and its functional inactivation is frequently observed in many tumor types. The tumor suppressive function of p53 is mainly attributed to its ability to regulate numerous target genes at the transcriptional level. While the mechanism of transcriptional induction by p53 is well characterized, p53-dependent repression is not understood in detail. Here, we review the manifold mechanisms of p53 as a transcriptional repressor. We classify two different categories of repressed genes based on the underlying mechanism, and novel mechanisms which involve regulation through noncoding RNAs are discussed. The complete elucidation of p53 functions is important for our understanding of its tumor-suppressor activity and, therefore, represents the key for the development of novel therapeutic approaches.

Dioxin toxicity, aryl hydrocarbon receptor signaling, and apoptosis-persistent pollutants affect programmed cell death

Exogenous ligands of the aryl hydrocarbon receptor (AhR) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related substances are highly toxic pollutants ubiquitously present in the environment. They cause a variety of toxic effects to different organs and tissues. Among other effects, TCDD exposure to laboratory animals leads to thymus atrophy and immunosuppression on the one hand, and to tumor formation on the other. Apoptosis appears to be involved in both these toxic effects: AhR activation by TCDD was discussed to induce apoptosis of immune cells, leading to the depletion of thymocytes and ultimately immunosuppression. This mechanism could help to explain the highly immunotoxic actions of TCDD but it is nevertheless under debate whether this is the mode of action for immunosuppression by this class of chemical substances. In other cell types, especially liver cells, TCDD inhibits apoptosis induced by genotoxic treatment. In initiation-promotion studies, TCDD was shown to be a potent liver tumor promoter. Among other theories it was hypothesized that TCDD acts as a tumor promoter by preventing initiated cells from undergoing apoptosis. The exact mechanisms of apoptosis inhibition by TCDD are not fully understood, but both in vivo and in vitro studies consistently showed an involvement of the tumor suppressor p53 in this effect. Various strings of evidence have been established linking apoptosis to the detrimental effects of exogenous activation of the AhR. Within this article, studies elucidating the effects of TCDD and related substances on apoptosis signaling, be it inducing or repressing, is to be reviewed.

DeltaNp63 transcriptionally regulates ATM to control p53 Serine-15 phosphorylation

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Activation of GPR30 inhibits the growth of prostate cancer cells through sustained activation of Erk1/2, c-jun/c-fos-dependent upregulation of p21, and induction of G(2) cell-cycle arrest

G-protein-coupled receptor-30 (GPR30) shows estrogen-binding affinity and mediates non-genomic signaling of estrogen to regulate cell growth. We here showed for the first time, in contrast to the reported promoting action of GPR30 on the growth of breast and ovarian cancer cells, that activation of GPR30 by the receptor-specific, non-estrogenic ligand G-1 inhibited the growth of androgen-dependent and androgen-independent prostate cancer (PCa) cells in vitro and PC-3 xenografts in vivo. However, G-1 elicited no growth or histological changes in the prostates of intact mice and did not inhibit growth in quiescent BPH-1, an immortalized benign prostatic epithelial cell line. Treatment of PC-3 cells with G-1 induced cell-cycle arrest at the G(2) phase and reduced the expression of G(2)-checkpoint regulators (cyclin-A2, cyclin-B1, cdc25c, and cdc2) and phosphorylation of their common transcriptional regulator NF-YA in PC-3 cells. With extensive use of siRNA-knockdown experiments and the MEK inhibitor PD98059 in this study, we dissected the mechanism underlying G-1-induced inhibition of PC-3 cell growth, which was mediated through GPR30, followed by sustained activation of Erk1/2 and a c-jun/c-fos-dependent upregulation of p21, resulting in the arrest of PC-3 growth at the G(2) phase. The discovery of this signaling pathway lays the foundation for future development of GPR30-based therapies for PCa.

Combination of nutlin-3 and VX-680 selectively targets p53 mutant cells with reversible effects on cells expressing wild-type p53

Chemotherapeutics (e.g., aurora kinase inhibitors) designed to target proliferative cells are often nonspecific for tumor cells as normal cycling cells are also susceptible. Indeed, one of the major dose-limiting toxicities of aurora kinase inhibitors is a dangerous depletion of neutrophils in patients. In this study we proposed a strategy to selectively target p53 mutant cells while sparing normal ones. The strategy is based on the understanding that normal cells have an intact p53 pathway but not tumor cells carrying p53 mutations. Nongenotoxic activation of p53 using nutlin led to a reversible activation of G1 and G2 arrest in normal cells, which prevents them from entering mitosis, thus protecting them from the side effects of aurora kinase inhibition (VX-680), namely endoreduplication and apoptosis. Cells carrying mutant p53 are selectively killed by the nutlin/VX-680 combination, whereas p53 wild-type cells retain their proliferative capacity. The major implications drawn from these results are: (1) reversible nongenotoxic activation of p53 may be used as a strategy for the chemoprotection of normal tissues, and (2) aurora kinase inhibitors may have alleviated side effects when used in combination with nutlin-like inhibitors. We highlight the distinct roles of p53 and p73 in mediating the cellular responses to VX-680 and suggest that dual protection by p53 and p73 are needed to guard against endoreduplication and polyploidy.

The cell line secretome, a suitable tool for investigating proteins released in vivo by tumors: application to the study of p53-modulated proteins secreted in lung cancer cells

Malignant processes such as metastasis, invasion, or angiogenesis are tightly dependent on the composition of the extracellular medium, which is itself affected by the release of proteins by the tumor cells. p53, a major tumor suppressor protein very frequently mutated and/or inactivated in cancer cells, is known to modulate the release of proteins by the tumor cells; however, while p53-modulated intracellular proteins have been extensively studied, little is known concerning their extracellular counterparts. Here, we characterized the p53-dependent secretome of a lung tumor model in vitro (H358 human nonsmall cell lung adenocarcinoma cell line with a homozygous deletion of p53) and demonstrate that the modulation of exported proteins can also be detected in vivo in the plasma of tumor-bearing mice. We used a clone of H358, stably transfected with a tetracycline-inducible wild-type p53-expressing vector. With the use of iTRAQ labeling and LC-MALDI-MS/MS analysis, we identified 909 proteins released in vitro by the cells, among which 91 are p53-modulated. Three proteins (GDF-15, FGF-19, and VEGF) were also investigated in H358/TetOn/p53 xenograft mice. The ELISA dosage on total tumor protein extracts confirmed the influence of p53 on the release of these proteins in vivo. Moreover, the GDF-15 concentration was measured in the plasma and its p53-dependent modulation was confirmed. To our knowledge, this is the first report establishing that the in vitro cell line secretome is reliable and reflects the extracellular release of proteins from tumor cells in vivo and could be used to identify putative tumor markers.

A rationally designed histone deacetylase inhibitor with distinct antitumor activity against ovarian cancer

Histone deacetylase inhibitors (HDACIs) are a class of antineoplastic agents previously demonstrating preclinical chemosensitizing activity against drug-resistant cancer cells and mouse xenografts. However, whereas clinical studies have shown efficacy against human hematologic malignancies, solid tumor trials have proved disappointing. We previously developed a novel HDACI, "OSU-HDAC42," and herein examine its activity against ovarian cancer cell lines and xenografts. OSU-HDAC42, (i) unlike most HDACIs, elicited a more than five-fold increase in G(2)-phase cells, at 2.5 microM, with G(2) arrest followed by apoptosis; (ii) at 1.0 microM, completely repressed messenger RNA expression of the cell cycle progression gene cdc2; (iii) at low doses (0.25-1.0 microM for 24 hours), induced tumor cell epithelial differentiation, as evidenced by morphology changes and a more than five-fold up-regulation of epithelium-specific cytokeratins; (iv) potently abrogated the growth of numerous ovarian cancer cells, with IC(50) values of 0.5 to 1.0 microM, whereas also remaining eight-fold less toxic (IC(50) of 8.6 microM) to normal ovarian surface epithelial cells; and (v) chemosensitizated platinum-resistant mouse xenografts to cisplatin. Compared with the clinically approved HDACI suberoylanilide hydroxamic acid (vorinostat), 1.0 microM OSU-HDAC42 was more biochemically potent (i.e., enzyme-inhibitory), as suggested by greater gene up-regulation and acetylation of both histone and nonhistone proteins. In p53-dysfunctional cells, however, OSU-HDAC42 was two- to eight-fold less inductive of p53-regulated genes, whereas also having a two-fold higher IC(50) than p53-functional cells, demonstrating some interaction with p53 tumor-suppressive cascades. These findings establish OSU-HDAC42 as a promising therapeutic agent for drug-resistant ovarian cancer and justify its further investigation.

Repression of SHP-1 expression by p53 leads to trkA tyrosine phosphorylation and suppression of breast cancer cell proliferation

The nerve growth factor (NGF) receptor, trkA, the tumour suppressor p53 and the phosphatase SHP-1 are critical in cell proliferation and differentiation. SHP-1 is a trkA phosphatase that dephosphorylates trkA at tyrosines (Y) 674 and 675. p53 can induce trkA activation and tyrosine phosphorylation in the absence of NGF stimulation. In breast cancer tumours trkA expression is associated with increased patient survival. TrkA protein expression is higher in breast-cancer cell lines than in normal breast epithelia. In cell lines (but not in normal breast epithelia) trkA is functional and can be NGF-stimulated to promote cell proliferation. This study investigates the functional relationship between trkA, p53 and SHP-1 in breast-cancer, and reveals that in wild-type (wt) trkA expressing breast-cancer cells both endogenous wtp53, activated by therapeutic agents, and transfected wtp53 repress expression of SHP-1 through the proximal CCAAT sequence of the SHP-1-P1-promoter and the transcription factor NF-Y. In these cells trkA-Y674/Y675 phosphorylation is detected when SHP-1 protein levels decrease in a wtp53-dependent manner. Proliferation and cell-cycle assays, with cells expressing endogenous or transfected wt-trkA and a temperature-sensitive p53 grown at 32 degrees C (when p53 is in the wt configuration), show suppressed cell proliferation. Suppression is not detected when grown at 37 degrees C (when p53 is in the mutant configuration). A release from suppression is observed when these cells are transiently transfected with wt-SHP-1 and grown at 32 degrees C. Suppression is also detected when, as control, wt-trkA-expressing cells are transiently transfected with SHP-1-siRNA, but not when a dominant-negative (DN) mutant trkA is used to abolish wt-trkA activity. Importantly, suppression is not seen with control trkA-negative breast-cancer cells (expressing wtp53, wt-SHP-1 and undetectable trkA), transfected with Y674F/Y675F mutant-trkA. BrdU-incorporation experiments reveal lack of incorporation in cells expressing wt-trkA and wtp53, or wt-trkA and SHP-1-siRNA. However, BrdU is incorporated in the presence of Y674F/Y675F mutant trkA or DN mutant trkA. These results indicate that p53 repression of SHP-1 expression leads to trkA-Y674/Y675 phosphorylation and trkA-dependent suppression of breast-cancer cell proliferation. These data provide an explanation as to why high trkA levels are associated with favourable prognosis.

Pro-proliferative FoxM1 is a target of p53-mediated repression

The p53 tumor suppressor protein acts as a transcription factor to modulate cellular responses to a wide variety of stresses. In this study we show that p53 is required for the downregulation of FoxM1, an essential transcription factor that regulates many G2/M-specific genes and is overexpressed in a multitude of solid tumors. After DNA damage, p53 facilitates the repression of FoxM1 mRNA, which is accompanied by a decrease in FoxM1 protein levels. In cells with reduced p53 expression, FoxM1 is upregulated after DNA damage. Nutlin, a small-molecule activator of p53, suppresses FoxM1 levels in two cell lines in which DNA damage facilitates only mild repression. Mechanistically, p53-mediated inhibition of FoxM1 is partially p21 and retinoblastoma (Rb) family dependent, although in some cases p21-independent repression of FoxM1 was also observed. The importance of FoxM1 to cell fate was indicated by the observation that G2/M arrest follows FoxM1 ablation. Finally, our results indicate a potential contribution of p53-mediated repression of FoxM1 for maintenance of a stable G2 arrest.

B-MYB is required for recovery from the DNA damage-induced G2 checkpoint in p53 mutant cells

In response to DNA damage, several signaling pathways that arrest the cell cycle in G(1) and G(2) are activated. The down-regulation of mitotic genes contributes to the stable maintenance of the G(2) arrest. The human LINC or DREAM complex, together with the B-MYB transcription factor, plays an essential role in the expression of G(2)-M genes. Here, we show that DNA damage results in the p53-dependent binding of p130 and E2F4 to LINC and the dissociation of B-MYB from LINC. We find that B-MYB fails to dissociate from LINC in p53 mutant cells, that this contributes to increased G(2)-M gene expression in response to DNA damage in these cells, and, importantly, that B-MYB is required for recovery from the G(2) DNA damage checkpoint in p53-negative cells. Reanalysis of microarray expression data sets revealed that high levels of B-MYB correlate with a p53 mutant status and an advanced tumor stage in primary human breast cancer. Taken together, these data suggest that B-MYB/LINC plays an important role in the DNA damage response downstream of p53.

Induction of G2/M arrest by pseudolaric acid B is mediated by activation of the ATM signaling pathway

AIM

The aim of this study was to investigate the mechanism of pseudolaric acid B (PLAB)-induced cell cycle arrest in human melanoma SK-28 cells.

METHODS

Cell growth inhibition was detected by MTT assay, the cell cycle was analyzed by flow cytometry, and protein expression was examined by Western blot analysis.

RESULTS

PLAB inhibited the growth of human melanoma cells and induced G(2)/M arrest in SK-28 cells, accompanied by an up-regulation of Cdc2 phosphorylation and a subsequent down-regulation of Cdc2 expression. Furthermore, PLAB decreased the expression of Cdc25C phosphatase and increased the expression of Wee1 kinase. Meanwhile, a reduction in Cdc2 activity was partly due to induction of the expression of p21(waf1/cip1) in a p53-dependent manner. In addition, PLAB activated the checkpoint kinase, Chk2, and increased the expression of p53, two major targets of ATM kinase. These effects were inhibited by caffeine, an ATM kinase inhibitor. We also found that PLAB significantly enhanced ATM kinase activity.

CONCLUSION

Taken together, these results suggest that PLAB induced G(2)/M arrest in human melanoma cells via a mechanism involving the activation of ATM, and the effect of PLAB on Cdc2 activity was mediated via interactions with the Chk2-Cdc25C and p53 signalling pathways, two distinct downstream pathways of ATM. PLAB may be a promising chemopreventive agent for treating human melanoma.

Cdk2 and Cdk4 activities are dispensable for tumorigenesis caused by the loss of p53

The loss of p53 induces spontaneous tumors in mice, and p53 mutations are found in approximately 50% of human tumors. These tumors are generally caused by a number of events, including genomic instability, checkpoint defects, mitotic defects, deregulation of transcriptional targets, impaired apoptosis, and G(1) deregulation or a combination of these effects. In order to determine the role of proteins involved in G(1) control in tumorigenesis, we focused on Cdk2 and Cdk4, two cyclin-dependent kinases that in association with cyclin E and cyclin D promote the G(1)/S phase transition. We analyzed the consequence of loss of Cdk2 in p53-null animals by generating Cdk2(-/-) p53(-/-) mice. These mice are viable and developed spontaneous tumors, predominantly lymphoblastic lymphomas, similar to p53(-/-) mice. In contrast, the genotypes Cdk4(-/-) p53(-/-) were mostly lethal, with few exceptions, and Cdk2(-/-) Cdk4(-/-) p53(-/-) mice die during embryogenesis at embryonic day 13.5. To study the oncogenic potential, we generated mouse embryonic fibroblasts (MEFs) and found that p53(-/-), Cdk2(-/-) p53(-/-), Cdk4(-/-) p53(-/-), and Cdk2(-/-) Cdk4(-/-) p53(-/-) MEFs grew at similar rates without entering senescence. Ras-transformed MEFs of these genotypes were able to form colonies in vitro and induce tumors in nude mice. Our results suggest that tumorigenicity mediated by p53 loss does not require either Cdk2 or Cdk4, which necessitates considering the use of broad-spectrum cell cycle inhibitors as a means of effective anti-Cdk cancer therapy.

DNA damage induced p53 downregulates Cdc20 by direct binding to its promoter causing chromatin remodeling

CDC20 is a critical molecule in the Spindle Assembly Checkpoint (SAC). It activates the Anaphase promoting complex and helps a dividing cell to proceed towards Anaphase. CDC20 is overexpressed in many tumor cells which cause chromosomal instability. There have been limited reports on the mechanism of SAC's response to genotoxic stress. We show that ectopically expressed p53 or DNA damage induced endogenous p53 can downregulate Cdc20 transcriptionally. We have identified a consensus p53-binding site on the Cdc20 promoter and have shown that it is being used by p53 to bind the promoter and bring about chromatin remodeling thereby repressing Cdc20. Additionally, p53 also downregulates Cdc20 promoter through CDE/CHR element, but in a p21 independent manner. This CDE/CHR element-mediated downregulation occurs only under p53 overexpressed condition but not in the context of DNA damage. The present results suggest that the two CCAAT elements in the Cdc20 promoter are not used by p53 to downregulate its activity, as reported earlier.

Upregulation of the cycline kinase subunit CKS2 increases cell proliferation rate in gastric cancer

PURPOSE

CKS2 was identified as an upregulated gene in gastric cancer via our DNA microarray. This study was to verify the upregulation of CKS2 in many gastric cancer patients and to examine the CKS2-mediated cellular response.

METHODS

CKS2 upregulation was analyzed using reverse transcriptase PCR, real-time PCR, and immunohistochemical and clinicopathological analyses. GFP-CKS2 or CKS2-siRNA was used to analyze the cellular localization and proliferation.

RESULTS

The strong upregulation of mRNA and protein levels of CKS2 was identified. In CKS2-overexpressing cells, tumor suppressor p53 and p21(cip1) were downregulated and cell growth was increased. In contrast, CKS2-siRNA-transfected cells showed an increased tumor suppressor expression and decreased cell growth.

CONCLUSIONS

We showed that CKS2 was significantly upregulated in gastric cancers and a high level of CKS2 was highly correlated with histologic tumor differentiation and pathological grade of the tumor size, lymph node, and metastasis stage. We suggest that the cell cycle regulator CKS2 might be deeply involved in gastric cancer progression.