p53 levels, functional domains, and DNA damage determine the extent of the apoptotic response of tumor cells

Arsenic exposure disrupts epigenetic regulation of SIRT1 in human keratinocytes

Arsenic is an environmental toxin which increases skin cancer risk for exposed populations worldwide; however the underlying biomolecular mechanism for arsenic-induced carcinogenesis is complex and poorly defined. Recent investigations show that histone deacetylase and DNA methyltransferase activity is impaired, and epigenetic patterns of gene regulation are consistently altered in cancers associated with arsenic exposure. Expression of the histone deacetylase SIRT1 is altered in solid tumours and haematological malignancies; however its role in arsenic-induced pathology is unknown. In this study we investigated the effect of arsenic on epigenetic regulation of SIRT1 and its targeting microRNA, miR-34a in primary human keratinocytes. Acetylation of histone H4 at lysine 16 (H4K16) increased in keratinocytes exposed to 0.5μM arsenite [As(III)]; and this was associated with chromatin remodelling at the miR-34a promoter. Moreover, although SIRT1 protein initially increased in these As(III)-exposed cells, after 24days expression was not significantly different from untreated controls. Extended exposure to low-dose As(III) (0.5μM; >5weeks) compromised the pattern of CpG methylation at SIRT1 and miR-34a gene promoters, and this was associated with altered expression for both genes. We have found that arsenic alters epigenetic regulation of SIRT1 expression via structural reorganisation of chromatin at the miR-34a gene promoter in the initial 24h of exposure; and over time, through shifts in miR-34a and SIRT1 gene methylation. Taken together, this investigation demonstrates that arsenic produces cumulative disruptions to epigenetic regulation of miR-34a expression, and this is associated with impaired coordination of SIRT1 functional activity.

Wig-1 regulates cell cycle arrest and cell death through the p53 targets FAS and 14-3-3σ

Wig-1, also known as ZMAT3, is a p53 target gene that encodes an RNA-binding zinc-finger protein involved in the regulation of mRNA stability through binding to AU-rich elements (AREs). We have used microarray analysis to identify novel Wig-1 target mRNAs. We identified 2447 transcripts with >fourfold differential expression between Wig-1 and control small interfering (si)RNA-treated HCT116 cells. Several p53 target genes were among the deregulated transcripts. We found that Wig-1 regulates FAS and 14-3-3σ mRNA independently of p53. We show that Wig-1 binds to FAS mRNA 3'-UTR and decreases its stability through an ARE in the 3'-UTR. Depletion of Wig-1 was associated with increased cell death and reduced cell cycle arrest upon DNA damage. Our results suggest a role of Wig-1 as a survival factor that directs the p53 stress response toward cell cycle arrest rather than apoptosis through the regulation of FAS and 14-3-3σ mRNA levels.

Identification and characterization of Dicer1e, a Dicer1 protein variant, in oral cancer cells

Background

The human dicer1 gene has been predicted to produce several mRNA variants that encode truncated Dicer1 proteins of varying lengths. One of these Dicer1 variants, Dicer1e, was recently found to be differentially expressed in breast cancer cells. Because the expression and function of the Dicer1e protein variant has not been well characterized and the underlying molecular mechanisms for the development of oral squamous cell carcinomas (OSCCs) are poorly understood, the present study sought to characterize the biological role of Dicer1e and determine its relationship, if any, to OSCC pathogenesis.

Methods

Western blot analyses were used to examine Dicer1e expression levels in a panel of oral cancer cells/tissues and during epithelial-mesenchymal transition (EMT), followed by 5′/3′-RACE analyses to obtain the full-length Dicer1e transcript. Biochemical fractionation and indirect immunofluorescent studies were performed to determine the cellular localization of Dicer1e and the effects of Dicer1e silencing on cancer cell proliferation, clonogenicity, and drug sensitivity were also assessed.

Results

Dicer1e protein levels were found to be overexpressed in OSCC cell lines of epithelial phenotype and in OSCC tissues with its levels downregulated during EMT. Moreover, the Dicer1e protein was observed to predominantly localize in the nucleus. 5′/3′-RACE analyses confirmed the presence of the Dicer1e transcript and silencing of Dicer1e impaired both cancer cell proliferation and clonogenicity by inducing either apoptosis and/or G2/M cell cycle arrest. Lastly, Dicer1e knockdown enhanced the chemosensitivity of oral cancer cells to cisplatin.

Conclusion

The expression levels of Dicer1e influence the pathogenesis of oral cancer cells and alter their response to chemosensitivity, thus supporting the importance of Dicer1e as a therapeutic target for OSCCs.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-190) contains supplementary material, which is available to authorized users.

Re-engineered p53 activates apoptosis in vivo and causes primary tumor regression in a dominant negative breast cancer xenograft model

Inactivation of p53 pathway is reported in more than half of all human tumors and can be correlated to malignant development. Missense mutation in the DNA binding region (DBD) of p53 is the most common mechanism of p53 inactivation in cancer cells. The resulting tumor-derived p53 variants, similar to wild-type (wt) p53, retain their ability to oligomerize via the tetramerization domain (TD). Upon hetero-oligomerization, mutant p53 enforces a dominant negative effect over active wt-p53 in cancer cells. To overcome this barrier, we have previously designed a chimeric superactive p53 (p53-CC) with an alternative oligomerization domain capable of escaping transdominant inhibition by mutant p53 in vitro. In this report, we demonstrate the superior tumor suppressor activity of p53-CC and its ability to cause tumor regression of the MDA-MB-468 aggressive p53-dominant negative breast cancer tumor model in vivo. In addition, we illustrate the profound effects of the dominant negative effect of endogenous mutant p53 over wt-p53 in cancer cells. Finally, we investigate the underlying differential mechanisms of activity for p53-CC and wt-p53 delivered using viral-mediated gene therapy approach in the MDA-MB-468 tumor model.

Lysine-specific modifications of p53: a matter of life and death?

Post-translational modifications provide a fine-tuned control of protein function(s) in the cell. The well-known tumour suppressor p53 is subject to many post-translational modifications, which alter its activity, localization and stability, thus ultimately modulating its response to various forms of genotoxic stress. In this review, we focus on the role of recently discovered lysine-specific modifications of p53, methylation and acetylation in particular, and their effects on p53 activity in damaged cells. We also discuss a possibility of mutual influence of covalent modifications in the p53 and histone proteins located in the vicinity of p53 binding sites in chromatin and propose important ramifications stemming from this hypothesis.

Spliced MDM2 isoforms promote mutant p53 accumulation and gain-of-function in tumorigenesis

Tumor suppressor p53 is frequently mutated in tumors. Mutant p53 (Mutp53) proteins often gain new activities in promoting tumorigenesis, defined as gain-of-function (GOF). Mutp53 often accumulates at high levels in tumors, which promotes mutp53 GOF in tumorigenesis. The mechanism of mutp53 accumulation is poorly understood. Here we find that MDM2 isoforms promote mutp53 accumulation in tumors. MDM2 isoform B (MDM2-B), the MDM2 isoform most frequently over-expressed in human tumors, interacts with full-length MDM2 to inhibit MDM2-mediated mutp53 degradation, promoting mutp53 accumulation and GOF in tumorigenesis. Furthermore, MDM2-B over-expression correlates with mutp53 accumulation in human tumors. In mutp53 knock-in mice, a MDM2 isoform similar to human MDM2-B is over-expressed in the majority of tumors, which promotes mutp53 accumulation and tumorigenesis. Thus, over-expression of MDM2 isoforms promotes mutp53 accumulation in tumors, contributing to mutp53 GOF in tumorigenesis. Furthermore, promoting mutp53 accumulation and GOF is an important mechanism by which MDM2 isoforms promote tumorigenesis.

Global analysis of p53-regulated transcription identifies its direct targets and unexpected regulatory mechanisms

The p53 transcription factor is a potent suppressor of tumor growth. We report here an analysis of its direct transcriptional program using Global Run-On sequencing (GRO-seq). Shortly after MDM2 inhibition by Nutlin-3, low levels of p53 rapidly activate ∼200 genes, most of them not previously established as direct targets. This immediate response involves all canonical p53 effector pathways, including apoptosis. Comparative global analysis of RNA synthesis vs steady state levels revealed that microarray profiling fails to identify low abundance transcripts directly activated by p53. Interestingly, p53 represses a subset of its activation targets before MDM2 inhibition. GRO-seq uncovered a plethora of gene-specific regulatory features affecting key survival and apoptotic genes within the p53 network. p53 regulates hundreds of enhancer-derived RNAs. Strikingly, direct p53 targets harbor pre-activated enhancers highly transcribed in p53 null cells. Altogether, these results enable the study of many uncharacterized p53 target genes and unexpected regulatory mechanisms.DOI: http://dx.doi.org/10.7554/eLife.02200.001.

Drugging the p53 pathway: understanding the route to clinical efficacy

The tumour suppressor p53 is the most frequently mutated gene in human cancer, with more than half of all human tumours carrying mutations in this particular gene. Intense efforts to develop drugs that could activate or restore the p53 pathway have now reached clinical trials. The first clinical results with inhibitors of MDM2, a negative regulator of p53, have shown efficacy but hint at on-target toxicities. Here, we describe the current state of the development of p53 pathway modulators and new pathway targets that have emerged. The challenge of targeting protein-protein interactions and a fragile mutant transcription factor has stimulated many exciting new approaches to drug discovery.

The chemosensitivity of testicular germ cell tumors

Although rare cancers overall, testicular germ cell tumors (TGCTs) are the most common type of cancer in young males below 40 years of age. Both subtypes of TGCTs, i.e., seminomas and non-seminomas, are highly curable and the majority of even metastatic patients may expect to be cured. These high cure rates are not due to the indolent nature of these cancers, but rather to their sensitivity to chemotherapy (and for seminomas to radiotherapy). The delineation of the cause of chemosensitivity at the molecular level is of paramount importance, because it may provide insights into the minority of TGCTs that are chemo-resistant and, thereby, provide opportunities for specific therapeutic interventions aimed at reverting them to chemosensitivity. In addition, delineation of the molecular basis of TGCT chemo-sensitivity may be informative for the cause of chemo-resistance of other more common types of cancer and, thus, may create new therapeutic leads. p53, a frequently mutated tumor suppressor in cancers in general, is not mutated in TGCTs, a fact that has implications for their chemo-sensitivity. Oct4, an embryonic transcription factor, is uniformly expressed in the seminoma and embryonic carcinoma components of non-seminomas, and its interplay with p53 may be important in the chemotherapy response of these tumors. This interplay, together with other features of TGCTs such as the gain of genetic material from the short arm of chromosome 12 and the association with disorders of testicular development, will be discussed in this paper and integrated in a unifying hypothesis that may explain their chemo-sensitivity.

Distinguishing luminal breast cancer subtypes by Ki67, progesterone receptor or TP53 status provides prognostic information

The objectives of this study were to determine the prognostic significance of subgrouping estrogen receptor (ER)-positive breast tumors into low- and high-risk luminal categories using Ki67 index, TP53, or progesterone receptor (PR) status. The study group comprised 540 patients with lymph node negative, invasive breast carcinoma. Luminal A subtype was defined as being ER positive, HER2 negative, and Ki67 low (<14% cells positive) and luminal B subtype as being ER positive, HER2 negative, and Ki67 high (≥ 14% cells positive). Luminal tumors were also subgrouped into risk categories based on the PR and TP53 status. Survival analysis was performed. Patients with luminal B tumors (n=173) had significantly worse disease-free survival compared to those with luminal A tumors (n=186) (log rank P-value=0.0164; univariate Cox regression relative risk 2.00; 95% CI, 1.12-3.58; P=0.0187). Luminal subtype remained an independent prognostic indicator on multivariate analysis including traditional prognostic factors (relative risk 2.12; 95% CI, 1.16-3.88; P=0.0151). Using TP53 status or PR negativity rather than Ki67 to classify ER-positive luminal tumors gave similar outcome results to those obtained using the proliferation index. However, it was a combination of the three markers, which proved the most powerful prognostically. Ki67 index, TP53 status, or PR negativity can be used to segregate ER-positive, HER2-negative tumors into prognostically meaningful subgroups with significantly different clinical outcomes. These biomarkers particularly in combination may potentially be used clinically to guide patient management.

The C terminus of p53 regulates gene expression by multiple mechanisms in a target- and tissue-specific manner in vivo

The C terminus of the tumor suppressor p53 is subjected to multiple post-translational modifications, suggesting that differing sets of modifications determine distinct cellular outcomes. Hamard et al. address this question by generating a Trp53 mutant mouse that constitutively expresses truncated p53. Intriguingly, the C terminus acts via three distinct mechanisms to control p53-dependent gene expression depending on the tissue. This study reconciles contradictory reports and delineates how regulation of target gene selectivity by p53 leads to alternate cellular outcomes.

The p53 tumor suppressor is a transcription factor that mediates varied cellular responses. The C terminus of p53 is subjected to multiple and diverse post-translational modifications. An attractive hypothesis is that differing sets of combinatorial modifications therein determine distinct cellular outcomes. To address this in vivo, a Trp53^ΔCTD/ΔCTD mouse was generated in which the endogenous p53 is targeted and replaced with a truncated mutant lacking the C-terminal 24 amino acids. These Trp53^ΔCTD/ΔCTD mice die within 2 wk post-partum with hematopoietic failure and impaired cerebellar development. Intriguingly, the C terminus acts via three distinct mechanisms to control p53-dependent gene expression depending on the tissue. First, in the bone marrow and thymus, the C terminus dampens p53 activity. Increased senescence in the Trp53^ΔCTD/ΔCTD bone marrow is accompanied by up-regulation of Cdkn1 (p21). In the thymus, the C-terminal domain negatively regulates p53-dependent gene expression by inhibiting promoter occupancy. Here, the hyperactive p53^ΔCTD induces apoptosis via enhanced expression of the proapoptotic Bbc3 (Puma) and Pmaip1 (Noxa). In the liver, a second mechanism prevails, since p53^ΔCTD has wild-type DNA binding but impaired gene expression. Thus, the C terminus of p53 is needed in liver cells at a step subsequent to DNA binding. Finally, in the spleen, the C terminus controls p53 protein levels, with the overexpressed p53^ΔCTD showing hyperactivity for gene expression. Thus, the C terminus of p53 regulates gene expression via multiple mechanisms depending on the tissue and target, and this leads to specific phenotypic effects in vivo.

Rotational positioning of nucleosomes facilitates selective binding of p53 to response elements associated with cell cycle arrest

The tumor suppressor protein p53 exhibits high affinity to the response elements regulating cell cycle arrest genes (CCA-sites), but relatively low affinity to the sites associated with apoptosis (Apo-sites). This in vivo tendency cannot be explained solely by the p53-DNA binding constants measured in vitro. Since p53 can bind nucleosomal DNA, we sought to understand if the two groups of p53 sites differ in their accessibility when embedded in nucleosomes. To this aim, we analyzed the sequence-dependent bending anisotropy of human genomic DNA containing p53 sites. For the 20 CCA-sites, we calculated rotational positioning patterns predicting that most of the sites are exposed on the nucleosomal surface. This is consistent with experimentally observed positioning of human nucleosomes. Remarkably, the sequence-dependent DNA anisotropy of both the p53 sites and flanking DNA work in concert producing strong positioning signals. By contrast, both the predicted and observed rotational settings of the 38 Apo-sites in nucleosomes suggest that many of these sites are buried inside, thus preventing immediate p53 recognition and delaying gene induction. The distinct chromatin organization of the CCA response elements appears to be one of the key factors facilitating p53-DNA binding and subsequent activation of genes associated with cell cycle arrest.

CSF1 is a novel p53 target gene whose protein product functions in a feed-forward manner to suppress apoptosis and enhance p53-mediated growth arrest

The p53 tumor suppressor gene has a common polymorphism at codon 72 that alters its function. We previously reported that the proline 72 polymorphic variant of p53 (P72) demonstrates increased ability to transactivate a subset of genes, relative to arginine 72 (R72); one of these genes is macrophage colony stimulating factor (CSF1). At present, the mechanism(s) underlying the increased transcriptional activity of P72 toward genes like CSF1 have not been completely elucidated. Additionally, the consequences of increased transcription of genes like CSF1 by the P72 variant to the downstream p53 pathway are unknown. In this report, we address these issues. We show that the CSF1 gene contains a conserved binding site for p53, and interestingly that the P72 variant shows increased ability to bind to this site. Moreover, we show that increased CSF1/CSF1R signaling in P72 cells feeds back on the p53 pathway to enhance p53 phosphorylation, levels, and transactivation of target genes, particularly the cyclin-dependent kinase inhibitor p21 (CDKN1A). This leads to an increase in p53-mediated growth arrest, along with a concomitant decrease in apoptosis. Notably, the CSF1/CSF1R signaling axis is overexpressed in several epithelial cancers, and there is clinical evidence that this pathway plays a role in radio-resistance of some cancers. We show that cells expressing CSF1 and CSF1R are indeed radio-resistant, and further, that this effect requires p53. These combined data are the first to implicate the CSF1/CSF1R pathway in the decision between p53-mediated growth arrest and apoptosis. They are also the first to highlight a cytokine as influential in cell fate determined by p53 in epithelial cells. Finally, these data may explain the association of the P72 variant and the CSF1/CSF1R pathway with increased senescence and radio-resistance in some epithelial tumor types.

Mitochondrial p53 mediates a transcription-independent regulation of cell respiration and interacts with the mitochondrial F₁F0-ATP synthase

We and others previously reported that endogenous p53 can be located at mitochondria in the absence of stress, suggesting that p53 has a role in the normal physiology of this organelle. The aim of this study was to characterize in unstressed cells the intramitochondrial localization of p53 and identify new partners and functions of p53 in mitochondria. We find that the intramitochondrial pool of p53 is located in the intermembrane space and the matrix. Of note, unstressed HCT116 p53(+/+) cells simultaneously show increased O₂ consumption and decreased mitochondrial superoxide production compared with their p53-null counterpart. This data was confirmed by stable H1299 cell lines expressing low levels of p53 specifically targeted to the matrix. Using immunoprecipitation and mass spectrometry, we identified the oligomycin sensitivity-conferring protein (OSCP), a subunit of the F₁F₀-ATP synthase complex, as a new partner of endogenous p53, specifically interacting with p53 localized in the matrix. Interestingly, this interaction seems implicated in mitochondrial p53 localization. Moreover, p53 localized in the matrix promotes the assembly of F₁F₀-ATP synthase. Taking into account that deregulations of mitochondrial respiration and reactive oxygen species production are tightly linked to cancer development, we suggest that mitochondrial p53 may be an important regulator of normal mitochondrial and cellular physiology, potentially exerting tumor suppression activity inside mitochondria.

CRHR1 mediates p53 transcription induced by high altitude hypoxia through ERK 1/2 signaling in rat hepatic cells

We have previously reported that hypoxia activates corticotrophin-releasing hormone (CRH) and the expression of its type-1 receptor (CRHR1) and induces disorders of the brain-endocrine-immune network. p53 is activated by hypoxia and involved in tumorigenesis and apoptosis. Whether CRHR1 regulates p53 transactivation to further influence apoptotic genes remains unclear. Here, we showed that hypoxia at a simulated altitude of 5km or 7km for 8 and 24h increased p53 protein and mRNA, and reduced apoptotic bax and IGFBP3 gene expression while upregulating the cell-arrest gene p21 for 8h in rat liver cells. The upregulation of p53 mRNA and downregulation of bax mRNA induced by hypoxia were blocked by pretreatment with the specific CRHR1 antagonist CP-154,526, but the downregulation of IGFBP3 and upregulation of p21 mRNA were not. Furthermore, CRH stimulated p53 mRNA via the ERK 1/2 pathway in the BRL-3A cell line and this was blocked by the ERK 1/2 antagonist U0126. These data provide novel evidence that the CRHR1-triggered ERK 1/2 pathway is involved in the activation of p53 and suppression of the apoptotic bax gene by hypoxia in rat liver.

p53 mediates autophagy and cell death by a mechanism contingent on Bnip3

Myocardial ischemia and angiotensin II activate the tumor suppressor p53 protein, which promotes cell death. Previously, we showed that the Bcl-2 death gene Bnip3 is highly induced during ischemia, where it triggers mitochondrial perturbations resulting in autophagy and cell death. However, whether p53 regulates Bnip3 and autophagy is unknown. Herein, we provide new compelling evidence for a novel signaling axis that commonly links p53 and Bnip3 for autophagy and cell death. p53 overexpression increased endogenous Bnip3 mRNA and protein levels resulting in mitochondrial defects leading to loss of mitochondrial ΔΨ(m). This was accompanied by an increase in autophagic flux and cell death. Notably, genetic loss of function studies, such as Atg7 knock-down or pharmacological inhibition of autophagy with 3-methyl adenine, suppressed cell death induced by p53--indicating that p53 induces maladaptive autophagy. Our previous work demonstrated that Bnip3 induces mitochondrial defects and autophagic cell death. Conversely, loss of function of Bnip3 in cardiac myocytes or Bnip3(-/-) mouse embryonic fibroblasts prevented mitochondrial targeting of p53, autophagy, and cell death. To our knowledge, these data provide the first evidence for the dual regulation of autophagy and cell death of cardiac myocytes by p53 that is mutually dependent on and obligatorily linked to Bnip3 gene activation. Hence, our findings may explain more fundamentally, how, autophagy and cell death are dually regulated during cardiac stress conditions where p53 is activated.

Another fork in the road--life or death decisions by the tumour suppressor p53

In response to cellular stress signals, the tumour suppressor p53 accumulates and triggers a host of antineoplastic responses. For instance, DNA damage activates two main p53-dependent responses: cell cycle arrest and attendant DNA repair or apoptosis (cell death). It is broadly accepted that, in response to DNA damage, the function of p53 as a sequence-specific transcription factor is crucial for tumour suppression. The molecular determinants, however, that favour the initiation of either a p53-dependent cell cycle arrest (life) or apoptotic (death) transcriptional programme remain elusive. Gaining a clear understanding of the mechanisms controlling cell fate determination by p53 could lead to the identification of molecular targets for therapy, which could selectively sensitize cancer cells to apoptosis. This review summarizes the literature addressing this important question in the field. Special emphasis is given to the role of the p53 response element, post-translational modifications and protein-protein interactions on cell fate decisions made by p53 in response to DNA damage.

Altered processing of amyloid precursor protein in cells undergoing apoptosis

Altered proteolysis of amyloid precursor protein is an important determinant of pathology development in Alzheimer's disease. Here, we describe the detection of two novel fragments of amyloid precursor protein in H4 neuroglioma cells undergoing apoptosis. Immunoreactivity of these 25-35 kDa fragments to two different amyloid precursor protein antibodies suggests that they contain the amyloid-β region and an epitope near the C-terminus of amyloid precursor protein. Generation of these fragments is associated with cleavage of caspase-3 and caspase-7, suggesting activation of these caspases. Studies in neurons undergoing DNA damage-induced apoptosis also showed similar results. Inclusion of caspase inhibitors prevented the generation of these novel fragments, suggesting that they are generated by a caspase-dependent mechanism. Molecular weight prediction and immunoreactivity of the fragments generated suggested that such fragments could not be generated by cleavage at any previously identified caspase, secretase, or calpain site on amyloid precursor protein. Bioinformatic analysis of the amino acid sequence of amyloid precursor protein revealed that fragments fitting the observed size and immunoreactivity could be generated by either cleavage at a novel, hitherto unidentified, caspase site or at a previously identified matrix metalloproteinase site in the extracellular domain. Proteolytic cleavage at any of these sites leads to a decrease in the generation of α-secretase cleaved secreted APP, which has both anti-apoptotic and neuroprotective properties, and thus may contribute to neurodegeneration in Alzheimer's disease.

Asperolide A, a marine-derived tetranorditerpenoid, induces G2/M arrest in human NCI-H460 lung carcinoma cells, is mediated by p53-p21 stabilization and modulated by Ras/Raf/MEK/ERK signaling pathway

Here we first demonstrate that asperolide A, a very recently reported marine-derived tetranorditerpenoid, leads to the inhibition of NCI-H460 lung carcinoma cell proliferation by G2/M arrest with the activation of the Ras/Raf/MEK/ERK signaling and p53-dependent p21 pathway. Treatment with 35 μM asperolide A (2 × IC₅₀) resulted in a significant increase in the proportion of G2/M phase cells, about a 2.9-fold increase during 48 h. Immunoblot assays demonstrated time-dependent inhibition of G2/M regulatory proteins. Moreover, asperolide A significantly activated MAP kinases (ERK1/2, JNK and p38 MAP kinase) by phosphorylation, and only the inhibition of ERK activation by PD98059 reversed downregulation of G2/M regulatory proteins CDC2, and suppressed upregulation of p21 and p-p53 levels. Transfection of cells with dominant-negative Ras (RasN17) mutant genes up-regulated asperolide A-induced the decrease of cyclin B1 and CDC2, suppressed Raf, ERK activity and p53-p21 expression, and at last, abolished G2/M arrest. This study indicates that asperolide A-induced G2/M arrest in human NCI-H460 lung carcinoma cells relys on the participation of the Ras/Raf/MEK/ERK signaling pathway in p53-p21 stabilization. An in vivo study with asperolide A illustrated a marked inhibition of tumor growth, and little toxcity compared to Cisplatin therapy. Overall, these findings provide potential effectiveness and a theoretical basis for the therapeutic use of asperolide A in the treatment of malignancies.

Tea polyphenols modulate antioxidant redox system on cisplatin-induced reactive oxygen species generation in a human breast cancer cell

Tea polyphenols (TPP) have potent antioxidant and anticancer properties, particularly in patients undergoing radiation or chemotherapy. However, few studies have been conducted on treatments using a combination of TPP and the conventional chemical anticancer drug cisplatin (CP). This study was designed to investigate the mechanism of the cytotoxicity of total TPP and CP, which may synergistically induce cell death in cancer cells. Here, breast cancer cells (MCF-7) were treated with various concentrations of TPP alone or in combination with the chemotherapeutic drug CP. The effect of TPP on cell growth, intracellular reactive oxygen species (ROS) level, apoptosis and gene expression of caspase-3, caspase-8 and caspase-9 and p53 was investigated. The MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay revealed that the MCF-7 cells were less sensitive to growth inhibition by TPP treatment than either CP or the combination therapy. Propidium iodide nuclear staining indicated that exposure to this combination increased the proportion of apoptotic nuclei compared with a single-agent treatment. Flow cytometry analysis was used to quantify changes in intracellular ROS. Detection of activated caspases by fluorescently labelled inhibitors of caspases (FLICA) combined with the plasma membrane permeability assay demonstrated that the percentage of early and late apoptotic/secondary necrotic cells was higher in the cells treated with the combination than in those treated with either TPP or CP alone. The combined TPP and CP treatment synergistically induced apoptosis through both caspase-8 and caspase-9 activation and p53 over-expression. This suggests that TPP plus CP may be used as an efficient antioxidant-based combination therapy for estrogen receptor (ER)-positive and p53-positive breast cancer.

A threshold mechanism mediates p53 cell fate decision between growth arrest and apoptosis

The p53 tumor suppressor responds to certain cellular stresses by inducing transcriptional programs that can lead to growth arrest or apoptosis. However, the molecular mechanisms responsible for choosing between these two cell fates are not well understood. Previous studies have suggested that p53 selectively activates proarrest target genes, due to the higher affinity of p53 for their promoters compared with proapoptotic genes. Here we show using microarray and chromatin immunoprecipitation that p53 binds to and transcriptionally activates both its proarrest and proapoptotic target genes proportionally to induced p53 expression levels. Further, we provide evidence that to trigger apoptosis, cells must overcome an apoptotic threshold, whose height is determined by expression levels of p53 and its targets, the duration of their expression and the cellular context. We demonstrate in multiple cells lines that below this threshold, expression levels of p53 and its targets were sufficient to induce arrest but not apoptosis. Above this threshold, p53 and its targets triggered extensive apoptosis. Moreover, lowering this threshold with inhibitors of antiapoptotic Bcl-2 family proteins sensitized cells to p53-induced apoptosis. These findings argue that agents that lower the apoptotic threshold should increase the efficacy of p53-mediated cancer therapy.

Markers of cellular senescence

Cellular senescence is a tumor suppression mechanism that evolved to limit duplication in somatic cells. Senescence is imposed by natural replicative boundaries or stress-induced signals, such as oncogenic transformation. Neoplastic cells can be forced to undergo senescence through genetic manipulations and epigenetic factors, including anticancer drugs, radiation, and differentiating agents. Senescent cells show distinct phenotypic and molecular characteristics, both in vitro or in vivo. These biomarkers might either cause or result from senescence induction, but could also be the byproducts of physiological changes in these non-replicating cells.

Autophagy, apoptosis, mitoptosis and necrosis: interdependence between those pathways and effects on cancer

Cell death is a fundamental ingredient of life. Thus, not surprisingly more than one form of cell death exists. Several excellent reviews on various forms of cell death have already been published but manuscripts describing interconnection and interdependence between such processes are uncommon. Here, what follows is a brief introduction on all three classical forms of cell death, followed by a more detailed insight into the role of p53, the master regulator of apoptosis, and other forms of cell death. While discussing p53 and also the role of caspases in cell death forms, we offer insight into the interplay between autophagy and apoptosis, or necrosis, where autophagy may initially serve pro-survival functions. The review moves further to present some details about less researched forms of programmed cell death, namely necroptosis, necrosis and mitoptosis. These "mixed" forms of cell death allow us to highlight the interconnected nature of cell death forms, particularly apoptosis and necrosis. The interdependence between apoptosis, autophagy and necrosis, and their significance for cancer development and treatment are also analyzed in further parts of the review. In the concluding parts, the afore-mentioned issues will be put in perspective for the development of novel anti-cancer therapies.

Pharmacological inhibition of GSK3 attenuates DNA damage-induced apoptosis via reduction of p53 mitochondrial translocation and Bax oligomerization in neuroblastoma SH-SY5Y cells

Glycogen synthase kinase-3 (GSK3) and p53 play crucial roles in the mitochondrial apoptotic pathway and are known to interact in the nucleus. However, it is not known if GSK3 has a regulatory role in the mitochondrial translocation of p53 that participates in apoptotic signaling following DNA damage. In this study, we demonstrated that lithium and SB216763, which are pharmacological inhibitors of GSK3, attenuated p53 accumulation and caspase-3 activation, as shown by PARP cleavage induced by the DNA-damaging agents doxorubicin, etoposide and camptothecin. Furthermore, each of these agents induced translocation of p53 to the mitochondria and activated the mitochondrial pathway of apoptosis, as evidenced by the release of cytochrome C from the mitochondria. Both mitochondrial translocation of p53 and mitochondrial release of cytochrome C were attenuated by inhibition of GSK3, indicating that GSK3 promotes the DNA damage-induced mitochondrial translocation of p53 and the mitochondrial apoptosis pathway. Interestingly, the regulation of p53 mitochondrial translocation by GSK3 was only evident with wild-type p53, not with mutated p53. GSK3 inhibition also reduced the phosphorylation of wild-type p53 at serine 33, which is induced by doxorubicin, etoposide and camptothecin in the mitochondria. Moreover, inhibition of GSK3 reduced etoposide-induced association of p53 with Bcl2 and Bax oligomerization. These findings show that GSK3 promotes the mitochondrial translocation of p53, enabling its interaction with Bcl2 to allow Bax oligomerization and the subsequent release of cytochrome C. This leads to caspase activation in the mitochondrial pathway of intrinsic apoptotic signaling.

A systematic screen reveals MicroRNA clusters that significantly regulate four major signaling pathways

MicroRNAs (miRNAs) are encoded in the genome as individual miRNA genes or as gene clusters transcribed as polycistronic units. About 50% of all miRNAs are estimated to be co-expressed with neighboring miRNAs. Recent studies have begun to illuminate the importance of the clustering of miRNAs from an evolutionary, as well as a functional standpoint. Many miRNA clusters coordinately regulate multiple members of cellular signaling pathways or protein interaction networks. This cooperative method of targeting could produce effects on an overall process that are much more dramatic than the smaller effects often associated with regulation by an individual miRNA. In this study, we screened 366 human miRNA minigenes to determine their effects on the major signaling pathways culminating in AP-1, NF-κB, c-Myc, or p53 transcriptional activity. By stratifying these data into miRNA clusters, this systematic screen provides experimental evidence for the combined effects of clustered miRNAs on these signaling pathways. We also verify p53 as a direct target of miR-200a. This study is the first to provide a panoramic view of miRNA clusters' effects on cellular pathways.

The transcriptional regulator gene E2 of the Human Papillomavirus (HPV) 16 influences the radiosensitivity of cervical keratinocytes

Background

Clinical studies have demonstrated that HPV induced tumors constitute a specific subclass of cancer with a better response to radiation treatment. The purpose of this study was to investigate meaning of viral E2-gene for radiosensitivity.

Methods

W12 cells contain episomal HPV 16 genomes, whereas S12 cells, which derive from the W12 line, contain HPV DNA as integrated copies. Clonogenic survival was analyzed using 96-well in vitro test. Using flow cytometry cell cycle analyses were performed. Expression of pRb and p53 were analyzed using intracellular staining.

Results

W12 cells (intact E2 gene) showed a lower survival fraction than S12 cells. W12 cells developed a G2/M block 24 h after irradiation with 2 Gy whereas S12 showed no G2/M bloc. After irradiation S12 cells developed polyploidy and pRb-positive cells decreased. W12 cells showed no change of pRb-positive cells.

Conclusions

Depending on E2 gene status differences in cell cycle regulation might cause radioresistance. The E2/E7/pRb pathway seems to influence HPV-induced radiosensitivity. Our experiments demonstrated an effect of HPV on radiosensitivity of cervical keratinocytes via viral transcription regulator E2 pathway.

Predictive biomarkers of sensitivity to the aurora and angiogenic kinase inhibitor ENMD-2076 in preclinical breast cancer models

PURPOSE

The Aurora kinases are a family of conserved serine-threonine kinases with key roles in mitotic cell division. As with other promising anticancer targets, patient selection strategies to identify a responsive subtype will likely be required for successful clinical development of Aurora kinase inhibitors. The purpose of this study was to evaluate the antitumor activity of the Aurora and angiogenic kinase inhibitor ENMD-2076 against preclinical models of breast cancer with identification of candidate predictive biomarkers.

EXPERIMENTAL DESIGN

Twenty-nine breast cancer cell lines were exposed to ENMD-2076 and the effects on proliferation, apoptosis, and cell-cycle distribution were evaluated. In vitro activity was confirmed in MDA-MB-468 and MDA-MB-231 triple-negative breast cancer xenografts. Systematic gene expression analysis was used to identify up- and downregulated pathways in the sensitive and resistant cell lines, including within the triple-negative breast cancer subset.

RESULTS

ENMD-2076 showed antiproliferative activity against breast cancer cell lines, with more robust activity against cell lines lacking estrogen receptor expression and those without increased HER2 expression. Within the triple-negative breast cancer subset, cell lines with a p53 mutation and increased p53 expression were more sensitive to the cytotoxic and proapoptotic effects of ENMD-2076 exposure than cell lines with decreased p53 expression.

CONCLUSIONS

ENMD-2076 exhibited robust anticancer activity against models of triple-negative breast cancer and the candidate predictive biomarkers identified in this study may be useful in selecting patients for Aurora kinase inhibitors in the future.

The effect of miR-338-3p on HBx deletion-mutant (HBx-d382) mediated liver-cell proliferation through CyclinD1 regulation

OBJECTIVE

Hepatitis B Virus (HBV) DNA integration and HBV X (HBx) deletion mutation occurs in HBV-positive liver cancer patients, and C-terminal deletion in HBx gene mutants are highly associated with hepatocarcinogenesis. Our previous study found that the HBx-d382 deletion mutant (deleted at nt 382-400) can down-regulate miR-338-3p expression in HBx-expressing cells. The aim of the present study is to examine the role of miR-338-3p in the HBx-d382-mediated liver-cell proliferation.

METHODS

We established HBx-expressing LO2 cells by Lipofectamine 2000 transfection. A miR-338-3p mimics or inhibitor was transfected into LO2/HBx-d382 and LO2/HBx cells using miR-NC as a control miRNA. In silico analysis of potential miR-338-3p targets revealed that miR-338-3p could target the cell cycle regulatory protein CyclinD1. To confirm that CyclinD1 is negatively regulated by miR-338-3p, we constructed luciferase reporters with wild-type and mutated CyclinD1-3'UTR target sites for miR-338-3p binding. We examined the CyclinD1 expression by real-time PCR and western blot, and proliferation activity by flow cytometric cell cycle analysis, Edu incorporation, and soft agar colony.

RESULTS

HBx-d382 exhibited enhanced proliferation and CyclinD1 expression in LO2 cells. miR-338-3p expression inhibited cell proliferation in LO2/HBx-d382 cells (and LO2/HBx cells), and also negatively regulated CyclinD1 protein expression. Of the two putative miR-338-3p binding sites in the CyclinD1-3'UTR region, the effect of miR-338-3p on the second binding site (nt 2397-2403) was required for the inhibition.

CONCLUSION

miR-338-3p can directly regulate CyclinD1 expression through binding to the CyclinD1-3'UTR region, mainly at nt 2397-2403. Down-regulation of miR-338-3p expression is required for liver cell proliferation in both LO2/HBx and LO2/HBx-d382 mutant cells, although the effect is more pronounced in LO2/HBx-d382 cells. Our study elucidated a novel mechanism, from a new miRNA-regulation perspective, underlying the propensity of HBx deletion mutants to induce hepatocarcinogenesis at a faster rate than HBx.

PIASy-mediated Tip60 sumoylation regulates p53-induced autophagy

Posttranslational modifications of p53 integrate diverse stress signals and regulate its activity, but their combinatorial contribution to overall p53 function is not clear. We investigated the roles of lysine (K) acetylation and sumoylation on p53 and their relation to apoptosis and autophagy. Here we describe the collaborative role of the SUMO E3 ligase PIASy and the lysine acetyltransferase Tip60 in p53-mediated autophagy. PIASy binding to p53 and PIASy-activated Tip60 lead to K386 sumoylation and K120 acetylation of p53, respectively. Even though these two modifications are not dependent on each other, together they act as a "binary death signal" to promote cytoplasmic accumulation of p53 and execution of PUMA-independent autophagy. PIASy-induced Tip60 sumoylation augments p53 K120 acetylation and apoptosis. In addition to p14(ARF) inactivation, impairment in this intricate signaling may explain why p53 mutations are not found in nearly 50% of malignancies.

Inhibition of nicotinamide phosphoribosyltransferase (NAMPT) activity by small molecule GMX1778 regulates reactive oxygen species (ROS)-mediated cytotoxicity in a p53- and nicotinic acid phosphoribosyltransferase1 (NAPRT1)-dependent manner

Cancer cells undergo mitosis more frequently than normal cells and thus have increased metabolic needs, which in turn lead to higher than normal reactive oxygen species (ROS) production. Higher ROS production increases cancer cell dependence on ROS scavenging systems to balance the increased ROS. Selectively modulating intracellular ROS in cancers by exploiting cancer dependence on ROS scavenging systems provides a useful therapeutic approach. Essential to developing these therapeutic strategies is to maintain physiologically low ROS levels in normal tissues while inducing ROS in cancer cells. GMX1778 is a specific inhibitor of nicotinamide phosphoribosyltransferase, a rate-limiting enzyme required for the regeneration of NAD(+) from nicotinamide. We show that GMX1778 increases intracellular ROS in cancer cells by elevating the superoxide level while decreasing the intracellular NAD(+) level. Notably, GMX1778 treatment does not induce ROS in normal cells. GMX1778-induced ROS can be diminished by adding nicotinic acid (NA) in a NA phosphoribosyltransferase 1 (NAPRT1)-dependent manner, but NAPRT1 is lost in a high frequency of glioblastomas, neuroblastomas, and sarcomas. In NAPRT1-deficient cancer cells, ROS induced by GMX1778 was not susceptible to treatment with NA. GMX1778-mediated ROS induction is p53-dependent, suggesting that the status of both p53 and NAPRT1 might affect tumor apoptosis, as determined by annexin-V staining. However, as determined by colony formation, GMX1778 long term cytotoxicity in cancer cells was only prevented by the addition of NA to NAPRT1-expressing cells. Exposure to GMX1778 may be a novel way of inducing ROS selectively in NAPRT1-negative tumors without inducing cytotoxic ROS in normal tissue.

Differentiated embryo-chondrocyte expressed gene 1 regulates p53-dependent cell survival versus cell death through macrophage inhibitory cytokine-1

Activation of p53 upon DNA damage induces an array of target genes, leading to cell cycle arrest and/or apoptosis. However, the mechanism by which the cell fate is controlled by p53 remains to be clarified. Previously, we showed that DEC1, a basic helix-loop-helix transcription factor and a target of p53, is capable of inducing cell cycle arrest and mediating DNA damage-induced premature senescence. Here, we found that ectopic expression of DEC1 inhibits, whereas knockdown of DEC1 enhances, DNA damage-induced cell death. Surprisingly, we showed that the anti-cell-death activity of DEC1 is p53 dependent, but DEC1 does not directly modulate p53 expression. Instead, we showed that DEC1 inhibits the ability of p53 to induce macrophage inhibitory cytokine-1 (MIC-1), but not other prosurvival/proapoptotic targets, including p21 and Puma. Importantly, we showed that upon binding to their respective response elements on the MIC-1 promoter, DEC1 and p53 physically interact on the MIC-1 promoter via the basic helix-loop-helix domain in DEC1 and the tetramerization domain in p53, which likely weakens the DNA-binding activity of p53 to the MIC-1 promoter. Finally, we found that depletion of MIC-1 abrogates the ability of DEC1 to attenuate DNA damage-induced cell death. Together, we hypothesize that DEC1 controls the response of p53-dependent cell survival vs. cell death to a stress signal through MIC-1.

Inhibition of tumor angiogenesis and growth by nanoparticle-mediated p53 gene therapy in mice

Mutation of the p53 tumor suppressor gene, the most common genetic alteration in human cancers, results in more aggressive disease and increased resistance to conventional therapies. Aggressiveness may be related to the increased angiogenic activity of cancer cells containing mutant p53. To restore wild-type p53 function in cancer cells, we developed polymeric nanoparticles (NPs) for p53 gene delivery. Previous in vitro and in vivo studies demonstrated the ability of these NPs to provide sustained intracellular release of DNA, thus sustained gene transfection and decreased tumor cell proliferation. We investigated in vivo mechanisms involved in NP-mediated p53 tumor inhibition, with focus on angiogenesis. We hypothesize that sustained p53 gene delivery will help decrease tumor angiogenic activity and thus reduce tumor growth and improve animal survival. Xenografts of p53 mutant tumors were treated with a single intratumoral injection of p53NPs. We observed intratumoral p53 gene expression corresponding to tumor growth inhibition, over 5 weeks. Treated tumors showed upregulation of thrombospondin-1, a potent antiangiogenic factor, and a decrease in microvessel density vs. controls (saline, p53 DNA alone, and control NPs). Greater levels of apoptosis were also observed in p53NP-treated tumors. Overall, this led to significantly improved survival in p53NP-treated animals. NP-mediated p53 gene delivery slowed cancer progression and improved survival in an in vivo cancer model. One mechanism by which this is accomplished is disruption of tumor angiogenesis. We conclude that the NP-mediated sustained tumor p53 gene therapy can effectively be used for tumor growth inhibition.

p53 basic C terminus regulates p53 functions through DNA binding modulation of subset of target genes

The p53 gene encodes a transcription factor that is composed of several functional domains: the N-terminal transactivation domain, the central sequence-specific DNA binding domain, the tetramerization domain, and the highly basic C-terminal regulatory domain (CTD). The p53 CTD is a nonspecific DNA binding domain that is subject to extensive post-translational modifications. However, the functional significance of the p53 CTD remains unclear. The role of this domain in the regulation of p53 functions is explored by comparing the activity of ectopically expressed wild-type (WT) p53 protein to that of a truncated mutant lacking the 24 terminal amino acids (Δ24). Using quantitative real time PCR and chromatin Immuno-Precipitation experiments, a p53 CTD deletion is shown to alter the p53-dependent induction of a subset of its target genes due to impaired specific DNA binding. Moreover, p53-induced growth arrest and apoptosis both require an intact p53 CTD. These data indicate that the p53 CTD is a positive regulator of p53 tumor suppressor functions.

Wild-type tumor repressor protein 53 (Trp53) promotes ovarian cancer cell survival

Loss of Pten in the Kras(G12D);Amhr2-Cre mutant mice leads to the transformation of ovarian surface epithelial (OSE) cells and rapid development of low-grade, invasive serous adenocarcinomas. Tumors occur with 100% penetrance and express elevated levels of wild-type tumor repressor protein 53 (TRP53). To test the functions of TRP53 in the Pten;Kras (Trp53+) mice, we disrupted the Trp53 gene yielding Pten;Kras(Trp53-) mice. By comparing morphology and gene expression profiles in the Trp53+ and Trp53- OSE cells from these mice, we document that wild-type TRP53 acts as a major promoter of OSE cell survival and differentiation: cells lacking Trp53 are transformed yet are less adherent, migratory, and invasive and exhibit a gene expression profile more like normal OSE cells. These results provide a new paradigm: wild-type TRP53 does not preferentially induce apoptotic or senescent related genes in the Pten;Kras(Trp53+) cancer cells but rather increases genes regulating DNA repair, cell cycle progression, and proliferation and decreases putative tumor suppressor genes. However, if TRP53 activity is forced higher by exposure to nutlin-3a (a mouse double minute-2 antagonist), TRP53 suppresses DNA repair genes and induces the expression of genes that control cell cycle arrest and apoptosis. Thus, in the Pten;Kras(Trp53+) mutant mouse OSE cells and likely in human TP53+ low-grade ovarian cancer cells, wild-type TRP53 controls global molecular changes that are dependent on its activation status. These results suggest that activation of TP53 may provide a promising new therapy for managing low-grade ovarian cancer and other cancers in humans in which wild-type TP53 is expressed.

Expression of the class II tumor suppressor gene RIG1 is directly regulated by p53 tumor suppressor in cancer cell lines

Recent studies indicated that the RIG1 (RARRES3/TIG3) plays an important role in cell proliferation, differentiation, and apoptosis. However, the regulatory mechanism of RIG1 gene expression has not been clearly elucidated. In this study, we identified a functional p53 response element (p53RE) in the RIG1 gene promoter. Transfection studies revealed that the RIG1 promoter activity was greatly enhanced by wild type but not mutated p53 protein. Sequence specific mutation of the p53RE abolished p53-mediated transactivation. Specific binding of p53 protein to the rig-p53RE was demonstrated using electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assay. Further studies confirmed that the expression of RIG1 mRNA and protein is enhanced through increased p53 protein in HepG2 or in H24-H1299 cells. In conclusion, our results indicated that RIG1 gene is a downstream target of p53 in cancer cell lines.

Induction of apoptosis by cytoplasmically localized wild-type p53 and the S121F mutant super p53

After DNA damage, p53 is accumulated in the nucleus and transactivates downstream genes and induces apoptosis. There are two pathways in p53-dependent apoptosis, the transactivation-dependent and -independent pathway. In this study, we constructed p53-inducible glioblastoma cell lines and analyzed them for the induction of apoptosis and transactivation of p53-downstream genes after the nuclear or cytoplasmic expression of p53. To sequester p53 in the cytoplasm, we used p53 mutant with arginine to glycine substitution at residue 306 (R306G). Wild-type p53 retained the ability to arrest the cell cycle, and a p53 mutant with serine to phenylalanine substitution at residue 121 (S121F), which has a strong ability to induce apoptosis, retained this ability even when both the wild-type and p53 and S121F mutant were exclusively sequestered from the nucleus into the cytoplasm. Notably, cytoplasmically sequestered wild-type p53 and S121F mutant transactivated the downstream genes with distinct expression profiles, and the strong apoptotic ability of S121F was not associated with its transactivation activity. These results underscore the existence of transactivation-independent apoptosis and cytoplasmic function of p53.

Reactive oxygen species and mitochondrial sensitivity to oxidative stress determine induction of cancer cell death by p21

p21(Waf1/Cip1/Sdi1) is a cyclin-dependent kinase inhibitor that mediates cell cycle arrest. Prolonged p21 up-regulation induces a senescent phenotype in normal and cancer cells, accompanied by an increase in intracellular reactive oxygen species (ROS). However, it has been shown recently that p21 expression can also lead to cell death in certain models. The mechanisms involved in this process are not fully understood. Here, we describe an induction of apoptosis by p21 in sarcoma cell lines that is p53-independent and can be ameliorated with antioxidants. Similar levels of p21 and ROS caused senescence in the absence of significant death in other cancer cell lines, suggesting a cell-specific response. We also found that cells undergoing p21-dependent cell death had higher sensitivity to oxidants and a specific pattern of mitochondrial polarization changes. Consistent with this, apoptosis could be blocked with targeted expression of catalase in the mitochondria of these cells. We propose that the balance between cancer cell death and arrest after p21 up-regulation depends on the specific effects of p21-induced ROS on the mitochondria. This suggests that selective up-regulation of p21 in cancer cells could be a successful therapeutic intervention for sarcomas and tumors with lower resistance to mitochondrial oxidative damage, regardless of p53 status.

Tumor suppressor genes and ROS: complex networks of interactions

Tumor suppressor genes regulate diverse cellular activities including DNA damage repair, cell cycle arrest, mitogenic signaling, cell differentiation, migration, and programmed cell death. In this review the tumor suppressor genes p53, FoxO, retinoblastoma (RB), p21, p16, and breast cancer susceptibility genes 1 and 2 (BRCA1 and BRCA2) and their roles in oxidative stress are summarized with a focus on the links and interplay between their pathways and reactive oxygen species (ROS). The results of a number of studies have demonstrated an antioxidant role for tumor suppressor proteins, activating the expression of some well-known antioxidant genes in response to oxidative stress. On the other hand, recent studies have revealed a pro-oxidant role for p53 by which cellular ROS are increased by enhanced transcription of proapoptotic genes. A tightly regulated feedback loop between ROS and FoxO proteins, with ROS regulating FoxO activity through posttranslational modifications and protein interactions and FoxO controlling intracellular ROS levels, has been demonstrated. Furthermore, these studies have shown that FoxO transcription factors and p38 mitogen-activated protein kinases may interact with the RB pathway under stress conditions. In addition, cellular senescence studies established an unexpected role for ROS in inducing and maintaining senescence-induced tumor suppression that blocks cytokinesis to ensure senescent cells never divide again. p21 and p16 have been shown to act as tumor suppressor proteins and this function extends beyond cell cycle control and includes important roles in regulating oxidative stress. Consequently, these important interactions indicate a critical potential role for tumor suppressor genes in the cellular response against oxidative stress and emphasize links between ROS and tumor suppressor genes that might be therapeutic targets in oxidative damage-associated diseases.

Feedback control of p53 translation by REDD1 and mTORC1 limits the p53-dependent DNA damage response

Exquisite control of the level and activity of p53 are required in order to preserve cellular homeostasis following DNA damage. How this regulation is integrated with other key metabolic pathways in vivo is poorly understood. Here, we describe an endogenous feedback circuit for regulation of p53 through its transcriptional target gene, Redd1, a stress-induced inhibitor of TOR complex 1 (TORC1) activity. Cells and tissues of Redd1(-/-) mice exhibit enhanced sensitivity to ionizing radiation and chemotherapy treatment, which we demonstrate is attributable to abnormally increased p53 protein level and activity in the absence of Redd1. We find that deregulation of p53 in this setting is not due to failed DNA repair or to increased p53 stabilization but, instead, to increased p53 translation. We show that Redd1 loss leads to elevated mammalian TORC1 (mTORC1) activity, which explains the increased p53 translation and protein levels. Together, these findings suggest that REDD1-mediated suppression of mTORC1 activity exerts feedback control on p53, thereby limiting the apoptotic response and contributing to cellular survival following DNA damage. This work therefore defines a role for REDD1 in the control of p53 in vivo, with potential therapeutic implications for cancer and for the variety of genetic diseases involving TOR pathway signaling components.

DECOD: fast and accurate discriminative DNA motif finding

MOTIVATION

Motif discovery is now routinely used in high-throughput studies including large-scale sequencing and proteomics. These datasets present new challenges. The first is speed. Many motif discovery methods do not scale well to large datasets. Another issue is identifying discriminative rather than generative motifs. Such discriminative motifs are important for identifying co-factors and for explaining changes in behavior between different conditions.

RESULTS

To address these issues we developed a method for DECOnvolved Discriminative motif discovery (DECOD). DECOD uses a k-mer count table and so its running time is independent of the size of the input set. By deconvolving the k-mers DECOD considers context information without using the sequences directly. DECOD outperforms previous methods both in speed and in accuracy when using simulated and real biological benchmark data. We performed new binding experiments for p53 mutants and used DECOD to identify p53 co-factors, suggesting new mechanisms for p53 activation.

AVAILABILITY

The source code and binaries for DECOD are available at http://www.sb.cs.cmu.edu/DECOD CONTACT: zivbj@cs.cmu.edu

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

S-phase-coupled apoptosis in tumor suppression

DNA replication is essential for accurate transmission of genomic information from parental to daughter cells. DNA replication is licensed once per cell division cycle. This process is highly regulated by both positive and negative regulators. Over-replication, under-replication, as well as DNA damage in a cell all induce the activation of checkpoint control pathways such as ATM/ATR, CHK kinases, and the tumor suppressor protein p53, which provide "damage controls" via either DNA repairs or apoptosis. This review focuses on accumulating evidence, with the emphasis on recently discovered Killin, that S-phase checkpoint control is crucial for a mammalian cell to make a life and death decision in order to safeguard genome integrity.

Blockade of Hsp90 by 17AAG antagonizes MDMX and synergizes with Nutlin to induce p53-mediated apoptosis in solid tumors

Strategies to induce p53 activation in wtp53-retaining tumors carry high potential in cancer therapy. Nutlin, a potent highly selective MDM2 inhibitor, induces non-genotoxic p53 activation. Although Nutlin shows promise in promoting cell death in hematopoietic malignancies, a major roadblock is that most solid cancers do not undergo apoptosis but merely reversible growth arrest. p53 inhibition by unopposed MDMX is one major cause for apoptosis resistance to Nutlin. The Hsp90 chaperone is ubiquitously activated in cancer cells and supports oncogenic survival pathways, many of which antagonize p53. The Hsp90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17AAG) is known to induce p53-dependent apoptosis. We show here that in multiple difficult-to-kill solid tumor cells 17AAG modulates several critical components that synergize with Nutlin-activated p53 signaling to convert Nutlin's transient cytostatic response into a cytotoxic killing response in vitro and in xenografts. Combined with Nutlin, 17AAG destabilizes MDMX, reduces MDM2, induces PUMA and inhibits oncogenic survival pathways, such as PI3K/AKT, which counteract p53 signaling at multiple levels. Mechanistically, 17AAG interferes with the repressive MDMX-p53 axis by inducing robust MDMX degradation, thereby markedly increasing p53 transcription compared with Nutlin alone. To our knowledge Nutlin+17AAG represents the first effective pharmacologic knockdown of MDMX. Our study identifies 17AAG as a promising synthetic lethal partner for a more efficient Nutlin-based therapy.

Loss of transforming growth factor β adaptor protein β-2 spectrin leads to delayed liver regeneration in mice

Liver regeneration, following partial hepatectomy (PHx), occurs through precisely controlled and synchronized cell proliferation, in which quiescent hepatocytes undergo one to two rounds of replication, with restoration of liver mass and function. We previously demonstrated that loss of the Smad3/4 adaptor protein β-2 spectrin (β2SP) is associated with faster entry into S phase, and hepatocellular cancer formation. These observations led us to further pursue the role of β2SP in cell cycle progression in vivo. Liver regeneration studies with PHx in β2SP(+/-) mice reveal a surprising and significant decrease in liver/body weight ratio at 48 hours after PHx in β2SP(+/-) mice in comparison to wildtype mice. At 48 hours after PHx we also observe decreased levels of cyclin E (2.4-fold, P < 0.05), Cdk1 (7.2-fold, P < 0.05), cyclin A, pRb (Ser249/Thr252), proliferative cell nuclear antigen (PCNA), cyclin D1 with elevated levels of pCdk1 (Thr14) (3.6-fold, P < 0.05). Strikingly, at 24 hours elevated levels of p53 (4-fold, P < 0.05), phospho-p53 (ser15 and ser20), and p21 (200-fold, P < 0.05) persisting to 48 hours after PHx further correlated with raised expression of the DNA damage markers pChk2 (Thr68) and γH2AX (S139). However, compromised cell cycle progression with loss of β2SP is not rescued by inhibiting p53 function, and that G(2) /M phase arrest observed is independent and upstream of p53.

CONCLUSION

β2SP deficiency results in dysfunctional hepatocyte cell cycle progression and delayed liver regeneration at 48 hours after PHx, which is p53-independent. β2SP loss may increase susceptibility to DNA damage, impair cell cycle progression, and ultimately lead to hepatocellular cancer.

Induction of apoptosis in colon cancer cells by a novel topoisomerase I inhibitor TopIn

The tumor suppressor p53 plays an important role in cellular emergency mechanisms through regulating the genes involved in cell cycle arrest and apoptosis. To identify small molecules that can activate p53-responsive transcription, we performed chemical screening using genetically engineered HCT116 reporter cells. We found that TopIn (7-phenyl-6H-[1,2,5]oxadiazolo[3,4-e]indole 3-oxide) efficiently activated p53-mediated transcriptional activity and induced phosphorylation of p53 at Ser15, thereby stabilizing the p53 protein. Furthermore, TopIn upregulated the expression of p21(WAF1/CIP1), a downstream target of p53, and suppressed cellular proliferation in various colon cancer cells. Additionally, TopIn induced DNA fragmentation, caspase-3/7 activation and poly ADP ribose polymerase cleavage, typical biochemical markers of apoptosis, in p53 wild-type and mutated colon cancer cells. Finally, we found that TopIn inhibited topoisomerase I activity, but not topoisomerase II, in vitro and induced the formation of the topoisomerase I-DNA complex in HCT116 colon cancer cells. Unlike camptothecin (CPT) and its derivative SN38, TopIn did not affect the activity of the ATP-binding cassette transporter breast cancer resistance protein (BCRP) or multidrug-resistant protein-1 (MDR-1). These results suggest that TopIn may present a promising new topoisomerase I-targeting anti-tumor therapeutics.

p53-mediated regulation of neuronal differentiation via regulation of dual oxidase maturation factor 1

The p53 transcription factor is involved in cell cycle, apoptosis and differentiation. However, the mechanism of p53 mediated differentiation is not fully understood. Here, we show that recently discovered dual oxidase maturation factor 1 (DUOXA1), which was implicated in neuronal differentiation, is regulated by p53 and may be an important factor in neuronal differentiation. We show that DUOXA1 is highly expressed in mouse neuronal stem cells with intensive nuclear localization. A strong interaction between DUOXA1 and p53 is observed in undifferentiated cells and declines in terminally differentiated neurons. Overexpressed p53 induces marked DUOXA1 expression in P19 cells and intensifies neuronal differentiation in the presence of retinoic acid, which suggests that p53 and DUOXA1 possess a neural differentiation potential. At day 3 of retinoic acid induced differentiation when cells showed a typical morphology of neuronal progenies, CD133 expression was down-regulated. The expression level of CD133 was significantly decreased in p53 over-expressing cells and was accompanied by a substantial increase in the expression level of neurofilament. In conclusion, DUOXA1 is a novel p53-regulated neurogenic factor involved in p53 dependent neuronal differentiation.

Accumulation of p53 determined by immunohistochemistry as a prognostic marker in node negative breast cancer; analysis according to St Gallen consensus and intrinsic subtypes

BACKGROUND

This study evaluated the prognostic impact of p53 accumulation by immunohistochemistry (IHC) in lymph node-negative breast cancer (LNN-BC), and in subgroups of St Gallen consensus and intrinsic subtypes.

METHODS

A total 845 with a pathologic diagnosis of LNN-BC patients that underwent surgery at the National Cancer Center, Korea between 2001 and 2005 were retrospectively reviewed.

RESULTS

The median age was 48 years (range: 25-85) and median follow-up period was 66.0 months (range: 9-101). Univariate analysis determined that tumor size, estrogen receptor (ER), progesterone receptor (PgR), p53, and Ki-67 were significant for disease free survival (DFS). Of these factors, PgR negativity (HR 3.57; 95% CI 1.26-10.09; P = 0.01) and p53 positivity (HR 3.17; 95% CI 1.51-6.65; P = 0.002) were independent prognostic factors in multivariate analysis. In the subanalysis for 4 intrinsic subtypes (luminal A, luminal B, HER2-overexpression, and triple-negative subtypes) and risk groups by St Gallen consensus, there were significant differences of DFS rates by p53 (5-year DFS rate, luminal A; 97.2% for p53 (-) vs 93.8% for p53 (+); P = 0.03, triple-negative subgroups; 94.1% vs 78.7%; P = 0.002, intermediate-risk group; 96.5% vs 90.7%; P = 0.003).

CONCLUSIONS

P53 has prognostic power in LNN-BC, and gives the additional prognostic information for intrinsic subtypes and St Gallen consensus.

Rescue of non-sense mutated p53 tumor suppressor gene by aminoglycosides

Mutation-based treatments are a new development in genetic medicine, in which the nature of the mutation dictates the therapeutic strategy. Interest has recently focused on diseases caused by premature termination codons (PTCs). Drugs inducing the readthrough of these PTCs restore the production of a full-length protein. In this study, we explored the possibility of using aminoglycoside antibiotics to induce the production of a full-length functional p53 protein from a gene carrying a PTC. We identified a human cancer cell line containing a PTC, for which high levels of readthrough were obtained in the presence of aminoglycosides. Using these cells, we demonstrated that aminoglycoside treatment stabilized the mutant mRNA, which would otherwise have been degraded by non-sense-mediated decay, resulting in the production of a functional full-length p53 protein. Finally, we showed that aminoglycoside treatment decreased the viability of cancer cells specifically in the presence of nonsense-mutated p53 gene. These results open possibilities of developing promising treatments of cancers linked with non-sense mutations in tumor suppressor genes. They show that molecules designed to induce stop-codon readthrough can be used to inhibit tumor growth and offer a rational basis for developing new personalized strategies that could diversify the existing arsenal of cancer therapies.