Shifting the paradigms for tumor suppression: lessons from the p53 field

The TP53 gene continues to hold distinction as the most frequently mutated gene in cancer. Since its discovery in 1979, hundreds of research groups have devoted their efforts toward understanding why this gene is so frequently selected against by tumors, with the hopes of harnessing this information toward improved therapy of cancer. The result is that this protein has been meticulously analyzed in tumor and normal cells, resulting in over one hundred thousand publications, with an average of five thousand papers published on p53 every year for the past decade. The journey toward understanding p53 function has been anything but straightforward; in fact, the field is notable for the numerous times that established paradigms not only have been shifted, but in fact have been shattered or reversed. In this review, we will discuss the manuscripts, or series of manuscripts, that have most radically changed our thinking about how this tumor suppressor functions, and we will delve into the emerging challenges for the future in this important area of research. It is hoped that this review will serve as a useful historical reference for those interested in p53, and a useful lesson on the need to be flexible in the face of established paradigms.

Association between TP53 genetic polymorphisms and the methylation and expression of miR-34a, 34b/c in colorectal cancer tissues

Colorectal cancer (CRC) is one of the most common types of cancers, as evidenced by the >1.2 million patient diagnoses and 600,000 mortalities globally each year. Recently, the microRNA (miR/miRNA)-34 miRNA precursor family was revealed to participate in the tumor protein (TP)-53 pathway, which is frequently involved in CRC. Furthermore, the expression of miR-34 is reportedly regulated by DNA methylation. Accordingly, the present study investigated the correlation between the methylation status of miR-34 miRNAs and miR-34 expression in paired CRC tumor and normal tissues. The methylation status of miR-34a and miR-34b/c was determined using the MethyLight assay, and the expression of miR-34a and miR-34b/c in the same paired tissues was analyzed by reverse transcription-quantitative polymerase chain reaction. The results revealed significantly elevated miR-34a (P=0.012) and miR-34b/c (P<0.0001) methylation levels in tumor tissues when compared with normal tissues, whereas only the expression of miR-34b/c differed (P=0.005) between the paired tissues. In addition, an association between TP53 haplotypes and miR-34 family expression levels was observed. The miR-34a methylation levels in the TP53 PIN A1A1 (48.56±36.49) and TP53 MSP GG (49.00±36.44) genotypes were increased in the tumor tissues when compared with normal tissues. In conclusion, it was determined that miR-34 promoter methylation and TP53 polymorphisms may be associated with CRC pathogenesis.

The Roles of p53 in Mitochondrial Dynamics and Cancer Metabolism: The Pendulum between Survival and Death in Breast Cancer?

Cancer research has been heavily geared towards genomic events in the development and progression of cancer. In contrast, metabolic regulation, such as aberrant metabolism in cancer, is poorly understood. Alteration in cellular metabolism was once regarded simply as a consequence of cancer rather than as playing a primary role in cancer promotion and maintenance. Resurgence of cancer metabolism research has identified critical metabolic reprogramming events within biosynthetic and bioenergetic pathways needed to fulfill the requirements of cancer cell growth and maintenance. The tumor suppressor protein p53 is emerging as a key regulator of metabolic processes and metabolic reprogramming in cancer cells&mdash;balancing the pendulum between cell death and survival. This review provides an overview of the classical and emerging non-classical tumor suppressor roles of p53 in regulating mitochondrial dynamics: mitochondrial engagement in cell death processes in the prevention of cancer. On the other hand, we discuss p53 as a key metabolic switch in cellular function and survival. The focus is then on the conceivable roles of p53 in breast cancer metabolism. Understanding the metabolic functions of p53 within breast cancer metabolism will, in due course, reveal critical metabolic hotspots that cancers advantageously re-engineer for sustenance. Illustration of these events will pave the way for finding novel therapeutics that target cancer metabolism and serve to overcome the breast cancer burden.

Different TP53 mutants in p53 overexpressed epithelial ovarian carcinoma can be associated both with altered and unaltered glycolytic and apoptotic profiles

Background

p53 is a tumor suppressor and key regulator of glycolysis in cancer cells, however highly mutated in tumors. In ovarian cancer, studies concerning p53 mutations focus on the DNA binding domain since the majority of hotspot mutations affects this region. Yet, mutations in other regions such as the proline rich domain may also affect the protein’s expression and activity. The aim of this study is to investigate the effect of various positions of mutations in TP53 gene on glycolysis, apoptosis and transcription of p53 target genes.

Methods

Mutations frequency and their effect on p53 expression were assessed by PCR-SSCP, sequencing and immunohistochemistry on 30 ovarian cancer biopsies. Six tumors were cultured, as well as SK-OV-3, OVCAR-3 and Igrov-1. SK-OV-3 cells were transfected with 2 TP53 mutants. p53 transcriptional activity was assayed by qPCR, apoptosis by flow cytometry and glycolysis by glucose and lactate measurements, with quantification of glycolytic enzymes expression.

Results

Our results showed a high frequency of the P72R mutant, associated with p53 overexpression in the ovarian biopsies. However, P72R mutant cells showed similar apoptosis and glycolysis as WT cells. DNA binding domain mutations decreased the transcriptional activity of the protein and increased glucose consumption and lactate production.

Conclusion

Despite the overexpression of the P72R mutated protein in the biopsies, it showed a similar apoptotic activity and glucose regulation ability as WT p53. Knowing that p53 expression status is used for chemotherapeutic approaches and prognosis in ovarian cancer, the results obtained highlight the importance of locating TP53 mutations.

Electronic supplementary material

The online version of this article (10.1186/s12935-018-0514-2) contains supplementary material, which is available to authorized users.

The Tumor Suppressor, P53, Decreases the Metal Transporter, ZIP14

Loss of p53’s proper function accounts for over half of identified human cancers. We identified the metal transporter ZIP14 (Zinc-regulated transporter (ZRT) and Iron-regulated transporter (IRT)-like Protein 14) as a p53-regulated protein. ZIP14 protein levels were upregulated by lack of p53 and downregulated by increased p53 expression. This regulation did not fully depend on the changes in ZIP14’s mRNA expression. Co-precipitation studies indicated that p53 interacts with ZIP14 and increases its ubiquitination and degradation. Moreover, knockdown of p53 resulted in higher non-transferrin-bound iron uptake, which was mediated by increased ZIP14 levels. Our study highlights a role for p53 in regulating nutrient metabolism and provides insight into how iron and possibly other metals such as zinc and manganese could be regulated in p53-inactivated tumor cells.

Inhibitory Effects of Metabolites of 5-Demethylnobiletin on Human Nonsmall Cell Lung Cancer Cells

5-Demethylnobiletin is a unique flavonoid found in citrus fruits with potential chemopreventive effects against human cancers. We previously identified three metabolites of 5DN, namely 5,4'-didemethylnobiletin (M1), 5,3',4'- tridemethylnobiletin (M2), and 5,3'-didemethylnobiletin (M3) in mice fed 5DN. Herein, we investigated the inhibitory effects of these three metabolites on NSCLC cells. Our results demonstrated that M1, M2, and especially M3 showed stronger inhibition on the growth and colony formation of H460 and H1299 cells compared to 5DN. Three metabolites significantly inhibited the tumorsphere formation of A549 cells. Flow cytometry analysis showed that all metabolites induced cell cycle arrest and cellular apoptosis, and these effects were also stronger than that of 5DN. The inhibitory effects of these metabolites were associated with their ability to modulate the key signaling proteins related to cell proliferation and apoptosis. Overall, our results provided a basis for utilizing 5DN and its metabolites for chemoprevention of lung cancer.

The MEK/ERK pathway is the primary conduit for Borrelia burgdorferi-induced inflammation and P53-mediated apoptosis in oligodendrocytes

Lyme neuroborreliosis (LNB) affects both the central and peripheral nervous systems. In a rhesus macaque model of LNB we had previously shown that brains of rhesus macaques inoculated with Borrelia burgdorferi release inflammatory mediators, and undergo oligodendrocyte and neuronal cell death. In vitro analysis of this phenomenon indicated that while B. burgdorferi can induce inflammation and apoptosis of oligodendrocytes per se, microglia are required for neuronal apoptosis. We hypothesized that the inflammatory milieu elicited by the bacterium in microglia or oligodendrocytes contributes to the apoptosis of neurons and glial cells, respectively, and that downstream signaling events in NFkB and/or MAPK pathways play a role in these phenotypes. To test these hypotheses in oligodendrocytes, several pathway inhibitors were used to determine their effect on inflammation and apoptosis, as induced by B. burgdorferi. In a human oligodendrocyte cell line (MO3.13), inhibition of the ERK pathway in the presence of B. burgdorferi markedly reduced inflammation, followed by the JNK, p38 and NFkB pathway inhibition. In addition to eliciting inflammation, B. burgdorferi also increased total p53 protein levels, and suppression of the ERK pathway mitigated this effect. While inhibition of p53 had a minimal effect in reducing inflammation, suppression of the ERK pathway or p53 reduced apoptosis as measured by active caspase-3 activity and the TUNEL assay. A similar result was seen in primary human oligodendrocytes wherein suppression of ERK or p53 reduced apoptosis. It is possible that inflammation and apoptosis in oligodendrocytes are divergent arms of MAPK pathways, particularly the MEK/ERK pathway.

Tumor suppressor p53 and estrogen receptors in nuclear-mitochondrial communication

Several gene transcription regulators considered solely localized within the nuclear compartment are being reported to be present in the mitochondria as well. There is growing interest in the role of mitochondria in regulating cellular metabolism in normal and disease states. Various findings demonstrate the importance of crosstalk between nuclear and mitochondrial genomes, transcriptomes, and proteomes in regulating cellular functions. Both tumor suppressor p53 and estrogen receptor (ER) were originally characterized as nuclear transcription factors. In addition to their individual roles as regulators of various genes, these two proteins interact resulting in major cellular consequences. In addition to its nuclear role, p53 has been localized to the mitochondria where it executes various transcription-independent functions. Likewise, ERs are reported to be present in mitochondria; however their functional roles remain to be clearly defined. In this review, we provide an integrated view of the current knowledge of nuclear and mitochondrial p53 and ERs and how it relates to normal and pathological physiology.

Review: apoptotic mechanisms in bacterial infections of the central nervous system

In this article we review the apoptotic mechanisms most frequently encountered in bacterial infections of the central nervous system (CNS). We focus specifically on apoptosis of neural cells (neurons and glia), and provide first an overview of the phenomenon of apoptosis itself and its extrinsic and intrinsic pathways. We then describe apoptosis in the context of infectious diseases and inflammation caused by bacteria, and review its role in the pathogenesis of the most relevant bacterial infections of the CNS.

Retinoic acid-related orphan receptor-α is induced in the setting of DNA damage and promotes pulmonary emphysema

RATIONALE

The discovery that retinoic acid-related orphan receptor (Rora)-α is highly expressed in lungs of patients with COPD led us to hypothesize that Rora may contribute to the pathogenesis of emphysema.

OBJECTIVES

To determine the role of Rora in smoke-induced emphysema.

METHODS

Cigarette smoke extract in vitro and elastase or cigarette smoke exposure in vivo were used to model smoke-related cell stress and airspace enlargement. Lung tissue from patients undergoing lung transplantation was examined for markers of DNA damage and Rora expression.

MEASUREMENTS AND MAIN RESULTS

Rora expression was induced by cigarette smoke in mice and in cell culture. Gene expression profiling of Rora-null mice exposed to cigarette smoke demonstrated enrichment for genes involved in DNA repair. Rora expression increased and Rora translocated to the nucleus after DNA damage. Inhibition of ataxia telangiectasia mutated decreased the induction of Rora. Gene silencing of Rora attenuated apoptotic cell death in response to cigarette smoke extract, whereas overexpression of Rora enhanced apoptosis. Rora-deficient mice were protected from elastase and cigarette smoke induced airspace enlargement. Finally, lungs of patients with COPD showed evidence of increased DNA damage even in the absence of active smoking.

CONCLUSIONS

Taken together, these findings suggest that DNA damage may contribute to the pathogenesis of emphysema, and that Rora has a previously unrecognized role in cellular responses to genotoxicity. These findings provide a potential link between emphysema and features of premature ageing, including enhanced susceptibility to lung cancer.

Mitochondrial remodeling in cancer metabolism and survival: potential for new therapies

Mitochondria are semi-autonomous organelles that play essential roles in cellular metabolism and programmed cell death pathways. Genomic, functional and structural mitochondrial alterations have been associated with cancer. Some of those alterations may provide a selective advantage to cells, allowing them to survive and grow under stresses created by oncogenesis. Due to the specific alterations that occur in cancer cell mitochondria, these organelles may provide promising targets for cancer therapy. The development of drugs that specifically target metabolic and mitochondrial alterations in tumor cells has become a matter of interest in recent years, with several molecules undergoing clinical trials. This review focuses on the most relevant mitochondrial alterations found in tumor cells, their contribution to cancer progression and survival, and potential usefulness for stratification and therapy.

Targeting the p53 signaling pathway in cancer therapy - the promises, challenges and perils

INTRODUCTION

Research over the past three decades has identified p53 as a multi-functional transcription factor. p53 influences myriad, highly diverse cellular processes, and represents one of the most important and extensively studied tumor suppressors. Activated by various stresses, p53 blocks cancer progression by provoking transient or permanent growth arrest, by enabling DNA repair, or by advancing cellular death programs. This anti-cancer activity profile, together with genomic and mutational analyses documenting inactivation of p53 in more than 50% of human cancers, motivated drug development efforts to (re-) activate p53 in established tumors.

AREAS COVERED

The complexities of p53 signaling in cancer are summarized, including current strategies and challenges to restore p53's tumor suppressive function in established tumors, to inactivate p53 inhibitors, and to restore wild type function of p53 mutant proteins.

EXPERT OPINION

p53 represents an attractive target for the development of anti-cancer therapies. Whether p53 is 'druggable', however, remains an area of active research and discussion, as p53 has pro-survival functions and chronic p53 activation accelerates aging, which may compromise the long-term homeostasis of an organism. The complex biology and dual functions of p53 in cancer prevention and age-related cellular responses pose significant challenges to the development of p53-targeting cancer therapies.

Specific TP53 mutations predict aggressive phenotype in head and neck squamous cell carcinoma: a retrospective archival study

BACKGROUND

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common malignancy in the world in developed countries. Despite the intense research in the area of squamous cell carcinomas of head and neck (HNSCC), long-term survival rate has not changed significantly in this malignancy during recent decades.

METHODS

In this study, we focused on TP53 mutations in specific regions, including DNA-binding surface, to determine whether mutations at specific locations of TP53 could be used to help in setting up prognosis and response to therapy of head and neck squamous cell carcinoma patients. We analysed TP53 mutations in 46 HNSCC by PCR-SSCP and sequencing and characterized how different TP53 mutations affect the patient outcome.

RESULTS

Tumours containing TP53 mutations in DNA-binding regions (L2, L3 and LSH motif) had a significantly poorer prognosis and response to radiotherapy than tumours outside those regions. Disease-specific 5-year survival of patients with TP53 mutations affecting DNA contacts was 43.5% while it was 77.8% (p < 0.05) in patients with TP53 mutations in other residues not involved in DNA contact. Moreover, nodal metastasis were more prevalent (although not statistically significantly) with TP53 mutations in DNA-binding surface regions. We noticed that the patients with TP53 mutations in L3/LSH motifs had a significantly poorer response (11.4% responding) to radiation than the patients with a wild type p53 (48.6%) or TP53 mutations outside the DNA-binding regions (40%) (p < 0.05).

CONCLUSIONS

These data indicate that a TP53 mutation in L2, L3 or LSH is worth pursuing as a marker for predicting prognosis and response to radiation among HNSCC patients.

Metabolism and brain cancer

Cellular energy metabolism is one of the main processes affected during the transition from normal to cancer cells, and it is a crucial determinant of cell proliferation or cell death. As a support for rapid proliferation, cancer cells choose to use glycolysis even in the presence of oxygen (Warburg effect) to fuel macromolecules for the synthesis of nucleotides, fatty acids, and amino acids for the accelerated mitosis, rather than fuel the tricarboxylic acid cycle and oxidative phosphorylation. Mitochondria biogenesis is also reprogrammed in cancer cells, and the destiny of those cells is determined by the balance between energy and macromolecule supplies, and the efficiency of buffering of the cumulative radical oxygen species. In glioblastoma, the most frequent and malignant adult brain tumor, a metabolic shift toward aerobic glycolysis is observed, with regulation by well known genes as integrants of oncogenic pathways such as phosphoinositide 3-kinase/protein kinase, MYC, and hypoxia regulated gene as hypoxia induced factor 1. The expression profile of a set of genes coding for glycolysis and the tricarboxylic acid cycle in glioblastoma cases confirms this metabolic switch. An understanding of how the main metabolic pathways are modified by cancer cells and the interactions between oncogenes and tumor suppressor genes with these pathways may enlighten new strategies in cancer therapy. In the present review, the main metabolic pathways are compared in normal and cancer cells, and key regulations by the main oncogenes and tumor suppressor genes are discussed. Potential therapeutic targets of the cancer energetic metabolism are enumerated, highlighting the astrocytomas, the most common brain cancer.

Restoration of DNA-binding and growth-suppressive activity of mutant forms of p53 via a PCAF-mediated acetylation pathway

Tumor-derived mutant forms of p53 compromise its DNA binding, transcriptional, and growth regulatory activity in a manner that is dependent upon the cell-type and the type of mutation. Given the high frequency of p53 mutations in human tumors, reactivation of the p53 pathway has been widely proposed as beneficial for cancer therapy. In support of this possibility p53 mutants possess a certain degree of conformational flexibility that allows for re-induction of function by a number of structurally different artificial compounds or by short peptides. This raises the question of whether physiological pathways for p53 mutant reactivation also exist and can be exploited therapeutically. The activity of wild-type p53 is modulated by various acetyl-transferases and deacetylases, but whether acetylation influences signaling by p53 mutant is still unknown. Here, we show that the PCAF acetyl-transferase is down-regulated in tumors harboring p53 mutants, where its re-expression leads to p53 acetylation and to cell death. Furthermore, acetylation restores the DNA-binding ability of p53 mutants in vitro and expression of PCAF, or treatment with deacetylase inhibitors, promotes their binding to p53-regulated promoters and transcriptional activity in vivo. These data suggest that PCAF-mediated acetylation rescues activity of at least a set of p53 mutations. Therefore, we propose that dis-regulation of PCAF activity is a pre-requisite for p53 mutant loss of function and for the oncogenic potential acquired by neoplastic cells expressing these proteins. Our findings offer a new rationale for therapeutic targeting of PCAF activity in tumors harboring oncogenic versions of p53.

p53 transcriptional pathways in breast cancer: the good, the bad and the complex

A p53 network immunohistochemically-based signature to discriminate between good and poor prognosis breast cancer would have clinical relevance, given the key role of p53 in malignancy and response to therapy. Utilizing a five-protein signature of p53/mdm2/mdm4/bcl2/p21 discriminates good-prognosis and poor-prognosis patient groups, based on the functionality of the p53 network. However, the relationship of this five-protein signature to p53 mutation, the wide range of breast cancer therapies now in use and the over-70 age group remain uncertain. Nonetheless, confirmation of the signature in two independent series suggests that this approach should be considered in further case series and in the context of clinical trials.

Regulation of the p53 transcriptional response by structurally diverse core promoters

p53 target promoters are structurally diverse and display pronounced differences in RNA polymerase II (RNAP II) occupancy even in unstressed cells, with higher levels observed on cell cycle arrest genes (p21) compared with apoptotic genes (Fas/APO1). This occupancy correlates well with their ability to undergo rapid or delayed stress induction. To understand the basis for such distinct temporal assembly of transcription complexes, we examined the role of core promoter structures in this process. We find that the p21 core promoter directs rapid, TATA box-dependent assembly of RNAP II preinitiation complexes (PICs), but permits few rounds of RNAP II reinitiation. In contrast, PIC formation at the Fas/APO1 core promoter is very inefficient but supports multiple rounds of transcription. We define a downstream element within the Fas/APO1 core promoter that is essential for its activation, and identify nuclear transcription factor Y (NF-Y) as its binding partner. NF-Y acts as a bifunctional transcription factor that regulates basal expression of Fas/APO1 in vivo. Thus, two critical parameters of the stress-induced p53 transcriptional response are the kinetics of gene induction and duration of expression through frequent reinitiation. These features are intrinsic, DNA-encoded properties of diverse core promoters that may be fundamental to anticipatory programming of p53 response genes upon stress.

p53 regulation of metabolic pathways

During the course of tumorigenesis, cells acquire a number of alterations that contribute to the acquisition of the malignant phenotype, allowing them to survive and flourish in increasingly hostile environments. Cancer cells can be characterized by perturbations in the control of cell proliferation and growth, resistance to death, and alterations in their interactions with the microenvironment. Underpinning many of these changes are shifts in metabolism that allow cancer cells to use alternative pathways for energy production and building the macromolecules necessary for growth, as well as regulating the generation of signaling molecules such as reactive oxygen species (ROS). In the past few years, it became clear that p53, the most studied, if not most important, tumor suppressor protein, can also directly control metabolic traits of cells.

DNA damage: from causes to cures

In order to maintain genome integrity, it is essential that any DNA damage is repaired. This is achieved in diverse ways in all cells to ensure cellular survival. There is a large repertoire of proteins that remove and repair DNA damage. However, sometimes these processes do not function correctly, leading to genome instability. Studies of DNA repair and genome instability and their causes and cures were showcased in the 2008 Biochemical Society Annual Symposium. The present article provides a summary of the talks given and the subsequent papers in this issue of Biochemical Society Transactions.