Inhibition of p53 and/or AKT as a new therapeutic approach specifically targeting ALT cancers

Intracellular metal ion-based chemistry for programmed cell death

Intracellular metal ions play essential roles in multiple physiological processes, including catalytic action, diverse cellular processes, intracellular signaling, and electron transfer. It is crucial to maintain intracellular metal ion homeostasis which is achieved by the subtle balance of storage and release of metal ions intracellularly along with the influx and efflux of metal ions at the interface of the cell membrane. Dysregulation of intracellular metal ions has been identified as a key mechanism in triggering programmed cell death (PCD). Despite the importance of metal ions in initiating PCD, the molecular mechanisms of intracellular metal ions within these processes are infrequently discussed. An in-depth understanding and review of the role of metal ions in triggering PCD may better uncover novel tools for cancer diagnosis and therapy. Specifically, the essential roles of calcium (Ca2+), iron (Fe2+/3+), copper (Cu+/2+), and zinc (Zn2+) ions in triggering PCD are primarily explored in this review, and other ions like manganese (Mn2+/3+/4+), cobalt (Co2+/3+) and magnesium ions (Mg2+) are briefly discussed. Further, this review elaborates on the underlying chemical mechanisms and summarizes these metal ions triggering PCD in cancer therapy. This review bridges chemistry, immunology, and biology to foster the rational regulation of metal ions to induce PCD for cancer therapy.

Hypoxia triggers cardiomyocyte apoptosis via regulating the m6A methylation-mediated LncMIAT/miR-708-5p/p53 axis

Long-time hypoxia induced cardiomyocyte apoptosis is an important mechanism of myocardial ischemia (MI) injury. Interestingly, long noncoding RNA myocardial infarction-associated transcript (LncMIAT) has been involved in the regulation of MI injury; however, the underlying mechanism by which LncMIAT affects the progression of hypoxia-induced cardiomyocyte apoptosis remains unclear. In the present study, hypoxia was found to promote cardiomyocyte apoptosis through an increased expression of LncMIAT in vitro. Biological investigations and dual-luciferase gene reporter assay further revealed that LncMIAT was able to bind with miR-708-5p to upregulate the p53-mediated cell death of the cardiomyocytes. Silencing of LncMIAT or overexpression of miR-708-5p led to a significant reduction in p53-mediated cardiomyocyte apoptosis. The methylated RNA immunoprecipitation (MeRIP)-qPCR results showed that hypoxia exerted its effects on LncMIAT through AKLBH5-N6-methyladenosine (m6A) methylation and therefore hypoxia was shown to trigger HL-1 cardiomyocyte apoptosis via the m6A methylation-mediated LncMIAT/miR-708-5p/p53 axis. Silencing of AKLBH5 significantly alleviated the m6A methylation-mediated LncMIAT upregulation and p53-mediated cardiomyocyte apoptosis, while promoted miR-708-5p expression. Taken together, the present study highlighted that LncMIAT could act as a key biological target during hypoxia-induced cardiomyocyte apoptosis. In addition, it was shown that hypoxia could promote cardiomyocyte apoptosis through regulation of the m6A methylation-mediated LncMIAT/miR-708-5p/p53 signaling axis.

Signalling inhibition by ponatinib disrupts productive alternative lengthening of telomeres (ALT)

Alternative lengthening of telomeres (ALT) supports telomere maintenance in 10-15% of cancers, thus representing a compelling target for therapy. By performing anti-cancer compound library screen on isogenic cell lines and using extrachromosomal telomeric C-circles, as a bona fide marker of ALT activity, we identify a receptor tyrosine kinase inhibitor ponatinib that deregulates ALT mechanisms, induces telomeric dysfunction, reduced ALT-associated telomere synthesis, and targets, in vivo, ALT-positive cells. Using RNA-sequencing and quantitative phosphoproteomic analyses, combined with C-circle level assessment, we find an ABL1-JNK-JUN signalling circuit to be inhibited by ponatinib and to have a role in suppressing telomeric C-circles. Furthermore, transcriptome and interactome analyses suggest a role of JUN in DNA damage repair. These results are corroborated by synergistic drug interactions between ponatinib and either DNA synthesis or repair inhibitors, such as triciribine. Taken together, we describe here a signalling pathway impacting ALT which can be targeted by a clinically approved drug.

Potential clinical treatment prospects behind the molecular mechanism of alternative lengthening of telomeres (ALT)

Normal somatic cells inevitably experience replicative stress and senescence during proliferation. Somatic cell carcinogenesis can be prevented in part by limiting the reproduction of damaged or old cells and removing them from the cell cycle [1, 2]. However, Cancer cells must overcome the issues of replication pressure and senescence as well as preserve telomere length in order to achieve immortality, in contrast to normal somatic cells [1, 2]. Although telomerase accounts for the bulk of telomere lengthening methods in human cancer cells, there is a non-negligible portion of telomere lengthening pathways that depend on alternative lengthening of telomeres (ALT) [3]. For the selection of novel possible therapeutic targets for ALT-related disorders, a thorough understanding of the molecular biology of these diseases is crucial [4]. The roles of ALT, typical ALT tumor cell traits, the pathophysiology and molecular mechanisms of ALT tumor disorders, such as adrenocortical carcinoma (ACC), are all summarized in this work. Additionally, this research compiles as many of its hypothetically viable but unproven treatment targets as it can (ALT-associated PML bodies (APB), etc.). This review is intended to contribute as much as possible to the development of research, while also trying to provide a partial information for prospective investigations on ALT pathways and associated diseases.

The repression of oncoprotein SET by the tumor suppressor p53 reveals a p53-SET-PP2A feedback loop for cancer therapy

The oncoprotein SET is frequently overexpressed in many types of tumors and contributes to malignant initiation and progression through multiple mechanisms, including the hijacking of the tumor suppressors p53 and PP2A. Targeting aberrant SET represents a promising strategy for cancer intervention. However, the mechanism by which endogenous SET is regulated in cancer cells remains largely unknown. Here, we identified the tumor suppressor p53 as a key regulator that transcriptionally repressed the expression of SET in both normal and cancer cells. In addition, p53 stimulated PP2A phosphatase activity via p53-mediated transcriptional repression of SET, whereby SET-mediated inhibition of PP2A was alleviated. Moreover, targeting the interaction between SET and PP2A catalytic subunit (PP2Ac) with FTY720 enhanced stress-induced p53 activation via PP2A-mediated dephosphorylation of p53 on threonine 55 (Thr55). Therefore, our findings uncovered a previously unknown p53-SET-PP2A regulatory feedback loop. To functionally potentiate this feedback loop, we designed a combined therapeutic strategy by simultaneously administrating a p53 activator and SET antagonist in cancer cells and observed a dramatic synergistic effect on tumor suppression. Our study reveals mechanistic insight into the regulation of the oncoprotein SET and raises a potential strategy for cancer therapy by stimulating the p53-SET-PP2A feedback loop.

The Cross Talk Between p53 and mTOR Pathways in Response to Physiological and Genotoxic Stresses

The tumor suppressor p53 is activated upon multiple cellular stresses, including DNA damage, oncogene activation, ribosomal stress, and hypoxia, to induce cell cycle arrest, apoptosis, and senescence. Mammalian target of rapamycin (mTOR), an evolutionarily conserved serine/threonine protein kinase, serves as a central regulator of cell growth, proliferation, and survival by coordinating nutrients, energy, growth factors, and oxygen levels. p53 dysfunction and mTOR pathway hyperactivation are hallmarks of human cancer. The balance between response to stresses or commitment to cell proliferation and survival is governed by various regulatory loops between the p53 and mTOR pathways. In this review, we first briefly introduce the tumor suppressor p53 and then describe the upstream regulators and downstream effectors of the mTOR pathway. Next, we discuss the role of p53 in regulating the mTOR pathway through its transcriptional and non-transcriptional effects. We further describe the complicated role of the mTOR pathway in modulating p53 activity. Finally, we discuss the current knowledge and future perspectives on the coordinated regulation of the p53 and mTOR pathways.

TASP1 Promotes Proliferation and Migration in Gastric Cancer via EMT and AKT/P-AKT Pathway

Threonine aspartase 1 (TASP1) was reported to function in the development of cancer. However, the regulatory mechanism of TASP1 in gastric cancer (GC) remains unclear. In this study, we determined the expression of TASP1 in tissues of GC patients, GC cells by qRT-PCR, and western blot and assessed the relationship between TASP1 and GC cell proliferation and migration via CCK-8 and transwell assay. It was found that the expression of TASP1 in GC tissues or GC cell lines was significantly higher than that in normal adjacent tissues or normal cells. The proliferation and migration of GC cells were inhibited upon TASP1 knockdown. Mechanism investigation revealed that TASP1 promoted GC cell proliferation and migration through upregulating the p-AKT/AKT expression. TASP1 induced GC cell migration via the epithelial -mesenchymal transition (EMT) pathway. In conclusion, TASP1 promotes GC progression through the EMT and AKT/p-AKT pathway, and it may serve as a new potential biomarker and therapeutic target for GC.

Stepwise crosstalk between aberrant Nf1, Tp53 and Rb signalling pathways induces gliomagenesis in zebrafish

The molecular pathogenesis of glioblastoma indicates that RTK/Ras/PI3K, RB and TP53 pathways are critical for human gliomagenesis. Here, several transgenic zebrafish lines with single or multiple deletions of nf1, tp53 and rb1 in astrocytes, were established to genetically induce gliomagenesis in zebrafish. In the mutant with a single deletion, we found only the nf1 mutation low-efficiently induced tumour incidence, suggesting that the Nf1 pathway is critical for the initiation of gliomagenesis in zebrafish. Combination of mutations, nf1;tp53 and rb1;tp53 combined knockout fish, showed much higher tumour incidences, high-grade histology, increased invasiveness, and shortened survival time. Further bioinformatics analyses demonstrated the alterations in RTK/Ras/PI3K, cell cycle, and focal adhesion pathways, induced by abrogated nf1, tp53, or rb1, were probably the critical stepwise biological events for the initiation and development of gliomagenesis in zebrafish. Gene expression profiling and histological analyses showed the tumours derived from zebrafish have significant similarities to the subgroups of human gliomas. Furthermore, temozolomide treatment effectively suppressed gliomagenesis in these glioma zebrafish models, and the histological responses in temozolomide-treated zebrafish were similar to those observed in clinically treated glioma patients. Thus, our findings will offer a potential tool for genetically investigating gliomagenesis and screening potential targeted anti-tumour compounds for glioma treatment.

The Interactions of DNA Repair, Telomere Homeostasis, and p53 Mutational Status in Solid Cancers: Risk, Prognosis, and Prediction

The disruption of genomic integrity due to the accumulation of various kinds of DNA damage, deficient DNA repair capacity, and telomere shortening constitute the hallmarks of malignant diseases. DNA damage response (DDR) is a signaling network to process DNA damage with importance for both cancer development and chemotherapy outcome. DDR represents the complex events that detect DNA lesions and activate signaling networks (cell cycle checkpoint induction, DNA repair, and induction of cell death). TP53, the guardian of the genome, governs the cell response, resulting in cell cycle arrest, DNA damage repair, apoptosis, and senescence. The mutational status of TP53 has an impact on DDR, and somatic mutations in this gene represent one of the critical events in human carcinogenesis. Telomere dysfunction in cells that lack p53-mediated surveillance of genomic integrity along with the involvement of DNA repair in telomeric DNA regions leads to genomic instability. While the role of individual players (DDR, telomere homeostasis, and TP53) in human cancers has attracted attention for some time, there is insufficient understanding of the interactions between these pathways. Since solid cancer is a complex and multifactorial disease with considerable inter- and intra-tumor heterogeneity, we mainly dedicated this review to the interactions of DNA repair, telomere homeostasis, and TP53 mutational status, in relation to (a) cancer risk, (b) cancer progression, and (c) cancer therapy.

Clinical Relevance of Alternative Lengthening of Telomeres in Cancer

The alternative lengthening of telomere (ALT) is a pathway responsible for cell immortalization in some kinds of tumors. Since the first description of ALT is relatively recent in the oncology field, its mechanism remains elusive, but recent works address ALT-related proteins or cellular structures as potential druggable targets for more specific and efficient antitumor therapies. Moreover, some new generation compounds for antitelomerase therapy in cancer were able to provoke acquisition of ALT phenotype in treated tumors, enhancing the importance of studies on this alternative lengthening of the telomere. However, ALT has been implicated in different - sometimes opposite - outcomes, according to the tumor type studied. Then, in order to design and develop new drugs for ALT+ cancer in an effective way, it is crucial to understand its clinical implications. In this review, we gathered works published in the last two decades to highlight the clinical relevance of ALT on oncology.

Role of POT1 in Human Cancer

Simple Summary

The segmentation of eukaryotic genomes into discrete linear chromosomes requires processes to solve several major biological problems, including prevention of the chromosome ends being recognized as DNA breaks and compensation for the shortening that occurs when linear DNA is replicated. A specialized set of six proteins, collectively referred to as shelterin, is involved in both of these processes, and mutations in several of these are now known to be involved in cancer. Here, we focus on Protection of Telomeres 1 (POT1), the shelterin protein that appears to be most commonly involved in cancer, and consider the clinical significance of findings about its biological functions and the prevalence of inherited and acquired mutations in the POT1 gene.

Abstract

Telomere abnormalities facilitate cancer development by contributing to genomic instability and cellular immortalization. The Protection of Telomeres 1 (POT1) protein is an essential subunit of the shelterin telomere binding complex. It directly binds to single-stranded telomeric DNA, protecting chromosomal ends from an inappropriate DNA damage response, and plays a role in telomere length regulation. Alterations of POT1 have been detected in a range of cancers. Here, we review the biological functions of POT1, the prevalence of POT1 germline and somatic mutations across cancer predisposition syndromes and tumor types, and the dysregulation of POT1 expression in cancers. We propose a framework for understanding how POT1 abnormalities may contribute to oncogenesis in different cell types. Finally, we summarize the clinical implications of POT1 alterations in the germline and in cancer, and possible approaches for the development of targeted cancer therapies.

SOX2 and p53 Expression Control Converges in PI3K/AKT Signaling with Versatile Implications for Stemness and Cancer

Stemness and reprogramming involve transcriptional master regulators that suppress cell differentiation while promoting self-renewal. A distinguished example thereof is SOX2, a high mobility group (HMG)-box transcription factor (TF), whose subcellular localization and turnover regulation in embryonic, induced-pluripotent, and cancer stem cells (ESCs, iPSCs, and CSCs, respectively) is mediated by the PI3K/AKT/SOX2 axis, a stem cell-specific branch of the PI3K/AKT signaling pathway. Further effector functions associated with PI3K/AKT induction include cell cycle progression, cellular (mass) growth, and the suppression of apoptosis. Apoptosis, however, is a central element of DNA damage response (DDR), where it provides a default mechanism for cell clearance when DNA integrity cannot be maintained. A key player in DDR is tumor suppressor p53, which accumulates upon DNA-damage and is counter-balanced by PI3K/AKT enforced turnover. Accordingly, stemness sustaining SOX2 expression and p53-dependent DDR mechanisms show molecular–functional overlap in PI3K/AKT signaling. This constellation proves challenging for stem cells whose genomic integrity is a functional imperative for normative ontogenesis. Unresolved mutations in stem and early progenitor cells may in fact provoke transformation and cancer development. Such mechanisms are also particularly relevant for iPSCs, where genetic changes imposed through somatic cell reprogramming may promote DNA damage. The current review aims to summarize the latest advances in the understanding of PI3K/AKT/SOX2-driven stemness and its intertwined relations to p53-signaling in DDR under conditions of pluripotency, reprogramming, and transformation.

METTL3 and N6-Methyladenosine Promote Homologous Recombination-Mediated Repair of DSBs by Modulating DNA-RNA Hybrid Accumulation

Double-strand breaks (DSBs) are the most deleterious DNA lesions, which, if left unrepaired, may lead to genome instability or cell death. Here, we report that, in response to DSBs, the RNA methyltransferase METTL3 is activated by ATM-mediated phosphorylation at S43. Phosphorylated METTL3 is then localized to DNA damage sites, where it methylates the N6 position of adenosine (m6A) in DNA damage-associated RNAs, which recruits the m6A reader protein YTHDC1 for protection. In this way, the METTL3-m6A-YTHDC1 axis modulates accumulation of DNA-RNA hybrids at DSBs sites, which then recruit RAD51 and BRCA1 for homologous recombination (HR)-mediated repair. METTL3-deficient cells display defective HR, accumulation of unrepaired DSBs, and genome instability. Accordingly, depletion of METTL3 significantly enhances the sensitivity of cancer cells and murine xenografts to DNA damage-based therapy. These findings uncover the function of METTL3 and YTHDC1 in HR-mediated DSB repair, which may have implications for cancer therapy.

RBMX is required for activation of ATR on repetitive DNAs to maintain genome stability

ATR is a master regulator of cell response to replication stress. Adequate activation of ATR is essential for preventing genome aberrance induced by replication defect. However, the mechanism underlying ATR activation is not fully understood. Here, we identify that RBMX is an ssDNA binding protein that orchestrates a novel pathway to activate ATR. Using super-resolution STORM, we observe that RBMX and RPA bind to adjacent but nonoverlapping sites on ssDNA in response to replication stress. RBMX then binds to and facilitates positioning of TopBP1, which activates nearby ATR associated with RPA. In addition, ATR activation by ssDNA-RBMX-TopBP1 is independent of ssDNA-dsDNA junction and 9-1-1 complex. ChIP-seq analysis reveals that RBMX/RPA are highly enriched on repetitive DNAs, which are considered as fragile sites with high replication stress. RBMX depletion leads to defective localization of TopBP1 to replication stressed sites and inadequate activation of ATR. Furthermore, cells with deficient RBMX demonstrate replication defect, leading to formation of micronuclei and a high rate of sister-chromatin exchange, indicative of genome instability. Together, the results identify a new ssDNA-RBMX-TopBP1 pathway that is specifically required for activation of ATR on repetitive DNAs. Therefore, RBMX is a key factor to ensure genome stability during replication.

Novel therapeutic strategies targeting telomere maintenance mechanisms in high-risk neuroblastoma

The majority of high-risk neuroblastomas can be divided into three distinct molecular subgroups defined by the presence of MYCN amplification, upstream TERT rearrangements or alternative lengthening of telomeres (ALT). The common defining feature of all three subgroups is altered telomere maintenance; MYCN amplification and upstream TERT rearrangements drive high levels of telomerase expression whereas ALT is a telomerase independent telomere maintenance mechanism. As all three telomere maintenance mechanisms are independently associated with poor outcomes, the development of strategies to selectively target either telomerase expressing or ALT cells holds great promise as a therapeutic approach that is applicable to the majority of children with aggressive disease.Here we summarise the biology of telomere maintenance and the molecular drivers of aggressive neuroblastoma before describing the most promising therapeutic strategies to target both telomerase expressing and ALT cancers. For telomerase-expressing neuroblastoma the most promising targeted agent to date is 6-thio-2'-deoxyguanosine, however clinical development of this agent is required. In osteosarcoma cell lines with ALT, selective sensitivity to ATR inhibition has been reported. However, we present data showing that in fact ALT neuroblastoma cells are more resistant to the clinical ATR inhibitor AZD6738 compared to other neuroblastoma subtypes. More recently a number of additional candidate compounds have been shown to show selectivity for ALT cancers, such as Tetra-Pt (bpy), a compound targeting the telomeric G-quadruplex and pifithrin-α, a putative p53 inhibitor. Further pre-clinical evaluation of these compounds in neuroblastoma models is warranted.In summary, telomere maintenance targeting strategies offer a significant opportunity to develop effective new therapies, applicable to a large proportion of children with high-risk neuroblastoma. In parallel to clinical development, more pre-clinical research specifically for neuroblastoma is urgently needed, if we are to improve survival for this common poor outcome tumour of childhood.