Brain Tumors: Development, Drug Resistance, and Sensitization - An Epigenetic Approach.

In this article, we describe contrasting developmental aspects of paediatric and adult brain tumours. We hypothesize that the formation of cancer progenitor cells, for both paediatric and adult, could be due to epigenetic events. However, the progression of adult brain tumours selectively involves more mutations compared to paediatric tumours. We further discuss epigenetic switches, comprising both histone modifications and DNA methylation, and how they can differentially regulate transcription and expression of oncogenes and tumour suppressor genes. Next, we summarize the currently available therapies for both types of brain tumours, explaining the merits and failures leading to drug resistance. We analyse different mechanisms of drug resistance and the role of epigenetics in this process. We then provide a rationale for combination therapy, which includes epigenetic drugs. In the end, we postulate a concept which describes how a combination therapy could be initiated. The timing, doses, and order of individual drug regimens will depend on the individual case. This type of combination therapy will be part of a personalized medicine which will differ from patient to patient.

Sensitization of Drug Resistant Cancer Cells: A Matter of Combination Therapy.

Cancer drug resistance is an enormous problem. It is responsible for most relapses in cancer patients following apparent remission after successful therapy. Understanding cancer relapse requires an understanding of the processes underlying cancer drug resistance. This article discusses the causes of cancer drug resistance, the current combination therapies, and the problems with the combination therapies. The rational design of combination therapy is warranted to improve the efficacy. These processes must be addressed by finding ways to sensitize the drug-resistant cancers cells to chemotherapy, and to prevent formation of drug resistant cancer cells. It is also necessary to prevent the formation of cancer progenitor cells by epigenetic mechanisms, as cancer progenitor cells are insensitive to standard therapies. In this article, we emphasize the role for the rational development of combination therapy, including epigenetic drugs, in achieving these goals.

Cancer Progenitor Cells: The Result of an Epigenetic Event?

The concept of cancer stem cells was proposed in the late 1990s. Although initially the idea seemed controversial, the existence of cancer stem cells is now well established. However, the process leading to the formation of cancer stem cells is still not clear and thus requires further research. This article discusses epigenetic events that possibly produce cancer progenitor cells from predisposed cells by the influence of their environment. Every somatic cell possesses an epigenetic signature in terms of histone modifications and DNA methylation, which are obtained during lineage-specific differentiation of pluripotent stem cells, which is specific to that particular tissue. We call this signature an epigenetic switch. The epigenetic switch is not fixed. Our epigenome alters with aging. However, depending on the predisposition of the cells of a particular tissue and their microenvironment, the balance of the switch (histone modifications and the DNA methylation) may be tilted to immortality in a few cells, which generates cancer progenitor cells.

The Effects of Histone Deacetylase Inhibitor and Calpain Inhibitor Combination Therapies on Ovarian Cancer Cells.

BACKGROUND: Ovarian cancer is difficult to treat due to absence of selective drugs and tendency of platinum drugs to promote resistance. Combination therapy using epigenetic drugs is predicted to be a beneficial alternative. MATERIALS AND METHODS: This study investigated the effects of combination therapies using two structurally different histone deacetylase (HDAC) inhibitors (HDACi), sodium butyrate and suberanilohydroxamic acid (SAHA), with the calpain inhibitor calpeptin on two characteristically different ovarian cancer cell lines, CAOV-3 and SKOV-3. RESULTS: Suboptimal doses of HDACi and calpeptin produced several effects. Growth inhibition was enhanced and the epigenetically silenced tumor suppressor genes ARHI, p21 and RARß2 were re-expressed. Methylation of specific CpG residues in ARHI were reduced. Cell-cycle progression was inhibited and apoptosis, as well as autophagy, were induced. The phosphorylation of ERK and Akt were differentially effected by these inhibitors. CONCLUSION: The re-expression of tumor suppressors may sensitize ovarian cancer cells, which then undergo apoptosis and autophagy for cell death.

The Evolution of Epigenetics: From Prokaryotes to Humans and Its Biological Consequences.

The evolution process includes genetic alterations that started with prokaryotes and now continues in humans. A distinct difference between prokaryotic chromosomes and eukaryotic chromosomes involves histones. As evolution progressed, genetic alterations accumulated and a mechanism for gene selection developed. It was as if nature was experimenting to optimally utilize the gene pool without changing individual gene sequences. This mechanism is called epigenetics, as it is above the genome. Curiously, the mechanism of epigenetic regulation in prokaryotes is strikingly different from that in eukaryotes, mainly higher eukaryotes, like mammals. In fact, epigenetics plays a significant role in the conserved process of embryogenesis and human development. Malfunction of epigenetic regulation results in many types of undesirable effects, including cardiovascular disease, metabolic disorders, autoimmune diseases, and cancer. This review provides a comparative analysis and new insights into these aspects.

A Comparative Analysis of Genetic and Epigenetic Events of Breast and Ovarian Cancer Related to Tumorigenesis.

Breast cancer persists as the most common cause of cancer death in women worldwide. Ovarian cancer is also a significant source of morbidity and mortality, as the fifth leading cause of cancer death among women. This reflects the continued need for further understanding and innovation in cancer treatment. Though breast and ovarian cancer usually present as distinct clinical entities, the recent explosion of large-scale -omics research has uncovered many overlaps, particularly with respect to genetic and epigenetic alterations. We compared genetic, microenvironmental, stromal, and epigenetic changes common between breast and ovarian cancer cells, as well as the clinical relevance of these changes. Some of the most striking commonalities include genetic alterations of BRCA1 and 2, TP53, RB1, NF1, FAT3, MYC, PTEN, and PIK3CA; down regulation of miRNAs 9, 100, 125a, 125b, and 214; and epigenetic alterations such as H3K27me3, H3K9me2, H3K9me3, H4K20me3, and H3K4me. These parallels suggest shared features of pathogenesis. Furthermore, preliminary evidence suggests a shared epigenetic mechanism of oncogenesis. These similarities, warrant further investigation in order to ultimately inform development of more effective chemotherapeutics, as well as strategies to circumvent drug resistance.

EMT and tumor metastasis.

EMT and MET comprise the processes by which cells transit between epithelial and mesenchymal states, and they play integral roles in both normal development and cancer metastasis. This article reviews these processes and the molecular pathways that contribute to them. First, we compare embryogenesis and development with cancer metastasis. We then discuss the signaling pathways and the differential expression and down-regulation of receptors in both tumor cells and stromal cells, which play a role in EMT and metastasis. We further delve into the clinical implications of EMT and MET in several types of tumors, and lastly, we discuss the role of epigenetic events that regulate EMT/MET processes. We hypothesize that reversible epigenetic events regulate both EMT and MET, and thus, also regulate the development of different types of metastatic cancers.

Use of epigenetic drugs in disease: an overview.

Epigenetic changes such as DNA methylation and histone methylation and acetylation alter gene expression at the level of transcription by upregulating, downregulating, or silencing genes completely. Dysregulation of epigenetic events can be pathological, leading to cardiovascular disease, neurological disorders, metabolic disorders, and cancer development. Therefore, identifying drugs that inhibit these epigenetic changes are of great clinical interest. In this review, we summarize the epigenetic events associated with different disorders and diseases including cardiovascular, neurological, and metabolic disorders, and cancer. Knowledge of the specific epigenetic changes associated with these types of diseases facilitates the development of specific inhibitors, which can be used as epigenetic drugs. In this review, we discuss the major classes of epigenetic drugs currently in use, such as DNA methylation inhibiting drugs, bromodomain inhibitors, histone acetyl transferase inhibitors, histone deacetylase inhibitors, protein methyltransferase inhibitors, and histone methylation inhibitors and their role in reversing epigenetic changes and treating disease.

Drug resistance in cancer: an overview.

Cancers have the ability to develop resistance to traditional therapies, and the increasing prevalence of these drug resistant cancers necessitates further research and treatment development. This paper outlines the current knowledge of mechanisms that promote or enable drug resistance, such as drug inactivation, drug target alteration, drug efflux, DNA damage repair, cell death inhibition, and the epithelial-mesenchymal transition, as well as how inherent tumor cell heterogeneity plays a role in drug resistance. It also describes the epigenetic modifications that can induce drug resistance and considers how such epigenetic factors may contribute to the development of cancer progenitor cells, which are not killed by conventional cancer therapies. Lastly, this review concludes with a discussion on the best treatment options for existing drug resistant cancers, ways to prevent the formation of drug resistant cancers and cancer progenitor cells, and future directions of study.

Genetic and epigenetic aspects of breast cancer progression and therapy.

Although breast cancer is a heterogeneous disease that is challenging to characterize and treat, the recent explosion of genetic and epigenetic research may help improve these endeavors. In the present review, we use genetic diversity to characterize and classify different types of breast cancer. We also discuss genetic and epigenetic changes that are involved in the development of different breast cancer types and examine how these changes affect prognosis. It appears that while a combination of mutations and copy number changes determine the type of breast cancer, epigenetic alterations may be the primary initiators of cancer development. Understanding these critical biomarkers and molecular changes will advance our ability to effectively treat breast cancer. Next, we examine potential drug therapies directed at epigenetic changes, as such epigenetic drug treatments may prove useful for treating patient-specific tumors, breast cancer progenitor cells, and drug-resistant cells. Lastly, we discuss on mechanisms of carcinogenesis, including a novel hypothesis outlining the role of epigenetics in the development of cancer progenitor cells and metastasis. Overall, breast cancer subtypes may have a similar epigenetic signal that promotes cancer development, and treatment may be most effective if both epigenetic and genetic differences are targeted.

MicroRNA let-7 downregulates STAT3 phosphorylation in pancreatic cancer cells by increasing SOCS3 expression.

Although dispensable for normal pancreatic function, STAT3 signaling is frequently activated in pancreatic cancers. Consistent downregulation of expression of microRNA let-7 is also characteristic of pancreatic ductal adenocarcinoma (PDAC) biopsy specimens. We demonstrate in this study that re-expression of let-7 in poorly-differentiated PDAC cell lines reduced phosphorylation/activation of STAT3 and its downstream signaling events and reduced the growth and migration of PDAC cells. Let-7 re-expression did not repress expression of STAT3 protein or its activator cytokine interleukin 6 (IL-6). However, let-7 re-expression enhanced cytoplasmic expression of suppressor of cytokine signaling 3 (SOCS3), which blocks STAT3 activation by JAK2. Our study thus identified a mechanism by which STAT3 signaling can be inhibited in pancreatic cancer cells by modifying let-7 expression.

Cancer development, progression, and therapy: an epigenetic overview.

Carcinogenesis involves uncontrolled cell growth, which follows the activation of oncogenes and/or the deactivation of tumor suppression genes. Metastasis requires down-regulation of cell adhesion receptors necessary for tissue-specific, cell-cell attachment, as well as up-regulation of receptors that enhance cell motility. Epigenetic changes, including histone modifications, DNA methylation, and DNA hydroxymethylation, can modify these characteristics. Targets for these epigenetic changes include signaling pathways that regulate apoptosis and autophagy, as well as microRNA. We propose that predisposed normal cells convert to cancer progenitor cells that, after growing, undergo an epithelial-mesenchymal transition. This process, which is partially under epigenetic control, can create a metastatic form of both progenitor and full-fledged cancer cells, after which metastasis to a distant location may occur. Identification of epigenetic regulatory mechanisms has provided potential therapeutic avenues. In particular, epigenetic drugs appear to potentiate the action of traditional therapeutics, often by demethylating and re-expressing tumor suppressor genes to inhibit tumorigenesis. Epigenetic drugs may inhibit both the formation and growth of cancer progenitor cells, thus reducing the recurrence of cancer. Adopting epigenetic alteration as a new hallmark of cancer is a logical and necessary step that will further encourage the development of novel epigenetic biomarkers and therapeutics.

Enhanced cytotoxicity from deoxyguanosine-enriched T-oligo in prostate cancer cells.

Prostate cancer represents approximately 10 percent of all cancer cases in men and accounts for more than a quarter of all cancer types. Advances in understanding the molecular mechanisms of prostate cancer progression, however, have not translated well to the clinic. Patients with metastatic and hormone-refractory disease have only palliative options for treatment, as chemotherapy seldom produces durable or complete responses, highlighting the need for novel therapeutic approaches. T-oligo, a single-stranded deoxyribonucleic acid with partial sequence homology to human telomeric DNA, has elicited cytostatic and/or cytotoxic effects in multiple cancer cell types. In contrast, normal primary cells of varying tissue types are resistant to cytotoxic actions of T-oligo, underscoring its potential utility as a novel targeted cancer therapeutic. Mechanistically, T-oligo is hypothesized to interfere with normal telomeric structure and form G-quadruplex structures, thereby inducing genomic stress in addition to aberrant upregulation of DNA damageresponse pathways. Here, we present data demonstrating the enhanced effectiveness of a deoxyguanosine-enriched sequence of T-oligo, termed (GGTT)4, which elicits robust cytotoxic effects in prostate cancer cells at lower concentrations than the most recent T-oligo sequence (5'-pGGT TAG GTG TAG GTT T 3') described to date and used for comparison in this study, while exerting no cytotoxic actions on nontransformed human prostate epithelial cells. Additionally, we provide evidence supporting the T-oligo induced activation of cJun N-terminal kinase (JNK) signaling in prostate cancer cells consistent with G-quadruplex formation, thereby significantly advancing the understanding of the T-oligo mechanism of action.

Telomere-homologous G-rich oligonucleotides sensitize human ovarian cancer cells to TRAIL-induced growth inhibition and apoptosis.

G-rich T-oligos (GT-oligos; oligonucleotides with homology to telomeres) elicit a DNA damage response in cells and induce cytotoxic effects in certain tumor cell lines. We have previously shown that GT-oligo inhibits growth, arrests cell cycle, and induces apoptosis in ovarian, pancreatic, and prostate cancer cells. However, not all ovarian cancer cell lines are susceptible to GT-oligo exposure. GT-oligo was found to induce transcript expression of the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors DR-4 and DR-5, which are generally silenced in ovarian cancer cells, rendering them insensitive to TRAIL. Exposure of TRAIL- and GT-oligo-resistant cell lines to GT-oligo rendered them sensitive to the cytotoxic effects of TRAIL, producing more than additive inhibition of growth. An intracellular inhibitor of the extrinsic apoptotic pathway, FLICE-like Inhibitory Protein-Short (FLIPs), was down-regulated and Jun kinase (JNK) was activated by exposure to GT-oligo. JNK inhibition partially reversed the growth inhibition caused by the combination of GT-oligo and TRAIL indicating partial involvement of the Jun kinase pathway in the resulting cytotoxic effect. Both capase-8 and caspases 3/7 were activated by exposure to GT-oligo plus TRAIL, consistent with activation of the extrinsic apoptotic pathway. These results demonstrate a novel way of sensitizing resistant ovarian cancer cells to TRAIL-mediated cytotoxicity.

Demethylation and re-expression of epigenetically silenced tumor suppressor genes: sensitization of cancer cells by combination therapy.

Epigenetic regulation in eukaryotic and mammalian systems is a complex and emerging field of study. While histone modifications create an open chromatin conformation allowing for gene transcription, CpG methylation adds a further dimension to the expression of specific genes in developmental pathways and carcinogenesis. In this review, we will highlight DNA methylation as one of the distinct mechanisms for gene silencing and try to provide insight into the role of epigenetics in cancer progenitor cell formation and carcinogenesis. We will also introduce the concept of a dynamic methylation-demethylation system and the potential for the existence of a demethylating enzyme in this process. Finally, we will explain how re-expression of epigenetically silenced tumor suppressor genes could be exploited to develop effective drug therapies. In particular, we will consider how a combination therapy that includes epigenetic drugs could possibly kill cancer progenitor cells and reduce the chance of relapse following chemotherapy.

Anti-breast cancer effects of histone deacetylase inhibitors and calpain inhibitor.

Development of new breast cancer therapies is needed, particularly as cells become refractory or develop increased drug resistance. In an effort to develop such treatments, class I and II histone deacetylases (HDACs), alone and in combination with other cytotoxic agents, are currently in clinical trial. Herein, we discuss the effects of histone deacetylase inhibitors (HDACi) when used in combination with calpeptin, an inhibitor of the regulatory protease, calpain. We present results of study in two breast cancer cells lines with distinct characteristics: MDA-MB-231 and MCF-7. When used in combination with calpeptin, two chemically distinct HDACi significantly inhibited growth and increased cell death by inducing cell-cycle arrest and apoptosis. MCF-7 cells exhibited a greater proportion of arrest at the G(1) phase, whereas triple-negative MDA-MB-231 cells exhibited increased cell cycle arrest at the S phase. Methylation of the imprinted and silenced proapoptoic tumor suppressor gene aplasia Ras homolog member I (ARHI) was reduced in both cell lines after treatment with HDACi. However, it was only re-expressed on such treatment in MDA-MB-231 cells, suggesting that re-expression operates under differential mechanisms in these two cell lines. Collectively, these results showed that the combination of HDACi and calpeptin inhibited the growth of two distinctly different types of breast cancer cells and could have wide clinical applications, though the mechanisms of inhibition are possibly different.

Mechanism of T-oligo-induced cell cycle arrest in Mia-PaCa pancreatic cancer cells.

DNA oligonucleotides with sequence homology to human telomeric DNA (T-oligo) induce cell cycle arrest, followed by apoptosis, senescence, or autophagy in a human cancer cell type-specific manner. T-oligo has potential as a new therapeutic strategy in oncology because of its ability to target certain types of tumor cells while sparing normal ones. In the present study, we demonstrate the T-oligo-induced S-phase cell cycle arrest in four pancreatic cancer cell lines. To further contribute to the mechanistic understanding of T-oligo, we also identify cyclin dependent kinase 2 (cdk2) as a functional mediator in the T-oligo-induced cell cycle arrest of pancreatic cancer cells. Ectopic expression of a constitutively active cdk2 mutant abrogates T-oligo-induced cell cycle arrest in these tumor cells while knockdown of cdk2 expression alone recapitulates the T-oligo effect. Finally, we demonstrate the dispensability of T-oligo-induced ATM/ATR-mediated DNA damage response-signaling pathways, which have long been considered functional in the T-oligo signaling mechanism.

Histone deacetylase inhibitors reverse CpG methylation by regulating DNMT1 through ERK signaling.

Methylation of CpG repeats in the upstream/promoter regions of genes is an established mechanism of gene silencing in many cell types. DNA methylation results in the recruitment of histone deacetylases (HDACs) to promoter regions, thereby repressing expression of genes. General inhibitors of class I and II HDACs (HDACi), such as sodium butyrate and suberoylanilide hydroxamic acid, suppress the growth of prostate cancer cells in vitro and in vivo. In this study, we investigated the mechanism of re-expression of silenced cell cycle inhibitors and retinoic acid receptor B2 (RARB2). HDACi inhibited cell cycle progression, and reversed promoter methylation and silencing of three tumor suppressor genes: RARB2 and the cell cycle regulating cyclin-dependent kinase inhibitors p16 and p21. HDACi repressed MAP kinase I (ERK) activation and down-regulated DNA (cytosine-5-)-methyltransferase 1 (DNMT1) levels. Direct inhibition of ERK activity similarly decreased DNMT1 protein levels and reversed the basal hypermethylation of the promoters and silencing of the RARB2, p21 and p16 tumor suppressor genes. Suppression of DNMT1 level by siRNA also reversed methylation of these tumor suppressor genes with similar kinetics. Collectively, these data demonstrate that HDACi, by inhibiting ERK activity, regulate DNMT1 and ultimately DNA methylation. These results demonstrate that HDACs regulate gene methylation, in addition to the established and reciprocal ability of CpG methylation to recruit HDACs to repress transcription.

T-oligos inhibit growth and induce apoptosis in human ovarian cancer cells.

Ovarian cancer remains a leading cause of death among women worldwide, and current treatment regimens for advanced disease are inadequate. Oligonucleotides with sequence homology to telomeres (called T-oligos) have been shown to mimic DNA damage responses in cells and induce cytotoxic effects in certain tumor cell lines. We studied the effects of 2 distinct 16 mer T-oligos in 4 human ovarian epithelial carcinoma cell lines. A T-oligo with perfect homology to the telomere overhang region demonstrated some cytotoxic activity in half of the cell lines. A G-rich T-oligo derivative showed more potency and broader cytotoxic activity in these lines than the parental T-oligo. Activation of apoptotic pathways in ovarian cancer cells by exposure to the T-oligo was demonstrated by multiple independent assays. T-oligo was shown to have additive, or more than additive, activity in combination with 2 different histone deacetylase drugs currently in clinical testing. T-oligos may therefore provide a new and tumor-targeted approach to ovarian cancers.