Journal of Radiation and Cancer Research

REVIEW ARTICLE
Year
: 2018  |  Volume : 9  |  Issue : 4  |  Page : 155--164

Breast cancer stem cells, epigenetics, and radiation


Garima Sinha1, Alejandra Ferrer2, Yahaira Naaldijk3, Caitlyn A Moore1, Qunfeng Wu4, Henning Ulrich3, Pranela Rameshwar5,  
1 Rutgers School of Graduate Studies; Department of Medicine, Hematology/Oncology, New Jersey Medical School, Rutgers School of Biomedical Health Science, São Paulo, Brazil
2 Rutgers School of Graduate Studies, New Jersey Medical School, São Paulo, Brazil
3 Department of Biochemistry, Institute of Chemistry, University of São Paulo, São Paulo, Brazil
4 Department of Pathology, Immunology and Laboratory Medicine, New Jersey Medical School, Rutgers School of Biomedical Health Science, Newark, NJ, USA
5 Department of Medicine, Hematology/Oncology, New Jersey Medical School, Rutgers School of Biomedical Health Science, Newark, NJ, USA

Correspondence Address:
Dr. Pranela Rameshwar
Department of Medicine, Hematology/Oncology, New Jersey Medical School, Rutgers School of Biomedical Health Science, Newark, NJ
USA

Abstract

Breast cancer remains a clinical problem despite advancements in the field. Cancer stem cells (CSCs) within the breast cancer population are implicated in cancer relapse. The dormant CSCs generally resist available treatment, thus challenging the current treatment paradigm. Radiation is an aggressive form of treatment typically used to reduce tumor mass in breast cancer patients. Several clinical and research-based studies have shown that radiation treatment cannot target all cancer cells, leaving behind radioresistant cells. The radioresistant cells have the potential to acquire stem cell-like features that render them untargetable with respect to the current technology. This review elaborates on cancer cells acquiring stem cell phenotype. In addition, we discuss the phenotype and function of cancer cells that are derived from radioresistant cells as well as indirect changes as a consequence to bystander effect. In addition, the epigenetic profile of the radioresistant cells plays a crucial role in the acquisition of cycling quiescence and stem cell-like phenotype and is detailed in this review.



How to cite this article:
Sinha G, Ferrer A, Naaldijk Y, Moore CA, Wu Q, Ulrich H, Rameshwar P. Breast cancer stem cells, epigenetics, and radiation.J Radiat Cancer Res 2018;9:155-164


How to cite this URL:
Sinha G, Ferrer A, Naaldijk Y, Moore CA, Wu Q, Ulrich H, Rameshwar P. Breast cancer stem cells, epigenetics, and radiation. J Radiat Cancer Res [serial online] 2018 [cited 2019 Mar 26 ];9:155-164
Available from: http://www.journalrcr.org/text.asp?2018/9/4/155/254002


Full Text



 Introduction



Breast cancer

Breast cancer (BC) is the second leading cause of cancer following lung malignancy.[1] In 2017, approximately, 250,000 new cases were estimated in the United States.[2] As longevity increases, the number of BC-related deaths rises, exemplifying age as a critical factor in BC development.[1],[2],[3] Aside from age, 5%–10% of female BC cases are related to germline mutations in BRCA1 and BRCA2 genes, while 7% of BRCA2 mutations are associated with male BC.[1],[4],[5],[6] Moreover, mutations in either BRCA1 or BRCA2 increase the susceptibility for the development of ovarian cancer.[4],[7],[8]

Histologically, BC is a heterogeneous disease that can be categorized broadly into invasive or in situ carcinomas. Invasive carcinoma can be subsequently divided into six subtypes: tubular, ductal lobular, invasive lobular, infiltrating ductal, mucinous, and medullary.[9] In situ carcinomas, on the other hand, can be divided into two subtypes: ductal and lobular.[9] At the molecular level, the different subtypes of BC can be branded as basal-like, luminal, human epidermal growth factor receptor 2/estrogen receptor (HER2+/ER−), claudin-low, normal breast-like, interferon-related, molecular apocrine, and triple-negative.[10],[11] The distinction between these subcategories is based on the expression levels of ER, HER2, vimentin, claudin, and epidermal growth factor receptor, among others.[12],[13] In addition, a correlation between histological analysis and the presence of genetic alterations has been reported. For example, a translocation leading to fusion of ETS-variant gene 6 and neurotrophic tyrosine receptor kinase 3 genes is predominantly observed in secretory breast carcinomas.[14]

Current treatment paradigm

Increased awareness of cancer screening technologies and the relative efficacy of currently available treatments have extended lifespans, reduced mortality rates, and improved quality of life for BC patients.[1] The present treatment paradigm involves first physically removing the malignant tissue, either partially (breast conserving) or fully (mastectomy).[1] Following surgery, the person is subjected to chemotherapy or radiation with the intent to eradicate remaining cancer cells. These therapies are chosen based on the BC subtype. For example, the preferential treatment for hormone receptor (HR)-related BC is to inhibit or suppress: (1) the production of hormones from ovary (i.e., goserelin), (2) the reuptake of estrogen (i.e., tamoxifen), and (3) the conversion of androgens to estrogens (i.e., letrozole).[10] Conversely, triple-negative BC, the most aggressive BC subtype, exhibits a 22% response rate to conventional chemotherapy treatments as well as a higher probability for metastasis and relapse.[10]

Radiation is used after the surgical removal of cancerous breast tissue, improving the lifespan of patients.[15] According to the BC organization, there are currently three essential radiotherapies used for the treatment of BC: External whole breast radiation, internal radiation, and intraoperative radiation. The effect these radiotherapies exert on BC is detailed in the following sections.

 Signaling Pathway Alterations After Radiation



Ionizing radiation (IR) directly kills cancer cells through the induction of double-strand DNA breaks, leading to apoptosis or inhibition of proliferative capacity through mitotic arrest.[16],[17],[18] In addition, radiation acts on the tumor microenvironment, impairing endothelial cell function and induced apoptosis.[19] Furthermore, an inflammatory response is known to be present after radiation, activating the innate immune response to which tumor cells respond evasively.[20],[21] IR acts through activation of p53 pathway, resulting in changes in cell cycling, senescence, apoptosis, or DNA repair.[17] IR can also target BC cells in p53-independent pathways such as activation of the death receptor pathway.[22] IR can also target BC cells through autophagy.[23] Autophagy is a mechanism by which a cell can replace its damaged organelles; therefore, this process can both support and inhibit cancer cell survival.[24] Autophagy regulates p53 through a feedback mechanism in which p53 activates autophagy which, subsequently, suppresses p53.[25] In a recent in vitro study, it was shown that radiation can also increase microRNA (miRNA)-199-5a which directly targets autophagy on IR treatment.[26] Radiotherapy can induce autophagy in BC cells through phosphatidylinositol-3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway.[23],[27] In a separate study, it was shown addition of mTOR inhibitor radiosensitized the BC cells by reducing the mitochondrial membrane potential.[28] Therefore, using radiotherapy alone can reduce its efficiency, but combining it with other drugs/treatment has the ability to increase its efficiency.

 Cancer Stem Cells and Radiation



BC is comprised of a relatively small population of cycling quiescent subpopulation with stem cell-like properties showing self-renewal and is responsible for tumor initiation with a heterogeneous population of tumor cells. This population is generally referred as cancer stem cells (CSCs). This population remains untargetable despite advancements made in BC treatment.[29],[30],[31],[32],[33] There are several markers reported in the literature for BC such as CD44+/CD24−.[34] However, it appears that this population has been shown to be heterogeneous, leading to the field selecting cells based on the expression of stem cell genes Nanog and Oct4.[35]

Increasing clinical evidence supports that CSCs are important for cancer resurgence and relapse.[32],[33],[36],[37],[38] As the name suggests, CSCs share phenotypic and functional similarity with normal, healthy stem cells.[39],[40],[41] However, the origin of CSCs has been a matter of debate in the field. One widely accepted theory suggests that normal stem cells undergo series of mutation to acquire CSC-like state, whereas a competing theory purports that a differentiated normal or cancer cell undergoes series of mutations to acquire stem cell-like state through the process of dedifferentiation.[35]

Cancer stem cells

Recent studies have shown that chemotherapy and radiation target a significant number of cancer cells within the tumor bulk, leaving behind a small radioresistant population of cancer cells.[37],[38],[42] CSCs are inherently radioresistant, but not all radioresistant cells are CSCs.[43],[44] Recent studies have shown that some of these resistant nonstem cell populations of cancer cells have potential to subsequently acquire a CSC-like state.[43],[44] The exact mechanism for radioresistant cells to transition into a CSC-like state remains unclear. However, there are several CSC-related processes that are upregulated in radioresistant cells.[44],[45],[46],[47] For instance, radioresistant cells generally show increased self-renewal and DNA damage repair potential.[31] Over the past decade, different genes and signaling cascades associated with the stem cell-like behavior have been thoroughly studied in CSCs, such as Oct4A, Nanog, aldehyde dehydrogenase (ALDH) genes, Notch, and genes linked to the Wnt pathway.[39],[43],[48],[49] Indeed, the literature indicated an increase in the aforementioned pathways in radioresistant cells.[44] In an in vitro study with BC cell lines and cells from primary source indicated that radiation was able to upregulate all reprogramming transcription factors that are used in generation of induced pluripotent stem cells – Oct4, Sox 2, Nanog and Klf4 through Notch-dependent pathway.[42] The findings in this paper suggested that radioresistance can reprogram cells into CSCs and this may occur in already transformed cells and by healthy nearby cells. Increased copy number of Oct4 and Sox 2 was thought to be responsible for radiation-induced CSC formation.[42] In an in vitro model, the application of a pan-ALDH inhibitor, disulfiram, was able to inhibit radiation-induced CSC formation through downregulation of nuclear factor kappa B (NF-κB).[50] The Notch signaling pathway plays an important role in stem cell renewal and maintenance.[51] γ-secretase is an enzyme responsible for cleaving the Notch receptor, which releases Notch intracellular domain (NICD).[51] Then, the NICD translocates to nucleus and transcriptionally regulates several genes that may play an important role in dormancy of a cell.[51] In a separate study with BC cells, combination of γ-secretase inhibitors with radiation was shown to reduce the CSC subpopulation,[46] thus suggesting combinatorial treatment approaches for targeting BC will be more productive way of targeting these cancer cells.

As indicated above, radiation can increase cancer clearance. However, based on the discussion above, it is likely that radioresistance could worsen the patient's prognosis. Hormones such as estrogen, progesterone, and herceptin have been studied to show clear roles for the development and maintenance of the healthy mammary gland.[52],[53],[54] As such, unbalanced hormone levels, either natural (due to mutation) or as a result of treatment, can lead to cancer development.[55] In these cases, the combination of radiation and hormone therapies can be more detrimental to the patient.[55] In a study performed with radiosensitive BC cells, stimulation with hormones lead to increased resistance of the CSC population.[55] The BC cell line T47D, upon stimulation with progesterone, either directly upregulated miRNAs such as 328 and 98-5p or downregulated miRNAs such as 22-3p and 29c-3p.[55] These miRNAs regulate transcription of cell cycle and stem cell-associated genes. Further, on irradiating the progesterone-stimulated T47D cells, the level of let-7 family of miRNA was downregulated in ALDH+ cells.[55] The downregulation/loss of let-7 family of miRNA has been known to upregulate the pluripotency genes.[56] Thus, these findings suggest,– at least in an in vitro two-dimensional model, that the BC cells can acquire a more stem cell-like state upon receiving a combination of radiation and hormone therapy.[55]

Epithelial-to-mesenchymal transition

The process of dedifferentiation of normal breast cells and BC cells occurs through epithelial-to-mesenchymal transition (EMT).[57],[58],[59],[60] During embryonic development, EMT is a process utilized by embryonic stem cells to change from epithelial to mesenchymal lineage.[60] Similarly, when normal breast epithelial cells undergo EMT, they acquire CSC-like properties by undergoing dedifferentiation.[60] Thus, EMT markers are upregulated in CSCs.[60] EMT also helps these BC/stem cells to metastasize to different organs where they either form a secondary tumor or remain dormant until conditions are favorable for them to undergo rigorous cell division.[61]

In a recent study, BC cells were first stimulated with transforming growth factor beta (TGF-β) to undergo EMT, and then, both the stem cells and nonstem cells were isolated and introduced to IR.[45] The CSCs resisted the radiation by reducing the polyploidy ratio and blocking the cells mostly at G2M phase, preventing mitotic catastrophe and increasing its clonogenicity.[45] In another study using different BC lines, MDA-MB-231, MCF-7, and T47D, radioresistant cells showed increased potential to form colonies compared to the parental cells and increased expression of stem cell and EMT-associated markers.[62] These findings further support the link between radioresistance and stemness. The role of oncogene Bmi-1, also a stem cell-associated gene, has been widely studied in the field of CSC maintenance. Overexpression of Bmi-1 can increase cell migration.[63] Studies have shown that radiation regulates EMT through Bmi-1 and the PI3K/Akt cascade.[47] EMT induction was reduced following radiation exposure when BC cells were treated with PI3K/Akt inhibitor, whereas EMT was completely hampered, with or without radiation exposure, in BC cells with shRNA knockdown of Bmi-1.[47] Further, it has been shown that women who received IR during childhood had an increased risk of developing BC, supporting early radiation as a risk factor.[64],[65]

 Radiation-Associated Nonspecific Effects



A major issue associated with radiation therapy is that it increases the already hypoxic tumor microenvironment, creating an even more suitable environment for CSC survival.[66] In this context, CSCs have reduced numbers of DNA breaks that could counter radiation-induced death.[66] However, nonspecific effects of radiation have also been reported to improve clearance of cancer cells within the tumor volume.[22] There are three distinct nonspecific long-term effects associated with radiation: bystander, abscopal, and cohort effects.[67]

Bystander effect

The bystander effects are limited to neighboring cells around the irradiated tumor.[67] The bystander effect on CSCs has been shown in many different forms of cancer, but this effect in BC needs more experimental validation.[68] It has been shown that exposure of BC cells to low-dose radiation leads to release of soluble TNF-related apoptosis-inducing ligand and tumor necrosis factor-alpha, resulting in changes in the tumor microenvironment and increased apoptosis in BC cells through bystander effect.[22] Furthermore, a study performed using MCF7 BC cells exposed to gradient irradiation showed better clearance of BC cells through TGF-β1, reactive oxygen species, and nitric oxide-mediated pathways.[69] However, a mathematical model based on the previous radiation data has shown that sparsely IR, such as g-irradiation, can promote the formation of CSCs, whereas dense radiation, such as neutrons, promotes the formation of preexisting CSC clones and this occurs through bystander effect.[68],[70],[71]

Abscopal effect

Radiation therapy can affect cells outside the irradiated volume, far from the treatment site, due to a phenomenon commonly known as the abscopal effect.[67] Since the abscopal effect targets distant cells, this limits the studies that can be conducted in a clinical setting. Thus, further studies are needed to dissect a mechanism for this effect. Based on the current clinical data, it is believed that the abscopal effect is helpful in improved clearance of cancer cells by activation of immune cells against cancer.[72] In a clinical trial (NCT02474186) within a cohort of BC patients, the combination of immunotherapy with radiation therapy significantly combatted BC through abscopal effect.[72] In a separate case report, it was shown that a 64-year-old women with metastatic BC showed regression of tumor mass 10 months after localized radiation therapy with no further reporting of tumor.[73]

Cohort effect

The cohort effect of radiation targets nonspecific cells within the tumor volume.[67] For instance, a high dose of localized radiation may affect the nearby tumor cells through transmission of low-dose signals into the surroundings.[69] Thus, the cohort effect increases the odds of clearance of tumor cells. In a recent study, MCF7 BC cells were subjected to a gradient of radiation in which the cells in the center of the dish receive the highest dose of irradiation and the dose decreases toward the edge of the plate. This radiation approach induced increased rates of cell death in MCF7 compared to uniform dose.[69] Similar to the abscopal effect, the molecular mechanism underlying this effect has yet to be elucidated.

 Radiation-Induced Autophagy



Autophagy is a process by which the cells use intracellular processes to survive. This can occur by the cells eliminating the damaged protein, molecules, and organelles as well as preserving energy during stress.[74] Cellular stress initiates autophagosome formation which then fuses with lysosome to form autophagolysosome.[74] The enzymes within lysosomal compartment lead to degradation of the damaged materials.[74] This prevents the cells from accumulating toxic agent, although autophagy can also induce cell death with excessive stress.[74] Thus, the complex molecular mechanisms of autophagy can also prevent cancer and benefit cancer cell survival.[24]

Similar to other source of stress, one of the common effects of radiation on BC cells is the induction of autophagy. This can either cause cancer cell death or maintain their survival.[24] Indeed, radiation-induced autophagy was experimentally demonstrated in the BC cell line, MCF-7.[28] Radiation, when introduced in combination with rapamycin, an mTOR pathway inhibitor (autophagy inhibitor), was able to increase cell death in MCF7. The mechanism by which this occurred is through mitochondrial hyperpolarization and p53 phosphorylation.[28]

A programmed cell death is through caspase − 3/7. The combinatorial treatment of the cell cycle inhibitor, tunicamycin, and radiation led to increased caspase 3.[75] Similar increase has been shown in MCF-7 cells through the activation of autophagy.[75] Overexpressing autophagy-associated genes, ATG5 and Beclin-1, in BC cells led to an increase in radiation-mediated cell death.[76] A possible mechanism by which autophagy induces cell death upon radiation is p53-mediated damage-regulated autophagy modulator (DRAM).[77],[78] Silencing of DRAM decreased radiation-induced cell death, independent of p53.[77],[78]

In contrast to its tumor suppressive role, autophagy can also support tumor survival.[79] An in vivo mouse model inhibited autophagy-associated genes ATG5, ATG7, and FIP200 and this led to reduced malignancy of BC.[79] FIP200 deletion drastically downregulated cell cycle proteins and this prevented early tumor initiation potential of BC cells.[79] Even in hereditary-associated BC mice model, autophagy disruption-reduced PALB2 (germline mutation) that is generally linked to tumor initiation.[80] These findings are in line with other studies showing autophagy being associated with stem cell survival.[80] During stress, stemness is maintained by induction of autophagic pathway to eliminate toxic molecules.[81] Within a tumor microenvironment, autophagy-associated gene ATG can maintain CSCs.[82],[83] Blocking of autophagy-associated genes by siRNA strategy has led to reduced radioresistance.[23]

A Phase I/II clinical trial in glioblastoma with autophagic inhibitor, hydroxychloroquine, when given as a combinatorial treatment with radiotherapy and/or chemotherapy, comparatively improved the patient prognosis.[84] One of the possible mechanisms by which autophagy induces BC dormancy is by regulating IL-6, since this cytokine has been shown to maintain the CSC population.[85] Radiation-mediated CSC survival has been discussed above, within another section. Radiation-mediated stress acts as inducer of autophagy which can be either tumor suppressor or initiator, depending on the tumor microenvironment. Thus, further dissecting the role of autophagy in BC progression needs to be done for improved treatment.

 Radiation-Mediated Epigenetics Changes



Introduction

The term “epigenetics” refers to heritable changes occurring within a cell without alterations of the DNA sequence. Chromatin modifications are implicated in cellular homeostasis by regulating gene expression and providing cellular identity. The “epigenome” has emerged as an important factor in the development of cancer and progression. Although alterations of the DNA sequence are a hallmark of oncogenesis, it has become evident that the epigenome is also involved in this process.[86],[87] Modifications to the DNA and the histones confer plasticity to cancer cells, thereby promoting survival within particular niches.[86] As epigenetic changes are reversible, they serve as a plausible avenue for the study of oncogenesis and possible eradication of cancer. The subsequent discussion in this section expands on the specific method of epigenetic links to radiation.

DNA methylation

DNA methylation is one of the modifications that have been extensively studied in the context of gene regulation. Insertions of a methyl group to cytosines, such as 5-methylcytosine, are associated with gene repression.[88] Methylation controls important cellular processes, such as cell cycle, DNA repair, and apoptosis. In cancer, these modifications work together to promote cancer cell survival and suitability for the metastatic microenvironment.[86],[89],[90] In addition, global hypermethylation is observed in cancer and in several tumor suppressor genes.[87] The proteins mediating these changes are termed methyltransferases or demethylases. Methylations usually occur in regions of the DNA consisting of cytosines followed by guanines, known as CpG islands.[86],[87],[88]

DNA methylation patterns have served as an approach to improve BC prognosis.[91],[92] Epigenetic signatures in DNA caused by the DNA methyltransferase protein 1 (DNMT1) mediate maintenance of the tumor by supporting breast CSCs.[93] Interactions between DNMT1 and enhancer of zeste homolog-2 lysine methyltransferase promote EMT in triple-negative BC cells by suppressing the gene wwc1, which participates in the Hippo pathway.[94] Furthermore, compelling evidence suggests that hypomethylation of stem cell-associated genes stimulates BC progression and migratory properties.[95] Hence, methylation is an important regulatory process that aids in the survival and advancement of BC cells.

Histone modifications

Chromatin is composed of histone octamers wrapped by 147 bp of DNA, forming the core unit, or nucleosome. The histones H2a, H2b, H3, and H4 are susceptible to epigenetic modifications that contribute to the diversity of the epigenome through 16 currently identified alterations.[87],[88] Histone modifications can cause either activation or repression of gene expression. For example, acetylation of lysine or arginine residues activates gene expression due to the neutralization of the positive charge of these amino acids, whereas methylation mostly correlates with transcriptional repression.[88] Importantly, the crosstalk between histone modifications and DNA methylation is relevant to cellular identity and control of gene expression, providing complexity to the role of epigenetics in physiological and pathophysiological processes.[86],[87]

The biological function of histone deacetylases (HDAC) has been widely examined with respect to cancer. Several studies have revealed a fundamental role of HDACs in the maintenance of a CSC-like phenotype in breast and ovarian tumors.[96] Importantly, it was demonstrated that HDAC6 is necessary for the immunosuppressive properties of BC-associated fibroblasts, leading to recruitment of regulatory T-cells and myeloid-derived suppressor cells.[97] Further, overexpression of HDAC1 causes reduction of ERα and increases proliferation of ER + BC.[98] Overall, these seminal studies prove the importance of epigenetic factors in BC dissemination.

 Relevant Therapy



Radiation remains a conventional regimen for cancer treatment. It is known that radiotherapy alters DNA methylation patterns, conferring radioresistance to cancer cells.[99] In addition, dose-dependent hypomethylation of DNA, mediated by radiotherapy, can lead to downregulation of DNMTs, resulting in genomic instability.[90],[100] Changes affecting the DNA damage response, apoptosis, and cell cycle pathways are prevalent after radiation, partly through methylation of the genes involved in these processes.[101] Moreover, immune signaling pathways in BC cells are affected by irradiation through NF-κB pathway regulation.[102]

Similar findings have been reported in other cancer models regarding the effects of radiation in epigenetics. In colon cancer cells, radiation-induced hypomethylation of genes known to be dysregulated during oncogenesis.[103] The hypomethylation marks encountered on the promoter regions of these genes were attributed to decreased levels of DNMT1/3b.[103] In addition, irradiation stimulates the acquisition of a CSC-like phenotype of prostate cancer cells.[104] Interestingly, radiation caused H3 methylation of the promoter region of ALDH1A1, preventing apoptosis and enhancing maintenance of the CSC phenotype.[104] Considering these findings, it is imperative to grasp the role of the epigenome during oncogenesis and cancer progression. Moreover, further investigation is required to fully understand the impact of radiation on epigenetic factors and how these alterations may enhance a CSC phenotype.

In light of the detrimental effects of radiotherapy on cancer epigenetics, discussed above, there are effects to use the scientific information to develop counteractive treatments. As an example, recent studies have demonstrated the benefits of combinatorial therapies that utilize both epigenetic inhibitors and radiation.[90] In addition, an epigenetic drug, consisting of dypiridamole and the antifolate TMCG, prevented H4K20 and H3K79 methylation marks, resulting in radiosensitization of BC cells by inhibition of DNA repair processes.[105]

Histone deacetylases inhibitors: Epigenetic-modifying drugs

HDAC inhibitors (HDACis) are therapeutic agents that promote gene transcription and chromatin relaxation by supporting the accumulation of acetylation marks. These “epi-drugs” have shown promising outcomes as anticancer therapeutics due to their cytotoxic activity, partly by inducing apoptosis, autophagy, and cell cycle arrest.[106] Four classes of HDACi have been described, such as hydroxamic acids, cyclic peptides, aliphatic acids, and benzamides.[107] Vorinostat, a hydroxamic acid, was the first HDACi approved by the FDA for the treatment of cutaneous T-cell lymphoma (CTCL).[108] This agent specifically inhibits Zn(II)-dependent Class I and II HDACs.[106] Cyclic peptides are the most structurally complex and diverse class of HDACi that allowed for interaction with multiple motifs within the HDACs enzymatic portion.[109] Currently, romidepsin (FK-228), a cyclic tetrapeptide, is being utilized for the treatment of CTCL.[110] Aliphatic acids are considered weak HDACi by excelling their function at millimolar concentrations.[106],[111] Furthermore, benzamides are mostly Class I HDACis, affecting primarily HDAC 1 and 3.[112] The structural composition of these compounds is essential for their interactions with different types of HDACs.

Histone deacetylases inhibitors as a therapeutic for breast cancer treatment

In BC, HDACis can modulate the epigenomic profile of cancer cells to promote apoptosis and cellular senescence. A recent study demonstrated that the HDACi, trichostatin A, increased miR-125a-5p, resulting in intrinsic apoptosis and inhibition of HDAC5.[113] Further, the HDACis butyrate and panobinostat have been shown to upregulate miR-31 by downregulating Bmi-1 to enhance cellular senescence in BC cells.[114] Furthermore, short-term exposure of vorinostat is implicated in the induction of DNA damage and halting of the replication machinery.[115] Importantly, a novel HDACi, YF479, displayed antitumor activity and prevented lung metastasis of BC cells.[116] These studies demonstrate the efficacy of HDACi's in the treatment of BC and their implications in possible eradication of the disease.

In addition, HDACis have become a promising therapy in combination with chemotherapy and radiation.[117] Valproic acid, a HDACi, coupled with whole-brain radiation therapy resulted in overall survival of BC patients with brain metastases.[118] Moreover, vorinostat sensitizes BC cells to radiotherapy and enhances cell death.[119] Combinatorial treatment also promoted autophagy and cytotoxicity and prevented cell survival.[99]

An ongoing double-blind Phase III clinical trial utilizes endocrine therapy in conjunction with the HDACi entinostat in BC patients with metastatic HR-positive and HER2−, previously treated with aromatase inhibitors.[102] This goal of this study is to improve the overall survival of metastatic BC by regulating resistance to endocrine therapy with HDACi.[120] This investigation is based on a Phase II clinical trial that improved progression-free and overall survival as well as patient's tolerance to the combinatorial therapy.[121] Despite the success of the use of HDACi's with adjuvant treatment, the application of these agents as a monotherapy for the treatment of cancers remains arguable. For example, a Phase II clinical trial used vorinostat as a therapeutic for Stage IV BC resulted in termination due to adverse outcomes (clinicaltrials.gov). Nonetheless, ten active clinical studies using HDACis are currently recruiting patients, demonstrating the potential of these agents in the treatment of BC (clinicaltrials.gov).

Prospects of epigenetic drugs on cancer stem cells

The “epigenome” is involved in the acquisition and maintenance of a CSC phenotype.[122] The use of epigenetic therapeutics has become relevant for the targeting of CSCs, preventing tumor formation and cancer relapse. These agents are essential for differentiating CSCs to enhance sensitivity to adjuvant therapy for cancer eradication. Recently, a study demonstrated that alterations of the epigenetic profile of ovarian CSCs, mediated by a DNA hypomethylating agent, resulted in the reduction of this population and enhanced sensitization to chemotherapy.[123] Another DNA-demethylating agent promoted chemosensitivity of CSCs by regulating the expression of miR-497.[124] Further, HDAC inhibition is implicated in the reduction of breast CSCs by induction of caspase-dependent apoptosis.[125] Taken together, these studies suggest the importance of epigenetic-modifying drugs in the targeting of CSCs.

 Conclusion



The CSC subpopulation of BC cells is primarily the cells responsible for cancer resurgence, leading to poor outcome. The CSCs are quiescent with respect to cell cycle and have been shown to resist conventional therapies, such as radiation.[44],[45],[46],[47] During resurgence, referred as reverse dormancy, the CSCs become cycling activated and develop into a heterogeneous population of cancer cells.[126] In addition, subsets of nonstem cells can be radioresistant and have the potential, as described in this review, to adopt CSC-like features that further contribute to their ability to persist in the body following treatment [Figure 1]. Furthermore, the epigenetic profile of radioresistant cells is critical to their ability to acquire a cycling quiescent, CSC-like phenotype [Figure 1]. Gaining a better understanding of the role of epigenetics in response to radiotherapy is an important parameter in developing more effective therapeutic strategies for the future.{Figure 1}

Financial support and sponsorship

This work was funded by the F.M. Kirby Foundation to PR and Fundação de Amparo à Pesquisa do Estado de São Paulo to YN and HU.

Conflicts of interest

There are no conflicts of interest.

References

1Kim CS, Algan O. Radiation Therapy – Breast Cancer Early Stage. Treasure Island (FL): StatPearls; 2018.
2Siegel RL, Miller KD, Jemal A. Cancer statistics, 2017. CA Cancer J Clin 2017;67:7-30.
3Becker S. A historic and scientific review of breast cancer: The next global healthcare challenge. Int J Gynaecol Obstet 2015;131 Suppl 1:S36-9.
4Mavaddat N, Peock S, Frost D, Ellis S, Platte R, Fineberg E, et al. Cancer risks for BRCA1 and BRCA2 mutation carriers: Results from prospective analysis of EMBRACE. J Natl Cancer Inst 2013;105:812-22.
5Peto J, Collins N, Barfoot R, Seal S, Warren W, Rahman N, et al. Prevalence of BRCA1 and BRCA2 gene mutations in patients with early-onset breast cancer. J Natl Cancer Inst 1999;91:943-9.
6Peshkin BN, Alabek ML, Isaacs C. BRCA1/2 mutations and triple negative breast cancers. Breast Dis 2010;32:25-33.
7Breast Cancer Linkage Consortium. Cancer risks in BRCA2 mutation carriers. J Natl Cancer Inst 1999;91:1310-6.
8Ford D, Easton DF, Bishop DT, Narod SA, Goldgar DE. Risks of cancer in BRCA1-mutation carriers. Breast cancer linkage consortium. Lancet 1994;343:692-5.
9Malhotra GK, Zhao X, Band H, Band V. Histological, molecular and functional subtypes of breast cancers. Cancer Biol Ther 2010;10:955-60.
10Tong CW, Wu M, Cho WC, To KK. Recent advances in the treatment of breast cancer. Front Oncol 2018;8:227.
11Weigelt B, Geyer FC, Reis-Filho JS. Histological types of breast cancer: How special are they? Mol Oncol 2010;4:192-208.
12Perou CM, Sørlie T, Eisen MB, van de Rijn M, Jeffrey SS, Rees CA, et al. Molecular portraits of human breast tumours. Nature 2000;406:747-52.
13Sørlie T, Perou CM, Tibshirani R, Aas T, Geisler S, Johnsen H, et al. Gene expression patterns of breast carcinomas distinguish tumor subclasses with clinical implications. Proc Natl Acad Sci U S A 2001;98:10869-74.
14Tognon C, Knezevich SR, Huntsman D, Roskelley CD, Melnyk N, Mathers JA, et al. Expression of the ETV6-NTRK3 gene fusion as a primary event in human secretory breast carcinoma. Cancer Cell 2002;2:367-76.
15Overgaard M, Hansen PS, Overgaard J, Rose C, Andersson M, Bach F, et al. Postoperative radiotherapy in high-risk premenopausal women with breast cancer who receive adjuvant chemotherapy. Danish breast cancer cooperative group 82b trial. N Engl J Med 1997;337:949-55.
16Mahaney BL, Meek K, Lees-Miller SP. Repair of ionizing radiation-induced DNA double-strand breaks by non-homologous end-joining. Biochem J 2009;417:639-50.
17Eriksson D, Stigbrand T. Radiation-induced cell death mechanisms. Tumour Biol 2010;31:363-72.
18Gewirtz DA. Growth arrest and cell death in the breast tumor cell in response to ionizing radiation and chemotherapeutic agents which induce DNA damage. Breast Cancer Res Treat 2000;62:223-35.
19Langley RE, Bump EA, Quartuccio SG, Medeiros D, Braunhut SJ. Radiation-induced apoptosis in microvascular endothelial cells. Br J Cancer 1997;75:666-72.
20Barker HE, Paget JT, Khan AA, Harrington KJ. The tumour microenvironment after radiotherapy: Mechanisms of resistance and recurrence. Nat Rev Cancer 2015;15:409-25.
21Dunn GP, Old LJ, Schreiber RD. The three es of cancer immunoediting. Annu Rev Immunol 2004;22:329-60.
22Luce A, Courtin A, Levalois C, Altmeyer-Morel S, Romeo PH, Chevillard S, et al. Death receptor pathways mediate targeted and non-targeted effects of ionizing radiations in breast cancer cells. Carcinogenesis 2009;30:432-9.
23Apel A, Herr I, Schwarz H, Rodemann HP, Mayer A. Blocked autophagy sensitizes resistant carcinoma cells to radiation therapy. Cancer Res 2008;68:1485-94.
24Jain K, Paranandi KS, Sridharan S, Basu A. Autophagy in breast cancer and its implications for therapy. Am J Cancer Res 2013;3:251-65.
25White E. Autophagy and p53. Cold Spring Harb Perspect Med 2016;6:a026120.
26Yi H, Liang B, Jia J, Liang N, Xu H, Ju G, et al. Differential roles of miR-199a-5p in radiation-induced autophagy in breast cancer cells. FEBS Lett 2013;587:436-43.
27Albert JM, Kim KW, Cao C, Lu B. Targeting the akt/mammalian target of rapamycin pathway for radiosensitization of breast cancer. Mol Cancer Ther 2006;5:1183-9.
28Paglin S, Lee NY, Nakar C, Fitzgerald M, Plotkin J, Deuel B, et al. Rapamycin-sensitive pathway regulates mitochondrial membrane potential, autophagy, and survival in irradiated MCF-7 cells. Cancer Res 2005;65:11061-70.
29Miller KD, Siegel RL, Lin CC, Mariotto AB, Kramer JL, Rowland JH, et al. Cancer treatment and survivorship statistics, 2016. CA Cancer J Clin 2016;66:271-89.
30Green BB, Taplin SH. Breast cancer screening controversies. J Am Board Fam Pract 2003;16:233-41.
31Aguirre-Ghiso JA. Models, mechanisms and clinical evidence for cancer dormancy. Nat Rev Cancer 2007;7:834-46.
32Uhr JW, Pantel K. Controversies in clinical cancer dormancy. Proc Natl Acad Sci U S A 2011;108:12396-400.
33Talmadge JE. Clonal selection of metastasis within the life history of a tumor. Cancer Res 2007;67:11471-5.
34Shao J, Fan W, Ma B, Wu Y. Breast cancer stem cells expressing different stem cell markers exhibit distinct biological characteristics. Mol Med Rep 2016;14:4991-8.
35Chen X, Liao R, Li D, Sun J. Induced cancer stem cells generated by radiochemotherapy and their therapeutic implications. Oncotarget 2017;8:17301-12.
36Braun S, Auer D, Marth C. The prognostic impact of bone marrow micrometastases in women with breast cancer. Cancer Invest 2009;27:598-603.
37Braun S, Pantel K, Müller P, Janni W, Hepp F, Kentenich CR, et al. Cytokeratin-positive cells in the bone marrow and survival of patients with stage I, II, or III breast cancer. N Engl J Med 2000;342:525-33.
38Braun S, Kentenich C, Janni W, Hepp F, de Waal J, Willgeroth F, et al. Lack of effect of adjuvant chemotherapy on the elimination of single dormant tumor cells in bone marrow of high-risk breast cancer patients. J Clin Oncol 2000;18:80-6.
39Patel SA, Ramkissoon SH, Bryan M, Pliner LF, Dontu G, Patel PS, et al. Delineation of breast cancer cell hierarchy identifies the subset responsible for dormancy. Sci Rep 2012;2:906.
40Velasco-Velázquez MA, Homsi N, De La Fuente M, Pestell RG. Breast cancer stem cells. Int J Biochem Cell Biol 2012;44:573-7.
41Reya T, Morrison SJ, Clarke MF, Weissman IL. Stem cells, cancer, and cancer stem cells. Nature 2001;414:105-11.
42Lagadec C, Vlashi E, Della Donna L, Dekmezian C, Pajonk F. Radiation-induced reprogramming of breast cancer cells. Stem Cells 2012;30:833-44.
43Bao B, Ahmad A, Azmi AS, Ali S, Sarkar FH. Overview of cancer stem cells (CSCs) and mechanisms of their regulation: Implications for cancer therapy. Curr Protoc Pharmacol 2013;Chapter 14:Unit:14.25.
44Pajonk F, Vlashi E, McBride WH. Radiation resistance of cancer stem cells: The 4 R's of radiobiology revisited. Stem Cells 2010;28:639-48.
45Konge J, Leteurtre F, Goislard M, Biard D, Morel-Altmeyer S, Vaurijoux A, et al. Breast cancer stem cell-like cells generated during TGFβ-induced EMT are radioresistant. Oncotarget 2018;9:23519-31.
46Lagadec C, Vlashi E, Alhiyari Y, Phillips TM, Bochkur Dratver M, Pajonk F, et al. Radiation-induced notch signaling in breast cancer stem cells. Int J Radiat Oncol Biol Phys 2013;87:609-18.
47Yuan W, Yuan Y, Zhang T, Wu S. Role of bmi-1 in regulation of ionizing irradiation-induced epithelial-mesenchymal transition and migration of breast cancer cells. PLoS One 2015;10:e0118799.
48Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF. Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci U S A 2003;100:3983-8.
49Woodward WA, Chen MS, Behbod F, Alfaro MP, Buchholz TA, Rosen JM, et al. WNT/beta-catenin mediates radiation resistance of mouse mammary progenitor cells. Proc Natl Acad Sci U S A 2007;104:618-23.
50Wang Y, Li W, Patel SS, Cong J, Zhang N, Sabbatino F, et al. Blocking the formation of radiation-induced breast cancer stem cells. Oncotarget 2014;5:3743-55.
51Liu J, Sato C, Cerletti M, Wagers A. Notch signaling in the regulation of stem cell self-renewal and differentiation. Curr Top Dev Biol 2010;92:367-409.
52Fillmore CM, Gupta PB, Rudnick JA, Caballero S, Keller PJ, Lander ES, et al. Estrogen expands breast cancer stem-like cells through paracrine FGF/Tbx3 signaling. Proc Natl Acad Sci U S A 2010;107:21737-42.
53Axlund SD, Sartorius CA. Progesterone regulation of stem and progenitor cells in normal and malignant breast. Mol Cell Endocrinol 2012;357:71-9.
54Rodríguez CE, Berardi DE, Abrigo M, Todaro LB, Bal de Kier Joffé ED, Fiszman GL, et al. Breast cancer stem cells are involved in trastuzumab resistance through the HER2 modulation in 3D culture. J Cell Biochem 2018;119:1381-91.
55Vares G, Cui X, Wang B, Nakajima T, Nenoi M. Generation of breast cancer stem cells by steroid hormones in irradiated human mammary cell lines. PLoS One 2013;8:e77124.
56Peter ME. Let-7 and miR-200 microRNAs: Guardians against pluripotency and cancer progression. Cell Cycle 2009;8:843-52.
57Ginestier C, Hur MH, Charafe-Jauffret E, Monville F, Dutcher J, Brown M, et al. ALDH1 is a marker of normal and malignant human mammary stem cells and a predictor of poor clinical outcome. Cell Stem Cell 2007;1:555-67.
58Wicha MS, Liu S, Dontu G. Cancer stem cells: An old idea – A paradigm shift. Cancer Res 2006;66:1883-90.
59Sell S. Stem cell origin of cancer and differentiation therapy. Crit Rev Oncol Hematol 2004;51:1-28.
60Mani SA, Guo W, Liao MJ, Eaton EN, Ayyanan A, Zhou AY, et al. The epithelial-mesenchymal transition generates cells with properties of stem cells. Cell 2008;133:704-15.
61Luo M, Brooks M, Wicha MS. Epithelial-mesenchymal plasticity of breast cancer stem cells: Implications for metastasis and therapeutic resistance. Curr Pharm Des 2015;21:1301-10.
62Ko YS, Jin H, Lee JS, Park SW, Chang KC, Kang KM, et al. Radioresistant breast cancer cells exhibit increased resistance to chemotherapy and enhanced invasive properties due to cancer stem cells. Oncol Rep 2018;40:3752-62.
63Guo BH, Feng Y, Zhang R, Xu LH, Li MZ, Kung HF, et al. Bmi-1 promotes invasion and metastasis, and its elevated expression is correlated with an advanced stage of breast cancer. Mol Cancer 2011;10:10.
64Oeffinger KC, Ford JS, Moskowitz CS, Diller LR, Hudson MM, Chou JF, et al. Breast cancer surveillance practices among women previously treated with chest radiation for a childhood cancer. JAMA 2009;301:404-14.
65John EM, Phipps AI, Knight JA, Milne RL, Dite GS, Hopper JL, et al. Medical radiation exposure and breast cancer risk: Findings from the breast cancer family registry. Int J Cancer 2007;121:386-94.
66Diehn M, Cho RW, Lobo NA, Kalisky T, Dorie MJ, Kulp AN, et al. Association of reactive oxygen species levels and radioresistance in cancer stem cells. Nature 2009;458:780-3.
67Wang R, Zhou T, Liu W, Zuo L. Molecular mechanism of bystander effects and related abscopal/cohort effects in cancer therapy. Oncotarget 2018;9:18637-47.
68Journy N, Mansouri I, Allodji RS, Demoor-Goldschmidt C, Ghazi D, Haddy N, et al. Volume effects of radiotherapy on the risk of second primary cancers: A systematic review of clinical and epidemiological studies. Radiother Oncol 2018. PMID: 30316563 & DOI: 10.1016/j.radonc.2018.09.017.
69Zhang D, Zhou T, He F, Rong Y, Lee SH, Wu S, et al. Reactive oxygen species formation and bystander effects in gradient irradiation on human breast cancer cells. Oncotarget 2016;7:41622-36.
70Shuryak I, Brenner DJ, Ullrich RL. Radiation-induced carcinogenesis: Mechanistically based differences between gamma-rays and neutrons, and interactions with DMBA. PLoS One 2011;6:e28559.
71Shuryak I, Hahnfeldt P, Hlatky L, Sachs RK, Brenner DJ. A new view of radiation-induced cancer: Integrating short – And long-term processes. Part I: Approach. Radiat Environ Biophys 2009;48:263-74.
72Golden EB, Chhabra A, Chachoua A, Adams S, Donach M, Fenton-Kerimian M, et al. Local radiotherapy and granulocyte-macrophage colony-stimulating factor to generate abscopal responses in patients with metastatic solid tumours: A proof-of-principle trial. Lancet Oncol 2015;16:795-803.
73Azami A, Suzuki N, Azami Y, Seto I, Sato A, Takano Y, et al. Abscopal effect following radiation monotherapy in breast cancer: A case report. Mol Clin Oncol 2018;9:283-6.
74Glick D, Barth S, Macleod KF. Autophagy: Cellular and molecular mechanisms. J Pathol 2010;221:3-12.
75Kim KW, Moretti L, Mitchell LR, Jung DK, Lu B. Endoplasmic reticulum stress mediates radiation-induced autophagy by perk-eIF2alpha in caspase-3/7-deficient cells. Oncogene 2010;29:3241-51.
76Kim KW, Mutter RW, Cao C, Albert JM, Freeman M, Hallahan DE, et al. Autophagy for cancer therapy through inhibition of pro-apoptotic proteins and mammalian target of rapamycin signaling. J Biol Chem 2006;281:36883-90.
77Crighton D, Wilkinson S, O'Prey J, Syed N, Smith P, Harrison PR, et al. DRAM, a p53-induced modulator of autophagy, is critical for apoptosis. Cell 2006;126:121-34.
78Cui L, Song Z, Liang B, Jia L, Ma S, Liu X, et al. Radiation induces autophagic cell death via the p53/DRAM signaling pathway in breast cancer cells. Oncol Rep 2016;35:3639-47.
79Wei H, Wei S, Gan B, Peng X, Zou W, Guan JL, et al. Suppression of autophagy by FIP200 deletion inhibits mammary tumorigenesis. Genes Dev 2011;25:1510-27.
80Huo Y, Cai H, Teplova I, Bowman-Colin C, Chen G, Price S, et al. Autophagy opposes p53-mediated tumor barrier to facilitate tumorigenesis in a model of PALB2-associated hereditary breast cancer. Cancer Discov 2013;3:894-907.
81Boya P, Codogno P, Rodriguez-Muela N. Autophagy in stem cells: Repair, remodelling and metabolic reprogramming. Development 2018;145. pii: dev146506.
82Sosa MS, Bragado P, Debnath J, Aguirre-Ghiso JA. Regulation of tumor cell dormancy by tissue microenvironments and autophagy. Adv Exp Med Biol 2013;734:73-89.
83Vera-Ramirez L, Vodnala SK, Nini R, Hunter KW, Green JE. Autophagy promotes the survival of dormant breast cancer cells and metastatic tumour recurrence. Nat Commun 2018;9:1944.
84Rosenfeld MR, Ye X, Supko JG, Desideri S, Grossman SA, Brem S, et al. Aphase I/II trial of hydroxychloroquine in conjunction with radiation therapy and concurrent and adjuvant temozolomide in patients with newly diagnosed glioblastoma multiforme. Autophagy 2014;10:1359-68.
85Maycotte P, Jones KL, Goodall ML, Thorburn J, Thorburn A. Autophagy supports breast cancer stem cell maintenance by regulating IL6 secretion. Mol Cancer Res 2015;13:651-8.
86Kim HJ, Bae SC. Histone deacetylase inhibitors: Molecular mechanisms of action and clinical trials as anti-cancer drugs. Am J Transl Res 2011;3:166-79.
87Bose P, Dai Y, Grant S. Histone deacetylase inhibitor (HDACI) mechanisms of action: Emerging insights. Pharmacol Ther 2014;143:323-36.
88Mann BS, Johnson JR, Cohen MH, Justice R, Pazdur R. FDA approval summary: Vorinostat for treatment of advanced primary cutaneous T-cell lymphoma. Oncologist 2007;12:1247-52.
89Pal D, Saha S. Hydroxamic acid – A novel molecule for anticancer therapy. J Adv Pharm Technol Res 2012;3:92-9.
90Mwakwari SC, Patil V, Guerrant W, Oyelere AK. Macrocyclic histone deacetylase inhibitors. Curr Top Med Chem 2010;10:1423-40.
91Duvic M, Bates SE, Piekarz R, Eisch R, Kim YH, Lerner A, et al. Responses to romidepsin in patients with cutaneous T-cell lymphoma and prior treatment with systemic chemotherapy. Leuk Lymphoma 2018;59:880-7.
92Chessum N, Jones K, Pasqua E, Tucker M. Recent advances in cancer therapeutics. Prog Med Chem 2015;54:1-63.
93Chou CJ, Herman D, Gottesfeld JM. Pimelic diphenylamide 106 is a slow, tight-binding inhibitor of class I histone deacetylases. J Biol Chem 2008;283:35402-9.
94Hsieh TH, Hsu CY, Tsai CF, Long CY, Wu CH, Wu DC, et al. HDAC inhibitors target HDAC5, upregulate microRNA-125a-5p, and induce apoptosis in breast cancer cells. Mol Ther 2015;23:656-66.
95Cho JH, Dimri M, Dimri GP. MicroRNA-31 is a transcriptional target of histone deacetylase inhibitors and a regulator of cellular senescence. J Biol Chem 2015;290:10555-67.
96Conti C, Leo E, Eichler GS, Sordet O, Martin MM, Fan A, et al. Inhibition of histone deacetylase in cancer cells slows down replication forks, activates dormant origins, and induces DNA damage. Cancer Res 2010;70:4470-80.
97Zhang T, Chen Y, Li J, Yang F, Wu H, Dai F, et al. Antitumor action of a novel histone deacetylase inhibitor, YF479, in breast cancer. Neoplasia 2014;16:665-77.
98Yeruva SL, Zhao F, Miller KD, Tevaarwerk AJ, Wagner LI, Gray RJ, et al. E2112: Randomized phase iii trial of endocrine therapy plus entinostat/placebo in patients with hormone receptor-positive advanced breast cancer. NPJ Breast Cancer 2018;4:1.
99Yardley DA, Ismail-Khan RR, Melichar B, Lichinitser M, Munster PN, Klein PM, et al. Randomized phase II, double-blind, placebo-controlled study of exemestane with or without entinostat in postmenopausal women with locally recurrent or metastatic estrogen receptor-positive breast cancer progressing on treatment with a nonsteroidal aromatase inhibitor. J Clin Oncol 2013;31:2128-35.
100Toh TB, Lim JJ, Chow EK. Epigenetics in cancer stem cells. Mol Cancer 2017;16:29.
101Wang Y, Cardenas H, Fang F, Condello S, Taverna P, Segar M, et al. Epigenetic targeting of ovarian cancer stem cells. Cancer Res 2014;74:4922-36.
102Liu L, Chen L, Wu X, Li X, Song Y, Mei Q, et al. Low-dose DNA-demethylating agent enhances the chemosensitivity of cancer cells by targeting cancer stem cells via the upregulation of microRNA-497. J Cancer Res Clin Oncol 2016;142:1431-9.
103Aztopal N, Erkisa M, Erturk E, Ulukaya E, Tokullugil AH, Ari F, et al. Valproic acid, a histone deacetylase inhibitor, induces apoptosis in breast cancer stem cells. Chem Biol Interact 2018;280:51-8.
104Dawson MA, Kouzarides T. Cancer epigenetics: From mechanism to therapy. Cell 2012;150:12-27.
105Sharma S, Kelly TK, Jones PA. Epigenetics in cancer. Carcinogenesis 2010;31:27-36.
106Allis CD, Jenuwein T. The molecular hallmarks of epigenetic control. Nat Rev Genet 2016;17:487-500.
107Szyf M, Pakneshan P, Rabbani SA. DNA methylation and breast cancer. Biochem Pharmacol 2004;68:1187-97.
108Kaushik N, Kim MJ, Kim RK, Kumar Kaushik N, Seong KM, Nam SY, et al. Low-dose radiation decreases tumor progression via the inhibition of the JAK1/STAT3 signaling axis in breast cancer cell lines. Sci Rep 2017;7:43361.
109Pouliot MC, Labrie Y, Diorio C, Durocher F. The role of methylation in breast cancer susceptibility and treatment. Anticancer Res 2015;35:4569-74.
110Pathania R, Ramachandran S, Elangovan S, Padia R, Yang P, Cinghu S, et al. DNMT1 is essential for mammary and cancer stem cell maintenance and tumorigenesis. Nat Commun 2015;6:6910.
111Liu X, Li C, Zhang R, Xiao W, Niu X, Ye X, et al. The EZH2- H3K27me3-DNMT1 complex orchestrates epigenetic silencing of the wwc1 gene, a hippo/YAP pathway upstream effector, in breast cancer epithelial cells. Cell Signal 2018;51:243-56.
112Hernandez-Vargas H, Ouzounova M, Le Calvez-Kelm F, Lambert MP, McKay-Chopin S, Tavtigian SV, et al. Methylome analysis reveals jak-STAT pathway deregulation in putative breast cancer stem cells. Epigenetics 2011;6:428-39.
113Witt AE, Lee CW, Lee TI, Azzam DJ, Wang B, Caslini C, et al. Identification of a cancer stem cell-specific function for the histone deacetylases, HDAC1 and HDAC7, in breast and ovarian cancer. Oncogene 2017;36:1707-20.
114Li A, Chen P, Leng Y, Kang J. Histone deacetylase 6 regulates the immunosuppressive properties of cancer-associated fibroblasts in breast cancer through the STAT3-COX2-dependent pathway. Oncogene 2018;37:5952-66.
115Kawai H, Li H, Avraham S, Jiang S, Avraham HK. Overexpression of histone deacetylase HDAC1 modulates breast cancer progression by negative regulation of estrogen receptor alpha. Int J Cancer 2003;107:353-8.
116Chiu HW, Yeh YL, Wang YC, Huang WJ, Ho SY, Lin P, et al. Combination of the novel histone deacetylase inhibitor YCW1 and radiation induces autophagic cell death through the downregulation of BNIP3 in triple-negative breast cancer cells in vitro and in an orthotopic mouse model. Mol Cancer 2016;15:46.
117Tharmalingam S, Sreetharan S, Kulesza AV, Boreham DR, Tai TC. Low-dose ionizing radiation exposure, oxidative stress and epigenetic programing of health and disease. Radiat Res 2017;188:525-38.
118Antwih DA, Gabbara KM, Lancaster WD, Ruden DM, Zielske SP. Radiation-induced epigenetic DNA methylation modification of radiation-response pathways. Epigenetics 2013;8:839-48.
119Halvorsen AR, Helland A, Fleischer T, Haug KM, Grenaker Alnaes GI, Nebdal D, et al. Differential DNA methylation analysis of breast cancer reveals the impact of immune signaling in radiation therapy. Int J Cancer 2014;135:2085-95.
120Bae JH, Kim JG, Heo K, Yang K, Kim TO, Yi JM, et al. Identification of radiation-induced aberrant hypomethylation in colon cancer. BMC Genomics 2015;16:56.
121Peitzsch C, Cojoc M, Hein L, Kurth I, Mäbert K, Trautmann F, et al. An epigenetic reprogramming strategy to resensitize radioresistant prostate cancer cells. Cancer Res 2016;76:2637-51.
122Montenegro MF, González-Guerrero R, Sánchez-del-Campo L, Piñero-Madrona A, Cabezas-Herrera J, Rodríguez-López JN, et al. Targeting the epigenetics of the DNA damage response in breast cancer. Cell Death Dis 2016;7:e2180.
123Groselj B, Sharma NL, Hamdy FC, Kerr M, Kiltie AE. Histone deacetylase inhibitors as radiosensitisers: Effects on DNA damage signalling and repair. Br J Cancer 2013;108:748-54.
124Reddy JP, Dawood S, Mitchell M, Debeb BG, Bloom E, Gonzalez-Angulo AM, et al. Antiepileptic drug use improves overall survival in breast cancer patients with brain metastases in the setting of whole brain radiotherapy. Radiother Oncol 2015;117:308-14.
125Baschnagel A, Russo A, Burgan WE, Carter D, Beam K, Palmieri D, et al. Vorinostat enhances the radiosensitivity of a breast cancer brain metastatic cell line grown in vitro and as intracranial xenografts. Mol Cancer Ther 2009;8:1589-95.
126Bliss SA, Greco SJ, Rameshwar P. Hierarchy of breast cancer cells: Key to reverse dormancy for therapeutic intervention. Stem Cells Transl Med 2014;3:782-6.