HDAC inhibition does not induce estrogen receptor in human triple-negative breast cancer cell lines and patient-derived xenografts

Lysine metabolism is a novel metabolic tumor suppressor pathway in breast cancer

The International Agency for Research on Cancer determined that obesity is the primary preventable cause of breast cancer. The nuclear receptor peroxisome proliferator activated receptor γ (PPARγ) binds inflammatory mediators in obesity and its expression is reduced in human breast cancer. We created a new model to better understand how the obese microenvironment alters nuclear receptor function in breast cancer. The obesity related cancer phenotype was PPARγ dependent; deletion of PPARγ in mammary epithelium which is a tumor suppressor in lean mice unexpectedly increased tumor latency, reduced the luminal progenitor (LP) tumor cell fraction, and increased autophagic and senescent cells. Loss of PPARγ expression in mammary epithelium of obese mice increased expression of 2-aminoadipate semialdehyde synthase (AASS) which regulates lysine catabolism to acetoacetate. PPARγ-associated co-repressors and activators regulated AASS expression via a canonical response element. AASS expression was significantly reduced in human breast cancer, and AASS overexpression or acetoacetate treatment inhibited proliferation and induced autophagy and senescence in human breast cancer cell lines. Genetic or pharmacologic HDAC inhibition promoted autophagy and senescence in mammary tumor cells in vitro and in vivo. We concluded that lysine metabolism is a novel metabolic tumor suppressor pathway in breast cancer.

Recent advancement of HDAC inhibitors against breast cancer

Recent studies highlight the great potential impact of HDAC inhibitors (HDACis) in suppressing TNBC, even though clinical trials including a single HDACis demonstrated unsatisfactory outcomes against TNBC. New compounds created to achieve isoform selectivity and/or a polypharmacological HDAC strategy have also produced interesting results. The current study discusses the HDACis pharmacophoric models and the structural alterations that produced drugs with strong inhibitory effects on TNBC progression. With more than 2 million new cases reported in 2018, breast cancer-the most common cancer among women worldwide-poses a significant financial burden on an already deteriorating public health system. Due to a lack of therapies being developed for triple-negative breast cancers and the development of resistance to the current treatment options, it is imperative to plan novel therapeutics in order to bring new medications to the pipeline. Additionally, HDACs deacetylate a large number of nonhistone cellular substrates that control a variety of biological processes, such as the beginning and development of cancer. The significance of HDACs in cancer and the therapeutic potential of HDAC inhibitor. Furthermore, we also reported molecular docking study with four HDAC inhibitors and performed molecular dynamic stimulation of the best dock score compound. Among the four ligands belinostat compound showed best binding affinity with histone deacetylase protein which was -8.7 kJ/mol. It also formed five conventional hydrogen bond with Gly 841, His 669, His 670, pro 809, and His 709 amino acid residues.

Inhibition of histone deacetylases attenuates tumor progression and improves immunotherapy in breast cancer

Breast cancer is one of the common malignancies with poor prognosis worldwide. The treatment of breast cancer patients includes surgery, radiation, hormone therapy, chemotherapy, targeted drug therapy and immunotherapy. In recent years, immunotherapy has potentiated the survival of certain breast cancer patients; however, primary resistance or acquired resistance attenuate the therapeutic outcomes. Histone acetyltransferases induce histone acetylation on lysine residues, which can be reversed by histone deacetylases (HDACs). Dysregulation of HDACs via mutation and abnormal expression contributes to tumorigenesis and tumor progression. Numerous HDAC inhibitors have been developed and exhibited the potent anti-tumor activity in a variety of cancers, including breast cancer. HDAC inhibitors ameliorated immunotherapeutic efficacy in cancer patients. In this review, we discuss the anti-tumor activity of HDAC inhibitors in breast cancer, including dacinostat, belinostat, abexinostat, mocetinotat, panobinostat, romidepsin, entinostat, vorinostat, pracinostat, tubastatin A, trichostatin A, and tucidinostat. Moreover, we uncover the mechanisms of HDAC inhibitors in improving immunotherapy in breast cancer. Furthermore, we highlight that HDAC inhibitors might be potent agents to potentiate immunotherapy in breast cancer.

HDAC Inhibitors for the Therapy of Triple Negative Breast Cancer

Triple negative breast cancer (TNBC) is an urgent as well as huge medical challenge, which is associated with poor prognosis and responsiveness to chemotherapies. Since epigenetic changes are highly implicated in TNBC tumorigenesis and development, inhibitors of histone deacetylases (HDACIs) could represent a promising therapeutic strategy. Although clinical trials involving single HDACIs showed disappointing results against TNBC, recent studies emphasize the high potential impact of HDACIs in controlling TNBC. In addition, encouraging results stem from new compounds designed to obtain isoform selectivity and/or polypharmacological HDAC approach. The present review provides a discussion of the HDACIs pharmacophoric models and of the structural modifications, leading to compounds with a potent activity against TNBC progression.

Positive Regulation of Estrogen Receptor Alpha in Breast Tumorigenesis

Estrogen receptor alpha (ERα, NR3A1) contributes through its expression in different tissues to a spectrum of physiological processes, including reproductive system development and physiology, bone mass maintenance, as well as cardiovascular and central nervous system functions. It is also one of the main drivers of tumorigenesis in breast and uterine cancer and can be targeted by several types of hormonal therapies. ERα is expressed in a subset of luminal cells corresponding to less than 10% of normal mammary epithelial cells and in over 70% of breast tumors (ER+ tumors), but the basis for its selective expression in normal or cancer tissues remains incompletely understood. The mapping of alternative promoters and regulatory elements has delineated the complex genomic structure of the ESR1 gene and shed light on the mechanistic basis for the tissue-specific regulation of ESR1 expression. However, much remains to be uncovered to better understand how ESR1 expression is regulated in breast cancer. This review recapitulates the current body of knowledge on the structure of the ESR1 gene and the complex mechanisms controlling its expression in breast tumors. In particular, we discuss the impact of genetic alterations, chromatin modifications, and enhanced expression of other luminal transcription regulators on ESR1 expression in tumor cells.

From Micro to Long: Non-Coding RNAs in Tamoxifen Resistance of Breast Cancer Cells

Breast cancer is the most commonly diagnosed cancer and the leading cause of cancer mortality among women. Two thirds of patients are classified as hormone receptor positive, based on expression of estrogen receptor alpha (ERα), the main driver of breast cancer cell proliferation, and/or progesterone receptor, which is regulated by ERα. Despite presenting the best prognosis, these tumors can recur when patients acquire resistance to treatment by aromatase inhibitors or antiestrogen such as tamoxifen (Tam). The mechanisms that are involved in Tam resistance are complex and involve multiple signaling pathways. Recently, roles for microRNAs and lncRNAs in controlling ER expression and/or tamoxifen action have been described, but the underlying mechanisms are still little explored. In this review, we will discuss the current state of knowledge on the roles of microRNAs and lncRNAs in the main mechanisms of tamoxifen resistance in hormone receptor positive breast cancer. In the future, this knowledge can be used to identify patients at a greater risk of relapse due to the expression patterns of ncRNAs that impact response to Tam, in order to guide their treatment more efficiently and possibly to design therapeutic strategies to bypass mechanisms of resistance.

Anticancer Therapy with HDAC Inhibitors: Mechanism-Based Combination Strategies and Future Perspectives

The increasing knowledge of molecular drivers of tumorigenesis has fueled targeted cancer therapies based on specific inhibitors. Beyond "classic" oncogene inhibitors, epigenetic therapy is an emerging field. Epigenetic alterations can occur at any time during cancer progression, altering the structure of the chromatin, the accessibility for transcription factors and thus the transcription of genes. They rely on post-translational histone modifications, particularly the acetylation of histone lysine residues, and are determined by the inverse action of histone acetyltransferases (HATs) and histone deacetylases (HDACs). Importantly, HDACs are often aberrantly overexpressed, predominantly leading to the transcriptional repression of tumor suppressor genes. Thus, histone deacetylase inhibitors (HDACis) are powerful drugs, with some already approved for certain hematological cancers. Albeit HDACis show activity in solid tumors as well, further refinement and the development of novel drugs are needed. This review describes the capability of HDACis to influence various pathways and, based on this knowledge, gives a comprehensive overview of various preclinical and clinical studies on solid tumors. A particular focus is placed on strategies for achieving higher efficacy by combination therapies, including phosphoinositide 3-kinase (PI3K)-EGFR inhibitors and hormone- or immunotherapy. This also includes new bifunctional inhibitors as well as novel approaches for HDAC degradation via PROteolysis-TArgeting Chimeras (PROTACs).

HDAC5 Inhibitors as a Potential Treatment in Breast Cancer Affecting Very Young Women

Background: Breast cancer in very young women (BCVY) defined as <35 years old, presents with different molecular biology than in older patients. High HDAC5 expression has been associated with poor prognosis in breast cancer (BC) tissue. We aimed to analyze HDAC5 expression in BCVY and older patients and their correlation with clinical features, also studying the potential of HDAC5 inhibition in BC cell lines. Methods: HDAC5 expression in 60 BCVY and 47 older cases were analyzed by qRT-PCR and correlated with clinical data. The effect of the HDAC5 inhibitor, LMK-235, was analyzed in BC cell lines from older and young patients. We performed time and dose dependence viability, migration, proliferation, and apoptosis assays. Results: Our results correlate higher HDAC5 expression with worse prognosis in BCVY. However, we observed no differences between HDAC5 expression and pathological features. Our results showed greatly reduced progression in BCVY cell lines and also in all triple negative subtypes when cell lines were treated with LMK-235. Conclusions: In BCVY, we found higher expression of HDAC5. Overexpression of HDAC5 in BCVY correlates with lower survival rates. LMK-235 could be a potential treatment in BCVY.

CDK2-mediated site-specific phosphorylation of EZH2 drives and maintains triple-negative breast cancer

Triple-negative breast cancer (TNBC), which lacks estrogen receptor α (ERα), progesterone receptor, and human epidermal growth factor receptor 2 (HER2) expression, is closely related to basal-like breast cancer. Previously, we and others report that cyclin E/cyclin-dependent kinase 2 (CDK2) phosphorylates enhancer of zeste homolog 2 (EZH2) at T416 (pT416-EZH2). Here, we show that transgenic expression of phospho-mimicking EZH2 mutant EZH2^T416D in mammary glands leads to tumors with TNBC phenotype. Coexpression of EZH2^T416D in mammary epithelia of HER2/Neu transgenic mice reprograms HER2-driven luminal tumors into basal-like tumors. Pharmacological inhibition of CDK2 or EZH2 allows re-expression of ERα and converts TNBC to luminal ERα-positive, rendering TNBC cells targetable by tamoxifen. Furthermore, the combination of either CDK2 or EZH2 inhibitor with tamoxifen effectively suppresses tumor growth and markedly improves the survival of the mice bearing TNBC tumors, suggesting that the mechanism-based combination therapy may be an alternative approach to treat TNBC.

EZH2 phosphorylation by CDK2 promotes progression of triple-negative breast cancer (TNBC). Here, the authors show that this signaling axis downregulates ERα, and thus combinatorial blockade of CDK2 and EZH2 sensitizes TNBC cells to tamoxifen.

Valproic acid promotes the epithelial-to-mesenchymal transition of breast cancer cells through stabilization of Snail and transcriptional upregulation of Zeb1

Histone deacetylases (HDACs) can regulate cancer progression and its inhibitors (HDACIs) have been widely used for cancer therapy. Valproic acid (VPA, 2-propylpentanoic acid) can inhibit the class I HDAC and suppress the malignancy of solid cancers. Our present study revealed that 1 mM VPA, which has no effect on cell proliferation, can significantly increase the migration and induce epithelial to mesenchymal transition (EMT) like properties of breast cancer cells. Further, VPA increased the expression of EMT-transcription factors (EMT-TFs) Snail and Zeb1. Knockdown of Snail and Zeb1 can attenuate VPA induced cell migration and EMT. Mechanistically, VPA increased the protein stability of Snail via suppression its phosphorylation at Ser 11. As to Zeb1, VPA can increase its promoter activity and transcription via a HDAC2 dependent manner. Over expression of HDAC2 can block VPA induced expression of Zeb1. Collectively, our data revealed that VPA can trigger the EMT of breast cancer cells via upregulation of Snail and Zeb1. It indicated that more attention should be paid to the effects of VPA on the clinical therapy of breast cancer.

Increased expression of the HDAC9 gene is associated with antiestrogen resistance of breast cancers

Estrogens play a pivotal role in breast cancer etiology, and endocrine therapy remains the main first line treatment for estrogen receptor‐alpha (ERα)‐positive breast cancer. ER are transcription factors whose activity is finely regulated by various regulatory complexes, including histone deacetylases (HDACs). Here, we investigated the role of HDAC9 in ERα signaling and response to antiestrogens in breast cancer cells. Various Michigan Cancer Foundation‐7 (MCF7) breast cancer cell lines that overexpress class IIa HDAC9 or that are resistant to the partial antiestrogen 4‐hydroxy‐tamoxifen (OHTam) were used to study phenotypic changes in response to ER ligands by using transcriptomic and gene set enrichment analyses. Kaplan–Meier survival analyses were performed using public transcriptomic datasets from human breast cancer biopsies. In MCF7 breast cancer cells, HDAC9 decreased ERα mRNA and protein expression and inhibited its transcriptional activity. Conversely, HDAC9 mRNA was strongly overexpressed in OHTam‐resistant MCF7 cells and in ERα‐negative breast tumor cell lines. Moreover, HDAC9‐overexpressing cells were less sensitive to OHTam antiproliferative effects compared with parental MCF7 cells. Several genes (including MUC1, SMC3 and S100P) were similarly deregulated in OHTam‐resistant and in HDAC9‐overexpressing MCF7 cells. Finally, HDAC9 expression was positively associated with genes upregulated in endocrine therapy‐resistant breast cancers and high HDAC9 levels were associated with worse prognosis in patients treated with OHTam. These results demonstrate the complex interactions of class IIa HDAC9 with ERα signaling in breast cancer cells and its effect on the response to hormone therapy.

Expression of calcium pumps is differentially regulated by histone deacetylase inhibitors and estrogen receptor alpha in breast cancer cells

Background

Remodeling of Ca²⁺ signaling is an important step in cancer progression, and altered expression of members of the Ca²⁺ signaling toolkit including the plasma membrane Ca²⁺ ATPases (PMCA proteins encoded by ATP2B genes) is common in tumors.

Methods

In this study PMCAs were examined in breast cancer datasets and in a variety of breast cancer cell lines representing different subtypes. We investigated how estrogen receptor alpha (ER-α) and histone deacetylase (HDAC) inhibitors regulate the expression of these pumps.

Results

Three distinct datasets displayed significantly lower ATP2B4 mRNA expression in invasive breast cancer tissue samples compared to normal breast tissue, whereas the expression of ATP2B1 and ATP2B2 was not altered. Studying the protein expression profiles of Ca²⁺ pumps in a variety of breast cancer cell lines revealed low PMCA4b expression in the ER-α positive cells, and its marked upregulation upon HDAC inhibitor treatments. PMCA4b expression was also positively regulated by the ER-α pathway in MCF-7 cells that led to enhanced Ca²⁺ extrusion capacity in response to 17β-estradiol (E2) treatment. E2-induced PMCA4b expression was further augmented by HDAC inhibitors. Surprisingly, E2 did not affect the expression of PMCA4b in other ER-α positive cells ZR-75-1, T-47D and BT-474. These findings were in good accordance with ChIP-seq data analysis that revealed an ER-α binding site in the ATP2B4 gene in MCF-7 cells but not in other ER-α positive tumor cells. In the triple negative cells PMCA4b expression was relatively high, and the effect of HDAC inhibitor treatment was less pronounced as compared to that of the ER-α positive cells. Although, the expression of PMCA4b was relatively high in the triple negative cells, a fraction of the protein was found in intracellular compartments that could interfere with the cellular function of the protein.

Conclusions

Our results suggest that the expression of Ca²⁺ pumps is highly regulated in breast cancer cells in a subtype specific manner. Our results suggest that hormonal imbalances, epigenetic modifications and impaired protein trafficking could interfere with the expression and cellular function of PMCA4b in the course of breast cancer progression.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4945-x) contains supplementary material, which is available to authorized users.

Co-inhibition of mTORC1, HDAC and ESR1α retards the growth of triple-negative breast cancer and suppresses cancer stem cells

Triple-negative breast cancer (TNBC) is the most refractory subtype of breast cancer. It causes the majority of breast cancer-related deaths, which has been largely associated with the plasticity of tumor cells and persistence of cancer stem cells (CSCs). Conventional chemotherapeutics enrich CSCs and lead to drug resistance and disease relapse. Development of a strategy capable of inhibiting both bulk and CSC populations is an unmet medical need. Inhibitors against estrogen receptor 1, HDACs, or mTOR have been studied in the treatment of TNBC; however, the results are inconsistent. In this work, we found that patient TNBC samples expressed high levels of mTORC1 and HDAC genes in comparison to luminal breast cancer samples. Furthermore, co-inhibition of mTORC1 and HDAC with rapamycin and valproic acid, but neither alone, reproducibly promoted ESR1 expression in TNBC cells. In combination with tamoxifen (inhibiting ESR1), both S6RP phosphorylation and rapamycin-induced 4E-BP1 upregulation in TNBC bulk cells was inhibited. We further showed that fractionated CSCs expressed higher levels of mTORC1 and HDAC than non-CSCs. As a result, co-inhibition of mTORC1, HDAC, and ESR1 was capable of reducing both bulk and CSC subpopulations as well as the conversion of fractionated non-CSC to CSCs in TNBC cells. These observations were partially recapitulated with the cultured tumor fragments from TNBC patients. Furthermore, co-administration of rapamycin, valproic acid, and tamoxifen retarded tumor growth and reduced CD44^high/+/CD24^low/− CSCs in a human TNBC xenograft model and hampered tumorigenesis after secondary transplantation. Since the drugs tested are commonly used in clinic, this study provides a new therapeutic strategy and a strong rationale for clinical evaluation of these combinations for the treatment of patients with TNBC.

Histone Deacetylases as New Therapeutic Targets in Triple-negative Breast Cancer: Progress and Promises

Triple-negative breast cancer (TNBC) lacks expression of estrogen receptor (ER), progesterone receptor (PR) and HER2 gene. It comprises approximately 15-20% of breast cancers (BCs). Unfortunately, TNBC's treatment continues to be a clinical problem because of its relatively poor prognosis, its aggressiveness and the lack of targeted therapies, leaving chemotherapy as the mainstay of treatment. It is essential to find new therapies against TNBC, in order to surpass the resistance and the invasiveness of already existing therapies. Given the fact that epigenetic processes control both the initiation and progression of TNBC, there is an increasing interest in the mechanisms, molecules and signaling pathways that participate at the epigenetic modulation of genes expressed in carcinogenesis. The acetylation of histone proteins provokes the transcription of genes involved in cell growth, and the expression of histone deacetylases (HDACs) is frequently up-regulated in many malignancies. Unfortunately, in the field of BC, HDAC inhibitors have shown limited effect as single agents. Nevertheless, their use in combination with kinase inhibitors, autophagy inhibitors, ionizing radiation, or two HDAC inhibitors together is currently being evaluated. HDAC inhibitors such as suberoylanilidehydroxamic acid (SAHA), sodium butyrate, mocetinostat, panobinostat, entinostat, YCW1 and N-(2-hydroxyphenyl)-2-propylpentanamide have shown promising therapeutic outcomes against TNBC, especially when they are used in combination with other anticancer agents. More studies concerning HDAC inhibitors in breast carcinomas along with a more accurate understanding of the TNBC's pathobiology are required for the possible identification of new therapeutic strategies.

Impact of genistein on the gut microbiome of humanized mice and its role in breast tumor inhibition

Since dietary polyphenols can have beneficial effects in prevention and treatment of cancer, we tested the hypothesis that breast cancer patients’ intestinal microbiota is modulated by genistein (GE), an isoflavone found in soy, and that microbial alterations may offset the side effects brought about by chemotherapy. We demonstrated successful humanization of germ-free mice by transplanting fecal samples from breast cancer patients before and after chemotherapy. Mice were then grouped based on chemotherapy status and GE or control diet. We did not find any significant differences between pre-chemotherapy and post-chemotherapy bacterial composition and abundances. Germ-free mice on a GE diet showed differences in microbial composition as compared to mice on control diet. Four weeks after introduction of the customized GE diet, there was distinct clustering of GE-fed mice as compared to the control-fed group. In the gut microbiome of GE-treated humanized mice, there was an increase in abundance of genera Lactococcus and Eubacterium. Phylum Verrucomicrobia showed statistically significant (p = 0.02) differences in abundances between the GE-fed and control-fed groups. There was an increase in bacteria belonging to family Lachnospiraceae and Ruminococcaceae in GE-fed mice. Marked changes were observed in GE catabolism in mice humanized with fecal material from two of three patients’ post-chemotherapy with complete disappearance of 4-ethylphenol and 2-(4-hydroxyphenol) propionic acid conjugates. The post-tumor samples did not show any distinct clustering of the gut microbiota between the two diet groups. There was an increase in latency of about 25% for tumor growth of the humanized mice that were on a GE diet as compared to humanized mice on a control diet. The average tumor size for the GE group was significantly decreased compared to the non-GE group. Collectively, our results suggest that the intestinal microbiota becomes altered with a GE diet before induction of tumor. Our findings indicate that GE modulates the microbiome in humanized mice that may contribute to its effects on increasing the latency of breast tumor and reducing tumor growth.

Targeting triple negative breast cancer with histone deacetylase inhibitors

INTRODUCTION

Triple negative breast cancer (TNBC) is a heterogeneous disease characterized by poor outcomes, higher rates of relapse, lack of biomarkers for rational use of targeted treatments and insensitivity to current available treatments. Histone deacetylase inhibitors (HDACis) perform multiple cytotoxic actions and are emerging as promising multifunctional agents in TNBC. Areas covered: This review focuses on the challenges so far addressed in the targeted treatment of TNBC and explores the various mechanisms by which HDACis control cancer cell growth, tumor progression and metastases. Pivotal preclinical trials on HDACis like panobinostat, vorinostat, and entinostat show that these epigenetic agents exert an anti-proliferative effect on TNBC cells and control tumor growth by multiple mechanisms of action, including apoptosis and regulation of the epithelial to mesenchimal transition (EMT). Combination studies have reported the synergism of HDACis with other anticancer agents. Expert opinion: In recent years, treatment of TNBC has recorded a high number of failures in the development of targeted agents. HDACis alone or in combination strategies show promising activity in TNBC and could have implications for the future targeted treatment of TNBC patients. Future research should identify which agent synergizes better with HDACis and which patient will benefit more from these epigenetic agents.

Both bulk and cancer stem cell subpopulations in triple-negative breast cancer are susceptible to Wnt, HDAC, and ERα coinhibition

Development of targeted therapies for triple-negative breast cancer (TNBC, a more aggressive subtype) is an unmet medical need. We analyzed data from 887 patients with invasive breast cancer and observed that increased Wnt and histone deacetylase (HDAC) activities are associated with estrogen receptor 1 (ESR1) and progesterone receptor (PGR) repression, poor survival, and increased relapse. The inverse correlation between Wnt signaling and repression of ESR1 and PGR expression was found to be magnified in cancer stem cell (CSC) subpopulations in TNBC cell lines. Cosuppression of Wnt, HDAC, and ESR1 using clinically relevant low-dose inhibitors effectively repressed both bulk and CSC subpopulations and converted CSCs to non-CSCs in TNBC cells without affecting MCF-10A mammary epithelial cells.

HDACs and HDAC Inhibitors in Cancer Development and Therapy

Over the last several decades, it has become clear that epigenetic abnormalities may be one of the hallmarks of cancer. Posttranslational modifications of histones, for example, may play a crucial role in cancer development and progression by modulating gene transcription, chromatin remodeling, and nuclear architecture. Histone acetylation, a well-studied posttranslational histone modification, is controlled by the opposing activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs). By removing acetyl groups, HDACs reverse chromatin acetylation and alter transcription of oncogenes and tumor suppressor genes. In addition, HDACs deacetylate numerous nonhistone cellular substrates that govern a wide array of biological processes including cancer initiation and progression. This review will discuss the role of HDACs in cancer and the therapeutic potential of HDAC inhibitors (HDACi) as emerging drugs in cancer treatment.

Trichostatin A enhances estrogen receptor-alpha repression in MCF-7 breast cancer cells under hypoxia

Estrogen receptor (ER) is a crucial determinant of resistance to endocrine therapy, which may change during the progression of breast cancer. We previously showed that hypoxia induces ESR1 gene repression and ERα protein degradation via proteasome-mediated pathway in breast cancer cells. HDAC plays important roles in the regulation of histone and non-histone protein post-translational modification. HDAC inhibitors can induce epigenetic changes and have therapeutic potential for targeting various cancers. Trichostatin A exerts potent antitumor activities against breast cancer cells in vitro and in vivo. In this report, we show that TSA augments ESR1 gene repression at the transcriptional level and downregulates ERα protein expression under hypoxic conditions through a proteasome-mediated pathway. TSA-induced estrogen response element-driven reporter activity in the absence of estrogen was synergistically enhanced under hypoxia; however, TSA inhibited cell proliferation under both normoxia and hypoxia. Our data show that the hypoxia-induced repression of ESR1 and degradation of ERα are enhanced by concomitant treatment with TSA. These findings expand our understanding of hormone responsiveness in the tumor microenvironment; however, additional in-depth studies are required to elucidate the detailed mechanisms of TSA-induced ERα regulation under hypoxia.