Inhibition of BET proteins impairs estrogen-mediated growth and transcription in breast cancers by pausing RNA polymerase advancement

Resistance of estrogen receptor function to BET bromodomain inhibition is mediated by transcriptional coactivator cooperativity

The bromodomain and extraterminal domain (BET) family of proteins are critical chromatin readers that bind to acetylated histones through their bromodomains to activate transcription. Here, we reveal that bromodomain inhibition fails to repress oncogenic targets of estrogen receptor because of an intrinsic transcriptional mechanism. While bromodomains are necessary for the transcription of many genes, bromodomain-containing protein 4 (BRD4) binds to estrogen receptor binding sites and activates transcription of critical oncogenes such as MYC, independently of its bromodomains. BRD4 associates with the Mediator complex and disruption of Mediator reduces BRD4's enhancer occupancy. Profiling changes of the post-initiation RNA polymerase II (Pol II)-associated factors revealed that BET proteins regulate interactions between Pol II and elongation factors SPT5, SPT6 and the polymerase-associated factor 1 complex, which associate with BET proteins independently of their bromodomains and mediate their transcription elongation effect. Our findings highlight the importance of bromodomain-independent functions and interactions of BET proteins in the development of future therapeutic strategies.

BET Bromodomain Inhibition Potentiates Ocular Melanoma Therapy by Inducing Cell Cycle Arrest

Purpose

Ocular melanoma is a common primary malignant ocular tumor in adults with limited effective treatments. Epigenetic regulation plays an important role in tumor development. The switching/sucrose nonfermentation (SWI/SNF) chromatin remodeling complex and bromodomain and extraterminal domain family proteins are epigenetic regulators involved in several cancers. We aimed to screen a candidate small molecule inhibitor targeting these regulators and investigate its effect and mechanism in ocular melanoma.

Methods

We observed phenotypes caused by knockdown of the corresponding gene and synergistic effects with BRD inhibitor treatment and SWI/SNF complex knockdown. The effect of JQ-1 on ocular melanoma cell cycle and apoptosis was analyzed with flow cytometry. Via RNA sequencing, we also explored the mechanism of BRD4.

Results

The best tumor inhibitory effect was observed for the BRD4 inhibitor (JQ-1), although there were no statistically obvious changes in the shBRD4 and shBRD9 groups. Interestingly, the inhibitory effect of JQ-1 was decrease in the shBRD4 group. JQ-1 inhibits the growth of melanoma in various cell lines and in tumor-bearing mice. We found 17 of these 28 common differentially expressed genes were downregulated after MEL270 and MEL290 cells treated with JQ-1. Four of these 17 genes, TP53I11, SH2D5, SEMA5A, and MDGA1, were positively correlated with BRD4. In TCGA database, low expression of TP53I11, SH2D5, SEMA5A, and MDGA1 improved the overall survival rate of patients. Furthermore, the disease-free survival rate was increased in the groups with low expression of TP53I11, SH2D5, and SEMA5A.

Conclusions

JQ-1 may act downstream of BRD4 and suppress ocular melanoma growth by inducing G1 cell cycle arrest.

Genome-Wide Estrogen Receptor Activity in Breast Cancer

The largest subtype of breast cancer is characterized by the expression and activity of the estrogen receptor alpha (ERalpha/ER). Although several effective therapies have significantly improved survival, the adaptability of cancer cells means that patients frequently stop responding or develop resistance to endocrine treatment. ER does not function in isolation and multiple associating factors have been reported to play a role in regulating the estrogen-driven transcriptional program. This review focuses on the dynamic interplay between some of these factors which co-occupy ER-bound regulatory elements, their contribution to estrogen signaling, and their possible therapeutic applications. Furthermore, the review illustrates how some ER association partners can influence and reprogram the genomic distribution of the estrogen receptor. As this dynamic ER activity enables cancer cell adaptability and impacts the clinical outcome, defining how this plasticity is determined is fundamental to our understanding of the mechanisms of disease progression.

The emerging role of BET inhibitors in breast cancer

Bromodomain and extraterminal domain (BET) proteins are epigenetic molecules that regulate the expression of multiple genes involved in carcinogenesis. Breast cancer is an heterogenous disease emerging from aberrant gene expression and epigenetic alteration patterns. Amplification or overexpression of BET proteins has been identified in breast tumors highlighting their clinical significance. Development of BET inhibitors that disrupt BET protein binding to acetylated lysine residues of chromatin and suppress transcription of various oncogenes has shown promising results in breast cancer cells and xenograft models. Currently, Phase I/II clinical trials explore safety and efficacy of BET inhibitors in solid tumors and breast cancer. Treatment-emergent toxicities have been reported, including thrombocytopenia and gastrointestinal disorders. Preliminary results demonstrated greater response rates to BET inhibitors in combination with already approved anticancer agents. Consistently, BET inhibition sensitized breast tumors to chemotherapy drugs, hormone therapy and PI3K inhibitors in vitro. This article aims to review all existing preclinical and clinical evidence regarding BET inhibitors in breast cancer.

Novel Pyrrolopyridone Bromodomain and Extra-Terminal Motif (BET) Inhibitors Effective in Endocrine-Resistant ER+ Breast Cancer with Acquired Resistance to Fulvestrant and Palbociclib

Acquired resistance to fulvestrant and palbociclib is a new challenge to treatment of estrogen receptor positive (ER+) breast cancer. ER is expressed in most resistance settings; thus, bromodomain and extra-terminal protein inhibitors (BETi) that target BET-amplified ER-mediated transcription have therapeutic potential. Novel pyrrolopyridone BETi leveraged novel interactions with L92/L94 confirmed by a cocrystal structure of 27 with BRD4. Optimization of BETi using growth inhibition in fulvestrant-resistant (MCF-7:CFR) cells was confirmed in endocrine-resistant, palbociclib-resistant, and ESR1 mutant cell lines. 27 was more potent in MCF-7:CFR cells than six BET inhibitors in clinical trials. Transcriptomic analysis differentiated 27 from the benchmark BETi, JQ-1, showing downregulation of oncogenes and upregulation of tumor suppressors and apoptosis. The therapeutic approach was validated by oral administration of 27 in orthotopic xenografts of endocrine-resistant breast cancer in monotherapy and in combination with fulvestrant. Importantly, at an equivalent dose in rats, thrombocytopenia was mitigated.

Role of BET Inhibitors in Triple Negative Breast Cancers

Bromodomain and extraterminal domain (BET) proteins have evolved as key multifunctional super-regulators that control gene expression. These proteins have been shown to upregulate transcriptional machinery leading to over expression of genes involved in cell proliferation and carcinogenesis. Based on favorable preclinical evidence of BET inhibitors in various cancer models; currently, 26 clinical trials are underway in various stages of study on various hematological and solid organ cancers. Unfortunately, preliminary evidence for these clinical studies does not support the application of BET inhibitors as monotherapy in cancer treatment. Furthermore, the combinatorial efficiency of BET inhibitors with other chemo-and immunotherapeutic agents remain elusive. In this review, we will provide a concise summary of the molecular basis and preliminary clinical outcomes of BET inhibitors in cancer therapy, with special focus on triple negative breast cancer.

BET Inhibition as a Rational Therapeutic Strategy for Invasive Lobular Breast Cancer

PURPOSE

Invasive lobular carcinoma (ILC) is a subtype of breast cancer accounting for 10% of breast tumors. The majority of patients are treated with endocrine therapy; however, endocrine resistance is common in estrogen receptor-positive breast cancer and new therapeutic strategies are needed. Bromodomain and extraterminal inhibitors (BETi) are effective in diverse types of breast cancer but they have not yet been assessed in ILC.

EXPERIMENTAL DESIGN

We assessed whether targeting the BET proteins with JQ1 could serve as an effective therapeutic strategy in ILC in both 2D and 3D models. We used dynamic BH3 profiling and RNA-sequencing (RNA-seq) to identify transcriptional reprograming enabling resistance to JQ1-induced apoptosis. As part of the RATHER study, we obtained copy-number alterations and RNA-seq on 61 ILC patient samples.

RESULTS

Certain ILC cell lines were sensitive to JQ1, while others were intrinsically resistant to JQ1-induced apoptosis. JQ1 treatment led to an enhanced dependence on antiapoptotic proteins and a transcriptional rewiring inducing fibroblast growth factor receptor 1 (FGFR1). This increase in FGFR1 was also evident in invasive ductal carcinoma (IDC) cell lines. The combination of JQ1 and FGFR1 inhibitors was highly effective at inhibiting growth in both 2D and 3D models of ILC and IDC. Interestingly, we found in the RATHER cohort of 61 ILC patients that 20% had FGFR1 amplification and we showed that high BRD3 mRNA expression was associated with poor survival specifically in ILC.

CONCLUSIONS

We provide evidence that BETi either alone or in combination with FGFR1 inhibitors or BH3 mimetics may be a useful therapeutic strategy for recurrent ILC patients.

Combining epigenetic drugs with other therapies for solid tumours - past lessons and future promise

Epigenetic dysregulation has long been recognized as a key factor contributing to tumorigenesis and tumour maintenance that can influence all of the recognized hallmarks of cancer. Despite regulatory approvals for the treatment of certain haematological malignancies, the efficacy of the first generation of epigenetic drugs (epi-drugs) in patients with solid tumours has been disappointing; however, successes have now been achieved in selected solid tumour subtypes, thanks to the development of novel compounds and a better understanding of cancer biology that have enabled precision medicine approaches. Several lines of evidence support that, beyond their potential as monotherapies, epigenetic drugs could have important roles in synergy with other anticancer therapies or in reversing acquired therapy resistance. Herein, we review the mechanisms by which epi-drugs can modulate the sensitivity of cancer cells to other forms of anticancer therapy, including chemotherapy, radiation therapy, hormone therapy, molecularly targeted therapy and immunotherapy. We provide a critical appraisal of the preclinical rationale, completed clinical studies and ongoing clinical trials relating to combination therapies incorporating epi-drugs. Finally, we propose and discuss rational clinical trial designs and drug development strategies, considering key factors including patient selection, tumour biomarker evaluation, drug scheduling and response assessment and study end points, with the aim of optimizing the development of such combinations.

Distinct Roles for BET Family Members in Estrogen Receptor α Enhancer Function and Gene Regulation in Breast Cancer Cells

The bromodomain family member proteins (BRD; BET proteins) are key coregulators for estrogen receptor alpha (ERα)-mediated transcriptional enhancers. The use of BRD-selective inhibitors has gained much attention as a potential treatment for various solid tumors, including ER-positive breast cancers. However, the roles of individual BET family members have largely remained unexplored. Here, we describe the role of BRDs in estrogen (E2)-dependent gene expression in ERα-positive breast cancer cells. We observed that chemical inhibition of BET family proteins with JQ1 impairs E2-regulated gene expression and growth in breast cancer cells. In addition, RNAi-mediated depletion of each BET family member (BRDs 2, 3, and 4) revealed partially redundant roles at ERα enhancers and for target gene transcription. Furthermore, we found a unique role of BRD3 as a molecular sensor of total BET family protein levels and activity through compensatory control of its own protein levels. Finally, we observed that BRD3 is recruited to a subset of ERα-binding sites (ERBS) that are enriched for active enhancer features, located in clusters of ERBSs likely functioning as "super enhancers," and associated with highly E2-responsive genes. Collectively, our results illustrate a critical and specific role for BET family members in ERα-dependent gene transcription. IMPLICATIONS: BRD3 is recruited to and controls the activity of a subset ERα transcriptional enhancers, providing a therapeutic opportunity to target BRD3 with BET inhibitors in ERα-positive breast cancers.

Compound C620-0696, a new potent inhibitor targeting BPTF, the chromatin-remodeling factor in non-small-cell lung cancer

Bromodomain PHD-finger transcription factor (BPTF) is the largest subunit of the nucleosome remodeling factor and plays an important role in chromatin remodeling for gene activation through its association with histone acetylation or methylation. BPTF is also involved in oncogene transcription in diverse progressions of cancers. Despite clinical trials for inhibitors of bromodomain and extra-terminal family proteins in human cancers, no potent and selective inhibitor targeting the BPTF bromodomain has been discovered. In this study, we identified a potential inhibitor, namely, C620-0696, by computational docking modeling to target bromodomain. Results of biolayer interferometry revealed that compound C620-0696 exhibited high binding affinity to the BPTF bromodomain. Moreover, C620-0696 was cytotoxic in BPTF with a high expression of non-small-cell lung cancer (NSCLC) cells. It suppressed the expression of the BPTF target gene c-MYC, which is known as an oncogenic transcriptional regulator in various cancers. C620-0696 also partially inhibited the migration and colony formation of NSCLC cells owing to apoptosis induction and cell cycle blockage. Thus, our study presents an effective strategy to target a bromodomain factor-mediated tumorigenesis in cancers with small molecules, supporting further exploration of the use of these inhibitors in oncology.

Inhibition of BET Proteins and Histone Deacetylase (HDACs): Crossing Roads in Cancer Therapy

Histone DeACetylases (HDACs) are enzymes that remove acetyl groups from histones and other proteins, regulating the expression of target genes. Pharmacological inhibition of these enzymes re-shapes chromatin acetylation status, confusing boundaries between transcriptionally active and quiescent chromatin. This results in reinducing expression of silent genes while repressing highly transcribed genes. Bromodomain and Extraterminal domain (BET) proteins are readers of acetylated chromatin status and accumulate on transcriptionally active regulatory elements where they serve as scaffold for the building of transcription-promoting complexes. The expression of many well-known oncogenes relies on BET proteins function, indicating BET inhibition as a strategy to counteract their activity. BETi and HDACi share many common targets and affect similar cellular processes to the point that combined inhibition of both these classes of proteins is regarded as a strategy to improve the effectiveness of these drugs in cancer. In this work, we aim to discuss the molecular basis of the interplay between HDAC and BET proteins, pointing at chromatin acetylation as a crucial node of their functional interaction. We will also describe the state of the art of their dual inhibition in cancer therapy. Finally, starting from their mechanism of action we will provide a speculative perspective on how these drugs may be employed in combination with standard therapies to improve effectiveness and/or overcome resistance.

Targeting bromodomain and extraterminal proteins in breast cancer

Breast cancer is a collection of distinct tumor subtypes that are driven by unique gene expression profiles. These transcriptomes are controlled by various epigenetic marks that dictate which genes are expressed and suppressed. During carcinogenesis, extensive restructuring of the epigenome occurs, including aberrant acetylation, alteration of methylation patterns, and accumulation of epigenetic readers at oncogenes. As epigenetic alterations are reversible, epigenome-modulating drugs could provide a mechanism to silence numerous oncogenes simultaneously. Here, we review the impact of inhibitors of the Bromodomain and Extraterminal (BET) family of epigenetic readers in breast cancer. These agents, including the prototypical BET inhibitor JQ1, have been shown to suppress a variety of oncogenic pathways while inducing minimal, if any, toxicity in models of several subtypes of breast cancer. BET inhibitors also synergize with multiple approved anti-cancer drugs, providing a greater response in breast cancer cell lines and mouse models than either single agent. The combined findings of the studies discussed here provide an excellent rationale for the continued investigation of the utility of BET inhibitors in breast cancer.

Targeting BRD4 proteins suppresses the growth of NSCLC through downregulation of eIF4E expression

Lung cancer is the leading cause of cancer-related death worldwide. Bromodomain and extraterminal domain (BET) proteins act as epigenome readers for gene transcriptional regulation. Among BET family members, BRD4 was well studied, but for its mechanism in non-small cell lung carcinoma has not been elucidated. eIF4E regulates gene translation and has been proved to play an important role in the progression of lung cancer. In this study, we first confirmed that BET inhibitors JQ1 and I-BET151 suppressed the growth of NSCLCs, in parallel with downregulated eIF4E expression. Then we found that knockdown of BRD4 expression using siRNAs inhibited the growth of NSCLCs as well as decreased eIF4E protein levels. Moreover, overexpression of eIF4E partially abrogated the growth inhibitory effect of JQ1, while knockdown of eIF4E enhanced the inhibitory effect of JQ1. Furthermore, JQ1 treatment or knockdown of BRD4 expression decreased eIF4E mRNA levels and inhibited its promoter activity by luciferase reporter assay. JQ1 treatment significantly decreased the binding of eIF4E promoter with BRD4. Finally, JQ1 inhibited the growth of H460 tumors in parallel with downregulated eIF4E mRNA and protein levels in a xenograft mouse model. These findings suggest that inhibition of BET by JQ1, I-BET151, or BRD4 silencing suppresses the growth of non-small cell lung carcinoma through decreasing eIF4E transcription and subsequent mRNA and protein expression. Considering that BET regulates gene transcription epigenetically, our findings not only reveal a new mechanism of BET-regulated eIF4E in lung cancer, but also indicate a novel strategy by co-targeting eIF4E for enhancing BET-targeted cancer therapy.

BET inhibitors as novel therapeutic agents in breast cancer

Tumoral cells not only depend on oncogenic abnormalities to maintain its malignant phenotype but on non-oncogenic vulnerabilities. Targeting epigenomics can modify specific cellular functions required for malignant transformation. The Bromodomain (BRD) family mediates their effect by recruiting proteins of the transcription machinery, recognizing acetylated-lysine residues in nucleosomal histones. Bromodomain and extra-terminal (BET) inhibitors have shown to produce growth inhibition in several tumors through the inhibition of the expression of several transcription factors. In this review we will discuss the current knowledge regarding BET inhibitors in breast cancer. Recent data demonstrates their antiproliferative effect in several cancer subtypes, including the triple negative subtype, or when combined with cell signaling inhibitors. We will also describe options for therapeutic combinations or potential mechanisms of resistance, with special emphasis on their future clinical development.

Transcriptional Dependencies in Diffuse Intrinsic Pontine Glioma

Diffuse intrinsic pontine glioma (DIPG) is a fatal pediatric cancer with limited therapeutic options. The majority of cases of DIPG exhibit a mutation in histone-3 (H3K27M) that results in oncogenic transcriptional aberrancies. We show here that DIPG is vulnerable to transcriptional disruption using bromodomain inhibition or CDK7 blockade. Targeting oncogenic transcription through either of these methods synergizes with HDAC inhibition, and DIPG cells resistant to HDAC inhibitor therapy retain sensitivity to CDK7 blockade. Identification of super-enhancers in DIPG provides insights toward the cell of origin, highlighting oligodendroglial lineage genes, and reveals unexpected mechanisms mediating tumor viability and invasion, including potassium channel function and EPH receptor signaling. The findings presented demonstrate transcriptional vulnerabilities and elucidate previously unknown mechanisms of DIPG pathobiology.

A Novel Epi-drug Therapy Based on the Suppression of BET Family Epigenetic Readers

Recent progress in epigenetic research has made a profound influence on pharmacoepigenomics, one of the fastest growing disciplines promising to provide new epi-drugs for the treatment of a broad range of diseases. Histone acetylation is among the most essential chromatin modifications underlying the dynamics of transcriptional activation. The acetylated genomic regions recruit the BET (bromodomain and extra-terminal) family of bromodomains (BRDs), thereby serving as a molecular scaffold in establishing RNA polymerase II specificity. Over the past several years, the BET epigenetic readers have become the main targets for drug therapy. The discovery of selective small-molecule compounds with capacity to inhibit BET proteins has paved a path for developing novel strategies against cancer, cardiovascular, skeletal, and inflammatory diseases. Therefore, further research into small chemicals impeding the regulatory activity of BRDs could offer therapeutic benefits for many health problems including tumor growth, heart disease, oral, and bone disorders.

AR Signaling in Breast Cancer

Androgen receptor (AR, a member of the steroid hormone receptor family) status has become increasingly important as both a prognostic marker and potential therapeutic target in breast cancer. AR is expressed in up to 90% of estrogen receptor (ER) positive breast cancer, and to a lesser degree, human epidermal growth factor 2 (HER2) amplified tumors. In the former, AR signaling has been correlated with a better prognosis given its inhibitory activity in estrogen dependent disease, though conversely has also been shown to increase resistance to anti-estrogen therapies such as tamoxifen. AR blockade can mitigate this resistance, and thus serves as a potential target in ER-positive breast cancer. In HER2 amplified breast cancer, studies are somewhat conflicting, though most show either no effect or are associated with poorer survival. Much of the available data on AR signaling is in triple-negative breast cancer (TNBC), which is an aggressive disease with inferior outcomes comparative to other breast cancer subtypes. At present, there are no approved targeted therapies in TNBC, making study of the AR signaling pathway compelling. Gene expression profiling studies have also identified a luminal androgen receptor (LAR) subtype that is dependent on AR signaling in TNBC. Regardless, there seems to be an association between AR expression and improved outcomes in TNBC. Despite lower pathologic complete response (pCR) rates with neoadjuvant therapy, patients with AR-expressing TNBC have been shown to have a better prognosis than those that are AR-negative. Clinical studies targeting AR have shown somewhat promising results. In this paper we review the literature on the biology of AR in breast cancer and its prognostic and predictive roles. We also present our thoughts on therapeutic strategies.

Targeted Degradation of BET Proteins in Triple-Negative Breast Cancer

Triple-negative breast cancers (TNBC) remain clinically challenging with a lack of options for targeted therapy. In this study, we report the development of a second-generation BET protein degrader, BETd-246, which exhibits superior selectivity, potency, and antitumor activity. In human TNBC cells, BETd-246 induced degradation of BET proteins at low nanomolar concentrations within 1 hour of exposure, resulting in robust growth inhibition and apoptosis. BETd-246 was more potent and effective in TNBC cells than its parental BET inhibitor compound BETi-211. RNA-seq analysis revealed predominant downregulation of a large number of genes involved in proliferation and apoptosis in cells treated with BETd-246, as compared with BETi-211 treatment that upregulated and downregulated a similar number of genes. Functional investigations identified the MCL1 gene as a critical downstream effector for BET degraders, which synergized with small-molecule inhibitors of BCL-xL in triggering apoptosis. In multiple murine xenograft models of human breast cancer, BETd-246 and a further optimized analogue BETd-260 effectively depleted BET proteins in tumors and exhibited strong antitumor activities at well-tolerated dosing schedules. Overall, our findings show that targeting BET proteins for degradation represents an effective therapeutic strategy for TNBC treatment. Cancer Res; 77(9); 2476-87. ©2017 AACR.

The BET inhibitor JQ1 selectively impairs tumour response to hypoxia and downregulates CA9 and angiogenesis in triple negative breast cancer

The availability of bromodomain and extra-terminal inhibitors (BETi) has enabled translational epigenetic studies in cancer. BET proteins regulate transcription by selectively recognizing acetylated lysine residues on chromatin. BETi compete with this process leading to both downregulation and upregulation of gene expression. Hypoxia enables progression of triple negative breast cancer (TNBC), the most aggressive form of breast cancer, partly by driving metabolic adaptation, angiogenesis and metastasis through upregulation of hypoxia-regulated genes (for example, carbonic anhydrase 9 (CA9) and vascular endothelial growth factor A (VEGF-A). Responses to hypoxia can be mediated epigenetically, thus we investigated whether BETi JQ1 could impair the TNBC response induced by hypoxia and exert anti-tumour effects. JQ1 significantly modulated 44% of hypoxia-induced genes, of which two-thirds were downregulated including CA9 and VEGF-A. JQ1 prevented HIF binding to the hypoxia response element in CA9 promoter, but did not alter HIF expression or activity, suggesting some HIF targets are BET-dependent. JQ1 reduced TNBC growth in vitro and in vivo and inhibited xenograft vascularization. These findings identify that BETi dually targets angiogenesis and the hypoxic response, an effective combination at reducing tumour growth in preclinical studies.

Bromodomain inhibitors and cancer therapy: From structures to applications

Aberrations in the epigenetic landscape are a hallmark of cancer. Alterations in enzymes that are “writers,” “erasers,” or “readers” of histone modification marks are common. Bromodomains are “readers” that bind acetylated lysines in histone tails. Their most important function is the regulation of gene transcription by the recruitment of different molecular partners. Moreover, proteins containing bromodomains are also epigenetic regulators, although little is known about the specific function of these domains. In recent years, there has been increasing interest in developing small molecules that can target specific bromodomains. First, this has helped clarify biological functions of bromodomain-containing proteins. Secondly, it opens a new front for combatting cancer. In this review we will describe the structures and mechanisms associated with Bromodomain and Extra-Terminal motif (BET) inhibitors and non-BET inhibitors, their current status of development, and their promising role as anti-cancer agents.

Induction of USP17 by combining BET and HDAC inhibitors in breast cancer cells

Members of the bromodomain and extra-C terminal (BET) domain protein family and the histone deacetylase (HDAC) enzyme family regulate the expression of important oncogenes and tumor suppressor genes. Here we show that the BET inhibitor JQ1 inhibits proliferation and induces apoptosis of both triple negative and estrogen receptor positive breast cancer cells. Consistent with the critical role of histone acetylation in the regulation of gene expression, treatment with JQ1 or the HDAC inhibitor mocetinostat was associated with global changes in gene expression resulting in suppression of genes involved in cell-cycle regulation. Combining JQ1 with mocetinostat, further decreased cell viability. This synergistic effect was associated with increased suppression of genes essential for cell-cycle progression. Furthermore, we detected dramatic increase in the expression of several members of the ubiquitin–specific protease 17 (USP17) family of deubiquitinating enzymes in response to the combination treatment. Increased expression of USP17 enzymes were able to attenuate the Ras/MAPK pathway causing decrease in cell viability, while, siRNA mediated depletion of USP17 significantly decreased cytotoxicity after the combination treatment. In conclusion, our study demonstrates that co-treatment with BET inhibitors and HDAC inhibitors reduces breast cancer cell viability through induction of USP17.

Small-Molecule Targeting of BET Proteins in Cancer

BET proteins have recently become recognized for their role in a broad range of cancers and are defined by the presence of two acetyl-histone reading bromodomains and an ET domain. This family of proteins includes BRD2, BRD3, BRD4, and BRDT. BRD4 is the most-studied BET protein in cancer, and normally serves as an epigenetic reader that links active chromatin marks to transcriptional elongation through activation of RNA polymerase II. The role of BRD3 and BRD4 first became known in cancer as mutant oncoproteins fused to the p300-recruiting NUT protein in a rare aggressive subtype of squamous cell cancer known as NUT midline carcinoma (NMC). BET inhibitors are acetyl-histone mimetics that specifically bind BET bromodomains, competitively inhibiting its engagement with chromatin. The antineoplastic effects of BET inhibitors were first demonstrated in NMC and have since been shown to be effective at inhibiting the growth of many different cancers, particularly acute leukemia. BET inhibitors have also been instrumental as tool compounds that have demonstrated the key role of BRD4 in driving NMC and non-NMC cancer growth. Many clinical trials enrolling patients with hematologic and solid tumors are ongoing, with encouraging preliminary findings. BET proteins BRD2, BRD3, and BRD4 are expressed in nearly all cells of the body, so there are concerns of toxicity with BET inhibitors, as well as the development of resistance. Toxicity and resistance may be overcome by combining BET inhibitors with other targeted inhibitors, or through the use of novel BET inhibitor derivatives.

The Bromodomain BET Inhibitor JQ1 Suppresses Tumor Angiogenesis in Models of Childhood Sarcoma

The bromodomain and extra-terminal domain inhibitor JQ1 has marked antitumor activity against several hematologic malignancies as well as solid tumor models. Here, we investigated its activity in vitro and in vivo against models of childhood rhabdomyosarcoma and Ewing sarcoma. In vitro, JQ1 (but not the inactive enantiomer JQ1R) inhibited cell proliferation and increased G1 fraction of cells, although there was no correlation between cell line sensitivity and suppression of c-MYC or MYCN. In vivo, xenografts showed significant inhibition of growth during the period of treatment, and rapid regrowth after treatment was stopped, activity typical of antiangiogenic agents. Furthermore, xenografts derived from cell lines intrinsically resistant or sensitive to JQ1 in vitro had similar sensitivity in vivo as xenografts. Further investigation showed that JQ1 reduced tumor vascularization. This was secondary to both drug-induced downregulation of tumor-derived growth factors and direct effects of JQ1 on vascular elements. JQ1 suppressed VEGF-stimulated vascularization of Matrigel plugs in mice, and in vitro suppressed differentiation, proliferation, and invasion of human umbilical cord vascular endothelial cells (HUVEC). In HUVECs, JQ1 partially suppressed c-MYC levels, but dramatically reduced AP-1 levels and activity through suppression of the AP-1-associated protein FOSL1. Our data suggest that the antitumor activity of JQ1 in these sarcoma models is largely a consequence of its antiangiogenic activity. Mol Cancer Ther; 15(5); 1018-28. ©2016 AACR.