Bromodomain And Extraterminal Protein Inhibition Research Articles

Unraveling the Role of Bromodomain and Extra-Terminal Proteins in Human Uterine Leiomyosarcoma.

Uterine leiomyosarcoma (uLMS) is the most common type of uterine sarcoma, associated with poor prognosis, high rates of recurrence, and metastasis. Currently, the molecular mechanism of the origin and development of uLMS is limited. Bromodomain and extra-terminal (BET) proteins are involved in both physiological and pathological events. However, the role of BET proteins in the pathogenesis of uLMS is unknown. Here, we show for the first time that BET protein family members, BRD2, BRD3, and BRD4, are aberrantly overexpressed in uLMS tissues compared to the myometrium, with a significant change by histochemical scoring assessment. Furthermore, inhibiting BET proteins with their small, potent inhibitors (JQ1 and I-BET 762) significantly inhibited the uLMS proliferation dose-dependently via cell cycle arrest. Notably, RNA-sequencing analysis revealed that the inhibition of BET proteins with JQ1 and I-BET 762 altered several critical pathways, including the hedgehog pathway, EMT, and transcription factor-driven pathways in uLMS. In addition, the targeted inhibition of BET proteins altered several other epigenetic regulators, including DNA methylases, histone modification, and m6A regulators. The connections between BET proteins and crucial biological pathways provide a fundamental structure to better understand uterine diseases, particularly uLMS pathogenesis. Accordingly, targeting the vulnerable epigenome may provide an additional regulatory mechanism for uterine cancer treatment.

A genome-wide CRISPR screen identifies BRD4 as a regulator of cardiomyocyte differentiation.

Human induced pluripotent stem cell (hiPSC) to cardiomyocyte (CM) differentiation has reshaped approaches to studying cardiac development and disease. In this study, we employed a genome-wide CRISPR screen in a hiPSC to CM differentiation system and reveal here that BRD4, a member of the bromodomain and extraterminal (BET) family, regulates CM differentiation. Chemical inhibition of BET proteins in mouse embryonic stem cell (mESC)-derived or hiPSC-derived cardiac progenitor cells (CPCs) results in decreased CM differentiation and persistence of cells expressing progenitor markers. In vivo, BRD4 deletion in second heart field (SHF) CPCs results in embryonic or early postnatal lethality, with mutants demonstrating myocardial hypoplasia and an increase in CPCs. Single-cell transcriptomics identified a subpopulation of SHF CPCs that is sensitive to BRD4 loss and associated with attenuated CM lineage-specific gene programs. These results highlight a previously unrecognized role for BRD4 in CM fate determination during development and a heterogenous requirement for BRD4 among SHF CPCs.

Inhibition of BET Proteins Regulates Fcγ Receptor Function and Reduces Inflammation in Rheumatoid Arthritis.

Overactivation of immune responses is a hallmark of autoimmune disease pathogenesis. This includes the heightened production of inflammatory cytokines such as Tumor Necrosis Factor α (TNFα), and the secretion of autoantibodies such as isotypes of rheumatoid factor (RF) and anticitrullinated protein antibody (ACPA). Fcγ receptors (FcγR) expressed on the surface of myeloid cells bind Immunoglobulin G (IgG) immune complexes. Recognition of autoantigen-antibody complexes by FcγR induces an inflammatory phenotype that results in tissue damage and further escalation of the inflammatory response. Bromodomain and extra-terminal protein (BET) inhibition is associated with reduced immune responses, making the BET family a potential therapeutic target for autoimmune diseases such as rheumatoid arthritis (RA). In this paper, we examined the BET inhibitor PLX51107 and its effect on regulating FcγR expression and function in RA. PLX51107 significantly downregulated expression of FcγRIIa, FcγRIIb, FcγRIIIa, and the common γ-chain, FcϵR1-γ, in both healthy donor and RA patient monocytes. Consistent with this, PLX51107 treatment attenuated signaling events downstream of FcγR activation. This was accompanied by a significant decrease in phagocytosis and TNFα production. Finally, in a collagen-induced arthritis model, PLX51107-treatment reduced FcγR expression in vivo accompanied by a significant reduction in footpad swelling. These results suggest that BET inhibition is a novel therapeutic approach that requires further exploration as a treatment for patients with RA.

The Role of Bromodomain and Extraterminal (BET) Proteins in Controlling the Phagocytic Activity of Microglia In Vitro: Relevance to Alzheimer’s Disease

The correct phagocytic activity of microglia is a prerequisite for maintaining homeostasis in the brain. In the analysis of mechanisms regulating microglial phagocytosis, we focused on the bromodomain and extraterminal domain (BET) proteins: Brd2, Brd3, and Brd4, the acetylation code readers that control gene expression in cooperation with transcription factors. We used pharmacological (JQ1) and genetic (siRNA) inhibition of BET proteins in murine microglial cell line BV2. Inhibition of BET proteins reduced the phagocytic activity of BV2, as determined by using a fluorescent microspheres-based assay and fluorescently labelled amyloid-beta peptides. Gene silencing experiments demonstrated that all brain-existing BET isoforms control phagocytosis in microglia. From a set of 84 phagocytosis-related genes, we have found the attenuation of the expression of 14: Siglec1, Sirpb1a, Cd36, Clec7a, Itgam, Tlr3, Fcgr1, Cd14, Marco, Pld1, Fcgr2b, Anxa1, Tnf, Nod1, upon BET inhibition. Further analysis of the mRNA level of other phagocytosis-related genes which were involved in the pathomechanism of Alzheimer's disease demonstrated that JQ1 significantly reduced the expression of Cd33, Trem2, and Zyx. Our results indicate the important role of BET proteins in controlling microglial phagocytosis; therefore, targeting BET may be the efficient method of modulating microglial activity.

Abstract 3267: ARID1A mutated bladder cancer cells are sensitized to BET protein inhibition with OTX-015

Abstract Background: Bladder cancer (BC) is a common and deadly disease. Inactivating ARID1A mutations occur in up to 30% of metastatic BC tumors, making it the most commonly mutated epigenetic gene and the 4th most commonly mutated gene overall in BC. ARID1A mutations are associated with decreased response to therapy and poor prognosis; thus, there is a need to develop therapies specifically targeting ARID1A mutant tumors. Using the UNC EpiG Diamond compound library, a set of well-annotated small molecule probes targeting chromatin regulatory proteins, we determined that inhibitors of bromodomain and extraterminal (BET) proteins potently inhibit the viability of BC cells. Here, we tested the hypothesis that ARID1Amut cells are particularly sensitized to the pan-BET inhibitor OTX-015. Methods: AR1D1A-competent (ARID1AWT) 5637 and ARID1A-mutated (ARID1Amut) HT1197 cells were treated with eight ascending concentrations of OTX-015 (0.1 nM-100 µM), and cell viability was measured using CellTiter-Glo™ after 72-120-hour incubations. IC50 values were calculated using a four-parameter non-linear regression model. Gene expression of BET inhibitor target genes, MYC, ARID1B, and RAD51, were evaluated by RT-PCR after a 48-hour incubation, normalized to an SDHA housekeeper and compared to a 0.1% DMSO control. CellTiter-Glo™ and RT-PCR experiments were conducted in technical and biologic triplicates. Western blotting evaluated OTX-015 effects on c-MYC, BAF250B (encoded by ARID1B), and RAD51 protein expression after 72-hour incubation. Results: OTX-015 was 8-times more potent in ARID1Amut HT1197 cells at 120 h than in ARID1AWT 5637 cells (IC50: 0.12 μM vs. 1.0 μM). OTX-015 treatment (1 µM) significantly reduced ARID1B and RAD51 mRNA expression versus 0.1% DMSO control (83% and 86% reduction, respectively, both P<0.0001). At the same concentration, OTX-015 did not significantly reduce MYC mRNA expression HT1197 cells (25% reduction, P=0.31), but did significantly reduce MYC expression in 5637 cells (57% reduction, P<0.0001) at 48 h. When comparing HT1197 to 5637 cells, OTX-015 treatment (1 µM) caused a greater reduction in ARID1B mRNA expression (83% vs. 62% reduction, P=0.02) and RAD51 mRNA expression (86% vs. 57%, P=0.001) at 48 h. Finally, OTX-015 treatment (1 µM) also resulted in more dramatically reduced c-MYC, ARID1B and RAD51 protein expression in HT1197 cells at 72 h, when compared to 5637 cells. Conclusions: These preliminary results support future lines of inquiry into the molecular mechanisms that underlie sensitization of ARID1Amut cells to BET protein inhibition. Citation Format: Ryan M. Kemper, Heemaja Mewada, Jeffrey S. Damrauer, Brian Hardy, Stephen F. Frye, Kenneth H. Pearce, Jesse Raab, William Y. Kim, Daniel James Crona. ARID1A mutated bladder cancer cells are sensitized to BET protein inhibition with OTX-015 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3267.

Favorable cognitive effects of the BET protein inhibitor apabetalone in patients 70 and older

AbstractBackgroundCognitive decline in late life including Alzheimer’s disease (AD) and vascular dementia (VaD) may be caused by epigenetic change. Bromodomain and extra‐terminal (BET) proteins are epigenetic transcriptional “readers” found to contribute to chronic disease. Apabetalone, a small molecule BET protein inhibitor for oral administration, was assessed for therapeutic effects on cognitive performance in a randomized trial of patients at high risk for cardiovascular disease (CVD).MethodIn the BETonMACE post‐Acute Coronary Syndrome trial in type 2 diabetes mellitus patients were randomized to apabetalone capsule 100 mg b.i.d. or placebo (n=2425). The Montreal Cognitive Assessment (MoCA) was performed on all patients 70 years or older at baseline (n=464) and yearly in the embedded cognition study. In a prespecified analysis, participants were assigned to one of three groups: MoCA score ≥ 26 (normal performance), MoCA score 25 – 22 (mild cognitive impairment), and MoCA score ≤ 21 (dementia).ResultApabetalone exposure was equivalent in each of the three MoCA‐score defined groups. Apabetalone treatment over approximately two years was associated with an increased total MoCA score in participants with baseline MoCA score of ≤ 21 (p = 0.02). Onset of cognition benefit appeared after 12 months treatment. There was no significant difference in change from baseline in the treatment groups with higher MoCA scores.ConclusionIn the BETonMACE trial epigenetic BET protein inhibition by apabetalone capsule 100 mg b.i.d. vs placebo was associated with improved cognition as measured by MoCA in patients with baseline scores of < 21. It is feasible to embed cognition assessment in randomized Phase 3 CVD endpoint studies. BET protein inhibitors warrant further investigation for late life cognitive disorders.

Inhibition of BET Proteins during Adolescence Affects Prefrontal Cortical Development: Relevance to Schizophrenia.

Background: The present study investigated the role of proteins from the bromodomain and extra-terminal (BET) family in schizophrenia-like abnormalities in a neurodevelopmental model of schizophrenia induced by prenatal methylazoxymethanol (MAM) administration (MAM-E17). Methods: An inhibitor of BET proteins, JQ1, was administered during adolescence on postnatal days (P) 23–P29, and behavioural responses (sensorimotor gating, recognition memory) and prefrontal cortical (mPFC) function (long-term potentiation (LTP), molecular and proteomic analyses) studies were performed in adult males and females. Results: Deficits in sensorimotor gating and recognition memory were observed only in MAM-treated males. However, adolescent JQ1 treatment affected animals of both sexes in the control but not MAM-treated groups and reduced behavioural responses in both sexes. An electrophysiological study showed LTP impairments only in male MAM-treated animals, and JQ1 did not affect LTP in the mPFC. In contrast, MAM did not affect activity-dependent gene expression, but JQ1 altered gene expression in both sexes. A proteomic study revealed alterations in MAM-treated groups mainly in males, while JQ1 affected both sexes. Conclusions: MAM-induced schizophrenia-like abnormalities were observed only in males, while adolescent JQ1 treatment affected memory recognition and altered the molecular and proteomic landscape in the mPFC of both sexes. Thus, transient adolescent inhibition of the BET family might prompt permanent alterations in the mPFC.

Inhibition of Epigenetic Reader BET Proteins by Apabetalone Counters Inflammation in Activated Innate Immune Cells from Fabry Disease Patients Receiving ERT

Abstract Fabry disease (FD) is a rare genetic disorder with a deficit in the degradation pathway of a glycolipid, globotriaocylceramide (Gb3). Gb3 deposits in various tissues evoke immune-mediated systemic inflammation, driving the progression of FD complications. In particular, cardiac and renal problems, leading causes of mortality in FD patients, are insufficiently managed by enzyme replacement therapy (ERT) in the long term. Here, we examined immune profiles in unstimulated peripheral blood mononuclear cells (PBMCs) from FD patients on ERT relative to untreated patients. The results showed an upregulation of the CCR2/MCP-1 axis, particularly in the presence of renal dysfunction. We also observed an increase of IL12B transcripts in unstimulated PBMCs when tracking the immune status over two years of ERT in patients with early-stage chronic kidney disease (CKD). Interestingly, pro-inflammatory responses in LPS-stimulated PBMCs were countered by apabetalone, a clinical-stage candidate that inhibits Bromodomain and Extra Terminal (BET) proteins, epigenetic readers. Apabetalone treatment dose dependently inhibited MCP-1 and IL-12 production by up to 90%. Apabetalone downregulated the transcription of other pro-inflammatory cytokines including TNF-α, IL-6 and IL-8 by 70%, 48% and 54%, respectively. Reactive oxygen species (ROS) are an indicator of oxidative damage caused by intracellular Gb3 deposits in FD patients. Apabetalone suppressed ROS levels by up to ~70% in LPS-stimulated neutrophils. Hence, apabetalone treatment may reduce pathological inflammation and oxidative stress in FD patients and thus complement ERT to optimize patient outcomes, warranting further investigation of apabetalone as a therapeutic for FD.

Abstract 14127: BET Protein Inhibitor Apabetalone Suppresses Inflammatory Hyperactivation of Monocytes From Patients With Cardiovascular Disease and Type 2 Diabetes

Introduction: Hyperactivation of the monocyte inflammatory response is partly controlled by epigenetic mechanisms and contributes to atherosclerotic plaque formation in patients with cardiovascular disease (CVD) and type 2 diabetes mellitus (DM2). Hypothesis: Monocyte activation in DM2+CVD is regulated by bromodomain and extraterminal (BET) epigenetic readers and can be inhibited by apabetalone - a clinical stage BET inhibitor. Methods: CD14+ monocytes from 14 DM2+CVD patients and 12 matched control subjects were treated ex vivo with 25 μM apabetalone ± 25 U/mL interferon γ (IFNγ) for 4h or 24h. Expressed genes (180) were analyzed with the Nanostring™ Innate Immunity Panel. Secreted cytokines were immunoprofiled with a Milliplex® array (42). Bioinformatics were performed with Ingenuity® Pathway Analysis (IPA®) software. Results: Unstimulated DM2+CVD monocytes had higher IL1A , IL1B and IL8 cytokine gene expression and Toll-like receptor (TLR) 2 surface abundance than control monocytes, indicating proinflammatory activation. Ex vivo apabetalone treatment reduced IL1A and IL8 mRNA (p<0.001), as well as MCP-1, MCP-3, GRO-α, and IL-8 secretion (p<0.01), countering the pro-inflammatory activation of DM2+CVD monocytes. Further, DM2+CVD monocytes were hyperresponsive to ex vivo stimulation with IFNγ, upregulating genes within cytokine and NF-κB pathways ( TNF , CCL7 , CCL8 , MYD88 , RELA ) >30% more than controls (p<0.05). Apabetalone countered IFNγ hyperresponsiveness by reducing expression of overexpressed genes by up to 30% more in DM2+CVD monocytes versus controls (p<0.01). Ex vivo apabetalone treatment also countered IFNγ induced secretion of IL-1β and TNFα in DM2+CVD monocytes (p<0.01). Consistently, IPA® analysis of immune gene signatures post IFNγ stimulation in both cohorts predicted a more prominent reduction of Toll-like receptor and cytokine pathways by apabetalone in the diseased state. Conclusions: Monocytes isolated from DM2+CVD patients receiving standard of care therapies are in a hyperinflammatory state and hyperactivate upon IFNγ stimulation. Apabetalone treatment diminishes this proinflammatory phenotype, providing mechanistic insight into how BET protein inhibition may reduce CVD risk in DM2 patients.

BET protein inhibitor apabetalone (RVX-208) suppresses pro-inflammatory hyper-activation of monocytes from patients with cardiovascular disease and type 2 diabetes

BackgroundPatients with cardiovascular disease (CVD) and type 2 diabetes (DM2) have a high residual risk for experiencing a major adverse cardiac event. Dysregulation of epigenetic mechanisms of gene transcription in innate immune cells contributes to CVD development but is currently not targeted by therapies. Apabetalone (RVX-208) is a small molecule inhibitor of bromodomain and extra-terminal (BET) proteins—histone acetylation readers that drive pro-inflammatory and pro-atherosclerotic gene transcription. Here, we assess the impact of apabetalone on ex vivo inflammatory responses of monocytes from DM2 + CVD patients.ResultsMonocytes isolated from DM2 + CVD patients and matched controls were treated ex vivo with apabetalone, interferon γ (IFNγ), IFNγ + apabetalone or vehicle and phenotyped for gene expression and protein secretion. Unstimulated DM2 + CVD monocytes had higher baseline IL-1α, IL-1β and IL-8 cytokine gene expression and Toll-like receptor (TLR) 2 surface abundance than control monocytes, indicating pro-inflammatory activation. Further, DM2 + CVD monocytes were hyper-responsive to stimulation with IFNγ, upregulating genes within cytokine and NF-κB pathways > 30% more than control monocytes (p < 0.05). Ex vivo apabetalone treatment countered cytokine secretion by DM2 + CVD monocytes at baseline (GROα and IL-8) and during IFNγ stimulation (IL-1β and TNFα). Apabetalone abolished pro-inflammatory hyper-activation by reducing TLR and cytokine gene signatures more robustly in DM2 + CVD versus control monocytes.ConclusionsMonocytes isolated from DM2 + CVD patients receiving standard of care therapies are in a hyper-inflammatory state and hyperactive upon IFNγ stimulation. Apabetalone treatment diminishes this pro-inflammatory phenotype, providing mechanistic insight into how BET protein inhibition may reduce CVD risk in DM2 patients.

Systems Biology Approach Identifies Prognostic Signatures of Poor Overall Survival and Guides the Prioritization of Novel BET-CHK1 Combination Therapy for Osteosarcoma.

Osteosarcoma (OS) patients exhibit poor overall survival, partly due to copy number variations (CNVs) resulting in dysregulated gene expression and therapeutic resistance. To identify actionable prognostic signatures of poor overall survival, we employed a systems biology approach using public databases to integrate CNVs, gene expression, and survival outcomes in pediatric, adolescent, and young adult OS patients. Chromosome 8 was a hotspot for poor prognostic signatures. The MYC-RAD21 copy number gain (8q24) correlated with increased gene expression and poor overall survival in 90% of the patients (n = 85). MYC and RAD21 play a role in replication-stress, which is a therapeutically actionable network. We prioritized replication-stress regulators, bromodomain and extra-terminal proteins (BETs), and CHK1, in order to test the hypothesis that the inhibition of BET + CHK1 in MYC-RAD21+ pediatric OS models would be efficacious and safe. We demonstrate that MYC-RAD21+ pediatric OS cell lines were sensitive to the inhibition of BET (BETi) and CHK1 (CHK1i) at clinically achievable concentrations. While the potentiation of CHK1i-mediated effects by BETi was BET-BRD4-dependent, MYC expression was BET-BRD4-independent. In MYC-RAD21+ pediatric OS xenografts, BETi + CHK1i significantly decreased tumor growth, increased survival, and was well tolerated. Therefore, targeting replication stress is a promising strategy to pursue as a therapeutic option for this devastating disease.

Bromodomain and extra-terminal domain (BET) proteins regulate melanocyte differentiation

BackgroundPharmacologic inhibition of bromodomain and extra-terminal (BET) proteins is currently being explored as a new therapeutic approach in cancer. Some studies have also implicated BET proteins as regulators of cell identity and differentiation through their interactions with lineage-specific factors. However, the role of BET proteins has not yet been investigated in melanocyte differentiation. Melanocyte inducing transcription factor (MITF) is the master regulator of melanocyte differentiation, essential for pigmentation and melanocyte survival. In this study, we tested the hypothesis that BET proteins regulate melanocyte differentiation through interactions with MITF.ResultsHere we show that chemical inhibition of BET proteins prevents differentiation of unpigmented melanoblasts into pigmented melanocytes and results in de-pigmentation of differentiated melanocytes. BET inhibition also slowed cell growth, without causing cell death, increasing the number of cells in G1. Transcriptional profiling revealed that BET inhibition resulted in decreased expression of pigment-specific genes, including many MITF targets. The expression of pigment-specific genes was also down-regulated in melanoma cells, but to a lesser extent. We found that RNAi depletion of the BET family members, bromodomain-containing protein 4 (BRD4) and bromodomain-containing protein 2 (BRD2) inhibited expression of two melanin synthesis enzymes, TYR and TYRP1. Both BRD4 and BRD2 were detected on melanocyte promoters surrounding MITF-binding sites, were associated with open chromatin structure, and promoted MITF binding to these sites. Furthermore, BRD4 and BRD2 physically interacted with MITF.ConclusionThese findings indicate a requirement for BET proteins in the regulation of pigmentation and melanocyte differentiation. We identified changes in pigmentation specific gene expression that occur upon BET inhibition in melanoblasts, melanocytes, and melanoma cells.

Bromodomain and Extra Terminal Protein Inhibitors Promote Pancreatic Endocrine Cell Fate.

Bromodomain and extraterminal (BET) proteins are epigenetic readers that interact with acetylated lysines of histone tails. Recent studies have demonstrated their role in cancer progression because they recruit key components of the transcriptional machinery to modulate gene expression. However, their role during embryonic development of the pancreas has never been studied. Using mouse embryonic pancreatic explants and human induced pluripotent stem cells (hiPSCs), we show that BET protein inhibition with I-BET151 or JQ1 enhances the number of neurogenin3 (NEUROG3) endocrine progenitors. In mouse explants, BET protein inhibition further led to increased expression of β-cell markers but in the meantime, strongly downregulated Ins1 expression. Similarly, although acinar markers, such as Cpa1 and CelA, were upregulated, Amy expression was repressed. In hiPSCs, BET inhibitors strongly repressed C-peptide and glucagon during endocrine differentiation. Explants and hiPSCs were then pulsed with BET inhibitors to increase NEUROG3 expression and further chased without inhibitors. Endocrine development was enhanced in explants with higher expression of insulin and maturation markers, such as UCN3 and MAFA. In hiPSCs, the outcome was different because C-peptide expression remained lower than in controls, but ghrelin expression was increased. Altogether, by using two independent models of pancreatic development, we show that BET proteins regulate multiple aspects of pancreatic development.

Suppression of YAP/TAZ-Notch1-NICD axis by bromodomain and extraterminal protein inhibition impairs liver regeneration.

Background and aims: Biological mechanisms that control liver regeneration remain poorly defined. However, these mechanisms are remarkable issues in the clinic that affect management of hepatic loss caused by liver surgery, traumatic injury, chronic infection, or liver poisoning. Increasing evidence has shown that various growth factors, cytokines, and metabolic signaling pathways affect the liver regenerative process. Our aim is to study the effect of bromodomain and extraterminal (BET) protein inhibition on liver regeneration and its mechanism.Methods: We studied the role of BET protein inhibitor, JQ1, in liver regeneration in a mouse model after 70% partial hepatectomy (PH). We evaluated yes-associated protein (YAP)/transcriptional co-activator with PDZ-binding motif (TAZ) and Notch signaling pathways, which were affected by BET protein inhibitor in mouse hepatic tissues and primary hepatocytes in vivo and AML12 cell lines in vitro. We evaluated the relationship of YAP/TAZ and Notch signaling pathway using YAP/TAZ pathway inhibitor in liver regeneration in vivo. Moreover, we analyzed the relationship of YAP/TAZ and Notch signaling pathways via overexpression or RNA silencing of Yap in AML12 cells. Furthermore, we used Yap overexpression mouse model to examine whether it can rescue liver regeneration damage caused by inhibition of BET proteins.Results: In this study, we report that BET protein inhibitor JQ1 molecule impairs the early phase of liver regeneration in a mouse model after 70% PH. Mechanistically, YAP/TAZ and Notch1-NICD pathways were suppressed by BET protein inhibitor in mouse hepatic tissues and primary hepatocytes in vivo and mouse AML12 cell lines in vitro. By using YAP/TAZ pathway inhibitor, we confirmed that the liver regeneration and the activation of Notch pathway were impaired by the inhibition of YAP/TAZ pathway in vivo. Furthermore, the study showed that Yap knockdown by shRNA in normal mouse hepatic cell line downregulated Notch1 signal transduction, whereas Yap overexpression promoted Notch1-NICD signals. Specific overexpression of Yap in mouse liver could rescue the effect of BET protein inhibition on liver regeneration injury.Conclusion: These results revealed the crucial role of the YAP/TAZ-Notch1-NICD axis in liver regeneration. Therefore, BET protein inhibitors must be used in caution in the treatment of hepatic diseases by reason of its suppressive roles in liver regeneration.

Synthetic Lethality of Combinatory Inhibition of BET and CDK7 in BET(i) Resistant Leukemia Cells

Bromodomain and extra terminal proteins (BETs) act as non-oncogene addiction targets1,2. BET inhibitors (BETi) are epigenetic drugs being explored as a promising therapeutic avenue in cancer, especially acute myeloid leukemia (AML)3,4,5. Although early clinical trials have highlighted the efficacy of this first-in-class therapy, chemotherapy resistance, which is commonly emerged during drug treatment, still hurdle the prevalence of BETi6,7.To seek for a strategy to overcome this hurdle, we carried out a combinatorial drug screening and discovered a synthetic lethality via combined inhibition of BRD4 and CDK7. Cyclin-dependent kinase 7 (CDK7), acting as a member of the general transcription factor H (TFIIH), can directly target the carboxyl-terminal domain (CTD) of the Rpb1 subunit of RNAPII for phosphorylation at serine 5 (Ser5) and serine 7 (Ser7), which is critical for transcription initiation. Recently, CDK7 has also been implicated in super-enhancer (SE) regulation8.Dual pharmacological inhibition of BRD4 and CDK7 caused a synergistic effect on the growth and apoptosis of K562 cells. Knockdown of BRD4 and CDK7 in the same cell line showed similar results as drug treatment, thus ruling out the off-target effects of inhibitors used in this study. Furthermore, we confirmed this synergistic effect in vivo using a xenograft mouse model. After 2 weeks of treatment, mice bearing BET-resistant cells showed prolonged lifespan with less leukemic burden and reduced leukemic blast infiltration in spleen, liver, and bone marrow when treated with combinatorial regimen compared with control and single agent groups. Furthermore, RNA-seq analysis showed that genes involved in cell cycle and proliferation were significantly down-regulated, while genes associated with apoptosis were up-regulated. Myc-associated genes were prominently altered, suggesting that c-Myc is one of the top targets of BRD4 and CDK7 inhibition. Ectopically expressing c-Myc partially rescued the synthetic lethality induced by BRD4 and CDK7 treatment, a finding further confirmed that Myc is potential targets of inhibition of BRD4 and CDK7. Further molecular analysis showed that BRD4 and CDK7 inhibition altered the RNAPII activity at actively transcribed genes. Interestingly, we also observed a strong reduction of RNAPII enrichment, but not H3K27Ac enrichment, at super-enhancers (SE). Loci-specific experiment on c-Myc SE regions will further reveal detailed mechanisms on how BRD4 and CDK7 inhibition affect RNAPII activity at SE to influence gene transcription.Congruently, by adopting a combinatorial drug screen approach, we identified the synthetic lethality of BRD4 and CDK7 inhibition in BRDi resistant leukemia cells both in vitro and in vivo. Our findings identify a new strategy that may enhance the clinical utility of BET inhibitors, especially for the treatment of BETi resistance.Refereneces1. Dawson, M. A., Kouzarides, T. & Huntly, B. J. Targeting epigenetic readers in cancer. N. Engl. J. Med. 367, 647-657 (2012)2. Shi, J. & Vakoc, C. R. The mechanisms behind the therapeutic activity of BET bromodomain inhibition. Mol. Cell 54, 728-736 (2014)3. Herait, P. E. et al. BET-bromodomain inhibitor OTX015 shows clinically meaningful activity at nontoxic doses: interim results of an ongoing phase I trial in hematologic malignancies. Cancer Res.74, CT231 (2014)4. Zuber, J. et al. RNAi screen identifies Brd4 as a therapeutic target in acute myeloid leukaemia. Nature 478, 524-528 (2011)5. Delmore, J. E. et al. BET bromodomain inhibition as a therapeutic strategy to target c-Myc. Cell 146, 904-917 (2011)6. Rathert, P. et al. Transcriptional plasticity promotes primary and acquired resistance to BET inhibition. Nature 525, 543-547 (2015)7. Fong, C. Y. et al. BET inhibitor resistance emerges from leukaemia stem cells. Nature 525, 538-542 (2015)8. E. Chipumuro, E. Marco, C.L. Christensen, et al. CDK7 inhibition suppresses super-enhancer-linked oncogenic transcription in MYCN-driven cancer. Cell, 159, 1126-1139 (2014) DisclosuresNo relevant conflicts of interest to declare.

Abstract A17: Bromodomain and extraterminal proteins regulate PD-L1/PD-1 signaling in breast cancer.

Abstract Introduction: Breast tumors are massively infiltrated immune cells of both lymphoid and myeloid origin that exert an anti-tumor pressure to limit cancer progression. Tumor cells can escape immune clearance by expressing several molecules that constitute the immune checkpoints. Antibodies targeting the PD-L1/PD-1 pathway are being evaluated clinically for several cancers and show early success. However, the realization that not all patients respond well to immunotherapy suggests other modalities could be combined to improve efficacy. The bromodomain and extraterminal (BET) proteins BRD2, BRD3 and BRD4 are epigenetically acting co-regulators of transcription, and are critical for cancer proliferation and metastasis. Little is known about the molecular mechanisms that regulate PD-L1 and PD-1 expression. BRD4 is known to bind directly to the PD-L1 promoter; its targeting suppresses PD-L1 expression while increasing CD8+ T cell activity to limit progression in ovarian tumor models. Hypotheses: BET proteins regulate PD-L1 and PD-L2 expression in breast tumors and PD-1 in CD8+ T cells. BET proteins are effectors of proinflammatory cytokine signaling, regulating PD-L1/PD-1 signaling in the breast tumor microenvironment. Methods: We used different cellular models of breast cancer. We also collected human peripheral blood mononuclear cells matched with mammary adipose tissue from mammoplasty patients at Boston Medical Center to assess expression of immune checkpoint proteins in the breast microenvironment. Pan-BET protein inhibition was performed by JQ1 treatment. Alternatively, BET proteins were individually and specifically depleted by siRNA. PD-1 and PD-L1 expression were determined by qRT-PCR and flow cytometry. Cytokines and chemokines present in the blood or secreted by the mammary adipose tissue were profiled by multiplexed antibody capture assay. Results: BET proteins regulate PD-L1 expression by breast cancer cells. BRD2 and BRD4 expression correlate with PD-1 in activated circulating T cells and BET inhibition ablates activation-induced PD-1 expression. Co-culture of breast cancer cells with activated circulating T cells enhances expression of immune checkpoint proteins in both compartments. BET protein inhibition rescues these phenotypes. Conditioned media from mammary adipose tissue increase PD-1 in PBMCs. Peripheral and mammary adipose tissue inflammatory signatures associate with PD-1 expression in PBMCs, suggesting that local inflammation in the breast tumor microenvironment contributes to diminishing anti-tumor immune responses. Conclusion: BET proteins regulate the PD-L1/PD-1 pathway in breast cancer. Next-generation BET protein inhibitors may combine well with PD-1 or PD-L1-targeted immunotherapies in difficult-to-treat cancers. Personalized BET profiles could inform individual patient responses to BET proteins and immune checkpoint inhibitors. Therapeutic approaches that treat the microenvironment should be leveraged to maximize efficacy of checkpoint inhibitor approaches for specific cancers. Citation Format: Guillaume Andrieu, Gerald V. Denis. Bromodomain and extraterminal proteins regulate PD-L1/PD-1 signaling in breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr A17.

Abstract 3936: Bromodomain testis-specific protein BRDT is expressed in a subset of esophageal squamous cell carcinomas and controls expression of differentiation-associated genes

Abstract Esophageal cancer is one of the most malignant cancers, ranking as the sixth leading cause of cancer-related deaths worldwide. The poor survival rate and prognosis highlight the limitations in the biological understanding of esophageal cancer and the urgent need for identification of novel targeted molecular therapies. Recently, large scale genomic analyses have revealed the extensive alternations of epigenetic regulators which may be used as a basis for developing new “epigenetic drugs “. BRDT, bromodomain testis-specific protein, is a member of the bromodomain and extra-terminal (BET) family of epigenetic reader proteins. BET proteins can regulate gene expression by recognizing acetylated lysines, thus playing important roles in both normal development and disease progression. Inhibition of BET proteins has emerged as a potential therapy for many types of cancer. In normal human tissues, unlike the other members of BET family, BRDT is exclusively highly expressed in testes where it drives the meiotic and post-meiotic gene expression to promote spermatogenesis. We have identified BRDT to be expressed in over 20% of esophageal squamous cell carcinoma (ESCC), a predominant subtype of esophageal cancer. Knockdown of BRDT does not affect cell proliferation, but leads to alterations in the expression of differentiation markers. In addition, RNA-seq following BRDT knockdown also supports a role of BRDT in cell differentiation. Depletion of BRDT does not alter the cellular response to BET inhibition. Surprisingly, we also identified BRDT transcripts encoding two truncated isoforms lacking the first bromodomain, which could potentially alter its epigenetic reader function. The genome occupancy profile of BRDT showed a significant enrichment at transcription start site (TSS), which is distinct from BRD4. It also showed potential interactions between BRDT and E2F family and SMAD family. Together, we showed that BRDT is overexpressed in ESCC cell lines and may influence ESCC development or progression in a subset of tumors by regulating cell differentiation-associated genes. Citation Format: Xin Wang, Feda H. Hamdan, Madhobi Sen, Ana P. Kutschat, Steven A. Johnsen. Bromodomain testis-specific protein BRDT is expressed in a subset of esophageal squamous cell carcinomas and controls expression of differentiation-associated genes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3936.

Abstract 3970: Therapeutic targeting of estrogen receptor positive breast cancer with the BET bromodomain inhibitor ODM-207

Abstract Introduction: The bromodomain and extraterminal (BET) family of proteins are chromatin readers that recognize and bind to specific acetylated histones and promote the transcription of several important cell identity genes. BET bromodomain inhibitors have shown promising antitumor activity in a variety of pre-clinical cancer models, as BET inhibition abrogates the transcription of several key oncogenes in a cell type-specific manner. It is known that inhibition of BET proteins effectively inhibits the proliferation of estrogen receptor positive (ER+) breast cancer cells, at least in part through repression of ER and MYC signaling. However, many additional cancer-associated genes are likely to underlie the growth inhibitory effects of BET inhibitors in breast cancer. The purpose of this study was to determine the anticancer activity of the novel BET bromodomain inhibitor ODM-207 in pre-clinical ER+ breast cancer models, and further, to look for cancer-associated signaling pathways suppressed by BET inhibitors. Methodology and results: ODM-207 is a novel, highly selective BET bromodomain inhibitor structurally distinct from JQ1 and its benzodiazepine-related derivatives. In this study, we show that ODM-207 effectively inhibits the proliferation of ER+ breast cancer cell lines when measured by cell viability assays as well as suppresses the growth of patient-derived xenograft tumors. Furthermore, we wanted to investigate the anticancer signaling pathways regulated by ODM-207 as well as the prototypical BET inhibitor JQ1 in breast cancer cells. For this purpose, we performed RNA sequencing on two ER+ breast cancer cell lines after 24h treatment with the aforementioned BET inhibitors. We found that both BET inhibitors targeted several genes and pathways important for breast cancer progression. For example, the targets included CDK4 and CDK6, two cell cycle kinases fundamental for the development and treatment of ER+ breast cancer. The RNA sequencing results were further validated in vitro, and were utilized as a basis for combination therapy assessment. Conclusions: Our results indicate that the novel BET bromodomain inhibitor ODM-207, which is currently in Phase I clinical trials for treating solid tumors, causes significant growth inhibition and cell cycle arrest in pre-clinical models of ER+ breast cancer, and regulates multiple crucial signaling pathways involved in breast cancer cell cycle and survival. Citation Format: Julia Lindqvist, Mari Björkman, Reetta Riikonen, Daniel Nicorici, Elina Mattila, Chandrasekhar Abbineni, Mahaboobi Jaleel, John Eriksson, Pekka Kallio, Anu-Maarit Moilanen. Therapeutic targeting of estrogen receptor positive breast cancer with the BET bromodomain inhibitor ODM-207 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3970.

Bromodomain and Extraterminal (BET) Proteins Regulate Hepatocyte Proliferation in Hepatocyte-Driven Liver Regeneration

Bromodomain and extraterminal (BET) proteins recruit key components of basic transcriptional machinery to promote gene expression. Aberrant expression and mutations in BET genes have been identified in many malignancies. Small molecule inhibitors of BET proteins such as JQ1 have shown efficacy in preclinical cancer models, including affecting growth of hepatocellular carcinoma. BET proteins also regulate cell proliferation in nontumor settings. We recently showed that BET proteins regulate cholangiocyte-driven liver regeneration. Here, we studied the role of BET proteins in hepatocyte-driven liver regeneration in partial hepatectomy (PHx) and acetaminophen-induced liver injury models in mice and zebrafish. JQ1 was injected 2 or 16 hours after PHx in mice to determine effect on hepatic injury, regeneration, and signaling. Mice treated with JQ1 after PHx displayed increased liver injury and a near-complete inhibition of hepatocyte proliferation. Levels of Ccnd1 mRNA and Cyclin D1 protein were reduced in animals injected with JQ1 16 hours after PHx and were even further reduced in animals injected with JQ1 2 hours after PHx. JQ1-treated zebrafish larvae after acetaminophen-induced injury also displayed notably impaired hepatocyte proliferation. In both models, Wnt signaling was prominently suppressed by JQ1. Our results show that BET proteins regulate hepatocyte proliferation-driven liver regeneration, and Wnt signaling is particularly sensitive to BET protein inhibition.

BET Inhibitors as Anticancer Agents: A Patent Review.

Bromodomain and Extra Terminal (BET) family of bromodomain proteins (BRDs), comprised of four members in humans (BRD2, BRD3, BRD4, and BRDT), has emerged as a promising new cancer target class for small-molecule drug discovery. This review discusses the patent literature of BET inhibitors (2010-2017) for the treatment of cancer and other related diseases. BET proteins act as 'epigenetic readers' and bind to acetylated lysine residues on the tails of histones H3 and H4. Inhibition of BET proteins for a wide array of therapeutic applications has led to the discovery and development of various BET inhibitors. The increasing significance of BET inhibitors as a potential anticancer therapeutic has led to an extensive patent activity both from academia and pharmaceutical industry. Several of the BET inhibitors are under clinical development for the treatment of various kinds of cancers. The unmet needs and challenges associated with BET inhibition for cancer treatment have been portrayed in this review. An insight into the current developments and future prospects has been described as well.