Role of BRD4 in hematopoietic differentiation of embryonic stem cells

Emerging roles of bromodomain protein 4 in regulation of stem cell identity

Understanding the mechanism of fate decision and lineage commitment is the key step for developing novel stem cell applications in therapeutics. This process is coordinately regulated through systematic epigenetic reprogramming and concomitant changes in the transcriptional landscape of the stem cells. One of the bromo- and extra-terminal domain (BET) family member proteins, bromodomain protein 4 (BRD4), performs the role of epigenetic reader and modulates gene expression by recruiting other transcription factors and directly regulating RNA polymerase II elongation. Controlled gene regulation is the critical step in maintenance of stem cell potency and dysregulation may lead to tumor formation. As a key transcriptional factor and epigenetic regulator, BRD4 contributes to stem cell maintenance in several ways. Being a druggable target, BRD4 is an attractive candidate for exploiting its potential in stem cell therapeutics. Therefore, it is crucial to elucidate how BRD4, through its interplay with pluripotency transcriptional regulators, control lineage commitment in stem cells. Here, we systemically review the role of BRD4 in complex gene regulatory network during three specific states of stem cell transitions: cell differentiation, cell reprogramming and transdifferentiation. A thorough understanding of BRD4 mediated epigenetic regulation in the maintenance of stem cell potency will be helpful to strategically control stem cell fates in regenerative medicine.

Design, synthesis and biological evaluation of imidazolopyridone derivatives as novel BRD4 inhibitors

Bromodomain containing protein 4 (BRD4) has been demonstrated to play critical roles in cellular proliferation and cell cycle progression. In this study, using the BRD4 inhibitor Fragment 9 as a lead compound, a series of imidazolopyridone derivatives were designed and tested for their inhibitory activity against BRD4 protein in vitro. Among them, HB100-A7 showed excellent BRD4(1) inhibitory activities with an IC₅₀ value of 0.035 μM in amplified luminescent proximity homogeneous assay (Alphascreen). The result of MTT assay showed that HB100-A7 could suppress the proliferation of pancreatic cancer cells. In addition, flow cytometry further illustrated that HB100-A7 treatment resulted in G0/G1 phase arrest and promoted apoptosis of BxPc3 cells. Furthermore, the in vivo study found that HB100-A7 displayed significant tumor growth inhibition in a pancreatic mouse tumor model (Panc-02). Moreover, IHC staining suggested that HB100-A7 induce cell apoptosis in pancreatic cancer tumor tissue. Together, this study revealed, for the first time, HB100-A7 is a promising lead compound for further development as a new generation of small molecule inhibitors targeting the BRD4 protein.

Isoform-specific involvement of Brpf1 in expansion of adult hematopoietic stem and progenitor cells

Bromodomain-containing proteins are known readers of histone acetylation that regulate chromatin structure and transcription. Although the functions of bromodomain-containing proteins in development, homeostasis, and disease states have been well studied, their role in self-renewal of hematopoietic stem and progenitor cells (HSPCs) remains poorly understood. Here, we performed a chemical screen using nine bromodomain inhibitors and found that the bromodomain and PHD finger-containing protein 1 (Brpf1) inhibitor OF-1 enhanced the expansion of Lin-Sca-1+c-Kit+ HSPCs ex vivo without skewing their lineage differentiation potential. Importantly, our results also revealed distinct functions of Brpf1 isoforms in HSPCs. Brpf1b promoted the expansion of HSPCs. By contrast, Brpf1a is the most abundant isoform in adult HSPCs but enhanced HSPC quiescence and decreased the HSPC expansion. Furthermore, inhibition of Brpf1a by OF-1 promoted histone acetylation and chromatin accessibility leading to increased expression of self-renewal-related genes (e.g. Mn1). The phenotypes produced by OF-1 treatment can be rescued by suppression of Mn1 in HSPCs. Our findings demonstrate that this novel bromodomain inhibitor OF-1 can promote the clinical application of HSPCs in transplantation.

Inhibition of BET selectively eliminates undifferentiated pluripotent stem cells

Embryonic stem cells (ESCs) maintain their cellular identity through the systematic regulation of master transcription factors and chromatin remodeling complexes. Recent work has shown that the unusually large-scale enhancers-namely super-enhancers (SEs), on which BRD4, a member of the bromodomain and extraterminal domain (BET) family is highly enriched-could regulate pluripotency-related transcription factors. Moreover, inhibition of BRD4 binding on SEs has been shown to induce the differentiation of ESCs. However, the underlying mechanism of BRD4 inhibition-mediated stem cell differentiation remains elusive. Here we show that both mouse and human ESCs lose their capacity for self-renewal upon treatment with JQ1, a selective inhibitor of BET family including BRD4, with rapid suppression of pluripotency-associated genes. Notably, a high concentration of JQ1 could selectively eliminate ESCs via apoptosis, without affecting the functionality of differentiated somatic cells from ESCs, suggesting that inhibition of BET may have a beneficial effect on the development of pluripotent stem cell-based cell therapy.

Translational Modeling of Drug-Induced Myelosuppression and Effect of Pretreatment Myelosuppression for AZD5153, a Selective BRD4 Inhibitor

In this work, we evaluate the potential risk of thrombocytopenia in man for a BRD4 inhibitor, AZD5153, based on the platelet count decreases from a Han Wistar rat study. The effects in rat were modeled and used to make clinical predictions for human populations with healthy baseline blood counts. At doses >10 mg, a dose-dependent effect on circulating platelets is expected, with similar predicted changes for both q.d. and b.i.d. dose schedules. These results suggest that at predicted efficacious doses, AZD5153 is likely to have some reductions in the clinical platelet counts, but within the normal range at projected efficacious doses. The model was then extended to incorporate preexisting myelosuppression where bone marrow function is inhibited by acute myeloid leukemia. Under these conditions, duration of platelet count recovery has the potential to be prolonged due to drug-induced myelosuppression.

The Essential Transcriptional Function of BRD4 in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is often initiated by genetic alterations of machineries that regulate chromatin and transcription, thereby blocking cell differentiation. Such mechanisms may also render leukemia cells vulnerable to perturbations of transcriptional regulators, which includes small molecules targeting the coactivator protein BRD4. Numerous studies have validated BRD4 as a therapeutic target in diverse subtypes of AML; however, the vital function of BRD4 in this disease is only beginning to be understood. Here we discuss the recent progress in elucidating the transcriptional function of BRD4 in AML cells, with an emphasis on the desirable attributes, but also the inherent limitations, of targeting general coactivator proteins as cancer therapy.

BRD4 is associated with raccoon polyomavirus genome and mediates viral gene transcription and maintenance of a stem cell state in neuroglial tumour cells

Polyomavirus infection often results in persistence of the viral genome with little or no virion production. However, infection of certain cell types can result in high viral gene transcription and either cytolysis or neoplastic transformation. While infection by polyomavirus is common in humans and many animals, major questions regarding viral persistence of most polyomaviruses remain unanswered. Specifically, identification of target cells for viral infection and the mechanisms polyomaviruses employ to maintain viral genomes within cells are important not only in ascribing causality to polyomaviruses in disease, but in understanding specific mechanisms by which they cause disease. Here, we characterize the cell of origin in raccoon polyomavirus (RacPyV)-associated neuroglial brain tumours as a neural stem cell. Moreover, we identify an association between the viral genome and the host cell bromodomain protein, BRD4, which is involved in numerous cellular functions, including cell cycle progression, differentiation of stem cells, tethering of persistent DNA viruses, and regulation of viral and host-cell gene transcription. We demonstrate that inhibition of BRD4 by the small molecule inhibitors (+)-JQ1 and IBET-151 (GSK1210151A) results in reduced RacPyV genome within cells in vitro, as well as significant reduction of viral gene transcripts LT and VP1, highlighting its importance in both maintenance of the viral genome and in driving oncogenic transformation by RacPyV. This work implicates BRD4 as a central protein involved in RacPyV neuroglial tumour cell proliferation and in the maintenance of a stem cell state.

Bromodomain 4: a cellular Swiss army knife

Bromodomain protein 4 (BRD4) is a transcriptional and epigenetic regulator that plays a pivotal role in cancer and inflammatory diseases. BRD4 binds and stays associated with chromatin during mitosis, bookmarking early G1 genes and reactivating transcription after mitotic silencing. BRD4 plays an important role in transcription, both as a passive scaffold via its recruitment of vital transcription factors and as an active kinase that phosphorylates RNA polymerase II, directly and indirectly regulating transcription. Through its HAT activity, BRD4 contributes to the maintenance of chromatin structure and nucleosome clearance. This review summarizes the known functions of BRD4 and proposes a model in which BRD4 actively coordinates chromatin structure and transcription.

Inhibition of Bromodomain and Extraterminal Domain Family Proteins Ameliorates Experimental Renal Damage

Renal inflammation has a key role in the onset and progression of immune- and nonimmune-mediated renal diseases. Therefore, the search for novel anti-inflammatory pharmacologic targets is of great interest in renal pathology. JQ1, a small molecule inhibitor of bromodomain and extraterminal (BET) proteins, was previously found to preserve renal function in experimental polycystic kidney disease. We report here that JQ1-induced BET inhibition modulated the in vitro expression of genes involved in several biologic processes, including inflammation and immune responses. Gene silencing of BRD4, an important BET protein, and chromatin immunoprecipitation assays showed that JQ1 alters the direct association of BRD4 with acetylated histone-packaged promoters and reduces the transcription of proinflammatory genes (IL-6, CCL-2, and CCL-5). In vivo, JQ1 abrogated experimental renal inflammation in murine models of unilateral ureteral obstruction, antimembrane basal GN, and infusion of Angiotensin II. Notably, JQ1 downregulated the expression of several genes controlled by the NF-κB pathway, a key inflammatory signaling pathway. The RelA NF-κB subunit is activated by acetylation of lysine 310. In damaged kidneys and cytokine-stimulated renal cells, JQ1 reduced the nuclear levels of RelA NF-κB. Additionally, JQ1 dampened the activation of the Th17 immune response in experimental renal damage. Our results show that inhibition of BET proteins reduces renal inflammation by several mechanisms: chromatin remodeling in promoter regions of specific genes, blockade of NF-κB pathway activation, and modulation of the Th17 immune response. These results suggest that inhibitors of BET proteins could have important therapeutic applications in inflammatory renal diseases.

Bromodomain Proteins Contribute to Maintenance of Bloodstream Form Stage Identity in the African Trypanosome

Trypanosoma brucei, the causative agent of African sleeping sickness, is transmitted to its mammalian host by the tsetse. In the fly, the parasite's surface is covered with invariant procyclin, while in the mammal it resides extracellularly in its bloodstream form (BF) and is densely covered with highly immunogenic Variant Surface Glycoprotein (VSG). In the BF, the parasite varies this highly immunogenic surface VSG using a repertoire of ~2500 distinct VSG genes. Recent reports in mammalian systems point to a role for histone acetyl-lysine recognizing bromodomain proteins in the maintenance of stem cell fate, leading us to hypothesize that bromodomain proteins may maintain the BF cell fate in trypanosomes. Using small-molecule inhibitors and genetic mutants for individual bromodomain proteins, we performed RNA-seq experiments that revealed changes in the transcriptome similar to those seen in cells differentiating from the BF to the insect stage. This was recapitulated at the protein level by the appearance of insect-stage proteins on the cell surface. Furthermore, bromodomain inhibition disrupts two major BF-specific immune evasion mechanisms that trypanosomes harness to evade mammalian host antibody responses. First, monoallelic expression of the antigenically varied VSG is disrupted. Second, rapid internalization of antibodies bound to VSG on the surface of the trypanosome is blocked. Thus, our studies reveal a role for trypanosome bromodomain proteins in maintaining bloodstream stage identity and immune evasion. Importantly, bromodomain inhibition leads to a decrease in virulence in a mouse model of infection, establishing these proteins as potential therapeutic drug targets for trypanosomiasis. Our 1.25Å resolution crystal structure of a trypanosome bromodomain in complex with I-BET151 reveals a novel binding mode of the inhibitor, which serves as a promising starting point for rational drug design.

Regulation of tumor cell plasticity by the androgen receptor in prostate cancer

Prostate cancer (PCa) has become the most common form of cancer in men in the developed world, and it ranks second in cancer-related deaths. Men that succumb to PCa have a disease that is resistant to hormonal therapies that suppress androgen receptor (AR) signaling, which plays a central role in tumor development and progression. Although AR continues to be a clinically relevant therapeutic target in PCa, selection pressures imposed by androgen-deprivation therapies promote the emergence of heterogeneous cell populations within tumors that dictate the severity of disease. This cellular plasticity, which is induced by androgen deprivation, is the focus of this review. More specifically, we address the emergence of cancer stem-like cells, epithelial-mesenchymal or myeloid plasticity, and neuroendocrine transdifferentiation as well as evidence that demonstrates how each is regulated by the AR. Importantly, because all of these cell phenotypes are associated with aggressive PCa, we examine novel therapeutic approaches for targeting therapy-induced cellular plasticity as a way of preventing PCa progression.

BRD4 promotes tumor growth and epithelial-mesenchymal transition in hepatocellular carcinoma

Bromodomain and extraterminal domain (BET) proteins are epigenetic readers that play an important role in chromatin remodeling and transcriptional regulation. In this study, we found that BRD4, a BET family member, is significantly upregulated in hepatocellular carcinoma (HCC) tissues compared with adjacent normal tissues. Furthermore, the overexpression of BRD4 in cancer tissues was correlated with poor prognosis in HCC patients. Using shRNA-mediated knockdown of BRD4 or lentivirus-mediated overexpression of BRD4 in HCC cells, we further showed that BRD4 was involved in HCC cell growth and invasion in vitro. Forced expression of BRD4 was sufficient to induce epithelial-mesenchymal transition (EMT) phenotypes in HCC cells. Additionally, BRD4 shRNA significantly inhibited HCC cell proliferation in vivo. Collectively, our study confirmed that BRD4 expression is a valuable predictor of recurrence and survival in patients with HCC. BRD4 can be further used as a potential therapeutic target of HCC.

KDM1 histone lysine demethylases as targets for treatments of oncological and neurodegenerative disease

Histone methylation and demethylation are important processes associated with the regulation of gene transcription, and alterations in histone methylation status have been linked to a large number of human diseases. Initially thought to be an irreversible process, histone methylation is now known to be reversed by two families of proteins containing over 30 members that act to remove methyl groups from specific lysine residues present in the tails of histone H3 and histone H4. A rapidly growing number of reports have implicated the FAD-dependent lysine specific demethylase (KDM1) family in cancer, and several small-molecule inhibitors are in development for the treatment of cancer. An additional role has emerged for KDM1 in brain function, offering additional opportunities for the development of novel therapeutic strategies in neurodegenerative disease. A decade after the identification of KDM1A as a histone demethylase, the first selective inhibitors have now reached the clinic.