BET bromodomain proteins are required for glioblastoma cell proliferation

Abstract

Epigenetic proteins have recently emerged as novel anticancer targets. Among these, bromodomain and extra terminal domain (BET) proteins recognize lysine-acetylated histones, thereby regulating gene expression. Newly described small molecules that inhibit BET proteins BRD2, BRD3, and BRD4 reduce proliferation of NUT (nuclear protein in testis)-midline carcinoma, multiple myeloma, and leukemia cells in vitro and in vivo. These findings prompted us to determine whether BET proteins may be therapeutic targets in the most common primary adult brain tumor, glioblastoma (GBM). We performed NanoString analysis of GBM tumor samples and controls to identify novel therapeutic targets. Several cell proliferation assays of GBM cell lines and stem cells were used to analyze the efficacy of the drug I-BET151 relative to temozolomide (TMZ) or cell cycle inhibitors. Lastly, we performed xenograft experiments to determine the efficacy of I-BET151 in vivo. We demonstrate that BRD2 and BRD4 RNA are significantly overexpressed in GBM, suggesting that BET protein inhibition may be an effective means of reducing GBM cell proliferation. Disruption of BRD4 expression in glioblastoma cells reduced cell cycle progression. Similarly, treatment with the BET protein inhibitor I-BET151 reduced GBM cell proliferation in vitro and in vivo. I-BET151 treatment enriched cells at the G1/S cell cycle transition. Importantly, I-BET151 is as potent at inhibiting GBM cell proliferation as TMZ, the current chemotherapy treatment administered to GBM patients. Since I-BET151 inhibits GBM cell proliferation by arresting cell cycle progression, we propose that BET protein inhibition may be a viable therapeutic option for GBM patients suffering from TMZ resistant tumors.

The Bromodomain protein BRD4 controls HOTAIR, a long noncoding RNA essential for glioblastoma proliferation

“…We previously demonstrated that the BET bromodomain inhibitor I-BET151 reduces cell proliferation in both GBM cell lines and patient-derived cells in vitro and in vivo (4). Therefore, we asked whether I-BET151 treatment reduced HOTAIR expression in GBM cells and found that treatment of LN18 cells with I-BET151 for 24 h reduces the expression of HOTAIR by ∼80% (Fig.…”

“…HOTAIR is a transcript necessary for glioblastoma proliferation, and it appears to be the most relevant lncRNA target of this drug. Our findings invoke a previously unidentified important mechanism of action of BET bromodomain inhibitors, which would in part explain the efficacy of this drug in blocking the proliferation of GBM cells in vivo and in vitro (4,15).…”

Epigenetic modification in chromatin machinery and its deregulation in pediatric brain tumors: Insight into epigenetic therapies

“…126 A number of preclinical studies has been performed in GBM models with the BRD inhibitors JQ1 and I-BET151, showing promising antitumor activity. 135,136 Moreover, OTX015 (MK-8628), a novel BET inhibitor, has been tested recently at the clinical level in dose-finding studies in GBM patients (NCT02296476), and displays in vitro and in vivo antitumor effects alone and in combination with conventional therapies in GBM models. 126 The safety and tolerability of another BET inhibitor, INCB057643, is also currently being tested in subjects with GBM (NCT02711137) ( Table 2).…”

“…BET I-BET151 I-BET151 specifically targets the recognition of acetylated lysine residues of BETs (131) Inhibits GBM cell proliferation (135) wild type for H3, a fifth pediatric glioblastoma line that carries the H3.3 G34V mutation, and isogenic cell lines derived from human astrocytes with and without transgene expression of the H3.3 K27M mutation. 108 Both patient-derived cell lines had global hypomethylation of H3K27 compared with the wild type models.…”

Effect and mechanism of BET bromodomain inhibition in macrophage transcriptional programming

“…In addition to recruitment and functioning around on gene promoters, Brd4 is also recruited to subsets of distal enhancers to promote gene transcription [14] . Various BET small molecule inhibitors successfully block BET protein recruitment and BET-mediated transcriptional activity [15][16][17] , and the function of these inhibitors has been explored in various disease settings including cancer [4,[18][19][20][21] , autoimmune response [4] , osteoporosis [22,23] and atherosclerosis [24] . Here we will focus our discussion on how BET inhibition affects inflammatory responses.…”