Abstract
Patients with neuroblastoma associated with MYCN oncogene amplification experience a very poor prognosis. BET bromodomain inhibitors are among the most promising novel anticancer agents as they block BRD3 and BRD4 from activating oncogene transcription. However, treatment with BET bromodomain inhibitors alone does not result in cancer remission in many murine models. MYCN-amplified neuroblastoma cells were treated with vehicle control, the BET bromodomain inhibitor JQ1, the histone deacetylase inhibitor panobinostat, or the combination of JQ1 and panobinostat. Genes modulated by JQ1, panobinostat, or the combination therapy were identified by Affymetrix microarray, and cell proliferation and apoptosis were examined by Alamar blue assays and flow cytometry analysis. Modulation of LIN28B promoter activity by BRD3 and BRD4 was examined by chromatin immunoprecipitation and luciferase assays. In addition, neuroblastoma-bearing mice were treated with vehicle control, JQ1, and/or panobinostat. LIN28B was one of the top genes synergistically reduced by JQ1 and panobinostat. BRD3 and BRD4 directly bound to the LIN28B gene promoter and activated LIN28B gene transcription, and knocking down LIN28B reduced the expression of N-Myc protein, but not N-Myc mRNA. JQ1 and panobinostat synergistically reduced LIN28B gene and N-Myc protein expression, and synergistically induced growth inhibition and apoptosis in neuroblastoma cells, but not normal nonmalignant cells in vitro In neuroblastoma-bearing mice, JQ1 and panobinostat synergistically and considerably reduced N-Myc protein expression in tumor tissues and blocked tumor progression. Our findings have identified a novel strategy to reduce the N-Myc oncoprotein expression and a novel therapeutic approach for the treatment of aggressive neuroblastoma. Clin Cancer Res; 22(10); 2534-44. ©2016 AACR.
Highlights
MYCN oncogene amplification occurs in approximately 25% to 33% of neuroblastoma cases, and results in MYCN gene and NMyc protein overexpression [1, 2]
BRD3 and BRD4 directly bound to the LIN28B gene promoter and activated LIN28B gene transcription, and knocking down LIN28B reduced the expression of N-Myc protein, but not N-Myc mRNA
We examined whether BRD3 regulated MYCN gene expression, and whether BRD3 and BRD4 cooperatively modulated MYCN gene expression
Summary
MYCN oncogene amplification occurs in approximately 25% to 33% of neuroblastoma cases, and results in MYCN gene and NMyc protein overexpression [1, 2]. The BET bromodomain proteins, such as BRD3 and BRD4, bind to acetylated lysine residues on histone proteins as chromatin readers, and play essential roles in the transcription of oncogenes such as C-MYC, MYCN, BCL2, and FOSL1 [3,4,5,6,7,8]. Small-molecule BET bromodomain inhibitors, such as JQ1 and I-BET151, competitively bind to acetyl–lysine recognition pockets, displace BET bromodomain proteins from chromatin, and reduce the expression of oncogenes, leading to cancer cell growth inhibition and apoptosis. Treatment with BET bromodomain inhibitors alone does not cause cancer remission in neuroblastoma-bearing mice
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