Perturbation of BRD4 Protein Function by BRD4-NUT Protein Abrogates Cellular Differentiation in NUT Midline Carcinoma

Junpeng Yan,Jason Diaz,Jing Jiao,Ranran Wang,Jianxin You

doi:10.1074/jbc.m111.246975

Junpeng Yan, Jason Diaz + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m111.246975

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Abstract

NUT midline carcinoma (NMC) belongs to a class of highly lethal and poorly differentiated epithelial cancers arising mainly in human midline organs. NMC is caused by the chromosome translocation-mediated fusion of the NUT (nuclear protein in testis) gene on chromosome 15 to a few other genes, most frequently the BRD4 gene on chromosome 19. The mechanism by which the BRD4-NUT fusion product blocks NMC cellular differentiation and contributes to oncogenesis remains elusive. In this study, we show that BRD4-NUT and BRD4 colocalize in discrete nuclear foci that are hyperacetylated but transcriptionally inactive. BRD4-NUT recruits histone acetyltransferases to induce histone hyperacetylation in these chromatin foci, which provide docking sites for accumulation of additional BRD4 and associated P-TEFB (positive transcription elongation factor b) complexes in the transcriptionally inactive BRD4-NUT foci. These molecular events lead to repression of a BRD4·P-TEFB downstream target gene c-fos, a component of activator protein 1 (AP-1), that directly regulates epithelial differentiation. Knockdown of BRD4-NUT in NMC cells disperses the transcriptionally inactive chromatin foci and releases the transcriptional activators to stimulate c-fos expression, leading to restoration of cellular differentiation. Our study provides a novel mechanism by which the BRD4-NUT oncogene perturbs BRD4 functions to block cellular differentiation and to contribute to the oncogenic progression in the highly aggressive NMC.

Highlights

AUGUST 5, 2011 VOLUME 286 NUMBER 31 the incorrect expression of a gene not normally expressed in the affected cells
BRD4-NUT Recruits BRD4 to Speckled Nuclear Foci—The NUT midline carcinoma (NMC) cell line HCC2429 established from a t(15;19) BRD4NUT translocation lung cancer patient was used in this study [7]
Because BRD4 and BRD4-NUT both carry the double bromodomains that can bind to acetylated histones on chromatin, we first investigated if the BRD4-NUT fusion is able to compete with BRD4 for chromatin binding or if both proteins can colocalize on cellular chromatin in HCC2429 cells

Summary

Introduction

AUGUST 5, 2011 VOLUME 286 NUMBER 31 the incorrect expression of a gene not normally expressed in the affected cells. This result suggests that the NUT protein does not have intrinsic HAT activity and that it associates with other HAT enzymes to induce histone hyperacetylation at the BRD4-NUT-specific nuclear foci. Expression of the BRD4 C-terminal domain (spanning BRD4 amino acids 1047–1362), which contains the P-TEFB interacting region and functions as a dominant negative inhibitor to block BRD41⁄7P-TEFB interaction, can disperse P-TEFB nuclear foci in HCC2429 cells (supplemental Fig. S6).

Results

Conclusion