P53-P21-RB pathway promotes BRD4 degradation in liver cancer through decreasing USP1 transcription

Liver cancer claims over 800,000 human deaths each year. Liver cancer is notoriously refractory to conventional therapeutics. Further insight into the etiology carries promise for innovative diagnostics and therapeutics. Tumor progression is governed by interplay between tumor promoting genes and suppressor genes. BRD4, an acetyl-lysine binding protein, plays a critical role in development and human diseases. In many cancer types, BRD4 is overexpressed and promotes activation of a pro-tumor gene network. But the underlying mechanism for BRD4 overexpression remains elusive. As BRD4 has risen as a promising therapeutic target, to understand the mechanism regulating BRD4 protein level will shed insight into BRD4-targeting therapeutics. In this study, we find BRD4 protein level in liver cancer is significantly regulated by P53, the most frequently dysregulated tumor suppressor. We identify a strong negative correlation between protein levels of P53 and BRD4 in liver cancer. We then show P53 promotes BRD4 protein degradation. Mechanistically, P53 represses the transcription of USP1, a deubiquitinase, through P21-RB. We show USP1 is a deubiquitinase of BRD4, which increases its stability. We show the pro-tumor role of USP1 is partially mediated by BRD4 and the USP1-BRD4 axis upholds expression of a group of cancer-related genes. In summary, we identify a functional P53-P21-RB-USP1-BRD4 axis in liver cancer. To examine the effect on transcriptome by USP1 and BRD4, we introduce tetracycline-inducible shRNA of USP1 and BRD4 respectively into HepG2 cells. Doxycycline was used to induce knockdown.