BRD4 is a key regulator of STING expression

The bromodomain and extra-terminal (BET) protein family, especially the bromodomain-containing protein 4 (BRD4), drives tumor progression. BET inhibitors (BETi) have shown therapeutic potential, although challenges remain. Exploring the downstream targets of BET inhibition is crucial for advancing precision cancer therapies. This study aimed to explore the downstream molecular targets of BETi using proteomics and to provide mechanistic insights that could support the development of precision cancer therapies. Reverse phase protein array (RPPA) and western blotting (WB) were performed to investigate the downstream molecular targets of BRD4 inhibitors (BRD4i). Chromatin immunoprecipitation (ChIP) sequencing in the GSE63581 dataset and quantitative reverse transcription polymerase chain reaction (RT-qPCR) were used to assess transcriptional regulation following BRD4i treatment. Methylation analyses of stimulator of interferon genes (STING) in the CCLE and TCGA datasets, WB, and chromatin immunoprecipitation-quantitative polymerase chain reaction (ChIP-qPCR) were performed to explore the epigenetic impacts after BRD4i treatment. In vivo and in vitro immune responses were evaluated using type I interferon assays, cytokine measurements, and syngeneic mouse models. BET inhibition significantly suppressed STING expression by targeting its promoter and enhancer regions, thereby reducing downstream innate immune signaling. DNA methylation was associated with altered BRD4 binding at STING regulatory regions. This suppression weakened the immune response triggered by poly (ADP-ribose) polymerase inhibitors (PARPi) through STING downregulation. Our findings revealed a complex, context-dependent regulation of tumor immunity by BETi, characterized by the suppression of STING-mediated responses. These results suggest that BETi may dampen immune activation induced by PARPi by decreasing STING expression. Optimizing therapeutic strategies for the combined use of BETi and PARPi are crucial to balance their effects and maximize clinical benefits.