Role of the Super-Enhancer Component Bromodomain Protein 4 in the Radiation Response of Human Head and Neck Squamous Cell Carcinoma Cells.

Radiotherapy is an effective treatment for cancer; however, radioresistant cancer cells result in recurrence. Therefore, elucidating the mechanisms of radioresistance is urgently needed. Super-enhancers (SEs) are clusters of enhancers occupied by a high density of master transcription factors, mediators, and bromodomain protein BRD4. Recently, we reported that ΔNp63, an oncogenic transcription factor, promotes radioresistance in human head and neck squamous cell carcinoma (HNSCC) cells. As ΔNp63 establishes SEs in HNSCC cells, SEs may be involved in radioresistance. Here, we investigated the role of the SE component BRD4 in the radiation responses of HNSCC cells using a BRD4 degrader ARV-771 or BRD4 knockdown. First, Western blotting confirmed that ARV-771 decreased BRD4 protein expression. ARV-771 treatment resulted in reduced cell proliferation and enhanced apoptosis in irradiated HNSCC cells. Moreover, colony formation assays revealed that both ARV-771 and BRD4 knockdown enhanced the radiosensitivity of HNSCC cells, suggesting BRD4 contributes to the radioresistance of HNSCC cells. Furthermore, fluorescence immunostaining revealed distinct localization patterns of γH2AX, a marker of DNA double-strand breaks, compared with BRD4 and ΔNp63 in irradiated cells. Notably, ARV-771 and BRD4 knockdown decreased ΔNp63 and BRD4 protein expression, whereas ΔNp63 knockdown had minimal impact on BRD4 expression. Taken together, these findings suggest that BRD4-dependent maintenance of ΔNp63 expression may contribute, at least in part, to the regulation of radioresistance in HNSCC cells.