242. BRD4 INHIBITION ENHANCES THE RADIOSENSITIVITY OF ESOPHAGEAL SQUAMOUS CELL CARCINOMA THROUGH REGULATING ATF3-MEDIATED SERINE AND NUCLEOTIDE SYNTHESIS

Background Radioresistance is a major culprit for radiotherapy failure in esophageal squamous cell carcinoma (ESCC). This study aimed to investigate the underlying mechanism of Brd4 in radiosensitivity of ESCC. Methods Brd2/3/4 proteins were assessed in radiosensitive and radioresistant ESCC tissues using IHC. A serial of functional experiments was performed to verify the significance of Brd4 in ESCC. RNA-seq and bioinformatics analyses were used to determine the potential downstream targets. The dual-luciferase reporter and ChIP assay were further examined the underlying regulatory mechanism among targets. Besides, we further verified the importance of ATF3-mediated serine and nucleotide metabolism in radiated ESCC cells. Results Brd4 is highly expressed in radio-resistant ESCC tissue. Knockdown of Brd4 led to increased DNA damage and cell apoptosis in irradiated ESCC cells. RNA-seq analyses exhibited that ATF3 was a potential downstream target of Brd4. The dual-luciferase reporter and ChIP assay demonstrated that Brd4 upregulated ATF3 expression via activation its promoter region. Besides, we found that ATF3 could facilitate the enzyme activities involved in serine and nucleotide biosynthesis pathway to promote radiation-induced DNA damage repair. Conclusion Brd4 facilitates ATF3 expression via binding to and activating ATF3 promoter region. Enhanced ATF3 further increases crucial enzymes activity in serine and nucleotide biosynthesis pathway to promote radiation-induced DNA damage repair. Targeting Brd4 is a promising treatment strategy to improve radiosensitivity in ESCC.