Pharmacological targeting of histone H3K27 acetylation/BRD4-dependent induction of ALDH1A3 for early-phase drug tolerance of gastric cancer

Drug-tolerant persister (DTP) cells arise in the early phase of chemotherapy and contribute to tumor relapse. In gastric cancer, we previously identified aldehyde dehydrogenase 1 family member A3 (ALDH1A3) as a DTP signature gene that contributes to survival after chemotherapy, while it remains elusive how the ALDH1A3-overexpressing DTP cells are enriched after chemotherapy and contribute to tumor promotion. Here, we demonstrated that ALDH1A3 was frequently overexpressed in clinical gastric cancer specimen after neoadjuvant chemotherapy and ALDH1A3 overexpression in gastric cancer patient-derived cells (PDCs) promoted tumor growth in vivo. Live imaging revealed that the ALDH1A3-overexpressing DTP cells were induced rather than selected after 5-fluorouracil (5-FU) treatment. We identified global elevation of histone H3 lysine 27 acetylation (H3K27ac) in the promoter regions of DTP cells including ALDH1A3 gene locus. Chemical screening further revealed bromodomain and extra-terminal motif (BET) protein inhibitors as efficient perturbators of the DTP phenotype. The BET inhibitors suppressed 5-FU-induced ADLH1A3 expression and selectively prevented the DTP cell proliferation. Importantly, ALDH1A3 downregulation and growth inhibition by BET inhibitors was rescued by exogenous ALDH1A3 expression. Among BET proteins, BRD4 was preferentially recruited to the ALDH1A3 promoter and promoted its expression in DTP cells. The BET inhibitor, Birabresib/OTX015, significantly suppressed ALDH1A3 expression and potentiated antitumor effect of 5-FU in gastric PDC xenograft model. These data indicate that the global H3K27ac induction and the BRD4-dependent ALDH1A3 induction is essential for gastric cancer drug tolerance. Examination of RNA-sequencing in (1) gastric cancer patient-derived cells (JSC15-3) treated with or without 5-FU for 5 days and (2) JSC15-3 cells were transfected with siNCT(negative control) and siBRD4 and treated with or without 3 µM 5-FU for 5 days.