Figure 5.

Enhanced synthetic lethality effect by the combination of bromodomain and extraterminal motif (BET) and HDAC inhibition in BRCA1 deficient breast cancer cells. (A)–(D) The effect of single or combination of the inhibitors of BET (OTX-015) and HDAC (entinostat) on ROS induction (A), DNA damage (B (Scale bar = 100 μm) and C), and cell viability (D) (Scale bar = 400 μm) in HCC1937 BRCA1 isogenic cells is shown. (E) Enhanced apoptosis induction in HCC1937 BRCA1^−/− cells by the combination of BET and HDAC inhibitors is shown. Western blots of cleaved caspase-3 and PARP1 were used as apoptosis indicators. (F) The synthetic lethality effect of the combination of BET and HDAC inhibitors in a panel of breast cancer cells with different BRCA1 status was tested. The cells were treated with single or combination of 0.25 × concentration of IC₅₀ of OTX-015 and entinostat for 72 h and alamarblue assay was performed to assess cell viability. The combination of other concentrations (4×, 2×, 1×, 0.5× IC₅₀) is presented in Supporting Information Fig. S2. (G)–(I) BRCA1 deficient breast cancer cells, including HCC1937 (G), SUM-149 (H) and MDA-MB-436 (I) were treated with the drug alone or combination of entinostat and OTX-015 at various concentrations for 72 h and the combination indices (CI) were determined with CompuSyn software. Data are mean ± SD, **P < 0.01 between two groups.