Small molecules targeting BRCA1-deficient cancer cells
BRCA1 mutation-carriers frequently develop triple-negative breast cancers that are resistant to cancer therapies. We used our established luciferase-expressing BRCA1-deficient human mammary epithelial-derived cell line series with stable shRNA-mediated BRCA1-knockdown, H-RasG12V-mediated transformation, and TP53-deficiency (PMID: 20038817) to perform a high-throughput-screen using the National Cancer Institute (NCI) chemical library Diversity Set II of 1,440 small molecules and noninvasive bioluminescence imaging. We selected five (NSC667251, NSC636948, NSC21683, NSC281383, and NSC136513) compounds that reproducibly induced cell death in engineered BRCA1-knockdown cells and in BRCA1-deficient human cancer cell lines. In p53-mutant MDA-MB-231 and SKBr3 lines, the compounds induced apoptosis and PARP cleavage by 12 hours. Sub-G1 flow cytometric analysis demonstrated maximal apoptosis induction by 48 hours of drug exposure. p53 protein stabilization and phosphorylation were observed with the selected compounds in BRCA1-deficient wild-type TP53-expressing cells. Drug-treated BRCA1-knockdown MCF10A cells and BRCA1-mutant HCC-1937 breast cancer cells showed gamma-H2AX foci and phospho-H2AX by immunoblotting indicating genotoxicity and DNA damage as contributing factors for apoptosis. NSC636948 was previously characterized as an inhibitory ligand of peroxisome proliferator-activated beta, delta; (PPAR beta, delta) and both NSC636948 and NSC667251 act as inverse agonists of PPAR beta, delta. Kaplan Meier survival analysis of a combined breast cancer cohort (n = 1,959) derived from the METABRIC, demonstrated that patients with high PPARD expression exhibited significantly worse overall survival compared with those with low PPARD expression (log-rank p = 0.008). The selected compounds provide candidate therapeutic leads as well as insights into molecular targets for further evaluation in BRCA1-deficient tumor models and for assessment in combination therapy strategies.