Targeting homologous recombination using imatinib results in enhanced tumor cell chemosensitivity and radiosensitivity

Abstract

RAD51 is a key protein in the homologous recombination (HR) pathway of DNA double-strand break repair, and HR represents a novel target for cancer therapy. Because imatinib (Gleevec) has been reported to reduce RAD51 protein levels, we tested the clonogenic survival for RT112, H1299, PANC1, and PC3 tumor cell lines of varying p53 status and normal GM05757 normal fibroblasts after exposure to single agent imatinib (0 -20 Mmol/ L; 0 -72 hours). We also combined imatinib with DNA damaging agents that are toxic to RAD51-deficient cells, including ionizing radiation, gemcitabine, and mitomycin C. We observed decreased nuclear expression and chromatin binding of RAD51 protein following imatinib treatment. Imatinib also resulted in decreased error-free HR as determined by a flow cytometry -based integrated direct repeat-green fusion protein reporter system; this correlated to reduced RAD51 expression. Clonogenic survival experiments revealed increased cell kill for imatinib-treated cells in combination with ionizing radiation, gemcitabine, and mitomycin C, due in part to mitotic catastrophe. In experiments using imatinib and gemcitabine, tumor cell lines were sensitized to a greater extent than normal fibroblasts. This preservation of the therapeutic ratio was confirmed in vivo using PC3 xenograft growth delay and intestinal crypt cell clonogenic assays. HR inhibition may be an additional mechanism of action for the chemosensitization and radiosensitization of solid tumors with imatinib with preservation of the therapeutic ratio.

Gene expression and mutation-guided synthetic lethality eradicates proliferating and quiescent leukemia cells

“…For example, BCR-ABL1 kinase inhibits BRCA1 and DNA-PKcs protein expression by suppression of translation and/or enhanced protein degradation (16,17). While expression of these proteins is partially restored in imatinib-treated cells, other key members of HR (RAD51) and D-NHEJ (LIG4, DCLRE1C, NHEJ1) are strongly downregulated by imatinib, thus promoting PARP1i-mediated synthetic lethality (38,39). The BRCA2-deficient phenotype of AML1-ETO-positive cells may depend on lack of stimulation of HOXA9 (22).…”

Synergistic Loss of Prostate Cancer Cell Viability by Coinhibition of HDAC and PARP

“…Indeed, stable cell lines of LNCaP engineered to have lower levels RAD51 demonstrated increased radiosensitivity (42). In addition, downregulation of RAD51 by imatinib, a receptor tyrosine kinase inhibitor, sensitized PC-3 cells to ionizing radiation, mitomycin C, and gemcitabine (43). RAD51 has been reported to be overexpressed in high-grade prostate cancer (Gleason score >7), both in sporadic and those with germline BRCA mutations (44).…”

Tumor Cell Kill by c-MYC Depletion: Role of MYC-Regulated Genes that Control DNA Double-Strand Break Repair

“…For all assays, cells were fixed and stained in 1% methylene blue in 50% ethanol 10 to 15 days later. Surviving clones with >50 cells were scored under a light microscope, and resulting survival was calculated as previously described (27,28). For apoptosis assays, H1299 cells (or Rat-1/myc controls) were treated with 0.5 μg/mL MMC for 1 hour or with 2 or 10 Gy IR at 48 hours after siRNA transfection and assessed for sub-G 1 populations by flow cytometry or nuclear apoptotic bodies, as previously described (21).…”