Abstract
Homologous recombination (HR)-directed DNA double-strand break (DSB) repair enables template-directed DNA repair to maintain genomic stability. RAD51 recombinase (RAD51) is a critical component of HR and facilitates DNA strand exchange in DSB repair. We report here that treating triple-negative breast cancer (TNBC) cells with the fatty acid nitroalkene 10-nitro-octadec-9-enoic acid (OA-NO(2)) in combination with the antineoplastic DNA-damaging agents doxorubicin, cisplatin, olaparib, and γ-irradiation (IR) enhances the antiproliferative effects of these agents. OA-NO(2) inhibited IR-induced RAD51 foci formation and enhanced H2A histone family member X (H2AX) phosphorylation in TNBC cells. Analyses of fluorescent DSB reporter activity with both static-flow cytometry and kinetic live-cell studies enabling temporal resolution of recombination revealed that OA-NO(2) inhibits HR and not nonhomologous end joining (NHEJ). OA-NO(2) alkylated Cys-319 in RAD51, and this alkylation depended on the Michael acceptor properties of OA-NO(2) because nonnitrated and saturated nonelectrophilic analogs of OA-NO(2), octadecanoic acid and 10-nitro-octadecanoic acid, did not react with Cys-319. Of note, OA-NO(2) alkylation of RAD51 inhibited its binding to ssDNA. RAD51 Cys-319 resides within the SH3-binding site of ABL proto-oncogene 1, nonreceptor tyrosine kinase (ABL1), so we investigated the effect of OA-NO(2)-mediated Cys-319 alkylation on ABL1 binding and found that OA-NO(2) inhibits RAD51-ABL1 complex formation both in vitro and in cell-based immunoprecipitation assays. The inhibition of the RAD51-ABL1 complex also suppressed downstream RAD51 Tyr-315 phosphorylation. In conclusion, RAD51 Cys-319 is a functionally significant site for adduction of soft electrophiles such as OA-NO(2) and suggests further investigation of lipid electrophile-based combinational therapies for TNBC.