621 Background: HER2 overexpression in breast cancer confers increased tumor aggressiveness. Targeted therapy against HER2 has improved outcomes, but resistance and disease progression ultimately occurs, thus necessitating novel therapeutic strategies. PARP inhibitors target homologous recombination (HR) deficient tumors, such as the BRCA-associated breast and ovarian cancers. In this study, we report unexpected susceptibility of HER2+ breast cancer cells to PARP inhibition alone independent of an inherent HR deficiency. Methods: Cell viability was measured using colony formation and ATPLite assays. Tumor growth delay was assessed in vivo in mice bearing breast cancer xenografts. Proteins were detected by immunoblotting. HR was assayed using radiation (IR)-induced Rad51 foci or a GFP-based HR assay. NFκB activity was measured using a NFκB-driven luciferase assay. Results: Surprisingly, PARP inhibition with ABT-888 alone reduced the colony forming ability and cell viability of the HER2+ breast cancer cell lines BT474, SKBR3, MDA-MB361, and HCC1954 (~70 – 99% reduction at 10μM). This susceptibility did not correlate with an inherent HR deficit. Interestingly, HER2 overexpression itself may be one mechanism of susceptibility to ABT-888 as evidenced by increased cellular cytotoxicity and in vivo tumor growth delay of MCF7 cells stably expressing HER2. Further dissection of the mechanism revealed that NFκB transcriptional activity was significantly inhibited by ABT-888 (>95%) which corresponded with reduced levels of phosphorylated p65 and total IKKα, and a concomitant increase in IkBα. Furthermore, overexpression of p65 abrogated cellular sensitivity to ABT-888. Conclusions: HER2+ breast cancer cells are highly susceptible to PARP inhibition despite being HR proficient. This may be, in part, due to inhibition of NFκB. Further investigation of whether the addition of PARP inhibition to HER2 targeted therapy will delay the onset of resistance to therapy is warranted to potentially improve outcomes in HER2+ breast cancer patients.
Aptamer-protamine-siRNA nanoparticles in targeted therapy of ErbB3 positive breast cancer cells