Abstract 5469: Trastuzumab resistant HER2+ breast cancer cells retain sensitivity to poly (ADP-ribose) polymerase (PARP) inhibition

Background: Overexpression of HER2 in breast cancer is associated with aggressive tumor features and poor patient survival. Current therapeutic agents targeting this receptor have increased the survival rate of patients with HER2+ breast cancer; however some patients eventually develop resistance to these therapies. We have previously reported that HER2+ breast cancers are susceptible to poly (ADP-Ribose) polymerase inhibitors (PARPi) alone independent of a basal or induced DNA repair deficiency but instead via suppression of NF-κB signaling. In this study, we investigated PARPi sensitivity in HER2+ breast cancer cells that are resistant to the HER2 targeted agent trastuzumab. Materials and Methods: HER2+ breast cancer cell lines BT-474, UACC812, SKBRR3 and their trastuzumab resistant counterparts were used in this study. Cells were treated with vehicle or 10µM ABT-888 or transfected with scrambled or PARP1 siRNA. NF-κB transcriptional activity was measured using a NF-κB-driven luciferase reporter assay. Protein expression was measured by Western blot analysis. Cellular cytotoxicity and viability following PARPi was assessed with colony formation and ATPlite assays, respectively. Results: HER2+ parent and trastuzumab resistant breast cancer cells showed similar susceptibility to the PARPi ABT-888. There was a dose response reduction in survival fraction (greater than 70% at 10μM) and a greater than 40% decrease in cell viability with 10μM ABT-888. This cytotoxicity was associated with more than 50% attenuation of NF-κB transcriptional activity and reduced expression of the NF-κB activator IKKα. Suppression of NF-κB signaling was also observed utilizing PARP1 siRNA. Conclusions: HER2+ breast cancer cells resistant to trastuzumab continue to be sensitive to PARP inhibition through attenuation of the NF-κB signaling pathway. These results support the use of PARPi as part of a therapeutic strategy for patients with HER2+ breast cancer. Citation Format: Monica E. Wielgos, Tiffiny Cooper, Andres Forero, James A. Bonner, Francisco J. Esteva, C K. Osborne, Rachel Schiff, Albert F. LoBuglio, Eddy S. Yang. Trastuzumab resistant HER2+ breast cancer cells retain sensitivity to poly (ADP-ribose) polymerase (PARP) inhibition. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5469. doi:10.1158/1538-7445.AM2014-5469