Data from Trastuzumab-Resistant HER2<sup>+</sup> Breast Cancer Cells Retain Sensitivity to Poly (ADP-Ribose) Polymerase (PARP) Inhibition

HER2-targeted therapies, such as trastuzumab, have increased the survival rates of HER2⁺ breast cancer patients. However, despite these therapies, many tumors eventually develop resistance to these therapies. Our lab previously reported an unexpected sensitivity of HER2⁺ breast cancer cells to poly (ADP-ribose) polymerase inhibitors (PARPi), agents that target homologous recombination (HR)–deficient tumors, independent of a DNA repair deficiency. In this study, we investigated whether HER2⁺ trastuzumab-resistant (TR) breast cancer cells were susceptible to PARPi and the mechanism behind PARPi induced cytotoxicity. We demonstrate that the PARPi ABT-888 (veliparib) decreased cell survival in vitro and tumor growth in vivo of HER2⁺ TR breast cancer cells. PARP-1 siRNA confirmed that cytotoxicity was due, in part, to PARP-1 inhibition. Furthermore, PARP-1 silencing had variable effects on the expression of several NF-κB–regulated genes. In particular, silencing PARP-1 inhibited NF-κB activity and reduced p65 binding at the IL8 promoter, which resulted in a decrease in IL8 mRNA and protein expression. Our results provide insight in the potential mechanism by which PARPi induces cytotoxicity in HER2⁺ breast cancer cells and support the testing of PARPi in patients with HER2⁺ breast cancer resistant to trastuzumab. Mol Cancer Ther; 17(5); 921–30. ©2018 AACR.