1063 Background: Poly (ADP-ribose) polymerase (PARP) inhibitors have emerged as promising targeted therapies for BRCA-mutated cancers by blocking repair of DNA double-strand breaks. However, resistance to PARP inhibitors have been described in some patients lowering overall response rates. The mechanisms underlying PARP inhibitor (PARPi) resistance are an area of active investigation. Methods: PARPi adaptive resistant clones (MDA-MB-468, MDA-MB-231, HCC1806) were generated in triple-negative breast cancer cell lines. Through morphologic observation and functional analysis, we evaluated epithelial-mesenchymal transition (EMT) and changes in immune checkpoint programmed death-ligand 1 (PD-L1). We also downregulated the expression of PD-L1 by shRNA to study the role of PD-L1 in PARPi resistance. We evaluated the immunology sensitivity to cytotoxic T cell upon PD-L1 change using a murine ex-vivo CD8+ T cell killing assay and a comparison of total killing cells percentage per well. Results: We demonstrated that inhibition of PARP enhances EMT, which induces phosphorylation of Akt at S473. This in turn upregulates the expression of PD-L1 by 2-3 fold in triple-negative breast cancer cells. In addition, PARPi–induced EMT occurred independent of PD-L1 upregulation in triple-negative breast cancer cells. Metformin administration (10µM) was found to reverse EMT by blocking the p-Akt S473 axis through activation of AMPK, resulting in downregulation of PD-L1 expression and sensitizing PARPi-resistant cancer cells to T cell killing. Conclusions: In summary, we identified that induction of EMT is a new mechanism for PARP inhibitor resistance. Metformin was able to reverse EMT and therefore a combination of metformin and PARP inhibitors may be a promising therapeutic strategy to increase the efficacy of PARP inhibitors and tumor sensitivity to T cells.
PI3K Inhibitor Increased Radiosensitizing Effect of PARP Inhibitor in BRCA-Proficient Triple Negative Breast Cancer Cells
