Abstract 4226: Histone deacetylase inhibitors sensitize cancer stem cells to PARP inhibitors in triple-negative breast cancer
Authors: Liu Yajing , Martin-Trevino Rachel , Shang Li , Davis April , Wicha Max , Liu Suling , Burness Monika 
Published: August 2015

Abstract
Background: Triple negative breast cancers (TNBCs) have a poor prognosis and are a therapeutic challenge due to resistance to multiple chemotherapy drugs. Growing evidence suggests that cancer stem cells (CSCs) may be promising therapeutic targets for treating TNBCs. Poly ADP ribose polymerase (PARP) inhibitors have shown striking activity in preclinical models of BRCA-deficient breast carcinomas. However they are less efficacious in tumors without germline BRCA mutations, which account for more than 80% of all TNBCs. Our data demonstrates that treatment of TNBC with PARP inhibitor increases CSCs, likely due to an elevated homologous recombination (HR) repair mechanism. Several small studies have suggested that histone deacetylase (HDAC) inhibitors, which decrease HR repair proteins, may sensitize non-BRCA-mutated TNBC to PARP inhibitors. Currently, little is known whether this synergistic effect can reverse the increase in CSCs caused by PARP inhibition. In this study, our aim was to investigate if HDAC inhibitors could sensitize CSCs to PARP inhibitors in TNBCs.
Results: Using four TNBC cell lines, we confirmed the synthetic lethality of HDAC and PARP inhibitors (Vorinostat and Olaparib respectively) in vitro. CompuSyn analysis revealed Combination Indices of 0.66, 0.44, 0.47 and 0.16 in SUM159, MDAMB231, SUM149 and HCC1937 cells respectively at ED50. An ALDEFLUOR assay revealed ∼1.9 fold increase in the CSC population after PARP inhibition in BRCA-mutant SUM149 and HCC1937 cells, while the absolute CSC number, (total cell number times percentage of ALDEFLUOR-positive cells) remained unchanged. Whereas in BRCA-wild type TNBC cell lines SUM159 and MDAMB231, there was no change in CSCs. Addition of the HDAC inhibitor decreased absolute CSC number by 85%, 78% and 40% in SUM149, SUM159 and HCC1937 cells respectively after 7 days of PARP inhibition. Although the mechanism is not fully understood at present, we demonstrated that Rad51, a key player in HR repair, mediates the sensitivity of TNBCs to PARP inhibition, and HDAC inhibitor prevents the formation of Rad51 foci at DNA damage site. This may indicate that HDAC inhibition sensitizes the CSCs of TNBCs to PARP inhibition via suppressing the HR pathway.
Conclusion: Our data suggests that PARP inhibition only targets the bulk cancer cells in BRCA-mutant TNBC cells while sparing CSCs and has no effect on BRCA WT cells. HDAC inhibition sensitized CSCs of TNBC to PARP inhibition regardless of BRCA status, possibly via interfering with Rad51 function. The inhibitors we used in the present study are either FDA-approved or at Phase III clinical trials, and the combination is predicted to have low toxicity. Thus, our findings are promising and could be rapidly translated into clinical use.
Citation Format: Yajing Liu, Rachel Martin-Trevino, Li Shang, April Davis, Max Wicha, Suling Liu, Monika Burness. Histone deacetylase inhibitors sensitize cancer stem cells to PARP inhibitors in triple-negative breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4226. doi:10.1158/1538-7445.AM2015-4226