Abstract S3-7: Treatment with Histone Deacetylase Inhibitors Creates ‘BRCAness’ and Sensitizes Human Triple Negative Breast Cancer Cells to PARP Inhibitors and Cisplatin
Authors: Bhalla KN , Rao R , Sharma P , Das Gupta S , Chauhan L , Stecklein S , Fiskus W 
Published: August 2014

Abstract
DNA damage induces DNA damage response (DDR), which regulates cell cycle transit, DNA repair and apoptosis. During DDR, stability and activity of ATR and CHK1 is essential for the cell cycle arrest and subsequent DNA repair through homologous recombination (HR) in which BRCA1 protein is involved. We have previously reported that ATR, CHK1 and BRCA1 are hsp90 client proteins and treatment with hsp90 inhibitor sensitizes cancer cells to DNA damage. We have also previously shown that treatment with pan-HDAC inhibitors, including vorinostat (VS) and panobinostat (PS), induces hyperacetylation of hsp90, thereby inhibiting its chaperone function. In the present studies we determined that treatment with VS or PS induced hyper-acetylation and inhibition of chaperone function of nuclear hsp90, leading to proteasomal degradation and depletion of ATR, CHK1 and BRCA1. This led to inhibition of DDR and DNA repair following ionizing radiation (IR) through destabilization of ATR-CHK1 and BRCA1 proteins. Based on this, we hypothesized that treatment with VS or PS would create ‘BRCAness’ in breast cancer cells. Specific siRNA-mediated knockdown of HDAC3 but not of HDAC1 or HDAC2 also induced hyper-acetylation of the nuclear hsp90 and depletion of ATR and CHK1, indicating that among the class I HDACs, HDAC3 is the deacetylase for the nuclear hsp90. We next determined whether, by depleting DDR proteins and BRCA1 and inducing ‘BRCAness’, treatment with VS would sensitize the triple negative breast cancer (TNBC) SUM59PT, MB-231 and HCC1937 cells to the PARP inhibitor ABT888 (veliparib). Indeed, combined treatment with VS or PS with ABT888 (10 to 20 μM) for 48 hours synergistically induced apoptosis (Combination indices by isobologram analysis being &lt; 1.0) of TNBC cells with (HCC1937) or without BRCA1 mutation (MB-231 and SUM159PT cells). As compared to treatment with each agent alone, co-treatment with VS and ABT888 also induced significantly more DNA strand breaks, as demonstrated by higher γ-H2AX levels. Combined treatment also induced markedly greater tail moment, as determined by the Comet assay that evaluates the SYBR green-stained DNA tails by fluorescent microscopy. Co-treatment with VS and ABT888 also induced more BH3 domain-only pro-death protein BIM, while knock-down of BIM by shRNA significantly reduced the apoptosis induced by co-treatment with VS and ABT888. It is noteworthy that treatment with VS also sensitized the TNBC cells to cisplatin (2.0 to 10 μM)-induced apoptosis. Moreover, co-treatment with VS and ciplatin synergistically induced apoptosis of TNBC cells (CIs &lt; 1.0). These findings indicate that treatment with pan-HDAC inhibitors VS or PS creates BRCAness, and in combination with a PARP inhibitor or cisplatin synergistically induces apoptosis in human TNBC cells. These findings support a compelling rationale to confirm whether the combination of VS or PS with ABT888 and cisplatin would be a highly active treatment in the in vivo models of human TNBC cells, irrespective of their expression of mutant BRCA1.
Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr S3-7.