The DNA damage mark pH2AX differentiates the cytotoxic effects of small molecule HDAC inhibitors in ovarian cancer cells

High grade epithelial ovarian cancers are relatively sensitive to DNA damaging platinum-based chemotherapy, suggesting that the dependencies of ovarian tumors on DNA damage response pathways can be harnessed for therapeutic purposes. Our goal was to determine if the DNA damage mark gamma-H2AX phosphorylation (pH2AX) could be used to identify suitable cytotoxic histone deacetylase inhibitors (HDACi) for ovarian cancer treatment. Nineteen chemically diverse HDACi compounds were tested in 7 ovarian cancer cell lines. Fluorescent, biochemical and cell-based assays were performed to assess DNA damage by induction of pH2AX and to measure cell viability and apoptosis. The relationships between pH2AX and the cellular effects of cell viability and apoptosis were calculated. Selected HDACi were tested in combination with cisplatin and other DNA damaging agents to determine if the HDACi improved upon the effects of the DNA damaging agents. The HDACi compounds induced differing levels of pH2AX expression. High levels of pH2AX in HDACi-treated ovarian cancer cells were tightly associated with decreased cell viability and increased apoptosis. Consequently, a ketone-based HDACi was chosen and found to enhance the effects of cisplatin, even in ovarian cancer cells with extreme resistance to DNA damaging drugs. In conclusion, a fluorescent-based assay for pH2AX can be used to determine cellular responses to HDACi in vitro and may be a useful tool to identify potentially more effective HDACi for the treatment of ovarian cancer. In addition, these results lend support to the inclusion of ketone-derived HDACi compounds for future development.