Ubiquitin Proteasome System Stress Underlies Synergistic Killing of Ovarian Cancer Cells by Bortezomib and a Novel HDAC6 Inhibitor

Purpose: Elevated metabolic activity of ovarian cancer cells causes increased ubiquitin-proteasome -system (UPS) stress, resulting in their greater sensitivity to the toxic effects of proteasomal inhibition. The proteasomes and a potentially compensatory histone deacetylase 6 (HDAC6)-dependent lysosomal pathway mediate eukaryotic protein turnover. We hypothesized that up-regulation of the HDAC6-dependent lysosomal pathway occurs in response to UPS stress and proteasomal inhibition, and thus, ovarian cancer cell death can be triggered most effectively by coinhibition of both the proteasome- and HDAC6-dependent protein degradation pathways. Experimental Design: To address this hypothesis, we examined HDAC6 expression patterns in normal and cancerous ovarian tissues and used a novel HDAC6-specific inhibitor, NK84, to address HDAC6 function in ovarian cancer. Results: Abnormally high levels of HDAC6 are expressed by ovarian cancer cells in situ and in culture relative to benign epithelium and immortalized ovarian surface epithelium, respectively. Specific HDAC6 inhibition acts in synergy with the proteasome inhibitor Bortezomib (PS-341) to cause selective apoptotic cell death of ovarian cancer cells at doses that do not cause significant toxicity when used individually. Levels of UPS stress regulate the sensitivity of ovarian cancer cells to proteasome/HDAC6 inhibition. Pharmacologic inhibition of HDAC6 also reduces ovarian cancer cell spreading and migration consistent with its known function in regulating microtubule polymerization via deacetylation of alpha-tubulin. Conclusion: Our results suggest the elevation of both the proteasomal and alternate HDAC6-dependent proteolytic pathways in ovarian cancer and the potential of combined inhibition of proteasome and HDAC6 as a therapy for ovarian cancer.