Targeting HDAC and proteostasis to eradicate quiescent cancer cells

Chemotherapy remains the primary treatment for ovarian cancer (OvCa), and chemoresistance drives patient mortality. Cellular quiescence, reversible exit from the cell cycle, increases chemotherapy resistance as chemotherapies primarily target rapidly proliferating cells. Here, we report that CHD4 and MBD3, components of the nucleosome remodeling and deacetylase (NuRD) complex, are downregulated in quiescent OvCa cells (qOvCa). We find that either CHD4 or MBD3 knockdown or histone deacetylase inhibitors (HDACi), induce quiescence in OvCa cells. RNA-Seq and ATAC-seq analysis of HDACi-treated cells confirmed expression changes consistent with induction of quiescence. Additionally, HDACi-treated cells revealed downregulation of the RHO/RAC pathways. Suggesting downregulation of the RAC pathway could play a role in HDACi-mediated quiescence, RAC inhibitors (RACi) similarly induced quiescence. Both HDACi and RACi resulted in nuclear-to-cytoplasmic shifting of the pro-proliferative transcription factor MRTFA. which has been linked to quiescence. Further analysis of HDACi qOvCa indicated multiple alterations in proteostasis, including increased proteasome activity and autophagy. We find qOvCa cells are dependent on these pathways for survival, such that there is profound synergistic OvCa cell death with HDACi and proteasome- or autophagy-inhibitor combination therapy. Combined, this work supports HDACi as pharmacologic means to induce a quiescent state in OvCa cells and sensitize them to proteostasis-targeting drugs.