Investigation of synergistic drug combinations involving HDAC6 inhibition and anti-cancer agents in triple negative breast cancer

Histone deacetylase 6 (HDAC6) is a large modular protein consisting of several domains including two tandem deacetylase domains and a ubiquitin binding domain (BUZ). Unlike other HDAC proteins which reside in the nucleus and deacetylate histones, HDAC6 locates in the cytoplasm and deacetylates cytoplasmic proteins. The role of HDAC6 in many cell signalling pathways prompted its development as a therapeutic target in various diseases including cancer. Substantial effort has been invested in the development of compounds that inhibit the catalytic activity of HDAC6 and are undergoing clinical trials. However, recent research has revealed a functional tolerance to HDAC6 inhibition in solid tumours. Therefore, in this study we undertook to investigate the synergistic effects of HDAC6 inhibition with anti-cancer therapies in triple negative breast cancer (TNBC). In silico analysis identified a statistically significant corelation between HDAC6 gene expression and growth inhibitory IC50 values of several anti-cancer drugs. Drug responses of the TNBC cell line, MDA-MB-231, which stably overexpressed wildtype full length (FL), catalytically dead (DD) or the BUZ domain truncated (BUZ) HDAC6 were analysed by MTT cell viability assay. Of the drugs tested, responses to olaparib, paclitaxel, or cisplatin were not affected by DD or BUZ overexpression. The TNBC cell line, Cal-51, demonstrated greater sensitivity to the combination of HDAC6 inhibition and cisplatin which was potentiated by HDAC6 degradation revealing differential responses to HDAC6 inhibitory therapy in TNBC. The overexpression of DD and BUZ HDAC6 sensitised cells to HSP90 inhibition and doxorubicin. Of note, unlike DD HDAC6, the overexpression of the BUZ truncated HDAC6 is catalytically active and diminished acetylated tubulin at similar levels to the overexpression of wildtype FL HDAC6. Ongoing research will further elucidate the underlying mechanism of action of drug synergies involving HDAC6 inhibition in TNBC.