Combination of pan-histone deacetylase inhibitor and autophagy inhibitor exerts superior efficacy against triple-negative human breast cancer cells

Histone deacetylase (HDAC) inhibitors (HDI) induce endoplasmic reticulum (ER) stress and apoptosis, while promoting autophagy, which promotes cancer cell survival when apoptosis is compromised. Here, we determined the in vitro and in vivo activity of the combination of the pan-HDI panobinostat and the autophagy inhibitor chloroquine against human estrogen/progesterone receptor and HER2 (triple)-negative breast cancer (TNBC) cells. Treatment of MB-231 and SUM159PT cells with panobinostat disrupted the hsp90/histone deacetylase 6/HSF1/p97 complex, resulting in the upregulation of hsp. This was accompanied by the induction of enhanced autophagic flux as evidenced by increased expression of LC3B-II and the degradation of the autophagic substrate p62. Treatment with panobinostat also induced the accumulation and colocalization of p62 with LC3B-II in cytosolic foci as evidenced by immunofluorescent confocal microscopy. Inhibition of panobinostat-induced autophagic flux by chloroquine markedly induced the accumulation of polyubiquitylated proteins and p62, caused synergistic cell death of MB-231 and SUM159PT cells, and inhibited mammosphere formation in MB-231 cells, compared with treatment with each agent alone. Finally, in mouse mammary fat pad xenografts of MB-231 cells, a tumor size-dependent induction of heat shock response, ER stress and autophagy were observed. Cotreatment with panobinostat and chloroquine resulted in reduced tumor burden and increased the survival of MB-231 breast cancer xenografts. Collectively, our findings show that cotreatment with an autophagy inhibitor and pan-HDI, for example, chloroquine and panobinostat results in accumulation of toxic polyubiquitylated proteins, exerts superior inhibitory effects on TNBC cell growth, and increases the survival of TNBC xenografts.