Treatment with Histone Deacetylase 6-Specific Inhibitor WT-161 Disrupts hsp90 Function, Abrogates Aggresome Formation and Sensitizes Human Mantle Cell Lymphoma Cells to Lethal ER Stress Induced by Proteasome Inhibitor Carfilzomib

Abstract 2856

The proteasome inhibitor bortezpmib has been shown to markedly increase the intracellular levels of misfolded proteins, induce aggresome formation and cause endoplasmic reticulum (ER) stress, resulting in apoptosis of human Mantle Cell Lymphoma (MCL) cells. Consistent with this, Bortezomib displays clinical efficacy in patients with relapsed and refractory MCL. We have recently reported that the pan-histone deacetylase (HDAC) inhibitor panobinostat, by also inhibiting HDAC6, abrogates aggresome formation and induces Endoplasmic Stress (ER) stress, as well as potentiates bortezomib-induced apoptosis of MCL cells. Here, we determined the anti-MCL cell activity of an HDAC6-specific inhibitor, WT-161 alone and in combination with the novel, orally bio-available, proteasome inhibitor carfilzomib (Proteolix Inc.) against human, cultured and primary, patient-derived MCL cells. Treatment with WT-161 (0.1 to 1.0 uM) resulted in a dose-dependent increase in the acetylation of alpha-tubulin and heat shock protein (hsp) 90, without any appreciable increase in the levels of acetylated histone (H) 3. Consistent with WT-161 mediated hyperacetylation and inhibition of hsp90 chaperone function, treatment with WT-161 increased the intracellular levels of polyubiuitylated proteins in the cultured MCL JeKo-1 and Z138 cells. WT-161 was also noted to dose-dependently deplete the levels of cyclin D1 in the cultured MCL cells. Treatment with WT-161 also induced ER stress response in the MCL cells, demonstrated by increase in the protein levels of Glucose regulated protein (GRP) 78, phosphorylated eIF2 (eukaryotic initation factor 2) α, and induction of the pro-apoptotic transcription factor CHOP (CAAT/Enhancer Binding Protein Homologous Protein). We next determined the effects of co-treatment with WT-161 on carfilzomib-induced aggresome formation, ER stress response and apoptosis of the cultured and primary MCL cells. Co-treatment with WT-161 (0.25 uM) abrogated carfilzomib-induced aggresome formation in MCL cells, as evidenced by confocal immunofluorescent staining of aggresomes with anti-HDAC6 and anti-ubiquitin antibodies. Compared to each agent alone, co-treatment with WT-161 and carfilzomib induced more intracellular polyubiquitylated proteins and induced higher levels of CHOP in the cultured MCL cells. Co-treatment with WT-161 and carfilzomib also synergistically induced apoptosis of the cultured MCL cells (combination indices < 1.0). Notably, co-treatment with WT-161 and carfilzomib also synergistically induced apoptosis of primary MCL cells (combination indices < 1.0). These findings strongly support the in vivo testing of the combination of an HDAC6-specific inhibitor such as WT-161 with the proteasome inhibitor carfilzomib against human MCL cells.