Class IIa HDAC inhibition enhances ER stress-mediated cell death in multiple myeloma

Abstract

Histone deacetylase (HDAC) inhibitors have been extensively investigated as therapeutic agents in cancer. However, the biologic role of class IIa HDACs (HDAC4, 5, 7 and 9) in cancer cells, including multiple myeloma (MM), remains unclear. Recent studies show HDAC4 interacts with activating transcription factor 4 (ATF4) and inhibits activation of endoplasmic reticulum (ER) stress associated proapoptotic transcription factor C/EBP homologous protein (CHOP). In this study, we hypothesized HDAC4 knockdown and/or inhibition could enhance apoptosis in MM cells under ER stress condition by upregulating ATF4, followed by CHOP. HDAC4 knockdown showed modest cell growth inhibition; however, it markedly enhanced cytotoxicity induced by either tunicamycin or carfilzomib (CFZ), associated with upregulating ATF4 and CHOP. For pharmacological inhibition of HDAC4, we employed a novel and selective class IIa HDAC inhibitor TMP269, alone and in combination with CFZ. As with HDAC4 knockdown, TMP269 significantly enhanced cytotoxicity induced by CFZ in MM cell lines, upregulating ATF4 and CHOP and inducing apoptosis. Conversely, enhanced cytotoxicity was abrogated by ATF4 knockdown, confirming ATF4 plays a pivotal role mediating cytotoxicity in this setting. These results provide the rationale for novel treatment strategies combining class IIa HDAC inhibitors with ER stressor, including proteasome inhibitors, to improve patient outcome in MM.