Abstract 4707: Inhibition of autophagy overcomes acquired resistance to vorinostat

Abstract

Abstract Histone deacetylase inhibitors (HDACi) have recently emerged as promising anticancer agents. Two HDACi, vorinostat (Zolinza®) and romidepsin (Istodax®), are currently FDA approved for use in cutaneous T-cell lymphoma and several others have advanced into Phase II and III clinical trials, both as single agents and in combination with cytotoxics, for a variety of malignancies. Encouraging results have been obtained in haematological malignancies. However, HDACi target many pathways and the full mechanism responsible for their anti-neoplastic activity is still far from clear. An understanding of the molecular mechanisms underlying resistance to HDACi may help to elucidate their mechanism of action and may be of relevance in an attempt to design more effective combination strategies. The purpose of this study is to understand the molecular alterations associated with resistance to vorinostat (Zolinza®). A vorinostat resistant clone (U937-VR) was derived from U937, a monocytic-like histiocytic lymphoma cell line, using a dose escalation protocol. Vorinostat-resistant cells (U937-VR) are able to grow in 2 µM vorinostat without induction of apoptosis. U937-VR cells are cross-resistant to the cytotoxic effects of similar HDACi's such as LBH589, but not to the structurally different benzamide MGCD0103, as measured by propidium iodide (PI) staining and caspase 3/7 activation. Also, resistance demonstrates a partial reversibility that could be indicative of a non-mutational mechanism of resistance. The LD50 of different chemotherapeutic drugs was evaluated by measuring apoptosis with PI staining. U937 parental and U937-VR cells have equivalent LD50 for the DNA damaging agent cisplatin and for the microtubule stabilizing agent taxol, indicating that the apoptotic machinery is intact in U937-VR cells. Interestingly, the resistant cells exhibit increased sensitivity toward chloroquine, an inhibitor of autophagy and to the proteasome inhibitors MG132 and bortezomib. This increased sensitivity correlates with an elevated accumulation of ubiquitinated proteins in U937-VR. These cells also have increased autophagic flux which can be inhibited by knock down of Beclin-1 or Lamp-2, which ultimately restores sensitivity to vorinostat. Autophagy is also activated in parental U937 cells upon treatment with vorinostat, however, in contrast to the resistant cells, its inhibition decreases sensitivity to vorinostat. We therefore propose that autophagy switches from a proapoptotic to a prosurvival signal through chronic exposure to vorinostat. The key players involved in that switch remain to be defined. These data are of importance in the design of combination strategies using inhibitors of autophagy and HDACi for the treatment of hematological malignancies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4707. doi:1538-7445.AM2012-4707