Abstract A15: HDAC6 inhibitor C1A abrogates the recruitment of the autophagic machinery and synergizes with proteasome, src kinase, and PI3K-mTOR inhibition.
Authors: Kaliszczak Maciej , Pardo Olivier E. , Seckl Michael J. , Aboagye Eric O. 
Published: August 2014

Abstract
The histone deacetylase (HDAC) family of proteins plays a profound role in transcriptional silencing and intracellular protein function, and HDAC inhibitors have shown promise for the treatment of human malignancies. HDAC6 activity is altered in tumorigenesis and its selective inhibition is likely to cause less toxicity compared to broad spectrum inhibitors. An important function of HDAC6 is the processing of misfolded proteins within cells during autophagy and formation of aggresomes. Prosurvival autophagy may represent a major impediment to successful cancer therapy and more potent and specific inhibitors of autophagy are needed. This study evaluated whether targeting HDAC6 could be used to impair autophagy and restore chemosensitivity. Treatment of colon cancer HCT-116 cells and osteosarcoma U2OS-LC3-GFP cells with HDAC6 inhibitor C1A for 24 hours (or tubastatin A, another HDAC6 inhibitor) caused a dose dependent increase in expression of lipid associated LC3 II, suggesting autophagosome formation or blockage. Using a pH-sensitive, double tagged mCherry-GFP-LC3 reporter, we could further demonstrate impairment of autophagosome-lysosome fusion. We hypothesized that the effect of autophagy tool compounds will be modulated by C1A. Increased LC3 II/p62 expression induced by chloroquine, 3-MA, bortezomib or BEZ-235 were inhibited by C1A (also seen with tubastatin A). Combination of C1A with bortezomib or BEZ-235 induced a greater proportion of cells undergoing apoptosis as depicted by an increase of caspase 3/7 activity in HCT-116 cells. Treatment with C1A (or tubastatin A) also impaired the formation of LC3 II following treatment with src inhibitor, dasatinib (or PP2, another src inhibitor but not PP3, a negative control compound); synergy as assessed by caspase 3/7 activity was seen in lung cancer A549 cells. Based on combination index values, we demonstrated that C1A synergistically enhanced proteasome, src kinase and PI3K-mTOR growth inhibition. In vivo, the anti-tumor activity of C1A in combination with bortezomib or BEZ-235 was confirmed in HCT-116 tumor bearing mice. Tumor growth was significantly delayed in animals treated with combination therapy. A similar observation was seen in A549 tumor bearing mice treated with C1A in combination with dasatinib. The efficacy of the combination C1A and BEZ-235 was also predicted as early as 48 h with [18F]fluorothymidine positron emission tomography ([18F]FLT-PET) in HCT-116 tumor bearing mice. These results provide a rationale for combination therapy with HDAC6 inhibitors and chemotherapy associated with pro-survival autophagy in the treatment of solid tumors. Imaging of cell proliferation with [18F]FLT-PET detected the mechanistic validity of this combination treatment approach and, thus, warrants further investigation in pre-clinical models and patients.
This work was supported in part by Cancer Research U.K.-Engineering and Physical Sciences Research Council Grant C2536/A10337.
Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A15.
Citation Format: Maciej Kaliszczak, Olivier E. Pardo, Michael J. Seckl, Eric O. Aboagye. HDAC6 inhibitor C1A abrogates the recruitment of the autophagic machinery and synergizes with proteasome, src kinase, and PI3K-mTOR inhibition. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr A15.