Abstract #4687: OSU-HDAC42, a novel HDAC inhibitor, exhibits biologic activity against human and canine malignant mast cells through both histone dependent and HSP90 dependent pathways

Epigenetic changes including DNA hypermethylation and histone hypoacetylation are found in many cancers. Evidence suggests that inhibition of histone deacetylation by histone deacetylase inhibitors (HDACis) can modulate some of these epigenetic changes in tumors, resulting in anti-tumor effects. In addition to histones, modification of non-histone targets such as HSP90 and transcription factors by HDACis may also play a role in their anti-cancer activities. Preliminary data from our laboratory and others indicate that HDACis have biologic activity against malignant mast cell disease, although the mechanism of this activity is not completely understood. The purpose of the following studies was to investigate the histone-dependent and histone-independent mechanisms of HDAC inhibition in malignant mast cells in vitro and ex vivo. Human and canine malignant mast cell lines and fresh canine malignant mast cells were treated with OSU-HDAC42 (a novel phenylbutyrate-based HDACi) and 17-AAG (an HSP90 inhibitor) and the effects on cell viability, cell cycling, cell invasion, histones and several signaling pathways were evaluated. OSU-HDAC42 induced hyperacetylation of H3, H4 and \#945;-tubulin and upregulation of p21. OSU-HDAC42 and 17-AAG downregulated the expression and/or phosphorylation of Kit, Akt, Her2/Neu, Stat3, and Stat5. Both drugs disrupted HSP90 chaperone function, as evidenced by dissociation of HSP90 and Kit in malignant mast cells as well as upregulation of HSP70 following treatment. 17-AAG, but not OSU-HDAC42, prevented Stat3 nuclear translocation. Treatment of malignant mast cell lines with OSU-HDAC42 induced loss of cell viability, cell cycle arrest, induction of caspase-3,7 activity and apoptosis that was Bcl-2 and Bcl-X_L independent. OSU-HDAC42 and 17-AAG blocked the invasion of malignant mast cell lines, possibly through downregulation of MMP9 and FAK. Lastly, OSU-HDAC42 induced the activation of caspase-3,7 in fresh canine malignant mast cells ex vivo (n=15) resulting in their apoptosis. As with the mast cell lines, downregulation of Kit and hyperacetylation of H3 and H4 were observed in the fresh malignant mast cells. In conclusion, the HDACi OSU-HDAC42 exhibits biologic activity against human and canine malignant mast cells in vitro and ex vivo through both histone-dependent and HSP90-dependent pathways, representing a promising new therapeutic approach for malignant mast cell disease.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 4687.