Molecular and biological analysis of histone deacetylase inhibitors with diverse specificities

Histone deacetylase inhibitors (HDACi) are anticancer agents that induce hyperacetylation of histones, resulting in chromatin remodeling and transcriptional changes. In addition, nonhistone proteins, such as the chaperone protein Hsp90, are functionally regulated through hyperacetylation mediated by HDACis. Histone acetylation is thought to be primarily regulated by HDACs 1, 2, and 3, whereas the acetylation of Hsp90 has been proposed to be specifically regulated through HDAC6. We compared the molecular and biologic effects induced by an HDACi with broad HDAC specificity (vorinostat) with agents that predominantly inhibited selected class I HDACs (MRLB-223 and romidepsin). MRLB-223, a potent inhibitor of HDACs 1 and 2, killed tumor cells using the same apoptotic pathways as the HDAC 1, 2, 3, 6, and 8 inhibitor vorinostat. However, vorinostat induced histone hyperacetylation and killed tumor cells more rapidly than MRLB-223 and had greater therapeutic efficacy in vivo. FDCP-1 cells dependent on the Hsp90 client protein Bcr-Abl for survival, were killed by all HDACis tested, concomitant with caspase-dependent degradation of Bcr-Abl. These studies provide evidence that inhibition of HDAC6 and degradation of Bcr-Abl following hyperacetylation of Hsp90 is likely not a major mechanism of action of HDACis as had been previously posited

Epigenetic activation of plasmacytoid DC drives IFNAR-dependent therapeutic differentiation of AML

Pharmacologic inhibition of epigenetic enzymes can have therapeutic benefit against hematologic malignancies. In addition to affecting tumor cell growth and proliferation, these epigenetic agents may induce antitumor immunity. Here, we discovered a novel immunoregulatory mechanism through inhibition of histone deacetylases (HDAC). In models of acute myeloid leukemia (AML), leukemia cell differentiation and therapeutic benefit mediated by the HDAC inhibitor (HDACi) panobinostat required activation of the type I interferon (IFN) pathway. Plasmacytoid dendritic cells (pDC) produced type I IFN after panobinostat treatment, through transcriptional activation of IFN genes concomitant with increased H3K27 acetylation at these loci. Depletion of pDCs abrogated panobinostat-mediated induction of type I IFN signaling in leukemia cells and impaired therapeutic efficacy, whereas combined treatment with panobinostat and IFNα improved outcomes in preclinical models. These discoveries offer a new therapeutic approach for AML and demonstrate that epigenetic rewiring of pDCs enhances antitumor immunity, opening the possibility of exploiting this approach for immunotherapies.Jessica M. Salmon, Izabela Todorovski, Kym L. Stanley, Claudia Bruedigam, Conor J. Kearney, Luciano G. Martelotto, Fernando Rossello, Timothy Semple, Gisela Mir Arnau, Magnus Zethoven, Michael Bots, Stefan Bjelosevic, Leonie A. Cluse, Peter J. Fraser, Veronique Litalien, Eva Vidacs, Kate McArthur, Antony Y. Matthews, Elise Gressier, Nicole A. de Weerd, Jens Lichte, Madison J. Kelly, Simon J. Hogg, Paul J. Hertzog, Lev M. Kats, Stephin J. Vervoort, Daniel D. De Carvalho, Stefanie Scheu, Sammy Bedoui, Benjamin T. Kile, Steven W. Lane, Andrew C. Perkins, Andrew H. Wei, Pilar M. Dominguez, and Ricky W. Johnston