Combining BET and HDAC inhibitors synergistically induces apoptosis of melanoma and suppresses AKT and YAP signaling

// Anja Heinemann 1, 2, 3 , Carleen Cullinane 4 , Ricardo De Paoli-Iseppi 2, 5 , James S. Wilmott 2, 5 , Dilini Gunatilake 1, 2, 3 , Jason Madore 2, 5 , Dario Strbenac 6 , Jean Y. Yang 6 , Kavitha Gowrishankar 1, 2 , Jessamy C. Tiffen 1, 2, 3 , Rab K. Prinjha 7 , Nicholas Smithers 7 , Grant A. McArthur 4 , Peter Hersey 1, 2, 3 , Stuart J. Gallagher 1, 2, 3 1 Melanoma Research Group, Kolling Institute of Medical Research, University of Sydney, St Leonards NSW, Australia 2 Melanoma Institute of Australia, North Sydney, NSW, Australia 3 Melanoma Immunology and Oncology Group, Centenary Institute, University of Sydney, Camperdown, NSW, Australia 4 Translational Research Laboratory, Peter MacCallum Cancer Centre, Melbourne, VIC, Australia 5 Sydney Medical School, University of Sydney, Sydney, NSW, Australia 6 School of Mathematics and Statistics, University of Sydney, Sydney NSW, Australia 7 Epinova Discovery Performance Unit, GlaxoSmithKline, Stevenage, United Kingdom Correspondence to: Stuart J. Gallagher, e-mail: stuart.gallagher@sydney.edu.au Keywords: panobinostat, I-BET151, melanoma, bromodomain, epigenetic Received: February 17, 2015      Accepted: May 23, 2015      Published: June 05, 2015 ABSTRACT Histone acetylation marks have an important role in controlling gene expression and are removed by histone deacetylases (HDACs). These marks are read by bromodomain and extra-terminal (BET) proteins and novel inhibitiors of these proteins are currently in clinical development. Inhibitors of HDAC and BET proteins have individually been shown to cause apoptosis and reduce growth of melanoma cells. Here we show that combining the HDAC inhibitor LBH589 and BET inhibitor I-BET151 synergistically induce apoptosis of melanoma cells but not of melanocytes. Induction of apoptosis proceeded through the mitochondrial pathway, was caspase dependent and involved upregulation of the BH3 pro-apoptotic protein BIM. Analysis of signal pathways in melanoma cell lines resistant to BRAF inhibitors revealed that treatment with the combination strongly downregulated anti-apoptotic proteins and proteins in the AKT and Hippo/YAP signaling pathways. Xenograft studies showed that the combination of inhibitors was more effective than single drug treatment and confirmed upregulation of BIM and downregulation of XIAP as seen in vitro . These results support the combination of these two classes of epigenetic regulators in treatment of melanoma including those resistant to BRAF inhibitors.