Non-epigenetic function of HDAC8 in regulating breast cancer stem cells by maintaining Notch1 protein stability.

// Min-Wu Chao 1, 2 , Po-Chen Chu 1, 3 , Hsiao-Ching Chuang 1 , Fang-Hsiu Shen 3 , Chih-Chien Chou 1 , En-Chi Hsu 1 , Lauren E. Himmel 1, 4 , Han-Li Huang 2 , Huang-Ju Tu 2 , Samuel K. Kulp 1 , Che-Ming Teng 2 , Ching-Shih Chen 1, 3 1 Division of Medicinal Chemistry, College of Pharmacy and Comprehensive Cancer Center, Columbus, Ohio, USA 2 Institute of Pharmacology, College of Medicine, National Taiwan University, Taipei, Taiwan 3 Institute of Biological Chemistry, Academia Sinica, Taipei, Taiwan 4 Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio, USA Correspondence to: Ching-Shih Chen, e-mail: chen.844@osu.edu Keywords: cancer stem cells, breast cancer, histone deacetylase 8, histone deacetylase inhibitors, Notch1 Received: July 28, 2015      Accepted: November 16, 2015      Published: November 28, 2015 ABSTRACT Here, we report a novel non-epigenetic function of histone deacetylase (HDAC) 8 in activating cancer stem cell (CSC)-like properties in breast cancer cells by enhancing the stability of Notch1 protein. The pan-HDAC inhibitors AR-42 and SAHA, and the class I HDAC inhibitor depsipeptide, suppressed mammosphere formation and other CSC markers by reducing Notch1 expression in MDA-MB-231 and SUM-159 cells. Interrogation of individual class I isoforms (HDAC1–3 and 8) using si/shRNA-mediated knockdown, ectopic expression and/or pharmacological inhibition revealed HDAC8 to be the primary mediator of this drug effect. This suppression of Notch1 in response to HDAC8 inhibition was abrogated by the proteasome inhibitor MG132 and siRNA-induced silencing of Fbwx7, indicating Notch1 suppression occurred through proteasomal degradation. However, co-immunoprecipitation analysis indicated that HDAC8 did not form complexes with Notch1 and HDAC inhibition had no effect on Notch1 acetylation. In a xenograft tumor model, the tumorigenicity of breast cancer cells was decreased by HDAC8 knockdown. These findings suggest the therapeutic potential of HDAC8 inhibition to suppress Notch1 signaling in breast cancer.