Histone Deacetylase Inhibitors Induce Apoptosis In Multiple Tumor Types Through Induction Of Atf3

Histone deacetylase inhibitors (HDACi) are approved for the treatment of cutaneous T-Cell lymphoma (CTCL) and Multiple Myeloma. In colon cancer cells HDACi-induced apoptosis is linked to induction of a specific transcriptional response involving upregulation of the immediate-early (IE) response genes FOS, JUN, EGR1, EGR3 and ATF3. To determine if this transcriptional response underpins HDACi-induced apoptosis across multiple tumour types, including CTCL and MM, 50 cell lines derived from common solid and haematological cancers were screened for sensitivity to HDACi-induced apoptosis, and response correlated with IE gene induction. HDACi treatment robustly induced IE gene mRNA and protein expression in multiple tumour lines. IE gene induction was sustained over 24 hours and dependent on the Sp1/Sp3 transcription factors and was also observed in vivo in CTCL patients treated with the HDACi panobinostat. The magnitude of induction of FOS, JUN and ATF3 expression correlated significantly with HDACi-induced apoptosis. The induction of ATF3 was critical for HDACi induced apoptosis as ATF3 downregulation markedly attenuated HDACi-induced apoptosis in multiple tumour lines, and ATF3 knockout MEFs were refractory to HDACi-induced apoptosis. To identify downstream targets of ATF3, the magnitude of ATF3 induction following HDACi treatment was correlated with altered expression of pro and anti-apoptotic genes involved in the intrinsic apoptotic pathway. The magnitude of ATF3 induction correlated inversely with repression of the anti-apoptotic gene Bcl-XL, and ATF3 knockdown attenuated HDACi-mediated Bcl-XL repression. Notably, combinatorial treatment of refractory cell lines with an HDACi and a Bcl-XL inhibitor significantly enhanced HDACi-induced apoptosis. This study demonstrates that IE gene induction is a universal feature of HDACi-induced apoptosis, and that ATF3 as a key driver of HDACi-induced apoptosis through repression of Bcl-XL. These findings establish a method for rapid assessment of HDACi response, and identify a combination strategy for enhancing the activity of these compounds. Citation Format: Anderly C. Chueh, Janson Tse, Paul Ioannidis, Lars Togel, Bee S. Tan, Mercedes Davlos-Salas, Rebecca Nightingale, Matthew R. Thompson, Bryan Williams, Michael Dickinson, Amardeep S. Dhillon, John M. Mariadason. Histone deacetylase inhibitors induce apoptosis in multiple tumor types through induction of ATF3. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4728.