Abstract 673: Targeting LSD1-HDACs crosstalk as a potential therapeutic strategy for triple negative breast cancer cells.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Triple negative breast cancer (TNBC) is the most aggressive type of breast tumor and one of the major clinical hurdles encountered in the development of specific treatment for this disease is lack of effective targeted therapy. Our recent studies demonstrate that lysine-specific demethylase 1 (LSD1) and histone deacetylases (HDACs) closely interact in controlling gene expression and growth of TNBC cells. However the underlying mechanisms are largely unknown. Here we show that knockdown of LSD1 expression (LSD1-KD) via RNA interference (RNAi) inhibits mRNA levels of most HDAC isozymes in a TNBC cell line, MDA-MB-231. HDAC5-targeted RNAi induces the most significant accumulation of H3K4me2, a specific substrate of LSD1, implicating a necessity for HDAC5 in LSD1 function. Combined treatment with LSD1 inhibitor, pargyline, and HDAC inhibitor, vorinostat, leads to superior growth inhibition and apoptosis in TNBC cells, but exhibits additive or antagonistic effect on growth inhibition in non-TNBC counterparts as well as non-tumorigenic mammary epithelial cells. Moreover, LSD1-KD augments vorinostat-induced re-expression of a subset of aberrantly silenced genes such as NR4A1, PCDH1, RGS16, BIK, and E-Cadherin whose re-expression may be tumor suppressive in TNBC cells, but not in other breast cancer subtypes. The re-expression of these genes is associated with increased H3K4me2 and acetyl-H3K9 marks at gene promoters, indicative of active chromatin. Integrative analysis of a panel of 36 human breast cancer cell lines of distinct subtypes shows that the expression of these genes is more profoundly suppressed in TNBC cells compared with other breast cancer subtypes. Genome-wide microarray study in MDA-MB-231 cells identifies a group of tumor suppressor genes (CDKN1A, CDKN1C, CRABP2, TNFAIP3, TP53TG1 and ING1) whose expression is induced by vorinostat and significantly enhanced by LSD1-KD. We also show that concurrent depletion of RGS16 by siRNA reduces overall cytotoxicity of vorinostat and blocks the re-expression of E-Cadherin, CDKN1C and ING1 in LSD1 deficient MDA-MB-231 cells. Furthermore, co-treatment with RGS16 siRNA reverses the down-regulation of NF-KB expression induced by combined inhibition of LSD1 and HDACs, suggesting a crucial role of RGS16 in controlling key apoptotic pathways in response to combination therapy in TNBC cells. Taken together, these results provide novel mechanistic insight into how LSD1 and HDACs modulate each other to enhance therapeutic efficacy of vorinostat in TNBC; and also suggest that inhibition of LSD1-HDACs axis-mediated histone modifications can be a potential therapeutic strategy to improve treatment of TNBC. This work is funded by: Breast Cancer Research Foundation, UPCI CCSG Shared Facilities & the Samuel and Emma Winters Foundation. Citation Format: Shauna Vasilatos, Tiffany A. Katz, Steffi Oesterreich, Nancy E. Davidson, Yi Huang. Targeting LSD1-HDACs crosstalk as a potential therapeutic strategy for triple negative breast cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 673. doi:10.1158/1538-7445.AM2013-673