Our previous studies demonstrated that lysine-specific demethylase 1 (LSD1) and histone deacetylases (HDACs) closely interact in controlling growth of breast cancer cells. However, the underlying mechanisms are largely unknown. In this study, we showed that knockdown of LSD1 expression (LSD1-KD) by RNAi decreased mRNA levels of HDAC isozymes in triple-negative breast cancer (TNBC) cells. Small interfering RNA (siRNA)-mediated depletion of HDAC5 expression induced the most significant accumulation of H3K4me2, a specific substrate of LSD1. Combined treatment with LSD1 inhibitor, pargyline, and HDAC inhibitor, SAHA (Vorinostat), led to superior growth inhibition and apoptotic death in TNBC cells, but exhibited additive or antagonistic effect on growth inhibition in non-TNBC counterparts or non-tumorigenic breast cells. Additionally, LSD1-KD enhanced SAHA-induced reexpression of a subset of aberrantly silenced genes, such as NR4A1, PCDH1, RGS16, BIK, and E-cadherin whose reexpression may be tumor suppressive. Genome-wide microarray study in MDA-MB-231 cells identified a group of tumor suppressor genes whose expression was induced by SAHA and significantly enhanced by LSD1-KD. We also showed that concurrent depletion of RGS16 by siRNA reduced overall cytotoxicity of SAHA and blocked the reexpression of E-cadherin, CDKN1C and ING1 in LSD1-deficient MDA-MB-231 cells. Furthermore, cotreatment with RGS16 siRNA reversed the downregulation of nuclear factor-kappaB expression induced by combined inhibition of LSD1 and HDACs, suggesting a crucial role of RGS16 in controlling key pathways of cell death in response to combination therapy. Taken together, these results provide novel mechanistic insight into the breast cancer subtype-dependent role of LSD1 in mediating HDAC activity and therapeutic efficacy of HDAC inhibitor.

Insulin-like growth factor 1 (IGF1) signaling regulates breast cancer initiation and progression and associated cancer phenotypes. We previously identified E-cadherin () as a repressor of IGF1 signaling and in this study examined how loss of E-cadherin affects IGF1R signaling and response to anti-IGF1R/insulin receptor (InsR) therapies in breast cancer. Breast cancer cell lines were used to assess how altered E-cadherin levels regulate IGF1R signaling and response to two anti-IGF1R/InsR therapies. proximity ligation assay (PLA) was used to define interaction between IGF1R and E-cadherin. TCGA RNA-seq and RPPA data were used to compare IGF1R/InsR activation in estrogen receptor-positive (ER+) invasive lobular carcinoma (ILC) and invasive ductal carcinoma (IDC) tumors. ER+ ILC cell lines and xenograft tumor explant cultures were used to evaluate efficacy to IGF1R pathway inhibition in combination with endocrine therapy. Diminished functional E-cadherin increased both activation of IGF1R signaling and efficacy to anti-IGF1R/InsR therapies. PLA demonstrated a direct endogenous interaction between IGF1R and E-cadherin at points of cell-cell contact. Increased expression of IGF1 ligand and levels of IGF1R/InsR phosphorylation were observed in E-cadherin-deficient ER+ ILC compared with IDC tumors. IGF1R pathway inhibitors were effective in inhibiting growth in ER+ ILC cell lines and synergized with endocrine therapy and similarly IGF1R/InsR inhibition reduced proliferation in ILC tumor explant culture. We provide evidence that loss of E-cadherin hyperactivates the IGF1R pathway and increases sensitivity to IGF1R/InsR targeted therapy, thus identifying the IGF1R pathway as a potential novel target in E-cadherin-deficient breast cancers. .