Abstract P3-07-12: Targeting SET to disrupt oncogenic CIP2A feedforward loop shows therapeutic potential in triple negative breast cancer cells
Authors: Huang T-T , Liu C-Y , Chen Y-T , Chu P-Y , Huang C-T , Wang W-L , Wu C-Y , Shiau C-W , Chen K-F , Tseng L-M 
Published: March 2017

Abstract
Triple-negative breast cancer (TNBC) remains a difficult-to-treat cancer with a need for new therapeutic target. The serine/threonine protein phosphatase 2A (PP2A) functions as a tumor suppressor and enhancing PP2A activity by PP2A-activating agents is a promising anti-cancer approach. There are intrinsic inhibitors of PP2A including SET and cancerous inhibitor of protein phosphatase 2A (CIP2A), both of which interact and inhibit PP2A, playing oncogenic roles. Here we studied the tumor promoting effects of SET and CIP2A and discovered a potent linkage between SET and CIP2A-feedforward pathway in TNBC. We further demonstrated the anticancer activity of a novel SET/PP2A protein-protein interaction inhibitor, TD-19. Analyzing TCGA and clinical TNBC cohort, SET and CIP2A overexpressions correlated with worse survival in TNBC patients. Tumors from TNBC patients with higher SET expressions were significantly associated with higher Ki-67 and p-Akt expressions.
Table 1 High SET H score (&gt;180) N=21Low SET H score (&lt;=180) N=70P valueAge [median (range)]54 (36-69)56 (30-88)0.185Primary tumor (T) 0.99916 (28.6)21 (30.0) 213 (61.9)43 (61.4) 3-42 (9.5)6 (8.6) Nodal status (N) 0.12208 (38.1)45 (64.3) 17 (33.3)12 (17.1) 2-36 (28.6)13 (18.6) Stage 0.49812 (9.5)14 (20.0) 213 (61.9)41 (58.6) 36 (28.6)15 (21.4) Grade 0.78311 (4.8)4 (5.7) 28 (38.1)21 (30.0) 312 (57.1)45 (64.3) Tumor necrosis13 (61.9)40 (57.1)0.698Lymphovascular invasion6 (30.0)15 (21.4)0.549CIP2A [median (range)]120 (40-220)100 (30-300)0.253p-Akt [median (range)]200 (65-300)160 (20-300)0.011Ki-67 [median (range)]30 (0-87.5)22.5 (0-85)0.004
Consistent with clinical results, TNBC cells with ectopic overexpression of SET and CIP2A showed significantly increased cell proliferation, migration and invasion, and colony-formation. Moreover, TD-19 demonstrated anti-tumor effects through inhibiting SET-PP2A interaction and CIP2A/PP2A/pAkt-mediated pathway in vitro and in a TNBC xenograft mouse model. Mechanistically, TD19 downregulated CIP2A mRNA via affecting Elk-1 amount in nucleus thereby decreased the binding of Elk-1 to the CIP2A-promotor. Furthermore, given that ERK-dependent phosphorylation of Elk-1 is essential for Elk-1 shuttling from cytoplasm to nucleus and PP2A controls ERK activation (phosphorylation), we hypothesized a CIP2A-feedforward loop consisting of pERK/pElk-1/CIP2A/PP2A may contribute to the oncogenesis in TNBC. Indeed, with TD19 treatment, SET/PP2A interaction was inhibited, PP2A activity was increased, and expressions of the CIP2A-feedforward loop members were suppressed. This loop was validated by knockdown of PP2A and ectopic expression of Elk-1. In addition, ectopic expression of SET increased pAkt, pERK, pElk-1 and CIP2A expressions, suggesting a positive linkage between SET and CIP2A feedforward signaling. Importantly, combining TD-19 with cisplatin demonstrated enhanced anti-proliferative and apoptotic effects in association with CIP2A downregulation in vitro. In summary, we discover a novel oncogenic CIP2A-feedforward loop in TNBC and targeting SET to disrupt oncogenic CIP2A feedforward loop shows therapeutic potential in TNBC.
Citation Format: Huang T-T, Liu C-Y, Chen Y-T, Chu P-Y, Huang C-T, Wang W-L, Wu C-Y, Shiau C-W, Chen K-F, Tseng L-M. Targeting SET to disrupt oncogenic CIP2A feedforward loop shows therapeutic potential in triple negative breast cancer cells [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-07-12.