Abstract P3-04-02: Invasive lobular carcinoma cell lines utilize WNT4 signaling to mediate estrogen-induced growth

Invasive lobular carcinoma (ILC) is a histological subtype of breast cancer representing 10-15% of newly diagnosed breast tumors. Over 90% of ILC are estrogen receptor (ER)-positive, however, endocrine response and estrogen signaling are not well understood in ILC. Retrospective analyses suggest that ILC patients treated with endocrine therapy have poorer outcomes than invasive ductal carcinoma (IDC) patients, and that ILC patients may not benefit from adjuvant tamoxifen. Based on these observations, we hypothesize that ER regulates unique signaling pathways in ILC cells that control growth and endocrine response. To identify putative targets that regulate endocrine response in ILC, we assessed genome-wide ER-mediated gene expression and ER genomic binding in the ILC cell lines MDA MB 134VI (MM134) and SUM44PE (SUM44). Among ILC-specific estrogen-regulated genes, the most strongly induced was the secreted ligand WNT4. Additionally, we identified an ILC-specific ER binding site (ERBS) at WNT4, suggesting that WNT4 is directly controlled by ER in ILC cells. Direct ER-regulation of WNT4 is in contrast to control of WNT4 by progesterone receptor (PR) during mammary gland development; however, further ER ChIP experiments suggest that ILC cells place WNT4 under ER control via unique use of the WNT4 ERBS. Thus, ILC cells may hijack a tightly regulated developmental program to drive estrogen-regulated cell phenotypes. Based on the role of WNT4 in mammary gland growth and expansion, we hypothesized that WNT4 is required for estrogen-induced growth in ILC cells. To test this, we used siRNA to knock down WNT4 in ILC and IDC cell lines. Using either of two siRNAs, WNT4 knockdown completely blocked estrogen-induced growth in ILC cells, but not IDC cells. Consistent with this, WNT4 knockdown abrogated estrogen-regulation of a subset of ER-target genes in MM134 cells, suggesting that these genes are downstream of WNT4 signaling. Wnt signaling typically acts via the canonical, β-catenin-dependent pathway; however, we observed that β-catenin is dysfunctional in ILC cells, and WNT4 cannot activate β-catenin signaling in cancer cell lines. This suggests that WNT4 regulates estrogen-induced growth in ILC cells via a novel non-canonical signaling pathway. Clinical observations suggest that ER regulates unique downstream pathways in ILC. We identified WNT4 as a putative downstream effector of ER signaling in ILC, as WNT4 is strongly-induced and directly regulated by ER specifically in ILC cells. Further, WNT4 is necessary for estrogen-induced growth in ILC cells, and likely signals via a non-canonical signaling pathway. Targeting WNT4 signaling represents a novel approach to modulate endocrine response specifically for ILC patients. Future studies will focus on characterizing the signaling pathway controlled by WNT4 in order to identify putative therapeutic targets. Citation Format: Sikora MJ, Oesterreich S. Invasive lobular carcinoma cell lines utilize WNT4 signaling to mediate estrogen-induced growth. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-04-02.