Abstract 1550: Identification of a novel squalene epoxidase function in breast cancer brain metastasis
Authors: Wang Shu-Ping , Yeh Jia-Yun , Lee Yu-Ling (Pony) , Lin Kun-Yuan , Chang Chao-Di , Yang Chih-Chieh , Cheng Shan-Yun , Hung Ya-Wen , Chang Yu-Hsien , Li Yao-Feng , Yu Yung-Lung 
Published: March 2024

Abstract
Brain metastases are serious complications of breast cancer, especially in triple-negative breast cancer (TNBC) patients. There is currently no effective treatment due to the unique brain microenvironment and limitation for drugs to cross the blood-brain barrier (BBB). BBB limits access of nutrients from the circulation and thereby makes the brain hypoxic and depleted of metabolites, growth factors and proteins. However, it remains poorly understood how breast cancer cells manipulate for adaption and colonization in the brain. To probe the molecular mechanism of breast cancer brain metastasis (BCBM), we compared gene signatures in primary breast tumors and BCBM tumors, and identified that SQLE (encodes squalene epoxidase, the second rate-limiting enzyme in the cholesterol biosynthesis) could be a leading-edge gene in breast cancer brain metastasis. To explore the role of SQLE in BCBM, we established the brain metastatic TNBC MDA-MB-231 (MDA231-BrM) cell line from brain metastatic subpopulation that originated from its parental MDA-MB-231 (MDA231) cell line. We found that SQLE expression was greatly upregulated in MDA231-BrM cells compared with the parental MDA231 cells. Using MDA231-BrM and MDA231 cell lines expressing SQLE shRNAs, we evaluated the effects of SQLE loss on cancer cell migration, invasion, and stemness by wound-healing, transwell invasion/migration, and tumorsphere formation assays. While loss of SQLE greatly attenuated cell invasiveness and stemness in both MDA231 and MDA231-BrM cells, loss of SQLE could only affect the cell migration activity on MDA231 cells but not MDA231-BrM cells. Our RNA-seq data further identified a subset of SQLE-affected genes that is uniquely enriched in MDA231-BrM cells and favors brain extravasation and colonization. To explore the potential function of SQLE in brain extravasation and colonization, we established in vitro BBB models and ex vivo mouse brain slice organotypic cultures. We showed that loss of SQLE inhibited the ability of MDA231-BrM cells to across the BBB-mimic astrocyte-endothelial structures as well as impaired the co-option with blood capillaries in the mouse brain slices. Although SQLE-deficient MDA231-BrM cells could still spread on the surface of the blood vessels, they seemed to undergo apoptosis. Since the ability to invade, migrate, and penetrate is critical for invasion of cancer cells, our results strongly imply the novel function of SQLE in breast cancer cell invasion, penetration, and even colonization in the brain through blood vessel co-option. In summary, our data reveal a novel role for SQLE in two critical requisites for breast-to-brain extravasation and colonization - the ability to penetrate through BBB and to co-opt brain vessels for metastatic expansion. Our findings indicate that targeting SQLE may represent a therapeutic opportunity for breast cancer brain metastases.
Citation Format: Shu-Ping Wang, Jia-Yun Yeh, Yu-Ling (Pony) Lee, Kun-Yuan Lin, Chao-Di Chang, Chih-Chieh Yang, Shan-Yun Cheng, Ya-Wen Hung, Yu-Hsien Chang, Yao-Feng Li, Yung-Lung Yu. Identification of a novel squalene epoxidase function in breast cancer brain metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1550.