Abstract GS5-08: Soluble E-cadherin: a novel prognostic biomarker and driver of brain metastasis in inflammatory breast cancer

Abstract Background: Inflammatory breast cancer (IBC) is a highly aggressive form of breast cancer with rapid onset and a strong propensity to spread to distant organs. Five-year overall survival (OS) rates remain poor, in part because of the high risk of brain metastasis: 19% of patients with IBC have brain metastases within the first 2 years after diagnosis. Our hypothesis for this study was that soluble E-cadherin (sEcad), an 80-kDa extracellular proteolytic fragment of full-length E-cadherin - a tumor promoter in IBC, is crucial for driving brain metastasis in IBC. Methods: We analyzed serum sEcad levels in from 348 IBC patients by ELISA. Four IBC cell lines [ER–/HER2+ (MDA-IBC3; SUM190) and ER–/HER2– (SUM149; BCX010)], human brain microvascular endothelial cells, and immortalized human astrocytes were used in this study. Stable overexpression of sEcad in IBC cell lines was achieved using lentiviral vectors. Mass spectrometry and Bio-ID-based proteomics assays, and RNA sequencing were used to identify sEcad-interacting proteins and potential mechanisms. In vivo, we studied tumor growth and brain metastasis in mice by injecting IBC cells into the mammary fatpad or tail vein, respectively, of SCID/Beige mice. Results: In IBC patients, higher serum sEcad levels correlated with poorer OS (p=0.02), earlier development of metastasis (p=0.006), and development of brain metastasis (p=0.04). On multivariable analysis, sEcad independently predicted OS (hazard ratio [HR]=2.07 [95% CI 1.19-3.60], p=0.01). In vitro, sEcad overexpression in IBC cell lines promoted anchorage-independent growth, migration, invasion, and resistance to anoikis. In vivo, sEcad-overexpressing SUM149 cells promoted primary tumor growth (p=0.007). Mice injected with sEcad-overexpressing MDA-IBC3 cells also had higher incidence of brain metastasis (100% vs 50%, p=0.03), metastatic burden (p=0.02) and number of metastases per mouse (p=0.0009), and had worse OS (p=0.0016), and brain metastasis-free survival (p=0.04), relative to controls. We further found that sEcad increased cancer cell adhesion to brain endothelial cells (p=0.01) and promoted induction of reactive astrocytes (as identified by high glial fibrillary acidic protein levels) in vitro and in vivo. Mechanistically, mass spectrometry and Bio-ID assays identified X-linked inhibitor of apoptosis protein (XIAP), a potent inhibitor of apoptotic cell death, as a novel binding partner of sEcad, which was validated through co-immunoprecipitation. Further analysis showed that sEcad bound to the BIR2 domain of XIAP. XIAP is the most potent and best-defined anti-apoptotic IAP family member, and it could induce NF-κB activation to inhibit tumor cell apoptosis Gene set enrichment analysis of RNA-seq profiling data showed activation of NF-kB signaling and downregulation of apoptotic pathways in the sEcad-overexpressing SUM149 cells compared with controls. Immunoblotting revealed that sEcad enhanced XIAP expression, activated NF-κB signaling, and inhibited cleavage of caspase-3 in IBC cells. Conclusions: We found that higher serum sEcad correlates with development of brain metastases and independently predicts poor OS in patients with IBC. We further found that sEcad promotes tumor growth and brain metastasis, perhaps via activation of XIAP/NF-κB signaling in breast cancer cells and promotion of endothelial cell adhesion and reactive astrocytosis in the brain microenvironment. These findings uncover a novel and crucial role for sEcad in brain metastasis and provide new insights and potential therapeutic targets for patients with metastatic IBC. Citation Format: Xiaoding Hu, Yun Xiong, Emilly S. Villodre, Juhee Song, Ganiraju C Manyam, Moises J Tacam, Jing Wang, Chandra Bartholomeusz, Debu Tripathy, Naoto T. Ueno, Wendy Woodward, Savitri Krishnamurthy, Junjie Chen, Bisrat Debeb. Soluble E-cadherin: a novel prognostic biomarker and driver of brain metastasis in inflammatory breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr GS5-08.