Abstract Background: Brain metastasis is a frequent and lethal site of relapse in inflammatory breast cancer (IBC), a rare, aggressive, and highly metastatic breast cancer subtype. We identified soluble E-cadherin (sEcad), an 80-kDa extracellular fragment of full-length E-cadherin, as being significantly associated with increased risk of brain metastasis and decreased overall survival (OS) in patients with metastatic IBC. Functional studies further indicate that sEcad promotes brain metastasis progression in both HER2+ and triple-negative IBC models. However, the underlying mechanism remains poorly understood. Methods: Stable overexpression of sEcad in IBC cell lines MDA-IBC3 (ER-/HER2+) and SUM149 (ER-/HER2-) was achieved using lentiviral transduction. PDIA4 knockdown was performed in the sEcad-overexpressing cells via shRNA. To evaluate brain metastasis and survival, MDA-IBC3-sEcad cells were injected via tail vein and SUM149-sEcad cells via intracardiac route into SCID/Beige mice. Mice were treated with 35G8, a blood-brain barrier-permeable PDIA4 inhibitor, administered subcutaneously. In silico analysis was performed using multiple independent breast cancer patient datasets. Results: High serum sEcad levels in IBC patients correlated with poorer OS (=0.02) and earlier development of metastasis (p=0.006). Overexpression of sEcad in IBC cell lines enhanced cell proliferation, and colony formation, migration, invasion, and reduced cell death in vitro. Mice injected with sEcad-overexpressing SUM149 and MDA-IBC3 cells had significantly higher tumor growth rates compared to controls (SUM149: p=0.007; MDA-IBC3: p=0.006). Mechanistically, mass spectrometry and Bio-ID assays identified Protein Disulfide Isomerase Family A Member 4 (PDIA4) as a novel binding partner of sEcad, which was validated through co-immunoprecipitation. PDIA4 expression was markedly elevated in sEcad-overexpressing cells in vitro and in vivo. Knockdown of PDIA4 in sEcad-overexpressing cells significantly reduced migration, invasion and survival. In silico analysis shows that PDIA4 is highly expressed in aggressive ER-negative, basal-like, and HER2+ tumors (p 0.0001), which have increased brain metastasis risk. PDIA4 levels are also higher in brain metastases than matched primary tumors (GSE125989, P=0.011). Treatment with the brain-permeable PDIA4 inhibitor 35G8 significantly decreased brain metastasis burden in SUM149-sEcad models, including a decrease in brain metastasis incidence (p = 0.05) and the number of brain metastatic lesions (p = 0.03). Conclusion: Our findings uncover a novel mechanism by which sEcad drives brain metastasis in IBC through its interaction with PDIA4. Inhibiting PDIA4 with the BBB-permeable compound 35G8 reduced brain metastatic burden, highlighting the sEcad-PDIA4 axis as a potential therapeutic target. Citation Format: X. Hu, I. Longa Rizzo, E. Schlee Villodre, T. Phi, K. Tesfamariam, J. Song, Y. Gong, K. Savitri, W. A. Woodward, B. G. Debeb. Therapeutic Inhibition of the sEcad-PDIA4 Axis Suppresses Brain Metastasis in Inflammatory Breast Cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-09-25.
Hu et al. (Tue,) studied this question.