Abstract P132: Promotion of E-cadherin-mediated tumor cell adhesion by COX-2/GSK3b signaling is a targetable mechanism of metastatic breast cancer

Purpose: Metastatic progression and treatment resistance of breast cancer has been associated with epithelial-mesenchymal transition (EMT), including downregulation of E-cadherin (CDH1) gene expression, which can be initiated by inflammatory signaling such as by COX-2 (PTGS2). However, E-cadherin expression is maintained in many advanced breast cancers, including inflammatory BC (IBC), where it plays an essential role in forming tumor cell emboli within the cancer parenchyma and dermal lymph vasculature and which predict poor outcomes. Thus, the study of IBC offers an opportunity to understand the mechanisms of IBC and other aggressive BCs that lead to E-cadherin-associated cluster-based metastasis, which has recently received heightened recognition. Here, we have investigated the mechanisms that sustain E-cadherin expression in metastatic breast cancer to identify new targeted treatment options. Study Design and Methods: In vitro emboli formation assays and gene/protein expression studies with IBC (SUM149, IBC-3) and non-IBC (SUM159, MCF-7) cell lines along with gene silencing, overexpression, and pharmacological interventions. Analysis of experimental (SUM149) and spontaneous (orthotopic patient-derived xenograft, PDX) lung metastases and of circulating tumor cells (CTCs) in xenograft models treated with celecoxib (Cxb) and/or paclitaxel by imaging cytometry, immunohistochemistry, and/or Western analysis. Results: By analyzing the transcription factor C/EBPδ (CEBPD) and cells derived from inflammatory breast cancer (IBC), an aggressive breast cancer subtype that often presents with E-cadherin-dependent tumor cell emboli, we discovered that COX-2, unexpectedly, sustained E-cadherin protein expression without changing its mRNA levels. Using an in vitro tumor cell emboli culture paradigm (3D), we found that COX-2 or its metabolite PGE2 increased AKT activity and the inhibitory phosphorylation on GSK3β that prevents degradation of p120 catenin (CTNND1), a stabilizer of E-cadherin adhesion complexes. Conversely, the COX-2 inhibitor celecoxib downregulated E-cadherin specifically at the protein level and caused cell death in 3D. Co-expression of E-cadherin and COX-2 was seen in breast cancer patients with poor outcomes and, along with inhibitory GSK3β phosphorylation, in patient-derived xenografts (PDX) of metastatic triple-negative breast cancers (TNBC). Celecoxib alone decreased E-cadherin protein expression within xenograft primary tumors, reduced circulating tumor cells (CTCs) and clusters, and sensitized lung metastases to paclitaxel treatment. Conclusions: Our study uncovered a novel function of COX-2/PGE2 in promoting E-cadherin protein expression and cell-cell adhesions that are relevant for tumor cell cluster-based metastasis. Indeed, COX-2 inhibition reduced CTC clusters in a xenograft model, and sentized established metastases to chemotherapy. These results suggest that patients with COX-2+/E-cadherin+ metastastic BC, including IBC, may specifically benefit from targeting the PGE2 pathway in cobniation therapy approaches. Citation Format: Balamurugan Kuppusamy, Saadiya Sehareen, Savitri Krishnamurthy, Shikha Sharana, Wei Tang, Naoto T. Ueno, Stefan Ambs, Dipak K. Poria, Esta Sternecka. Promotion of E-cadherin-mediated tumor cell adhesion by COX-2/GSK3b signaling is a targetable mechanism of metastatic breast cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P132.