Abstract 5118: Ectopic expression of cathepsin G in mammary tumor cells and its role in tumor growth and osteolytic bone metastasis

Breast cancer has a high propensity to metastasize to the bone microenvironment. The resulting lesions are predominantly osteolytic and are associated with patient morbidity and mortality. We have demonstrated that tumor-bone interaction plays an important role in osteolytic bone metastasis mediated by the dysregulation of a unique set of genes. Among these genes, several proteases are up-regulated at the tumor-bone interface, including Cathepsin G, a serine protease. These proteases act as mediators between the tumor and stroma. Previously, we have shown that both malignant cells and osteoclasts at the TB interface express higher levels of Cathepsin G, which is critical for osteolytic bone metastasis. The precise role of tumor and/or host-derived Cathepsin G in osteolytic bone metastasis is unknown. In this report, we examined the contribution of tumor-derived Cathepsin G by testing the hypothesis that ectopic expression of Cathepsin G modulates malignant cell phenotype, osteoclast recruitment and activation, and osteolytic metastasis. We generated stable mammary tumor cell lines constitutively expressing Cathepsin G (Cl66-CTSG). Cathepsin G-expressing tumor cells showed diminished in vitro cell proliferative potential and in vivo mammary fat pad tumor growth as compared to vector control transfected Cl66-CMV cells. We demonstrated that Cathepsin G-overexpressing tumor cells induced osteoclast activation and differentiation in vitro and tumor-induced osteolysis in vivo. To observe the morphological changes in tumor cells by CTSG, we performed phalloidin staining and live cell imaging using CTSG-treated tumor cells. We observed that CTSG induced the formation of homotypic aggregations of tumor cells. We found that increasing the concentration of CTSG decreases tumor cell proliferation and increases cell cycle arrest. Our data suggest that tumor-derived Cathepsin G might have distinct tumor-modulating activities in the bone microenvironment by regulating cell proliferation and osteoclast recruitment and activation. Citation Format: Ridhi Bhola, Reegan Sturgeon, Rakesh K. Singh. Ectopic expression of cathepsin G in mammary tumor cells and its role in tumor growth and osteolytic bone metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5118.