LAUSR

Circulating tumor cells (CTCs) seed metastases which eventually kill most of the breast cancer patients; however, CTCs have not been effectively targeted partially due to the unclear cellular and molecular mechanisms. Emerging evidence and our studies demonstrate that CTCs are detectable as singles as well as multicellular clusters in cancer patients; and presence of the latter is associated with unfavorable progression-free survival (PFS) and poor overall survival (OS). Compared to single CTCs, CTC clusters show enriched stemness properties, including enhanced tumorigenic potential and polyclonal metastases, more efficient mammosphere formation, and upregulated expression of CD44 and stemness-related signaling pathways. Using intravital multiphoton microscopic imaging of patient-derived-xenograft (PDX) models and mouse tumor models, we found that clustered tumor cells in migration and circulation resulted from aggregation of individual tumor cells rather than collective migration and cohesive shedding. Molecularly, the intercellular CD44-CD44 homophilic interactions are essential for the multicellular CTC aggregation, recruitment of downstream PAK2, and activation of the FAK signaling. Depletion of CD44 or antibody-mediated blockade of CD44 interactions effectively prevent tumor cell aggregation and abolish polyclonal metastases. In addition to CD44, we have identified other surface receptors and adhesion molecules which also contribute to CTC cluster formation. Our studies highlight that CTC clusters, whose presence is correlated with a poor prognosis of breast cancer patients, can serve as a liquid biopsy-based biomarker as well as novel therapeutic targets of polyclonal metastasis. Citation Format: H Liu. CTC Clusters, Cancer Stemness, and New Treatment Strategies [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr TS2-1.