Breast cancer cell coculture induces normal lung fibroblast transition to CAFs, promoting tumor cell dormancy and therapy resistance.

Cancer-associated fibroblasts (CAFs) shape the tumor microenvironment of primary breast tumors to promote tumor progression and therapy resistance. While the lung is a top metastatic site in breast cancer, the origins of lung metastasis-associated fibroblasts and their influence on disseminating tumor cell outgrowth and chemoresistance are poorly understood. Here, we demonstrate the applicability of 2-dimensional and 3-dimensional cocultures of primary human lung fibroblasts (LF) and breast cancer cells (BCC) as models of tumor-stromal interactions in lung metastatic breast cancer. Using these models, we find that BCC lines representing clinically relevant molecular subtypes differentially induce CAF-like phenotypes in primary LFs corresponding with their propensity for lung metastasis. Furthermore, we identify a mechanism by which juxtacrine signaling from LFs to triple negative breast cancer (TNBC) cells promotes expansion of prognostic dormant-like cell subpopulations and instigates autophagy-dependent therapy resistance via integrin and Janus kinase1/2 signaling. A high content kinase inhibitor compound library screen using this model identifies vacuolar protein sorting 34 as a therapeutic vulnerability unique to BCC-LF interaction whose inhibition can resensitize TNBC cells to chemotherapy and relieve LF-mediated extrinsic therapy resistance. Therefore, we propose coculture of primary human LFs and BCC as a reductionist model of interactions between tumor cells and lung-resident stroma and a tool for therapeutic and mechanistic discovery in lung metastatic breast cancer.