Blocking cancer-fibroblast mutualism inhibits proliferation of endocrine therapy resistant breast cancer.

In early-stage estrogen receptor-positive (ER + ) breast cancer, resistance to endocrine therapy (ET) and CDK4/6 inhibitors (CDK4/6i) often involve a shift away from estrogen-driven proliferation. The nature and source of compensatory growth signals driving cancer proliferation remain unknown but represent direct therapeutic targets of resistant cells. By analyzing single-cell RNA-sequencing data from serial biopsies of patient tumors, we elucidated compensatory growth signaling pathways activated in ET + CDK4/6i-resistant cancer cells, along with the intercellular growth signal communications within the tumor microenvironment. In most patient tumors, resistant cancer cells increased ERBB growth pathway activity during treatment, only partially through ERBB receptor upregulation. Concurrently, fibroblasts within the tumor increased ERBB ligand communication with cancer cells, as they differentiated to a proliferative and mesenchymal phenotype in response to TGF $\beta$ signals from cancer cells. In vitro model systems demonstrated molecularly how therapy induces a mutualistic cycle of crosstalk between cancer cells and fibroblasts, fostering a growth factor-rich tumor microenvironment circumventing estrogen reliance. We show that ERBB inhibition can break this cancer-fibroblasts mutualism, targeting an acquired sensitivity of resistant cancer cells.