Cancer-associated fibroblasts rewire the estrogen receptor response in luminal breast cancer, enabling estrogen independence

Advanced breast cancers represent a major therapeutic challenge due to their refractoriness to treatment. Cancer-associated fibroblasts (CAFs) are the most abundant constituents of the tumor microenvironment and have been linked to most hallmarks of cancer. However, the influence of CAFs on therapeutic outcome remains largely unchartered. Here, we reveal that spatial coincidence of abundant CAF infiltration with malignant cells was associated with reduced estrogen receptor (ER)-a expression in luminal breast tumors. Notably, CAFs mediated estrogen-independent tumor growth by selectively regulating ER-a signaling. Whereas most prototypical estrogen-responsive genes were suppressed, CAFs maintained gene expression related to therapeutic resistance. A high-throughput drug screen in co-cultures identified effector pathways involved in the CAF-induced regulation of ER-a signaling. Among these, the transforming growth factor-ß and the janus kinase signaling cascades were validated as actionable targets to counteract the CAF-induced ER-a modulation. Finally, genes that were downregulated in cancer cells by CAFs were predictive of poor response to endocrine treatment. In conclusion, our work reveals that CAFs directly control the luminal breast cancer phenotype by selectively modulating ER-a expression and transcriptional function, and further proposes novel targets to disrupt the crosstalk between CAFs and tumor cells to reinstate treatment response to endocrine therapy in patients. Overall design: Gene expression profiling (RNA-seq) and differentail genes expression analyses of MCF-7 cells following estrogen stimulation (after 6 and 24 hours) either alone or precultured in transwells with CAFs (for 48 hours). MCF-7 cells alone and without estrogen stimulation is the control condition. In total 24 samples divided in 6 different conditions/timepoints, each one with 4 replicates.