In vivo exposure to CXCL12-high CAFs and chronic CXCL12 stimulation in vitro is sufficient to enhance metastasis of ER-positive breast cancer.

[PURPOSE] Cancer-associated fibroblasts (CAFs) play important roles in breast cancer (BC) progression and metastasis. Here we investigated whether CAFs from indolent vs. aggressive BCs differ in gene expression profiles and how they impact metastasis.

[EXPERIMENTAL DESIGN] Genotypic differences in CAF lines from basal-like (CAF23) and luminal-A BC (CAF19), were compared and effects on CAF-induced phenotypes of estrogen receptor (ER) positive BC models evaluated.

[RESULTS] Co-injection of MCF7 with CAF23 cells enhanced tumor metastasis in vivo, while CAF19 did not. CXCL12 was strongly overexpressed in CAF23. BC cells isolated from MCF7 + CAF23 tumors were enriched in epithelial-mesenchymal transition (EMT) genes and cancer stem cell (CSC)-like behavior. Chronic CXCL12 exposure in vitro, as may occur in BC with high CXCL12-secreting CAFs, phenocopied CAF23-enhanced metastasis. Single cell analysis of primary human BC revealed CAFs are the major source of CXCL12 in breast tumors. A high CXCL12-CAF gene expression profile was prognostic of poor BC outcome and was strongly over-represented in CAFs within BC metastases. Finally, gene expression changes induced in MCF7 cells by co-injection with CAF23 in vivo correlated significantly with gene expression differences between normal and malignant epithelial cells in BC containing high CXCL12 CAFs.

[CONCLUSIONS] These findings suggest that CXCL12 overexpressing CAFs can induce gene expression changes in breast cancer that promote breast cancer metastasis, potentially through expansion of the CSC population. Targeting the CAF CXCL12/CXCR4 axis may offer a novel treatment strategy for metastatic breast cancer and warrants further investigation.

[STATEMENT OF TRANSLATIONAL RELEVANCE] Cancer associated fibroblasts (CAF) within the breast tumor microenvironment influence breast cancer behavior. Our study indicates that high CXCL12-expressing CAFs can induce a stable metastatic phenotype in estrogen receptor positive breast cancer models. Gene expression similarities between a high CXCL12 CAF line and high CXCL12-expressing CAFs from primary and metastatic human breast cancers define a CXCL12-high CAF signature that is prognostic of poor BC patient outcome. Furthermore, gene expression changes induced in MCF7 cells by CXCL12 high CAFs in vivo were similar to the gene expression differences between normal and malignant breast epithelial cells in breast cancers containing CXCL12 high CAFs. Disruption of CAF-driven, CXCL12-mediated reprogramming of breast cancer cells might provide an opportunity to prevent or treat breast cancer metastasis.