Microenvironmental Factors Shape Tumor Heterogeneity and Routes of Metastatic Dissemination in Breast Cancer

Introduction: The axes CCR7-CCL21 and CXCR4-CXCL12 determine organ-specific metastatic spread in lymph node (LN) and distant organs. Here, we investigated the regulation of chemokine-induced metastatic dissemination by TME factors, focusing on luminal-A breast tumors. Methods: Using the METABRIC dataset, we determined the expression patterns of CCR7 and CXCR4, and the association between CCR7 and LN metastasis, in different breast cancer subtypes. In parallel, luminal-A MCF-7 cells over-expressing CCR7 or CXCR4 were stimulated by a combination of factors typically residing in the TME of luminal-A tumors (=TME stimulation). Downstream signaling and migratory abilities in response to chemotactic signals were assessed in vitro using migration assays and a novel 3D hydrogel system in which directional protrusions towards chemokines were determined. In vivo mouse models were used to examine the metastatic spread. Results: METABRIC analyses indicated that CCR7 and CXCR4 expression levels were lower in luminal-A tumors than in more aggressive subtypes of breast cancer. TME stimulation down-regulated the migration of CCR7- and CXCR4-expressing MCF-7 cells in response to their respective ligands, CCL21 and CXCL12, and inhibited the formation of directional protrusions in response to CCL21. TME stimulation has shut-off CCL21-induced PI3K and MAPK signaling in CCR7-expressing cells (not by inducing receptor internalization). In vivo, TME stimulation alone elevated LN metastasis but in its presence the signals delivered by CCR7 could not come into effect, in terms of LN spread. Rather, when the TME stimulus was imposed on CCR7-expressing cells, the metastatic spread in bones was considerably increased. Conclusions: Microenvironmental signals induce an aggressive metastatic phenotype in luminal-A breast tumor cells, providing a personalized context to the potential use of chemokines in targeting metastasis.