Abstract
<p style="margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font-stretch: normal; font-size: 12px; line-height: normal; font-family: "Helvetica Neue";">Breast cancer (BC) metastasis is the foremost cause of BC-related mortality. Interactions between BC cells and non-cancer cells of the tumor microenvironment (TME) are pivotal in BC metastasis. Cancer-associated fibroblasts (CAFs) are critical orchestrators and drivers of these interactions. BC metastasis occurs via the transport of circulating tumor cells (CTCs) in the vasculature. In animal models, CTC clusters have a greater capability to establish metastasis rather than CTCs by themselves. We have demonstrated that CAFs also circulate (cCAFs) in the blood of BC patients, both individually and in clusters with CTCs. We have established primary BC and primary CAF cell lines from dissociated BC tumors of different molecular subtypes. Preliminary data from co-injection of CAF and BC cells in xenograft mouse models, suggests that cCAFs can egress prior to and independent of BC cells <i>in vivo</i>. We were able to detect clusters of cCAF and CTCs in the blood of mice as early as two weeks post-injection. <i>In vitro</i>, we found that metastatic BC cells can cluster with CAF cells, whereas non-metastatic MCF7 BC cells do not<i>. </i>We also found that the co-injection of CAF cells isolated from an “aggressive” basal-like BC (CAF23) can transform MCF7 cells and make them metastatic <i>in vivo</i>. These transformed MCF7 cells can now form heterotypic clusters with CAFs. Furthermore, gene expression analysis of CAF23 transformed MCF7 cells shows that they are enriched in BC stem cells and pathways of EMT. Taken together, we hypothesized that CAFs in the primary TME can reprogram BC cells, and as cCAFs, they originate from the primary tumor, and cluster with CTCs to facilitate metastatic seeding of BC cells. These heterotypic clusters of cCAFs and CTCs are enriched in CD44+ breast cancer stem cells and are critical drivers of BC metastasis. We also demonstrate a direct mechanism via the CXCL12/CXCR4 axis being critical to the CAF function in the transformation of MCF7 cells by CAF23. Therefore, targeting cCAFs-CTC clusters in breast cancer may be a novel therapeutic strategy to abrogate BC metastasis.</p>