Integrative analysis of mesenchymal stromal cell conversion by breast cancer secretomes in blank slate 3D microenvironments

The ability of primary tumour cells to invade and metastasise is responsible for over 90% of cancer patient deaths. During tumour growth and progression, cancer cells secrete factors that recruit and reprogram otherwise healthy cells to facilitate the formation of a favourable tumour microenvironment. Here, we have investigated how breast cancer cells convert normal mesenchymal stromal cells (MSCs) into tumour-associated MSCs (TA-MSCs) using unbiased global approaches. We compared the secretomes produced by non-invasive MCF-7 cells with invasive MDA-MB-231 cells, and identified extracellular matrix and exosome components associated with invasion. We then treated MSCs cultured in fully-defined, synthetic 3D hydrogels with invasive/non-invasive cancer secretomes and analysed their responses by kinase activity profiling and RNA sequencing, which led us to identify an invasion-associated signature of MSC conversion. Finally, we investigated whether there is an organ-specific metastasis-associated reprogramming of MSCs, and found that breast cancer cells from different metastatic sites have different secretome profiles that induce reprogramming of MSCs. These data describe at a systems-level how breast cancer cells with different invasion and metastatic abilities secrete molecules that activate MSCs and convert them into TA-MSCs.