The first end point of this study was to identify specific pro-apoptotic or anti-proliferative factors in the breast cancer cell secretome. To this end, we designed an in vitro screen that effectively cross-cultured 20 breast cancer cell lines in each other's conditioned media. We selected the strongest pro-apoptotic hits and performed further proteomic and biochemical characterization in order to analyze their composition. We determined that the pro-apoptotic activity resided in the soluble, exosome-free secreted fraction of triple negative breast cancer cell conditioned medium and used proteomic insights in order to narrow down the list of possible candidate molecules responsible for the apoptotic effect. The second endpoint of this study was to evaluate the particulate fraction found in breast cancer cell conditioned media for diagnostically significant molecules. We isolated cancer exosomes, employing a serial ultracentrifugation protocol, and were able to establish that the exosome cell surface receptors identically reflect the molecular identity of their cell lines of origin. However, downstream protein kinases within exosomes display patterns of depletion or enrichment in comparison to the corresponding cell lines. Overall, we found that the exosome protein composition in breast cancer is informative enough to guide the choice of specific inhibitor treatment in a clinical setting.
