Abstract 1988: Metastatic v non-metastatic cancer exosomes in brain metastasis

Brain metastasis affects 10-30% of breast cancer patients, but it is unclear how breast cancer cells can colonize the unique and mostly cellular brain environment. Exosomes, endosomal-derived extracellular vesicles, have been reported to support tumorigenesis, mediate tumor-stromal interactions, and drive pre-metastatic niche formation, ultimately creating sites conducive to cancer development. Still, their role in brain metastasis remains to be fully understood. Glial cells, namely astrocytes, are important contributors to brain metastasis and secrete exosomes that support metastatic outgrowth in the brain. Thus, our study aims to investigate if breast cancer-derived exosomes can transform astrocytes towards a pro-metastatic phenotype. To do this, we isolated exosomes from 4T1 metastatic and 67NR non-metastatic mouse mammary carcinoma cell conditioned media using ultracentrifugation. Exosomes were characterized using Western immunoblot, dynamic light scatter analysis, and transmission electron microscopy. Using confocal microscopy, we found that mouse astrocytes can internalize 4T1 exosomes in vitro. Preliminary studies using qPCR analysis revealed that mouse astrocytes treated with 4T1 exosomes compared to those treated with 67NR exosomes exhibit differential gene expression, suggesting that there is a unique response profile associated with metastatic versus non-metastatic breast cancer-derived exosomes. To further assess this, we are investigating changes in the astrocyte secretome in response to 4T1 and 67NR exosome treatment using protein mass spectrometry. Taking a global approach, we have also been assessing differences between 4T1 and 67NR exosomal miRNA, and protein that could promote a shift in astrocytes towards a pro-metastatic phenotype. For these studies, we have implemented the Nanostring nCounter Analysis System and protein mass spectrometry, respectively. Proteomics assessment revealed distinct differences between the 4T1 metastatic and 67NR non-metastatic exosomes, whereby we identified 259 proteins that were enriched in 4T1 exosomes and 71 proteins that were decreased in 4T1 exosomes compared to 67NR’s. Proteins enriched in 4T1 exosomes were found to be associated with RNA binding and translation machinery, as well as abnormal cell adhesion, and BRCA1 interactions. Proteins identified to be decreased in 4T1 exosomes were found to play a role in IL-6 and IL-12 signaling events, the estrogen signaling pathway, and syndecan-2 signaling. These findings indicate that 4T1 and 67NR exosomes differ in protein cargo and these differences may be important in facilitating brain metastasis. Altogether, these studies will provide a more comprehensive assessment of the differences between metastatic versus non-metastatic cancer exosomes, and ultimately, provide novel insight into understanding breast cancer metastasis to the brain. Citation Format: Megan Sayyad, Madhavi Puchalapalli, Chris Canal, Theresa Swift-Scanlan, Andrew Ottens, Jennifer E. Koblinski. Metastatic v non-metastatic cancer exosomes in brain metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1988.