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Breast cancer metastasizes to the brain in 15-30% of all breast cancer cases, and metastasis is the predominant cause of breast cancer-related deaths. Patients with HER2-enriched and triple-negative breast cancers (TNBCs) are more likely to develop brain metastases. While targeted therapies exist for HER2-enriched breast cancers, there are no effective treatments for TNBCs. Thus, a greater understanding of how these cancers spread to the brain is critical. In order to spread to the brain, disseminated breast cancer cells must overcome 2 major steps—crossing the blood-brain barrier (BBB) and survival and successful colonization of the distinctive and mostly cellular brain environment. Here, we report a novel role for breast cancer cell surface receptor, Syndecan-1 (Sdc1), a heparan sulfate proteoglycan, in promoting breast cancer cell transmigration across the BBB. We found that when we silenced Sdc1 expression in a highly metastatic TNBC cell line, MDA-MB-231, these cells exhibited reduced migration across an in vitro BBB model system. Further, in an in vivo experimental model of metastasis, mice injected with MDA-MB-231 Sdc1 KD (knock-down) cells developed less brain metastases than mice injected with control non-silencing (NS1) cells. Conversely, we found that overexpression of Sdc1 in a metastatic triple-negative mouse mammary carcinoma cell line, 4T1, led to an increase in brain metastases compared to empty vector control-treated mice. We predicted that a secreted factor(s) facilitated BBB disruption that allowed for Sdc1-mediated BBB transmigration, and found that silencing Sdc1 led to decreases in the production and/or release of various cytokines and chemokines implicated in BBB permeability and transmigration. In addition to supporting BBB transmigration, through an in vitro tissue section adhesion assay, we found that Sdc1 also facilitates adhesion of breast cancer cells to the brain, and not to the liver or lungs, revealing specificity for the brain. Further, we report that Sdc1 is expressed in 81% of breast cancer patient brain metastases in our tissue microarray study and that patients with TNBC and high Sdc1 expression have shorter disease-free survival based on a study performed using data from The Cancer Genome Atlas. Taken together, we predict that breast cancer cell Sdc1-regulated cytokines and chemokines promote BBB permeability and/or support transmigration to facilitate breast cancer metastasis to the brain. We also provide evidence for breast cancer-secreted extracellular vesicles, namely exosomes, in supporting the formation of a pro-metastatic brain environment. We compared exosomes derived from the metastatic 4T1 mouse mammary carcinoma cell line to a non-metastatic counterpart, the 67NR cell line, to assess their microRNA and protein composition and their effect(s) on recipient astrocytes, known mediators of brain metastasis. We found that there are inherent differences in both the microRNA and protein cargo from the metastatic 4T1 cells compared to the non-metastatic 67NR cells, whereby the metastatic 4T1 cells contained various tumor-promoting microRNAs and proteins, and also contained 4.5-fold more protein than the non-metastatic 67NR cells. Mouse astrocytes treated with the metastatic 4T1 exosomes exhibited a shift towards a pro-metastatic phenotype, characterized by upregulation of pro-inflammatory genes, and genes associated with astrocyte reactivity and cancer, whereby 67NR exosome-treated astrocytes exhibited a response profile that overlapped with untreated controls. Overall, these findings reveal an important role for exosomes in driving changes in the brain microenvironment to create a site conducive for cancer growth. Together, both studies help to elucidate how breast cancer cells can invade and colonize the unique brain environment. |
