Brain infiltration of breast cancer stem cells is facilitated by paracrine signaling by inhibitor of differentiation 3 to nuclear respiratory factor 1.

Treatment options for brain metastatic breast cancer are limited because the molecular mechanism for how breast cancer cells infiltrate the brain is not fully understood. For breast tumors to metastasize to the brain first, cells need to detach from the primary tumor, enter in the blood circulation, survive within the microvascular niche, and then cross the blood-brain barrier (BBB) to colonize into the brain. It is critical to understand how breast cancer cells transmigrate through the BBB to prevent brain metastasis. Nuclear respiratory factor 1 (NRF1) transcription factor has been reported to be highly active in several human cancers and its aberrant expression facilitates in the acquisition of breast cancer stem cells (BCSCs). Inhibitor of differentiation protein 3 (ID3), a transcription regulating protein, induces pluripotent endothelial stem cells (ESCs). Herein, we investigated if NRF1-induced BCSCs could cross a BBB model and guiding of BCSCs by ID3-induced ESCs across the BBB. BCSCs and ESCs were subjected to functional gain/loss experiments to determine if NRF1/ID3 contributed to lineage-specific BCSCs organ entry. First, we tested whether NRF1 promoted migration of breast cancer using a BBB model consisting of BCSCs or MDA-MB231 cells, brain endothelial cell layer, and astrocytes. NRF1 overexpression increased the propensity for BCSCs and NRF1-induced MDA-MB231 cells to adhere to brain endothelial cells and migrate across a human BBB model. Increased adhesion of NRF1-induced BCSCs to ESCs was detected. NRF1-induced BCSCs crossed through the BBB model and this was promoted by ESCs. We also showed that environmental relevant exposure to PCBs (PCB153 and PCB77) produced differential effects. Treatment with PCB153 showed increased growth of NRF1-induced BCSCs tumor spheroids and increased in vivo migration of ESCs. Exosomal ID3 released from endothelial cells also supported the growth of NRF1-induced BCSCs and provide the basis for paracrine effects by ESCs associated with breast tumors. Xenograft experiments showed that ID3 overexpressing brain ESCs not only supported the growth of BCSC tumor spheroids but guided them to the neural crest in zebrafish. These findings show for the first time a novel role for ID3 and NRF1 by which ESCs help guide BCSCs to distant metastatic sites where they most likely facilitate the colonization, survival, and proliferation of BCSCs. This knowledge is important for pre-clinical testing of NRF1/ID3 modifying agents to prevent the spread of breast cancer to the brain.