A molecular analysis of blood-tumor barrier permeability in three experimental models of brain metastasis from breast cancer
Brain metastases from breast cancer are increasing in incidence. Thirty-seven percent of HER2+ or triple negative metastatic breast cancer patients develop brain metastases, which confer a median survival of 13.8 months. Because the brain is protected by the blood-brain barrier (BBB), most conventional breast cancer chemotherapeutics do not penetrate the brain at high enough concentrations to elicit cytotoxic responses in brain metastases. The BBB is heterogeneously altered following the influx of metastatic tumor cells into the brain, forming the blood-tumor barrier (BTB). A comprehensive understanding of the molecular biology of the BTB in metastatic disease is critical to identify molecular determinants of BTB permeability with the ultimate goal of delivering an effective amount of cytotoxic chemotherapy. I hypothesize that there are consistent alterations in the BTB between highly permeable and poorly permeable brain metastases from breast cancer. ^ Functional components of the BBB/BTB and the neuroinflammatory response were characterized in highly permeable and poorly permeable experimental brain metastases from breast cancer to facilitate identification of molecular pathways that may be used to increase chemotherapeutic efficacy. Brain seeking variants of three human breast cancer cell lines, MDA-MB-231-BR6, JIMT1-BR3, and SUM190-BR3 were injected into the left cardiac ventricle of mice to produce brain metastases. Following a peri-mortem intravenous infusion of 3kd Texas Red dextran (TRD) and perfusion, permeable metastases were characterized by diffusion of red dye within and around the metastasis. When brain metastases and uninvolved brain were compared changes in endothelial cells, pericytes, astrocytes, and the neuroinflammatory response were observed. The most striking changes were observed in highly permeable and poorly permeable brain metastases. Increased expression of desmin+ pericytes was observed in permeable metastases in the 231-BR6 (p=0.0002), JIMT-1-BR3 (p=0.004), and SUM190-BR3 (p=0.008) model systems. However, there was a significant decrease in CD13+ pericytes in the 231-BR6 (p=0.014) and JIMT-1-BR3 (p=0.002) model systems. Trends were corroborated in human craniotomy specimens. These findings were validated functionally in in vitro models of the BBB and BTB. Permeability was measured with transendothelial electrical resistance (TEER), and there was an increase in permeability in the desmin+ pericyte BBB model compared to the CD13+ BBB model, this difference was also observed in the BTB models and was significant (p=0.0074). ^ I have provided the first comprehensive characterization of the BTB in brain metastases from breast cancer. A correlation of desmin+ and CD13+ pericyte subpopulations and BTB permeability was observed in vivo. I have demonstrated that the desmin+ pericyte subpopulation increased permeability in in vitro models of the BBB and BTB. Pericyte subpopulations may affect specific aspects of BBB/BTB permeability. Further, these findings provide testable hypotheses for effective delivery of cytotoxic chemotherapy for brain metastases from breast cancer.^
Margaret A. Miller, Purdue University, Patricia S. Steeg, Purdue University.