ASTROCYTIC S1P3 SIGNALING MODULATES BLOOD-TUMOR BARRIER PERMEABILITY IN BRAIN METASTASES

Breast cancer brain metastases are incurable. The blood-brain barrier (BBB) is a multicellular dynamic structure regulating exchanges between the blood and the central nervous system. As cancer cells colonize the brain, the BBB evolves into a blood-tumor barrier (BTB) and remains a potent barrier to compound penetration, contributing to poor efficacy of CNS chemotherapy. Identifying the molecular underpinnings of BBB/BTB permeability will lead to the development of efficacious treatments. Three mouse models of brain metastasis of breast cancer (BMBC) were developed: a triple negative (231-BR) and two HER2 overexpressing (SUM190-BR, JIMT-1-BR) subtypes. Paracellular permeability of the BTB was assessed using 3kDa Texas Red dextran (TRD). Combining fluorescent imaging techniques with laser capture microdissection, highly vs. poorly permeable metastatic lesions were dissected. Subsequently, gene expression profiling was performed on extracted RNA. Only 10% of the metastatic lesions harbored a TRD diffusion correlating with chemotherapy efficacy (i.e. highly permeable lesions). Several genes were differentially expressed between highly and poorly permeable metastases. S1PR3, sphingosine-1-phosphate receptor 3, showed a 2.4-fold increase (P=0.02) in RNA expression in highly permeable lesions. We confirmed this result at a protein level using immunostaining: S1PR3 was overexpressed by neuroinflammatory astrocytes surrounding metastases in the highly permeable metastases in the three BMBC models (231-BR, P=0.034; JIMT-1-BR, P=0.010 and SUM190-BR, P=0.0162). S1P3+ astrocytes were detected in human craniotomies of brain metastases, validating the clinical relevance of our findings. To investigate the functional role of S1PR3 in BTB permeability, mice were treated with an S1P3 antagonist for four days at 10mg/kg twice a day. S1P3 antagonist significantly decreased the diffusion of TRD by 3-fold (P=0.0118). This study represents an important proof of concept for the modulation of BTB peremability through reactive astrocytes. These data may identify new strategies to selectively permeabilize the BTB and enhance chemotherapeutic efficacy.