The endothelial barrier of the brain is characterized by specialized endothelial cells with low paracellular permeability. Along with the basement membrane and other cell types, the blood brain barrier (BBB) is an important regulator of normal brain metabolism. VEGF is a growth factor secreted by tumors that can induce vascular permeability (VP) and disrupt the integrity of the BBB. Our previous studies have demonstrated a Src requirement in the host compartment for VEGF-induced vascular permeability (VP), particularly in the lung and brain. We have exploited the 'leakage-resistant1 phenotype of Src-knockout mice to examine the characteristics of tumor growth and metastasis. These studies have revealed that an absence of Src leads to reduced VEGF-induced VP of blood vessels providing protection from lung metastases. In the brain, a reduction in VEGF-induced VP has also been associated with Src-mediated changes in FAK phosphorylation. Recent data indicate that FAK is enriched in brain blood vessels compared to surrounding cell types, and is a target of Src-mediated VP. We hypothesize that FAK is a key intermediate in the maintenance of the integrity of the BBB. In Aim 1 we will characterize the role of Src in regulating VEGF- induced FAK activation and VP of brain blood vessels. These studies will examine the molecular basis for the VP phenotype and will include the characterization of BMK1 and eNOS. In Aim 2 we will determine whether Src regulates BMK1 in VEGF-stimulated brain blood vessels. In Aim 3 we will examine the role of eNOS in Src- vs. BMK1-mediated VEGF-induced VP. While these studies will provide novel and important insights into the regulation of the BBB, relatively few studies have examined the role of the BBB in tumor metastasis. Therefore, we will use animal models to determine the function of FAK, BMK1 and eNOS in the host compartment during breast tumor metastasis to the brain. These studies will build upon our previous studies of the role of Src in the host compartment in Src-mediated metastases to the lung and provide a comparison for the characterization of the growth and invasion of intracranial tumors. A panel of human astrocytomas is available for xenograft mouse models of human glioma growth and infiltration. The results of these studies will be validated in human brain tumor samples of varying malignancy to determine the clinicopathological relevance of VP and the BBB in various stages of tumor growth and invasion.