Abstract: Gliomas, particularly glioblastoma multiforme, perturb the blood-brain barrier and cause brain edema that contributes to morbidity and mortality. The mechanisms underlying this vasogenic edema are poorly understood. We examined the effects of cocultured primary cultured human glioblastoma cells and glioma-derived growth factors on the endothelial cell tight junction proteins claudin 1, claudin 5, occludin, and zonula occludens 1 of brain-derived microvascular endothelial cells and a human umbilical vein endothelial cell line. Cocultured glioblastoma cells and glioma-derived factors (e.g. transforming growth factor β2) enhanced the paracellular flux of endothelial cell monolayers in conjunction with downregulation of the tight junction proteins. Neutralizing anti-transforming growth factor β2 antibodies partially restored the barrier properties in this in vitro blood-brain barrier model. The involvement of endothelial cell-derived matrix metalloproteinases (MMPs) was demonstrated by quantitative reverse-transcriptase-polymerase chain reaction analysis and by the determination of MMP activities via zymography and fluorometry in the presence or absence of the MMP inhibitor GM6001. Occludin, claudin 1, and claudin 5 were expressed in microvascular endothelial cells in nonneoplastic brain samples but were significantly reduced in anaplastic astrocytoma and glioblastoma samples. Taken together, these in vitro and in vivo results indicate that glioma-derived factors may induce MMPs and downregulate endothelial tight junction protein and, thus, play a key role in glioma-induced impairment of the blood-brain barrier.