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P. Turowski, Z. Ockrim, R. Crawford, J. Greenwood; Molecular Mechanisms of Vascular Permeability in Microvascular Endothelial Cells. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2650.
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Vascular endothelial growth factor (VEGF) is an angiogenic growth factor with a strong vasoactive potential which appears to be central to the pathogenesis of diabetic macular edema. The present study aimed at characterizing the mechanisms underlying VEGF-induced permeability changes in microvascular endothelial cells. Particular emphasis was put on concomitant changes in cell signaling and paracellular junction architecture.
Primary microvascular endothelial cell cultures were established from rat brains or retinae. Such in vitro cultures were then exposed to VEGF or other vasoactive substances. Subsequent changes in the phosphorylation status of intracellular signal transducers were monitored by immunoblotting. Changes in tight (TJ) or adherens junction (AJ) protein were assessed by immunoblotting or by indirect immunocytochemistry. Barrier changes were measured by cellular impedance or fluorescent-dextran flux.
In vitro cultures of cerebral or retinal microvascular endothelial cells formed tight barriers and the AJ and TJ proteins VE-cadherin, catenins, occludin, claudin-5 and ZO-1 were found to be expressed and correctly localised. VEGF stimulation induced signaling through MAP kinases and the PI3K/Akt pathway and affected the subcellular distribution of many junctional proteins. Long-term treatment also induced significant reduction in transmembrane components of AJ and TJ. Surprisingly, no vasoactive response was observed when VEGF was applied to the apical side of microvascular endothelial cells. However, basolateral addition led to enhanced paracellular permeability. In contrast, other vasoactive stimuli such as lysophosphatidic acid (LPA) or the activation of Intercellular Cell Adhesion Molecule 1 (ICAM-1) induced permeability from the apical side.
Primary cultures of microvascular endothelial cells from either brain or retina appear to be polarized, at least in their vasoactive responsiveness to VEGF and this may be an important feature to distinguish between circulating and tissue-borne VEGF. Further dissection of the underlying molecular mechanism will undoubtedly refine our understanding of anti-VEGF therapeutic strategies.
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