Molecular mechanisms underlying the vascular lesions in proliferative diabetic retinopathy

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In diabetic retinopathy, the visual acuity of patients is impaired by the formation of fibrovascular tissue as well as retinal edema. Through analysis of clinical samples, we had obtained the data to suggest that the fibrovascular tissue formation is attributed to the induction of vascular endothelial growth factor (VEGF), especially its isoform VEGF_<165>, and membrane-type 1 matrix metalloproteinase(MT1-MMP) in retinal glial cells. In this study, the mechanisms underlying the induction of these molecules were investigated, with reference to tissue hypoxia. Retinal glial cells were isolated from the rabbit retina, and cultured under either normoxic(20% O_2) or hypoxic(1% O_2) condition. RT-PCR and real-time PCR analyses showed that hypoxia induces the expression of MT1-MMP and VEGF in retinal glial cells. As concerns VEGF, the isoform VEGF_<165> was selectively up-regulated in hypoxic retinal glial cells. One of the high-affinity receptors for VEGF, VEGFR-2, was also found to be expre … More ssed in retinal glial cells under hypoxia, while negligible in those under normoxia. Furthermore, the hypoxia-induced MT1-MMP expression was inhibited in the presence of the VEGFR-2 inhibitor SU1498 or the anti-VEGF antibody, indicating that the expression of MT1-MMP in hypoxic retinal glial cells is mediated by VEGF-VEGFR-2 system in an autocrine fashion. On the other hand, the mechanisms of retinal edema formation due to the breakdown of blood-retinal barrier were analyzed in reference to the expression of tight junction molecules, claudins. First, retinal vascular endothelial cells were shown to express claudin-5 at their cell borders. Using cultured bEND.3 endothelial cells, it was demonstrated that the localization of claudin-5 at the cell borders of endothelial cells disappears under hypoxic condition with close correlation to the breakdown of barrier function. MAP kinase cascades were involved in the hypoxia-induced changes in claudin-5 expression. The data obtained in this study suggest the role of tissue hypoxia in the formation of fibrovascular tissue and retinal edema through alteration of VEGF_<165>, MT1-MMP and claudin-5 expression. Less