Inhibition of protein kinase C is not sufficient to prevent or reverse effects of VEGF165 on claudin-1 and permeability in microvascular retinal endothelial cells.

PURPOSE: Pathogenesis of diabetic macular edema is driven by deregulated expression of VEGF. A study of long-term exposure of immortalized bovine retinal endothelial cells (iBRECs) to VEGF(165) clearly confirmed the role of the tight junction protein claudin-1, which almost completely disappeared within 24 hours, an effect that was completely reversed by addition of the VEGF-binding Fab fragment ranibizumab. This study was conducted to investigate whether the VEGF(165)-induced loss of claudin-1 is regulated by protein kinase C (PKC) and indeed affects the barrier function of iBRECs.
METHODS: The effects of various PKC inhibitors on claudin-1 expression and cellular localization in iBRECs treated with VEGF(165) for up to 2 days were studied by Western blot analyses and immunofluorescence microscopy. The permeability of the cell layers was determined by transendothelial electrical resistance measurements.
RESULTS: Activation of PKC led to decreased expression of claudin-1, which was blocked by inhibitors of PKCdelta. However, none of the PKC inhibitors significantly affected VEGF(165)-induced effects on cellular localization or expression of claudin-1. Also VEGF(165)-induced higher permeability of iBREC layers could be reversed or prevented by ranibizumab but not by PKC inhibitors. In addition, low claudin-1 expression and its delocalization from the plasma membrane were significantly associated with elevated permeability.
CONCLUSIONS: In iBRECs, PKC isoforms are not crucially involved in the VEGF(165)-initiated signal transduction that affects permeability and expression of claudin-1. This finding is in contrast to published results concerning only short-term effects of VEGF(165). The results also confirmed that claudin-1 is a highly relevant component of functional tight junctions in retinal endothelial cells.