Inhibition of Single Routes of Intracellular Signaling is Not Sufficient to Neutralize the Biphasic Disturbance of a Retinal Endothelial Cell Barrier Induced by VEGF-A165.

Background/aims: Hallmark of diabetic macular edema is the enhanced permeability of retinal endothelial cells (REC) induced by vascular endothelial growth factor (VEGF-A165), which acts through activating specific receptors. To improve the predictability of inhibitors' potentials to block harmful effects of VEGF-A165, we investigated if its signaling pathways triggered in REC are redundant.

Methods: Immortalized bovine REC monolayers were treated with inhibitors specific for various protein kinases in combination with VEGF-A165. Permeability was monitored continuously by measurements of the cell index (CI) to reveal even subtle and transient changes. Expression of tight junction (TJ) proteins was determined as additional indicator of barrier stability.

Results: After a sharp but transient CI drop caused by VEGF-A165 early after its addition, further exposure resulted in a continuous CI decline over several days associated with loss of TJ protein claudin-1. Both phases were blocked by inhibition of VEGF receptor 2. Tested inhibitors of intracellular kinases had a limited or no effect, or were efficient only in certain phases of exposure to VEGF-A165, e.g. inhibiting protein kinase C only prevented the early response. High concentrations of some inhibitors even resulted in VEGF-independent barrier destabilization.

Conclusions: Specific kinase inhibitors differently affect VEGF-A165-triggered processes in distinct phases of its action. VEGF-A165-initiated signaling is redundant and blocking of key proteins of single pathways is not sufficient to suppress REC barrier breakdown.