Activin A prevents hyper-responsiveness to VEGF in pathological blood vessels by perturbing trafficking of activated VEGFR2

While quality of life compromising afflictions such as diabetic retinopathy (DR) are driven by vascular endothelial growth factor (VEGF), there is a growing appreciation that the concentration of VEGF is not the only influencer of vascular dysfunction within the retina. Activin A (activin), a ligand of the transforming growth factor β superfamily (TGF-β), attenuated VEGF-induced VE-cadherin disorganization, pore formation and permeability of primary human retinal endothelial cells (HRECs). Efforts to further investigate the mechanism of this phenomenon revealed that activin reduced expression of Rab11, which was required for the activin effect. The activin effect was not observed in cells with suppressed expression of the endosomally-localized protein tyrosine phosphatase, (PTP1b), whereas PTPs present on the plasma membrane were dispensable. Activin attenuated VEGF-mediated phosphorylation of VEGF receptor 2 (VEGFR2) at time points 30 min and longer post stimulation. Together these data support the concept that activin suppressed VEGF-induced barrier relaxation by perturbing trafficking of activated VEGFR2. This activin-based suppression of responsiveness to VEGF was compromised in the endothelium of pathological blood vessels from patients who developed end-stage proliferative diabetic retinopathy (PDR). This defect rendered HRECs hyper-responsive to VEGF and was not observed in retinal vessels of diabetic mice, which do not develop the angiogenic forms of DR. These data provide novel insights regarding the pathogenesis of PDR in patients.