Constitutive and oxidative-stress-induced expression of VEGF in the RPE are differently regulated by different Mitogen-activated protein kinases

Abstract

Vascular endothelial growth factor (VEGF) is a fundamental factor for angiogenesis. It plays important roles in pathological conditions (e.g. the development of wet AMD), but also in the healthy organism) e.g. in maintaining the vasculature and supporting the retina). Recent therapies to treat the wet AMD focus on neutralizing VEGF indiscriminately. VEGF is constitutively expressed in the retina, but its expression is upregulated by various (noxious) stimuli, e.g. oxidative stress or hypoxia. Discrimination between constitutive expression of VEGF and its pathological upregulation might provide the possibility of focusing on inhibiting the pathological expression only. Here, we focused on the influence of different mitogen-activated protein kinase (MAPK) (p38, Erk, JNK) on the secretion and expression of VEGF, with or without being challenged by oxidative stress. VEGF secretion was measured using a perfusion organ culture model; expression was examined in primary RPE culture and Western blotting. Constitutive VEGF expression and secretion can be diminished by inhibiting p38, while inhibiting Erk or JNK does not show a significant effect. When challenged with oxidative stress (250 microM t-butylhydroperoxide), VEGF expression and secretion increases and the influence of the MAPK changes: While p38 still accounts for about 30% of the secretion, Erk shows a similar influence. Inhibiting JNK presents conflicting results. In organ culture, inhibiting JNK significantly increases VEGF secretion after stimulation with 250 microM tBH, while with regard to VEGF expression in RPE cell culture, this effect could not be seen. Constitutive and oxidative stress induced VEGF secretion, and expression is differently regulated, which might offer an opportunity to selectively inhibit pathological VEGF expression only.