MAPK3 and diabetic retinopathy: In diabetic retinopathy and retinopathy of prematurity the presence of hypoxia causes upregulation of the expression of G protein-coupled receptor 91 (GPR91) and consequently augmented expression of VEGF through ERK1/2-C/EBP β (c-Fos) and/or ERK1/2-COX-2/PGE2 signaling pathways [62] Figure 1.