Receptors for VEGF (flt-1 and flk-1) are expressed in nonvascular cells in the eye

Purpose. Vascular endothelial growth factor (VEGF) has been thought to exclusively target vascular endothelial cells to induce vascular permeability and endothelial cell proliferation. It has been suggested that the receptors for VEGF, flt-1 and flk-1, are found only on endothelial cells. However, immunohistochemical staining for flt-1 and flk-1 shows positivity in nonvascular cells of the retina and in epiretinal membranes. The purpose of this study was to determine if retinal pigmented epithelial cells (RPE) and retinal glia, possess receptors for VEGF. Methods. RT-PCR was performed on total RNA harvested from cultured human RPE cells and retinal glia, and from RPE from post-mortem eyes using unique oligonucleotide pairs for human flt-1 and flk-1. Immunoblots were done on RPE cell lysates using affinity purified anti-peptide polyclonal antibodies for flt-1 and flk-1. Human RPE and retinal glia cultures were grown to confluence in slide wells, air dried, fixed in methanol, and immunocytochemically stained for flt-1, flk-1, and VEGF. Results. Expression of mRNA for both flt-1 and flk-1 was detected by RT-PCR in cultured RPE and retinal glia, and freshly isolated RPE. Immunoblots of RPE cell lysates demonstrated a 150kb band recognized by anti-flt-1 and a 180kb band recognized by anti-flk-1. These antibodies also immunocytochemically stained cultured RPE and retinal glia. This staining was eliminated by pre-incubation of each antibody with its purified antigen. Immunocytochemical staining was also detected in RPE and retinal glia cultures using a polyclonal antibody specific for VEGF. Conclusions. RPE and retinal glia express VEGF and both known receptors for VEGF. This suggests that VEGF may have functions other than those mediated through vascular endothelium.