Muller cells stimulate endothelial cell growth by releasing VEGF

Purpose. Muller cells are thought 10 have an important role in regulating retinal microvascular function and growth. Our goal is to identify the factors produced and released by Muller cells to mediate their function on target cells. Method. Muller cells isolated from rat retina were maintained in a serum-free medium for 3 days and the resulting conditioned medium (MCM) was clarified by centrifugation and filtration. The growth rate of the bovine retinal microvascular endothelial (BRE), bovine aortic endothelial (BAE) and mink lung epithelial CCL-64 cells were evaluated in the presence of various concentrations of MCM. Results. Addition of MCM to exponentially growing cells stimulated the proliferation of retinal microvascular endothelial cells but not that of the mink lung epithelial cells. When target cells were incubated with low concentrations of VEGF, BRE cells responded to mitogenic action of this specific factor. The BAE cells were less sensitive to either VCM or VEGF. Muller cell stimulation of BRE cells via MCM was partially inhibited by the addition of and VEGF polyclonal antibody to the culture and by adsorption of MCM to heparin-conjugated sepharose column. Western blot analysis demonstrated the presence of a protein band co-migrating with purified VEGF. Conclusion. We have direct evideice that Muller cell conditioned medium contain VEGF which is probably the primary source of the mitogenic activity directed specifically to capillary endothelial cells of the retina.