Expression of LRP1 in retinal pigment epithelial cells and its regulation by growth factors

The retinal pigment epithelial (RPE) cells are mitotically inactive under normal conditions, but play a pivotal role in the pathogenesis of proliferative vitreoretinopathy (PVR). Triggered by changes in the concentrations of growth factors, RPE cells reenter the cell cycle, proliferate, and migrate onto the retinal surface, into the subretinal space, and into the vitreous. The receptor for alpha(2)-macroglobulin (low-density lipoprotein receptor-related protein [LRP1], or CD91) is known to be involved in the processes of cell migration and invasion, as well as in the regulation of growth factor homeostasis. The purpose of this study was to investigate the expression of this receptor and its regulation, at the protein and mRNA levels, in human (h)RPE cells.

The cell surface expression of the receptor was studied by immunocytochemistry and flow cytometry. The endocytosis-related activity of LRP1 in hRPE cells was examined by assessing the uptake of FITC-labeled, methylamine (MA)-treated alpha(2)-M (alpha(2)-M-MA). LRP1 mRNA expression was analyzed by means of the RNase protection assay (RPA) after the hRPE cells were stimulated with the growth factors TGF-beta1, TGF-beta2, PDGF, VEGF (each 10 ng/mL), or bFGF (5 ng/mL).

hRPE cells expressed LRP1 on their cell surface. The receptor mediated rapid binding and endocytosis of FITC-labeled alpha(2)-M-MA. The expression of LRP1 mRNA strongly increased on stimulation of the cells with TGF-beta1, TGF-beta2, or VEGF, whereas PDGF or bFGF elicited only minor effects.

The expression of functionally active LRP1 in hRPE cells suggests that the receptor may be involved in cell migration and invasion, as reported for other LRP1-expressing cells. Thus, certain growth factors may control RPE cell migration and invasion in vivo through a regulation of LRP1 expression. As LRP1 mediates the clearance of alpha(2)-M, known to regulate the homeostasis of many cytokines and growth factors, this receptor may be a promising target for therapeutic intervention in PVR.