Receptor mediated uptake of apo B and apo E rich lipoproteins by human glomerular epithelial cells

Abstract

Various pathological disorders are accompanied by the deposition of lipids into glomerular cells. To gain insight into these disorders, it is essential to know if glomerular cells possess lipoprotein receptors. We therefore characterized the activity of lipoprotein receptors in cultured epithelial cells of the human glomerulus. Podocytes were chosen as they are directly exposed to lipoproteins in pathological states like in glomerular proteinuria (such as, nephrotic syndrome). Isolated human glomeruli (purity greater than 95%) were incubated in buffered RPMI 1640 medium supplemented with 20% heat-inactivated fetal bovine serum at 37 degrees C and 5% CO2. Outgrowing cells were vimentin and keratin positive. Monolayer cultures of human glomerular epithelial cells upon incubation in lipoprotein deficient serum for 48 hours expressed a receptor-dependent uptake of lipoproteins. These cells showed about 10% of the maximal capacity for LDL uptake as compared to fibroblasts; however, the Km values for binding, internalization and degradation were similar in the cultures of glomerular epithelial cells and fibroblasts. The Km values for degradation of LDL, chylomicron remnants, beta-VLDL from cholesterol-fed rabbits and VLDL from familial LCAT-deficiency patients were 14.2, 4.9, 2.9, 4.5 micrograms protein/ml medium, respectively, for glomerular epithelial cells. The avid uptake of 125I-labeled apo E-containing lipoproteins was further substantiated by their poor displacement by a 25-fold excess of unlabeled LDL and their ability to down regulate the apo B,E receptor activity. LDL as well as beta-VLDL were able to suppress the incorporation of 14C acetate into sterols and to stimulate 3H-cholesterylester formation. These experiments show that cultured glomerular epithelial cells express lipoprotein receptor activity. Plasma concentrations of apo E-containing lipoproteins are increased in certain renal diseases (such as, nephrotic syndrome); these lipoproteins could be rapidly removed by glomerular epithelial cells and lead to lipid deposition in glomeruli.