Rat liver sinusoidal endothelial cells (LSECs) express functional low density lipoprotein receptor-related protein-1 (LRP-1)

Abstract

The low density lipoprotein receptor-related protein-1 (LRP-1) is a large, multifunctional endocytic receptor from the LDL receptor family, highly expressed in liver parenchymal cells (PCs), neurons, activated astrocytes, and fibroblasts. The aim of the study was to investigate if liver sinusoidal endothelial cells (LSECs), highly specialized scavenger cells, express LRP-1. To address this question, experiments were performed in vivo and in vitro to determine if receptor associated protein (RAP) and trypsin-activated α2-macroglobulin (α2M∗) were endocytosed in LSECs. Both ligands were cleared from the circulation mainly by the liver. Hepatocellular distribution of intravenously administered ligands, assessed after magnetic bead cell separation using LSEC- and KC-specific antibodies, showed that PCs contained 93% and 82% of liver-associated 125I-RAP and 125I-α2M∗, whereas 5% and 11% were associated with LSECs. Uptake of RAP and α2M∗ in the different liver cell population in vitro was specific and followed by degradation. The uptake of 125I-RAP was not inhibited by ligands to known endocytosis receptors in LSECs, while uptake of 125I-α2M∗ was significantly inhibited by RAP, suggesting the involvement of LRP-1. Immunofluorescence using LRP-1 antibody showed positive staining in LSECs. Ligand blot analyses using total cell proteins and 125I-RAP followed by mass spectrometry further confirmed and identified LRP-1 in LSECs. LSECs express functional LRP-1. An important implication of our findings is that LSECs contribute to the rapid removal of blood borne ligands for LRP-1 and may thus play a role in lipid homeostasis.