title: High-density lipoprotein endocytosis in endothelial cells.
abstract: OBJECTIVE
  To describe the way stations of high-density lipoprotein (HDL) uptake and its lipid exchange in endothelial cells in vitro and in vivo.
  
  
  METHODS
  A combination of fluorescence microscopy using novel fluorescent cholesterol surrogates and electron microscopy was used to analyze HDL endocytosis in great detail in primary human endothelial cells. Further, HDL uptake was quantified using radio-labeled HDL particles. To validate the in vitro findings mice were injected with fluorescently labeled HDL and particle uptake in the liver was analyzed using fluorescence microscopy.
  
  
  RESULTS
  HDL uptake occurred via clathrin-coated pits, tubular endosomes and multivesicular bodies in human umbilical vein endothelial cells. During uptake and resecretion, HDL-derived cholesterol was exchanged at a faster rate than cholesteryl oleate, resembling the HDL particle pathway seen in hepatic cells. In addition, lysosomes were not involved in this process and thus HDL degradation was not detectable. In vivo, we found HDL mainly localized in mouse hepatic endothelial cells. HDL was not detected in parenchymal liver cells, indicating that lipid transfer from HDL to hepatocytes occurs primarily via scavenger receptor, class B, type I mediated selective uptake without concomitant HDL endocytosis.
  
  
  CONCLUSIONS
  HDL endocytosis occurs via clathrin-coated pits, tubular endosomes and multivesicular bodies in human endothelial cells. Mouse endothelial cells showed a similar HDL uptake pattern in vivo indicating that the endothelium is one major site of HDL endocytosis and transcytosis.
authors: Stefanie Fruhwürth, Clemens Röhrl, Herbert Stangl, Center for Pathobiochemistry and Genetics, Institute of Medical Chemistry, Medical University of Vienna, 1090 Vienna, Austria.
authors: Stefanie
authors: S
authors: Fruhwürth
authors: Margit
authors: M
authors: Pavelka
authors: Robert
authors: R
authors: Bittman
authors: Werner J
authors: WJ
authors: Kovacs
authors: Katharina M
authors: KM
authors: Walter
authors: Clemens
authors: C
authors: Röhrl
authors: Herbert
authors: H
authors: Stangl
keywords: Cholesterol; Endocytosis; Endothelium; High-density lipoprotein; Human coronary artery endothelial cells; Human umbilical vein endothelial cells