Binding and internalization of lipopolysaccharide by Cla-1, a human orthologue of rodent scavenger receptor B1

J Biol Chem. 2003 Jun 20;278(25):22771-80. doi: 10.1074/jbc.M211032200. Epub 2003 Mar 21.

Abstract

Scavenger receptor, class B, type I (SR-BI) mediates selective uptake of high density lipoprotein (HDL) cholesteryl ester. SR-BI recognizes HDL, low density lipoprotein (LDL), exchangeable apolipoproteins, and protein-free lipid vesicles containing negatively charged phospholipids. Lipopolysaccharides (LPS) are highly glycosylated anionic phospholipids contributing to septic shock. Despite significant structural similarities between anionic phospholipids and LPS, the role of SR-BI in LPS uptake is unknown. Cla-1, the human SR-BI orthologue, was determined to be a LPS-binding protein and endocytic receptor mediating the binding and internalization of lipoprotein-free, monomerized LPS. LPS strongly competed with HDL, lipidfree apoA-I and apoA-II for HDL binding to the mouse RAW cells. Stably transfected HeLa cells expressing Cla-1-bound LPS with a Kd of about 16 microg/ml, and had a 3-4-fold increase in binding capacity and LPS uptake. Bodipy-labeled LPS uptake was found to initially accumulate in the plasma membrane and subsequently in a perinuclear region identified predominantly as the Golgi complex. Bodipy-LPS and Alexa-apoA-I had staining that colocalized on the cell surface and intracellularly indicating similar transport mechanisms. When associated with HDL, LPS uptake was increased in Cla-1 overexpressing HeLa cells by 5-10-fold. Cla-1-associated 3H-LPS uptake exceeded 125I-apolipoprotein uptake by 5-fold indicating a selective LPS uptake. Upon interacting with Cla-1 overexpressing HeLa cells, the complex (Bodipy-LPS/Alexa 488 apolipoprotein-labeled HDL) bound and was internalized as a holoparticle. Intracellularly, LPS and apolipoproteins were sorted to different intracellular compartments. With LPS-associated HDL, intracellular LPS co-localized predominantly with transferrin, indicating delivery to an endocytic recycling compartment. Our study reveals a close similarity between Cla-1-mediated selective LPS uptake and the recently described selective lipid sorting by rodent SR-BI. In summary, Cla-1 was found to bind and internalize monomerized and HDL-associated LPS, indicating that Cla-1 may play important role in septic shock by affecting LPS cellular uptake and clearance.

Publication types

  • Comparative Study

MeSH terms

  • Animals
  • Binding Sites
  • Biological Transport
  • CD36 Antigens / chemistry
  • CD36 Antigens / metabolism*
  • Cell Line
  • Cholesterol, HDL / pharmacokinetics*
  • Cloning, Molecular
  • Escherichia coli
  • HeLa Cells
  • Humans
  • Kinetics
  • Lipopolysaccharides / pharmacokinetics*
  • Macrophages
  • Membrane Proteins*
  • Mice
  • Receptors, Immunologic / metabolism*
  • Receptors, Lipoprotein / chemistry
  • Receptors, Lipoprotein / metabolism*
  • Receptors, Scavenger
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / metabolism
  • Salmonella
  • Scavenger Receptors, Class B
  • Transfection

Substances

  • CD36 Antigens
  • Cholesterol, HDL
  • Lipopolysaccharides
  • Membrane Proteins
  • Receptors, Immunologic
  • Receptors, Lipoprotein
  • Receptors, Scavenger
  • Recombinant Proteins
  • SCARB1 protein, human
  • Scarb1 protein, mouse
  • Scavenger Receptors, Class B