title: Cubilin- and megalin-mediated uptake of albumin in cultured proximal tubule cells of opossum kidney.
abstract: BACKGROUND
  Reabsorption of albumin from the glomerular filtrate occurs via receptor-mediated endocytosis in the proximal tubule. This process is initiated by binding of albumin in apical clathrin-coated pits, followed by endocytosis and degradation in lysosomes. Although binding sites have been characterized by kinetic studies, the receptors responsible for the binding of albumin have not been fully identified. Two giant glycoproteins, cubilin and megalin, constitute important endocytic receptors localized to the kidney proximal tubule.
  
  
  METHODS
  In the present study, we examined the colocalization of cubilin and megalin in the endocytic pathway and the relationship between the uptake of albumin and the expression of cubilin and megalin in opossum kidney (OK) proximal tubule cells by immunocytochemistry and immunoblotting.
  
  
  RESULTS
  OK cells expressed both cubilin and megalin. The light microscope labeling patterns for cubilin and megalin were almost identical and were mainly located at the surface area of the cells. Cubilin and megalin were also shown to colocalize on cell surface microvilli, in coated pits, and in endocytic compartments at the electron microscope level. Endocytosed bovine serum albumin (BSA) was identified exclusively in cells expressing megalin and cubilin. Uptake of BSA-FITC was saturable and inhibited by receptor-associated protein (RAP) and by intrinsic factor-vitamin B12 complex (IF-B12) at high concentrations. Significant inhibition was also observed by specific antibodies to cubilin, and megalin and cubilin antisense oligonucleotides likewise significantly reduced albumin uptake. Egg albumin did not affect the uptake of BSA.
  
  
  CONCLUSIONS
  The present observations suggest that the two receptors cubilin and megalin are both involved in the endocytic uptake of albumin in renal proximal tubule cells.
authors.affiliation: Department of Cell Biology, Institute of Anatomy and Department of Medical Biochemistry, University of Aarhus, Aarhus, Denmark.
authors.forename: X Y
authors.initials: XY
authors.lastname: Zhai
authors.forename: R
authors.initials: R
authors.lastname: Nielsen
authors.forename: H
authors.initials: H
authors.lastname: Birn
authors.forename: K
authors.initials: K
authors.lastname: Drumm
authors.forename: S
authors.initials: S
authors.lastname: Mildenberger
authors.forename: R
authors.initials: R
authors.lastname: Freudinger
authors.forename: S K
authors.initials: SK
authors.lastname: Moestrup
authors.forename: P J
authors.initials: PJ
authors.lastname: Verroust
authors.forename: E I
authors.initials: EI
authors.lastname: Christensen
authors.forename: M
authors.initials: M
authors.lastname: Gekle
mesh_terms: D000418:Albumins / Q000493:pharmacokinetics*; D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D002478:Cells, Cultured; D004705:Endocytosis / Q000187:drug effects / Q000502:physiology; D004847:Epithelial Cells / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure; D016650:Fluorescein-5-isothiocyanate / Q000032:analysis / Q000493:pharmacokinetics; D016336:Gene Products, vpr / Q000494:pharmacology; D026541:Heymann Nephritis Antigenic Complex; D007150:Immunohistochemistry; D007437:Intrinsic Factor / Q000494:pharmacology; D007687:Kidney Tubules, Proximal / Q000166:cytology* / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000032:analysis / Q000378:metabolism*; D016253:Microscopy, Immunoelectron; D009893:Opossums; D011956:Receptors, Cell Surface / Q000032:analysis / Q000378:metabolism*; D012710:Serum Albumin, Bovine / Q000032:analysis / Q000493:pharmacokinetics; D014805:Vitamin B 12 / Q000494:pharmacology
chemical_list: D000418:Albumins; D016336:Gene Products, vpr; D026541:Heymann Nephritis Antigenic Complex; D008562:Membrane Glycoproteins; D011956:Receptors, Cell Surface; C029486:intrinsic factor-cobalamin receptor; D012710:Serum Albumin, Bovine; D007437:Intrinsic Factor; D016650:Fluorescein-5-isothiocyanate; D014805:Vitamin B 12