title: Albumin endocytosis is regulated by heterotrimeric GTP-binding protein G alpha i-3 in opossum kidney cells.
abstract: Proteinuria is an adverse feature in patients with renal disease, possibly due to toxicity of albumin to proximal tubular cells. Albumin is reabsorbed from tubular fluid by receptor-mediated endocytosis. The mechanism of regulation of the endocytosis is unknown. The large quantities of G proteins in proximal tubular cell apical membranes suggests that they may have a regulatory role in endocytosis. 125I-labeled albumin uptake was measured in opossum kidney (OK) cells. This is a saturable process with high-affinity [apparent dissociation constant (Kd) = 24.3 mg/l] and low-affinity (Kd = 15.9 g/l) components. The endocytic uptake of gold-albumin into OK cells was confirmed by electron microscopy. 125I-albumin endocytosis in OK cells was inhibited by pertussis toxin, but cholera toxin had no effect. Pertussis toxin also inhibited uptake of [3H]inulin. OK cells were stably transfected with a cDNA for the G protein subunit G alpha i-3 and transfectants were screened by immunoblotting. Several G alpha i-3-overexpressing clones were detected. OK cells overexpressing G alpha i-3 demonstrate increased 125I-albumin uptake, which is abolished by pertussis toxin, in both a concentration- and time-dependent manner. These results suggest that albumin endocytosis in OK cells is regulated by the G protein G alpha i-3.
authors.affiliation: Department of Nephrology, Leicester University Medical School, United Kingdom.
authors.forename: N J
authors.initials: NJ
authors.lastname: Brunskill
authors.forename: N
authors.initials: N
authors.lastname: Cockcroft
authors.forename: S
authors.initials: S
authors.lastname: Nahorski
authors.forename: J
authors.initials: J
authors.lastname: Walls
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D002772:Cholera Toxin / Q000494:pharmacology; D004705:Endocytosis / Q000502:physiology*; D019204:GTP-Binding Proteins / Q000235:genetics / Q000502:physiology*; D006046:Gold; D007444:Inulin / Q000037:antagonists & inhibitors / Q000493:pharmacokinetics; D007668:Kidney / Q000166:cytology / Q000502:physiology*; D007700:Kinetics; D008854:Microscopy, Electron; D009893:Opossums; D037342:Pertussis Toxin; D012709:Serum Albumin / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014162:Transfection; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D012709:Serum Albumin; D010566:Virulence Factors, Bordetella; D006046:Gold; D007444:Inulin; D002772:Cholera Toxin; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins