title: Hemoglobin inhibits albumin uptake by proximal tubule cells: implications for sickle cell disease.
abstract: Proximal tubule (PT) dysfunction, including tubular proteinuria, is a significant complication in young sickle cell disease (SCD) that can eventually lead to chronic kidney disease. Hemoglobin (Hb) dimers released from red blood cells upon hemolysis are filtered into the kidney and internalized by megalin/cubilin receptors into PT cells. The PT is especially sensitive to heme toxicity, and tubular dysfunction in SCD is thought to result from prolonged exposure to filtered Hb. Here we show that concentrations of Hb predicted to enter the tubule lumen during hemolytic crisis competitively inhibit the uptake of another megalin/cubilin ligand (albumin) by PT cells. These effects were independent of heme reduction state. The Glu7Val mutant of Hb that causes SCD was equally effective at inhibiting albumin uptake compared with wild-type Hb. Addition of the Hb scavenger haptoglobin (Hpt) restored albumin uptake in the presence of Hb, suggesting that Hpt binding to the Hb αβ dimer-dimer interface interferes with Hb binding to megalin/cubilin. BLAST searches and structural modeling analyses revealed regions of similarity between Hb and albumin that map to this region and may represent sites of Hb interaction with megalin/cubilin. Our studies suggest that impaired endocytosis of megalin/cubilin ligands, rather than heme toxicity, may be the cause of tubular proteinuria in SCD patients. Additionally, loss of these filtered proteins into the urine may contribute to the extra-renal pathogenesis of SCD.
authors.affiliation: Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; and.
authors.forename: Megan L
authors.initials: ML
authors.lastname: Eshbach
authors.affiliation: Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; and.
authors.forename: Amandeep
authors.initials: A
authors.lastname: Kaur
authors.affiliation: Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; and.
authors.forename: Youssef
authors.initials: Y
authors.lastname: Rbaibi
authors.affiliation: Division of Pulmonary, Allergy and Critical Care Medicine, Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania.
authors.forename: Jesús
authors.initials: J
authors.lastname: Tejero
authors.affiliation: Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; and weisz@pitt.edu.
authors.forename: Ora A
authors.initials: OA
authors.lastname: Weisz
mesh_terms: D000595:Amino Acid Sequence; D000755:Anemia, Sickle Cell / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002460:Cell Line; D002461:Cell Line, Transformed; D005260:Female; D006242:Haptoglobins / Q000737:chemistry* / Q000378:metabolism; D006418:Heme / Q000737:chemistry; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism; D006461:Hemolysis; D006801:Humans; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000378:metabolism; D008024:Ligands; D026561:Low Density Lipoprotein Receptor-Related Protein-2 / Q000737:chemistry* / Q000378:metabolism; D008297:Male; D009893:Opossums; D010084:Oxidation-Reduction; D011485:Protein Binding; D000072756:Protein Conformation, alpha-Helical; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012709:Serum Albumin / Q000737:chemistry* / Q000378:metabolism
chemical_list: C485359:HP protein, human; D006242:Haptoglobins; D006454:Hemoglobins; D008024:Ligands; D026561:Low Density Lipoprotein Receptor-Related Protein-2; D012709:Serum Albumin; D006418:Heme
keywords: megalin; proteinuria; proximal tubule; sickle cell disease; vitamin D