title: Proteinuria causes dysfunctional autophagy in the proximal tubule.
abstract: Proteinuria is a major risk factor for chronic kidney disease progression. Furthermore, exposure of proximal tubular epithelial cells to excess albumin promotes tubular atrophy and fibrosis, key predictors of progressive organ dysfunction. However, the link between proteinuria and tubular damage is unclear. We propose that pathological albumin exposure impairs proximal tubular autophagy, an essential process for recycling damaged organelles and toxic intracellular macromolecules. In both mouse primary proximal tubule and immortalized human kidney cells, albumin exposure decreased the number of autophagosomes, visualized by the autophagosome-specific fluorescent markers monodansylcadaverine and GFP-LC3, respectively. Similarly, renal cortical tissue harvested from proteinuric mice contained reduced numbers of autophagosomes on electron micrographs, and immunoblots showed reduced steady-state LC3-II content. Albumin exposure decreased autophagic flux in vitro in a concentration-dependent manner as assessed by LC3-II accumulation rate in the presence of bafilomycin, an H+-ATPase inhibitor that prevents lysosomal LC3-II degradation. In addition, albumin treatment significantly increased the half-life of radiolabeled long-lived proteins, indicating that the primary mechanism of degradation, autophagy, is dysfunctional. In vitro, mammalian target of rapamycin (mTOR) activation, a potent autophagy inhibitor, suppressed autophagy as a result of intracellular amino acid accumulation from lysosomal albumin degradation. mTOR activation was demonstrated by the increased phosphorylation of its downstream target, S6K, with free amino acid or albumin exposure. We propose that excess albumin uptake and degradation inhibit proximal tubule autophagy via an mTOR-mediated mechanism and contribute to progressive tubular injury.
authors: Renal Section, Department of Medicine, Boston University, Boston, Massachusetts.
authors: Angela C
authors: AC
authors: Nolin
authors: Renal Section, Department of Medicine, Boston University, Boston, Massachusetts.
authors: Ryan M
authors: RM
authors: Mulhern
authors: Department of Pathology, Boston University, Boston, Massachusetts; and.
authors: Maria V
authors: MV
authors: Panchenko
authors: Renal Section, Department of Medicine, Boston University, Boston, Massachusetts.
authors: Anna
authors: A
authors: Pisarek-Horowitz
authors: Renal Section, Department of Medicine, Boston University, Boston, Massachusetts.
authors: Zhiyong
authors: Z
authors: Wang
authors: Department of Medicine, Boston University School of Medicine, Boston, Massachusetts.
authors: Orian
authors: O
authors: Shirihai
authors: Renal Section, Department of Medicine, Boston University, Boston, Massachusetts.
authors: Steven C
authors: SC
authors: Borkan
authors: Renal Section, Department of Medicine, Boston University, Boston, Massachusetts; ahavasi@bu.edu.
authors: Andrea
authors: 0000-0002-8758-0823
authors: A
authors: Havasi
mesh_terms: D000818:Animals; D000071182:Autophagosomes / Q000378:metabolism*; D001343:Autophagy / Q000502:physiology*; D002460:Cell Line; D006801:Humans; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D051379:Mice; D010766:Phosphorylation; D011507:Proteinuria / Q000378:metabolism*; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism
chemical_list: C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases
keywords: albumin; autophagy; mTOR; proteinuria; reabsorption