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Evidence for the role of alveolar epithelial gp60 in active transalveolar albumin transport in the rat lung

Authors

  • Theresa A. John,

    1. Department of Pharmacology, University of Illinois College of Medicine and Division of Pulmonary and Critical Care Medicine, Northwestern University Medical School, Chicago, IL, USA
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  • Stephen M. Vogel,

    1. Department of Pharmacology, University of Illinois College of Medicine and Division of Pulmonary and Critical Care Medicine, Northwestern University Medical School, Chicago, IL, USA
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  • Richard D. Minshall,

    1. Department of Pharmacology, University of Illinois College of Medicine and Division of Pulmonary and Critical Care Medicine, Northwestern University Medical School, Chicago, IL, USA
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  • Karen Ridge,

    1. Department of Pharmacology, University of Illinois College of Medicine and Division of Pulmonary and Critical Care Medicine, Northwestern University Medical School, Chicago, IL, USA
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  • Chinnaswamy Tiruppathi,

    1. Department of Pharmacology, University of Illinois College of Medicine and Division of Pulmonary and Critical Care Medicine, Northwestern University Medical School, Chicago, IL, USA
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  • Asrar B. Malik

    1. Department of Pharmacology, University of Illinois College of Medicine and Division of Pulmonary and Critical Care Medicine, Northwestern University Medical School, Chicago, IL, USA
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Corresponding author A. B. Malik: Department of Pharmacology, University of Illinois College of Medicine, 835 South Wolcott Avenue (M/C 868), Chicago, IL 60612, USA. Email: abmalik@uic.edu

Abstract

  • Transcytosis of albumin, involving the 60 kDa albumin-binding glycoprotein, gp60, was studied in cultured type II alveolar epithelial cells obtained from rat lungs.

  • Type II cells internalized the interfacial fluorescent dye RH 414, which marks for plasmalemma vesicles. Fluorescent forms of albumin and anti-gp60 antibody colocalized in the same plasmalemma vesicles.

  • Antibody (100 μg ml−1) cross-linking of gp60 for brief periods (15 min) markedly stimulated vesicular uptake of fluorescently tagged albumin. The caveolar disrupting agent, filipin (10 nm), abolished the stimulated internalization of albumin.

  • The vast majority of plasmalemmal vesicles carrying albumin also immunostained for caveolin-1; however, lysosomes did not stain for caveolin-1. Filipin depleted the epithelial cells of the caveolin-1-positive, albumin-transporting plasmalemma vesicles.

  • Prolonged (> 1 h) stimulation of type II cells with cross-linking anti-gp60 antibody produced loss of cell-surface gp60 and abolished endocytic albumin uptake.

  • Transalveolar transport of albumin was also studied in the isogravimetric rat lung preparation perfused at 37°C. 125I-labelled albumin was instilled into distal airspaces of lungs, and the resulting 125I-labelled albumin efflux into the vascular perfusate was determined.

  • Unlabelled albumin (studied over a range of 0–10 g (100 instilled ml)−1) inhibited 40% of the transport of labelled albumin ((5.7 ± 0.4) × 105 counts (instilled ml)−1) with an IC50 value of 0.34 g (100 ml)−1.

  • Filipin blocked the displacement-sensitive component of 125I-labelled albumin transport, but had no effect on the transport of the paracellular tracer 3[H]mannitol.

  • Displacement-sensitive 125I-labelled albumin transport had a significantly greater Q10 (27–37 °C) than the non-displaceable component.

  • Cross-linking of gp60 by antibody instillation stimulated only the displacement-sensitive 125I-labelled albumin transalveolar transport in intact rat lungs.

  • To estimate the transport capacity of the displacement-sensitive system, the percentage of instilled 125I-labelled albumin counts remaining in lung tissue was compared in lungs treated with instillates containing either 0.05 g (100 ml)−1 unlabelled albumin or 5 g (100 ml)−1 unlabelled albumin. Approximately 25% of instilled 125I-labelled albumin was cleared from the lung preparations per hour by the displacement-sensitive transport pathway. This component was blocked by filipin.

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