Low-Affinity Transport of FITC-Albumin in Alveolar Type II Epithelial Cell Line RLE-6TN

Abstract

  FITC-albumin uptake by cultured alveolar type II epithelial cells, RLE-6TN, is mediated by high- and low-affinity transport systems.¹⁾ In this study, characteristics of the low-affinity transport system were evaluated. The uptake of FITC-albumin was time and temperature dependent and was inhibited by metabolic inhibitors and bafilomycin A₁. Confocal laser scanning microscopic analysis showed punctate localization of the fluorescence in the cells, which was partly localized in lysosomes. FITC-albumin taken up by the cells gradually degraded over time, as shown by fluoroimage analyzer after SDS-PAGE. The uptake of FITC-albumin by RLE-6TN cells was not inhibited by caveolae-mediated endocytosis inhibitors such as nystatin, but was inhibited by clathrin-mediated endocytosis inhibitors such as phenylarsine oxide. The uptake was also inhibited by potassium depletion and hypertonicity, conditions known to inhibit clathrin-mediated endocytosis. In addition, macropinocytosis inhibitors such as 5-(N-ethyl-N-isopropyl) amiloride inhibited the uptake. These results indicate that the low-affinity transport of FITC-albumin in RLE-6TN cells is at least in part mediated by clathrin-mediated endocytosis, but not by caveolae-mediated endocytosis. Possible involvement of macropinocytosis was also suggested.