Endocytic uptake of FITC-albumin by human alveolar epithelial cell line A549.
The uptake mechanism of FITC-labeled albumin (FITC-albumin) was examined in human alveolar epithelial cell line A549. FITC-albumin uptake by A549 cells was time- and temperature-dependent, and was markedly suppressed at 4°C compared with that at 37°C. The uptake was saturable, and was mediated by a high-affinity, low-capacity system and by a low-affinity, high-capacity system. In the following experiments, we focused on the low-affinity system. FITC-albumin uptake was markedly inhibited by metabolic inhibitors and by a vacuolar H⁺-ATPase, bafilomycin A₁. The uptake was inhibited by clathrin-mediated endocytosis inhibitors (phenylarsine oxide and chlorpromazine). Potassium depletion and hypertonicity that inhibit clathrin-mediated endocytosis also decreased FITC-albumin uptake. On the other hand, caveolae-mediated endocytosis inhibitors (indomethacin and nystatin) did not affect FITC-albumin uptake. In addition, FITC-albumin uptake was inhibited by macropinocytosis inhibitors such as 5-(N-ethyl-N-isopropyl) amiloride. These results suggest that the low-affinity system of FITC-albumin uptake is mediated by endocytosis in A549 cells, predominantly via a clathrin-mediated pathway. Macropinocytosis, but not caveolae-mediated endocytosis, may also be involved. Considering our previous findings, albumin may be transported by a similar mechanism and/or pathway in rat and human alveolar epithelial cells.
Department of Pharmaceutics and Therapeutics, Graduate School of Biomedical Sciences, Hiroshima University, Hiroshima, Japan.
Ryoko
R
Yumoto
Sayuri
S
Suzuka
Keisuke
K
Oda
Junya
J
Nagai
Mikihisa
M
Takano
D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism; D056809:Alveolar Epithelial Cells / Q000187:drug effects / Q000378:metabolism*; D021941:Caveolae / Q000187:drug effects / Q000378:metabolism; D002460:Cell Line; D002966:Clathrin / Q000037:antagonists & inhibitors / Q000378:metabolism*; D003080:Cold Temperature; D004705:Endocytosis / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016650:Fluorescein-5-isothiocyanate / Q000031:analogs & derivatives* / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D049990:Membrane Transport Modulators / Q000494:pharmacology; D009994:Osmolar Concentration; D010873:Pinocytosis / Q000187:drug effects; D011188:Potassium / Q000378:metabolism; D012709:Serum Albumin / Q000378:metabolism*; D012710:Serum Albumin, Bovine / Q000378:metabolism
D002966:Clathrin; D004791:Enzyme Inhibitors; C518288:FITC-albumin; D049990:Membrane Transport Modulators; D012709:Serum Albumin; D012710:Serum Albumin, Bovine; D000251:Adenosine Triphosphatases; D016650:Fluorescein-5-isothiocyanate; D011188:Potassium