Internalization of FITC-albumin in Human Adipose-derived Stem Cells: Involvement of Clathrin and Caveolin

Adipose tissue-derived stem cells (AdSCs) are one of the most promising cell types for cell-based therapies. In addition, AdSCs systematically injected into the body have been reported to localize to damaged tissues and certain types of tumor. As an important part of establishing a potent drug delivery system with AdSCs, the mechanism and efficiency of uptake into AdSCs has drawn much research attention. However, this remains to be fully clarified. The aim of this study was to examine the characteristics of endocytosis-mediated uptake in human AdSCs. We used fluorescein isothiocyanate-labeled albumin (FITC-albumin) as a potent marker of endocytosis. FITC-albumin uptake was time- and temperature-dependent. Confocal microscopy showed punctate localization of fluorescence in the cytoplasm. FITC-albumin uptake was inhibited by human serum albumin in a concentration-dependent manner. FITC-albumin uptake was inhibited by a metabolic inhibitor (2,4-dinitrophenol), a microtubule polymerization inhibitor (colchicine), an actin polymerization inhibitor (cytochalasin D), endosomal acidification inhibitors (chloroquine and bafilomycin A₁), clathrin-dependent endocytosis inhibitors (chloropromazine, phenylarsine oxide, and Pitstop2), and caveolin-dependent endocytosis inhibitors (nystatin and methyl-β-cyclodextrin). Furthermore, the knockdown of the clathrin heavy chain and caveolin-1 significantly reduced FITC-albumin uptake. These findings suggest that AdSCs take up albumin via endocytic pathways in which clathrin and caveolin are involved.