Mechanism underlying insulin uptake in alveolar epithelial cell line RLE-6TN

For the development of efficient pulmonary delivery systems for protein and peptide drugs, it is important to understand their transport mechanisms in alveolar epithelial cells. In this study, the uptake mechanism for FITC-insulin in cultured alveolar epithelial cell line RLE-6TN was elucidated. FITC-insulin uptake by RLE-6TN cells was time-dependent, temperature-sensitive, and concentration-dependent. The uptake was inhibited by metabolic inhibitors, cytochalasin D, clathrin-mediated endocytosis inhibitors, and dynasore, an inhibitor of dynamin GTPase. On the other hand, no inhibitory effect was observed with caveolae-mediated endocytosis inhibitors and a macropinocytosis inhibitor. Intracellular FITC-insulin was found to be partly transported to the basal side of the epithelial cell monolayers. In addition, colocalization of FITC-insulin and LysoTracker Red was observed on confocal laser scanning microscopy, indicating that FITC-insulin was partly targeted to lysosomes. In accordance with these findings, SDS–PAGE/fluoroimage analysis showed that intact FITC-insulin in the cells was eliminated with time. The possible receptor involved in FITC-insulin uptake by RLE-6TN cells was examined by using siRNA. Transfection of the cells with megalin or insulin receptor siRNA successfully reduced the corresponding mRNA expression. FITC-insulin uptake decreased on the transfection with insulin receptor siRNA, but not that with megalin siRNA. These results suggest that insulin is taken up through endocytosis in RLE-6TN cells, and after the endocytosis, the intracellular insulin is partly degraded in lysosomes and partly transported to the basal side. Insulin receptor, but not megalin, may be involved at least partly in insulin endocytosis in RLE-6TN cells.