The Establishment of Polarity and Enhanced Transcytosis of Transferrin Receptors in Enterocyte-like Caco-2 Cells

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Abstract
The effect of enterocyte-like differentiation on the transferrin receptor (TfR) polarity in filter-grown Caco-2 cells was studied. The ratio of apical to basolateral TfRs which was found to be approximately 1:1 on the first day after the cells had reached confluence, changed to 1:40 eight days after reaching confluence. The transepithelial electrical resistance (TEER), transport of horseradish peroxidase (HRP) across the monolayer, and total cellular TfR number remained constant over this period. However, the activity of brush border membrane-associated alkaline phosphatase, an established marker for enterocyte differentiation, increased over this 8-day period concurrent with a decrease in apical TfR number. These results suggest that enterocyte-like differentiation rather than tight junction formation is most likely responsible for the polarized distribution of TfRs in Caco-2 cells. The effects of the fungal metabolite brefeldin A (BFA) on TfR distribution and TfR-mediated transcytosis in Caco-2 cells were also studied. BFA caused a marked decrease in the number of basolateral TfRs along with a slight increase in the number of apical TfR. BFA enhanced the TfR-mediated transcytosis of both 125I-Tf and the horseradish peroxidase-Tf conjugate across Caco-2 cells in both apical-to-basolateral and basolateral-to-apical directions. These findings imply a potential application of BFA as an enhancer for TfR-mediated delivery of protein drugs across the intestinal epithelium.