THE ESTABLISHMENT OF POLARITY AND ENHANCED TRANSCYTOSIS OF TRANSFERRIN RECEPTORS IN ENTEROCYTE-LIKE CACO-2 CELLS

The effect of enterocyte-like differentiation on the transferrin recep tor (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 electric al resistance (TEER), transport of horseradish peroxidase (HRP) across the monolayer, and total cellular TfR number remained constant over t his period. However, the activity of brush border membrane-associated alkaline phosphatase, an established marker for enterocyte differentia tion, increased over this 8-day period concurrent with a decrease in a pical TfR number. These results suggest that enterocyte-like different iation rather than tight junction formation is most likely responsible for the polarized distribution of TfRs in Caco-2 cells. The effects o f 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 tr anscytosis of both I-125-Tf and the horseradish peroxidase-Tf conjugat e across Caco-2 cells in both apical-to-basolateral and basolateral-to -apical directions. These findings imply a potential application of BF A as an enhancer for TfR-mediated delivery of protein drugs across the intestinal epithelium.