Transferrin stimulates iron absorption, exocytosis, and secretion in cultured intestinal cells

The cellularmechanism by which basolateral transferrin (Tf) produces an increase inapical-to-basolateral Fe flux in Caco-2 cells was analyzed. After apulse of ⁵⁹Fe from the apicalmedium, three types of basolateral⁵⁹Fe efflux were found: a⁵⁹Fe efflux that was independentof the presence of Tf in the basolateral medium, a⁵⁹Fe efflux in which⁵⁹Fe left the cell bound to Tf,and a Tf-dependent ⁵⁹Fe efflux inwhich ⁵⁹Fe came off the cell notbound to Tf. Furthermore, addition of Tf to the basolateral mediumdoubled the exocytosis rate of Tf and increased the secretion ofapolipoprotein A, a basolateral secretion marker. Both apotransferrinand Fe-containing Tf produced similar increases in⁵⁹Fe efflux, Tf exocytosis, andapolipoprotein A secretion. The Ca²⁺ channel inhibitor SKF-96365inhibited both the Tf-mediated increase in transepithelial Fe transportand the secretion of apolipoprotein A. Thus the activation oftransepithelial Fe transport by Tf seems to be mediated byCa²⁺ entry into the cells.