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

The cellular mechanism by which basolateral transferrin (Tf) produces an increase in apical-to-basolateral Fe flux in Caco-2 cells was analyzed. After a pulse of59Fe from the apical medium, three types of basolateral59Fe efflux were found: a59Fe efflux that was independent of the presence of Tf in the basolateral medium, a59Fe efflux in which59Fe left the cell bound to Tf, and a Tf-dependent59Fe efflux in which59Fe came off the cell not bound to Tf. Furthermore, addition of Tf to the basolateral medium doubled the exocytosis rate of Tf and increased the secretion of apolipoprotein A, a basolateral secretion marker. Both apotransferrin and Fe-containing Tf produced similar increases in59Fe efflux, Tf exocytosis, and apolipoprotein A secretion. The Ca2+channel inhibitor SKF-96365 inhibited both the Tf-mediated increase in transepithelial Fe transport and the secretion of apolipoprotein A. Thus the activation of transepithelial Fe transport by Tf seems to be mediated by Ca2+entry into the cells.