REGULATION OF IRON UPTAKE AND TRANSPORT BY TRANSFERRIN IN CACO-2 CELLS, AN INTESTINAL-CELL LINE

Caco-2 cells grown in bicameral chambers, a model of intestinal epithe lial iron transport (Biochim. Biophys. Acta (1991) 1070, 205-208), wer e used to study the effect of apo-transferrin (apo-Tf) in the basal ch amber on Fe-59 uptake from the apical surface, intracellular Fe-59 dis tribution, and Fe-59 transport into the basal chamber. Caco-2 cells we re grown with varying amounts of iron to achieve cells that were eithe r iron-deficient (FeD), of normal iron status (FeN), or iron-loaded (F eH). The effect of apo-Tf was most marked in FeD cells with the transp ort of Fe-59 from 1 mu M Fe-59-ascorbate on the apical side to the bas al chamber measured as (22.2 +/- 3.0) . 10(4), (8.2 +/- 0.6) . 10(4), and (2.7 +/- 0.4) . 10(4) atoms Fe-59/cell/min in the presence of apo- Tf, BSA, and no added protein, respectively. Unexpectedly in FeD cells total Fe-59 uptake (i.e., both Fe-59 in the cells and that transporte d into the basal chamber) was decreased by basolateral apo-Tf with tot al uptake of (2.6 +/- 0.3) . 10(5), (4.8 +/- 0.6) . 10(5), and (4.8 +/ - 0.7) . 10(5) atoms/cell/min with apo-Tf, BSA, and no additions, resp ectively. Analysis of intracellular Fe-59 by isoelectrofocusing in pol yacrylamide gels demonstrated Fe-59 migrating both with a basic pI and with the pI values of ferritin (Ft) at a ratio of 200:1 (basic pI moi ety: ferritin) in FeD cells. The presence of Tf further decreased the small amount of Fe-59 in Ft. These studies demonstrate that basolatera l Tf affects the apical uptake of Fe-59, the intracellular distributio n of Fe-59, and the transport of Fe-59 across intestinal epithelium, t he latter effect occurring even when cellular content of ferritin is h igh.