Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/16/1996
Publication Date: N/A
Interpretive Summary: Iron deficiency anemia is generally considered the most prevalent nutritional deficiency in both the USA and the world. The factors that regulate iron absorption and bioavailability of iron from foods and meals are poorly understood. This report describes research on Caco-2 cells, a human intestinal cell line that provides an attractive model for studies of fhuman iron absorption. For this model to be useful in studies of iron absorption, we need to establish conditions which mimic clinical and nutritional observations using human subjects. Iron absorption involves two processes, uptake of iron into the cell and transfer of iron out of the cell into the blood. We found that iron uptake by Caco-2 cells mimics that observed in humans, in that reduced iron (ferrous) is taken up more readily than oxidized (ferric) iron. Also, we found that cells of low iron status take up more iron than those of normal iron status if iron is provided in the ferrous form, but not if it is provided in the ferric form We did not observe any differences in the transfer component between low- iron status and normal-iron status cells. Development and validation of this model for determining mechanisms that regulate iron absorption and factors affecting iron availability could contribute, significantly, to efforts to reduce the incidence of iron-deficiency anemia.
Technical Abstract: Caco-2 cells in culture provide an attractive model for the study of human iron absorption. Since iron status has a marked effect on human iron absorption, we devised serum-free growth conditions which allow manipulation of Caco-2 cell iron stores while maintaining growth. Caco-2 cells were cultured in serum- free media containing 0-20 micro mol/L added iron. Intracellular ferritin, measured by radioimmunoassay, increased 100-fold with the addition of 20 micro mol/L iron to the serum-free growth medium. Iron uptake and transfer across Caco-2 cell monolayers were measured from balanced salt solutions of ferrous and ferric forms of iron. Uptake from ferrous, but not ferric, iron was inversely proportional to cell ferritin concentration and culture media iron concentration. Kinetic analysis of uptake data from solutions of ferrous and ferric iron revealed saturable and nonsaturable components for ferrous iron, but only a nonsaturable component for ferric iron. Uptake by the nonsaturable pathwa was not affected by cell ferritin concentration for either form of iron. Maximal uptake of ferrous iron via the saturable pathway was nearly two-fold greater in cells cultured under low versus high iron conditions. Iron transfer across Caco-2 cell monolayers was not proportional to iron uptake, but was related to monolayer permeability. Iron uptake by Caco-2 cells is a reliable indicator of relative iron availability. In contrast, iron transfer across Caco-2 cell monolayers cultured under serum-free conditions on microporous membranes does not mimic absorption patterns in humans.