FERROUS IRON UPTAKE BUT NOT TRANSFER IS DOWN-REGULATED IN CACO-2 CELLS GROWN IN HIGH IRON SERUM-FREE MEDIUM

Caco-2 cells in culture provide an attractive model for the study of h uman iron absorption. Because iron status has a marked effect on human iron absorption, we devised serum-free growth conditions that allow m anipulation of Caco-2 cell iron stores while maintaining growth. Caco- 2 cells were cultured in serum-free media containing 0-20 mu mol/L add ed iron. Intracellular ferritin, measured by radioimmunoassay, increas ed 100-fold with the addition of 20 mu mol/L iron to the serum-free gr owth medium. Iron uptake and transfer across Caco-2 cell monolayers we re measured from balanced salt solutions of ferrous and ferric forms o f iron. Uptake from ferrous, but not ferric, iron was inversely relate d to cell ferritin concentration and culture medium 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 nons aturable pathway was not affected by cell ferritin concentration for e ither form of iron. Maximal uptake from a ferrous iron solution via th e saturable pathway was nearly 100% greater in cells cultured under lo w compared with high iron conditions. Iron transfer across Caco-2 mono layers was not proportional to iron uptake, but was related to monolay er permeability. Iron uptake by Caco-2 cells was a reliable indicator of relative iron availability. We observed no difference in iron trans fer that was related to the iron status of the cell monolayer. The lac k of this effect suggests that this model may be inadequate for studie s of iron transfer.