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Ferrous Iron Uptake but Not Transfer Is Down-Regulated in Caco-2 Cells Grown in High Iron Serum-Free Medium^1,2,

Institute of Food Science, Cornell University, Ithaca, NY 14853 ^* U.S. Plant, Soil and Nutrition Laboratory, USDA-Agricultural Research Service, Ithaca, NY 14853

Caco-2 cells in culture provide an attractive model for the study of human iron absorption. Because iron status has a marked effect on human iron absorption, we devised serum-free growth conditions that allow manipulation of Caco-2 cell iron stores while maintaining growth. Caco-2 cells were cultured in serum-free media containing 0–20 µmol/L added iron. Intracellular ferritin, measured by radioimmunoassay, increased 100-fold with the addition of 20 µmol/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 related 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 nonsaturable pathway was not affected by cell ferritin concentration for either form of iron. Maximal uptake from a ferrous iron solution via the saturable pathway was nearly 100% greater in cells cultured under low compared with high iron conditions. Iron transfer across Caco-2 monolayers was not proportional to iron uptake, but was related to monolayer permeability. Iron uptake by Caco-2 cells was a reliable indicator of relative iron availability. We observed no difference in iron transfer that was related to the iron status of the cell monolayer. The lack of this effect suggests that this model may be inadequate for studies of iron transfer.

KEY WORDS: • Caco-2 • iron uptake • iron transfer • ferritin • serum-free

¹ Proprietary or brand names are provided for the convenience of the reader; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 4 March 1996. Revision accepted 30 August 1996.

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