|Briske Anderson, Mary|
Submitted to: Journal of Nutritional Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/25/2001
Publication Date: 12/1/2001
Citation: Reeves, P.G., Briske-Anderson, M.J., Johnson, L. 2001. Pre-treatment of caco-2 cells with zinc during the differentiation phase alters the kinetics of zinc uptake and transport. Journal of Nutritional Biochemistry. 12:674-684. Interpretive Summary: It is important to understand how components of the diet affect the absorption of nutrients from the intestine. Diets containing zinc at amounts equal to that found in an over-the-counter mineral supplement may cause a reduction in the amount of zinc absorbed into the body. Specific proteins in the membrane of cells aid in the transport (absorption) of the trace minerals. These proteins are called transporters, and they can be made or degraded depending on how much of the mineral is in the diet. We tested the hypothesis that zinc might be inhibiting the function of its own transport proteins by using a single cell type called Caco-2 that acts like the intestinal cell and will grow in culture dishes. We grew these cells in normal and high amounts of zinc in the growth media, and then determined whether uptake of zinc into the cells and transport of zinc across the cell layer was affected. We found that both zinc uptake and transport were significantly decreased when the cells were grown in high zinc conditions. These studies provide evidence that dietary zinc can regulate its own transporters for getting zinc into the cell as well as for getting it out of the cell. We postulate further that the affected transporters, recently discovered by others and named hZip-2 and ZnT-1, might be regulated at the level of the gene by the amount of dietary zinc consumed.
Technical Abstract: The Caco-2 cell model was used to study the efficiency of absorption and endogenous excretion of zinc (Zn) regulated by dietary Zn concentration. Cells were seeded onto high pore-density membranes and maintained in medium supplemented with 10% FBS. After confluence, cells were treated with 5 or 25 uM Zn for 7 d, and Zn uptake and transport were measured in both apical (AP) and basolateral (BL) directions by using 65Zn. Similar cells were labeled with 65**Zn and the release of Zn to the AP and BL sides was measured. The AP uptake of Zn in cells exposed to 25 uM Zn was slower (p<0.05) than that in cells exposed to 5 uM Zn. The AP to BL transport rate in the 25 uM Zn group was only 40% (p<0.05) of that in the 5 uM group. In contrast, the rate of BL Zn uptake was 4-fold higher in cells treated with 25 uM Zn than in those treated with 5 uM Zn (p<0.05). The BL to AP transport rate was 2-fold higher in cells treated with 25 uM Zn than in those treated with 5 uM Zn (p<0.05). Basolateral uptake was 6 to 25 times greater (p<0.05) than AP uptake for cells treated with 5 and 25 uM Zn, respectively. The rate of Zn release was enhanced about 4-fold (p<0.05) by 25 uM Zn treatment. Release to the BL side was 10 times greater than to the AP side. Zn-induced metallothionein, thought to down- regulate AP to BL Zn transport, was 4-fold higher (p<0.001) in the 25 uM Zn group than in the 5 uM group, but the rate of BL Zn release was higher in cells treated with 25 uM Zn than in those treated with 5 uM Zn (p<0.05). Induced changes in transport rates by media Zn concentrations could involve the up- and/or down-regulation of Zn influx and efflux proteins such as hZip-2 and ZnT-1.