Dose Effects of Apical versus Basolateral Zinc Supplementation on Epithelial Resistance, Viability, and Metallothionein Expression in Two Intestinal Epithelial Cell Lines.

Zinc supplementation is used to reduce diarrhea incidence in piglets and it has been shown in vitro that the antisecretory effects are maximal after basolateral zinc application. To examine whether the application site and dose of zinc also influence passive ion permeability and viability, porcine (IPEC-J2) and human (Caco-2) intestinal epithelial cells were treated with increasing zinc concentrations (0-200 µM) at either the apical or basolateral side. Transepithelial electrical resistance and viability decreased and expression of metallothionein and the efflux zinc transporter 1 increased most prominently when zinc was added in high concentrations at the basolateral side of IPEC-J2 cells. Zinc transporter 4, a zinc importer, was not affected. Heat shock protein 70 mRNA expression increased only after basolateral addition of 200 µM zinc in IPEC-J2 cells. Thus, zinc can elicit toxic effects especially when added at the basolateral side, with IPEC-J2 cells being more susceptible than Caco-2 cells.

Regulation of intracellular Zn homeostasis in two intestinal epithelial cell models at various maturation time points.

After weaning, piglets are often fed diets supplemented with high concentrations of zinc (Zn) to decrease post-weaning diarrhea. The aim of this study was to elucidate the regulation of Zn homeostasis within intestinal epithelial cells during excessive Zn exposure. High Zn concentrations elevated the intracellular Zn level in IPEC-J2 and Caco-2 cells which was influenced by differentiation status and time of exposure. With increasing Zn concentrations, mRNA and protein levels of metallothionein (MT) and zinc transporter 1 (ZnT1) were upregulated, whereas zinc transporter 4 (ZIP4) expression was downregulated. Metal-regulatory transcription factor-1 (MTF1) mRNA expression was upregulated at high Zn concentrations in IPEC-J2 cells, which corresponded to higher intracellular Zn concentrations. Based on these results, we suggest that intestinal epithelial cells adapt the expression of these genes to the amount of extracellular Zn available in order to maintain Zn homeostasis. Cell line-dependent differences in the regulation of Zn homeostasis were detected.

A high amount of dietary zinc changes the expression of zinc transporters and metallothionein in jejunal epithelial cells in vitro and in vivo but does not prevent zinc accumulation in jejunal tissue of piglets.

High dietary zinc concentrations are used to prevent or treat diarrhea in piglets and humans, but long-term adaptation to high zinc supply has yet not been assessed. Intestinal zinc uptake is facilitated through members of zinc transporter families SLC30 (ZnT) and SLC39 (ZIP). Whereas in rodents, regulation of zinc homeostasis at low or adequate zinc supply has been described, such mechanisms are unclear in piglets. A total of 54 piglets were fed diets containing 57 [low dietary zinc (LZn)], 164 [normal dietary zinc (NZn)], or 2425 [high dietary zinc (HZn)] mg/kg dry matter zinc. After 4 wk, 10 piglets/group were killed and jejunal tissues taken for analysis of zinc transporters SLC30A1 (ZnT1), SLC30A2 (ZnT2), SLC30A5 (ZnT5), SLC39A4 (ZIP4), divalent metal transporter 1 (DMT1), and metallothionein-1 (MT). Weight gain was higher (P < 0.05) in pigs fed HZn than in the LZn and NZn groups during the first 2 wk. Food intake did not differ between groups. The digesta and jejunal tissue zinc concentrations were higher (P < 0.05) in the HZn pigs than in NZn and LZn pigs. Expression of ZnT1 was higher (P < 0.05) and ZIP4 lower (P < 0.05) in HZn pigs than in the 2 other groups, whereas expression of ZnT5 and DMT1 did not differ between treatments. Expression of ZnT2 was lower (P < 0.05) in the LZn group than in the HZn and NZn groups. The mRNA expression and protein abundance of MT was higher (P < 0.05) in the HZn group than in the NZn and LZn groups. Studies with intestinal porcine cell line intestinal epithelial cell-J2 confirmed the dose-dependent downregulation of ZIP4 and upregulation of ZnT1 and MT (P < 0.05) with increasing zinc concentration within 24 h. In conclusion, high dietary zinc concentrations increase intracellular zinc, promote increased zinc export from intestinal tissues into extracellular compartments, and decrease zinc uptake from the gut lumen. The adaptive process appears to be established within 24 h; however, it does not prevent tissue zinc accumulation.