Responsive transporter genes within the murine intestinal-pancreatic axis form a basis of zinc homeostasis

Zn homeostasis in animals is a consequence of avid uptake and retention, except during conditions of limited dietary availability, and/or factors such as parasites, which compete for this micronutrient or compromise retention by the host. Membrane proteins that facilitate Zn transport constitute the SLC30A (ZnT) and SLC39A (Zip) gene families. Because dietary recommendations are based on the balance between intestinal absorption and endogenous losses, we have studied Zn transporter expression of the murine intestinal-pancreatic axis to identify transporters that are likely to be involved in homeostatic control of Zn metabolism. Marked tissue specificity of expression was observed in Zn-depleted vs. Zn-adequate mice. As shown by quantitative PCR, Western blot analysis, and immunohistochemistry, intestinal Zip4 was markedly up-regulated in response to Zn-depletion conditions. The increased abundance of Zip4 is concentrated at the apical membrane of enterocytes. There are 16 ZnT and Zip transporters expressed in pancreas. Only two, ZnT1 and ZnT2 (both cellular Zn exporters), show a progressive down-regulation under Zn-depleted conditions. In Zn-adequate mice, ZnT1 is diffusely distributed in acinar cell cytoplasm and colocalizes with alpha-amylase but is not detected in pancreatic islets. In acinar cells during Zn depletion, ZnT1 is localized to the plasma membrane. Intestinal Zip4 up-regulation by Zn-depletion conditions is dampened in metallothionein knockout mice, suggesting that intracellular Zn pools influence these responses. The results show that Zn transporter expression in the intestinal-pancreatic axis is a component of the homeostatic regulation of this micronutrient.

Regulation of zinc metabolism and genomic outcomes

Differential mRNA display and cDNA array analysis have identified zinc-regulated genes in small intestine, thymus and monocytes. The vast majority of the transcriptome is not influenced by dietary zinc intake, high or low. Of the genes that are zinc regulated, most are involved in signal transduction (particularly influencing the immune response), responses to stress and redox, growth and energy utilization. Among the genes identified are uroguanylin (UG), cholecystokinin, lymphocyte-specific protein tyrosine kinase (LCK), T-cell cytokine receptor, heat shock proteins and the DNA damage repair and recombination protein-23B. Zinc transporters (ZnT) help regulate the supply of this micronutrient to maintain cellular functions. Expression of ZnT-1 and -2 is regulated by dietary zinc in many organs including small intestine and kidney. ZnT-4 is ubiquitously expressed but is refractory to zinc intake. Expression of ZnT-1, -2 and -4 changes markedly during gestation and lactation from highly abundant to undetectable. Each ZnT has an endosomal-like appearance in the tissues examined. Upregulation of ZnT-1 and ZnT-2 by dietary zinc strongly implicates these transporters in zinc acquisition and/or storage for subsequent systemic needs. THP-1 cells were used as a model to examine the response of human cells to changes in zinc status. Based on mRNA quantities, Zip1 and ZnT-5 were the most highly expressed. Zinc depletion of these cells decreased expression of all transporters except Zip2, where expression increased markedly. Collectively, these findings provide a genomic footprint upon which to address the biological and clinical significance of zinc and new avenues for status assessment.

Zinc transporters 1, 2 and 4 are differentially expressed and localized in rats during pregnancy and lactation

Zinc metabolism is controlled within relatively restricted limits throughout the life cycle. Expression and localization of zinc transporters 1, 2 and 4 during pregnancy and lactation in small intestine, mammary gland and liver of the rat were investigated using Northern analysis, Western blotting and immunohistochemistry. In maternal tissues, zinc transporter 4 was the most widely expressed among these zinc transporters in the tissues examined. In small intestine and liver, zinc transporter 4 increased from levels found during late gestation, but zinc transporter 1 did not. Zinc transporter 2 expression in small intestine was transient, being highest around parturition, and was not detected in liver. Immunohistochemistry revealed unique patterns of zinc transporter localization at different stages of development. In the placenta, zinc transporters 1 and 4 were found concentrated along the villous visceral splanchnopleure. In the mammary gland, zinc transporter 4 was most abundant in cells surrounding the alveolar ducts and oriented to the basement lamina. All three transporters were highly expressed in neonatal small intestine, principally near the apical surface, but zinc transporters 1 and 4 increased in abundance at the basolateral surface during development. Zinc transporter 2 was oriented apically, directly adjacent to the microvilli of enterocytes. Within the intestine, expression of each transporter was limited to enterocytes. These results support a role for these transporters in maintaining an adequate zinc supply derived from the maternal diet for zinc acquisition and use by the fetus and neonate.

Effects of intracellular zinc depletion on metallothionein and ZIP2 transporter expression and apoptosis

Zinc is critical for the functional and structural integrity of cells. We have used the monocytic cell line THP-1 as a model in which to study both the responsiveness of metallothionein and ZIP2 transporter expression to zinc depletion induced by the intracellular zinc chelator TPEN [N,N,N',N'-tetrakis(2-pyridylmethyl) ethylenediamine] and the extent of concomitant apoptosis. Metallothionein expression increased proportionately with the addition of zinc to the medium and decreased with TPEN treatment. When treated with TPEN, both THP-1 cells and human peripheral blood mononuclear cells exhibited marked decreases in cellular zinc concentrations and increases in ZIP2 mRNA expression. These results suggest that cells attempt to homeostatically adjust to zinc depletion. When THP-1 cells were treated with >5 microM TPEN, cell viability decreased, and cells entered the early stages of apoptosis. These data show that metallothionein and ZIP2 expression are inversely related during zinc depletion and that apoptosis is concurrent with these changes.