Dietary Zinc Absorption Research Articles

Simple in vitro method to evaluate ZIP zinc transport ability through zinc transporter 1 and metallothionein expression measurements.

Measuring the cellular zinc content and examining the alteration of zinc status are critical for investigating the cellular homeostasis and dynamics of zinc and its involvement in patho-physiological functions. Many Zrt- and Irt-related protein (ZIP) transporters uptake zinc from the extracellular space. Among Zn transporters (ZNTs), ZNT1 effluxes cytosolic zinc. As cytosolic zinc-binding proteins, metallothioneins (MTs) also contribute to the control of cellular zinc homeostasis. Systemic and cellular zinc homeostasis is considered to be maintained by balancing expression and functional activities of these proteins. The zinc transport ability of ZIPs is typically measured by evaluating cellular zinc content with various zinc-detection methods and systems. Many small-molecule fluorescent probes and fluorescence resonance energy transfer-based protein sensors have been exploited for this purpose. Although powerful analytical methods using special instruments have been developed to quantify zinc, they are often not easily accessible. Here, we present a simplified and inexpensive method to estimate the zinc transport ability of ZIP transporters using the expression responses of ZNT1 and MT. This protocol should be effective in several applications because ZNT1 and MT expression are easily evaluated by immunoblotting and immunofluorescence staining as basic biochemical techniques available in most laboratories. This method is advantageous for examining the relative zinc status or alterations mediated by expression changes of ZIPs in cells cultured in normal medium without zinc supplementation. As zinc is an essential micronutrient, extensive research is necessary to improve dietary zinc absorption to promote health. Therefore, we also propose a simple screening method of foods to improve zinc absorption as an application of measuring ZIP-mediated MT expression.

Slc39a4 in the small intestine predicts zinc absorption and utilization: a comprehensive analysis of zinc transporter expression in response to diets of varied zinc content in young mice

Zinc homeostasis is primarily maintained by zinc transporters that regulate zinc uptake and efflux in the small intestine; however, the relative contribution of the many zinc transporters identified (Slc39a1-14, Slc30a1-10) to dietary zinc absorption and utilization remains unknown. The objective of this study was to determine the expression of Slc39a1-14 and Slc30a1-10 in the small intestine and their relative contribution to dietary zinc absorption in mice. Five-week-old male C57BL/6J mice were fed modified AIN-93G diets containing <1, 30, or 100ppm zinc (n=15 mice/diet). Following 1 week of feeding, mice were given an oral gavage containing 67Zn and liver and plasma isotope appearance was determined 6-h later by ICP-MS. Expression of Slc39a1-14 and Slc30a1-10 was determined in mucosa from duodenum, jejunum, and ileum. Plasma and liver total zinc concentrations were not different after one week of feeding (P>.05). Liver and plasma appearance of 67Zn was greater in mice fed <1ppm compared to the 30ppm (P<.0001) and 100ppm (P<.0001) zinc diets. With the exception of Slc39a2, Slc39a12, Slc30a3, and Slc30a8, the remaining zinc transporters were expressed across all diets and intestinal segments. Expression of Slc39a4, Slc39a11, and Slc30a6 changed with diet (Pdiet<.05 for all); expression of Slc39a5, Slc39a7, Slc39a11, Slc39a14, Slc30a1, Slc30a2, Slc30a4, Slc30a5, Slc30a7, and Slc30a10 changed by intestinal segment (Psegment<.05 for all). Slc39a4 was the only transporter positively associated with liver (r2=0.316, P<.001) and plasma (r2=0.189, P<.01) 67Zn appearance. Although most zinc transporters are expressed in the small intestine, intestinal Slc39a4 predicts fractional zinc absorption and utilization in young mice.

Influence of Dairy Products on Bioavailability of Zinc from Other Food Products: A Review of Complementarity at a Meal Level.

In this paper, we reviewed the role of dairy products in dietary zinc absorption. Dairy products can have a reasonable contribution for dietary zinc intake in Western diets, where dairy consumption is high. However, the co-ingestion of dairy products can also improve zinc absorption from other food products. Such improvements have been observed when dairy products (e.g., milk or yoghurt) were ingested together with food such as rice, tortillas or bread products, all of which are considered to be high-phytate foods with low inherent zinc absorption. For foods low in phytate, the co-ingestion of dairy products did not improve zinc absorption. Improved zinc absorption of zinc from high-phytate foods following co-ingestion with dairy products may be related to the beneficial effects of the citrate and phosphopeptides present in dairy products. Considering that the main dietary zinc sources in areas in the world where zinc deficiency is most prevalent are typically high in phytate, the inclusion of dairy products in meals may be a viable dietary strategy to improve zinc absorption.

Drosophila ZnT1 is essential in the intestine for dietary zinc absorption

Zinc is an essential trace element and participates in a variety of biological processes. ZnT (SLC30) family members are generally responsible for zinc efflux across the membrane regulating zinc homeostasis. In mammals, the only predominantly plasma membrane resident ZnT has been reported to be ZnT1, and ZnT1-/ZnT1- mice die at the embryonic stage. In Drosophila, knock down of ZnT1 homologue (dZnT1//ZnT63C/CG17723) results in growth arrest under zinc-limiting conditions. To investigate the essentiality of dZnT1 for zinc homeostasis, as well as its role in dietary zinc uptake especially under normal physiological conditions, we generated dZnT1 mutants by the CRISPER/Cas9 method. Homozygous mutant dZnT1 is lethal, with substantial zinc accumulation in the iron cell region, posterior midgut as well as gastric caeca. Expression of human ZnT1 (hZnT1), in the whole body or in the entire midgut, fully rescued the dZnT1 mutant lethality, whereas tissue-specific expression of hZnT1 in the iron cell region and posterior midgut partially rescued the developmental defect of the dZnT1 mutant. Supplementation of zinc together with clioquinol or hinokitiol conferred a limited but observable rescue upon dZnT1 loss. Our work demonstrated the absolute requirement of dZnT1 in Drosophila survival and indicated that the most essential role of dZnT1 is in the gut.

An extracellular histidine-containing motif in the zinc transporter ZIP4 plays a role in zinc sensing and zinc-induced endocytosis in mammalian cells

Zinc is an essential trace element that serves as a cofactor for enzymes in critical biochemical processes and also plays a structural role in numerous proteins. Zinc transporter ZIP4 (ZIP4) is a zinc importer required for dietary zinc uptake in the intestine and other cell types. Studies in cultured cells have reported that zinc stimulates the endocytosis of plasma membrane-localized ZIP4 protein, resulting in reduced cellular zinc uptake. Thus, zinc-regulated trafficking of ZIP4 is a key means for regulating cellular zinc homeostasis, but the underlying mechanisms are not well understood. In this study, we used mutational analysis, immunoblotting, HEK293 cells, and immunofluorescence microscopy to identify a histidine-containing motif (398HTH) in the first extracellular loop that is required for high sensitivity to low zinc concentrations in a zinc-induced endocytic response of mouse ZIP4 (mZIP4). Moreover, using synthetic peptides with selective substitutions and truncated mZIP4 variants, we provide evidence that histidine residues in this motif coordinate a zinc ion in mZIP4 homodimers at the plasma membrane. These findings suggest that 398HTH is an important zinc-sensing motif for eliciting high-affinity zinc-stimulated endocytosis of mZIP4 and provide insight into cellular mechanisms for regulating cellular zinc homeostasis in mammalian cells.

Zinc Deficiency and Long-Term Outcome in Cases After Isolated Intestinal Transplantation in Taiwan

ObjectivesThe small intestine is the primary site for absorption of dietary zinc. Intestinal transplant recipients are at high risk for zinc deficiency because of the long process of posttransplant adaptation. We initiated an intestinal transplant program in Taiwan in 2007. In this study, we aimed to retrospectively investigate the incidence of zinc deficiency in recipients after intestinal transplantation. MethodsTwenty-one isolated intestinal transplants were performed in 20 patients with 1 retransplantation. The level of serum zinc was monitored periodically, and zinc supplements were administered when zinc level was below 700 ng/mL. Twelve patients with graft above 1-year survival and with available related data were enrolled for the analysis of zinc deficiency. The levels of serum zinc were tracked, and the protocol of zinc supplementation is discussed herein. ResultsThe survival rates of 20 transplant recipients for 1 year, 3 years, and 5 years were 85%, 75%, and 65%, respectively. In the 12 grafts that survived longer than 1 year, we found that zinc deficiency was highest during the third (41.7%) to sixth (50%) month after transplantation. Sustained supplementation of zinc was required for over 70% of patients throughout the 3-year period to maintain their zinc level around the lower normal limit. ConclusionThe outcome of isolated small bowel transplantation is promising. Periodical monitoring and sufficient dosing of zinc supplements should be considered into the posttransplant protocol to prevent zinc deficiency after intestinal transplantation.

Zinc Absorption from Representative Diet in a Chinese Elderly Population Using Stable Isotope Technique

ObjectiveTo determine the dietary zinc absorption in a Chinese elderly population and provide the basic data for the setting of zinc (Zn) recommended nutrient intakes (RNI) for Chinese elderly people. MethodsA total of 24 elderly people were recruited for this study and were administered oral doses of 3 mg 67Zn and 1.2 mg dysprosium on the fourth day. The primary macronutrients, energy, and phytic acid in the representative diet were examined based on the Chinese National Standard Methods. Fecal samples were collected during the experimental period and analyzed for zinc content, 67Zn isotope ratio, and dysprosium content. ResultsThe mean (± SD) zinc intake from the representative Chinese diet was 10.6 ± 1.5 mg/d. The phytic acid-to-zinc molar ratio in the diet was 6.4. The absorption rate of 67Zn was 27.9% ± 9.2%. The RNI of zinc, which were calculated by the absorption rate in elderly men and women, were 10.4 and 9.2 mg/d, respectively. ConclusionThis study got the dietary Zn absorption in a Chinese elderly population. We found that Zn absorption was higher in elderly men than in elderly women. The current RNI in elderly female is lower than our finding, which indicates that more attention is needed regarding elderly females' zinc status and health.

A role for the Drosophila zinc transporter Zip88E in protecting against dietary zinc toxicity.

Zinc absorption in animals is thought to be regulated in a local, cell autonomous manner with intestinal cells responding to dietary zinc content. The Drosophila zinc transporter Zip88E shows strong sequence similarity to Zips 42C.1, 42C.2 and 89B as well as mammalian Zips 1, 2 and 3, suggesting that it may act in concert with the apically-localised Drosophila zinc uptake transporters to facilitate dietary zinc absorption by importing ions into the midgut enterocytes. However, the functional characterisation of Zip88E presented here indicates that Zip88E may instead play a role in detecting and responding to zinc toxicity. Larvae homozygous for a null Zip88E allele are viable yet display heightened sensitivity to elevated levels of dietary zinc. This decreased zinc tolerance is accompanied by an overall decrease in Metallothionein B transcription throughout the larval midgut. A Zip88E reporter gene is expressed only in the salivary glands, a handful of enteroendocrine cells at the boundary between the anterior and middle midgut regions, and in two parallel strips of sensory cell projections connecting to the larval ventral ganglion. Zip88E expression solely in this restricted subset of cells is sufficient to rescue the Zip88E mutant phenotype. Together, our data suggest that Zip88E may be functioning in a small subset of cells to detect excessive zinc levels and induce a systemic response to reduce dietary zinc absorption and hence protect against toxicity.

Functional studies of Drosophila zinc transporters reveal the mechanism for zinc excretion in Malpighian tubules

BackgroundZinc is an essential metal involved in many physiological processes. Previous work has identified a set of zinc transporters involved in Drosophila dietary zinc absorption. However, zinc excretion and reabsorption, the other two important processes to maintain zinc homeostasis, are not as well understood. In this work, we screened all the potential zinc transporter Zip (SLC39) and ZnT (SLC30) members for their likely roles in zinc excretion in Malpighian tubules, an insect organ functionally analogous to mammalian kidneys.ResultsZip71B (CG10006, most homologous to hZIP5), in addition to the previously characterized ZnT35C (CG3994), was identified as being critical in zinc excretion. Tubule-specific knockdown of Zip71B/dZip5 reduces zinc accumulation in the tubules, but increases zinc levels in the body, resulting in survival defect under zinc excess conditions. Zip71B/dZip5 is localized to the plasma membrane at the basolateral side of the tubules, and is functionally epistatic to the apically localized ZnT35C in regulating the tubule zinc homeostasis. Our results indicate that Zip71B/dZip5 is involved in zinc import into the tubular cells from the circulation, and ZnT35C in turn effluxes the tubular zinc out. Notably, mammalian ZIP5, which is expressed in the kidney, functions analogously to Zip71B/dZip5 in the fly while hZIP4 cannot complement the loss of Zip71B/dZip5 function. Furthermore, Zip71B/dZip5 expression is regulated by zinc so that, in response to toxic levels of zinc, the tubules can increase zinc efflux capability. We also characterized the role of dZnT1 (CG17723) in zinc reabsorption in Malpighian tubules. Finally, using a tubule calcification model, we were able to show that knockdown of Zip71B/dZip5 or ZnT35C was able to mitigate stone formation, consistent with their roles in tubular zinc homeostasis.ConclusionsOur results start to sketch out a relatively complete picture of the zinc excretion process in Drosophila Malpighian tubules, and may provide a reference for relevant mammalian studies.

Fractional Zinc Absorption for Men, Women, and Adolescents Is Overestimated in the Current Dietary Reference Intakes

Background: The fractional zinc absorption values used in the current Dietary Reference Intakes (DRIs) for zinc were based on data from published studies. However, the inhibitory effect of phytate was underestimated because of the low phytate content of the diets in the studies used.Objective: The objective of this study was to estimate the fractional absorption of dietary zinc from the US diet by using 2 published algorithms.Methods: Nutrient intake data were obtained from the NHANES 2009–2010 and the corresponding Food Patterns Equivalents Database. Data were analyzed with the use of R software by taking into account the complex survey design. The International Zinc Nutrition Consultative Group (IZiNCG; Brown et al. Food Nutr Bull 2004;25:S99–203) and Miller et al. (Br J Nutr 2013;109:695–700) models were used to estimate zinc absorption.Results: Geometric means (95% CIs) of zinc absorption for all subjects were 30.1% (29.9%, 30.2%) or 31.3% (30.9%, 31.6%) with the use of the IZiNCG model and Miller et al. model, respectively. For men, women, and adolescents, absorption values obtained in this study with the use of the 2 models were 27.2%, 31.4%, and 30.1%, respectively, for the IZiNCG model and 28.0%, 33.0%, and 31.6%, respectively, for the Miller et al. model, compared with the 41%, 48%, and 40%, respectively, used in the current DRIs. For preadolescents, estimated absorption values (31.1% and 32.8% for the IZiNCG model and Miller et al. model, respectively) compare well with the conservative estimate of 30% used in the DRIs. When the new estimates of zinc absorption were applied to the current DRI values for men and women, the results suggest that the Estimated Average Requirement (EAR) and RDA for these groups need to be increased by nearly one-half of the current values in order to meet their requirements for absorbed zinc.Conclusions: These data suggest that zinc absorption is overestimated for men, women, and adolescents in the current DRI. Upward adjustments of the DRI for these groups are recommended.

Zinc Absorption Is Not Related to Dietary Phytate Intake in Infants and Young Children Based on Modeling Combined Data from Multiple Studies1–3

Background: It is widely understood that the 2 primary factors affecting dietary zinc absorption in adults are the quantities of zinc and phytate in the diet. Although a similar association of absorption to dietary zinc and phytate is presumed to exist in children, to our knowledge, no large-scale examination of the relation of zinc absorption to dietary and growth factors has been conducted.Objective: The goal was to apply an adult absorption model and related models to data from zinc absorption studies of infants and children in order to determine the nature of the relation of zinc absorption to dietary zinc and phytate, age, body size, and zinc homeostatic variables.Methods: Data from 236 children between 8 and 50 mo of age were obtained from stable-isotope studies of zinc absorption. Statistical and mechanistic models were fit to the data using linear and nonlinear regression analysis.Results: The effect of dietary phytate on zinc absorption when controlling for dietary zinc was very small and not statistically discernable (P = 0.29). A 500-mg/d increase in dietary phytate reduced absorbed zinc by <0.04 mg/d. Absorption was observed to vary with age, weight, and height (P < 0.0001) when controlling for dietary zinc. For example, absorption from 6 mg/d of dietary zinc increased by as much as 0.2 mg/d with a 12-mo increase in age. Absorption varied with weight and exchangeable zinc pool size (0.01 <P < 0.05) when controlling for dietary zinc and age.Conclusions: The absence of a detectable phytate effect on zinc absorption raises caution about use of dietary phytate:zinc molar ratios to predict zinc bioavailability and does not support phytate reduction as a strategy to improve zinc status of young children. The effect of age on zinc absorption and the absence of a phytate effect should facilitate estimations of dietary zinc needs in young children.

Gastric and Colonic Zinc Transporter ZIP11 (Slc39a11) in Mice Responds to Dietary Zinc and Exhibits Nuclear Localization

Zinc transporters have been characterized to further understand the absorption and metabolism of dietary zinc. Our goal was to characterize zinc transporter Slc39a11 (ZIP11) expression and its subcellular localization within cells of the murine gastrointestinal tract of mice and to determine if dietary zinc regulates ZIP11. The greatest ZIP11 expression was in the stomach, cecum, and colon. Both Zip11 mRNA and ZIP11 protein were shown to be downregulated during dietary zinc restriction (<1 mg Zn/kg) in the murine stomach tissue but were unaffected in the colon. Acute repletion with zinc did not restore Zip11 mRNA levels in the stomach. Immunohistochemistry (IHC) revealed high ZIP11 levels in the lower regions of gastric glands and parietal cells of the stomach. IHC analysis of the colon showed a marked ZIP11 abundance within the cytoplasm of the colonic epithelial cells. IHC also showed an increase in ZIP11 expression in the colon during zinc restriction. There is a robust abundance of ZIP11 in the nuclei of cells of both stomach and colon. Our experiments suggest that when dietary zinc intake is compromised, the colon may increase zinc transporter expression to improve the efficiency for absorption via increased expression of specific zinc transporters, including ZIP11 and also zinc transporter Slc39a4. In conclusion, ZIP11 is highly expressed within the murine stomach and colon and appears to be partially regulated by dietary zinc intake within these tissues. ZIP11 may play a specialized role in zinc homeostasis within these tissues, helping to maintain mucosal integrity and function.

Functional studies of Drosophila zinc transporters reveal the mechanism for dietary zinc absorption and regulation.

BackgroundZinc is key to the function of many proteins, but the process of dietary zinc absorption is not well clarified. Current knowledge about dietary zinc absorption is fragmented, and mostly derives from incomplete mammalian studies. To gain a comprehensive picture of this process, we systematically characterized all zinc transporters (that is, the Zip and ZnT family members) for their possible roles in dietary zinc absorption in a genetically amenable model organism, Drosophila melanogaster.ResultsA set of plasma membrane-resident zinc transporters was identified to be responsible for absorbing zinc from the lumen into the enterocyte and the subsequent exit of zinc to the circulation. dZip1 and dZip2, two functionally overlapping zinc importers, are responsible for absorbing zinc from the lumen into the enterocyte. Exit of zinc to the circulation is mediated through another two functionally overlapping zinc exporters, dZnT1, and its homolog CG5130 (dZnT77C). Somewhat surprisingly, it appears that the array of intracellular ZnT proteins, including the Golgi-resident dZnT7, is not directly involved in dietary zinc absorption. By modulating zinc status in different parts of the body, we found that regulation of dietary zinc absorption, in contrast to that of iron, is unresponsive to bodily needs or zinc status outside the gut. The zinc transporters that are involved in dietary zinc absorption, including the importers dZip1 and dZip2, and the exporter dZnT1, are respectively regulated at the RNA and protein levels by zinc in the enterocyte.ConclusionsOur study using the model organism Drosophila thus starts to reveal a comprehensive sketch of dietary zinc absorption and its regulatory control, a process that is still incompletely understood in mammalian organisms. The knowledge gained will act as a reference for future mammalian studies, and also enable an appreciation of this important process from an evolutionary perspective.

Dietary Taurine Reduces Zinc‐Induced Toxicity in Male Wistar Rats

Taurine is an agent for treating the heavy metal intoxication and presence of metals such as zinc, copper, and iron may have a role in heavy metal toxicity, a study was undertaken to investigate the effect of taurine on the toxicity of zinc in male Wistar rats. The rats were divided into 8 groups and fed different diets with or without supplement of 5% taurine and 150 to 600 ppm zinc for 2 mo. It was found that the body weight of rats, the ratios of liver and kidney weight to body weight, and the level of glutathione in the liver were decreased with increasing the dose of zinc. The levels of zinc in the liver, kidney, and plasma, the levels of malondialdehyde in the plasma, the levels of thiobarbiture acid-reactive substances in the liver, the activities of aspartate transaminase, alanine transaminase in the plasma, the levels of blood urea nitrogen and creatinine in the plasma of rats were increased with the increasing dose of zinc. Hence, symptoms of zinc toxicity in rats included loss of body weight, hepatotoxicity, and nephrotoxicity. However, these toxic effects of zinc were significantly reduced when the rats fed diet with supplement of taurine. Furthermore, the level of zinc in the feces of rats treated with taurine and zinc was higher than that of rats treated with zinc alone. It indicated that taurine thereby leading to a decreased absorption of dietary zinc and promoted excretion.

Effect of Bioactive Dietary Polyphenols on Zinc Transport across the Intestinal Caco-2 Cell Monolayers

Polyphenolic compounds are known to possess many beneficial health effects, including the antioxidative activities of scavenging reactive oxygen species and chelating metals, such as iron and zinc. Tea and red wine are thought to be important sources of these compounds. However, some polyphenolic compounds can also reduce the absorption of iron, and possibly other trace metals, when included in a diet. There is very little information on the effect of dietary polyphenolic compounds on the status of trace elements other than iron. The effects of epigallocatechin-3-gallate (EGCG), green tea extract (GT), and grape seed extract (GSE) on the absorption of (65)Zn were examined and compared with their effects on (55)Fe absorption in human intestinal Caco-2 cells grown on microporous membrane inserts. The levels of EGCG, GT, and GSE used in this study were within physiological ranges and did not affect the integrity of the Caco-2 cell monolayers. GSE significantly (P < 0.05) reduced zinc transport across the cell monolayer, and the decreased zinc transport was associated with a reduction in apical zinc uptake. However, EGCG and GT did not alter zinc absorption. In contrast, the polyphenolic compounds in EGCG, GT, and GSE almost completely blocked transepithelial iron transport across the cell monolayer. The effect of GSE on zinc absorption was very different from that on iron absorption. Whereas GSE decreased zinc absorption by reducing apical zinc uptake, the polyphenolic compounds inhibited iron absorption by enhancing apical iron uptake. GSE inhibited zinc absorption similarly to that observed for phytate. Phytate significantly (P < 0.05) decreased transepithelial zinc transport by reducing apical zinc uptake. The inhibition of zinc absorption may be due to the presence of procyanidins in GSE, which bind zinc with high affinity and block the transport of zinc across the apical membrane of enterocytes. Further research on the absorption of zinc as zinc-polyphenol complexes and free zinc should provide further insight into the process of dietary zinc absorption in the presence of GSE and other bioactive dietary polyphenols. The present study suggests that some individuals should consider their zinc status if they regularly consume procyanidin-containing foods in their diet. However, further studies, especially in vivo studies, are needed to confirm these results.

Dietary zinc intake and its effects on zinc nutrition in healthy Japanese living in the central area of Japan

In the present study, we first examined the dietary zinc intake from food groups in 109 healthy Japanese (24-82 years old, 45 male and 64 female) by means of the 72-h recall method. We then used the ratio of apo/holo-activities of angiotensin converting enzyme (ACE ratio) that is a more sensitive index of zinc nutrition than zinc concentration in the serum and examined the correlation between their zinc intake and ACE ratio. Dietary zinc intake in healthy Japanese was maximal from rice and rice products. There were significant inverse correlations between the ACE ratio and dietary zinc intake from rice and rice products and shellfish, and a significant positive correlation between ACE ratio and dietary zinc intake from other beans and bean processed foods. On the other hand, there were no significant correlations between serum zinc concentrations and dietary zinc intake from any food group except processed fish. These findings suggested that rice is a major source of dietary zinc intake in healthy Japanese. It is also suggested that shellfish also has a major impact on zinc nutrition, although dietary zinc intake from this source is minimal. Since beans contain phytic acid, which inhibits the absorption of dietary zinc, it is suggested that intake of beans causes impairment of zinc nutrition.

Dietary zinc absorption: A play of Zips and ZnTs in the gut

Studies on dietary zinc absorption are of fundamental nutritional significance, owing to the ubiquity of zinc in biological processes and the severe outcomes of zinc deficiency in humans. Insights into the molecular basis of dietary zinc absorption have advanced in recent years through functional characterization of zinc transporters in cell culture, immunohistochemical studies on rodent intestine and analysis of gene knockout mice. Zinc transporters with manifested expression in enterocytes include ZnT1, ZnT2, ZnT4, ZnT5, ZnT6, ZnT7, Zip4, and Zip5. Among them, ZIP4, the gene responsible for Acrodermatitis enteropathica, an inherited human zinc deficiency, mediates dietary zinc uptake into enterocytes across the apical membrane, while ZnT1 is involved in zinc efflux from enterocytes across the basolateral membrane into circulation. The intracellular trafficking pathways for zinc retention and movement between apical and basolateral sides of the enterocytes have yet to be defined. The utilization of Drosophila model in elucidating molecular mechanisms of dietary zinc absorption is also discussed in this review.

Dietary zinc absorption is mediated by ZnT1 in Drosophila melanogaster

Zinc is an essential nutritional factor involved in many key biological processes. However, the physiological function of zinc transporters at the organismal level is not well characterized. Early embryonic lethality of Znt1 knockout mice precludes functional analysis of the role of ZnT1 in dietary zinc absorption. Here, we report the identification and characterization of the Drosophila ZnT1 orthologue, dZnT1, for its role in Drosophila dietary zinc absorption. In cell culture, dZnT1 promoted zinc transport to reduce cytoplasmic zinc levels. Ubiquitous RNA interference of dZnT1 in Drosophila resulted in developmental arrest under restriction of dietary zinc, while dZnT1-overexpressing flies exhibited hypersensitivity to zinc. dZnT1 was prominently expressed in restricted regions of the midgut and exhibited a distribution on the basolateral membrane of the enterocytes. Gut-specific silencing of dZnT1 was sufficient to evoke lethality under zinc scarcity. Human ZnT1, but not ZnT7 or ZnT4, could rescue the zinc-acquiring defects caused by dZnT1 silencing. Taken together, our results proved that dZnT1 is a key zinc transporter in dietary zinc absorption, functioning by pumping zinc out of the enterocytes across the basolateral membrane. This study will be helpful in understanding the fundamental process of acquiring dietary zinc in higher eukaryotes.

Regulation and function of Zip4, the acrodermatitis enteropathica gene

The SLC39A (solute carrier 39A) [ZIP (Zrt-Irt-like protein)] family consists of 14 members which are thought to control zinc uptake into the cytoplasm. Among these, ZIP4 is known to be particularly important for zinc homoeostasis. Mutations in this gene cause acrodermatitis enteropathica, a rare recessive-lethal human genetic disorder. In the present paper, our studies of the regulation and function of the mouse Zip4 gene are briefly reviewed. Mouse Zip4 is expressed at highest levels in tissues involved in absorption of dietary or maternal zinc, and the gene and protein are dynamically regulated by multiple post-transcriptional mechanisms in response to zinc availability. ZIP4 accumulates at the apical surface of enterocytes and endoderm cells when zinc is deficient, because of increased stability of the mRNA and stabilization of the protein. In contrast, when zinc is replenished, the mRNA is destabilized and the protein is internalized and degraded rapidly. The critical importance of ZIP4 in zinc homoeostasis is revealed in mice with targeted deletions of this gene. Homozygous Zip4-knockout embryos die during early morphogenesis and heterozygous offspring are significantly underrepresented and display an array of developmental defects, including exencephalia, anophthalmia and severe growth retardation. Mice heterozygous for Zip4-knockout are hypersensitive to zinc deficiency, which suggests that humans heterozygous for this gene may also be very sensitive to zinc deficiency.

Adaptation in human zinc absorption as influenced by dietary zinc and bioavailability

BackgroundAn understanding of the relations among dietary zinc intake, bioavailability, and absorption is necessary for making dietary intake recommendations. ObjectivesWe aimed to assess adaptation in human zinc absorption to controlled differences in zinc and phytate intakes and to apply the results to predictive models. DesignIn 3 experiments, radiotracers were used to assess zinc absorption by healthy adults (n = 109) from controlled diets, before and after 4 or 8 wk of dietary equilibration. Subjects consumed 4–29 mg Zn/d from 1 of 10 diets, 5 with molar ratios of phytate to zinc from 2 to 7 and 5 with ratios from 15 to 23. ResultsAbsorptive efficiency was inversely related to dietary zinc from both low- and high-phytate diets. In response to low zinc intakes (<11 mg/d) for 4–8 wk, zinc absorption was up-regulated to as high as 92%, but only if the diets were low in phytate. The results help validate and refine a published saturable transport model that predicts zinc absorption from dietary zinc and phytate. Possible biomarkers of impaired zinc status, including erythrocyte osmotic fragility, in vitro erythrocyte 65Zn uptake, and leukocyte expression of the zinc transport proteins Zip1 and ZnT1, were unresponsive to dietary zinc content. ConclusionsHumans absorbed zinc more efficiently from low-zinc diets and adapted to further increase zinc absorption after consuming low-zinc, low-phytate diets for several weeks. Such adaptation did not occur with higher phytate diets. Zinc absorption can be predicted from dietary zinc and phytate after allowing for dietary equilibration.