cDNA array analysis identifies thymic LCK as upregulated in moderate murine zinc deficiency before T-lymphocyte population changes.

The detrimental sequelae of severe zinc deficiency on the thymus and T-lymphocyte compartment of the mammalian immune system have been established, but underlying mechanisms remain unknown. Hypothesizing that the alterations in T-lymphocyte number and function observed during zinc deficiency may result from changes in gene expression, we sought to compare thymic mRNA expression profiles of zinc-deficient and zinc-normal mice utilizing cDNA arrays. For our murine model described herein, 3 wk of dietary zinc deficiency did not perturb food intake or growth rate in young adult, outbred mice, but significantly depressed multiple parameters of zinc status. Furthermore, fluorescence-activated cell sorting (FACS) analysis demonstrated no changes in thymocyte populations expressing the cell surface markers CD3, CD4 or CD8, establishing that observed changes in mRNA abundances were not attributable to different thymocyte populations. Yet notably, at this moderate level of zinc deficiency, cDNA array analysis identified four potentially zinc-regulated mRNAs whose modulation was confirmed independently, twice, using both semiquantitative and real-time quantitative reverse transcription-polymerase chain reaction. Expression of one of these genes (myeloid cell leukemia sequence-1) was depressed, whereas the others [DNA damage repair and recombination protein 23B, the mouse laminin receptor and the lymphocyte-specific protein tyrosine kinase (LCK)] were elevated in the zinc-deficient mice. Further Western analysis demonstrated that the zinc binding protein LCK was elevated in these zinc-deficient mice. Results demonstrate that 3 wk of dietary zinc insufficiency can alter specific thymic mRNA and protein abundances before alterations occur in thymocyte development as detectable by FACS analysis.

Modulation of intestinal gene expression by dietary zinc status: effectiveness of cDNA arrays for expression profiling of a single nutrient deficiency.

Mammalian nutritional status affects the homeostatic balance of multiple physiological processes and their associated gene expression. Although DNA array analysis can monitor large numbers of genes, there are no reports of expression profiling of a micronutrient deficiency in an intact animal system. In this report, we have tested the feasibility of using cDNA arrays to compare the global changes in expression of genes of known function that occur in the early stages of rodent zinc deficiency. The gene-modulating effects of this deficiency were demonstrated by real-time quantitative PCR measurements of altered mRNA levels for metallothionein 1, zinc transporter 2, and uroguanylin, all of which have been previously documented as zinc-regulated genes. As a result of the low level of inherent noise within this model system and application of a recently reported statistical tool for statistical analysis of microarrays [Tusher, V.G., Tibshirani, R. & Chu, G. (2001) Proc. Natl. Acad. Sci. USA 98, 5116-5121], we demonstrate the ability to reproducibly identify the modest changes in mRNA abundance produced by this single micronutrient deficiency. Among the genes identified by this array profile are intestinal genes that influence signaling pathways, growth, transcription, redox, and energy utilization. Additionally, the influence of dietary zinc supply on the expression of some of these genes was confirmed by real-time quantitative PCR. Overall, these data support the effectiveness of cDNA array expression profiling to investigate the pleiotropic effects of specific nutrients and may provide an approach to establishing markers for assessment of nutritional status.

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.

Dietary zinc deficiency increases uroguanylin accumulation in rat kidney.

BACKGROUND: Zinc deficiency in humans produces a secretory diarrhea that is corrected by zinc supplementation. In rats, differential mRNA display analysis has shown that intestinal uroguanylin gene expression is increased in zinc deficiency. An endocrine axis involving intestinal uroguanylin and the kidney may exist. Therefore, we conducted this study to examine whether zinc deficiency would affect uroguanylin expression in the kidney of rats. METHODS: A purified diet, deficient or adequate in zinc content, was fed to rats. Preprouroguanylin mRNA was localized in kidney by in situ hybridization, and prouroguanylin/uroguanylin peptides were localized in the kidney by immunohistochemistry. Abundance was measured by Western blotting and slot blotting analyses. RESULTS: In situ hybridization demonstrated that preprouroguanylin mRNA-expressing cells were localized in the proximal tubules, being primarily limited to the cortical-medullary junction. Zinc deficiency did not alter the abundance or distribution of the mRNA. Immunohistochemistry, using a uroguanylin peptide-specific, affinity-purified antibody, demonstrated that immunoreactive uroguanylin peptide was localized to the same cells but that the staining was stronger in zinc-deficient rats. Western blotting analysis of kidney extracts showed that there was no difference in abundance of prouroguanylin between zinc adequate and deficient rats. However, slot blotting analysis demonstrated that the abundance of a low molecular weight immunoreactive peptide, presumably uroguanylin, was higher in extracts of zinc-deficient rats. CONCLUSION: The results suggest that production of prouroguanylin by the kidney, in contrast to the intestine, is not influenced by dietary zinc intake, but that higher amounts of uroguanylin in kidney extracts may reflect renal processing of the hormone obtained from the systemic circulation.

Differential regulation of zinc transporter 1, 2, and 4 mRNA expression by dietary zinc in rats.

Zinc metabolism is well regulated over a wide range of dietary intakes to help maintain cellular zinc-dependent functions. Expression of transporter molecules, which influence zinc influx and efflux across the plasma and intracellular membranes, contributes to this regulation. We have examined in rats the comparative response of zinc transporters 1, 2, and 4 (ZnT-1, ZnT-2 and ZnT-4) to dietary zinc. ZnT-1 and ZnT-4 are expressed ubiquitously, whereas ZnT-2 is limited to small intestine, kidney, placenta and, in some cases, the liver. When zinc intake was low (<1 mg Zn/kg), ZnT-2 mRNA was extremely low in small intestine and kidney compared with an adequate intake (30 mg Zn/kg). ZnT-1 and ZnT-2 mRNAs were markedly greater in both tissues when a supplemental zinc intake (180 mg Zn/kg) was provided. ZnT-4 was refractory to changes in zinc intake. When zinc was provided as a single oral dose (70 mg/kg body), ZnT-1 and ZnT-2 mRNA levels were increased many fold in small intestine, liver and kidney, whereas ZnT-4 gene expression was not changed. The expression of ZnT-1 and ZnT-2 is comparable to zinc-induced changes in metallothionein mRNA levels, suggesting a similar mode of regulation for these genes. The relative differential in regulation by zinc is ZnT-2 > ZnT-1 > ZnT-4. These data provide evidence that, in an animal model, zinc transporter expression is responsive to zinc under physiologically relevant conditions.

Metallothionein expression protects against carbon tetrachloride-induced hepatotoxicity, but overexpression and dietary zinc supplementation provide no further protection in metallothionein transgenic and knockout mice.

Metallothionein and zinc have been implicated in cellular defense against a number of cytotoxic agents. With respect to the free radical-generating hepatotoxin carbon tetrachloride, conclusions about a defensive role were reached from in vitro studies, in vivo studies using inducers of metallothionein and studies using injections of pharmacological amounts of zinc. Metallothionein knockout (null) and metallothionein transgenic mice are more direct models to examine the effects of metallothionein expression on induced cytotoxicity. Similarly, zinc presented via the diet is a more physiological model than that presented via injection. We examined whether metallothionein-overexpressing mice or metallothionein knockout mice had altered sensitivity to carbon tetrachloride and whether supplemental dietary zinc reduced sensitivity to carbon tetrachloride in these genotypes. Metallothionein knockout mice produced no metallothionein and were unable to sequester additional hepatic zinc in response to elevated dietary zinc. Hepatotoxicity, as measured by serum alanine aminotransferase activity, histological analyses and hepatic thiol levels, was greater in the knockout mice than in controls 12 h after carbon tetrachloride treatment but not at later time points (up to 48 h). In contrast, metallothionein-overexpressing mice produced more metallothionein and sequestered more liver zinc than control mice, but hepatotoxicity was similar between genotypes. Supplemental dietary zinc had no effect on hepatotoxicity with either genotype. These data suggest metallothionein null mice were more susceptible to carbon tetrachloride-induced hepatotoxicity than were control mice. However, neither metallothionein overexpression nor supplemental dietary zinc provided further protection.

L-type Ca(2+) channel-mediated Zn(2+) toxicity and modulation by ZnT-1 in PC12 cells.

In view of evidence that Zn(2+) neurotoxicity contributes to some forms of pathological neuronal death, we developed a model of Zn(2+) neurotoxicity in a cell line amenable to genetic manipulations. Exposure to 500 microM ZnCl(2) for 15 min under depolarizing conditions resulted in modest levels of PC12 cell death, that was reduced by the L-type Ca(2+) channel antagonist, nimodipine, and increased by the L-type Ca(2+) channel opener, S(-)-Bay K 8644. At lower insult levels (200 micrometer Zn(2+)+Bay K 8644), Zn(2+)-induced death appeared apoptotic under electron microscopy and was sensitive to the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-CH(2)F (Z-VAD); at higher insult levels (1000 microM+Bay K 8644), cells underwent necrosis insensitive to Z-VAD. To test the hypothesis that the plasma membrane transporter, ZnT-1, modulates Zn(2+) neurotoxicity, we generated stable PC12 cell lines overexpressing wild type or dominant negative forms of rat ZnT-1 (rZnT-1). Clones T9 and T23 overexpressing wild type rZnT-1 exhibited enhanced Zn(2+) efflux and reduced vulnerability to Zn(2+)-induced death compared to the parental line, whereas clones D5 and D16 expressing dominant negative rZnT-1 exhibited the opposite characteristics.

Prouroguanylin overproduction and localization in the intestine of zinc-deficient rats.

Identification of the upregulation of preprouroguanylin mRNA in the rat small intestine during zinc deficiency provides a potential mechanistic link between production of the intestinal hormone uroguanylin and the diarrhea that may accompany zinc deficiency. In the current study, in situ hybridization demonstrated that the number of preprouroguanylin mRNA-expressing cells was significantly higher in zinc-deficient rats than in zinc-adequate rats. Immunohistochemical studies, with a uroguanylin peptide affinity-purified antibody, demonstrated that immunoreactivity was localized to the tips of villi of the duodenum and jejunum in zinc-adequate rats. However, positive cells were scattered throughout the villus of zinc-deficient rats. A subset of cells, perhaps enterochromaffin cells, exhibited the predominant staining, whereas no specific staining was found in goblet cells or lymphocytes of the lamina propria. Western blotting demonstrated that the expression of prouroguanylin in both duodenum and jejunum was elevated by dietary zinc depletion. These results show that dietary zinc deficiency upregulates prouroguanylin in intestinal cells, which is consistent with a role for uroguanylin in the etiology of diarrhea observed in human zinc deficiency.

Metallothionein mRNA in monocytes and peripheral blood mononuclear cells and in cells from dried blood spots increases after zinc supplementation of men.

A specific, sensitive and reliable index for assessment of human zinc status has not been developed, and continues to present a considerable challenge for nutritionists in the trace element field. We have focused on metallothionein (MT) expression as a potential index. A protocol involving 16 men and a 10-d supplementation period plus a 4-d postsupplementation period was used to examine the relative response of MT expression in erythrocytes, monocytes, peripheral blood mononuclear cells (PBMC) and cells from a dried blood spot (DBS). Zinc was supplemented at the current adult male recommended dietary allowance (RDA) of 15 mg. Erythrocyte MT protein, as measured by ELISA, increased gradually to about twofold over the placebo group during zinc supplementation and remained elevated for 4 d postsupplementation. Competitive reverse transcriptase-polymerase chain reaction showed that MT mRNA levels in both monocytes and PBMC increased (up to 4.7- and 2.7-fold, respectively) after 2 d of supplementation, with greater expression in monocytes compared with PBMC. Total RNA extracted from dried blood spots, representing cells from 50 microL of blood, showed a comparable change in MT mRNA upon zinc supplementation. In each leukocyte population isolated, when zinc supplementation was withdrawn, MT mRNA levels decreased. Collectively, these experiments show that, in men, MT gene expression increases during supplementation at the RDA, and that the DBS sampling method will be of value in measuring MT expression in a variety of clinical and survey situations.

Regulation of cysteine-rich intestinal protein, a zinc finger protein, by mediators of the immune response.

Cysteine-rich intestinal protein (CRIP), a member of the LIM protein family, has a unique double zinc finger motif as the defining feature. CRIP is highly expressed in intestine and immune cells. CRIP transgenic (Tg) mice and nontransgenic controls were challenged with lipopolysaccharide (LPS). Serum concentrations of interferon-gamma and tumor necrosis factor-alpha were less while those of interleukin-6 and -10 were greater in the Tg mice following LPS administration. CRIP-overexpressing splenocytes produce the same cytokine profile. These responses are consistent with a regulatory role for this protein in cell differentiation, which produces an imbalance in Th1 and Th2 cytokines. Stimulation of CRIP protein levels by LPS is eliminated in metallothionein knockout mice, suggesting metallothionein is the source of zinc for this zinc finger protein and, further, that this could reflect a relationship to the zinc nutritional status and to the aberrant Th1/Th2 cytokine balance observed in zinc deficiency.

Metallothionein expression in animals: a physiological perspective on function.

An integration of knowledge concerning regulation of metallothionein expression with research on metallothionein's proposed functions is necessary to delineate how this metalloprotein affects cellular processes, especially zinc metabolism. Metallothionein expression is driven by a number of physiological mediators through several response elements in the metallothionein gene promoter. Cellular accumulation of metallothionein depends on both gene expression and protein degradation. Both depend largely on availability of cellular zinc derived from the dietary zinc supply. Metallothionein expression is related to zinc accumulation in certain organs. Evidence has been produced, which suggests that metallothionein could act in a number of biochemical processes. It may act in zinc trafficking and/or zinc donation to apoproteins, including zinc finger proteins that act in cellular signaling and transcriptional regulation. As a result, metallothionein expression may affect a number of cellular processes including gene expression, apoptosis, proliferation and differentiation. The ability of metallothionein to exchange other metals with zinc in these proteins may explain a role in metal toxicity. Similarly, mobilization of zinc from metallothionein by oxidative stresses may explain its proposed antioxidant function. Apparent good health of metallothionein-deficient mice argues against a critical biological role for metallothionein; however, expression may be critical in times of stress.

Integrative aspects of zinc transporters.

Cells maintain zinc concentrations with relatively narrow limits. Nevertheless, physiologically relevant changes in free Zn(II) pools or changes in Zn bound to specific ligands or within vesicles may occur without a major change in total cellular zinc concentrations. The task of maintaining such levels rests in part with zinc transporter proteins. The genes for some putative zinc transporters have recently been cloned. As of this time, most have not been directly shown to transport zinc in functional studies, albeit evidence is strong that they have such a function. Zinc transporter (ZnT)-1 was identified as a rescue agent for cells maintained in very high extracellular zinc conditions; therefore, ZnT-1 has been suggested to function as an exporter. ZnT-1 is expressed in a variety of tissues, including intestine, kidney and liver. Intestinal expression is regional, being much greater in duodenum and jejunum and in villus versus crypt cells. Immunolocalization places ZnT-1 at the basolateral membrane of intestinal enterocytes and epithelial cells of the distal renal tubules. Regulation of ZnT-1 mRNA and ZnT-1 protein does not change markedly with changes in dietary zinc level except when a large single oral zinc supplement is provided. ZnT-1 is induced by transient ischemia of the forebrain. ZnT-2 and ZnT-3 may function in tissue-specific vesicular zinc transport. ZnT-4 is believed to be abundant in mammary gland and may be associated with zinc secretion into milk. A mutation of the ZnT-4 gene may account for the lethal milk (lm) syndrome. The putative zinc transporters identified thus far appear to have characteristics commensurate with functions in integrative zinc acquisition and homeostasis.

Regulation of intestinal gene expression by dietary zinc: induction of uroguanylin mRNA by zinc deficiency.

The regulation of gene expression by nutrients plays an important role in the overall manifestations of nutritional deficiencies. Insufficient intakes of dietary micronutrients, such as zinc, produce profound effects in multiple organs and tissues. One of the major challenges, however, is to identify genes affected by changes in nutritional status. Differential display of mRNA has proved to be a valuable technique in meeting this challenge. In our ongoing search for genes responsive to dietary zinc, we compared small intestinal mRNA from rats that were fed zinc-deficient or -adequate diets using differential display to generate 3' anchored expressed sequence tags (EST). EST for intestinal mRNAs with altered expression due to zinc deficiency include two peptide hormones, intestinal fatty acid binding protein, intestinal alkaline phosphatase II, a proteasomal ATPase, cis-Golgi p28 and two subunits of the ubiquinone oxidoreductase. The EST for one of the hormones yielded the sequence for the 3' end of an mRNA encoding preprouroguanylin and was used to clone the remaining portion of the rat cDNA via 5' rapid amplification of cDNA ends. Northern blot analysis of RNA from rat intestine demonstrated that preprouroguanylin mRNA was 2.5-fold more abundant during zinc deficiency. Uroguanylin, a natriuretic peptide hormone, is an endogenous ligand for the same guanylate cyclase C that the Escherichia coli heat-stable enterotoxin (STa) binds when it causes secretory diarrhea by activating the cystic fibrosis transmembrane conductance regulator, thus altering fluid balance in the intestine. This suggests a mechanism whereby zinc deficiency could induce uroguanylin levels in the intestine and cause or potentiate diarrhea.

Nutritional regulation of gene expression.

Genes are regulated by complex arrays of response elements that influence the rate of transcription. Nutrients and hormones either act directly to influence these rates or act indirectly through specialized signaling pathways. Metabolites of vitamins A and D, fatty acids, some sterols, and zinc are among the nutrients that influence transcription directly. Components of dietary fiber may influence gene expression indirectly through changes in hormonal signaling, mechanical stimuli, and metabolites produced by the intestinal microflora. In addition, consumption of water-soluble fibers may lead to changes in gene expression mediated through indirect mechanisms that influence transcription rates. In the large intestine, short-chain fatty acids, including butyric acid, are produced by microflora. Butyric acid can indirectly influence gene expression. Some sources of fiber limit nutrient absorption, particularly of trace elements. This could have direct or indirect effects on gene expression. Identification of genes in colonic epithelial cells that are differentially regulated by dietary fiber will be an important step toward understanding the role of dietary factors in colorectal cancer progression.

A role of zinc in the regulation of gene expression.

Zn, without question, has important functions related to gene expression. Newer technologies applied to address these functions are providing answers relating to the importance of this micronutrient in human and animal health.

Regulation of the zinc transporter ZnT-1 by dietary zinc.

The understanding of mechanisms controlling zinc absorption and metabolism at the molecular level has advanced recently. Kinetics of zinc transport have been investigated for many years, but only recently have genes coding for proteins thought to be involved in the transport process been cloned. Four putative zinc transporters, known as ZnT-1 through ZnT-4, have now been described. Among these transporters, only ZnT-1 is ubiquitously expressed. In this report, we examine the pattern of ZnT-1 expression in the intestine and analyze the regulation of ZnT-1 by dietary zinc in both the intestine and liver. Immunofluorescence demonstrated that intestinal ZnT-1 was most abundant at the basolateral surface of enterocytes lining the villi of the duodenum and jejunum. By Western blot analysis, intestinal and liver ZnT-1 protein migrated as a 42- and 36-kDa protein, respectively. Dietary zinc supplementation elevated the level of intestinal ZnT-1 mRNA and protein approximately 50% and 10%, respectively, but had no effect in the liver. In response to an acute oral zinc dose, the level of intestinal ZnT-1 mRNA was up-regulated 8-fold, without a corresponding increase in ZnT-1 protein. Conversely, the acute oral dose did not affect liver ZnT-1 mRNA, but resulted in a 5-fold increase in liver ZnT-1 protein. These results represent studies on the expression of intestinal and hepatic ZnT-1 in an intact animal model. The data suggest that ZnT-1 is at least part of the mechanism by which dietary zinc is absorbed and that, despite the zinc responsiveness of the ZnT-1 gene, additional factors may be regulating the steady-state level of ZnT-1 transporter protein.

Metallothionein knockout and transgenic mice exhibit altered intestinal processing of zinc with uniform zinc-dependent zinc transporter-1 expression.

A role for metallothionein in intestinal zinc absorption has been the subject of considerable debate. If metallothionein affects zinc absorption, then those factors that induce metallothionein synthesis (e.g., heavy metals, hormones) should alter zinc absorption and homeostasis. The present studies used metallothionein transgenic mice (overexpressing) and metallothionein knockout mice (no expression of metallothionein-1 or metallothionein-2) to examine directly the effects of metallothionein on zinc absorption, independent of secondary effects that could be caused by metallothionein inducers. Zinc absorption was examined by administering a single oral zinc dose (0.5 mmol/kg) by feeding tube to metallothionein transgenic and metallothionein knockout mice and measuring the serum zinc concentration. Two hours after the dose, the serum zinc concentration was 2.3 times higher in metallothionein knockout mice than in their control strain. Conversely, the concentration was elevated only one third as much in the metallothionein transgenic mice as in their controls after the zinc dose. We found that the serum zinc concentration was inversely related to the level of metallothionein protein. The intestinal zinc content was higher in the metallothionein knockout mice, however, suggesting that metallothionein did not reduce zinc absorption by simply sequestering zinc in the mucosa. The expression of the zinc transporter ZnT-1 was directly related to the serum zinc level and was independent of the level of metallothionein. These results further support metallothionein as an important component for reducing the efficiency of zinc absorption at elevated zinc intakes.

Mammalian zinc transporters.

Genes that are involved in mammalian zinc transport recently have been cloned. These all predict proteins with multiple membrane spanning regions, and most have a histidine-rich intracellular loop. ZnT-1 was the first cloned and is associated with zinc efflux. It is found in all tissues examined, and, at least in some, ZnT-1 expression is regulated by dietary zinc intake. In enterocytes of the small intestine and renal tubular cells, ZnT-1 is localized to the basolateral membrane, suggesting an orientation that is consistent with zinc absorption/retention. ZnT-2 is also an exporter and may be involved in zinc efflux or uptake into vesicles in intestine, kidney, and testis. ZnT-3 is involved in zinc uptake into vesicles in neurons and possibly in testis. ZnT-4 is also an exporter and is highly expressed in mammary gland and brain. The divalent cation transporter 1 (DCT1) is regulated by iron, but exhibits transport activity for a number of trace elements including zinc. Description of a family of zinc transporters bridges the integrative and reductionist approach to the study of zinc metabolism. Other members of this transporter family may emerge. Many of these may be regulated by zinc, and some may respond to immune challenge, oxidative stress, and competing metals in the dietary supply. Collectively, description of transporters that influence cellular zinc uptake and efflux will provide a clearer understanding of the molecular events that regulate zinc absorption and homeostasis.

Metallothionein expression is increased in monocytes and erythrocytes of young men during zinc supplementation.

The metallothionein gene is transcriptionally regulated by zinc. Consequently, metallothionein has potential for serving as an index of dietary zinc status in humans. To examine this possibility, an enzyme-linked immunoassay (ELISA) based on a sandwich approach that utilizes monoclonal and chicken egg yolk antibodies was used to compare the response of erythrocyte metallothionein protein levels with the response of monocyte metallothionein mRNA levels as measured by competitive reverse transcriptase-polymerase chain reaction (CRT-PCR) during zinc supplementation. Young male subjects participated in an 18-d supplementation study in which zinc was provided at 50 mg/d. Control subjects received a placebo. The zinc supplement resulted in significantly greater erythrocyte metallothionein levels by d 8 of supplementation compared with controls. Monocyte metallothionein mRNA levels were significantly greater than those of controls by d 2 of supplementation. Both remained elevated through d 18. They returned to base line by 8 and 4 d after supplementation, respectively. The plasma zinc concentration was significantly greater than in controls by d 6 and had returned to control levels by d 22 of supplementation. The results presented here show that both monocyte metallothionein mRNA and erythrocyte metallothionein protein concentrations change in human subjects in response to elevated dietary zinc intake and that monocyte metallothionein mRNA responds more rapidly to elevation of dietary zinc status than erythrocyte metallothionein protein. Consequently, both erythrocyte metallothionein and monocyte metallothionein mRNA may prove to be measures useful for assessment of either zinc depletion or the bioavailability of zinc supplements.