Distinct mechanisms of zinc uptake at the apical and basolateral membranes of caco-2 cells

Plasma non-transferrin-bound iron uptake by the small intestine leads to intestinal injury and intestinal flora dysbiosis in an iron overload mouse model and Caco-2 cells

Iron overload often occurs during blood transfusion and iron supplementation, resulting in the presence of non-transferrin-bound iron (NTBI) in host plasma and damage to multiple organs, but effects on the intestine have rarely been reported. In this study, an iron overload mouse model with plasma NTBI was established by intraperitoneal injection of iron dextran. We found that plasma NTBI damaged intestinal morphology, caused intestinal oxidative stress injury and reactive oxygen species (ROS) accumulation, and induced intestinal epithelial cell apoptosis. In addition, plasma NTBI increased the relative abundance of Ileibacterium and Desulfovibrio in the cecum, while the relative abundance of Faecalibaculum and Romboutsia was reduced. Ileibacterium may be a potential microbial biomarker of plasma NTBI. Based on the function prediction analysis, plasma NTBI led to the weakening of intestinal microbiota function, significantly reducing the function of the extracellular structure. Further investigation into the mechanism of injury showed that iron absorption in the small intestine significantly increased in the iron group. Caco-2 cell monolayers were used as a model of the intestinal epithelium to study the mechanism of iron transport. By adding ferric ammonium citrate (FAC, plasma NTBI in physiological form) to the basolateral side, the apparent permeability coefficient (Papp) values from the basolateral to the apical side were greater than 3×10-6 cm s-1. Intracellular ferritin level and apical iron concentration significantly increased, and SLC39A8 (ZIP8) and SLC39A14 (ZIP14) were highly expressed in the FAC group. Short hairpin RNA (shRNA) was used to knock down ZIP8 and ZIP14 in Caco-2 cells. Transfection with ZIP14-specific shRNA decreased intracellular ferritin level and inhibited iron uptake. These results revealed that plasma NTBI may cause intestinal injury and intestinal flora dysbiosis due to the uptake of plasma NTBI from the basolateral side into the small intestine, which is probably mediated by ZIP14.

Toxicity of palytoxin, purified ovatoxin-a, ovatoxin-d and extracts of Ostreopsis cf. ovata on the Caco-2 intestinal barrier model

Human intoxications in the Mediterranean Sea have been linked to blooms of the dinoflagellate Ostreopsis cf. ovata, producer of palytoxin (PlTX)-like toxins called ovatoxins (OVTXs). Exposure routes include only inhalation and contact, although PlTX-poisoning by seafood has been described in tropical regions. To address the impact of OVTXs on the intestinal barrier, dinoflagellate extracts, purified OVTX-a and -d and PlTX were tested on differentiated Caco-2 cells. Viability, inflammatory response and barrier integrity were recorded after 24 h treatment. OVTX-a and -d were not cytotoxic up to 20 ng/mL but increased IL-8 release, although to a lesser extent compared to PlTX. While PlTX and OVTX-a (at 0.5 and 5 ng/mL respectively) affected intestinal barrier integrity, OVTX-d up to 5 ng/mL did not. Overall, OVTX-d was shown to be less toxic than OVTX-a and PlTX. Therefore, oral exposure to OVTX-a and -d could provoked lower acute toxicity than PlTX.

A Guide to Human Zinc Absorption: General Overview and Recent Advances of In Vitro Intestinal Models

Zinc absorption in the small intestine is one of the main mechanisms regulating the systemic homeostasis of this essential trace element. This review summarizes the key aspects of human zinc homeostasis and distribution. In particular, current knowledge on human intestinal zinc absorption and the influence of diet-derived factors on bioaccessibility and bioavailability as well as intrinsic luminal and basolateral factors with an impact on zinc uptake are discussed. Their investigation is increasingly performed using in vitro cellular intestinal models, which are continually being refined and keep gaining importance for studying zinc uptake and transport via the human intestinal epithelium. The vast majority of these models is based on the human intestinal cell line Caco-2 in combination with other relevant components of the intestinal epithelium, such as mucin-secreting goblet cells and in vitro digestion models, and applying improved compositions of apical and basolateral media to mimic the in vivo situation as closely as possible. Particular emphasis is placed on summarizing previous applications as well as key results of these models, comparing their results to data obtained in humans, and discussing their advantages and limitations.

The impact of apical and basolateral albumin on intestinal zinc resorption in the Caco-2/HT-29-MTX co-culture model

The molecular mechanisms of intestinal zinc resorption and its regulation are still topics of ongoing research. To this end, the application of suitable in vitro intestinal models, optimized with regard to their cellular composition and medium constituents, is of crucial importance. As one vital aspect, the impact of cell culture media or buffer compounds, respectively, on the speciation and cellular availability of zinc has to be considered when investigating zinc resorption. Thus, the present study aims to investigate the impact of serum, and in particular its main constituent serum albumin, on zinc uptake and toxicity in the intestinal cell line Caco-2. Furthermore, the impact of serum albumin on zinc resorption is analyzed using a co-culture of Caco-2 cells and the mucin-producing goblet cell line HT-29-MTX. Apically added albumin significantly impaired zinc uptake into enterocytes and buffered its cytotoxicity. Yet, undigested albumin does not occur in the intestinal lumen in vivo and impairment of zinc uptake was abrogated by digestion of albumin. Interestingly, zinc uptake, as well as gene expression studies of mt1a and selected intestinal zinc transporters after zinc incubation for 24 h, did not show significant differences between 0 and 10% serum. Importantly, the basolateral application of serum in a transport study significantly enhanced fractional apical zinc resorption, suggesting that the occurrence of a zinc acceptor in the plasma considerably affects intestinal zinc resorption. This study demonstrates that the apical and basolateral medium composition is crucial when investigating zinc, particularly its intestinal resorption, using in vitro cell culture.

Effect of food matrices on the in vitro bioavailability and oxidative damage in PC12 cells of lead

The bioavailability and oxidative damage toxicity of lead (Pb) in seven food matrices, including rice, milk, tomato, garlic, apple, kelp and pork, were determined using an in vitro digestion/Caco-2 cell model and a rat pheochromocytoma (PC12) oxidative damage model. Results showed that Pb bioaccessibility and bioavailability in the apple and kelp groups were significantly lower than other food matrix groups, with corresponding values of 11.05-28.31% and 1.57-8.81%, respectively. Oxidative damage assays showed that digestion products of apple polyphenol extract, which was selected from seven food matrices, could increase the oxidation resistance and the levels of glutathione (GSH), superoxide dismutase (SOD), catalase (CAT) and acetyl cholinesterase (AChE) by 32.23%, 39.02%, 27.14% and 30.90%, respectively. Additionally, malondialdehyde (MDA) and reactive oxygen species (ROS) levels could be decreased by 59.66% and 40.21%, respectively. In conclusion, phenolics were an important food matrix that could decrease the bioavailability and oxidative damage of Pb.

Zinc strengthens the jejunal barrier by reversibly tightening the paracellular route

During the postweaning period, piglets are prone to gastrointestinal infections. The resulting impairment of intestinal barrier function may cause diarrhea associated with growth retardation or even death of piglets. Orally applied Zn is commonly used to prevent and treat diarrhea, but its mode of action still needs to be elucidated. To analyze the molecular mechanism whereby Zn acts on porcine intestinal barrier function, ex vivo studies on piglet jejunum and accompanying in vitro studies on a porcine jejunal epithelial cell line, IPEC-J2/PS, were performed with electrophysiological tools. Feeding pharmacological Zn doses exerted no significant electrophysiologically ascertainable short- and long-term effects on jejunal barrier function ex vivo. However, in IPEC-J2/PS, basolateral Zn was cytotoxic since its application caused a release of lactate dehydrogenase and an irreversible breakdown of the epithelial barrier. In contrast, apical Zn application caused an immediate increase in paracellular resistance and a decrease in permeability to the paracellular marker fluorescein, reflecting overall barrier strengthening in vitro. Apical effects were fully reversible upon washout. This indicates that Zn supplemented to feed was completely washed out during ex vivo jejunum preparation. We conclude that there is no evidence for long-term barrier effects through prophylactic Zn supplementation and that extracellular Zn acts acutely and reversibly from the apical side via tightening the paracellular route, thus counteracting leak-flux diarrhea.NEW & NOTEWORTHY Therapeutically administered Zn successfully treats diarrhea in veterinary and human medicine. Here we present data that Zn strengthens the porcine jejunal epithelial barrier by reversibly tightening the paracellular route for inorganic ions and small solutes. Acute or long-lasting Zn effects on transcellular transport (Cl^- secretion) were not detected. We therefore conclude that Zn is useful for acutely treating leak-flux diarrhea rather than secretory diarrhea. Suitability as prophylactic feed supplement, however, is questionable.

Soybean extracts increase cell surface ZIP4 abundance and cellular zinc levels: a potential novel strategy to enhance zinc absorption by ZIP4 targeting

Dietary zinc deficiency puts human health at risk, so we explored strategies for enhancing zinc absorption. In the small intestine, the zinc transporter ZIP4 functions as an essential component of zinc absorption. Overexpression of ZIP4 protein increases zinc uptake and thereby cellular zinc levels, suggesting that food components with the ability to increase ZIP4 could potentially enhance zinc absorption via the intestine. In the present study, we used mouse Hepa cells, which regulate mouse Zip4 (mZip4) in a manner indistinguishable from that in intestinal enterocytes, to screen for suitable food components that can increase the abundance of ZIP4. Using this ZIP4-targeting strategy, two such soybean extracts were identified that were specifically able to decrease mZip4 endocytosis in response to zinc. These soybean extracts also effectively increased the abundance of apically localized mZip4 in transfected polarized Caco2 and Madin-Darby canine kidney cells and, moreover, two apically localized mZip4 acrodermatitis enteropathica mutants. Soybean components were purified from one extract and soyasaponin Bb was identified as an active component that increased both mZip4 protein abundance and zinc levels in Hepa cells. Finally, we confirmed that soyasaponin Bb is capable of enhancing cell surface endogenous human ZIP4 in human cells. Our results suggest that ZIP4 targeting may represent a new strategy to improve zinc absorption in humans.

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.

Characterization of basolateral-to-apical transepithelial transport of cadmium in intestinal TC7 cell monolayers

Cadmium (Cd) is a toxic metal with an extremely long half-life in humans. The intestinal absorption of Cd has been extensively studied but the role the intestinal epithelium may play in metal excretion has never been considered. The basolateral (BL)-to-apical (AP) transepithelial transport of Cd was characterized in TC7 human intestinal cells. Both AP and BL uptakes varied with days in culture, and BL uptake was twofold higher compared to AP in differentiated cultures. A 50% increase in the BL uptake of 0.5 μM (109)Cd was observed at pH 8.5 in a chloride but not nitrate medium, suggesting the involvement of a pH-sensitive mechanism of transport for chloro-complexes. Fe and Zn inhibited the BL uptake of Cd whereas complexation by albumin had no effect, but the stimulatory effect of pH 8.5 was lost in the presence of albumin. The BL uptake of [(3)H]-MPP(+) and (109)Cd were both inhibited by decynium22 without reciprocal inhibition. MRP2 and MDR1 mRNA levels increased as a function of days in culture. A 25 and 20% decrease in the cellular AP efflux of Cd was observed in the presence of verapamil and probenecid, respectively. In cells treated with BSO, which lowered by 26% the total cellular thiol content, the inhibitory effect of verapamil increased, whereas that of probenecid decreased. These results reveal the existence of a decynium22-sensitive mechanism of transport for Cd at the BL membrane, and suggest the involvement of MDR1 and MRP2 in cellular Cd efflux at the AP membrane. It is conceivable that the intestinal epithelium may contribute to Cd blood excretion.

The content of elements in rainwater and its relation to the frequency of hospitalization for atrial fibrillation in Opole Voivodship, Poland, during 2000-2002

The elemental composition of rainwater is a suitable indirect indicator of the presence of chemical elements in airborne dust. As such, rainwater is considered a suitable monitor for environmental or natural pollution. The yearly content of chemical elements in rainwater may be considered a good indicator for determining the influence of these environmental factors on human body. We decided to investigate the relationship between chemical elements in rainwater and the frequency of hospitalizations for atrial fibrillation (I48, ICD10) on the population of the region of Opole Voivodship, Poland during the years 2000-2002. The voivodship or province has a population of one million and is divided into 12 counties that have between 44,000 and 151,000 inhabitants. We calculated the yearly average number of hospitalizations by reasons of atrial fibrillation per 10,000 inhabitants in particular counties. The average content of the chosen chemical elements in kilograms per hectare per year was calculated for each county individually and the data evaluated by means of Spearman's correlation coefficient. There is a high positive correlation between chromium in rainwater and hospital admissions for atrial fibrillation (r = 0.62) and mild positive correlation with cadmium (r = 0.57), lead (r = 0.57), zinc (r = 0.50), and chloride (r = 0.48). There are no significant differences between male and female patients. The biological activities of these elements, which may include enhanced adrenergic stimulation, remodeling of calcium or other ion channels, or the cytotoxic effects of hexavalent chromium may explain the positive correlations here reported.

Differentiation- and polarization-dependent zinc tolerance in Caco-2 cells

PURPOSE

Enterocytes are feasibly confronted with enormous zinc concentrations especially as a result of oral zinc supplementation. In the present study, we investigated the mechanisms underlying the exceptional ability to withstand this usually toxic load using the enterocytic cell line Caco-2.

METHODS

By MTT test analysis, we compared zinc tolerance in undifferentiated Caco-2 cells (udCaco-2) and differentiated Caco-2 cells (dCaco-2). By RT-PCR, we compared the respective baseline expression levels of certain zinc transporters and metallothioneins (MTs) as well as the regulation of these components in response to high zinc concentrations. Moreover, using dCaco-2 cells cultured on porous membranes, we explored zinc tolerance in dependence of the side of zinc administration: apical versus basolateral.

RESULTS

dCaco-2 cells tolerate significantly higher levels of zinc compared to udCaco-2 cells. This adaptation was accompanied by upregulated ZnT-1 and downregulated ZIP1 levels. The expression of metallothioneins MT1A and MT1X was also significantly downregulated during differentiation. Moreover, apparent from profound differences in zinc tolerance between apical and basolateral zinc application, polarization was concluded to have substantial impact on cellular zinc tolerance.

CONCLUSIONS

The profound increase in zinc tolerance we found in differentiated enterocyte-type Caco-2 cells and in particular, the impact of polarization are likely to reflect the physiologically indispensable capability of enterocytes to cope with remarkable concentrations of intestinal zinc.

Effect of dietary ligands and food matrices on zinc uptake in Caco-2 cells: implications in assessing zinc bioavailability

The kinetics, depletion/repletion of zinc, and effects of dietary ligands/food matrices on (65)Zn uptake was studied in Caco-2 cells. The uptake of zinc showed a saturable and nonsaturable component, depending upon the media zinc concentrations. Intracellular depletion increased zinc uptake, whereas zinc loading did not. Phytic acid and histidine inhibited zinc uptake, while tannic acid, tartaric acid, arginine, and methionine increased zinc uptake. Tannic acid at a 1:50 molar ratio promoted zinc uptake from wheat- and rice-based food matrices. Further, Caco-2 cells responded similarly with zinc and iron uptake when fed Indian bread prepared from low- and high-extraction wheat flour, representing low and high phytate content. However, inclusion of tea extract or red grape juice as a source of polyphenols enhanced the uptake of zinc while decreasing that of iron. These results suggest that the Caco-2 cells predict the correct direction of response to dietary ligands even from complex foods.

Kinetics of zinc absorption by in situ ligated intestinal loops of broilers involved in zinc transporters

Two experiments were conducted to study the kinetics of Zn absorption and Zn transporter mRNA levels in the duodenum, jejunum, and ileum of broilers to investigate the main site of Zn absorption in the small intestine, the absorption mechanisms, and whether those transporters were involved. In experiment 1, we compared Zn absorption in 3 segments and at different post-perfusion time points, and found that Zn absorption increased linearly within 30 min and was higher (P < 0.05) in the ileum than in the other 2 segments. In experiment 2, intestinal loops were perfused with solutions containing 0, 0.077, 0.154, 0.308, 0.616, 1.232, or 2.464 mmol/L of ZnSO(4) . 7H(2)O, and Zn concentrations in perfusates were determined at 30 min after perfusion. The mRNA levels of transporters in 3 intestinal loops from the control group and the 0.616 mmol of Zn/L group were analyzed. The kinetic curves showed that Zn transported to the duodenum and jejunum depended on a saturable carrier-mediated process. The Michaelis-Menten constant (K(m)) value was higher in the duodenum than in the jejunum (1.44 +/- 0.33 vs. 0.51 +/- 0.17 mM, P = 0.012). Similarly, the maximum absorption velocity (J(max)) value was greater in the duodenum than in the jejunum (5.32 +/- 1.46 vs. 2.57 +/- 0.39 nmol/min per cm, P = 0.069), whereas absorption in the ileum occurred with a nonsaturable diffusion process and had a diffusive constant (P) of 5.72 x 10(-3) +/- 1.1 x 10(-4) cm(2)/min. Moreover, the mRNA levels of metallothionein, zinc transporter 1, and Zn transporter 5 were lower in the ileum than in the duodenum or jejunum in the Zn-supplemented group, further indicating that Zn absorption in the ileum occurred mainly by a nonsaturable diffusive pathway. The Zn fluxes were significantly higher (P < 0.005) in the ileum than in the other 2 segments at different Zn concentrations. This research suggests that the ileum is the main site of Zn absorption and that the mechanism involved is nonsaturable diffusion, which is different from that in the other 2 segments, depending on the regulation of Zn transporter expression.

Zinc supplementation or regulation of its homeostasis: advantages and threats

To accomplish its multifunctional biological roles, zinc requires precise homeostatic mechanisms. There are efficient mechanisms that regulate zinc absorption from the alimentary tract and its excretion by the kidney depending on the organism demands. The regulatory mechanisms of cellular zinc inflow, distribution, and zinc outflow are so efficient that symptoms of zinc deficiency are rare, and symptoms connected with its massive accumulation are even more rare. The efficiency of homeostatic mechanisms that prevent zinc deficiency or excessive zinc accumulation in the organism is genetically conditioned. It seems that an essential element of zinc homeostasis is the efficiency of zinc transmembrane exchange mechanisms. Intracellular free zinc concentration is higher than in extracellular space. Physiologically, the active outflow of zinc ions from the cell depends on the increase of its concentration in extracellular space. The ion pumps activity depends on the efficiency by which the cell manages energy. Considering the fact that zinc deficiency accelerates apoptosis and that excessive zinc accumulation inside cells results in a toxic effect that forces its death brings about several questions: Is intensification and acceleration of changes in zinc metabolism with age meaningful? Is there a real zinc deficiency occurring with age or in connection with the aforementioned pathological processes, or is it just a case of tissue and cell redistribution? When discussing factors that influence zinc homeostasis, can we consider zinc supplementation or regulation of zinc balance in the area of its redistribution? To clarify these aspects, an essential element will also be the clear understanding of the nomenclature used to describe changes in zinc balance. Zinc homeostasis can be different in different age groups and depends on sex, thus zinc dyshomeostasis refers to changes in its metabolism that deviate from the normal rates for a particular age group and sex. This concept is very ample and implies that zinc deficiency may result from a low-zinc diet, poor absorption, excessive loss of zinc, zinc redistribution in intra- and extracellular compartments, or a combination of these factors that is inadequate for the given age and sex group. Such factor or factors need to be considered for preventing particular homeostasis disorders (or dyshomeostasis). Regulation of zinc metabolism by influencing reversal of redistribution processes ought to be the main point of pharmacologic and nonpharmacologic actions to reestablish zinc homeostasis. Supplementation and chelation are of marginal importance and can be used to correct long-term dietary zinc deficiency or zinc poisoning or in some cases in therapeutic interventions. In view of its biological importance, the problem posed by the influence of zinc metabolism requires further investigation. To date, one cannot consider, for example, routine zinc supplementation in old age, because changes of metabolism with age are not necessarily a cause of zinc deficiency. Supplementation is warranted only in cases in which deficiency has been established unambiguously. An essential element is to prevent sudden changes in zinc metabolism, which lead to dyshomeostasis in the terms defined here. The primary prophylaxes, regular physical activity, efficient treatment of chronic diseases, are all elements of such prevention.

Zinc ions efflux from lymphocytes in vitro in the presence of a calcium and magnesium ionic environment and its changes following administration of verapamil

The total and ouabain-dependent rate constants of efflux of zinc (Zn) ions from lymphocytes isolated from healthy subjects were measured in vitro in an environment containing calcium (Ca) and magnesium (Mg) ions. Both the total (ERCt-Zn) and ouabain-dependent (ERCos-Zn) rate constants were higher in the presence of Mg2+, with the the oubain-dependent efflux significantly different 0.29+/-0.07 vs 0.13+/-0.02 with and without Mg2+, respectively (p<0.001). After the addition of verapamil, an increase of ERCE-Zn was observed in both ionic environments and was higher and statistically significant in the presence of Mg2+: 1.94+/-0.64 vs 2.97+/-1.16 (p<0.025). These results suggest that verapamil has an enhancing effect on Zn efflux from isolated lymphocytes, suggesting that calcium channel blockers might result in better Zn homeostatic regulation in diseases of the cardiovascular system.

Pharmacokinetic Characterization of Transcellular Transport and Drug Interaction of Digoxin in Caco-2 Cell Monolayers

To characterize the intestinal absorption of digoxin, its transcellular transport and drug interaction activity was investigated using Caco-2 cell monolayers. We examined digoxin transport in the presence and absence of ouabain to determine whether digoxin binding to Na+,K(+)-ATPase affects its transcellular digoxin transport, and evaluated its influx and efflux clearance by model-dependent pharmacokinetic analysis. Transcellular transport in the basal-to-apical direction was greater than that in the opposite direction. In addition, ouabain decreased the cellular accumulation of digoxin, but it did not alter its transcellular transport profile. The observations for transcellular transport and cellular accumulation in the presence of ouabain were used for the pharmacokinetic analysis, which showed that the efflux clearance of digoxin on the apical side of the monolayer was 15 times greater than that on the basal side. Apical-to-basal transport was increased by carvedilol and pimobendan, and these compounds suppressed the efflux clearance on the apical side and the influx clearance on the basal side. These findings indicate that the intestinal absorption of digoxin is primarily dominated by the efflux process on the luminal side of the intestine, and that carvedilol and pimobendan may vary the rate of intestinal digoxin absorption mainly by inhibiting its exsorptive transport.

Mammalian zinc transporters

New insights into mammalian zinc metabolism have been acquired through the identification and characterization of zinc transporters. These proteins all have transmembrane domains, and are encoded by two solute-linked carrier (SLC) gene families: ZnT (SLC30) and Zip (SLC39). There are at least 9 ZnT and 15 Zip transporters in human cells. They appear to have opposite roles in cellular zinc homeostasis. ZnT transporters reduce intracellular zinc availability by promoting zinc efflux from cells or into intracellular vesicles, while Zip transporters increase intracellular zinc availability by promoting extracellular zinc uptake and, perhaps, vesicular zinc release into the cytoplasm. Both the ZnT and Zip transporter families exhibit unique tissue-specific expression, differential responsiveness to dietary zinc deficiency and excess, and differential responsiveness to physiologic stimuli via hormones and cytokines.

Zinc uptake across the apical membrane of freshwater rainbow trout intestine is mediated by high affinity, low affinity, and histidine-facilitated pathways

Zinc is both a vital nutrient and an important toxicant to aquatic biota. In order to understand the interplay between nutrition and toxicity, it will be important to determine the mechanisms and the factors that regulate zinc uptake. The mechanism of apical intestinal Zn(II) uptake in freshwater rainbow trout and its potential modification by the complexing amino acid histidine was investigated using brush-border membrane vesicles (BBMVs). Following characterisation of the BBMV preparation, zinc uptake in the absence of histidine was both time- and concentration-dependent and consisted of two components. A saturable phase of uptake was described by an affinity constant of 57+/-17 microM and a transport capacity of 1867+/-296 nmol mg membrane protein(-1) min(-1). At higher zinc levels (>500 microM) a linear, diffusive component of uptake was evident. Zinc transport was also temperature-dependent, with Q10 values suggesting zinc uptake was a carrier-mediated process. Zinc uptake by vesicles in the presence of histidine was correlated to a mono-histidine species (Zn(His)+) at all Zn(II) concentrations examined.

Influence of zinc deficiency on the mRNA expression of zinc transporters in adult rats

The accumulation of zinc in the cell is a sum of influx and efflux processes via transporter proteins, like the four Zn transporters (ZnT1-4), the divalent cation transporter 1 (DCT1) and of storage processes mainly bound to metallothionein (MT). To study the effect of Zn deficiency on mRNA expression levels, adult rats were used as an animal model. Food intake was restricted to 8 g/day containing 2 microg Zn/g fortified with pure phytate in Zn deficiency rats and 58 microg Zn/g in controls (n = 7). At day 1, 2, 4, 7, 11, 16, 22, and 29 of Zn deficiency, 3 animals were sacrificed, respectively (n = 24). Zn deficiency was evident from reduced plasma Zn, plasma alkaline phosphatase activity and severe mobilization of Zn from tissue stores (mainly skeleton), while food intake and body weight remained unaffected. Tissues representing Zn absorption (jejunum, colon), Zn storage and utilization (muscle, liver), and Zn excretion (kidney) were retrieved. Total RNA contents increased in colon (p = 0.003) and trend to decrease in liver (p = 0.086). Zn deficiency was without effect on tissue total RNA concentrations in muscle tissue and kidney. Real-time reverse transcription (RT) polymerase chain reaction (PCR) assays were developed and a relative quantification on the basis of GAPDH was applied. Assays allowed a relative and accurate quantification of mRNA molecules with a sufficiently high sensitivity and repeatability. All known Zn transporter subtypes were found in the tissues. ZnT3 was newly elucidated and sequenced in rat tissues. Expression patterns and reactions to Zn deficiency were specific for the tissue analysed. Expression results imply that some transporters are expressed constitutively, whereas others are highly regulated in tissues responsible for Zn homeostasis. The most distinct changes of expression levels were shown in colon which can therefore be postulated as a highly Zn sensitive tissue. MT was down-regulated in all tissues, massively in liver (p < 0.001) and in colon (p = 0.002) and in tendency also in the jejunum and kidney. In parallel with intracellular Zn status it is a potent candidate gene for Zn deficiency. ZnT1 and ZnT2 showed a significant up-regulation of mRNA expression in colon (p = 0.032 and p = 0.026) and for ZnT2 a trend of down regulation in jejunum (p = 0.098). This study provides the first comparative view of regulation of gene expression and fully quantitative expression analysis of all known Zn transporters in a non growing adult rat model on a constant platform and therefore allows a direct comparison.

Pre-treatment of Caco-2 cells with zinc during the differentiation phase alters the kinetics of zinc uptake and transport(2)

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 &mgr;mol Zn/L for 7 d, and Zn uptake and transport were measured in both apical (AP) and basolateral (BL) directions by using (65)Zn. 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 &mgr;mol Zn/L was slower (p < 0.05) than that in cells exposed to 5 &mgr;mol Zn/L. The AP to BL transport rate in the 25 &mgr;mol Zn/L group was only 40% (p < 0.05) of that in the 5 &mgr;M group. In contrast, the rate of BL Zn uptake was 4-fold higher in cells treated with 25 &mgr;mol Zn/L than in those treated with 5 &mgr;mol Zn/L (p < 0.05). The BL to AP transport rate was 2-fold higher in cells treated with 25 &mgr;mol Zn/L than in those treated with 5 &mgr;mol Zn/L (p < 0.05). Basolateral uptake was 6 to 25 times greater (p < 0.05) than AP uptake for cells treated with 5 and 25 &mgr;mol Zn/L, respectively. The rate of Zn release was enhanced about 4-fold (p < 0.05) by 25 &mgr;mol Zn/L treatment. Release to the BL side was 10 times greater than to the AP side. Zn-induced metallothionein (MT), thought to down-regulate AP to BL Zn transport, was 4-fold higher (p < 0.001) in the 25 &mgr;mol Zn/L group than in the 5 &mgr;M group, but the rate of BL Zn release was higher in cells treated with 25 &mgr;mol Zn/L than in those treated with 5 &mgr;mol Zn/L (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 the ZIP and ZnT families of Zn transporters.

Zinc regulates the function and expression of the iron transporters DMT1 and IREG1 in human intestinal Caco-2 cells

Trace metals influence the absorption of each other from the diet and it has been suggested that the divalent metal transporter (DMT1) represents a common uptake pathway for these important micronutrients. However, compelling evidence from our laboratory suggests that DMT1 is predominantly an iron transporter, with lower affinity for other metals. Several studies have shown that increasing dietary iron downregulates DMT1. Interestingly, our current data indicate that zinc upregulates DMT1 protein and mRNA expression and also pH-dependent iron uptake. Transepithelial flux of iron was also increased and was associated with a rise in IREG1 mRNA expression.

Calcium, iron, and zinc uptake from digests of infant formulas by Caco-2 cells

Our aim was to estimate the bioavailability of calcium, iron, and zinc from infant formulas using a model that includes in vitro digestion and a Caco-2 cell culture to estimate the uptake. The cell culture conditions were selected, and uptake assays were carried out first with calcium, iron, and zinc standard solutions, and then with the soluble fraction of enzymatic digests of an adapted milk-based and a soy-based infant formula. It was not possible to measure the uptake of calcium, iron, and zinc from standard solutions added to the cell cultures in amounts similar to those present in infant formula digests with our method. The fact that it was, however, possible in the case of enzymatic digests suggests the presence of components in the digests that enhance mineral uptake. When mineral uptakes were expressed as percentages of the mineral present, statistically significant differences were found in the case of calcium between the uptake from the milk- and the soy-based formulas. For iron and zinc no such differences were observed.

Influence of heat treatment of casein in presence of reducing sugars on Zn solubility and Zn uptake by Caco-2 cells after in vitro digestion

The effect of the heat treatment of casein in presence of reducing sugars on some aspects of Zn availability was investigated. Samples were prepared by mixing casein with glucose-fructose, and were used unprocessed (C) or heated (HC). Changes in Zn speciation after the in vitro digestion of the samples, both as part of a diet and in isolation, were studied. The uptake of soluble Zn from the digested samples was investigated in Caco-2 cells. After in vitro digestion, the percentage of precipitated Zn was significantly higher with the HC sample, both when digested alone and as a part of the diet. In assays with Caco-2 cells, a significant decrease in Zn uptake was observed when the uptake buffer contained the sample C digest, by comparison with the control buffer, without casein digest. When the digested heated mixture was added, Zn uptake by the cells was significantly lower than in either of the two other cases. It may be concluded that the heat treatment of casein in the presence of glucose-fructose has a negative effect on Zn availability because, after in vitro digestion, Zn insolubilization was enhanced and Zn uptake by the enterocyte was impaired, compared with the unheated mixture. In addition, the usefulness of Caco-2 cells in this kind of research has been shown.

Nramp2 expression is associated with pH-dependent iron uptake across the apical membrane of human intestinal Caco-2 cells

The absorption of dietary non-heme iron by intestinal enterocytes is crucial to the maintenance of body iron homeostasis. This process must be tightly regulated since there are no distinct mechanisms for the excretion of excess iron from the body. An insight into the cellular mechanisms has recently been provided by expression cloning of a divalent cation transporter (DCT1) from rat duodenum and positional cloning of its human homologue, Nramp2. Here we demonstrate that Nramp2 is expressed in the apical membrane of the human intestinal epithelial cell line, Caco 2 TC7, and is associated with functional iron transport in these cells with a substrate preference for iron over other divalent cations. Iron transport occurs by a proton-dependent mechanism, exhibiting a concurrent intracellular acidification. Taken together, these data suggest that the expression of the Nramp2 transporter in human enterocytes may play an important role in intestinal iron absorption.

Cadmium uptake and transepithelial transport in control and long-term exposed Caco-2 cells: the role of metallothionein

Exposure of humans to cadmium, a common environmental pollutant, is mainly through food intake. However, the mechanisms of intestinal absorption have not been clearly elucidated for this toxic metal ion. In order to investigate the effects of long-term exposure to this metal and the role of metallothioneins in cadmium absorption, we used human-derived Caco-2 cells cultured on porous membrane filters. We first validated this model by quantifying metal uptake and transepithelial transport on control cells and cells adapted to grow for 2 to 5 weeks in the presence of low doses of cadmium in the culture medium. The nontoxic doses of cadmium (0.1, 1.0, and 5 microM), in which Caco-2 cells could be cultured for many passages without deleterious effects, were determined by evaluating transepithelial resistance of the cells and lactate dehydrogenase leakage. After 24 h of 1 microM Cd exposure, intracellular cadmium levels were 3- and 6-fold higher for cells exposed for extended periods to 1 and 5 microM cadmium, respectively, compared to control cells. In control and long-term exposed cells, this accumulation was inhibited by zinc, copper, and pCMBS, but not by verapamil or ouabain. Intracellular metallothionein content was increased 1.5-, 5-, and 12-fold for the cells grown in the presence of 0.1, 1.0, and 5 microM cadmium, respectively, in the culture medium. The amount of metallothionein synthesized and released by the cells was highly correlated with cadmium accumulation and transport. Our results suggest that Caco-2 cell monolayers are a good predictive model for the study of cadmium intestinal absorption following exposure to repeated low doses of cadmium, and confirm the essential role of metallothionein in the regulation of cadmium intestinal absorption.

Genistein increases metallothionein expression in human intestinal cells, Caco-2

Flavonoids found in common vegetables, fruits, and legumes have been shown to possess antioxidant property. This study is the first to demonstrate that one member of the flavonoid family, genistein, can induce the expression of metallothionein (a metal-binding protein with antioxidant property). We found the effect of genistein to be time- and dose-dependent (10-100 microM). The effect can be observed at both protein and mRNA levels and was synergistic to that of 30 microM zinc. Genistein was shown previously to interact with the estrogen receptor and induce gene expression similar to estrogens at a lower affinity. We thus tested the hypothesis that the effect of genistein on metallothionein expression was mediated through the steroid hormone pathway. We found that various glucocorticoids do not affect metallothionein expression in Caco-2 cells. 17Beta-estradiol at 10-100 microM (concentrations much higher than needed to activate the estrogen response element) induced metallothionein expression in Caco-2 cells. However, a synthetic estrogen, diethylstilbestrol, did not increase metallothionein level at 10 microM. 17Beta-estradiol also did not act synergistically with zinc. Thus, genistein may enhance metallothionein expression through an uncharacterized mechanism. Further studies are needed to delineate the molecular mechanism and to determine whether the expression of other genes is also affected by genistein.

Purification and characterization of a major zinc binding protein from renal brush border membrane of rat

In spite of the fact that zinc is an essential trace element, mechanisms that contribute to zinc homeostasis in mammals are poorly understood. An attempt has been made to identify and purify zinc binding components from renal brush border membrane (BBM), which could be involved in the binding of zinc and the subsequent translocation across the BBM. A 40 kDa major zinc binding protein has been identified and purified from renal BBM, which showed a dissociation constant (Kd) of 211 microM and maximal binding (Bmax) of 207 nmol/mg protein. 8 g zinc atoms could interact with 1 mol of protein. Specificity of the protein for zinc was checked by metal displacement and UV-absorption assay. It was found that only Cd2+ could displace the zinc bound to the protein. Other metals tested (Cu2+, Mg2+, Ca2+) did not show any interaction with the protein. These data indicated that purified protein is highly specific and has a high affinity for zinc. The carbohydrate content was found to be 7.85 mg% in the purified protein. Immunofluorescence localization of this protein in kidney sections revealed that this major zinc binding protein is exclusively localized in the proximal convoluted tubules. These results suggested that the 40 kDa major zinc binding transmembrane glycoprotein is highly specific for zinc and has a high affinity for zinc.

Specific 1,25(OH)2D3-mediated regulation of transcellular calcium transport in Caco-2 cells

Calcium transport in the apical-to-basolateral (A-to-B) or B-to-A direction was examined in cells treated with 10 nM 1, 25-dihydroxyvitamin D3 [1,25(OH)2D3, calcitriol] for up to 72 h. Net A-to-B calcium transport was positive at all time points and increased from 0.14 +/- 0.06 to 0.50 +/- 0.01 nmol. well-1. min-1 after 72 h of calcitriol treatment. Neither phenol red transport nor transepithelial electrical resistance was altered by calcitriol treatment, suggesting that the increase in net A-to-B calcium transport was not due to paracellular movement. Neither 25-hydroxyvitamin D3 nor 24,25-dihydroxyvitamin D3 (100 nM, 48 h) alters basal or calcitriol-stimulated A-to-B calcium transport. Treatment with the calmodulin antagonist trifluoperazine (50 microM) reduced calcitriol-stimulated A-to-B Ca transport by 56%. The transcription inhibitor actinomycin D inhibited calcitriol-regulated A-to-B calcium transport as well as calbindin D9k and 24-hydroxylase mRNA accumulation. These data demonstrate that calcitriol-mediated A-to-B calcium transport in Caco-2 cells is a specific, transcellular process that requires transcriptional events normally mediated through the vitamin D receptor.

Manganese transport by Caco-2 cells

The uptake and transport kinetics of manganese (Mn) were investigated in the human intestinal Caco-2 cell line both from the absorption side (apical to basolateral) and from the exsorption side (basolateral to apical). With regard to the former, transport versus time revealed (as uptake) a biphasic pattern with an initial transient phase followed by steady-state conditions. Uptake versus Mn concentrations showed saturation-type kinetics with a 100% increase of Mn binding capacity when measurements were made from 0.5 to 2 h of incubation. The transport characteristics in steady-state conditions exhibited two components, saturable (Vmax = 3.70+/-0.07 nmol/cm2/h, K(m) = 32.2+/-3.4 microM) and nonsaturable (slope = [1.4+/-0.2] x 10(-6)cm(-2)/h) usually presumed to reflect transcellular (carrier mediated) and paracellular (diffusional) pathways, respectively. Mn fluxes were decreased by calcium and calcium antagonists, almost 100% inhibited at 4 degrees C, and affected by quinacrine and ouabain. The inhibition of ATP synthesis was apparently ineffective. From the exsorption side, the Mn fluxes, without a transient period, had an approx 20-fold smaller rate than in the absorptive direction and showed mainly a nonsaturable route (slope = [0.6+/-0.1] x 10(-6) cm(-2)/h). The mechanisms participating in the Mn movements through the monolayer are discussed and proposed to be in common, at least partly, with other divalent cations such as calcium, zinc, or iron.

Transport of nickel across monolayers of human intestinal Caco-2 cells

The passage of nickel across monolayers of intestinal epithelial Caco-2 cells, originally derived from a human colonic adenocarcinoma, was studied in bicameral chambers. The results showed that the transport and accumulation of nickel were depressed in iron-loaded monolayers, indicating that the metal participates in an absorptive process for iron in the Caco-2 cells. No detectable transport of nickel in either the apical to basal or basal to apical direction occurred at 4 degreesC. Since cellular metabolism is inhibited at 4 degreesC, these data indicate that there is no passive transcellular or paracellular passage of the nickel across the monolayers. Studies in ATP-depleted monolayers showed an increased permeability of nickel, and concomitantly there was a similar increase in the permeability of the paracellular marker mannitol. These results indicate that the metabolic inhibition results in a loosening of the junctional complexes between the Caco-2 cells, resulting in a paracellular leakage of the nickel. Additional experiments showed that the transport of nickel in the basal to apical direction occurred at a higher rate than in the apical to basal direction. This indicates the presence of an extrusion mechanism that secretes the nickel from the basal to the apical side of the Caco-2 cells. Studies with Caco-2 cells and in vivo studies by other authors have shown similar results for other metals, indicating that colonic epithelial cells may have the ability to secrete some metals.

Effect of transmucosal fluid movement on zinc and copper absorption from the rat small intestine

1. The effects of transmucosal fluid movement on zinc and copper absorption from rat small intestine were investigated using an in situ recirculating perfusion method. 2. Transmucosal fluid movement was obtained by using hypo-, iso- or hyperosmotic perfusate containing various concentrations of sodium chloride. Linear correlations were obtained between net fluid movement and absorption rate constants, which were calculated from the perfusion decline of zinc (n = 15; r = 0.77) and copper (n = 15; r = 0.75). 3. The changes in serum levels of these metals did not correlate with the increase in the absorption rate. 4. The present study indicates that zinc and copper absorption is dependent on transmucosal fluid movement.

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.

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.

Equivalent uptake of organic and inorganic zinc by monkey kidney fibroblasts, human intestinal epithelial cells, or perfused mouse intestine

Zinc (Zn) is recognized as an essential nutrient, and is added as a supplement to animal and human diets. There are claims that zinc methionine (ZnMet) forms a stable complex that is preferentially transported into tissues, and this has contributed to uncertainty about conflicting reports on the bioavailability of various Zn compounds. This study evaluated the cellular and intestinal uptake of inorganic and organic forms of Zn. Steady-state uptake of 65Zn by human intestine epithelial cells, and monkey kidney fibroblasts was not significantly different with zinc chloride (ZnCl2), ZnMet, or zinc propionate (ZnProp) (P > 0.05). Uptake of 65Zn from zinc chelated with EDTA was significantly lower (P < 0.01). In live mice, 65Zn uptake by perfused intestine and deposition in intestine and liver showed no significant difference between ZnCl2 and ZnMet. Equimolar [65Zn]methionine and zinc[35S]methionine were prepared according to a patented method that yields "complexed" Zn. Cellular uptake of the radiolabeled methionine was <0.1% of the radiolabeled Zn from these complexes, indicating separate uptake of the Zn and methionine. Gel filtration did not distinguish between 65Zn in ZnCl2, ZnProp, or reagent ZnMet, though feed-grade ZnMet containing >10% protein did give a higher-mol-wt form of 65Zn. Results of this study show equivalent uptake of Zn from inorganic and organic compounds, and support recent feed trials on Zn bioavailability.

Effect of bile/pancreatic secretions on absorption of radioactive or stable zinc. In vivo and in vitro studies

Biliary/pancreatic (B/P) secretions are a major component of endogenous secretions, and endogenously secreted Zn is a primary means of Zn homeostasis. This study examined whether B/P fluid alters the absorption/reabsorption of Zn and, in doing so, whether this contributes to homeostatic control of Zn. Animal experiments utilized rats fed 10 or 300 micrograms Zn/kg diet. An open-ended gut perfusion study in which 65Zn-labeled B/P fluid or 67Zn-labeled and digested diet found significantly decreased Zn absorption from B/P fluid. Although Zn absorption from both sources was less in animals fed diets higher in Zn, there was no interaction of treatment and diet. Further studies utilizing cultured human colon carcinoma cells (CACO-2) as in vitro models of gut enterocytes found that the presence of B/P fluid significantly decreased Zn retention and/or transport and resulted in a redistribution of cellular Zn after 1200 min of incubation. These studies show that a substance in B/P fluid can decrease the absorption of Zn and also suggest that dietary Zn and Zn associated with B/P secretions are absorbed from distinct pools. However, the lack of an interactive effect with diet, and the amount of time required to see differences in CACO-2 cells, suggest that differences in absorption are not a major contributor to Zn homeostasis.

Distinct molecular mechanisms for protein sorting within immature secretory granules of pancreatic beta-cells

In the beta-cells of pancreatic islets, insulin is stored as the predominant protein within storage granules that undergo regulated exocytosis in response to glucose. By pulse-chase analysis of radiolabeled protein condensation in beta-cells, the formation of insoluble aggregates of regulated secretory protein lags behind the conversion of proinsulin to insulin. Condensation occurs within immature granules (IGs), accounting for passive protein sorting as demonstrated by constitutive-like secretion of newly synthesized C-peptide in stoichiometric excess of insulin (Kuliawat, R., and P. Arvan. J. Cell Biol. 1992. 118:521-529). Experimental manipulation of condensation conditions in vivo reveals a direct relationship between sorting of regulated secretory protein and polymer assembly within IGs. By contrast, entry from the trans-Golgi network into IGs does not appear especially selective for regulated secretory proteins. Specifically, in normal islets, lysosomal enzyme precursors enter the stimulus-dependent secretory pathway with comparable efficiency to that of proinsulin. However, within 2 h after synthesis (the same period during which proinsulin processing occurs), newly synthesized hydrolases are fairly efficiently relocated out of the stimulus-dependent pathway. In tunicamycin-treated islets, while entry of new lysosomal enzymes into the regulated secretory pathway continues unperturbed, exit of nonglycosylated hydrolases from this pathway does not occur. Consequently, the ultimate targeting of nonglycosylated hydrolases in beta-cells is to storage granules rather than lysosomes. These results implicate a post-Golgi mechanism for the active removal of lysosomal hydrolases away from condensed granule contents during the storage process for regulated secretory proteins.

Zinc transport and metallothionein induction in primary cultures of rabbit kidney proximal cells

Primary cultures of isolated rabbit renal proximal cells were grown on collagen-coated permeable supports. The confluent epithelia were polarized, making possible the measurement of uptakes and effluxes across the apical and the basolateral membranes. Uptakes of 65Zn were assessed under initial rate conditions, after 0.5 min incubation. The kinetic parameters of apical uptake were a Jmax of 25.1 +/- 5.3 pmol min-1 (micrograms DNA)-1, a Km of 43.3 +/- 7.3 microM and an unsaturable constant of 0.105 +/- 0.029 (n = 7) at 37 degrees C. Cadmium competitively inhibited the zinc uptake, with a Ki value of 24.5 +/- 7.3 microM. Basolateral uptake was characterized by a high capacity (Jmax = 227.9 +/- 46.6 pmol min-1 (micrograms DNA)-1) and an affinity similar to that of the apical uptake (Km = 35.4 +/- 14.2 microM). Cadmium had no effect on the basolateral zinc uptake. Effluxes across the basolateral face of the epithelium always exceeded those across the apical face. Excess zinc in the culture medium induced the synthesis of metallothionein in the epithelia, as judged by the rate of [35S]cysteine incorporation into a fraction of cytosolic proteins. Metallothionein induction did not appear to modify the kinetic parameters of the apical zinc uptake. These data suggest that separate saturable transport systems are responsible for the apical and basolateral zinc uptakes in proximal renal cells. Induction of metallothionein had no apparent effect on apical zinc uptake in this system.

Glutathione mercaptides as transport forms of metals

Among the many cellular functions of GSH, the roles of this tripeptide in metal transport, storage, and metabolism have recently received considerable attention. Although these roles had often been overlooked, they are critical for normal cellular metabolism and for protection from xenobiotics. Indeed, a number of the protective and regulatory functions of GSH are related to its ability to chelate reactive metals. GSH functions in the mobilization and delivery of metals between ligands, in the transport of metals across cell membranes, as a source of cysteine for metal binding, and as a reductant or cofactor in redox reactions involving metals. However, the interaction between GSH and metals can also produce or exacerbate cell injury. For example, GSH appears to be involved in the renal accumulation and toxicity of a number of metals, and in the carcinogenicity of chromium. Additional work is clearly needed to identify the mechanisms involved, and to better define the roles of GSH in metal homeostasis.