The influence of zinc-binding ligands in fetal circulation on zinc clearance across the in situ perfused guinea pig placenta

Role of zinc in female reproduction

Zinc is a critical component in a number of conserved processes that regulate female germ cell growth, fertility, and pregnancy. During follicle development, a sufficient intracellular concentration of zinc in the oocyte maintains meiotic arrest at prophase I until the germ cell is ready to undergo maturation. An adequate supply of zinc is necessary for the oocyte to form a fertilization-competent egg as dietary zinc deficiency or chelation of zinc disrupts maturation and reduces the oocyte quality. Following sperm fusion to the egg to initiate the acrosomal reaction, a quick release of zinc, known as the zinc spark, induces egg activation in addition to facilitating zona pellucida hardening and reducing sperm motility to prevent polyspermy. Symmetric division, proliferation, and differentiation of the preimplantation embryo rely on zinc availability, both during the oocyte development and post-fertilization. Further, the fetal contribution to the placenta, fetal limb growth, and neural tube development are hindered in females challenged with zinc deficiency during pregnancy. In this review, we discuss the role of zinc in germ cell development, fertilization, and pregnancy with a focus on recent studies in mammalian females. We further detail the fundamental zinc-mediated reproductive processes that have only been explored in non-mammalian species and speculate on the role of zinc in similar mechanisms of female mammals. The evidence collected over the last decade highlights the necessity of zinc for normal fertility and healthy pregnancy outcomes, which suggests zinc supplementation should be considered for reproductive age women at risk of zinc deficiency.

The Role of Fe, Zn, and Cu in Pregnancy

Iron (Fe), copper (Cu), and zinc (Zn) are microelements essential for the proper functioning of living organisms. These elements participatein many processes, including cellular metabolism and antioxidant and anti-inflammatory defenses, and also influence enzyme activity, regulate gene expression, and take part in protein synthesis. Fe, Cu, and Zn have a significant impact on the health of pregnant women and in the development of the fetus, as well as on the health of the newborn. A proper concentration of these elements in the body of women during pregnancy reduces the risk of complications such as anemia, induced hypertension, low birth weight, preeclampsia, and postnatal complications. The interactions between Fe, Cu, and Zn influence their availability due to their similar physicochemical properties. This most often occurs during intestinal absorption, where metal ions compete for binding sites with transport compounds. Additionally, the relationships between these ions have a great influence on the course of reactions in the tissues, as well as on their excretion, which can be stimulated or delayed. This review aims to summarize reports on the influence of Fe, Cu, and Zn on the course of single and multiple pregnancies, and to discuss the interdependencies and mechanisms occurring between Fe, Cu, and Zn.

Association of Cord Blood Zinc Level and Birth Weight in a Sample of Iranian Neonates

Background:

In addition to its short-term effects, low birth weight increases the risk of noncommunicable diseases (NCDs) in adult life. The quality of maternal diet including the macronutrient intake is very important in this regard. This study aims to evaluate the possible associations between maternal zinc and neonatal anthropometric measures.

Method:

This cross-sectional study was conducted on 226 pairs of mothers-neonates in Isfahan, Iran. Maternal characteristics including the history of the disease, age, preconceptional weight, weight gain during pregnancy, as well as, anthropometric characteristics of neonates such as weight, height, length, and circumferences of head, belly, chest, and thigh were documented. Cord blood zinc was measured by atomic absorption spectrophotometer in three groups of neonates depending on their weights.

Results:

The gestational age of neonates was 35 to 38 weeks with a mean weight of 3.13 ± 0.42 kg. The mean of zinc concentration was 0.81 ± 0.18 and it was higher in neonates with appropriate weight than in those with high or low birth weight (0.82 ± 0.18, 0.75 ± 0.19, and 0.65 ± 0.12 ng/ml, respectively). Bivariate correlation analysis showed significant weak correlation between cord blood zinc and neonatal weight (r = 0.16, P = 0.04).

Conclusions:

The cord blood zinc concentration of normal-weight neonates was higher than others. Our findings suggest that maternal zinc may influence neonatal birth weight, and it should be considered in the primordial prevention of NCDs.

Cadmium exposure during pregnancy and lactation: materno-fetal and newborn repercussions of Cd(ii), and Cd-metallothionein complexes

Cadmium (Cd) is a non-physiological heavy metal that can be harmful at low concentrations. Increasing anthropogenic activities are incrementing the risk of accumulation of this heavy metal in different organs and tissues of the body. In the case of pregnant women, the threat is more serious due to the implications affecting not only their own health but also fetal development as well. Metallothioneins (MTs), small cysteine-rich proteins, are involved in zinc (Zn) and copper homeostasis in mammals but can, however, also bind with Cd if present. The accumulation of Cd in maternal tissues (e.g. placenta, maternal blood, and mammary glands) induces the synthesis of MTs, preferably MT2, in an attempt to sequester the metal to avoid toxicity. The formed Cd-MT complexes will avoid the Cd transport from the placenta to the fetus and end up accumulating in the maternal kidneys. At the same time, high concentrations of MTs will increase the formation of Zn-MT complexes, therefore decreasing the amount of Zn ions available to be transported to the fetus by means of Zn transporters such as ZnT2, ZIP14 and DMT1. Although MTs cannot transport Cd from the mother to the fetus, the divalent DMT1 transporter is suggested to carry the metal to the fetus. As a consequence, the low levels of Zn(ii) in the fetus, together with the presence of Cd(ii) coming from the mother either via the placenta and cord blood or via breast milk induce changes in the fetal development including fetal growth retardation, and low weight or height of the newborn. Likewise, the concentrations of Cd(ii) in the newborn can cause alterations such as cognitive disabilities. In summary, the presence of Cd(ii) in the maternal tissues will induce MT synthesis in an attempt to detoxify these tissues and reduce the possible toxicity of Cd in fetal and newborn tissues.

Comparison of porous and nano zinc oxide for replacing high-dose dietary regular zinc oxide in weaning piglets

The aim of this study was to compare the effect of dietary supplementation with low dose of porous and nano zinc oxide (ZnO) on weaning piglets, and to evaluate the possibility of using them as an alternative to high dose of regular ZnO. Piglets were randomly allocated into four treatment groups fed with four diets: (1) basal diet (NC), (2) NC+ 3000 mg/kg ZnO (PC), (3) NC + 500 mg/kg porous ZnO (HiZ) and (4) NC + 500 mg/kg nano ZnO (ZNP). The result showed that piglets in HiZ group had less diarrhea than ZNP group (P < 0.05). Besides, there was no significant difference between PC, HiZ and ZNP groups in terms of serum malondialdeyhde (MDA) concentration and glutathione peroxidase (GSH-Px) activity (P > 0.05). Analysis of trace metal elements revealed that piglets fed with high dose of regular ZnO had the highest Zn level in kidney (P < 0.05), which may induce kidney stone formation. Additionally, a decrease in ileal crypt depth was observed in PC, HiZ and ZNP group, suggesting an effective protection against intestinal injury. Results of mRNA analysis in intestine showed that ZNP supplementation had better effects on up-regulated trefoil factor 3 (TFF3) and nuclear factor erythroid 2-related factor 2 (Nrf2) levels in duodenum and jejunum than HiZ did (P < 0.05), implying that nano ZnO may possess higher anti-inflammatory capacity than porous ZnO. In conclusion, dietary supplementation with low dose of porous and nano ZnO had similar (even better) effect on improving growth performance and intestinal morphology, reducing diarrhea and intestinal inflammatory as high dose of regular ZnO in weaning piglets. Compared with nano ZnO, porous ZnO had better performance on reducing diarrhea but less effect on up-regulation of intestinal TFF3 and Nrf2.

Zinc hyperaccumulation in squirrelfish (Holocentrus adscenscionis) and its role in embryo viability

Female squirrelfish (Fam. Holocentridae) can accumulate and temporarily sequester copious amounts of zinc (Zn) in their livers. There, it is initially compartmentalized before a subsequent, estrogen-triggered redistribution to the ovaries. Here we show that cellular uptake of Zn is also influenced by estrogen signaling, and that estrogen increases concentrations of the plasma Zn-binding protein vitellogenin (VTG). However, estrogen-mediated increases in VTG are not sufficient to accommodate the magnitude of hepato-ovarian Zn transfer in female squirrelfish (Holocentrus adscensionis). These findings suggest that holocentrids have acquired the ability to use hormonal cues to drive hepatic uptake and storage of Zn, signal for its physiological redistribution, and influence the capacity for systemic transport of Zn beyond the mediation of increased plasma VTG concentrations. Such specific adaptations suggest an advantage for the oocyte, which is corroborated in further studies where we determined that oocyte Zn concentrations are positively correlated with egg viability in captive-spawned squirrelfish. The novel nature of these findings underlies the importance of Zn in squirrelfish reproductive biology.

Protective effect of zinc on related parameters to bone metabolism in common carp fish (Cyprinus carpio L.) intoxified with cadmium

The short term effects of waterborne cadmium (Cd(+2)) on the levels of serum parameters related to bone metabolism including calcium (Ca), inorganic phosphorus (P(i)) and alkaline phosphatase (ALP) in common carp fish (Cyprinus carpio L.) were studied. Fish were treated with varying concentrations of Cd(+2) (0.22, 1.1 and 2.2 mg l(-1)) daily for 14 days. The results obtained show that serum P(i) and ALP concentrations were elevated by increasing Cd(+2) concentration in water containing fish whereas serum Ca level was decreased. At the same time, the protective role of waterborne zinc (Zn(+2), 1 mg l(-1)) on the same parameters was also investigated. Results showing that daily treatment of fish with Zn(+2), increased the concentrations of Ca and ALP in serum by 2.07 and 1.86 fold and decreased serum P(i) level by 57.7% in comparison with Cd(+2) treatment. The combination of Cd(+2) and Zn(+2) on the same parameters was studied next. There was a significant (P < 0.05) elevation in serum Ca and ALP levels in comparison with Cd(+2) treatment. Decreasing in serum P(i) level was not significant in comparison with Cd(+2) treatments. The protective effect of Zn(+2) on Cd(+2) disturbances in serum parameters related to bone metabolism in this manuscript has been also discussed.

Transport pathways for cadmium in the intestine and kidney proximal tubule: focus on the interaction with essential metals

Cadmium (Cd) is a toxic metal with a propensity to accumulate in the proximal tubules cells (PTC) of the kidney where it can lead to tubular dysfunction and eventually renal failure. Although Cd(2+)-induced nephrotoxicity has been well described there is still uncertainty about how this metal gains entry into these cells to induce toxicity. As a non-essential metal, specific transport proteins for Cd are unlikely to exist. Rather transport proteins/channels used by essential metals (iron, zinc, calcium) are thought to be responsible. When these dietary essential metals are in short supply and deficiencies develop, Cd absorption and toxicity are enhanced. This is primarily due to increased expression of essential metal transport proteins such as divalent metal transporter 1 (DMT1) which can transport Cd in the intestine and enhance toxicity in the kidney. The zinc/bicarbonate sympoters ZIP8 and 14 are expressed at the apical membrane of enterocytes and PTC, and can transport Cd into cells. TRPV5 and 6 are major transporters for calcium in intestine and kidney and may be involved in Cd transport in these locations. Cd in the circulation is bound to proteins such as metallothioneins (MT) which are readily filtered. Two multiligand receptors, megalin and cubulin, reabsorb filtered proteins including albumin and MT by the process of receptor-mediated endocytosis. This review summarises the transport pathways for Cd in the intestine and kidney proximal tubule focusing in particular at how Cd uses essential metal transport processes to gain entry to the circulation and the kidney.

Increased levels of metallothionein in placenta of smokers

Experiments were designed to evaluate and compare metallothionein (MT), zinc and cadmium levels in human placentas of smoking and non-smoking women. Smoking was assessed by self-reported cigarette consumption and urine cotinine levels before delivery. Smoking pregnant women with urine cotinine levels higher than 130 ng/ml were included in the smoking group. Determination of placental MT was performed by western blot analysis after tissue homogenization and saturation with cadmium chloride (1000 ppm). Metallothionein was analyzed with a monoclonal antibody raised against MT-1 and MT-2 and with a second anti mouse antibody conjugated to alkaline phosphatase. Zinc and cadmium were determined by neutron activation analysis and atomic absorption spectrometry respectively. Smokers showed higher placental MT and cadmium levels, together with decreased newborn birth weights, as compared to non-smokers. The semi-quantitative analysis of western blots by band densitometry indicated that darker bands corresponded to MT present in smokers' samples. This study confirms that cigarette smoking increases cadmium accumulation in placental tissue and suggests that this element has a stimulatory effect on placental MT production.

Cadmium uptake in rat hepatocytes in relation to speciation and to complexation with metallothionein and albumin

Cadmium (Cd) uptake has been studied in primary cultures of rat hepatocytes focusing on the impact of inorganic and organic speciation. Uptake time-course studies over a 60-min exposure to 0.3 microM (109)Cd revealed a zero-time uptake and a slower process of accumulation which proceeds within minutes. (109)Cd uptake showed saturation kinetics (K(m) = 3.5 +/- 0.8 microM), and was highly sensitive to inhibition by Zn and Hg. There was no evidence for sensitivity to the external pH nor for any preferential transport of the free cation Cd(2+) over CdCl(n) (2-n) chloro-complexes. According to the assumption that only inorganic metal species are available, metal uptake decreased upon albumin (BSA) addition to the exposure media. In contrast, higher levels of (109)Cd accumulation were obtained under optimal conditions for Cd complexation by MT. Comparison among uptake data obtained under inorganic and organic conditions revealed that Cd-MT would be taken up 0.4 times as rapidly as Cd(inorg). We conclude that uptake of Cd in rat hepatocytes involves specific transport mechanism(s) subjected to Zn or Hg interactions. Uptake of inorganic Cd is not proportional to the levels of free Cd(2+) and does not involve the divalent cation transporter DCT1 nor the co-transporter Fe(2+)-H(+) NRAMP2. We found Cd-MT but not Cd-BSA to be available for the liver cells, and have estimated a binding affinity four orders of magnitude higher for Cd complexation with MT compared to BSA; MT may have a significant role in Cd delivery to the liver.

Zinc, copper, and iron metabolism during porcine fetal development

Zinc, copper, and iron levels in maternal and fetal pig tissues and fluids were measured starting on d 30 of gestation and continuing to term (d 114) at 10-d intervals. Fetal hematocrit increased from a low of 19% on d 30 to 32% by d 50, after which it remained above 30% to term. Amniotic fluid zinc, copper, and iron all reached maximal levels by d 60 of gestation. Maternal serum zinc levels fluctuated little during gestation, but fetal serum zinc concentration was significantly elevated above maternal levels during the second trimester. Fetal serum copper levels were significantly lower than maternal values throughout gestation and this was also the case for ceruloplasmin oxidase activity. Maternal serum iron reached its lowest level by d 80 of gestation when rate of transfer of iron to the developing fetuses was high. Fetal serum iron declined throughout gestation, reaching its lowest level on d 100. In general, fetal liver concentrations of zinc, copper, and iron were higher than the corresponding maternal values throughout gestation. Distinct increases were noted for fetal hepatic zinc and copper concentrations during the second trimester of pregnancy and these were accompanied by increases in cytosolic and metallothionein-bound zinc and copper levels. Maternal hepatic iron declined during the second trimester, reaching its lowest point on d 80, indicative of the shunting of maternal iron reserves to fetal tissues. Fetal kidney metal levels did not demonstrate any distinctive developmental patterns with respect to zinc, copper, or iron concentrations, but a general accumulation of each metal was observed as gestation progressed. The results of this study highlight some of the distinct changes occurring in the metabolism of zinc, copper, and iron in both maternal and fetal tissues and fluids during gestation in the pig.

Identification of an alpha2-macroglobulin receptor in human mammary epithelial cells

Several cases of zinc (Zn) deficiency in human infants caused by abnormally low concentrations of Zn in breast milk were recently reported, the underlying mechanism of which is not known. Alpha2-macroglobulin (alpha2-M), a major Zn-binding ligand in serum, presents a potential vehicle for mammary Zn uptake. This study was conducted to determine if an alpha2-M receptor is present in human mammary epithelial cells, where it may be involved in the endocytosis of alpha2-M into the mammary gland. Normal human mammary epithelial cells were grown to confluency in serum-free medium. For all binding and uptake studies, alpha2-M, preactivated with methylamine and labeled with 125I, was added to cells for varied lengths of time to determine saturation over time and at varied concentrations to determine saturation over increasing concentration of ligand. Nonspecific and competitive binding were measured by addition of a 100-fold molar excess of unlabeled alpha2-M and serum albumin or lactoferrin, respectively. Binding at 4 degreesC was specific for alpha2-M and approached saturation kinetics at 56 nmol/L. Scatchard plot analysis of the binding data demonstrated more than one binding site: a high affinity, saturable binding site and a low affinity, nonsaturable binding site. Uptake of alpha2-M at 37 degreesC was rapid and continuous over increasing concentrations of alpha2-M, and internalized alpha2-M was rapidly degraded. Results from this study present evidence for receptor-mediated uptake of alpha2-M in human mammary epithelial cells, which in turn, provides a potential mechanism for Zn acquisition by the cell.

1H and (113)Cd NMR Investigations of Cd(2+) and Zn(2+) Binding Sites on Serum Albumin: Competition with Ca(2+), Ni(2+), Cu(2+), and Zn(2+)

1H and (113)Cd NMR studies are used to investigate the Cd(2+) binding sites on serum albumin (67 kDa) in competition with other metal ions. A wide range of mammalian serum albumins possess two similar strong Cd(2+) binding sites (site A 113-124 ppm; site B 24-28 ppm). The two strong sites are shown not to involve the free thiol at Cys34. Ca(2+) influences the binding of Cd(2+) to isolated human albumin, and similar effects due to endogenous Ca(2+) are observed for intact human blood serum. (1)H NMR studies show that the same two His residues of human serum albumin are perturbed by Zn(2+) and Cd(2+) binding alike. Zn(2+) displaces Cd(2+) from site A which leads to Cd(2+) occupation of a third site (C, 45 ppm). The N-terminus of HSA is not the locus of the two strong Cd(2+) binding sites, in contrast to Cu(2+) and Ni(2+). After saturation of the N-terminal binding site, Cu(2+) or Ni(2+) also displaces Cd(2+) from site A to site C. The effect of pH on Cd(2+) binding is described. A common Cd(2+)/Zn(2+) binding site (site A) involving interdomain His residues is discussed.

Zinc transport across an endothelium includes vesicular cotransport with albumin

Bovine pulmonary arterial endothelial cells (BPAEC) were grown on permeable polycarbonate membrane filters suspended between two compartments representing the blood vessel lumen and the interstitium. This in vitro model of an endothelium was subjected to a battery of tests to unravel the mechanisms of zinc transport from the blood into peripheral tissues. Transport of 65Zn across BPAEC from media containing zinc concentrations up to 50 mumol/L exhibited both saturable and nonsaturable kinetics. Vmax of the saturable component was 246 +/- 43 pmol/(h x cm2) and Km was 2.3 +/- 1.3 mumol/L. Transport was pH and temperature sensitive and substantially influenced by albumin and histidine concentrations, but not influenced by analogous minerals or metabolic inhibitors. Inhibition of coated vesicle formation by depletion of intracellular potassium reduced 65Zn transport. Albumin carrying a zinc ion crossed the endothelium more rapidly than zinc-free albumin. When evaluated together, this body of evidence supports the existence of two major pathways of zinc transport across the pulmonary endothelium, but neither involves entry into the endothelial cells. One pathway involves receptor-mediated cotransport with albumin by transcytotic vesicles. The other is nonsaturable and involves cotransport with albumin and low molecular weight ligands, principally histidine, through intercellular junctions and nonselective, bulk-fluid transcytosis.

Human placental transfer of zinc: normal characteristics and role of ethanol

The fetal alcohol syndrome is primarily an impairment of growth and development. Zinc deficiency also causes abnormal fetal growth. Moreover, alcohol has been shown in some rodent studies to impair placental transport of zinc. The purpose of this investigation was to define better normal human placental zinc transport and the effects of alcohol on this process. To do this we employed the isolated perfused single cotyledon human term placental model, as well as the cultured human cytotrophoblast. In the perfused placental studies, it was shown that zinc is transferred by the placenta very slowly, about 6% of the rate of transport of antipyrine, a freely diffusible marker. The transfer is comparable in both directions, maternal to fetal and the reverse. Zinc does not cross the placenta against a zinc concentration gradient, in either direction. Rather there is good evidence of significant uptake (storage) of the zinc by the placenta on the recirculating compartment side of gradient studies. Moreover, when the perfusion fluid was low (0.2 g/100 ml) in albumin, about twice as much zinc accumulated in the perfused cotyledon and there was less zinc in the maternal compartment, as compared to perfusion with ten-fold higher (2.0 g/100 ml) albumin concentrations. Thus, ligand binding in the perfusate importantly influences placental zinc uptake. Interestingly, however, the increased placental binding of zinc did not translate into greater transfer of zinc to the fetal compartment. Thus, normal zinc transfer is slow, equal bidirectionally, and dependent on ligand binding in perfusate and placenta.(ABSTRACT TRUNCATED AT 250 WORDS)

Cadmium effect on zinc metabolism in human trophoblast cells: involvement of cadmium-induced metallothionein

Isolated trophoblast cells from human placenta at term were used to investigate the effect of cadmium exposure on the placental transport of zinc. Cells were exposed to cadmium acetate (0-2 microM) for 18 h. These concentrations were found to be non-cytotoxic as determined by protein recovery and the leakage of lactate dehydrogenase. Primary trophoblast cultures showed a cadmium-dependent increase in metallothionein content. After incubation with 65Zn for different times 65Zn contents were compared between control and cadmium-exposed cells. Zinc uptake by the cells after cadmium pre-exposure was increased in a cadmium dose-dependent manner. The additional amount of zinc that was taken up by the cadmium pre-exposed cells with respect to the control cells remained constant during a 30-min incubation of the same cells in a metal-free medium. The cytosol of cadmium-exposed cells showed a higher content of zinc than that of control cells. Most of the zinc in the cytosol is in the metallothionein fraction. We have thus demonstrated that cadmium-induced metallothionein binds zinc in the trophoblast, making this essential element less available to the fetal circulation.