Toward a molecular understanding of zinc metabolism

The absorption of zinc is increased when the dietary zinc supply is low. This is caused by increased intestinal transport and reduced secretion of endogenous zinc into the intestine. Kinetic analysis of zinc transport, based on data from either the isolated perfused intestine or brush border membrane vesicles, demonstrates uptake velocity is increased homeostatically by a carrier-mediated phase of transport in response to low dietary zinc. Zinc within intestinal cells binds to high molecular weight proteins and metallothionein. Expression of the metallothionein gene is altered by zinc status and the protein appears to have a function in intestinal cells. Zinc transport across the basolateral membrane is also carrier-mediated and may be ATP-driven. Newly absorbed zinc is transported via albumin, first to the liver and then is redistributed to other tissues, particularly muscle and bone which provide the greatest reserves. Plasma zinc levels remain relatively constant except during periods of dietary zinc depletion and acute responses to stress, infection or inflammation where they are depressed. Experiments with intact rats and isolated rat liver parenchymal cells have shown that hepatic zinc turnover is rapid. Stimulation of liver cells by glucocorticoids, glucagon, epinephrine, cAMP or interleukin-1-like factors alters uptake/exchange kinetics such that there is a net accumulation of cellular zinc. Metallothionein gene expression is enhanced by these hormonal signals, and a considerable portion of the newly accumulated zinc is accounted for as that associated with this zinc-binding protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

The level of free intracellular zinc mediates programmed cell death/cell survival decisions in plant embryos

Zinc is a potent regulator of programmed cell death (PCD) in animals. While certain, cell-type-specific concentrations of intracellular free zinc are required to protect cells from death, zinc depletion commits cells to death in diverse systems. As in animals, PCD has a fundamental role in plant biology, but its molecular regulation is poorly understood. In particular, the involvement of zinc in the control of plant PCD remains unknown. Here, we used somatic embryos of Norway spruce (Picea abies) to investigate the role of zinc in developmental PCD, which is crucial for correct embryonic patterning. Staining of the early embryos with zinc-specific molecular probes (Zinquin-ethyl-ester and Dansylaminoethyl-cyclen) has revealed high accumulation of zinc in the proliferating cells of the embryonal masses and abrupt decrease of zinc content in the dying terminally differentiated suspensor cells. Exposure of early embryos to a membrane-permeable zinc chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine led to embryonic lethality, as it induced ectopic cell death affecting embryonal masses. This cell death involved the loss of plasma membrane integrity, metacaspase-like proteolytic activity, and nuclear DNA fragmentation. To verify the anti-cell death effect of zinc, we incubated early embryos with increased concentrations of zinc sulfate. Zinc supplementation inhibited developmental PCD and led to suppression of terminal differentiation and elimination of the embryo suspensors, causing inhibition of embryo maturation. Our data demonstrate that perturbation of zinc homeostasis disrupts the balance between cell proliferation and PCD required for plant embryogenesis. This establishes zinc as an important cue governing cell fate decisions in plants.     

补锌对酒精性肝病的影响及其分子机制

目的:本研究是为了观察饮食补充锌减轻酒精性肝病损伤的作用及与HNF-4α的关系。方法:选用成年C57BL/6小鼠40只,按随机数字表分为4组(n=10):正常对照组、酒精中毒组、正常补锌组及酒精补锌组,用不同饮食喂养6个月处死,在正常补锌组和酒精补锌组小鼠饮用水中加入硫酸锌,使锌的含量达到75 mg/L。取各组小鼠肝组织进行病理切片及增殖细胞核抗原(PCNA)免疫组织化学染色,RT-PCR检测肝细胞核因子-4α(HNF-4α)含量,Western blot检测肝组织HNF-4α蛋白表达,检测"肝组织超氧化物歧化酶(SOD)活性及丙二醛(MDA)含量"。结果:酒精中毒组小鼠HNF-4α转录及表达均明显低于正常对照组,差异具有统计学意义(P<0.05),该组小鼠MDA含量增高,SOD活性下降与正常对照组相比差异有统计学意义(P<0.05);而酒精补锌组小鼠PCNA阳性肝细胞数目及HNF-4α蛋白表达水平明显高于酒精中毒组,差异有统计学意义(P<0.05),该组小鼠SOD活性增加,MDA下降,与酒精中毒组相比差异有统计学意义(P<0.05)。结论:长期酒精喂养导致小鼠氧化还原失衡,而补锌可逆转该状态。我们推测饮食补锌可能是通过增加HNF-4α的转录及表达而增强酒精喂养小鼠的肝再生,因此,饮食补锌可能对酒精性肝病有较好的影响。     

Zinc transport in rabbit tissues. Some hormonal aspects of the turnover of zinc in female reproductive organs, liver and body fluids

1. To investigate the influence of hormonal conditions upon the kinetics of zinc transport, specific radioactivity of (65)Zn was determined in certain tissues and fluids from unmated or pregnant rabbits during the first half of gestation. 2. Compartmental analysis was used to find the simplest mathematical model that simulated satisfactorily tracer behaviour. Models were fitted to experimental results by a numerical procedure using a computer. 3. The kinetics of zinc exchange in most tissues investigated could adequately be described by a three-compartment model, in which total tissue zinc content was divided into a rapidly exchanging pool, with a turnover time of about 1h, and a slowly exchanging pool, the turnover time of which was in liver 15h, in peak-stage corpus luteum 8h, and in the other tissues 30-70h. 4. In rabbit endometrium zinc transport varied with hormonal conditions, the turnover rate being higher in non-pregnant than pregnant endometrium. 5. Uptake of (65)Zn by uterine fluid was slow, and in the free-lying embryos (blastocysts) slower still, in keeping with uterine fluid acting as carrier of zinc into the unimplanted embryos. 6. In placental tissue zinc transport varied with gestational stage. Foetal placenta exchanged zinc with blood plasma four times faster than maternal placenta. In foetuses zinc turnover time and flux equalled that of the slow zinc compartment in foetal placenta. 7. Corpus luteum on days 5-6 of gestation showed peak specific radioactivity and zinc flux values, which exceeded those of all other tissues. 8. In liver the slow zinc compartment had a higher rate of turnover than corresponding compartments in tissues other than peak-stage corpus luteum, but no hormone-dependent changes were observed. 9. Zinc uptake by erythrocytes was the slowest of all examined.     

High levels of zinc ions induce loss of mitochondrial potential and degradation of antiapoptotic Bcl-2 protein in in vitro cultivated human prostate epithelial cells

Prostate epithelial cells contain the highest levels of zinc among all organs and tissues in the human body. Zinc is accumulated primarily in the mitochondria, where it is responsible for inhibition of mitochondrial aconitase activity, thereby increasing citrate production. The present study was designed to clarify the role of zinc for human prostate epithelial cell growth and apoptosis. Apoptosis of in vitro cultivated human prostate epithelial cells exposed to ZnCl(2) was analyzed by determination of phospholipid membrane asymmetry, nuclear fragmentation, DNA strand breaks, changes of mitochondrial potential and cellular pro/antiapoptotic proteins. Zinc induced apoptosis without involvement of p53 by decreasing mitochondrial transmembrane potential (DeltaPsi(m)) and Bcl-2 protein levels in proliferating epithelial cells. Thus, the high local concentrations of zinc ions in the prostatic lumen seem to be necessary to regulate proliferative activities and to enforce epithelial differentiation processes.     

The acrodermatitis enteropathica gene ZIP4 encodes a tissue-specific,zinc-regulated zinc transporter in mice

The human ZIP4 gene (SLC39A4) is a candidate for the genetic disorder of zinc metabolism acrodermatitis enteropathica. To understand its role in zinc homeostasis, we examined the function and expression of mouse ZIP4. This gene encodes a well conserved eight-transmembrane protein that can specifically increase the influx of zinc into transfected cells. Expression of this gene is robust in tissues involved in nutrient uptake, such as the intestines and embryonic visceral yolk sac, and is dynamically regulated by zinc. Dietary zinc deficiency causes a marked increase in the accumulation of ZIP4 mRNA in these tissues, whereas injection of zinc or increasing zinc content of the diet rapidly reduces its abundance. Zinc can also regulate the accumulation of ZIP4 protein at the apical surface of enterocytes and visceral endoderm cells. These results provide compelling evidence that ZIP4 is a zinc transporter that plays an important role in zinc homeostasis, a process that is defective in acrodermatitis enteropathica in humans.     

Hepatic zinc response via metallothionein induction after tumor transplantation

Based on previous findings that liver zinc and metallothionein (MT) levels increase after tumor transplantation, zinc metabolism in tumor-bearing mice was studied to clarify the role of zinc-MT in host defense systems. Zinc in the hepatic cytosolic MT fraction did not increase in tumor-bearing mice fed a zinc-deficient diet, suggesting that dietary zinc is necessary for apo-MT induction in the liver after tumor transplantation and is then incorporated into the apo-MT. When (65)ZnCl(2) was intravenously injected, liver (65)Zn levels in the tumor-bearing mice were higher than those in control mice for 72 h after the injection. Pancreatic and blood (65)Zn levels in tumor-bearing mice were lower than those in controls for 24 h (pancreas) and 6 h (blood) after the injection. These findings indicate that the hepatic zinc response via MT induction influences zinc metabolism in the body after tumor transplantation. Moreover, (65)Zn uptake in the liver of MT-deficient tumor-bearing mice was lower than that in control tumor-bearing mice 1 h after injection. (65)Zn uptake in the tumor and blood (65)Zn levels in the MT-deficient tumor-bearing mice were higher than those in the control tumor-bearing mice. Tumor weight increased more in MT-deficient mice than in control mice. The formation of zinc-MT in the liver of tumor-bearing mice might decrease blood zinc availability for tumors and other tissues, such as the pancreas.     

Zinc protection against lipid peroxidation from cadmium.

Effect of zinc and cadmium on lipid peroxidation and catalase activity in liver, heart, brain and testis was determined in order to characterise the interaction of zinc with cadmium. Zinc and cadmium both increased lipid peroxidation significantly in the tissues studied. In animals pretreated with zinc prior to cadmium administration, significant decrease in lipid peroxidation in liver was observed. Lipid peroxidation was not affected significantly in testis but a significant increase was observed in heart and brain tissues. Catalase activity in testis increased significantly by zinc treatment with or without cadmium administration.     

The effect of zinc on the oxidation of krebs cycle intermediates by rat liver and kidney homogenates

Zinc ions in small concentrations (2 × 10^?5M) inhibit rat liver and kidney succinoxidase systems by forming an inactive complex with the enzyme. Under conditions which favor oxidative phosphorylation (isotonic KCl homogenates, Mg ion, and muscle adenylate or adenosine triphosphate), inhibition by zinc occurs, but this inhibition is transitory. The rate of oxidation subsequently rises to a level even higher than shown by the control data. A later fall to a lower level of activity can be attributed to the zinc inhibition of other Krebs cycle intermediates, particularly the oxidation of α-ketoglutarate and citrate. The addition of a protein such as insulin has no protective action against zinc inhibition, and crystalline zinc insulin causes the same inhibition as does zinc sulfate. With the exception of succinoxidase, the oxidizing enzymes of rat kidney are more readily inhibited by zinc ions than are those of rat liver.Amorphous insulin, in itself, inhibits citrate oxidation by rat kidney homogenates.     

Effects of zinc on cell proliferation and proteoglycan characteristics of epiphyseal chondrocytes

Zinc has been postulated as an important nutritional factor involved in growth promotion; however, the cellular mechanisms involved in the effects of zinc on linear growth remain to be elucidated. This study was conducted to evaluate the effects of zinc on the proliferation rate of epiphyseal growth plate chondrocytes and on the structural characteristics of the proteoglycans synthesized by these cells. For these purposes, hypertrophic and proliferating chondrocytes were isolated from the tibiae of 1- and 5-week-old chickens, respectively. Chondrocytes were cultured under serum-free conditions and primary cultures were used. The results showed that zinc stimulated proliferation by 40-50% above the baseline in the case of proliferating chondrocytes, but it had no effect on hypertrophic chondrocytes. Zinc had neither any effects on mean charge density of proteoglycans synthesized by hypertrophic chondrocytes nor in their hydrodynamic size. In contrast, zinc induced an increase in mean charge density and a decrease of hydrodynamic size of proteoglycans synthesized by proliferating chondrocytes. In both cell types zinc had no effect on the composition and hydrodynamic size of the glycosaminoglycan chains. The increased ability of proliferating chondrocytes cultured in the presence of zinc to synthesize 3'-phosphoadenosine 5'-phosphosulfate (PAPS) could be explained by the induction of enzymes participating in the sulfation pathway of proteoglycans. Therefore, the increase in mean charge density of proteoglycans observed in this study may be explained by an increase of the degree of sulfation of proteoglycan molecules. We speculate that the effect of zinc on linear growth may be explained at a cellular level by: a) an increase in proliferation rates of proliferating chondrocytes, and b) increased synthesis of highly charged proteoglycan molecules which decreases mineralization.     

Effect of zinc supplementation on type 2 diabetes parameters and liver metallothionein expressions in Wistar rats

Zinc is a trace metal and acts as an active component of various enzymes. Zinc deficiency has been suggested to be associated with the development of diabetes. The present study investigated the role of zinc supplementation on prevention of diabetic conditions. A double-disease model mimicking hyperlipidemia and type 2 diabetes was created by applying high-fat diet and streptozotocin (STZ) to Wistar rats. We demonstrated that zinc supplementation improved symptoms of diabetes such as polydipsia and increased serum level of high-density lipoprotein cholesterol, indicating that zinc supplementation has a potential beneficial effect on diabetic conditions. The level of maldondialdehyde (MDA), an oxidative stress marker, was reduced in liver by zinc supplementation in high fat-fed rats with or without STZ injection. Meanwhile, we observed an increase in the expression of metallothioneins (MTs) in liver of rats treated with zinc. This suggests that the induction of MTs in liver, which has been shown to be important in scavenging free radicals, could be one of the underlying mechanisms of zinc supplementation on reducing MDA levels in liver. Finally, we found that zinc levels in liver were increased while there was no change in serum zinc levels, indicating that local zinc level might be a critical factor for the induction of MTs. Also, the level of MTs could potentially be an index of zinc bioavailability. Taken together, these results suggest that both zinc and MT could play an important role in balancing nutrition and metabolism to prevent diabetic development.     

Zinc and copper of fetal organs during the second trimester of pregnancy

In fetus with a mean gestational age of 18 weeks (range 15-25, n = 14), zinc and copper concentrations in liver, femur, rib, and skeletal muscle were measured. Zinc and copper concentrations are highest in liver. A trend of decreasing liver zinc concentrations during gestational age is suggested. Zinc concentrations are significantly correlated with copper concentrations in liver and in femur, suggesting steady growth in both organs. Femur zinc values rank ca. 30% of those in liver, femur copper, ca. 2%. Zinc or copper concentrations in rib are of the same levels as in skeletal muscle. Their concentration for zinc ranks ca. 20%, for copper, ca. 5% of the values in liver. All zinc and copper values are lower than reported in third trimester fetal organs. Calculated zinc/copper molar ratios are distinctive for the various organs: in liver, 6 +/- 1, in femur, 73 +/- 8, and in soft tissues, 26 +/- 3. Calculated ratios from published values obtained from the third trimester of pregnancy show that the ratios in liver and skeletal muscle maintain these levels. The zinc/copper molar ratio can serve as an internal reference in zinc and/or copper measurements.     

Nucleotide excision repair rates in rat tissues.

We have determined and compared nucleotide excision repair capability of several rat tissues by a method based on restoration of the transformation activity of UV-irradiated pBlueScript by incubation in repair-competent protein extracts. After 3 h of incubation, plasmid DNA was isolated and used to transform competent Escherichia coli cells. Damaged plasmids showed low transformation efficiency prior to incubation in repair-competent extracts. After incubation the transformation efficiency was restored to different extents permitting calculation of the repair capacity of the extracts. Our results showed that rapidly proliferating tissues such as liver, kidney and testis showed higher nucleotide excision repair capacity than slowly proliferating tissues such as heart, muscle, lung and spleen. When liver and splenocytes were stimulated to proliferation by partial hepatectomy and mitogen stimulation, their repair capability increased in parallel with the respective proliferative rates.     

Inhibitor of pyrimidine metabolism from tumor tissues

Inhibitors of normal rat liver 5′-nucleotidase and dUMP kinase $\text{in vitro}$ were found in rapidly proliferating tissues, such as Yoshida sarcoma. Two inhibitors were separated from Yoshida sarcoma by zone electrophoresis, gel filtration on Sephadex G-200 and DEAE-cellulose column chromatography. One inhibited both 5′-nucleotidase and dUMP kinase, while the other inhibited only dUMP kinase. These inhibitors were not detectable in normal rat liver. They were induced in regenerating rat liver and present in rapidly proliferating tissues, such as Yoshida sarcoma and Ehrlich ascites tumor and rat marrow cells. These inhibitors were heat labile. One had a large molecular weight (500,000>) and the other a small molecular weight ($\text{Ca}\text{.}$ 50,000).     

Zinc accumulation and metallothionein gene expression in the proliferating epidermis during wound healing in mouse skin

Metallothionein (MT), a low molecular weight metal-binding protein, has been related to zinc and copper metabolism, the acute-phase response, and cellular proliferation. In this study, we investigated changes in zinc metabolism and MT gene expression occurring in tissue damage and repair during wound healing in mouse skin. Northern blot analysis revealed that a significant increase of MT mRNA was observed in the liver for 18 h after wounding, and serum zinc downfall and hepatic zinc uptake were observed. In situ hybridization analysis showed that no significant expression of MT mRNA was detected within the first 9 h after wounding. However, it was expressed restrictively in the proliferating epidermis of the wound margin after 12 h. Zinc began to accumulate in wounded skin after MT gene expressed. Northern blotting and immunocytochemical staining revealed that MT has been synthesized actively during the growth phase compared with the stationary phase in normal human epidermal keratinocytes in vitro. Intracellular zinc accumulation was observed in the proliferating cells. We concluded that hepatic MT plays an important role as an acute phase protein against host damage, and epidermal MT contributes in the supply of zinc to wounded tissue and activates proliferation for the regeneration of epidermis. Accepted: 2 July 1999     

The movement of zinc and copper from the fertilized egg into metallothionein-like proteins in developing chick hepatic tissue

The movement of copper and zinc from the fertilized egg into the developing embryo and the subsequent association of the two metals with metallothionein-like proteins (MT) of chick liver was investigated. Hepatic levels of MT in 1-d-old chicks, estimated from specifically bound^115mCd, were increased eightfold over endogenous levels of these copper- and zinc-binding proteins in adult liver. Copper was assimilated from the oocyte into the embryo more rapidly than zinc in the early stages of embryogenesis. Between 15 and 16 d of development, during the time that (MT) was detected, there was a significant increase in copper concentration, whereas hepatic zinc concentration remained relatively constant.     

The effect of heavy metal accumulation on metallothionein content in selected tissues of bank voles and yellow-necked mice caught near a steelworks and zinc smelter

The effect of cadmium, zinc, and copper accumulation on metallothionein content in the selected tissues of bank voles and yellow-necked mice trapped near the Sendzimir Steelworks in Krakow and the zinc smelter in Bukowno were analysed. The Borecka Forest was chosen as a control area. The highest cadmium concentration, 32.98 microg g(-1) dry weight, was detected in the kidneys of the bank voles caught in the Bukowno area. Zinc and copper concentrations in the tissues did not exceed the critical values. Metallothionein content in the liver and kidneys was associated with heavy metal accumulation in the tissues. The highest content of sulphydryl groups was detected in the livers of the bank voles trapped within the neighbourhood of the zinc smelter in Bukowno. The highest level of disulphide bonds was found in the kidneys of the bank voles from the same area.     

一种新型丝氨酸苏氨酸激酶CPK的初步克隆及功能

从人胎肝cDNA文库中分离到一种cDNA ,推测的蛋白氨基酸序列具有明显的丝氨酸苏氨酸激酶结构域,可能编码一种新的丝氨酸苏氨酸激酶(命名为CPK ,cellproliferationassociationkinase) .CPKmRNA全长约3 0kb .RT PCR检测表明CPKmRNA在增殖活跃组织或细胞如人胚胎组织、肿瘤细胞株中呈高丰度表达,在成人组织则呈低丰度或不表达.利用PCR技术扩增大鼠同源cDNA ,以此为探针,Northern杂交发现CPK表达于多种成年小鼠组织,以脑组织丰度最高.小鼠2 3肝部分切除可迅速诱导CPKmRNA的表达,在术后2 4～4 8h达高峰,与2 3肝部分切除后细胞增殖期吻合.以小鼠同源cDNA片段为模板,合成RNA探针,原位杂交显示在胚胎发育期CPK低丰度表达在大多数组织,以神经系统组织变化最大,在第8d胚胎神经管内皮细胞中即出现表达,11～13d在端脑、脑膜、间脑、脊神经节表皮细胞、海马、小脑神经胶质细胞等多种神经细胞中表达且丰度较高,16d后这些组织的表达迅速下降到较低水平,表明CPK可能与神经系统的生长发育有一定关联.利用信号通路检测技术,观察到CPK的表达对MAPK ,p38MAPK途径的激活有明显的影响,并可显著增强表皮生长因子对这两条途径的激活,提示该激酶可能参与细胞因子的信号转导.     

Some physiological and biochemical indices of zinc toxicity in two freshwater fishes, Clarias lazera and Tilapia zilli

1. The effects of lethal zinc concentrations on some physiological and biochemical parameters in Clarias lazera and Tilapia zilli were investigated. 2. The analyses of lactate, pyruvate and glycogen in both liver and muscle tissues and the relation among them have been studied in detail. 3. Significant increases were observed in liver and serum proteins, serum alkaline phosphatase (ALP), erythrocyte count (RBCs), haematocrit or packed cell volume (PCV) and haemoglobin (HB) concentrations. 4. Zinc exposure reduced liver and serum acid phosphatase (ACP) as well as liver alkaline phosphatase (ALP).     

Supplementation of zinc mitigates the altered uptake and turnover of <Superscript>65</Superscript>Zn in liver and whole body of diabetic rats

Diabetes is a life threatening disease and its onset is linked with both environmental and genetic factors. Zinc metabolism gets altered during diabetes and results in many complications. The present study was designed to elucidate the effects of zinc supplementation on the biokinetics of ⁶⁵Zn in whole body, liver and its biodistribution in diabetic rats. The animals were divided into four groups viz; normal control; diabetic (single intraperitoneal injection of alloxan 150 mg/kg body weight); zinc treated (227 mg/l in drinking water); and diabetic + zinc treated. To carry out biokinetics study, each rat was injected intraperitoneally with 0.74 MBq radioactivity of ⁶⁵Zn following 4 weeks of different treatments and the radioactivity was determined by using a suitably shielded scintillation counter. Alloxan induced diabetic rats showed a significant decrease in both the fast (Tb₁) and slow (Tb₂) components of biological half-life of ⁶⁵Zn which, however, were normalized in whole body (P > 0.05) following zinc supplementation. In case of liver, Tb₂ component was brought back to the normal but Tb₁ component was not increased significantly. The present study indicates that the paucity of zinc in the tissues of the diabetic animals was due to decreased retention of tissue zinc as evidenced by increased serum Zn, hyperzincuria and increased rate of uptake of ⁶⁵Zn by the liver. Zinc supplementation caused a significant improvement in the retention of zinc in the tissues and is therefore likely to be of benefit in the treatment of diabetes.