Pregnancy-associated changes in plasma metallothionein concentration and renal cadmium accumulation in rats

Patterns of trace element deposition in beluga whale teeth reflect early life history

Determination of trace element concentrations in continuously growing biological structures such as otoliths, whiskers, and teeth can provide important insight into physiological and ontogenetic processes. We examined concentrations of 11 trace elements (Li, Mg, Mn, Cu, Zn, Se, Rb, Sr, Cs, Ba, Pb) in the annual dentine growth layer groups (GLGs) of teeth of 66 Eastern Canadian Arctic belugas (Delphinapterus leucas). Several of these trace elements displayed clear and consistent patterns in early life, though few longer term trends or signals were present in trace element data for either females or males. Large changes in Sr and Ba concentrations in fetal dentine reflected in utero shifts in element deposition in the teeth of developing belugas. Marked changes in these elements during the first years after birth were likely associated with the onset of nursing and subsequent weaning. Mg, Mn, and Zn also displayed clear and consistent patterns in early life that correlated strongly with dentine stable nitrogen isotope (δ15N) data, suggesting these elements merit further study as potential tools for studying nursing and weaning. Depositional patterns of Zn and Pb, which have been linked to sexual maturation in female Pacific walruses (Odobenus rosmarus divergens), were inconsistent in beluga teeth. Some individuals (including males) displayed patterns strongly resembling those observed in female walruses, whereas many animals did not, perhaps because they had not yet reached sexual maturity. The lack of clear patterns in trace element deposition after the first few years of life may have resulted from pooling samples from multiple populations/regions collected across more than two decades, but may also indicate that elemental concentrations are primarily driven by other, extrinsic processes later in life, and might be useful as biomonitors of environmental element concentrations or tools for delineating population structure.

Estimation of health risks associated with dietary cadmium exposure

In much of the world, currently employed upper limits of tolerable intake and acceptable excretion of cadmium (Cd) (ECd/Ecr) are 0.83 µg/kg body weight/day and 5.24 µg/g creatinine, respectively. These figures were derived from a risk assessment model that interpreted β2-microglobulin (β2MG) excretion > 300 μg/g creatinine as a "critical" endpoint. However, current evidence suggests that Cd accumulation reduces glomerular filtration rate at values of ECd/Ecr much lower than 5.24 µg/g creatinine. Low ECd/Ecr has also been associated with increased risks of kidney disease, type 2 diabetes, osteoporosis, cancer, and other disorders. These associations have cast considerable doubt on conventional guidelines. The goals of this paper are to evaluate whether these guidelines are low enough to minimize associated health risks reliably, and indeed whether permissible intake of a cumulative toxin like Cd is a valid concept. We highlight sources and levels of Cd in the human diet and review absorption, distribution, kidney accumulation, and excretion of the metal. We present evidence for the following propositions: excreted Cd emanates from injured tubular epithelial cells of the kidney; Cd excretion is a manifestation of current tissue injury; reduction of present and future exposure to environmental Cd cannot mitigate injury in progress; and Cd excretion is optimally expressed as a function of creatinine clearance rather than creatinine excretion. We comprehensively review the adverse health effects of Cd and urine and blood Cd levels at which adverse effects have been observed. The cumulative nature of Cd toxicity and the susceptibility of multiple organs to toxicity at low body burdens raise serious doubt that guidelines concerning permissible intake of Cd can be meaningful.

Zinc concentrations in teeth of female walruses reflect the onset of reproductive maturity

Age at maturity is an important parameter in many demographic models and, for some species, can be difficult to obtain using traditional methods. Incremental growth structures act as biological archives, recording information throughout an organism's life and possibly allowing for the reconstruction of life history events. Concentrations of zinc (Zn) in animal tissues are known to be linked to life history, physiology and reproduction and may be retained in incremental growth structures. This study reconstructed lifetime Zn concentrations in teeth (n = 93) of female Pacific walruses (Odobenus rosmarus divergens) collected from 1932-2016. Zn displayed a characteristic pattern of accumulation, with a change point marking the beginning of a lifelong, linear increase in Zn concentrations. We hypothesized that this change point marks the onset of reproductive maturity. The age at which the change point occurred (age_cp) was estimated by counting tooth cementum growth layers. These estimates closely matched literature values of timing of first ovulation in female walruses. Total number of ovulations (estimated from ovary corpora counts from paired tooth/ovary specimens) was closely related to reproductive lifespan (total lifespan - age_cp; R² = 0.70). Further, age_cp tracked changes in Pacific walrus population size as a proportion of carrying capacity, decreasing when the population was depleted by commercial hunting and peaking when carrying capacity was exceeded. This novel approach will aid walrus management, and is likely applicable to other species, offering a potentially powerful tool for research, management and conservation of wildlife populations.

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.

Iron transport in the kidney: implications for physiology and cadmium nephrotoxicity

The kidney has recently emerged as an organ with a significant role in systemic iron (Fe) homeostasis. Substantial amounts of Fe are filtered by the kidney, which have to be reabsorbed to prevent Fe deficiency. Accordingly Fe transporters and receptors for protein-bound Fe are expressed in the nephron that may also function as entry pathways for toxic metals, such as cadmium (Cd), by way of "ionic and molecular mimicry". Similarities, but also differences in handling of Cd by these transport routes offer rationales for the propensity of the kidney to develop Cd toxicity. This critical review provides a comprehensive update on Fe transport by the kidney and its relevance for physiology and Cd nephrotoxicity. Based on quantitative considerations, we have also estimated the in vivo relevance of the described transport pathways for physiology and toxicology. Under physiological conditions all segments of the kidney tubules are likely to utilize Fe for cellular Fe requiring processes for metabolic purposes and also to contribute to reabsorption of free and bound forms of Fe into the circulation. But Cd entering tubule cells disrupts metabolic pathways and is unable to exit. Furthermore, our quantitative analyses contest established models linking chronic Cd nephrotoxicity to proximal tubular uptake of metallothionein-bound Cd. Hence, Fe transport by the kidney may be beneficial by preventing losses from the body. But increased uptake of Fe or Cd that cannot exit tubule cells may lead to kidney injury, and Fe deficiency may facilitate renal Cd uptake.

Human retinal cadmium accumulation as a factor in the etiology of age-related macular degeneration

Cadmium is a naturally occurring, highly toxic, metallic element. It pollutes the environment as a result of industrial activity and accumulates in human tissues with a long biological half-life. Cadmium content has been demonstrated to increase in human retinal tissues as a function of age and tobacco smokers have approximately twice as much cadmium in retinal tissues than non-smokers. Smoking is also a key environmental risk factor for the retinal disease age-related macular degeneration (AMD). Recent studies have shown that urinary cadmium levels (a measure of Cd body burden) are higher in smokers who have AMD. We now report the Cd measurements in human retinal tissues from eyes afflicted with AMD compared to non-diseased eyes (controls) from age-matched donors. Human donor eyes frozen under argon gas were assessed for AMD severity using color stereoscopic fundus photographs and the Minnesota Grading System. Cadmium, zinc and, copper levels were measured in retinal tissues (neural retina, retinal pigment epithelium and choroid) using inductively coupled plasma mass spectrometry and graphite furnace spectrophotometry and values were normalized to tissue protein levels. Higher Cd levels were found in the neural retina and RPE for eyes afflicted with AMD compared to controls in males, differences were not statistically significant in females. The results indicate that higher retinal cadmium burdens are associated with the presence of AMD at least in males and suggest possible gender differences in the metabolism of metals in the human retina.

The effects of air pollution and smoking on placental cadmium, zinc concentration and metallothionein expression

This study is designed to determine the placental zinc (Zn) and cadmium (Cd) levels in mothers who were smokers, mothers who were thought to be exposed to air pollution, and mothers who were non-smokers and to investigate the relationship between the expression of placental metallothionein (MT) binding these metals and blood progesterone level. Placental Zn and Cd levels were measured by atomic absorption spectrometry. Presence of placental MT was determined immunohistochemically. Placental changes were examined by light microscope after H&E and PAS staining. Immunohistochemical MT staining of syncytiotrophoblastic and villous interstitial cells were scored as positive or negative. Among the 92 mothers included in the study, 33 were smokers (Group I), 29 had been exposed to air pollution (Group II) and 30 were non-smoker rural residents who had never been exposed to air pollution (Group III). Mean off-spring birth weight of 3198.62+/-380.01 g and mean placenta weight of 561.38+/-111.55 g of Group II were lower when compared with those of other two groups. In Group I, mean placental Cd and Zn were 0.063+/-0.022 microg/g and 39.84+/-15.5 microg/g, respectively, being higher than in other groups. In Group II, mean placental Cd and Zn levels were higher than those of Group III. Blood progesterone levels of subjects in Group I (121 ng/ml) were the lowest of all groups. While the mean count of villi was the highest in Group III; the highest mean count of syncytial knots was in Group II. Thickening of vasculo-syncytial membrane was most prominent in Group I. Similarly, MT staining was positive and very dense in 72.7% (24/33) of cases in Group I (p<or=0.05). MT staining was positive in 69.0% (29/20) and denser in Group II cases compared to 36% (11/30) in Group III (p<or=0.05). This study showed that smoking increased Cd levels in placenta and accompanied an increase in placental MT expression immunohistochemically. The effects of exposure to air pollution are equally harmful as smoking related effects.

Trace elements in tissues of phocid seals harvested in the Alaskan and Canadian Arctic: influence of age and feeding ecology

Concentrations of selected trace elements (Ag, Cu, Cd, Se, Zn, THg, and MeHg) were measured in tissues of subsistence-harvested ringed (Pusa hispida (Schreber, 1775)), bearded (Erignathus barbatus (Erxleben, 1777)), and spotted (Phoca largha Pallas, 1811) seals from Alaska and ringed seals from Canada. Most variables differed significantly in tissues of phocids analyzed. Concentration of renal Cd was highest in ringed seals from Canada and bearded seals from Alaska, while spotted seals had the lowest concentrations. Concentrations of Cd increased with age to a maximum in ringed and bearded seals, followed by a slow decline with increasing age. Spotted seals had the highest proportion of MeHg to THg (%MeHg) in liver and bearded seals the lowest ratio. THg in seal tissues followed the opposite trend. %MeHg in ringed and bearded seals followed a hyperbolic decay function with age, but was highly variable in spotted seals. Seals with lesions had a higher relative occurrence of MeHg in liver. The molar ratio of Se:THg in liver exceeded 1:1 in most seals and was negatively correlated with age in ringed and spotted seals. Hepatic Ag was higher in bearded seals than in ringed and spotted seals. A correlation of Ag with age was not documented.

Increased spontaneous motor activity in offspring after maternal cadmium exposure during lactation

Lactating rats were exposed to 0, 5 or 25ppm cadmium as cadmium chloride in the drinking water. A battery of neurobehavioural tests was applied to the male offspring after weaning at 5 weeks until 4 months of age. The mean cadmium levels in exposed pup kidneys ranged from 0.006 to 0.030mg/kg wet weight at weaning, with the corresponding maternal kidney cadmium levels ranging from 4 to 13mg/kg wet weight. Significantly increased activity during the initial 20min of the spontaneous motor activity test was observed in the highest-dose group compared to the control group. The activity outcome was significantly positively correlated with cadmium levels in the pup kidneys. No cadmium-related changes in performance were observed in the Morris water maze, the E-shaped maze or the elevated plus-maze. The results indicate that neurobehavioural effects during development may be a more sensitive parameter for Cd toxicity than renal dysfunction.

Maternal-fetal distribution of cadmium in the guinea pig following a low dose inhalation exposure

Pregnant guinea pigs in their last trimester of gestation were exposed by inhalation to cadmium (Cd) chloride level (50 microg/m3 Cd) for 1 and 5 days. Cd content was evaluated by atomic absorption spectrophotometry. Maternal blood Cd concentration increased by 127 and 223% of control for 1 and 5 days of exposure. Maternal lung Cd concentration increased significantly by 11.66- and by 48.24-fold after 1 and 5 days of treatment, while maternal liver showed an increase of 188 and 227% for 1 and 5 days of exposure. Also, fetal Cd concentration significantly increased in brain (156 and 192%), liver (159 and 174%) and heart (201 and 157%) after 1 and 5 days of exposure, compared to unexposed females. Placental calcium content decreased significantly by 16% of control after 5 days of exposure. These results suggest that low-level inhalation of Cd may pass through the guinea pig placenta and accumulate in fetal brain, liver and heart.

Increased metallothionein in mouse liver, kidneys, and duodenum during lactation

Lactation-induced increases in cadmium absorption and retention have been demonstrated in mid-lactating mice, but no systematic measurements of endogenous metal-binding protein concentrations during lactation have been reported. Using Cd/hemoglobin radioassay, this study detected significant increases in metallothionein (MT) concentrations in liver (4-fold), kidneys (2-fold), and duodenum (2-fold), but not jejunum, of mouse dams on days 13 and 20 of lactation. These increases occurred in the absence of cadmium exposure and were specific to the lactation period; dams 5 days after weaning showed MT levels that were similar to those of nonpregnant (NP) mice. Similarly, Northern blot analyses of livers from lactating mice demonstrated that MT mRNA concentrations in maternal liver during mid-lactation were 6-fold higher than those observed 5 days after pups were weaned. Gel filtration of final supernatants from the Cd/hemoglobin assay confirmed that the Cd-binding molecule induced during lactation was indeed metallothionein. In addition, chromatographic analyses of cytosols from tissues taken from dams administered small amounts of Cd (66 ng/mouse) showed that the trace amounts of Cd absorbed through the maternal gastrointestinal tract during mid-lactation were also bound to the MT. These results indicate MT induction in mouse dams occurs as a physiological consequence of lactation, requiring no external stimulus. This induced MT participates in binding low levels of dietary cadmium consumed by the dam. During lactation, elevated maternal MT may affect pathways for essential trace metals as well as sequester toxic metals harmful to the neonate. Multiparous humans may have increased risk of accumulating environmental Cd.

Accumulation of cadmium, zinc, and copper in maternal blood and developmental placental tissue: differences between Finland, Estonia, and St. Petersburg

Cadmium, zinc, and copper from placental tissue and blood samples at the first trimester (n = 64) and at term (n = 152) were analyzed; the welfare of newborns and placental 7-ethoxycoumarin O-deethylase (ECOD) activities in vitro were determined. The study material was collected from Finland, Estonia, and Russia. The results demonstrate that Cd starts to accumulate in the placenta during the first trimester and that Zn and Cu contents were significantly higher at the first trimester than at term. Among nonsmokers a negative correlation was found between placental Cu content and birth weight of neonates. Among smokers a positive correlation between placental Zn content and birth weight and ECOD activity was found. The birth weights correlated inversely with the length of time the mothers smoked. The highest Cd concentrations were detected in the samples collected from St. Petersburg. The data demonstrate an inverse accumulation of Zn and Cd throughout the pregnancy in the placenta and maternal blood samples. Zn may act as a positive marker or even an enzymatic enhancement for the human placental vital functions. Smoking, parity, age, and especially the place of residence affect the Cd, Zn, and Cu contents and ratios in placenta and mother's blood.

Role of placental metallothionein in maternal to fetal transfer of cadmium in genetically altered mice

The role of placental metallothionein (MT) as a barrier for maternal to fetal transfer of cadmium (Cd) was investigated using mice which overexpressed the MT-1 isoform (MT-1*), mice which did not express the MT-1 and 2 isoforms (MT-null) and control mice (C57BL/6). In addition, immunohistochemical localization of MT in the placenta was determined in these mice. Two days prior to parturition, the mice were injected with radioactive 109Cd chloride (4 microCi, 0.6 ng Cd/mouse) and killed 24 h later. Organs and fetuses were collected and radioactivity, MT and metal levels were measured. Cd accumulated mainly in the liver and kidney (80% of administered dose) with very low levels (0.1-0.3%) detected in fetuses. When analyzed on a per organ or per gram basis, MT-null fetuses accumulated significantly more Cd (3-10-fold) than the control fetuses and there was no significant difference in fetal Cd accumulation in the MT-1* and control fetuses. As expected, MT and zinc levels were higher in MT-1* than C57BL/6 mice and no MT was detected in MT-null mice. Most striking was the high hepatic MT levels in MT-1* dams (4 mg/g). Immunohistochemical analysis showed MT staining in spongiotrophoblasts, glycogen cells, visceral yolk sac, trophoblast giant cells and maternal decidual cells with the MT-1* placenta staining much more intensely as compared to control placenta. The results suggest that placental MT reduces maternal to fetal Cd transfer, however the low doses of Cd administered in the present experiment resulted in high levels of Cd accumulation in liver and kidney in all groups of mice with a low concentration of Cd reaching the placenta. Thus, the role of placental MT as a barrier for Cd is inconclusive.

Erythrocyte metallothionein in relation to other biochemical zinc indices in pregnant and nonpregnant women

Erythrocyte metallothionein (E-MT) is considered a promising index of zinc status in humans, since it may be more sensitive than other biochemical indices to changes in dietary zinc. However, conditions of high zinc demand with substantial redistribution of tissue zinc and specific changes in hormone profile, such as pregnancy, may have an influence on E-MT levels in addition to dietary zinc. In this study, we compared E-MT concentrations in relation to other biochemical zinc indices in healthy pregnant women at delivery (n = 40) and non-pregnant women (n = 22) with similar habitual dietary zinc intakes (average 13.3 mg/d). Pregnant women had lower serum zinc and albumin-bound serum zinc, but higher levels of alpha 2-macroglobulin-bound serum zinc than the nonpregnant women. Erythrocyte zinc (E-Zn) was similar in both groups, but E-MT (mean +/- SE) was slightly but significantly (p < 0.05) higher in the pregnant women (2.9 +/- 0.09 nmol/g protein) compared to nonpregnant women (2.6 +/- 0.06 nmol/g protein). A significant correlation was observed between E-MT and E-Zn in the nonpregnant women (r = 0.70; p < 0.001), consistent with the role of intracellular zinc in the regulation of metallothionein synthesis. However, such correlation was not observed in the pregnant women, suggesting that E-MT levels in pregnancy may be influenced by factors related to the pregnant state.

Urinary cadmium levels during pregnancy and postpartum. A longitudinal study

It is well established that pregnancy induces physiological, metabolical and hormonal changes. As a consequence, trace metal metabolism can be affected. The aim of the present study was to assess the urinary cadmium levels in women during gestation and postpartum. The survey was conducted in a group of nonoccupationally cadmium-exposed women from Southern Catalonia (NE, Spain). Urine samples were obtained before pregnancy, during the 6th, 10th, 26th, and 30th wk of gestation, as well as during the 5th and 24th wk after delivery. Urinary cadmium levels were determined by graphite furnace atomic absorption spectrophotometry. The concentrations of zinc and copper in plasma were also measured. Moreover, to assess the effect of the diet during pregnancy, dietary ingestions of zinc, iron, and calcium were also determined. A significant decrease of plasma zinc levels could be observed during the last two trimesters of pregnancy, while plasma copper concentrations significantly increased during the same period. Urinary cadmium concentrations ranged from 0.05 to 3.79 micrograms/g creatinine (geometric mean 0.49 +/- 2.26 micrograms/g creatinine). No significant changes in urinary cadmium concentrations during pregnancy and postpartum could be observed.

Cellular adaptation to chronic cadmium exposure: intracellular localization of metallothionein protein in human trophoblast cells (JAr)

Trophoblast cells are the first embryonic cells that modulate the transfer of a variety of compounds (oxygen, amino acids, xenobiotics, metals) from the maternal to the fetal circulation in the human placenta. Human placental exposure to the toxic metal, cadmium (Cd) results in a decrease in the production of human chorionic gonadotropin (hCG), a decrease in the maternal to fetal transport of zinc (Zn), and trophoblastic necrosis. Thus, the ability of trophoblast cells to adapt to exposure to the toxic metal Cd has been considered crucial. In this study, the expression and intracellular localization of metallothionein (MT), a small molecular weight, metal binding protein, was examined in trophoblast cells (JAr) grown in normal media and in cells exposed chronically (6 months) to 2 microM CdCl2. Conventional and confocal fluorescence microscopy were used to examine the intracellular localization of MT protein in control cells and cells grown chronically in Cd. In unexposed trophoblast cells, MT protein was primarily perinuclear with low level, punctate expression in the cytosol. Following both chronic and 24 hour exposure to Cd, MT protein levels were increased (at least 3-fold in both chronic and acute exposures) and the protein was now concentrated inside the nucleus with a lacy, cytoskeletal pattern of expression in the cytosol. To determine if the nuclear accumulation of MT protein was dependent on new protein synthesis, control cells were exposed to CdCl2 (2 microM) and cycloheximide (2 micrograms/. ml) for 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)