The placental transfer and toxicity of selenite relative to cadmium in the human term perfused placenta

Selenium, Selenoproteins, and Female Reproduction: A Review

Selenium (Se) is an essential micronutrient that has several important functions in animal and human health. The biological functions of Se are carried out by selenoproteins (encoded by twenty-five genes in human and twenty-four in mice), which are reportedly present in all three domains of life. As a component of selenoproteins, Se has structural and enzymatic functions; in the latter context it is best recognized for its catalytic and antioxidant activities. In this review, we highlight the biological functions of Se and selenoproteins followed by an elaborated review of the relationship between Se and female reproductive function. Data pertaining to Se status and female fertility and reproduction are sparse, with most such studies focusing on the role of Se in pregnancy. Only recently has some light been shed on its potential role in ovarian physiology. The exact underlying molecular and biochemical mechanisms through which Se or selenoproteins modulate female reproduction are largely unknown; their role in human pregnancy and related complications is not yet sufficiently understood. Properly powered, randomized, controlled trials (intervention vs. control) in populations of relatively low Se status will be essential to clarify their role. In the meantime, studies elucidating the potential effect of Se supplementation and selenoproteins (i.e., GPX1, SELENOP, and SELENOS) in ovarian function and overall female reproductive efficiency would be of great value.

Ex Vivo Dual Perfusion of the Human Placenta: Disease Simulation, Therapeutic Pharmacokinetics and Analysis of Off-Target Effects

In recent years ex vivo dual perfusion of the human placental lobule is seeing an international renaissance in its application to understanding fetal health and development. Here, we discuss the methods and uses of this technique in the evaluation of (1) vascular function, (2) transplacental clearance, (3) hemodynamic and oxygenation changes associated with pregnancy complications on placental structure and function, and (4) placental toxicology and post-perfusion evaluation of tissue architecture.

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.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Cytotoxicant-induced trophoblast dysfunction and abnormal pregnancy outcomes: role of zinc and metallothionein

Normal trophoblast function, including implantation, hormone production, and formation of the selectively permeable maternofetal barrier, is essential for the establishment and maintenance of the fetoplacental unit and proper fetal development. Maternal cytotoxicant exposure causes the destruction of these cells, especially the terminally differentiated syncytiotrophoblasts, and results in a myriad of poor pregnancy outcomes. These outcomes range from intrauterine growth retardation and malformation to spontaneous abortion or stillbirth. There is recent evidence that the metal-binding protein, metallothionein, is involved in the protection of human trophoblastic cells from heavy metal-induced and severe oxidative stress-induced apoptosis. Metallothionein, with its unique biochemical structure, can both bind essential metal ions, such as the transcription modulator zinc, and yet allow their ready displacement by toxic nonessential metal ions or damaging free radicals. These properties suggest that metallothionein may be responsible not only for sequestering the cytotoxic agents, but also for altering signal transduction in the affected cells. Here, we review several identified causes of adverse pregnancy outcomes (specifically, prenatal exposure to cigarette smoke and alcohol, gestational infection, and exposure to environmental contaminants), discuss the role of zinc in modulating the cellular response to these toxic insults, and then propose how metallothionein may function to mediate this protective response.

Selenium in pregnancy: is selenium an active defective ion against environmental chemical stress?

Transportation of selenium from mother to fetus and its possible effects on mother's zinc, copper, cadmium, and mercury levels were studied together during the first trimester and at term in 216 mothers. Mothers came from three geographical places with different selenium intakes. The role of selenium as a biomarker for the vital function was estimated by studying the associations between tissue or blood selenium content and placental cytochrome P450 enzyme activities and the newborn's birth weight. Regardless of the selenium intake of the mothers, higher concentrations were found in the cord blood than in mother's blood reflecting active transportation of selenium to the fetus. Active smoking was associated with higher placental selenium concentrations like it is associated with higher placental zinc concentrations. When the cadmium concentrations were high in placenta, as in smokers, the transfer of selenium from blood to placenta was increased, decreasing the selenium levels in blood. On the other hand, the high selenium concentrations in blood were connected to lower cadmium concentrations in placenta also in nonsmokers. Selenium had correlations with copper and zinc. ECOD activity in placental tissue, mercury in mothers' hair, mothers' age, and selenium concentrations in cord blood and placental selenium all seem to have connections with xenobiotic-metabolizing enzymes linked effects among mothers. These data suggest that selenium has an active role in the mother's defense systems against the toxicity of environmental pollutants and the constituents of cigarette smoke.

Cadmium accumulation and effects on progesterone release by cultured human trophoblast cells

This study was designed to examine the characteristics of cadmium bioaccumulation by human trophoblast cells in culture and the subsequent effect of cadmium exposure on progesterone production and syncytial formation. The accumulation of cadmium suggested a time- and dose-dependent relationship, although it was not significant. The rate of metal accumulation was similar in all cadmium-treated groups. After 72 h of continuous exposure to cadmium concentrations of 5, 10, and 20 microM, progesterone release was diminished to 69, 51, and 38% of control values (P < 0.05), respectively. When cells were exposed to cadmium from 72 to 96 h (after syncytial development), progesterone release exhibited the same pattern of decline in response to increasing cadmium concentrations. Histologic evaluation of whole mounts of trophoblast cells exposed to 20 microM CdCl2 for 96 h revealed that syncytial formation seemed to be uninhibited. The pattern of cadmium-accumulation by normal cultured human trophoblast cells suggests a time- and dose-relationship with a concomitant decrease in progesterone release that occurs without apparent inhibition of syncytial development.

Pharmacokinetic and toxicity studies of AZT (zidovudine) following perfusion of human term placenta for 14 hours

Multiple exposures to AZT (Zidovudine) for 14 hr were examined in the dually perfused human term placental lobule in order to determine the pharmacokinetics of transfer, as well as several viability parameters of toxicity. In each experiment, three separate additions of AZT at a concentration of 3.8 mM was added to the maternal reservoir, and perfusate samples were obtained from both the maternal and the fetal compartments for determinations of AZT, glucose, lactate, oxygen, and human chorionic gonadotropin (hCG) concentrations. During 14 hr of continuous exposure to this high concentration of AZT, the production of hCG was significantly reduced by 75% when compared to the 2-hr control period before the administration of AZT. In addition, lactate production was reduced by 45% after AZT administration. Such changes in hCG and lactate production were not observed in separate experiments conducted over the same time interval, but with no AZT added. Based upon a lack of total perfusion fluid loss, changes in fetal arterial pressure, and histopathology, placental lobule integrity was maintained throughout the perfusion period. Further, AZT readily crossed the placenta into the fetal compartment reaching equilibrium with maternal levels within 60-90 min after addition of each administration of AZT. Based upon AZT levels in the fetal perfusate, AZT does not accumulate against a concentration gradient and therefore appears to be diffusion limited. Placental tissues obtained from perfused, partially perfused, and nonperfused regions at the conclusion of the experiment were analyzed for AZT levels. Substantial AZT levels in the nonperfused tissues indicated that AZT is a freely diffusible compound. The results of the current study demonstrate that high concentrations of AZT alter placental function resulting in reduced production of hCG and lactate.

Energy charge monitoring via magnetic resonance spectroscopy 31P in the perfused human placenta: effects of cadmium, dinitrophenol and iodoacetate

Phosphorus 31 nuclear magnetic resonance spectroscopy as a non-invasive technique was applied to monitor the metabolic activity of the human placenta during perfusion in vitro. During control perfusions (n = 3) there was an initial increase in adenosine triphosphate (ATP) and a fall in inorganic phosphate (Pi). Thereafter, however, the level of both ATP and Pi remained constant throughout the perfusion period (11 h). Additional biochemical parameters such as glucose consumption, lactate production and the release of hormones, human chorionic gonadotrophin (hGC). measured in the perfusate samples, were also used to assess the viability of the placental tissue. As with ATP, all these biochemical parameters under the control conditions showed a stable rate of metabolic activity throughout the length of the experiments. In additional experiments, the effect of the metabolic inhibitor dinitrophenol (n = 2) and dinitrophenol (DNP) together with iodoacetic acid (IOA, n = 2) were studied. DNP (0.1 mM) alone showed a slight decrease of all parameters. In contrast, the addition of IOA (0.1 mM) with DNP (0.1 mM) not only blocked the production of ATP but also produced a substantial impact on placental metabolic activity. The effect of a toxic dose of cadmium (20 nmol/ml) was studied also (n = 3). This dose of cadmium demonstrated no effect on phosphorus metabolism. However, the rate of glucose consumption and the release of hCG were significantly reduced.