Teratogenic evaluation of tributyltin chloride in rats following oral exposure

Maternal exposure to tributyltin during early gestation increases adverse pregnancy outcomes by impairing placental development

Tributyltin (TBT) is a persistent organotin pollutant widely used as agricultural and wood biocides, exhibiting well-documented toxicity to reproductive functions in aquatic organisms. However, the effect of TBT on early pregnancy and placental development has been rarely studied in mice. Pregnant mice were fed with 0, 0.2, and 2 mg/kg/day TBT from gravid day 1 to day 8 or 13. TBT exposure led to an increase in the number of resorbed embryo and a reduction in the weight of fetus at gestational days 13. Further study showed that TBT significantly decreased placental weight and area, lowered laminin immunoreactivity and the expressions of placental development-related molecules including Fra1, Eomes, Hand1, and Ascl2. Moreover, TBT treatment markedly inhibited the placental proliferation and induced up-regulation of p53 and cleaved caspase-3 proteins, and down-regulation of Bcl-2 protein. In addition, TBT administration increased levels of malondialdehyde and H₂ O₂ and decreased activities of catalase and superoxide dismutase. Collectively, these results suggested TBT-induced adverse pregnancy outcomes during early pregnancy might be involved in developmental disorders of the placenta via dysregulation of key molecules, proliferation, apoptosis, and oxidative stress.

Overview of the Pathophysiological Implications of Organotins on the Endocrine System

Organotins (OTs) are pollutants that are used widely by industry as disinfectants, pesticides, and most frequently as biocides in antifouling paints. This mini-review presents the main evidences from the literature about morphophysiological changes induced by OTs in the mammal endocrine system, focusing on the metabolism and reproductive control. Similar to other toxic compounds, the main effects with potential health risks to humans and experimental animals are not only related to dose and time of exposure but also to age, gender, and tissue/cell exposed. Regarding the underlying mechanisms, current literature indicates that OTs can directly damage endocrine glands, as well as interfere with neurohormonal control of endocrine function (i.e., in the hypothalamic-pituitary axis), altering hormone synthesis and/or bioavailability or activity of hormone receptors in the target cells. Importantly, OTs induces biochemical and morphological changes in gonads, abnormal steroidogenesis, both associated with reproductive dysfunctions such as irregular estrous cyclicity in female or spermatogenic disorders in male animals. Additionally, due to their role on endocrine systems predisposing to obesity, OTs are also included in the metabolism disrupting chemical hypothesis, either by central (e.g., accurate nucleus and lateral hypothalamus) or peripheral (e.g., adipose tissue) mechanisms. Thus, OTs should be indeed considered a major endocrine disruptor, being indispensable to understand the main toxic effects on the different tissues and its causative role for endocrine, metabolic, and reproductive dysfunctions observed.

The problem of species comparison of developmental toxicity: can we extrapolate human developmental toxicity induced by environmental chemicals from the data on rodents?

Rodent models have great utility for evaluating the potential of environmental chemicals to alter human reproductive development. However, animal studies have some problems of species differences in extrapolating to human developmental toxicity induced by xenobiotics, because the placental endocrine functions in particular vary considerably among different species. For example, estrogen biosynthesis during pregnancy in humans is much different from that in rodents. In humans, ovarian function gradually declines after fertilization, as the placenta becomes the primary site of estrogen biosynthesis during pregnancy. In contrast to the process in humans, the ovary (not the placenta) is the main source of estrogen during pregnancy in rodents, because the placenta of rodents does not express the catalytic enzymes for estrogen biosynthesis, such as aromatase. The regulation of estrogen biosynthesis in the placenta is very important for human embryos because altering placental function can cause permanent effects on embryos. It has been suggested that rodents are therefore unsuitable for evaluating the potential effects of xenobiotics on the human reproductive system and developmental toxicity induced by the alteration of placental endocrine functions. Consequently, there is an urgent need to establish effective tools to evaluate the in vivo reproductive and developmental toxicity of environmental contaminants that disrupt the placental endocrine functions, including maintenance of local estrogen concentrations in the placenta. To resolve the problems, in this review we propose using transgenic mice, in which the transgene is controlled by placental-specific promoters, and local transgene systems into the placenta using viral vectors.

In utero exposure to tributyltin chloride differentially alters male and female fetal gonad morphology and gene expression profiles in the Sprague–Dawley rat

Tributyltin (TBT) is an environmental contaminant commonly used in anti-fouling agents for boats, as well as a by-product from several industrial processes. It has been shown to accumulate in organisms living in areas with heavy maritime traffic thereby entering the food chain. Here, we determined the consequences of in utero exposure to TBT on the developing fetal gonads in the Sprague-Dawley rat. Timed pregnant rats were gavaged either with vehicle or TBT (0.25, 2.5, 10 or 20 mg/kg) from days 0 to 19 or 8 to 19 of gestation. On gestational day 20, dams were sacrificed; fetal testes and ovaries were processed for light (LM) or electron microscopic (EM) evaluation and RNA was prepared for gene expression profiling. At the highest doses of TBT the number of Sertoli cells and gonocytes was reduced, there were large intracellular spaces between Sertoli cells and gonocytes and there was an increased abundance of lipid droplets in the Sertoli cells; EM studies revealed abnormally dilated endoplasmic reticulum in Sertoli cells and gonocytes. In the intertubular region between adjacent interstitial cells, immunostaining for the gap junctional protein connexin 43 was strong in controls, whereas it was reduced or completely absent in treated rats. In the ovaries, TBT (20 mg/kg, days 0-19; 10 mg/kg, days 8-19) reduced the number of germ cells by 44% and 46%, respectively. On examining gene expression profiles in the testis, 40 genes out of 1176 tested were upregulated more than two-fold over control. While no genes were upregulated in the TBT exposed fetal ovary, eight genes were downregulated. In conclusion, in utero exposure to TBT resulted in gender-specific alterations in gonadal development and gene expression profiles suggesting that there may be different adaptive changes to toxicity in developing male and female rats.

Effects of in utero tributyltin chloride exposure in the rat on pregnancy outcome

Tributyltin, an organotin, is ubiquitous in the environment. The consumption of contaminated marine species leads to human dietary exposure to this compound. Tributyltin is an endocrine disruptor in many wildlife species and inhibits aromatase in mammalian placental and granulosa-like tumor cell lines. We investigated the effects of tributyltin chloride exposure on pregnancy outcome in the Sprague-Dawley rat. Timed pregnant rats were gavaged either with vehicle (olive oil) or tributyltin chloride (0.25, 2.5, 10, or 20 mg/kg) from days 0-19 or 8-19 of gestation. On gestational day 20, dams were sacrificed, and pregnancy outcome was determined. Tributyltin and its metabolites (dibutyltin, monobutyltin) were measured in maternal blood by gas chromatography. Both tributyltin and dibutyltin were present in maternal blood at approximately equal concentrations, whereas monobutyltin contributed minimally to total organotins. Organotin concentrations increased in a dose-dependent pattern in dams, independent of the window of exposure. Tributyltin chloride administration significantly reduced maternal weight gain only at the highest dose (20 mg/kg); a significant increase in post-implantation loss and decreased litter sizes, in addition to decreased fetal weights, was observed in this group. Tributyltin chloride exposure did not result in external malformations, nor was there a change in sex ratios. However, exposure to 0.25, 2.5, or 10 mg/kg tributyltin chloride from gestation days (GD) 0-19 resulted in a significant increase in normalized anogenital distances in male fetuses; exposure from days 8-19 had no effect. There was a dramatic increase in the incidence of low weight (< or =0.75 of the mean) fetuses after exposure to 20 mg/kg tributyltin chloride. Delayed ossification of the fetal skeleton was observed after in utero exposure to either 10 mg/kg or 20 mg/kg tributyltin chloride. Serum thyroxine and triiodothyronine levels were reduced significantly in dams exposed to 10 and 20 mg/kg tributyltin chloride throughout gestation; in dams treated with tributyltin from GD 8-19, serum thyroxine concentrations, but not triiodothyronine, were significantly decreased at both the 2.5 and 10 mg/kg exposures. Thus, maternal thyroid hormone homeostasis may be important in mediating the developmental toxicity of organotins.

Continuous Measurement of Glutamate and Hydrogen Peroxide Using a Microfabricated Biosensor for Studying the Neurotoxicity of Tributyltin

We first measured the effects of trace levels of an endocrine disruptor, tributyltin (TBT), on the secretion response from nerve cells using a microfabricated biosensor designed for the continuous measurement of L-glutamate and hydrogen peroxide. We observed higher and long-lasting glutamate and hydrogen peroxide concentrations from the cells when cultured rat cortical neurons were exposed to TBT. Glutamate and hydrogen peroxide release was induced even when we reduced the TBT concentration to 10 nM. This concentration is about two orders of magnitude lower than the concentration that induced apoptosis-like cell death. We also report on the effects of NMDA and non-NMDA receptor antagonists, which can help us to understand the mechanism of TBT neurotoxicity.

Distribution, biomagnification, and elimination of butyltin compound residues in common cormorants (Phalacrocorax carbo) from Lake Biwa, Japan

Concentrations of butyltin compounds (BTs) were determined in various body tissues of common cormorants (Phalacrocorax carbo) collected from the Lake Biwa, Japan. Elevated concentrations of butyltins were detected in the feathers of cormorants. Among other organs and tissues, butyltin levels were also higher in the kidney (290+/-150 ng/g) and liver (270+/-260 ng/g), ranging from 115 to 544 ng/g and 142 to 1007 ng/g (wet wt basis), respectively. The accumulation of BTs in cormorant bodies was in the order of MBT>DBT>TBT and their organ specific burdens were in the order of muscle>/=feathers>skin>liver>rest of the tissues and organs. The higher levels of BTs residues in feather suggested the excretion of about one fourth of their body burden during a complete molting cycle, which has been a natural detoxification mechanism in these birds. Based on the whole body concentrations of BTs in cormorants (42-160 ng/g wet wt) and fish (10-55 ng/g wet wt) biomagnification factors were assessed to be in the range of 1.1-4.1. To our knowledge, this is the first fundamental study to substantially indicate the contamination and kinetics of BTs in wild birds.

Oral bis(tri-n-butyltin) oxide in pregnant mice. I. Potential influence of maternal behavior on postnatal mortality

Pregnant Swiss mice were treated with 0, 5, 10, 20, and 30 mg/kg body weight of bis(tri-n-butyltin) oxide (TBTO) on d 6-15 of gestation. At birth litters were normalized to eight pups, and postnatal evaluation of pup growth rate and behavioral observations of dams were carried out. Litters were sacrificed on postnatal days (pnd) 7, 14, and 21, to perform hematological analysis, in connection with another study. Dam weight gain was impaired in all the treated groups (except at the lowest dose level) in the late phase of gestation. A high incidence of anticipated or delayed parturitions, without any correlation with fetal mass, was observed in the treated groups. All the treated dams showed a significant increase in resorptions, and a decrease in body weight gain between gestational day (gd) 6 and pnd 1. At birth, only the 20 and 30 mg/kg dose groups showed reduced litter size and reduced pup weight. Body weight gain reduction of pups persisted in wk 1 of life only in the 10 and 20 mg/kg dose groups. In addition, the maternal weight trend was affected during the lactation period in the higher dose groups. Postnatal death rate and growth rate of treated pups were affected by an altered maternal behavior; pups, apparently viable and with normal weight, were found often scattered throughout the cage with signs of wounds, and the percentage of dams that had not built a nest increased in the 10, 20, and 30 mg/kg dose groups. Total absence of parental care was noted in many litters, and many infanticidal events were reported. Our results seem to confirm low TBTO embryofetotoxicity, and strongly support the assumption that TBTO's toxicity to the mother is much stronger than its embryo-fetotoxic potential. Most of the reproductive parameters examined in this study were unaffected in the low-dose group, while some indices, such as gestation length and maternal weight gain between gd 6 and pnd 1, were markedly altered also at the 5 mg/kg dose level and appear to be sensitive parameters in assessing maternal toxicity.

Comparative developmental toxicity of butyltin trichloride, dibutyltin dichloride and tributyltin chloride in rats

Butyltin trichloride (BT), dibutyltin dichloride (DBT) and tributyltin chloride (TBT) were compared for their developmental toxicity including teratogenic potential following administration during the susceptible period for the teratogenesis of DBT. Pregnant rats were given either BT at a dose of 1000, 1500 or 2000 mg kg-1, DBT at a dose of 10 or 15 mg kg-1 or TBT at a dose of 40 or 80 mg kg-1 by gastric intubation on days 7 and 8 of pregnancy. Although maternal toxicity occurred, as evidenced by a significantly increased maternal lethality at 1500 and 2000 mg kg-1 and decreased maternal weight gain at 1000 and 1500 mg kg-1, no significant increase in the incidences of postimplantation loss and malformed fetuses were observed after treatment with BT. Treatment with DBT resulted in a significantly lower maternal weight gain, lower fetal weight and higher postimplantation embryolethality. A significantly and markedly increased incidence of fetuses with malformations, such as exencephaly, cleft jaw, cleft lip, ankyloglossia, club foot, deformity of the vertebral column in the cervical and thoracic regions and of the ribs and ano- or microphthalmia, was observed in both groups treated with DBT. While treatment with TBT at 40 and 80 mg kg-1 caused a significantly decreased maternal weight gain and increased postimplantation embryolethality, no significantly increased incidence of malformed fetuses occurred. It could be concluded that BT, DBT and TBT are different in the susceptibility and spectrum of developmental toxicity.

Further evaluation of the developmental toxicity of tributyltin chloride in rats

The objective of this study was to further evaluate the developmental toxicity of tributyltin chloride (TBTCl) in rats. Pregnant rats were given TBTCl by gastric intubation at a dose of 25, 50 or 100 mg/kg on days 7-9, days 10-12 or days 13-15 of pregnancy. A significant increase in the incidence of post-implantation loss was found in the groups treated with TBTCl on days 7-9 at 25 and 50 mg/kg and on days 10-12 at 100 mg/kg, but not in the groups treated with TBTCl on days 13-15. No significant increase in the incidence of malformed fetuses was observed after treatment with TBTCl on days 7-9. A significant increase incidence of malformed fetuses was detected when TBTCl was given on days 10-12 at 100 mg/kg and on days 13-15 at 25, 50 and 100 mg/kg. The most predominant malformation was cleft palate. It could be concluded that the manifestation of deviant development induced by TBTCl varies with the developmental stage at the time of administration and TBTCl possesses teratogenic potential with developmental phase specificity.

Susceptible period for the teratogenicity of di-n-butyltin dichloride in rats

Pregnant rats were given di-n-butyltin dichloride (DBT) by gastric intubation at a dose of 20 mg/kg on days 7-9, 10-12 or 13-15 of pregnancy or at a dose of 20 or 40 mg/kg on day 6, 7, 8 or 9 of pregnancy. While treatment with DBT on days 7-9 was significantly and highly teratogenic, no evidence of teratogenicity was detected when DBT was given on days 10-12 or 13-15. Treatment on day 7 or 8 with both doses of DBT, but neither on day 6 or 9, resulted in an increased incidence of fetuses with malformations. The highest incidence of malformed fetuses occurred after treatment on day 8. The incidence of malformed fetuses was proportional to the dose of DBT. Anomaly of tail, anal atresia, club foot, omphalocele, deformity of the vertebral column, defect of the ribs and anophthalmia or microphthalmia were predominantly observed. It could be concluded that, following maternal exposure to DBT in rats, developing offspring are not susceptible to teratogenic effects of DBT on day 6 and that day 7 is the earliest susceptible period, day 8 is the highest susceptible period and day 9 is no longer a susceptible period for teratogenesis of DBT.

Teratogenicity of di-n-butyltin dichloride in rats

Pregnant rats were given di-n-butyltin dichloride (DBT) by gastric intubation at a dose of 0, 2.5, 5.0, 7.5 or 10.0 mg/kg on days 7-15 of pregnancy. Maternal toxicity occurred in the 7.5 and 10.0 mg/kg groups as evidenced by a significant increase in maternal death and decrease in food consumption and body weight gain. The incidence of fetuses with malformations was roughly proportional to the dose of DBT, and was significantly increased in the 5.0, 7.5 and 10.0 mg/kg groups. Cleft jaw, ankyloglossia, defects of the mandible, fusion of the ribs and deformity of the vertebral column were predominantly found. It is concluded that DBT produced teratogenic effects in the absence of maternal toxicity.