Interactions of halogenated industrial chemicals with transthyretin and effects on thyroid hormone levels in vivo

Endocrine disrupting chemicals: interference of thyroid hormone binding to transthyretins and to thyroid hormone receptors

We examined the effects of industrial, medical and agricultural chemicals on 3,5,3'-L-[125I]triiodothyronine ([125I]T(3)) binding to transthyretins (TTRs) and thyroid hormone receptors (TRs). Among the chemicals investigated diethylstilbestrol (DES) was the most powerful inhibitor of [125I]T(3) binding to chicken and bullfrog TTR (cTTR and bTTR). Inhibition of [125I]T(3) binding was more apparent in cTTR than bTTR. Scatchard analysis revealed DES, pentachlorophenol and ioxynil as competitive inhibitors of [125I]T(3) binding to cTTR and bTTR. However, cTTR's affinity for the three chemicals was higher than its affinity for T(3). A miticide dicofol (10(-10)-10(-7) M) activated [125I]T(3) binding to bTTR up to 170%. However, at 4x10(-5) M it inhibited [125I]T(3) binding by 83%. Dicofol's biphasic effect upon [125I]T(3) binding was not detected in TTRs from other species. DES and pentachlorophenol, in the presence of plasma, increased cellular uptake of [125I]T(3) in vitro, by displacing [125I]T(3) from its plasma binding sites. These chemicals did not, however, affect the association of cTTR with chicken retinol-binding protein. All chemicals investigated had little or no influence on [125I]T(3) binding to chicken TR (cTR) and bullfrog TR (bTR). Several endocrine disrupting chemicals that were tested interfered with T(3) binding to TTR rather than to TR. Binding of the endocrine disrupting chemicals to TTR may weaken their intrinsic effects on target cells by depressing their free concentrations in plasma. However, this may affect TH homeostasis in vivo by altering the free concentrations of plasma THs.

Metabolites of polychlorinated biphenyls in human liver and adipose tissue

A method for analysis of hydroxylated metabolites of polychlorinated biphenyls (OH-PCBs) was developed and adopted to the previously described method for determination of PCBs and methylsulfonyl-PCBs (MeSO2-PCBs) in human liver and adipose tissue. The concentrations of OH-PCBs (14 congeners), MeSO2-PCBs (24 congeners), and PCBs (17 congeners) in five paired samples of human liver and adipose tissue are reported. The sum of OH-PCB congeners was higher in liver (7-175 ng/g lipids) than in adipose tissue (0.3-9 ng/g lipids). In both liver and adipose tissue, 2,2',3,4,4',5'-hexachloro-3'-biphenylol (3'-OH-CB138) and 2,2',3,3',4,5'-hexachloro-4'-biphenylol (4'-OH-CB130) were the predominant hydroxylated PCB metabolites. The sum of OH-PCB congeners were of the same order of magnitude as those of methylsulfonyl metabolites of PCB in the same samples, 12 to 358 ng/g lipids and 2 to 9 ng/g lipids in liver and adipose tissue, respectively. The levels of PCBs were similar in liver and adipose tissue, 459 to 2,085 ng/g lipids and 561 to 2,343 ng/g lipids, respectively. The sum of OH-PCBs and the sum of MeSO2-PCBs correlated to the sum of PCBs. The determined PCB metabolites constituted 3 to 26% of total PCB concentration in the liver and 0.3 to 0.8% of total PCBs in the adipose tissue samples.

Burden of organochlorine pesticides in blood and its effect on thyroid hormones in women

Man has utilized a wide variety of pesticides to combat the crop pests and vectors of human diseases. However, in this process, he has overlooked the darker side of these noxious chemicals, the concentrations of which have reached the environment and pose serious threats, such as mutagenesis, teratogenesis, carcinogenesis and endocrine dysfunction in various components of the ecosystem. The present study was planned to assess the burden of organochlorine pesticides and their influence on thyroid function in women. The study included a total of 123 women from Jaipur City who visited the Thyroid clinic in SMS Medical College and Hospital. One hundred women showed normal thyroid hormone levels while the remaining 23 women had depleted T4 and high TSH levels. The qualitative and quantitative estimation of organochlorine pesticides was carried out by gas chromatography. Out of the analyzed pesticides, the concentration of p,p'-DDT and its metabolites was higher in all the subjects, but dieldrin was found to be significantly high in the hypothyroid women. The correlation analysis for dieldrin and depleted T4 levels in hypothyroid women elicited an inverse relationship between them.

In vitro effects of polychlorinated biphenyls and hydroxy metabolites on nitric oxide synthases in rat brain

Nitric oxide synthases (NOS) play a key role in motor activity in the cerebellum, hormonal regulation in the hypothalamus, and long-term potentiation (LTP), learning, and memory processes in the hippocampus. Developmental exposure to polychlorinated biphenyls (PCBs) has been shown to affect psychomotor functions, learning and memory processes, and to inhibit LTP. We hypothesized that PCBs may disrupt the regulation of such neurological functions by altering NOSs. We have studied the in vitro effects of several PCB congeners and some hydroxy PCBs on NOS activity in cytosolic (presumably neuronal NOS [nNOS]) and membrane (presumably endothelial NOS [eNOS]) fractions in different brain regions of young and adult rats. Among the two selected dichloro PCBs, the ortho-PCB, 2,2'-dichlorobiphenyl (DCB), inhibited both cytosolic and membrane NOS activity at low micromolar concentrations (3-10 microM) in the selected brain regions of all age groups while the non-ortho-PCB, 4,4'-DCB, did not. 2,2'-DCB inhibited cytosolic NOS to a greater extent than membrane NOS. Pentachloro-PCBs (PeCBs) and hexachloro-PCBs (HCBs) did not have a significant effect on adult cerebellar cytosolic or membrane NOS. However, mono- and dihydroxy derivatives of HCBs significantly decreased cytosolic NOS (IC50s: 16.33 +/- 0.47 and 33.65 +/- 4.33 microM, respectively) but resulted in a marginal effect on membrane NOS in the cerebellum. Among three adult rat brain regions, the hypothalamic cytosolic NOS was the most sensitive to 2,2'-DCB. Also, cytosolic NOS in the cerebellum and hypothalamus of young rats was less sensitive than in the older rats. In summary, these results indicate that only di-ortho-PCB inhibited both NOS and hydroxy substitution of one or more chlorine molecules significantly increased the potency of both ortho- and non-ortho-HCBs. The selective sensitivity of NOS to dichloro- ortho-PCB and hydroxy metabolites suggests that the inhibition of NOS could play a role in the neuroendocrine effects as well as learning and memory deficits caused by exposure to PCBs.

Long-term exposure to wood-preserving chemicals containing pentachlorophenol and lindane is related to neurobehavioral performance in women

BACKGROUND

The adverse neurobehavioral effects of long-term low exposure to wood-preserving chemicals (WPC) containing solvents, pentachlorophenol (PCP) and gamma-hexachlorocyclohexane (gamma-HCH; lindane), and other neurotoxicants were investigated in a neuropsychological group study.

METHODS

Out of a population of 2,000 women visiting the outpatient practice of a gynecological department, a sample of 15 women aged 31-56 (mean 43) with long-term exposure to WPC verified by self-report, biological monitoring, and environmental samples was investigated. Fifteen controls aged 42 (31-56) years were drawn from the same population and pair-wise matched with respect to sex, age, education, and estimated intelligence.

RESULTS

For the exposed group, mean PCP serum level was 43.6 micrograms/l and mean gamma-HCH blood level was 0.085 microgram/l. Mean duration of exposure was 10 (5-17) years. Intellectual functioning, attention, memory, and visuo-motor performance were examined, suggesting significant group differences in visual short-term memory (Benton Test; d = 1.5, P = .005), verbal memory (paired associate learning and Peterson paradigm; d = 4.3 and 1.6, P < .001), and an incidental learning task (d = 2.3; P = .001). Frequent subjective complaints as assessed by questionnaire were attenuated motivation (d = 1.7; P = .001), increased fatigue (d = 1.6; P = .001), distractibility (d = 1.0; P = .003), and depressed mood (d = 1.9; P = .004). PCP blood level was significantly associated with paired-associate learning, Benton Test, and reading/naming speed.

CONCLUSIONS

Long-term low-dose exposure to WPC in the domestic environment could be related to subjective complaints (attention, mood, and motivation) and to subtle alterations of neurobehavioral performance (e.g., working memory) in women.

Pentachlorophenol exposure in women with gynecological and endocrine dysfunction

Exposure to wood preservatives containing pentachlorophenol (PCP) was detected in 65 women who consulted the Endocrinological Department of the University Hospital of Obstetrics and Gynecology, Heidelberg, Germany, because of gynecological problems. Blood PCP levels ranged from 20.7 to 133 microg per liter of serum. One hundred and six women with similar clinical conditions, corresponding age and body weight, no PCP exposure in history, and PCP levels below 20 microg per liter of serum served as control group. Significant associations were found between serum PCP concentrations, age, and different parameters of the endocrine system. PCP may act centrally on a hypothalamic or suprahypothalamic level which may result in mild ovarian and adrenal insufficiency. PCP may, therefore, play a role in the increasing infertility problem.

Effects of environmental synthetic chemicals on thyroid function

Synthetic chemicals are released into the environment by design (pesticides) or as a result of industrial activity. It is well known that natural environmental chemicals can cause goiter or thyroid imbalance. However, the effects of synthetic chemicals on thyroid function have received little attention, and there is much controversy over their potential clinical impact, because few studies have been conducted in humans. This article reviews the literature on possible thyroid disruption in wildlife, humans, and experimental animals and focuses on the most studied chemicals: the pesticides DDT, amitrole, and the thiocarbamate family, including ethylenethiourea, and the industrial chemicals polyhalogenated hydrocarbons, phenol derivatives, and phthalates. Wildlife observations in polluted areas clearly demonstrate a significant incidence of goiter and/or thyroid imbalance in several species. Experimental evidence in rodents, fish, and primates confirms the potentiality for thyroid disruption of several chemicals and illustrates the mechanisms involved. In adult humans, however, exposure to background levels of chemicals does not seem to have a significant negative effect on thyroid function, while exposure at higher levels, occupational or accidental, may produce mild thyroid changes. The impact of transgenerational, background exposure in utero on fetal neurodevelopment and later childhood cognitive function is now under scrutiny. There are several studies linking a lack of optimal neurological function in infants and children with high background levels of exposure to polychlorinated biphenyls (PCBs), dioxins, and/or co-contaminants, but it is unclear if the effects are caused by thyroid disruption in utero or direct neurotoxicity.

Pesticides: multiple mechanisms of demasculinization

Many pesticides are known to produce reproductive and developmental effects in chronically exposed non-target organisms, including humans. Recent evidence suggests that demasculinization may be an important mechanism responsible for some of these effects. Some pesticides have been shown to interact with the androgen receptor and to act as antagonists, while others have been shown to interact with the estrogen receptor and function as estrogens in both in vitro and in vivo. Many pesticides can also lower serum androgen levels by altering rates of synthesis or metabolism. Given the ubiquity of pesticides in the environment and the multiple mechanisms whereby they can elicit demasculinizing effects, synergy between such compounds may produce clinical endocrine dysfunction at current human exposure levels.

Binding of a 3,3', 4,4'-tetrachlorobiphenyl (CB-77) metabolite to fetal transthyretin and effects on fetal thyroid hormone levels in mice

The present study was conducted in order to study the effect of the PCB congener 3,3', 4,4'-tetrachlorobiphenyl (CB-77) on fetal thyroxin homeostasis in the mouse, and to examine a possible underlying mechanism behind the effect. C57BL mice were treated with 14C-labelled or unlabelled CB-77 (1 or 10 mg/kg body wt.) on day 13 of gestation, and control animals were treated with corn oil. The experiment was terminated at 4 days after exposure. Maternal and fetal plasma and livers, and whole fetuses for homogenate preparation, were collected and analysed for total radioactivity, in vitro binding of 125I-thyroxin to plasma transthyretin (TTR; a thyroxin-transporting protein), and free and total thyroxin (FT4, TT4) levels. Maternal plasma, fetal plasma and homogenates were also analyzed for presence of CB-77 and metabolites. Results showed a dose-dependent uptake of radioactivity in plasma and liver, fetal plasma 14C-levels being about five-times higher in 10 mg/kg dosed animals as after 1 mg/kg. Fetal; plasma levels of total radioactivity were four- to nine-times above maternal levels and corresponded to only one compound, the metabolite 4-OH-3,3', 4',5-tetrachlorobiphenyl (4-OH-tCB). 4-OH-tCB was the major metabolite also in whole fetuses, with only small amounts of the parent compound (approximately 15% of the 4-OH-tCB) and traces (approximately 6%) of two other metabolites, 2-OH-3,3, 4,4'-tetrachlorobiphenyl and 5-OH-3,3', 4,4'-tetrachlorobiphenyl. Polyacrylamide gel electrophoresis confirmed that the 14C-radioactivity in fetal plasma was bound to TTR, and revealed that in vitro binding of 125I-T4 to fetal TTR was reduced to 50% of control values in treated animals (10 mg/kg body wt.). Fetal plasma FT4 and TT4 levels were significantly decreased (64 and 55% of control fetuses) after 10 mg/kg treatment. In conclusion, exposure of pregnant mice to CB-77 results in the accumulation of the metabolite 4-OH-tCB in fetal mouse plasma. The metabolite binds to TTR and is accompanied by a significant decrease in fetal plasma T4 levels. A causative correlation between TTR binding and effects on T4 levels is suggested.

Reduction of thyroxine uptake into cerebrospinal fluid and rat brain by hexachlorobenzene and pentachlorophenol

In the present study the effects of hexachlorobenzene (HCB) and the metabolite pentachlorophenol (PCP) were investigated with respect to uptake of thyroxine (T4) into cerebrospinal fluid (CSF) and brain structures of rats. [125I]T4 was taken up into CSF of control rats by a relatively slow process, reaching a steady state after about 3 h. Both repeated dosing of HCB and single doses of PCP caused decreased uptake of [125I]T4 into CSF, total brain tissue as well as specific brain structures, such as occipital cortex, thalamus, and hippocampus. Although HCB-treatment caused a build-up of HCB and PCP levels in serum in brain only HCB was present in significant amounts (16% of the serum level). In CSF, both HCB and PCP concentrations were below detection levels. Separate experiments with PCP showed, however, a dose- and time-dependent uptake of PCP into CSF. The present results indicate that PCP and the parent compound HCB are able to affect brain supply of T4. This may have consequences for an adequate development of the brain or proper brain function in adults. The exact mechanisms of interference of PCP and/or HCB in brain uptake of T4 remain to be established.

Hexachlorobenzene-induced hypothyroidism. Involvement of different mechanisms by parent compound and metabolite

Rats received repeated oral treatment with different doses of hexachlorobenzene (HCB) (0-3.5 mmol/kg) for 2 or 4 weeks. Measurements of thyroid hormone status after 2 weeks showed a dose-dependent decrease of total thyroxine (TT4) levels, decreased free thyroxine (FT4) levels and little change of total triiodothyronine (TT3) levels. The effects on thyroid hormone status were more pronounced after 4 weeks and also included increased thyroid stimulating hormone (TSH) levels. These conditions suggest that HCB had induced hypothyroidism in these animals. Indications for occupation of thyroid hormone binding proteins were found in serum of exposed animals. The major metabolite pentachlorophenol (PCP) also caused, by competitive interactions with thyroid hormone binding proteins in serum, a rapid and dose-dependent decrease of TT4 and FT4 levels, but not of TT3 levels in serum. The decrease of serum TT4 levels by repeated dosing with 3.5 mmol HCB/kg for 4 weeks could be attributed to competitive interactions of PCP with hormone serum binding proteins and to increased metabolism induced by HCB to an equal degree. At lower dose levels or with shorter dosing periods, increased metabolism of T4 is the main cause of decreased TT4 serum levels. This is the first indication that a similar effect is caused simultaneously by the parent compound and its metabolite through different and independent mechanisms.

Effects on thyroid hormone metabolism and depletion of lung vitamin A in rats by airborne particulate matter

Thyroxine (T4) and vitamin A are important regulators of normal epithelial differentiation and proliferation and might act in the promotion phase of carcinogenesis. Thyroid hormone and vitamin A metabolism are linked by a common plasma carrier protein, transthyretin (TTR). Polychlorinated biphenyls (PCBs) and related organochlorine compounds deplete vitamin A and thyroxine by interaction with TTR and alteration of their metabolism in hepatic and other organs. In the present report an outdoor airborne particulate matter (APM) extract was tested for both interaction with thyroid hormone and vitamin A metabolism, in order to address the question of whether APM has the potency to deplete vitamin A and thyroid hormones. Furthermore, studies were performed to characterize compounds present in APM that interact with TTR. A third aim was to compare the interaction of APM extracts with TTR and thyroxine-binding globulin (TBG), the major carrier protein for thyroxine in humans. Results showed that a single treatment of rats with an outdoor APM extract depleted plasma thyroxine and triiodothyronine levels and increased plasma retinol levels gradually over the time period studied, while liver retinol, lung retinol, and retinyl palmitate levels were depleted by 30-50%. As outdoor APM was able to inhibit T4-TTR binding in vitro, this suggests that the reduction in thyroxine levels in vivo is caused by the same phenomenon. Experiments showed that the neutral fraction of the APM extract accounted for most of the inhibitory activity on T4-TTR binding. Polycyclic aromatic hydrocarbons and nitrated derivatives are not likely to be responsible for the activity of the neutral fraction, because several representatives of these compounds showed no or very little interaction with TTR. Pentachlorophenol, a compound with known inhibitory activity on T4-TTR binding, was detected in the organic acid fraction of both a cigarette smoke sample and an outdoor APM sample. Finally, it was shown that several indoor and outdoor APM extracts only interact with TTR, but not with TBG. As APM has the potency to deplete lung vitamin A in vivo and vitamin A might have a protective effect in the process of lung carcinogenesis, APM might increase the susceptibility for the development of lung cancer.

Enrichment of metabolites in the cerebrospinal fluid of cod (Gadus morhua) following oral administration of hexachlorobenzene and 2,4',5-trichlorobiphenyl

The disposition of 14C-labelled hexachlorobenzene (HCB) and 2,4',5-trichlorobiphenyl (triCB) was studied in cod (Gadus morhua) and rainbow trout (Oncorhynchus mykiss). For both compounds tape section autoradiography revealed substantial amounts of radiolabelled material in the central nervous system (CNS) of cod, whereas only traces of radioactivity were observed in the CNS of rainbow trout. Furthermore, an enrichment of radiolabelled compound in the cerebrospinal fluid (CSF) was observed in the cod, whereas no radioactivity could be detected in the CSF of rainbow trout. According to autoradiography, the CNS of cod dosed with HCB contained the parent compound, whereas the major part of radioactivity in CSF was due to HCB metabolites. Thin-layer chromatography of extracts from cod dosed with triCB showed the presence of parent compound in the CNS, whereas part of the radioactivity in the CSF was due to triCB metabolites. The activities of cytochrome P-450 and UDP-glucuronosyltransferase in the CNS of cod and rainbow trout were determined in microsomal and mitochondrial fractions. Both species expressed activities which were in the same order of magnitude as those reported for the corresponding fractions from rat brain. Incubation of triCB with cod brain mitochondria and microsomes resulted in the formation of two polar metabolites. It is suggested that cod may be more vulnerable than rainbow trout regarding neurotoxicological effects of HCB, triCB and related environmental pollutants.

Hexachlorobenzene and its metabolites pentachlorophenol and tetrachlorohydroquinone: interaction with thyroxine binding sites of rat thyroid hormone carriers ex vivo and in vitro

Previous results have indicated that hexachlorobenzene (HCB)-induced hypothyroidism may be caused by its main metabolite pentachlorophenol (PCP), and by tetrachlorohydroquinone (TCHQ), rather than by the parent compound. In the present experiments it was investigated whether hormone displacement from serum carriers could be a factor in the development of this hypothyroidism. In an in vitro competition assay PCP was an effective competitor for the thyroxine (T4)-binding sites of serum carriers, whereas HCB was ineffective. Ex vivo experimental results demonstrated occupation of T4-binding sites in sera from PCP-exposed animals but not in sera from HCB- or TCHQ-treated animals. Competing ability for T4-binding sites was still present in sera of PCP-exposed animals but was absent in HCB- or TCHQ-exposed animals. The results suggest that thyroid hormone displacement by the major metabolite PCP may play a role in HCB-induced hypothyroidism.

Effects of hexachlorobenzene and its metabolites pentachlorophenol and tetrachlorohydroquinone on serum thyroid hormone levels in rats

Effects of administration of equimolar doses of hexachlorobenzene (HCB) and its metabolites pentachlorophenol (PCP) and tetrachlorohydroquinone (TCHQ) on serum thyroxine (TT4) and triiodothyronine (TT3) levels in rats were studied. Furthermore, it was investigated whether the observed effects were related to the serum levels of HCB or PCP. Rats received either corn oil (controls) or HCB, PCP or TCHQ in a single equimolar intraperitoneal dose of 0.056 mmol/kg. Results indicated that HCB did not alter serum TT4 and TT3 levels for a period up to 96 h after dosing. In contrast, PCP and TCHQ were both capable of reducing serum TT4 levels with a maximum effect between 6 and 24 h after exposure. TCHQ was more effective in repressing TT3 than TT4 blood levels. Dose-response experiments were carried out in order to obtain insight into the sensitivity of the observed effects. Rats received different doses of PCP or TCHQ intraperitoneally. The reductions of TT4 levels by PCP were inversely related to serum PCP levels in exposed animals, based on the toxicokinetics and dose-response profiles. Furthermore, PCP serum levels after HCB administration appeared too low to cause an effect. The results of this study indicate that not HCB itself, but rather its metabolites PCP and TCHQ may be involved in reduced serum thyroid hormone levels after HCB administration.