Upstream eventTH synthesis, Decreased
T4 in serum, Decreased
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Inhibition of Thyroperoxidase and Subsequent Adverse Neurodevelopmental Outcomes in Mammals||adjacent||High||Moderate|
|XX Inhibition of Sodium Iodide Symporter and Subsequent Adverse Neurodevelopmental Outcomes in Mammals||adjacent||High||Moderate|
|Sodium Iodide Symporter (NIS) Inhibition and Subsequent Adverse Neurodevelopmental Outcomes in Mammals||adjacent||High||High|
|Inhibition of Na+/I- symporter (NIS) leads to learning and memory impairment||adjacent||High||Moderate|
|Thyroperoxidase inhibition leading to reduced young of year survival via anterior swim bladder inflation||adjacent|
|Xenopus laevis||Xenopus laevis||High||NCBI|
Life Stage Applicability
|All life stages||High|
Key Event Relationship Description
Thyroid hormones (THs), thyroxine (T4) and triiodothyronine (T3) are synthesized by NIS and TPO in the thyroid gland as iodinated thyroglobulin (Tg) and stored in the colloid of thyroid follicles. Secretion from the follicle into serum is a multi-step process. The first involves thyroid stimulating hormone (TSH) stimulation of the separation of the peptide linkage between Tg and TH. The next steps involve endocytosis of colloid, fusion of the endosome with the basolateral membrane of the thyrocyte, and finally release of TH into blood. More detailed descriptions of this process can be found in reviews by Braverman and Utiger (2012) and Zoeller et al. (2007).
NIS main physiological function is to transport one iodide ion along with two sodium ions across the basolateral membrane of thyroid follicular cells. Once inside the follicular cells, the iodide diffuses to the apical membrane, where it is metabolically oxidized through the action of TPO to iodinium (I+), which in turn iodinates tyrosine residues of the Tg proteins in the follicle colloid. Therefore, NIS and TPO are both essential for the synthesis of thyroid hormones (T3 and T4).
Compounds, such as triclosan, triclocarban, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), and bisphenol A (BPA) have been reported to decrease TH synthesis by inducing an inhibition of NIS-mediated iodide uptake and decreasing the expression of genes involved in TH synthesis, as shown in rat thyroid follicular FRTL-5 cells, and on the activity of TPO, using rat thyroid microsomes (Wu Y et al., 2016).
Perchlorate, thiocyanate, nitrate, and iodide, competitive inhibitors of iodide uptake, have been shown to inhibit radioactive iodide uptake by NIS (Tonacchera et al. 2004), consequentially causing inhibition of TH synthesis. In particular, perchlorate blocks iodide uptake into the thyroid and decreases the production of TH (Steinmaus, 2016a).
Evidence Supporting this KER
The weight of evidence linking these two KEs of decreased TH synthesis and decreased T4 in serum is strong. It is commonly accepted dogma that decreased synthesis in the thyroid gland will result in decreased circulating TH (serum T4).
The biological relationship between two KEs in this KER is well understood and documented fact within the scientific community.
There is limited direct evidence supporting the relationship between decreased TH synthesis and lowered circulating hormone levels during development. Lu and Anderson (1994) followed the time course of TH synthesis, measured as thyroxine secretion rate, in non-treated pregnant rats and correlated it with serum T4 levels. However, while empirical data is scarce, it is widely accepted dogma the TPO or NIS inhibition leads to declines in serum TH levels. This is due to the fact that the sole source for circulating T4 derives from hormone synthesis in the thyroid gland. Indeed, it has been known for decades that insufficient dietary iodine (or NIS inhibition) will lead to decreased serum TH concentrations due to inadequate synthesis. Furthermore, a wide variety of drugs and chemicals that inhibit TPO are known to result in decreased release of TH from the thyroid gland, as well as decreased circulating TH concentrations. This is evidenced by a very large number of studies that employed a wide variety of techniques, including thyroid gland explant cultures, tracing organification of 131-I and in vivo treatment of a variety of animal species with known TPO inhibitors (King and May,1984; Atterwill et al., 1990; Brown et al., 1986; Brucker-Davis, 1998; Hornung et al., 2010; Hurley et al., 1998; Kohrle, 2008).
In Sui and Gilbert, 2003, developing rats were exposed in utero and postnatally to 0, 3, or 10 ppm propylthiouracil (PTU, TPO inhibitor), administered in the drinking water of dams from GD6 until PND30. Excitatory postsynaptic potentials and population spikes (indicative of neuronal network function) were recorded in area CA1 of hippocampal slices from offspring between PND21 and PND30. PTU caused at PND30 a decrease of maternal total T4 (43.9% with 3-ppm, and 65.0% with 10-ppm) compared to controls, with no change in total T3. Maternal TSH was increased above control levels in a dose-dependent manner. In pups, total T4 was depressed (by 75%) in both treated groups relative to control pups, and total T3 was depressed (35.8% in the 3-ppm group and 66.5% in the 10-ppm group) relative to controls. TH insufficiency also compromised synaptic communication in area CA1 of developing rat hippocampus (involved in learning and memory).
Similarly, in Gilbert, 2004 developing rats were transiently exposed to PTU (0 or 15 ppm), through the drinking water of pregnant dams beginning on GD18 until PND21. This regimen markedly reduced circulating levels of TH in pups (T3: ∼50% lower than control at PND21; T4: T4 ∼40% below control levels). TH insufficiency also compromised synaptic function in the hippocampus and neuronal network function.
In Dong et al., 2005, dam rats were administered through gestation and lactation with either iodine-deficient diet or MMI (TPO inhibitor) added to drinking water. Exposure was terminated on PND30. Both treated groups showed lower concentrations of serum FT3 (~60% decrease on PND30) and FT4 (~80% decrease on PND30), and a reduction of LTP in hippocampus.
Similarly, other studies have shown associations between NIS inhibition and decreased TH serum levels (Dong et al., 2017; Calil-Silveira et al., 2016; Tang et al., 2013; Liu et al., 2012; Pearce et al., 2012).
Temporal Evidence: For the current AOP, the temporal nature of this is developmental (Seed et al., 2005). There are currently no studies that measured both TPO synthesis and TH production during development. However, the impact of decreased TH synthesis on serum hormones is similar across all ages. Good evidence for the temporal relationship comes from thyroid system modeling (e.g., Degon et al., 2008; Fisher et al., 2013). In addition, recovery experiments have demonstrated that serum thyroid hormones recovered in athyroid mice following grafting of in-vitro derived follicles (Antonica et al., 2012).
Dose-response Evidence: Dose-response data is lacking from studies that include concurrent measures of both TH synthesis and serum TH concentrations. However, data are available demonstrating correlations between thyroidal TH and serum TH concentrations during gestation and lactation during development (Gilbert et al., 2013). These data were used to develop a rat quantitative biologically-based dose-response model for iodine deficiency (Fisher et al., 2013), which support the role of NIS activity inhibition in relation to TH levels (T3 and T4) in serum.
Uncertainties and Inconsistencies
There are no inconsistencies in this KER, but there are some uncertainties. The first uncertainty stems from the paucity of data for quantitative modeling of the relationship between the degree of synthesis decrease and resulting changes in circulating T4 concentrations. In addition, most of the data supporting this KER comes from inhibition of TPO or NIS, and there are a number of other processes (e.g., endocytosis, lysosomal fusion, basolateral fusion and release) that are not as well studied.
Quantitative Understanding of the Linkage
- Fisher et al. (2013) published a quantitative biologically-based dose-response model for iodine deficiency in the rat, which mimics NIS inhibition. This model provides quantitative relationships for thyroidal T4 synthesis (iodine organification) and predictions of serum T4 concentrations in developing rats. In particular, HPT axis adaptations to dietary iodide intake in euthyroid (4.1-39 µg iodide/day) and iodide-deficient (0.31 and 1.2 µg iodide/day) conditions were evaluated. Alterations in T4 homeostasis were more apparent than for T3. In rat pups that were iodide deficient during gestation and lactation, decreases in serum T4 levels were associated with declines in TH levels in the fetal brain and a suppression of synaptic responses in the hippocampal region of the brain of the adult offspring (Gilbert et al., 2013).
There are other computational models that include thyroid hormone synthesis. Ekerot et al. (2012) modeled TPO, T3, T4 and TSH in dogs and humans based on exposure to myeloperoxidase inhibitors that also inhibit TPO and has recently adapted for rat (Leonard et al., 2016). While the original model predicted serum TH and TSH levels as a function of oral dose, it was not used to explicitly predict the relationship between serum hormones and TPO inhibition, or thyroidal hormone synthesis. Leonard et al. (2016) recently incorporated TPO inhibition into the model. Degon et al (2008) developed a human thyroid model that includes TPO, but does not make quantitative prediction of organification changes due to inhibition of the TPO enzyme.
- Steinmaus et al., 2016b Authors of this study showed that the presence of high level of perchlorate, thiocyanate, nitrate, and iodide (NIS inhibitors) in water supply induced a decrease of total T4 [regression coefficient (β) = –0.70; 95% CI: –1.06, –0.34], a decrease of free thyroxine (fT4) (β = –0.053; 95% CI: –0.092, –0.013), and an increase of TSH, as shown in 1880 pregnant women from San Diego County, California, during 2000–2003.
- Wu F et al., 2012 Authors of this study found that high dose perchlorate (NIS inhibitor) (520 mg/kg body weight) in Sprague-Dawley rats (28-day old) caused a decrease of Tg (~ 50% lower than control), and TPO (~ 45% lower than control) gene expression, indicative of reduced TH biosynthesis, together with a decrease of free T3 (~ 50% lower than control) and free T4 levels (~ 50% lower than control), and a remarkable increase of TSH levels (125% higher than control).
- Dong et al., 2017 Wistar rats exposed to di-(2-ethylhexyl)phthalate (DEHP) (at 0, 150, 300, and 600 mg/kg/day for 3 and 6 months), a chemical known to elicit a reduction of serum TH levels, underwent decreased of serum TH levels (FT3, FT4 in the DEHP-dosed group were lower than those in the control (p < 0.05)). With the increase of DEHP-treated dose, serum total T3, total T4 and TSH level in all groups of different DEHP doses were lower than the control group at 6 months (p < 0.05), and treated rats presented also a general increase of the expression of thyrotropin releasing hormone receptor (TRHr), Deiodinases 1 (D1), thyroid stimulating hormone beta (TSHβ), NIS, thyroid stimulating hormone receptor (TSHr), TPO, thyroid transcription factor 1 (TTF-1), and thyroglobulin (TG) genes. In particular, NIS protein, after 3 month exposure to middle (300 mg/kg/day) and high-dose (600 mg/kg/day) DEHP, resulted decreased (by ~ 30% with 300 mg/kg/day, and by ~ 85% with 600 mg/kg/day), whilst after 6 months a progressive protein increase was observed. Globally these data indicate that DEHP-dependent reduction of TH serum levels occurs via a strong perturbation of the HPT axis.
- Calil-Silveira et al., 2016 Male Wistar rats treated for two months with NaI (0.05% and 0.005%) or NaI+NaClO4 (0.05%) (NIS inhibitors), underwent high levels of urine iodine, increased serum thyrotropin levels, slightly decreased serum TH levels, and a decreased expression of NIS, thyrotropin receptor, and TPO mRNA and protein, indicating a primary thyroid dysfunction.
- Tang et al., 2013 showed that 2,3',4,4',5-pentachlorobiphenyl (PCB118) (i.p. injected in male Wistar rats at 10, 100, or 1000 μg/kg/day, 5 days/week for 13 weeks) caused a progressive decrease of free T4, free T3 and TSH levels in serum (e.g., serum free T3, free T4 and TSH were reduced to 75%, 31% and 52%, respectively, at 1000 μg/kg/day PCB118), together with a downregulation of NIS (~ 30% at 1000 μg/kg/day PCB118) and Tg (~ 40% at 1000 μg/kg/day PCB118) mRNA expression. Moreover, PCB118 also led to histopathological deterioration of the thyroid (i.e., follicular hyperplasia and expansion).
- Liu et al., 2012 This in vivo study reported that PCB153 (i.p. injected in Sprague-Dawley rats at 0, 4, 16 and 32 mg/kg/day for 5 consecutive days) caused a decrease of NIS, TPO and Tg, deiodinases, the receptors (TSHr and TRHr), together with a decrease of serum total T4, total T3, and thyrotropin releasing hormone (TRH).
- Pearce et al., 2012 A cross-sectional study conducted on 134 first-trimester pregnant women from Athens, Greece, showed an inverse correlation between the level of urinary perchlorate (NIS inhibitor) and free T3 and free T4 plasma values.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
While the majority of the empirical evidence comes from work with laboratory rodents, there is a large amount of supporting data from humans (with anti-hyperthyroidism drugs including PTU and MMI), some amphibian species (e.g., frog) (Hornung et al. 2010), and some avian species (e.g, chicken) (Van Herck et al. 2013). The following are samples from a large literature that supports this concept: Cooper et al. (1982; 1983); Hornung et al.(2010); Van Herck et al. (2013); Paul et al. (2013); Alexander et al. (2017). Despite many physiological similarities, humans and rats exhibit significantly different susceptibilities to thyroid perturbation. For instance, dose-response data for changes in serum T3, T4, and TSH levels have been analysed from studies in humans, rats, mice, and rabbits. It was found that thyroid homeostasis in the rat appeared to be strikingly more sensitive to perchlorate (NIS inhibitor) than any of the other species. Therefore, data obtained from rat NIS studies should be critically evaluated for their relevance to humans (Lewandowski et al., 2004).
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