Upstream eventInhibition, Deiodinase 1
Decreased, Triiodothyronine (T3) in serum
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Deiodinase 1 inhibition leading to reduced young of year survival via posterior swim bladder inflation||adjacent|
|Deiodinase 1 inhibition leading to reduced young of year survival via anterior swim bladder inflation||adjacent|
|fathead minnow||Pimephales promelas||NCBI|
Life Stage Applicability
|All life stages|
Key Event Relationship Description
The two major thyroid hormones are thyroxine (T4) and the more biologically active triiodothyronine (T3), both iodinated derivatives of tyrosine. Active and inactive THs are tightly regulated by enzymes called iodothyronine deiodinases (DIO). The activation occurs via outer ring deiodination (ORD), i.e. removing iodine from the outer, phenolic ring of T4 to form T3, while inactivation occurs via inner ring deiodination (IRD), i.e. removing iodine from the inner tyrosol ring of T4 or T3.
Three types of iodothyronine deiodinases (DIO1-3) have been described in vertebrates that activate or inactivate THs and are therefore important mediators of TH action. All deiodinases are integral membrane proteins of the thioredoxin superfamily that contain selenocysteine in their catalytic centre. Type I deiodinase is capable of converting T4 into T3, as well as to convert rT3 to the inactive thyroid hormone 3,3’ T2, through outer ring deiodination. rT3, rather than T4, is the preferred substrate for DIO1. furthermore, DIO1 has a very high Km (µM range, compared to nM range for DIO2) (Darras and Van Herck, 2012). Type II deiodinase (DIO2) is only capable of ORD activity with T4 as a preferred substrate (i.e., activation of T4 tot T3). DIO3 can inner ring deiodinate T4 and T3 to the inactive forms of THs, reverse T3, (rT3) and 3,3’-T2 respectively. (Darras and Van Herck, 2012)
Because of the high Km and preference for rT3 as a substrate, the importance of DIO1 in activating T4 to T3 in a physiological situation is likely limited.
Evidence Supporting this KER
Inhibition of DIO1 activity is widely accepted to directly decrease T3 levels, since the conversion of T4 to T3 is inhibited. The importance of DIO1 inhibition in altering serum T3 levels depends on the relative role of different deiodinases in regulating serum versus tissue T3 levels and in negative feedback within the HPT axis. Both aspects appear to differ to some extent among vertebrate taxa.
Inhibition of DIO1 activity is widely accepted to directly decrease T3 levels, since the conversion of T4 to T3 is inhibited.
- In the study of Cavallin et al. (2017) fathead minnow larvae were exposed to IOP, a model iodothyronine deiodinase inhibitor that is assumed to inhibit all three deiodinase enzymes (DIO1,2,3) and pronounced decreases of whole body T3 concentrations were observed.
- Stinckens et al. (submitted) showed that IOP reduced T3 levels in zebrafish in 21 and 32 day old larvae that had been exposed starting from fertilization.
Uncertainties and Inconsistencies
Since in fish early life stages THs are typically measured on a whole body level, it is currently uncertain whether T3 level changes occur at the serum and/or tissue level. Pending more dedicated studies, whole body TH levels are considered a proxy for serum TH levels.
The importance of DIO1 inhibition in altering serum T3 levels depends on the relative role of different deiodinases in regulating serum versus tissue T3 levels and in negative feedback within the HPT axis. Both aspects appear to differ to some extent among vertebrate taxa, but the details are not understood yet.
Another uncertainty lies in the relative importance of the different T4 actvating iodothyronine deiodinases (DIO1, DIO2) in the conversion of T4 to T3. It has been previously suggested that DIO2 is the major contributor to TH activation in developing zebrafish embryos (Darras et al., 2015; Walpita et al., 2010). It has been shown that a morpholino knockdown targeting DIO1 mRNA alone did not affect embryonic development in zebrafish, while knockdown of DIO2 delayed progression of otic vesicle length, head-trunk angle and pigmentation index (Houbrechts et al., 2016; Walpita et al., 2010, 2009). DIO1 inhibition may only become essential in hypothyroidal circumstances, for example when DIO2 is inhibited or in case of iodine deficiency, in zebrafish (Walpita et al., 2010) and mice (Galton et al., 2009; Schneider et al., 2006).
In the study of Cavallin et al. (2017) fathead minnow larvae were exposed to IOP, a model iodothyronine deiodinase inhibitor that is assumed to inhibit all three deiodinase enzymes (DIO1,2,3). Transcriptional analysis showed that especially DIO2, but also DIO3 mRNA levels (in some treatments), were increased in 10 to 21 day old larvae exposed to IOP as of the age of 6 days. This suggests that IOP effectively inhibited DIO2 and DIO3 in the larvae and that mRNA levels increased as a compensatory response. The authors also observed pronounced decreases of whole body T3 concentrations and increases of whole body T4 concentrations. It is not clear whether inhibition of DIO1 also played a role in the decrease of T3 levels.
Quantitative Understanding of the Linkage
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Thyroid hormone levels are regulated via negative feedback, influencing this KER. Additionally, deiodinases regulate the activity of thryoid hormones, not only in serum and target organs, but also in the thryoid gland. Deiodinases themselves are known to be involved in the negative feedback system that results in increased TSH levels when the levels of T4 (and also T3) in serum are low (Schneider et al., 2001), resulting in an even more complicated impact on this KER. Increased TSH levels then stimulate increased T4 release from the thyroid gland, resulting in a compensatory increase of serum T4 levels. In DIO2 knockout mice it seemed that the negative feedback system was blocked resulting in increased levels of T4 and TSH and in normal rather than decreased T3 levels compared to WT. By inhibiting DIO1 using a PTU exposure, Schneider et al. (2001) showed that DIO2 played a role in the increased TSH levels in response to T3 or T4 injection.
Domain of Applicability
Mol et al. (1998) concluded that deiodinases in teleosts were more similar to mammalian deiodinases than had been generally accepted, based on the similarities in susceptibility to inhibition and the agreement of the Km values.
It is generally assumed that deiodinase 1 in liver is the main supplier of T3 to circulation in mammals (Leonard et al., 1986), and the same appears to be true for birds (Freeman et al., 1991), while DIO2 is assumed to regulate intracellular concentrations of T3. In contrast to the general assumptions however, Maia et al. (2005) determined that in a normal physiological situation in humans the contribution of DIO2 to plasma T3 levels is twice that of DIO1. By contrast, DIO1 function in teleostean and amphibian T3 plasma regulation is less clear (Finnson et al. 1999, Kuiper et al. 2006). The presence of DIO1 in the liver of teleosts has been a controversial issue, and both the high level of DIO2 activity and its expression in the liver of teleosts are unique among vertebrates (Orozco and Valverde, 2005). These differences make it difficult to exactly evaluate the importance of DIO1 in regulating serum/tissue T3 levels across vertebrates.
Cavallin, J.E., Ankley, G.T., Blackwell, B.R., Blanksma, C.A., Fay, K.A., Jensen, K.M., Kahl, M.D., Knapen, D., Kosian, P.A., Poole, S.T., Randolph, E.C., Schroeder, A.L., Vergauwen, L., Villeneuve, D.L., 2017. Impaired swim bladder inflation in early life stage fathead minnows exposed to a deiodinase inhibitor, iopanoic acid. Environmental Toxicology and Chemistry 36, 2942-2952.
Darras, V.M., Van Herck, S.L.J., 2012. Iodothyronine deiodinase structure and function: from ascidians to humans. Journal of Endocrinology 215, 189-206.
Mol, K.A., Van der Geyten, S., Burel, C., Kuhn, E.R., Boujard, T., Darras, V.M., 1998. Comparative study of iodothyronine outer ring and inner ring deiodinase activities in five teleostean fishes. Fish Physiology and Biochemistry 18, 253-266.
Finnson, K.W., McLeese, J.M., Eales, J.G., 1999. Deiodination and deconjugation of thyroid hormone conjugates and type I deiodination in liver of rainbow trout, Oncorhynchus mykiss. General and Comparative Endocrinology 115, 387-397.
Kuiper, G., Klootwijk, W., Dubois, G.M., Destree, O., Darras, V.M., Van der Geyten, S., Demeneix, B., Visser, T.J., 2006. Characterization of recombinant Xenopus laevis type I iodothyronine deiodinase: substitution of a proline residue in the catalytic center by serine (Pro132Ser) restores sensitivity to 6-propyl-2-thiouracil. Endocrinology 147, 3519-3529.
Stinckens, E., Vergauwen, L., Blackwell, B.R., Ankley, G.T., Villeneuve, D.L., Knapen, D., The effect of thyroperoxidase and deiodinase inhibition on anterior swim bladder inflation in the zebrafish. Environmental Science & Technology submitted.
Schneider, M.J., Fiering, S.N., Pallud, S.E., Parlow, A.F., St Germain, D.L., Galton, V.A., 2001. Targeted disruption of the type 2 selenodeiodinase gene (D102) results in a phenotype of pituitary resistance to T-4. Molecular Endocrinology 15, 2137-2148.