**2.3. By thyroid hormone-sulfotransferases (Figure 1)**

130 Thyroid Hormone

are still a subject for research.

OATP1A4 OATP1A5

OATP6B1 OATP6C1

multispecific (+).

LAT2

OATP1A2 T4, T3, rT3 OATP1A3 T4, T3

OATP1B1 T4, T3, T3S, T4S, rT3S

OATP1B3 rT3, T4S, T3S, rT3S

OATP1B2 T3, T4

OATP4C1 T3, T4

LAT1 T3, T4, rT3, T2

that there is continued/ increased maternal to fetal supply of TH in the 3rd trimester despite increasing fetal TH production [57]. It is also likely that increased expression of these transporters with gestation may also fulfil the increased need for other biological substances for fetal growth and development, such as amino acids. The most factors regulating the placental expression of these transporters are unknown until now. There are suggestions in rodents that the activity of system-L and the expression of MCT8 in non-placental tissues are influenced by thyroid status [58] suggesting that TH may be a regulator of its own transporters [50]. During the passage of THs from the maternal circulation to the fetal circulation, each THT is likely to have a specific role in each different plasma membrane layer, which might include cellular influx, efflux, or both [59]. To sum, THTs of the various placental cell types serve as channels that help to maintain the differences in the composition of THs and their metabolites between maternal and fetal circulations (figure1 [2,45-47] and tables 2 & 3 [51,52,55,59,60,61]). The relative contributions of these THTs to the transplacental transport of thyroid hormones

Transporter*<sup>a</sup>* Iodothyronine derivates Specificity*<sup>b</sup>*

MCT8 T3, T4, rT3, T2 +++ MCT10 T3, T4 ++ OATP1A1 T3, T4, rT3, T2, T4S, T3S, rT3S, T2S +

OATP1C1 T4, rT3, T3, T4S ++ OATP2B1 T4 + OATP3A1 (V1/V2) ++ OATP4A1 T3, T4, rT3 +

NTCP T4, T3, T4S, T3S ++

**Table 2.** Types of thyroid hormone transporters and their iodothyronine derivates.

*<sup>a</sup>* The human protein symbol is presented, if TH transport has been demonstrated in different species including humans. *<sup>b</sup>* If a transporter only transports iodothyronine derivatives, specificity is high (+++). If fewer than five other ligands are known, specificity is moderate (++). If more than five ligands are known, the transporter is denoted as

Sulfation (S) appears to be an important pathway for the reversible inactivation of THs during fetal development [2,13,45-47]. Monique Kester and the group from Erasmus University have used a rat model to study the regulation of fetal TH status and have also extended their studies to human pregnancy [62]. The sulfotransferases catalyze the sulfation of the hydroxyl group of compounds, using 3'-phosphoadenosine-5'-phosphsulfate (PAPS) as the universal sulfate donor [63]. This co-factor PAPS is synthesized from two ATP molecules and inorganic sulfate. Neither the DII or DIII iodothyronines catalyze the deiodination of sulfated iodothyronines nor sulfation strongly facilitates the inner ring deiodination of T4 and T3 by DI, but blocks the outer ring deiodination of T4 (activation) [13,64]. The outer ring deiodination of rT3 by DI is not affected by sulfation [64]. Sulfation thus induces the irreversible degradation of TH. Thus, rapid inner ring deiodinations of T4S, T3S and out ring deiodination of rT3S lead to high concentrations of these sulfates in plasma of adult humans [13,65].

High concentrations of the different iodothyronine sulfates, T4S (thyroxine sulfate), T3S (triiodothyronine sulfate), rT3S (reverse triiodothyronine sulfate) and T2S (diiodothyronine sulfate), have been documented in human fetal and neonatal plasma as well as in amniotic fluid [65,66], and similar findings have been reported for sheep [67]. This has classically been explained by the low hepatic DI expression in the human fetus until the postnatal period [68] and lack of hepatic DI expression until birth in rats [69]. Also, in the rat placenta, where there are insignificant sulfotransferases activities but high DIII activity, irreversible inactivation of DIII appears to be the predominant pathway of iodothyronine metabolism [13]. In the rat fetal liver, sulfotransferase activity is present from the end of the third trimester (GD 17), a time when DI activity is relatively absent [69]. The TH-sulfates may accumulate under such circumstances to form a 'reservoir' of inactive TH from which active hormone may be liberated, in a tissue specific and gestational dependent manner by the action of arylsulfases [13]. To date, six members of this family (ARSAeARSF) have been identified in humans [13,70]. It is interesting that DIII is abundantly expressed in the human placenta [39] and deiodinates T4 and T3 to 3,3'-T2 and rT3, respectively, thus providing substrates for these actions. In the human fetal circulation, T4S and in particular T3S, may represent a reservoir of inactive TH, from which active hormone may be liberated as required (vide supra) [13]. The iodothyronine sulfates in human fetal circulation and amniotic fluid may be derived, at least in part, from sulfation of THs by thermostabile phenol sulfotransferases in the uterus and placenta [13,45]. This may provide a route for the supply of maternal TH to the fetus in addition to placental transfer. Alternatively, iodothyronine sulfates may accumulate in the fetal circulation because of the absence of hepatic transporters which mediate their removal from plasma. It has been demonstrated recently that hepatic uptake of the different iodothyronine sulfates in rats is mediated at least in part through the NTCP and OATP families [71]. Thus, the TH-sulfation mechanism might be useful for non-invasive prenatal diagnostics of fetal thyroid function which is autonomously regulated. The overviews presented here are consistent with the evolving view that sulfation is a major chemical defense system in the maternal-fetal thyroid axis and will hopefully provide a basis for understanding more about these enzymes.

Maternal-Fetal Thyroid Interactions 133

Abbreviations: T3 is triiodothyronine, T4 is thyroxine, TR is thyroid hormone receptor, RXR is retinoid X receptors, TRE is T3-responsive element, nTRE is none T3-responsive element, Ds is deiodonases, S is sulfotransferases, MCT is monocarboxylate transporter, OATP is organic anion transporter, MAPK/ERK1/2 is mitogen-activated protein kinase,

**Figure 2.** (A) Schematic representation of major thyroid hormone receptors (TRα, β) domains and functional sub-regions. (B) General model for genomic and non-genomic actions of TH in both adult and fetus; Schematic representation of thyroid hormones (THs; T4 and T3) genomic actions, initiated at the nuclear receptors (TRβ), and non-genomic actions, initiated at cytoplasmatic receptors (TRβ, TRα) and at the plasma membrane on the membrane receptors, particularly integrin αvβ3 receptor. T4 binding (but not T3) to cytoplasmic TRα may cause a change of state of actin. T3 binding (but not T4) to cytoplasmic TRβ activates the phosphatidylinositol 3-kinase (PI-3K) pathway leading to alteration in membrane ion pumps and to transcription of specific genes. TH binding to the integrin receptor results in activation of mitogen-activated protein kinase (MAPK/ERK1/2). Phosphorylated MAPK (pMAPK) translocates to the nucleus where it phosphorylates transcription factors including thyroid receptors (TRβ), estrogen receptor (ER) and signal transducer activators of transcription (STAT). Generally, activity is regulated by an exchange of corepressor (CoR) and coactivator (CoA) complexes.

P is phosphorylation and PI-3K is phosphatidylinositol 3-kinase.

**(A)**

**(B)**
