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Endocrinology, doi:10.1210/en.2008-0430
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Endocrinology Vol. 149, No. 10 5307-5314
Copyright © 2008 by The Endocrine Society

Thyroid Hormone Transport and Metabolism by Organic Anion Transporter 1C1 and Consequences of Genetic Variation

Wendy M. van der Deure, Pia Skov Hansen, Robin P. Peeters, Kirsten Ohm Kyvik, Edith C. H. Friesema, Laszlo Hegedüs and Theo J. Visser

Department of Internal Medicine (W.M.v.d.D., R.P.P., E.C.H.F., T.J.V.), Erasmus University Medical Center, and Institute of Regional Health Research (O.K.), 3015-GE Rotterdam, The Netherlands; Department of Endocrinology and Metabolism (S.H., L.H.), Odense University Hospital, DK-5000 Odense, Denmark; and The Danish Twin Registry (S.H., O.K.), Epidemiology, Institute of Public Health, University of Southern Denmark, DK-5230 Odense, Denmark

Address all correspondence and requests for reprints to: Theo J. Visser, Ph.D., Department of Internal Medicine, Erasmus University Medical Center, Room Ee 502, Dr. Molewaterplein 50, 3015 GE Rotterdam, The Netherlands. E-mail: t.j.visser{at}erasmusmc.nl.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Organic anion transporting polypeptide (OATP) 1C1 has been characterized as a specific thyroid hormone transporter. Based on its expression in capillaries in different brain regions, OATP1C1 is thought to play a key role in transporting thyroid hormone across the blood-brain barrier. For this reason, we studied the specificity of iodothyronine transport by OATP1C1 in detail by analysis of thyroid hormone uptake in OATP1C1-transfected COS1 cells. Furthermore, we examined whether OATP1C1 is rate limiting in subsequent thyroid hormone metabolism in cells cotransfected with deiodinases. We also studied the effect of genetic variation in the OATP1C1 gene: polymorphisms were determined in 155 blood donors and 1192 Danish twins and related to serum thyroid hormone levels. In vitro effects of the polymorphisms were analyzed in cells transfected with the variants. Cells transfected with OATP1C1 showed increased transport of T4 and T4 sulfate (T4S), little transport of rT3, and no transport of T3 or T3 sulphate, compared with mock transfected cells. Metabolism of T4, T4S, and rT3 by cotransfected deiodinases was greatly augmented in the presence of OATP1C1. The OATP1C1-intron3C>T, Pro143Thr, and C3035T polymorphisms were not consistently associated with thyroid hormone levels, nor did they affect transport function in vitro. In conclusion, OATP1C1 mediates transport of T4, T4S, and rT3 and increases the access of these substrates to the intracellular active sites of the deiodinases. No effect of genetic variation on the function of OATP1C1 was observed.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
ORGANIC ANION transporting polypeptides (OATPs) are multispecific sodium-independent transport proteins that are expressed in many tissues (1). In general, OATPs exhibit broad substrate specificity as they facilitate transport of a large variety of amphipathic organic compounds such as bile salts, lipid-lowering drugs, but also thyroid hormones (TH) (2, 3, 4, 5, 6). In contrast to most OATPs, OATP1C1 shows high substrate specificity. It has been characterized as a high-affinity T4 and rT3 transporter with Michaelis constant (Km) values in the nanomolar range, whereas other OATPs display Km values for TH transport far above the serum TH concentration (2, 3, 7). Based on the expression of OATP1C1 in capillaries in multiple brain regions, this protein is thought to play an important role in delivering serum TH to the brain (7, 8). Additional support for a significant role for OATP1C1 in TH transport in the brain comes from a study from Sugiyama et al. (8), in which they show that OATP1c1 is up-regulated in hypothyroid rats and down-regulated in hyperthyroid rats.

In this study, we examined the specificity of iodothyronine transport by OATP1C1 in transfected COS1 cells and analyzed whether OATP1C1 is rate limiting in subsequent iodothyronine metabolism in cells cotransfected with different deiodinases. In addition, we studied the effect of genetic variation in the OATP1C1 gene because polymorphisms in different OATPs have been shown to alter the transport function of these proteins (9, 10). Identified polymorphisms were analyzed for association with serum thyroid parameters in two populations. In addition, the effect of the polymorphisms on the transport function of OATP1C1 was tested by analysis of uptake and metabolism of iodothyronines in cells transfected with OATP1C1 variants.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Plasmids and construction of OATP1C1 variants
Human µ-crystallin (pSG5-hCRYM), rat type 1 deiodinase (pcDNA3-rD1), human type 2 deiodinase (pcDNA3-hD2-rD1-SECIS), and human type 3 deiodinase (pCIneo-hD3) plasmids were constructed as previously described (11). An OATP1C1 IMAGE clone (IMAGE 4801171) was purchased from the German Resource Center for Genome Research (www.rzpd.de) and subcloned into pcDNA3.1– (Invitrogen, Breda, The Netherlands) using NotI and Acc65I restriction sites.

The Pro143Thr polymorphism was introduced into the OATP1C1 cDNA using the QuikChange site-directed mutagenesis protocol (Stratagene, Amsterdam, The Netherlands) with forward primer CAA ATA TGA GAG ATA TTC TAC TTC CTC CAA TTC CAC TCT CAG C and the complementary reverse primer. The OATP1C1–3'-untranslated region (UTR) variants were constructed as follows: a PCR was performed on pcDNA3.1-OATP1C1 with forward primer 5'-TGTGTGGAGCTGCAAAACTC-3' and reverse primer 5'-TGCAAAATG TCAACCAATTAGAAG-3', generating a PCR product of 926 bp. A second PCR was performed on genomic DNA from a subject homozygous for the wild-type or for the variant 3'UTR with forward primer 5'-TGGGCACAGTGTCAATTCTC-3' and reverse primer 5'-CGTCGTGTA TAAGTAGGAAGTTGC-3', generating a product of 1107 bp. The final sequence of 1791 bp was generated by a PCR of a 1:1 mixture of the above PCR products using the first forward primer and second reverse primer. This PCR fragment was cloned into pCR-Blunt II TOPO (Invitrogen), excised with BstEII and Acc65I and shuttled into pcDNA3.1, yielding pcDNA3.1-OATP1C1-WT-3'UTR or VA-3'UTR.

Cell culture
COS1 cells were cultured in 6- or 24-well dishes (Corning, Schiphol, The Netherlands) with DMEM/F12 medium (Invitrogen), containing 9% heat-inactivated fetal bovine serum (Invitrogen) and 100 nM sodium selenite (Sigma, St. Louis, MO).

Iodothyronine transport and metabolism
Materials.
[125I]NaI, [125I]T4, and [125I]T3 were obtained from Amersham Biosciences (Little Chalfont, Buckinghamshire, UK). [125I]rT3 was purchased from PerkinElmer (Boston, MA), and unlabeled T4, T3, and rT3 from Henning GmbH (Berlin, Germany). Unlabeled and 125I-labeled T4 sulfate (T4S) and T3 sulfate (T3S) were synthesized as described previously (12). FuGENE6 transfection reagent was obtained from Roche Diagnostics (Indianapolis, IN).

Uptake studies.
COS1 cells were cultured in 6-well plates and cotransfected with 500 ng empty pcDNA3.1 or pcDNA3.1-OATP1C1-Pro143, Thr143, WT-3'UTR, or VA-3'UTR plus 500 ng pSG5-hCRYM, coding for a high-affinity cytosolic TH-binding protein that prevents efflux of internalized iodothyronines (13). This protein binds not only T4 and T3 but also T4S and T3S [Visser, W. E., personal communication (14)]. However, the affinity of hCRYM for rT3 and rT3 sulfate is much lower, compared with the other iodothyronines.

After 24 h culturing, cells were washed with incubation medium (DMEM-F12 with 0.1% BSA) and incubated for 15, 30, or 60 min at 37 C with 1 nM (2 x 105 cpm) [125I]T4, [125I]T3, [125I]rT3, [125I]T4S, or [125I]T3S in 1.5 ml incubation medium. After incubation, cells were harvested and analyzed as described previously (11). For determination of kinetic parameters, cells were incubated with 0.01–1 µM [125I]T4 or 0.1–5 µM [125I]T4S. Data were corrected for background uptake in cells transfected with empty vector instead of OATP1C1.

Metabolism studies.
COS1 cells were cultured in 24-well dishes and transfected with 100 ng empty vector plus 100 ng rD1, hD2, or hD3 plasmid or with 100 ng deiodinase plus 100 ng OATP1C1 plasmid. After 24 h culturing, cells were incubated for 24 h at 37 C with 1 nM (1 x 106 cpm) 125I-labeled T4, rT3, or T4S in 0.5 ml incubation medium. After incubation, medium was harvested and analyzed by HPLC as described previously (11). Production of the metabolites is presented as percentage of total radioactivity in the medium, which represents greater than 90% of added radioactivity.

Identification of polymorphisms and haplotypes in the OATP1C1 gene
The structure of the OATP1C1 gene was determined using the National Center for Biotechnology Information gene database (Bethesda, MD). The polymorphisms in the region were obtained from databases of the International Hapmap Project (http://www.hapmap.org) (15) and the National Center for Biotechnology Information dbSNP (http://www.ncbi.nlm.nih.gov). In addition, DNA of 25 randomly selected Caucasian subjects was used for sequence analysis of the complete coding region, including intron/exon boundaries, of the OATP1C1 gene to verify polymorphisms found in different databases and to identify novel polymorphisms. Primers used for amplification and sequencing are available on request.

The haploblock structure of the gene was determined using Haploview (version 3.32) (16) according to the method of Gabriel et al. (17). A haploblock is a set of statistically associated polymorphisms, and Haploview is a program designed to analyze and visualize patterns of linkage disequilibrium, i.e. nonrandom associations between polymorphisms.

Study populations
We analyzed a population of 154 healthy blood donors from the Sanquin Blood Bank South West region (Rotterdam, The Netherlands) (18). Informed consent was given by all donors. Serum TSH, T4, T3, and free T4 (FT4) were determined as described previously (18). T4S was determined by a specific RIA (19).

The second population consisted of participants of a nationwide project (GEMINAKAR) investigating the relative influence of genetic and environmental factors on a variety of traits among Danish twins. Based on a questionnaire survey concerning physical health and health-related behavior performed in 1994, a representative sample of twin pairs was recruited from the population-based Danish Twin Registry (20). A detailed description of the ascertainment procedure can be found elsewhere (21, 22, 23). In the GEMINAKAR study, 1512 individuals (756 twin pairs) were examined. Blood samples for thyroid measurements and genotype information were available in 1266 individuals, distributed in 554 monozygotic (MZ), 474 same sex dizygotic, and 238 opposite sex twin individuals. In the MZ twin pairs, in which only one of the twins was genotyped, we assumed identical genotypes. Twin pairs in which one or both twins had self-reported thyroid disease (22 twin pairs) or biochemical thyroid disease (15 twin pairs) were excluded, leaving 1192 (524 MZ, 442 dizygotic, and 226 opposite sex twin individuals). Serum thyroid parameters were determined as described previously (21, 24). Written informed consent was obtained from all participants, and the study was approved by all regional Danish scientific-ethical committees (case file 97/25 PMC).

In the case of significant associations, the use of twins would allow us to assess the contribution of these polymorphisms to the trait variation and the genetic variance.

Genotyping
The polymorphisms OATP1C1-intron3C>T (rs10770704), OATP1C1-Pro143Thr (rs36010656), and OATP1C1-C3035T (rs10444412) were determined by 5'fluorogenic Taqman assays. Reactions were performed in 384-wells format on ABI9700 PCR machines with end point reading on the ABI 7900HT Taqman machine (Applied Biosystems, Nieuwerkerk a/d IJssel, The Netherlands) (25).

Statistical analysis
Data were analyzed using SPSS 10.0.7 for Windows (SPSS, Inc., Chicago, IL) and STATA statistical software (STATA Corp., College Station, TX). P values are two sided throughout and were considered significant if P < 0.05.

For the in vitro experiments, unpaired Student’s t tests were used to test whether iodothyronine uptake and metabolism induced by wild-type OATP1C1 was significantly different from cells transfected with variant OATP1C1 or empty vector.

The polymorphisms were analyzed for deviation from Hardy-Weinberg equilibrium (HWE) proportions using a {chi}2 test. In the cohort of blood donors, the distribution of serum TSH was skewed and therefore transformed by the natural logarithm. Differences between genotypes were adjusted for age and gender and tested by analysis of covariance. In case of an allele dose effect for either one of the polymorphisms, a linear regression analysis was performed to quantify the association.

In the Danish twins, associations between the OATP1C1-C3035T polymorphism and serum thyroid parameters were assessed using regression analysis. The statistical inference measures were computed using a technique (the cluster option in STATA) that takes the dependency of twin data into account. The genotype information was incorporated into the regression analyses by coding the noncarriers as 0, heterozygous carriers as 1, and homozygote carriers as 2. Due to nonnormal distribution of most of the serum, parameters were transformed by the natural logarithm, and adjustment for age and gender was performed.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Transport of (sulfated) iodothyronines by OATP1C1
Incubation of OATP1C1-transfected COS1 cells with [125I]T4 resulted in a significant stimulation of the uptake of this iodothyronine over cells transfected with empty vector (Fig. 1AGo). In addition, cellular uptake of [125I]rT3 was induced in cells transfected with OATP1C1, compared with mock transfected cells; however, this failed to reach significance (Fig. 1AGo). The low affinity of rT3 for CRYM may at least in part explain why its uptake was lower than that of T4 and not linear with incubation time (13). Cellular uptake of [125I]T3 was not stimulated after transfection with OATP1C1 (Fig. 1AGo). The high basal [125I]T3 uptake in cells transfected with empty vector and CRYM is probably due to T3 uptake by endogenous transporters in COS1 cells.


Figure 1
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  		FIG. 1. A, [125I]T4, [125I]rT3, [125I]T3, [125I]T4S, and [125I]T3S uptake by COS1 cells transfected with empty vector or OATP1C1. Cells were cotransfected with CRYM, an intracellular TH-binding protein. Cells were incubated for 15, 30, or 60 min at 37 C with 1 nM (2 x 105 cpm) 125I-labeled T4, rT3, T3, T4S, or T3S. Data are expressed as percentage uptake of added radioactivity. Results are the means ± SEM of at least two experiments. *, P < 0.05, **, P < 0.01 of cells transfected with OATP1C1 vs. mock-transfected cells. B, Ligand concentration-dependent uptake of T4 and T4S by OATP1C1-transfected COS1 cells. Cells were incubated for 30 min with 0.01–1 µM [125I]T4 or 0.1–5 µM [125I]T4S. Data were corrected for background uptake in cells transfected with empty vector instead of OATP1C1. Curve fitting was performed using the Michaelis-Menten equation, v = Vmax/(1+Km/S). Results are the means ± SEM of four to six observations.

 
In addition to T4, rT3, and T3, OATP1C1-transfected cells were tested for uptake of sulfated iodothyronines T4S and T3S. OATP1C1 did not facilitate uptake of [125I]T3S. However, [125I]T4S uptake was increased in OATP1C1-transfected cells, compared with mock transfected cells (Fig. 1AGo). In addition, the saturation kinetics of T4 and T4S uptake by OATP1C1 were studied by incubation of OATP1C1-transfected COS1 cells during 30 min with 0.01–1 µM T4 or 0.1–5 µM T4S. The results are presented in Fig. 1BGo, showing that both T4 and T4S uptake were saturable with apparent Km values of 0.12 µM for T4 and 2.6 µM for T4S. maximal velocity (Vmax) values were similar for T4 and T4S (0.25 vs. 0.40 nmol/min).

Iodothyronine metabolism in cells cotransfected with OATP1C1 and deiodinases
Incubation of cells transfected with deiodinase (D)-2 or D3 led to some conversion of T4. However, this was markedly and significantly increased, when cells were cotransfected with OATP1C1 (Fig. 2Go).


Figure 2
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  		FIG. 2. [125I]T4, [125I]rT3, and [125I]T4S metabolism by COS1 cells transfected with either D1, D2, or D3 alone or together with OATP1C1. Cells were incubated for 24 h at 37 C with 1 nM (1 x 106 cpm) 125I-labeled T4, rT3, or T4S. Metabolism is shown as percentage of metabolites in the medium after 24 h incubation. Results are the means ± SEM of at least three experiments. *, P < 0.05; **, P < 0.01 of cells cotransfected with OATP1C1 and deiodinase vs. cells cotransfected with empty vector and deiodinase.

 
Based on the transport assays, it is not entirely clear whether OATP1C1 transports [125I]rT3 in a significant manner. However, metabolism of rT3 by either D1 or D2 was increased 2-fold in cells cotransfected with OATP1C1 (Fig. 2Go). In addition, T4S metabolism by D1 was increased 10-fold in cells cotransfected with OATP1C1 (Fig. 2Go).

Identification of polymorphisms in the OATP1C1 gene
Sequence analyses of 25 Caucasian subjects revealed 13 polymorphisms, among which one novel sequence variation in the OATP1C1 gene (Fig. 3Go). For this study, we preferentially selected nonsynonymous polymorphisms or polymorphisms in the 3'UTR. One nonsynonymous polymorphism, OATP1C1-Pro143Thr, and four polymorphisms in the 3'UTR were selected. By analyzing the linkage disequilibrium block structure across the gene from the Hapmap data (phase II, release 22) using the Haploview program (16), the identified polymorphisms in the 3'UTR were found to be part of a haploblock spanning from exon 10 to the end of the 3'UTR in exon 15. Therefore, the C3035T polymorphism was genotyped as a tagging polymorphism for the other polymorphisms in this haploblock. In an attempt to cover the remainder of the gene, a C>T polymorphism in intron 3 (rs10770704) was genotyped as a tagging polymorphism for the first haploblock encompassing the first four exons (Fig. 3Go).


Figure 3
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  		FIG. 3. Schematic overview of the OATP1C1 gene (chromosome 12p12.2). Nucleotide numbers are based on GenBank accession no. NM 017435.2. The vertical black boxes represent the 15 exons of the OATP1C1 gene, and the vertical lines represent the polymorphisms found by direct sequencing of 25 Caucasian blood donors. The numbers between parentheses represent the minor allele frequencies of the different polymorphisms. The horizontal lines above the gene structure are a schematic representation of the three linkage disequilibrium (LD) blocks according to Hapmap data (Hapmap release II, phase 21).

 
OATP1C1 polymorphisms in healthy blood donors
The baseline characteristics of the population of blood donors are shown in Table 1Go. One subject failed genotyping for the OATP1C1-intron3C>T polymorphism. All polymorphisms were in Hardy-Weinberg equilibrium.


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  		TABLE 1. Baseline characteristics of the study populations

 
The OATP1C1-intron3C>T polymorphism was not associated with serum thyroid parameters (data not shown). For the OATP1C1-Pro143Thr polymorphism, all serum thyroid parameters were similar between carriers and noncarriers, except for rT3, which was higher in carriers of the OATP1C1-Thr143 allele (Table 2Go). The OATP1C1-C3035T polymorphism was in a dose-dependent manner associated with a higher serum FT4 and a higher rT3 (Table 2Go).


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  		TABLE 2. Serum thyroid parameters by OATP1C1 genotypes in a population of Dutch blood donors

 
OATP1C1 polymorphisms in healthy Danish twins
Based on the results of the association analysis of the OATP1C1 polymorphisms in the cohort of blood donors, we genotyped only the OATP1C1-C3035T polymorphism in the population of Danish twins. The baseline characteristics of this population are shown in Table 1Go. The allele frequency of the OATP1C1-C3035T polymorphism was 41.7%, which is almost 6% lower than the frequency observed in the population of healthy blood donors.

In contrast to our findings in the blood donors, no significant differences in serum thyroid parameters were observed between subjects carrying no, one, or two copies of this polymorphism (Table 3Go). Therefore, we refrained from assessing the contribution of this polymorphism to the variation in serum thyroid parameters and the genetic variance.


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  		TABLE 3. Serum thyroid parameters by OATP1C1-C3035T genotype in a population of Danish twins

 
Effect of OATP1C1 polymorphisms in vitro
The in vitro effect of OATP1C1-Pro143Thr and the polymorphisms in the 3'UTR was tested by transfecting COS1 cells with these variants and analyzing them for differences in T4 and rT3 uptake and metabolism from wild-type OATP1C1. For OATP1C1-Pro143Thr, no significant differences in uptake and metabolism of T4, compared with wild-type OATP1C1, were observed (Fig. 4Go, A and B). In addition, no difference in T4 uptake and T4 metabolism was observed between cells transfected with OATP1C1-WT-3'UTR or VA-3'UTR (Fig. 4Go, A and B). Neither could we detect any differences in rT3 uptake and metabolism between the different OATP1C1 variants (data not shown).


Figure 4
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  		FIG. 4. A, Uptake of [125I]T4 by COS1 cells transfected with empty vector or one of the OATP1C1 variants. Cells were cotransfacted with CRYM. Cells were incubated for 15, 30, or 60 min at 37 C with 1 nM (2 x 105 cpm) [125I]T4. Data are expressed as percentage uptake of added radioactivity. Results are the means ± SEM of at least three experiments. No significant differences in T4 uptake were observed between cells transfected with wild-type OATP1C1 and cells transfected with wild-type OATP1C1 and cells transfected with variant OATP1C1. B, Metabolism of [125I]T4 by COS1 cells transfected with either D2 or D3 alone or together with one of the OATP1C1 variants. Cells were incubated for 24 h at 37 C with 1 nM (1 x 106 cpm) 125I-labeled T4. Metabolism is shown as percentage of metabolites in the medium after 24 h incubation. Results are the means ± SEM of at least three experiments. No significant differences in T4 metabolism were observed between cells cotransfacted with wild-type OATP1C1 and deiodinase vs. cells cotransfacted with variant OATP1C1 and deiodinase.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
In this study we demonstrated that T4, T4S, and to some extent rT3 uptake was induced in cells transfected with OATP1C1, compared with mock transfected cells. In addition, metabolism of T4, T4S, and rT3 by transfected deiodinases was markedly stimulated in the presence of OATP1C1. Although, the OATP1C1-Pro143Thr and OATP1C1-C3035T polymorphisms were associated with serum thyroid parameters in 155 blood donors, we could not replicate these findings in a much larger cohort of Danish twins. Nor did we observe any difference in uptake and metabolism of T4 and rT3 between these variants and wild-type OATP1C1.

Based on its lipophilic structure, it was assumed that thyroid hormone enters the cell through passive diffusion (26). However, with the discovery of monocarboxylate transporter-8 as a specific thyroid hormone transporter and the fact that mutations in this transporter lead to the Allan-Herndon-Dudley syndrome (OMIM 300523), the interest in this area of research has greatly increased (27, 28, 29). In this paper we focused on OATP1C1, which was first described by Pizzagalli et al. (7) as a specific TH transporter. OATP1C1 is expressed in multiple brain regions and the testis. In line with findings by Pizzagalli et al., we found a clear induction of T4 uptake and to some extent also of rT3 uptake by cells transfected with OATP1C1. Furthermore, metabolism of T4 and rT3 by D1, D2, or D3 was markedly increased if OATP1C1 was cotransfected. This demonstrates that OATP1C1 expression is rate limiting in iodothyronine metabolism by the deiodinases. It thus supports the hypothesis that OATP1C1 indeed increases the intracellular availability of these iodothyronines because the deiodinases are membrane proteins with their active sites located intracellularly (35).

It should be noted that cotransfection with deiodinases was done to monitor the increase in intracellular iodothyronine concentration by OATP1C1 rather than to mimic particular tissue cells. OATP1C1 is probably coexpressed only with D2 in vivo. Both OATP1C1 and D2 are expressed in tanycytes that line the third ventricle (44). Conversion of T4 to T3 in these cells plays an important role in the negative feedback of thyroid hormone at the hypothalamus. OATP1C1 and D2 are also coexpressed in the testis, which may also facilitate local T3 formation in this tissue (30).

We further extended the findings from Pizzagalli et al. (7) by showing that OATP1C1 also facilitates transport and subsequent metabolism of T4S but not T3S. The serum concentrations of T4S and T3S are low under normal conditions because sulfation of TH accelerates its degradation by D1 (19, 31, 32, 33, 34, 35). However, serum iodothyronine sulfate levels are high in preterm infants and during critical illness (31, 33, 36). Under these conditions, the sulfates might serve as a reservoir of inactive, from which active TH can be recruited when necessary. It is therefore interesting that T4S is a ligand for OATP1C1-mediated transport. Because Oatp1c1 is localized on both the luminal and abluminal membrane of brain capillaries in rats and mice, this protein could play a role in uptake as well as efflux of T4S across the blood-brain barrier (8). During embryonic development, uptake of T4S across the blood-brain barrier by OATP1C1 may serve as source of T4 for the brain after local hydrolysis by sulfatases. OATP1C1 might, however, also serve as an export pump for T4S. Several sulfotransferases are expressed in different brain regions (37, 38). Among these are the most potent iodothyronine sulfotransferases SULT1A1 and SULT1E1 (39, 40). In addition to the action of the deiodinases, these enzymes might tightly regulate T4 levels in different brain regions.

The Michaelis-Menten transport kinetics of T4 and T4S transport by OATP1C1 were determined. Both were saturable with apparent Km values of 0.12 µM for T4 and 2.6 µM for T4S. It is important to realize that these are approximate values because in addition to OATP1C1, binding of T4 and T4S to BSA and CRYM are also saturable processes. Nevertheless, our Km value for T4 is in good agreement with that previously reported by Pizzagalli et al. (90 nM) (7).

Based on the function of OATP1C1, polymorphisms in the OATP1C1 gene might be associated with T4, T4S, and rT3 levels (7). Because the serum FT4 concentration (~15 pM) is orders of magnitude lower than the apparent Km value (~100 nM), the rate of T4 transport by OATP1C1 in vivo is linearly dependent on the Vmax to Km ratio. Polymorphisms that change the Vmax or Km value would directly affect tissue T4 uptake by OATP1C1.

Carriers of the OATP1C1-Pro143Thr polymorphism indeed had higher rT3 levels than noncarriers in a population of healthy blood donors. Furthermore, the OATP1C1-C3035T polymorphism was in a dose-dependent manner associated with higher serum FT4 and serum rT3 in this same cohort. In contrast to our findings in the Dutch blood donors, no significant differences in serum thyroid parameters were observed between carriers of no, one, or two copies of the OATP1C1-C3035T polymorphism in the Danish twins. In addition, in a third population of approximately 1000 Caucasian subjects, the OATP1C1-C3035T polymorphism was not significantly associated with serum TH levels (van der Deure, W. M., R. P. Peeters, and T. J. Visser, unpublished data).

Furthermore, uptake and metabolism of T4 and rT3 by COS1 cells transfected with the OATP1C1 variants was similar to cells transfected with cDNA coding for the wild-type form of the OATP1C1 protein. It is therefore likely that our initial findings in the blood donors were chance findings, which might be caused by the small sample size of this cohort (41). However, differences in age and environmental factors such as iodine intake between the two populations might also in part explain the inconsistent findings.

Although we did not observe an effect of polymorphisms in the OATP1C1 gene on serum thyroid parameters, this might not rule out local effects of genetic variation. For instance, the Thr92Ala polymorphism in D2 is not associated with serum thyroid parameters or differences in iodothyronine metabolism in transiently transfected COS1 cells (18) but has been associated with insulin resistance in different populations (42, 43). Therefore, we cannot exclude that polymorphisms in the OATP1C1 gene could be associated with brain-related phenotypes, such as depression or cognition.

In conclusion, our findings indicate that OATP1C1 mediates plasma membrane transport of T4, T4S, and rT3 and is rate limiting in iodothyronine metabolism by the deiodinases. Polymorphisms in the OATP1C1 gene are not associated with serum thyroid parameters, nor do they alter the transport function of OATP1C1.


   Acknowledgments
 
We thank T. I. A. Sørensen for his help in exploitation of the Danish twins data and M. Fenger for his help in making several of the analysis of the Danish twin samples. PerkinElmer/Wallac (Turku, Finland) kindly provided the kits for determination of thyroid peroxidase antibodies, Tgab, TSH, and FT4. We are indebted to Ole Blaabjerg and Esther Jensen for supervising the biochemical thyroid analyses. We thank Ronald van der Wal and Bianca de Graaf for help with the sequence analysis. We thank Wim Klootwijk and Edward Visser for excellent assistance and synthesis of the iodothyronine sulfates.


   Footnotes
 
This work was supported by The Netherlands Organization of Scientific Research Institute for Diseases in the Elderly Grants 6730040 (to W.M.v.d.D.) and 014-93-015; the Danish Research Agency; the Foundation of 17-12-1981; the Agnes and Knut Mørk Foundation; the Novo Nordisk Foundation; the Foundation of Medical Research in the County of Funen; Else Poulsens Mindelegat; the Foundation of Direktør Jacob Madsen and Hustru Olga Madsen; the Foundation of Johan Boserup and Lise Boserup; the A. P. Møller and Hustru Chastine McKinney Møllers Foundation; the A. P. Møller Relief Foundation; and the Clinical Research Institute, Odense University.

Disclosure Statement: W.M.v.d.D., P.S.H., R.P.P., K.O.K., E.C.H.F., L.H., and T.J.V. have no conflicts of interest.

First Published Online June 19, 2008

Abbreviations: D, Deiodinase; FT4, free T4; HWE, Hardy-Weinberg equilibrium; Km, Michaelis constant; MZ, monozygotic; OATP, organic anion transporting polypeptide; TH, thyroid hormone; T3S, T3 sulfate; T4S, T4 sulfate; UTR, untranslated region; Vmax, maximal velocity.

Received March 28, 2008.

Accepted for publication June 12, 2008.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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