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Endocrinology, doi:10.1210/en.2008-0169
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Endocrinology Vol. 149, No. 9 4695-4701
Copyright © 2008 by The Endocrine Society

Organic Anion Transporter 1B1: An Important Factor in Hepatic Thyroid Hormone and Estrogen Transport and Metabolism

Wendy M. van der Deure, Edith C. H. Friesema, Frank Jan de Jong, Yolanda B. de Rijke, Frank H. de Jong, André G. Uitterlinden, Monique M. B. Breteler, Robin P. Peeters and Theo J. Visser

Departments of Internal Medicine (W.M.v.d.D., E.C.H.F., F.H.d.J., A.G.U., R.P.P., T.J.V.), Epidemiology and Biostatistics (F.J.d.J., A.G.U., M.M.B.B.), Neurology (F.J.d.J.), and Clinical Chemistry (Y.B.d.R., A.G.U.), Erasmus University Medical Center, 3015 GE Rotterdam, The Netherlands

Address all correspondence and requests for reprints to: Theo J. Visser, Ph.D., Erasmus University Medical Center, Department of Internal Medicine, 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
 
Sulfation is an important pathway in the metabolism of thyroid hormone and estrogens. Sulfation of estrogens is reversible by estrogen sulfatase, but sulfation of thyroid hormone accelerates its degradation by the type 1 deiodinase in liver. Organic anion transporters (OATPs) are capable of transporting iodothyronine sulfates such as T4 sulfate (T4S), T3S, and rT3S or estrogen sulfates like estrone sulfate (E1S), but the major hepatic transporter for these conjugates has not been identified. A possible candidate is OATP1B1 because model substrates for this transporter include the bilirubin mimic bromosulfophthalein (BSP) and E1S, and it is highly and specifically expressed in liver. Therefore, OATP1B1-transfected COS1 cells were studied by analysis of BSP, E1S, and iodothyronine sulfate uptake and metabolism. Two Caucasian populations (155 blood donors and 1012 participants of the Rotterdam Scan Study) were genotyped for the OATP1B1-Val174Ala polymorphism and associated with bilirubin, E1S, and T4S levels. OATP1B1-transfected cells strongly induced uptake of BSP, E1S, T4S, T3S, and rT3S compared with mock-transfected cells. Metabolism of iodothyronine sulfates by cotransfected type 1 deiodinase was greatly augmented in the presence of OATP1B1. OATP1B1-Val174 showed a 40% higher induction of transport and metabolism of these substrates than OATP1B1-Ala174. Carriers of the OATP1B1-Ala174 allele had higher serum bilirubin, E1S, and T4S levels. In conclusion, OATP1B1 is an important factor in hepatic transport and metabolism of bilirubin, E1S, and iodothyronine sulfates. OATP1B1-Ala174 displays decreased transport activity and thereby gives rise to higher bilirubin, E1S, and T4S levels in carriers of this polymorphism.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
SULFATION IS AN important pathway in the metabolism of different hormones, such as thyroid hormone and estrogens. This reaction is catalyzed by a group of cytosolic sulfotransferases (SULTs) located in different tissues such as the liver, kidney and brain (1). All members of the human SULT1 family catalyze the sulfation of thyroid hormone (2, 3, 4, 5). Sulfation of estrogen is, however, mainly carried out by SULT1E1 (1).

The serum concentrations of sulfated iodothyronines, such as T4 sulfate (T4S), T3S, and rT3S, are low under normal conditions because they are rapidly degraded by the type 1 deiodinase (D1) (6, 7, 8, 9, 10, 11). However, serum iodothyronine sulfate levels are high in preterm infants and during critical illness, possibly due to a decreased metabolism (7, 9, 12). Serum concentrations of estrone sulfate (E1S) are 10–20 times higher than those of the unconjugated estrogens, thereby reflecting the primary role of sulfation in estrogen metabolism (13). The formation of E1S serves as a reservoir for active estrogen, because the sulfation of estrogens is readily reversible by estrogen sulfatase (14).

Different organic anion transporters (OATPs) are capable of transporting iodothyronine and estrogen sulfates (15, 16, 17), but the major hepatic transporter for these conjugates has not yet been identified. A possible candidate is OATP1B1 because model substrates for this transporter include the bilirubin mimic bromosulfophthalein (BSP) and E1S, and it is highly and specifically expressed in liver (16, 18). Therefore, OATP1B1-transfected COS1 cells were studied by analysis of BSP, E1S, T4S, T3S, and rT3S uptake and metabolism.

Recently, various polymorphisms in the SLCO1B1 gene, encoding OATP1B1, have been identified (19). Especially, the OATP1B1-Val174Ala polymorphism is of interest, because the valine at position 174 is conserved in all members of the OATP1 subfamily and is located in a putative transmembrane region of OATP1B1, which is important for substrate recognition and transport (20) (Fig. 1Go). In addition, this polymorphism is associated with higher serum bilirubin levels due to an altered transport function of the protein (21). We therefore analyzed the effect of this polymorphism on the function of OATP1B1 in vivo and in vitro as a transporter for sulfated iodothyronines and estrogens.


Figure 1
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  		FIG. 1. A putative two-dimensional model of the human OATP1B1 protein with localization of valine to alanine amino acid change caused by the Val174Ala polymorphism in the SLCO1B1 gene (Chr 12p12) [Adapted with permission from R. G. Tirona et al. J Biol Chem276:35669–35675, 2001 (19 ). © American Society for Biochemistry and Molecular Biology.]

 

   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Constructs
The pCMV6-XL4-OATP1B1 plasmid was kindly provided by Prof. Dr. Peter J. Meier (Institute of Clinical Pharmacology and Toxicology, University Hospital Zürich, Zürich, Switzerland). The Val174Ala polymorphism was introduced into the OATP1B1 expression vector by site-directed mutagenesis according to the QuikChange site-directed mutagenesis protocol of Stratagene (Amsterdam, The Netherlands). The sense and antisense primers containing the nucleotide change needed to produce the OATP1B1-Ala174 variant (sense 5'-CTG GGT CAT ACA TGT GGA TAT ATG CGT TCA TGG GTA ATA TGC-3') were used in circular mutagenesis reactions with 100 ng plasmid template and 1 U Pfu Turbo high-fidelity DNA polymerase. The construct with the OATP1B1-Val174Ala polymorphism was fully sequenced on an automated ABI 3100 capillary sequencer, using the Big Dye terminator cycle sequencing method (Applied Biosystems, Nieuwerkerk aan den Ijssel, The Netherlands) to verify that the desired mutation had been generated and that no other mutations had occurred during amplification.

Human µ-crystallin (CRYM) cDNA was purchased from the RZPD German Resource Center for Genome Research (www.rzpd.de) and subcloned into pSG5 (Stratagene) using EcoRI and BamHI sites. pcDNA3-hMCT8 and pcDNA3-ratD1 (rD1) plasmids were constructed as previously described (22).

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

BSP, E1S, and iodothyronine (sulfate) transport and metabolism by OATP1B1
Materials.
FuGENE6 transfection reagent was obtained from Roche Diagnostics (Indianapolis, IN). Na125I, [125I]T4, and [125I]T3 were obtained from Amersham Biosciences (Little Chalfont, Buckinghamshire, UK). [125I]rT3 and [3H]E1S were purchased from PerkinElmer (Boston, MA) and unlabeled T4, T3, rT3 and E1S from Henning GmbH (Berlin, Germany). BSP was obtained from Fluka Chemica (Buchs, Switzerland). [125I]BSP was prepared by radioiodination of BSP using the chloramine-T method and purified on a small (~1 ml bed volume) Sephadex LH20 column by successive rinsing with 0.1 M HCl and water and final elution with 0.1 M NH4OH in ethanol. The purity of [125I]BSP was checked by HPLC analysis. For [125I]T4S, [125I]T3S, and [125I]rT3S synthesis, solutions of [125I]T4, [125I]T3, or [125I]rT3 were evaporated to complete dryness under a stream of N2. Ice-cold chlorosulfonic acid (0.2 ml) in dimethylformamide (1:4, vol/vol) was added to the residues. The mixtures were incubated for 2 h at 37 C, and the reactions were stopped by adding 0.8 ml ice-cold water. Sulfates thus synthesized were separated on a Sephadex LH-20 column (23). The final purified preparations contained virtually no contamination from free iodide or native hormone as checked by HPLC analysis.

Uptake studies.
COS1 cells were cultured in six-well culture dishes and transfected in duplicate with 500 ng pCMV6-XL4 plasmid without or with OATP1B1-Val174 or Ala174 insert using FuGENE6 according to the manufacturer’s guidelines. After 36 h, cells were washed with incubation medium (Dulbecco’s PBS, 0.1% BSA, and 0.1% glucose) and incubated for 5, 10, or 30 min at 37 C with 1 nM (2 x 105 cpm) 125I-labeled BSP, T4, T3, rT3, T4S, T3S, or rT3S in 1.5 ml incubation medium. After incubation, medium was aspirated and cells were washed with incubation medium, lysed with 0.1 M NaOH and counted for radioactivity in a {gamma}-scintillation counter. In the experiments with [3H]E1S, cells were washed with incubation medium (Dulbecco’s PBS with 0.1% glucose) and incubated for 1, 2, 5, or 10 min at 37 C with 100 nM [3H]E1S in 1.5 ml incubation medium. Cells were lysed with 0.1% sodium dodecyl sulfate and subsequently counted for radioactivity. Except for the experiments with [125I]BSP and [3H]E1S, uptake was determined in cells cotransfected with the high-affinity cytosolic thyroid hormone-binding protein CRYM to prevent efflux of internalized iodothyronines (24). For T4, T3, and rT3 uptake experiments, pcDNA3-MCT8 plasmid was used as positive control.

Metabolism studies.
COS1 cells were cultured in 24-well culture dishes and transfected with 100 ng pcDNA3-rD1 and 100 ng pCMV6-XL4 plasmid without or with OATP1B1-Val174 or Ala174 insert. Thirty-six hours after transfection, cells were incubated for 24 h at 37 C with 1 nM (1 x 106 cpm) [125I]T4S, T3S, or rT3S in 0.5 ml incubation medium. After incubation, medium was harvested and analyzed by HPLC as described previously (22).

Study populations
Genotypes of the OATP1B1-Val174Ala polymorphism were determined in 158 healthy blood donors from the Sanquin Blood Bank South West region (Rotterdam, The Netherlands) (25). Informed consent was given by all donors.

Participants of the Rotterdam Scan Study, an ongoing prospective population-based cohort study designed to study causes and consequences of age-related brain changes on MRI (26), were also genotyped. Data on serum thyroid parameters and the use of thyroid medication were available in 1045 participants. E1S levels were determined in a subgroup of 430 participants. The Rotterdam Scan Study was conducted in accordance with the tenets of the Declaration of Helsinki. The Medical Ethics Committee of the Erasmus University approved the study.

Serum analyses
Total bilirubin was determined by the Jendrassik-Grof diazo method on the Hitachi 912 (Roche Diagnostics, Mannheim, Germany), and E1S was measured by a RIA from Diagnostic Systems Laboratories (Webster, TX). TSH, free T4 (FT4), and T3 were measured by chemiluminescence assays (Vitros ECI Immunodiagnostic System; Ortho-Clinical Diagnostics, Amersham, Rochester, NY). rT3 was measured with an in-house RIA (27), whereas T4S was determined by a specific RIA as previously described (6). T4S data were available only in healthy blood donors, whereas E1S levels were only determined in participants of the Rotterdam Scan Study.

Genotyping
DNA was isolated from peripheral leukocytes by standard procedures. The OATP1B1-Val174Ala (rs4149056) genotypes were determined by 5'-fluorogenic TaqMan assays. Reactions were performed in 384-well format on ABI9700 two 384-well PCR machines with endpoint reading on the ABI 7900HT TaqMan machine (Applied Biosystems, Nieuwerkerk aan den IJssel, The Netherlands) (28). All healthy blood donors were successfully genotyped. In the Rotterdam Scan Study, DNA was available from 989 of the 1012 participants. The genotyping success rate was 98.8%, with 12 samples that failed genotyping.

Statistical analysis
Data were analyzed using SPSS 10.0.7 for Windows (SPSS, Inc., Chicago, IL). P values are two-sided throughout and were considered significant if less than 0.05.

For the in vitro experiments, unpaired Student’s t tests were used to test whether the iodothyronine uptake and metabolism induced by OATP1B1-Val174 was significantly different from that in cells transfected with OATP1B1-Ala174 or empty vector.

For the OATP1B1-Val174Ala polymorphism, deviation from Hardy-Weinberg equilibrium proportions was analyzed using a {chi}2 test. Differences between genotypes were adjusted for age and gender and tested by analysis of covariance. The analysis of the effect of the polymorphism on serum thyroid parameters was carried out after exclusion of subjects taking thyroid medication or any other drugs known to interfere with thyroid hormone metabolism and/or subjects with serum thyroid parameters indicating overt thyroid disease. Hypothyroidism was defined by a serum TSH concentration above the reference range (0.4–4.3 mU/liter) and FT4 below the reference range (11–25 pmol/liter). Hyperthyroidism was defined by serum TSH less than 0.4 mU/liter and FT4 more than 25 pmol/liter. Based on these exclusion criteria, three subjects were excluded in the population of healthy blood donors and 33 subjects in the Rotterdam Scan Study. Therefore, 155 healthy blood donors and 1012 subjects of the Rotterdam Scan Study were eligible for analysis. For the analysis of the effect of the OATP1B1-Val174Ala polymorphism on E1S levels, subjects on estrogen drug therapy (n = 6) were excluded, leaving 424 subjects eligible for analysis. Due to nonnormal distribution, bilirubin, E1S, TSH, and T4S levels were logarithmically transformed.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Transport of BSP, E1S, and (sulfated) iodothyronines by OATP1B1
Previously, BSP and E1S have been used as model substrates to characterize OATP1B1 (16, 18, 19). In line with results from earlier studies (16, 18, 19), uptake of BSP and E1S was strongly induced in cells transfected with OATP1B1 compared with mock-transfected cells. Maximum uptake of E1S was already obtained after 1 min, pointing toward a rapidly reached equilibrium between uptake and efflux of E1S in a short time period. Subsequently, the percent uptake remained the same even when cells were incubated for longer time periods.

Induction of BSP and E1S uptake was approximately 50% lower for the OATP1B1-Ala174 variant in comparison with the Val174 variant (Fig. 2Go).


Figure 2
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  		FIG. 2. [125I]BSP and [3H]E1S uptake by COS1 cells transfected with empty vector, OATP1B1-Val174, or OATP1B1-Ala174. Cells were incubated at 37 C with 1 nM [125I]BSP or 100 nM [3H]E1S for different time periods. Data are expressed as percent uptake of added radioactivity. Results are the means ± SEM of three experiments. a, P < 0.05 for OATP1B1-Val174 vs. OATP1B1-Ala174 and vs. empty vector; b, P < 0.05 for OATP1B1-Ala174 vs. empty vector.

 
For uptake experiments with (sulfated) iodothyronines, cells were cotransfected with CRYM (24). This protein binds not only iodothyronines but also sulfated iodothyronines (Visser, W. E., personal communication). Cotransfection of cells with CRYM and OATP1B1 did not result in significant uptake of [125I]T4 and [125I]T3, in contrast to cells cotransfected with CRYM and MCT8, a very active T3 and T4 transporter (29) (Fig. 3Go). In addition, OATP1B1-Val174 and OATP1B1-Ala174 showed little transport of [125I]rT3 in comparison with MCT8. However, uptake of the sulfated iodothyronines [125I]T4S, [125I]T3S, and [125I]rT3S was clearly induced in cells transfected with OATP1B1 (Fig. 4Go) compared with cells transfected with the empty vector. Induction of uptake of the iodothyronine sulfates was 50–70% lower for the Ala174 variant than for the Val174 variant.


Figure 3
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  		FIG. 3. [125I]T4, [125I]T3, and [125I]rT3 uptake by COS1 cells transfected with empty vector, OATP1B1-Val174, OATP1B1-Ala174, or MCT8. Cells were cotransfected with CRYM, an intracellular thyroid hormone-binding protein. Cells were incubated for 5, 10, or 30 min at 37 C with 1 nM (2 x 105 cpm) 125I-labeled T4, T3, or rT3. Data are expressed as percent uptake of added radioactivity. Results are the means ± SEM of three experiments. a, P < 0.05 for OATP1B1-Val174 vs. OATP1B1-Ala174 and vs. empty vector; b, P < 0.05 for OATP1B1-Ala174 vs. empty vector.

 

Figure 4
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  		FIG. 4. [125I]T4S, [125I]T3S, and [125I]rT3S uptake by COS1 cells transfected with empty vector, OATP1B1-Val174, or OATP1B1-Ala174. Cells were cotransfected with CRYM. Cells were incubated for 5, 10, or 30 min at 37 C with 1 nM [125I]T4S or T3S. Data are expressed as percent uptake of added radioactivity. Results are the means ± SEM of three experiments.

 
Iodothyronine sulfate metabolism in cells cotransfected with OATP1B1 and D1
T4S, T3S, and rT3S metabolism by rD1 was markedly increased when cells were cotransfected with OATP1B1 (Fig. 5Go). About 20% of T4S and T3S was deiodinated by rD1 when OATP1B1-Val174 was cotransfected, whereas no iodide was produced in cells transfected with rD1 alone. Only 3% of rT3S was deiodinated in D1-transfected cells, whereas almost 60% of rT3S was deiodinated by D1 in the presence of OATP1B1. Induction of T4S, T3S and rT3S metabolism was 40–50% lower for the OATP1B1-Ala174 than for the Val174 variant (Fig. 5Go).


Figure 5
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  		FIG. 5. [125I]T4S and [125I]T3S metabolism by COS1 cells transfected with either rD1 alone or rD1 together with OATP1B1-Val174 or OATP1B1-Ala174. Cells were incubated for 24 h at 37 C with 1 nM (1 x 106 cpm) [125I]T4S or T3S. Metabolism is shown as percentage of metabolites in the medium after 24 h incubation. Results are the means ± SEM of four experiments with T4S and three experiments with T3S and rT3S. a, P < 0.05 for OATP1B1-Val174 vs. OATP1B1-Ala174 and vs. empty vector; b, P < 0.05 for OATP1B1-Ala174 vs. empty vector.

 
Association of OATP1B1-Val174Ala with serum bilirubin and hormone levels
Table 1Go shows the characteristics of the population of blood donors and of the subjects participating in the Rotterdam Scan Study. In the cohort of healthy blood donors, 103 subjects were wild-type, 48 subjects were heterozygous, and three subjects were homozygous for the OATP1B1-Val174Ala polymorphism, resulting in a minor-allele frequency of 17.7%. In the Rotterdam Scan Study, the genotype distribution was quite similar with 714 wild-type, 236 heterozygous, and 27 homozygous subjects. In this cohort, the minor allele frequency was 14.8%, which is similar to previous studies in Caucasians (19). As only few homozygous subjects were identified, we combined heterozygotes and homozygotes into variant carriers.


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  		TABLE 1. Baseline characteristics of the study populations

 
In line with the in vitro findings, we found that bilirubin levels were 20% higher in carriers of the OATP1B1-Ala174 allele compared with noncarriers (Table 2Go). Moreover, carriers had almost 40% higher E1S and 24% higher T4S than noncarriers.


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  		TABLE 2. Serum parameters by OATP1B1-Val174Ala genotype in healthy blood donors and in participants of the Rotterdam Scan Study

 
Carriers of the Ala174 allele had higher rT3 levels in the cohort of healthy blood donors; however, this association failed to reach significance in the larger population of elderly Caucasians (Table 2Go). There was no association of the OATP1B1-Val174Ala polymorphism with serum TSH, FT4, and T3 levels (Table 2Go).


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
In this study, we demonstrate that transfection of COS1 cells with OATP1B1 strongly induces uptake of BSP, E1S, and the iodothyronine sulfates T4S, T3S, and rT3S compared with mock-transfected cells. Metabolism of iodothyronine sulfates by rD1 is greatly stimulated in the presence of OATP1B1. Moreover, OATP1B1-Val174 showed a 40% higher induction of transport and metabolism of these substrates than OATP1B1-Ala174. These in vitro findings were supported by in vivo results showing that carriers of the OATP1B1-Ala174 allele had higher serum bilirubin, E1S, and T4S levels.

Sulfation is an important pathway in both thyroid hormone and estrogen metabolism. The physiological role of sulfated iodothyronines is not completely clear. The main function is probably the irreversible degradation of thyroid hormone by D1 (10). However, iodothyronine sulfates might also represent a reservoir of reversibly inactivated thyroid hormone when they are not degraded by D1, for instance in the fetus and during severe illness (30). Serum supply of estrogen sulfates and desulfation by estrogen sulfatase, on the other hand, plays an important role in the local formation of estrogens in different tissues (1, 14).

Although different OATPs are capable of sulfate conjugate transport, the major hepatic transporter for these conjugates has not been identified. In this study, we showed that transfection of cells with OATP1B1 induced an approximately 20-fold increased uptake rate of BSP. Moreover, OATP1B1-transfected cells showed considerable uptake of E1S, similar to previously published findings regarding uptake of these substrates (16). In the same way, transport of T4S, T3S, and rT3S was greatly stimulated by OATP1B1 expression. Likewise, metabolism of these iodothyronine sulfates by D1 showed a marked increase when OATP1B1 was cotransfected. OATP1B1-Val174 showed a 40% higher induction of transport and metabolism of these substrates than OATP1B1-Ala174. However, OATP1B1 did not induce T3 or T4 uptake, and we only found a minor increase in rT3 uptake. This is in contrast with findings published by Abe et al. (16), who showed T3 and T4 uptake in Xenopus oocytes injected with OATP1B1 cRNA. The discrepancy between their and our findings might be explained by differences in expression efficiency of the two systems, or the higher intracellular thyroid hormone-binding capacity of Xenopus oocytes compared with COS1 cells. Therefore, we cotransfected with CRYM, a high-affinity cytosolic thyroid hormone-binding protein (24), which has recently been shown to greatly augment net T3 and T4 uptake by inhibition of T3 and T4 efflux (31). Nevertheless, no significant uptake of T3 and T4 by OATP1B1 could be detected in the presence of CRYM either. Taken together, we conclude that in COS1 cells, OATP1B1 has no T4 and T3 and only little rT3 transport activity but does facilitate transport of iodothyronine sulfates and the previously used model substrates E1S and BSP. However, in a different transport assay system, OATP1B1-specific T4 and T3 transport has been demonstrated (16).

The physiological relevance of this transporter in vivo is underlined by the observation that carriers of the OATP1B1-Ala174 allele had higher bilirubin, E1S, and T4S levels than noncarriers. This is in line with the in vitro data showing that uptake and metabolism of these substrates by the OATP1B1-Ala174 variant was less efficient compared with the OATP1B1-Val174 variant. Previously, Tirona and colleagues (19) have also shown that the OATP1B1-Ala174 variant shows decreased uptake of E1S and estradiol-17β-glucuronide. A possible explanation for this altered transport function comes from a study performed by Kameyama and colleagues (32), who showed by immunocytochemistry that OATP1B1-Ala174 protein is localized not only at the plasma membrane but also in the intracellular space, whereas OATP1B1-Val174 variant is only expressed at the plasma membrane. Therefore, the decreased activity of OATP1B1-Ala174 variant could be explained by a sorting error, resulting in decreased expression at the plasma membrane and thus decreased transport activity. In addition, kinetic analyses in transfected HEK293 cells have been performed showing that the differences in Km values were not apparent between OATP1B1-Val174 and OATP1B1-Ala174 variants. However, both the Vmax value and the intrinsic clearance were significantly decreased in cells expressing OATP1B1-Ala174 compared with OATP1B1-Val174, probably due to the sorting error of the OATP1B1-Ala174 variant.

Breast cancer is one of the major causes of death in European and American women, occurring most frequently in postmenopausal women. Because estrogens have an important role in breast cancer, therapies aim to block interaction with the estrogen receptor by use of an antiestrogen or by inhibiting the conversion of androstenedione to estrone with an aromatase inhibitor (33, 34). Local formation of estrogens in breast tumors might be more important than circulating estrogens for growth and survival of estrogen-dependent breast cancer in postmenopausal women (35). Serum E1S is a major source of estrogens for breast tissue through local sulfatase activity (1). Because carriers of the OATP1B1-Val174Ala polymorphism have life-long higher serum E1S levels, the possible association of this polymorphism with risk and prognosis of breast cancer should be investigated.

In conclusion, OATP1B1 is an important factor in the hepatic transport and metabolism of bilirubin, E1S, and iodothyronine sulfates. The OATP1B1-Ala174 variant allele displays decreased transport activity and thereby gives rise to higher serum bilirubin, E1S, and T4S levels in carriers of this polymorphism.


   Acknowledgments
 
We are very grateful to the participants of the Rotterdam Scan Study. Furthermore, we acknowledge all the participating general practitioners, pharmacies, and the many field workers in the research center in Ommoord, Rotterdam, The Netherlands.


   Footnotes
 
This work was supported by the Netherlands Organization of Scientific Research (NWO) Research Institute for Diseases in the Elderly Grant 6730040 (W.M.v.d.D.).

Disclosure Statement: The authors have nothing to disclose.

First Published Online May 22, 2008

Abbreviations: BSP, Bromosulfophthalein; CRYM, µ-crystallin; D1, type 1 deiodinase; E1S, estrone sulfate; FT4, free T4; OATP, organic anion transporter; rT3S, rT3 sulfate; SULT, sulfotransferase; T3S, T3 sulfate; T4S, T4 sulfate.

Received February 5, 2008.

Accepted for publication May 9, 2008.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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W. M. van der Deure, P. S. Hansen, R. P. Peeters, K. O. Kyvik, E. C. H. Friesema, L. Hegedus, and T. J. Visser
Thyroid Hormone Transport and Metabolism by Organic Anion Transporter 1C1 and Consequences of Genetic Variation
Endocrinology, October 1, 2008; 149(10): 5307 - 5314.
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