Identification of thyroid hormone transporters in humans: different molecules are involved in a tissue-specific manner

A role for thyroid hormone transporters in transcriptional regulation by thyroid hormone receptors

Thyroid hormones (THs) must be taken up by target cells to act at the genomic level through binding to nuclear thyroid hormone receptors (TRs). Extensive study has been made of mechanisms by which TH-bound TRs regulate transcription, yet little is known about the critical upstream step, i.e. how THs enter the cell. Growing evidence suggests that saturable transport mechanisms mediate the greater part of TH movement across the plasma membrane and have important roles in the regulation of TH bioavailability. For example, System L is a multifunctional transport system serving as a plasma membrane transporter of THs and amino acids in mammalian cells. We have used two complementary systems, the Xenopus oocyte (which has negligible basal System L activity) and the mammalian BeWo cell line (which has System L activity for TH transport), to investigate the role of this representative TH transporter in nuclear action of THs. We demonstrate that overexpression of System L in Xenopus oocytes increases both cytoplasmic and nuclear delivery of THs from external medium and also enhances transcriptional activation by TRs. Conversely, blocking endogenous System L activity in BeWo cells with specific inhibitors reduces both TH uptake and TR function. These results indicate that plasma membrane TH transporters such as System L may have important roles in gene regulation by TRs.

Characterization and identification of steroid sulfate transporters of human placenta

Human trophoblasts depend on the supply of external precursors, such as dehydroepiandrosterone-3-sulfate (DHEA-S) and 16 alpha-OH-DHEA-S, for synthesis of estrogens. The aim of the present study was to characterize the uptake of DHEA-S by isolated mononucleated trophoblasts (MT) and to identify the involved transporter polypeptides. The kinetic analysis of DHEA-(35)S uptake by MT revealed a saturable uptake mechanism (K(m) = 26 microM, V(max) = 428 pmol x mg protein(-1) x min(-1)), which was superimposed by a nonsaturable uptake mechanism (diffusion constant = 1.2 microl x mg protein(-1) x min(-1)). Uptake of [(3)H]DHEA-S by MT was Na(+) dependent and inhibited by sulfobromophthalein (BSP), steroid sulfates, and probenecid, but not by steroid glucuronides, unconjugated steroids, conjugated bile acids, ouabain, p-aminohippurate (PAH), and bumetanide. MT took up [(35)S]BSP, [(3)H]estrone-sulfate, but not (3)H-labeled ouabain, estradiol-17beta-glucuronide, taurocholate, and PAH. RT-PCR revealed that the organic anion-transporting polypeptides OATP-B, -D, -E, and the organic anion transporter OAT-4 are highly expressed, and that OATP-A, -C, -8, OAT-3, and Na(+)-taurocholate cotransporting polypeptide (NTCP) are not or are only lowly expressed in term placental tissue and freshly isolated and cultured trophoblasts. Immunohistochemistry of first- and third-trimester placenta detected OAT-4 on cytotrophoblast membranes and at the basal surface of the syncytiotrophoblast. Our results indicate that uptake of steroid sulfates by isolated MT is mediated by OATP-B and OAT-4 and suggest a physiological role of both carrier proteins in placental uptake of fetal-derived steroid sulfates.

Active influx transport is mediated by members of the organic anion transporting polypeptide family in human epidermal keratinocytes

Normal human epidermal keratinocytes have been shown to express a cell-type-specific pattern of extrahepatic cytochrome P450 enzymes and efflux transport proteins showing that these cells metabolize and excrete a variety of xenobiotics. Recently transport proteins involved in the uptake of xenobiotics have been detected and here we analyzed the mRNA and protein expression profiles and functional activities of these proteins in human keratinocytes in comparison to primary liver cells. The transporters studied included the subtypes A, B, C, D, and E of the organic anion transporting polypeptide (OATP) family, which are responsible for the uptake of various anionic and neutral molecules and especially organic cations - including drugs. Constitutive expression of OATP-B, OATP-D, and OATP-E was shown for the first time in normal human epidermal keratinocytes on a molecular level using reverse transcription polymerase chain reaction and northern blot analysis, as well as in human skin tissue shown by tissue blot hybridization and immunohistochemistry. Expression of OATP-A and OATP-C was not detected in any of the keratinocyte samples. In contrast, liver tissue showed a significant expression of OATP-A and OATP-B as well as OATP-C, a weak expression of OATP-D, and no expression of OATP-E. These data revealed that normal human epidermal keratinocytes express a specific profile of transporters involved in drug influx. Using a newly developed uptake-transport assay, uptake of known and well-characterized OATP substrates like estradiol-17beta-glucuronide and estrone sulfate was inhibited in normal human epidermal keratinocytes by specific inhibitors such as taurocholate, verifying the functional capacity of the expressed OATPs. Human dermal fibroblasts seem to have a lower influx transport activity for estradiol-17beta-glucuronide, which correlates with the immunohistologic data. Even though the substrate specificity of the OATP isoforms is only partially known until now, our findings support the concept that uptake of large organic cations like drugs in keratinocytes is an active transport process mediated by members of the OATP family.

Expression of organic anion transporting polypeptide E (OATP-E) in human placenta

PURPOSE

To examine the expression of multifunctional Na(+)-independent organic anion transporting polypeptide, termed OATP-E, involved in the transport of thyroid hormone in human placenta.

METHODS

Western blot analysis was performed using a specific antibody against OATP-E in human placenta to confirm the expression of OATP-E at the protein level. Immunohistochemistry was also performed using the specific antibody against OATP-E on frozen sections of human placenta.

RESULTS

By Western blot analysis, a single band for OATP-E was observed in human placenta. Immunohistochemistry revealed that OATP-E was predominantly expressed at the apical surface of the syncytiotrophoblasts in placenta.

CONCLUSION

These results reveal that OATP-E is expressed in human placenta, suggesting a functional role for the transplacental transfer of thyroid hormone.

The superfamily of organic anion transporting polypeptides

Organic anion transporting polypeptides (Oatps/OATPs) form a growing gene superfamily and mediate transport of a wide spectrum of amphipathic organic solutes. Different Oatps/OATPs have partially overlapping and partially distinct substrate preferences for organic solutes such as bile salts, steroid conjugates, thyroid hormones, anionic oligopeptides, drugs, toxins and other xenobiotics. While some Oatps/OATPs are preferentially or even selectively expressed in one tissue such as the liver, others are expressed in multiple organs including the blood-brain barrier (BBB), choroid plexus, lung, heart, intestine, kidney, placenta and testis. This review summarizes the actual state of the rapidly expanding OATP superfamily and covers the structural properties, the genomic classification, the phylogenetic relationships and the functional transport characteristics. In addition, we propose a new species independent and open ended nomenclature and classification system, which is based on divergent evolution and agrees with the guidelines of the Human Genome Nomenclature Committee.

Effects of thyroid state on the expression of hepatic thyroid hormone transporters in rats

Liver uptake of thyroxine (T4) is mediated by transporters and is rate limiting for hepatic 3,3',5-triiodothyronine (T3) production. We investigated whether hepatic mRNA for T4 transporters is regulated by thyroid state using Xenopus laevis oocytes as an expression system. Because X. laevis oocytes show high endogenous uptake of T4, T4 sulfamate (T4NS) was used as an alternative ligand for the hepatic T4 transporters. Oocytes were injected with 23 ng liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats, and after 3-4 days uptake was determined by incubation of injected and uninjected oocytes for 1 h at 25 degrees C or for 4 h at 18 degrees C with 10 nM [125I]T4NS. Expression of type I deiodinase (D1), which is regulated by thyroid state, was studied in the oocytes as an internal control. Uptake of T4NS showed similar approximately fourfold increases after injection of liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats. A similar lack of effect of thyroid state was observed using reverse T3 as ligand. In contrast, D1 activity induced by liver mRNA from hyperthyroid and hypothyroid rats in the oocytes was 2.4-fold higher and 2.7-fold lower, respectively, compared with euthyroid rats. Studies have shown that uptake of iodothyronines in rat liver is mediated in part by several organic anion transporters, such as the Na+/taurocholate-cotransporting polypeptide (rNTCP) and the Na-independent organic anion-transporting polypeptide (rOATP1). Therefore, the effects of thyroid state on rNTCP, rOATP1, and D1 mRNA levels in rat liver were also determined. Northern analysis showed no differences in rNTCP or rOATP1 mRNA levels between hyperthyroid and hypothyroid rats, whereas D1 mRNA levels varied widely as expected. These results suggest little effect of thyroid state on the levels of mRNA coding for T4 transporters in rat liver, including rNTCP and rOATP1. However, they do not exclude regulation of hepatic T4 transporters by thyroid hormone at the translational and posttranslational level.

Thyroid hormone actions on neural cells

1. In addition to its role in cellular metabolic activity, thyroid hormone (TH) is critically involved in growth, development, and function of the central nervous system. In the brain, as in other structures, TH is described to exert its major action by the binding of L-3,5,3'-triiodothyronine (T3), considered as the bioactive form of the hormone, to nuclear thyroid hormone receptors (TR) that function as ligand-dependent transcription factors. 2. The transcription of numerous brain genes was indeed shown to be positively or negatively regulated by TH, turning these TR-mediated effects one explanation for the physiological effects of TH. In this context, the knowledge from TR-knockout studies provides some surprising results, since neonatal hypothyroidism is associated to more significant abnormalities than is TR deficiency. Some (nonexclusive) hypotheses include a permissive effect of TH, allowing derepression of unliganded-TR effects and non-TR-mediated effects of the hormone, further emphasizing the importance of a controlled accessibility of neural cells to TH. 3. On the other hand, T3 was demonstrated to directly act not only on neuronal but also on glial cells proliferation and differentiation, contributing to the harmonious development of the brain. Interestingly, in addition to these direct actions on neuronal and glial cells, several lines of evidence, notably developped in our laboratory, point out the role of thyroid hormone in neuronal-glial interactions.

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.