Comparison of mechanisms mediating uptake and efflux of thyroid hormones in the human choriocarcinoma cell line, JAR

Expression and function of thyroid hormone transporters in the microvillous plasma membrane of human term placental syncytiotrophoblast

The transplacental passage of thyroid hormones (THs) from mother to fetus in humans has been deduced from observational clinical studies and is important for normal fetoplacental development. To investigate the transporters that regulate TH uptake by syncytiotrophoblast (the primary barrier to maternal-fetal exchange, which lies in direct contact with maternal blood), we isolated the microvillous plasma membrane (MVM) of human term syncytiotrophoblasts. We have demonstrated that MVM vesicles express plasma membrane TH transporter proteins, including system-L (L-type amino acid transporter 1 and CD98), monocarboxylate transporters (MCTs) 8 and 10, organic anion-transporting polypeptides 1A2 and 4A1. We provide the first definitive evidence that the human syncytiotrophoblast MVM is capable of rapid, saturable T(4) and T(3) uptake at similar rates and in a Na(+)-independent manner. These two major forms of THs could not significantly inhibit each others' uptake, suggesting that each is mediated by largely different transporters. No single transporter was noted to play a dominant role in either T(4) or T(3) uptake. Using combinations of transporter inhibitors that had an additive effect on TH uptake, we provide evidence that 67% of saturable T(4) uptake is facilitated by system-L and MCT10 with a minor role played by organic anion-transporting polypeptides, whereas 87% of saturable T(3) uptake is mediated by MCT8 and MCT10. Our data demonstrate that syncytiotrophoblast may control the quantity and forms of THs taken up by the human placenta. Thus, syncytiotrophoblast could be critical in regulating transplacental TH supply from the mother to the fetus.

Iodothyronine Interactions with the System L1 Amino Acid Exchanger in 3T3-L1 Adipocytes

Thyroid hormones enter isolated white adipocytes largely by a System L1-type amino acid transporter en route to exerting genomic actions. Differentiated 3T3-L1 mouse adipocytes in culture express mRNA for LAT1 (the catalytic subunit of high-affinity System L1). L-[¹²⁵I]-T₃ uptake into 3T3-L1 adipocytes included a substantial saturable component inhibited by leucine. L-[³H]phenylalanine uptake into 3T3-L1 cells was saturable (K_m of 31 μM), competitively inhibited by T₃ (K_i of 1.2 μM) and blocked by leucine, BCH, and rT₃ as expected for substrate interactions of System L1. Efflux of preloaded L-[³H]phenylalanine from 3T3-L1 adipocytes was trans stimulated by external leucine, demonstrating the obligatory exchange mechanism of System L1 transport. T₃ (10 μM) did not significantly trans stimulate L-[³H]phenylalanine efflux, but did competitively inhibit the trans stimulatory effect of 10 μM leucine. The results highlight strong competitive interactions between iodothyronines (T₃, rT₃) and amino acids for transport by System L1 in adipocytes, which may impact cellular iodothyronine exchanges during altered states of protein nutrition.

Tissue uptake of thyroid hormone by amino acid transporters

Thyroid hormones (THs) -- thyroxine (T4) and tri-iodothyronine (T3) -- are iodinated derivatives of the amino acid tyrosine, which regulates growth, development and critical metabolic functions. THs are taken up by target cells and act at the genomic level via nuclear thyroid receptors. Saturable transport mechanisms mediate the greater part of TH movement across the plasma membrane. System L1 permease is a transporter of THs and amino acids in mammalian adipose tissue, placenta and brain. T(3) is also a substrate of a putative System T transporter, which is selective for aromatic amino acids. The activity and functional mechanisms of these transporters can be crucial to cells in determining both their hormone sensitivity and their responses to change in circulating hormone concentrations or availability of competing substrates (e.g. amino acids). TH transporters are potentially important pharmacological targets in the design of novel or improved therapies for thyroid-related disorders.

Synthesis of thyroid hormone binding proteins transthyretin and albumin by human trophoblast

CONTEXT

Mechanisms regulating materno-fetal transfer of thyroid hormone are not well understood. Modulation of trophoblast type 3 iodothyronine deiodinase (D3) may play an important role.

OBJECTIVE

The objective of this study was to investigate trophoblast thyroid hormone binding proteins that may modulate interactions between D3 and T4.

DESIGN

Placentas were obtained by informed consent from women delivering normal infants by repeat cesarean section at 38-40 wk gestation. T4 and T3 binding was examined in human placenta. Serum thyroid hormone binding proteins were identified by Western blotting, and their mRNA was examined by RT-PCR. Presence of these proteins in trophoblast was determined by immunocytochemistry and immunofluorescence. Cytosol was progressively purified to reveal additional thyroid hormone binding proteins that were identified by matrix-assisted laser desorption/ionization time of flight mass spectrometry. Effects of mefenamic acid on placental deiodination were examined by HPLC.

RESULTS

We detected high-affinity T4 and T3 binding in human placental cytosol. All three major serum-binding proteins, T4 binding globulin (TBG), transthyretin (TTR), and albumin, were present in cytosol. TTR mRNA and albumin mRNA were detected in human placenta, and TTR and albumin were identified histochemically in syncytiotrophoblasts. Neither TBG mRNA nor TBG was detected, suggesting that plasma TBG had contaminated the cytosol preparation. Low-affinity thyroid hormone binding proteins alpha-1-antitrypsin and alpha-1-acid glycoprotein were also identified. Addition of mefenamic acid, a potent inhibitor of thyroid hormone binding, to placental cytosol significantly enhanced deiodination of T4 by D3.

CONCLUSIONS

Placenta produces a series of thyroid hormone binding proteins that may modify thyroid hormone deiodination and materno-fetal thyroid hormone transport.

Enhanced non-esterified fatty acids and corticosterone in blood plasma of chickens treated with insulin are significantly depleted by reverse T: minor changes in hypoglycaemia

Previously, it has been observed that dexamethasone or adrenaline-induced hyperlipaemia in blood of chicken was significantly reduced after administration of reverse triiodothyronine (rT3). The present experiment was performed on chicken to determine the altered circulating non-esterified fatty acids (NEFA) induced by physiologically enhanced endogenous corticosterone and catecholamines may also be influenced by rT3. Rise of both hormones were induced by insulin administration. Changes in circulating glucose, corticosterone and catecholamines were additionally measured. Following insulin injection blood glucose fell on the average by 32.7% below control at 2 h of the experiment. Additional treatment with rT3 (rT3 + insulin group) gradually attenuated this decrease and at 4 and 6 h of the experiment it was 17.1% and 12.9% below control, respectively, suggesting on slight inhibition by rT3 of insulin-stimulated glucose utilization. Exposure to insulin significantly increased NEFA levels to about 670% above control group. Additional treatment with rT3 reduced this increase to 309% of control, suggesting inhibition of lipolysis by rT3. Similar alterations were observed in plasma corticosterone levels. Insulin treatment peaked the corticosterone levels maximally by 507.6% above control. Additional treatment with rT3 abolished this rise in the averages to 194.2% above control, possibly by interaction of rT3 with hypothalamo-adrenal axis. Insulin injection increased plasma catecholamines on the average by 21.5% and 53.4% for adrenaline and noradrenaline respectively. Supplementary treatment with rT3 intensified this rise by 55.6% and 71.6% respectively. The obtained results suggest on inhibitory effect of rT3 on hypoglycaemia, hyperlipaemia and plasma corticosterone concentrations in chickens treated with insulin. Contrary to this, rT3 enhanced the rise of plasma catecholamines due to insulin treatment. The obtained data favour the assumption that hypometabolic properties of rT3 depends mainly upon reduced supply of NEFA as a result of restricted lipolysis and to a lesser extent upon the supply of glucose.

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.

Complex regulation of thyroid hormone action: multiple opportunities for pharmacological intervention

The thyroid hormone (TH; 3,3',5,5'-tetra-iodothyronine and 3,3',5'-triiodothyronine) regulates growth, development, and critical metabolic functions. Thyroid diseases are among the most prevalent group of metabolic disorders in the Western world. TH exerts effects through complex biological pathways, which offer a wealth of opportunities to pharmacologically intervene in TH signalling at numerous steps. These include biosynthesis, cell-specific uptake or export (involving L-type amino acid transporter, organic anion transporter, organic cation transporter, or multidrug resistance transporter), as well as nuclear targeting and actions (the latter including TH receptor binding and histone acetylation/deacetylation). Such processes represent potentially important pharmacological targets for the design of novel or improved therapies for TH disorders, obesity, and cardiovascular diseases.

Thyroid hormone export varies among primary cells and appears to differ from hormone uptake

We characterized T3 efflux in primary cultures of cells derived from human placenta, neonatal rat cardiac myocytes, and rat inner medullary collecting ducts (IMCD). The T3 efflux rate was highest in placenta cells, followed by ventriculocytes, atriocytes, and IMCD cells. Verapamil reversibly blocked [125I]T3 efflux in these cells in a manner that correlated with their T3 efflux rate. Thus, verapamil inhibition of [125I]T3 efflux in placenta cells led to a 432% increase in the [125I]T3 content compared with 33% increase in IMCD cells. Several unlabeled iodothyronines, but not TRIAC, differentially blocked [125I]T3 efflux such as (T4 > T3 > rT3 = D-T3 > D-T4) in placenta cells and (T4 > rT3 = D-T4 = T3 > D-T3) in ventriculocytes, suggesting tissue-specific differences in the carriers/transporters responsible for T3 efflux. This hypothesis draws further support from the fact that D-T3 inhibited [125I]T3 efflux in placenta cells, but not in ventriculocytes. TRIAC did not affect T3 efflux in ventriculocytes or placenta cells, but it greatly inhibited [125I]T3 uptake in these cells, suggesting that [125I]T3 uptake and efflux mechanisms are distinct and appear to be mediated by distinct carrier/transporter proteins. Collectively, these data suggest that differences in thyroid hormone transport in target cells may provide an important mechanism for regulating hormone action in a tissue-specific fashion.

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.

Thyroid hormone export in rat FRTL-5 thyroid cells and mouse NIH-3T3 cells is carrier-mediated, verapamil-sensitive, and stereospecific

Export of L-T3 out of the cell is one factor governing the cellular T3 content and response. We previously observed in liver-derived cells that T3 export was inhibited by verapamil, suggesting that it is due to either ATP-binding cassette/multidrug resistance (MDR1/mdr1b) or multidrug resistance-related (MRP1/mrp1) proteins. To test this hypothesis we measured T3 export in FRTL-5, NIH-3T3, and rat hepatoma (HTC) cells that varied in expression of these proteins. FRTL-5 and NIH-3T3 cells were found to contain a T3 efflux mechanism that is verapamil inhibitable, saturable, and stereospecific. By contrast, T3 efflux in HTC cells was slow and unaffected by verapamil. Neither FRTL-5 nor NIH-3T3 cells express mdrlb, but all three cell types express mrpl, as assessed by immunoblotting. Overexpression of MDR1 in NIH-3T3 cells did not enhance verapamil-inhibitable T3 efflux. Photoaffinity labeling of FRTL-5 and NIH-3T3 cells with [125I]L-T3 revealed a labeled 90- to 100-kDa protein that was not present in HTC cells. Verapamil and excess nonradioactive L-T3, but not D-T3, inhibited labeling of this protein. The lack of correlation between T3 efflux and MDR1 and mrpl expression and the finding of a photoaffinity-labeled putative transport protein smaller than MDR1 or mrp1 protein (approximately 170 kDa) suggest that a novel protein is involved in the transport of T3 out of cells.