Uptake of L-tri-iodothyronine by human cultured trophoblast cells

Pathophysiological aspects of thyroid hormone disorders/thyroid peroxidase autoantibodies and reproduction

BACKGROUND

Thyroid hormone disorders and thyroid peroxidase autoantibodies (TPO-Ab) in women are associated with subfertility and early pregnancy loss. Here, we aim to provide a comprehensive overview of the literature on the pathophysiology of these associations.

METHODS

A review of the literature in the English language was carried out. Relevant studies were identified by searching Medline, EMBASE and the Cochrane Controlled Trials Register from 1975 until March 2014.

RESULTS

From a total of 6108 primary selected articles from the literature search, 105 articles were selected for critical appraisal. Observational data indicate that altered thyroid hormone levels are associated with disturbed folliculogenesis, spermatogenesis, lower fertilization rates and lower embryo quality. Triiodothyronine (T3) in combination with FSH enhances granulosa cell proliferation and inhibits granulosa cell apoptosis by the PI3K/Akt pathway. T3 is considered a biological amplifier of the stimulatory action of gonadotrophins on granulosa cell function. T3 increases the expression of matrix metalloproteinases (MMP), MMP-2, MMP-3, fetal fibronectin and integrin α5β1T3 in early placental extravillous trophoblasts. Thyroid hormone transporters and receptors are expressed in the ovary, early embryo, endometrium, uterus and placenta. No other data explaining the associations could be retrieved from the literature. The presence of TPO-Ab is negatively associated with spermatogenesis, fertilization and embryo quality, but no data are available on the potential pathophysiological mechanisms.

CONCLUSIONS

Thyroid hormone disorders and TPO-Ab are associated with disturbed folliculogenesis, spermatogenesis, fertilization and embryogenesis. The pathophysiology of these associations remains largely unknown, as evidence is limited and includes studies using small sample sizes, and often restricted to animal models. There are no studies on the pathophysiology underlying the association between TPO-Ab and reproduction. The available evidence, although limited, supports a role of thyroid hormone in fertility and early pregnancy. This justifies clinical intervention studies on the effects of thyroid hormone supplementation in women with subclinical hypothyroidism and in women prone to develop hypothyroidism due to the presence of TPO-Ab. In addition, more research is needed to identify the underlying mechanisms. This would be of particular interest in women undergoing IVF to pinpoint the effects of thyroid hormone on different parameters of reproduction.

The Role of Thyroid Hormone in Trophoblast Function, Early Pregnancy Maintenance, and Fetal Neurodevelopment

OBJECTIVE

To review the literature on the roles of thyroid hormone in trophoblast function, early pregnancy maintenance, and fetal neurodevelopment.

METHODS

MEDLINE was searched for English-language papers published from 1971 to 2003, using the key words "brain," "hypothyroidism," "placenta," "pregnancy," "threatened abortion," "thyroid hormone," "thyroid hormone receptor," "thyroid hormone replacement therapy," "thyroid hormone-responsive gene," and "trophoblast."

RESULTS

Transplacental transfer of thyroid hormone occurs before the onset of fetal thyroid hormone secretion. Thyroid hormone receptors and iodothyronine deiodinases are present in the placenta and the fetal central nervous system early in pregnancy, and thyroid hormone plays a crucial role both in trophoblast function and fetal neurodevelopment. Maternal hypothyroxinemia is associated with a high rate of spontaneous abortion and long-term neuropsychological deficits in children born of hypothyroid mothers. Maternal iodine deficiency also causes a wide spectrum of neuropsychological disorders in children, ranging from subclinical deficits in cognitive motor and auditory functions to hypothyroid-induced cognitive impairment in infants. However, these conditions are preventable when iodine supplementation is initiated before the second trimester. Although thyroid hormone replacement therapy is effective for reducing the adverse effects complicated by maternal hypothyroidism, the appropriate dose of thyroid hormone is mandatory in protecting the early stage of pregnancy.

CONCLUSIONS

Close monitoring of maternal thyroid hormone status and ensuring adequate maternal thyroid hormone levels in early pregnancy are of great importance to prevent miscarriage and neuropsychological deficits in infants.

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.

Lack of membrane transport of l-thyroxine sulphate in the human choriocarcinoma cell line, JAr

We examined uptake of l -thyroxine sulphate (T(4)S) and possible interactions between T(4)S and thyroxine (T(4)) uptake in the choriocarcinoma cell line JAr. Cells were incubated with 50 p m(125)I-T(4)S in the absence (total uptake) and in the presence (non-specific uptake) of 10 microm T(4)S. Cells were also incubated at 37 degrees C for 2 min with 50 p m(125)I-T(4)in the presence of an increasing amount of unlabelled T(4)(0-10 microm) or T(4)S (0-30 microm). There was negligible total uptake of(125)I-T(4)S (1.14+/-0. 05 fmol/mg cellular protein, mean+/-sem) and no specific uptake after 120 min incubation. Minor inhibition of(125)I-T(4)uptake by T(4)S could be explained entirely by a low level of residual T(4)(0. 2 per cent) in the T(4)S preparation. These findings indicate that T(4)S does not share the T(4)membrane transporter.

Thyroid hormone efflux from placental tissue is not stimulated during cell volume regulation

The effects of cell swelling induced by hyposmotic shock on efflux of hybrid hormones and selected amino acids from human placental tissue were examined. Decreasing the osmolarity of external medium from 290 to 140 mOsm/kg stimulated release of taurine, tryptophan and glutamine from placental tissue fragments. The efflux rate constant for taurine increased from 0.0069 +/- 0.0012/min to 0.0646 +/- 0.0217/min (n = 6) (P < 0.001), for tryptophan from 0.016 +/- 0.0010/min to 0.0295 +/- 0.0016/min (n = 6) (P < 0.001), and for glutamine from 0.0267 +/- 0.0027/min to 0.0659 +/- 0.0043/min (n = 4) (P < 0.001). In contrast, hyposmotic challenge did not affect release of triiodothyronine, thyroxine and leucine. These results indicate that transport processes involved in the regulation of cellular volume are unlikely to facilitate efflux of thyroid hormones from placental tissue, and therefore are unlikely to mediate transfer of thyroid hormones across the placenta. In addition, it is unlikely that the transport system facilitating the release of amino acids from placental tissue during regulatory volume decrease is one of the known amino acid carriers.

Guanidine transport in a human choriocarcinoma cell line (JAR)

PURPOSE

Many endogenous substances and xenobiotics are organic cations. Transplacental transport of organic cations is an important determinant of the delivery of these compounds to the fetus. The aim of this study was to determine the mechanisms of organic cation transport using the human choriocarcinoma cell line (JAR) as a model system with [14C]guanidine as a ligand.

METHODS

Uptake studies of [14C]guanidine were carried out in JAR cell monolayers on day 2 after plating.

RESULTS

[14C]guanidine uptake was temperature dependent, saturable (Km = 167 microM) and inhibited by many organic cations including amiloride, cimetidine, quinine, quinidine and nicotine. [14C]guanidine uptake exhibited a counterflux phenomenon indicative of a carrier-mediated process. The uptake of [14C]guanidine was sodium and pH-independent and could be driven by an inside-negative membrane potential difference.

CONCLUSIONS

This is the first demonstration of an electrogenic guanidine transporter in a human cell culture model. This transporter may play a role in the transplacental transport of many clinically used drugs and xenobiotics.

Interactions between transport of triiodothyronine and tryptophan in JAR cells

We studied the effect of a number of amino acids on uptake of L-triiodothyronine (T3) in the human choriocarcinoma cell line, JAR. Tryptophan inhibited saturable T3 uptake by about 57% without any significant effect on the non-saturable uptake. Michaelis constant (Km) for T3 uptake was 1.06 +/- 0.15 microM (n = 15) with the corresponding maximum velocity (Vmax) of 24.2 +/- 3.1 pmol/min/mg cellular protein. For tryptophan uptake the Km was 1.31 +/- 0.26 microM (n = 7) and Vmax was 166.4 +/- 35.7 pmol/min/mg protein. The kinetic parameters for both uptake processes were similar to those reported in normal placenta. Uptake of T3 was inhibited by tryptophan but not phenylalanine, but tryptophan uptake was inhibited both by T3 and phenylalanine. Inhibition of T3 uptake by tryptophan was dose dependent, with an inhibition constant (Ki) of 2.9 +/- 0.5 mM. Similarly, tryptophan uptake was inhibited by T3 and phenylalanine in a dose dependent way with Ki values of 4.9 +/- 0.5 microM and 15.6 +/- 4.8 microM respectively. Km for T3 uptake was significantly increased to 1.86 +/- 0.42 microM (n = 4) in the presence of 3 mM unlabelled tryptophan and, similarly, Km for tryptophan uptake was significantly increased to 9.91 +/- 2.57 microM (n = 3) in the presence of 5 microM unlabelled T3. Efflux of T3 was progressively inhibited by increasing concentrations of both ligands, i.e. was saturable. We conclude that there is mutual competitive inhibition between uptake systems for T3 and tryptophan in JAR cells, but the kinetic parameters of cross-inhibition of uptake by the substrates suggest that the carriers are distinct. T3 may be transported in JAR cells by at least two transport systems with differing substrate specificities. We also demonstrated the presence of a saturable membrane carrier mediating the efflux of T3 from the cells which was subject to trans-inhibition by T3 and tryptophan.