Characterization of lysosomal monoiodotyrosine transport in rat thyroid cells. Evidence for transport by system h

Emerging role of thyroid hormone metabolites

Thyroid hormones (THs) are essential for the regulation of development and metabolism in key organs. THs produce biological effects both by directly affecting gene expression through the interaction with nuclear receptors (genomic effects) and by activating protein kinases and/or ion channels (short-term effects). Such activations can be either direct, in the case of ion channels, or mediated by membrane or cytoplasmic receptors. Short-term-activated signalling pathways often play a role in the regulation of genomic effects. Several TH intermediate metabolites, which were previously considered without biological activity, have now been associated with a broad range of actions, mostly attributable to short-term effects. Here, we give an overview of the physiological roles and mechanisms of action of THs, focusing on the emerging position that TH metabolites are acquiring as important regulators of physiology and metabolism.

Role of amino acid transporters in amino acid sensing

Amino acid (AA) transporters may act as sensors, as well as carriers, of tissue nutrient supplies. This review considers recent advances in our understanding of the AA-sensing functions of AA transporters in both epithelial and nonepithelial cells. These transporters mediate AA exchanges between extracellular and intracellular fluid compartments, delivering substrates to intracellular AA sensors. AA transporters on endosomal (eg, lysosomal) membranes may themselves function as intracellular AA sensors. AA transporters at the cell surface, particularly those for large neutral AAs such as leucine, interact functionally with intracellular nutrient-signaling pathways that regulate metabolism: for example, the mammalian target of rapamycin complex 1 (mTORC1) pathway, which promotes cell growth, and the general control non-derepressible (GCN) pathway, which is activated by AA starvation. Under some circumstances, upregulation of AA transporter expression [notably a leucine transporter, solute carrier 7A5 (SLC7A5)] is required to initiate AA-dependent activation of the mTORC1 pathway. Certain AA transporters may have dual receptor-transporter functions, operating as "transceptors" to sense extracellular (or intracellular) AA availability upstream of intracellular signaling pathways. New opportunities for nutritional therapy may include targeting of AA transporters (or mechanisms that upregulate their expression) to promote protein-anabolic signals for retention or recovery of lean tissue mass.

In vitro characterization of the thyroidal uptake of O-(2-[18F]fluoroethyl)-l-tyrosine

OBJECTIVES

Positron emission tomography (PET) using O-(2-[(18)F]fluoroethyl)-l-tyrosine (FET) has been successfully employed in the diagnostic workup of brain tumors. Knowledge on the mechanisms of the uptake of radiolabeled amino acids into thyroidal tissues and well-differentiated thyroid carcinomas is limited. We therefore studied several factors potentially governing the uptake of FET in the rat thyroid cell line FRTL-5 in comparison with thyroid tumor cell lines of human origin.

METHODS

FET uptake was determined in thyroid-stimulating hormone (TSH)-stimulated and TSH-deprived FRTL-5 cells, as well as in the cell lines U-138 MG (human glioblastoma), Onco DG-1 (human papillary thyroid carcinoma) and ML-1 (human follicular thyroid carcinoma). The TSH responsiveness of cells was measured by the incubation of TSH-treated and untreated control cells with 2-[(18)F]fluoro-2-deoxyglucose (FDG). All cellular tracer uptake values were related to total protein mass and expressed as percentage per milligram. For countertransport studies, FRTL-5 cells were exposed to 10-300 microM tyrosine methyl ester. TSH-stimulated and TSH-deprived FRTL-5 cells were incubated with 100 kBq/ml FET for 20 min. 2-Aminobicyclo-[2,2,1]heptane-2-carboxylic acid (BCH), alpha-(methylamino)-isobutyric acid, L-serine and tryptophan were used as competitive inhibitors of FET uptake. All inhibition experiments were repeated with the human thyroid carcinoma cell lines to obtain comparative FET uptake values.

RESULTS

The FET uptake was 155+/-30%/mg in FRTL-5 cells (n=6), 108+/-14%/mg in U-138 MG cells (n=6), 194+/-60%/mg in ML-1 cells (n=9) and 64+/-23%/mg in Onco DG-1 cells (n=6) under identical incubation conditions. Preloading with tyrosine methyl ester increased cellular FET uptake dose dependently in FRTL-5 cells (165+/-25%, n=6). While TSH increased the uptake of FDG in FRTL-5 cells by sixfold, there was no TSH effect on FET accumulation. FET uptake by TSH-treated FRTL-5 cells was sodium independent and significantly inhibited by BCH (91.4+/-3.0%, n=9), tryptophan (94.8+/-1.6%, n=8) and serine (83.2+/-10.8%, n=12). TSH-starved FRTL-5 cells had a sodium-dependent component with a similar inhibition pattern. Onco DG-1 mainly confirmed the inhibition pattern of FET uptake in FRTL-5 cells, reflecting System-L-mediated FET uptake that was blocked by BCH and serine (72-85%, n=9). ML-1 cells revealed a pronounced sodium-dependent FET uptake that was inhibited by tryptophan (70+/-10%, n=9, P<.05) in the presence and in the absence of sodium, suggesting a contribution of alternative amino acid carriers.

CONCLUSION

FET uptake by FRTL-5 cells is not TSH dependent. FET uptake by FRTL-5 cells seems to be mainly mediated by a carrier exhibiting the characteristics of the System L amino acid transporter. FET uptake in thyroid cells and thyroid carcinoma cells was in the same range as that in a glioblastoma cell line. This encourages further research efforts towards the clinical evaluation of FET for the diagnostic workup of well-differentiated thyroid carcinomas.

Halometabolites and cellular dehalogenase systems: an evolutionary perspective

We review the role of iodothyronine deiodinases (IDs) in the evolution of vertebrate thyroidal systems within the larger context of biological metabolism of halogens. Since the beginning of life, the ubiquity of organohalogens in the biosphere has provided a major selective pressure for the evolution and conservation of cellular mechanisms specialized in halogen metabolism. Among naturally available halogens, iodine emerged as a critical component of unique developmental and metabolic messengers. Metabolism of iodinated compounds occurs in the three major domains of life, and invertebrate deuterostomes possess several biochemical traits and molecular homologs of vertebrate thyroidal systems, including ancestral homologs of IDs identified in urochordates. The finely tuned cellular regulation of iodometabolite uptake and disposal is a remarkable event in evolution and might have been decisive for the explosive diversification of ontogenetic strategies in vertebrates.

Lysosomal amino acid transporter LYAAT-1 in the rat central nervous system: an in situ hybridization and immunohistochemical study

A first mammalian lysosomal transporter (LYAAT-1) was recently identified and functionally characterized. Preliminary immunocytochemical data revealed that LYAAT-1 localizes to lysosomes in some neurons. In order to determine whether it is expressed in specific neuron populations and other cell types, and to confirm whether it is localized at the membrane of lysosomes, we used in situ hybridization and immunohistochemistry methods in adult rat central nervous system (CNS). We found that LYAAT-1 is expressed in most areas of the CNS, specifically in neurons, but also in choroid plexus and ependymal epithelium cells. LYAAT-1-IR (immunoreactivity) levels varied among different neuroanatomical structures but were present in neurons independently of the neurotransmitter used (glutamate, GABA, acetylcholine, noradrenaline, serotonin, or glycine). Light and confocal microscopy demonstrated that LYAAT-1 and the lysosomal marker cathepsin D colocalized throughout the brain and electron microscopy showed that LYAAT-1-IR was associated with lysosomal membranes. In addition, LYAAT-1-IR was also found associated with other membranes belonging to the Golgi apparatus and lateral saccules and less frequently with multivesicular bodies, endoplasmic reticulum, and occasionally with the plasma membrane. The localization of LYAAT-1 at the lysosomal membrane is consistent with the view that it mediates amino acid efflux from lysosomes. Furthermore, its cell expression pattern suggests that it may contribute to specialized cellular function in the rat CNS such as neuronal metabolism, neurotransmission, and control of brain amino acid homeostasis.

Identification and characterization of a lysosomal transporter for small neutral amino acids

In eukaryotic cells, lysosomes represent a major site for macromolecule degradation. Hydrolysis products are eventually exported from this acidic organelle into the cytosol through specific transporters. Impairment of this process at either the hydrolysis or the efflux step is responsible of several lysosomal storage diseases. However, most lysosomal transporters, although biochemically characterized, remain unknown at the molecular level. In this study, we report the molecular and functional characterization of a lysosomal amino acid transporter (LYAAT-1), remotely related to a family of H+-coupled plasma membrane and synaptic vesicle amino acid transporters. LYAAT-1 is expressed in most rat tissues, with highest levels in the brain where it is present in neurons. Upon overexpression in COS-7 cells, the recombinant protein mediates the accumulation of neutral amino acids, such as gamma-aminobutyric acid, l-alanine, and l-proline, through an H+/amino acid symport. Confocal microscopy on brain sections revealed that this transporter colocalizes with cathepsin D, an established lysosomal marker. LYAAT-1 thus appears as a lysosomal transporter that actively exports neutral amino acids from lysosomes by chemiosmotic coupling to the H+-ATPase of these organelles. Homology searching in eukaryotic genomes suggests that LYAAT-1 defines a subgroup of lysosomal transporters in the amino acid/auxin permease family.

Lysosomal transport disorders

In the group of lysosomal storage diseases, transport disorders occupy a special place because they represent rare examples of inborn errors of metabolism caused by a defect of an intracellular membrane transporter. In particular, two disorders are caused by a proven defect in carrier-mediated transport of metabolites: cystinosis and the group of sialic acid storage disorders (SASD). The recent identification of the gene mutations for both disorders will improve patient diagnosis and shed light on new physiological mechanisms of intracellular trafficking.

Thyroglobulin regulates follicular function and heterogeneity by suppressing thyroid-specific gene expression

Thyroglobulin (TG) is the primary synthetic product of the thyroid and the macromolecular precursor of thyroid hormones. TG synthesis, iodination, storage in follicles, and lysosomal degradation can each modulate thyroid hormone formation and secretion into the circulation. Thyrotropin (TSH), via its receptor (the TSHR), increases thyroid hormone levels by upregulating expression of the sodium iodide symporter (NIS), thyroid peroxidase (TPO), and TG genes. TSH does this by modulating the expression and activity of the thyroid-specific transcription factors, thyroid transcription factor (TTF)-1, TTF-2, and Pax-8, which coordinately regulate NIS, TPO, TG, and the TSHR. Major histocompatibility complex (MHC) class I gene expression, which is also regulated by TTF-1 and Pax-8 in the thyroid, is simultaneously decreased; this maintains self tolerance in the face of TSH-increased gene products necessary for thyroid hormone formation. We now show that follicular TG, 27S > 19S > 12S, counter-regulates TSH-increased thyroid-specific gene transcription by suppressing the expression of the TTF-1, TTF-2, and Pax-8 genes. This decreases expression of the TG, TPO, NIS and TSHR genes, but increases class I expression. TG action involves an apical membrane TG-binding protein; however, it acts transcriptionally, targeting, for example, a sequence within 1.15 kb of the start of TTF-1 transcription. TG does not affect ubiquitous transcription factors regulating TG, TPO, NIS and/or TSHR gene expression. TG activity is not duplicated by thyroid hormones or iodide. We hypothesize that TG-initiated, transcriptional regulation of thyroid-restricted genes is a normal, feedback, compensatory mechanism which regulates follicular function, regulates thyroid hormone secretion, and contributes to follicular heterogeneity.

Species difference in radioactivity elimination from liver parenchymal cells after injection of radiolabeled proteins

To elucidate the cause for the different levels of hepatic radioactivity among mammals after injection of protein radiopharmaceuticals, the metabolism of radiolabeled proteins and the fate of their radiometabolites in the parenchymal cells of rat liver were investigated and compared with those of mice. We used galactosyl-neoglycoalbumin (NGA) as a carrier protein, and NGA was labeled with 111In via 1-(4-isothiocyanatobenzyl)ethylenediaminetetraacetic acid (SCN-Bz-EDTA) or 1-[p-(5-maleimidopentyl)aminobenzyl]ethylenediaminetetraacetic acid (EMCS-Bz-EDTA) and with 125I via direct iodination. All radiolabeled NGAs exhibited rapid accumulation in liver parenchymal cells after intravenous injection into rats. Radioactivity was eliminated following NGA-125I injection at similar rates from rat and mouse liver. In contrast, both 111In-labeled NGAs demonstrated much slower elimination of radioactivity in rat when compared with mouse liver. Analyses of radioactivity in bile and liver indicated that both SCN-Bz-EDTA and EMCS-Bz-EDTA rendered mono-amino acid adducts as the final radiometabolites, which were generated in rat liver within 1 h postinjection. Subcellular distribution studies suggested that these radiometabolites were copurified with lysosome in rat liver. Because similar results were observed in mice previously, the difference between rats and mice in radioactivity elimination from liver parenchymal cells would be predominantly attributable to the different efflux rate of the 111In-labeled metabolites from the lysosome between these species. Such differences in the efflux rates of radiometabolites from the lysosome among mammals may also account for the different hepatic radioactivity levels of radiolabeled proteins between animal and clinical studies.

Characterization of a melanosomal transport system in murine melanocytes mediating entry of the melanogenic substrate tyrosine

In this study, we identify a transport system for tyrosine, the initial precursor of melanin synthesis, in the melanosomes of murine melanocytes. Melanosomes preloaded with tyrosine demonstrated countertransport of 10 microM [3H]tyrosine, indicating carrier-mediated transport. Melanosomal tyrosine transport was saturable, with an apparent Km for tyrosine transport of 54 microM and a maximal velocity of 15 pmol of tyrosine/unit of hexosaminidase/min. Transport was temperature-dependent (Ea = 7.5 kcal/mol) and showed stereospecificity for the l-isomer of tyrosine. Aromatic, neutral hydrophobic compounds (such as tryptophan and phenylalanine), as well as the small, bulky neutral amino acids (such as leucine, isoleucine, and methionine) competed for tyrosine transport. Tyrosine transport was inhibited by the classical system L analogue, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and by monoiodotyrosine, but not by cystine, lysine, glutamic acid, or 2-(methylamino)-isobutyric acid. Tyrosine transport showed no dependence on Na+ or K+, and did not require an acidic environment or the availability of free thiols. These results demonstrate the existence of a neutral amino acid carrier in murine melanocyte melanosomes which resembles the rat thyroid FRTL-5 lysosomal system h. This transport system is critical to the function of the melanosome since tyrosine is the essential substrate required for the synthesis of the pigment melanin.

Ultrastructural evidence of thyroid damage in amiodarone-induced thyrotoxicosis

Amiodarone-induced thyrotoxicosis occurs in 2-12.1% of patients on chronic amiodarone treatment. In most cases its pathogenesis is related to iodine overload in the presence of preexisting thyroid abnormalities, such as multinodular or diffuse goiter or autonomous nodule. A minority of patients show apparently normal glands or pictures of non-autoimmune thyroiditis. However, there is recent evidence of a direct toxic effect of amiodarone, with consequent release of iodothyronines into the circulation. We report a patient with amiodarone-induced thyrotoxicosis with toxic thyroid effects demonstrated by electron microscopy in a fine-needle aspiration biopsy. There were three main pathologic findings: multilamellar lysosomal inclusions, intramitchondrial glycogen inclusions--both ultrastructural findings indicating thyroid cell damage--and a microscopic morphological pattern of thyroid cell hyperfunction. No inflammatory changes were found. Plasma thyroglobulin levels were high. The patient proved to be a non responder to simultaneous administration of methimazole (starting dose 30 mg/day) and potassium perchlorate (1000 mg/day for 40 days), while still taking amiodarone, thus providing evidence against a possible pathogenetic role of iodine overload. Dexamethasone (starting dose 3 mg/day) was added to methimazole. After three months euthyroidism had been restored and plasma thyroglobulin level substantially decreased. Subsequent subclinical hypothyroidism developed, which persisted after stopping antithyroid treatment and required substitution treatment with levothyroxine. In view of the primary role of lysosome function in the proteolysis of thyroglobulin molecules and of the energy-requiring carrier-mediated transport of monoiodotyrosine across the lysosomal membrane for iodine salvage and reutilization, we suggest that the pathological lysosomal and mitochondrial changes observed could be an ultrastructural marker for subsequent hypothyroidism in amiodarone-induced thyrotoxicosis. Our observations suggest the usefulness of ultrastructural thyroid evaluation and serial plasma thyroglobulin determinations to thoroughly evaluate the underlying pathogenetic mechanisms in amiodarone-associated thyrotoxicosis with apparently normal thyroid glands. Moreover, more knowledge of its pathogenesis could improve both prognostic stratification and treatment guides.

Detection and characterization of a transport system mediating cysteamine entry into human fibroblast lysosomes. Specificity for aminoethylthiol and aminoethylsulfide derivatives

The uptake of [3H]cysteamine by Percoll-purified human fibroblast lysosomes was investigated to determine whether lysosomes contain a transport system recognizing cysteamine. Lysosomal cysteamine uptake is a Na(+)-independent process which rapidly attains a steady state within 1 min at pH 7.0 and 37 degrees C. A biphasic Arrhenius plot is observed for cysteamine uptake, giving a Q10 of 2.2 from 17 to 26 degrees C and a Q10 of 1.2 from 27 to 35 degrees C. The rate of lysosomal cysteamine uptake is maximal at pH 8.2, half-maximal at pH 6.8, and declines approximately 50-fold from the maximum to show very little transport at pH 5.0. Cysteamine uptake into fibroblast lysosomes displays complete saturability with a Km of 0.88 mM and Vmax of 1410 pmol of beta-N-acetylhexosaminidase/min at pH 7.0 and 37 degrees C. Analog inhibition studies demonstrated that all analogs recognized thus far by the cysteamine carrier are either aminothiols or aminosulfides and contain an amino group and sulfur atom separated by a carbon chain, 2 carbon atoms in length. The Ki constants for these analogs as competitive inhibitors of lysosomal cysteamine uptake are 2-(ethylthio)ethylamine (0.64 mM), 1-amino-2-methyl-2-propanethiol (0.74 mM), 2-dimethylaminoethanethiol (0.87 mM), thiocholine (1.6 mM), and bis(2-aminoethyl)sulfide (4.9 mM). L-Cysteine, D-penicillamine, and analogs lacking either a sulfur atom or amino group are not recognized by the cysteamine carrier including ethanolamine, choline, taurine, beta-mercaptoethanol, ethylenediamine, cadaverine, spermine, spermidine, histamine, dopamine, and 3-hydroxytyramine. In a cystine-depletion assay, a 2-h exposure of cystinotic fibroblasts to 1 mM 1-amino-2-methyl-2-propanethiol lowers cell cystine levels to the same low level obtained with cysteamine. Thus, all four aminothiols, known to deplete cystinotic fibroblasts of their accumulated cystine, are recognized as substrates by the lysosomal cysteamine carrier, suggesting the importance of this transporter in the delivery of aminothiols to the lysosomal compartment.

The thyrotropin receptor

This chapter has outlined the complex process required for thyroid growth and function. Both events are regulated by TSHR via a multiplicity of signals, with the aid of and requirement for a multiplicity of hormones that regulate the TSHR via receptor cross-talk: insulin, IGF-I, adrenergic receptors, and purinergic receptors. Cross-talk appears to regulate G-protein interactions or activities induced by TSH as well as TSHR gene expression. The TSHR structure and its mechanism of signal transduction is being rapidly unraveled in several laboratories, since the recent cloning of the receptor. In addition, the epitopes for autoantibodies against the receptor that can subvert the normal regulated synthesis and secretion of thyroid hormones, causing hyper- or hypofunction, have been defined. Studies of regulation of the TSHR minimal promotor have uncovered a better understanding of the mechanisms by which TSH regulates both growth and function of the thyroid cell. A key novel component of this phenomenon involves TSH AMP positive and negative regulation of the TSHR. Negative transcriptional regulation is a common feature of MHC class I genes in the thyroid. Subversion of negative regulation or too little negative regulation is suggested to result in autoimmune disease. Methimazole and iodide at autoregulatory levels may be important in reversing this process and returning thyroid function to normal. Their action appears to involve factors that react with the IREs on both the TSHR and the TG promoter. Too much negative regulation, as in the case of ras transformation, results in abnormal growth without function. TTF-1 is implicated as a critical autoregulatory component in both positive and negative regulation of the TSHR and appears to be the link between TSH, the TSHR, TSHR-mediated signals, TG and TPO biosynthesis, and thyroid hormone formation. Differentially regulated expression of the TSHR and TG by cAMP and insulin depend on differences in the specificity of the TTF-1 site, that is, the lack of Pax-8 interactions with the TSHR, and the IRE sites. Single-strand binding proteins will become important in determining how TSHR transcription is controlled mechanistically.