The thyroid hormone secretory pathway--current dogmas and alternative hypotheses

[18F]tetrafluoroborate-PET/CT enables sensitive tumor and metastasis in vivo imaging in a sodium iodide symporter-expressing tumor model

Cancer cell metastasis is responsible for most cancer deaths. Non-invasive in vivo cancer cell tracking in spontaneously metastasizing tumor models still poses a challenge requiring highest sensitivity and excellent contrast. The goal of this study was to evaluate if the recently introduced PET radiotracer [18F]tetrafluoroborate ([18F]BF4-) is useful for sensitive and specific metastasis detection in an orthotopic xenograft breast cancer model expressing the human sodium iodide symporter (NIS) as a reporter. In vivo imaging was complemented by ex vivo fluorescence microscopy and γ-counting of harvested tissues. Radionuclide imaging with [18F]BF4- (PET/CT) was compared to the conventional tracer [123I]iodide (sequential SPECT/CT). We found that [18F]BF4- was superior due to better pharmacokinetics, i.e. faster tumor uptake and faster and more complete clearance from circulation. [18F]BF4--PET was also highly specific as in all detected tissues cancer cell presence was confirmed microscopically. Undetected comparable tissues were similarly found to be free of metastasis. Metastasis detection by routine metabolic imaging with [18F]FDG-PET failed due to low standard uptake values and low contrast caused by adjacent metabolically active organs in this model. [18F]BF4--PET combined with NIS expressing disease models is particularly useful whenever preclinical in vivo cell tracking is of interest.

Interdependence of thyroglobulin processing and thyroid hormone export in the mouse thyroid gland

Thyroid hormone (TH) target cells need to adopt mechanisms to maintain sufficient levels of TH to ensure regular functions. This includes thyroid epithelial cells, which generate TH in addition to being TH-responsive. However, the cellular and molecular pathways underlying thyroid auto-regulation are insufficiently understood. In order to investigate whether thyroglobulin processing and TH export are sensed by thyrocytes, we inactivated thyroglobulin-processing cathepsins and TH-exporting monocarboxylate transporters (Mct) in the mouse. The states of thyroglobulin storage and its protease-mediated processing and degradation were related to the levels of TH transporter molecules by immunoblotting and immunofluorescence microscopy. Thyroid epithelial cells of cathepsin-deficient mice showed increased Mct8 protein levels at the basolateral plasma membrane domains when compared to wild type controls. While the protein amounts of the thyroglobulin-degrading cathepsin D remained largely unaffected by Mct8 or Mct10 single-deficiencies, a significant increase in the amounts of the thyroglobulin-processing cathepsins B and L was detectable in particular in Mct8/Mct10 double deficiency. In addition, it was observed that larger endo-lysosomes containing cathepsins B, D, and L were typical for Mct8- and/or Mct10-deficient mouse thyroid epithelial cells. These data support the notion of a crosstalk between TH transporters and thyroglobulin-processing proteases in thyroid epithelial cells. We conclude that a defect in exporting thyroxine from thyroid follicles feeds back positively on its cathepsin-mediated proteolytic liberation from the precursor thyroglobulin, thereby adding to the development of auto-thyrotoxic states in Mct8 and/or Mct10 deficiencies. The data suggest TH sensing molecules within thyrocytes that contribute to thyroid auto-regulation.

A plasminogen-like protein, present in the apical extracellular environment of thyroid epithelial cells, degrades thyroglobulin in vitro

The prothyroid hormone, thyroglobulin (Tg), is stored at high concentrations in the thyroid follicular lumen as a soluble 19S homo-dimer and as heavier soluble (27S and 37S) and insoluble (Tgm) forms. Follicular degradation of Tg may contribute to maintaining Tg concentrations compatible with follicle integrity. Here, we report on the presence of a plasminogen-like protein in the follicular lumen of normal human thyroids and its synthesis and apical secretion by cultured epithelial thyroid cells. Since all the main luminal forms of Tg are cleaved by this plasminogen-like protein, we suggest that it contributes to Tg degradation in the follicular lumen.

ABCA5 resides in lysosomes, and ABCA5 knockout mice develop lysosomal disease-like symptoms

ABCA5 is a member of the ABC transporter A subfamily, and a mouse orthologue (mABCA5) in newborn mouse brain and neural cells was identified by reverse transcription-PCR. Full-length cDNA cloning revealed that mABCA5 consists of 1,642 amino acid residues and that its putative structure is that of a full-type ABC transporter having two sets of six transmembrane segments and a nucleotide binding domain. Immunohistochemical studies revealed that mABCA5 is expressed in brain, lung, heart, and thyroid gland. A subcellular localization analysis showed that mABCA5 is a resident of lysosomes and late endosomes. Abca5(-)(/)(-) mice exhibited symptoms similar to those of several lysosomal diseases in heart, although no prominent abnormalities were found in brain or lung. They developed a dilated cardiomyopathy-like heart after reaching adulthood and died due to depression of the cardiovascular system. In addition, Abca5(-)(/)(-) mice also exhibited exophthalmos and collapse of the thyroid gland. Therefore, ABCA5 is a protein related to a lysosomal disease and plays important roles, especially in cardiomyocytes and follicular cells.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Targeting of thyroglobulin to transcytosis following megalin-mediated endocytosis: evidence for a preferential pH-independent pathway

TG internalized from the colloid by megalin, bypasses the lysosomal pathway and is transported across thyrocytes by transcytosis. Although most of the intracellular mechanisms responsible for targeting of ligands to transcytosis are unknown, for certain ligands a role of lysosomal pH has been established. Thus, ligands that undergo lysosomal degradation dissociate from their receptors due to the low pH of endosomes, whereas certain ligands that undergo transcytosis fail to dissociate because they bind to their receptors at acidic pH. Here we studied the role of pH in TG transcytosis. We first investigated the effect of pH on megalin binding to TG in solid phase assays and found that, although megalin bound to TG at various pH values (ranging from 4-8), optimal binding was seen at acidic pH (ranging from 4.5-6). We then studied the effect of chloroquine (CQ) and ammonium chloride (AC), which increase endosomal pH, on transcytosis of TG across Fisher rat thyroid (FRTL-5 cells). Transcytosis assays were performed using FRTL-5 cells cultured on filters in dual chambered devices, with megalin expression only on the upper surface of the layers. TG was added to the upper chamber and transcytosed TG was measured in fluids collected from the lower chamber after incubation at 37 C. Treatment of FRTL-5 cells with CQ or AC did not affect binding and uptake of TG, but it did reduce T3 release from exogenously added TG, used as a measure of TG degradation in the lysosomal pathway. Treatment with CQ or AC resulted in an increase of transcytosis of TG across FRTL-5 cells, but only to a minimal extent (15-20%). The effects of CQ or AC and those of a megalin competitor (the monoclonal antibody 1H2, which reduced transcytosis) were not additive, suggesting that CQ and AC act on the megalin-mediated pathway. In conclusion, because TG binding to megalin is greatest at acidic pH, it is possible that TG does not dissociate from megalin in the lysosomal pathway. However, the pH-dependence of TG binding to megalin does not account for much of transcytosis, which probably occurs largely because of other mechanisms of targeting.

Thyroid stimulating hormone upregulates secretion of cathepsin B from thyroid epithelial cells

Constant levels of thyroid hormones in the blood are principal requirements for normal vertebrate development. Their release depends on the regulated proteolysis of thyroglobulin which is extracellularly stored in the follicle lumen under resting conditions. Thyroglobulin is proteolytically degraded to a major part in lysosomes, but in part also extracellularly leading to the release of thyroxine. Extracellularly occurring lysosomal enzymes are most probably involved in the proteolytic release of thyroxine. In this study we have analyzed the secretion of cathepsin B by thyroid follicle cells (primary cells as well as FRTL-5 cells) and its regulation by thyroid stimulating hormone, which stimulated the secretory release of the proenzyme as well as of mature cathepsin B. Within one to two hours of stimulation with thyroid stimulating hormone, the cathepsin B activity associated with the plasma membrane increased significantly. This increase correlated closely with the localization of lysosomes in close proximity to the plasma membrane of cultured thyrocytes as well as with the thyroxine liberating activity of thyrocyte secretion media. These observations indicate that thyroid stimulating hormone induces the secretion of cathepsin B, which contributes to the extracellular release of thyroxine by thyrocytes.

Polarized trafficking of thyrocyte proteins in MDCK cells

Recent studies suggest striking similarities between polarized protein sorting in thyrocytes and MDCK epithelial cells, including apical trafficking of thyroglobulin (Tg), thyroid peroxidase, and aminopeptidase N; as well as basolateral targeting of heparan sulfate proteoglycans, thrombospondin 1 (TSP1), type 1 5'-deiodinase, sodium-potassium ATPase, and the thyrotropin receptor. In this report, we have firstly expressed in stably transfected MDCK II cells a range of truncation mutants lacking up to 78% of the C-terminus of TSP1; these studies indicate that the N-terminal region containing the heparin binding domain is sufficient for basolateral targeting of TSP1. Secondly, we have stably transfected MDCK II cells with both Tg and sodium-iodide symporter (NIS) cDNAs, obtaining clones that simultaneously express both thyroid-specific proteins at the apical and basolateral cell surfaces, respectively. These studies represent promising early steps towards designing artificial thyrocytes by thyroid gene transfer into MDCK cells.

Cysteine proteinases mediate extracellular prohormone processing in the thyroid

Thyroglobulin, the precursor of thyroid hormones, is extracellularly stored in a highly condensed and covalently cross-linked form. Solublization of thyroglobulin is facilitated by cysteine proteinases like cathepsins B and K which are proteolytically active at the surface of thyroid epithelial cells. The cysteine proteinases mediate the processing of thyroglobulin by limited extracellular proteolysis at the apical plasma membrane, thereby rapidly liberating thyroxine. The trafficking of cysteine proteinases in thyroid epithelial cells includes their targeting to lysosomes where they become maturated before being transported to the apical plasma membrane and, thus, into the extracellular follicle lumen. We propose that thyroid stimulating hormone regulates extracellular proteolysis of thyroglobulin in that it enhances the rate of exocytosis of lysosomal proteins at the apical plasma membrane. Later, thyroid stimulating hormone upregulates thyroglobulin synthesis and its secretion into the follicle lumen for subsequent compaction by covalent cross-linking. Hence, cycles of thyroglobulin proteolysis and thyroglobulin deposition might result in the regulation of the size of the luminal content of thyroid follicles. We conclude that the biological significance of extracellularly acting cysteine proteinases of the thyroid is the rapid utilization of thyroglobulin for the maintenance of constant thyroid hormone levels in vertebrate organisms.

Megalin in thyroid physiology and pathology

Megalin, a member of the low density lipoprotein endocytic receptor family, is expressed on the apical surface of thyroid epithelial cells, directly facing the follicle lumen, where colloid is stored in high concentrations. Studies in vivo and with cultured thyroid cells have provided evidence that megalin expression on thyroid cells is TSH-dependent. Thyroglobulin (Tg), the major protein component of the colloid and the precursor of thyroid hormones, binds to megalin with high affinity and megalin mediates in part its uptake by thyrocytes. Tg internalized by megalin avoids the lysosomal pathway and is delivered by transepithelial transport (transcytosis) to the basolateral membrane of thyrocytes, from which it is released into the bloodstream. This process competes with pathways leading to thyroid hormone release from Tg molecules, which occurs following internalization of Tg molecules from the colloid by other means of uptake (fluid phase endocytosis or endocytosis mediated by low affinity receptors) that result in proteolytic cleavage in the lyosomes. During transcytosis of Tg, a portion of megalin (secretory component) remains complexed with Tg and enters the circulation, where its detection may serve as a tool to identify the origin of serum Tg in patients with thyroid diseases. Tg endocytosis via megalin is facilitated by the interaction of Tg with cell surface heparan sulfate proteoglycans, which occurs via a carboxyl terminal heparin binding site of Tg functionally related with a major megalin binding site. Although autoantibodies against megalin can be found in the serum of approximately 50% of patients with autoimmune thyroiditis, a role of megalin in this and other thyroid diseases remains to be established.

Role of thyroglobulin endocytic pathways in the control of thyroid hormone release

Thyroglobulin (Tg), the thyroid hormone precursor, is synthesized by thyrocytes and secreted into the colloid. Hormone release requires uptake of Tg by thyrocytes and degradation in lysosomes. This process must be precisely regulated. Tg uptake occurs mainly by micropinocytosis, which can result from both fluid-phase pinocytosis and receptor-mediated endocytosis. Because Tg is highly concentrated in the colloid, fluid-phase pinocytosis or low-affinity receptors should provide sufficient Tg uptake for hormone release; high-affinity receptors may serve to target Tg away from lysosomes, through recycling into the colloid or by transcytosis into the bloodstream. Several apical receptors have been suggested to play roles in Tg uptake and intracellular trafficking. A thyroid asialoglycoprotein receptor may internalize and recycle immature forms of Tg back to the colloid, a function also attributed to an as yet unidentified N-acetylglucosamine receptor. Megalin mediates Tg uptake by thyrocytes, especially under intense thyroid-stimulating hormone stimulation, resulting in transcytosis of Tg from the colloid to the bloodstream, a function that prevents excessive hormone release.

Circulating thyroglobulin transcytosed by thyroid cells in complexed with secretory components of its endocytic receptor megalin

After its endocytosis from the colloid, some thyroglobulin (Tg) is transcytosed intact across thyrocytes, accounting in part for its presence in the circulation. We previously showed that megalin (gp330), an endocytic Tg receptor, mediates apical to basolateral Tg transcytosis. Here we investigated whether a portion of megalin remains combined with Tg after its transcytosis, using studies with cultured thyroid cells and in vivo observations. FRTL-5 cells, a rat thyroid cell line, cultured on filters in dual chambers form tight junctions and exhibit features of polarity, with expression of megalin exclusively on the upper (apical) surface. After the addition of unlabeled Tg to the upper chamber and incubation at 37 C, some Tg was transcytosed intact across FRTL-5 cells into the lower chamber. Two antimegalin ectodomain antibodies precipitated transcytosed Tg in fluids collected from the lower chamber. After the addition of Tg to surface-biotinylated FRTL-5 cells, an anti-Tg antibody and the two antimegalin ectodomain antibodies precipitated high molecular mass biotinylated material in fluids collected from the lower chamber, corresponding to much of the megalin ectodomain, as well as smaller amounts of lower molecular mass material. The results indicate that Tg transcytosed across FRTL-5 cells remains complexed with megalin ectodomain components, which we refer to as megalin secretory components. In aminotriazole-treated rats, which develop increased megalin-mediated Tg transcytosis, antimegalin antibodies precipitated some of the Tg in the serum. Tg was also precipitated by antimegalin antibodies in sera from patients with Graves' disease, in which we found increased megalin expression on the apical surface of thyrocytes. In contrast, in thyroidectomized patients with metastatic papillary thyroid carcinoma, in whom Tg is directly secreted by neoplastic thyroid cells into the circulation rather than transcytosed, serum Tg was not precipitated by antimegalin antibodies. The detection of Tg-megalin complexes may help identify the source of serum Tg in patients with thyroid diseases.

Role of megalin (gp330) in transcytosis of thyroglobulin by thyroid cells. A novel function in the control of thyroid hormone release

When thyroglobulin (Tg) is endocytosed by thyrocytes and transported to lysosomes, thyroid hormones (T4 and T3) are released. However, some internalized Tg is transcytosed intact into the bloodstream, thereby avoiding proteolytic cleavage. Here we show that megalin (gp330), a Tg receptor on thyroid cells, plays a role in Tg transcytosis. Following incubation with exogenous rat Tg at 37 degrees C, Fisher rat thyroid (FRTL-5) cells, a differentiated thyroid cell line, released T3 into the medium. However, when cells were incubated with Tg plus either of two megalin competitors, T3 release was increased, suggesting that Tg internalized by megalin bypassed the lysosomal pathway, possibly with release of undegraded Tg from cells. To assess this possibility, we performed experiments in which FRTL-5 cells were incubated with either unlabeled or (125)I-labeled Tg at 37 degrees C to allow internalization, treated with heparin to remove cell surface-bound Tg, and further incubated at 37 degrees C to allow Tg release. Intact 330-kDa Tg was released into the medium, and the amount released was markedly reduced by megalin competitors. To investigate whether Tg release resulted from transcytosis, we studied FRTL-5 cells cultured as polarized layers with tight junctions on permeable filters in the upper chamber of dual chambered devices. Following the addition of Tg to the upper chamber and incubation at 37 degrees C, intact 330-kDa Tg was found in fluids collected from the lower chamber. The amount recovered was markedly reduced by megalin competitors, indicating that megalin mediates Tg transcytosis. We also studied Tg transcytosis in vivo, using a rat model of goiter induced by aminotriazole, in which increased release of thyrotropin induces massive colloid endocytosis. This was associated with increased megalin expression on thyrocytes and increased serum Tg levels, with reduced serum T3 levels, supporting the conclusion that megalin mediates Tg transcytosis. Tg transcytosis is a novel function of megalin, which usually transports ligands to lysosomes. Megalin-mediated transcytosis may regulate the extent of thyroid hormone release.

Identification of a heparin-binding region of rat thyroglobulin involved in megalin binding

We recently showed that thyroglobulin (Tg) is a heparin-binding protein and that heparin inhibits binding of Tg to its endocytic receptor megalin (gp330). Here we have identified a heparin-binding region in the carboxyl-terminal portion of rat Tg and have studied its involvement in megalin binding. Rat thyroid extracts, obtained by ammonium sulfate precipitation, were separated by column fractionation into four Tg polypeptides, with apparent masses of 660, 330, 210, and 50 kDa. As assessed by enzyme-linked immunoadsorbent assays and ligand blot binding assays, megalin bound to intact Tg (660 and 330 kDa) and, to a even greater extent, to the 210-kDa Tg polypeptide. Furthermore, the 210-kDa Tg polypeptide inhibited megalin binding to intact Tg by approximately 70%. Solid phase assays showed binding of biotin-labeled heparin to intact Tg and to the 210-kDa Tg polypeptide. We characterized the 210-kDa Tg polypeptide by matrix-assisted laser desorption/ionization mass spectrometry analysis and found that it corresponds to the carboxyl-terminal portion of rat Tg. We developed a synthetic peptide corresponding to a 15-amino acid sequence in the carboxyl-terminal portion of rat Tg (Arg(689)-Lys(703)), containing a heparin-binding consensus sequence (SRRLKRP) and demonstrated heparin binding to this peptide. A rabbit antibody raised against the peptide recognized intact Tg in its native conformation and under denaturing conditions. This antibody markedly reduced heparin-binding to intact Tg, indicating that the region of native Tg corresponding to the peptide is involved in heparin binding. Furthermore, the anti-Tg peptide antibody almost completely inhibited binding of megalin to Tg, suggesting that the Tg region containing the peptide sequence is required for megalin binding. Physiologically, Tg binding to megalin on thyroid cells may be facilitated by Tg interaction with heparin-like molecules (heparan sulfate proteoglycans) via adjacent binding sites.

Megalin (gp330) is an endocytic receptor for thyroglobulin on cultured fisher rat thyroid cells

We recently reported that megalin (gp330), an endocytic receptor found on the apical surface of thyroid cells, binds thyroglobulin (Tg) with high affinity in solid phase assays. Megalin-bound Tg was releasable by heparin. Here we show that Fisher rat thyroid (FRTL-5) cells, a differentiated rat thyroid cell line, can bind and endocytose Tg via megalin. We first demonstrated that FRTL-5 cells express megalin in a thyroid-stimulating hormone-dependent manner. Evidence of Tg binding to megalin on FRTL-5 cells and on an immortalized rat renal proximal tubule cell line (IRPT cells), was obtained by incubating the cells with 125I-Tg, followed by chemical cross-linking and immunoprecipitation of 125I-Tg with antibodies against megalin. To investigate cell binding further, we developed an assay in which cells were incubated with unlabeled Tg at 4 degrees C, followed by incubation with heparin, which released almost all of the cell-bound Tg into the medium. In solid phase experiments designed to illuminate the mechanism of heparin release, we demonstrated that Tg is a heparin-binding protein, as are several megalin ligands. The amount of Tg released by heparin from FRTL-5 and IRPT cells, measured by enzyme-linked immunosorbent assay (ELISA), was markedly reduced by two megalin competitors, receptor-associated protein (RAP) and 1H2 (monoclonal antibody against megalin), indicating that much of the Tg released by heparin had been bound to megalin ( approximately 60-80%). The amount inhibited by RAP was considered to represent specific binding to megalin, which was saturable and of high affinity (Kd approximately 11.2 nM). Tg endocytosis by FRTL-5 and IRPT cells was demonstrated in experiments in which cells were incubated with unlabeled Tg at 37 degrees C, followed by heparin to remove cell-bound Tg. The amount of Tg internalized (measured by ELISA in the cell lysates) was reduced by RAP and 1H2, indicating that Tg endocytosis is partially mediated by megalin.

Enhanced binding to the molecular chaperone BiP slows thyroglobulin export from the endoplasmic reticulum

To examine how binding of BiP (a molecular chaperone of the hsp70 family that resides in the endoplasmic reticulum) influences the conformational maturation of thyroglobulin (Tg, the precursor for thyroid hormone synthesis), we have developed a system of recombinant Tg stably expressed in wild-type Chinese hamster ovary (CHO) cells and CHO-B cells genetically manipulated for selectively increased BiP expression. The elevation of immunoreactive BiP in CHO-B cells is comparable to that seen during the unfolded protein response in the thyrocytes of certain human patients and animals suffering from congenital hypothyroid goiter with defective Tg. However, in CHO-B cells, we expressed Tg containing no mutations that induce misfolding (i.e. no unfolded protein response), so that levels of all other endoplasmic reticulum chaperones were normal. Increased availability of BiP did not accelerate Tg secretion; rather, the export of newly synthesized Tg was delayed. Tg detained intracellularly was concentrated in the endoplasmic reticulum. By coimmunoprecipitation, BiP exhibited enhanced binding to Tg in CHO-B cells. Moreover, two-dimensional gel analysis showed that BiP associated especially well with intracellular Tg containing mispaired disulfide bonds, thought to represent early Tg folding intermediates. An endoplasmic reticulum chaperone of the hsp90 family, GRP94, was also associated in Tg-chaperone complexes. The results suggest that increased binding of BiP to Tg leads to its delayed conformational maturation in the endoplasmic reticulum.

Chapter 5 Role of lysosomes in cell injury

Lysosomes are acidic intracellular vacuoles of heterogeneous shape, size, and content. Lysosomes contain hydrolytic enzymes that degrade proteins, lipids, carbohydrates, and nucleic acids derived from intracellular (through autophagy) and extracellular (through heterophagy) sources. Lysosomal degradation regulates several physiological cell functions. These include turnover of cellular organelles and extracellular constituents; amino acid and glucose homeostasis; processing of proteins; lipid metabolism; cell growth, differentiation, and involution; host defenses against microorganisms and other pathogens; and removal of necrotic and foreign material from the circulation and from tissues.

Lysosomal degradation also plays an important role in the pathophysiology of acute and chronic cell injury, inflammation and repair, and tumor growth and metastasis. The participation of the lysosomes in the specific types of cell injury we have discussed is due to altered regulation of one or more of the following processes: turnover of cellular organelles by autophagic degradation; levels and activities of lysosomal hydrolases; levels of intracellular and extracellular lysosomal hydrolase inhibitors; transport of degradation products from the lysosomal matrix to the cytosol; permeability of the lysosomal membrane to hydrolases; lysosomal vacuolar acidification; transport of degradable substrates and of pathogens to the lysosomes; transport and processing of secretory proteins and lysosomal hydrolases during biogenesis; traffic and fusion of lysosomal vacuoles and vesicles; secretion of lysosomal hydrolases; and accumulation of metals, particularly iron, acidotropic agents, and undegraded and/or undegradable materials in lysosomes.

The type-1 repeats of thyroglobulin regulate thyroglobulin degradation and T3, T4 release in thyrocytes

Thyroglobulin (Tg) proteolytic steps are central phenomena in Tg processing and thyroid hormone release in thyrocytes. Based on recent literature data, we propose that the type-1 repetitive units present in the Tg sequence could act as binders and reversible inhibitors of the proteases implicated in Tg processing. The pH-dependent interactions of proteases with the repeats could permit (i) protection from degradation of low iodinated Tg to be recycled; (ii) restriction of early proteolytic attacks to N- and C-terminal hormone formation sites; (iii) increase of the half-time of acidic proteases necessary for the final, extensive degradation steps of Tg.

Variability of serum thyroglobulin levels is determined by a major gene

OBJECTIVE

There are large variations in the circulating concentrations of thyroglobulin. The purpose of this study was to explore the possibility of a genetic basis for the variability of serum concentration of thyroglobulin (Tg) in euthyroid individuals.

DESIGN

The serum concentration of thyroglobulin (Tg) varies several-fold in euthyroid individuals. Other circulating proteins also show wide normal ranges of concentration and these variations have been shown to have a genetic as well as an environmental basis. To explore the possibility of a genetic basis for variability in serum Tg levels, an analysis was made of serum Tg levels in 44 pairs of identical twins and 66 nuclear families who were euthyroid and thyroid autoantibody negative (thereby eliminating subclinical autoimmune thyroid disease and Tg autoantibody interference with the Tg assay).

RESULTS

Each pair of identical twins tended to have a similar Tg level and the overall correlation was highly significant (r = 0.734, P < 0.001). There was no relation between Tg and TSH levels in the twins (r = 0.119; P = 0.366). Segregation analysis of the 66 families showed that where both parents had Tg levels above the overall median for the subjects (males, 19 micrograms/l; females, 33 micrograms/l), 73% of the offspring also had concentrations above these levels, compared with 30% of the offspring when one parent had a high Tg level and only 16% in families where neither parent had a high Tg level.

CONCLUSIONS

Complex segregation analysis using the computer program Pointer suggested that variability in Tg levels was the result of a major dominant-like gene effect (accounting for 80% of the variability) combined with a multifactorial component. Thyroglobulin, a template for thyroid hormone production, is also a major thyroid autoantigen and inherited variations in serum Tg levels may have implications for the pathogenesis of autoimmune thyroid disease.

Extrathyroidal release of thyroid hormones from thyroglobulin by J774 mouse macrophages

Thyroglobulin appears in the circulation of vertebrates at species-specific concentrations. We have observed that the clearance of thyroglobulin from the circulation occurs in the liver by macrophages. Here we show that the thyroid hormones T3 and T4 were released by incubation of mouse macrophages (J774) with thyroglobulin. Thyroid hormone release was a fast process, with an initial rate of approximately 20 pmol T4/mg per min and approximately 0.6 pmol T3/mg per min, indicating that macrophages preferentially release T4. The bulk of released thyroid hormones appeared after 5 min of incubation of macrophages with thyroglobulin, whereas degradation of the protein was detectable only after several hours. During internalization of thyroglobulin, endocytic vesicles and endosomes were reached at 5 min and lysosomes at 60 min. T4 release started extracellularly by secreted proteases and continued along the endocytic pathway of thyroglobulin, whereas T3 release occurred mainly intracellularly when thyroglobulin had reached the lysosomes. This shows that the release of both hormones occurred at distinct cellular sites. Our in vitro observations suggest that macrophages in situ represent an extrathyroidal source for thyroid hormones from circulating thyroglobulin.

Radioiodine uptake as a function of stored iodine and thyroglobulin in thyroid follicles: in vitro quantitative imaging approach with secondary ion mass spectrometry (SIMS) microscopy in human nontoxic goiter

We studied by an imaging technique, secondary ion mass spectrometry (SIMS) microscopy, the ability of thyroid follicles to pick-up and organify radioiodine (125I) according to the amounts of iodine (127I) and sulfur previously stored in thyroglobulin (Tg). After incubation with radioiodine in miniorgan culture, the SIMS analysis of 14 fragments from 12 goitrous patients permitted the observation of 3 iodine distribution profiles. In group 1 125I and 127I were easily detected by imaging; 125I concentration in follicular lumen was 80-fold lower than that of 127I. In group 2 125I images were obtained after long-term exposure and image processing. 125I concentration was not measurable while that of 127I was about 1.7-fold higher than that observed in group 1. In group 3 it was not possible to detect 125I while only traces of 127I were detected. This last profile can coexist with profile 1 in the same specimen. Tg sulfur concentration was 2-fold lower in group 3 than in the other 2 groups in which they were almost the same and significantly correlated with those of 127I. These data provide new insight into Tg and 127I traffic in the development of nonfunctioning goiters.