Regulation of thyroglobulin glycosylation. A comparative study of the thyroglobulins from porcine thyroid glands and follicles in serum-free culture

Capitalizing glycomic changes for improved biomarker-based cancer diagnostics

Cancer serum biomarkers are valuable or even indispensable for cancer diagnostics and/or monitoring and, currently, many cancer serum markers are routinely used in the clinic. Most of those markers are glycoproteins, carrying cancer-specific glycan structures that can provide extra-information for cancer monitoring. Nonetheless, in the majority of cases, this differential feature is not exploited and the corresponding analytical assays detect only the protein amount, disregarding the analysis of the aberrant glycoform. Two exceptions to this trend are the biomarkers α-fetoprotein (AFP) and cancer antigen 19-9 (CA19-9), which are clinically monitored for their cancer-related glycan changes, and only the AFP assay includes quantification of both the protein amount and the altered glycoform. This narrative review demonstrates, through several examples, the advantages of the combined quantification of protein cancer biomarkers and the respective glycoform analysis, which enable to yield the maximum information and overcome the weaknesses of each individual analysis. This strategy allows to achieve higher sensitivity and specificity in the detection of cancer, enhancing the diagnostic power of biomarker-based cancer detection tests.

Glycosylation in the Thyroid Gland: Vital Aspects of Glycoprotein Function in Thyrocyte Physiology and Thyroid Disorders

The key proteins responsible for hormone synthesis in the thyroid are glycosylated. Oligosaccharides strongly affect the function of glycosylated proteins. Both thyroid-stimulating hormone (TSH) secreted by the pituitary gland and TSH receptors on the surface of thyrocytes contain N-glycans, which are crucial to their proper activity. Thyroglobulin (Tg), the protein backbone for synthesis of thyroid hormones, is a heavily N-glycosylated protein, containing 20 putative N-glycosylated sites. N-oligosaccharides play a role in Tg transport into the follicular lumen, where thyroid hormones are produced, and into thyrocytes, where hyposialylated Tg is degraded. N-glycans of the cell membrane transporters sodium/iodide symporter and pendrin are necessary for iodide transport. Some changes in glycosylation result in abnormal activity of the thyroid and alteration of the metabolic clearance rate of hormones. Alteration of glycan structures is a pathological process related to the progression of chronic diseases such as thyroid cancers and autoimmunity. Thyroid carcinogenesis is accompanied by changes in sialylation and fucosylation, β1,6-branching of glycans, the content and structure of poly-LacNAc chains, as well as O-GlcNAcylation, while in thyroid autoimmunity the main processes affected are sialylation and fucosylation. The glycobiology of the thyroid gland is an intensively studied field of research, providing new data helpful in understanding the role of the sugar component in thyroid protein biology and disorders.

Identification, Characterization, and X-ray Crystallographic Analysis of a Novel Type of Mannose-Specific Lectin CGL1 from the Pacific Oyster Crassostrea gigas

A novel mannose-specific lectin, named CGL1 (15.5 kDa), was isolated from the oyster Crassostrea gigas. Characterization of CGL1 involved isothermal titration calorimetry (ITC), glycoconjugate microarray, and frontal affinity chromatography (FAC). This analysis revealed that CGL1 has strict specificity for the mannose monomer and for high mannose-type N-glycans (HMTGs). Primary structure of CGL1 did not show any homology with known lectins but did show homology with proteins of the natterin family. Crystal structure of the CGL1 revealed a unique homodimer in which each protomer was composed of 2 domains related by a pseudo two-fold axis. Complex structures of CGL1 with mannose molecules showed that residues have 8 hydrogen bond interactions with O1, O2, O3, O4, and O5 hydroxyl groups of mannose. The complex interactions that are not observed with other mannose-binding lectins revealed the structural basis for the strict specificity for mannose. These characteristics of CGL1 may be helpful as a research tool and for clinical applications.

Glycan-Dependent Mutual and Reversible Sequestration of Two Thyroid Cancer Biomarkers

BACKGROUND

Thyroglobulin (Tg), the major thyroidal protein, plays important roles in thyroid hormone biosynthesis and in autoimmune thyroid diseases (AITD). Tg also serves as a pre- and postoperative biomarker of differentiated thyroid cancer (DTC). The endogenous β-galactoside binding lectin galectin-3 (Gal-3), secreted by transformed thyroid cells, has been shown to be another useful biomarker of DTC. Tg contains covalently linked complex-type glycans that can serve as binding epitopes of Gal-3. The objective of the study is to investigate the interaction between Tg and Gal-3 and discuss its potential consequences.

METHODS

Binding interaction between Tg and Gal-3 was first studied by hemagglutination inhibition assays. Subsequently, a detailed analysis of binding thermodynamics was carried out by isothermal titration calorimetry. Quantitative precipitation was performed to study the complex formation between Tg and Gal-3 and to determine the binding stoichiometry. The concentration-dependent rate and amount of complex formation between Tg and Gal-3 was examined spectrophotometrically. A similar approach was taken to study the effect of free Tg and Gal-3 on preformed Tg-Gal-3 complex.

RESULTS

Quantitative biochemical and biophysical data show that these two biomarkers produced by thyroid cancer cells interact with each other with submicromolar affinity and form an insoluble complex at their stoichiometric concentration. One Tg molecule could bind up to 14 molecules of Gal-3. Such complex formation mutually sequestered both Tg and Gal-3, decreasing the concentration of their freely available forms. Formation of the Tg-Gal-3 complex was reversible as the preformed complex was dissolved by free Tg as well as free Gal-3. While free Tg rapidly dissolved preformed Tg-Gal-3 complex in a concentration-dependent manner, Gal-3 was found to be much less efficient and slowly dissolved only a fraction of the preformed complex at a relatively higher Gal-3 concentration.

CONCLUSIONS

Complex formation between Tg and Gal-3 through high affinity binding and the sensitivity of the complex to free Tg and Gal-3 can potentially influence their biological functions. Interactions between Tg and Gal-3 might also interfere with their clinical detection, the same way Tg autoantibody (TgAb) is reported to interfere with Tg assays. The data support a model of Gal-3-mediated homeostatic process of Tg.

Modulation of TSHR signaling by posttranslational modifications

Posttranslational modifications of seven transmembrane receptors (7TMRs) affect their function to a large extent. Many studies of glycosylation or phosphorylation of 7TMRs have shown that these modifications influence the cell-surface expression or signaling of the receptor. Recently, other types of posttranslational modifications of the thyrotropin-stimulating hormone receptor (TSHR) have been characterized, including sialylation and dimerization. Increased TSHR sialylation results in increased TSHR cell-surface expression. Furthermore, TSHR oligomerization and the probable modification of TSHR signaling in lipid rafts require further clarification with regard to their functional consequences. In addition to its known coupling to Galphas and Galphaq, and possibly other G proteins, the TSHR also couples to further signaling pathways, such as the mitogen-activated protein kinase (MAPK) pathway, which involves G-protein-coupled receptor kinases (GRKs) and arrestins. We discuss these emerging new findings and their implications for signaling of the TSHR.

Sialylation of human thyrotropin receptor improves and prolongs its cell-surface expression

Glycosylation of the thyrotropin receptor (TSHR) has been shown to be essential for correct protein folding and for cell-surface targeting. In a recent study, we detected increased expression of beta-galactoside alpha(2,6)-sialyltransferase (SIAT1) in toxic thyroid adenomas where gain-of-function mutations of the TSHR have been invoked as one of the major causes. To investigate the physiological meaning of these findings, we designed experiments to evaluate the consequences of sialylation for the expression of the TSHR. Hence, we investigated the effect of coexpressing the TSHR and different sialyltransferases (SIAT1, SIAT4a, and SIAT8a) for cell-surface expression of the receptor. Coexpression of each of the three SIAT isoforms and the TSHR in COS-7 cells increased TSHR expression on the cell surface in the range of 50 to 100%. Moreover, Western blot analysis with lectins specific for alpha(2,3) and alpha(2,6)-linked sialic acids and lectin-binding enzyme-linked immunosorbent assay support a direct effect on TSHR cell-surface expression mediated by sialic acid transfer to the TSHR. Finally, we treated living COS-7 cells after cotransfection of TSHR and SIAT8a with neuraminidase for 30 min to remove covalently linked sialic acid. Subsequent loss of TSHR cell-surface expression suggests that sialylation prolongs the resting time of the TSHR on the cell surface. Our data demonstrate for the first time that the transfer of sialic acid can improve and prolong cell-surface expression of a transmembrane receptor.

Thyroid sialyltransferase mRNA level and activity are increased in Graves' disease

Sialylation of cell components is an important immunomodulating mechanism affecting cell response to hormones and adhesion molecules. To study alterations in sialic acid metabolism in Graves' disease (GD) we measured the following parameters in various human thyroid tissues: lipid-bound sialic acid (LBSA) content, ganglioside profile, total sialyltransferase activity, and the two major sialyltransferase mRNAs for sialyltransferase-1 (ST6Gal I) and for sialyltransferase-4A (ST3Gal I). Fragments of toxic thyroid nodules (TN), nontoxic thyroid nodules (NN) and nontumorous tissue from patients with nodular goiter or thyroid cancer were used as a control (C). The LBSA content and sialyltransferase activity were the highest in the GD group (164 +/- 4.44 versus 120 +/- 2.00 nmoL/g, p = 0.005 and 1625 +/- 283.5 versus 324 +/- 54.2 cpm/mg of protein, p < 0.005 compared to control group C). Ganglioside profile in the GD group was similar to that in control tissues. Sialyltransferase- 1 mRNA and sialyltransferase-4A mRNA levels were significantly higher in the GD group than in the control group (12.52 +/- 6.90 versus 2.54 +/- 1.24 arbitrary units, p < 0.005 and 2,49 +/- 1.16 versus 1.23 +/- 0.46 arbitrary units, p < 0.05, respectively). There was a positive correlation between the increased sialyltransferase-1 mRNA level and the TSH-receptor antibody titer determined by the TRAK test. These results indicate that sialyltransferases expression and activity are increased in GD. Exact mechanism of this upregulation remains unknown, though one of possible explanations is the activation of the thyrotropin (TSH) receptor.

Thyroid autoantibody profiles in ophthalmic dominant and thyroid dominant Graves' disease differ and suggest ophthalmopathy is a multiantigenic disease

BACKGROUND

Thyroid-associated ophthalmopathy (TAO) occurs in 25-50% of patients with Graves' disease (GD) and is occasionally seen in hypothyroid Hashimoto's disease or euthyroid individuals. The link between TAO and hyperthyroidism remains unclear. We hypothesized that qualitative or quantitative differences in thyroid antibodies might determine individual predisposition to these features.

METHODS

In a prospective study over 3 years, thyroid antibody levels were measured in all patients diagnosed at the Singapore National Eye Centre to have GD. These patients had no known history of thyroid disease, presented with eye complaints and diagnosis was made by an ophthalmologist. A total of 31 patients were identified. Antibody levels were compared against 71 consecutive patients referred to a thyroid clinic (TC) for thyrotoxic symptoms in whom the diagnosis of GD was confirmed by a thyroidologist.

FINDINGS

Thyroid autoantibody profiles of patients diagnosed at the ophthalmology centre (OC) and TC differed markedly. OC patients had significantly higher TSI (P = 0.003) but lower TPOAb (P = 0.008) and TgAb levels (P < 0.001). In contrast, TC patients had higher free T4 (P = 0.048) and higher TBII levels (P < 0.001). Antibody levels were correlated with four parameters of ophthalmopathy--chronic lid retraction, lid swelling, proptosis and extraocular myopathy (EOM). On univariate logistic regression analysis, TSI was a positive predictor and TPOAb and TgAb negative predictors of all four features. In the absence of TgAb, the odds ratios for individual TAO features ranged from 2.8 to 7.9, with corresponding values of 3.9-10.2 when TPOAb was absent. In stepwise logistic regression analysis, TSI was the strongest independent predictor of all aspects studied: lid fullness P = 0.001, proptosis P = 0.001, lid retraction P = 0.008, EOM P = 0.009. Among smokers, TPOAb were significantly lower (P = 0.044) but no association between smoking and the other antibodies was observed.

INTERPRETATION

The study demonstrates markedly different thyroid autoantibody profiles in newly diagnosed GD patients with ophthalmic dominant as opposed to thyroid dominant features. It suggests differing antibody patterns are associated with predisposition to hyperthyroidism and orbitopathy. In addition, an association between smoking and low TPOAb levels was noted.

Depletion of divalent cations within the secretory pathway inhibits the terminal glycosylation of complex carbohydrates of thyroglobulin

Newly synthesized thyroglobulin transiting the secretory pathway is posttranslationally modified by addition of oligosaccharides to asparagine N-linked residues. The effect of divalent cation depletion on oligosaccharide processing of Tg was studied in FRTL-5 cells. Treatment with an ionophore, A23187, or thapsigargin, an inhibitor of the sarcoplasmic/endoplasmic reticulum ATPases delayed Tg secretion. These effects were accompanied by a normal distribution of the marker of the endoplasmic reticulum protein disulfide isomerase. Analysis of the thyroglobulin oligosaccharides by Bio-gel P4 chromatography showed that in the presence of A23187 and thapsigargin the addition of peripheral sialic acid and possibly galactose is inhibited. These findings were strengthened by experiments of exoglycosidase digestion and SDS-PAGE analysis of the resulting products. These results reveal a cellular mechanism of production of thyroglobulin with incompletely processed complex chains, i.e., the ligand of the recently described GlcNAc and asialoglycoprotein receptors of the thyroid. Since A23187 and thapsigargin inhibit biosynthetically the addition of peripheral sugars on N-linked oligosaccharides chains, the thyroglobulin molecules secreted in the presence of A23187 and thapsigargin should greatly facilitate studies on the function of the GlcNAc and asialoglycoprotein receptors of the thyroid.

A comparative review of the structure and biosynthesis of thyroglobulin

Thyroglobulin, the major iodoglycoprotein of the thyroid (Mr 669 kDa) has a sedimentation coefficient of 19 S and an isoelectric point (pI) of 4.4-4.7. The protein has been isolated and purified from saline extracts of the gland of several animal species, by methods such as ammonium sulfate fractionation, DEAE-cellulose chromatography and Sepharose 4B/6B gel-filtration. DEAE-cellulose chromatography of thyroglobulin from many species, by linear gradient, yielded a complex elution pattern, while camel thyroglobulin showed only a major and minor peak. As an iodoprotein, the protein has 0.1-2.0% iodine. The amino acid and iodoamino acid composition of thyroglobulins, in general, is similar. However, a high thyroxine content (15 mol/mol protein) has been noted for buffalo species. Asparagine or aspartic acid has been reported as the major N-terminal amino acid for thyroglobulins of several animal species whereas glutamic acid is the sole N-terminal amino acid for buffalo thyroglobulin. As a glycoprotein, thyroglobulin contains 8-10% total carbohydrate with galactose, mannose, fucose, N-acetyl glucosamine and sialic acid residues. The carbohydrate in the protein is distributed as two distinct units, A and B. In addition, human thyroglobulin has carbohydrate unit C. The occurrence of sulfate and phosphate as Gal-3-SO4 and Man-6-PO4, respectively, has been reported in few species. The quaternary structure of native thyroglobulin is comprised of two equal sized subunits of 330 kDa. However, the protein appears to contain 4-8 non-identical units in few species. The synthesis of thyroid hormones occurs in the matrix of the protein and is regulated by pituitary thyrotropin. The role of tyrosine residues 5 and 130 in thyroxine synthesis has been well documented.

Carbohydrate analysis of porcine thyroglobulin isoforms with different iodine contents

To further validate the relationship between thyroid hormone formation and the carbohydrate structure of thyroglobulin (Tg), we reinvestigated the relationship between the iodine content and the asparagine-linked oligosaccharide structures of porcine Tg. Purified porcine Tg was further separated into isoforms (Tg-F1, -F2 and -F3) with a DEAE-cellulose ion-exchange chromatography column. The iodine residues, neutral sugar and sialic acid were analyzed for the separated Tg isoforms and their asparagine-linked oligosaccharide structures were analyzed. The asparagine-linked oligosaccharides were released from Tg-F1, -F2 and -F3 by hydrazinolysis and each oligosaccharide was labeled with p-aminobenzoic acid octyl ester (ABOE). The ABOE-labeled oligosaccharides from Tg-F1, -F2 and -F3 were analyzed for their relative content in oligosaccharides of each structure type by chemical methods and DEAE- and ConA high-performance liquid chromatography (HPLC) columns. As a result, it was revealed that the Tg fraction eluted at higher ionic strength from a DEAE-cellulose column is apt to contain more of each iodoamino acid, as well as total content of iodine, larger negative zeta-potential, conforming to sialic acid content in the Tg molecule and to a higher content of di-sialo-bi-antennary complex and to high mannose type oligosaccharides. These results support the conclusion that iodine organification of the Tg molecule is correlated with asparagine-linked oligosaccharide completion.

Perturbation of cellular calcium delays the secretion and alters the glycosylation of thyroglobulin in FRTL-5 cells

Treatment of FRTL-5 cells with a Ca2+ ionophore, A23187, or a specific inhibitor of the endoplasmic reticulum Ca2+ ATPases, thapsigargin, delayed thyroglobulin secretion. The secreted thyroglobulin showed an increased electrophoretic mobility and a reduced sensitivity to neuraminidase. Only thyroglobulin that was still in the endoplasmic reticulum was sensitive to the Ca(2+)-perturbant drugs as shown by experiments in which the drugs were added at different times during a chase. Analysis of the carbohydrate chains by BioGel P4 showed that thyroglobulin secreted in the presence of the Ca(2+)-perturbants displayed an increased ratio high mannose/complex chains.

Intracellular protein transport to the thyrocyte plasma membrane: potential implications for thyroid physiology

We present a snapshot of developments in epithelial biology that may prove helpful in understanding cellular aspects of the machinery designed for the synthesis of thyroid hormones on the thyroglobulin precursor. The functional unit of the thyroid gland is the follicle, delimited by a monolayer of thyrocytes. Like the cells of most simple epithelia, thyrocytes exhibit specialization of the cell surface that confronts two different extracellular environments-apical and basolateral, which are separated by tight junctions. Specifically, the basolateral domain faces the interstitium/bloodstream, while the apical domain is in contact with the lumen that is the primary target for newly synthesized thyroglobulin secretion and also serves as a storage depot for previously secreted protein. Thyrocytes use their polarity in several important ways, such as for maintaining basolaterally located iodide uptake and T4 deiodination, as well apically located iodide efflux and iodination machinery. The mechanisms by which this organization is established, fall in large part under the more general cell biological problem of intracellular sorting and trafficking of different proteins en route to the cell surface. Nearly all exportable proteins begin their biological life after synthesis in an intracellular compartment known as the endoplasmic reticulum (ER), upon which different degrees of difficulty may be encountered during nascent polypeptide folding and initial export to the Golgi complex. In these initial stages, ER molecular chaperones can assist in monitoring protein folding and export while themselves remaining as resident proteins of the thyroid ER. After export from the ER, most subsequent sorting for protein delivery to apical or basolateral surfaces of thyrocytes occurs within another specialized intracellular compartment known as the trans-Golgi network. Targeting information encoded in secretory proteins and plasma membrane proteins can be exposed or buried at different stages along the export pathway, which is likely to account for sorting and specific delivery of different newly-synthesized proteins. Defects in either burying or exposing these structural signals, and consequent abnormalities in protein transport, may contribute to different thyroid pathologies.

Thyrotropin controls dolichol-linked sugar pools and oligosaccharyltransferase activity in thyroid cells

We previously showed that thyroglobulin (Tg) glycosylation is enhanced 1.5-fold under thyrotropin (TSH) stimulation, corresponding to an increased number of oligosaccharide chains per molecule of Tg. Now the steps involving dolichol components and oligosaccharyltransferase activity have been studied. Porcine thyroid cells were cultured on porous bottom filters with or without TSH and incubated with [14C]mevalonate. Under TSH regulation, the level of the whole of dolichol components was increased 1.25-fold without modifying their distribution. Dolichol, and free and monosaccharide-linked dolichyl-phosphate, represented respectively 40% and 45% of total dolichol components while dolichyl-pyrophosphate-oligosaccharide represented 3% only. A marked enhancement (4.2-fold) of oligosaccharyltransferase activity occurred in stimulated cells, which could correspond to the addition of the two TSH effects: stimulation of Tg synthesis (3-fold) and of Tg glycosylation (1.5-fold). The amount of lipid carriers appeared to be insufficiently increased but no component is a limiting step, suggesting that the turnover of dolichol derivatives may be increased under TSH control through their use by higher amounts of Tg.

N-glycans modulate in vivo and in vitro thyroid hormone synthesis. Study at the N-terminal domain of thyroglobulin

Thyroglobulin (Tg) is the substrate for thyroid hormone biosynthesis, which requires tyrosine iodination and iodotyrosine coupling and occurs at the apical membrane of the thyrocytes. Tg glycoconjugates have been shown to play a major role in Tg routing through cellular compartments and recycling after endocytosis. Here we show that glycoconjugates also play a direct role in hormonosynthesis. The N-terminal domain (NTD; Asn1-Met171) of human Tg, which bears the preferential hormonogenic site, brings two N-glycans (Asn57 and Asn91). NTD preparations were purified from Tg with low and mild iodine content in vivo and from poorly iodinated Tg after in vitro iodination and coupling. NTD separated from poorly iodinated Tg was also submitted to iodination and coupling after desialylation and deglycosylation. The various NTD isoforms were analyzed for their N-glycan structures and hormone contents. Our results show that 1) in vivo as well as in vitro unglycosylated isoforms did not synthesize hormones, whereas fully or partially (at Asn91) glycosylated isoforms did; 2) high mannose type structures enhanced the hormone content; and 3) desialylation did not affect in vitro hormone synthesis. Evidence of a direct involvement in hormonosynthesis adds to the role of N-glycans in Tg function and opens the way to new mechanisms for regulation (e.g. TSH modulation of N-glycan) or alteration (e.g. Asn91 mutation) of thyroid hormone synthesis.

Analysis of protein glycosylation by mass spectrometry

There is a growing pharmaceutical market for protein-based drugs for use in therapy and diagnosis. The rapid developments in molecular and cell biology have resulted in production of expression systems for manufacturing of recombinant proteins and monoclonal antibodies. These proteins are glycosylated when expressed in cell systems with glycosylation ability. For glycoproteins intended for therapeutic administration it is important to have knowledge about the structure of the carbohydrate side chains to avoid cell systems that produce structures, which in humans can cause undesired reactions, e.g., immunological and unfavorable serum clearance rate. Structural analysis of glycoprotein oligosaccharides requires sophisticated instruments like mass spectrometers and nuclear magnetic resonance spectrometers. However, before the structural analysis can be conducted, the carbohydrate chains have to be released from the protein and purified to homogeneity, and this is often the most time-consuming step. Mass spectrometry has played and still plays an important role in analysis of protein glycosylation. The superior sensitivity compared to other spectroscopic methods is its main asset. Structural analysis of carbohydrates faces several problems, however, due to the chemical nature of the constituent monosaccharide residues. For oligosaccharides or glycoconjugates, the structural information from mass spectrometry is essentially limited to monosaccharide sequence, molecular weight, an only in exceptional cases glycosidic linkage positions can be obtained. In order to completely establish an oligosaccharide structure, several other structural parameters have to be determined, e.g., linkage positions, anomeric configuration and identification of the monosaccharide building blocks. One way to address some of these problems is to work on chemical pretreatment of the glycoconjugate, to specifically modify the carbohydrate chain. In order to introduce specific modifications, we have used periodate oxidation and trifluoroacetolysis with the objective of determining glycosidic linkage positions by mass spectrometry.

Hormonal regulation of some steps of thyroglobulin synthesis and secretion in bicameral cell culture

Porcine thyroid cells were cultured for 15 days on porous bottom chambers with or without different mixtures of hormones added to serum-free basal medium. Assays with 10% serum were also performed for comparison with previously published results. The effects of the hormones, particularly insulin, TSH and hydrocortisone, were studied on total RNA content, thyroglobulin mRNA level, the amount of thyroglobulin secreted into the apical medium and on glycosylation. Insulin and TSH similarly increased the total RNA content, and their effects were additive. Thyroglobulin mRNA content was increased twofold by insulin and threefold by TSH. When they were added simultaneously, the maximal level of thyroglobulin mRNA was reached, showing that TSH and insulin effects on thyroglobulin gene expression were additive. Hydrocortisone alone did not modify total RNA or thyroglobulin mRNA content but the hormone amplified total RNA when insulin and TSH were present together. The basal level of thyroglobulin secreted into the apical medium was increased threefold by insulin and fourfold by TSH. The effects of these two hormones added together appeared to be additive. Hydrocortisone had no effect alone or even when combined with insulin or TSH. However, when the three hormones were added together, the hormonal response was amplified. TSH effect and insulin effect on the incorporation of 3H-mannose into thyroglobulin as well as on the anionic residue content of the molecule were additive.

TSH-induced galactose incorporation at the NH2 terminus of thyroglobulin secreted by FRTL-5 cells

FRTL-5 cells were cultured in media containing standard growth factors with or without TSH, plus labeled precursors of N-linked oligosaccharide chains. The thyroglobulin secreted in the medium was purified and fragmented with CNBr. Three peptides were identified by NH2-terminal sequencing, that were labeled mainly with D-[2-3H]mannose, independent of TSH. One of them, corresponding to the NH2-terminus of thyroglobulin, incorporated both more D-[2-3H]mannose and more D-[1-3H]galactose upon TSH addition. These data likely reflect a TSH-induced increment of N-linked glycosylation at the NH2-terminus of thyroglobulin, mostly with the maturation of high-mannose to complex chains.

Carbohydrate binding properties of the envelope glycoproteins of human immunodeficiency virus type 1

Here, we confirm and extend our previous findings on human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein N-acetylglucosaminyl binding properties. We show the occurrence of saturable, temperature, pH, and calcium dependent carbohydrate-specific interactions between recombinant precursor gp160 (rgp160) and two affinity matrices: D-mannose-divinylsulfone-agarose, and natural glycoprotein, fetuin, also coupled to agarose. Binding of rgp160 to the matrices was inhibited by soluble mannosyl derivatives, alpha-D-Man17-BSA and mannan, by beta-D-GlcNAc47-BSA and by glycopeptides from Pronase-treated porcine thyroglobulin, which produces oligomannose and complex N-linked glycans. Glycopeptides from Endoglycosidase H-treated thyroglobulin partially inhibited rgp160 binding, as did the asialo-agalacto-tetraantennary precursor oligosaccharide of human alpha 1-acid glycoprotein for binding to fetuin-agarose. beta-D-Glucan and beta-D-Gal17-BSA had no or only limited effect. Also, surface unit rgp120 specifically interacted with fetuin-agarose and soluble fetuin, but in the latter case with a twofold reduced affinity relative to rgp160. After affinity chromatography, rgp160 was specifically retained by the two matrices and eluted by mannan in both cases, while rgp120 was not retained by fetuin-agarose but only eluted as a significantly retarded peak, which confirms its specific but weak interaction. Thus, rgp160 interacts with both oligomannose type, and the mannosyl core of complex type N-linked glycans, and its gp120 region plays a role in this interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of carbohydrate residues on human thyroid peroxidase (TPO) and thyroglobulin (Tg) and effects of deglycosylation, reduction and unfolding on autoantibody binding

The contribution of carbohydrate residues and peptide chain conformation to autoantibody binding sites on human thyroid peroxidase (TPO) and thyroglobulin (Tg) has been investigated. In addition the nature of carbohydrate residues associated with human TPO has been studied. 125I-labelled human TPO and Tg were treated with the following glycosidases: EndoD, EndoH, neuraminidase, O-glycanase, neuraminidase followed by O-glycanase and PNGaseF. Thereafter binding to different sera containing TPO autoantibodies and Tg autoantibodies was assessed using solid phase protein A to separate antibody-bound and free labelled antigens. In addition, labelled Tg and TPO were treated with reducing agent (dithiothreitol) or sodium acetate buffer pH 7.5, 5.5 and 3.2 (followed by neutralisation with 2 M Tris pH 8.3) prior to antibody binding studies. Furthermore, the effect of deglycosylation and treatment with acid buffers on TPO enzyme activity was studied. The nature of carbohydrate residues associated with hTPO was analysed by assessment of the effects of different glycosidases on 125I-TPO mobility on SDS-PAGE followed by autoradiography and by the use of lectins. Deglycosylation of labelled Tg and TPO had no clear effect on Tg and TPO autoantibody binding. Reduction of labelled Tg and TPO resulted in almost complete loss of autoantibody binding with all sera studied. Furthermore, adjusting the pH of labelled TPO or Tg transiently to pH 5.5 lowered autoantibody binding in the case of all the sera and the effect was more marked at pH 3.2. TPO enzyme activity (guaiacol assay) of unlabelled TPO was decreased after treatment with EndoH but not with other glycosidases. The low pH buffers affected unlabelled TPO enzyme activity measured by iodide assay. Treatment of 125I-labelled TPO with EndoH, neuraminidase and PNGaseF caused marked changes in the double band pattern characteristic of TPO on analysis by SDS gel electrophoresis (TPO doublet). Analysis of changes in the mobility of the 2 bands of the doublet after treatment with different glycosidases and binding studies with lectins indicated that both high mannose and complex type sugar residues were associated with hTPO. The high mannose type residues were associated mostly with the lower band of the hTPO doublet whereas complex type residues were associated mostly with the upper band. Overall, our studies indicate that (1) the major autoantibody binding sites on hTPO and hTg are conformational, (2) sugar residues do not appear to be important in forming the autoantibody binding sites on hTPO and hTg, and (3) both high mannose type and complex type sugar residues are associated with hTPO.

Environmental effects on protein glycosylation

Cultured mammalian cells are being used to produce proteins for therapeutic and diagnostic use because of their ability to perform complex post-translational modifications, including glycosylation. The oligosaccharide moieties can play an important role in defining several biological properties of glycoproteins, including clearance rate, immunogenicity, and biological specific activity. There is a growing interest in defining the factors that influence glycosylation, including the cell culture environment. In this review we organize the published data from in vitro cell culture and tissue culture studies that demonstrate direct effects of the culture environment on N-linked glycosylation.

Sulfated N-linked carbohydrate chains in porcine thyroglobulin

N-linked carbohydrate chains of porcine thyroglobulin were released by the hydrazinolysis procedure. The resulting mixture of oligosaccharide-alditols was fractionated by high-voltage paper electrophoresis, the acidic fractions were further separated by high-performance liquid chromatography on Lichrosorb-NH2, and analyzed by 500-MHz 1H-NMR spectroscopy and, partially, by permethylation analysis. Of the acidic oligosaccharide-alditols, the following sulfated carbohydrate chains could be identified: NeuAc alpha 2----6Gal beta 1----4GlcNAc beta 1----2Man alpha 1----3[(SO3Na----3)Gal beta 1----4GlcNAc beta1----2-Mana alpha 1----6]Man beta 1----4GlcNAc beta 1----4[Fuc alpha 1----6]GlcNAc-ol and NeuAc alpha 2----6Gal beta 1----4(SO3Na----)0-1 GlcNAc beta 1----2-Man alpha 1----3[NeuAc alpha 2----6Gal beta 1----4(SO3Na----6)1-0GlcNAc beta 1----2Man alpha 1----6]Man beta 1----4GlcNAc beta 1----4[Fuc alpha 1----6]GlcNAc- ol. The sulfated structural elements for porcine thyroglobulin form novel details of N-linked carbohydrate chains. They contribute to the fine structure of these oligosaccharides and are another type of expression of microheterogeneity.

Blood appearance of rat alkaline phosphatase originating from the duodenum in vitro

The major source of rat serum alkaline phosphatase (ALP) is well known to be from the intestinal enzyme, but it is still unclear whether it is from the duodenal or the ileal enzyme. The organic origin was investigated by means of two-dimensional electrophoresis. Major isoelectric points and molecular masses for activities of duodenal enzyme treated with both phosphatidylinositol-specific phospholipase C and neuraminidase were identified apparently with those of the major serum enzyme. In organ culture, the normal duodenal enzyme was released in the highest amounts to the culture medium. These results indicate that the major source of serum ALP in adult rats is basically from the duodenal enzyme. On the other hand, lectin affinity chromatography for ALPs showed that the ALP in the medium from culture duodenum and liver had the same complex-type sugar chain as with the ALP in the duodenal tissue. Although the duodenal ALP induced by glucosamine in vitro had the hybrid-type chain, sugar chains of the induced ALP in the culture medium were of the complex type, indicating that medial ALPs possessing the same sugar chain as the native duodenal enzyme, complex type, are mainly released from their tissues in normal conditions.