Enzymatic deglycosylation of the subunits of chorionic gonadotropin. Effects on formation of tertiary structure and biological activity

Biochemical roles of the oligosaccharide chains in thyrostimulin, a heterodimeric hormone of glycoprotein hormone subunits alpha 2 (GPA2) and beta 5 (GPB5)

Thyrostimulin is a heterodimeric hormone composed of GPA2 and GPB5, and shares the thyroid-stimulating hormone receptor (TSHR). Thyrostimulin has three N-linked oligosaccharide chains, two in GPA2 and one in GPB5. The roles of these N-linked oligosaccharides in secretion, heterodimer formation and signal transduction were analyzed. Recombinant GPA2s lacking either of the two oligosaccharides were obtained from conditioned medium, whereas dual site-disrupted GPA2 and the GPB5 mutant were not expressed in either the conditioned medium or cell lysate. The binding between GPA2 and GPB5 was weaker than that between TSH subunits GPA1 and TSH beta. Neither of the oligosaccharides in GPA2 had significant effects on heterodimerization. Disruption of either of the oligosaccharides in GPA2 significantly decreased receptor activation, suggesting their critical role in receptor activation.

Glycosylation patterns of human chorionic gonadotropin revealed by liquid chromatography-mass spectrometry and bioinformatics

Due to their extensive structural heterogeneity, the elucidation of glycosylation patterns in glycoproteins such as the subunits of human chorionic gonadotropin (hCG), hCG-alpha, and hCG-beta, remains one of the most challenging problems in the proteomic analysis of post-translational modifications. In consequence, glycosylation is usually studied after decomposition of the intact proteins to the proteolytic peptide level. However, by this approach all information about the combination of the different glycopeptides in the intact protein is lost. In this study we have, therefore, attempted to combine the results of glycan identification after tryptic digestion with molecular mass measurements on the native starting material of the new first WHO Reference Reagents (RR) for hCG-alpha (99/720) and hCG-beta (99/650). Despite the extremely high number of possible combinations of the glycans identified in the tryptic peptides by HPLC-MS (>1000 for hCG-alpha and >10 000 for hCG-beta), the mass spectra of intact hCG-alpha and hCG-beta revealed only a limited number of glycoforms present in hCG preparations from pools of pregnancy urines. Peak annotations for hCG-alpha were performed with the help of a bioinformatic algorithm that generated a database containing all possible modifications of the proteins, including modifications possibly introduced during sample preparation such as oxidation or truncation, for subsequent searches for combinations fitting the mass difference between the polypeptide backbone and the measured molecular masses. Fourteen different glycoforms of hCG-alpha, containing biantennary, partly sialylized hybrid-type glycans, including methionine-oxidized and N-terminally truncated forms, were identified. Mass spectra of high quality were also obtained for hCG-beta, however, a database search mass accuracy of +/-5 Da was insufficient to unambiguously assign the possible combinations of post-translational modifications. In summary, mass spectrometric fingerprints of intact molecules were shown to be highly useful for the characterization of glycosylation patterns of different hCG preparations such as the new first WHO RR for immunoassays and could be the first step in establishing biophysical reference methods for hCG and related molecules.

Efficient preparation of glycoprotein hormones lacking an alpha-subunit oligosaccharide

The oligosaccharide on alpha-subunit loop 2 (alpha 2) is needed for full glycoprotein hormone efficacy. Efforts to prepare glycoprotein hormone antagonists usually involve removing the alpha 2 oligosaccharide and are hampered by its requirement for efficient heterodimer secretion from mammalian cells. Here we show that hormones lacking this oligosaccharide can be produced by treating them at low pH to dissociate the heterodimer and permitting the subunits to re-associate in the presence of peptide N-glycosidase F (PNGase F). Re-assembly of human choriogonadotropin, human follitropin, and bovine lutropin occurred rapidly and efficiently following removal of the alpha 2 oligosaccharide by PNGase F. Consequently, virtually all heterodimers formed in the presence of this enzyme lacked this oligosaccharide. These findings support the notion that heterodimer assembly in vitro occurs by a threading mechanism that is impeded by the presence of the alpha 2 oligosaccharide. This procedure should facilitate the study of glycoprotein hormone structure and function.

Inositol phosphate stimulation by LH requires the entire alpha Asn56 oligosaccharide

Lentil lectin-bound, fucose-enriched hTSH was reported to stimulate both cAMP and inositol phosphate (IP) intracellular signalling pathways, whereas fucose-depleted hTSH stimulated only the cAMP pathway. Gonadotropins activate the cAMP pathway and in several studies higher concentrations activate the IP pathway. Since only the 10% of alpha subunit Asn(56) oligosaccharides (Asn(52) in humans) are fucosylated, the higher glycoprotein hormone concentrations required for IP pathway activation might be related to the abundance of competent hormone isoforms. Lentil lectin-fractionated equine (e)LHalpha and eFSHalpha preparations were combined with a truncated, des(121-149)eLHbeta preparation. All four hybrid hormone preparations induced IP accumulation in porcine theca cells, suggesting that activation of the IP pathway was not dependent on fucosylation at alpha subunit Asn(56). However, the presence of Asn(56) carbohydrate was necessary for increased IP accumulation. Intact, rather than Asn(56)-deglycosylated eLH preparations provoked a biphasic steroidogenic response by rat testis Leydig cells, suggesting that Galpha(i) stimulation was also sensitive to loss of Asn(56) carbohydrate. While rat granulosa cells responded to human FSH preparations in a biphasic manner, a classical sigmoidal response was obtained to eFSH and Asn(56)-deglycosylated eFSH, suggesting that the equine preparations did not activate Galpha(i). Purified oLHalpha Asn(56) oligosaccharides inhibited FSH-stimulated steroidogenesis in rat granulosa cell cultures indicating a direct role for carbohydrate in FSH action. The same carbohydrate preparation inhibited hCG-stimulated fluorescence energy transfer suggesting oligosaccharide involvement in activated LH receptor self-association.

Equine follicle-stimulating hormone: molecular cloning of beta subunit and biological role of the asparagine-linked oligosaccharide at asparagine(56) of alpha subunit

Equine FSH (eFSH) and eCG are members of the glycoprotein hormone family. These proteins are heterodimeric, composed of noncovalently associated alpha and beta subunits. We have previously reported that recombinant eCG has potent LH- and FSH-like activities and that the oligosaccharide at Asn(56) of the alpha subunit plays an indispensable role in expressing LH- but not FSH-like activity. In the present study, we cloned eFSH beta subunit cDNA and expressed wild-type recombinant eFSH and a partially deglycosylated mutant FSH (eFSH alpha56/beta) to investigate the biological role of the oligosaccharide at Asn(56) in FSH activity. The wild-type eFSH and eCG stimulated estradiol production in a dose-dependent manner in the primary cultures of rat granulosa cells, indicating that these equine gonadotropins have FSH activity. Partially deglycosylated eCG (eCG alpha56/beta) also stimulated estradiol production, confirming that the FSH-like activity of eCG is resistant to the removal of the N-linked oligosaccharide. Partially deglycosylated eFSH (eFSH alpha56/beta), however, did not show any FSH activity, indicating that the oligosaccharide at Asn(56) was necessary for eFSH. Thus, FSH-like activities of two gonadotropins, eCG and eFSH, are evoked through the distinct molecular mechanisms regarding the biological role of oligosaccharide at Asn(56) of the alpha subunit.

Structure, pathology and function of the N-linked sugar chains of human chorionic gonadotropin

Human chorionic gonadotropin (hCG) contains five acidic N-linked sugar chains, which are derived from three neutral oligosaccharides by sialylation. Each of the two subunits (hCGalpha and hCGbeta) of hCG contain two glycosylated Asn residues. Glycopeptides, each containing a single glycosylated Asn, were obtained by digestion of hCGalpha with trypsin, and of hCGbeta with chymotrypsin and lysyl endopeptidase. Comparative study of the sugar chains of the four glycopeptides revealed the occurrence of site-directed glycosylation. Studies of the sugar chains of hCGs, purified from urine of patients with various trophoblastic diseases, revealed that choriocarcinoma hCGs contain sialylated or non-sialylated forms of eight neutral oligosaccharides. In contrast, hCGs from invasive mole patients contain sialyl derivatives of five neutral oligosaccharides. The structural characteristics of the five neutral oligosaccharides, detected in choriocarcinoma hCGs but not in normal placental hCGs, indicate that N-acetylglucosaminyltransferase IV (GnT-IV) is abnormally expressed in the malignant cells. This supposition was confirmed by molecular biological study of GnT-IV in placenta and choriocarcinoma cell lines. The appearance of tumor-specific sugar chains in hCG has been used to develop a diagnostic method of searching for malignant trophoblastic diseases. In addition, a summary of the current knowledge concerning the functional role of N-linked sugar chains in the expression of the hormonal activity of hCG has been presented.

Mutants of human choriogonadotropin lacking N-glycosyl chains in the alpha-subunit. 1. Mechanism for the differential action of the N-linked carbohydrates

Analogs of human choriogonadotropin (hCG) lacking N-glycosyl chains at alpha52Asn and alpha78Asn were purified from the culture media of insect cells by immunoaffinity chromatography using a monoclonal antibody column. As previously reported, while analogs lacking carbohydrate at alpha52Asn and alpha78Asn had similar receptor binding activities compared with the wild type recombinant hCG (hCGwt), they differed in their signal transduction properties. The mutant lacking carbohydrate at alpha78Asn had 20% less cAMP-stimulating activity than hCGwt, but the absence of glycosylation at alpha52Asn resulted in the reduction of cAMP accumulation by 90-95%. A similar effect of the mutations was observed on the stimulation of steroidogenesis. Circular dichroism spectra of the two mutants showed significant differences. The mutant lacking carbohydrate at alpha52Asn had a much higher negative mean residue ellipticity (MRE) at 200 nm and a lower negative MRE at 220 nm than that lacking carbohydrate at alpha78Asn and hCGwt. The dissociation rates of the alpha52Asn and alpha78Asn carbohydrate deficient mutants at pH 3 and room temperature, measured by using 1-anilino-8-naphthalenesulfonate, were 9.4 x 10(-5) and 3.8 x 10(-5) s-1, respectively, as compared with 1.5 x 10(-5) s-1 for hCGwt. The results of both CD measurements and dissociation studies strongly suggest that the absence of carbohydrate at alpha52Asn results in conformational changes in the mutant which might explain the loss in its signal transduction function. This is further supported by indirect evidence from two other lines of experimentation. Unlike the mutant lacking carbohydrate at alpha78Asn, the one lacking carbohydrate at alpha52Asn cross-reacted with the two subunit specific monoclonal antibodies, anti-hCGalpha and anti-hCGbeta, which normally did not cross-react with the native or the hCGwt. Also, polyclonal anti-hCGbeta but not anti-hCGalpha was able to restore the cAMP-producing activity of the alpha52Asn carbohydrate deficient mutant. From all the data taken together, it appears that the loss of second messenger-producing activity of hCG with the absence of the glycosyl chain at alpha52Asn was probably due to a conformational change in the heterodimer rather than due to the loss of the alpha52Asn-carbohydrate-receptor interaction.

NMR studies of the free alpha subunit of human chorionic gonadotropin. Structural influences of N-glycosylation and the beta subunit on the conformation of the alpha subunit

Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hormone that is involved in the maintenance of the corpus luteum in early pregnancy. Glycosylation at Asn52 of its alpha subunit (alpha hCG) is essential for signal transduction, whereas the N-glycan at Asn78 stabilizes the structure of the protein. In this study, an almost complete 1H-NMR and a partial 13C-NMR spectral assignment for the amino acids and the N-glycans of alpha hCG and of an enzymatically deglycosylated form, which had a single GlcNAc residue at each of its two glycosylation sites, has been achieved. The secondary structure of alpha hCG is solution, which was determined based on NOE data, is partially similar to that of the alpha subunit in the crystal structure of hCG, but large structural differences are found for amino acid residues 33-58. In the crystal structure of hCG, residues 33-37 and 54-58 of the alpha subunit are part of an intersubunit seven-stranded beta-barrel and residues 41-47 constitute a 3(10)-helix. In contrast, in free alpha hCG in solution, amino acids 33-58 are part of a large disordered loop, indicating that in intact hCG interactions with the beta subunit of hCG stabilize the conformation of the alpha subunit. The NMR data of alpha hCG and its deglycosylated counterpart are very similar, indicating that removal of carbohydrate residues other than GlcNAc-1 does not notably affect the conformation of the protein part. However, numerous 1H-NOEs between the GlcNAc-1 residue at Asn78 and several amino acid residues show that this GlcNAc residue is tightly packed against the protein, being an integral part of the structure of the alpha subunit. 1H-NOEs across the glycosidic linkages of the glycan, resonance-line widths, and 1H and 13C chemical shifts of the other monosaccharides suggest that the remainder of the glycans at Asn78, and the glycans at Asn52 are largely extended in solution.

Bacterial expression of human chorionic gonadotropin alpha subunit: studies on refolding, dimer assembly and interaction with two different beta subunits

Human chorionic gonadotropin (hCG) is a member of a family of heterodimeric glycoprotein hormones that have a common alpha subunit but differ in their hormone-specific beta subunit. The common alpha subunit contains two asparagine (N)-linked oligosaccharides. To study the function of carbohydrates on in vitro refolding of alpha subunit and dimer assembly, we generated recombinant non-glycosylated hCG alpha subunit (rNGl-hCGalpha) from E. coli. The expression vector was constructed by inserting hCGalpha cDNA coding for the mature form in-frame into a pQE-30 vector, which contains a 6 x His sequence immediately before the 5'-end of hCGalpha cDNA for subsequent purification of rNG-hCGalpha. The rNG-hCGalpha expressed in inclusion bodies was efficiently purified by immobilized metal chelate affinity chromatography on Ni-NTA resin. SDS-PAGE, solid-phase binding assay and immunoblotting demonstrated the expression of rNG-hCG. Its alpha molecular weight on SDS-PAGE was 14.7 kDa under reducing conditions and 15 kDa for a monomer accompanied with some higher molecular weight oligomer under non-reducing conditions. Reconstitution of rNG-hCGalpha with native hCGbeta and oFSHbeta occurred in very low yield under standard conditions. However, the oxidation-reduction system cystamine (1.34 mM) and cysteamine (7.3 mM) facilitated both the refolding of rNG-hCGalpha and reconstitution of rNG-hCGalpha with native hCGbeta to regain partially correct conformation. These were revealed by conformationally sensitive antibody and receptor binding assays. Cystamine and cysteamine were more effective in the recombination of rNG-hCGalpha with oFSHbeta as indicated by a 22-36-fold decrease in the amount required to cause a 50% competitive inhibition in radioreceptor assay. They have no effect on assembly of rNG-hCGalpha with oLHbeta. Our results suggest the carbohydrate moieties confer greater conformational flexibility to the backbone of the beta subunit and the relative rigidity of the beta subunit may serve as a conformational template of the alpha subunit. The present approach has made it possible to prepare the non-glycosylated gonadotropin alpha subunit in adequate amounts for further study on their biological and topographical features in complete absence of carbohydrate.

Site-specific and complete enzymic deglycosylation of the native human chorionic gonadotropin alpha-subunit

Numerous studies have shown that glycosylation of the alpha-subunit of human chorionic gonadotropin (alpha hCG) is essential for the biological activity of this hormone. To obtain detailed insight into the function of N-glycosylation, the availability of site-specifically and fully deglycosylated alpha-subunits obtained under non-denaturing conditions is a prerequisite. NMR spectroscopy in combination with FAB-mapping demonstrates that only Asn52 of the alpha-subunit is accessible to digestion by peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F under native conditions. Treatment of native alpha hCG with endo-beta-N-acetylglucosaminidase B results in full deglycosylation yielding alpha hCG with one GlcNAc residue at both Asn52 and Asn78.

The asparagine-linked oligosaccharides of the human chorionic gonadotropin beta subunit facilitate correct disulfide bond pairing

The role of asparagine (N)-linked oligosaccharide chains in intracellular folding of the human chorionic gonadotropin (hCG)-beta subunit was determined by examining the kinetics of folding in Chinese hamster ovary (CHO) cells transfected with wild-type or mutant hCG-beta genes lacking one or both of the asparagine glycosylation sites. The half-time for folding of p beta 1 into p beta 2, the rate-determining step in beta folding, was 7 min for wild-type beta but 33 min for beta lacking both N-linked glycans. The p beta 1-->p beta 2 half-time was 7.5 min in CHO cells expressing the beta subunit missing the Asn13-linked glycan and 10 min for the beta subunit missing the Asn30-linked glycan. The inefficient folding of hCG-beta lacking both N-linked glycans correlated with the slow formation of the last three disulfide bonds (i.e. disulfides 23-72, 93-100, and 26-110) to form in the hCG-beta-folding pathway. Unglycosylated hCG-beta was slowly secreted from CHO cells, and beta subunit-folding intermediates retained in cells for more than 5 h were degraded into a hCG-beta core fragment-like protein. However, coexpression of the hCG-alpha gene enhanced folding and formation of disulfide bonds 23-72, 93-100, and 26-110 of hCG-beta lacking N-linked glycans. In addition, the molecular chaperones BiP, ERp72, and ERp94, but not calnexin, were found in a complex with unglycosylated, unfolded hCG-beta and may be involved in the folding of this beta form. These data indicate that N-linked oligosaccharides assist hCG-beta subunit folding by facilitating disulfide bond formation.

Structural analysis of N-linked oligosaccharides of equine chorionic gonadotropin and lutropin beta-subunits

Equine chorionic gonadotropin (eCG) and lutropin (eLH) are composed of alpha- and beta-subunits with an identical amino acid sequence but show different biological activities. To elucidate the molecular difference between these gonadotropins, the structure of the N-linked oligosaccharides of each beta-subunit was determined. N-linked sugar chains, liberated as tritum-labeled oligosaccharides by hydrazinolysis followed by N-acetylation and reduction with NaB3H4, were neutralized by sialidase digestion and/or methanolytic desulfation. Neutralized oligosaccharides were fractionated by sequential chromatography on serial lectin affinity columns and on a Bio-Gel P-4 column. Each oligosaccharide structure was determined by sequential exoglycosidase digestion in conjunction with elution profiles on lectin columns and methylation analysis. Each beta-subunit contained a single N-glycosylation site, but a high degree of microheterogeneity was observed in the structure of its N-linked oligosaccharides. eCG beta contained mono-, bi-, tri-, and tetraantennary complex-type oligosaccharides in a ratio of 3:63:13:1. eCG beta oligosaccharides contained about 16% of the bisecting GlcNAc and about 20% of poly-N-acetyllactosamine structures. Elongation of N-acetyllactosamine units showed a preference to the Man alpha 1-->6 side rather than the Man alpha 1-->3 side. Triantennary chains had only a C-2, 4-branched structure. eLH beta contained only mono- and biantennary complex-type and hybrid-type oligosaccharides in a ratio of approximately 18:67:10. eLH beta also contained bisected structures in about 18%. Oligosaccharides derived from the sulfated fraction of eLH beta contained GalNAc residues at nonreducing termini. Oligosaccharides from the sialylated/sulfated fraction of eLH beta contained both Gal and GalNAc residues at nonreducing termini, and those GalNAc residues were preferentially distributed to the Man alpha 1-->3 side of the trimannosyl core. These results clearly indicate that eCG beta and eLH beta possess structurally distinct N-linked oligosaccharides in addition to different charge groups even though they have a protein moiety identical to each other. Our results suggest that the biological activity of these hormones might be modulated by its terminal charge groups and stem structures of carbohydrate moiety synthesized in different organs.

Topographical differences in human choriogonadotropin antagonist as revealed by monoclonal antibodies

Human choriogonadotropin (hCG), a highly glycosylated hormone loses its biological activity following deglycosylation. The deglycosylated hormone (DG-hCG) binds to gonadal cell receptors without causing complete signal transduction. In order to analyze the molecular features of DG-hCG, we have prepared and studied five monoclonal antibodies (Mab). Two DG-hCG Mabs were highly specific while three others showed some but low cross-reactivity with hCG. Only those that recognized hCG albeit weakly, could inhibit binding of 125I-hCG to receptors or hCG action in MA-10 (mouse Leydig tumor cells). All five Mabs were highly reactive with the free alpha subunit. As these alpha subunit favoring antibodies easily recognized DG-hCG but not the intact hCG configuration, we suggest that loss of sugars in alpha and beta subunits of DG-hCG was responsible for these alterations. Thus, in the molecular topography of DG-hCG, the orientation of the alpha subunit would be different from that in the native hormone and this may be further perturbed following binding to the specific receptor, leading to inefficient coupling to the signal transducing machinery.

Endo beta-N-acetylglucosaminidase F cleavage specificity with peptide free oligosaccharides

Endo beta-N-acetylglucosaminidase activities were determined based on conversion of oligosaccharides containing two N-acetylglucosamines to the oligosaccharides with a single N-acetylglucosamine at the reducing terminal and following their separation on a carbohydrate analyzer. The oligosaccharides eluted from the high performance anion exchange column in the order of fucosyl-N,N'-diacetylchitobiose, N,N'-diacetylchitobiose and N-acetylglucosamine containing reducing terminals. Using this assay, differences in cleavage specificity of the endo beta-N-acetylglucosaminidase F (Endo F) activity on various free oligosaccharides obtained from the standard glycoproteins was determined. The commercial Endo F-peptide N-glycosidase/glycanyl amidase (PNGase) mixture readily cleaved high mannose and complex oligosaccharides (neutral and sialyated) with common core alpha 1-6 linked fucose found in porcine thyroglobulin including the trimannosyl-chitobiose core structure. However, the same Endo F mixture did not cleave the non-fucosylated complex oligosaccharides found in human transferrin and also the common core structure. Glycopeptide counterparts with and without fucose were good substrates for the endoglycosidases. These results show that the specificity of these enzymes is such that they can recognize the conformational differences between free oligosaccharides and glycopeptides with and without the common core alpha 1-6 linked fucose. In contrast, highly purified Endo F cleaved only the high mannose type oligosaccharides and was unable to cleave ovalbumin hybrid type oligosaccharides.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of sugar chains in the in-vitro activity of recombinant human interleukin 5

The structures of O-linked sugar chains of recombinant human interleukin 5 (rhIL-5) produced by Chinese hamster ovary cells were determined employing high-performance anion-exchange column chromatography under high pH conditions. The core sequence was elucidated to be Gal beta 1-->3-GalNAc by its susceptibility to endo-alpha-N-acetylgalactosaminidase and sialic acid linkages were determined using sialidases of different specificities. To investigate the role of sugar chains in the in-vitro activity of rhIL-5, it was digested with various glycosidases. While removal of N-linked sugar chains resulted in 2.8-fold increase of the activity, de-O-glycosylated rhIL-5 showed 10-fold higher activity than intact rhIL-5, suggesting that the presence of O-linked sugar chains suppresses the activity more effectively than that of N-linked chains. While incubation of de-N-glycosylated rhIL-5 at 70 degrees C for 30 min decreased the activity, intact and de-O-glycosylated rhIL-5 lost little activity, suggesting that N-linked sugar chains contributed to the thermostability of the molecule.

Structures and functions of the sugar chains of glycoproteins

Most proteins within living organisms contain sugar chains. Recent advancements in cell biology have revealed that many of these sugar chains play important roles as signals for cell-surface recognition phenomena in multi-cellular organisms. In order to elucidate the biological information included in the sugar chains and link them with biology, a novel scientific field called 'glycobiology' has been established. This review will give an outline of the analytical techniques for the structural study of the sugar chains of glycoproteins, the structural characteristics of the sugar chains and the biosynthetic mechanism to produce such characteristics. Based on this knowledge, functional aspects of the sugar chains of glycohormones and of those in the immune system will be described to help others understand this new scientific field.

Modification of blood group A antigen expression in a pancreatic cancer cell line (PC-1) by inhibitors of N-glycan processing

Pancreatic adenocarcinomas induced in Syrian hamsters by treatment with N-nitrosobis(2-oxopropyl) amine express blood group A antigen, which is absent in normal pancreatic cells. On membrane glycoproteins purified from tumors, blood group A antigen has been found to be expressed on multiantennary Asn-linked complex glycans. In this study, we investigated the effect of inhibitors of Asn-glycan processing on blood group A antigen bearing glycan structures in a cell line (PC-1) established from a primary induced pancreatic cancer. Expression of blood group A antigen on cells and in membrane preparations was blocked by treatment with 1-deoxymannojirimycin, an inhibitor of mannosidase I, but was retained after treatment with swainsonine, an inhibitor of mannosidase II. However, swainsonine treatment altered the glycan structure associated with blood group A antigen from an endoglycosidase H resistant type to a sensitive type, indicating that the blood group A structure might shift from a complex type to a hybrid type glycan by this treatment. These results demonstrate that Asn-linked glycans carry the major blood group A antigens in PC-1 cells.

Comparison of the biological and immunological properties of glycosylation deficient human chorionic gonadotropin variants produced by site directed mutagenesis and chemical deglycosylation

Appropriate glycosylation of gonadotropins is essential for the full expression of their biological activity. In this investigation we have compared the properties of glycosylation deficient human chorionic gonadotropin (hCG) produced by chemical deglycosylation (HF treatment--DG-hCG) or recombinant techniques (site directed mutagenesis). Among the recombinant hCG molecules secreted into the culture medium, the following variants containing selective N-glycosylation deletions at delta alpha 1 or delta alpha 1,2 or in both subunits could not stimulate steroidogenesis in mouse Leydig tumor cells (MA-10 cell line) and in this respect were very similar to DG-hCG. The other variants were fully active like native hCG, but the alpha + delta beta 1,2 recombinant hCG was a partial agonist. In radioimmunoassay with antibodies against native hCG, the DG-hCG as well as all recombinant hCG variants, including the wild type (WT), were similar. However, with antisera against DG-hCG or affinity purified antibodies specific to DG-hCG the alpha/delta 1,2 beta mutant, the WT hormone and native hCG were less active. In this assay mutants containing N-glycosylation deletions in the alpha subunit as well as the delta alpha 1,2/delta beta 1,2, variant showed higher activity. A similar pattern was evident in reaction with a selected monoclonal antibody showing preferential binding of 125I-labeled DG-hCG (antagonist). Affinity purified antibody directed against native hCG conformation was successful in converting DG-hCG and some inactive glycosylation deficient variants into an active form by stimulating progesterone in MA-10 cells. These data suggest that there are similarities as well as subtle differences in conformation of DG-hCG and various recombinant hCG molecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Follicle-stimulating hormone signal transduction: role of carbohydrate in aromatase induction in immature rat Sertoli cells

Receptor activated adenylate cyclase acts as a major transmembrane signalling system. It is widely accepted that upon binding to its receptor, follicle-stimulating hormone (FSH) activates the cAMP-dependent pathway which in turn mediates FSH-induced estradiol production in Sertoli cells. Studies utilizing several chemically derived variants of FSH have demonstrated that these variants bind to the FSH receptors with equal avidity but differ in their ability to activate cAMP-dependent pathways. Since cAMP is believed to be the second messenger responsible for FSH signal transduction, we tested two hypotheses: (1) that the effects of different oFSH variants on cAMP production and aromatase induction (as measured by estradiol production) would be in parallel; and (2) that deglycosylated ovine FSH (DG-oFSH) would antagonize the ability of intact oFSH to stimulate aromatase induction, similar to its reported antagonistic effect on cAMP production. Immature rat (7- to 10-day-old) Sertoli cells were cultured and the effects of several different oFSH variants on cAMP production and/or aromatase induction were tested. The variants tested were native oFSH, DG-oFSH, asialo oFSH (AS-oFSH), a recombinant of intact LH alpha and FSH beta (alpha + beta) and a recombinant of deglycosylated LH alpha and intact FSH beta (DG alpha + beta). Both native oFSH and alpha + beta recombinant at relatively large doses (10 ng) elicited a significant increase in extracellular cAMP accumulation as well as total cAMP production. In contrast, DG-oFSH did not produce an increase in cAMP even at 10-fold higher doses than native oFSH. Intracellular cAMP concentrations did not increase following stimulation with native oFSH, DG-oFSH or DG alpha + beta. In contrast to the divergent effects of oFSH and DG-oFSH on cAMP production all variants of oFSH stimulated estradiol production from Sertoli cells albeit with varying potencies. The sensitivity (minimal effective dose) and ED50 (dose at which half maximal response is achieved) of the estradiol (E2) response curve to increasing concentrations of native oFSH were 0.025 +/- 0.01 and 0.33 +/- 0.05 ng, respectively. Asialo-oFSH (AS-oFSH) increased E2 production with a potency (comparative dose required for effect) similar to that of native oFSH.(ABSTRACT TRUNCATED AT 400 WORDS)

Structures of the asparagine-linked sugar chains of human chorionic gonadotropin from a patient with extragonadal germ cell tumour

Human chorionic gonadotropin (hCG) purified from the urine of a male patient with extragonadal germ cell tumour contained four asparagine-linked sugar chains in one molecule. The sugar chains were quantitatively released from the polypeptide moiety by hydrazinolysis and recovered as oligosaccharides after N-acetylation. The oligosaccharide mixture was separated into a neutral (N) and three acidic (A1, A2 and A3) fractions by anion-exchange column chromatography. By sequential exoglycosidase digestion, methylation analysis and lectin column chromatography, the structures of these oligosaccharides were found to be the same as those of female gestational choriocarcinoma hCGs. Both contain eight kinds of sugar chains: triantennary, abnormal and normal biantennary, and monoantennary complex-type sugar chain with or without a fucosylated core portion.

The role of carbohydrate in human choriogonadotropin (hCG) action. Effects of N-linked carbohydrate chains from hCG and other glycoproteins on hormonal activity

Deglycosylation of gonadotropins and thyrotropin results in a major loss of hormonal bioactivity, while not impairing receptor-binding activity. However, a direct role of the glycan moieties in hormonal signal transduction has not been demonstrated. The addition of carbohydrate chains together with the deglycosylated hormone does not restore the hormonal activity. In contrast, glycopeptides were found to inhibit human choriogonadotropin (hCG)-stimulated adenylyl cyclase activity and hCG binding to its receptor. An inhibition of hCG-stimulated adenylyl cyclase activity but not hCG binding to receptor by glycopeptides specifically from hCG, has previously been reported as a lectin-like membrane component has been implicated in hCG action. In the present study we have shown that glycopeptides and oligosaccharides prepared from hCG, transferrin, fetuin, alpha 1-acid glycoprotein and ovalbumin inhibit the binding of hCG to its receptor. The inhibition was also observed with a highly purified preparation of the receptor, thus suggesting a lack of involvement of other lectin-like membrane components as previously proposed. We suggest that a lectin-like interaction with the hormone, if any, involves the receptor itself. Adenylyl cyclase activity stimulated by hCG, isoproterenol or forskolin was inhibited by oligosaccharides, indicating a non-specific interaction. Our results suggest that Asn-linked oligosaccharide chains from various glycoproteins perturb hCG-receptor interactions through a putative carbohydrate binding site on the receptor.

Carbohydrate moieties of small placental hCG: requirement of mannose structure for biological activity

The 43 kDa human chorionic gonadotropin (hCG) (SP-hCG) was purified from human placenta and analyzed for sugar moieties. The low hexosamine content suggests that SP-hCG probably lacks O-linked sugar chains in the beta-subunit and incompletely formed N-linked sugar chains in the alpha- and beta-subunits. In the present study SP-hCG was hydrolyzed with various glycosidases. Treatment of hCG or SP-hCG with O-glycan peptide hydrolase increased the mobility of asialo-hCG beta in reduced sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) while that of SP-hCG beta was unaffected, indicating that SP-hCG beta does not contain NeuNAc-Gal-GalNAc unit. Alpha-Mannosidase and endoglycosidase H hydrolyzed mannose and the high mannose-GlcNAc moieties, respectively, from alpha- and beta-subunits of SP-hCG, but not from the subunits of authentic hCG. Glycopeptidase F hydrolyzed completely the N-linked sugar chains from SP-hCG subunits, producing alpha- and beta-subunits with estimated Mr of 15,000 and 18,500, respectively. The biological activity of purified SP-hCG is about 50-80% of highly purified authentic hCG. In an in vitro system SP-hCG increased cAMP accumulation and testosterone production by rat Leydig cells to the same levels as that induced by hCG. However, the biological activity of SP-hCG was markedly reduced, following treatment with endoglycosidase H or alpha-mannosidase. To attain the level of testosterone production equivalent to that induced with untreated SP-hCG, 10-20 times higher dose of treated SP-hCG was required. On the other hand, cAMP accumulation induced with treated SP-hCG even at a very high concentration was substantially lower than that attained with untreated SP-hCG. In conclusion, the mannose moieties are essential structural components of the hormone in stimulating cAMP accumulation and steroidogenesis by rat Leydig cells.

Evidence for impaired subunit interaction in chemically deglycosylated human choriogonadotropin

Using three different methods evidence was obtained that native and deglycosylated choriogonadotropin show differences in their conformations which might account for the antagonistic properties of deglycosylated choriogonadotropin: 1. In the deglycosylated hormone additional peptide bonds were susceptible to chymotrypsin. 2. In the far ultraviolet circular dichroism only small differences existed between native and deglycosylated choriogonadotropin. However, in 80% hexafluoropropanol the deglycosylated hormone adopted a higher degree of ordered structure. 3. At 37 degrees C the deglycosylated hormone showed a 13 fold increase of the dissociation rate into subunits at pH 3 in comparison to native choriogonadotropin. The results provide evidence that in chemically deglycosylated choriogonadotropin the subunit interactions are disturbed due to conformational changes.

Alterations in antigenic structure of gonadotropins following deglycosylation

Radioimmunological techniques were utilized to probe possible changes in conformation of gonadotropins (human chorionic gonadotropin--hCG; and ovine luteinizing hormone--oLH) following chemical deglycosylation (DG-hCG and DG-LH). All antisera produced in rabbits, rats or mice contained antibodies that were specific to the deglycosylated hormones with the native hormones showing weak and non-parallel cross-reaction (less than 5%), but with rabbit antibodies to native hormones the deglycosylated hormones were fully reactive. Using hCG, asialo-hCG (A-hCG) and DG-hCG, we have shown that removal of sugars internal to sialic acid is required to produce these specific antibodies. These are in complete agreement with the observations that extensive deglycosylation of these hormones is necessary to induce changes in biological activity at the cellular level. Based on these data, we suggest that chemical deglycosylation results in changes in antigenic structure of these hormones by generation of new determinants or exposure of previously buried sites and these changes are of no consequence to receptor recognition.

Structures, function, and transformational changes of the sugar chains of glycohormones

Human chorionic gonadotropin (hCG), human luteinizing hormone, human thyroid-stimulating hormone, and human follicle-stimulating hormone are closely related family of proteins which share a common alpha-subunit. However, their sugar moieties are quite different. hCG contains five acidic asparagine-linked sugar chains. These five sugar chains are derived by sialylation from three neutral oligosaccharides: two biantennary (N-1 and N-2) and one monoantennary (N-3) complex-type oligosaccharides. Although hCG purified from the urine of pregnant women is more enriched in sialylated sugar chains than that purified from placenta, the molar ratio of N-1, N-2, and N-3 of these two hCGs are the same (1:2:1). Comparative study of the sugar moieties of the alpha- and beta-subunits of hCG revealed that alpha contains 1 mol each of N-2 and N-3, while beta contains 1 mol each of N-1 and N-2. This specific distribution of oligosaccharides at the four asparagine loci of the hCG molecule is now helping us to consider the functional role of the sugar moiety of glycohormones. hCG is produced not only by the trophoblast but also by various trophoblastic diseases. The hCGs purified from the urine of patients with hydatidiform mole contain the same oligosaccharides as normal hCG. However, those from the urine of choriocarcinoma patients contain five additional neutral oligosaccharides. In contrast, hCGs from invasive-mole patients contain three of the five oligosaccharides, specifically found in choriocarcinoma hCGs. The malignant transformational change of the sugar moiety of hCG can be explained by an increase of a fucosyltransferase, which forms the Fuc alpha 1----6GlcNAc group and by ectopic expression and subsequent modification of N-acetylglucosaminyltransferase IV. The appearance of tumor-specific sugar chains of hCG has been used to develop a new diagnostic method for invasive mole and choriocarcinoma.

Production of human thyroid-stimulating hormone in Chinese hamster ovary cells

Human thyroid-stimulating hormone (hTSH) has been produced in Chinese hamster ovary (CHO) cells co-transformed with two plasmids: one carrying the alpha subunit cDNA with mouse dihydrofolate reductase gene and the other carrying hTSH beta subunit cDNA. Each cDNA was driven to expression under the control of SV40 early promoter. hTSH and its alpha subunit were secreted into culture media, and their secretion increased with exposure of the cells to increasing concentrations of methotrexate. Gel filtration analysis revealed that the molecular size of the hTSH was the same as that of natural hTSH. Furthermore, the CHO cell-produced hTSH elevated the cyclic AMP level in the rat thyroid cell line FRTL-5 in the same manner as natural hTSH does.

The subcellular sites of sulfation of mouse thyrotropin and free alpha subunits: studies employing subcellular fractionation and inhibitors of the intracellular translocation of proteins

To determine the subcellular sites of sulfation of thyrotropin (TSH) and free alpha-subunits, mouse thyrotropic tumor minces were incubated simultaneously with [3H]Met and [35S]SO4 for 1 or 3h, homogenized, and fractionated by discontinuous sucrose gradient ultracentrifugation. Dual-labeled TSH or free alpha-subunits were immunoprecipitated, and analyzed by SDS-gel electrophoresis. Endoglycosidase F released all [35S], but little [3H], from the dual-labeled species, indicating that [35S]SO4 was incorporated into oligosaccharides of TSH and free alpha-subunits. Both [35S]TSH and [35S] free alpha-subunits were predominantly in Golgi fractions at 1 and 3 h, but small amounts were also detected in fractions enriched in rough endoplasmic reticulum (RER). Similar distributions of [35S]SO4-labeled species were noted in cell fractions prepared from mouse pituitaries. Pituitaries from hypothyroid mice were incubated with [3H]Met and [35S]SO4 for 2 h, then chased for 4 or 16 h in the absence or presence of 2 uM monensin (Mon) or 10 uM carboxyl cyanide m-chlorophenylhydrazone (CCCP). At 4h, release into the medium of [3H]TSH was inhibited 59% and 86% by Mon and CCCP, respectively; release of [35S]TSH was inhibited 28% and 46%. At 4h, release of [3H]free alpha-subunits was inhibited 58% and 81% by these drugs, respectively; release of [35S]free alpha-subunits was inhibited 6% and 50%. Thus, Mon and CCCP inhibited the release of each [3H] species more than the [35S] species, indicating that most sulfation occurred in Golgi.

Enzymatic deglycosylation of thyroid-stimulating hormone with peptide N-glycosidase F and endo-beta-N-acetylglucosaminidase F

We investigated the ability of two enzymes, peptide N-glycosidase F (PNGase F) and endo-beta-N-acetylglucosaminidase F (Endo F), to deglycosylate microgram quantities of bovine TSH and its subunits under nondenaturing conditions. One oligosaccharide chain could be selectively removed from the alpha subunit by PNGase F, and all the oligosaccharide chains from both subunits could be removed by Endo F. These methods of enzymatic deglycosylation should permit study of the functional role of each N-linked carbohydrate chain of various glycoprotein hormones.

Deglycosylated human follitropin: characterization and effects on adenosine cyclic 3',5'-phosphate production in porcine granulosa cells

Chemical deglycosylation of gonadotropins with hydrogen fluoride (HF) has facilitated the investigation of the structure-function relationship of the individual peptide and oligosaccharide moieties in the mechanism of hormone action. These studies have dealt almost exclusively with lutropin or human choriogonadotropin. We report here the chemical characterization and biological properties of deglycosylated human follitropin (degly hFSH). Results indicate that deglycosylation of hFSH by HF removes 89% of the total carbohydrate without disruption of the peptide chain or significant loss of amino acid residues. However, a change in the conformation of the molecule was observed by measurement of the far-ultraviolet circular dichroic spectrum. The degly hFSH showed a 44% reduction in binding when tested in a FSH radioimmunoassay utilizing a polyclonal antibody. Binding of the degly hFSH to FSH-responsive tissues showed that the altered hormone bound with equal or better avidity than the intact hormone while the association constants were approximately the same for both preparations. The degly hFSH alone did not stimulate the FSH-stimulatable adenylyl cyclase (AC) activity of cellular homogenates of small follicle porcine granulosa cells. Furthermore, degly hFSH was a potent antagonist of hFSH-stimulatable AC activity when coincubated with intact hFSH. In intact granulosa cells, both the hFSH and the degly hFSH stimulated cAMP production and release by these cells. However, the degly hFSH was one-tenth as effective as the intact hormone.(ABSTRACT TRUNCATED AT 250 WORDS)

Activities of deglycosylated thyrotropin at the thyroid membrane receptor-adenylate cyclase system

A bovine thyrotropin (bTSH) preparation was deglycosylated by treatment with anhydrous hydrogen fluoride (HF) in the presence of anisole. The resulting material consisted of TSH derivatives that exhibited different molecular sizes, all smaller than the native hormone. The majority (62%) of the deglycosylated TSH derivatives did not bind to the lectin concanavalin A, while 98% of the native TSH was able to bind. The deglycosylated TSH derivatives bound to the high affinity-high specificity TSH binding sites in human thyroid membranes with a potency more than twice that of equivalent immunological amounts of the native bTSH. Despite the enhanced binding affinity for the TSH receptor, the deglycosylated TSH derivatives were unable to stimulate adenylate cyclase fully. Maximal stimulation achieved with bTSH derivatives was only 9 to 17% of the maximal stimulation achieved with native bTSH. Further, the deglycosylated derivatives competitively inhibited stimulation of the thyroidal adenylate cyclase by native bTSH. We conclude that HF treatment of bTSH results in partially deglycosylated TSH derivatives that exhibit enhanced ability to bind to the TSH receptor and markedly diminished adenylate cyclase-stimulating activity.

A role for glycosylation of the alpha subunit in transduction of biological signal in glycoprotein hormones

The biological properties of recombinants of glycoprotein hormones in which the alpha and beta subunits were differentially deglycosylated have been investigated. Specific deglycosylation of the alpha subunit generated a recombinant that had more receptor-binding activity but did not produce hormone response in the target cells. The deglycosylated alpha + beta recombinant was also an antagonist of the action of the native hormone. Thus, the carbohydrates in the alpha subunit play a dominant role in the transduction of the hormone signal into the cell.

Inhibition of adenylyl cyclase activity in rat corpora luteal tissue by glycopeptides of human chorionic gonadotropin and the alpha-subunit of human chorionic gonadotropin

Indirect evidence has indicated that the carbohydrate moieties of the glycoprotein hormones are involved in the activation of the receptor-adenylyl cyclase system of reproductive tissues. In the present study, we have isolated the glycopeptides (GP) from human chorionic gonadotropin (hCG), the alpha-subunit of hCG, fetuin, and bovine gamma-globulin (b gamma G). These along with a number of synthetic oligosaccharides were tested for their ability to inhibit adenylyl cyclase (AC). There was less than 0.001% cross-reactivity of the GP from hCG, hCG alpha, fetuin, and b gamma G when tested in a double-antibody hCG radioimmunoassay or rat corpora lutea radioreceptor assay. The GP of fetuin, b gamma G, and the synthetic oligosaccharides did not inhibit AC activity of 2000 g corpora lutea membranes when coincubated with 100 ng of hCG/mL (ED50). However, when the GP of hCG and hCG alpha were included with intact hCG, there was a dose-related inhibition. Inhibition of cyclase activity was enhanced when the hCG GP were desialylated. This occurred without a change in the lag time of hCG activation which was calculated to be 1-1.5 min. Changing the concentration of ATP and Mg2+ did not affect the inhibitory effects of the hCG alpha GP on hCG-stimulated AC activity. Inhibition by hCG GP followed uncompetitive kinetics. The inhibition by the GP of hCG seems to be restricted to the LH/hCG-stimulatable AC system because the same dosage of hCG GP which inhibited the rat luteal AC system did not have any effect on the rat hepatocyte AC system when coincubated with glucagon or on NaF-stimulated activity in luteal membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

The common alpha subunit of bovine glycoprotein hormones: limited formation of native structure by the totally nonglycosylated polypeptide chain

The folding of the bovine glycoprotein hormone alpha subunit, synthesized in bacteria following insertion of the nucleotide sequence coding for this polypeptide, has been studied to determine the effect that a complete lack of carbohydrate has on this process. The bacterially derived alpha polypeptide (bac-alpha), extracted from E. coli in the presence of reductant and denaturant, had an estimated 0.2% native structure as determined by a conformationally sensitive radioimmunoassay. Upon reduction of disulfide bonds and reoxidation in air, the amount of native structure increased about 18-fold. Approximately 2% of the refolded bac-alpha preparation combines with the beta subunit of human chorionic gonadotropin (hCG beta) to form a complex that binds to the gonadotropin receptor and elicits a biological response. Since the correct folding (by immunological criteria) of bac-alpha (ca 3%) is significantly greater than expected from a random formation of disulfide bonds (0.1%), it appears that correct folding of alpha subunit can occur in the complete absence of carbohydrate, though in very low yield. Native bovine lutropin alpha subunit (LH alpha) and chemically deglycosylated LH alpha (which retains two asparagine-linked N-acetyl glucosamine residues per alpha oligosaccharide) were subjected to the same reduction/reoxidation regimen as the bacterially produced alpha subunit. As has been reported previously [Giudice LC, Pierce, JG, J Biol Chem 251: 6392, 1976] intact LH alpha fully regained its native structure. The partially deglycosylated LH alpha also refolds to a native-like structure in high yield as assessed by immunological assays and by its ability to combine with HCG beta to form a biologically active complex. The data show that carbohydrate, while not obligatory for correct folding, greatly facilitates the formation of functional alpha subunit.