Antibodies against human chorionic gonadotropin convert the deglycosylated hormone from an antagonist to an agonist

Model of Glycoprotein Hormone Receptor Ligand Binding and Signaling

Studies described here were initiated to develop a model of glycoprotein hormone receptor structure and function. We found that the region that links the lutropin receptor leucine-rich repeat domain (LRD) to its transmembrane domain (TMD) has substantial roles in ligand binding and signaling, hence we term it the signaling specificity domain (SSD). Theoretical considerations indicated the short SSDs in marmoset lutropin and salmon follitropin receptors have KH domain folds. We assembled models of lutropin, follitropin, and thyrotropin receptors by aligning models of their LRD, TMD, and shortened SSD in a manner that explains how substitutions in follitropin and thyrotropin receptors distant from their apparent ligand binding sites enable them to recognize lutropins. In these models, the SSD is parallel to the concave surface of the LRD and makes extensive contacts with TMD outer loops 1 and 2. The LRD appears to contact TMD outer loop 3 and a few residues in helices 1, 5, 6, and 7. We propose that signaling results from contacts of the ligands with the SSD and LRD that alter the LRD, which then moves TMD helices 6 and 7. The positions of the LRD and SSD support the notion that the receptor can be activated by hormones that dock with these domains in either of two different orientations. This would account for the abilities of some ligands and ligand chimeras to bind multiple receptors and for some receptors to bind multiple ligands. This property of the receptor may have contributed significantly to ligand-receptor co-evolution.

Antibody to immunoglobulin G and polyethylene glycol augment cyclic adenosine monophosphate production by TSH receptor antibody bound to porcine thyroid cells

Anti-immunoglobulin G (IgG) augments cyclic adenosine monophosphate (cAMP) production by thyroid-blocking antibody (TBAb) bound to porcine thyroid cells (PTC). This is described as a conversion phenomenon. We reported the effect of polyethylene glycol (PEG) to augment thyroid-stimulating antibody (TSAb) activity in a PTC assay. In the present experiment we examined the effect of anti-immunoglobulin G (IgG) and PEG on cAMP production from TBAb or TSAb bound to PTC. TBAb bound to PTC was separated from unbound TBAb by centrifugation after a first incubation (0.5 hour at 37 degrees C) of TBAb-IgG with PTC. TBAb bound to PTC were incubated with anti-human (h)IgG or hIgG fragments [F(ab')(2), Fc, Fd, H chain or L-chain] for 4 hours at 37 degrees C in the second incubation. Anti-IgG or anti-IgG fragments increased cAMP production. No conversion was caused by protein A, protein L, or PEG (5%). PEG did not augment cAMP production by these IgG antibodies. PEG augmented cAMP production during incubation of TSAb-IgG bound to PTC, but anti-IgG did not. PEG significantly augmented cAMP production by coincubation of TSAb-IgG bound to PTC and the unbound TSAb-IgG (obtained from the first incubation). A similar augmentative effect of PEG was also observed in experiments using TSAb-F(ab')(2) and TSAb-Fab. cAMP production by PTC bound by both TBAb- and TSAb-IgG was increased by co-incubation with anti-IgG, but was not increased by PEG. In conclusion, anti-IgG specifically increased cAMP production from TBAb bound to PTC (conversion phenomenon) and PEG specifically increased cAMP production by TSAb bound to PTC. Different mechanisms enhance cAMP production by TSAb and conversion of TBAb.

Modification of the sialic acid residues of choriogonadotropin affects signal transduction

Human choriogonadotropin (hCG) is a glycoprotein hormone that activates adenylyl cyclase. The carbohydrate moieties of hCG are required for biological activity, but not for binding to the gonadotropin receptors. We modified N-acetylneuraminic acid (NeuAc) on the oligosaccharide moieties of hCG, and determined the effect on its biological activity by measuring hormone-stimulated adenylyl cyclase. Treating hCG with sodium periodate to remove two carbon atoms from NeuAc or quantitatively removing NeuAc from hCG reduced its biological activity by 36% and 50%, respectively. The galactose residues of asialo-hCG were reacted with NeuAc-hydrazone or a hydrazone of the oligosaccharide from the ganglioside GM1 (Gal(beta 1-3)GalNAc(beta 1-4) [NeuAc(alpha 2-3)]Gal(beta 1-4)Glc). The gonadotropin receptor had high affinity for both derivatives, but their biological activity was less than that of hCG. These results suggest that several structural aspects of NeuAc including carbon side chain, an intact ring structure, and the position of NeuAc relative to other carbohydrate residues are important for full biological activity of hCG.

Structural predictions for the ligand-binding region of glycoprotein hormone receptors and the nature of hormone-receptor interactions

BACKGROUND

Glycoprotein hormones influence the development and function of the ovary, testis and thyroid by binding to specific high-affinity receptors. The extracellular domains of these receptors are members of the leucine-rich repeat (LRR) protein superfamily and are responsible for the high-affinity binding. The crystal structure of a glycoprotein hormone, namely human choriogonadotropin (hCG), is known, but neither the receptor structure, mode of hormone binding, nor mechanism for activation, have been established.

RESULTS

Despite very low sequence similarity between exon-demarcated LRRs in the receptors and the LRRs of porcine ribonuclease inhibitor (RI), the secondary structures for the two repeat sets are found to be alike Constraints on curvature and beta-barrel geometry from the sequence pattern for repeated beta alpha units suggest that the receptors contain three-dimensional structures similar to that of RI. With the RI crystal structure as a template, models were constructed for exons 2-8 of the receptors. The model for this portion of the choriogonadotropin receptor is complementary in shape and electrostatic characteristics to the surface of hCG at an identified focus of hormone-receptor interaction.

CONCLUSIONS

The predicted models for the structures and mode of hormone binding of the glycoprotein hormone receptors are to a large extent consistent with currently available biochemical and mutational data. Repeated sequences in beta-barrel proteins are shown to have general implications for constraints on structure. Averaging techniques used here to recognize the structural motif in these receptors should also apply to other proteins with repeated sequences.

Model of human chorionic gonadotropin and lutropin receptor interaction that explains signal transduction of the glycoprotein hormones

The goal of these studies was to devise a model that explains how human chorionic gonadotropin (hCG) interacts with lutropin (LH) receptors to elicit a hormone signal. Here we show that alpha-subunit residues near the N terminus, the exposed surface of the cysteine knot, and portions of the first and third loops most distant from the beta-subunit interface were recognized by antibodies that bound to hCG-receptor complexes. These observations were combined with similar data obtained for the beta-subunit (Cosowsky, L., Rao, S.N.V., Macdonald, G.J., Papkoff, H., Campbell, R.K., and Moyle, W.R. (1995) J. Biol. Chem. 270, 20011-20019), information on residues of hCG that can be changed without disrupting hormone function, the crystal structure of deglycosylated hCG, and the crystal structure of a leucine-repeat protein to devise a model of hCG-receptor interaction. This model suggest that the extracellular domain of the LH receptor is "U-" or "J"-shaped and makes several contacts with the transmembrane domain. High affinity hormone binding results from interactions between residues in the curved portion of the extracellular domain of the receptor and the groove in the hormone formed by the apposition of the second alpha-subunit loop and the first and third beta-subunit loops. Most of the remainder of the hormone is found in the large space between the arms of the extracellular domain and makes few, if any, additional specific contacts with the receptor needed for high affinity binding. Signal transduction is caused by steric or other influences of the hormone on the distance between the arms of the extracellular domain, an effect augmented by the oligosaccharides. Because the extracellular domain is coupled at multiple sites to the transmembrane domain, the change in conformation of the extracellular domain is relayed to the transmembrane domain and subsequently to the cytoplasmic surface of the plasma membrane. While the model does not require the hormone to contact the transmembrane domain to initiate signal transduction, small portions of both subunits may be near the transmembrane domain and assist in initiating the hormonal signal. This is the first model that is consistent with all known information on the activity of the gonadotropins including the amounts of the hormone that are exposed in the hormone-receptor complex, the apparent lack of specific contacts between much of the hormone and the receptor, and the roles of the oligosaccharides in signal transduction.(ABSTRACT TRUNCATED AT 400 WORDS)

Number and topography of epitopes of human chorionic gonadotropin (hCG) are shared by desialylated and deglycosylated hCG

A previously established map of the surface epitopes of human chorionic gonadotropin (hCG) served as template for the present study in which we investigated the antigenic surfaces of two glycosylation variants of hCG, i.e. desialylated hCG (asialo-hCG) and deglycosylated hCG (degly-hCG). This map allocates five epitopes to the alpha subunit, five to the beta subunit and four alpha beta epitopes to structures formed only by the alpha/beta heterodimer holo-hCG (Schwarz et al. (1986) Endocrinology 118, 189-197; Berger et al. (1990) J. Endocrinol. 125, 301-309). Here it is described that both variants complied with this template: each of the 14 distinct monoclonal antibodies with which the epitopes of hCG were defined reacted with radiolabeled asialo-hCG and degly-hCG as well and generally bound degly-hCG with greater affinity than hCG. Moreover, every combination of capture and radiolabeled detection antibody that was either compatible or incompatible on unlabeled hCG was so also on unlabeled asialo-hCG and degly-hCG. It thus appears that alterations of the carbohydrate structure of hCG can be associated with a change in affinity between some antibodies and their respective epitopes but not with a loss of an epitope or with a change in the topographical relationships of the 14 epitopes.

Reversal of the antagonism of deglycosylated human chorionic gonadotropin by aggregation with wheat germ agglutinin

Human chorionic gonadotropin (hCG) behaves as an antagonist upon chemical deglycosylation with hydrogen fluoride. In this study it was found that the alpha- and beta-subunits of deglycosylated hCG (DGhCG) still retained their ability to bind to wheat germ agglutinin (WGA) but not to concanavalin A. The antagonism of DGhCG against activation of adenylate cyclase and steroidogenesis was reversed, when WGA was bound to the N-linked carbohydrate moieties of hCG alpha- and beta-subunits followed by addition of purified Leydig cells. The complex formed by incubation at an approximately equimolar ratio induced the maximal reversal of antagonism. This reversal of antagonism was diminished by addition of N-acetylglucosamine. No increase of steric hindrance at the receptor sites was seen in binding studies of the [125I]DGhCG-WGA complex, indicating that the receptor-binding domain of hCG may not be adjacent to the carbohydrate moieties. Kinetic studies of the hormonal response showed that the DGhCG-WGA complex terminated cAMP accumulation after 30 min of incubation, but not testosterone production. Our results suggest that tetravalent WGA can also reverse the antagonism of DGhCG, as in bivalent antibodies to hCG beta.

Enhancement of the activity of thyroid-stimulating antibodies by anti-human IgG antibodies in vitro

In an attempt to determine whether or not anti-human IgG antibodies could influence the activity of thyroid-stimulating antibodies (TSAb), we investigated the effects of anti-human Fc antibody (anti-Fc antibody) and IgG from rheumatoid arthritis patients (RA-IgG) on TSAb activity using FRTL-5 cells. It was found that these anti-human IgG antibodies enhanced the TSAb activity in vitro. FRTL-5 cells were first incubated with Graves' disease IgG for 30 min at 37 degrees C, then washed and incubated in Hanks' balanced salt solution with anti-human IgG antibodies for 60 min at 4 degrees C, and then for a further 120 min at 37 degrees C. The level of cAMP accumulated in the medium was determined by RIA. Anti-Fc antibody significantly augmented the cAMP formation stimulated by 16 out of 24 Graves' IgGs, whereas anti-F(ab')2 antibody did not potentiate cAMP accumulation. Three of five RA-IgGs, which are usually defined as specific antibodies for the Fc fragment of human IgG, mimicked these stimulatory effects. Protein A also potentiated the cAMP formation stimulated by Graves' IgGs. Furthermore, there was a significant correlation between the TSAb titres of these Graves' IgGs and the potentiating effects of anti-Fc antibody (r = 0.495, P less than 0.05, n = 21). These results suggest that the interaction of TSH receptor antibody with anti-human IgG antibodies might modulate thyroid function in Graves' disease.

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.

Conformation of the beta subunit of deglycosylated human chorionic gonadotropin in the interaction at receptor sites

The present study was performed to evaluate the relationship between the antagonist effect and the conformation of deglycosylated human chorionic gonadotropin (DGhCG) on the Leydig cell lutropin receptors. The maximum binding of [125I]DGhCG to anti-hCG beta antibody was decreased by 50%, while its binding profile to anti-hCG, anti-DGhCG and anti-hCG alpha antibodies remained unchanged, suggesting conformational changes in the beta subunit of DGhCG. However, the association of [125I]DGhCG to the binding sites of the receptors was much faster than that of [125I]hCG, and the ligand reached the binding equilibrium at 4 and 37 degrees C for 3 h and 15-30 min, respectively. Thus, the conformational changes in the beta subunit were not accompanied by loss of its receptor binding ability. The agonist properties of DGhCG which was bound to the receptors was fully restored by the addition of anti-hCG beta subunit antibody, while anti-hCG or anti-DGhCG restored only about 30% of the full agonist activity. This was probably due to a change of the conformation of the beta subunit to make it similar to that of the intact hormone. This restoration caused by anti-hCG beta was partially prevented by anti-hCG alpha. These facts indicate that some conformational change only in the beta subunit, not in the alpha subunit, of the deglycosylated hormone bound to the receptors is essential for the restoration of agonist properties.

In-vitro conversion of blocking type anti-TSH receptor antibody to the stimulating type by anti-human IgG antibodies

We examined the relationship between blocking type anti-TSH receptor antibody (TRAb) and stimulating type TRAb by trying to convert blocking type TRAb to the stimulatory type in vitro. Immunoglobulins (IgGs) purified from sera of six patients with primary hypothyroidism blocked bovine TSH (100 microU/ml)-induced cAMP production (58.3-82.1% inhibition) in cultured porcine thyroid cells. None of these IgGs showed stimulating activity even after their dilution. In the conversion experiment, thyroid cells were first incubated with these IgGs at 34 degrees C for 30 min and then washed with incubation buffer. They were then incubated with various kinds of anti-human IgG antibodies first for 1.5 h at 4 degrees C and then for 18 h at 34 degrees C, and the cAMP concentrations in the supernatants were measured. All six IgGs showed strong, dose-dependent thyroid-stimulating activity after addition of antibodies against human whole IgG, or Fab or Fc fragments of IgG. The Fab or F(ab')2 fragments of goat anti-human IgG antibody also had these converting activities, although less than whole IgG. Addition of normal IgG in the first incubation or anti-human IgG antibody alone had no thyroid-stimulating activity. Anti-human IgA or IgM antibodies did not have these converting activities. These results show that blocking type anti-TSH receptor antibodies can be converted to the stimulating type by anti-human IgG antibodies in vitro. The results suggest that the blocking and stimulating types bind to the same epitope(s) of TSH-receptor related antigens. The same anti-TSH receptor antibody may act as a stimulator or blocker by the influence of other factors, such as anti-idiotype antibody.

Current ideas on the significance of protein glycosylation

Carbohydrate has been removed from a number of glycoproteins without major effect on the structure or enzyme activity of the protein. Thus carbohydrate has been suggested to underly a non-primary function for proteins, such as in relatively non-specific interactions with other carbohydrates or macromolecules, stabilization of protein conformation, or protection from proteolysis. This non-specific concept is consistent with both the general similarity in carbohydrate structure on very diverse glycoproteins and the frequent structural microheterogeneity of carbohydrate chains at given sites. The concept is supported in a general sense by the viability of cells whose glycosylation processes have been globally disrupted by mutation or pharmacological inhibitors. In contrast to the above observations, other studies have revealed the existence of specific, selective receptors for discrete oligosaccharide structures on glycoproteins which seem to be important for compartmentalization of the glycoprotein, or the positioning of cells on which the glycoprotein is concentrated. Sometimes multivalency in the carbohydrate-receptor interaction is crucial. There are additional possible roles for carbohydrate in the transduction of information upon binding to a receptor. The possibility of specific roles for carbohydrate is supported by the existence of numerous unique carbohydrate structures, many of which have been detected as glycoantigens by monoclonal antibodies, with unique distributions in developing and differentiated cells. This article attempts to summarize and rationalize the contradictory results. It appears that in general carbohydrate does in fact underlie only roles secondary to a protein's primary function. These secondary roles are simple non-specific ones of protection and stabilization, but often also satisfy the more sophisticated needs of spatial position control and compartmentalization in multicellular eukaryotic organisms. It is suggested that there are advantages, evolutionarily speaking, for the shared use of carbohydrate for non-specific roles and for specific roles primarily as luxury functions to be executed during the processes of cell differentiation and morphogenesis.

The role of the carbohydrate moiety in thyrotropin action

The relative binding affinity of deglycosylated human TSH was 6-fold higher than that of native TSH. Although deglycosylated human TSH significantly stimulated adenylate cyclase, it was less effective than the native hormone. When deglycosylated human TSH was added with bovine TSH, however, a dose-dependent antagonism was observed. In particular, submaximal and maximal concentrations of bovine TSH and deglycosylated human TSH resulted in cAMP values much lower than the sum of activities of the individual hormones. The data suggest that although the effects of TSH deglycosylation are not as dramatic as with the gonadotropins, the carbohydrates of TSH appear to be required for maximal activation of adenylate cyclase by the hormone.

Gonadotropin releasing hormone activation is mediated by dimerization of occupied receptors

A dinitrophenyl (DNP)-derivative of a gonadotropin releasing hormone (GnRH) antagonist was prepared by chemical modification of the epsilon amino group in position 6 of [D-pGlu1,D-Phe2,D-Trp3,D-Lys6]GnRH with 1-fluoro-2, 4-dinitrobenzene. The DNP-antagonist D-pGlu-D-Phe-D-Trp-Ser-Tyr-D-Lys(N epsilon-DNP)-Leu-Arg-Pro-Gly-NH2, retained high affinity binding to the GnRH receptor of pituitary membrane preparations and exhibited antagonistic activity when assayed in cultured pituitary cells. Both antibodies against DNP and their Fab fragments were able to bind the DNP-antagonist. However, only the addition of bivalent antibodies (and not the Fab fragments) converted the DNP-antagonist to an agonist. These results suggest that divalency is a critical factor in GnRH action.

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.