Receptor-binding activity of highly purified bovine luteinizing hormone and thyrotropin, and their subunits

A New Perspective on Thyroid Hormones: Crosstalk with Reproductive Hormones in Females

Accumulating evidence has shown that thyroid hormones (THs) are vital for female reproductive system homeostasis. THs regulate the reproductive functions through thyroid hormone receptors (THRs)-mediated genomic- and integrin-receptor-associated nongenomic mechanisms, depending on TH ligand status and DNA level, as well as transcription and extra-nuclear signaling transduction activities. These processes involve the binding of THs to intracellular THRs and steroid hormone receptors or membrane receptors and the recruitment of hormone-response elements. In addition, THs and other reproductive hormones can activate common signaling pathways due to their structural similarity and shared DNA consensus sequences among thyroid, peptide, and protein hormones and their receptors, thus constituting a complex and reciprocal interaction network. Moreover, THs not only indirectly affect the synthesis, secretion, and action of reproductive hormones, but are also regulated by these hormones at the same time. This crosstalk may be one of the pivotal factors regulating female reproductive behavior and hormone-related diseases, including tumors. Elucidating the interaction mechanism among the aforementioned hormones will contribute to apprehending the etiology of female reproductive diseases, shedding new light on the treatment of gynecological disorders.

The Human TSHβ Subunit Proteins and Their Binding Sites on the TSH Receptor Using Molecular Dynamics Simulation

To gain further insight into the binding of the normal and variant human TSHβ subunits (TSHβ and TSHβv), we modeled the 2 monomeric proteins and studied their interaction with the TSH receptor ectodomain (TSHR-ECD) using molecular dynamics simulation Furthermore, analyzed their bioactivity in vitro using recombinant proteins to confirm that such binding was physiologically relevant. Examining the interaction of TSHβ and TSHβv with the TSHR-ECD model using molecular dynamic simulation revealed strong binding of these proteins to the receptor ECD. The specificity of TSHβ and TSHβv binding to the TSHR-ECD was examined by analyzing the hydrogen-bonding residues of these subunits to the FSH receptor ECD, indicating the inability of these molecules to bind to the FSH receptors. Furthermore, the modelling suggests that TSHβ and TSHβv proteins clasped the concave surface of the leucine rich region of the TSHR ECD in a similar way to the native TSH using dynamic hydrogen bonding. These mutually exclusive stable interactions between the subunits and ECD residues included some high-affinity contact sites corresponding to binding models of native TSH. Furthermore, we cloned TSHβ and TSHβv proteins using the entire coding ORF and purified the flag-tagged proteins. The expressed TSHβ subunit proteins retained bioactivity both in a coculture system as well as with immune-purified proteins. In summary, we showed that such interactions can result in a functional outcome and may exert physiological or pathophysiological effects in immune cells.

Novel insights on thyroid-stimulating hormone receptor signal transduction

The TSH receptor (TSHR) is a member of the glycoprotein hormone receptors, a subfamily of family A G protein-coupled receptors. The TSHR is of great importance for the growth and function of the thyroid gland. The TSHR and its endogenous ligand TSH are pivotal proteins with respect to a variety of physiological functions and malfunctions. The molecular events of TSHR regulation can be summarized as a process of signal transduction, including signal reception, conversion, and amplification. The steps during signal transduction from the extra- to the intracellular sites of the cell are not yet comprehensively understood. However, essential new insights have been achieved in recent years on the interrelated mechanisms at the extracellular region, the transmembrane domain, and intracellular components. This review contains a critical summary of available knowledge of the molecular mechanisms of signal transduction at the TSHR, for example, the key amino acids involved in hormone binding or in the structural conformational changes that lead to G protein activation or signaling regulation. Aspects of TSHR oligomerization, signaling promiscuity, signaling selectivity, phenotypes of genetic variations, and potential extrathyroidal receptor activity are also considered, because these are relevant to an understanding of the overall function of the TSHR, including physiological, pathophysiological, and pharmacological perspectives. Directions for future research are discussed.

Structure-function relations of the thyrotropin receptor

The thyrotropin (thyroid-stimulating hormone or TSH) receptor is an amphiphilic membrane component with a relative molecular mass of about 200,000 as judged by gel filtration and an isoelectric point close to pH 5. Analyses with chemical, enzymic and affinity probes indicate that the receptor is a glycoprotein containing a disulphide bridge and that the integrity of the disulphide bond is essential for maintaining the structure of the TSH-binding site. Serum from patients with Graves' disease contains antibodies which inhibit the binding of TSH to its receptor and there is considerable evidence that this effect is due to a direct interaction between the antibodies and the receptor. The antibody-receptor interaction is probably responsible for the TSH agonist properties of Graves' serum and, similarly, the TSH antagonist properties of the sera from a small number of patients can be explained on the basis of antibody-receptor binding. Although TSH and IgG from Graves' disease patients appear to bind to the same receptor, the relationship between the sites for the two substances is not clearly understood. However, Fab fragments of Graves' IgG are as effective as intact IgG in competing with TSH for the receptor and gel filtration and immunoprecipitation studies indicate that the binding of hormone and antibody to the receptor is mutually exclusive. Current evidence suggests therefore that the binding sites for TSH and TSH receptor antibodies are very closely related and may well be identical.

Human alpha-subunit analogs act as partial agonists to the thyroid-stimulating hormone receptor: differential effects of free and yoked subunits

The alpha-subunit is common to the heterodimeric glycoprotein hormones and has been highly conserved throughout vertebrate evolution. In an effort to determine if wild-type and engineered human alpha analogs can serve as agonists or antagonists to the human thyroid-stimulating hormone (TSH) receptor (TSHR), a potent alpha mutant, obtained by replacing four amino acid residues with lysine (alpha4K), was assayed and compared with the wild-type alpha-subunit. When added to CHO cells expressing TSHR, alpha4K, and to a very limited extent the fused homodimer, alpha4K-alpha4K, but not alpha, exhibited agonist activity as judged by cAMP production. When yoked to TSHR to yield fusion proteins, neither alpha, alpha4K, alpha-alpha, nor alpha4K-alpha4K activated TSHR, although yoked alpha4K and alpha4K-alpha4K were weak inhibitors of TSH binding to TSHR. The yoked subunit-receptor complexes were, however, functional as evidenced by increased cAMP production in cells co-expressing human TSHbeta and alpha-TSHR, alpha4K-TSHR, alpha-alpha-TSHR, and alpha4K-alpha4K-TSHR. These results demonstrate that agonists to TSHR can be obtained with alpha-subunit analogs and suggest that rational protein engineering may lead to more potent alpha-based derivatives. The differences found between the experimental paradigms of adding free alpha analogs to TSHR and covalent attachment are attributed to con-formational constraints imposed by fusion of the alpha-subunit analog and receptor, and may suggest an important role for a free (C-terminal) alpha-carboxyl in the absence of the beta-subunit.

Yoked complexes of human choriogonadotropin and the lutropin receptor: evidence that monomeric individual subunits are inactive

Human choriogonadotropin (hCG) contains an alpha-subunit, common to other members of the glycoprotein hormone family, and a unique beta-subunit that determines hormone specificity. It is generally thought that heterodimer formation is obligatory for full hormonal activity, although other studies have indicated that individual subunits and homodimeric hCGbeta were capable of low affinity binding to the LH receptor (LHR) and subsequent activation. Previously, we constructed two yoked hormone (hCG)-LHR complexes, where the two hormone subunits and the heptahelical receptor were engineered to form single polypeptide chains, i.e. N-beta-alpha-LHR-C and N-alpha-beta-LHR-C. Expression of both complexes led to constitutive stimulation of cAMP production. In the present study, we investigated whether the human alpha-subunit and hCGbeta can act as functional agonists when covalently attached to or coexpressed with the LH receptor. Our initial results showed that hCGbeta, but not alpha, was able to activate LHR with an increase in intracellular cAMP in human embryonic kidney 293 cells but not in Chinese hamster ovary or COS-7 cells. Further examination of this apparent cell-specific agonist activity of hCGbeta revealed that low levels of endogenous alpha-subunit were expressed in human embryonic kidney 293 cells, thus enabling sufficient amounts of active heterodimer to form with the transfected hCGbeta to activate LHR. The studies in Chinese hamster ovary and COS-7 cells clearly demonstrate that, even under experimental conditions where hormone-receptor interactions are maximized, individual subunits of hCG can not act as functional agonists, at least in their monomeric form.

Risk factors for goiter and thyroid nodules

The occurrence of thyroid diseases is determined by interplay between genetic and environmental factors. The major environmental factor that determines goiter prevalence is iodine status, but other environmental factors influencing entire populations have been identified such as goitrogens in food and drinking water. Less focus has been on individual environmental factors and the interplay between factors. The goiter prevalence is higher in certain groups in the population. The variation in goiter prevalence between the genders is well known with a higher occurrence among women. The association with age is probably dependent on iodine status, because it seems that the zenith of goiter prevalence appears earlier in life the more severe iodine deficiency the population is exposed to. The association with individual risk factors has been investigated in some studies, especially the association with tobacco smoking. In iodine-deficient areas, a strong association between tobacco smoking and goiter prevalence is found, whereas the association is less pronounced in iodine-replete areas. This was predictable from experimental studies showing thiocyanate to be the mediator of the goitrogenic effect of tobacco smoke acting as a competitive inhibitor of iodine uptake. The association with alcohol intake has only been investigated in few studies, but a low occurrence of goiter among alcohol consumers has been found. The mechanism of this association is not known. Increased goiter prevalence during pregnancy has been reported, and recently a long-term goitrogenic effect of pregnancies has also been shown. As demonstrated for tobacco smoking, this association is dependent on iodine status, because the association has only been found in areas with a suboptimal iodine intake. This indicates pregnancy-induced goiter to be the result of exacerbation of existing iodine deficiency. Recently, the use of oral contraceptives has been shown to be associated with a markedly reduced prevalence of goiter, although experimental studies have previously shown proliferative effects of estrogens on thyrocytes. Some implications for prevention of thyroid disease could be suggested. Discussion of smoking habits should be included in a consultation for goiter with a motivation to quit smoking. Iodine deficiency has particularly strong goitrogenic effects during pregnancy and for the sake of the mother as well as the fetus, sufficient iodine supply should be ensured to all pregnant women. The difference in age maximum in goiter prevalence suggests that monitoring of iodine deficiency disorders should ideally include a spectrum of age groups.

Low goitre prevalence among users of oral contraceptives in a population sample of 3712 women

OBJECTIVE

Goitre occurs primarily in women. As oestrogens may be an aetiologic factor, the use of oral contraceptives (OC) could be associated with an increased risk of goitre. This is supported by experimental data, but a recent population study suggested decreased thyroid volumes among users of OC.

DESIGN

Cross-sectional, observational study.

PARTICIPANTS

A random population sample of 3712 women from two Danish cities.

MEASUREMENTS

Ultrasound and clinical examination of the thyroid, measurement of serum TSH, T3, T4 and thyroglobulin, and personal interviews with registration of use of OC or post-menopausal oestrogen therapy. Data were analysed in linear models and logistic regression adjusting for age and iodine status.

RESULTS

Thyroid volume was lower among users than non-users of OC, 11.1 ml vs. 12.1 ml (P < 0.001). Use of OC was correspondingly associated with a reduced prevalence of thyroid enlargement on ultrasound (odds ratio 0.53, 95% confidence interval 0.37-0.74) and of palpable and visible goitre (OR 0.24, 95% CI 0.07-0.81). Serum TSH was 1.24 mU/l in non-users of OC and 1.35 mU/l in users (P = 0.002). The percentage of goitre cases prevented by OC compared to a hypothetical population without use of OC was 29%. Post-menopausal oestrogen therapy showed similar, but statistically non-significant, associations.

CONCLUSION

Use of oral contraceptives was associated with a lower thyroid volume and reduced risk of goitre, and clinically evident goitre was four times more frequent among non-users than among users of oral contraceptives.

Thyrotropic action of human chorionic gonadotropin

Hyperthyroidism or increased thyroid function has been reported in many patients with trophoblastic tumors. In these cases, greatly increased human chorionic gonadotropin (hCG) levels and suppressed TSH levels suggest that hCG has thyrotropic activity. Recent investigations have clarified the structural homology not only in the hCG and TSH molecules but also in their receptors, and this homology suggests the basis for the reactivity of hCG with the TSH receptor. The clinical significance of the thyrotropic action of hCG is now also recognized in normal pregnancy and hyperemesis gravidarum. Highly purified hLH binds to recombinant hTSH receptor and is about 10 times as potent as purified hCG in increasing cAMP. The beta-subunits of hCG and hLH share 85% sequence identity in their first 114 amino acids but differ in the carboxy-terminal peptide because hCG beta contains a 31-amino acid extension (beta-CTP). A recombinant mutant hCG that lacks beta-CTP showed almost identical potency to LH on stimulation of recombinant hTSH receptor. If intact hCG were as potent as hLH in regard to its thyrotropic activity, most pregnant women would become thyrotoxic. One of the roles of the beta-CTP may be to prevent overt hyperthyroidism in the first trimester of pregnancy when a large amount of hCG is produced by the placenta. Nicked hCG preparations, obtained from patients with trophoblastic disease or by enzymatic digestion of intact hCG, showed approximately 1.5- to 2-fold stimulation of recombinant hTSH receptor compared with intact hCG. This suggests that the thyrotropic activity of hCG may be influenced by the metabolism of the hCG molecule itself. Deglycosylation and/or desialylation of hCG enhances its thyrotropic potency. Basic hCG isoforms with lower sialic acid content extracted from hydatidiform moles were more potent in activating adenylate cyclase, and showed high bioactivity/immunoactivity (B/I) ratio in CHO cells expressing human TSH receptors. This is consistent with the finding that the beta-CTP truncated hCG with higher thyrotropic potency is substantially deglycosylated and desialylated in the beta-subunit relative to intact hCG because all four O-linked glycosylation sites occur within the missing C-terminal extension. The desialylated hCG variant also interacts directly with recombinant hTSH receptors transfected into human thyroid cancer cells. There is thyroid-stimulating activity in sera of normal pregnant women, and this correlates with serum hCG levels. The thyroid gland of normal pregnant women may be stimulated by hCG to secrete slightly excessive quantities of T4 and induce a slight suppression of TSH, perhaps being about 1 mU/L less than nongravid levels, but not high enough to induce overt hyperthyroidism. Maternal thyroid glands may secrete more thyroid hormone during early pregnancy in response to the thyrotropic activity of hCG that overrides the normal operation of the hypothalamic-pituitary-thyroid feedback system. Biochemical hyperthyroidism associated with hyperemesis gravidarum has been attributed to hCG. In patients with hyperemesis gravidarum, thyrotropic in serum correlated with hCG immunoreactivity, and the severity of vomiting as indicated by clinical and biochemical parameters correlated with the degree of thyroid stimulation. To understand the thyrotropic action of hCG, it is necessary to know whether hCG activates the same domain of the TSH receptor as does TSH. The identification of the molecular structure of the hCG isoform with the highest thyrotropic potency will resolve the enigma of gestational thyrotoxicosis and the hyperthyroidism associated with trophoblastic disease and hCG-producing tumors.

Absence of lutropin (LH) receptor mRNA in the rat thyroid: further evidence for specificity cross-over at the thyroid-stimulating hormone receptor level

Chorionic gonadotropin (CG) and purified lutropin (LH) activate intact thyroid tissue and isolated thyroid cells. A recent report has suggested that the presence of aberrant LH/CG receptors in human and rat thyroid tissue may interact with gonadotropin thus explaining the mechanisms of thyroid cell stimulation. To detect putative thyroidal LH receptor mRNA, a segment of the transmembrane region containing domains 3 through 6 of the rat (r) LH receptor was targeted for amplification using the polymerase chain reaction (PCR). cDNA prepared from a rLH receptor-positive control tissue (testis) was efficiently amplified under stringent annealing conditions giving a 486 bp product as predicted. However, cDNAs from thyroidal tissue and from the thyroid-stimulating hormone (TSH)- and hCG-responsive 1B-6 subclone of Fisher rat thyroid cells (FRTL-5) yielded no detectable 486 bp product. A smaller (non-LH) fragment amplified to similar extents from both testis and thyroidal cDNAs provided a useful internal control for amplification. This allowed the conclusion that specificity cross-over between LH/CG and TSH occurs at the TSH receptor and that the LH/CG receptor gene is transcriptionally silent in rat thyroidal cells.

Structure of the lutropin/choriogonadotropin receptor

In summary, the LH/CG receptor is a single polypeptide which contains a large hydrophilic domain that is situated extracellularly, attached to a region that spans the plasma membrane seven times, the carboxy-terminal region being intracellular. This topology was predicted by the amino acid sequence and has been confirmed by our immunofluorescence studies. The extracellular domain, which is related to a family of leucine-rich glycoproteins, is presumably involved in binding the large glycoprotein hormones hCG and LH. The carboxy-terminal half of the receptor, which is related to the family of rhodopsinlike receptors, is (by analogy with these receptors) presumably involved in the coupling of the receptor to the G protein. Our transfection studies confirm that this single polypeptide is capable of binding hormone and activating adenylyl cyclase. Therefore, not only is the structure of the LH/CG receptor unique compared to other cell surface receptors characterized to date, but also its structure suggests that the mechanism of the translation of hormone binding to G protein coupling in this receptor is different from other G protein-coupled receptors whose ligands are much smaller and intercalcate among the transmembrane helices. We predict that, due to the homology among the glycoprotein hormones, the structures of the FSH and TSH receptors share extensive amino acid and structural homology with the LH/CG receptor. Last, our newly acquired knowledge about the structure of the LH/CG receptor, and the development of a cDNA and antibodies for this receptor, should enable more detailed studies on the function and regulation of the LH/CG receptor, not previously possible.

Fetal hypophysis as the main source of serum TSH in fetal rat

Decapitation performed at days 17-18 leads to a drastic drop (82%) in blood TSH of 19 and 21-day-old rat fetuses below the mother's level. 125I-TSH injected at 21 days into the mother's bloodstream is not found in fetal blood. The fetal hypophysis is the main source of fetal plasmatic TSH.

Direct demonstration of intrinsic follicle-stimulating hormone receptor-binding activity in acid-treated equine luteinizing hormone

After dissociating equine gonadotropins as a function of time at pH 3, we examined them by radioligand assay and sodium dodecyl sulfate polyacrylamide gel electrophoresis under nondissociating conditions (low, 0.1% SDS). Equine follicle-stimulating hormone (FSH) rapidly lost its receptor-binding activity, and low SDS-polyacrylamide gels demonstrated dissociation into subunits. Maximum dissociation occurred after 20-30 min of pH 3 incubation. Equine luteinizing hormone (LH), however, retained most biologic activity and was largely intact after 72 h of pH 3 incubation. Dose-response curves of acid-treated equine LH and FSH and intact equine LH and FSH were compared in five types of radioligand receptor assays. LH and FSH receptor-binding activities of equine LH were unaffected by pH 3. Equine LH showed 19- and 32-times more activity in the rat testis FSH assay than it did in chicken or horse FSH assays, respectively, directly demonstrating the intrinsic FSH receptor-binding activity of equine LH and the relative lack of specificity for these hormone preparations of the rat FSH receptor. Acid-treated 95% of its biologic activity in FSH assays. In LH assays, the slight (0.2%) activity of equine FSH was relatively unaffected by acid treatment, suggesting that contamination by equine LH accounts for this activity.

Comparison of newt lens regeneration stimulating activity in preparations of mammalian thyrotropin and fibroblast growth factor purified by various methods

Irises of the newt Notophthalmus viridescens will regenerate a new lens in organ culture in the presence of the bovine thyrotropin preparation NIH-TSH-B8. It is not certain, however, whether thyrotropin itself is responsible for this stimulatory effect. To elucidate this problem further we compared the lens regeneration stimulating activity of thyrotropin preparations from several species, prepared by various methods. The lowest effective concentrations were approximately 3.0 micrograms ml-1 for the bovine NIH-TSH-B8 and 1.4 micrograms ml-1 for the ovine NIAMDD-oTSH-9 preparations. At those lowest concentrations, lenses with elongating lens fiber cells (stage 6) and enlarged lens fiber core (stage 8) were obtained, respectively, and the lens-fiber-specific protein gamma-crystallin was present in both cases. The crude bovine thyrotropin fraction, Sigma-TS-10, stimulated lens regeneration only at the highest concentration, 1400 micrograms ml-1. Bovine Pierce-bTSH, the purest thyrotropin preparation, stimulated lens regeneration sporadically at the lower concentration of 0.04 micrograms ml-1 up to the advanced stage 9 with large lens fiber core and flattened lens epithelium in one of nineteen irises. The pituitary fibroblast growth factor is a known contaminant of thyrotropin preparations. The preparation CR-FGF-40002 at concentrations between 0.001 and 0.1 microgram ml-1 did not promote lens regeneration. Therefore, the lens regeneration stimulating activity in thyrotropin preparations is not attributable to the fibroblast growth factor, and may also be independent of thyrotropin because the lens regeneration stimulating activity is not proportional to the thyrotropic activity in the preparations examined.

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.

Solubilized human thyrotrophin receptors behave as one class of high-affinity binding sites

Evidence is presented justifying the conclusion that, in contrast to the dextran-coated charcoal technique, the widely used technique of separating bound and free TSH with polyethylene glycol is inadequate and yields inaccurate results. Optimum values for the concentration of Triton X-100, pH, salts, temperature and time of incubation were established for the TSH-TSH receptor interaction. According to Scatchard analysis, soluble TSH receptors behaved as one class of binding sites. The affinity constant for this class of binding sites (Ka 1.3 X 10(9) M-1) is identical to that for the high-affinity binding sites found in human thyroid membranes (Ka 1.2 X 10(9) M-1). No low-affinity binding sites could be detected after solubilization of membrane receptors. Chromatography experiments on Sepharose CL-6B indicated that, in excess TSH, each micelle containing TSH receptors (molecular weight 150 000) binds 4 [125I]TSH molecules. These data, together with the absence of low-affinity binding sites, led to the hypothesis that high-affinity TSH binding sites may be formed by the clustering of 4 low-affinity binding sites. Cross-reactivity experiments showed that both alpha- and beta-subunits are involved in the binding of TSH to its receptor; the TSH beta-subunit showed an increased cross-reactivity with soluble receptors.

Spatial relationships of the human chorionic gonadotropin (hCG) subunits in the assembly of the hCG-receptor complex in the luteinized rat ovary

In an attempt to examine the spatial relationships of the human chorionic gonadotropin (hCG) subunits in the assembly of the hCG-receptor complex, the recombined 125I-labeled hCG, with label in either the alpha subunit or the beta subunit, was cross-linked to the luteinizing hormone (LH)/hCG receptor. The efficacy of the cross-linking of the 125I-alpha subunit or the 125I-beta subunit of hCG to the LH/hCG receptor was then examined. The autoradiographic profile of 125I-hCG-receptor complex containing the label in the alpha subunit of hCG showed that the alpha subunit can cross-link with all four subunits of the LH/hCG receptor. However, only one faint labeled band, corresponding to Mr = 68,000, was detected when the 125I-hCG-receptor complex with label in the beta subunit was subjected to sodium dodecyl sulfate/polyacrylamide gel electrophoresis under reducing conditions. When the electrophoresis was performed under nonreducing conditions, the Mr 68,000 band disappeared concomitantly with the accumulation of radioactivity in the high molecular weight region. These results indicated that the beta subunit of hCG, unlike the alpha subunit, can cross-link only weakly with the smallest subunit of the LH/hCG receptor. A comparison of the differential effectiveness of the cross-linking of 125I-alpha subunit with 125I-beta subunit of hCG to the LH/hCG receptor suggests that both alpha and beta subunits of hCG are intimately associated with the receptor, but the bulk of the beta subunit of hCG is buried in between the receptor and the alpha subunit of hCG. On the basis of our data, a model for the spatial arrangement of hCG subunits in the hCG-receptor complex is proposed.

Preferential masking by the receptor of immunoreactive sites on the alpha subunit of human choriogonadotropin

125I-Labeled human choriogonadotropin (125I-hCG) bound to rat ovarian receptor was solubilized in Triton X-100. By using increasing concentrations of nine different antisera specific for the individual subunits of human choriogonadotropin (hCG), free 125I-hCG or 125I-hCG-receptor complex was precipitated by double-antibody technique. The ability of any antiserum to bind to the hormone-specific beta subunit was not affected by hCG binding to receptor, suggesting that this subunit is not directly involved with the receptor in the final state of the hormone-receptor complex. In contrast, every antiserum specific for the alpha subunit was dramatically inhibited in binding to the solubilized 125I-hCG-receptor complex. These results suggest that the alpha subunit directly interacts with the receptor, thereby masking immunoreactive sites normally available on the free hormone. Because a number of reports describe binding activity of high concentrations of immunopurified beta subunits of hCG, we propose a two-step model for the binding of hCG to receptor and postulate separate and distinct roles for the subunits. We propose that the binding of hCG to the receptor involves a specific low-affinity initial interaction of the beta subunit with the receptor that activates a second site for the high-affinity binding of alpha subunit and stabilization of the hormone-receptor complex.

A role for luteinizing hormone and follicle-stimulating hormone in the etiology of simple goiters in the pediatric population

In light of the progressive, pubertal increases in serum gonadotropin (FSH, LH) levels coinciding with the peak incidence of simple goiters; the common alpha subunits associated with the gonadotropins, TSH and hCG; the ability of LH and hCG to bind to thyroid TSH receptors and affect thyroid function; and, the probable interactions between thyroid hormones and gonadotropins, a role for gonadotropins in the etiology of simple goiters merits consideration.