Thyrotropin binding to and adenylate cyclase activity of porcine thyroid plasma membranes

The cloning of the human follicle stimulating hormone receptor and its expression in COS-7, CHO, and Y-1 cells

Follicle stimulating hormone (FSH) receptor clones were isolated from a human testis cDNA library. Characterization of the cDNA clones showed that the DNA and predicted amino acid sequences of the long open reading frame differed from a previously published human ovarian FSH receptor sequence (Minegish et al. (1991) Biochem. Biophys. Res. Commun. 175, 1125-1130) by seven nucleotides and five amino acids. A human FSH receptor splice variant was also identified and characterized. A full-length human FSH receptor cDNA was engineered for expression in COS-7, CHO, and Y-1 cells. In transient transfections of COS-7 cells and stable transfections of Y-1 cells, efficient FSH receptor mRNA accumulation and isolation of FSH-responsive cell lines occurred only when an intron was included in the 5' untranslated region of the FSH receptor transcription unit. Y-1 cells stably transfected with the FSH receptor responded to FSH treatment by rounding up and by synthesizing increased amounts of progesterone. Stably transfected CHO cell lines, which responded to FSH by synthesizing increased amounts of cAMP, were isolated irrespective of the presence of the heterologous intron. The FSH-responsive CHO and Y-1 cell lines may be suitable for the development of better in vitro FSH bioassays. These cells also constitute a convenient source of human FSH receptor protein for use in radioreceptor assays and in studies of receptor-ligand interactions.

Identification and properties of a novel type of Na+-permeable amiloride-sensitive channel in thyroid cells

Amiloride-sensitive cationic channels are present in the apical membrane of porcine thyroid cells in primary culture. An amiloride-sensitive (K0.5 = 150 +/- 28 nM where K0.5 is the concentration of unlabelled ligand which reduces the specific binding of the same labelled ligand by 50%) 22Na+-flux component (Km for Na+ at 18 mM) has been identified which was also blocked by the potent amiloride derivative phenamil (K0.5 = 47 +/- 21 nM). The most potent inhibitor of Na+/H+ exchange, ethylisopropyl-amiloride, hardly inhibited this 22Na+-influx component at a concentration of 21 microM. Amiloride binding sites were characterized using [3H]phenamil. The tritiated ligand binds to a single family of binding sites in thyroid membranes with a Kd value of 50 +/- 10 nM and a maximal binding capacity of 5 +/- 1 pmol/mg protein. Patch-clamp experiments have directly demonstrated the existence of a phenamil- and amiloride-sensitive cationic channel, with a conductance of 2.6 pS, which is permeable to sodium, but not very selective (PNa+/PK+ = 1.2). This channel is an important element in the regulation of the resting membrane potential of thyroid cells.

Means for the assessment of radioligand quality and its importance in receptor-binding studies. Observations with radiolabelled formylmethionyl-leucyl-phenylalanine

Various methods for testing the quality of radioligands were applied to two different radiolabelled forms of formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe). The purpose of the study was both to examine the value of these methods for assessing radioligand quality and to determine the suitability of these particular radioligands for studying the chemotactic formylpeptide receptors on the rabbit neutrophil. It is useful in this context to distinguish two different aspects of radioligand quality: these are purity and equivalence to the native ligand. The two methods described for measuring receptor-reactivity (or 'bindability'), by measuring binding to an increasing excess of receptors and by a re-incubation procedure, provide a reliable measure of purity that should readily be applicable to other radioligands. Equivalence to the native ligand is more difficult to establish, and any uncertainty about the specific radioactivity of the radioligand can pose serious problems with this assessment. Commercial preparations of both tritiated and 35S-labelled fMet-Leu-Phe were found to be inadequately pure for detailed receptor studies. Repurification by t.l.c., however, consistently yielded radioligand preparations of high purity and close equivalence to the native ligand. Other radioligands may often also require a suitable repurification step before use for detailed receptor studies; this is especially important whenever a complex receptor-binding pattern is envisaged.

Separation and analysis of two plasma membrane fractions from bovine thyroid which differ in TSH binding and TSH activation of adenylate cyclase

A tissue disruption technique leading to the separation of thyroid epithelial cell components from interfollicular material has been used to study the distribution and the properties of membrane adenylate cyclase originating from intraglandular thyroid and non-thyroid cells. Bovine thyroid fragments were forced through a metallic sieve. The material which filtrates was composed of open cells and cell debris (fraction A); the material remaining on the sieve contained the basal lamina and the interfollicular material as shown by photon and electron microscopic observations (fraction B). Homogenates (HA and HB) were prepared from fractions A and B and centrifuged on a 41% sucrose layer to prepare membrane fractions: MA and MB, which were tested for the presence of adenylate cyclase, TSH-responsive adenylate cyclase and 125I-labelled TSH binding activity. HA and HB contained respectively 70% and 30% of the total thyroid adenylate cyclase activity. MA and MB were similarly enriched in 5'-nucleotidase and adenylate cyclase: 8- to 10-fold as compared to the corresponding homogenates. MA and MB exhibited a marked difference in the response to TSH: TSH either alone or in the presence of Gpp(NH)p stimulated the adenylate cyclase of MA and did not have any effect on MB. Fractionation of MA by isopycnic centrifugation on Percoll gradients yielded a membrane peak exhibiting a TSH-responsive adenylate cyclase activity and a 125I-labelled TSH binding activity displaceable by an excess of unlabelled TSH. A membrane peak at the same density was obtained from MB but its adenylate cyclase did not respond to TSH and there was no specific binding of labelled TSH. Our data indicate that an important fraction of membrane adenylate cyclase of the thyroid does not seem to be coupled with TSH receptor; the major part of this fraction (MB) likely originates from intraglandular non-thyroid epithelial cells. The separation of this membrane fraction from the thyroid cell plasma membrane fraction (MA) allows to increase the response of this latter fraction to TSH.

Multicomponent structure of the thyrotropin receptor: relationship to Graves' disease

The thyrotropin receptor is proposed to contain both a glycoprotein and a ganglioside component. Monoclonal antibodies have been developed against soluble thyroid TSH receptor preparations and using Graves' lymphocytes. These show that initial recognition of thyrotropin requires the glycoprotein component, but that monoclonal antibodies to this component block thyrotropin function (blocking antibodies) rather than mimic thyrotropin. Monoclonal antibodies which stimulate thyroid activity in cultured cell systems (cAMP increase) or mouse bioassays, all interact with gangliosides. Using monoclonal antibodies to the glycoprotein component of the thyrotropin receptor, we show that two protein bands, molecular weights 18,000-23,000 and 50,000-55,000, are precipitated from detergent-solubilized preparations. Using a crosslinking procedure with 125I-labeled thyrotropin, we show that thyrotropin binding is related to the disappearance of the 18,000-23,000 molecular weight band on sodium dodecylsulfate gels and the appearance of a 30,000-33,000 molecular weight thyrotropin-membrane component complex. Higher molecular weight thyrotropin-membrane complexes of 75,000-80,000 and 250,000 are visualized when binding studies are performed at pH 7.4 in physiologic medium; larger amounts of the 30,000-33,000 complex are evident at pH 6.0 in a low salt medium. It is thus proposed that the glycoprotein component of the thyrotropin receptor is composed of two subunits with apparent molecular weights of 18,000-23,000 and 50,000-55,000; that the 18,000-23,000 subunit interacts with thyrotropin; and that different receptor subunits can exist depending on in vitro binding conditions. Using monoclonal-stimulating antibodies or natural autoimmune IgG preparations from patients' sera, we show that stimulating antibodies exhibit species-specific binding to human thyroid ganglioside preparations. Individual components or determinants of the thyrotropin receptor structure with specific autoimmune immunoglobulins.

Biological activity of anti-thyrotropin anti-idiotypic antibody

Rabbit anti-rat anti-human thyrotropin anti-idiotypic antibodies have been raised. These antibodies were active at the thyrotropin (TSH) receptor, in that they inhibited 125I-labeled bovine TSH binding to thyroid plasma membranes, stimulated adenylate cyclase activity through a guanyl nucleotide-dependent mechanism, augmented radioiodide transport into isolated porcine thyroid follicular cells and induced such cultured cells to organize into follicles. Aside from substantiating the expectation that anti-hormone anti-idiotypic antibodies may possess properties of the original hormone, this work raised the possibility that thyroid-stimulating antibodies which cause the hyperthyroidism of Graves' disease may be anti-TSH anti-idiotypic antibodies.

Resolution of radioiodinated thyrotropin into receptor active and inactive components by column chromatography

Following radioiodination by the lactoperoxidase method and subsequent purification on Sephadex G100, it was found that [125I]TSH exhibited varying degrees of binding activities to the thyrotropin receptor. In order to further purify the radiolabeled hormone, the [125I]TSH preparation was chromatographed on Sepharose 6B. Two peaks of radioactive material (Peaks I and II) were recovered, containing approx. 60% of the applied radioactivity. Upon elution with Mg2+, the remainder of the radiolabeled material was recovered as a single peak (Peak III). Characterization of these 3 peaks by radioimmunoassay demonstrated that all 3 were immunocompetent, although Peaks I and III were 3-4-fold more immunoreactive than Peak II. Analysis by radioreceptor assay indicated that Peak III showed an increase in receptor-binding capacity (in comparison with the [125I]TSH preparation purified by Sephadex G100 alone), while both Peaks I and II exhibited significantly reduced binding activity. In contrast, human TSH (NIH) chromatographed mainly as a receptor inactive peak, although it was fully immunocompetent. Scatchard analysis of receptor binding to bovine [125I]TSH from Peak III yielded a curvilinear plot with affinities similar to those we have previously reported for [125I]TSH purified by Sephadex G100 chromatography. The total number of binding sites, however, increased proportionally with the active fraction of the [125I]TSH preparation. Since the mass of bound hormone is calculated from the percent bound of total radioactivity and only a fraction of the measured total participates in the binding, it is therefore necessary to correct for the inactive fraction when calculating the total receptor number.

Thyrotropin binding to porcine thyroid plasma membranes: kinetic and thermodynamic analyses

Evaluation of TSH binding to plasma membranes of porcine thyroid revealed unique sensitivity to pH and temperature. Analysis of apparent equilibrium binding yielded a linear Scatchard plot at the optimal pH of 6.0, indicating one class of binding sites. At physiological pH 7.4 a curvilinear Scatchard plot was obtained, resolved by computer analysis into two classes of binding sites of different affinities and capacities. Treatment of membranes with phospholipase C resulted in a 20% decrease in the number of high affinity sites, but no change occurred in binding affinity. In contrast, low affinity sites were not altered. To evaluate the significance of the curvilinear Scatchard plot, the kinetics of association were examined. The intrinsic Kd (kd/ka) was 0.20 nM, a value essentially equivalent to that of the high affinity binding component. The 'negative cooperativity' model of hormone binding was evaluated by examining the effect of excess unlabeled TSH on dissociation rate. Dissociation of bound 125I-labeled TSH was biphasic, and was enhanced by unlabeled hormone, regardless of whether the membranes were prelabeled at pH 6.0 or 7.4. This effect was not correlated with curvilinear Scatchard plots, and therefore not proof of negative cooperativity. Binding sites for TSH were further distinguished by their sensitivity to temperature. A van't Hoff plot of temperature dependence of the apparent Kd of the high affinity site was linear from 4 to 37 degrees C. In contrast, the apparent Kd of low affinity binding did not vary with respect to temperature. These results demonstrate that there are at least two independent binding sites for TSH on porcine thyroid plasma membranes, distinguishable by their equilibrium binding properties.

Direct visualization of membrane clustering and endocytosis of thyrotropin into cultured thyroid cells

We prepared a highly fluorescent conjugate of thyrotropin (TSH) which retains approximately 25% of the binding affinity of native TSH towards TSH receptor and approximately 25% of the potency of the native hormone in stimulating the accumulation of cAMP in thyroid cells. Using an image-intensified microscopy system, we observed that our fluorescent TSH bound specifically to diffusely distributed membrane receptors on live rat or bovine embryo thyroid cells grown in culture. At 37 degrees C the fluorescent hormone formed visible patches which were internalized and subsequently degraded. Hence, TSH, like other polypeptide hormones, is internalized by a process of receptor-mediated endocytosis. The addition of bivalent antibodies against the fluorophore rhodamine, to thyroid cells that were previously exposed to sub-optimal concentrations of rhodamine TSH, elicited maximal stimulation of the thyroid adenyl cyclase. Monovalent Fab' fragments did not enhance the response of rhodamine TSH. Therefore we conclude that enhanced surface clustering of TSH molecules increases their capacity to stimulate the adenyl cyclase of thyroid cells.

Biologic activity of anti-thyrotropin anti-idiotypic antibody

We raised an antihuman thyrotropin anti-idiotypic antibody and showed that it was active at the thyrotropin receptor. Thus this antibody inhibited 125I b-TSH binding to thyroid plasma membranes, stimulated adenylate cyclase activity through a guanyl nucleotide-dependent mechanism, increased radioiodide entry rate into isolated porcine thyroid follicular cells, and induced such cultured cells to organize into follicles. All these parameters are typical of thyrotropin action. This work raises the possibility that thyroid stimulating antibodies that cause the hyperthyroidism of Graves disease may be, at least in some patients, anti-thyrotropin anti-idiotypic antibodies. It also offers a novel method whereby antireceptor antibodies used in the isolation and characterization of the receptor may be raised from ligands.

Thyrotropin receptor-adenylate cyclase system in plasma membranes from normal and diseased human thyroid glands

Thyrotropin binding characteristics and adenylate cyclase (AC) activity of thyroid plasma membranes were studied in 52 tissues from normal and diseased human thyroids. Data from normal glands, Graves' goiters, non toxic multinodular goiters and nodular and perinodular tissue of toxic nodular goiters show the same basal, TSH- and NaF- stimulated adenylate cyclase activities (no. = 45; 34.1 +/- 3.2 (m +/- SE), 378 +/- 43, 298 +/- 48 pmol cAMP x min-1 x mg membrane protein-1), the same stimulability of AC by TSH (11.3 +/- 1.4--fold over basal level) and by NaF (8.1 +/- 1.8-fold), the same apparent TSH binding equilibrium constants (5.6 +/- 0.7 and 406 +/- 57 nM) and the same TSH binding site concentrations (2.2 +/- 0.4, 27.8 +/- 5.9 pmol x mg membrane protein-1). Alterations of the TSH receptor and of the AC were detected in membranes from tumoral and metastatic lymph node tissues from thyroid papillary carcinoma and in the thyroid tissue from post-radioiodide therapy thyroiditis. These observations suggest that: (i) hyperthyroidism in Graves' disease or toxic nodular goiter does not result in and is not a consequence of an alteration in the TSH receptor-adenylate cyclase system; (ii) there is no evidence supporting a relationship between the studied membrane properties and clinical or histological status; (iii) membrane abnormalities detected in thyroid carcinoma vary widely; (iv) studies of these membrane alterations might be of interest in the therapeutic management of thyroid carcinoma and may lead to a better understanding of the receptor-adenylate system.