Multiple expressions of the activity of guanine nucleotide regulatory protein in human thyroid adenylate cyclase

Measurement of cAMP accumulation in Chinese hamster ovary cells transfected with the recombinant human TSH receptor (CHO-R): a new bioassay for human thyrotropin

Circulating TSH bioactivity may vary in several clinical and experimental conditions. Since the reliability of the current methods for the measurement of TSH bioactivity is limited, a new bioassay based on cAMP accumulation in Chinese Hamster Ovary cells transfected with recombinant human TSH receptor (CHO-R) was set up. The sensitivity was 0.3 +/- 0.1, 0.4 +/- 0.1 and 0.01 +/- 0.01 micrograms/L for TSH IRP 80/558, recombinant human TSH and bovine TSH, respectively. Standard curves were parallel, and the intra- and inter-assay coefficients of variation were 13 +/- 1.1% and 22 +/- 1.9%, respectively. LH, FSH, CG and TSH subunits did not stimulate cAMP accumulation up to high concentrations. Circulating TSH was partially purified by immunoaffinity separation and concentrated before being bioassayed. However, plain sera with high TSH levels, such as those from primary hypothyroid patients (PH), could be directly tested in CHO-R bioassay, provided that sera were added at concentrations lower than 10%. TSH from 6 normal subjects had biological to immunological ratio (B/I) ranging from 0.6 to 2.1 (mean +/- SD = 1.4 +/- 0.5). TSH from 6 patients with PH showed bioactivity significantly lower than in normals (B/I = 0.6 +/- 0.3; p < 0.001; range = 0.3-1.1). TSH from 5 patients with central hypothyroidism of hypothalamic origin (CH) had undetectable basal bioactivity (B/I < 0.2), which normalized in only one patient after acute TRH and in all patients after chronic TRH administration. In conclusion, CHO-R cells provide an excellent tool for evaluating TSH bioactivity, owing to high sensitivity, specificity, reproducibility and feasibility of the assay.(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.

Cell membrane coating with glutaraldehyde: application to a versatile solid-phase assay for thyroid membrane proteins and molecules interacting with thyroid membranes

In defined conditions, glutaraldehyde was shown to tightly bind cell membranes to flexible microtiter plates without significant alteration of the antigenic and functional properties of membrane proteins. In the presence of 0.06% glutaraldehyde, human thyroid membranes were bound to plastic firmly enough to resist numerous washing and flicking steps; the coated membranes remained almost unaltered with regard to monoclonal antibody and thyrotropin binding as well as adenylate cyclase and peroxidase activities. Based on the use of thyroid membrane-coated microtiter plates, a versatile solid-phase assay was developed which allowed screening of anti-membrane monoclonal antibodies, detection of thyrotropin-displacing activity in hormone and antibody preparations, and monitoring of fractionation experiments of solubilized membrane antigens and thyrotropin receptor. It was concluded that the use of glutaraldehyde for coating cell membranes to flexible microtiter plates enabled the establishment of simple, rapid, and reliable assays for detection and quantitation of membrane proteins and molecules interacting with membranes.

Decreased receptor binding of biologically inactive thyrotropin in central hypothyroidism. Effect of treatment with thyrotropin-releasing hormone

Previous studies have suggested that certain cases of idiopathic central hypothyroidism of hypothalamic origin may result from the secretion of biologically inactive thyrotropin. To investigate this possibility and to define the mechanism of defective hormone action, we measured the adenylate cyclase-stimulating bioactivity (B) and receptor-binding (R) activity of purified immunoreactive serum thyrotropin (I) from seven patients with hypothalamic hypothyroidism. We found a strikingly decreased R/I ratio (less than 0.15) in patients as compared with controls (0.6 to 2.7) and a similarly decreased B/I ratio (less than 0.2 vs 2.8 to 5.6). After acute injection of thyrotropin-releasing hormone (TRH, 200 micrograms intravenously), the R/I ratio increased in two of three patients, but the B/I ratio became normal in only one. After administration of TRH for 20 to 30 days, an increase in immunoreactive serum thyrotropin was observed in all patients. Moreover, both ratios returned to normal in all but one patient, who had apparent desensitization. The increase in the amount and bioactivity of secreted thyrotropin after long-term TRH therapy resulted in enhanced secretion of serum thyroid hormones in all patients studied. We conclude that in certain cases of hypothalamic hypothyroidism, secreted thyrotropin lacks biologic activity because of impaired binding to its receptor; TRH treatment can correct both defects. These data suggest that TRH regulates not only the secretion of thyrotropin but also its specific molecular and conformational features required for hormone action.

Hormone receptor-coupling factor-adenylate cyclase interaction: theoretical considerations

The steady-state response properties of two current models for receptor/nucleotide coupling protein/adenylate cyclase systems are examined by computer modeling techniques. In the model of Levitzki (Trends Pharmac. Sci., May, 1982, pp. 203-208), a ligand may give rise to full or partial agonist behavior only. In the model of Stadel, DeLean, and Lefkowitz (1982) configurations of the rate constants can be found which lead not only to full or partial agonist behavior, but also to varying degrees of inhibition at sufficiently high concentrations of ligand, as observed experimentally in a variety of adenylate cyclase systems. In the latter model, it is also possible to find configurations of the rate constants for which addition of a ligand will lead to inhibition of adenylate cyclase activity. The nature of partial agonism and reasons as to why it may be expected to occur for a wide variety of ligands are discussed.

Role of the carbohydrate moiety of human choriogonadotropin in its thyrotropic activity

To investigate the role of the carbohydrate moiety of human choriogonadotropin (hCG) in its thyrotropic activity, highly purified hCG and its desialylated subunits were treated with anhydrous HF/anisole (1 h, 0 degree C). The deglycosylated alpha- and beta-subunits were recombined with their native complementary subunits, and the interactions of these hCG congeners with the thyrotropin (TSH) receptor-adenylate cyclase system were investigated using human thyroid membranes. Deglycosylated hCG (dghCG) bound to the high affinity-low capacity TSH-binding sites of thyroid membranes; its equilibrium dissociation constant was lower than that of asialo-hCG (ashCG) (ED50: 2.6 and 6 microM, respectively). Like ashCG, dghCG did not stimulate thyroidal adenylate cyclase, but rather inhibited TSH stimulation of this enzyme in a dose-dependent manner. Thus, dghCG behaved as an antagonist and exhibited an inhibition constant of 0.78 microM while ashCG exhibited a constant of 1.50 microM. As might be predicted from the behavior of dghCG, absence of carbohydrate from either subunit enhanced the ability of the hCG hybrid recombinants to interact with the TSH receptor-adenylate cyclase system. However, only the hybrid recombinant lacking carbohydrate on its alpha-subunit lacked intrinsic thyrotropic activity; the hybrid recombinant lacking carbohydrate on its beta-subunit not only displayed intrinsic thyrotropic activity, but was of even higher potency than intact hCG in stimulating thyroidal adenylate cyclase. These results demonstrate that the carbohydrate moieties of both hCG subunits impede the process of recognition of hCG by the TSH receptor, while the carbohydrate moiety of the alpha-subunit, but not that of the beta-subunit, is essential for the process of hCG activation of thyroidal adenylate cyclase.