Effect of thyrotropin-releasing hormone on the carbohydrate structure of secreted mouse thyrotropin. Analysis by lectin affinity chromatography

Recombinant human thyroid-stimulating hormone: pharmacology, clinical applications and potential uses

The major functions of pituitary thyroid-stimulating hormone (TSH) are to maintain the biosynthesis and secretion of the thyroid hormones L-thyroxine (T4) and L-3,5,3'triidothyronine (T3). The TSH core contains two apoproteins, the alpha and beta subunits. The alpha subunit is identical to that of pituitary follitropin, pituitary lutropin and placental chorionic gonadotropin, whereas the beta subunit is unique. TSH is a glycoprotein; the glycoprotein components of the alpha and beta subunits account for more than 10% of their mass and are essential for normal thyrotropic action and intravascular kinetics. The hypothalamic tripeptide, TSH-releasing hormone (TRH) is required for optimum TSH biosynthesis, particularly as far as addition of the glycoprotein components is concerned. TRH deficiency is associated with secretion of TSH molecules that are appropriately measured in most assays but have reduced bioactivity. In previous years the TSH used in clinical practice was obtained and purified from bovine pituitaries. Bovine TSH was used to test thyroid function and to augment the uptake of radioiodine in patients with thyroid cancer. Bovine TSH has been largely abandoned as a clinical agent because of adverse immune reactions. A recombinant human TSH (rhTSH; Thyrogen), has been approved by the US FDA for diagnostic use in patients with thyroid cancer. The alpha and beta subunits of Thyrogen are identical to those of human pituitary TSH. Thyrogen has a specific activity of approximately 4 IU/mg and is a potent stimulator of T4, T3 and thyroglobulin (Tg) secretion in healthy volunteers. It also increases thyroid iodide uptake in patients with thyroid cancer or multinodular goitre and in volunteers, even those exposed to large amounts of stable iodide. Thyroid cancer patients who have been treated by thyroidectomy and radioiodine ablation but are at risk of harbouring residual thyroid cancer are candidates for Thyrogen administration to prepare them for whole body iodide scans and serum Tg measurements. In thyroidectomised thyroid cancer patients who are unable to secrete pituitary TSH upon thyroid hormone withdrawal, Thyrogen is the only acceptable method to prepare them for these procedures. Thyrogen has been used on a compassionate basis to prepare patients for radioiodine ablation. rhTSH, in addition to being useful in the management of patients with thyroid cancer, is potentially useful to test thyroid reserve and to aid in thyroid-related nuclear medicine procedures. In the future, TSH analogues that have superagonist or antagonist properties may become available as therapeutic agents.

Investigations on the carbohydrate moieties of glycoprotein allergens

Many allergens are glycoproteins and their carbohydrate structure can contribute to the IgE reactivity. Therefore it is of great interest to study the carbohydrate structures of these particular antigens. Here, we present an overview of methods combining basic procedures in glycochemistry with various applications of electrophoresis that allow investigating single allergens in crude extracts. Various allergen extracts, e.g. from tomato, grass pollen and bacteria were analysed and the suitability of the tests are discussed.

The role of sialic acid in opsonin-dependent and opsonin-independent adhesion of Listeria monocytogenes to murine peritoneal macrophages

The adhesion of listeriae to host cells employs mechanisms which are complex and not well understood. Listeria monocytogenes is a facultative intracellular pathogen responsible for meningoencephalitis, septicemia, and abortion in susceptible and immunocompromised individuals. Subsequent to colonization and penetration of the gut epithelium, the organism attaches to resident macrophages and replicates intracellularly, thus evading the humoral immune system of the infected host. The focus of these studies was to investigate the attachment of the organism to murine peritoneal macrophages in an opsonin-dependent and opsonin-independent fashion. Assessment of competitive binding experiments by immunofluorescence and enzyme-linked immunosorbent assays showed that adhesion of the organism to macrophages in the presence or absence of opsonins was inhibited (90%) by N-acetylneuraminic acid (NAcNeu). In addition, the lectin from Maackia amurensis, with affinity for NAcNeu-alpha(2,3)galactose, blocked binding of L. monocytogenes to host cells. Oxidation of the surface carbohydrates on the organism by using sodium metaperiodate resulted in a dose-dependent reduction (up to 98%) in adherence to macrophages. Monoclonal antibody to complement receptor 3 did not prevent listeriae from binding to mouse macrophages or from replicating within the infected cells whether or not normal mouse serum was present. Based on our results, we propose the involvement of NAcNeu, a member of the sialic acid group, in the attachment of L. monocytogenes to permissive murine macrophages.

Studies on the carbohydrate moieties of the timothy grass pollen allergen Phl p I

Timothy grass pollen was investigated in order to determine the carbohydrate moieties of its major grass group I (Phl p I) and to study its impact on allergenicity. Based on computer calculations one N-glycosylation site was deduced from the cDNA data of Phl p I. After two-dimensional polyacrylamide gel electrophoresis, followed by blotting of pollen extract and by use of the monoclonal antibody IG 12 we identified at least six isoallergens of Phl p I with the main spots at a molecular mass of 35-37 kDa and a pI range of 6.5-7.3. Deglycosylation by trifluoromethanesulfonic acid resulted in a decrease of about 2 kDa. Treatment with N-glycosidase A resulted in a partial deglycosylation, while N-glycosidase F and O-glycosidase had no effect. Ten lectins were investigated for their binding to Phl p I components: Aleuria aurantia agglutinin showed strong reactivity (indicating fucose residues), while Galanthus nivalis agglutinin (indicating mannose residues) and concanavalin A (indicating mannose, glucose or N-acetylglucosamine residues) showed weak binding. By neutral sugar analysis we determined similar contents of the monosaccharides in the isoallergens. In order to study the influence of the carbohydrate structures of Phl p I on IgE reactivity we tested some patient sera for their reactivity with intact and deglycosylated Phl p I. Even though most of the IgE antibodies bind at the protein core, we detected one serum that recognized carbohydrate moieties on the Phl p I.

Structures of high-mannose and complex oligosaccharides of mouse TSH and free alpha-subunits after in vitro incubation of thyrotropic tissue with TRH

To determine whether incubation of mouse thyrotropic tissue with TRH in vitro influenced the oligosaccharide structure of TSH, thyrotropic tumor tissue or pituitary tissue was incubated in vitro with [3H]mannose or with [35S]sulfate and [3H]methionine, in the absence or presence of TRH for times up to 24 h. [3H]mannose-labeled oligosaccharides from intracellular TSH and free alpha-subunits were analyzed by paper chromatography, and were predominantly Man9GlcNAc and Man8GlcNAc units both in the absence and presence of TRH. The [35S]sulfate/[3H]methionine ratio in secreted molecules was greater for TSH than for free alpha-subunits; within TSH heterodimers the ratio was greater for beta-subunits than alpha-subunits. The [35S]/[3H] ratio was not altered in TSH or free alpha-subunits by TRH. Analyses of [3H]mannose-labeled charged oligosaccharides by HPLC anion-exchange chromatography revealed similar types of oligosaccharides present on TSH subunits and free alpha-subunits (having one or two sulfate residues, one or two sialic acid residues, or both a sulfate and a sialic acid residue). These charged oligosaccharides occurred in different proportions on TSH subunits compared to free alpha-subunits, and also differed depending on whether the tissue source was tumorous or nontumorous. The proportions of oligosaccharide unit types were not altered by TRH. Thus, while this study provided information concerning the high-mannose and complex oligosaccharides of mouse TSH, there was no evidence that short incubations of tissues with TRH in vitro caused modulation of TSH oligosaccharide structures.

Environmental effects on protein glycosylation

Cultured mammalian cells are being used to produce proteins for therapeutic and diagnostic use because of their ability to perform complex post-translational modifications, including glycosylation. The oligosaccharide moieties can play an important role in defining several biological properties of glycoproteins, including clearance rate, immunogenicity, and biological specific activity. There is a growing interest in defining the factors that influence glycosylation, including the cell culture environment. In this review we organize the published data from in vitro cell culture and tissue culture studies that demonstrate direct effects of the culture environment on N-linked glycosylation.

Carbohydrate-dependent epitope mapping of human thyrotropin

To probe possible effects of carbohydrate chains in the conformation of pituitary glycoprotein hormones, two radiolabeled derivatives of human thyroid-stimulating hormone (hTSH), either partially deglycosylated in the beta-subunit or fully deglycosylated in both the alpha- and beta-subunits, were compared to the native hormone for binding to monoclonal as well as polyclonal antibodies. Monoclonal antibodies were screened for their ability to bind the intact hormone (anti-hTSH), hTSH and its free alpha-subunit (anti-alpha) or its free beta-subunit (anti-beta). A panel of 14 monoclonal antibodies directed against at least eight out of the 12 epitopes known to be present in the hormone was tested in solid-phase assays for their capacity to bind intact and deglycosylated forms of hTSH. All of them displayed identical recognition of native and partially deglycosylated 125I-hTSH. In contrast, binding of fully deglycosylated 125I-hTSH to anti-hTSH and anti-beta antibodies was dramatically lost while that of anti-alpha was preserved. This clearly indicates that most of the epitopes specific for subunit association as well as those present on the beta-subunit are glycosylation dependent. No alteration was found in antibody recognition following deglycosylation of free individual subunits, indicating that the carbohydrate effect can only occur in the combined dimer. Using polyclonal antisera raised against the International Reference Preparations, we found that the deglycosylated hormone could be bound by the anti-beta antiserum although at a much lower dilution than the native antigen, suggesting the presence of at least one glycosylation-independent epitope in the beta-subunit. Competitive binding assays revealed that deglycosylated hTSH is 5 times less immunoreactive toward the anti-beta compared to the anti-alpha antiserum. The current data thus demonstrate the presence of the glycosylation-independent epitopes in the alpha-subunit of hTSH and the localization of most of the glycosylation-dependent domains in the beta-subunit.

Differential effect of inhibitors of oligosaccharide processing on the secretion of thyrotropin from dispersed rodent pituitary cells

We examined the effect of various inhibitors of oligosaccharide processing on the content and secretion of newly synthesized thyroid-stimulating hormone (TSH) from dispersed hypothyroid rodent pituitary cells. 1-deoxynojirimycin and N-methyl-1-deoxynojirimycin, both inhibitors of glucosidases I and II, decreased intracellular TSH (to 60-76% of control) and secreted TSH (to 60-63% of control) after a 1-hour incubation (pulse) with [35S]methionine and an 8-hour incubation (chase) in isotope-free media. In contrast, deoxymannojirimycin and swainsonine, inhibitors of mannosidase I and II, respectively, increased both intracellular TSH (to 267-309% of control) and secreted TSH (to 192% of control) at 8 hours. TSH oligosaccharides synthesized in the presence of these glucosidase and mannosidase inhibitors were largely sensitive to endo-beta-N-acetylglucosaminidase H (endo H), confirming inhibition of processing. Despite differences in oligosaccharide structure, the in vitro bioactivities of these secreted TSH isoforms were nearly identical. These data confirm and extend previous work performed with 1-deoxynojirimycin suggesting that glucosylated high mannose forms of TSH are more susceptible to intracellular degradation. The novel finding that deoxymannojirimycin and swainsonine increase secreted and total TSH above control levels suggests that non-glucosylated high mannose forms as well as hybrid-type oligosaccharides may facilitate secretion and direct TSH away from a natural degradation pathway.

Physiological regulation of thyrotropin

The pattern of TSH secretion in man in pulsatile in addition to the well known circadian variation. The mechanism triggering TSH pulses remains unclear to date. Infusions of somatostatin or dopamine rapidly lowering basal TSH levels without suppressing the pulsatile pattern suggest that an episodic disinhibition exerted by a physiological inhibitor is not a likely cause. On the same basis, thyroid hormones do not appear to be candidates, since they similarly inhibit basal TSH levels after a time lag of several hours but again do not suppress pulsatile release of the hormone. In contrast, bolus injections of dexamethasone completely abolish pulsatile release of TSH for several hours despite a normal sensitivity of the pituitary to exogenous TRH, suggesting a hypothalamic action of the drug. The hypothesis that pulsatile TSH release might be governed by a pulsatile mode of a hypothalamic stimulator is supported by the observation that an infusion of nifedipine, a calcium channel blocker, which in vitro selectively inhibits the TRH effect on TSH but not prolactin secretion, exerts a comparable effect when it is infused in vivo.

Importance of glycosylation for hepatic clearance of renal renin

Three differently glycosylated forms of renin (renin A, B-1, and B-2) were highly purified from rat kidneys by pepstatin-aminohexyl-Sepharose affinity chromatography and by serial lectin affinity chromatography on concanavalin A (con A) and lentil lectin-Sepharose, and the role of glycosylation of renin was investigated. Renin A and renin B-1 were loosely and tightly bound to con A, respectively, but did not bind to lentil lectin. Renin B-2 bound to both con A and lentil lectin. These three forms of renin were all similar in their physicochemical characteristics, including molecular weight, isoelectric point, specific activity, Km, optimum pH, and antigenicity. Each form of renin, labeled with 125I and given intravenously to anesthetized rats, disappeared from the circulation at different rates (metabolic clearance rates of 5.05 +/- 1.02, 17.1 +/- 2.5, and 36.0 +/- 4.1 ml.min-1.kg-1 for renins A, B-1, and B-2, respectively). Labeled renin A distributed to a similar extent in the liver and kidney (21.2 +/- 0.2 and 15.2 +/- 0.8% of the injected dose, respectively), whereas renins B-1 and B-2 were distributed predominantly in the liver (56.3 +/- 1.2 and 72.3 +/- 3.7% of the injected dose, respectively) and to a lesser extent in the kidney (4.3 +/- 0.3 and 2.1 +/- 0.2%, respectively). Deglycosylation of renin B-1 with endoglycosidase F resulted in no loss of its enzymatic activity or antigenicity but greatly reduced the metabolic clearance rate to 18% (from 17.1 +/- 2.5 to 3.09 +/- 0.17 ml.min-1.kg-1). Deglycosylation of renin B-1 greatly decreased its uptake by the liver (from 56.3 +/- 1.2 to 3.3 +/- 0.2%) and increased its uptake by the kidney (from 4.3 +/- 0.3 to 23.9 +/- 0.9%). These studies indicate the importance of glycosylation of renin for its hepatic uptake and metabolic clearance rate.