Ricin and lentil lectin-affinity chromatography reveals oligosaccharide heterogeneity of thyrotropin secreted by 12 human pituitary tumors

[Clinical and morphological characteristics, diagnostic criteria, and outcomes of surgical treatment of TSH-secreting pituitary adenomas]

Thyrotropinomas (TSH-secreting tumors) are a rare type of pituitary adenomas, which account for about 0.5-2.0% of all pituitary tumors. The criterion of thyrotropinoma is visualization of the tumor in the presence of a normal or elevated level of the thyroid-stimulating hormone (TSH) in the blood and elevated concentrations of free T4 (fT4) and free T3 (fT3).

OBJECTIVE

To study the clinical, diagnostic, and morphological characteristics and treatment outcomes of TSH-secreting pituitary tumors.

MATERIAL AND METHODS

The study included 21 patients aged from 15 to 67 years with pituitary adenoma and a normal or elevated blood TSH level combined with elevated fT4 and fT3 levels who were operated on at the Neurosurgical Institute in the period between 2002 and 2015. Before surgery, in the early postoperative period, and 6 months after surgery, the patients were tested for levels of TSH, fT4, fT3, prolactin, cortisol, the luteinizing hormone (LH), the follicle-stimulating hormone (FSH), estradiol/testosterone, and the insulin-like growth factor (IGF-1). The thyroid status was evaluated using the following reference values: TSH, 0.4-4.0 mIU/L; fT4, 11.5-22.7 pmol/L; fT3, 3.5-6.5 pmol/L. An immunohistochemical study of material was performed with antibodies to TSH, PRL, GH, ACTH, LH, FSH, and Ki-67 (MiB-1 clone); in 13 cases, we used tests with antibodies to somatostatin receptors type 2 and 5 and to D2 subtype dopamine receptors.

RESULTS

Thyrotropinomas were detected in patients aged from 15 to 67 years (median, 39 years), with an equal rate in males (48%) and females (52%). Before admission to the Neurosurgical Institute, 11 (52%) patients were erroneously diagnosed with primary hyperthyroidism; based on the diagnosis, 7 of these patients underwent surgery on the thyroid gland and/or received thyrostatics (4 cases). Hyperthyroidism symptoms were observed in 16 (76%) patients. The blood level of TSH was 2.47-38.4 mIU/L (median, 6.56); fT4, 22.8-54.8 nmol/L (median, 36); fT3, 4.24-12.9 pmol/L (median, 9.66). Tumors had the endosellar localization in 4 (19%) cases and the endo-extrasellar localization in 17 (91%) cases. Total tumor resection was performed in 7 (33%) patients. All these tumors had the endosellar and endo-suprasellar localization. No total resection was performed in patients with infiltrative growth of adenoma (invading the skull base structures). An immunohistochemical study of tumor resection specimens detected only TSH expression in 3 (14%) cases; 18 (86%) tumors were plurihormonal and secreted TSH and GH and/or PRL. Of 13 tumors, expression of the type 2 dopamine receptor was detected in 9 (69%) cases; expression of somatostatin receptors type 5 and type 2 was found in 6 (46%) and 2 (15%) cases, respectively.

CONCLUSION

The criterion for total tumor resection was a postoperative decrease in the TSH level to 0.1 mIU/L or less. Total resection was performed in 33% of patients with tumors of only the endosellar and endo-suprasellar localization. In most cases, tumors were plurihormonal and secreted TSH and GH and/or PRL.

Stability of recombinant human thyrotropin potency based on bioassay in FRTL-5 cells

BACKGROUND

Recombinant human thyrotropin (rhTSH; Thyrogen®) is approved for use in a 0.9 mg dose/day for 2 consecutive days for diagnosis and treatment of differentiated thyroid cancer. It is recommended that it be injected immediately after reconstitution in the distilled water diluent supplied by the manufacturer. However, Thyrogen has been used off-label in doses less than the standard 0.9 mg dose for stimulation of radioiodine uptake in the treatment of multinodular goiter. To determine whether the biologic activity of Thyrogen can be preserved after dilution, we designed experiments to assess the biologic stability of Thyrogen under different durations and temperatures of storage.

METHODS

rhTSH was diluted in 1% bovine serum albumin in phosphate-buffered saline to a concentration of 0.9 mg /mL and further diluted to 0.1 mg/mL. Aliquots of 0.5 mL were stored at room temperature, 4°C, -11°C, and -60°C for various lengths of time. In addition, rhTSH aliquots were also subjected to incubation for 1 hour at 50°C and to 10 cycles of freezing in dry ice alternating with thawing at 37°C. Bioassays were performed in FRTL-5 cells. rhTSH was added to the media at a final concentration of either 5 ng/mL or 20 ng/mL, and the cells were then incubated for 48 hours. Potency was assessed by measurement of ¹²⁵I-iodide uptake in comparison to cells treated with perchlorate to block iodide uptake. Samples were immunoassayed at day 185 of storage.

RESULTS

Samples stored at 4°C, -11°C, -60°C, and room temperature retained activity after storage periods of up to 204 days. Samples subjected to 10 freeze-thaw cycles or heated to 50°C for 1 hour retained full biologic activity. Immunoassay at day 185 showed no difference in immunoactivity in relation to the storage condition.

CONCLUSION

rhTSH kept at 4°C, -11°C, -60°C, and room temperature maintained good biologic potency for more than 6 months of storage when tested in vitro, indicating that the biologic activity is very stable. However, altered sialylation occurring during storage could have altered the half-life of rhTSH. Nevertheless, the data provide reassurance that storage in the cold for a few months does not result in significant loss of biologic activity.

Different responses to chronic somatostatin analogues in patients with central hyperthyroidism

OBJECTIVE

Central hyperthyroidism is mainly due to two different causes, TSH-secreting pituitary adenoma (TSH-oma) and resistance to thyroid hormone in its pituitary variant, i.e. patients presenting with signs and symptoms of hyperthyroidism (PRTH). Because therapeutic approach and the clinical follow-up are extremely different in these two disorders, a correct differential diagnosis is mandatory. Unfortunately, no definite pathognomonic tool is presently available and an extensive biochemical and instrumental workup is frequently needed in order to reach the correct diagnosis. Aim of the present study was to investigate the use of somatostatin analogues in the differential diagnosis between TSH-omas and PRTH, as well as the possible treatment of PRTH with these analogues.

DESIGN AND PATIENTS

Eight patients with TSH-oma and four with PRTH underwent the acute test with somatostatin analogue Octreotide (0.1 mg subcutaneously), as well as long-acting Octreotide-LAR (30 mg intramuscularly every 28 days) for two months.

MEASUREMENTS

Serum TSH, FT3 and FT4 were evaluated in basal condition, at time 0 and every hour for 6 h during acute test, and every 15 days for 2 months during chronic treatment.

RESULTS

During acute test, in both patients with PRTH and TSH-oma, a similar reduction in immunoreactive TSH and FT3 levels was observed, while no variations were found in FT4 concentrations. In contrast, during the administration of Octreotide-LAR, no significant variations of all tested parameters were observed in PRTH group, whereas FT3 and FT4 concentrations normalized or presented a significant reduction (> 30% of pretreatment values) in seven of eight patients with TSH-oma, despite minor variation of immunoreactive TSH levels.

CONCLUSIONS

Chronic administration of long-acting somatostatin analogues in patients with central hyperthyroidism caused a marked decrease of FT3 and FT4 levels in all patients but one with TSH-oma, while patients with PRTH did not respond at all. Thus, administration of long acting somatostatin analogues for at least 2 months can be useful in the differential diagnosis in problematic cases of central hyperthyroidism. Furthermore, the present findings exclude the possibility of a beneficial effect of chronic administration of somatostatin analogues in controlling thyrotoxic symptoms in PRTH patients.

Glycosylation variants of human TSH selectively activate signal transduction pathways

The oligosaccharide chains of pituitary glycoprotein hormones such as human thyroid-stimulating hormone (hTSH) have been shown to be important in biosynthesis, subunit association, secretion and bioactivity. However, the exact biological significance of these glycosylation variants (isoforms) remains controversial. The aim of this paper is to investigate the role of hTSH glycosylation variants in signal transduction. Human pituitary standard TSH (2nd International Reference Preparation 80/558; IRP-hTSH) was treated with neuraminidase, fractionated by isoelectric focusing (IEF) and affinity chromatography using the lectins concanavalin A (Con A) and lentil. To determine the in vitro bioactivity of these hTSH isoforms, simultaneous measurement of cAMP formation and inositol phosphates release was applied in two different cell systems (CHO cells stably and Cos-7 cells transiently transfected with hTSHR cDNA). Desialylated TSH variants showed a significantly increased ratio of bioactivity to immunoreactivity for cAMP production in CHO-R cells (B/I ratio desialylated variants: 3.54 +/- 0.005; B/I ratio sialylated variants: 2.84 +/- 0.01 P < 0.05). Testing the bioactivity of hTSH glycosylation variants isolated by IEF, we found basic variants to be significantly more active than acidic ones in stimulating the cAMP formation in CHO-R cells (B/I ratio basic variants: 9.92 +/- 0.64; neutral variants: 5.98 +/- 0.07; acidic variants: 2.80 +/- 0.12; P < 0.01). There were no differences in stimulation of IP-release. High-mannose TSH variants (firmly bound to Con A) showed greater potency to stimulate cAMP formation and IP-release in both CHO-R and Cos-7 cells than biantennary TSH variants (weakly bound to Con A). Both core-fucosylated (lentil-bound) and core-unfucosylated (lentil-unbound) TSH variants proved to be strong stimulators of cAMP release in CHO and Cos-7 cells. In CHO-R (Cos-7) cells, 400 microU/ml core-fucosylated TSH stimulated cAMP formation 14(2.6)-fold, core-unfucosylated TSH 7.3(2.3)-fold over control values. In contrast to our findings of cAMP activation by both core-fucosylated and core-unfucosylated TSH variants, release of IPs was stimulated only by, core-fucosylated (lentil-bound) TSH variants and not by TSH variants lacking core-fucose residues (lentil-unbound TSH). This was true for both CHO-R and Cos-7 cells. The lentil-unbound TSH therefore showed an identical differential activation of signal transduction pathways in two different cell systems: strong stimulation of the cAMP-cascade without activation of IPs release (P < 0.05). In conclusion, we showed for the first time for TSH that the two dominant intracellular signal transduction systems (cAMP formation and IPs release) are activated to different degrees by hTSH glycosylation variants.

Transiently decreased sialylation of thyrotropin (TSH) in a patient with the euthyroid sick syndrome

A 60-year old woman was admitted to the medical intensive care unit with respiratory distress and worsening renal function, and was found to have bilateral renal artery occlusion. Aggressive nutrition per nasogastric tube was begun on day 8 of her illness, and she eventually recovered after bilateral renal artery bypass surgery, which was performed on day 15. She developed the euthyroid sick syndrome. Levels of serum TSH, T3, and T4 fell during the first few days of her illness, then all trended to normal levels by day 28. The TSH level declined from 1.6 microU/mL on day 2 to 0.2 microU/mL on day 5, then rose to 4.5 microU/mL on day 10, and was 3.8 microU/mL on day 14. The ratios of free T4/TSH, a crude measure of the bioactivity of TSH, were 1.4, 8.0, 0.16, 0.32, and 1.14 on days 2, 5, 10, 14 and 28, respectively. TSH was immunoaffinity concentrated from serum collected on four dates. The TSH oligosaccharides were enzymatically released, treated with or without neuraminidase, labeled with a fluorescent probe, and analyzed by gel electrophoresis. The TSH oligosaccharides were found to be transiently less sialylated on day 13 as compared to days 2, 4, and 24. Three gel bands representing poorly sialylated oligosaccharides represented a mean of 20.6% of TSH oligosaccharides on days 2, 4, and 24, but represented 33.7% of TSH oligosaccharides on day 13. This is the first report of altered TSH oligosaccharide sialylation in the euthyroid sick syndrome. If confirmed by studies of additional patients, altered TSH sialylation may, in part, explain the altered TSH bioactivity that has been described in the euthyroid sick syndrome.

Glycoprotein-secreting pituitary adenomas

In the past, pituitary tumours that produce one or more of the glycoproteins (TSH, LH, FSH and alpha subunit) were thought to be rare. However, using modern immunocytochemical and molecular biology techniques, these tumours are being recognized with increasing frequency. Many of these tumours produce glycoprotein alpha and beta subunits in addition to intact glycoproteins. Hormone production is often low compared with tumour size, and serum hormone levels may not be elevated in these patients. Tumours that produce the gonadotrophins (LH or FSH) or alpha subunit account for the majority of clinically non-functioning pituitary adenomas. They do not cause a specific clinical syndrome, and usually present with symptoms of a large mass lesion and/or hypopituitarism. Optimal treatment of these tumours is often difficult. The initial approach is usually transsphenoidal surgery, followed by radiation therapy if there are symptoms due to residual tumour. Medical therapy of gonadotrophin and alpha subunit tumours may include the use of dopamine agonists or somatostatin analogues, although neither has been shown to consistently decrease tumour size. Preliminary trials with experimental GnRH antagonists suggest that these agents may be useful as adjuvant therapy of gonadotrophin tumours. Tumours that produce TSH are rare. Patients present with hyperthyroidism, which is often misdiagnosed as Graves' disease, as well as with symptoms of a pituitary mass lesion. Almost all TSH tumours secrete excess amounts of free alpha subunit. Optimal treatment of these tumours includes transsphenoidal surgery, followed by radiation therapy for residual tumour. The somatostatin analogue octreotide is effective in reducing excess TSH secretion from these tumours, and causes a reduction in tumour volume in a significant minority of patients.

Binding of thyrotropin to lentil lectin is unchanged by thyrotropin-releasing hormone administration in three patients with thyrotropin-producing pituitary adenomas

Glycoproteins have increased affinity for lentil lectin when fucose residues are bound to N-acetylglucosamine in the "core region" of their asparagine-linked oligosaccharides. In three patients with thyrotropin (TSH)-producing pituitary tumors, the proportion of serum TSH isoforms that bound to lentil (70.8% +/- 15%) was higher than that seen for TSH from normal persons (32.5 +/- 8%). Unlike normal subjects, the concentration of TSH circulating in the tumor patients after acute administration of TSH-releasing hormone (TRH) did not rise, and the TSH did not exhibit increased binding to lentil compared to basal TSH. The TSH binding to lentil in one tumor patient decreased after metoclopramide, but TSH binding to lentil generally remained unchanged after metoclopramide or L-dopa administration. We conclude that human thyrotropic tumor tissue, unlike normal thyrotrophs, generally fails to release more highly fucosylated isoforms of TSH after pharmacologic stimulation, perhaps because the tumor tissue is less readily modulated by endocrine stimuli, or because the TSH is already relatively highly fucosylated.

Fucosylation of glycoproteins begins in the rough endoplasmic reticulum of mouse active thyrotrophs

Our aim was to determine whether fucosylation of glycoproteins begins in the rough endoplasmic reticulum (RER) of active thyrotrophs. This would contrast with most cells studied, in which fucosylation generally is associated with the Golgi apparatus. Mouse thyrotropic tumor tissue was incubated with [35S]methionine for 2, 5, 7, 10, 30, and 90 minutes. TSH and free alpha-subunits were immunoprecipitated from cell lysates, and they displayed a time-dependent increase in affinity for lentil lectin (which binds oligosaccharides having core fucose), even at short times. Since no 20-30 minute lag in onset of TSH- and free alpha-subunit-lentil binding was appreciated, as might have been expected had fucosylation begun only in the Golgi, it appeared that fucosylation was beginning in the RER of thyrotrophs. Pituitary tissue from euthyroid and hypothyroid mice was incubated with [3H]fucose, then subjected to electron microscopic autoradiography. The pituitaries of hypothyroid mice had numerous "thyroidectomy cells," which had 40% of silver grains over dilated cisternae of RER. "Nonthyroidectomy" cells had few silver grains over RER; most were over secretory granules and Golgi areas. Thus, active mouse thyrotrophs appear to shift the subcellular site of fucosylation partially from Golgi to RER, and this phenomenon may represent one cellular mechanism whereby the endocrine regulation of the structure of TSH oligosaccharides is accomplished.