Role of Subunit Sialic Acid in Hepatic Binding, Plasma Survival Rate, and In Vivo Thyrotropic Activity of Human Chorionic Gonadotropin

Hoermann, Rudolf; Kubota, Ken; Amir, Syed M.

doi:10.1089/thy.1993.3.41

Cited by 26 publications

(18 citation statements)

References 22 publications

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“…For example, terminal sialic acid residues may affect bioactivity of closely related glycoprotein hormones in different ways, related to both species-and hormone-specific receptor interactions (23)(24)(25)(26)(27). As reported, highly sialylated (3-subunit carbaoydrate chains of hCG are important in prolonging plasma' half-life (10,23). Moreover, deglycosylation of the hCG a subunit resulted in a dimer that had a plasma half-life similar to that of the fully glycosylated hCG, whereas deglycosylation of the hCG (3 subunit decreased plasma halflife of the dimer (10 of the phTSH a subunit by a specific liver receptor.…”

Section: Discussionmentioning

confidence: 99%

Subunit-specific functions of N-linked oligosaccharides in human thyrotropin: role of terminal residues of alpha- and beta-subunit oligosaccharides in metabolic clearance and bioactivity.

Szkudlinski

Thotakura²,

Weintraub³

1995

Proc. Natl. Acad. Sci. U.S.A.

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The recombinant human thyroid stimulating hormone (rhTSH) containing oligosaccharides terminated with NeuAc(a2-3)Gal(q1-4)GlcNAc(31 showed higher in vivo activity and lower metabolic clearance rate (MCR) SO44GalNAc(031-4)GlcNAcI31 but also with NeuAc(a2-3 or a2-6)Gal(j31-4)GlcNAcl31 (6, 7). Recombinant human TSH (rhTSH) expressed in Chinese hamster ovary (CHO) cells differs from the phTSH in its glycosylation pattern; the N-linked glycans on the rhTSH contain only NeuAc(a2-3)Gal(f31-4)GlcNAcf31-terminal sequence (8,9). We have shown that rhTSH with highly sialylated oligosaccharide chains had longer plasma half-life and higher in vivo bioactivity despite lower in vitro bioactivity compared to the predominantly sulfated phTSH or enzymatically desialylated rhTSH (asialo-rhTSH; asrhTSH) (3).Previous studies investigating the role of oligosaccharides on individual subunits in biological function of glycoprotein hormones have used two major approaches. First, dimerization of chemically or enzymatically deglycosylated subunits has been used (10-12). Second, in vitro mutagenesis of the glycosylation sites of different gonadotropins has been used to elucidate the role of each individual oligosaccharide chain (13-15). However, these strategies permit elucidation of differences related only to the presence or absence of an entire carbohydrate chain in a specific subunit or glycosylation site. Moreover, it is known, for example, that desialylation may have a more pronounced effect on the metabolic clearance rate (MCR) and in vivo bioactivity than deglycosylation (16). Thus, oligosaccharide removal by deglycosylation or mutagenesis was not adequate to determine biological function of a particular oligosaccharide structure on each subunit. Furthermore, previous studies in this field have not used dimerization of TSH subunits with defined carbohydrate composition to address this question.We have exploited the availability of exclusively sialylated rhTSH preparations expressed in CHO cells as well as highly purified phTSH with known oligosaccharide structures to assess the contribution of sialylation and sulfation in each of the subunits in biological properties of TSH. Present studies of phTSH and rhTSH subunit hybrids indicated that the N-linked carbohydrate residues of the 3 subunit play a more important role than those of the a subunit in determining MCR, which has a major impact on in vivo bioactivity. Moreover, our data demonstrate that terminal sialic acid residues on the a but not on the 13 subunit attenuate hTSH intrinsic potency, further emphasizing a distinct role of terminal sialylation in each subunit.MATERIALS AND METHODS Materials. hTSH (AFP-8600B) was obtained from the National Hormone and Pituitary Program (Baltimore). rhTSH was produced by Genzyme as described (8,17).Isolation of rhTSH and phTSH Subunits. Purified phTSH, rhTSH, and asrhTSH were dissolved to 10 mg/ml in 0.1 M sodium phosphate buffer (pH 7.0) that contained 6 M guanidine hydrochloride and 0.02% sodium azide and was incubated overnight...

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Section: Discussionmentioning

confidence: 99%

Subunit-specific functions of N-linked oligosaccharides in human thyrotropin: role of terminal residues of alpha- and beta-subunit oligosaccharides in metabolic clearance and bioactivity.

Szkudlinski

Thotakura²,

Weintraub³

1995

Proc. Natl. Acad. Sci. U.S.A.

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“…The desialylated hCG variant also interacts directly with recombinant hTSH receptors transfected into human thyroid cancer cells (31). Although removal of sialic acid from hCG enhanced its TSH receptor binding, it also sharply reduced its plasma half-life because sialic acid blocks the high affinity for and rapid uptake of hCG by hepatic receptors for asialoglycoproteins (32). It is likely that the in vivo thyrotropic activity of hCG is regulated by two factors: the amount of desialylated hCG produced from trophoblast cells and its plasma half-life (17 It has been proposed that hCG is a putative thyroid stimulator in normal pregnancy because (i) serum TSH levels are suppressed when hCG levels are highest at 8-12 weeks of gestation (Fig.…”

Section: ) (5)mentioning

confidence: 99%

Thyrotropic Action of Human Chorionic Gonadotropin

Yoshimura¹,

Hershman²

1995

Thyroid

314

146

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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 att...

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“…The underlying mechanism remains hypothetical and many causes have been advocated: dysfunctional folliculogenesis with early oocyte atresia despite apparently normal hormonal response (2,5); genetic or chromosomal factor (6,7); defect in the in vivo bioactivity of hCG (8,9) related to desialylation of the molecule with rapid metabolic clearance by the liver (10). Finally, the cause might be commonly linked to human errors in hCG administration (3,11,12):…”

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confidence: 99%

“Empty follicle syndrome” after human error: pregnancy obtained after repeated oocyte retrieval in a gonadotropin-releasing hormone antagonist cycle

Snaifer

Hugues

Poncelet

et al. 2008

Fertility and Sterility

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Role of Subunit Sialic Acid in Hepatic Binding, Plasma Survival Rate, and In Vivo Thyrotropic Activity of Human Chorionic Gonadotropin

Cited by 26 publications

References 22 publications

Subunit-specific functions of N-linked oligosaccharides in human thyrotropin: role of terminal residues of alpha- and beta-subunit oligosaccharides in metabolic clearance and bioactivity.

Subunit-specific functions of N-linked oligosaccharides in human thyrotropin: role of terminal residues of alpha- and beta-subunit oligosaccharides in metabolic clearance and bioactivity.

Thyrotropic Action of Human Chorionic Gonadotropin

“Empty follicle syndrome” after human error: pregnancy obtained after repeated oocyte retrieval in a gonadotropin-releasing hormone antagonist cycle

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