Thyrotropic Activity of Acidic Isoelectric Variants of Human Chorionic Gonadotropin from Trophoblastic Tumors

Abstract

Previous studies have indicated that hCG has a weak intrinsic thyroid-stimulating activity. Differences in the molecular composition and biological activity of hCG in patients with trophoblastic diseases and pregnant women occur, but are not well defined. Therefore, we have studied the effect of serum samples and purified hCG preparations from patients with trophoblastic diseases on T3 release from human and porcine thyroid slices in vitro. We examined 30 serum samples from 13 patients with nonseminomatous testicular germ cell tumors, 3 from women with choriocarcinoma, and 5 from patients with hydatidiform moles. In all but 1 serum sample from the tumor patients, but in none of 11 serum samples of pregnant women, T3-releasing activity was found. Two patients with testicular cancer and 1 patient with molar pregnancy experienced episodes of frank hyperthyroidism. Isoelectric focusing on polyacrylamide gels of tumor sera (n = 15) revealed substantial amounts of acidic isoelectric variants, pI 3.3-3.9, which were only barely detectable in pregnancy sera. The percentage of acidic hCG variants with pI 3.3-4.0 to total hCG with pI 3.3-5.2, as determined by hCG (+hCG beta) RIA of the eluted fractions of polyacrylamide gel isoelectric focussing, varied from 12-45% in sera of tumor patients and from 0-4% in pregnant sera. We purified the acidic variants of hCG with pI 3.6-3.8 (hCGav) from the urine of our patients. The beta-subunit of purified hCGav had a slightly higher mol wt (35,750) than that of hCG CR 119 (34,190) on polyacrylamide gel electrophoresis. The hCGav showed a dose-dependant stimulation of T3 release and cAMP generation from human thyroid slices, whereas the other hCG fractions on isoelectric focussing had no thyrotropic effect in similar dose levels. The TSH-like activity of hCGav could be roughly estimated as 10 mIU TSH/IU hCGav. Anti-hCG (+hCG beta) antiserum, but not anti-hTSH antiserum, neutralized the biological activity of hCGav. These findings strongly suggest that acidic hCG variants act as functional stimulators of the human thyroid in vitro. Since these molecular variants of hCG can exist in patients with trophoblastic diseases in significant amounts, they could be responsible for some cases of hyperthyroidism in trophoblastic diseases.

Tumor- and Pregnancy-Derived Isoforms of Human Chorionic Gonadotropin: Biological and Diagnostic Relevance

“…It thus represents an artificial alkaline and markedly smaller variant with a higher pI than naturally occurring pI variants. In contrast, patients with hCG-producing tumors such as choriocarcinoma or testicular cancer often excrete exceptionally overglycosylated, and thus hyperacidic isoforms of hCG [9]. …”

“…Carbohydrate moieties differ in branching (number and length of antennae) and terminal sialylation, resulting in different degrees of negative charges. In particular, hCG variants from tumor patients are often highly acidic (hCGav), presumably because of being hyperglycosylated and/or oversialylated [9]. In previous investigations, degly-hCG and asialo-hCG [10, 11]displayed higher receptor affinities for the LH/CG-R of rat testis membranes (r-LH/CG-R) [12, 13]than non-modified highly purified urinary preg-hCG [11, 14, 15].…”

Asialoagalacto-human chorionic gonadotropin, a carbohydrate-modified variant of human chorionic gonadotropin, antagonizes the stimulatory actions of bovine thyroid-stimulating hormone on thyroid function and HLA-DR expression in human thyroid in vitro and in vivo.

“…IEF was performed on polyacrylamide gels as described in detail elsewhere (19) Immunoradiometric assays for hCG and its free beta-subunit. The immunological activities of hCG preparations were assessed by immunoradiometric assays (IRMAs), specific for holo-hCG and free betasubunit of hCG, respectively.…”

“…To study the influence of various inhibitors on the binding of '25I-asialo-hCG (3 X 101 M) a membrane fraction was prepared from rat liver, as described in detail previously (13 Atkinson and Kendall-Taylor (27), as described in detail previously (19). Thyroid slices (1 X 1 x 0.5 mm) were incubated with the various inhibitors in a two chamber dialysis system in modified Earles' buffer containing in addition 2 g/liter gelatine, 2 g/liter glucose, 100 IU/liter insulin and antibiotics (penicilline and streptomycine) under 95% OJ 5% CO2 atmosphere, for 5 h at 370C.…”

Human Chorionic Gonadotropin Promotes Thyroid Growth via Thyrotropin Receptors in FRTL-5 Cells.

“…The result of positive TGA in patient M3 was consistent with her high serum free T4 and low TSH. It is conceivable that, in patient M3, the existence of abnormal hCG molecules, such as acidic variants of hCG reported by Mann et al(1986), might have induced high serum free T4 and low TSH.…”