Crude urinary human chorionic gonadotropin contains variant forms of HCG with low sialic acid content that exhibit an increased thyrotropic activity in CHO cells expressing the human TSH receptor

Summary

Hyperthyroidism occurs in association with pregnancy or trophoblastic tumours. This is due to the secretion of thyroid stimulators by trophoblastic cells, most likely hCG or a variant form of hCG In the present studies we sought to identify hCG variants with enhanced thyrotropic activity contained in crude hCG extract from pregnancy urine (hCGc). Such studies seem now feasible, because highly sensitive assays employing CHO cells transfected with the recombinant human TSH receptor recently became available. Initially, we found the activity of hCGc to both inhibit the binding of ¹²⁵I-bTSH to CHO-TSHr cells and to stimulate the cAMP release by the cells to be increased, compared to highly purified hCG (hCGp), which was tested in comparable immunological concentrations. We then processed hCGc on a DEAE-52 anionexchange column to separate materials of interest, termed hCGv, from hCGp. HCGv was further purified by gel chromatography, and found to be enriched in terms of both, its hold hCG immunoactivity and its TSH binding inhibiting activity, compared to hCGc where it was derived from. It also proved more potent than hCGp to bind to recombinant hTSH receptor and to stimulate adenylate cyclas activity in CHO-TSHr cells. Enzymatic desialylation was able to increase the potency of both hCGv and hCGp and rendered the two desialylated hCG forms near equipotent. Isoelectric focusing and direct measurement of sialic acid contents revealed hCGv to be less sialylated than hCGp.

In conclusion, the present studies show that crude urinary hCG extract of pregnancy origin contains less sialylated variant forms of hCG that are more potent stimulators of cAMP release in CHO-TSHr cells than hCG itself. An in vivo bioactivity of these variants, however may not be directly inferred from the present data because lack of sialic acid may result in a more rapid plasma clearance of such substances.

Key words

Thyroid - TSH receptor - hCG - pregnancy urine - sialic acid