Desialylated and Deglycosylated Human Chorionic Gonadotropin Are Superagonists of Native Human Chorionic Gonadotropin in Human Thyroid Follicles

Highly purified human chorionic gonadotropin (hCG) interacts with the thyrotropin (TSH) receptor and stimulates triiodothyronine (T₃) secretion, iodide uptake and organification, and cyclic adenosine monophosphate (cAMP) formation in human thyroid follicles. Because of interest in the role of the carbohydrate component in the structure-function relationships of hCG we undertook to deplete hCG of its sialic acid or carbohydrate residues and assess the thyrotropic activity of the carbohydrate-modified forms. For this purpose, we used our assay system consisting of human thyroid follicles cultured and suspended in collagen gel in serum-free medium. Under these conditions, the cells are organized as follicular three-dimensional structures with normal polarity, enabling enhanced responsiveness to hormonal stimulation, and T₃ secretion can be measured as a response parameter. Desialylated (ds)-hCG and deglycosylated (dg)-hCG dose-dependently stimulated T₃ secretion, iodide uptake and organification, and in each case did so with about twice the intrinsic activity of native hCG. Indeed, removal of the sialic acid or carbohydrate residues from native hCG transformed it into a thyroid stimulator that elicited a maximal response in terms of iodide uptake, organification and T₃ secretion by human thyroid follicles as high as TSH and almost twice as high as native hCG. Not only were ds-hCG and dg-hCG more intrinsically active than hCG, they were more than five times as potent. As with hCG, both ds-hCG and dg-hCG managed to elicit such responses in human thyrocytes while evoking minimal amounts of cAMP, illustrating the concept of cAMP superfluity and highlighting the potential pitfalls of using cAMP as a measure of hormonal bioactivity. hCG, and to a greater extent ds-hCG and dg-hCG, inhibited, as did TSH, γ-interferon-induced human leukocyte antigen-DR (HLA-DR) expression in human thyrocytes, again reflecting the intrinsic thyrotropic activity of native hCG and its variants depleted of sialic acid or carbohydrate residues. In conclusion, this is the first report on the thyrotropic activity of ds-hCG and dg-hCG using the physiologically relevant hormonal end-point response, thyroid hormone secretion. The study was conducted in a serum-free culture system of human thyroid follicles and shows that removal of the sialic acid or carbohydrate residues from native hCG transform hCG variants into thyroid stimulating superagonists. The hCG variants inhibited, as did TSH, γ-interferon-induced HLA-DR expression.