Variation in the Thyrotropic Activity of Human Chorionic Gonadotropin in Chinese Hamster Ovary Cells Arises from Differential Expression of the Human Thyrotropin Receptor and Microheterogeneity of the Hormone.

The role of hCG as a stimulator of the human thyroid has been a
subject of controversy, because discrepant results have been
obtained in different in vitro assays. In an attempt to explain the
variation observed in the thyroid response to hCG, we investigated
the ability of hCG and that of its isoforms and glycosylation
variants to inhibit [125I]bovine (b) TSH binding and stimulate
adenylate cyclase in two clones, JP09 and JP26, of Chinese hamster
ovary cells stably transfected with the human TSH receptor (hTSHr).
The two clones differed with respect to the number of hTSHr
expressed per cell (34,000 in JP09 and 2,000 in JP26 cells). Both
responded extremely well to bTSH; the cAMP response to 0.001 IU/L
bTSH was distinguishable from basal values. Interestingly, JP09
cells were readily stimulated by hCG (20-100 mg/L; 0.52-2.6 x
10(-6) mol/L) to release cAMP, whereas JP26 cells showed little if
any response. Also, cAMP stimulation produced by asialo-hCG was
12-fold in JP09 cells and only 4-fold in JP26 cells compared to 45-
and 67-fold stimulations by bTSH, respectively. Stimulation by
asialo-hCG was approximately 30% that of bTSH in JP09 cells, but
less than 6% in JP26 cells. When assessing the thyrotropic activity
of the microheterogeneous isoforms of hCG, more alkaline pI forms
were found to be more active than those of a more acidic pI
regardless of whether they were derived from normal or molar
pregnancy urine. Further studies with hCG, asialo-hCG,
asialoagalacto-hCG, and deglycosylated hCG revealed that removal of
sialic acid caused a marked increase in both its affinity for hTSHr
and its cAMP-releasing potency, whereas removal of further
carbohydrate, although it slightly enhanced receptor binding, was
detrimental to adenylate cyclase activation. In conclusion,
differences in hTSHr expression may cause a variation in the cAMP
response to hCG or its glycosylation variants, as does the
microheterogeneity of the hormone itself. These mechanisms may be
responsible at least in part for the divergent responses of
different cell types to hCG and render interpretation of the
physiological meaning of the data obtained in recombinant receptor
systems difficult.