The relative importance of the oligosaccharide units in human chorionic gonadotropin (CG) for LH/CG receptor activation in rat Leydig cells and mouse Leydig tumor cells

Abstract

Abstract The biological properties of deglycosylated human chorionic gonadotropin (DhCG), obtained by hydrogen fluoride treatment (HF-DhCG) of intact hCG or by oligonucleotide-directed mutagenesis (CHO-DhCG), and that of their fully glycosylated counterparts, were tested in terms of cAMP and steroid production in rat Leydig cells and in mouse Leydig tumor cells (MA-10 cells). In both cell types, HF-DhCG and CHO-DhCG possessed comparable biological activities. The maximum for DhCG-induced cAMP production was approximately 12% of that of intact hCG when tested in rat Leydig cells, and only 2% when tested in MA-10 cells. DhCG possessed significant steroidogenic activity in both cell types. In MA-10 cells the maximum for DhCG-induced steroidogenesis was 30–50% of that of intact hCG, while in rat Leydig cells DhCG and hCG induced similar steroidogenic maxima. Based on its ED50, DhCG possessed 10–17% of the steroidogenic potency of intact hCG in rat Leydig cells, while in MA-10 cells DhCG was only 2-fold less potent than hCG. When accurate hormone-receptor binding data are absent, the intrinsic receptor-stimulating activity of a ligand can still be estimated at full receptor occupancy, provided that over the whole dose range the biological response is proportional to receptor stimulation. The present data show that in transfected MA-10(P+29) cells which over-express rat phosphodiesterase, the hormone-induced stimulation of cAMP and steroid production is directly coupled to receptor activation up to maximal occupation of the LH/CG receptor. The intrinsic receptor-stimulating activity of DhCG, measured in MA-10(P+29) cells in terms of DhCG-induced steroidogenesis, appeared to be 7- to 10-fold lower than that of intact hCG. As it is known from the literature that DhCG possesses 2- to 3-fold higher affinity to the LH/CG receptor than intact hCG, this increased binding affinity of DhCG may partly compensate for the 7- to 10-fold reduction in the intrinsic receptor-stimulating activity, resulting in only a 2-fold reduction in steroidogenic potency of DhCG. In terms of adenylyl cyclase stimulation in MA-10(P+29) cells, DhCG possessed approximately 50-fold lower receptor-stimulating activity than intact hCG, being similar to that observed in wild-type MA-10 cells. This study clearly shows that the oligosaccharide units in hCG are not essential for LH/CG receptor activation, and that the relative receptor-stimulating activity of DhCG to that of hCG is highly dependent on whether cAMP or steroid production is measured as an index for bioactivity, and whether bioactivity is tested in rat Leydig cells or MA-10 cells. Journal of Endocrinology (1995) 147, 367–375