THE RELATIVE IMPORTANCE OF THE OLIGOSACCHARIDE UNITS IN HUMAN CHORIONIC-GONADOTROPIN (CG) FOR LH CG RECEPTOR ACTIVATION IN RAT LEYDIG-CELLSAND MOUSE LEYDIG TUMOR-CELLS/

The biological properties of deglycosylated human chorionic gonadotrop in (DhCG), obtained by hydrogen fluoride treatment (HF-DhCG) of intact hCG or by oligonucleotide-directed mutagenesis (CHO-DhCG), and that o f their fully glycosylated counterparts, were tested in term of cAMP a nd steroid production in rat Leydig cells and in mouse Leydig tumor ce lls (MA-10 cells). In both cell types, HF-DhCG and CHO-DhCG possessed comparable biological activities. The maximum for DhCG-induced cAMP pr oduction was approximately 12% of that of intact hCG when tested in ra t 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 in tact hCG, while in rat Leydig cells DhCG and hCG induced similar stero idogenic maxima. Based on its ED(50), DhCG possessed 10-17% of the ste roidogenic potency of intact hCG in rat Leydig cells, while in MA-10 c ells DhCG was only a-fold less potent than hCG. When accurate hormone- receptor binding data are absent, the intrinsic receptor-stimulating a ctivity of a ligand can still be estimated at full receptor occupancy, provided that over the whole dose range the biological response is pr oportional to receptor stimulation. The present data show that in tran sfected MA-10(P(+)29) cells which over-express rat phosphodiesterase, the hormone-induced stimulation of cAMP and steroid production is dire ctly coupled to receptor activation up to maximal occupation of the LH /CG receptor. The intrinsic receptor-stimulating activity of DhCG, mea sured 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 affi nity to the LH/CG receptor than intact hCG, this increased binding aff inity of DhCG may partly compensate for the 7- to 10-fold reduction in the intrinsic receptor-stimulating activity, resulting in only a 2-fo ld reduction in steroidogenic potency of DhCG. In terms of adenylyl cy clase stimulation in MA-10(P(+)29) cells, DhCG possessed approximately 50-fold lower receptor-stimulating activity than intact hCG, being si milar to that observed in wild-type MA-10 cells. This study clearly sh ows that the oligosaccharide units in hCG are not essential for LH/CG receptor activation, and that the relative receptor-stimulating activi ty of DhCG to that of. hCG is highly dependent on whether cAMP or ster oid production is measured as an index for bioactivity, and whether bi oactivity is tested in rat Leydig cells or MA-10 cells.