DEGLYCOSYLATED HUMAN CHORIONIC-GONADOTROPIN (HCG) ANTAGONIZES HCG STIMULATION OF 3',5'-CYCLIC ADENOSINE-MONOPHOSPHATE ACCUMULATION THROUGH A NONCOMPETITIVE INTERACTION WITH RECOMBINANT HUMAN LUTEINIZING-HORMONE RECEPTORS

In the rat, the antagonistic properties of deglycosylated (dg) gonadotropins in vitro are characterized by high affinity receptor binding but impaired ability to stimulate cAMP accumulation. In human, the functional role of N-linked sugars in human CG (hCG) action is unclear because of the unavailability of totally deglycosylated hCG and because of the difficulty involved in obtaining human gonadal tissues. We have recently prepared completely deglycosylated hCG using site-directed mutagenesis and expressed functional human LH (hLH) receptors using cloned complementary DNA. Since hLH receptor shows distinct ligand specificity from that of rat LH receptor, we examined binding kinetics and signal transduction of recombinant dg-hCG using recombinant hLH receptors. In embryonic human kidney cells (293) transfected with hLH receptor complementary DNA, I-125-hCG binding to its receptor was studied in the presence of varying amounts of unlabeled dg-hCG or wild type (WT)-hCG. Lineweaver-Burk analysis of the binding kinetics showed that the displacement of I-125-hCG by dg-hCG was noncompetitive whereas that seen for WT-hCG was competitive. The noncompetitive nature of dg-hCG binding was further confirmed using rat LH receptors present in testis membrane preparations. After preincubation of LH receptor-expressing 293 cells with WT-hCG, inclusion of I-125-hCG competitively displaced WT-hCG. In contrast, preincubation with dg-hCG prevented subsequent I-125-hCG binding to human LH receptor for at least 46 h. WT-hCG caused a dose-dependent increase in cAMP accumulation in the 293 cells with an ED50 of 10 ng/ml. However, dg-hCG was ineffective in inducing cAMP production with a maximal effect of only 12% of that stimulated by WT-hCG. In the presence of increasing doses of dg-hCG, stimulation of cAMP by WT-hCG was antagonized in a dose-dependent manner. In contrast, forskolin stimulation of cAMP was not antagonized by dg-hCG, indicating receptor-mediation of dg-hCG action. Similar to binding studies, preincubation with dg-hCG also dose-dependently blocked the subsequent stimulatory effect of WT-hCG on cAMP production. Thus, the noncompetitive binding of dg-hCG to hLH receptors and its antagonism of hCG stimulation of cAMP accumulation suggest that dg-hCG is an irreversible receptor blocker with unique antagonistic properties.