STIMULATION OF N-GLYCOSYLATION OF HUMAN CHORIONIC GONADOTROPIN BY cAMP

Abstract

It is known that cAMP exerts multiple effects on the biosynthesis of human chorionic gonadotropin (hCG) at the levels of gene activation and mRNAstabilization. We have investigated the influence of 8-bromo-cAMP on the N-glycosylation and processing of the hCG-o-subunit in first trimester placenta. By means of pulse-chase experiments (30 min pulse with [°SS]Met, 5-120 min chase) three intracellular precursors of the a-subunit of secreted hCG were observed with apparent molecular weights of 11 kDa (non-glycosylated), 16.5 kDa (one N-glycosyl residue), and 19.5 kDa (two N-glycosyl residues)). HCG secreted in vitro as well as purified from the urine of pregnant women contained a sialylated a-subunit (20.6 kDa) which was digested by endo-@-N-glucosaminidase H (Endo H) yielding a 16.5 kDa form which indicates that one carbohydrate residue was probably of the hybride type. 8-bromo-cAMP caused the 16.6 kDa form to be converted faster into the 19.5 kDa precursor than in control cultures. In addition, the Endo sensitivity of the a-subunit of secreted hCG was almost completely abolished in cultures treated with 0.5 mM 8-bromo-cAMP. This seems to indicate that cAMP influences the velocity as well as the extent of N-glycosylation of the hCG-g-subunit. The specificity of this cAMP-action was investigated by means of the N-glycosyl acceptor peptide (octanoyl-tripeptide, OTP) N-octanoyl- asparagyl-tyrosyl-threonine amide. In JEG-3 cells N-glycosylation of OTP and its secretion was significantly stimulated by 8-bromo-cAMP.In cells treated with 1 mM 8-bromo-cAMP a 5-22-fold higher amount of glycosylated OTP was secreted into the culture media. Whereas in the control cultures more than 70% of N-glycosyl-OTP was accumulated in the cells, about 60% was secreted in the presence of 1 mM 8-bromo-cAMP. In placenta tissue similar results were obtained. These results show that the stimulation of N-glycosylation by cAMP occurs in a general way and is not confined to the synthesis of the hCG-a-subunit.