Inositol phosphate stimulation by LH requires the entire alpha Asn56 oligosaccharide

Lentil lectin-bound, fucose-enriched hTSH was reported to stimulate both cAMP and inositol phosphate (IP) intracellular signalling pathways, whereas fucose-depleted hTSH stimulated only the cAMP pathway. Gonadotropins activate the cAMP pathway and in several studies higher concentrations activate the IP pathway. Since only the 10% of alpha subunit Asn(56) oligosaccharides (Asn(52) in humans) are fucosylated, the higher glycoprotein hormone concentrations required for IP pathway activation might be related to the abundance of competent hormone isoforms. Lentil lectin-fractionated equine (e)LHalpha and eFSHalpha preparations were combined with a truncated, des(121-149)eLHbeta preparation. All four hybrid hormone preparations induced IP accumulation in porcine theca cells, suggesting that activation of the IP pathway was not dependent on fucosylation at alpha subunit Asn(56). However, the presence of Asn(56) carbohydrate was necessary for increased IP accumulation. Intact, rather than Asn(56)-deglycosylated eLH preparations provoked a biphasic steroidogenic response by rat testis Leydig cells, suggesting that Galpha(i) stimulation was also sensitive to loss of Asn(56) carbohydrate. While rat granulosa cells responded to human FSH preparations in a biphasic manner, a classical sigmoidal response was obtained to eFSH and Asn(56)-deglycosylated eFSH, suggesting that the equine preparations did not activate Galpha(i). Purified oLHalpha Asn(56) oligosaccharides inhibited FSH-stimulated steroidogenesis in rat granulosa cell cultures indicating a direct role for carbohydrate in FSH action. The same carbohydrate preparation inhibited hCG-stimulated fluorescence energy transfer suggesting oligosaccharide involvement in activated LH receptor self-association.