DOES OLIGOSACCHARIDE-PHOSPHATIDYLINOSITOL (GLYCOSYL-PHOSPHATIDYLINOSITOL) HYDROLYSIS MEDIATE PROLACTIN SIGNAL-TRANSDUCTION IN GRANULOSA-CELLS

Initial biosynthetic radiolabelling experiments with cultured granulos a cells revealed the presence of an oligosaccharide-phosphatidylinosit ol (glycosyl-phosphatidylinositol; (Ose)(n)PtdIns) structurally relate d to (Ose)(n)PtdIns-lipids isolated from other cell types. Prolactin ( PRL) stimulated [H-3]glucosamine-(Ose)(n)PtdIns turnover and the rapid generation of [H-3]myristoyl-diacylglycerol in cultured follicle-stim ulating hormone-(FSH)-primed granulosa cells endowed with PRL receptor s. In parallel experiments performed with [H-3]myo-inositol-labelled g ranulosa cells, treatment with PRL stimulated (Ose)(n)PtdIns hydrolysi s in a similar manner, whereas no effect on phosphoinositide (PtdIns, PtdInsP and PtdInsP2) turnover could be observed. These results strong ly suggest that the cleavage of (Ose)(n)PtdIns by phosphodiesterase fo llowed by the subsequent generation of diacylglycerol and a soluble ph osphoinositol-oligosaccharide (inositol-phosphoglycan; (Ose)(n)InsP) m oiety could be part of the signal-transduction mechanism linking PRL r eceptors to their biological effects in granulosa cells. To test this hypothesis, we examined the effect of PRL and purified (Ose)(n)InsP mo iety (from rat liver membranes) on granulosa cell 3beta-hydroxysteroid dehydrogenase/DELTA5-4 isomerase (3beta-HSD) enzyme activity. Results presented show that, in FSH-primed granulosa cells, PRL (40 nM) and ( Ose)(n)InsP (5 muM) prevented gonadotropin-stimulated 3beta-HSD activi ty. Furthermore, in undifferentiated granulosa cells where PRL recepto rs are absent, no effect of the hormone on 3beta-HSD activity could be observed, whereas (Ose)(n)InsP (1-10 muM) inhibited enzyme activity i n a dose-dependent manner.