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

Initial biosynthetic radiolabelling experiments with cultured granulosa cells revealed the presence of an oligosaccharide-phosphatidylinositol (glycosyl-phosphatidylinositol; (Ose)(n)PtdIns) structurally related 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-stimulating hormone-(FSH)-primed granulosa cells endowed with PRL receptors. In parallel experiments performed with [H-3]myo-inositol-labelled granulosa cells, treatment with PRL stimulated (Ose)(n)PtdIns hydrolysis in a similar manner, whereas no effect on phosphoinositide (PtdIns, PtdInsP and PtdInsP2) turnover could be observed. These results strongly suggest that the cleavage of (Ose)(n)PtdIns by phosphodiesterase followed by the subsequent generation of diacylglycerol and a soluble phosphoinositol-oligosaccharide (inositol-phosphoglycan; (Ose)(n)InsP) moiety could be part of the signal-transduction mechanism linking PRL receptors to their biological effects in granulosa cells. To test this hypothesis, we examined the effect of PRL and purified (Ose)(n)InsP moiety (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 activity. Furthermore, in undifferentiated granulosa cells where PRL receptors are absent, no effect of the hormone on 3beta-HSD activity could be observed, whereas (Ose)(n)InsP (1-10 muM) inhibited enzyme activity in a dose-dependent manner.