Does oligosaccharide‐phosphatidylinositol (glycosyl‐phosphatidylinositol) hydrolysis mediate prolactin signal transduction in granulosa cells?

Abstract

Initial biosynthetic radiolabelling experiments with cultured granulosa cells revealed the presence of an oligosaccharide-phosphatidylinositol (glycosyl-phosphatidylinositol ; (Ose),PtdIns) structurally related to (Ose),,PtdIns-lipids isolated from other cell types. Prolactin (PRL) stimulated[3H]glu~~samine-(Ose),PtdIns turnover and the rapid generation of ['Hlmyristoyl-diacylglycerol in cultured follicle-stimulating hormone-(FSH)-primed granulosa cells endowed with PRL receptors.In parallel experiments performed with [7H]myo-inositol-labelled granulosa cells, treatment with PRL stimulated (Ose),PtdIns hydrolysis in a similar manner, whereas no effect on phosphoinositide (PtdIns, PtdInsP and PtdInsP,) turnover could be observed. These results strongly suggest that the cleavage of (Ose),PtdIns by phosphodiesterase followed by the subsequent generation of diacylglycerol and a soluble phosphoinositol-oligosaccharide (inositol-phosphoglycan; (Ose),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),InsP moiety (from rat liver membranes) on granulosa cell 3P-hydroxysteroid dehydr~genase/A~-~ isomerase (3P-HSD) enzyme activity. Results presented show that, in FSH-primed granulosa cells, PRL (40 nM) and (Ose),,InsP (5 pM) prevented gonadotropin-stimulated 3P-HSD activity. Furthermore, in undifferentiated granulosa cells where PRL receptors are absent, no effect of the hormone on 3P-HSD activity could be observed, whereas (Ose),,InsP (1 -10 pM) inhibited enzyme activity in a dose-dependent manner.Ovarian granulosa cell proliferation and differentiation appears to be initiated by the binding of the gonadotropin follicle stimulating hormone (FSH) to specific plasma membrane receptors, resulting in the elevation of intracellular CAMP and the subsequent activation of protein-kinase-a-mediated events implicated in the transformation of the immature cell to its fully mature counterpart (for a review see Hsueh et al., 1985;Amsterdam et al., 1989). This process is accompanied by the development of functional prolactin (PRL) receptors that are further increased thereafter by homologous up-regulation and by luteinizing hormone (Wang et al., 1979;Wang et al., 1980). In differentiated granulosa cells, PRL-receptor activation prevents gonadotropin-stim- Correspondence to