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

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Abstract
Initial biosynthetic radiolabelling experiments with cultured granulosa cells revealed the presence of an oligosaccharide‐phosphatidylinositol (glycoyl‐phosphatidylinositol; (Ose)nPtdIns) structurally related to (Ose)nPtdIns‐lipids isolated from other cell types. Prolactin (PRL) stimulated [3H]glucosamine‐(Ose)nPtdIns turnover and the rapid generation of [3H]myristoyl‐diacylglycerol in cultured follicle‐stimulating hormone‐(FSH)‐primed granulosa cells endowed with PRL receptors. In parallel experiments performed with [3H]myo‐inositol‐labelled granulosa cells, treatment with PRL stimulated (Ose)nPtdIns 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)nPtdIns by phosphodiesterase followed by the subsequent generation of diacylglycerol and a soluble phosphoinositol‐oligosaccharide (inositol‐phosphoglycan; (Ose)nInsP) 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)nInsP moiety (from rat liver membranes) on granulos cell 3β‐hydroxysteroid dehydrogenase/Δ5–4 isomerase (3β‐HSD) enzyme activity. Results presented show that, in FSH‐primed granulosa cells, PRL (40 nM) and (Ose)nInsP (5 μM) prevented gonadotropin‐stimulated 3β‐HSD activity. Further‐more, in undifterentiated granulosa cells where PRL receptors are absent, no effect of the hormone on 3β‐HSD activity could be observed, whereas, (Ose)nInsP (1–10 μM) inhibited enzyme activity in a dose‐dependent manner.