GnRH analogs stimulate phospholipase C activity in mammary tumor membranes: modulation by GTP

An in vitro assay for phospholipase C activity was developed, employing exogenously added ³²P-labelled phosphatidylinositol 4, 5-bisphosphate as substrate. This enzymatic assay was used to analyse the direct effect of GnRH on mammary tumors. GnRH agonists stimulate membranal phosphoinositide-specific phospholipase C activity. The increase in inositoltrisphosphate production is dose dependent, and is inhibited by the GnRH antagonist Org-30276. We took advantage of this non-cellular assay system for evaluating the role of G-binding proteins in the phosphoinositide transducing system in mammary tumors. GTP γ S stimulates the basal and GnRH-dependent phospholipase C activity. This effect was abolished by GDPβS. The cytosolic phospholipase C activity was also stimulated by GTPγS but was not affected by the hormone. These results suggest that GnRH may affect the growth of mammary tumors directly and not only through the reduction of blood gonadotropin level.