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 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 gamma S stimulates the basal and GnRH-dependent phospholipase C activity. This effect was abolished by GDP beta S. The cytosolic phospholipase C activity was also stimulated by GTP gamma S 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.