Gonadotropin-Releasing Hormone Stimulates Phospholipid Labeling in Cultured Granulosa Cells*

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Abstract
Cultured ovarian granulosa cells from preantral and preovulatory follicles were incubated with [32P]Pi to label endogenous phospholipids. Labeled cells were then incubated with FSH, GnRH, or a GnRH agonist analog [D-Ala6]GnRH (GnRHa), cellular phospholipids were separated by two-dimensional thin layer chromatography, and the radioactivity was determined. Phosphatidylcholine was the major labeled phospholipid accounting for 64% of the total radioactivity. The remaining labeling was distributed among choline plasmalogen (8.4%), phosphatidylinositol (6.3%), lyso phosphatidylcholine (3.7%), phosphatidylethanolamine (3.4%), phosphatidic acid (1.75%), phosphatidylserine (1.65%), and cardiolipin (1.3%). GnRH and its agonist analog GnRHa, but not FSH, increased 32P incorporation into phospholipids by 2-fold. Analysis of the several phospholipids revealed that GnRHa (10-7 M) increased 32P labeling of phosphatidylcholine and lyso phosphatidylcholine by 1.5- and 2.5-fold respectively, and that of phosphatidic acid and phosphatidylinositol by 5- and 7-fold, respectively, during 60 min of incubation. The natural decapeptide GnRH was 30 times less potent than its agonist analog. Labeling of other phospholipids was not affected by GnRHa treatment, and FSH had no effect on 32P incorporation under similar conditions. The stimulatory effect of GnRHa was blocked by the potent GnRH antagonist [D-pGlu1,pClPhe2, D-Trp3,6]GnRH. The minimal stimulating dose of GnRHa was 10-12 M, and increased phospholipid labeling could be detected after 10 min of incubation with the analog. These results indicate that phospholipids, in particular phosphatidylinositol and phosphatidic acid, might be involved in the mechanism by which GnRH exerts its gonadal effects.