Gonadotropin-releasing hormone directly induces apoptotic cell death in the rat ovary: biochemical and in situ detection of deoxyribonucleic acid fragmentation in granulosa cells

Abstract

The majority of ovarian follicles undergo atresia through a mechanism involving apoptotic cell death. Although GnRH and its agonists have been shown to suppress ovarian growth and differentiation in hypophysectomized rats, studies on the induction of follicle atresia by GnRH are contradictory. In the present study, the direct effect of GnRH on the occurrence of apoptosis in the ovary was investigated in hypophysectomized estrogen-treated immature rats. Starting 2 days after operation and estrogen capsule implantation, rats were treated with a GnRH agonist (GnRHa; [desGly10,D-Phe6,Pro9-N-ethylamide] GnRH; 50 micrograms/injection, twice daily). Total ovarian DNA was isolated 48 h later, labeled at the 3'ends with [32P]dideoxy ATP, and size-fractionated. Compared to that in control animals, treatment with GnRHa increased DNA fragmentation in multiples of 180 basepairs, a hallmark of apoptosis, demonstrating that GnRH directly increases ovarian apoptotic cell demise. In contrast, FSH treatment (10 micrograms/injection, twice daily) decreased apoptotic DNA fragmentation, and the antiapoptotic effect of FSH was partially blocked by concomitant treatment with GnRHa. The apoptosis-inducing effect of GnRHa was time and dose dependent, with a significant increase seen after 24 h of treatment and a maximal 5.5-fold increase with 10 micrograms GnRHa/injection. Similar to studies using DNA isolated from whole ovaries, DNA obtained from isolated granulosa cells also showed a time- and dose-dependent increase in DNA fragmentation after GnRHa treatment. The effect on DNA fragmentation by GnRHa was mediated by ovarian GnRH receptors, because a potent GnRH receptor blocker, Azaline B, prevented GnRHa action. In addition, in situ end labeling of DNA using digoxigenin-dideoxy-UTP showed that DNA fragmentation was confined to the granulosa cells of preantral and antral follicles. No GnRHa-induced apoptosis was detected in granulosa cells of primary follicles or in thecal and interstitial cells. These data suggest that GnRH directly increases apoptotic cell death in the ovary, and the GnRH action is confined to the granulosa cells. These data provide a basis for future studies on the mechanism of follicular atresia and the regulation of ovarian endonuclease by GnRH.