GROWTH-HORMONE SUPPRESSION OF APOPTOSIS IN PREOVULATORY RAT FOLLICLES AND PARTIAL NEUTRALIZATION BY INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEIN

A growing body of evidence suggests that growth hormone IGH) plays a role in regulating ovarian function by augmenting gonadotropin stimulation of granulosa cell differentiation and folliculogenesis. The majority of follicles in the mammalian ovary do not ovulate, but instead undergo a degenerative process (atresia) involving apoptotic cell death. The objective of the present study was to investigate the role of GH in regulating follicle apoptosis and to determine whether or not insulin-like growth factor-I (IGF-I) mediates GH action in this process. Preovulatory follicles obtained from eCG-primed rats were cultured for 24 h in serum-free conditions with or without hormone treatments. After culture, follicular apoptotic DNA fragmentation was analyzed by autoradiography of size-fractionated DNA labeled at 3' ends with [P-32]dideoxy-ATP. Culture of preovulatory follicles resulted in a spontaneous onset of apoptotic DNA fragmentation that was suppressed by ovine GH (oGH) in a dose-dependent manner, reaching a maximum of 65% suppression. To rule out the effect of residual gonadotropin in the oGH preparation, follicles were also cultured with recombinant bovine growth hormone (rbGH). Like oGH, rbGH suppressed apoptotic DNA fragmentation. Our earlier study indicated that hCG and FSH treatment also suppress apoptosis in the present model system, but no additive effect of GH and either hCG or FSH on the suppression of apoptosis was observed. To determine whether the observed effect of GH action on follicle apoptosis is mediated by IGF-I, three types of studies were carried out. First, follicles were cultured with rbGH in the presence of insulin-like growth factor binding protein-3 (IGFBP-3), which is known to neutralize IGF-I action. Cotreatment with IGFBP-3 reversed the suppressive effect of GH in a dose-dependent manner by up to 75%, suggesting that endogenous IGF-I is partially responsible for the effects of GH in this system. Follicles were also cultured in the presence of GH and IGF-I together, and no additive effect of these hormones was observed. To further support this hypothesis, levels of IGF-I mRNA in the follicles were measured; rbGH treatment increased IGF-I mRNA levels by 1.5-fold. Taken together, these data suggest that GH acts as an ovarian follicle survival factor by preventing follicle cell apoptosis and that part of the GH action is mediated by locally produced IGF-I.