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

A growing body of evidence suggests that growth hormone IGH) plays a r ole in regulating ovarian function by augmenting gonadotropin stimulat ion of granulosa cell differentiation and folliculogenesis. The majori ty of follicles in the mammalian ovary do not ovulate, but instead und ergo a degenerative process (atresia) involving apoptotic cell death. The objective of the present study was to investigate the role of GH i n regulating follicle apoptosis and to determine whether or not insuli n-like growth factor-I (IGF-I) mediates GH action in this process. Pre ovulatory 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 autora diography of size-fractionated DNA labeled at 3' ends with [P-32]dideo xy-ATP. Culture of preovulatory follicles resulted in a spontaneous on set of apoptotic DNA fragmentation that was suppressed by ovine GH (oG H) 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 ear lier study indicated that hCG and FSH treatment also suppress apoptosi s 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 med iated by IGF-I, three types of studies were carried out. First, follic les were cultured with rbGH in the presence of insulin-like growth fac tor binding protein-3 (IGFBP-3), which is known to neutralize IGF-I ac tion. Cotreatment with IGFBP-3 reversed the suppressive effect of GH i n a dose-dependent manner by up to 75%, suggesting that endogenous IGF -I is partially responsible for the effects of GH in this system. Foll icles 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 togethe r, these data suggest that GH acts as an ovarian follicle survival fac tor by preventing follicle cell apoptosis and that part of the GH acti on is mediated by locally produced IGF-I.