Ovarian follicular growth and atresia: The relationship between cell proliferation and survival

ABSTRACT Growth factors and steroids play an important role in the regulation of ovarian follicular development. In cattle, two of the earliest detectable differences between the healthy dominant follicle selected for development to the ovulatory stage and subordinate follicles destined to undergo atresia are the greater availability of IGF and the greater capacity to produce estradiol in the dominant follicle. We have shown that IGF-I and estradiol stimulate the proliferation of bovine granulosa cells in vitro and promote granulosa cell survival by increasing resistance to apoptosis. Furthermore, the ability of IGF-I and estradiol to increase resistance to apoptosis is tied to their ability to promote progression through the cell cycle. Blocking the cell cycle at the transition between the first gap phase and the DNA synthesis phase using a specific inhibitor prevented the protective effects of IGF-I and estradiol against apoptosis. Further experiments showed that the protective effect of IGF-I against apoptosis is mediated by the stimulation of phosphatidylinositol 3-kinase and its downstream target, protein kinase B/Akt. Constitutive activation of Akt by the infection of granulosa cells with a recombinant Akt adenovirus protected against apoptosis, and this effect also depended on cell cycle progression. These experiments show that the protective effect of estradiol and IGF-I against apoptosis depends on unperturbed progression through the cell cycle. Once follicles have developed to the preovulatory stage, the LH surge induces terminal differentiation of granulosa cells and withdrawal from the cell cycle. Bovine granulosa cells withdraw from the cell cycle by 12 h after the LH surge and become resistant to apoptosis, even in the absence of growth factors. Treatment with a progesterone receptor antagonist in vitro caused reentry of granulosa cells into the cell cycle and susceptibility to apoptosis, suggesting that induction of progesterone receptor expression by the LH surge is required for cell cycle withdrawal and resistance to apoptosis. In summary, the susceptibility of granulosa cells to apoptosis depends on the cell cycle. Proliferating granulosa cells in growing follicles depend on growth factors for survival, whereas cells that have terminally differentiated in response to the LH surge are resistant to apoptosis and relatively independent of growth factors for survival.

Key Words: Apoptosis, Atresia, Bovine, Follicle, Ovary, Proliferation

© 2004, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2004. 82(E. Suppl.):E40-E52

Implications

Survival factors for growing follicles in vivo include insulin-like growth factor and estradiol, which are available at elevated levels in the dominant relative to the subordinate follicles at the time of follicle selection. Insulin-like growth factor and estradiol protect cells from apoptosis in vitro, an effect dependent on unperturbed progression through the cell cycle. Because granulosa cells seem to undergo apoptosis predominantly at the G1 to S transition of the cell cycle and insulin-like growth factor and estradiol increase G1 to S transition, it is likely that the protective effect of these factors stems from their ability to facilitate G1 to S transition (Figure 5). Exit from the cell cycle by granulosa cells of preovulatory follicles in response to the LH surge induces resistance to apoptosis, a property that may be important for long-term survival in the corpus luteum (Figure 6). Future studies should investigate mechanisms for the interactions between pathways for cell proliferation and apoptosis.