CDK5

Abi

CDK5

DARPP32

Ovarian follicle development is dependent on growth factors that stimulate cell proliferation and act as survival factors to prevent apoptosis of follicle cells. We examined the mechanism of the protective effect of IGF-I against Fas ligand (FasL)-induced apoptosis of granulosa cells and its relationship to cell proliferation. IGF-I activated both the phosphoinositide 3'-OH kinase (PI3K) and the mitogen-activated protein kinase (MAPK) pathways. Experiments using specific inhibitors of these pathways showed that protection by IGF-I was mediated by the PI3K pathway and not the MAPK pathway. Recombinant adenoviruses were used to test whether the downstream target of PI3K activation, Akt kinase, was required for protection against apoptosis. Expression of dominant negative Akt (dnAkt) prevented protection by IGF-I while expression of constitutively active Akt (myrAkt) mimicked the effect of IGF-I. Treatment with IGF-I, or expression of myrAkt, increased progression from G0/G1 to S phase of the cell cycle while expression of dnAkt inhibited G0/G1 to S phase progression and prevented the stimulatory effect of IGF-I. We tested whether cell cycle progression was required for protection from apoptosis using the cyclin dependent kinase-2 inhibitor roscovitine, which blocks cells at the G1/S transition. Roscovitine prevented the protective effect of IGF-I and myrAkt expression against apoptosis. Therefore, activation of Akt is not sufficient to protect granulosa cells from apoptosis in the absence of cell cycle progression. In summary, IGF-I protects granulosa cells from apoptosis by activation of the PI3K/Akt pathway. This protective effect can only occur when progression from G1 to S phase of the cell cycle regulated by the PI3K/Akt pathway is unperturbed.