Cell Cycle Progression and Activation of Akt Kinase Are Required for Insulin-Like Growth Factor I-Mediated Suppression of Apoptosis in Granulosa Cells

Abstract

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-induced apoptosis of granulosa cells and its relationship to cell proliferation. IGF-I activated both the phosphoinositide 3'-OH kinase (PI3K) and the 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 prevented protection by IGF-I whereas expression of constitutively active Akt (myrAkt) mimicked the effect of IGF-I. Treatment with IGF-I, or expression of myrAkt, increased progression from G(0)/G(1) to S phase of the cell cycle whereas expression of dominant negative Akt inhibited G(0)/G(1) 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 G(1)/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 occur only when progression from G(1) to S phase of the cell cycle regulated by the PI3K/Akt pathway is unperturbed.

Akt Down-Regulates ERK1/2 Nuclear Localization and Angiotensin II-induced Cell Proliferation through PEA-15

“…Whereas Akt overexpression is mainly associated with an oncogenic phenotype, this pathological context is usually associated with mutation or modification of the expression of other components of the Akt pathway, such as PI3K and/or phosphatase and tensin homologue deleted on chromosome 10 (Luo et al, 2003). Besides, the dual involvement of Akt in mediating proliferative and/or antiapoptotic responses has to be considered inasmuch as both responses are intricate (Hu et al, 2004). Zhu et al (2004) reported that Gab2 overexpression, which enhanced the phosphorylation of Akt but not of ERK1/2, reduced cell proliferation induced by granulocyte colony-stimulated factor (Zhu et al, 2004).…”

Transforming growth factor-β1 inhibits luteinization and promotes apoptosis in bovine granulosa cells

“…During the first gap phase or G1, cells are quiescent but potentially receptive to mitogenic stimuli, which stimulate cells to embark into the proliferative phases of the cell cycle consisting of the DNA synthesis phase (S), the second gap phase (G2), and the mitotic phase (M). In granulosa cells of the developing follicle, the G1/S transition phase is a critical checkpoint, as progression from the quiescent stage (G0/G1) to the S phase requires the action of cell proliferation and survival factors such as insulin-like growth factor-1 (IGF1) and estradiol (E 2 ; Hu et al 2004.…”

PTEN and Akt expression during growth of human ovarian follicles

“…Such activation is correlated with cell survival in a wide variety of cells, including those of epithelial, mesenchymal and neuronal origin [9,10], as well as cancer cells [11]. In bovine granulosa cells, Akt, a downstream substrate of PI3K, was necessary for the protective effect of IGF-1 against FasL-induced apoptosis [12]. Akt has been also demonstrated to be involved in granulosa cell survival in hens [6] and pigs [13].…”