Cell cycle progression and activation of Akt kinase are required for insulin-like growth factor I-mediated suppression of apoptosis in granulosa cells.
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.
Department of Animal Science, Cornell University, Ithaca, New York 14853, USA.
Che-Lin
CL
Hu
Robert G
RG
Cowan
Rebecca M
RM
Harman
Susan M
SM
Quirk
D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D002417:Cattle; D002453:Cell Cycle / Q000187:drug effects / Q000502:physiology*; D002478:Cells, Cultured; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D053222:Fas Ligand Protein; D005260:Female; D006107:Granulosa Cells / Q000166:cytology* / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology / Q000502:physiology*; D020935:MAP Kinase Signaling System / Q000187:drug effects; D008562:Membrane Glycoproteins / Q000187:drug effects / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000187:drug effects / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000187:drug effects / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D011687:Purines / Q000494:pharmacology; D000077546:Roscovitine; D015398:Signal Transduction; D019014:fas Receptor / Q000187:drug effects / Q000378:metabolism
D004791:Enzyme Inhibitors; D053222:Fas Ligand Protein; D007334:Insulin-Like Growth Factor I; D008562:Membrane Glycoproteins; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D011687:Purines; D000077546:Roscovitine; D019014:fas Receptor