Activation of the Akt/protein kinase B signaling pathway is associated with granulosa cell survival.
Follicles from the hen ovary that have been selected into the preovulatory hierarchy are committed to ovulation and rarely become atretic under normal physiological conditions. In part, this is attributed to the resistance of the granulosa layer to apoptosis. The present studies were conducted to evaluate the role of the phosphatidylinositol (PI) 3-kinase/Akt signaling pathway in hen granulosa cell survival and, by implication, follicle viability. Cloning of the chicken akt2 homologue revealed a high degree of amino acid homology to its mammalian counterparts within the catalytic domain, plus complete conservation of the putative Thr(308) and Ser(474) phosphorylation sites. Treatment of granulosa cells from the three largest preovulatory follicles with insulin-like growth factor (IGF)-I and, to a lesser extent, transforming growth factor (TGF)-alpha induces rapid phosphorylation of Akt, and such phosphorylation is effectively blocked by the PI 3-kinase-inhibitor LY294006. Serum withdrawal from cultured cells for 33-44 h initiates oligonucleosome formation, an indicator of apoptotic cell death, whereas cotreatment with IGF-I prevents this effect. Moreover, treatment of cultured cells for 20 h with LY294006 induces apoptosis. The potential for nonspecific cell toxicity following LY294006 treatment is considered unlikely because of the ability of either LH or 8-bromo cAMP cotreatment to block LY294006-induced cell death. Finally, both IGF-I and TGF-alpha also activate mitogen-activated protein (MAP) kinase signaling, at least in part, through the phosphorylation of ERK: However, treatment with neither U0126 nor PD98059 (inhibitors of MAP kinase kinase) induced cell death in cultured granulosa cells, despite the ability of each inhibitor to effectively block Erk phosphorylation. Taken together, these results provide evidence for a role of the Akt signaling pathway in promoting cell survival within the preovulatory follicle granulosa layer. In addition, the data indicate the importance of an alternative survival pathway mediated via gonadotropins and protein kinase A independent of Akt signaling.
Johnson
A L
AL
Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, USA. johnson.128@nd.edu
Bridgham
J T
JT
Swenson
J A
JA
D000818:Animals; D002070:Butadienes / Q000494:pharmacology; D002470:Cell Survival; D002645:Chickens; D018076:DNA, Complementary / Q000737:chemistry; D004789:Enzyme Activation; D005260:Female; D015870:Gene Expression; D006107:Granulosa Cells / Q000737:chemistry / Q000502:physiology*; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D008969:Molecular Sequence Data; D009570:Nitriles / Q000494:pharmacology; D009707:Nucleosomes / Q000378:metabolism; D006080:Ovarian Follicle / Q000502:physiology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger / Q000032:analysis; D017385:Sequence Homology; D015398:Signal Transduction; D013799:Theca Cells / Q000737:chemistry
D002070:Butadienes; D018076:DNA, Complementary; D007334:Insulin-Like Growth Factor I; D009570:Nitriles; D009707:Nucleosomes; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger; C113580:U 0126; C494918:AKT1 protein, human; C494921:AKT2 protein, human