Stem cell factor/c-kit up-regulates cyclin D3 and promotes cell cycle progression via the phosphoinositide 3-kinase/p70 S6 kinase pathway in spermatogonia

Stem cell factor (SCF)/c-kit plays an important role in the regulation of hematopoiesis, melanogenesis, and spermatogenesis, In the testis, the SCF/c-kit system is believed to regulate germ cell proliferation, meiosis, and apoptosis, Studies with type A spermatogonia in vivo and in vitro have indicated that SCF induces DNA synthesis and proliferation. However, the signaling pathway for this function of SCF/c-kit has not been elucidated. We now demonstrate that SCF activates phosphoinositide 3-kinase (P13-K) and p70 S6 kinase (p70S6K) and that rapamycin, a FRAP/mammalian target of rapamycin-dependent inhibitor of p70S6K, completely inhibited bromodeoxyuridine incorporation induced by SCF in primary cultures of spermatogonia, SCF induced cyclin D3 expression and phosphorylation of the retinoblastoma protein through a pathway that is sensitive to both wortmannin and rapamycin, Furthermore, AKT, but not protein kinase C-zeta is used by SCF/c-kit/P13-K to activate p70S6K, Dominant negative AKT-K179M: completely abolished p70S6K phosphorylation induced by the constitutively active P13-K catalytic subunit p110, Constitutively active v-AKT highly phosphorylated p70S6K, which was totally inhibited by rapamycin, Thus, SCF/c-kit uses a rapamycin-sensitive P13-K/AKT/p70S6K/cyclin D3 pathway to promote spermatogonial cell proliferation.