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 h ematopoiesis, melanogenesis, and spermatogenesis, In the testis, the SCF/c- kit system is believed to regulate germ cell proliferation, meiosis, and ap optosis, Studies with type A spermatogonia in vivo and in vitro have indica ted that SCF induces DNA synthesis and proliferation. However, the signalin g pathway for this function of SCF/c-kit has not been elucidated. We now de monstrate that SCF activates phosphoinositide 3-kinase (P13-K) and p70 S6 k inase (p70S6K) and that rapamycin, a FRAP/mammalian target of rapamycin-dep endent inhibitor of p70S6K, completely inhibited bromodeoxyuridine incorpor ation induced by SCF in primary cultures of spermatogonia, SCF induced cycl in 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 p 70S6K, Dominant negative AKT-K179M: completely abolished p70S6K phosphoryla tion induced by the constitutively active P13-K catalytic subunit p110, Con stitutively active v-AKT highly phosphorylated p70S6K, which was totally in hibited by rapamycin, Thus, SCF/c-kit uses a rapamycin-sensitive P13-K/AKT/ p70S6K/cyclin D3 pathway to promote spermatogonial cell proliferation.