Xp42Mpk1 activation is not required for germinal vescicle breakdown but for Raf complete phosphorylation in insulin-stimulated Xenopus oocytes

Fully grown G(2)-arrested Xenopus oocytes resume meiosis in vitro upon exposure to hormonal stimulation. Progesterone triggers oocyte meiosis resumption through a Ras-independent pathway that involves a p39(Mos)-dependent activation of the mitogen-activated protein (MAP) kinases. Insulin also triggers meiosis resumption through a tyrosine kinase receptor that activates a Ras-dependent pathway leading to the MAP kinases activation. Antisense phosphorothioate oligonucleotides were used to prevent p39(Mos) accumulation and Erk-like Xp42(Mpk1) activation during insulin-induced Xenopus oocytes maturation. In contrast to previous works, prevention of p39(Mos)-induced activation of Xp42(Mpk1) in insulin-treated oocytes did not inhibit but delayed meiotic resumption, like in progesterone-stimulated oocytes. Activations of Xp42(Mpk1), the unique Erk of the oocyte, and of its downstream target p90(Rsk), were impaired and phosphorylation of the MAPKK kinase Raf was partially inhibited. Similarly, oocytes treated with the MEK inhibitor U0126, stimulated by insulin exhibited delayed germinal vesicle breakdown, absence of Xp42(Mpk1) activation, and partial phosphorylation of Raf. To summarize, whereas p39(Mos)-induced activation of MEK/MAPK pathway is dispensable for insulin-induced germinal vesicle breakdown, Xp42(Mpk1) activation induced by insulin is dependent upon p39(Mos) synthesis. Raf complete phosphorylation appears to require the MEK/MAPK pathway activation both in progesterone and insulin-stimulated oocytes.