Mitogen-activated protein kinase (MAPK) (extracellular signal-regulated kinase) prevents DNA replication and parthenogenesis in maturing oocytes. After the meiotic cell cycle in starfish eggs, MAPK activity is maintained until fertilization. When eggs are fertilized, inactivation of MAPK occurs, allowing development to proceed. Without fertilization, highly synchronous apoptosis of starfish eggs starts 10 h after germinal vesicle breakdown, which varies according to season and individual animals. For induction of the apoptosis, MAPK should be activated for a definite period, called the MAPKdependent period, during which eggs develop competence to die, although the exact duration of the period was unclear. In this study, we show that the duration of the MAPK-dependent period was ϳ8 h. Membrane blebbing occurred ϳ2 h after the MAPK-dependent period. Surprisingly, when MAPK was inhibited by U0126 after the MAPK-dependent period, activation of caspase-3 occurred earlier than in the control eggs. Thus, inactivation of MAPK is a prerequisite for apoptosis. Also, even in the absence of the inhibitor, MAPK was inactivated spontaneously when eggs began to bleb, indicating that inactivation of MAPK after the MAPK-dependent period acts upstream of caspase-3. Inactivation of MAPK also resulted in the activation of p38MAPK, which may contribute to apoptotic body formation.
INTRODUCTIONApoptosis plays critical roles in development and in the maintenance of homeostasis. Once triggered, the apoptotic program induces activation of a series of biochemical events. The best characterized pathway of apoptosis involves the release of cytochrome c from mitochondria, leading to the activation of caspase-9. The caspase-9 cleaves and activates caspase-3, which is the key enzyme to execute apoptosis (reviewed by Chang and Yang, 2000). Caspase-3 cleaves a large number of proteins within the cell, leading to the orderly dismantling of the apoptotic cell (reviewed by Porter and Janicke, 1999).A mitogen-activated protein kinase kinase kinase phosphorylates and activates a MAP kinase kinase (MAPKK), which phosphorylates and activates mitogen-activated protein kinase (MAPK). In mammals, there are at least three genetically distinct groups of MAPK pathways, including extracellular signal-regulated kinase (ERK: MAPK), the c-Jun NH 2 -terminal kinase (JNK), and the p38MAPK (reviewed by Widmann et al., 1999;Davis, 2000). The JNK and the p38MAPK cascades are activated by many agents that induce apoptosis such as oxidative stress, UV radiation, transforming growth factor- treatment, and anticancer drugs (Zanke et al., 1996;Huot et al., 1998;Tournier et al., 2000;Edlund et al., 2003). Inhibition of JNK and p38MAPK suppresses apoptosis induced by these agents. Although JNK and p38MAPK seem to be involved in modulating apoptosis, ERK is generally considered as a survival factor. In Drosophila, the Ras-MAPK signaling pathway promotes cell survival by inhibiting the expression and activity of the proapoptotic protein Hid (Kurada and White, 1998). In rat ...