The mitogen-activated protein kinase (MAPK) cascade is one of the most
important signal transduction pathways that regulate the cell cycle in somatic
cells. The present study examined the phosphorylation states of components in
the MAPK cascade, Raf-1, MEK-1, and extracellular signal regulated kinases
(ERKs), which are activated by mitogens, throughout early mouse embryo
development and in cultured somatic cells generally. In somatic cells, Raf-1
and MEK-1 were phosphorylated at M-phase and dephosphorylated during
interphase. ERKs were not phosphorylated at any stage during the cell cycle.
These results were similar to previous findings for the first and second cell
cycles of early mouse embryos. In contrast, after the four-cell stage, not
only ERKs, but also Raf-1 and MEK-1, were not phosphorylated at any stage
during the cell cycle in mouse early embryos. These results suggest that the
MAPK cascade in mouse embryos is regulated by the same mechanism as in somatic
cells before the two-cell stage, and that regulation is changed to an
embryo-specific mechanism after the four-cell stage.