Extracellular signal-regulated kinase (ERK)1/2 activity is reported to be required in mammalian cells for timely entry into and exit from mitosis (i.e., the G 2 -mitosis [G 2 /M] and metaphase-anaphase [M/A] transitions). However, it is unclearwhether this involvement reflects a direct requirement for ERK1/2 activity during these transitions or for activating gene transcription programs at earlier stages of the cell cycle. To examine these possibilities, we followed live cells in which ERK1/2 activity was inhibited through late G 2 and mitosis. We find that acute inhibition of ERK1/2 during late G 2 and through mitosis does not affect the timing of the G 2 /M or M/A transitions in normal or transformed human cells, nor does it impede spindle assembly, inactivate the p38 stress-activated checkpoint during late G 2 or the spindle assembly checkpoint during mitosis. Using CENP-F as a marker for progress through G 2 , we also show that sustained inhibition of ERK1/2 transiently delays the cell cycle in early/mid-G 2 via a p53-dependent mechanism. Together, our data reveal that ERK1/2 activity is required in early G 2 for a timely entry into mitosis but that it does not directly regulate cell cycle progression from late G 2 through mitosis in normal or transformed mammalian cells.
INTRODUCTIONThe extracellular signal-regulated (ERK)1/2 pathway consists of Raf1, mitogen-activated protein kinase kinase (MEK)1/2, and ERK1/2 kinases, which upon pathway activation are sequentially and specifically phosphorylated . Once activated, ERK1/2 phosphorylates numerous cytoplasmic and nuclear substrates that lead to diverse cellular responses, including proliferation and differentiation, via both transcription-dependent and independent mechanisms (Pearson et al., 2001;Yoon and Seger, 2006). Basically, ERK1/2 activity is essential for cell growth; it mediates the G 1 /S transition by facilitating nucleotide and protein syntheses as well as the transcription of cell cycle regulators, including cyclins D and E (Widmann et al., 1999;Roovers and Assoian, 2000). Not unexpectedly, constitutive activation of ERK1/2 via mutations in Raf or its upstream Ras G protein leads to uncontrolled cell growth, whereas the inability to activate ERK1/2 is lethal in utero (Wojnowski et al., 1998;Giroux et al., 1999) and inhibits the growth of cultured cells (Pages et al., 1993).In addition to its essential role in promoting the G 1 /S transition, enhanced ERK1/2 activity is also required in mammalian cells for a timely G 2 -mitosis (G 2 /M) transition. Long-term (hours to days) suppression of ERK1/2 by using pharmacological inhibitors of MEK1/2, dominant-negative MEK1, or RNA interference (RNAi) produces a delay in "G 2 /M" (Wright et al., 1999;Hayne et al., 2000;Roberts et al., 2002;Liu et al., 2004;Knauf et al., 2006). Gene expression profiling of nontransformed mammary epithelial cells also reveals that constitutive activation of ERK1/2 via MEK1 leads to increased levels of mRNAs that encode mitotic proteins such as Cyclin B, CDK1, CENP-E, Bub1, Mad...