Extracellular signal-regulated kinase 1/2 activity is not required in mammalian cells during late G₂ for timely entry into or exit from mitosis

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 G₂-mitosis [G₂/M] and metaphase-anaphase [M/A] transitions). However, it is unclear whether 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 G₂ and mitosis. We find that acute inhibition of ERK1/2 during late G₂ and through mitosis does not affect the timing of the G₂/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 G₂ or the spindle assembly checkpoint during mitosis. Using CENP-F as a marker for progress through G₂, we also show that sustained inhibition of ERK1/2 transiently delays the cell cycle in early/mid-G₂ via a p53-dependent mechanism. Together, our data reveal that ERK1/2 activity is required in early G₂ for a timely entry into mitosis but that it does not directly regulate cell cycle progression from late G₂ through mitosis in normal or transformed mammalian cells.