Activation of extracellular signal‐regulated kinase (ERK) in G2 phase delays mitotic entry through p21CIP1

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Abstract
Extracellular signal‐regulated kinase activity is essential for mediating cell cycle progression from G1 phase to S phase (DNA synthesis). In contrast, the role of extracellular signal‐regulated kinase during G2 phase and mitosis (M phase) is largely undefined. Previous studies have suggested that inhibition of basal extracellular signal‐regulated kinase activity delays G2‐ and M‐phase progression. In the current investigation, we have examined the consequence of activating the extracellular signal‐regulated kinase pathway during G2 phase on subsequent progression through mitosis. Using synchronized HeLa cells, we show that activation of the extracellular signal‐regulated kinase pathway with phorbol 12‐myristate 13‐acetate or epidermal growth factor during G2 phase causes a rapid cell cycle arrest in G2 as measured by flow cytometry, mitotic indices and cyclin B1 expression. This G2‐phase arrest was reversed by pre‐treatment with bisindolylmaleimide or U0126, which are selective inhibitors of protein kinase C proteins or the extracellular signal‐regulated kinase activators, MEK1/2, respectively. The extracellular signal‐regulated kinase‐mediated delay in M‐phase entry appeared to involve de novo synthesis of the cyclin‐dependent kinase inhibitor, p21CIP1, during G2 through a p53‐independent mechanism. To establish a function for the increased expression of p21CIP1 and delayed cell cycle progression, we show that extracellular signal‐regulated kinase activation in G2‐phase cells results in an increased number of cells containing chromosome aberrations characteristic of genomic instability. The presence of chromosome aberrations following extracellular signal‐regulated kinase activation during G2‐phase was further augmented in cells lacking p21CIP1. These findings suggest that p21CIP1 mediated inhibition of cell cycle progression during G2/M phase protects against inappropriate activation of signalling pathways, which may cause excessive chromosome damage and be detrimental to cell survival.