MSK1 is required for CREB phosphorylation in response to mitogens in mouse embryonic stem cells

Mouse embryonic stem (ES) cells homozygous for disruption of the MSK1 gene had no detectable MSK1 activity. However, their activators (extracellular signal related kinase (ERK)1/ERK2) were stimulated normally in mitogen- and stress-activated protein kinase (MSK)1-/- and wild type cells in response to tetradecanoylphorbol acetate (TPA) and epidermal growth factor (EGF). TPA and EGF induced the phosphorylation of cyclic AMP-responsive element binding protein (CREB) at Ser-133 and ATF1 at Ser-63 in wild type cells and this was abolished by inhibition of the mitogen-activated protein kinase cascade. In contrast, the TPA- and EGF-induced phosphorylation of CREB/ATF1 was barely detectable in MSK1-/- cells. However, basal and forskolin-induced phosphorylation was similar, indicating that the MSK1 'knockout' did not prevent CREB phosphorylation by cyclic AMP-dependent protein kinase. Thus MSK1 is required for CREB and ATF1 phosphorylation after mitogenic stimulation of ES cells.