Regulation of cyclin-dependent kinase (Cdk) 2 Thr-160 phosphorylation and activity by mitogen-activated protein kinase in late G1 phase

Mitogen-activated protein (MAP) kinases, p42(MAPK) and p44(MAPK), are central components of growth-promoting signalling pathways. However, how stimulation of MAP kinases culminates in cell-cycle progression is still poorly understood. Here we show that mitogenic stimulation of NIH 3T3 cells causes a sustained activation of MAP kinases, which lasts until cells begin progressing through the G(1)/S boundary. Furthermore, we observed that disruption of the MAP-kinase pathway with a selective MEK (MAP kinase/extracellular-signal-regulated protein kinase kinase) inhibitor, PD98059, prevents the activation of cyclin-dependent kinase (Cdk) 2 and DNA synthesis, even when added during late G(1) phase, once the known mechanisms by which MAP kinase controls G(1) progression, accumulation of G(1) cyclins and degradation of Cdk inhibitors have already taken place. Moreover, we provide evidence indicating that MAP kinases control Cdk2 Thr-160 activating phosphorylation and function, possibly by regulating the activity of a Cdk-activating kinase, thus promoting the re-initiation of DNA synthesis. These findings suggest the existence of a novel mechanism whereby signal-transducing pathways converging on MAP kinases can affect the cell-cycle machinery and, ultimately, participate in cell-growth control.