Regulation of cyclin-dependent kinase (Cdk) 2 Thr-160 phosphorylation and activity by mitogen-activated protein kinase in late G(1) phase

Mitogen-activated protein (MAP) kinases, p42(MAPK) and p44(MAPK), are centr al components of growth-promoting signalling pathways. However, how stimula tion of MAP kinases culminates in cell-cycle progression is still poorly un derstood. Here we show that mitogenic stimulation of NIH 3T3 cells causes a sustained activation of MAP kinases, which lasts until cells begin progres sing through the G(1)/S boundary. Furthermore, we observed that disruption of the MAP-kinase pathway with a selective MEK (MAP kinase/extracellular-si gnal-regulated protein kinase kinase) inhibitor, PD98059, prevents the acti vation of cyclin-dependent kinase (Cdk) 2 and DNA synthesis, even when adde d during late G(1) phase, once the known mechanisms by which MAP kinase con trols G(1) progression, accumulation of G(1) cyclins and degradation of Cdk inhibitors have already taken place. Moreover, we provide evidence indicat ing that MAP kinases control Cdk2 Thr-160 activating phosphorylation and fu nction, possibly by regulating the activity of a Cdk-activating kinase, thu s promoting the re-initiation of DNA synthesis. These findings suggest the existence of a novel mechanism whereby signal-transducing pathways convergi ng on MAP kinases can affect the cell-cycle machinery and, ultimately, part icipate in cell-growth control.