c-Fos (green) is more stable (top to bottom) in PDGF-treated cells.

Blenis/Macmillan

Duration of kinase signaling determines how a cell responds to external cues like growth factors. But how a cell measures signaling time has been a mystery. Now, Leon Murphy, John Blenis (Harvard Medical School, Boston, MA), and colleagues have found that immediate early gene (IEG) products have a sensor mechanism that makes these distinctions. Their findings may provide a target for cancer treatment drugs.

Growth factors PDGF and EGF elicit different responses in fibroblasts, with only PDGF inducing cells to enter S-phase. The group found that EGF transiently activated ERK MAP kinases, whereas PDGF stimulated prolonged activation. The IEG product c-Fos is known to be transcribed with similar kinetics in response to either growth factor, but c-Fos was only phosphorylated when ERK activity was sustained. This not only stabilized c-Fos, but also primed...

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