Negative Feedback Regulation and Desensitization of Insulin- and Epidermal Growth Factor-stimulated p21 Activation

Insulin and epidermal growth factor receptors transmit signals for cell proliferation and gene regulation through formation of active GTP-bound p21 mediated by the guanine nucleotide exchange factor Sos. Sos is constitutively bound to the adaptor protein Grb2 and growth factor stimulation induces association of the Grb2/Sos complex with Shc and movement of Sos to the plasma membrane location of p21 . Insulin or epidermal growth factor stimulation induces a rapid increase in p21 levels, but after several minutes levels decline toward basal despite ongoing hormone stimulation. Here we show that deactivation of p21 correlates closely with phosphorylation of Sos and dissociation of Sos from Grb2, and that inhibition of mitogen-activated protein (MAP) kinase kinase (also known as extracellular signal-related kinase (ERK) kinase, or MEK) blocks both events, resulting in prolonged p21 activation. These data suggest that a negative feedback loop exists whereby activation of the Raf/MEK/MAP kinase cascade by p21 causes Sos phosphorylation and, therefore, Sos/Grb2 dissociation, limiting the duration of p21 activation by growth factors. A serine/threonine kinase downstream of MEK (probably MAP kinase) mediates this desensitization feedback pathway.