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Negative feedback regulation and desensitization of insulin- and epidermal growth factor-stimulated p21ras activation
W J Langlois1, T Sasaoka, A R Saltiel
1Department of Medicine, University of California, San Diego, La Jolla 92093, USA.
Abstract:
Insulin and epidermal growth factor receptors transmit signals for cell proliferation and gene regulation through formation of active GTP-bound p21ras 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 p21ras. Insulin or epidermal growth factor stimulation induces a rapid increase in p21ras levels, but after several minutes levels decline toward basal despite ongoing hormone stimulation. Here we show that deactivation of p21ras 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 p21ras activation. These data suggest that a negative feedback loop exists whereby activation of the Raf/MEK/MAP kinase cascade by p21ras causes Sos phosphorylation and, therefore, Sos/Grb2 dissociation, limiting the duration of p21ras activation by growth factors. A serine/threonine kinase downstream of MEK (probably MAP kinase) mediates this desensitization feedback pathway.
Insights
Growth factor signaling involves p21ras activation. A feedback loop, mediated by MAP kinase, phosphorylates Sos, limiting p21ras activation duration and ensuring signal termination.
Area of Science:
- Cellular signaling pathways
- Molecular biology
- Signal transduction
Background:
- Insulin and epidermal growth factor receptors initiate cell proliferation and gene regulation.
- Signal transmission relies on p21ras activation, facilitated by the guanine nucleotide exchange factor Sos.
- Sos forms a complex with Grb2, translocating to the plasma membrane upon growth factor stimulation.
Purpose of the Study:
- To investigate the mechanism of p21ras deactivation following growth factor stimulation.
- To identify the role of Sos phosphorylation and its dissociation from Grb2 in regulating p21ras activity.
- To elucidate the negative feedback loop controlling the duration of p21ras activation.
Main Methods:
- Monitoring p21ras levels after insulin or epidermal growth factor stimulation.
- Analyzing Sos phosphorylation and its dissociation from Grb2.
- Utilizing mitogen-activated protein (MAP) kinase kinase (MEK) inhibition to assess its impact on p21ras activation.
Main Results:
- p21ras levels transiently increase upon growth factor stimulation and then decline.
- p21ras deactivation correlates with Sos phosphorylation and dissociation from Grb2.
- Inhibition of MEK prolongs p21ras activation by preventing Sos phosphorylation and dissociation.
Conclusions:
- A negative feedback mechanism limits the duration of p21ras activation by growth factors.
- Activation of the Raf/MEK/MAP kinase cascade leads to Sos phosphorylation, causing dissociation from Grb2.
- This feedback loop, likely mediated by MAP kinase, desensitizes the signaling pathway.