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The Journal of Neuroscience, March 16, 2005, 25(11):2838-2852; doi:10.1523/JNEUROSCI.5060-04.2005

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Cellular/Molecular
Insulin-Like Growth Factor 1 Inhibits Extracellular Signal-Regulated Kinase to Promote Neuronal Survival via the Phosphatidylinositol 3-Kinase/Protein Kinase A/c-Raf Pathway

Srinivasa Subramaniam,¹ Neelam Shahani,² Jens Strelau,¹ Christine Laliberté,³ Roland Brandt,² David Kaplan,^3,4 and Klaus Unsicker¹

¹Neuroanatomy and Interdisciplinary Center for Neurosciences, University of Heidelberg, D-69120 Heidelberg, Germany, ²Department of Neurobiology, University of Osnabrueck, D-49076 Osnabrueck, Germany, ³The Hospital for Sick Children, Toronto, Ontario, Canada M5G 1X8, and ⁴Department of Medical Genetics and Microbiology and Institute of Medical Sciences, University of Toronto, Toronto, Ontario, Canada M5S 1A8

Extracellular signal-regulated kinase (ERK) activation has been shown to promote neuronal death in various paradigms. We demonstrated previously that the late and sustained ERK activation in cerebellar granule neurons (CGNs) cultured in low potassium predominantly promotes plasma membrane (PM) damage. Here, we examined the effects of a well established neuronal survival factor, insulin-like growth factor 1 (IGF-1), on the ERK cell death pathway. Stimulation of CGNs with IGF-1 induced an early and transient ERK activation but abrogated the appearance of late and sustained ERK. Withdrawal or readdition of IGF-1 after 4 h in low potassium failed to prevent sustained ERK activation and cell death. IGF-1 activated the protein kinase A (PKA) to mediate ERK inhibition via c-Raf phosphorylation at an inhibitory site (Ser259). Phosphatidylinositol 3-kinase (PI3K) or PKA inhibitors, but not a specific Akt inhibitor, abrogated PKA signaling. This suggests that the PI3K/PKA/c-Raf-Ser259 pathway mediates ERK inhibition by IGF-1 independent of Akt. In addition, adenoviral-mediated expression of constitutively active MEK (mitogen-activated protein kinase kinase) or Sindbis viral-mediated expression of mutant Raf Ser259Ala both attenuated IGF-1-mediated prevention of PM damage. Activation of caspase-3 promoted DNA damage. Its inhibition by IGF-1 was both PI3K and Akt dependent but PKA independent. 8-Br-cAMP, an activator of PKA, induced phosphorylation of c-Raf-Ser259 and inhibited ERK activation without affecting caspase-3. This indicates a selective role for PKA in ERK inhibition through c-Raf-Ser259 phosphorylation. Together, these data demonstrate that IGF-1 can positively and negatively regulate the ERK pathway in the same neuronal cell, and provide new insights into the PI3K/Akt/PKA signaling pathways in IGF-1-mediated neuronal survival.

Key words: ERK; neuronal death; Sindbis virus; Raf-S259A; PI3K; PKA

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Received June 22, 2004; revised January 28, 2005; accepted January 31, 2005.

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