Neuronal Death by Oxidative Stress Involves Activation of FOXO3 through a Two-Arm Pathway That Activates Stress Kinases and Attenuates Insulin-like Growth Factor I Signaling

Abstract

Oxidative stress kills neurons by stimulating FOXO3, a transcription factor whose activity is inhibited by insulin-like growth factor I (IGF-I), a wide-spectrum neurotrophic signal. Because recent evidence has shown that oxidative stress blocks neuroprotection by IGF-I, we examined whether attenuation of IGF-I signaling is linked to neuronal death by oxidative stress, as both events may contribute to neurodegeneration. We observed that in neurons, activation of FOXO3 by a burst of oxidative stress elicited by 50 muM hydrogen peroxide (H(2)O(2)) recruited a two-pronged pathway. A first, rapid arm attenuated AKT inhibition of FOXO3 through p38 MAPK-mediated blockade of IGF-I stimulation of AKT. A second delayed arm involved activation of FOXO3 by Jun-kinase 2 (JNK2). Notably, blockade of IGF-I signaling through p38 MAPK was necessary for JNK2 to activate FOXO3, unveiling a competitive regulatory interplay between the two arms onto FOXO3 activity. Therefore, an abrupt rise in oxidative stress activates p38 MAPK to tilt the balance in a competitive AKT/JNK2 regulation of FOXO3 toward its activation, eventually leading to neuronal death. In view of previous observations linking attenuation of IGF-I signaling to other causes of neuronal death, these findings suggest that blockade of trophic input is a common step in neuronal death.

Two-step activation of FOXO3 by AMPK generates a coherent feed-forward loop determining excitotoxic cell fate

“…However, it is possible that FOXO3 dephosphorylation and its subsequent nuclear translocation is a requirement for subsequent post-translational modifications (that is, FOXO3 phosphorylation by AMPK). 17 Indeed, this is a (2) in (a)) reduced Bim expression following short-term pAMPK activity (40 min, gray bars). This network structure represents a CFL and, by preventing Bim expression in times of short-term stress, may allow pAMPK to exert its pro-survival effects.…”

“…Excellent candidates are the FOXO3 and AP-1 transcription factors, which have been implicated in Bim expression during neuronal apoptosis. 13,[16][17][18] We transfected CGNs with a reporter construct bearing the wild-type (wt) bim promoter sequence, or bim promoter constructs harboring mutations in the FOXO3 or AP-1 binding sites. Luciferase activity was significantly increased in GLUT-treated neurons expressing the wt bim promoter.…”

“…In addition to FOXO3 nuclear translocation, further post-translational modifications may be necessary to stimulate its transcriptional activity. 17,22 This hypothesis was tested by co-expressing the bim promoter plasmid with a FOXO3 construct, mutated at the AKT phosphorylation sites and therefore permanently localized to the nucleus ('FOXO3-nuclear' 21 ). GLUT/GLY (100/10 mM) exposure significantly increased luciferase activity in CGNs expressing 'FOXO3-nuclear' (Supplementary Figure 4A), suggesting that FOXO3 required further post-translational modifications to increase its binding to the bim promoter.…”

The role of IGF-1 in neurodegenerative diseases

“…IGF-1 and insulin act as opponents of these processes by activating both insulin and IGF-1 receptors located in the brain tissue. Their downstream signaling cascade leads to activation of PI3K/Akt pathway, which in turn inactivates FOXO3 and many other mediators participating in cell destruction 18 . Glycogen synthase kinase 3ß (GSK-3ß), caspase-3 and p38 mitogen-activated protein kinase (p38 MAPK) are just some of the examples of enzymes involved in apoptosis, cell death and energy impairment of the cell that are inhibited via insulin and IGF-1 signaling.…”

“…That had major consequences on the brain plasticity, prevention of neuronal apoptosis and delaying neurodegeneration processes 22 . Although IGF-1(and insulin) acts neuroprotective by mitigating oxidative damage, an abrupt rise in oxidative stress reduces IGF-1 signaling through Akt blockage and leads to neuronal death, suggesting that interference with trophic input is the key in understanding neurodegeneration processes 18 .…”