The widely used
cholesterol-lowering drugs,
statins, were reported to reduce the incidence of
stroke and the progression of
Alzheimer's disease. However, little is known on how
statins exert these beneficial effects. In this study, we investigated the molecular mechanisms underlying the neuroprotective actions of
statins in primary cultured cortical neurons. We found that chronic treatment of neurons with a low dosage of two CNS-permeable
statins (
lovastatin and
simvastatin) selectively reduced
NMDA-induced cell death but not the
caspase-mediated apoptosis. The protective effects of stains were inhibited by
mevalonate, a PI3K inhibitor, and
tyrphostin AG538, suggesting roles for
cholesterol and
insulin/IGF-1 signaling in the neurotoxic response. We further demonstrate that
statins block
calcium-dependent
calpain activation, resulting in complete suppression of
protein truncation events on multiple
calpain substrates that are involved in neuronal death including CDK5 coactivator p35 cleavage to p25, GSK3 and
beta-catenin. This is followed by reduced and increased nuclear translocation of p25 and
beta-catenin, respectively. Under excitotoxic conditions, the activities of CDK5 and
beta-catenin are exclusively regulated by
calpain-mediated cleavage while apoptosis modulates
beta-catenin mainly through phosphorylation. Strikingly, our data demonstrate that the
calpain-blocking effect of
statins is largely mediated by stimulation of
alpha-secretase cleavage of APP, resulting in increased secretion of its soluble form, sAPP. Finally, our data suggest that
statin-regulated sAPP secretion occurs via activation of the PI3K pathway and inhibition of ROCK signaling. Altogether, our study provides novel insights into
statin-mediated neuronal excitoprotection through both
cholesterol-dependent and -independent mechanisms and links them to
calpain-mediated neuronal death.