N-acetylcysteine prevents β-amyloid toxicity by a stimulatory effect on p35/cyclin-dependent kinase 5 activity in cultured cortical neurons

Although previous studies have indicated that the neuroprotective effect of N-acetylcysteine (NAC) required activation of the Ras-extracellular-signal-regulated kinase (ERK) pathway, the detailed mechanisms and signal cascades leading to activation ERK are not clear. In the present study, we investigated the effect of NAC on Aβ_25-35-induced neuronal death. Pretreatment of neurons with NAC 1 hr before application of Aβ prevented Aβ-mediated cell death. NAC increased cyclin-dependent kinase 5 (Cdk5) phosphorylation, an effect that was blocked by Cdk5 inhibitor. The neuroprotective effect of NAC was significantly attenuated by Cdk5 inhibitors or in neurons transfected with Cdk5 or p35 small interfering RNA (siRNA). Conversely, pretreatment of neurons with the calpain inhibitors calpeptin or MDL28170 enhanced the neuroprotective effect of NAC. Aβ_25-35 caused a significant decrease in the level of p35, with a concomitant increase in p25, which was completely prevented by NAC. This effect of NAC was blocked by the Cdk5 inhibitors roscovitine and butyrolactone. In addition, NAC increased Cdk5/p35 kinase activity but reduced Cdk5 kinase activity. Aβ _25-35 treatment decreased phosphorylated levels of ERK, which could be reversed by NAC. The effect of NAC was completely blocked by Cdk5 inhibitors. NAC reversed the Ab25-35-induced decrease in the expression of Bcl-2, which could be blocked by the MAPK kinase (MEK) inhibitor or Cdk5 inhibitors. These results suggest that NAC-mediated neuroprotection against Aβ toxicity is likely mediated by the p35/ Cdk5-ERKs-Bcl-2 signal pathway.