Inhibition of glycogen synthase kinase-3 reduces l-DOPA-induced neurotoxicity

The neurotoxicity of l-3,4-dihydroxyphenylalanine (l-DOPA), used for the treatment of Parkinson's disease, remains controversial. Although there are many reports suggesting that long-term treatment of l-DOPA causes neuronal death, an increasing body of recent evidence has proposed that l-DOPA might be neuroprotective rather than neurotoxic. We investigated the effect of l-DOPA on neuronally differentiated PC12 (nPC12) cells by treating cells with various concentrations of l-DOPA for 24 h. We also studied whether glycogen synthase kinase (GSK)-3 activation is related to l-DOPA-induced neurotoxicity by simultaneously treating cells with several concentrations of l-DOPA and a GSK-3 inhibitor for 24 h. MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay, trypan blue staining, cell counting kit-8, and DAPI staining all showed that l-DOPA decreased nPC12 cell viability at high concentrations. In addition, 100 μM l-DOPA treatment significantly increased the activity of GSK-3 and death signals including cytochrome c, activated caspase-3 and cleaved PARP, and decreased survival signals including heat shock transcription factor-1 in a concentration-dependent manner. Treatment with GSK-3 inhibitor VIII or lithium chloride prevented l-DOPA-induced cell death. Together, these results suggest that l-DOPA induces neuronal cell death at high concentrations and that the neurotoxic effect of l-DOPA might be mediated in part by GSK-3 activation.