Interactions among nitric oxide and Bcl-family proteins after MPP+ exposure of SH-SY5Y neural cells II: exogenous NO replicates MPP+ actions

Differential Effects of Methyl-4-Phenylpyridinium Ion, Rotenone, and Paraquat on Differentiated SH-SY5Y Cells

Paraquat (PQ), a cationic nonselective bipyridyl herbicide, has been used as neurotoxicant to modulate Parkinson's disease in laboratory settings. Other compounds like rotenone (ROT), a pesticide, and 1-methyl-4-phenylpyridinium ion (MPP(+)) have been widely used as neurotoxicants. We compared the toxicity of these three neurotoxicants using differentiated dopaminergic SH-SY5Y human cells, aiming to elucidate their differential effects. PQ-induced neurotoxicity was shown to be concentration and time dependent, being mitochondrial dysfunction followed by neuronal death. On the other hand, cells exposure to MPP(+) induced mitochondrial dysfunction, but not cellular lyses. Meanwhile, ROT promoted both mitochondrial dysfunction and neuronal death, revealing a biphasic pattern. To further elucidate PQ neurotoxic mechanism, several protective agents were used. SH-SY5Y cells pretreatment with tiron (TIR) and 2-hydroxybenzoic acid sodium salt (NaSAL), both antioxidants, and N ω -nitro-L-arginine methyl ester hydrochloride (L-NAME), a nitric oxide synthase inhibitor, partially protected against PQ-induced cell injury. Additionally, 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenyl-propyl)piperazine (GBR 12909), a dopamine transporter inhibitor, and cycloheximide (CHX), a protein synthesis inhibitor, also partially protected against PQ-induced cell injury. In conclusion, we demonstrated that PQ, MPP(+), and ROT exerted differential toxic effects on dopaminergic cells. PQ neurotoxicity occurred through exacerbated oxidative stress, with involvement of uptake through the dopamine transporter and protein synthesis.

Antidepressants reveal differential effect against 1-methyl-4-phenylpyridinium toxicity in differentiated PC12 cells

Treatment of depression may ameliorate the cognitive disability and motor slowness in Parkinson's disease. It has been shown that antidepressants, including fluoxetine, may attenuate or exacerbate neuronal cell death. The present study assessed the effect of antidepressants (amitriptyline, tranylcypromine and fluoxetine) against the toxicity of 1-methyl-4-phenylpyridinium (MPP(+)) in relation to the mitochondria-mediated cell death process in differentiated PC12 cells. Amitriptyline and tranylcypromine attenuated the MPP(+)-induced cell death that may be associated with mitochondrial membrane permeability change and oxidative stress. Both compounds prevented the loss of the mitochondrial transmembrane potential, over-expression of Bax, reduction in Bcl-2 level, cytochrome c release, caspase-3 activation, formation of reactive oxygen species and depletion of GSH. The inhibitory effect of tranylcypromine was greater than that of amitriptyline on the basis of concentration. In contrast, fluoxetine revealed a toxic effect and exhibited an additive effect against the toxicity of MPP(+). Results show that amitriptyline and tranylcypromine may attenuate the MPP(+) toxicity by suppressing the mitochondrial membrane permeability change that leads to cytochrome c release and subsequent caspase-3 activation. The effects seem to be associated with the inhibitory action on the formation of reactive oxygen species and the depletion of GSH. In contrast, fluoxetine seems to exert an additive toxic effect against neuronal cell damage by increasing mitochondrial damage and oxidative stress.

Nitric oxide-induced transcriptional up-regulation of protective genes by Nrf2 via the antioxidant response element counteracts apoptosis of neuroblastoma cells

Nitric oxide (NO) is a signaling molecule that in excess causes cell death. Here we report a mechanism of NO-induced transcriptional up-regulation of genes encoding detoxifying enzymes and protective proteins and their role in counteracting NO-induced apoptosis of neuroblastoma cells. Promoter analysis using reporter assays identified the antioxidant response element (ARE) located in the promoter region of NAD(P)H:quinone oxidoreductase 1 (Nqo1) and other detoxifying enzyme genes as responsible for NO-mediated gene induction. The transcription factors NF-E2-related factor 2 (Nrf2) and small maf proteins were detected in NO-induced nuclear protein-ARE complexes. Nrf2 augmented NO-induced, ARE-dependent gene expression, which was blocked by dominant-negative Nrf2 (DN-Nrf2) lacking the transcriptional activation domain. Consistent with these results, Nrf2 was localized in the cytoplasm in unstimulated cells, and NO triggered its rapid nuclear accumulation. Neuroblastoma cells were stably transfected with DN-Nrf2, which repressed both the expression of protective genes and their induction by NO. These DN-Nrf2 cells exhibited reduced NQO1 enzymatic activity and were sensitized to NO-induced apoptosis. Similar results were obtained when Nrf2 expression was blocked by RNA interference. Conversely, stable cells expressing higher levels of Nrf2 protein had elevated NQO1 activity and were protected from NO. Finally, NO-mediated ARE-dependent gene induction occurred well before apoptosis as judged by caspase activation. These results together suggest that NO signals the transcriptional up-regulation of NQO1 and other detoxifying enzyme and protective genes through Nrf2 via the ARE to counteract NO-induced apoptosis of neuroblastoma cells.