Cellular antioxidant adaptive survival response to 6-hydroxydopamine-induced nitrosative cell death in C6 glioma cells

Parkinson's disease (PD) is a progressive neurodegenerative movement disorder characterized by selective loss of dopaminergic neurons in the substantia nigra. 6-Hydroxydopamine (6-OHDA) is a catecholaminergic neurotoxin widely used to produce experimental models of PD and has been reported to cause oxidative and/or nitrosative stress. In this study, we have investigated 6-OHDA-induced nitrosative cell death and its self-defense mechanism in C6 glioma cells. Treatment of C6 cells with 6-OHDA increased expression of inducible nitric oxide synthase (iNOS) and subsequent production of nitric oxide (NO). Furthermore 6-OHDA treatment led to peroxynitrite generation and nitrotyrosine formation. 6-OHDA-induced nitrosative stress ultimately caused apoptotic cell death as determined by decreased Bcl-2/Bax ratio, activation of c-Jun N-termianl kinase (JNK), and cleavage of caspase-3 and poly(ADP-ribose)polymerase (PARP), which were attenuated by peroxynitrite decomposition catalyst, 5,10,15,20-tetrakis(4-sulfonatophenyl)prophyrinato iron(III) (FeTPPS). In another experiment, exposure of C6 glioma cells to 6-OHDA resulted in an increased expression of heme oxygenase-1 (HO-1) and 6-OHDA-induced cytotoxicity was effectively suppressed by the HO-1 inducer SnCl ₂ and aggravated by HO-1 inhibitor zinc protoporphyrin (ZnPP), supporting the cytoprotective role of HO-1. To elucidate the molecular mechanism underlying 6-OHDA-mediated HO-1 induction, we have examined the possible involvement of NF-E2-related factor 2 (Nrf2), which plays an important role in the transcriptional regulation of phase II detoxifying and antioxidant enzymes. 6-OHDA treatment increased nuclear translocation and transcriptional activity of Nrf2, which seemed to be partly mediated by activation of upstream kinases such as Akt/protein kinase B (PKB). Taken together these findings suggest that HO-1 up-regulation via Nrf2 activation may mediate the cellular adaptive survival response to 6-OHDA-induced nitrosative cell death in C6 glioma cells.