Glutaredoxin protects cerebellar granule neurons from dopamine-induced apoptosis by dual activation of the ras-phosphoinositide 3-kinase and jun n-terminal kinase pathways

Glutaredoxin 2 (Grx2) from Escherichia coli protects cerebellar neurons from dopamine-induced apoptosis via nuclear factor kappa B (NF-kappaB) activation, which is mediated by the expression of redox factor-1 (Ref-1). An analysis of the mechanisms underlying Grx2 protective activity revealed dual activation of signal transduction pathways. Grx2 significantly activated the Ras/phosphoinositide 3-kinase/Akt/NF-kappaB cascade in parallel to the Jun N-terminal kinase (JNK)/AP1 cascade. Dopamine, in comparison, down-regulated both pathways. Treatment of neurons with Ref-1 antisense oligonucleotide reduced the ability of Grx2 to activate Akt and AP-1 but had no effect on the phosphorylation of JNK1/2, suggesting that Akt/NF-kappaB and AP-1 are regulated by Ref-1. Exposure of the neurons to JNK1/2 antisense oligonucleotide in the presence of Grx2 significantly reduced AP-1 and NF-kappaB DNA binding activities and abolished Grx2 protection. These results demonstrate that dual activation of Ras/phosphoinositide 3-kinase and AP-1 cascades, which are mediated by Ref-1, is an essential component of the Grx2 mechanism of action.

The molecular mechanism of dopamine-induced apoptosis: identification and characterization of genes that mediate dopamine toxicity

Parkinson's disease (PD) is a progressive neurological disorder caused by rather selective degeneration of the dopaminergic (DA) neurons in the substantia nigra. Though subject to intensive research, the etiology of this nigral neuronal loss is still enigmatic and treatment is basically symptomatic. The current major hypothesis suggests that nigral neuronal death in PD is due to excessive oxidative stress generated by auto- and enzymatic oxidation of the endogenous neurotransmitter dopamine (DA), the formation of neuromelanin and presence of high concentrations of iron. We have found that DA toxicity is mediated through its oxidative metabolites. Whereas thiol-containing antioxidants provided marked protection against DA toxicity, ascorbic acid accelerated DA-induced death. Using the differential display approach, we sought to isolate and characterize genes whose expression is altered in response to DA toxicity. We found an upregulation of the collapsin response mediator protein (CRM) and TCP-1delta in sympathetic neurons, which undergo dopamine-induced apoptosis. The isolation of these genes led us to examine the expression and activity of CRM and TCP-1delta related genes. Indeed, we found a significant induction of mRNAs of the secreted collapsin-1 and the mitochondrial stress protein HSP60. Antibodies directed against collapsin-1 provided marked and prolonged protection of several neuronal cell types from dopamine-induced apoptosis. In a parallel study, using antisense technology, we found that inhibition of TCP-1delta expression significantly reduced DA-induced neuronal death. These findings suggest a functional role for collapsin-1 and TCP-1delta as positive mediators of DA-induced neuronal apoptosis.

Activation of nuclear transcription factor kappa B (NF-kappaB) is essential for dopamine-induced apoptosis in PC12 cells

The etiology of Parkinson's disease is still unknown, though current investigations support the notion of the pivotal involvement of oxidative stress in the process of neurodegeneration in the substantia nigra (SN). In the present study, we investigated the molecular mechanisms underlying cellular response to a challenge by dopamine, one of the local oxidative stressors in the SN. Based on studies showing that nuclear factor kappa B (NF-kappaB) is activated by oxidative stress, we studied the involvement of NF-kappaB in the toxicity of PC12 cells following dopamine exposure. We found that dopamine (0.1-0.5 m M) treatment increased the phosphorylation of the IkappaB protein, the inhibitory subunit of NF-kappaB in the cytoplasm. Immunoblot analysis demonstrated the presence of NF-kappaB-p65 protein in the nuclear fraction and its disappearance from the cytoplasmic fraction after 2 h of dopamine exposure. Dopamine-induced NF-kappaB activation was also evidenced by electromobility shift assay using radioactive labeled NF-kappaB consensus DNA sequence. Cell-permeable NF-kappaB inhibitor SN-50 rescued the cells from dopamine-induced apoptosis and showed the importance of NF-kappaB activation to the induction of apoptosis. Furthermore, flow cytometry assay demonstrated a higher level of translocated NF-kappaB-p65 in the apoptotic nuclei than in the unaffected nuclei. In conclusion, our findings suggest that NF-kappaB activation is essential to dopamine-induced apoptosis in PC12 cells and it may be involved in nigral neurodegeneration in patients with Parkinson's disease.

Increased oxidative stress in ataxia telangiectasia evidenced by alterations in redox state of brains from Atm-deficient mice

Ataxia-telangiectasia (A-T) is a genetic disorder caused by mutational inactivation of the ATM gene. A-T patients display a pleiotropic phenotype and suffer primarily from progressive ataxia caused by degeneration of cerebellar Purkinje and granule neurons. Disruption of the mouse Atm locus creates a murine model of A-T that exhibits most of the clinical features of the human disease. We previously hypothesized that some aspects of A-T, such as the preferential loss of certain neurons, could result from a continuous state of increased oxidative stress (G. Rotman and Y. Shiloh, Cancer Surv., 29: 285-304, 1997; G. Rotman and Y. Shiloh, BioEssays, 19: 911-917, 1997). The present work tests this hypothesis by analyzing markers of redox state in brains of Atm-deficient mice. We found alterations in the levels of thiol-containing compounds in Atm (-/-) brains, as well as significant changes in the activities of thioredoxin, catalase, and manganese superoxide dismutase in Atm (-/-) cerebella. These changes are indicative of increased levels of reactive oxygen species, which are seen primarily in the cerebellum of Atm-deficient mice. Our findings support the hypothesis that the absence of functional ATM results in oxidative stress, which may be an important cause of the degeneration of cerebellar neurons in A-T.

Glutaredoxin protects cerebellar granule neurons from dopamine-induced apoptosis by activating NF-kappa B via Ref-1

The neurotransmitter dopamine (DA) induces apoptosis via its oxidative metabolites. This study shows that glutaredoxin 2 (Grx2) from Escherichia coli and human glutaredoxin could protect cerebellar granule neurons from DA-induced apoptosis. E. coli Grx2, which catalyzes glutathione-disulfide oxidoreduction via its -Cys-Pro-Tyr-Cys- active site, penetrates into cerebellar granule neurons and exerts its activity via NF-kappaB activation. Analysis of single and double cysteine to serine substitutions in the active site of Grx2 showed that both cysteine residues were essential for activity. Although DA significantly reduced NF-kappaB binding activity, Grx2 could stimulate the binding of NF-kappaB to DNA by: (i) translocating NF-kappaB from the cytoplasm to the nucleus after promoting the phosphorylation and degradation of I-kappaBalpha, and (ii) activating the binding of pre existing nuclear NF-kappaB. The DNA binding activity of NF-kappaB itself was essential for neuronal survival. Overexpression of I-kappaB dominant negative gene (I-kappaB-DeltaN) in granule neurons significantly reduced their viability, irrespective of the presence of Grx2. Ref-1 expression was down-regulated by DA but up-regulated by Grx2, while treatment of neurons with Ref-1 antisense oligonucleotide reduced the ability of Grx2 to activate NF-kappaB binding activity. These results show that Grx2 exerts its anti apoptotic activity through the activation of Ref-1, which then activates NF-kappaB.

The involvement of p53 in dopamine-induced apoptosis of cerebellar granule neurons and leukemic cells overexpressing p53

1. The pathogenesis of the selective degeneration of the dopaminergic neurons in Parkinson's disease is still enigmatic. Recently we have shown that dopamine can induce apoptosis in postmitotic neuronal cells, as well as in other cellular systems, thus suggesting a role for this endogenous neurotransmitter and associated oxidative stress in the neuronal death process. 2. Dopamine has been shown to be capable of inducing DNA damage through its oxidative metabolites. p53 is a transcription factor that has a major role in determining cell fate in response to DNA damage. We therefore examined the possible correlation between dopamine-triggered apoptosis, DNA damage and levels of total phosphorylated p53 protein in cultured mouse cerebellar granule neurons. 3. Marked DNA damage and apoptotic nuclear condensation and fragmentation were detected within several hours of exposure to dopamine. An associated marked threefold increase in p53 phosphorylation was observed within this time window. Using a temperature-sensitive p53 activation system in leukemia LTR6 cells, were found that p53 inactivation dramatically attenuated dopamine toxicity. 4. We therefore conclude that DNA damage and p53 activation may have a role in mediating dopamine-induced apoptosis. Modulation of the p53 system may therefore have a protective role against the toxicity of this endogenous neurotransmitter and associated oxidative stress.

Biochemical and temporal analysis of events associated with apoptosis induced by lowering the extracellular potassium concentration in mouse cerebellar granule neurons

We analyzed biochemically and temporally the molecular events that occur in the programmed cell death of mouse cerebellar granule neurons deprived of high potassium levels. An hour after switching the neurons to a low extracellular K+ concentration ([K+]o), a significant part of the genomic DNA was already cleaved to high-molecular-weight fragments. This phenomenon was intensified with the progression of the death process. Addition of cycloheximide to the neurons 4 h after high [K+]o deprivation resulted in no cell loss and complete recovery of the damaged DNA. DNA margination and nuclear fragmentation as assessed by 4,6-diaminodiphenyl-2-phenylindole staining were observable in a few cells beginning approximately 4 h after the removal of high [K+]o and developed to nuclear condensation 4 h later. Six hours after high [K+]o deprivation, the DNA was fragmented into oligonucleosome-sized fragments. Within 6 h after removal of the extracellular K+, 50% of the neurons were committed to die and lost their ability to be rescued by readministration of 25 mM [K+]o. Similar to high [K+]o deprivation, inhibition of RNA or protein synthesis failed to halt neuronal degeneration of a similar percentage of cells 6 h after the onset of the death process. Mitochondrial function steadily decreased after [K+]o removal. An approximately 40% decrease in RNA and protein synthesis was detected by 6 h of [K+]o removal during the period of cell death commitment; rates continued to decline gradually thereafter. The temporal characteristics of the DNA damage and recovery, DNA cleavage to oligonucleosome-sized fragments, and the reduction in mitochondrial activity-events that occurred within the critical time-may indicate that these processes have an important part in the mechanism that committed the neurons to die.