Ion dependence of cytotoxicity of carmustine against PC12 cells

Carmustine-induced phosphatidylserine translocation in the erythrocyte membrane

The nitrosourea alkylating agent, carmustine, is used as chemotherapeutic drug in several malignancies. The substance triggers tumor cell apoptosis. Side effects of carmustine include myelotoxicity with anemia. At least in theory, anemia could partly be due to stimulation of eryptosis, the suicidal death of erythrocytes, characterized by cell shrinkage and breakdown of phosphatidylserine asymmetry of the cell membrane with phosphatidylserine exposure at the erythrocyte surface. Stimulators of eryptosis include increase of cytosolic Ca²⁺ activity ([Ca²⁺]i). The present study tested whether carmustine triggers eryptosis. To this end [Ca²⁺]i was estimated from Fluo3 fluorescence, cell volume from forward scatter, phosphatidylserine exposure from annexin V binding, and hemolysis from hemoglobin release. As a result a 48 h exposure to carmustine (≥25 µM) significantly increased [Ca²⁺]i, decreased forward scatter and increased annexin V binding. The effect on annexin V binding was significantly blunted in the absence of extracellular Ca²⁺. In conclusion, carmustine stimulates eryptosis at least partially by increasing cytosolic Ca²⁺ activity.

Schisandrin B as a hormetic agent for preventing age-related neurodegenerative diseases

Oxidative stress and mitochondrial dysfunction have been implicated in the pathogenesis of neurodegenerative diseases, with the latter preceding the appearance of clinical symptoms. The energy failure resulting from mitochondrial dysfunction further impedes brain function, which demands large amounts of energy. Schisandrin B (Sch B), an active ingredient isolated from Fructus Schisandrae, has been shown to afford generalized tissue protection against oxidative damage in various organs, including the brain, of experimental animals. Recent experimental findings have further demonstrated that Sch B can protect neuronal cells against oxidative challenge, presumably by functioning as a hormetic agent to sustain cellular redox homeostasis and mitoenergetic capacity in neuronal cells. The combined actions of Sch B offer a promising prospect for preventing or possibly delaying the onset of neurodegenerative diseases, as well as enhancing brain health.

(-)Schisandrin B ameliorates paraquat-induced oxidative stress by suppressing glutathione depletion and enhancing glutathione recovery in differentiated PC12 cells

Exposure to paraquat (PQ; N,N'-dimethyl-4-4'-bipyridium), a potent herbicide, can lead to neuronal cell death and increased risk of Parkinson's disease because of oxidative stress. In this study, we investigated the effect of (-)schisandrin B [(-)Sch B, a potent enantiomer of schisandrin B] on PQ-induced cell injury in differentiated pheochromocytoma cells (PC12). PQ treatment caused cell injury in PC12 cells, as indicated by the significant increase in lactate dehydrogenase (LDH) leakage. Pretreatment with (-)Sch B (5 μM) protected against PQ-induced toxicity in PC12 cells, as evidenced by the significant decrease in LDH leakage. (-)Sch B induced the cytochrome P-450-mediated reactive oxygen species generation in differentiated PC12 cells. The cytoprotection afforded by (-)Sch B pretreatment was associated with an increase in cellular reduced glutathione (GSH) level as well as the enhancement of γ-glutamylcysteine ligase (GCL) and glutathione reductase (GR) activity in PQ-challenged cells. Both GCL and GR inhibitors abrogated the cytoprotective effect of (-)Sch B in PQ-challenged cells. The biochemical mechanism underlying the GSH-enhancing effect of (-)Sch B was further investigated in PC12 cells subjected to an acute peroxide challenge. Although the initial GSH depletion induced by peroxide was reduced through GR-catalyzed regeneration of GSH in (-)Sch B-pretreated cells, the later enhanced GSH recovery was mainly mediated by GCL-catalyzed GSH synthesis. The results suggest that (-)Sch B treatment may increase the resistance of dopaminergic cells against PQ-induced oxidative stress through reducing the extent of oxidant-induced GSH depletion and enhancing the subsequent GSH recovery.

Lack of congruence between cysteine dioxygenase activity and S-carboxymethyl-l-cysteine S-oxidation activity in rat cytosol

The identity of the enzyme(s) responsible for the S-oxidation of the mucoactive drug S-carboxy-methyl-l-cysteine (SCMC) is unknown but the protein(s) are a susceptibility factor for a number of chronic degenerative diseases. The structural similarities between the amino acid l-cysteine and SCMC have raised the possibility that cysteine dioxygenase (CDO) may be responsible for this biotransformation reaction. Both CDO and SCMC S-oxygenase were found to require Fe2+ for enzymatic activity, and both enzyme activities were inhibited by Fe2+ and Fe3+ chelators. However, sulphydryl group modification of the enzymes resulted in the activation of the S-oxidation of SCMC but inhibition of the S-oxidation of l-cysteine. When the two enzyme activities were quantified in 20 female hepatic cytosolic fractions no linear correlation in the production of their respective metabolites was seen. The results of this investigation indicate that CDO is not responsible for the S-oxidation of SCMC in the rat.

Glutathione disulfide induces neural cell death via a 12-lipoxygenase pathway

Oxidized glutathione (GSSG) is commonly viewed as a byproduct of GSH metabolism. The pathophysiological significance of GSSG per se remains poorly understood. Adopting a microinjection approach to isolate GSSG elevation within the cell, this work identifies that GSSG can trigger neural HT4 cell death via a 12-lipoxygenase (12-Lox)-dependent mechanism. In vivo, stereotaxic injection of GSSG into the brain caused lesion in wild-type mice but less so in 12-Lox knockout mice. Microinjection of graded amounts identified 0.5 mM as the lethal [GSSG]i in resting cells. Interestingly, this threshold was shifted to the left by 20-fold (0.025 mM) in GSH-deficient cells. This is important because tissue GSH lowering is commonly noted in the context of several diseases as well as in aging. Inhibition of GSSG reductase by BCNU is known to result in GSSG accumulation and caused cell death in a 12-Lox-sensitive manner. GSSG S-glutathionylated purified 12-Lox as well as in a model of glutamate-induced HT4 cell death in vitro where V5-tagged 12-Lox was expressed in cells. Countering glutamate-induced 12-Lox S-glutathionylation by glutaredoxin-1 overexpression protected against cell death. Strategies directed at improving or arresting cellular GSSG clearance may be effective in minimizing oxidative stress-related tissue injury or potentiating the killing of tumor cells, respectively.

Zinc stimulates the production of toxic reactive oxygen species (ROS) and inhibits glutathione reductase in astrocytes

The release of zinc (Zn) from glutamatergic synapses contributes to the neuropathology of ischemia, traumatic brain injury, and stroke. Astrocytes surround glutamatergic synapses and are vulnerable to the toxicity of Zn, which impairs their antioxidative glutathione (GSH) system and elevates the production of reactive oxygen species (ROS). It is not known whether one or both of these actions are the primary cause of Zn-induced cell death in astrocytes. Using primary rat astrocyte cultures we have examined whether Zn-mediated impairment of GSH redox cycling is the main source of its toxicity. Zn acetate at concentrations of 100 microM or greater were found to inactivate glutathione reductase (GR) via an NADPH-dependent mechanism, while concentrations of 150 microM and above caused substantial cell death. Furthermore, Zn increased the ratio of GSSG:GSH in astrocytes, increased their export of GSSG, slowed their clearance of exogenous H2O2, and promoted the intracellular production of ROS. In contrast, the GR inhibitor, carmustine, did not induce cell death, cause the production of ROS, or alter the GSH thiol redox balance. Taken together these results indicate that Zn toxicity in astrocytes is primarily associated with the generation of intracellular ROS, rather than the inhibition of GR.

The glutathione reductase inhibitor carmustine induces an influx of Ca2+ in PC12 cells

We studied the effects of carmustine (1,3-bis(2-chloroethyl)-1-nitrosourea) on the intracellular Ca(2+) concentration ([Ca(2+)](i)) in PC12 cells using fura-2 fluorescence imaging. Carmustine (100 microM) caused a delayed increase in [Ca(2+)](i) that developed within approximately 3 h. This effect was enhanced in cells that were pretreated with an inhibitor of glutathione (GSH) synthesis, buthionine sulfoximine (BSO, 200 microM, 24 h), and was suppressed in cells that were treated with an antioxidant deferoxamine (50 microM). The carmustine-induced increase in [Ca(2+)](i) was absolutely dependent on the presence of extracellular Ca(2+) and could be inhibited by dihydropyridine blockers of L-type voltage-gated Ca(2+) channels (nimodipine or nitrendipine, 10 microM). The increase in [Ca(2+)](i) was also suppressed in Cl(-)-free solution and in the presence of the Cl(-) channel blockers, indanyloxyacetic acid 94 (IAA-94, 100 microM) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 100 microM). The inhibition was complete when the blockers were applied simultaneously with carmustine and was partial when the blockers were applied after the initial increase in [Ca(2+)](i). We conclude that carmustine induces an influx of extracellular Ca(2+) through L-type Ca(2+) channels and that this effect is mediated by oxidative stress that results from the depletion of GSH following the inhibition by carmustine of glutathione reductase.

Phenylalanine 4-Monooxygenase and the S-Oxidation of S-Carboxymethyl-L-cysteine

The identity of the enzyme responsible for the S-oxidation of the mucolytic S-substituted L-cysteine drug, S-carboxymethyl-L-cysteine (SCMC), has been actively investigated for the last 10 years. A genetic polymorphism exists in the oxidation of the thioether moiety that has been identified as a disease susceptibility factor in a number of degenerative diseases. This polymorphism has also been implicated in the wide variation in clinical response to SCMC therapy in man. To date little is known about the molecular enzymology of this reaction but a previous investigation revealed that rat activated phenylalanine 4-monooxygenase (PAH) could S-oxidise both Met- and S-methyl-L-cysteine (SMC) to their S-oxide metabolites. We have investigated the hypothesis that SCMC was also a substrate for activated PAH in the cytosolic faction of the Wistar rat. 1. Substrate and inhibitor investigation revealed that SCMC was a substrate for activated PAH activity in vitro. 2. The large aromatic amino acid hydroxylase monoclonal antibody and the Fe3+ chelator, deferoxamine, completely inhibited both Phe and SCMC oxidation to their respective metabolites. 3. Analysis of the Dixon plots revealed that both Phe and SCMC competitively inhibited each other's oxidation. 4. Correlation studies showed that the rate of production of Tyr was positively correlated to the production of both SCMC and SMC S-oxides in 20 female Wistar rat hepatic cytosolic fractions. These results strongly support the hypothesis that PAH is the enzyme responsible for SCMC S-oxidation in the rat.