Detection and measurement of drug-induced oxygen radical formation

Enhancement of DMNQ-induced hepatocyte toxicity by cytochrome P450 inhibition

Two mechanisms have been proposed to explain quinone cytotoxicity: oxidative stress via the redox cycle and the arylation of intracellular nucleophiles. As the redox cycle is catalyzed by NADPH cytochrome P450 reductase, cytochrome P450 systems are expected to be related to the cytotoxicity induced by redox-cycling quinones. Thus, we investigated the relationship between cytochrome P450 systems and quinone toxicity for rat primary hepatocytes using an arylator, 1,4-benzoquinone (BQ), and a redox cycler, 2,3-dimethoxy-1,4-naphthoquinone (DMNQ). The hepatocyte toxicity of both BQ and DMNQ increased in a time- and dose-dependent manner. Pretreatment with cytochrome P450 inhibitors, such as SKF-525A (SKF), ketoconazole and 2-methy-1,2-di-3-pyridyl-1-propanone, enhanced the hepatocyte toxicity induced by DMNQ but did not affect BQ-induced hepatocyte toxicity. The production of superoxide anion and the levels of glutathione disulfide and thiobarbituric-acid-reactive substances were increased by treatment with DMNQ, and SKF pretreatment further enhanced their increases. In addition, NADPH oxidation in microsomes was increased by treatment with DMNQ and further augmented by pretreatment with SKF, and a NADPH cytochrome P450 reductase inhibitor, diphenyleneiodonium chloride completely suppressed NADPH oxidations increased by treatment with either DMNQ- or DMNQ + SKF. Pretreatment with antioxidants, such as alpha-tocopherol, reduced glutathione, N-acetyl cysteine or an iron ion chelator deferoxamine, totally suppressed DMNQ- and DMNQ + SKF-induced hepatocyte toxicity. These results indicate that the hepatocyte toxicity of redox-cycling quinones is enhanced under cytochrome P450 inhibition, and that this enhancement is caused by the potentiation of oxidative stress.

Effect of cypermethrin on isolated male and female rat hepatocytes

Cypermethrin is a synthetic pyrethroid that belongs to a group of insecticides with low mammalian toxicity but high insecticidal activity. The present study was designed to investigate the toxicity of cypermethrin on freshly isolated hepatocytes from male and female rats. Hepatocytes were harvested by a collagenase perfusion technique and were exposed to different concentrations of cypermethrin (100, 200, 400, or 800 ng/2 x 10(6) cells) for up to 2 h. Cell viability and the leakage of aspartate transaminase (AST) and alanine transaminase (ALT) were determined throughout the incubation period. The cell viability of the hepatocytes from male and female rats exposed to 400 ng and 800 ng was significantly reduced after 60 and 30 min of incubation, respectively. With cells from female rats, viability was also reduced upon exposure to 200 ng cypermethrin for 2 h. The decrease in cell viability was dose and time dependent. The leakage of ALT and AST was significantly increased with 400 and 800 ng concentrations at 60 and 30 min, respectively. ALT leakage from female hepatocytes was significantly increased at 60 min of incubation with the 200-ng dose, whereas 2 h of incubation was required for the leakage of ALT from the cells of male rats. The present data indicate that cypermethrin has toxic effects on male and female rat hepatocytes in a dose- and time-dependent manner. The data suggest that female rat hepatocytes may be more sensitive to the toxic effects of cypermethrin than male cells.

Reactive oxygen species generation by human spermatozoa is induced by exogenous NADPH and inhibited by the flavoprotein inhibitors diphenylene iodonium and quinacrine

Human spermatozoa possess a specialized capacity to generate reactive oxygen species (ROS) that is thought to be of significance in the redox regulation of sperm capacitation (De Lamirande and Gagnon, 1993; Aitken et al., 1995). However, the mechanisms by which ROS are generated by these cells are not understood. In this study we have examined the possible significance of NADPH as a substrate for ROS production by human spermatozoa. Addition of NADPH to viable populations of motile spermatozoa induced a sudden dose-dependent increase in the rate of superoxide generation via mechanisms that could not be disrupted by inhibitors of the mitochondrial electron transport chain (antimycin A, rotenone, carbonyl cyanide m-chlorophenylhydrazone [CCCP], and sodium azide), diaphorase (dicoumarol) xanthine oxidase (allopurinol), or lactic acid dehydrogenase (sodium oxamate). However, NADPH-induced ROS generation could be stimulated by permeabilization and was negatively correlated with sperm function. Both NADH and NADPH were active electron donors in this system, while NAD+ and NADP+ exhibited little activity. Stereo-specificity was evident in the response in that only the beta-isomer of NADPH supported superoxide production. The involvement of a flavoprotein in the electron transfer process was indicated by the high sensitivity of the oxidase to inhibition by diphenylene iodonium and quinacrine. These results indicate that NAD(P)H can serve as an electron donor for superoxide generation by human spermatozoa and present a simple strategy for the production of motile populations of free radical generating cells with which to study the significance of these molecules in the control of normal and pathological sperm function.

Antitumor effects of human recombinant interleukin-1 alpha and etoposide against human tumor cells: mechanism for synergism in vitro and activity in vivo

Recombinant human interleukin 1 alpha (rh IL-1 alpha) and etoposide (VP-16) synergize for direct growth inhibition of several human tumor cell lines in vitro. Our previous studies demonstrated that VP-16 increased the number of membrane-associated IL-1 receptors (IL-1Rs) and also enhanced the internalization of receptor-bound rh IL-1 alpha. The purposes of this study were to test our hypotheses that these events were critical to the synergy between rhIL-1 alpha and VP-16, to determine whether rhIL- 1 alpha and VP-16 synergize to increase superoxide (SO) anion radical production in vitro since SO anion has been implicated in the toxic effects of IL-1, and to investigate the antitumor efficacy of the combination against tumors in vivo. A375/C6 melanoma cells and OVCAR-3 ovarian carcinoma cells were tested with IL-1 receptor antagonist (IL-1 ra) before exposure to rhIL-1 alpha, VP-16 and rhIL-1 alpha plus VP-16. The synergistic or antagonistic effects were assessed by MTT assay. SO production was measured by reduction of cytochrome C. Athymic female mice bearing the A375/C6 melanoma were treated by rhIL-1 alpha, VP-16, and rhIL- 1 alpha+VP-16. The antitumor effects were evaluated by quantitating tumor growth and survival time. Pretreatment with the IL-1ra abrogated the synergistic effects of rhIL-1 alpha and VP-16. The production of SO radical by A375/C6 cells was increased 2.5 fold by the combination of rhIL-1 alpha and VP-16, and the addition of exogenous SOD blocked the synergy between rhIL-1 alpha and VP-16. However, when A375/SOD15 cells which over-expressed manganese superoxide dismutase (MnSOD) after MnSOD cDNA transfection were exposed to rhIL-1 alpha and VP-16, in vitro antagonism was observed. In vivo studies demonstrated that the combination of rhIL-1 alpha and VP-16 delayed tumor growth better than either agent alone, although long-term survival was not improved because of substantial toxicity. Our results suggest that the synergistic antitumor effects of IL-1 alpha and VP-16 may be due to IL-1R modulation and increased internalization of IL-1-IL-1R complex by VP-16 treatment, as well as to a subsequent increase in SO anion radical production from the tumor cells exposed to both drugs. Thus, the combination of IL-1 alpha and VP-16 might prove useful for the treatment of malignant disease in vivo, if the increased toxicity can be reduced or managed.

Diquat-induced oxidative damage in hepatic microsomes: effects of antioxidants

The ability of the redox cycling compound, diquat, to induce lipid peroxidation and oxidative damage was investigated using hepatic microsomes. Antioxidants, with demonstrated efficacy in physical models of oxidative stress, were examined in a diquat model. Diquat (10 microM-3 mM) induced lipid peroxidation (TBARS) in hepatic microsomes prepared from Fischer 344 rats. Diquat (1 mM) also increased protein carbonyl formation, NADPH oxidation and superoxide anion radical production (acetylated cytochrome c reduction). The novel antioxidants U-74,006F, U-78,517G and the known antioxidant, DPPD, decreased diquat-induced lipid peroxidation to levels below that of the control. These antioxidants also decreased protein carbonyl formation caused by diquat. U-74,006F and U-78,517G reduced NADPH oxidation slightly; although this inhibition was statistically significant, the biological significance is questionable. DPPD had no effect on this parameter. U-78,517G inhibited the reduction of acetylated cytochrome c slightly, whereas the other antioxidants had little effect. Thus overall, the increase in NADPH oxidation and the production of superoxide anion by redox cycling of diquat were not substantially affected by antioxidants. Neither did the test compounds show evidence of activity as iron chelators. This leads to the suggestion that antioxidants are preventing diquat-induced oxidative damage by scavenging lipid peroxyl radicals and preventing the propagation of the lipid peroxidation process.

Studies on the anti-inflammatory activity of ebselen. Ebselen interferes with granulocyte oxidative burst by dual inhibition of NADPH oxidase and protein kinase C?

Ebselen (PZ51, 2-phenyl-1,2-benzoisoselenazol-3-(2H)-one) was shown to be an inhibitor of human granulocyte oxidative burst stimulated by phorbol myristate acetate (IC50 25 microM). Estimation of the primary oxygen metabolites of the burst was complicated by the redox chemistry of Ebselen. Ebselen inhibited NADPH-stimulated superoxide generation by a partially purified NADPH oxidase preparation with an IC50 of 0.5-1.0 microM. Ebselen was also shown to inhibit the activity of partially purified Ca2+- and phospholipid-dependent protein kinase C (IC50 ca. 0.5 microM). Phorbol ester-stimulated phosphorylation of protein in intact cells was inhibited by Ebselen (IC50 ca. 50 microM). These pharmacodynamic properties of Ebselen are discussed in terms of its anti-inflammatory activity in vivo. The findings are also discussed in terms of Ebselen's known ability to interact with sulfhydryl components of cells, particularly critical thiol components of the enzymes studied.

Role of redox cycling and lipid peroxidation in bipyridyl herbicide cytotoxicity. Studies with a compromised isolated hepatocyte model system

The role of active oxygen species and lipid peroxidation in the toxic effects of diquat, paraquat and other bipyridyl herbicides remains controversial. In vitro studies have shown that these compounds are potent generators of active oxygen species by redox cycling and that they stimulate lipid peroxidation. In vivo studies have failed, however, to show clear evidence of lipid peroxidation resulting from toxic exposures to these compounds. We have directly compared the abilities of three bipyridyl herbicides, diquat (DQ), paraquat (PQ) and benzyl viologen (BV), to generate superoxide anion radical (O2-.) in rat liver microsomes and H2O2 in hepatocytes and correlated this with their relative toxicities to a compromised isolated hepatocyte system. DQ was the most potent generator of O2-. and H2O2, being slightly more potent than BV and much better than PQ. This ability of the bipyridyls to generate active oxygen was positively correlated with the ability to induce toxicity in hepatocytes pretreated with 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) to inhibit their glutathione reductase activity, i.e. DQ greater than BV greater than PQ. DQ caused a rapid depletion of cellular GSH and a concomitant increase in GSSG in this system. Toxicity, measured as loss of plasma membrane integrity, was pronounced after only 30-60 min of incubation and was accompanied by a significant increase in lipid peroxidation. The onset of lipid peroxidation could not be separated temporally from the expression of toxicity. However, the total inhibition of lipid peroxidation by the antioxidants Trolox C, promethazine and N,N'-diphenyl-p-phenylenediamine only delayed toxicity, indicating that, even though lipid peroxidation may play some role in enhancing bipyridyl herbicide toxicity, it is not essential for the toxicity to manifest itself.

Comparative studies on the mechanisms of paraquat and 1-methyl-4-phenylpyridine (MPP+) cytotoxicity

1-methyl-4-phenylpyridine (MPP+) is the putative toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and is structurally similar to the herbicide paraquat (PQ++). We have therefore compared the effects of MPP+ and PQ++ on a well characterized experimental model, namely isolated rat hepatocytes. PQ++ generates reactive oxygen species within cells by redox cycling and its toxicity to hepatocytes was potentiated by pretreatment with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of glutathione reductase. In BCNU-treated cells, PQ++ caused GSH depletion, lipid peroxidation and cell death. These cytotoxic effects were prevented by the antioxidant N,N'-diphenyl-p-phenylenediamine (DPPD) and the iron-chelating agent desferrioxamine. MPP+ also caused GSH depletion in BCNU-treated hepatocytes but its cytotoxicity was not markedly affected by BCNU, nor was it accompanied by significant lipid peroxidation. DPPD and desferrioxamine also failed to prevent MPP+-induced cell death. We conclude that the production of active oxygen species is likely to play a major role in PQ++ cytotoxicity, while MPP+-induced cell damage may involve additional, more important toxic mechanisms.

Mutagenicity of quinones: pathways of metabolic activation and detoxification

The mutagenicity of various quinones, a class of compounds widely distributed in nature, is demonstrated in the Salmonella TA104 tester strain. The metabolic pathways by which four quinones, menadione, benzo[a]pyrene 3,6-quinone, 9,10-phenanthrenequinone, and danthron, caused mutagenicity in this test system were investigated in detail as were the detoxification pathways. The two-electron reduction of these quinones by NAD(P)H-quinone oxidoreductase (DT-diaphorase) was not mutagenic, whereas the one-electron reduction, catalyzed by NADPH-cytochrome P-450 reductase, was mutagenic, except for danthron, which was only slightly mutagenic. The mutagenicity of the quinones via this pathway was found to be attributable to the generation of oxygen radicals. The cytochrome P-450 monooxygenase also played a significant role in the detoxification and bioactivation of these quinones. For example, phenanthrenequinone was converted to a nonmutagenic metabolite in a cytochrome P-450-dependent reaction, whereas danthron was converted to a highly mutagenic metabolite. These studies show the complexity of metabolic pathways involved in the mutagenicity of quinones.