N-Nitrosofenfluramine induces cytotoxicity via mitochondrial dysfunction and oxidative stress in isolated rat hepatocytes

Effects of N-acetyl-L-cysteine on target sites of hydroxylated fullerene-induced cytotoxicity in isolated rat hepatocytes

The effects of N-acetyl-L-cysteine (NAC) on cytotoxicity caused by a hydroxylated fullerene [C60(OH)24], which is known a nanomaterial and/or a water-soluble fullerene derivative, were studied in freshly isolated rat hepatocytes. The exposure of hepatocytes to C60(OH)24 at a concentration of 0.1 mM caused time (0-3 h)-dependent cell death accompanied by the formation of cell blebs, loss of cellular ATP, and reduced glutathione (GSH) and protein thiol levels, as well as the accumulation of glutathione disulfide and malondialdehyde (MDA), indicating lipid peroxidation. Despite this, C60(OH)24-induced cytotoxicity was effectively prevented by NAC pretreatment ranging in concentrations from 1 to 5 mM. Further, the loss of mitochondrial membrane potential (MMP) and generation of oxygen radical species in hepatocytes incubated with C60(OH)24 were inhibited by pretreatment with NAC, which caused increases in cellular and/or mitochondrial levels of GSH, accompanied by increased levels of cysteine via enzymatic deacetylation of NAC. On the other hand, severe depletion of cellular GSH levels caused by diethyl maleate at a concentration of 1.25 mM led to the enhancement of C60(OH)24-induced cell death accompanied by a rapid loss of ATP. Taken collectively, these results indicate that pretreatment with NAC ameliorates (a) mitochondrial dysfunction linked to the depletion of ATP, MMP, and mitochondrial GSH level and (b) induction of oxidative stress assessed by reactive oxygen species generation, losses of intracellular GSH and protein thiol levels, and MDA formation caused by C60(OH)24, suggesting that the onset of toxic effects is at least partially attributable to a thiol redox-state imbalance as well as mitochondrial dysfunction related to oxidative phosphorylation.

Mitochondrial and liver oxidative stress alterations induced by N-butyl-N-(4-hydroxybutyl)nitrosamine: relevance for hepatotoxicity

The most significant toxicological effect of nitrosamines like N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) is their carcinogenic activity, which may result from exposure to a single large dose or from chronic exposure to relatively small doses. However, its effects on mitochondrial liver bioenergetics were never investigated. Liver is the principal organ responsible for BBN metabolic activation, and mitochondria have a central function in cellular energy production, participating in multiple metabolic pathways. Therefore any negative effect on mitochondrial function may affect cell viability. In the present work, ICR male mice were given 0.05% of BBN in drinking water for a period of 12 weeks and were sacrificed one week later. Mitochondrial physiology was characterized in BBN- and control-treated mice. Transmembrane electric potential developed by mitochondria was significantly affected when pyruvate-malate was used, with an increase in state 4 respiration observed for pyruvate-malate (46%) and succinate (38%). A decrease in the contents of one subunit of mitochondrial complex I and in one subunit of mitochondrial complex IV was also observed. In addition, the activity of both complexes I and II was also decreased by BBN treatment. The treatment with BBN increases the susceptibility of liver mitochondria to the opening of the mitochondrial permeability transition pore. This susceptibility could be related with the increase in the production of H2 O2 by mitochondria and increased oxidative stress confirmed by augmented susceptibility to lipid peroxidation. These results lead to the conclusion that hepatic mitochondria are one primary target for BBN toxic action during liver metabolism.

Cytotoxic effects of hydroxylated fullerenes on isolated rat hepatocytes via mitochondrial dysfunction

The cytotoxic effects of hydroxylated fullerenes, also termed fullerenols or fullerols [C(60)(OH)( n )], which are known nanomaterials and water-soluble fullerene derivatives, were studied in freshly isolated rat hepatocytes. The exposure of hepatocytes to C(60)(OH)(24) caused not only concentration (0-0.25 mM)- and time (0-3 h)-dependent cell death accompanied by the formation of cell blebs, loss of cellular ATP, reduced glutathione (GSH), and protein thiol levels, but also the accumulation of glutathione disulfide and malondialdehyde, indicating lipid peroxidation. Of the other analogues examined, the cytotoxic effects of C(60)(OH)(12) and fullerene C(60) at a concentration of 0.125 mM were less than those of C(60)(OH)(24). The loss of mitochondrial membrane potential and generation of oxygen radical species in hepatocytes incubated with C(60)(OH)(24) were greater than those with C(60)(OH)(12) and fullerene C(60). In the oxygen consumption of mitochondria isolated from rat liver, the ratios of state-3/state-4 respiration were more markedly decreased by C(60)(OH)(24) and C(60)(OH)(12) compared with C(60). In addition, C(60)(OH)(24) and C(60)(OH)(12) resulted in the induction of the mitochondrial permeability transition (MPT), and the effects of C(60)(OH)(12) were less than those of C(60)(OH)(24). Taken collectively, these results indicate that (a) mitochondria are target organelles for fullerenols, which elicit cytotoxicity through mitochondrial failure related to the induction of the MPT, mitochondrial depolarization, and inhibition of ATP synthesis in the early stage and subsequently oxidation of GSH and protein thiols, and lipid peroxidation through oxidative stress at a later stage; and (b) the toxic effects of fullerenols may depend on the number of hydroxyl groups participating in fullerene in rat hepatocytes.

Detection of sildenafil analogues in herbal products for erectile dysfunction

Sildenafil, the active ingredient in Viagra (Pfizer), is a prescription medicine used for erectile dysfunction. Compounds with chemical structures similar to that of sildenafil were isolated and purified during the analysis of some herbal products marketed for treatment of erectile dysfunction. Structural elucidation using liquid chromatography-diode array detection, infrared spectroscopy, liquid chromatography-tandem mass spectrometry, and nuclear magnetic resonance spectroscopy confirmed that the compounds were homosildenafil, hydroxyhomosildenafil, and acetildenafil. The implications of adulteration by compounds structurally related to prescription drugs are discussed. Unlike established drugs, the efficacy and safety of such analogues are largely unknown. This poses a great challenge for safety and health administrators to detect these modified structures and to regulate them. Consumers who use such adulterated products are at risk of developing serious adverse reactions, potentially leading to death. Greater collaboration and exchange of information between various health authorities, health professionals, academics, researchers, and industry, as well as public education, are key steps in the efforts to stem the growing trend of adulteration of herbal products by analogues of prescription drugs.

ATP-generating glycolytic substrates prevent N-nitrosofenfluramine-induced cytotoxicity in isolated rat hepatocytes

The relationship between cytotoxicity induced by N-nitrosofenfluramine and mitochondrial or glycolytic adenosine triphosphate (ATP) synthesis-dependent intracellular bioenergetics was studied in isolated rat hepatocytes. The supplementation of fructose, an ATP-generating glycolytic substrate, to hepatocyte suspensions prevented N-nitrosofenfluramine-induced cell injury accompanied by the formation of cell blebs, abrupt loss of intracellular ATP and reduced glutathione and mitochondrial membrane potential (DeltaPsi), and the accumulation of oxidized glutathione and malondialdehyde, indicating lipid peroxidation, during a 2h incubation period. Fructose (1-20mM) resulted in concentration-dependent protection against the cytotoxicity of N-nitrosofenfluramine at a concentration of 0.6mM, a low toxic dose. Pretreatment with xylitol, another glycolytic substrate, at concentration of 15mM also prevented the cytotoxicity caused by the nitroso compound, but neither glucose nor sucrose exhibited protective effects. In addition, fructose inhibited N-nitrosofenfluramine (0.5 and 0.6mM)-induced DNA damage, as evaluated in the comet assay, indicating that nuclei as well as mitochondria are target sites of the compound. These results indicate that (a) the onset of N-nitrosofenfluramine-induced cytotoxicity in rat hepatocytes is linked to mitochondrial failure, and that (b) the insufficient supply of ATP in turn limits the activities of all energy-requiring reactions and consequently leads to acute cell death.

Role of mitochondrial membrane permeability transition in N-nitrosofenfluramine-induced cell injury in rat hepatocytes

The role of mitochondrial membrane permeability transition in N-nitrosofenfluramine-induced cell injury was studied in mitochondria and hepatocytes isolated from rat liver. Mitochondrial permeability transition has been proposed as a common final pathway in acute cell death through mitochondrial dysfunction. In isolated mitochondria, N-nitrosofenfluramine (0.25 to 1.0 mM) in the presence of Ca(2+) (50 microM) elicited a concentration-dependent induction of mitochondrial swelling dependent on mitochondrial permeability transition and the release of cytochrome c, both of which were prevented by pretreatment with a specific inhibitor of mitochondrial permeability transition, cyclosporin A (0.2 microM). The effects of N-nitrosofenfluramine on mitochondria were more potent than those of fenfluramine, which is a sympathomimetic amine with anorectic action. The pretreatment of isolated hepatocytes with cyclosporin A (2 microM) partially but not completely prevented N-nitrosofenfluramine (0.6 mM; a low toxic dose)-induced cell death, loss of cellular ATP, formation of cell blebs and decrease in mitochondrial membrane potential. These results suggest that the onset of N-nitrosofenfluramine-induced cytotoxicity is linked to mitochondrial failure dependent upon induction of mitochondrial permeability transition accompanied by mitochondrial depolarization, the release of cytochrome c and depletion of intracellular ATP through uncoupling of oxidative phosphorylation.