2-Nitrosofluorene and N-hydroxy-2-aminofluorene react with the ubiquinone-reduction center (center N) of the mitochondrial cytochrome bc1 complex

Aromatic Amines in Experimental Cancer Research: Tissue-Specific Effects, an Old Problem and New Solutions

Carcinogenic aromatic amines usually produce tumors in specific target tissue, such as 2-acetylaminofluorene (AAF) producing liver tumors in rats, in contrast to some other structurally related arylamines. A hypothesis is presented that explains the mode of action in this rat liver model. Genotoxic and nongenotoxic effects work together and make AAF a complete rat liver carcinogen. The cytotoxic, promoting effects are particularly important. N-Hydroxy-2-aminofluorene and 2-nitrosofluorene, two metabolites of AAF, are able to uncouple the mitochondrial respiratory chain. They entertain a redox cycle that removes electrons from the respiratory chain and impairs ATP production. The dose-dependent opening of the mitochondrial permeability transition pore signals the viability of the cell. If the pore is opened to a certain extent, the cell is eliminated by apoptosis. As a consequence, oval cells proliferate, and as this process is overloaded, the liver transforms into a cirrhosis-like situation and thus provides the conditions under which initiated liver cells develop tumors. Such an interpretation is based on assumptions that have been debated for a long time. Some of these often forgotten developments are reviewed in support of the hypothesis, which allows a more comprehensive view of the complex in vivo situation at a time when in vitro models prevail.

The mitochondrial permeability transition from in vitro artifact to disease target

The mitochondrial permeability transition pore is a high conductance channel whose opening leads to an increase of mitochondrial inner membrane permeability to solutes with molecular masses up to approximately 1500 Da. In this review we trace the rise of the permeability transition pore from the status of in vitro artifact to that of effector mechanism of cell death. We then cover recent results based on genetic inactivation of putative permeability transition pore components, and discuss their meaning for our understanding of pore structure. Finally, we discuss evidence indicating that the permeability transition pore plays a role in pathophysiology, with specific emphasis on in vivo models of disease.

Early resistance to cell death and to onset of the mitochondrial permeability transition during hepatocarcinogenesis with 2-acetylaminofluorene

A hallmark of tumorigenesis is resistance to apoptosis. To explore whether resistance to cell death precedes tumor formation, we have studied the short-term effects of the hepatocarcinogen 2-acetylaminofluorene (AAF) on liver mitochondria, on hepatocytes, and on the response to bacterial endotoxin lipopolysaccharide (LPS) in albino Wistar rats. We show that after as early as two weeks of AAF feeding liver mitochondria developed an increased resistance to opening of the permeability transition pore (PTP), an inner membrane channel that is involved in various forms of cell death. Consistent with a mitochondrial adaptive response in vivo, (i) AAF feeding increased the expression of BCL-2 in mitochondria, and (ii) hepatocytes isolated from AAF-fed rats became resistant to PTP-dependent depolarization, cytochrome c release, and cell death, which were instead observed in hepatocytes from rats fed a control diet. AAF-fed rats were fully protected from the hepatotoxic effects of the injection of 20-30 microg of LPS plus 700 mg of d-galactosamine (d-GalN) x kg-1 of body weight, a treatment that in control rats readily caused a large increase of terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling-positive cells in liver cryosections and release of alanine and aspartate aminotransferase into the bloodstream. Treatment with LPS and d-GalN triggered cleavage of BID, a BCL-2 family member, in the livers of both control- and AAF-fed animals, whereas caspase 3 was cleaved only in control-fed animals, indicating that the mitochondrial proapoptotic pathway had been selectively suppressed during AAF feeding. Phenotypic reversion was observed after stopping the carcinogenic diet. These results underscore a key role of mitochondria in apoptosis and demonstrate that regulation of the mitochondrial PTP is altered early during AAF carcinogenesis, which matches, and possibly causes, the increased resistance of hepatocytes to death stimuli in vivo. Both events precede tumor formation, suggesting that suppression of apoptosis may contribute to the selection of a resistant phenotype, eventually increasing the probability of cell progression to the transformed state.

Mitochondrial targets of drug toxicity

Mitochondria have long been recognized as the generators of energy for the cell. Like any other power source, however, mitochondria are highly vulnerable to inhibition or uncoupling of the energy harnessing process and run a high risk for catastrophic damage to the cell. The exquisite structural and functional characteristics of mitochondria provide a number of primary targets for xenobiotic-induced bioenergetic failure. They also provide opportunities for selective delivery of drugs to the mitochondrion. In light of the large number of natural, commercial, pharmaceutical, and environmental chemicals that manifest their toxicity by interfering with mitochondrial bioenergetics, it is important to understand the underlying mechanisms. The significance is further underscored by the recent identification of bioenergetic control points for cell replication and differentiation and the realization that mitochondria play a determinant role in cell signaling and apoptotic modes of cell death.

Impairment of respiration and oxidative phosphorylation by redox cyclers 2-nitrosofluorene and menadione

The present study was designed to investigate the effects of 2-nitrosofluorene (NOF), a metabolite of carcinogenic 2-acetylaminofluorene, on mitochondrial respiration and oxidative phosphorylation. NOF reacts with the NADH:ubiquinone oxidoreductase (complex I) and consumes oxygen in a rotenone-insensitive manner. Unlike menadione, which is able to bypass the rotenone-block and to restore ATP-formation, NOF-induced electron flow was almost completely uncoupled. In normal respiration both redox-cyclers decreased the respiratory control and P/O ratios at low concentrations (2-20 nmol/mg) in NADH-dependent oxidation. With succinate as substrate, only NOF was significantly active. In contrast to NOF, the hydroxamic acid N-hydroxy-2-acetylaminofluorene (N-OH-AAF) impaired mitochondrial energy conversion only at much higher concentrations (80 nmol/mg). At concentrations > 10 nmol/mg, NOF inhibited electron flow through the respiratory chain in NADH- and succinate-dependent oxidation, as determined by dinitrophenolate-uncoupled respiration. The small protective effect of L-cysteine indicates that covalent binding of the nitroso-compound to SH-groups may not explain sufficiently the inhibitory effect of NOF. The results support the notion that redox cyclers impair oxidative phosphorylation by establishing alternative pathways for electron transport in the respiratory chain.