Genotoxic and chronic toxic effects in the carcinogenicity of aromatic amines

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 debate on carcinogenicity of permanent hair dyes: new insights

Oxidative (permanent) hair dyes contain one or several "primary intermediates" (e.g., p-phenylenediamines, p-aminophenols) and "couplers" (e.g., m-aminophenols, m-hydroxyphenols). In the presence of peroxide, the primary intermediate(s) and the coupler(s) undergo a chemical reaction to form colored oligomers. In the 1970s a number of aromatic amines used in oxidative hair dyes were identified as mutagenic and/or carcinogenic in rodents after lifetime oral administration. In response, regulatory action was taken, and some hair dye ingredients were banned in the European Union. Although recent results suggest that the main "primary intermediate" of oxidative hair dyes, p-phenylenediamine, has a weak genotoxic potential in vitro, it was not mutagenic in a mixture with nonmutagenic couplers, if tested under conditions comparable to those of practical use. Under conditions of use of permanent hair dyes, between 0.1 and 0.5% of the applied p-phenylenediamine may be absorbed through the skin. Acetylation in the skin is a key metabolic step for the primary intermediates p-phenylenediamine and p-aminophenol. Because of the involvement of aromatic amines, the discussion on the carcinogenicity of hair dyes in humans has been focused on urothelial cancer. Numerous epidemiological studies have investigated the risk of bladder cancer associated with the profession as a hairdresser, as well as the risk to consumers of hair dyes. Although some earlier studies suggested an overrepresentation of bladder cancer in male hairdressers, the majority of modern studies do not show an increase in relevant bladder cancer risk for professional or personal use of oxidative hair dyes. Today, there seems to be no relevant bladder cancer risk from the use of oxidative hair dyes. Such a conclusion can be derived from new toxicokinetic and metabolism investigations and is in general accordance with current epidemiological data. Human urothelial cancers, chemically induced by aromatic amines, have typical latency times often longer than 20 years. Since earlier exposures could have an impact decades later, the possibility of bladder cancer in hairdressers having intensively worked with permanent hair dyes during earlier decades (prior to the 1980s) should be taken into account.

c-fos gene expression in rat liver is induced by phenobarbital

In the present study we investigated c-fos expression in rat livers, that was initiated with the three arylamines, 2-acetylaminofluorene, 2-acetylaminophenanthrene and trans-4-acetylaminostilbene. The tumor promoter phenobarbital was applied chronically for 26, 52 and 100 weeks. Gene expression, determined by the mRNA level, and FOS protein were increased after 52 weeks of treatment in arylamine initiated as well as in phenobarbital only treated animals. Expression of c-fos seems to be a phenobarbital induced effect that is independent of additional initiator treatments. This finding was supported by immunohistochemical studies demonstrating increased FOS levels to be localized around the central vein. The results indicate that phenobarbital, a widely used tumor promoter, induces c-fos expression. In addition, we demonstrated enhanced FOS in GST-P-positive foci and in tumors.

The dual role of 2-acetylaminofluorene in hepatocarcinogenesis: specific targets for initiation and promotion

2-Acetylaminofluorene (AAF) is one of the most widely studied model carcinogens. It produces liver tumors in rats. Comparison with other arylamides shows that promutagenic DNA lesions are necessary but not sufficient to explain this tissue-specific effect. Mutagenicity of AAF was studied in AS52 cells and compared with that of 2-acetylaminophenanthrene and trans-4-acetylaminostilbene which are incomplete carcinogens in rat liver. The major mutations were G to T transversions in all cases. All three acetamides acted as initiators in an initiation-promotion experiment with phenobarbital as a promoter. Chronic toxic effects of AAF were attributed to specific effects of AAF metabolites on mitochondrial respiration. Electron drainage by 2-nitrosofluorene causes an uncoupling effect on oxidative phosphorylation in vitro. Corresponding compensatory effects were observed in vivo. Initiating as well as promoting properties of AAF are therefore considered responsible for the generation of rat liver tumors. The results support the hypothesis that genotoxic effects generate initiated cells which begin to proliferate only when microcirculation is disturbed due to cirrhotic alterations. These are triggered by non-genotoxic interference with mitochondrial respiration and oxidative phosphorylation.