Domizio Serra

Genotoxicity of chromium compounds. A review

This article reviews approximately 700 results reported in the literature with 32 chromium compounds assayed in 130 short-term tests, using different targets and/or genetic end-points. The large majority of the results obtained with Cr(VI) compounds were positive, as a function of Cr(VI) solubility and bioavailability to target cells. On the other hand, Cr(III) compounds, although even more reactive than Cr(VI) with purified nucleic acids, did not induce genotoxic effects in the majority of studies using intact cells. Coupled with the findings of metabolic studies, the large data-base generated in short-term test systems provides useful information for predicting and interpreting the peculiar patterns of Cr(VI) carcinogenicity.

Inhibition of urethan-induced lung tumors in mice by dietary N-acetylcysteine

The thiol N-acetylcysteine (NAC), a precursor of intracellular glutathione (GSH), efficiently prevented the induction of lung tumors in Swiss albino mice, when supplemented to the diet (0.2%) both before and after an i.p. injection of the carcinogen urethan (ethyl carbamate). Irrespective of urethan administration, NAC also significantly enhanced GSH S-transferase activity in liver preparations of the same animals. These data show that, under certain conditions, it is possible to prevent chemically induced cancer by increasing the levels of physiological trapping agents.

Metabolic reduction of chromium by alveolar macrophages and its relationships to cigarette smoke.

Pulmonary alveolar macrophages (PAM), obtained by bronchoalveolar lavage from 47 individuals, reduced hexavalent chromium [Cr(VI)] and decreased its mutagenicity. Their specific activity--mostly mediated by cytosolic, enzyme-catalyzed mechanisms--was significantly higher than in corresponding preparations of mixed-cell populations from human peripheral lung parenchyma or bronchial tree, or from rat lung or liver. At equivalent number of PAM, Cr(VI) reduction, total protein, and some oxidoreductase activities were significantly increased in smokers. No appreciable variation could be detected between lung cancer and noncancer patients. In rats, the Cr(VI)-reducing activity of PAM preparations was induced by Aroclor 1254. Thus, alveolar macrophages provide crucial defense mechanisms not only by phagocytizing metals, but also by metabolically reducing Cr(VI). The epithelial-lining fluid (ELF) also displayed some Cr(VI) reduction. Together with already investigated metabolic processes occurring inside lung cells, these mechanisms are expected to determine thresholds in the pulmonary carcinogenicity of chromium.

Circadian reduction of chromium in the gastric environment

Samples of gastric juice from variously treated subjects efficiently reduced hexavalent chromium and decreased its mutagenicity. Chromium reduction was due to thermostable components of gastric secretions and was favoured by the acidity of the intragastric environment. The circadian monitoring of pH and of chromium reduction, as assessed by colorimetric analysis at hourly intervals, showed a basal activity (less than 10 micrograms/ml gastric juice) during the night and interdigestive periods, and peaks (tens of micrograms/ml) during the 3-4-h periods after each meal. Assays in the Ames reversion test confirmed that the decrease in mutagenicity of sodium dichromate produced by gastric juice was significantly enhanced after meals. This physiological mechanism is expected to provide an important protective barrier against the oral toxicity of this metal, and may explain its lack of oral carcinogenicity.

Metabolic reduction of chromium, as related to its carcinogenic properties

At variance with Cr(III), Cr(VI) compounds easily cross cell membranes and exert genotoxic effects. No metabolic oxidation of Cr(III) could be detected, whereas Cr(VI) reduction was observed in the presence of body fluids and subcellular fractions of various tissues from several animal species. The differential efficiency of this process may account for the selection of target tissues in Cr(VI) carcinogenesis. For instance, reduction by saliva and gastric juice may explain a lack of carcinogenicity by the oral route; reduction inside erythrocytes may explain a lack of carcinogenicity at a distance from administration sites; reduction by the epithelial-lining fluid of terminal airways and by alveolar macrophages may be consistent with the occurrence of thresholds in lung carcinogenesis. Liver preparations displayed the top efficiency in reducing Cr(VI), whereas skeletal muscle, i.e., a typical target in experimental Cr(VI) carcinogenesis, had no detectable activity. Bronchial tree and peripheral lung parenchyma preparations from almost 100 individuals reduced Cr(VI) to a variable extent. The efficiency of lung parenchyma and of isolated alveolar macrophages was enhanced in cigarette smokers. In rats, Cr(VI) reduction by lung preparations was significantly stimulated by the repeated i.t. instillation of Cr(VI) itself. Among the electron donors (chiefly GSH) and enzymatic mechanisms responsible for the intracellular Cr(VI) reduction, such as cytochrome P-450 reductase, glutathione redactase, and aldehyde oxidase, an important role can be ascribed to cytosolic DT diaphorase activity, usually catalyzing a 2-electron reduction.

Metabolic activation of hepatocarcinogens in chronic hepatitis B

S9 fraction pools of liver biopsy samples, collected from 129 patients in two consecutive studies, were comparatively assayed for their ability to activate aflatoxin B1 (AFB1) and a tryptophan pyrolysate product (Trp-P-2) in a miniaturized Salmonella mutagenicity test system. Metabolic activation was not affected to a significant extent by most of the monitored variability factors, such as sex, alcohol, cigarette smoking and liver histology (minimal changes, chronic persistent (CPH) or active (CAH) hepatitis, CAH steatosis, or cirrhosis). Conversely, a significant enhancement of activation was observed for AFB1 in cases of mild CAH and especially for Trp-P-2 in hepatitis B virus carriers, irrespective of their histologic diagnosis.

Mechanistic aspects of chromium carcinogenicity.

There is no doubt that chromium has been the most extensively investigated metal with respect to the assessment of long-term effects and to the evaluation of the underlying mechanisms. Notwithstanding the huge data-base available in the literature, many aspects, bearing both scientific and practical interest, are still unclear or controversial. This paper will briefly discuss some relevant problems, including carcinogenicity and genotoxicity of chromium compounds as related to their valency and solubility, interactions with other compounds, bioavailability to target cells and molecules, interconversion processes between different oxidation states in various body areas and cell compartments, mechanisms of chromium metabolism and carcinogenicity, and existence of thresholds in chromium carcinogenesis.

Genotoxicity and metabolism of chromium compounds

This article reviews studies and new experimental data concerning the genotoxicity of chromium compounds of different oxidation state in a variety of short‐term test systems, as well as their metabolic fate. The chemical or biochemical reduction of Cr(VI) has been investigated in several body fluids and cell preparations of different tissues from various animal species, including humans. The variability factors involved and the underlying mechanisms have been explored. The selective reduction of Cr(VI) in different sites of the organism may be consistent with its lack of carcinogenicity by the oral route or at distance from administration sites. Also in the lung, Cr(VI) carcinogenicity is likely to be limited by defense mechanisms of the lower respiratory tract, such as those provided by the epithelial‐lining fluid and by alveolar macrophages.

Antigenotoxic and Anticarcinogenic Effects of Thiols. In Vitro Inhibition of the Mutagenicity of Drug Nitrosation Products and Protection of Rat Liver ADP-Ribosyl Transferase Activity

Reduced glutathione (GSH) is well-known to play a fundamental role in the protection of the organism against toxic, mutagenic and/or carcinogenic agents (see e.g. refs. 1–3 for reviews). Among synthetic aminothiols, acting as analogs and precursors of GSH, N-acetyl-L-cysteine (NAC) is of particular interest, because this molecule is already extensively used in the treatment of chronic respiratory diseases, and is extremely well tolerated in humans4. In addition, NAC is known to possess various antitoxic and antioxidant properties3–5.

Inhibition of Mutagenesis and Carcinogenesis by N-Acetylcysteine

N-acetylcysteine (NAC), a precursor of intracellular cysteine and glutathione (GSH)) (1), is extensively used in the treatment of patients suffering from respiratory diseases. Moreover, this synthetic aminothiol has been shown to possess a variety of antitoxic properties in humans, animals and in vitro test systems (2).