F. Vigagni

Cytogenetic effects of Metalaxyl on human and animal chromosomes

The purpose of this study was to assess the cytogenetic effects in vitro and in vivo of a commonly used fungicide, Metalaxyl. Chromosome damage in vitro, quantified by cultured human peripheral blood lymphocytes, demonstrated dose-related effects not associated with mitotic inhibition or cell death. Significant induction of chromosomal aberrations was observed with between 300 and 1000 micrograms/ml Metalaxyl in the absence of microsomal activation. Incubation in the presence of S9 mix produced less cytogenetic damage. Single i.p. injections of 75-300 mg/kg Metalaxyl had no effect on the frequency of micronuclei, detected in murine polychromatic erythrocytes. Micronuclei results were not compromised by direct evidence of cytotoxicity in the bone marrow of treated animals. The results in the present study indicated that genotoxicity of Metalaxyl was detected only in vitro and not in vivo. Available data reported that Metalaxyl was non-carcinogenic and gave negative results in a battery of genotoxicity tests. So, clastogenicity of Metalaxyl may not be evidence for DNA reactivity, but it may indicate alterations in cell homeostasis which are well implicated in the process of carcinogenesis.

Genetic safety evaluation of pesticides in different short-term tests

Cyanazine, cyhexatin, dicamba and DNOC are pesticides commonly and broadly used in agriculture pest control. However, there is little information on their toxicity and mutagenicity in human cells and in whole animals. Therefore, UDS assay and SCE assay in human peripheral lymphocytes, and chromosome aberration analysis in bone marrow of rats have been used to assess the DNA-damaging activity of the above pesticides. Cyanazine proved non-genotoxic in all the test systems. Cyhexatin showed only weakly positive results for SCE induction in human lymphocytes, providing no concern for genotoxicological hazard. While dicamba did not show clastogenic effects in rodents, DNOC gave significant dose-related increases of structural chromosome aberrations in rat bone marrow cells. Female animals showed increased sensitivity to the toxic effects by DNOC at the highest dose. The results provide further information on the intrinsic genotoxic activity of the tested pesticides, which may contribute to the toxicological assessment of the risk associated with human exposure.

Genetic and non-genetic biomarkers related to carcinogenesis in evaluating toxicological risk from Fenarimol

A multibiomarker approach based on the study of toxicity mechanisms at both genetic and metabolic levels has been applied to Fenarimol. With regard to genotoxicity, particular attention was given to assays for chromosomal aberration and micronuclei; clastogenic potential was assessed in human peripheral blood lymphocytes in vitro, while the induction of micronuclei was studied in male CD1 mouse bone marrow polychromatic erythrocytes (PCE). Fenarimol did not induce any significant dose-related increase in micronucleated PCEs, up to 4-fold above the control level at a single dose of 75 mg/kg b.w., was observed 24 h after treatment. Using selective biochemical markers of effect Fenarimol was found to induce CYP 2B1 isoforms in liver, kidney and lung microsomes of Swiss Albino CD1 male and female mice, as shown by the significant increase in specific 2B1-probe pentoxyresorufin O-dealkylase activity. On the contrary, CYP 3A, probed by N-demethylation of aminopyrine, were only induced in the liver. Results were corroborated by means of Western immunoblotting using rabbit polyclonal antibodies anti-CYP 2B1 and 3A. Northern blotting analysis with CYP 2B1 and 3A cDNA biotinylated probes showed that the expression of such isoforms is regulated at mRNA level. Taken as a whole, these data indicate the possible (mutagenic) cotoxic/cocarcinogenic and promoting potential of this fungicide.

Mechanism of genotoxicity and electron density distribution by NMR of 5-nitro-3-thiophenecarboxamides, a novel group of direct-acting mutagens in Salmonella typhimurium.

The mutagenic activity of 23 5-nitro-3-thiophenecarboxanilides and of 5-nitro-3-thiophenecarboxamide, the prototype, (NTCAs) have been evaluated in the Ames test on Salmonella typhimurium strains TA100 ad TA98 with and without metabolic activation. Effects of different substituents (electron-donating and electron-withdrawing) were studied to evaluate structural features that affect the metabolism and the bacterial mutagenic potency. All the derivatives were direct-acting mutagens, the mutagenic potency ranging from 0.7 to 142 revertants (rev.)/nmol in TA100 and from 0.09 to 68 rev./nmol in TA98 strain. Results obtained with strains TA98NR and TA98/1,8-DNP6 indicated that the mutagenic activity was largely dependent on bacterial nitroreductase, whereas the O-acetylation step was not critical for mutagenic potency. Superoxide (O2-.) and hydroxyl (OH.) scavengers as well as other radical scavengers and enzymes inhibited NTCAs mutagenicity to different extents. In particular, O2-. seemed to be involved in NTCAs mutagenicity, showing a free radical pathway for NTCA metabolism. [1H]- and [13C]NMR data indicated that the effects of different substituents on genotoxicity are probably not exerted on the electron density distribution. The importance of factors such as extent of nitration, reduction potential, orientation of nitrosubstituent and planarity of the molecule are discussed.

Murine α/β Interferons Inhibit Benzo(A)Pyrene Activation and Mutagenesis in Mice

In addition to their antiviral and immune regulatory properties, interferons (IFNs) are known to depress hepatic cytochrome P450-dependent metabolism. As many chemical mutagens and carcinogens require bioactivation by the mixed-function monooxygenase (MFO) system in order to be genotoxic, a combined genetic and biochemical approach was used to establish whether IFNs could inhibit the activation of benzo(a)pyrene (BaP) to the ultimate clastogenic metabolite(s) in vivo.