Role of mutagenicity in asbestos fiber-induced carcinogenicity and other diseases.

The cellular and molecular mechanisms of how asbestos fibers induce cancers and other diseases are not well understood. Both serpentine and amphibole asbestos fibers have been shown to induce oxidative stress, inflammatory responses, cellular toxicity and tissue injuries, genetic changes, and epigenetic alterations in target cells in vitro and tissues in vivo. Most of these mechanisms are believe to be shared by both fiber-induced cancers and noncancerous diseases. This article summarizes the findings from existing literature with a focus on genetic changes, specifically, mutagenicity of asbestos fibers. Thus far, experimental evidence suggesting the involvement of mutagenesis in asbestos carcinogenicity is more convincing than asbestos-induced fibrotic diseases. The potential contributions of mutagenicity to asbestos-induced diseases, with an emphasis on carcinogenicity, are reviewed from five aspects: (1) whether there is a mutagenic mode of action (MOA) in fiber-induced carcinogenesis; (2) mutagenicity/carcinogenicity at low dose; (3) biological activities that contribute to mutagenicity and impact of target tissue/cell type; (4) health endpoints with or without mutagenicity as a key event; and finally, (5) determinant factors of toxicity in mutagenicity. At the end of this review, a consensus statement of what is known, what is believed to be factual but requires confirmation, and existing data gaps, as well as future research needs and directions, is provided.

Mutant human tumor suppressor p53 modulates the activation of mitogen-activated protein kinase and nuclear factor-kappaB, but not c-Jun N-terminal kinase and activated protein-1.

The roles of the mitogen-activated kinase protein (MAPK) pathway, nuclear factor-kappa B (NF-kappaB), and activator protein-1 (AP-1) in cellular responses to growth factors and mitogen are well established. However, the manner by which these proliferative pathways are affected by the tumor suppressor protein p53 is not fully understood. We report here the results of an investigation of the status of p53 on two human melanoma cell lines with wild-type p53 (SK-Mel-186) or mutant p53 (SK-Mel-110). The basal levels of the activated extracellular-signal regulated kinases 1 and 2 (ERK1/2) were high in cells with wild-type p53, but low in cells with mutant p53. The 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced activation of ERK1/2 through the phosphorylation of threonine and tyrosine at 202 and 204, respectively, was demonstrated in both cell lines, however, in a discrete manner. TPA-induced activation of ERK1/2 was sustained in wild-type p53 cells, while only a transient activation was seen in mutant p53 cells. Inhibition of MAPK kinase (MEK), an upstream kinase, by U0126, blocked TPA-induced activation of ERK1/2 in wild-type p53 cells and in mutant p53 cells. Treatment of wild-type p53 (SK-Mel 186) cells with small interfering RNA (siRNA) of p53 displayed a transient induction of activation of ERK1/2 following TPA treatment, indicating that p53 has a role in the regulation of the activation of ERK1/2. NF-kappaB activity decreased significantly in cells with wild-type p53, while enhanced NF-kappaB activity was evident in cells with mutant p53. The expression of either wild-type or mutant p53 had a similar effect on TPA-induced Jun N-terminal kinase (JNK) activation, indicating specificity for the ERK pathway. Similarly, AP-1 binding activity showed a transient variation in both cell lines after TPA treatment but with different kinetics. These observations suggest that both wild-type and mutant p53 can modulate the activation pathways for ERK1/2, and NF-kappaB distinctively, while modulating the pathways of JNK and AP-1 similarly. These differences may influence cellular processes such as proliferation, differentiation, and apoptosis.

Genotoxicity of benzene and its metabolites.

The potential role of genotoxicity in human leukemias associated with benzene (BZ) exposures was investigated by a systematic review of over 1400 genotoxicity test results for BZ and its metabolites. Studies of rodents exposed to radiolabeled BZ found a low level of radiolabel in isolated DNA with no preferential binding in target tissues of neoplasia. Adducts were not identified by 32P-postlabeling (equivalent to a covalent binding index <0.002) under the dosage conditions producing neoplasia in the rodent bioassays, and this method would have detected adducts at 1/10,000th the levels reported in the DNA-binding studies. Adducts were detected by 32P-postlabeling in vitro and following high acute BZ doses in vivo, but levels were about 100-fold less than those found by DNA binding. These findings suggest that DNA-adduct formation may not be a significant mechanism for BZ-induced neoplasia in rodents. The evaluation of other genotoxicity test results revealed that BZ and its metabolites did not produce reverse mutations in Salmonella typhimurium but were clastogenic and aneugenic, producing micronuclei, chromosomal aberrations, sister chromatid exchanges and DNA strand breaks. Rodent and human data were compared, and BZ genotoxicity results in both were similar for the available tests. Also, the biotransformation of BZ was qualitatively similar in rodents, humans and non-human primates, further indicating that rodent and human genotoxicity data were compatible. The genotoxicity test results for BZ and its metabolites were the most similar to those of topoisomerase II inhibitors and provided less support for proposed mechanisms involving DNA reactivity, mitotic spindle poisoning or oxidative DNA damage as genotoxic mechanisms; all of which have been demonstrated experimentally for BZ or its metabolites. Studies of the chromosomal translocations found in BZ-exposed persons and secondary human leukemias produced by topoisomerase II inhibitors provide some additional support for this mechanism being potentially operative in BZ-induced leukemia.

Mutagenicity of benzo[b]phenanthro[2,3-d]thiophene (BPT) and its metabolites in TA100 and base-specific tester strains (TA7001-TA7006) of Salmonella typhimurium: evidence of multiple pathways for the bioactivation of BPT.

Benzo[b]phenanthro[2,3-d]thiophene (BPT), and a number of its metabolites, including BPT-3,4-diol, BPT sulfoxide, BPT sulfone, and 3-hydroxyBPT were assessed for their mutagenic activity in Salmonella typhimurium strain TA100, and S. typhimurium base-specific strains TA7001, TA7002, TA7003, TA7004, TA7005, and TA7006. Among the compounds tested in strain TA100, BPT, BPT sulfone, and 3-hydroxyBPT did not show any significant mutagenic response in the presence of S9. In contrast BPT sulfoxide and BPT-3,4-diol (a precursor to the bay-region diol epoxide of BPT) showed significant mutagenic activity in the presence of S9. Surprisingly, BPT sulfoxide was nearly 3.3-fold more mutagenic than BPT-3,4-diol in the presence of S9. BPT sulfoxide also displayed intrinsic mutagenic activity, which was nearly 1.5-fold less than that displayed by BPT-3,4-diol in the presence of S9. In base specific tester strains, BPT sulfoxide was the most active metabolite in strains TA7002, TA7004, and TA7005 with S9 activation. In these strains, BPT-3,4-diol was 2- to 7-fold less mutagenic than BPT sulfoxide in the presence of S9. Only in strain TA7006, BPT-3,4-diol was four-fold more mutagenic than BPT sulfoxide. The fact that BPT sulfoxide is significantly more mutagenic than BPT-3,4-diol in S. typhimurium strain TA100 suggests that the formation of sulfoxide may be the principal pathway for the metabolic activation of BPT to mutagenic products. Based on the results from Tester Strain TA7005, it indicate that BPT and its most mutagenic metabolite BPT sulfoxide induce predominantly CG --> AT transversion, which is observed as the most frequent base substitution mutation of p53 tumor-suppressor gene in human lung cancer.

N-nitroso compounds and mutagens in Chinese fermented (sour) corn pancakes.

Stomach cancer rates in rural Linqu County, Shandong Province, China, are exceptionally high. A previous case-control study revealed that the risk of stomach cancer was 30% higher among those who consumed sour (fermented) corn pancakes at least daily. A previous study of the sour pancakes reported volatile nitrosamines in most specimens, and almost half reportedly showed mutagenic activity. Few households currently consume sour pancakes, and the duration of fermentation has been shortened. We tested specimens of pancake batter and sour pancakes from Linqu County for mutagenic activity using the Ames test; for N-nitroso compounds (NOC) we used the Nitrolite-thermal energy analysis (TEA) method. Results of the Ames test were inconclusive: only 1 out of 15 cooked pancakes showed a positive mutagenic response, and all 15 batter specimens were negative; however, several batter specimens showed a weakly positive trend of mutagenicity with extract concentration. Our assay for total nitroso compounds was weakly positive in only 1 out of 15 specimens of sour pancake batter. That specimen was also tested by gas chromatography-TEA for nitrosaminoacids and volatile nitrosamines, but none were detected. It seems unlikely that the Chinese sour pancakes are significantly contaminated by NOC or other mutagens.

Inhibition of benzo(a)pyrene-induced lung tumorigenesis in A/J mice by dietary N-acetylcysteine conjugates of benzyl and phenethyl isothiocyanates during the postinitiation phase is associated with activation of mitogen-activated protein kinases and p53 activity and induction of apoptosis.

Recent studies in cell culture have shown that isothiocyanates (ITCs) induce apoptosis via activation of mitogen-activated protein (MAP) kinases and p53 pathways, suggesting a potential for ITCs or their conjugates to inhibit tumorigenesis during the postinitiation phase. To evaluate whether ITC compounds administered after carcinogen treatment inhibit lung tumorigenesis, we investigated in A/J mice the effects of the N-acetylcysteine (NAC) conjugates of benzyl (BITC-NAC) and phenethyl ITC (PEITC-NAC) in the diet (15 micromol/g) administered after a single dose of 20 micromol benzo(a)pyrene [B(a)P]. The formation of lung adenomas was examined 140 days after B(a)P dosing. Both the BITC-NAC and PEITC-NAC-treated groups showed a significant reduction in lung tumor multiplicity from 6.1 +/- 3.1 tumors/mouse in the B(a)P group fed the control diet to 3.7 +/- 2.9 and 3.4 +/- 2.7 tumors/mouse (P = 0.018 and 0.006, respectively). To investigate the mechanisms of tumor inhibition, lung tissues were obtained at 21, 84, and 140 days at interim sacrifices during the bioassay. These tissues showed a significant increase in apoptosis as determined by in situ end-labeling for both ITC-NAC-treated groups. The MAP kinase pathway was activated in the ITC-NAC-treated groups. The activation of c-Jun NH(2)-terminal kinase was higher in the BITC-NAC and PEITC-NAC groups when compared with B(a)P-treated control. The phosphorylation of p38 and extracellular signal-regulated kinases (ErKs) 1 and 2 was also induced by these treatments. To determine the downstream target of MAP kinases, activator protein-1 (AP-1) and nuclear factor-kappaB activities were evaluated by gel shift assay. The AP-1 binding activity was remarkably increased in lung tissue from both the BITC-NAC and PEITC-NAC groups. No change in nuclear factor-kappaB binding activity was found, however. Phosphorylation of p53 was also higher than the constitutive levels in both ITC-NAC-treated groups, but no induction of p53 expression was detected. This study demonstrates the chemopreventive efficacy of the NAC conjugates of PEITC and BITC administered in the diet after a single dose of B(a)P for lung tumorigenesis and provides the first in vivo evidence that activation of MAP kinases, AP-1 transcription factors, p53 phosphorylation, and the induction of apoptosis may be involved in the chemopreventive activity of these compounds.