The micronucleus test with peripheral reticulocytes from phenacetin-treated mice

Effect of the metabolic capacity in rat liver S9 on the positive results of in vitro micronucleus tests

A high incidence of positive results is obtained with in vitro genotoxicity tests, which do not correlate with the in vivo negative results in many cases. To address this issue, the metabolic profile of rat liver 9000 × g supernatant fraction (S9) pretreated with phenobarbital (PB) and 5,6-benzoflavone (BNF) was characterized. Furthermore, the in vitro micronucleus tests of 10 compounds were performed with PB-BNF-induced rat S9. PB-BNF increased cytochrome P450 (CYP) activity and CYP1A1, CYP1A2, CYP2B1/2, CYP2C6, CYP3A1, and CYP3A2 expression in rat S9, whereas it decreased CYP2C11 and CYP2E1 expression. PB-BNF-induced S9 enhanced the micronucleus induction (MI) of benzo[a]pyrene (BaP), cyclophosphamide (CPA), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine hydrochloride (PhIP), which are metabolized by CYP1A1, CYP2C6, and CYP1A2, respectively. In contrast, coumarin and chlorpheniramine showed MI with PB-BNF-induced S9 despite the fact that they show negative results in the in vivo studies. Furthermore, diclofenac, piroxicam, lansoprazole, and caffeine showed MI regardless of the enzyme induction by PB-BNF, whereas phenacetin did not show MI. These results indicate that PB-BNF-induced rat S9 is effective in detecting the genotoxic potential of promutagens, such as BaP, CPA, and PhIP, but not of coumarin and chlorpheniramine, probably due to the differences in the in vitro and in vivo metabolic profile and its exposure levels of the drugs.

Nasal cytotoxic and carcinogenic activities of systemically distributed organic chemicals

Toxicity and carcinogenicity in the mucosa of the nasal passages in rodents has been produced by a variety of organic chemicals which are systemically distributed. In this review, 14 such chemicals or classes were identified that produced rodent nasal cytotoxicity, but not carcinogenicity, and 11 were identified that produced nasal carcinogenicity. Most chemicals that affect the nasal mucosa were either concentrated in that tissue or readily activated there, or both. All chemicals with effects in the nasal mucosa that were DNA-reactive, were also carcinogenic, if adequately tested. None of the rodent nasal cytotoxins has been identified as a human systemic nasal toxin. This may reflect the lesser biotransformation activity of human nasal mucosa compared to rodent and the much lower levels of human exposures. None of the rodent carcinogens lacking DNA reactivity has been identified as a nasal carcinogen or other cancer hazard to humans. Some DNA-reactive rodent carcinogens that affect the nasal mucosa, as well as other tissues, have been associated with cancer at various sites in humans, but not the nasal cavity. Thus, findings in only the rodent nasal mucosa do not necessarily predict either a toxic or carcinogenic hazard to that tissue in humans.

Mouse Polycomb M33 is required for splenic vascular and adrenal gland formation through regulating Ad4BP/SF1 expression

Mice with disrupted mammalian PcG (Polycomb group) genes commonly show skeletal transformation of anterior-posterior identities. Disruption of the murine M33 gene, a PcG member, displayed posterior transformation of the vertebral columns and sternal ribs. In addition, failure of T-cell expansion and hypoplasia and sex-reversal of the gonads, have been observed. In the present study, we identified defects in the splenic and adrenal formation of M33-knock-out (KO) mice on a C57BL/6 genetic background. The spleen in these animals was smaller than in the wild-type mice and was spotted red because of nonuniform distribution of blood cells. Histologic examination revealed disorganization of the vascular endothelium and its surrounding structures, and immunohistochemistry demonstrated disturbances in vascular formation and colonization of immature hematopoietic cells. These splenic phenotypes observed in the M33-KO mice were quite similar to those seen in Ad4BP/SF1 (Nr5a1) knock-outs. Moreover, the adrenal glands of M33-KO and Ad4BP/SF1 heterozygous KO mice were smaller than those of the wild-type mice. Western blot, immunohistochemistry, and reverse transcriptase-polymerase chain reaction (RT-PCR) analyses of the M33 knock-outs all indicated significantly low expression of adrenal 4 binding protein/steroidogenic factor-1 (Ad4BP/SF-1), indicating that M33 is an essential upstream regulator of Ad4BP/SF1. In agreement with these observations, chromatin immunoprecipitation assays with adrenocortical Y-1 cells revealed direct binding of the M33-containing PcG to the Ad4BP/SF1 gene locus.

Ectopic mitotic recombination in Drosophila probed with bacterial beta-galactosidase gene-based reporter transgenes

Plasmids were constructed to investigate homologous mitotic recombination in Drosophila cells. Heteroalleles containing truncated but overlapping segments of the bacterial beta-galactosidase gene (lacZ) were positioned either on separate plasmids or as direct repeats on the same chromosome. Recombination reconstituted a functional lacZgene leading to expression of LacZ+activity detectable by histochemical staining. High extrachromosomal recombination (ECR) frequencies between unlinked heteroalleles were observed upon transient co-transfection into Drosophila melanogaster Schneider line 2 (S2) cells. Stably transfected cells containing the lacZ heteroalleles linked on a chromosome exhibited intrachromosomal recombination (ICR) frequencies two orders of magnitude lower than ECR frequencies. Recombination was inducible by exposing the cells to ethyl methanesulphonate or mitomycin C. Recombination products were characterized by multiplex PCR analysis and unequal sister chromatid recombination was found as the predominant mechanism reconstituting the lacZ gene. To investigate recombination in vivo imaginal disc cells from transgenic larvae carrying the reporter gene on the X chromosome were isolated and stained for LacZ+ activity. The presence of a few LacZ+ clones indicated that mitotic recombination events occurred at frequencies two orders of magnitude lower than the corresponding event in cultured cells and late during larval development.

Achievements by CSGMT/JEMS.MMS: the Collaborative Study Group for the Micronucleus Test in the Mammalian Mutagenesis Study Group of the Environmental Mutagen Society of Japan

The Collaborative Study Group for the Micronucleus Test (CSGMT) is one of the task groups in the Mammalian Mutagenesis Study Group (MMS) of the Environmental Mutagen Society of Japan (JEMS). It was established in 1982 and has made efforts to understand what the micronucleus test is, what are the advantages and disadvantages of the test as an in vivo detection system for mutagens/carcinogens, and to establish a standard protocol applicable to numerous chemicals. Members of the CSGMT have published more than 75 papers as part of collaborative studies and have contributed to the understanding of the nature of the micronucleus test and to setting guidelines for testing of medicinal and other chemicals. The CSGMT held some workshops to share up-to-date knowledge and techniques on the micronucleus test. Through workshops and collaborative studies, the CSGMT contributed to the maintaining of a high standard of knowledge and techniques among Japanese researchers of the micronucleus test. This paper reviews achievements made by the CSGMT until now.

Micronucleus induction in mouse peripheral blood reticulocytes by 2,7-diaminophenazine and related compounds

The peripheral blood micronucleus test using acridine orange (AO) supravital staining was performed with phenazine (Pz), 1-aminophenazine (1-APz), 2-aminophenazine (2-APz) and 2,7-diaminophenazine (2,7-DAPz). Male ICR mice and BDF1 mice were treated intraperitoneally once or twice 24 h apart with each chemical. Irrespective of the number of administrations, Pz and 1-APz were not clastogenic in either mouse strain. In the single treatment regime, 2-APz slightly increased micronucleated reticulocytes (MNRETs) 48 h after 200 mg/kg dosing in ICR mice and 72 h after 400 mg/kg dosing in BDF1 mice. The MNRET frequencies were not enhanced after double treatment in either mouse strain. The single treatment with 2,7-DAPz strongly and dose-dependently induced MNRETs in BDF1 mice, and the incidences of MNRETs, which peaked 48 or 72 h after treatment, were much higher than those in ICR mice. In both mouse strains, the frequencies of MNRETs observed after double treatment with 2,7-DAPz were higher than those after single treatment. The MNRET incidence induced 72 h after first 200 mg/kg dosing in BDF1 mice was 2.21%.