Mutagenicity of the human bladder carcinogen 4-aminobiphenyl to the bladder of MutaMouse transgenic mice

Influence of arylamine N-acetyltransferase, sex, and age on 4-aminobiphenyl-induced in vivo mutant frequencies and spectra in mouse liver

One model for cancer initiation by 4-aminobiphenyl (ABP) involves N-oxidation by cytochrome P450 CYP1A2 followed by O-conjugation by N-acetyltransferase(s) NAT1 and/or NAT2 and decomposition to a DNA-binding nitrenium ion. We recently observed that neonatal ABP exposure produced liver tumors in male but not in female mice, and that NAT deficiency reduced liver tumor incidence. However, ABP-induced liver tumor incidence did not correlate with liver levels of N-(deoxyguanosin-8-yl)-ABP adducts 24 hr after exposure. In this study, we compared in vivo ABP-induced DNA mutant frequencies and spectra between male and female wild-type and NAT-deficient Muta™Mouse using both the tumor-inducing neonatal exposure protocol and a 28-day repetitive dosing adult exposure protocol. ABP produced an increase in liver DNA mutant frequencies in both neonates and adults. However, we observed no sex or strain differences in mutant frequencies in neonatally exposed mice, and higher frequencies in adult females than males. Neonatal ABP exposure of wild-type mice increased the proportion of G-T transversions in both males and females, while exposure of Nat1/2(-/-) mice produced increased G-T transversions in males and a decrease in females, even though females had higher levels of N-(deoxyguanosin-8-yl)-4-ABP adducts. There was no correlation of mutant frequencies or spectra between mice dosed as neonates or as adults. These results suggest that observed sex- and NAT-dependent differences in ABP-induced liver tumor incidence in mice are not due to differences in either mutation rates or mutational spectra, and that mechanisms independent of carcinogen bioactivation, covalent DNA binding and mutation may be responsible for these differences.

An improved liquid chromatography-tandem mass spectrometry method for the quantification of 4-aminobiphenyl DNA adducts in urinary bladder cells and tissues

Exposure to 4-aminobiphenyl (4-ABP), an environmental and tobacco smoke carcinogen that targets the bladder urothelium, leads to DNA adduct formation and cancer development [1]. Two major analytical challenges in DNA adduct analysis of human samples have been limited sample availability and the need to reach detection limits approaching the part-per-billion threshold. By operating at nano-flow rates and incorporating a capillary analytical column in addition to an online sample enrichment step, we have developed a sensitive and quantitative HPLC-MS/MS method appropriate for the analysis of such samples. This assay for the deoxyguanosine adduct of 4-ABP (dG-C8-4-ABP) gave mass detection limits of 20amol in 1.25microg of DNA (5 adducts in 10(9) nucleosides) with a linear range of 70amol to 70fmol. 4-ABP-exposed human bladder cells and rat bladder tissue were analyzed in triplicate, and higher dose concentrations led to increased numbers of detected adducts. It was subsequently established that sample requirements could be further reduced to 1microg digestions and the equivalent of 250ng DNA per injection for the detection of low levels of dG-C8-4-ABP in a matrix of exfoliated human urothelial cell DNA. This method is appropriate for the characterization and quantification of DNA adducts in human samples and can lead to a greater understanding of their role in carcinogenesis and also facilitate evaluation of chemopreventive agents.

Discovery of potent, selective, and orally active carboxylic acid based inhibitors of matrix metalloproteinase-13

The matrix metalloproteinase enzyme MMP-13 plays a key role in the degradation of type II collagen in cartilage and bone in osteoarthritis (OA). An effective MMP-13 inhibitor would therefore be a novel disease modifying therapy for the treatment of arthritis. Our efforts have resulted in the discovery of a series of carboxylic acid inhibitors of MMP-13 that do not significantly inhibit the related MMP-1 (collagenase-1) or tumor necrosis factor-alpha (TNF-alpha) converting enzyme (TACE). It has previously been suggested (but not proven) that inhibition of the latter two enzymes could lead to side effects. A promising carboxylic acid lead 9 was identified and a convergent synthesis developed. This paper describes the optimization of 9 and the identification of a compound 24f for further development. Compound 24f is a subnanomolar inhibitor of MMP-13 (IC(50) value 0.5 nM and K(i) of 0.19 nM) having no activity against MMP-1 or TACE (IC(50) of >10000 nM). Furthermore, in a rat model of MMP-13-induced cartilage degradation, 24f significantly reduced proteoglycan release following oral dosing at 30 mg/kg (75% inhibition, p < 0.05) and at 10 mg/kg (40% inhibition, p < 0.05).

4-Aminobiphenyl and DNA Reactivity: Case Study Within the Context of the 2006 IPCS Human Relevance Framework for Analysis of a Cancer Mode of Action for Humans

The IPCS Human Relevance Framework was evaluated for a DNA-reactive (genotoxic) carcinogen, 4-aminobiphenyl, based on a wealth of data in animals and humans. The mode of action involves metabolic activation by N-hydroxylation, followed by N-esterification leading to the formation of a reactive electrophile, which binds covalently to DNA, principally to deoxyguanosine, leading to an increased rate of DNA mutations and ultimately to the development of cancer. In humans and dogs, the urinary bladder urothelium is the target organ, whereas in mice it is the bladder and liver; in other species, other tissues can be involved. Differences in organ specificity are thought to be due to differences in metabolic activation versus inactivation. Based on qualitative and quantitative considerations, the mode of action is possible in humans. Other biological processes, such as toxicity and regenerative proliferation, can significantly influence the dose response of 4-aminobiphenyl-induced tumors. Based on the IPCS Human Relevance Framework, 4-aminobiphenyl would be predicted to be a carcinogen in humans, and this is corroborated by extensive epidemiologic evidence. The IPCA Human Relevance Framework is useful in evaluating DNA-reactive carcinogens.

4-Aminobiphenyl induces liver DNA adducts in both neonatal and adult mice but induces liver mutations only in neonatal mice

The mechanisms underlying the susceptibility of neonatal mice to genotoxic carcinogens were investigated by analyzing the DNA adducts and mutations induced in the livers of neonatal and adult Big Blue transgenic mice by 4-aminobiphenyl (4-ABP), a potent human and rodent carcinogen. Neonatal and adult mice were treated with a regimen of 4-ABP known to induce tumors in neonatal mice. Animals were sacrificed 1 day after the last treatment for DNA adduct analysis and 8 weeks after the last treatment for analysis of lacI and cII mutant frequency (MF). N-(Deoxyguanosin-8-yl)-4-ABP was the major DNA adduct identified in the livers of the 4-ABP-treated mice and levels of this adduct were significantly higher in treated animals than in the controls for both the neonates and adults. Adduct levels for adult females (44.0 +/- 4.8 adducts/10(6) nucleotides) were higher than in neonatal females (25.9 +/- 2.2 adducts/10(6) nucleotides), while adduct levels in adult males (13.5 +/- 2.0 adducts/10(6) nucleotides) were lower than in neonatal males (33.8 +/- 4.1 adducts/10(6) nucleotides). 4-ABP treatment significantly increased the liver cII MFs in both sexes of neonatal mice but not in adult mice. Sequence analysis of cII mutant DNA revealed that 4-ABP induced a unique spectrum of mutations in neonatal mice, characterized by a high frequency of G:C-->T:A transversion, while the mutation spectrum in 4-ABP-treated adults was similar to that of control mice. Our results indicate that DNA adduct formation by 4-ABP depends as much on sex as it does on age, whereas the conversion of DNA adducts into mutations differed with animal age. These observations suggest that neonates are more sensitive than adults to genotoxic carcinogens because the relatively high levels of cell division in the developing animal facilitate the conversion of DNA damage into mutation. Supplementary material for this article can be found on the International Journal of Cancer website at http://www.interscience.wiley.com/jpages/0020-7136/suppmat/index.html

Detailed review of transgenic rodent mutation assays

Induced chromosomal and gene mutations play a role in carcinogenesis and may be involved in the production of birth defects and other disease conditions. While it is widely accepted that in vivo mutation assays are more relevant to the human condition than are in vitro assays, our ability to evaluate mutagenesis in vivo in a broad range of tissues has historically been quite limited. The development of transgenic rodent (TGR) mutation models has given us the ability to detect, quantify, and sequence mutations in a range of somatic and germ cells. This document provides a comprehensive review of the TGR mutation assay literature and assesses the potential use of these assays in a regulatory context. The information is arranged as follows. (1) TGR mutagenicity models and their use for the analysis of gene and chromosomal mutation are fully described. (2) The principles underlying current OECD tests for the assessment of genotoxicity in vitro and in vivo, and also nontransgenic assays available for assessment of gene mutation, are described. (3) All available information pertaining to the conduct of TGR assays and important parameters of assay performance have been tabulated and analyzed. (4) The performance of TGR assays, both in isolation and as part of a battery of in vitro and in vivo short-term genotoxicity tests, in predicting carcinogenicity is described. (5) Recommendations are made regarding the experimental parameters for TGR assays, and the use of TGR assays in a regulatory context.

Mutagenicity testing with transgenic mice. Part I: Comparison with the mouse bone marrow micronucleus test

As part of a larger literature study on transgenic animals in mutagenicity testing, test results from the transgenic mutagenicity assays (lacI model; commercially available as the Big Blue(R) mouse, and the lacZ model; commercially available as the Mutatrade markMouse), were compared with the results on the same substances in the more traditional mouse bone marrow micronucleus test. 39 substances were found which had been tested in the micronucleus assay and in the above transgenic mouse systems. Although, the transgenic animal mutation assay is not directly comparable with the micronucleus test, because different genetic endpoints are examined: chromosome aberration versus gene mutation, the results for the majority of substances were in agreement. Both test systems, the transgenic mouse assay and the mouse bone marrow micronucleus test, have advantages and they complement each other. However, the transgenic animal assay has some distinct advantages over the micronucleus test: it is not restricted to one target organ and detects systemic as well as local mutagenic effects.

Recent advances in the protocols of transgenic mouse mutation assays

Transgenic mutation assays were developed to detect gene mutations in multiple organs of mice or rats. The assays permit (1) quantitative measurements of mutation frequencies in all tissues/organs including germ cells and (2) molecular analysis of induced and spontaneous mutations by DNA sequencing analysis. The protocols of recently developed selections in the lambda phage-based transgenic mutation assays, i.e. cII, Spi(-) and 6-thioguanine selections, are described, and a data set of transgenic mutation assays, including those using Big Blue and Muta Mouse, is presented.