N-sulfoöxy-2-aminofluorene is the major ultimate electrophilic and carcinogenic metabolite of N-hydroxy-2-acetylaminofluorene in the livers of infant male C57BL/6J x C3H/HeJ F1 (B6C3F1) mice

N-OH-AABP Modifications in Human DNA May Lead to Auto-Antibodies in Bladder Cancer Subjects

4-Aminobiphenyl (4-ABP) and other related arylamines have emerged to be responsible for human urinary bladder tumors and cancers. Hemoglobin-ABP adducts have been recognized in the blood of smokers, and it builds up in the circulatory system over the period of years that might lead to a bladder tumor. N-hydroxy-Acetyl 4-Aminobiphenyl (N-OH-AABP) is one of the reactive forms of 4-ABP which has a potential to initiate tumor growth and causes cancer rapidly. In the present study, commercially available human DNA was modified by N-OH-AABP, and its modifications were analyzed biophysically from fluorescence spectroscopy and thermal denaturation studies. Further, Sera and IgG from bladder cancer patients' blood were assessed for affinity to native and N-OH-AABP modified human DNA using ELISA. The study showed N-OH-AABP caused damage in the structure of the DNA macromolecule and the perturbations resulting from damage leads to change in the Tm of the DNA molecule. Bladder cancer auto-antibodies, particularly in smoker group, showed preferential binding to N-OH-AABP modified human DNA. This study shows that N-OH-AABP modified DNA could be an antigenic stimulus for the generation of autoantibodies in the sera of bladder cancer patients.

Mutagenicity of N-acyloxy-N-alkoxyamides as an indicator of DNA intercalation: The role of fluorene and fluorenone substituents as DNA intercalators

N-Acyloxy-N-alkoxyamides are direct-acting mutagens in S. typhimurium TA100 and TA98. A reliable QSAR for their activity in TA100 has been developed, which indicates reversible intercalation into the DNA helix through naphthalene substituents. In this paper, we show that fluorene as a substituent does not facilitate intercalation while fluorenone does, although the efficacy is determined by the position of substitution on the fluorenone as well as the N-acyloxy-N-alkoxyamide side chain. Where intercalation is evident, the increased binding to DNA is similar to that of naphthalene and is worth the equivalent of ca four LogP hydrophobicity units. 4-Substituted fluorenones, where the anomeric amide group is in the bay region do not intercalate, which is attributed to the requirement for a weaker edge-on, rather than an end-on intercalation. Mutagencity in S. typhimurium TA98, which detects frame shifts through intercalation, supports the findings. Fluorene appears not to intercalate, which points to the fact that the charge delocalised 2-fluorenylnitrenium ion, the ultimate metabolite from 2-aminofluorene (AF) and 2-acetylaminofluorene (AAF) is the itercalating agent responsible for frameshift mutations leading to their carcinogenicity.

Genotoxic effect of N-hydroxy-4-acetylaminobiphenyl on human DNA: implications in bladder cancer

BACKGROUND

The interaction of environmental chemicals and their metabolites with biological macromolecules can result in cytotoxic and genotoxic effects. 4-Aminobiphenyl (4-ABP) and several other related arylamines have been shown to be causally involved in the induction of human urinary bladder cancers. The genotoxic and the carcinogenic effects of 4-ABP are exhibited only when it is metabolically converted to a reactive electrophile, the aryl nitrenium ions, which subsequently binds to DNA and induce lesions. Although several studies have reported the formation of 4-ABP-DNA adducts, no extensive work has been done to investigate the immunogenicity of 4-ABP-modified DNA and its possible involvement in the generation of antibodies in bladder cancer patients.

METHODOLOGY/PRINCIPAL FINDINGS

Human DNA was modified by N-hydroxy-4-acetylaminobiphenyl (N-OH-AABP), a reactive metabolite of 4-ABP. Structural perturbations in the N-OH-AABP modified DNA were assessed by ultraviolet, fluorescence, and circular dichroic spectroscopy as well as by agarose gel electrophoresis. Genotoxicity of N-OH-AABP modified DNA was ascertained by comet assay. High performance liquid chromatography (HPLC) analysis of native and modified DNA samples confirmed the formation of N-(deoxyguanosine-8-yl)-4-aminobiphenyl (dG-C8-4ABP) in the N-OH-AABP damaged DNA. The experimentally induced antibodies against N-OH-AABP-modified DNA exhibited much better recognition of the DNA isolated from bladder cancer patients as compared to the DNA obtained from healthy individuals in competitive binding ELISA.

CONCLUSIONS/SIGNIFICANCE

This work shows epitope sharing between the DNA isolated from bladder cancer patients and the N-OH-AABP-modified DNA implicating the role of 4-ABP metabolites in the DNA damage and neo-antigenic epitope generation that could lead to the induction of antibodies in bladder cancer patients.

Strategy for genotoxicity testing—Metabolic considerations

The report from the 2002 International Workshop on Genotoxicity Tests (IWGT) Strategy Expert Group emphasized metabolic considerations as an important area to address in developing a common strategy for genotoxicity testing. A working group convened at the 2005 4th IWGT to discuss this area further and propose practical strategy recommendations. To propose a strategy, the working group reviewed: (1) the current status and deficiencies, including examples of carcinogens "missed" in genotoxicity testing, established shortcomings of the standard in vitro induced S9 activation system and drug metabolite case examples; (2) the current status of possible remedies, including alternative S9 sources, other external metabolism systems or genetically engineered test systems; (3) any existing positions or guidance. The working group established consensus principles to guide strategy development. Thus, a human metabolite of interest should be represented in genotoxicity and carcinogenicity testing, including evaluation of alternative genotoxicity in vitro metabolic activation or test systems, and the selection of a carcinogenicity test species showing appropriate biotransformation. Appropriate action triggers need to be defined based on the extent of human exposure, considering any structural knowledge of the metabolite, and when genotoxicity is observed upon in vitro testing in the presence of metabolic activation. These triggers also need to be considered in defining the timing of human pharmaceutical ADME assessments. The working group proposed two strategies to consider; a more proactive approach, which emphasizes early metabolism predictions to drive appropriate hazard assessment; and a retroactive approach to manage safety risks of a unique or "major" metabolite once identified and quantitated from human clinical ADME studies. In both strategies, the assessment of the genotoxic potential of a metabolite could include the use of an alternative or optimized in vitro metabolic activation system, or direct testing of an isolated or synthesized metabolite. The working group also identified specific areas where more data or experiences need to be gained to reach consensus. These included defining a discrete exposure action trigger for safety assessment and when direct testing of a metabolite of interest is warranted versus the use of an alternative in vitro activation system, a universal recommendation for the timing of human ADME studies for drug candidates and the positioning of metabolite structural knowledge (through in silico systems, literature, expert analysis) in supporting metabolite safety qualification. Lastly, the working group outlined future considerations for refining the initially proposed strategies. These included the need for further evaluation of the current in vitro genotoxicity testing protocols that can potentially perturb or reduce the level of metabolic activity (potential alterations in metabolism associated with both the use of some solvents to solubilize test chemicals and testing to the guidance limit dose), and proposing broader evaluations of alternative metabolic activation sources or engineered test systems to further challenge the suitability of (or replace) the current induced liver S9 activation source.

Identification of new DNA adducts in human bladder epithelia exposed to the proximate metabolite of 4-aminobiphenyl using 32P-postlabeling method

The DNA adducts were analyzed by 32P-postlabeling method following exposure of human uroepithelial cells (HUC) to N-hydroxy-4-aminobiphenyl (N-OH-ABP), the proximate metabolite of the human bladder carcinogen 4-aminobiphenyl (ABP). TLC of the postlabeled products on the first dimension revealed several products, the majority of which stayed close to the origin and were earlier identified as the 3',5' -bisphospho derivatives of N-(deoxyguanosin-8-yl)-4-aminobiphenyl and N-(deoxyadenosin-8-yl)-4-aminobiphenyl (Carcinogenesis 13 (1993) 955; Carcinogenesis 16 (1995) 295). Here we report characterization of two additional adducts that amounted to less than 5% of the total adducts. Autoradiography of D1 chromatogram of the postlabeled products of calf thymus DNA chemically interacted with N-OH-ABP under acidic conditions revealed two adducts, #1 and #2, with R(f) values of about 0.2 and 0.3, respectively. Two adducts with D1 thin layer chromatographic properties similar to those of adducts #1 and #2 were obtained on postlabeling analyses of products generated by chemical interaction of N-acetoxy-4-aminobiphenyl (N-OAc-ABP) with deoxyguanosine-3' -monophosphate (dGp). Based on proton NMR and mass spectroscopic analyses of the synthetic products derived from N-OAc-ABP, the chemical structures of adducts #1 and #2 have been identified as 3-(deoxyguanosin-N(2)-yl)-4-aminobiphenyl, and N-(deoxyguanosin-N(2)-yl)-4-aminobiphenyl, respectively. Both of these adducts were insensitive to digestion with nuclease P1. 32P-Postlabeling analysis of the nuclease P1 enriched DNA hydrolysate of HUC cells treated with N-OH-ABP showed the presence of adduct #2 but not adduct #1. Adduct #2 was also detected in calf thymus DNA incubated with HUC cytosol and N-OH-ABP in the presence of acetyl CoA. These results suggest that in the target cells for ABP carcinogenesis in vivo, N-OH-ABP is bioactivated by acetyl CoA-dependent acyltransferases to reactive arylnitrenium ions that covalently interact at N(2)-position of deoxyguanosine in DNA.

Identification of N-(deoxyguanosin-8-yl)-4-azobiphenyl by (32)P-postlabeling analyses of DNA in human uroepithelial cells exposed to proximate metabolites of the environmental carcinogen 4-aminobiphenyl

DNA adducts formed in human uroepithelial cells (HUC) following exposure to N-hydroxy-4-aminobiphenyl (N-OH-ABP), the proximate metabolite of the human bladder carcinogen 4-aminobiphenyl (ABP), were analyzed by the (32)P-postlabeling method. Two adducts detected by (32)P-postlabeling were previously identified as the 3',5'-bisphospho derivatives of N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-ABP) and N-(deoxyadenosin-8-yl)-4-aminobiphenyl (dA-C8-ABP) (Frederickson S et al. [1992] Carcinogenesis 13: 955-961; Hatcher and Swaminathan [1995b] Carcinogenesis 16: 295-301). In contrast to the dG-C8-ABP adduct, which was 3'-dephosphorylated by nuclease P1, dA-C8-ABP was resistant to nuclease P1, thus providing an enrichment step before postlabeling. Autoradiography of the two-dimensional thin-layer chromatogram of the postlabeled products obtained following nuclease P1 digestion revealed several minor adducts, one of which has been identified in the present study. Postlabeling analyses following nuclease P1 digestion of the products obtained from the reaction of N-acetoxy-4-aminobiphenyl with deoxyguanosine-3'-monophosphate (dGp) demonstrated the presence of this minor adduct. The 3'-monophosphate derivative of the adduct was subsequently chromatographically purified and subjected to spectroscopic analyses. Based on proton NMR and mass spectroscopic analyses of the synthetic product, the chemical structure of the adduct has been identified as N-(deoxyguanosin-N(2)-yl)-4-azobiphenyl (dG-N==N-ABP). (32)P-Postlabeling analysis of the nuclease P1-enriched DNA hydrolysate of HUCs treated with N-OH-ABP or N-hydroxy-4-acetylaminobiphenyl (N-OH-AABP) showed the presence of the dG-N==N-ABP adduct. It was also detected in calf thymus DNA incubated with HUC cytosol and N-OH-ABP in the presence of acetyl-CoA, or incubated with HUC microsomes and N-OH-AABP. These results demonstrate that in the target cells for ABP carcinogenesis in vivo, N-OH-ABP and N-OH-AABP are bioactivated by acyltransferases to reactive arylnitrenium ions that covalently interact at the N2 position of deoxyguanosine in DNA.

Metabolic activation capacity of neonatal mice in relation to the neonatal mouse tumorigenicity bioassay

The neonatal mouse tumorigenicity bioassay is a well-developed animal model that has recently been recommended as an alternative tumorigenicity bioassay by the International Conference on Harmonization (ICH) for Technical Requirements for the Registration of Pharmaceuticals for Human Use. There are sufficient data to conclude that this animal model is highly sensitive to genotoxic chemical carcinogens that exert their tumorigenicity through mechanisms involving the formation of covalently bound exogenous DNA adducts that lead to mutation. On the other hand, it is not sensitive to chemical carcinogens that exert tumorigenicity through a secondary mechanism. The metabolizing enzymes present in the neonatal mouse, particularly the cytochromes P450, are critical factors in determining the tumorigenic potency of a chemical tested in this bioassay. However, compared to the metabolizing enzymes of the adult mouse and rat, the study of the metabolizing enzymes in neonatal mouse tissues has been relatively limited.

Comparison of three different in vitro mutation assays used for the investigation of cytochrome P450-mediated mutagenicity of nitro-polycyclic aromatic hydrocarbons

Three different in vitro mutation assays were used to investigate the involvement of cytochrome P450 enzymes in the activation of the nitro-polycyclic aromatic hydrocarbons (nitroPAHs) 1-nitropyrene and 2-nitrofluorene and their reduced metabolites amino-polycyclic aromatic hydrocarbons (aminoPAHs) 1-aminopyrene and 2-aminofluorene. Mutagenicity was investigated at the HPRT locus in Chinese hamster V79 cells with (V79-NH) or without (V79-MZ) endogenous acetyltransferase activity, stably expressing human cytochrome P450 cDNAs; in NIH/3T3 control or stably expressing human CYP1A2 cells, in combination with a shuttle vector containing a reporter gene; and in Salmonella typhimurium TA98, by inhibition of cytochrome P450 enzymes in rat liver S9 mix. Both the HPRT assay and the Ames test did not show any involvement of CYP3A in the activation of 1-nitropyrene to a mutagenic metabolite. In addition, a clear involvement of CYP1A2 in the activation of the nitroPAH 1-nitropyrene was demonstrated in both mutation assays using eukaryotic cells. However, no activation of 1-nitropyrene was seen in the eukaryotic cell lines when expressing only CYP1A2 (V79-MZ1A2) or acetyltransferase (V79-NH, 3T3-LNCX). The reduced metabolite of 1-nitropyrene, 1-aminopyrene, was also shown to be activated to a mutagenic metabolite by CYP1A2, using 3T3-1A2 cells in combination with a shuttle vector, and the Amestest in combination with the specific CYP1A2 inhibitor furafylline. No clear involvement of cytochrome P450 could be demonstrated for activation of 2-nitrofluorene to a mutagenic metabolite, whereas a role for CYP1A2 in the bioactivation of 2-aminofluorene is suggested. In the present study, we have demonstrated the complementary value of the three in vitro mutation assays in the examination of promutagen activation pathways.

Mutagenicity of 4-aminobiphenyl and 4-acetylaminobiphenyl in Salmonella typhimurium strains expressing different levels of N-acetyltransferase

4-Aminobiphenyl (4-ABP), an aromatic amine present in tobacco smoke, is an animal and human carcinogen. 4-ABP can undergo several biotransformation reactions to yield DNA-binding species. The role of acetylation in the biotransformation of 4-ABP to reactive intermediates was investigated by determining mutagenicity in Salmonella typhimurium strains expressing various levels of acetyltransferases (NAT/OAT). Strain YG1029, which has multiple copies of the NAT/OAT gene, was the most sensitive to 4-ABP. With rat S9 activation, 4-ABP (5 micrograms/plate) induced 789 +/- 98 revertants/plate. At that concentration, an average of 200 revertants/plate was seen in both TA100, which has a single copy of the NAT/OAT gene, and in TA100/1,8DNP6, which is NAT/OAT deficient. This pattern was also present when the bacteria were exposed to the acetylated derivative, 4-acetylaminobiphenyl (4-AABP). At 10 micrograms/plate, 4-AABP induced 855 +/- 47 revertants/plate in YG1029 while 169 +/- 39 and 149 +/- 28 revertants/plate were observed in strains TA100 and TA100/1,8DNP6, respectively. The mutagenic profiles of 4-ABP and 4-AABP observed with the mouse S9 activating system were similar to that seen with the rat. These data establish a correlation between increased bacterial NAT/OAT activity and increased mutagenicity of 4-ABP. Results with both 4-ABP and 4-AABP support acetylation of the oxygen to be a key step in activation.

Electrochemical detection and quantification of the acetylated and deacetylated C8-deoxyguanosine DNA adducts induced by 2-acetylaminofluorene

The genotoxic agent 2-acetylaminofluorene induces, upon metabolic activation, two main types of DNA adducts in animal tissue, i.e., (deoxyguanine-8-yl)-aminofluorene (dG-C8-AF) and N-(deoxyguanine-8-yl)-acetylaminofluorene (dG-C8-AAF). Quantification of the frequency of these adducts usually relies on the use of radioactively labeled 2-acetylaminofluorene. Here, we report the development of a sensitive, non-radioactive method for the quantification of dG-C8-AF and dG-C8-AAF. Essentially, the modified DNA bases are separated by high-performance liquid chromatography (HPLC) and quantified by electrochemical detection. We established that both modified bases guanine-C8-aminofluorene and guanine-C8-acetylaminofluorene are electrochemically active. Subsequently, a procedure was developed to quantify dG-C8-AF and dG-C8-AAF in genomic DNA. Following DNA hydrolysis the adducted bases were extracted by ethyl acetate, separated by HPLC, and detected electrochemically. This procedure has been applied in the analysis of dG-C8-AAF in N-acetoxy-2-acetylaminofluorene-modified calf thymus DNA and in the detection of dG-C8-AAF and dG-C8-AF in liver DNA of mice injected intraperitoneally with 150-450 mg N-hydroxy-2-acetylaminofluorene/kg. The quantification of relatively low dG-C8-AF and dG-C8-AAF adduct levels (i.e., 0.1-1 adduct/10(6) nucleotides) in mouse liver DNA demonstrates the sensitivity of this electrochemical detection procedure. The detection limit of the method is 1 adduct per 10(6) nucleotides for both adducts using 20 micrograms of DNA and 4 adducts per 10(8) nucleotides using 500 micrograms DNA.

Aryl sulfotransferase IV deficiency in rat liver carcinogenesis initiated with diethylnitrosamine and promoted with N-2-fluorenylacetamide or its C-9-oxidized metabolites

Down regulation of aryl sulfotransferase IV (AST IV) in promotion/progression of liver carcinogenesis by N-2-fluorenylacetamide (2-FAA) has been established. This study examined whether the C-9 oxidized metabolites of 2-FAA, which have recently been shown to promote diethylnitrosamine (DEN)-initiated liver carcinogenesis in male Sprague-Dawley rats, effect the above change. Hence, in DEN-initiated rats, the effects of promoting regimens of 9-OH-2-FAA or 9-oxo-2-FAA, 15 oral doses at 50 and 100 mumol/kg of body weight, were compared to those of 2-FAA at 50 mumol/kg of body weight and of the vehicle on the activity of N-hydroxy(OH)-2-FAA sulfotransferase (ST), an isozyme of AST IV and AST IV expression and distribution. Relative to the vehicle, treatment with the fluorenyl compounds led to decreased levels in hepatic N-OH-2-FAA ST activity and development of hepatic nodules and tumors which had still lower levels of the ST activity than the respective remnant livers. At approximately 8 months after treatment with the C-9-oxidized compounds at doses twice that of 2-FAA, the extents of decreases in the hepatic N-OH-2-FAA ST activity and cytosolic AST IV protein in tumors were comparable to those with 2-FAA. Immunocytochemical analysis showed close association of AST IV deficiency with neoplastic liver lesions. In comparison to N-OH-2-FAA, 9-OH-2-FAA had only low and 9-oxo-2-FAA lacked sulfate acceptor activity in the presence of male rat liver cytosol or AST IV. At 3.3-fold greater concentration than N-OH-2-FAA, 9-oxo-2-FAA inhibited (27%) the sulfate acceptor activity of N-OH-2-FAA in the presence of AST IV, which suggested interference by 9-oxo-2-FAA at the active site. Although the C-9-oxidized compounds do not appear to be substrates for N-OH-2-FAA ST, their ability to cause a decrease in N-OH-2-FAA ST activity and protein similar to that of 2-FAA supports their role in hepatocarcinogenesis. Whereas 9-OH-2-FAA had a 3.9-fold greater sulfate acceptor activity in the presence of female than male rat liver cytosol and inhibited dehydroepiandrosterone ST activity of female rat liver, N-OH-2-FAA and 9-oxo-2-FAA inhibited estrone ST activity of male rat liver, suggesting that the C-9-oxidized compounds as well as N-OH-2-FAA are substrates for STs other than AST IV.

Homodimeric and heterodimeric aryl sulfotransferases catalyze the sulfuric acid esterification of N-hydroxy-2-acetylaminofluorene

Three aryl sulfotransferases (ASTs) isolated from rat liver catalyze the sulfuric acid esterification of the carcinogen N-hydroxy-2-acetylaminofluorene (N-OH-2AAF). These three ASTs were separated by high resolution anion exchange chromatography and were designated Q1, Q2, and Q3. Q1 and Q2 had high N-OH-2AAF sulfonation activity, whereas Q3 showed low activity. Reversed phase high performance liquid chromatography/mass spectrometry analysis showed Q1-Q3 to be comprised of 33,945- and 35,675-Da protein subunits. Q1 contained only the 35,675-Da protein subunit, Q2 contained equal quantities of 33,945- and 35,675-Da subunits, and Q3 contained only the 33,945-Da subunit. The subunit compositions of Q1-Q3 were confirmed by immunochemical analysis. Size exclusion high performance liquid chromatography confirmed that the active quaternary structure of the three isoenzymes was dimeric. Analysis of liver cytosols for the relative contributions of Q1-Q3 to total cytosolic N-OH-2AAF sulfotransferase activity indicated the Q1, Q2, and Q3 accounted for 44, 46, and 10% of the activity, respectively. These results demonstrate the existence of both homodimeric and heterodimeric aryl sulfotransferases and show that two ASTs, a homodimer of 35,675-Da subunits and a heterodimer of a 33,945- and a 35,675-Da subunit, are primarily responsible for hepatic N-OH-2AAF sulfotransferase activity.

Rat phenol-preferring sulfotransferase genes (Stp and Stp2): localization to mouse chromosomes 7 and 17

The phenol-preferring sulfotransferases aryl sulfotransferase IV and N-hydroxyarylamine sulfotransferase catalyze sulfate conjugation of N-hydroxy-2-acetylaminofluorene, a metabolite capable of causing hepatocarcinogenesis in rats. We utilized published cDNA sequences of these sulfotransferases to type the progeny of two multilocus crosses and determined that the genes, aryl sulfotransferase (Stp) and N-hydroxyarylamine sulfotransferase (Stp2), map to positions on mouse chromosomes 7 and 17.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Roles of electrophilic sulfuric acid ester metabolites in mutagenesis and carcinogenesis by some polynuclear aromatic hydrocarbons

Hydroxylation of meso-methyl groups with subsequent formation of reactive benzylic esters bearing a good leaving group (e.g. sulfate) was proposed as a possible biochemical mechanism of activation and tumorigenicity of methyl-substituted polycyclic aromatic hydrocarbons (PAHs). In support of this postulation, recent studies have demonstrated the formation by rodent hepatic sulfotransferase activity of electrophilic, mutagenic, and carcinogenic sulfuric acid esters of several hydroxymethyl aromatic hydrocarbons including hydroxymethyl derivatives of benz[a]anthracene, 6-hydroxymethylbenzo[a]pyrene, 5-hydroxymethylchrysene, 9-hydroxymethyl-10-methylanthracene, and 1-hydroxymethylpyrene. Besides these hydroxymethyl PAHs containing a primary benzylic alcoholic group, some aromatic hydrocarbons with secondary benzylic hydroxyl functional group(s) are also metabolically activated through sulfuric acid esterification.

Sulfonation in chemical carcinogenesis--history and present status

Many laboratories have characterized the electrophilic metabolites of chemical carcinogens and their covalently bound adducts with genomic DNA in vivo. Recent studies from our laboratory have shown that enzymatic sulfonation of members of several classes of proximate carcinogens containing C- or N-hydroxy groups converts them to electrophilic, mutagenic, and carcinogenic sulfuric acid ester metabolites in mouse liver. These compounds form the subject of this report.

Molecular specificity of aryl sulfotransferase IV (tyrosine-ester sulfotransferase) for xenobiotic substrates and inhibitors

Studies on the interactions of benzylic alcohols, aldehydes, and carboxylic acids with homogeneous preparations of aryl sulfotransferase (AST) IV have yielded information about the nature of the active site of the enzyme. Lipophilicity and stereochemical configuration of benzylic alcohols are key factors in determining their interaction with the active site of AST IV. Furthermore, aldehydes and carboxylic acids corresponding to the subsequent oxidation states derived from benzylic alcohols are inhibitors of the enzyme. Additional investigations on the catalytic specificity of AST IV indicate that both primary and secondary N-hydroxy arylamines can serve as substrates for the enzyme. These results with benzylic alcohols, aldehydes, carboxylic acids, and N-hydroxy arylamines have yielded insight into some of the parameters important in recognition of substrates and inhibitors by the active site of the enzyme and should be useful both in understanding in vivo metabolic interactions and in designing appropriate new inhibitors to use as selective probes for the role of sulfation in metabolism of specific xenobiotics.

Carcinogen activation by sulfate conjugate formation

The foregoing pages presented a substantial body of data that established that sulfotransferase conjugation can transform many xenobiotics into agents that can modify cellular macromolecules. However, activation by sulfation is rarely the only metabolic pathway that is open to these compounds; other pathways can become more important in response to a variety of factors. This metabolic switching can be produced by substrate concentration, cofactor availability, kinetic factors that dictate the velocity of the various possible conjugation reactions, and, in some cases, competition between Phase-I and Phase-II metabolism. Also, it is important to realize that demonstration of activation by sulfate ester formation in vitro does not necessarily mean that a similar activation process will occur in vivo. Experience also teaches that argument by analogy can be very misleading in the case of sulfate activation. Small structural differences can upset the delicate balance between sulfate activation and the various other competing pathways. Nevertheless, sulfation is an important mechanism by which a number of chemicals are transformed to their activated forms.

Metabolic activation of carcinogens

Most chemical carcinogens are not active in themselves but require bioactivation to electrophiles that bind covalently to DNA and often act by producing mutations. In recent years it has been realized that mutations can be important at many stages of carcinogenesis. A variety of different enzymes are involved in bioactivation reactions, which include oxidation, reduction, thiol conjugation, acetyl transfer, sulfur transfer, methyl transfer, glucuronosyl transfer, and epoxide hydrolysis. These processes often occur in concert with a single carcinogen. Humans vary considerably in activities of these enzymes and this variation may contribute to differences in risk.

Age- and sex-related differences in activation of the carcinogen 7-hydroxymethyl-12-methylbenz[a]anthracene to an electrophilic sulfuric acid ester metabolite in rats. Possible involvement of hydroxysteroid sulfotransferase activity

Metabolic activation of 7-hydroxymethyl-12-methylbenz[a]anthracene (HMBA) and related hydroxymethyl polycyclic aromatic hydrocarbons to electrophilic and mutagenic sulfuric acid esters has been demonstrated previously (Watabe et al., In: Xenobiotic Metabolism and Disposition (Eds. Kato R, Estabrook RW and Cayen MN), pp. 393-400. Taylor & Francis, London, 1989). In the present study, the rat hepatic sulfotransferase activity catalyzing the formation of such reactive sulfuric acid esters was inhibited strongly by dehydroepiandrosterone, a typical substrate hydroxysteroid sulfotransferases (HSSTs). Pentachlorophenol, a potent phenol sulfotransferase inhibitor, had little effect in this regard. A marked sex difference was observed for the hepatic cytosolic sulfotransferase activity for HMBA in rats. This sex difference was age-related; no significant difference was observed in preweanling rats, whereas in adult rats female rat liver showed a much higher enzyme activity. These age- and sex-related differences in the sulfonation of HMBA reflect the regulation of HMBA sulfotransferase activity by gonadal hormones as previously demonstrated with HSSTs. Thus, pretreatment with estradiol benzoate significantly enhanced the sulfotransferase activity for HMBA in both male and female rats, (P less than 0.01 and P less than 0.05 respectively), whereas testosterone propionate pretreatment decreased this activity. Castration of male rats increased the HMBA sulfotransferase activity 2- to 3-fold compared with that in control animals. By contrast, ovariectomy reduced the enzyme activity 38% in females. These results imply that rat liver HSST activity is responsible for the sulfonation of HMBA. Intraperitoneal injection of HMBA (0.25 mumol/g body wt) into infant rats produced benzylic DNA adducts in the liver which were chromatographically identical with those obtained from incubations of HMBA with deoxyguanosine and deoxyadenosine in the presence of hepatic cytosolic sulfotransferase activity. Intraperitoneal administration of sodium 7-sulfooxymethyl-12-methylbenz[a]anthracene resulted in much higher levels of these adducts and the deoxycytidine adduct in the liver DNA than did an equimolar amount of the parent hydroxymethyl hydrocarbon. The levels of hepatic benzylic DNA adducts formed from HMBA were reduced markedly by pretreatment of rats with dehydroepiandrosterone, a strong inhibitor of hepatic sulfotransferase activity for this hydrocarbon.

Cell proliferation in carcinogenesis

Chemicals that induce cancer at high doses in animal bioassays often fail to fit the traditional characterization of genotoxins. Many of these nongenotoxic compounds (such as sodium saccharin) have in common the property that they increase cell proliferation in the target organ. A biologically based, computerized description of carcinogenesis was used to show that the increase in cell proliferation can account for the carcinogenicity of nongenotoxic compounds. The carcinogenic dose-response relationship for genotoxic chemicals (such as 2-acetylaminofluorene) was also due in part to increased cell proliferation. Mechanistic information is required for determination of the existence of a threshold for the proliferative (and carcinogenic) response of nongenotoxic chemicals and the estimation of risk for human exposure.

Proliferative and genotoxic cellular effects in 2-acetylaminofluorene bladder and liver carcinogenesis: biological modeling of the ED01 study

The development of tumors in relationship to 2-acetylaminofluorene (AAF) dose and time on study has been evaluated in an experiment conducted by the National Center for Toxicological Research (NCTR) using more than 24,000 female BALB/c mice. By using a biologically based model of two-event carcinogenesis accounting explicitly for both genotoxic and nongenotoxic proliferative effects at the cellular level, we provide a unifying explanation for the apparently disparate dose-response results observed in the urinary bladder and liver. Experimental observations of dose-related DNA adduct levels in both tissues and hyperplasia in the bladder were utilized in estimation of model parameters. Analyses demonstrate that tumor prevalence in the liver can be explained entirely by the influence of AAF on the first of two genetic events, and in the bladder by the synergy between AAF genotoxicity affecting both genetic events and cellular proliferation at higher doses. These results are consistent across the entire ED01 data set.

Reduction of intestinal carcinogen absorption by carcinogen-specific secretory immunity

A secretory immune response to the carcinogen 2-acetylaminofluorene (AAF) was elicited in rabbits by directly immunizing the small intestine with an AAF-cholera toxin conjugate. High-titer, high-affinity secretory immunoglobulin A (IgA) antibody to AAF was secreted into the intestinal lumen in response to this immunogen. Immune secretions reduced the transepithelial absorption of a 125I-labeled derivative of AAF by more than half. This reduction of absorption by hapten-specific IgA suggests that oral vaccines against carcinogens and toxicants could be developed for humans.

Proto-oncogene activation during chemically induced hepatocarcinogenesis in rodents

The liver is a frequent site for the development of chemically induced cancer in rodents. This is primarily owing to the capability of the liver to activate a large variety of exogenous chemicals metabolically to reactive electrophilic species that can covalently interact with cellular DNA and other macromolecules (Miller and Miller, 1966; Miller, 1978). It is the potential alteration of the hepatocellular genome by mutational events that forms the theoretical basis for the heritable nature of cancer as well as, at least in part, the altered phenotype of neoplastic cells; however, our understanding of the exact nature of these heritable genetic alterations remains fragmentary. Within the last decade the delineation of the molecular basis of viral oncogenesis, especially by retroviruses, has revealed potential targets in the cell genome for the reactive forms of chemical agents in relation to their carcinogenic action (Bishop, 1987). Primary among such potential targets are proto-oncogenes, homologous to the transforming genes of oncogenic retroviruses from which they have evolved (Temin, 1974). The objective of this brief review is to consider the evidence that induced alterations in the structure and/or regulation of expression of proto-oncogenes may play one or more roles in rodent hepatocarcinogenesis, especially in relation to the stages of initiation, promotion, and progression (Pitot et al., 1988).

Changes in rat liver N-hydroxy-2-acetylaminofluorene aryl sulfotransferase activity at early and late stages of hepatocarcinogenesis resulting from dietary administration of 2-acetylaminofluorene

The ability of 2-acetylaminofluorene (AAF) to mediate a loss in N-hydroxy-AAF (N-OH-AAF) aryl sulfotransferase activity when fed to male Sprague-Dawley rats was examined at early and late stages of hepatocarcinogenesis. Administration of 0.05% AAF in the diet for 1 week caused liver N-OH-AAF aryl sulfotransferase activity to decrease to 15 +/- 5% of that for liver from non-carcinogen-fed rats, and the activity remained low throughout 19 weeks of AAF feeding. When rats were fed AAF diet for 3 weeks, then placed on a control diet, liver N-OH-AAF aryl sulfotransferase activity returned to normal levels within 3 weeks. In contrast, when rats were fed AAF for 19 weeks, then placed on control diet for an additional 10 weeks, little or no recovery of N-OH-AAF aryl sulfotransferase activity was observed in cytosols from whole livers or isolated hyperplastic nodules, respectively. These findings suggest two types of AAF-mediated decreases in sulfotransferase activity: (a) a decrease observed early in the initial stages of AAF feeding which returns to normal levels when AAF is removed from diet, and (b) a persistent decrease in activity following long term AAF administration.

A rapid and sensitive screening method for the detection of anti-2-acetylaminofluorene immunoglobulins

A method is described in which anti-2-acetylaminofluorene immunoglobulins may be detected using a simple and sensitive screening procedure. The method is based on immunoglobulin binding of an 125I derivatized 2-aminofluorene radiotracer. Tracer binding is not isotype specific, and thus the method is useful for the detection of either IgG or IgA. Competitive binding experiments with the radiotracer were used to determine the specificity of immunoglobulin response by measurement of cross-reactivity with related ligands. This method allows quantitation of the immune response to the carcinogen in serum and other biological fluids (i.e., intestinal secretions).

Irreversible inhibition of the cytosolic metabolism of N-hydroxy-2-acetylaminofluorene by its glycolyl analog

The glycolyl hydroxamic acid derivative of 2-aminofluorene was found to be a potent inhibitor of its own metabolism and the metabolism of N-hydroxy-2-acetylaminofluorene by rat liver cytosol. The inhibition was irreversible, as well as time and concentration dependent, which indicates a suicide-inhibition type of metabolism. There was a direct correlation between the inhibition of N-hydroxy-2-acetylaminofluorene disappearance and 2-acetylaminofluorene formation. In contrast, both the glycolyl and acetyl hydroxamic acid derivatives were metabolized to a similar extent by enzymes in the microsomal fraction.

Species variation in bladder cell and liver cell activation of acetylaminofluorene

The metabolism and mutagenicity of 2-acetylaminofluorene were measured using freshly prepared intact bladder and liver cells from the cow, dog and rat. High pressure liquid chromatography was used to separate 2-acetylaminofluorene metabolites, and Salmonella typhimurium, strain TA98, was used to detect mutagenic intermediates. Species differences as well as animal-to-animal variation within a species were observed. Mutagenic activity with 2-acetylaminofluorene was greater with cow bladder cells than with dog or rat bladder cells. However, dog bladder cells were most active in metabolizing 2-acetylaminofluorene, and rat bladder cells were least active. Liver cells from all three species metabolized 2-acetylaminofluorene to mutagens for Salmonella, with dog and cow cells being more active than rat liver cells. However, cow liver cells were the most active in metabolizing 2-acetylaminofluorene, followed by rat and dog cells. With all cell types studied, except rat bladder cells, aminofluorene was the major metabolite detected. Carbon and N-hydroxylated products were produced by liver and bladder cells of the three species and glucuronide and sulfate conjugates of the metabolites were detected from both cell types. Correlations between mutagenic activity and the level of metabolism or any individual metabolite were not apparent. The data suggest that the relative contribution of bladder cell metabolism in aromatic amine induced bladder cancer may vary with the species.

Enzymatic denitrosation of diphenylnitrosamine: activation or inactivation?

Nitrosodiphenylamine was tested for induction of DNA single strand breaks in rat hepatocytes and Chinese hamster V 79 cells with the alkaline filter elution assay. While in rat hepatocytes DNA damage could be observed, negative results were obtained in V 79 cells. In view of the metabolic capacity of hepatocytes and the chemical structure of nitrosodiphenylamine, it seems likely that cytochrome P-450-dependent, reductive denitrosation might be necessary for exerting this effect. Therefore the metabolism of nitrosodiphenylamine was investigated in phenobarbital-induced mouse liver microsomes. Various metabolites were determined by HPLC. One metabolite was identified as diphenylamine, whereas the others were characterized as p-hydroxydiphenylamine and its corresponding quinoneimine. It is postulated that diphenylhydroxylamine, which is not found as a metabolite, might be involved in exerting the observed genetoxic effects.

Hepatic DNA and RNA adduct formation from the carcinogen 7-hydroxymethyl-12-methylbenz[a]anthracene and its electrophilic sulfuric acid ester metabolite in preweanling rats and mice

DNA and RNA adducts that were chromatographically identical to those formed in vitro on reaction of 7-sulfooxymethyl-12-methyl-benz[a]anthracene with guanine and adenine nucleosides were formed in the livers of rats and mice given i.p. injections of 7-hydroxymethyl- or 7-sulfooxymethyl-12-methyl-benz[a]anthracene. Considerably higher levels of these hepatic adducts were obtained from the latter short-lived electrophilic ester than from the hydroxymethyl compound. These observations are consistent with the finding of rat liver cytosolic sulfotransferase activity for 7-hydroxymethyl-12-methylbenz[a]anthracene (Watabe et al., Science 215, 403, 1982). Formation of these hepatic adducts from 7-hydroxymethyl-12-methylbenz[a]anthracene was inhibited by prior administration to rats of dehydroepiandrosterone, an inhibitor of the sulfotransferase activity for this hydroxymethyl hydrocarbon.

Metabolic denitrosation of diphenylnitrosamine: a possible bioactivation pathway

Nitrosodiphenylamine was tested for induction of DNA single strand breaks in rat hepatocytes and Chinese hamster V 79 cells with the alkaline filter elution assay. While in rat hepatocytes DNA damage was observed, negative results were obtained in V 79 cells. In view of the metabolic capacity of hepatocytes and the chemical structure of nitrosodiphenylamine it seems likely that cytochrome P-450-dependent, reductive denitrosation might be necessary for exerting this effect. Therefore the metabolism of nitrosodiphenylamine was investigated in phenobarbital-induced mouse liver microsomes and some of the metabolites were also tested. One metabolite was identified as diphenylamine whereas the others were identified as a ring-hydroxylated derivative of diphenylamine and its corresponding quinoneimine. Diphenylhydroxylamine which was not detected in the microsomes as a metabolite produced a significant amount of DNA single strand breaks in V 79 cells. When diphenylhydroxylamine was incubated with microsomes electron spin resonance spectrum was observed which indicated the formation of the diphenylnitroxide radical. This radical seems to be mediated by auto-oxidation rather than by enzymatic catalysis. Whether diphenylhydroxylamine might be responsible for the observed genetoxic effects of nitrosodiphenylamine assumed to be produced via active oxygen species is discussed.

Variables influencing DNA-binding in mouse liver

The suitability of certain mouse strains for carcinogenicity testing has been questioned. Some chemicals increase the incidence of liver tumors above a relatively high background, an effect not seen in rats. This raises the question whether species and tissue specific effects are involved which are reflected in the DNA binding of metabolites. DNA binding indices in mouse liver have been determined in only a few instances. They are comparable to those found for rat liver DNA with aniline, benzo(a)-pyrene, butadiene, dimethylnitrosamine, methylnitrosourea and they are lower in the mouse with aflatoxin B1, trans-4-acetylaminostilbene and 2-aminofluorene derivatives. The available data on DNA binding in mouse liver suggest that the same adducts are formed as in rats but that metabolism and repair are variables which can modify the extent of DNA damage. However, the extent of DNA binding does not always correlate with the susceptibility of this tissue to carcinogenesis. But mouse liver is no exception in this respect. It is concluded that the formation of mouse liver tumors in long term studies with genotoxic chemicals indicates tumor initiating potential. In contrast, there are other chemicals such as chlorinated hydrocarbon insecticides which do not bind to DNA to any extent and which are not genotoxic in common short term tests and yet give rise to liver tumors in mice but not in rats. Positive results in long term studies are suggested to indicate promoting properties of such compounds.