Comparison of the in vitro metabolisms and mutagenicities of dibenzo[a,c]anthracene, dibenzo[a,h]anthracene and dibenzo[a,j]anthracene: influence of norharman

Investigation of the Complete Suite of the Leucine and Isoleucine Isomers: Toward Prediction of Ion Mobility Separation Capabilities

In this study we investigated 11 isomers with the molecular formula C₆H₁₃NO₂ (m/z 131) to ascertain the potential of utilizing drift tube ion mobility mass spectrometry to aid in the separation of isomeric mixtures. This study of small molecules provides a detailed examination of the application of uniform field ion mobility for a narrow scope of isomers with variations in both bond coordination and stereochemistry. For small molecules, it was observed that in general constitutional isomers are more readily separated by uniform field mobility in comparison to stereoisomers such as enantiomers or diastereomers. Diastereomers exhibited differences in their collision cross section (CCS), but were unresolvable in a mixture, whereas the enantiomers studied did not exhibit statistically different CCS values. A mathematical relationship relating the CCS to resolving power was developed in order to predict the required ion mobility resolving power needed to separate the various isomer classes. For the majority of isomers evaluated in this study, a uniform field-based resolving power of 100 was predicted to be sufficient to resolve over half (∼60%) of all hypothetical isomer pairs, including leucine and isoleucine, whereas their stereoisomers (d- and l-forms) are predicted to be significantly more challenging, if not impossible, to separate by conventional drift tube techniques.

Dibenzanthracenes and benzochrysenes elicit both genotoxic and nongenotoxic events in rat liver ‘stem-like’ cells

Polycyclic aromatic hydrocarbons (PAHs) with molecular weight 278 are a group of PAHs that are mostly not covered by the current monitoring programs, despite their relative abundance in environmental samples and possible carcinogenicity. Although benzo[g]chrysene (BgChry) and dibenz[a,h]anthracene (DBahA) have been for a long time studied as genotoxic, tumour-initiating compounds, little is known about the potential tumour-promoting effects of this group of PAHs. In the present study, we investigated their impact on activation of the aryl hydrocarbon receptor (AhR), induction of enzymes involved in metabolic activation of PAHs, disruption of cell cycle control in confluent cell population and inhibition of gap junctional intercellular communication (GJIC), using the rat liver epithelial cell line WB-F344 as a model of liver progenitor cells. We found that BgChry was the weakest inducer of the AhR-mediated activity, while relative potencies of benzo[b]chrysene (BbChry) and benzo[c]chrysene (BcChry) were comparable to the previously reported values for dibenzanthracenes. All compounds increased expression of cytochromes P450 1A1 and 1B1, and aldo-keto reductase 1C9. BgChry was found to induce high amounts of DNA adducts, which corresponded with induction of p53 phosphorylation at Ser15, apoptosis and accumulation of cells in S-phase of cell cycle, leading to a decrease in cell numbers. All other compounds were found to stimulate cell proliferation in contact-inhibited WB-F344 cells in a dose-dependent manner. We found that only BgChry, and to a lesser extent also BcChry, inhibited GJIC at high concentrations. Taken together, dibenzanthracenes and benzochrysenes, with exception of BgChry, seem to act primarily through deregulation of cell proliferation in liver epithelial cells, which is related to their relatively high AhR-mediated activity. The disruption of cell cycle control might contribute to their carcinogenic effects, as well as to carcinogenicity of complex environmental mixtures containing high levels of PAHs with molecular weight 278.

ATSDR evaluation of health effects of chemicals. IV. Polycyclic aromatic hydrocarbons (PAHs): understanding a complex problem

Polycyclic Aromatic Hydrocarbons (PAHs) are a group of chemicals that are formed during the incomplete burning of coal, oil, gas, wood, garbage, or other organic substances, such as tobacco and charbroiled meat. There are more than 100 PAHs. PAHs generally occur as complex mixtures (for example, as part of products such as soot), not as single compounds. PAHs are found throughout the environment in the air, water, and soil. As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals, including PAHs (ATSDR, 1995), found at facilities on the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) and which pose the most significant potential threat to human health, as determined by ATSDR and the Environmental Protection Agency (EPA). These profiles include information on health effects of chemicals from different routes and durations of exposure, their potential for exposure, regulations and advisories, and the adequacy of the existing database. Assessing the health effects of PAHs is a major challenge because environmental exposures to these chemicals are usually to complex mixtures of PAHs with other chemicals. The biological consequences of human exposure to mixtures of PAHs depend on the toxicity, carcinogenic and noncarcinogenic, of the individual components of the mixture, the types of interactions among them, and confounding factors that are not thoroughly understood. Also identified are components of exposure and health effects research needed on PAHs that will allow estimation of realistic human health risks posed by exposures to PAHs. The exposure assessment component of research should focus on (1) development of reliable analytical methods for the determination of bioavailable PAHs following ingestion, (2) estimation of bioavailable PAHs from environmental media, particularly the determination of particle-bound PAHs, (3) data on ambient levels of PAHs metabolites in tissues/fluids of control populations, and (4) the need for a critical evaluation of current levels of PAHs found in environmental media including data from hazardous waste sites. The health effects component should focus on obtaining information on (1) the health effects of mixtures of PAHs particularly their noncarcinogenic effects in humans, and (2) their toxicokinetics. This report provides excerpts from the toxicological profile of PAHs (ATSDR, 1995) that contains more detailed information.

Genotoxic potential of beta-carbolines: a review

The mutagenic and co-mutagenic properties of harman, norharman and of some of their pharmacologically important derivatives are reviewed. These compounds do not behave as true mutagens, but rather interact, directly or indirectly with DNA, leading to various consequences. This unusual behaviour is most probably related to the particular structure of the chemical nucleus common to all beta-carbolines which confers to the different derivatives the property to interact with various macromolecules and enzymatic systems. These interactions are compiled and discussed in this review. The alterations, by beta-carbolines, of some important enzymatic systems, e.g. cytochrome P-450, have been clearly demonstrated, yet many discrepancies and contradictions exist so that an interpretation of the results and the definition of some common mechanism appears premature. Since beta-carbolines are widely distributed in tissues and since they may modify and increase genotoxic and toxic consequences of other compounds, these interactions need to be clarified.

HPLC separation of 32P-postlabelled DNA adducts formed from dibenz[a,h]anthracene in skin

Mouse skin and human skin have been treated in vivo or in short-term organ culture with dibenz[a,h]anthracene (DB[a,h]A), the related 3,4- or 5,6-diols or the anti- or syn-3,4-diol 1,2-oxides. DNA hydrolysates have been 32P-postlabelled and the adducts present examined by HPLC using a phenyl-modified reverse phase column and, for comparison, by PEI-cellulose TLC and autoradiography. The adducts formed when the diol-epoxides were reacted with salmon sperm DNA were also examined. The results show that in mouse skin treated in vivo, the major adducts formed from DB[a,h]A and the 3,4-diol were the same and that two of them were more polar than those formed in skin or in DNA that had been treated with the related anti- or syn-diol epoxides. Human skin treated with DB[a,h]A in culture yielded an adduct profile that was qualitatively similar to the profiles obtained with mouse skin.

Enhancing effects of harman and norharman on induction of preneoplastic and neoplastic kidney lesions in rats initiated with N-ethyl-N-hydroxyethylnitrosamine

The modifying potential of two nephrotoxic agents, harman and norharman, on N-ethyl-N-hydroxyethylnitrosamine (EHEN)-induced renal and hepatic carcinogenesis was investigated in male F344/DuCrj rats. Animals were given 0.1% EHEN in their drinking water for the first 2 weeks as an initiator. Subsequently, starting 3 weeks from the commencement, they were fed diet containing these compounds at concentrations of 1000, 500 or 0 ppm until week 26, and then killed for light microscopic examination. The mean numbers of renal tubular cell hyperplasias/cm2 and those of tumors/cm2 in rats given harman and norharman at 1000 ppm after initiation, but not at 500 ppm, were significantly increased as compared to the control values. However, neither compound modified liver carcinogenesis. It is concluded that harman and norharman show enhancing effects on rat kidney carcinogenesis, when ingested at dose levels which cause renal tubular damage.

HPLC separation of 32P-postlabelled benzo[b]fluoranthene-DNA adducts

Analysis using 32P-postlabelling and a recently developed HPLC method resolved the adduct formed by reaction of the benzo[b]fluoranthene (BbF) anti-bay-region diol-epoxide with DNA from the more polar major adduct produced by the hydrocarbon in three different biological systems. In each case, the adduct formed from the anti-bay-region diol-epoxide constituted only a minor proportion of the total DNA modification. Comparisons of the DNA adducts formed from the hydrocarbon with those formed in microsomal incubations from the putative metabolites BbF-9,10-diol, anti-BbF-9,10-diol-11,12-oxide and the 5,9,10- and 6,9,10-BbF-triols indicate that the predominant pathway for BbF activation in skin probably involves a bay-region triol-epoxide possessing a phenolic OH-group on the peninsula ring.

Dose-dependent renal tubular toxicity of harman and norharman in male F344 rats

The renal toxicity of harman and norharman, administered for 2 or 4 weeks at dietary levels of 1,000, 500, or 0 parts per million (ppm), was investigated in 6-week-old male F344/DuCrj rats. Although rats fed 1,000 ppm harman or norharman, but not the 500 ppm level, demonstrated marked body weight retardation from 1 week to termination, no mortalities occurred. Marked elevation of water consumption was evident in rats given harman or norharman at 1,000 ppm, but not at 500 ppm, together with large increases in urine of low specific gravity. Urinary lysosomal enzymes (N-acetyl-beta-D-glucosaminidase, NAG, and lactate dehydrogenase, LDH) and sugar levels were increased, and the brush border enzymes (gamma-glutamyl transpeptidase, GGT, and alkaline phosphatase, ALP) decreased. Furthermore, serum biochemistry revealed clear elevation of parameters indicating renal toxicity in these rats. Histopathologically, rats fed 1,000 ppm harman or norharman, but not 500 ppm, demonstrated focal toxic renal degenerative/necrotic and regenerative lesions in proximal, distal, and collecting tubules. These changes were associated with a clearly increased labeling index (LI) of the nuclei of renal tubular epithelial cells on immunohistochemical staining for 5-bromo-2'-deoxyuridine (BrdU). Chemical specific crystal formation within tubular lumina was evident in rats fed 1,000 ppm, but not 500 ppm, this being considered the cause of the renal tubular lesions. It was concluded that harman and norharman exert renal toxicity at the dietary level of 1,000 ppm, but not 500 ppm, in male F344 rats.

Analysis of point mutations in murine c-Ha-ras of skin tumors initiated with dibenz[a,j]anthracene and derivatives

This study was designed to evaluate the point mutations in the murine c-Ha-ras gene of skin papillomas induced by initiation with dibenz[a,j]anthracene (DB[a,j]A), its bay-region anti-diol epoxide ((+/-)anti-DB[a,j]A-DE), and a 7,14-dimethyl analogue (7,14-diMeDB[a,j]A). Recent studies (Nair RV, et al., Chem Res Toxicol 4:115-122, 1991) in our laboratory have revealed both deoxyguanosine (dGuo) and deoxyadenosine (dAdo) adducts formed from the anti- and syn-diol epoxides of DB[a,j]A in cultured mouse epidermal cells after exposure to this hydrocarbon. Using PCR amplification and direct sequencing, we found specific A182----T transversion mutations (eight of 10 tumors) in codon 61 of c-Ha-ras in papillomas induced by initiation with DB[a,j]A. Analysis of papillomas generated by initiation with the more biologically potent analogue 7,14-diMeDB[a,j]A revealed that five of five tumors exhibited A182----T transversions in codon 61. The nature of the changes in the two DB[a,j]A tumors not showing codon 61 mutations in Ha-ras is currently not known since these tumor DNAs also did not possess c-Ha-ras mutations at codons 12, 13, or 59. Interestingly, papillomas produced by initiation with (+/-)anti-DB[a,j]A-DE also possessed A182----T transversion mutations in codon 61 of c-Ha-ras (five of five tumors). These data suggest that dAdo adducts derived from both parent hydrocarbons may play an important role in their tumor-initiating activity and possibly implicate a specific diol epoxide-dAdo adduct in this process.

Microsomal metabolism of dibenz[a,c]anthracene, dibenz[a,h]anthracene and dibenz[a,j]anthracene to bis-dihydrodiols and polyhydroxylated products

Polar, ethyl acetate soluble metabolites formed in incubations of dibenz[a,c]anthracene (DB[a,c]A), dibenz[a,h]anthracene (DB[a,h]A) and the related DB[a,h]A 3,4-diol and dibenz[a,j]anthracene (DB[a,j]A) with 3-methylcholanthrene (3-MC)-induced rat liver microsomal preparations have been separated by HPLC and examined using fluorescence, UV and NMR spectroscopy. Metabolites with spectral properties consistant with their identification as the 3,4:8,9-bis-diol of DB[a,j]A and a 1,2,3,4,12,13-hexol derived from DB[a,c]A were found. DB[a,h]A was metabolized to three polar products identified as the 3,4:10,11-bis-diol and the related 1,2,3,4,8,9- and 1,2,3,4,10,11-hexols, which were also formed, together with the related 1,2,3,4-tetrol, from the DB[a,h]A 3,4-diol. The possible role of bis-diols in the metabolic activation of these three dibenzanthracenes is discussed.

Mutagenicity of dibenz[a,c]anthracene and its derivatives in Salmonella typhimurium TA100

The mutagenic activities of dibenz[a,c]anthracene (DB[a,c]A), and its 11 derivatives, including 3 diols, 6 phenols and 2 oxepines, were studied in the TA100 strain of Salmonella typhimurium at doses varying from 0 to 20 micrograms/plate in the presence of a rat-liver S9 (9000 x g) preparation. Among the diols of DB[a,c]A tested DB[a,c]A-10,11-diol was the most mutagenic compound. However, it was consistently less mutagenic than the parent hydrocarbon. Oxepine-1 and oxepine-2 which are believed to be the photoisomerized products of DB[a,c]A-1,2 oxide and DB[a,c]A-3,4-oxide, respectively, were also less mutagenic than DB[a,c]A. In contrast to these results, 4-hydroxyDB[a,c]A was almost twice as active as DB[a,c]A, and 2-hydroxy- and 3-hydroxyDB[a,c]A were even more (4-6-fold) mutagenic than DB[a,c]A. The remaining phenols were relatively inactive or weakly active in this mutagenicity assay. These results provide initial evidence that the bay-region theory may not be applicable to the mutagenesis of DB[a,c]A, and that the angular ring substituted phenols of DB[a,c]A may be involved in the metabolic activation of this highly mutagenic hydrocarbon.

The metabolism of the carcinogen dibenz[a,j]acridine in isolated rat hepatocytes and in vivo in rats

1. 3H-Dibenz[a,j]acridine (DBAJAC) metabolism occurred readily in vitro in incubations with hepatocytes from phenobarbital-pretreated, 3-methylcholanthrene-pretreated and untreated rats, with the formation of water-soluble conjugates and unconjugated metabolites. 2. For incubations of 3H-DBAJAC with hepatocytes the major organic solvent-soluble metabolites found with and without beta-glucuronidase/arylsulphatase hydrolysis were the phenols, 3-hydroxy-DBAJAC, and 4-hydroxy-DBAJAC, and the proposed proximate carcinogen, trans-3,4-dihydroxy-3,4-dihydro-DBAJAC. The latter comprised 34-66% of the total organic solvent-soluble metabolites. 3. In contrast to results previously reported for rat hepatic microsomes, the K-region 5,6-oxide, and its dihydrodiol were minor metabolites detected after hepatocyte incubations. 4. Faecal excretion accounted for the bulk of radioactivity after i.p. doses of 3H-DBAJAC (0.5 mg/kg), and i.v. doses (0.5 mg/kg) were rapidly excreted into the 6 h bile. The organic solvent-soluble fraction obtained after enzymic hydrolysis of bile (approximately 25% of excreted radioactivity) was subjected to h.p.l.c. It contained polar secondary oxidation products and virtually no 3,4-dihydrodiol. 5. Experiments conducted with greater hepatocyte densities (10(7) cells/ml) and longer incubation times showed increased extents of metabolism, DNA and protein binding of radioactivity which paralleled the extent of metabolism. Very considerable metabolism of the 3,4-dihydrodiol occurred by the end of the incubation period.

Lack of hepatotoxicity or promotion of enzyme-altered liver foci development in rats treated with harman or norharman

The modifying effects of harman or norharman on liver carcinogenesis were investigated in male F344/DuCrj rats initially treated with N-nitrosodiethylamine (DEN). Two weeks after a single dose of DEN (200 mg/kg, intraperitoneally), rats were given harman or norharman at dietary levels of 1000 and 200 parts per million (ppm), or sodium phenobarbital (PB) at 500 ppm as a positive control, for 6 wk. At wk 3 following DEN administration, all animals were subjected to partial hepatectomy. Marked retardation of body weight gain was observed in rats treated with harman or norharman at 1000 ppm, but not at 200 ppm. Increased relative kidney but not liver weights were associated with harman or norharman treatment, especially in the higher dose groups. Although no toxicity-related hepatocyte lesions were found, severe renal toxic tubular lesions and regeneration were evident. Neither harman nor norharman significantly increased the numbers or areas of glutathione S-transferase placental form (GST-P) positive foci observed after DEN initiation, in clear contrast to PB. The results thus demonstrated that harman and norharman are nontoxic for the liver and lack modifying potential for liver carcinogenesis in our medium-term bioassay system.