Identification of metabolites of benzo[b]fluoranthene

Dose-related Mutagenic and Clastogenic Effects of Benzo[b]fluoranthene in Mouse Somatic Tissues Detected by Duplex Sequencing and the Micronucleus Assay

Polycyclic aromatic hydrocarbons (PAHs) are common environmental pollutants that originate from the incomplete combustion of organic materials. We investigated the clastogenicity and mutagenicity of benzo[ b ]fluoranthene (BbF), one of 16 priority PAHs, in MutaMouse males after a 28-day oral exposure. BbF causes robust dose-dependent increases in micronucleus frequency in peripheral blood, indicative of chromosome damage. Duplex Sequencing (DS), an error-corrected sequencing technology, reveals that BbF induces dose-dependent increases in mutation frequencies in bone marrow (BM) and liver. Mutagenicity is increased in intergenic relative to genic regions, suggesting a role for transcription-coupled repair of BbF-induced DNA damage. At higher doses, the maximum mutagenic response to BbF is higher in liver, which has a lower mitotic index but higher metabolic capacity than BM; however, mutagenic potency is comparable between the two tissues. BbF induces primarily C:G>A:T mutations, followed by C:G>T:A and C:G>G:C, indicating that BbF metabolites mainly target guanines and cytosines. The mutation spectrum of BbF correlates with cancer mutational signatures associated with tobacco exposure, supporting its contribution to the carcinogenicity of combustion-derived PAHs in humans. Overall, BbF's mutagenic effects are similar to benzo[ a ]pyrene, a well-studied mutagenic PAH. Our work showcases the utility of DS for effective mutagenicity assessment of environmental pollutants.

Synopsis

We used Duplex Sequencing to study the mutagenicity of benzo[ b ]fluoranthene across the mouse genome. Dose-dependent changes in mutation frequency and spectrum quantify its role in PAH-induced carcinogenicity.

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.

Inhibition of cis-diamminedichloroplatinum (II)-induced DNA interstrand cross-link removal by 7-ethyl-10-hydroxy-camptothecin in HST-1 human squamous-carcinoma cells

The combination of cis-diamminedichloroplatinum(II) (CDDP) and 7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (CPT-11), a topoisomerase-I inhibitor, has been shown to be synergistic in vitro and clinically active against several human cancers refractory to chemotherapy. To elucidate the mechanism of the synergistic cytotoxicity of CDDP and 7-ethyl-10-hydroxycamptothecin (SN-38), an active metabolite of CPT-11, we studied the interaction of these agents using an HST-1 human squamous-carcinoma cell line. Cells were exposed to the IC50 concentration of SN-38 (5.0 ng/ml) for 1 hr and various concentrations of CDDP for 1 hr in several different treatment schedules. SN-38 augmented the anti-tumor activity of CDDP in all schedules, with maximal synergy observed with simultaneous administration. Evaluation of the kinetics of the removal of DNA interstrand cross-links, measured by alkaline elution, showed significant reduction of this removal in the cells exposed to SN-38 and CDDP, as compared with the cells exposed to CDDP alone. No differences, however, were found in the initially attained level of DNA interstrand cross-links induced by CDDP between these cells. Moreover, the intracellular accumulation of platinum measured by atomic-absorption spectrophotometry, was virtually identical between these cells. These results indicate that SN-38 can modulate the removal of platinum-DNA adducts, thereby potentiating the cytotoxicity of CDDP, suggesting a critical role for topoisomerase I in the repair of DNA interstrand cross-links.

Evaluation of the tumor-initiating activity of 4-, 5-, 6-, and 7-fluorobenzo[b]fluoranthene in mouse skin

Benzo[b]fluoranthene and 4-, 5-, 6-, and 7-fluorobenzo[b]fluoranthene were evaluated for tumor-initiating activity in mouse skin. These fluorinated benzo[b]fluoranthene derivatives were assayed at doses of 400, 120, 40, and 10 nmol per mouse. Similar tumorigenic activity was observed for benzo[b]fluoranthene and 5-fluorobenzo[b]fluoranthene. While 4-fluorobenzo[b]fluoranthene did produce a significant tumorigenic response at each dose assayed, substantially fewer tumors per mouse were observed compared to benzo[b]fluoranthene at initiator doses at or above 120 nmol. Only moderate tumorigenic activity was observed for 6- and 7-fluorobenzo[b]fluoranthene. Both of these fluorinated derivatives were significantly less tumorigenic (P < 0.05) than 4-fluorobenzo[b]fluoranthene when administered at initiator doses at or below 120 nmol. These results were unanticipated in view of data which indicate that metabolism of trans-9,10-dihydro-9,10-dihydroxybenzo[b]-fluoranthene to trans-9,10-dihydro-5,9,10-trihydroxybenzo[b]fluoranthene represents a principal activation mechanism of benzo[b]fluoranthene in mouse skin. The potential of fluorine substitution not only to inhibit metabolism, but also to alter the genotoxic activity of those metabolites which do form could explain the tumorigenic activity observed with these fluorinated derivatives of benzo[b]fluoranthene. These data suggest caution in the interpretation of results based exclusively upon the assumption that the only influence of fluorine substitution is inhibition of the formation of specific metabolites.

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.

Relative tumor initiating activity of benzo[a]fluoranthene, benzo[b]fluoranthene, naphtho[1,2-b]fluoranthene and naphtho[2,1-a]fluoranthene on mouse skin

The tumor-initiating activities of benzo[a]fluoranthene (BaF), benzo[b]fluoranthene (BbF), naphtho[1,2-b]fluoranthene (NbF) and naphtho[2,1-a]fluoranthene (NaF) were evaluated on the skin of female CD-1 mice. Each of these polycyclic aromatic hydrocarbons was assayed at total initiation doses of 1.0 and 4.0 mumol/mouse. These hydrocarbons were applied in 10 subdoses administered every other day. Promotion commenced 10 days after the last initiator dose and consisted of thrice weekly application of 2.5 micrograms of tetradecanoylphorbol acetate for 20 weeks. BbF was the most potent tumor initiator inducing a 100% incidence of tumor-bearing mice with an average of 8.5 tumors per mouse at a total initiator dose of 1.0 mumol. NaF was slightly more active as a tumor initiator than either NbF or BaF. NaF induced a 90% incidence of tumor-bearing mice with an average of 5.9 tumors per mouse at a total initiator dose of 1.0 mumol. BaF and NbF at a total initiator dose of 4.0 mumol exhibited similar tumor-initiating activity with both inducing a 90% incidence of tumor-bearing mice with an average of 4.3 and 6.6 tumors per mouse, respectively. However, at a total initiator dose of 1.0 mumol, BaF and NbF induced a 95% and 65% incidence of tumor-bearing mice with an average of 3.3 and 2.5 tumors per mouse, respectively.

Effects of fluorine substitution on the DNA binding and tumorigenicity of benzo[b]fluoranthene in mouse epidermis

The effects of fluorine substitution on benzo[b]fluoranthene (B[b]F) DNA adduct formation and tumorigenicity in mouse epidermis were investigated. Fluoro derivatives studied included 1-, 6-, 7-, 8-, 9- and 11-fluoroB[b]F as well as 1,9- and 6,9-difluoroB[b]F. Each compound was applied topically to mice and hydrocarbon/DNA adduct formation was assessed using the 32P-post-labelling technique. All of the fluorinated compounds bound to DNA to a lesser extent than B[b]F. Among the fluorinated compounds, the greatest binding was observed for 8-fluoroB[b]F. Lowest levels of hydrocarbon/DNA adduct formation from the fluoro derivatives were observed for 1-, 7-, 11- and 6,9-difluoroB[b]F. The tumor-initiating activities on mouse skin of 7-, 9- and 11-fluoroB[b]F were determined. All three compounds were significantly less tumorigenic than B[b]F. The results of this study are discussed with respect to possible mechanisms of metabolic activation of B[b]F.

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action

Polychlorinated and polybrominated biphenyls are industrial chemical mixtures which have been implicated in numerous human poisonings in Taiwan and Japan (PCBs) and Michigan (PBBs). Moreover, these polyhalogenated biphenyls have been widely detected in the environment including the air, water, fish, wildlife, human adipose tissue, and blood and breast milk. A major problem associated with the analysis and toxicology of this group of chemicals is their chemical complexity (e.g., there are 209 possible PCB isomers and congeners) and the remarkable effects of structure on activity. This article will discuss the effects of structure on the biologic and toxic effects of individual PCB and PBB congeners as well as reconstituted mixtures. The results clearly show that like "dioxin" (or 2,3,7,8-TCDD), the PCBs and PBBs elicit their effects through a cytosolic receptor protein which preferentially binds with the toxins which are approximate isostereomers of 2,3,7,8-TCDD. The evidence for this mechanism of action will be discussed in detail.

Benzofluoranthenes: calculated diol epoxide reactivities

The benzo- and dibenzofluoranthenes, widespread environmental contaminants, have been subjects of continued interest. Calculations of the reactivity of potential ultimate carcinogenic metabolites of the benzofluoranthenes predict unexpected behavior for these non-alternant hydrocarbons.