Role of cytochrome P4501A1 in biotransformation of a tissue specific sarcomagen N-methyldibenzo[c,g]carbazole

Mechanism of 7H-Dibenzo[c,g]carbazole metabolism in cytochrome P450 1A1: Insights from computational studies

7H-Dibenzo[c,g]carbazole (DBC) is a prevalent environmental contaminant that induces tumorigenesis in several experimental animals. Recently, it has been utilized to develop high-performance solar cells and organic phosphorescent materials. It is imperative to strengthen investigations of DBC metabolism to understand its potential risks to human health. In this study, human CYP1A1 was employed as the metabolic enzyme to investigate the metabolic mechanism of DBC by molecular docking, molecular dynamics (MD) simulation, and quantum mechanical (QM) calculation. The results indicate that DBC binds to CYP1A1 in two modes (mode 1 and mode 2) mainly through nonpolar solvation energies (ΔGnonpolar). The formation of the two binding modes is attributed to the anchoring effect of the hydrogen bond formed by DBC with Asp320 (mode 1) or Ser116 (mode 2). Mode 1 is a "reactive" conformation, while mode 2 is not considered a "reactive" conformation. C5 is identified as the dominant site, and the pyrrole nitrogen cannot participate in the metabolism. DBC is metabolized mainly by a distinct electrophilic addition-rearrangement mechanism, with an energy barrier of 21.74 kcal/mol. The results provide meaningful insights into the biometabolic process of DBC and contribute to understanding its environmental effects and health risks.

7H-Dibenzo[c,g]carbazole: Metabolic pathways and toxicity

7H-Dibenzo[c,g]carbazole (DBC), a local and systemic carcinogen in animal studies, is a common environmental pollutant. It generally co-occurs in a variety of organic complex mixtures derived from incomplete combustion of organic matter. Despite high lipophilicity, DBC is more water-soluble and faster metabolized than the homocyclic aromatics. Moreover, greater polarity, high bioaccumulation potential, and persistence in the environment may imply DBC's higher biological significance and impact on human health, even at lower concentrations. The biotransformation pathways of DBC are incompletely known and the ultimate carcinogenic metabolite(s) are not clearly identified as yet. Structure-biological studies suggest two ways of activation: at the ring carbon atoms and at the pyrrole nitrogen. It is supposed that the particular pathway of biotransformation might be connected with the tissue/organ specificity of DBC. Cytochrome P450 (CYP) family of enzymes plays a pivotal role in the metabolism of DBC; though, the one-electron activation and the aldo-keto reductase-catalyzed oxidation are also involved in metabolic activation. Additionally, DBC can be photoactivated even at physiologically relevant doses of UVA light due to the extended aromatic ring system resulting in strong genotoxicity and oxidative stress. The goal of this review is to summarize current knowledge on mechanisms of DBC activation and possible implications for toxicity, genotoxicity, and carcinogenicity.

Ultraviolet A radiation potentiates the cytotoxic and genotoxic effects of 7 H-dibenzo[c,g]carbazole and its methyl derivatives

7H-Dibenzo[c,g]carbazole (DBC) is a heterocyclic aromatic hydrocarbon that is carcinogenic in many species and tissues. DBC is a common environmental pollutant, and is therefore constantly exposed to sunlight. However, there are limited data exploring the toxicity of DBC photoexcitation products. Here, we investigated the impact of ultraviolet (UV) A radiation on the biological activity of DBC and its methyl derivatives, 5,9-dibenzo[c,g]carbazole and N-methyl dibenzo[c,g]carbazole, on human skin HaCaT keratinocytes. Co-exposure of HaCaT cells to UVA and DBC derivatives resulted in a sharp dose-dependent decrease in cell survival and apparent changes in cell morphology. Under the same treatment conditions, significant increases in DNA strand breaks, intracellular reactive oxygen species, and oxidative damage to DNA were observed in HaCaT cells. Consistent with these results, an apparent inhibition in superoxide dismutase, but not glutathione peroxidase activity, was detected in cells treated with DBC and its derivatives under UVA irradiation. The photoactivation-induced toxicity of individual DBC derivatives correlated with the electron excitation energies approximately expressed as the energy difference between the highest occupied and the lowest vacant molecular orbital. Our data provide the first evidence that UVA can enhance the toxicity of DBC and its derivatives. Photoactivation-induced conversion of harmless chemical compounds to toxic photoproducts associated with reactive oxygen species generation may substantially amplify the adverse health effects of UVA radiation and contribute to increased incidence of skin cancer.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Genotoxicity of 7H-dibenzo[c,g]carbazole and its methyl derivatives in human keratinocytes

Differences between tissues in the expression of drug-metabolizing enzymes may substantially contribute to tissue-specificity of chemical carcinogens. To verify this hypothesis, the spontaneously immortalized human keratinocytes HaCaT were used, in order to evaluate the genotoxic potential of 7H-dibenzo[c,g]carbazole (DBC), a known hepatocarcinogen and sarcomagen, and its synthetic tissue-specific derivatives, 5,9-dimethyl-DBC (DiMeDBC) and N-methyl-DBC (N-MeDBC), which manifest specific tropism to the liver and skin, respectively. HaCaT cells mainly express cytochrome P4501A1 (CYP1A1), which is involved in metabolism of DBC and N-MeDBC, but not DiMeDBC [10]. Both DBC and the sarcomagen N-MeDBC induced significant levels of DNA strand-breaks, micronuclei, and DNA adducts followed by the phosphorylation of the p53 protein and histone H2AX in HaCaT cells. In contrast, the specific hepatocarcinogen DiMeDBC was devoid of any significant genotoxic activity in this cell line. Our study demonstrates that the absence of drug-metabolizing enzyme(s) involved in DiMeDBC metabolism may contribute substantially to the tissue-specific genotoxicity of this hepatocarcinogen.

Genotoxicity of 7H-dibenzo[c,g]carbazole and its tissue-specific derivatives in human hepatoma HepG2 cells is related to CYP1A1/1A2 expression

The goal of this study was to investigate the genotoxicity of 7H-dibenzo[c,g]carbazole (DBC), a ubiquitous environmental pollutant, and its methyl derivatives, 5,9-dimethylDBC (DiMeDBC), a strict hepatocarcinogen, and N-methylDBC (N-MeDBC), a specific sarcomagen in human hepatoma HepG2 cells, and to infer potential mechanisms underlying the biological activity of particular carcinogen. All dibenzocarbazoles, regardless the tissue specificity, induced significant DNA strand break levels and micronuclei in HepG2 cells; though a mitotic spindle dysfunction rather than a chromosome breakage was implicated in N-MeDBC-mediated micronucleus formation. While DBC and N-MeDBC produced stable DNA adducts followed with p53 protein phosphorylation at Ser-15, DiMeDBC failed. A significant increase in DNA strand breaks following incubation of exposed cells with a repair-specific endonuclease (Fpg protein) suggested that either oxidative DNA damage or unstable DNA-adducts might underlie DiMeDBC genotoxicity in human hepatoma cells. DiMeDBC and N-MeDBC increased substantially also the amount of CYP1A1/2 expression in HepG2 cells. Pretreatment of cells with substances affecting AhR-mediated CYP1A family of enzymes expression; however, diminished DiMeDBC and N-MeDBC genotoxicity. Our data clearly demonstrated differences in the mechanisms involved in the biological activity of DiMeDBC and N-MeDBC in human hepatoma cells; the genotoxicity of these DBC derivatives is closely related to CYP1A1/2 expression.

Differences in DNA damage and repair produced by systemic, hepatocarcinogenic and sarcomagenic dibenzocarbazole derivatives in a model of rat liver progenitor cells

Liver progenitor (oval) cells are a potential target cell population for hepatocarcinogens. Our recent study showed that the liver carcinogens 7H-dibenzo[c,g]carbazole (DBC) and 5,9-dimethyldibenzo[c,g]carbazole (DiMeDBC), but not the sarcomagen N-methyldibenzo[c,g]carbazole (N-MeDBC), induced several cellular events associated with tumor promotion in WB-F344 cells, an in vitro model of liver oval cells [J. Vondracek, L. Svihalkova-Sindlerova, K. Pencikova, P. Krcmar, Z. Andrysik, K. Chramostova, S. Marvanova, Z. Valovicova, A. Kozubik, A. Gabelova, M. Machala, 7H-Dibenzo[c,g]carbazole and 5,9-dimethyldibenzo[c,g]carbazole exert multiple toxic events contributing to tumor promotion in rat liver epithelial 'stem-like' cells, Mutat. Res. Fundam. Mol. Mech. Mutagen. 596 (2006) 43-56]. In this study, we focused on the genotoxic effects generated by these dibenzocarbazoles in WB-F344 cells to better understand the cellular and molecular mechanisms involved in hepatocarcinogenesis. Lower IC(50) values determined for DBC and DiMeDBC, as compared with N-MeDBC, indicated a higher sensitivity of WB-F344 cells towards hepatocarcinogens. Accordingly, DBC produced a dose-dependent DNA-adduct formation resulting in substantial inhibition of DNA replication and transcription. In contrast, DNA-adduct number detected in DiMeDBC-exposed cells was almost negligible, whereas N-MeDBC produced a low level of DNA adducts. Although all dibenzocarbazoles significantly increased the level of strand breaks (p<0.05) and micronuclei (p<0.001) after 2-h treatment, differences in the kinetics of strand break rejoining were found. The strand break level in DiMeDBC- and N-MeDBC-exposed cells returned to near the background level within 24h after treatment, whereas a relatively high DNA damage level was detected in DBC-treated cells up to 48h after exposure. Additional breaks detected after incubation of DiMeDBC-exposed WB-F344 cells with a repair-specific endonuclease, along with a nearly 3-fold higher level of reactive oxygen species found in these cells as compared with control, suggest a possible role of oxidative stress in DiMeDBC genotoxicity. We demonstrated qualitative differences in the DNA damage profiles produced by hepatocarcinogens DBC and DiMeDBC in WB-F344 cells. Different lesions may trigger distinct cellular pathways involved in hepatocarcinogenesis. The low amount of DNA damage, together with an efficient repair, may explain the lack of hepatocarcinogenicity of N-MeDBC.

7H-Dibenzo[c,g]carbazole and 5,9-dimethyldibenzo[c,g]carbazole exert multiple toxic events contributing to tumor promotion in rat liver epithelial 'stem-like' cells

Immature liver progenitor cells have been suggested to be an important target of hepatotoxins and hepatocarcinogens. The goal of the present study was to assess the impact of 7H-dibenzo[c,g]carbazole (DBC) and its tissue-specific carcinogenic N-methyl (N-MeDBC) and 5,9-dimethyl (DiMeDBC) derivatives on rat liver epithelial WB-F344 cells, in vitro model of liver progenitor cells. We investigated the cellular events associated with both tumor initiation and promotion, such as activation of aryl hydrocarbon receptor (AhR), changes in expression of enzymes involved in metabolic activation of DBC and its derivatives, effects on cell cycle, cell proliferation/apoptosis and inhibition of gap junctional intercellular communication (GJIC). N-MeDBC, a tissue-specific sarcomagen, was only a weak inhibitor of GJIC or inducer of AhR-mediated activity, and it did not affect either cell proliferation or apoptosis. DBC was efficient GJIC inhibitor, while DiMeDBC manifested the strongest AhR inducing activity. Accordingly, DiMeDBC was also the most potent inducer of cytochrome P450 1A1 (CYP1A1) and CYP1A2 expression among the three compounds tested. Both DBC and DiMeDBC induced expression of CYP1B1 and aldo-keto reductase 1C9 (AKR1C9). N-MeDBC failed to significantly upregulate CYP1A1/2 and it only moderately increased CYP1B1 or AKR1C9. Only the potent liver carcinogens, DBC and DiMeDBC, caused a significant increase of p53 phosphorylation at Ser15, an increased accumulation of cells in S-phase and apoptosis at micromolar concentrations. In addition, DiMeDBC was found to stimulate cell proliferation of contact-inhibited WB-F344 cells at 1 microM concentration, which is a mode of action that might further contribute to its hepatocarcinogenicity. The present data seem to suggest that the AhR activation, induction of enzymes involved in metabolic activation, inhibition of GJIC or stimulation of cell proliferation might all contribute to the hepatocarcinogenic effects of DBC and DiMeDBC.

DNA adduct formation by 7H-dibenzo[c,g]carbazole and its tissue- and organ-specific derivatives in Chinese hamster V79 cell lines stably expressing cytochrome P450 enzymes

The cytochrome P4501A subfamily (CYP1A) is involved in the metabolic activation of 7H-dibenzo[c,g]carbazole (DBC) and its tissue- and organ-specific derivatives, N-methyldibenzo[c,g]carbazole (MeDBC)and 5,9-dimethyldibenzo[c,g]carbazole (diMeDBC). In this study, we have evaluated the relationship between the tissue specificity and (32)P-postlabeled adduct patterns produced by these compounds by using a panel of Chinese hamster V79 cell lines stably expressing human CYP1A1 and CYP1A2 and/or N-acetyltransferase. Treatment of the parental cell lines V79MZ and V79NH, which are devoid of any CYP activity, with DBC and its derivatives did not result in detectable adducts. The highest DNA adduct levels were found in CYP1A1-expressing V79MZh1A1 cells after DBC and MeDBC treatment (24.5 +/- 7.2 and 16.2 +/- 3.6 adducts/10(8) nucleotides, respectively). Exposure of this cell line to DBC resulted in five distinct spots, while six spots with different chromatographic mobilities were detected in MeDBC-treated cells. DiMeDBC produced only very low levels of DNA adducts in V79MZh1A1 cells. DBC and MeDBC formed relatively low levels of DNA adducts in CYP1A2-expressing V79MZh1A2 cells (0.7 +/- 0.2 and 2.1 +/- 1.2 adducts/10(8) nucleotides, respectively). DBC formed three weak spots and MeDBC five spots in V79MZh1A2 cells, and all the spots had different chromatographic mobilities. In contrast, diMeDBC did not induce any DNA adducts in these cells, although diMeDBC induced a significant dose-dependent increase in micronucleus frequency under similar treatment conditions (r = 0.76; P < 0.001). The significant increase in DNA damage in the Comet assay following incubation of exposed cells with a repair-specific endonuclease (Fpg protein) suggests that base modifications such as 8-oxodG or Fapy-adducts might be responsible for the genotoxicity of diMeDBC in V79MZh1A2 cells. The similarities between the DNA adduct patterns produced by DBC and MeDBC in V79MZh1A1 and V79MZh1A2 cells suggest that biotransformation mediated via CYP1A1 and CYP1A2 might depend on a PAH-type pathway involving the aromatic ring system.

In vitro toxicity of 7H-dibenzo[c,g]carbazole in human liver cell lines

7H-Dibenzo[c,g]carbazole (DBC) is a model N-heterocyclic aromatic compound (NHA) which is both a hepatotoxin and hepatocarcinogen in rodents. The focus of this investigation was to determine whether human liver cell lines display differential sensitivities to DBC-induced toxicity. Treatment of cell lines with increasing DBC concentrations produced apoptosis only in HepG2 cells. Although DBC inhibited the clonogenic growth of all cell lines at high concentrations, only the survival of HepG2 cells was reduced at lower concentrations. DBC inhibited DNA synthesis in two (HepG2, HLF) of the three cell lines at lower concentrations and was effective only at a high concentration in Mahlavu cells. Differences in DBC uptake were not observed in any of the cell lines, suggesting that bioavailability was not a limiting factor. DBC-DNA adducts were not detected in HLF or Mahlavu cells at either low or high concentrations of DBC. Consistent with the DNA adduct data, RP-HPLC analysis indicated that DBC was metabolized to a lesser degree in the HLF and Mahlavu cells. These results suggest that human liver cell lines differ markedly in the ability to metabolize DBC to toxic species and that DBC-induced apoptosis is only observed in cells that produce detectable metabolites and DBC-DNA adducts.

Mutagenicity of 7H-dibenzo[c,g]carbazole and its tissue specific derivatives in genetically engineered Chinese hamster V79 cell lines stably expressing cytochrome P450

Genetically engineered Chinese hamster V79 cell lines with stable expression of human cytochrome P4501A1 and 1A2 were used to characterize the particular form of P450 enzymes capable of activating 7H-dibenzo[c,g]carbazole (DBC) and its tissue- and organ-specific derivatives, N-methylDBC (N-MeDBC) and 5,9-dimethylDBC (diMeDBC). In addition, a V79 cell line with co-expression of CYP1A2 together with a phase II enzyme, N-acetyltransferase was utilized to study the role of an entire metabolic activation system in biotransformation of these carbazoles. The rise of 6-thioguanine resistant (6-TG(r)) mutations was followed as a marker of biological activity of these agents. None of the carbazoles elevated significantly the frequency of mutations in the parental V79MZ cell line lacking any cytochrome P450 (CYP) activity or in the V79NH cells expressing N-acetyltransferase activity. A variable, however, increase of mutations was found in the cell lines expressing CYP activity. Both DBC, a potent liver and skin carcinogen, and N-MeDBC, a specific sarcomagen, increased significantly (P<0.001) the frequency of 6-TG(r) mutations in V79MZh1A1 cells, expressing the human CYP1A1; in contrast, a strict hepatocarcinogen diMeDBC was devoid of any activity. All carbazoles elevated significantly the level of mutations in the V79MZh1A2 cell line expressing the human CYP1A2, N-MeDBC was most efficient. Co-expression of CYP1A2 together with NAT activity significantly reduced or totally eliminated the mutagenicity of all carbazoles. These data confirm that CYP1A1 is explicitly involved in the activation of sarcomagenic DBC derivatives, whereas CYP1A2 is included in biotransformation of all DBC derivatives. Reactive intermediates formed due to CYP1A2 activation are substrate for conjugation reactions mediated by N-acetyltransferase.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.